Articles de revues : « Hf-Nd-Pb isotopic compositions »

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Vezinet, Adrien, Emilie Thomassot, Yan Luo, Chiranjeeb Sarkar et D. Graham Pearson. « Diachronous Redistribution of Hf and Nd Isotopes at the Crystal Scale—Consequences for the Isotopic Evolution of a Poly-Metamorphic Crustal Terrane ». Geosciences 12, n^o 1 (12 janvier 2022) : 36. http://dx.doi.org/10.3390/geosciences12010036.

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In metamorphic rocks, mineral species react over a range of pressure–temperature conditions that do not necessarily overlap. Mineral equilibration can occur at varied points along the metamorphic pressure–temperature (PT) path, and thus at different times. The sole or dominant use of zircon isotopic compositions to constrain the evolution of metamorphic rocks might then inadvertently skew geological interpretations towards one aspect or one moment of a rock’s history. Here, we present in-situ U–Pb/Sm–Nd isotope analyses of the apatite crystals extracted from two meta-igneous rocks exposed in the Saglek Block (North Atlantic craton, Canada), an Archean metamorphic terrane, with the aim of examining the various signatures and events that they record. The data are combined with published U–Pb/Hf/O isotope compositions of zircon extracted from the same hand-specimens. We found an offset of nearly ca. 1.5 Gyr between U-Pb ages derived from the oldest zircon cores and apatite U–Pb/Sm–Nd isotopic ages, and an offset of ca. 200 Ma between the youngest zircon metamorphic overgrowths and apatite. These differences in metamorphic ages recorded by zircon and apatite mean that the redistribution of Hf isotopes (largely hosted in zircon) and Nd isotopes (largely hosted in apatite within these rocks), were not synchronous at the hand-specimen scale (≤~0.001 m3). We propose that the diachronous redistribution of Hf and Nd isotopes and their parent isotopes was caused by the different PT conditions of growth equilibration between zircon and apatite during metamorphism. These findings document the latest metamorphic evolution of the Saglek Block, highlighting the role played by intra-crustal reworking during the late-Archean regional metamorphic event.

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Tong, Jie, Haibo Zou, Zipei Guo, Liwen Chang, Lizhu Wang et Yongwei Zhao. « Geochronology and Origin of Quaternary Dacites from the Daliuchong Volcano in the Tengchong Volcanic Field (TVF), SE Tibetan Plateau ». Minerals 14, n^o 10 (30 septembre 2024) : 990. http://dx.doi.org/10.3390/min14100990.

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Quaternary volcanoes from the southeastern Tibetan Plateau occur at the Tengchong volcanic field (TVF). The Daliuchong volcano is the largest volcano in the TVF, which has the most felsic compositions with explosive eruptions. The eruption history and origin of the Daliuchong volcano are a matter of debate. In the present paper, we report the groundmass K-Ar ages, whole-rock Sr-Nd-Pb-Hf isotopes, zircon U-Pb ages, and Hf-O isotopic compositions for the Daliuchong volcano to constrain its eruption history and petrogenesis. The groundmass K-Ar ages and zircon U-Pb ages indicate mid-Pleistocene (0.6 Ma to 0.3 Ma) eruptions. The presence of zircon phenocrysts with enriched mantle-like O-Hf isotopes (δ18O < 6‰, and εHf about −2) suggests the involvement of mantle-derived basaltic magmas. The whole-rock Pb isotope compositions and Sr-Nd isotope modeling reveal the involvement of magma from the lower crust. The zircon xenocrysts reveal previously unrecognized 20-Ma magmatic activity at the TVF and contamination of late Cretaceous (66–80 Ma) S-type granites during the formation of the Daliuchong dacites. The dacite magma at Daliuchong was formed by mixing of the mantle-derived magma and lower-crust-derived magma and subsequently contaminated by upper crustal materials, including late Cretaceous S-type granitic rocks.

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Linghu, Miaomiao, Zimu Li, Jinfeng Sun et Jiheng Zhang. « Magma Source and Petrogenesis of the Early Cretaceous Granites in The Liaodong Peninsula : Evidence from In Situ Apatite Sr-Nd and Zircon Hf-O Isotopes ». Minerals 13, n^o 4 (12 avril 2023) : 545. http://dx.doi.org/10.3390/min13040545.

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Apatite Sr-Nd and zircon Hf-O isotopes are broadly used to trace magma sources and constrain magma evolution processes, further improving our understanding of the origin of granitoids. We present zircon U-Pb ages, whole-rock major and trace elements, and whole-rock Sr-Nd-Hf, zircon Hf-O, and apatite Sr-Nd isotopic data for the coarse-grained quartz monzonite, biotite monzogranite, and granite porphyry in the Yushulinzi pluton in the Liaodong Peninsula, the eastern North China Craton, to establish their magma sources and petrogenesis. The coarse-grained quartz monzonite, biotite monzogranite, and granite porphyry were formed contemporaneously, with zircon U-Pb ages of 123–119 Ma. They share enriched whole-rock Sr-Nd-Hf and zircon Hf isotopic compositions, and the coarse-grained quartz monzonite has crust-like δ18O values (5.7–6.7‰). The coarse-grained quartz monzonite and biotite monzogranite have variable apatite (87Sr/86Sr)i ratios and negative apatite εNd(t) values. These isotopic characteristics indicate that the different rock types in the Yushulinzi pluton were derived from the partial melting of ancient crustal material in the North China Craton. Their geochemical and petrographic characteristics indicate that the crystal-melt segregation model can be employed to elucidate the genetic links among different rock types, with the coarse-grained quartz monzonite representing crystal accumulation and the biotite monzogranite and granite porphyry representing interstitial melts extracted from a crystal-rich magma chamber. Furthermore, the variable apatite Sr isotopic compositions and subtle differences in the peak zircon εHf(t) values of the studied rock samples confirm the possibility of a contribution from shallow crustal components and materials with high εHf(t) values during magma evolution, which is not readily revealed by their whole-rock Sr-Nd-Hf isotopic compositions. These results demonstrate that in situ apatite Sr-Nd and zircon Hf-O isotopic analyses have the potential to provide distinctive insights into the magma sources and evolution of magmatic systems.

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Roulleau, Emilie, et Ross Stevenson. « Geochemical and isotopic (Nd–Sr–Hf–Pb) evidence for a lithospheric mantle source in the formation of the alkaline Monteregian Province (Quebec) ». Canadian Journal of Earth Sciences 50, n^o 6 (juin 2013) : 650–66. http://dx.doi.org/10.1139/cjes-2012-0145.

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We present new major element and isotopic (Nd–Sr–Hf–Pb) data and modelling from alkaline rocks of the Monteregian Igneous Province of southern Quebec (Canada) that constrain the mantle source and the magmatic origin of these rocks. The whole-rock chemical composition of the intrusions is consistent with fractional crystallization of an assemblage of olivine ± clinopyroxene (± plagioclase) derived from ocean island basalts (OIB)-like magmas, and variations in the Sr and Nd isotope compositions suggest as much as 20% crustal contamination. The bulk of the Nd–Sr–Hf and Pb isotopic data form a tight cluster between a depleted mantle end-member (HIMU, high-U/Pb mantle) and an enriched mantle (EMI) end-member and are thought to reflect a sub-continental lithospheric mantle that was metasomatized by a convecting asthenospheric plume. Variations in these isotopic compositions along the west–east axis of the Monteregian Province (from the Oka carbonatite to the Mount Shefford intrusion) may reflect various degrees of mixing between HIMU and EMI enriched mantle reservoirs. Anomalously low 207Pb/204Pb and 208Pb/204Pb isotopic ratios from some of the intrusions likely indicate incorporation of an Archean component within the lithospheric mantle. We propose a model in which Monteregian magmatism formed from melting of a predominantly Proterozoic metasomatized lithospheric mantle in response to lithospheric extension during the opening of the North Atlantic Ocean at ca. 124 Ma.

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Bogina, Maria, Boris Belyatsky, Evgenii Sharkov, Alexey Chistyakov et Robert Krymsky. « Origin of the Middle Paleoproterozoic Tiksheozero Ultramafic-Alkaline-Carbonatite Complex, NE Fennoscandian Shield : Evidence from Geochemical and Isotope Sr-Nd-Hf-Pb-Os Data ». Minerals 11, n^o 6 (27 mai 2021) : 570. http://dx.doi.org/10.3390/min11060570.

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This article reports new geochemical, Sr-Nd-Hf-Pb and Re-Os data on the rocks of the Middle Paleoproterozoic (1.99 Ga) Tiksheozero ultramafic-alkaline-carbonatite complex confined to the northeastern margin of the Karelian Craton. We focus on the poorly studied silicate rocks. Based on petrographic and geochemical research, the silicate rocks are subdivided into two groups: an ultramafic-mafic series depleted in REE, and other incompatible elements and an alkaline series enriched in these elements. Isotope studies showed that all rocks have juvenile isotope signatures and were likely derived from a primitive OIB-type mantle source with possible contributions of the subcontinental lithospheric mantle (SCLM). Insignificant crustal contamination is recorded by Pb and Os isotopic compositions. The incompatible element enrichment in the alkaline rocks and depletion in ultramafic-mafic rocks of the mildly alkaline series with allowance for insignificant crustal contamination confirm their derivation from different primary melts. However, a narrow range of Sr, Nd, Hf, and Pb isotope compositions and compact clusters in 207Pb/204Pb-206Pb/204Pb, Nd-87Sr/86Sr and Hf-Nd isotope diagrams indicate their origination from a common mantle source. A model of subsequent two-stage melting is being most consistent with the geochemical data for this complex.

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Bell, K., A. N. Zaitsev, J. Spratt, S. Fröjdö et A. S. Rukhlov. « Elemental, lead and sulfur isotopic compositions of galena from Kola carbonatites, Russia – implications for melt and mantle evolution ». Mineralogical Magazine 79, n^o 2 (avril 2015) : 219–41. http://dx.doi.org/10.1180/minmag.2015.079.2.01.

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AbstractGalena from four REE-rich (Khibina, Sallanlatvi, Seblyavr, Vuoriyarvi) and REE-poor (Kovdor) carbonatites, as well as hydrothermal veins (Khibina) all from the Devonian Kola Alkaline Province of northwestern Russia was analysed for trace elements and Pb and S isotope compositions. Microprobe analyses show that the only detectable elements in galena are Bi and Ag and these vary from not detectable to 2.23 and not detectable to 0.43 wt.% respectively. Three distinct galena groups can be recognized using Bi and Ag contents, which differ from groupings based on Pb isotope data. The Pb isotope ratios show significant spread with 206Pb/204Pb ratios (16.79 to 18.99), 207Pb/204Pb (15.22 to 15.58) and 208Pb/204Pb ratios (36.75 to 38.62). A near-linear array in a 207Pb/204Pb vs.206Pb/204Pb ratio diagram is consistent with mixing between distinct mantle sources, one of which formed during a major differentiation event in the late Archaean or earlier. The S isotopic composition (δ34S) of galena from carbonatites is significantly lighter (–6.7 to –10.3% Canyon Diablo Troilite (CDT) from REE-rich Khibina, Seblyavr and Vuoriyarvi carbonatites, and – 3.2% CDT from REE-poor Kovdor carbonatites) than the mantle value of 0%. Although there is no correlation between S and any of the Pb isotope ratios, Bi and Ag abundances correlate negatively with δ34S values. The variations in the isotopic composition of Pb are attributed to partial melting of an isotopically heterogeneous mantle source, while those of δ34S (together with Bi and Ag abundances) are considered to be process driven. Although variation in Pb isotope values between complexes might reflect different degrees of interaction between carbonatitic melts and continental crust or metasomatized lithosphere, the published noble gas and C, O, Sr, Nd and Hf isotopic data suggest that the variable Pb isotope ratios are best attributed to isotopic differences preserved within a sub-lithospheric mantle source. Different Pb isotopic compositions of galena from the same complex are consistent with a model of magma replenishment by carbonatitic melts/fluids each marked by quite different Pb isotopic compositions.

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CINTRON FRANQUI, NADJA OMARA, SUNG HI CHOI et DER-CHUEN LEE. « Peridotites and basaltic rocks within an ophiolitic mélange from the SW igneous province of Puerto Rico : relation to the evolution of the Caribbean Plate ». Geological Magazine 154, n^o 1 (2 février 2016) : 96–118. http://dx.doi.org/10.1017/s001675681500093x.

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AbstractThe geology of Puerto Rico is divided into three regions: the north, central and SW igneous provinces. Characterized by its Jurassic ophiolitic mélange basement, lithology of the SW Igneous Province (SIP) is not related to either of the other two provinces. The ophiolitic mélange is exposed in three peridotite belts: Monte del Estado, Rio Guanajibo and Sierra Bermeja. We present geochemical data to identify the tectonic setting of the SIP peridotite formation and its relation to the evolution of the Caribbean Plate. Comparisons of spinel Cr no. (13–21), Mg no. (63.3–69.6) and TiO2suggest an abyssal peridotite origin; however, only Sierra Bermeja presents high TiO2characteristics of a mid-ocean-ridge-basalt- (MORB-) like melt reaction. Temperatures determined with two-pyroxene geothermometers indicated a cold thermal regime ofc. 800–1050°C, with characteristics of large-offset transform fault abyssal peridotites. The geochemistry and Sr–Nd–Hf–Pb isotopic compositions of basalts within the mélange were also analysed. Las Palmas amphibolites exhibited normal-MORB-like rare earth element (REE) and trace-element patterns, whereas metabasalts and Lower Cajul basalts exhibited island-arc tholeiitic-like patterns. Highly radiogenic Sr isotopes (0.70339–0.70562) of the basalts suggest seawater alteration; however, Pb–Pb and Nd–Hf isotope correlations represent the primary compositions of a Pacific/Atlantic MORB source for the amphibolites, metabasalts and Lower Cajul basalts. We propose that the SIP ophiolitic mélange was formed along a large-offset transform fault, which initiated subduction and preserved both proto-Pacific and proto-Caribbean lithospheric mantle. Younger Upper Cajul basalts exhibited enriched-MORB-like geochemical and isotopic signatures, which can be attributed to a tectonized Caribbean ocean plateau.

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Xu, Wang-Chun, Hong-Fei Zhang, Li-Ran Chen, Bi-Ji Luo, Liang Guo et Jing-Liang Guo. « Transition from the lithospheric to asthenospheric mantle-derived magmatism in the Early Jurassic along eastern Bangong–Nujiang Suture, Tibet : Evidence for continental arc extension induced by slab rollback ». GSA Bulletin 133, n^o 1-2 (2 juin 2020) : 134–48. http://dx.doi.org/10.1130/b35554.1.

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Abstract The transition of the geochemical signature in mafic rocks along the eastern Bangong–Nujiang suture in Tibet contains important information about geodynamic processes in the upper mantle. This study recognized two episodes of Early Jurassic gabbros from the Kaqiong terrane, a microblock within the Bangong–Nujiang suture zone. Early gabbros (ca. 197–191 Ma) appear as lenses in the basement complex and were overprinted by amphibolite/granulite-facies metamorphism at ca. 180 Ma. Later undeformed hornblende gabbros (ca. 177–175 Ma) occur as dikes intruding into the basement complex. The early metagabbros are characterized by arc-like geochemical features and enriched Nd-Hf isotopic compositions (whole rock ∑Nd(t) = –0.7 to +0.3; zircon ∑Hf(t) = –5.7 to –2.2), which suggests formation by partial melting of an enriched lithospheric mantle source. In contrast, the later hornblende gabbros have depleted Nd-Hf isotopic compositions (whole rock ∑Nd(t) = +6.1 to +7.1; zircon ∑Hf(t) = +10.7 to +16.8) and normal mid–oceanic–ridge basalt (N–MORB)-type rare earth element (REE) features. They also show variable enrichments of fluid mobile elements (e.g., Rb, U, Pb), indicative of the input of slab-derived fluids in their mantle source. Thus, the hornblende gabbros were most likely originated from the asthenospheric mantle metasomatized by subducted oceanic slab-derived fluids. The transition in geochemical and isotopic compositions of these mantle-derived magmas reveals a long-lasting lithosphere extension and thinning along the southern margin of the Qiangtang terrane in the Early Jurassic. Combined with geological observations, we propose that this transition has resulted from the southward rollback of the subducting Bangong–Nujiang Tethyan oceanic slab. The slab rollback could have initiated the overriding plate extension and the asthenosphere upwelling. Wider implications of this study are that an onset of slab rollback could be an important trigger for the transition of magmatic geochemistry in subduction zones.

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Garçon, Marion, Catherine Chauvel, Christian France-Lanord, Mara Limonta et Eduardo Garzanti. « Which minerals control the Nd–Hf–Sr–Pb isotopic compositions of river sediments ? » Chemical Geology 364 (janvier 2014) : 42–55. http://dx.doi.org/10.1016/j.chemgeo.2013.11.018.

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Xu, Wen-Liang, Jia-Hui Chen, Ai-Hua Weng, Jie Tang, Feng Wang, Chun-Guang Wang, Peng Guo, Yi-Ni Wang, Hao Yang et Andrey A. Sorokin. « Stagnant slab front within the mantle transition zone controls the formation of Cenozoic intracontinental high-Mg andesites in northeast Asia ». Geology 49, n^o 1 (25 août 2020) : 19–24. http://dx.doi.org/10.1130/g47917.1.

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Abstract The geochemistry of Cenozoic intracontinental high-Mg andesites (HMAs) in northeast Asia, together with regional geophysical data, offers an opportunity to explore the genetic relationship between the formation of intracontinental HMAs and subduction of the Pacific plate. Compared with primary HMAs in arcs, Cenozoic intracontinental HMAs in northeast Asia have lower Mg# [100 × Mg/(Mg + Fe2+)] values (53–56) and CaO contents (5.8–6.6 wt%), higher alkali (Na2O + K2O) contents (5.15–6.45 wt%), and enriched Sr-Nd-Hf isotopic compositions (87Sr/86Sr = 0.7056–0.7059; εNd = −4.9 to −3.4; εHf = −4.7 to −2.6) as well as lower Pb isotope ratios (206Pb/204Pb = 16.76–19.19; 207Pb/204Pb = 15.42–15.45; 208Pb/204Pb = 36.71–37.11). These Cenozoic intracontinental HMAs are similar to Cenozoic potassic basalts in northeast China with respect to their Sr-Nd-Pb-Hf isotopic compositions but have higher SiO2 and Al2O3 contents and lower K2O, MgO, and light rare earth element contents. These features indicate that these Cenozoic intracontinental HMAs originated from the mantle, where recycled ancient sediments and water contributed to partial melting of peridotite. Combined with the presence of a large low-resistivity anomaly derived from the mantle transition zone (MTZ) near these intracontinental HMAs, and their occurrence above the stagnant slab front within the MTZ (at 600 km depth) in northeast Asia, we conclude that the stagnant slab front, with high contents of recycled ancient sediments and water, has controlled the formation of Cenozoic intracontinental HMAs in northeast Asia.

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Jweda, Jason, Louise Bolge, Cornelia Class et Steven L. Goldstein. « High Precision Sr-Nd-Hf-Pb Isotopic Compositions of USGS Reference Material BCR-2 ». Geostandards and Geoanalytical Research 40, n^o 1 (19 mars 2015) : 101–15. http://dx.doi.org/10.1111/j.1751-908x.2015.00342.x.

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Wang, Jing, Jun He, Jingxin Zhao, Yizeng Yang et Fukun Chen. « Multiple-Stage Neoproterozoic Magmatism Recorded in the Zhangbaling Uplift of the Northeastern Yangtze Block : Evidence from Zircon Ages and Geochemistry ». Minerals 13, n^o 4 (17 avril 2023) : 562. http://dx.doi.org/10.3390/min13040562.

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The Yangtze Block records Neoproterozoic magmatism and sedimentation related to the breakup of Rodinia and is an important piece in the reconstruction of the supercontinent. However, the tectonic setting and position of this block in Rodinia remain a subject of debate. In the present study, we report the zircon U-Pb ages and Hf isotopic composition of zircon and geochemical and Nd-Pb isotopic compositions for meta-volcanic rocks exposed in the Zhangbaling uplift of the NE Yangtze Block. The volcanic rocks, dominated by rhyolite and dacite, belong to the calc-alkaline series and show geochemical characteristics of arc rocks. Zircon U-Pb isotopic ages show that volcanic rocks in the Xileng Formation formed at ca. 790 Ma and ca. 760–700 Ma peaking at ~740 Ma. The late-stage volcanism was widely exposed in the uplift, characterized by a temporal-spatial trend becoming younger southwards. The old volcanic rocks have low initial εNd (−11.0) and εHf (−19.7 to −8.2) values and low Pb isotopic ratios, likely indicating an origin from ancient basement rocks underneath the Yangtze Block. The younger ones, being similar to continental arc andesite in trace element compositions, have relatively high initial εNd (mostly −4.6 to 0.5) and εHf (−0.4 to 8.8) values and high Pb isotopic ratios. These isotopic features point to an origin from the partial melting of juvenile crustal rocks. Sedimentary rocks of the Xileng Formation and the overlying strata also contain numerous zircon grains of ~700 Ma to ~630 Ma. The volcanic rocks in the Zhangbaling uplift might demonstrate long-lasting subduction along the northeastern margin of the Yangtze Block, probably active until ca. 700 Ma.

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Zhong, Yun, Xu Zhang, Zhilei Sun, Jinnan Liu, Wei Li, Yaoliang Ma, Weiliang Liu, Bin Xia et Yao Guan. « Sr–Nd–Pb–Hf Isotopic Constraints on the Mantle Heterogeneities beneath the South Mid-Atlantic Ridge at 18–21°S ». Minerals 10, n^o 11 (13 novembre 2020) : 1010. http://dx.doi.org/10.3390/min10111010.

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In an attempt to investigate the nature and origin of mantle heterogeneities beneath the South Mid-Atlantic Ridge (SMAR), we report new whole-rock Sr, Nd, Pb, and Hf isotopic data from eight basalt samples at four dredge stations along the SMAR between 18°S and 21°S. Sr, Nd, and Pb isotopic data from SMAR mid-ocean ridge basalts (MORBs) at 18–21°S published by other researchers were also utilized in this study. The SMAR MORBs at 18–21°S feature the following ratio ranges: 87Sr/86Sr = 0.70212 to 0.70410, 143Nd/144Nd = 0.512893 to 0.513177, 206Pb/204Pb = 18.05 to 19.50, 207Pb/204Pb = 15.47 to 15.71, 208Pb/204Pb = 37.87 to 38.64, and 176Hf/177Hf = 0.283001 to 0.283175. The 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, and 176Hf/177Hf ratios of these MORBs varied considerably along the SMAR axis. The variable compositions of the Sr–Nd–Pb–Hf isotopes, combined with the corresponding whole-rock major and trace elemental abundances reported in previous studies, suggest that the SMAR MORBs at 18–21°S were probably derived from a heterogeneous mantle substrate related to a mixture of depleted mantle (DM) materials with a small amount (but variable input) of HIMU (high-μ, where μ = 238U/204Pb)- and enriched (EMII)-type materials. The HIMU-type materials likely originated from the proximal St. Helena plume and may have been transported through “pipe-like inclined sublithospheric channels” into the SMAR axial zone. The EMII-type materials possibly originated from a recycled metasomatized oceanic crust that may have been derived from the early dispersion of other plume heads into the subcontinental asthenosphere prior to the opening of the South Atlantic Ocean. In addition, the contributions of subducted sediments, continental crust, and subcontinental lithospheric mantle components to the formation of the SMAR MORBs at 18–21°S may be nonexistent or negligible.

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Filipov, Petyo. « Mafic adakite-like magmatism at the Cretaceous–Paleogene boundary from the Northwest Rhodopes, Bulgaria ». Review of the Bulgarian Geological Society 84, n^o 3 (décembre 2023) : 81–84. http://dx.doi.org/10.52215/rev.bgs.2023.84.3.81.

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New findings of monzogabbro and gabbro-monzodiorite dykes aged 62 Ma reveal the existence of magmatism in the gap between termination of the Late Cretaceous subduction-related and start of the early–middle Eocene post-collisional magmatic stages in the Rhodope Massif. Their geochemical compositions strongly resemble adakitic signature as well as hornblende and epidote geobarometers predicted lower crustal conditions (20–30 km depth) of crystallization. Detailed studies on U-Pb geochronology of zircon populations and Sr-Nd-Hf isotopic compositions point to significant assimilation of the local basement and mixing of mantle-crust sources in the regime of a thickened crust.

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Wang, Xiao-Jun, Li-Hui Chen, Albrecht W. Hofmann, Takeshi Hanyu, Hiroshi Kawabata, Yuan Zhong, Lie-Wen Xie et al. « Recycled ancient ghost carbonate in the Pitcairn mantle plume ». Proceedings of the National Academy of Sciences 115, n^o 35 (13 août 2018) : 8682–87. http://dx.doi.org/10.1073/pnas.1719570115.

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The extreme Sr, Nd, Hf, and Pb isotopic compositions found in Pitcairn Island basalts have been labeled enriched mantle 1 (EM1), characterizing them as one of the isotopic mantle end members. The EM1 origin has been vigorously debated for over 25 years, with interpretations ranging from delaminated subcontinental lithosphere, to recycled lower continental crust, to recycled oceanic crust carrying ancient pelagic sediments, all of which may potentially generate the requisite radiogenic isotopic composition. Here we find that δ26Mg ratios in Pitcairn EM1 basalts are significantly lower than in normal mantle and are the lowest values so far recorded in oceanic basalts. A global survey of Mg isotopic compositions of potentially recycled components shows that marine carbonates constitute the most common and typical reservoir invariably characterized by extremely low δ26Mg values. We therefore infer that the subnormal δ26Mg of the Pitcairn EM1 component originates from subducted marine carbonates. This, combined with previously published evidence showing exceptionally unradiogenic Pb as well as sulfur isotopes affected by mass-independent fractionation, suggests that the Pitcairn EM1 component is most likely derived from late Archean subducted carbonate-bearing sediments. However, the low Ca/Al ratios of Pitcairn lavas are inconsistent with experimental evidence showing high Ca/Al ratios in melts derived from carbonate-bearing mantle sources. We suggest that carbonate–silicate reactions in the late Archean subducted sediments exhausted the carbonates, but the isotopically light magnesium of the carbonate was incorporated in the silicates, which then entered the lower mantle and ultimately became the Pitcairn plume source.

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Tan, Jun, Jun-Hao Wei, Shao-Qing Zhao, Yan-Jun Li, Yan Liu, Xiao-Yang Liu, Fei Zhang, Jin-Rong Gan et Zhi-Hua Wang. « Petrogenesis of Late Triassic high-Mg diorites and associated granitoids with implications for Paleo-Tethys evolution in the northeast Tibetan Plateau ». GSA Bulletin 132, n^o 5-6 (17 septembre 2019) : 955–76. http://dx.doi.org/10.1130/b35225.1.

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Abstract Recent research on Paleo-Tethys tectonics has identified a huge late Paleozoic to Mesozoic igneous belt that extends more than 2500 km in the northeast Tibetan Plateau. However, the magma genesis and evolution in this belt remains a subject of considerable debate. This paper presents a combination of zircon U-Pb ages, mineral compositions, major and trace element concentrations, and Sr-Nd-Hf isotopic data for the plutons across the Zhiduo arc belt that marks the site connecting different tectonic-magmatic units. The studied rocks from one quartz diorite, two granodiorite plutons, and their mafic enclaves define a continuous compositional evolution varying from high- to medium-K calc-alkaline gabbroic diorite to granodiorite. Laser ablation–inductively coupled plasma–mass spectroscopy U-Pb analyses of zircons from these three plutonic suites and one mafic enclave yield Late Triassic ages of 222–217 Ma, establishing that the mafic and felsic magmas were nearly coeval. All these rocks are featured by zoned hornblende and plagioclase with Mg- and Ca-rich mantles or oscillatory change in compositions. They exhibit high and variable MgO (up to 4.88–5.66 wt%), Cr, and Ni contents except that one granitoid pluton (Dangjiangrong) possesses high Co (up to 145.0 ppm). They are characterized by subduction-type trace element patterns, with prominent positive Rb, Th, Pb, and K anomalies and negative Ba, Nb, P, and Ti. Together with continuous and heterogeneous Sr-, Nd-, and zircon Hf-isotopic compositions, it suggests that these Late Triassic high-Mg diorites and associated granitoids were generated through magma mixing and fractional crystallization accompanied by chemical exchange. Taking into account the magmatic record from nearby regions, we suggest that double-sided subduction and rollback of the subducting Paleo-Tethys oceanic slab is the main mechanism to generate geochemically-varied magmatism in the northeast Tibetan Plateau, and eventually close the Paleo-Tethys Ocean during much of the Late Triassic.

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Hopkinson, Thomas, Nigel Harris, Nick M. W. Roberts, Clare J. Warren, Sam Hammond, Christopher J. Spencer et Randall R. Parrish. « Evolution of the melt source during protracted crustal anatexis : An example from the Bhutan Himalaya ». Geology 48, n^o 1 (19 novembre 2019) : 87–91. http://dx.doi.org/10.1130/g47078.1.

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Abstract The chemical compositions of magmatic zircon growth zones provide powerful insight into evolving magma compositions due to their ability to record both time and the local chemical environment. In situ U-Pb and Hf isotope analyses of zircon rims from Oligocene–Miocene leucogranites of the Bhutan Himalaya reveal, for the first time, an evolution in melt composition between 32 and 12 Ma. The data indicate a uniform melt source from 32 Ma to 17 Ma, and the progressive addition of an older source component to the melt from at least ca. 17 Ma. Age-corrected ɛHf ratios decrease from between −10 and −15 down to values as low as −23 by 12 Ma. Complementary whole-rock Nd isotope data corroborate the Hf data, with a progressive decrease in ɛNd(t) from ca. 18 to 12 Ma. Published zircon and whole-rock Nd data from different lithotectonic units in the Himalaya suggest a chemical distinction between the younger Greater Himalayan Series (GHS) and the older Lesser Himalayan Series (LHS). The time-dependent isotopic evolution shown in the leucogranites demonstrates a progressive increase in melt contribution from older lithologies, suggestive of increasing LHS involvement in Himalayan melting over time. The time-resolved data are consistent with LHS material being progressively accreted to the base of the GHS from ca. 17 Ma, facilitated by deformation along the Main Central thrust. From 17 Ma, decompression, which had triggered anatexis in the GHS since the Paleogene, enabled melting in older sources from the accreted LHS, now forming the lowermost hanging wall of the thrust.

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Tao, Lu, Fa-Bin Pan, Rong Liu, Chong Jin, Bao-Jian Jia et Xiaobo He. « Petrogenesis of the Cretaceous granitoids in Zhejiang, northeast South China Block and their implications for episodic retreat and roll-back of the Paleo-Pacific Plate ». GSA Bulletin 132, n^o 7-8 (20 novembre 2019) : 1514–36. http://dx.doi.org/10.1130/b35426.1.

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Abstract Two Cretaceous granitoid belts (i.e., the northwest and southeast belts) have been identified in Zhejiang, northeast South China Block. In this study, seven granitoid plutons from both the two belts were collected for zircon U-Pb dating, whole-rock geochemistry, Sr-Nd isotope, and zircon Hf isotope analyses. Chronologically, the Longyou (132 Ma), Sucun (136 Ma), Shanghekou (131 Ma), and Huangshitan (ca. 126 Ma) plutons from the northwest belt display older magma crystallization age than those of the Xiaoxiong (100 Ma), Zhujiajian (108 Ma), and Qingbang island (108 Ma) plutons from the southeast belt. The Sucun quartz monzonite and the Longyou, Shanghekou, Zhujiajian, and Qingbang island granites therein are fractionated I-type granites (i.e., partial melting of meta-igneous rocks) with relatively moderate-low Zr saturation temperature (723–823 °C) and pronouncedly evolved Nd and Hf isotopic compositions (εNd(t) = –8.17 to –5.67 and εHf(t) = –15.07 to –5.67), indicating that they are derivatives of ancient crustal melt-dominated magmas. The Huangshitan granite shows A-type granitic (i.e., granites that are alkaline and anhydrous and from anorogenic setting) features with high Ga/Al (3.47–5.58), rare earth element (REE) content (271–402 ppm), and Zr saturation temperature (781–889 °C). It holds less enriched Nd and Hf isotopic compositions (εNd(t) = –4.13 to –3.60 and εHf(t) = –5.90 to –2.16) and is attributed to partial melting of mature crustal materials with minor basaltic magma incorporation. The Xiaoxiong (quartz) syenitic porphyry is characterized by moderate SiO2 content (60.68–69.92 wt%), high alkali (9.03–11.66 wt%) and REE contents with fractionated REE pattern [(La/Yb)N = 13.8–26.1]. Its relatively depleted Nd and Hf isotopic compositions (εNd(t) = –3.67 to –3.42 and εHf(t) = –5.76 to –2.25) imply that it could be a derivative of basaltic magma from K-rich metasomatized mantle. Available geochronological data indicate that there were two episodic magmatic pulses at ca. 140–120 Ma and ca. 110–85 Ma associated with the Paleo-Pacific Plate underthrusting beneath the northeast South China Block. Here we put forward an episodic slab retreat and roll-back model to account for generation of these magmatic rocks. Firstly, the subducting Paleo-Pacific slab roll-back initiated at ca. 140 Ma and reached climax at ca. 130–120 Ma, which led to formation of the Longyou, Sucun, and Shanghekou I-type granites and the Huangshitan A-type granite, respectively. Subsequently, a flat slab subduction stage occurred with eastward trench retreat, causing a period of magmatic quiescence from ca. 120 to 110 Ma. The following second slab roll-back started at ca. 110 Ma and reached climax at ca. 100 Ma, giving rise to the earlier Zhujiajian and Qingbang island I-type granites and the later Xiaoxiong (quartz) syenitic porphyry.

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Delavault, Hélène, Catherine Chauvel, Emilie Thomassot, Colin W. Devey et Baptiste Dazas. « Sulfur and lead isotopic evidence of relic Archean sediments in the Pitcairn mantle plume ». Proceedings of the National Academy of Sciences 113, n^o 46 (24 octobre 2016) : 12952–56. http://dx.doi.org/10.1073/pnas.1523805113.

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The isotopic diversity of oceanic island basalts (OIB) is usually attributed to the influence, in their sources, of ancient material recycled into the mantle, although the nature, age, and quantities of this material remain controversial. The unradiogenic Pb isotope signature of the enriched mantle I (EM I) source of basalts from, for example, Pitcairn or Walvis Ridge has been variously attributed to recycled pelagic sediments, lower continental crust, or recycled subcontinental lithosphere. Our study helps resolve this debate by showing that Pitcairn lavas contain sulfides whose sulfur isotopic compositions are affected by mass-independent fractionation (S-MIF down to Δ33S = −0.8), something which is thought to have occurred on Earth only before 2.45 Ga, constraining the youngest possible age of the EM I source component. With this independent age constraint and a Monte Carlo refinement modeling of lead isotopes, we place the likely Pitcairn source age at 2.5 Ga to 2.6 Ga. The Pb, Sr, Nd, and Hf isotopic mixing arrays show that the Archean EM I material was poor in trace elements, resembling Archean sediment. After subduction, this Archean sediment apparently remained stored in the deep Earth for billions of years before returning to the surface as Pitcairn´s characteristic EM I signature. The presence of negative S-MIF in the deep mantle may also help resolve the problem of an apparent deficit of negative Δ33S anomalies so far found in surface reservoirs.

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Yang, Hang, Peng Wu, Anlin Liu et Feng Wang. « Petrogenesis of the Eocene Highly Fractionated Granite Porphyry with REE Tetrad Effect : An Example from Western Yunnan, Southeastern Tibetan Plateau ». Minerals 13, n^o 11 (30 octobre 2023) : 1390. http://dx.doi.org/10.3390/min13111390.

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Highly fractionated granites are widely distributed in the crust and provide unique windows into magmatic evolution. This study reports petrography, zircon U–Pb ages, trace elemental, and Hf isotopic, as well as whole-rock elemental and Nd isotopic data of highly fractionated granite porphyries from the Shiguanshan area in western Yunnan, southeastern Tibet. The granite porphyries were formed at 34.0 ± 0.3 Ma in a post-collisional setting. They are strongly peraluminous (A/CNK = 1.95–2.80), have high SiO2 content (SiO2 = 78.16–79.13 wt.%) and zircon saturation temperatures (803–829 °C, average 819 °C), and low MgO, with pronounced enrichment in Pb, U, Th, and Rb, and depletion in Ti, Eu, P, Sr, and Ba, and belong to highly fractionated A-type granites. These rocks define linear trends on Harker diagrams and display similar enriched whole-rock Nd isotopic (εNd(t) = −12.8 to −12.3) and zircon Hf isotopic (εHf(t) = −10.4 to −8.8) compositions compared to the published data of coeval mantle-derived syenite porphyries, which can be attributed to fractional crystallization processes. A quantitative model suggests that the Shiguanshan granite porphyries likely formed through the fractionation process of a mineral assemblage consisting of plagioclase, K-feldspar, biotite, and amphibole (in a ratio of 40:30:25:5), with fractionation degrees of 50%–55%. The magmatic textures and zircons, decoupling between the REE tetrad effect and fractionation of twin-elements, along with the modeling result of Rayleigh fractionation, suggest that the REE tetrad effect in the Shiguanshan granite porphyries may be caused by fractionation of accessory minerals. Our data, along with regional observations, propose that the generation of these granite porphyries is possibly related to lithospheric removal following the Indo–Asia collision.

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Martin, Adam P., Alan F. Cooper, Richard C. Price, Philip R. Kyle et John A. Gamble. « Chapter 5.2b Erebus Volcanic Province : petrology ». Geological Society, London, Memoirs 55, n^o 1 (2021) : 447–89. http://dx.doi.org/10.1144/m55-2018-80.

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AbstractIgneous rocks of the Erebus Volcanic Province have been investigated for more than a century but many aspects of petrogenesis remain problematic. Current interpretations are assessed and summarized using a comprehensive dataset of previously published and new geochemical and geochronological data. Igneous rocks, ranging in age from 25 Ma to the present day, are mainly nepheline normative. Compositional variation is largely controlled by fractionation of olivine + clinopyroxene + magnetite/ilmenite + titanite ± kaersutite ± feldspar, with relatively undifferentiated melts being generated by <10% partial melting of a mixed spinel + garnet lherzolite source. Equilibration of radiogenic Sr, Nd, Pb and Hf is consistent with a high time-integrated HIMUsensu strictosource component and this is unlikely to be related to subduction of the palaeo-Pacific Plate around 0.5 Ga. Relatively undifferentiated whole-rock chemistry can be modelled to infer complex sources comprising depleted and enriched peridotite, HIMU, eclogite-like and carbonatite-like components. Spatial (west–east) variations in Sr, Nd and Pb isotopic compositions and Ba/Rb and Nb/Ta ratios can be interpreted to indicate increasing involvement of an eclogitic crustal component eastwards. Melting in the region is related to decompression, possibly from edge-driven mantle convection or a mantle plume.

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Fedorov, P. I., A. V. Koloskov, B. V. Beliatsky et L. B. Golovneva. « SPATIOTEMPORAL CHANGE OF DEEP SOURCES FOR CENOZOIC VOLCANIC ROCKS IN THE EASTERN KORYAK HIGHLANDS ». Tikhookeanskaya Geologiya 41 (2022) : 3–19. http://dx.doi.org/10.30911/0207-4028-2022-41-3-3-19.

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A spatiotemporal analysis of the trace element ratios and the isotopic composition of Sr, Nd and Pb in Cenozoic post-accretionary volcanic rocks in the east of the Koryak Highlands (Northeastern Russia) has been carried out. The Early Paleogene volcanic complex is shown to be represented by moderately Ti-rich aluminous tholeiites with elevated contents of high field strength elements (HFSE) (except for Ta and Nb), which makes them similar to E-MORBs. Low (La/Yb)n and high Zr/Nb (25–35) ratios characterize one of the source components as depleted and similar to MORBs. At the same time, low ratios of Nb/Ta, Ce/Pb, Nb/La with a high ratio of K/Nb were determined in basalts, which indicates the presence of a subduction component in the source. Low Zr/Hf and (Dy/Yb)n ratios indicate melting of the garnet-free substratum. The second (Miocene) stage is represented by the subalkaline basalt flows and dacite extrusions and dikes forming a bimodal series. Basalts are characterized by low concentrations of large ion lithophile elements (LILE), enrichment in HFSE, and fractionated distribution of REE, with (La/Yb)n ratios varying from the values characteristic of E-MORBs to the values corresponding to intraplate tholeiites. The Nb/Ta, Ce/Pb, La/Ta, and Hf/Th ratios tend to exhibit compositions of intraplate tholeiites. The (Dy/Yb)n and (La/Yb)pm - Ybpm ratios indicate the formation of Miocene basalts during the selective melting of garnet peridotite with varying garnet contents in the source. Dikes of dacites, in comparison with effusive facies, differ in calc-alkaline differentiation trends and other absolute concentrations of a number of elements. Early Quaternary alkaline olivine basalts and basanites of the Navarin area are high in HFSE, LILE, and REE and are compositionally similar to intraplate volcanics of oceanic islands and continental rifts. Data points of these rocks plotted on discriminant diagrams are confined to the field of the garnet-rich intraplate source with low degrees of partial melting. The initial isotopic ratios of Sr, Nd, and Pb in the Cenozoic basaltoids characterize the deep sources of Cenozoic rocks in the eastern part of the Koryak Highlands as depleted. The similarity of isotope ratios in the basalts of the Miocene and Early Quaternary stages of volcanism suggests that the mantle region was isotopically homogenized as a result of local convection.

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Drolma, Yangchen, Kaijun Li, Yubin Li, Jinshu Zhang, Chengye Yang, Gen Zhang, Ruoming Li et Duo Liu. « Geochronology, Geochemistry, and In Situ Sr-Nd-Hf Isotopic Compositions of a Tourmaline-Bearing Leucogranite in Eastern Tethyan Himalaya : Implications for Tectonic Setting and Rare Metal Mineralization ». Minerals 14, n^o 8 (26 juillet 2024) : 755. http://dx.doi.org/10.3390/min14080755.

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Himalayan leucogranite is an excellent target for understanding the orogenic process of the India–Asia collision, but its origin and tectonic significance are still under debate. An integrated study of geochronology, geochemistry, and in situ Sr-Nd-Hf isotopes was conducted for a tourmaline-bearing leucogranite in the eastern Tethyan Himalaya using LA-ICP-MS, X-ray fluorescence spectroscopy, and ICP-MS and LA-MC-ICP-MS, respectively. LA-ICP-MS U-Pb dating of zircon and monazite showed that it was emplaced at ~19 Ma. The leucogranite had high SiO2 and Al2O3 contents ranging from 73.16 to 73.99 wt.% and 15.05 to 15.24 wt.%, respectively. It was characterized by a high aluminum saturation index (1.14–1.19) and Rb/Sr ratio (3.58–6.35), which is characteristic of S-type granite. The leucogranite was enriched in light rare-earth elements (LREEs; e.g., La and Ce) and large ion lithophile elements (LILEs; e.g., Rb, K, and Pb) and depleted in heavy rare-earth elements (e.g., Tm, Yb, and Lu) and high field strength elements (HFSEs; e.g., Nb, Zr, and Ti). It was characterized by high I Sr (t) (0.7268–0.7281) and low ε Nd (t) (−14.6 to −13.2) and ε Hf (t) (−12.6 to −9.47), which was consistent with the isotopic characteristics of the Higher Himalayan Sequence. Petrogenetically, the origin of the leucogranite is best explained by the decompression-induced muscovite dehydration melting of an ancient metapelitic source within the Higher Himalayan Sequence during regional extension due to the movement of the South Tibetan Detachment System (STDS). The significantly high lithium and beryllium contents of the leucogranite and associated pegmatite suggest that Himalayan leucogranites possess huge potential for lithium and beryllium exploration.

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Li, Min, Houtian Xin, Bangfang Ren, Yunwei Ren et Wengang Liu. « Early–Middle Permian post-collisional granitoids in the northern Beishan orogen, northwestern China : evidence from U–Pb ages and Sr–Nd–Hf isotopes ». Canadian Journal of Earth Sciences 57, n^o 6 (juin 2020) : 681–97. http://dx.doi.org/10.1139/cjes-2019-0088.

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The geochemistry and Sr–Nd isotope, zircon U–Pb, and zircon Hf isotope compositions are reported for monzogranites and granodiorites from the Hazhu area in the northern Beishan orogen, northwestern China. Zircon U–Pb dating yielded ages of 270.1 ± 1.1 and 277.4 ± 1.2 Ma for the monzogranites and 263.6 ± 1.2 and 262.2 ± 1.1 Ma for the granodiorites. These monzogranites and granodiorites are metaluminous to weakly peraluminous I-type and belong to mid-K calc-alkaline and high-K calc-alkaline series. They exhibit high Mg# values and moderate degrees of differentiation (D.I. = 70.7–88.1). They are enriched in large-ion lithophile elements and light rare earth elements and depleted in high field strength elements. They show high (87Sr/86Sr)i ratios of 0.6995–0.7070 and high εNd(t) values of 4.37–5.70 with Nd model ages (TDM) of 522–789 Ma, suggesting a juvenile crustal origin. Furthermore, their εHf(t) values are all positive, and Hf isotopic crustal model ages ([Formula: see text] = 394–1097 Ma) also indicate a juvenile crustal origin. According to the data obtained in this study and other regional geological data acquired recently, the Hazhu granitoids were derived from common sources of melting from the Neoproterozoic to late Paleozoic juvenile crusts. The younger intrusions (granodiorites) are more basic, likely as a result of more juvenile lower crust being melted along with asthenospheric upwelling, which led to the addition of more basic components. These granitoids formed in a post-collisional setting. The tectonic regime transformed from an arc-related compressional setting to post-collisional extension, likely as a result of lithospheric extension and thinning in response to oceanic lithospheric delamination. These granitoids in the northern Beishan orogen were probably emplaced in a post-collisional extensional setting and suggest vertical continental crustal growth in the southern Central Asian Orogenic Belt.

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Jagoutz, Oliver, Pierre Bouilhol, Urs Schaltegger et Othmar Müntener. « The isotopic evolution of the Kohistan Ladakh arc from subduction initiation to continent arc collision ». Geological Society, London, Special Publications 483, n^o 1 (19 septembre 2018) : 165–82. http://dx.doi.org/10.1144/sp483.7.

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AbstractMagmatic arcs associated with subduction zones are the dominant active locus of continental crust formation, and evolve in space and time towards magmatic compositions comparable to that of continental crust. Accordingly, the secular evolution of magmatic arcs is crucial to the understanding of crust formation processes. In this paper we present the first comprehensive U–Pb, Hf, Nd and Sr isotopic dataset documenting c. 120 myr of magmatic evolution in the Kohistan-Ladakh paleo-island arc. We found a long-term magmatic evolution that is controlled by the overall geodynamic of the Neo-Tethys realm. Apart from the post-collisionnal melts, the intra-oceanic history of the arc shows two main episodes (150–80 Ma and 80–50 Ma) of distinct geochemical signatures involving the slab and the sub-arc mantle components that are intimately linked to the slab dynamics.

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Szilas, Kristoffer, J. Elis Hoffmann, Christina Hansmeier, Julie A. Hollis, Carsten Münker, Sebastian Viehmann et Haino U. Kasper. « Sm-Nd and Lu-Hf isotope and trace-element systematics of Mesoarchaean amphibolites, inner Ameralik fjord, southern West Greenland ». Mineralogical Magazine 79, n^o 4 (août 2015) : 857–76. http://dx.doi.org/10.1180/minmag.2015.079.4.02.

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AbstractFragmented supracrustal rocks are typical components of Archaean high-grade gneiss terranes, such as those in the North Atlantic Craton. Here we present the first major, trace element and Nd-Hf isotope data for amphibolites collected in the yet poorly studied southern inner Ameralik fjord region of southern West Greenland. In addition, new U-Pb zircon ages were obtained from the surrounding TTG gneisses.Based on their trace-element patterns, two different groups of amphibolites can be distinguished. Following screening for post-magmatic alteration and outlying ε values, a reduced sample set defines a147Sm/143Nd regression age of 3038 Ma ±310 Ma (MSWD = 9.2) and a176Lu/176Hf regression age of 2867 ±160 Ma (MSWD = 5.5). Initial εNd2970Mavalues of the least-altered amphibolites range from 0.0 to +5.7 and initial εHf2970Ma range from +0.7 to +10.4, indicating significant isotopic heterogeneity of their mantle sources with involvement of depleted domains as well as crustal sources.Surprisingly, the amphibolites which are apparently most evolved and incompatible element-rich have the most depleted Hf-isotope compositions. This apparent paradox may be explained by the sampling of a local mantle source region with ancient previous melt depletion, which was re-enriched by a fluid component during subduction zone volcanism or alternatively by preferential melting of an ancient pyroxenite component in the mantle source of the enriched rocks.

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Wang, Yuejun, Yuzhi Zhang, Xin Qian, Vongpaseuth Senebouttalath, Yang Wang, Yukun Wang, Chengshi Gan et Khin Zaw. « Ordo-Silurian assemblage in the Indochina interior : Geochronological, elemental, and Sr-Nd-Pb-Hf-O isotopic constraints of early Paleozoic granitoids in South Laos ». GSA Bulletin 133, n^o 1-2 (17 juin 2020) : 325–46. http://dx.doi.org/10.1130/b35605.1.

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Abstract In order to verify the early Paleozoic accretionary assemblage in the Indochina interior and constrain the Prototethyan tectonic evolution in Southeast Asia, this study presents a set of new U-Pb geochronological, elemental, and Sr-Nd-Pb-Hf-O isotopic data for the fifty-two representative granitoids in South Laos. The granitoids from the Kontum terrane, Tam Ky-Phuoc Son tectonic zone, and southern Truong Son igneous zone in South Laos yield the crystallization ages of 464–485 Ma, 455–471 Ma, and 427–446 Ma, respectively, with a northerly younging trend within the Indochina interior. They are mainly monzogranite with A/CNK = 0.96–1.99 and K2O > Na2O, which are marked by enrichment in large-ion lithophile elements and depletion in high field strength elements with remarkable Nb-Ta, Sr-P, and Ti negative anomalies. Their initial 87Sr/86Sr ratios range from 0.70510 to 0.71559, εNd(t) from −9.5 to −3.0, (206Pb/204Pb)i from 18.65 to 19.72, (207Pb/204Pb)i from 15.66 to 15.80, and (208Pb/ 204Pb)i from 38.84 to 39.79. The corresponding zircon ɛHf(t) and δ18O values are in the range of −10.6 to +1.0 and 6.88‰ to 8.94‰, respectively. In addition, their Sr-Nd-Pb and Hf-O isotopic compositions are generally similar with those of time-equivalent granitoids in South Tibet and SW Yunnan, China, and synchronous mafic-intermediate igneous rocks in South Laos, but distinctive from those of the supracrustal sedimentary-derived South China Paleozoic granite and Lincang-Sukhothai S-type granite. The early Paleozoic granitoids in South Laos might have originated from a mixed source of the wedge-derived juvenile crust coupled with supracrustal materials. All these data synthetically suggest the southward subduction of the Tam Ky-Phuoc Son Ocean and the northerly on-growing Ordo-Silurian accretionary orogenesis within the previously defined “single-ancient” Indochina block. The assemblage of the Indochina block might initiate at ca. 430 Ma in the Silurian and terminate in the Early-Middle Devonian.

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Choi, Sung Hi, Samuel B. Mukasa, Alexandre V. Andronikov et Maria C. Marcano. « Extreme Sr–Nd–Pb–Hf isotopic compositions exhibited by the Tinaquillo peridotite massif, Northern Venezuela : implications for geodynamic setting ». Contributions to Mineralogy and Petrology 153, n^o 4 (12 décembre 2006) : 443–63. http://dx.doi.org/10.1007/s00410-006-0159-3.

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Lu, Jia, Chen Zhang et Dongdong Liu. « Geochronological, Geochemical and Sr-Nd-Hf Isotopic Studies of the A-type Granites and Adakitic Granodiorites in Western Junggar : Petrogenesis and Tectonic Implications ». Minerals 10, n^o 5 (29 avril 2020) : 397. http://dx.doi.org/10.3390/min10050397.

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Late Carboniferous magmatism in the Western Junggar region of the Central Asian Orogenic Belt (CAOB) provides a critical geological record of regional tectonic and geodynamic history. In this study, we determined the zircon U-Pb isotopic compositions, bulk-rock Sr-Nd-Hf isotopic compositions, and major and trace element geochemistry of two granitic bodies in the Western Junggar, with the aim of constraining their emplacement ages, magmatic origin, and geodynamic significance. Radiometric ages indicate that the plutons were emplaced during the Late Carboniferous (322–307 Ma). Plutons in the North Karamay region are characterized by high Sr content (347–362 ppm) and low Y content (15.3–16.7 ppm), yielding relatively high Sr/Y ratios (20.8–23.7). They show consistent Yb (1.68–1.85 ppm), Cr (16–19 ppm), Co (7.5–8.1 ppm) and Ni (5.9–6.6 ppm) content, similar to that of modern adakites. The Hongshan plutons are characterized by high SiO2 (69.95–74.66 wt%), Na2O (3.26–3.64 wt%), and K2O (4.84–5.16 wt%) content, low Al2O3 (12.02–12.84 wt%;) and MgO (0.13–0 18 wt%) content, and low Mg# values (0.16–0.22). This group shows a clear geochemical affinity with A-type granites. All of the studied granitoids have positive εNd(t) (+4.89 to +7.21) and εHf(t) (+7.70 to +13.00) values, with young TDM(Nd) 806–526 Ma) and TDM(Hf) (656–383 Ma) ages, indicating a substantial addition of juvenile material. The adakitic granodiorites in the North Karamay region were likely generated via partial melting of thickened lower crust, while the A-type granites in the Hongshan area may have been derived from the melting of lower-middle crust in an intra-oceanic arc, which consists mainly of oceanic crust. The emplacement of these granitoids represents a regional magmatic “flare up”, which can be explained by the rollback of a subducting slab.

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Sun, Jungang, Ting Liang, Xiaohuang Liu, Xiong Zhang, Bei Liu et Guorong Quan. « Triassic Thermal Pulse of TARIM Mantle Plume : Evidence from Geochronology, Geochemistry, and Nd Isotopes of the Mafic Dikes from the Halaqi Area, Xinjiang, China ». Minerals 14, n^o 3 (8 mars 2024) : 283. http://dx.doi.org/10.3390/min14030283.

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Owing to the paucity of research on synchronous mafic rocks in the Tarim Basin, the late Paleozoic–early Mesozoic tectonic development of this region is not well defined. The Halaqi region is situated on Tarim’s northwest edge, and numerous mafic dikes can be found cross-cutting the Permian strata. The whole-rock geochemistry, zircon U–Pb age, and Sr–Nd isotopic signature of these mafic rocks have never been reported before, and this contribution can offer geochronological and petrogenetic investigations that provide fresh insight into the geodynamic development of the area. Major oxide contents in the Halaqi mafic rocks vary, including SiO2 (45.74–50.31 wt.%), Al2O3 (13.28–14.8 wt.%), FeOT (16.48–19.19 wt.%), MgO (7.58–10.32 wt.%), CaO (7.19–12.39 wt.%), Na2O (2.97–4.50 wt.%), K2O (0.24–0.63 wt.%), TiO2 (1.11–1.29 wt.%), MnO (0.14–0.16 wt.%), and P2O5 (0.13–0.17 wt.%). The mafic rocks are enriched in high-field-strength elements (e.g., Zr and Hf) and large-ion lithophile elements (e.g., Sr, Th, and U) but depleted in Nb, Ta, and P. The total REEs in the rocks are lower (ΣREE = 72.80–86.85 ppm), and HREEs are somewhat depleted in comparison to LREEs, with positive Eu anomalies (Eu/Eu* = 1.05–1.17) but weak negative Ce anomalies (Ce/Ce* = 0.91–0.93). Zircon U–Pb ages of 201–247 Ma were obtained from a total of 18 magmatic zircon grains found in the mafic rocks that were studied. These results point to a middle-to-late Triassic emplacement. The mafic dikes exhibit somewhat enriched Nd isotopic compositions (εNd(t) = –1.6~–0.2) and an older Nd model age (TDM = 1.24–1.37 Ga). The Halaqi middle–late Triassic mafic dikes are thought to have originated from the same tectonic background as the Permian Tarim Large Igneous Province, along with similar geochemical and isotopic compositions. This suggests that they are all products of the interaction between asthenospheric and lithospheric mantles in an intraplate extensional environment. Research indicates that the Triassic mafic magmatism in northwest Tarim could be the product of the continuous thermal pulse of the Tarim mantle plume and be a part of the Tarim LIP.

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Mikuni, Kazuto, Naoto Hirano, Shiki Machida, Hirochika Sumino, Norikatsu Akizawa, Akihiro Tamura, Tomoaki Morishita et Yasuhiro Kato. « Contribution of carbonatite and recycled oceanic crust to petit-spot lavas on the western Pacific Plate ». Solid Earth 15, n^o 2 (8 février 2024) : 167–96. http://dx.doi.org/10.5194/se-15-167-2024.

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Abstract. Petit-spot volcanoes, occurring due to plate flexure, have been reported globally. As the petit-spot melts ascend from the asthenosphere, they provide crucial information of the lithosphere–asthenosphere boundary. Herein, we examined the lava outcrops of six monogenetic volcanoes formed by petit-spot volcanism in the western Pacific. We then analyzed the 40Ar/39Ar ages, major and trace element compositions, and Sr, Nd, and Pb isotopic ratios of the petit-spot basalts. The 40Ar/39Ar ages of two monogenetic volcanoes were ca. 2.6 Ma (million years ago) and ca. 0 Ma. The isotopic compositions of the western Pacific petit-spot basalts suggest geochemically similar melting sources. They were likely derived from a mixture of high-μ (HIMU) mantle-like and enriched mantle (EM)-1-like components related to carbonatitic/carbonated materials and recycled crustal components. The characteristic trace element composition (i.e., Zr, Hf, and Ti depletions) of the western Pacific petit-spot magmas could be explained by the partial melting of ∼ 5 % crust bearing garnet lherzolite, with 10 % carbonatite flux to a given mass of the source, as implied by a mass-balance-based melting model. This result confirms the involvement of carbonatite melt and recycled crust in the source of petit-spot melts. It provides insights into the genesis of tectonic-induced volcanoes, including the Hawaiian North Arch and Samoan petit-spot-like rejuvenated volcanoes that have a similar trace element composition to petit-spot basalts.

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Liu, Huanan, Feng Yuan, Shengjin Zhao, Mingjing Fan et Xiangguo Guo. « SHRIMP U–Pb Zircon Ages, Geochemistry and Sr–Nd–Hf Isotope Systematics of the Zalute Intrusive Suite in the Southern Great Xing’an Range, NE China : Petrogenesis and Geodynamical Implications ». Minerals 10, n^o 10 (20 octobre 2020) : 927. http://dx.doi.org/10.3390/min10100927.

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An integrated zircon geochronological, elemental geochemical, and Sr–Nd–Hf isotopic investigation was carried out on a suite of dioritic–granitic rocks at Zalute in the southern Great Xing’an Range (SGXR), NE China, in order to probe the source and petrogenesis of these granitoid rocks and further constrain the geodynamical setting of early Early Cretaceous magmatism. The results of Sensitive High-Resolution Ion Micro Probe (SHRIMP) zircon U–Pb dating reveal that the Zalute dioritic–granitic rocks have a consistent crystallization age of ca. 137–136 Ma, consisting of quartz diorite (136 ± 1.4 Ma), monzogranite (136 ± 0.8 Ma), and granite porphyry (137 ± 1.3 Ma), which record an early Early Cretaceous magmatic intrusion. Geochemically, the quartz diorites, monzogranites, and granite porphyries are mostly high-K calc-alkaline and show features of typical I-type affinity. They possess uniform and depleted Sr–Nd–Hf isotopic compositions (e.g., initial 87Sr/86Sr ratios of 0.7035 to 0.7049, εNd(t) of −0.02 to +2.61, and εHf(t) of +6.8 to +9.6), reflecting a common source, whose parental magma is best explained as resulting from the partial melting of juvenile source rocks in the lower crust produced by underplating of mantle-derived mafic magma, with minor involvement of ancient crustal components. Evidence from their close spatio–temporal relationship, common source, and the compositional trend is consistent with a magmatic differentiation model of the intermediate-felsic intrusive suite, with continued fractional crystallization from quartz diorites, towards monzogranites, then to granite porphyries. Combined with previously published data in the SGXR, our new results indicate that the Zalute intermediate-felsic intrusive suite was formed during the post-collisional extension related to the closure of the Mongol–Okhotsk Ocean and subsequent slab break-off.

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Chen, Wei, Xinbiao Lü, Xiaofeng Cao et Wenjia Ai. « Petrogenesis and tectonic setting of the Dapingliang Late Neoproterozoic magmatic rocks in the eastern Kuluketage Block : geochronological, geochemical and Sr–Nd–Pb–Hf isotopic implications ». Geological Magazine 157, n^o 2 (17 juin 2019) : 173–200. http://dx.doi.org/10.1017/s0016756819000530.

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AbstractIn the past ten years, a great deal of geological study has been reported on the magmatic rocks exposed in the central and western region of the Kuluketage Block, while similar research in the eastern region has rarely been reported. In this paper, we report zircon U–Pb geochronological, zircon Lu–Hf isotopic, whole-rock elemental and Sr–Nd–Pb isotopic data for the Dapingliang intermediate-acid intrusive rocks in the eastern Kuluketage Block, in order to evaluate its petrogenesis and tectonic significance. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating provided a weighted mean 206Pb/238U age of 735 ± 3 Ma for the albitophyre (D1), 717 ± 2 Ma for the granite porphyry (D2) and 721 ± 1 Ma for the diorite porphyrite (D3). Geochemical analyses reveal that D1 and D2 belong to Na-rich alkaline A-type granites, and D3 shows the features of high-K calc-alkaline I-type granite. D1 and D2 are characterized by light rare earth element (LREE) enrichment and relative depletion of high field strength element (HFSE), with relatively flat heavy rare earth element (HREE) patterns and obviously negative Eu anomalies. D3 is characterized by the enrichment of LREE and depletion of HFSE, with negative slope HREE patterns and slightly negative Eu anomalies. In tectonic discrimination diagrams, D1 and D2 fall in the within-plate granite (WPG) field, indicating a rift setting. Although D3 falls within the volcanic arc granite (VAG) field, it most likely formed in a rift setting, as inferred from its petrology, Sr–Nd–Hf isotopes and regional tectonic evolution. Based on pronounced εNd(t), εHf(t), Pb isotopic data, TDM2 and high (87Sr/86Sr)i and elemental compositions, D1 was derived from the partial melting of basement amphibolites of the old lower crust. D2 originated from a mixture of the old lower crust and depleted mantle-derived magmas and was dominated by partial melting of the basement amphibolites of the lower crust. D3 could have been formed by partial melting of K-rich hornblende in the lower crust. Combining previous studies, we think that the c. 745–710 Ma magmatic rocks were formed in a continental rift setting. A partial melting scheme, triggered by underplating of mantle plume-derived magmas, is proposed to interpret the formation of c. 745–710 Ma A-type and I-type granitoids, mantle-derived mafic dykes, bimodal intrusive rocks, adakitic granites and volcanic rocks. These magmatic activities were probably a reflection of the break-up of the Rodinia supercontinent.Highlights(1)Circa 720 Ma magmatism in the eastern Kuluketage Block.(2)Na-rich granite was derived from partial melting of basement amphibolites.(3)The c. 745–710 Ma magmatic rocks were formed in a continental rift setting.(4)The underplating of mantle plume-derived magmas is proposed.

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WU, FU-YUAN, ROGER H. MITCHELL, QIU-LI LI, CHANG ZHANG et YUE-HENG YANG. « Emplacement age and isotopic composition of the Prairie Lake carbonatite complex, Northwestern Ontario, Canada ». Geological Magazine 154, n^o 2 (12 février 2016) : 217–36. http://dx.doi.org/10.1017/s0016756815001120.

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AbstractAlkaline rock and carbonatite complexes, including the Prairie Lake complex (NW Ontario), are widely distributed in the Canadian region of the Midcontinent Rift in North America. It has been suggested that these complexes were emplaced during the main stage of rifting magmatism and are related to a mantle plume. The Prairie Lake complex is composed of carbonatite, ijolite and potassic nepheline syenite. Two samples of baddeleyite from the carbonatite yield U–Pb ages of 1157.2±2.3 and 1158.2±3.8 Ma, identical to the age of 1163.6±3.6 Ma obtained for baddeleyite from the ijolite. Apatite from the carbonatite yields the same U–Pb age of ~1160 Ma using TIMS, SIMS and laser ablation techniques. These ages indicate that the various rocks within the complex were synchronously emplaced at about 1160 Ma. The carbonatite, ijolite and syenite have identical Sr, Nd and Hf isotopic compositions with a 87Sr/86Sr ratio of ~0.70254, and positive εNd(t)1160 and εHf(t)1160 values of ~+3.5 and ~+4.6, respectively, indicating that the silicate and carbonatitic rocks are co-genetic and related by simple fractional crystallization from a magma derived from a weakly depleted mantle. These age determinations extend the period of magmatism in the Midcontinent Rift in the Lake Superior area to 1160 Ma, but do not indicate whether the magmatism is associated with passive continental rifting or the initial stages of plume-induced rifting.

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Wei, Pengfei, Dapeng Li, Ke Geng, Yan Zhang, Qiang Liu, Wei Xie, Yingxin Song, Na Cai, Chao Zhang et Zhigang Song. « Petrogenesis of the Weideshan Pluton in Jiaodong and Its Implications for Gold Polymetallic Mineralization : Constraints from Zircon U-Pb-Hf Isotopes, Petrogeochemistry, and Whole-Rock Sr-Nd Isotopes ». Minerals 14, n^o 1 (19 décembre 2023) : 7. http://dx.doi.org/10.3390/min14010007.

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The Early Cretaceous Weideshan granites are associated with large-scale Au and polymetallic Cu-Mo-Pb-Zn mineralization. To investigate the petrogenesis of the Weideshan granite and constrain its tectonic setting during the Early Cretaceous, we conducted a zircon U-Pb-Hf isotope and whole-rock geochemical and Sr-Nd isotopic study of the granite. In situ zircon U-Pb dating of three granite samples yielded Early Cretaceous ages of 112.83 ± 0.80, 112.64 ± 0.91, and 111.82 ± 0.78 Ma. The samples had high-K calc-alkaline compositions and were enriched in the light rare earth and large-ion lithophile elements (e.g., K, Rb, Ba, Th, and U) and depleted in high-field-strength elements (e.g., Nb, Ti, and P). The samples had small negative Eu anomalies and initial 87Sr/86Sr and εNd(t) values of 0.70853–0.71029 and –18 to –14, respectively. The zircon εHf(t) values varied between −16 and −12, with corresponding two-stage model ages (tDM2) of 2180–2000 Ma. These characteristics indicated that the Weideshan pluton was formed in a back-arc extensional environment caused by subduction of the Paleo-Pacific Plate toward the Asian continent during the early Cretaceous. The magma was generated by crust–mantle interaction during lithospheric thinning. The diagenetic age of the Weideshan granites was the same as the formation age of gold and polymetallic ores in the Jiaodong area. The extensive fluid circulation induced by the magma emplacement may be the main source of ore-forming materials for the gold and polymetallic Cu-Mo-Pb-Zn deposits.

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Zheng, Hao, Li-Feng Zhong, Argyrios Kapsiotis, Guan-Qiang Cai, Zhi-Feng Wan et Bin Xia. « Post-spreading Basalts from the Nanyue Seamount : Implications for the Involvement of Crustal- and Plume-Type Components in the Genesis of the South China Sea Mantle ». Minerals 9, n^o 6 (23 juin 2019) : 378. http://dx.doi.org/10.3390/min9060378.

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Fresh samples of basalts were collected by dredging from the Nanyue intraplate seamount in the Southwest sub-basin of the South China Sea (SCS). These are alkali basalts displaying right-sloping, chondrite-normalized rare earth element (REE) profiles. The investigated basalts are characterized by low Os content (60.37–85.13 ppt) and radiogenic 187Os/188Os ratios (~0.19 to 0.21). Furthermore, 40Ar/39Ar dating of the Nanyue basalts showed they formed during the Tortonian (~8.3 Ma) and, thus, are products of (Late Cenozoic) post-spreading volcanism. The Sr–Nd–Pb–Hf isotopic compositions of the Nanyue basalts indicate that their parental melts were derived from an upper mantle reservoir possessing the so-called Dupal isotopic anomaly. Semiquantitative isotopic modeling demonstrates that the isotopic compositions of the Nanyue basalts can be reproduced by mixing three components: the average Pacific midocean ridge basalt (MORB), the lower continental crust (LCC), and the average Hainan ocean island basalt (OIB). Our preferred hypothesis for the genesis of the Nanyue basalts is that their parental magmas were produced from an originally depleted mantle (DM) source that was much affected by the activity of the Hainan plume. Initially, the Hainan diapir caused a thermal perturbation in the upper mantle under the present-day Southwest sub-basin of the SCS that led to erosion of the overlying LCC. Eventually, the resultant suboceanic lithospheric mantle (SOLM) interacted with OIB-type components derived from the nearby Hainan plume. Collectively, these processes contributed crustal- and plume-type components to the upper mantle underlying the Southwest sub-basin of the SCS. This implies that the Dupal isotopic signature in the upper mantle beneath the SCS was an artifact of in situ geological processes rather than a feature inherited from a Southern Hemispheric, upper mantle source.

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Quelhas, Pedro, João Mata et Ágata Alveirinho Dias. « Evidence for mixed contribution of mantle and lower and upper crust to the genesis of Jurassic I-type granites from Macao, SE China ». GSA Bulletin 133, n^o 1-2 (30 avril 2020) : 37–56. http://dx.doi.org/10.1130/b35552.1.

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Abstract Much controversy has occurred in the past few decades regarding the nature of the sources, the petrogenetic processes, and the tectonic regime(s) of the Jurassic magmatism within the Southeast China magmatic belt. This study aims to contribute to the discussion with mineral chemistry, and whole-rock element and Sr-Nd-Hf-Pb isotopic geochemical data from granitic rocks and microgranular mafic enclaves from Macao, where two discrete groups of I-type biotite granites have been identified (referred to as Macao Group I [MGI] and Macao Group II [MGII]). It is proposed that the granitic magmas were generated by partial melting of infracrustal medium- to high-K, basaltic Paleoproterozoic to Mesoproterozoic protoliths (Nd depleted mantle model age [TDM2] = 1.7–1.6 Ga and Hf TDM2 = 1.8–1.6 Ga), triggered by underplating of hot mantle-derived magmas in an extensional setting related to the foundering of a previously flat slab (paleo–Pacific plate) beneath the SE China continent. The main differences between the two groups of Macao granites are attributed to assimilation and fractional crystallization processes, during which upper-crustal Paleozoic metasediments were variably assimilated by MGI magmas. This is evidenced by an increase in initial 87Sr/86Sr ratios with degree of evolution, presence of metasedimentary enclaves, and high percentage of zircon xenocrysts with Paleozoic ages. In addition, other processes like late-stage fluid/melt interaction and magma mixing also left some imprints on granite compositions (rare earth element tetrad effect plus non–charge-and-radius-controlled behavior of trace elements and decoupling between different isotope systems, respectively). The distribution of isotopically distinct granites in SE China reflects the nature of the two Cathaysia crustal blocks juxtaposed along the Zhenghe-Dapu fault.

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Wang, Hao, Shaohua Wang, Zhaowen Xu, Bin Fu, Zengxia Zhao, Zhenggang Li, Yanhui Dong, Limei Tang et Jie Li. « Geochemical and Sr–Nd–Pb–Hf–O isotopic compositions of the Tiezhai complex : Implications for lithosphere destruction of the North China Craton ». Gondwana Research 61 (septembre 2018) : 203–21. http://dx.doi.org/10.1016/j.gr.2018.04.020.

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LI, ZHEN, JIAN-SHENG QIU et XI-SHENG XU. « Geochronological, geochemical and Sr–Nd–Hf isotopic constraints on petrogenesis of Late Mesozoic gabbro–granite complexes on the southeast coast of Fujian, South China : insights into a depleted mantle source region and crust–mantle interactions ». Geological Magazine 149, n^o 3 (16 septembre 2011) : 459–82. http://dx.doi.org/10.1017/s0016756811000793.

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AbstractThe Quanzhou (QZ) and Huacuo (HC) gabbro–granite complexes on the southeast coast of Fujian, South China, are important components of a Late Mesozoic calc-alkaline volcanic–plutonic belt in the region. The complexes provide an excellent opportunity to investigate the genetic relationships between acid and basic magmas, and their interactions within the intrusive environment. The complexes are composed mainly of monzogranite and biotite granodiorite in the QZ complex, and biotite granite in the HC complex, with lesser amounts of hornblende gabbro. Zircon U–Pb dating provides consistent crystallization ages of 109 ± 1 Ma and 108 ± 1 Ma for the QZ gabbros and monzogranites, and an age of 111 ± 1 Ma for the HC gabbro, which is contemporaneous with the spatially associated HC granites. Both the mafic and felsic intrusions in these complexes are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), and are depleted in high-field-strength elements (HFSEs; e.g. Nb and Ta). They show similarly homogeneous Sr–Nd isotopic compositions. All these factors indicate a close genetic relationship between the gabbroic and granitic rocks in the QZ and HC complexes. Although the enriched Sr–Nd isotopic signatures of the QZ and HC gabbros seemingly point to an enriched mantle source (EM-1), they have highly variable zircon Hf isotopic compositions, with εHf(t) values ranging from negative to positive (specifically –4.6 to +6.1 for the QZ gabbros and –4.8 to +11.6 for the HC gabbros). We interpret the parental basic magmas of these gabbros to have received contributions from a depleted mantle source and crustal components. Contributions from such a depleted mantle source resulted in the growth of juvenile basaltic lower crust, the partial melting of which generated the parental felsic magmas of the QZ and HC complexes. Furthermore, based on a synthesis of petrography, geochronology, elemental and isotopic geochemistry and tectonics, we propose that break-off and rollback of the Late Mesozoic subducted Palaeo-Pacific Plate triggered the upwelling of asthenospheric mantle below the coastal area of the South China Block, which induced extension of the overlying continental lithosphere, and finally initiated the large-scale Late Yanshanian magmatism in the study area.

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Wu, Tao, Zhilong Huang, Mu Yang, Dexian Zhang, Jiawei Zhang et Chen Wei. « Petrogenesis and Tectonic Setting of the Highly Fractionated Junye Granitic Intrusion in the Yiliu Tungsten Polymetallic Deposit, Guangdong Province, South China : Constraints from Geochemistry and Sr-Nd-Pb-Hf Isotopes ». Minerals 10, n^o 7 (16 juillet 2020) : 631. http://dx.doi.org/10.3390/min10070631.

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The Yiliu tungsten polymetallic deposit, located in the south central portion of the Nanling nonferrous metal metallogenic province in South China, is an area with common Yanshanian tectonothermal events. Early Yanshanian magmatism leads to the emplacement of voluminous tungsten-bearing granite intrusions, such as the Baoshan, Benggangling and Junye plutons, which are considered temporally and spatially associated with W-polymetallic mineralization in the Yiliu region. Here, we investigate the basic geological and petrological characteristics of the Junye granites, and present major and trace element geochemical data and bulk-rock Sr-Nd-Pb-Hf isotopic data to gain insight into the petrogenesis and tectonic setting of granitic intrusions in the region. The Junye granites are high-K calc-alkaline and metaluminous to weakly peraluminous [A/CNK = molar ratios of Al2O3/(CaO + Na2O + K2O) = 0.97–1.02] with enrichment in SiO2 (75.68–76.44 wt.%), relatively high total alkalis (K2O + Na2O = 8.06–8.45 wt.%) with K2O/Na2O ratios ranging from 1.12 to 1.42, and moderate Al2O3 (12.62–13.00 wt.%), but low in P2O5 (<0.01 wt.%), MgO (0.02–0.04 wt.%), CaO (0.78–0.95 wt.%) and Fe2O3T (0.93–1.07 wt.%). They show spectacular tetrad effect REE (rare earth element) patterns with low ΣREE content (53.2–145.3 ppm), negative Eu anomalies (δEu = 0.09–0.17) and slight enrichment of LREEs (light rare earth elements) relative to HREEs (heavy rare earth elements). The granites are enriched in Rb (481–860 ppm), Th (16.2–46.1 ppm) and U (25.4–40.8 ppm) but depleted in Ba (1.0–5.8 ppm), Sr (11.1–23.4 ppm), P (9.5–26.7 ppm) and Ti (241–393 ppm). All geochemical features lead us to interpret the Junye granites as highly fractionated I-type granites. These granites underwent intense interaction between highly evolved magma and volatile-rich hydrothermal fluids during the late stage of formation, and accompanied fractional crystallization of biotite, plagioclase and accessory minerals, such as apatite, monazite and allanite. Additionally, the granites show uniform Nd isotopic ratios with calculated εNd (152 Ma) values of −8.28 to −8.91 and Nd model age (TDM2) of 1645 to 1698 Ma, stable age-corrected initial Pb isotopic compositions with (206Pb/204Pb)i of 18.646–19.010, (207Pb/204Pb)i of 15.767–15.786 and (208Pb/204Pb)i of 39.113–39.159, respectively, and homogeneous Hf isotopic values yielding εHf (152 Ma) values from −6.9 to −9.5 with TDM2 ages of 1680 to 2214 Ma, collectively suggesting that the granitic magma was probably derived from the remelting of ancient infracrustal materials in the basement of the Nanling region. Consequently, we consider that the Junye granites are the products of partial melting of Paleoproterozoic infracrustal medium- to high-K metamorphic basaltic rocks in the Cathaysia Block, which was caused by the underplating of coeval mantle basaltic magmas that provided abundant heat energy for melting in a tectonic setting, with lithospheric extension and thinning during the late Jurassic period.

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Nelson, Wendy R., Barry B. Hanan, David W. Graham, Steven B. Shirey, Gezahegn Yirgu, Dereje Ayalew et Tanya Furman. « Distinguishing Plume and Metasomatized Lithospheric Mantle Contributions to Post-Flood Basalt Volcanism on the Southeastern Ethiopian Plateau ». Journal of Petrology 60, n^o 5 (1 mai 2019) : 1063–94. http://dx.doi.org/10.1093/petrology/egz024.

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Abstract Magmatism in the East African Rift System (EARS) contains a spatial and temporal record of changing contributions from the Afar mantle plume, anciently metasomatized lithosphere, the upper mantle and the continental crust. A full understanding of this record requires characterizing volcanic products both within the rift valley and on its flanks. In this study, three suites of mafic, transitional to alkaline lavas, were collected over a northeast-southwest distance of ∼150 km along the southeastern Ethiopian Plateau, adjacent to the Main Ethiopian Rift. Specifically, late Oligocene to Quaternary mafic lavas were collected from Chiro, Debre Sahil and the Bale Mountains. New major element, trace element, 40Ar/39Ar ages and isotopic results (Sr, Nd, Pb, Hf, Os, He) show spatial and temporal variation in the lavas caused by dynamical changes in the source of volcanism during the evolution of the EARS. The trace element compositions of Oligocene and Miocene Chiro lavas indicate derivation from mildly depleted and nominally anhydrous lithospheric mantle, with variable inputs from the crust. Further south, Miocene Debre Sahil and alkaline Bale Mountains lavas have enriched incompatible trace element ratios (e.g. Ba/Nb = 12–43, La/SmN = 3·1–4·9, Tb/YbN = 1·6–2·4). Additionally, their 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf and 206Pb/204Pb values trend toward a radiogenic Pb (HIMU) component. Radiogenic 187Os/188Os in these lavas correlates positively with 206Pb/204Pb and trace element indicators consistent with ancient metasomatic enrichment of their mantle source. In contrast, transitional Miocene Bale Mountains lavas have lower incompatible trace element abundances, less enriched trace element ratios (Ba/Nb ∼7, La/SmN = 2·3–2·5) and less radiogenic isotopic signatures that originate from melting garnet-bearing, anhydrous lithospheric mantle (Tb/YbN = 2·5–2·9). Pliocene and Quaternary Bale Mountains basaltic lavas are chemically and isotopically similar to Main Ethiopian Rift lavas. Trace element and isotopic indicators in both of these suites denote an amphibole-bearing source distinct from that sampled by the older Bale Mountains lavas. Isotopically, Pliocene and Quaternary Bale lavas have notably less radiogenic Sr–Nd–Pb–Hf isotopic ratios. Quaternary Bale Mountains lavas have the strongest mantle plume contribution (3He/4He = 12·1–12·5 RA), while other Bale Mountains, Debre Sahil and Chiro lavas were derived dominantly by melting of lithospheric or upper mantle sources (3He/4He = 5·1–9·1 RA). A multi-stage, regional-scale model of metasomatism and partial melting accounts for the spatial and temporal variations on the southeastern Ethiopian Plateau. Early Debre Sahil and alkaline Bale Mountains mafic lavas are melts derived from Pan-African lithosphere containing amphibole-bearing metasomes, while later transitional Bale basalts are melts of lithosphere containing anhydrous, clinopyroxene-rich veins. These ancient metasomatized domains were eventually removed through preferential melting, potentially during thermal erosion of the lithosphere or lithospheric foundering. Pliocene and Quaternary Bale Mountains lavas erupted after tectonic extension progressed throughout Ethiopia and was accompanied by increased plume influence on the volcanic products.

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Chen, Jiajie, Lebing Fu, Chengbiao Leng, Xu Zhao, Jian Ma, Hongze Gao et Yu Xia. « Origin of Quartz Diorite and Mafic Enclaves in the Delong Gold-Copper Deposit and Evaluation of the Gold-Copper Mineralization Potential ». Minerals 13, n^o 9 (13 septembre 2023) : 1202. http://dx.doi.org/10.3390/min13091202.

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The Triassic Paleo-Tethyan magmatic belt in the East Kunlun Orogen (EKO) hosts a small number of porphyry-skarn deposits. The controls of these deposits, especially those in the eastern EKO, are poorly understood. In this contribution, we report new petrological, zircon U-Th-Pb-Hf isotopic, whole-rock elemental with Sr-Nd isotopic, and mineral chemistry data of the Delong quartz diorite and mafic enclaves to constrain their petrogenesis and metal fertility. The quartz diorite and mafic enclaves are emplaced in the Late Triassic (ca. 234 Ma). They are medium-K, metaluminous, enriched in large-ion lithophile elements (e.g., Rb, Ba, Th) and light rare earth elements (e.g., La, Ce, Nd), and relatively depleted in high field strength elements (e.g., Nb, Ta, Ti, P) and heavy rare earth elements (e.g., Gd, Er, Tm, Yb). The quartz diorite show similar (87Sr/86Sr)i (0.712584~0.713172) and more depleted εNd(t) (−6.4~−5.7) and εHf(t) (−2.3~+2.6) to those of mafic enclaves ((87Sr/86Sr)i = 0.712463~0.713093; εNd(t) = −6.4~−6.0; εHf(t) = −9.4~−4.8). Geochemical compositions of zircon, amphibole, and biotite yield high water content (5.3 wt.%~6.9 wt.% and 6.1 wt.%~7.3 wt.% based on amphibole, respectively) and high redox state for both the quartz diorite and mafic enclaves. These data, together with petrography, indicate the Delong intrusion was formed by mingling of magmas from enriched mantle and lower continental crust with juvenile materials. The oxidized and water-rich features of these magmas denote they have potential for porphyry Cu (±Au ± Mo) deposits, as do some Triassic magmatic rocks in the eastern EKO that show similar geochemical and petrographic characteristics with the Delong intrusion.

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Gao, Nan, Yingkui Xu, Dan Zhu, Yang Li, Xiongyao Li, Jianzhong Liu et Jin-Cheng Luo. « Isotopic decoupling of K from Sr and Nd in the Saima alkaline complex, NE China : interactions of cratonic roots and asthenosphere ». Geological Magazine, 5 avril 2023, 1–8. http://dx.doi.org/10.1017/s001675682300016x.

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Abstract We report high-precision K isotopes, apatite U–Pb ages, whole-rock elements and Sr–Nd isotopes for the Saima nephelite syenite in the North China Craton. Trace-element and Sr–Nd–Hf–O isotope data indicate the presence of subducting sediments in the source region, while K isotopic compositions show a narrow range between –0.54 ‰ and –0.28 ‰, with an average of –0.41 ± 0.06 ‰, identical to the value of the asthenosphere. The nearly identical K isotopic compositions are low probability events compared with the K isotopic compositions of island arc lavas reported previously (–1.55 ‰ to +0.2 ‰). Although crustal contamination is consistent with the Sr–Nd–K isotopic data, alternatively we propose that the isotopic data also reconcile with the interaction between cratonic roots and the underlying convective asthenosphere, if this interaction is over prolonged periods of time. Numerical simulations successfully reproduced the observed data, if the metasomatism of the lithospheric mantle root, the source of the Saima alkaline rocks, occurred 500 Ma ago. Our study reveals that the isotopic compositions of fast-diffusion components in a lithospheric mantle metasomatized by ancient subducting melts can be effectively homogenized by convective asthenosphere through diffusion over a long time interval.

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Yang, Lei, Jia-Min Wang, Xiao-Chi Liu, Gautam P. Khanal et Fu-Yuan Wu. « Sr-Nd-Hf Isotopic Disequilibrium During the Partial Melting of Metasediments : Insight From Himalayan Leucosome ». Frontiers in Earth Science 10 (5 mai 2022). http://dx.doi.org/10.3389/feart.2022.891960.

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Radiogenic isotopes of granitoids are widely applied to fingerprint the source of granitoids and study the magma mingling and assimilation processes, aiming to decipher the planetary differentiation. This weapon is based on the assumption that crustal melts inherit the radiogenic isotopes of protoliths. However, complicated melting processes in the crust would drive the radiogenic isotopes of melt away from the source, thus calling for a clear understanding of the behaviors of radiogenic isotopes during crustal melting. This study investigated the joint behavior of Rb-Sr, Sm-Nd, and Lu-Hf isotope systems during the melting of metasediments. Leucosome, schist, and leucogranite samples were collected from the Nyalam, South Tibet, where the leucosome was produced by muscovite dehydration melting of metapelites. Results show that the leucosome has a broad range of Sr-Nd-Hf isotopes (87Sr/86Sri: 0.763,48–0.875321, εNd(t): 14.6–11.6, εHf(t): 15.0–4.4) that deviate from the metasediments. We attributed it to the non-modal or disequilibrium melting of metapelites. The variation of Sr isotopes of leucosome is formed due to preferential entry into the melt of muscovite relative to plagioclase during melting. The changing 87Sr/86Sri and 87Rb/86Sr of leucosome define an errorchron at ∼ 500 Ma indicating an early Paleozoic tectono-thermal event supported by the U-Pb age of zircon cores (∼460 Ma) in the leucosome. The Nd isotopes of leucosome are mainly controlled by the preferential dissolution of apatite relative to monazite. The inadequate dissolution of zircon caused more depleted initial Hf isotopes compositions in the leucosome than the source. However, the leucosome with a higher crystallization temperature has radiogenic isotopic composition closer to the source. It indicates that the melting temperature greatly affects the isotope equilibrium between source and melt.

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Wei, Wei, Xin-Biao Lv et Xiang-Dong Wang. « The granite porphyry hidden in the Shuangjianzishan deposit, southern Great Xing’an Range, NE China : geochronology, isotope geochemistry and tectonic implications ». Geological Magazine, 8 octobre 2020, 1–15. http://dx.doi.org/10.1017/s0016756820001065.

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Abstract The Shuangjianzishan vein-type Ag-Pb-Zn deposit in the southern Great Xing’an Range (GXR), NE China, is hosted in the slate of the Lower Permian Dashizhai Formation intruded by granite porphyry. In this paper, U–Pb zircon ages and bulk-rock and isotope (Sr, Nd, Pb and Hf) compositions are reported to investigate the derivation, evolution and geodynamic setting of this granite porphyry. It is closely associated with Pb-Zn-Ag mineralization in the southern GXR and contains important geological information relating to regional tectonic evolution. Laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) zircon U–Pb dating yields an emplacement age of 131 ± 1 Ma for the granite porphyry. Bulk-rock analyses show that the Shuangjianzishan granite porphyry is characterized by high Si, Na and K contents but low Mg and Fe contents, and that the enrichment of Zr, Y and Ga suggests an A-type granite affinity. Most of the studied samples have relatively low 87Sr/86Sr values (0.70549–0.70558), with positive ϵNd(t) (0.71–0.88) and ϵHf(t) (4.9–6.9) values. The Sr–Nd isotope modelling results, in combination with the young TDM2 ages of Nd and Hf (850–864 and 668–778 Ma, respectively), reveal that the Shuangjianzishan granite porphyry may be derived from the melting of mantle-derived juvenile component, with minor lower crustal components; this finding is also supported by Pb isotopic compositions. Considering the widespread presence of granitoids with coeval volcanic rocks and regional geology data, we propose that the Shuangjianzishan granite porphyry formed in a post-orogenic extensional environment related to the upwelling of asthenospheric mantle following the closure of the Mongol–Okhotsk Ocean.

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Tao, Lu, Biji Luo, Hongfei Zhang, Wangchun Xu, Hua Huang, He Yang, Huaning Qiu et al. « Multiple mantle metasomatism recorded by Triassic post-collisional ultrapotassic and tholeiitic magmatism in West Qinling, NW China ». Journal of the Geological Society, 20 août 2024. http://dx.doi.org/10.1144/jgs2024-020.

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Mantle properties and metasomatic processes, together with interlayer interactions at orogenic belts are still under debate. The post-collisional magmatic rocks could provide secondhand constraints on these issues. Hereby, an integrated geochronologic, geochemical, and isotopic study was conducted on post-collisional Zhanwa dolerites, Dashui granodiorite–monzonite-monzogabbro complex, and Jiuzigou pyroxenite–syenite complex at West Qinling. The Zhanwa dolerites, being tholeiitic with flat trace elemental pattern, were mixtures of melts from asthenospheric and oceanic slab-fluid-metasomatized lithospheric mantles. The Dashui monzonite-monzogabbro suite is ultrapotassic with strongly right-inclined trace elemental pattern and evolved isotopic composition (( 87 Sr/ 86 Sr) i = 0.7066–0.7068, ε Nd ( t ) = -4.56 to -4.35, ε Hf ( t ) = -3.14 to -2.35, and ( 206 Pb/ 204 Pb) i = 17.99–18.12)). It derived from melting of continent-crustal materials-metasomatized mantle. In contrast, the Dashui granodiorites are shoshonitic–calc-alkaline with more evolved isotopic compositions with ( 87 Sr/ 86 Sr) i , ε Nd ( t ), ε Hf ( t ), and ( 206 Pb/2 04 Pb) i of 0.7074–0.7091, -7.56 to -5.51, -5.61 to -3.53, and 18.18–18.46, respectively. They represent hybrids of mantle-derived K-rich and crustal-derived felsic melts. The Jiuzigou complex holds extremely high trace elemental contents. Its mantle source was metasomatized by both oceanic crustal fluids and sedimentary melts. Generation of these rocks (ca. 235–223 Ma) not only recorded multiple mantle metasomatism, but also captured collective asthenospheric and lithospheric magmatic responses at a post-collisional setting.

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Bollen, Elizabeth M., Harold H. Stowell, Ruth F. Aronoff, Sara V. Stotter, Christopher G. Daniel, Christopher R. M. McFarlane et Jeffrey D. Vervoort. « Reconciling garnet Lu-Hf and Sm-Nd, and monazite U-Pb ages for a prolonged metamorphic event, northern New Mexico ». Journal of Petrology, 4 avril 2022. http://dx.doi.org/10.1093/petrology/egac031.

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Abstract Garnet Sm-Nd and Lu-Hf isotopic data are increasingly used in tandem to characterize tectono-metamorphic events and their duration because the two chronometers may yield a progression from older Lu-Hf to younger Sm-Nd ages. In this contribution, we utilize garnet compositional zoning and monazite U-Pb ages to evaluate explanations for differences in garnet Sm-Nd and Lu-Hf ages for low- to mid-amphibolite facies rocks from the Picuris and Tusas Mountains, New Mexico. Ten Sm-Nd ages, eight Lu-Hf garnet ages (6 previously published), and four matrix monazite U-Pb dates span 1422–1370 Ma, 1456–1399 Ma, and 1377–1356 Ma, respectively. The weighted means of garnet ages are 1398±11 Ma for Sm-Nd (N=10/10) and 1402±4 Ma for Lu-Hf (N=6/8); however, Sm-Nd and Lu-Hf ages from three samples differ by 29, 34, and 55 m.y. with differences between 2σ age envelopes of 12, 9, and 38 m.y., respectively. In these samples, primary Lu and Sm zoning in garnet, diffusion modification of zoning, or disequilibrium is sufficient to cause older Lu-Hf ages and younger Sm-Nd ages. Alternatively, two of the three Lu-Hf ages, c. 1450 Ma, may reflect the onset of a prolonged regional event or an earlier thermal event and the three c. 1370 Ma garnet Sm-Nd ages may reflect diffusional re-equilibration. These three c. 1370 Ma ages are indistinguishable from the monazite ages and garnet have trace element zoning consistent with diffusional alteration of growth zoning in Sm and Nd but not Lu and Hf. Garnet from the other seven samples have prograde zoning with little to no evidence for diffusional relaxation of zoning in these elements. We conclude that the majority of garnet preserve primary compositions and indistinguishable garnet ages of c. 1400 Ma indicating a c. 22 m.y. garnet growth history based on weighted mean. The two oldest c. 1450 Lu-Hf and three youngest c. 1370 Sm-Nd garnet ages require an earlier thermal history and modification of garnet composition, respectively. Results of this study re-define the youngest extent of amphibolite-facies metamorphism during the Picuris Orogeny to at least c. 1370 Ma.

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Lei, Ming, Jianlin Chen et Maoliang Zhang. « Origin of Late Cenozoic Basaltic Magmatism in Inner Mongolia, NE China : Constraints From Sr–Nd–Hf–Pb–Mo–He Isotopes ». Geochemistry, Geophysics, Geosystems 24, n^o 12 (décembre 2023). http://dx.doi.org/10.1029/2023gc011166.

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AbstractThis paper presents a study of the late Cenozoic Chifeng basalts (CBs) of NE China, including their olivine He isotopic compositions, whole‐rock major‐ and trace‐element contents, and whole‐rock Sr–Nd–Hf–Pb–Mo isotopic compositions, with the aim of constraining their mantle source. Results show that the basalts have high MgO, low CaO contents, and high FeOT/MnO values, which indicate that their mantle lithology was most likely pyroxenite. The CBs also exhibit ocean‐island‐basalt‐like trace‐element patterns (e.g., enrichment in light rare earth elements and high‐field‐strength elements) and have depleted Sr–Nd–Hf and relatively radiogenic Pb isotopic compositions, requiring both depleted and enriched components in their mantle. The low olivine He (3He/4He = 0.8–5.5 Ra) and whole‐rock Mo (δ98/95Mo = −0.71‰ to −0.18‰) isotopic values of the CBs, together with geophysical evidence, indicate that the rocks were derived from a depleted MORB mantle (DMM) enriched by recycled oceanic crust that was sourced from the mantle transition zone (MTZ). During the late Cenozoic, ascending wet mantle plumes triggered by dehydration of a stagnant Pacific oceanic slab are inferred to have transported preexisting recycled Pacific oceanic crust from the MTZ into the overlying asthenosphere mantle. The upwelling Pacific oceanic crust reacted with asthenospheric mantle peridotite (i.e., DMM) to produce mantle pyroxenite, whose partial melting at shallow depths generated the CBs. Considering the low δ98/95Mo values of both the CBs and coeval potassic basalts from NE China, we speculate that there may be a low δ98/95Mo reservoir in the MTZ beneath NE China.

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Williamson, Nicole M. B., Dominique Weis, James S. Scoates et Michael O. Garcia. « Emergence of the Loa Mantle Component in the Hawaiian Islands Based on the Geochemistry of Kauaʻi Shield‐Stage Basalts ». Geochemistry, Geophysics, Geosystems 24, n^o 10 (octobre 2023). http://dx.doi.org/10.1029/2023gc010980.

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AbstractKauaʻi shield‐stage lavas are central to understanding the origin of the distinct Kea and Loa Hawaiian geochemical trends in Hawaiian basalts. These trends reflect two geochemically distinct sides in the Hawaiian plume, with Loa to the southwest and Kea to the northeast. The geochemistry and Sr‐Nd‐Hf isotopic compositions of shield‐stage lavas from Kauaʻi show a transition from Kea to Loa across the island with the Loa mantle source becoming dominant as the volcano grew. This geochemical transition is gradual from west to east Kauaʻi and supports the hypothesis that the Kauaʻi volcano sampled both sides of the bilateral Hawaiian plume, a phenomenon that is unusual for a Hawaiian volcano. Notably, Kauaʻi marks the arrival of progressively larger volumes of Loa compositions within the Hawaiian mantle plume. The new data from Kauaʻi, combined with an updated and comprehensive database of Hawaiian shield‐stage major element oxides, trace element concentrations, and isotopic compositions normalized to the same standard values, allows for the Pb‐Sr‐Nd‐Hf isotopic compositions of the Average Loa (‘ALOA’) common geochemical component to be estimated. Despite the bilateral Loa‐Kea geochemical trend beginning at Molokaʻi, Loa compositions dominate the erupted volume of Hawaiian volcanoes younger than 3 Ma, validating the volumetric importance of the Loa source in the lower mantle portion of the Hawaiian plume.

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Jung, S., J. A. Pfänder, O. Nebel, M. Willbold, S. Hoernes, J. Berndt et A. Pack. « High-K andesites as witnesses of a continental arc system in the Western Alps, Italy : constraints from HFSE and Hf–Nd–Sr–Pb–O isotope systematics ». Contributions to Mineralogy and Petrology 178, n^o 2 (24 janvier 2023). http://dx.doi.org/10.1007/s00410-022-01983-w.

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AbstractGeochemical and isotopic data are presented for ~ 32 Ma-old high-K andesites and dacites from the Alpine Chain. The samples consist of plagioclase, amphibole, titanomagnetite and rare biotite and quartz. Geochemical and isotope data indicate that slab-derived fluids, sediment melts and presumably AFC processes involving continental crust played a key role in the petrogenesis of the high-K rocks. A contribution of fluids is suggested based on the overall enrichment of large-ion lithophile elements and related high Ba/La, Ba/Zr, Ba/Th, Ba/Nb and Pb/Nd, sometimes distinctively higher than average continental crust. Positively correlated Ba/Nb–Th/Nb relationships, low Ce/Pb, low Nb/U and a negative correlation of Pb isotopes with Ce/Pb and Nb/U and positive ∆ 7/4 and ∆ 8/4 values similar to GLOSS imply the additional involvement of a sediment-derived melt. Negatively correlated Nb/Ta–Zr/Hf ratios at overall low Nb/Ta (13–7.5) are best explained by parental magma differentiation involving amphibole and biotite in a continental arc system. The samples have moderately unradiogenic Nd (εNd: – 2.0 to – 6.7) and radiogenic 87Sr/86Sr isotope compositions (0.7085–0.7113), moderately radiogenic Pb isotope compositions (206Pb/204Pb: 18.50–18.72; 207Pb/204Pb: 15.59–15.65; 208Pb/204Pb: 38.30–38.67), and elevated δ18O values (+ 6.5 to + 9.1 ‰). Epsilon Hf isotope values range from + 2.5 to – 4.0. Negative εHf(t) and εNd(t) values and 206Pb/204Pb ratios are correlated with elevated K2O abundances that indicate enrichment in K2O is related to AFC processes. The offset of εHf at a given εNd points to involvement of aged garnet-bearing crustal lithologies. The latter feature is qualitatively consistent with modification of unexposed primary basaltic andesites by AFC processes involving deep crustal material. In conclusion, in an Alpine context, inferred unexposed primitive high-K basaltic to andesitic melts are generated in the mantle wedge through fluid infiltration from the descending slab where fluids may have caused also partial melting of sedimentary rocks that mixed with evolving andesite–dacite compositions towards shallow-level intrusive and extrusive rocks. High-K and related trace element and isotope features thus result from a combination of already elevated values with participation of fluids and melts and probably AFC processes.

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