Journal articles on the topic 'Arc Crust'
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LEAT, P. T., R. D. LARTER, and I. L. MILLAR. "Silicic magmas of Protector Shoal, South Sandwich arc: indicators of generation of primitive continental crust in an island arc." Geological Magazine 144, no. 1 (October 27, 2006): 179–90. http://dx.doi.org/10.1017/s0016756806002925.
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Protector Shoal, the northernmost and most silicic volcano of the South Sandwich arc, erupted dacite–rhyolite pumice in 1962. We report geochemical data for a new suite of samples dredged from the volcano. Geochemically, the dredge and 1962 samples form four distinct magma groups that cannot have been related to each other, and are unlikely to have been related to a single basaltic parent, by fractional crystallization. Instead, the silicic rocks are more likely to have been generated by partial melting of basaltic lower crust within the arc. Trace element and Sr–Nd isotope data indicate that the silicic volcanics have compositions that are more similar to the volcanic arc than the oceanic basement formed at a back-arc spreading centre, and volcanic arc basalts are considered to be the likely source for the silicic magmas. The South Sandwich Islands are one of several intra-oceanic arcs (Tonga–Kermadec, Izu–Bonin) that have: (1) significant amounts of compositionally bimodal mafic–silicic volcanic products and (2) 6.0–6.5 km s−1P-wave velocity layers in their mid-crusts that have been imaged by wide-angle seismic surveys and interpreted as intermediate-silicic plutons. Geochemical and volume considerations indicate that both the silicic volcanics and plutonic layers were generated by partial melting of basaltic arc crust, representing an early stage in the fractionation of oceanic basalt to form continental crust.
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Davidson, Jon, Simon Turner, Heather Handley, Colin Macpherson, and Anthony Dosseto. "Amphibole “sponge” in arc crust?" Geology 35, no. 9 (2007): 787. http://dx.doi.org/10.1130/g23637a.1.
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Tomlinson, K. Y., R. P. Hall, D. J. Hughes, and P. C. Thurston. "Geochemistry and assemblage accretion of metavolcanic rocks in the Beardmore–Geraldton greenstone belt, Superior Province." Canadian Journal of Earth Sciences 33, no. 11 (November 1, 1996): 1520–33. http://dx.doi.org/10.1139/e96-115.
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The Beardmore–Geraldton greenstone belt lies between the Wabigoon volcanic arc (Onaman–Tashota terrane) and the Quetico metasedimentary subprovince and thus has an important bearing on the accretionary model that has been proposed for the amalgamation of these terranes. This paper presents geochemical evidence for the petrogenetic affinities of the volcanic units of the western half of the Beardmore–Geraldton greenstone belt. These data suggest that the metavolcanic rocks of the greenstone belt form a series of distinct packages. Trace element data are used to demonstrate the similarities and differences of each unit of lavas and to characterize their source region and likely tectono-magmatic setting. The data indicate that three separate fragments of volcanic crust representing oceanic crust, arc crust, and back-arc crust formed in a small arc system and were juxtaposed prior to collision with the Wabigoon arc. These fragments of crust were then accreted to the Wabigoon arc where sedimentation was followed by thrusting and folding resulting in shortening of the belt. Delamination of the volcanic units is thought to have been responsible for the preservation of just the pillow lava sequence of oceanic crust. Lower crustal or crust–mantle delamination of the Wawa arc and underplating of the Quetico are thought to have been responsible for the late and long-lived, high-grade metamorphic event in the Quetico and such "flake tectonics" are thought to have been an important process in the interaction between the Wabigoon, Quetico, and Wawa subprovinces.
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Lee, Cin-Ty A., Peter Luffi, Emily J. Chin, Romain Bouchet, Rajdeep Dasgupta, Douglas M. Morton, Veronique Le Roux, Qing-zhu Yin, and Daphne Jin. "Copper Systematics in Arc Magmas and Implications for Crust-Mantle Differentiation." Science 336, no. 6077 (April 5, 2012): 64–68. http://dx.doi.org/10.1126/science.1217313.
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Arc magmas are important building blocks of the continental crust. Because many arc lavas are oxidized, continent formation is thought to be associated with oxidizing conditions. On the basis of copper’s (Cu’s) affinity for reduced sulfur phases, we tracked the redox state of arc magmas from mantle source to emplacement in the crust. Primary arc and mid-ocean ridge basalts have identical Cu contents, indicating that the redox states of primitive arc magmas are indistinguishable from that of mid-ocean ridge basalts. During magmatic differentiation, the Cu content of most arc magmas decreases markedly because of sulfide segregation. Because a similar depletion in Cu characterizes global continental crust, the formation of sulfide-bearing cumulates under reducing conditions may be a critical step in continent formation.
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Lee, Cin-Ty, and Boda Liu. "Thick crust, hydrous magmas, and the paradox of voluminous cold magmatism." Volcanica 4, no. 2 (October 25, 2021): 227–38. http://dx.doi.org/10.30909/vol.04.02.227238.
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Andesites are refined and “cold” magmas compared to their basaltic parents, yet large volumes of andesites are generated at continental arcs. We show that large andesitic plutons are favored when arc crust attains a thickness of ~60 km while mafic plutons are small and favored when arc crust is thin. Using simple thermal models, we show that large, long-lived and relatively cold partially molten zones, sustained by recharge of hydrous basaltic magmas, are favored at depth when arc crust is thick due to the reduced efficiency of heat loss with increasing crustal thickness. Thin crust and drier magmas favor hotter and thinner partially molten zones. Our study provides an explanation for the apparent paradox that the most voluminous magmas in continental arc settings are cold. The origin of andesites may be linked to the interplay between magmatic differentiation, the availability of water, and the processes that control crustal thickness.
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Jagoutz, Oliver, Pierre Bouilhol, Urs Schaltegger, and Othmar Müntener. "The isotopic evolution of the Kohistan Ladakh arc from subduction initiation to continent arc collision." Geological Society, London, Special Publications 483, no. 1 (September 19, 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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Kent, Adam J. R. "Quaternary Volcanism in the Cascade Arc." Elements 18, no. 4 (August 1, 2022): 232–38. http://dx.doi.org/10.2138/gselements.18.4.232.
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The Cascade arc has produced a remarkable diversity of volcanic rocks over the Quaternary period. The major stratovolcanoes that define the arc front are dominated by eruptions of andesitic and dacitic intermediate magmas, produced largely by fractionation, melting, assimilation, and mixing within the crust. In addition, relative to many other subduction zones, the arc has produced significant mafic volcanism. These more primitive magmas reveal complexity in mantle wedge dynamics, sources, and magma production processes, and suggest that there are significant differences along the arc in the amount of magma that enters the lower Cascade crust from the underlying mantle.
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Liu, Xue Long, Na Zhang, and Jian Kang. "The Lead Isotope Characteristics and Tracing Significance of Ore Metallogenic Material in the Geza Arc,Yunnan." Advanced Materials Research 1073-1076 (December 2014): 2054–57. http://dx.doi.org/10.4028/www.scientific.net/amr.1073-1076.2054.
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Geza arc is the important parts of Yidun island arc in southwest of Sanjiang tectonic magmatic belts, it located in the southern tip of the Yidun island arc, which is a newly discovered copper polymetallic ore concentration area in the recently years in China. Based on the development stage of island arc orogenic, the distribution of intrusive rocks, composition, geochemical characteristics, Geza island arc granits belt can be divided into three belts. Geza island arc several typical porphyry deposits Pb isotopic data show that Pb206/Pb204 17.680~19.165, Pb207/Pb204 15.453~15.773,change in scope, Pb208/Pb204 37.730~39.654. Most of samples are normal lead, Pb isotopes focused on the side of orogenic evolution line and the lower crust range,with the characteristics of crust-mantle mixed source.
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CHEN, BIN, and BOR-MING JAHN. "Geochemical and isotopic studies of the sedimentary and granitic rocks of the Altai orogen of northwest China and their tectonic implications." Geological Magazine 139, no. 1 (January 2002): 1–13. http://dx.doi.org/10.1017/s0016756801006100.
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The Altai orogen (northwest China) represents the southwestern margin of the Central Asian Orogenic Belt. Geochemical and Nd–Sr isotope analyses were carried out on the Palaeozoic sedimentary and granitic rocks in order to trace their sources and to evaluate the pattern of continental growth of the orogen. Nd isotopic data for both the granites and sediments suggest a significant proportion of middle Proterozoic crust beneath the Altai orogen. However, addition of juvenile material (arc/back-arc oceanic crust) during Palaeozoic times is also significant. Trace elements and isotopic data of sediments suggest their sources were immature. They represent mixtures between a Palaeozoic juvenile component and an evolved continental crust. The early Palaeozoic sediments show εNd(T) = −3.4 to −5.0, TDM = 1.5–1.8 Ga, and ISr = 0.710–0.712. They represent a passive margin setting, with a predominance of evolved crustal material in the source. The Devonian sequences, however, might have been deposited in a back-arc basin setting, produced by subduction of the Junggar oceanic crust along the Irtysh fault. A significant addition of arc material into the sedimentary basin is responsible for the highly variable εNd values (−6 to 0) and ISr (0.711–0.706). The Carboniferous rocks were also deposited in a back-arc basin setting but with predominantly arc material in the source as suggested by an abrupt increase in εNd(T) (+6 to +3) and decrease in ISr (0.7045–0.7051). Voluminous syn-orogenic granitoids have εNd(T) = +2.1 to −4.3, ISr = 0.705–0.714 and TDM = 0.7–1.6 Ga. They were not derived by melting of local metasedimentary rocks as suggested by previous workers, but by melting of a more juvenile source at depth. Post-orogenic granites have higher εNd(T) (∼ +4.4) than the syn-orogenic granitoids, indicating their derivation from a deeper crustal level where juvenile crust may predominate.
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Kepezhinskas, Pavel, Nikita Kepezhinskas, and Nikolai Berdnikov. "Gold, platinum and palladium enrichments in arcs: role of mantle wedge, arc crust and halogen-rich slab fluids." E3S Web of Conferences 98 (2019): 08010. http://dx.doi.org/10.1051/e3sconf/20199808010.
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Arc-related magmas are frequently enriched in Au, Pt and Pd in respect to MORB and OIB igneous suites. Magmatic arcs commonly host large-scale hydrothermal Au and Au-Cu and PGE mineralization related to young volcanic systems and zoned ultramafic complexes respectively. Island-arc mantle xenoliths show Au, Pt, Pd enrichments related to mantle wedge metasomatism by slab-derived fluids. Long-lived plumbing systems in arc crust (arc magma chambers) show further enhancement of Au, Pt and Pd enrichments through subduction-related metamorphic and metasomatic processes in the presence of halogen-rich, aqueous fluids. We propose that Au-Pt-Pd enrichments in arcs are caused by mantle wedge-slab interactions followed by differentiation and metamorphism of magmatic conduits in arc crust.
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Niitsuma, Nobuaki. "Rupture and delamination of arc crust rupture and delamination of island arc crust due to the arc-arc collision in the South Fossa Magna, central Japan." Island Arc 8, no. 4 (December 1999): 441–58. http://dx.doi.org/10.1046/j.1440-1738.1999.00255.x.
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DRAUT, AMY E., PETER D. CLIFT, DAVID M. CHEW, MATTHEW J. COOPER, REX N. TAYLOR, and ROBYN E. HANNIGAN. "Laurentian crustal recycling in the Ordovician Grampian Orogeny: Nd isotopic evidence from western Ireland." Geological Magazine 141, no. 2 (March 2004): 195–207. http://dx.doi.org/10.1017/s001675680400891x.
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Because magmatism associated with subduction is thought to be the principal source for continental crust generation, assessing the relative contribution of pre-existing (subducted and assimilated) continental material to arc magmatism in accreted arcs is important to understanding the origin of continental crust. We present a detailed Nd isotopic stratigraphy for volcanic and volcaniclastic formations from the South Mayo Trough, an accreted oceanic arc exposed in the western Irish Caledonides. These units span an arc–continent collision event, the Grampian (Taconic) Orogeny, in which an intra-oceanic island arc was accreted onto the passive continental margin of Laurentia starting at ∼ 475 Ma (Arenig). The stratigraphy corresponding to pre-, syn- and post-collisional volcanism reveals a progression of εNd(t) from strongly positive values, consistent with melt derivation almost exclusively from oceanic mantle beneath the arc, to strongly negative values, indicating incorporation of continental material into the melt. Using εNd(t) values of meta-sediments that represent the Laurentian passive margin and accretionary prism, we are able to quantify the relative proportions of continent-derived melt at various stages of arc formation and accretion. Mass balance calculations show that mantle-derived magmatism contributes substantially to melt production during all stages of arc–continent collision, never accounting for less than 21% of the total. This implies that a significant addition of new, rather than recycled, continental crust can accompany arc–continent collision and continental arc magmatism.
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Chiaradia, Massimo, Lluís Fontboté, and Agustín Paladines. "Metal Sources in Mineral Deposits and Crustal Rocks of Ecuador (1° N–4° S): A Lead Isotope Synthesis." Economic Geology 99, no. 6 (September 1, 2004): 1085–106. http://dx.doi.org/10.2113/econgeo.99.6.1085.
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Abstract Ecuador consists of terranes having both continental (Chaucha, Tahuin, Loja terranes) and oceanic (Macuchi, Alao, Salado terranes) affinity, which were accreted to the Amazon craton from Late Jurassic to Eocene. Four main magmatic arcs were formed by the subduction of the Farallon/Nazca plate since the Jurassic: a Jurassic continental arc on the western margin of the Amazon craton, a Jurassic island arc (Alao terrane), an early Tertiary island arc (Macuchi terrane), and a middle-late Tertiary continental arc encompassing the terranes of Macuchi, Chaucha, Tahuin, Loja, and Alao after complete assembly of the Ecuadorian crust. Mineral deposits formed during these magmatic arc activities include porphyry-Cu and gold skarn deposits in association with the Jurassic continental arc, polymetallic volcanic-hosted massive sulfide deposits (VHMS) in association with the Jurassic island arc of Alao, Au-Cu-Zn VHMS deposits in association with the early Tertiary island arc of Macuchi, and porphyry-Cu and precious-metal epithermal deposits in association with the middle-late Tertiary continental-arc magmatism on the newly assembled crust of Ecuador (Macuchi, Chaucha, Tahuin, Loja, and Alao terranes). In this study, we have compiled 148 new and 125 previously published lead isotope analyses on Paleozoic to Miocene metamorphic, intrusive, volcanic, and volcanosedimentary rocks, as well as on Jurassic to Miocene magmatic-related ore deposits of Ecuador. Lead isotope compositions of the magmatic rocks of the four main arc events derive from mixing of various sources including mantle, variably enriched by pelagic sediments and/or by a high 238U/204Pb component, and heterogeneous continental crust rocks. Lead isotope compositions of the Ecuadorian ore deposits display a broad range of values (206Pb/204Pb = 18.3–19.3, 207Pb/204Pb = 15.54–15.74, 208Pb/204Pb = 38.2–39.2), which is as large as the range previously reported for all magmatic-related ore deposits of the Central Andean provinces I and II combined. Ore deposits formed before complete assembly of the Ecuadorian crust through complete accretion of the several terranes (i.e., pre-Eocene) have lead isotope compositions overlapping those of the associated magmatic rocks, suggesting a largely magmatic origin for their lead. In contrast, post-assembly ore deposits (i.e., post-Eocene) have lead isotope compositions that only partly overlap those of the coeval magmatic rocks of the continental arc. In fact, several ore deposits have lead isotope compositions shifted toward those of the basement rocks that host them, suggesting that lead derives from a mixture of magmatic lead and basement-rock lead leached by hydrothermal fluids. Most Ecuadorian ores have high 207Pb/204Pb values (>15.55), suggesting a dominant continental crust or pelagic sediment origin of the lead. However, we caution against concluding that chalcophile metals (for example, Cu and Au) also have a continental crust origin. Ore deposits of the different terranes of Ecuador, irrespective of their age, plot in distinct isotopic fields, which are internally homogeneous. This suggests that lithologic factors had an important control on the lead isotope compositions. Ultimately, lead isotope compositions of the ore deposits of Ecuador mirror the isotopic compositions of the rocks of the host terranes and are consistent with the multiterrane nature of the Ecuadorian crust.
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Zhang, Pingchuan, Changqing Yu, and Xiangzhi Zeng. "Crustal Electrical Structure of the Zhaheba Complex Imaged by Magnetotelluric Data and Its Tectonic Implications." Applied Sciences 11, no. 21 (October 26, 2021): 10013. http://dx.doi.org/10.3390/app112110013.
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A Magnetotelluric profile stretching northward from the Wulungu Depression (on the northern margin of the Junggar Basin) to the Dulate arc (crossing the Zhaheba–Aermantai ophiolite belt) was carried out in an attempt to probe the crustal structure and properties of the East Junggar, NW China. Along the profile, the inversion model was used to determine the electrical structure of the crust and uppermost mantle. The results revealed that the crust of the eastern Junggar Basin is composed of the shallow low resistivity layer and underlying high resistivity bodies. There is a crustal detachment in the basement: the upper layer is a Hercynian folded basement and the lower is a Precambrian basement. The Zhaheba complex is characterized by relatively high resistivity, with a thickness of ~5 km, the bottom controlled by the Zhaheba–Aermantai fault. The crust of the Yemaquan arc is composed of the residual continental crust, characterized by stable resistance. The exposed intrusive rocks are characterized by irregular resistors. The crust of the Dulate arc is characterized by relatively low resistivity. The shallow low resistivity layers represent the Zhaheba depression composed of the Devonian-Permian volcanic and sedimentary rocks. The crustal conductive anomalies are related to the magmatism and mechanism of metal deposits in the post-collision period.
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Demina, L. I., V. S. Zakharov, and M. Yu Promyslova. "Introduction of the Stanovsky ophiolites of Faddesky block of Northeasten Taimyr according to geological data and numerical modeling results." Moscow University Bulletin. Series 4. Geology, no. 1 (December 15, 2022): 24–34. http://dx.doi.org/10.33623/0579-9406-2022-1-24-34.
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The geological position, morphology of serpentinite bodies, limitation by faults, the presence of surrounding metamorphites as enclosing strata fragments within the Stanovsky ophiolite complex, the nature of metamorphism, high deformation of rocks of both ophiolites and contact zone strata, and the mineral parageneses of secondary transformations fully correspond to the signs of Stanovsky ophiolites introduction into the deeply metamorphosed Faddeevsky block strata of Northeastern Taimyr, and not their obduction. The modeling results showed that fragments of the oceanic crust introduced into the metamorphosed complexes of the continental crust during the collision can have a dual origin — from the primary oceanic crust, and from the newly formed crust of the back-arc basin. A detailed chemical composition analysis of the Stanovsky ophiolites indicates their formation in the suprasubduction setting of the back-arc basin, which justifies the possibility of the second scenario.
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MOUMBLOW, R. M., G. A. ARCURI, A. P. DICKIN, and C. F. GOWER. "Nd and Pb isotope mapping of crustal domains within the Makkovik Province, Labrador." Geological Magazine 156, no. 5 (April 3, 2018): 833–48. http://dx.doi.org/10.1017/s0016756818000195.
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AbstractThe Makkovik Province of eastern Labrador represents part of an accretionary orogen active during an early stage in the development of the Palaeoproterozoic southern Laurentian continental margin. New Nd isotope data for the eastern Makkovik Province suggest that accreted juvenile Makkovik crust was generated in the Cape Harrison domain during a single crust-forming event at c. 2.0 Ga. Pb isotope data support this model, and show a strong similarity to radiogenic crustal signatures in the juvenile Palaeoproterozoic crust of the Ketilidian mobile belt of southern Greenland. As previously proposed, an arc accretion event at c. 1.9 Ga triggered subduction-zone reversal and the development of an ensialic arc on the composite margin. After the subduction flip, a temporary release of compressive stress at c. 1.87 Ga led to the development of a retro-arc foreland basin on the downloaded Archean continental edge, forming the Aillik Group. Unlike previous models, a second arc is not envisaged. Instead, a compressive regime at c. 1.82 Ga is attributed to continued ensialic arc plutonism on the existing margin. The tectonic model for the Makkovikian orogeny proposed here is similar to that for the Ketilidian orogeny. Major- and trace-element analyses suggest that much of the magmatism in the Makkovik orogen results from post-accretionary ensialic arc activity, and that few vestiges remain of the original accreted volcanic arc. This pattern of arc accretion and intense post-accretion reworking is common to many accretionary orogens, such as the South American Andes and North American Cordillera.
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Suda, Yoshimitsu, Yasutaka Hayasaka, and Kosuke Kimura. "Crustal Evolution of a Paleozoic Intra-oceanic Island-Arc-Back-Arc Basin System Constrained by the Geochemistry and Geochronology of the Yakuno Ophiolite, Southwest Japan." Journal of Geological Research 2014 (May 28, 2014): 1–10. http://dx.doi.org/10.1155/2014/652484.
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The Yakuno ophiolite in southwest Japan is considered to have been obducted by the collision between an intra-oceanic island-arc-back-arc basin (intra-OIA-BAB) system and the East Asian continent during the late Paleozoic. New SIMS (SHRIMP) zircon U-Pb determinations for amphibolite and metagabbro of BAB origin within the Yakuno ophiolite yield ages of 293.4 ± 9.5 Ma and 288 ± 13 Ma, respectively. These ages are slightly older (however, overlapping within analytical errors) than the magmatic age of arc granitoids (ca. 285–282 Ma) that intruded into the mafic rocks of BAB origin. Results from geochronological and geochemical data of the Yakuno ophiolite give rise to the following tentative geotectonic model for the Paleozoic intra-OIA-BAB system: the initial stage of BAB rifting (ca. 293–288 Ma) formed the BAB crust with island-arc basalt (IAB) signatures, which was brought to the OIA setting, and generated the arc granitoids (ca. 285–282 Ma) by anatexis of the BAB crust. A later stage of BAB rifting (<ca. 285 Ma) formed the BAB crust with IAB to MORB signatures, on which the Permian sediments were conformably deposited. These components collided with the eastern margin of the East Asian continent during the early Mesozoic.
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Ketchum, John W. F., Nicholas G. Culshaw, and Sandra M. Barr. "Anatomy and orogenic history of a Paleoproterozoic accretionary belt: the Makkovik Province, Labrador, Canada." Canadian Journal of Earth Sciences 39, no. 5 (May 1, 2002): 711–30. http://dx.doi.org/10.1139/e01-099.
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The Makkovik Province is a segment of a Paleoproterozoic accretionary belt (the MakkovikKetilidian orogen) that developed on the southern margin of Laurentia at 1.91.7 Ga. In contrast to coeval Laurentian orogenic belts that mainly resulted from collision of Archean plates, MakkovikianKetilidian orogenesis was dominated by active-margin processes including continental margin arc plutonism and juvenile terrane accretion, both of which were accompanied by regional transpression. In the Makkovik Province, earliest deformation and amphibolite-facies metamorphism of Paleoproterozoic riftdrift assemblages (Post Hill and Moran Lake groups) and the Archean foreland (Nain Province) occurred at 1.9 Ga in response to accretion of a Paleoproterozoic island arc. Following this collision, cratonward-dipping subduction was established, resulting in the formation of the 18951870 Ma Island Harbour Bay Plutonic Suite, a calc-alkaline magmatic arc built on reworked Archean crust. Crust formation continued between ca. 1860 and 1850 Ma with deposition of the Aillik Group on a largely juvenile basement in a rifted-arc or back-arc setting. Sometime before 1802 Ma this depositional basin was tectonically inverted, with resultant northwestward thrusting of the Aillik Group over reworked Archean crust. This phase of deformation may have been driven by accretion of a second island arc potentially represented by the Cape Harrison Metamorphic Suite. Regional transpression and amphibolite-facies metamorphism at ca. 18151780 Ma were accompanied by widespread granitoid plutonism. These events were mainly concentrated in the juvenile domains and are thought to reflect processes in a broad continental back-arc setting. A final orogenic pulse, marked by regional greenschist-facies transpression and emplacement of A-type granitoid plutons, occurred between 1740 and 1700 Ma, with deformation and plutonism potentially linked to crustmantle detachment and incursion of mafic magmas at the base of the crust, respectively. The record of crustal development suggests that the coeval themes of spatially and temporally linked structural and plutonic activity, oceanward migration of this activity over time, and a trend toward increasingly more localized deformation occurred throughout the orogenic history of the Makkovik Province. These characteristics are thought to broadly reflect oceanward crustal growth of the orogen over time. In the correlative Ketilidian mobile belt of southern Greenland, these themes were also operative but appear to have been less pronounced, most likely due to minimal or a complete absence of accretion of island-arc material.
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Tang, Yu-Wei, Long Chen, Zi-Fu Zhao, and Yong-Fei Zheng. "Geochemical evidence for the production of granitoids through reworking of the juvenile mafic arc crust in the Gangdese orogen, southern Tibet." GSA Bulletin 132, no. 7-8 (November 7, 2019): 1347–64. http://dx.doi.org/10.1130/b35304.1.
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Abstract Although continental crust is characterized by the widespread occurrence of granitoids, the causal relationship between continental crust growth and granitic magmatism still remains enigmatic. While fractional crystallization of basaltic magmas (with or without crustal contamination) and partial melting of mafic lower crust are two feasible mechanisms for the production of granitoids in continental arc regions, the problem has been encountered in discriminating between the two mechanisms by whole-rock geochemistry. This can be resolved by an integrated study of zircon U-Pb ages and Hf-O isotopes together with whole-rock major-trace elements and Sr-Nd-Pb isotopes, which is illustrated for Mesozoic granitoids from the Gangdese orogen in southern Tibet. The results provide geochemical evidence for prompt reworking of the juvenile mafic arc crust in the newly accreted continental margin. The target granitoids exhibit high contents of SiO2 (65.76–70.75 wt%) and Na2O + K2O (6.38–8.15 wt%) but low contents of MgO (0.19–0.98 wt%), Fe2O3 (0.88–3.13 wt%), CaO (2.00–3.82 wt%), Ni (<5.8 ppm), and Cr (≤10 ppm). They are enriched in large ion lithophile elements, Pb, and light rare earth elements but depleted in high field strength elements. The granitoids are relatively depleted in whole-rock Sr-Nd isotope compositions with low (87Sr/86Sr)i ratios of 0.7043–0.7048 and positive εNd(t) values of 0.5–2.6, and have relatively low 207Pb/204Pb and 208Pb/204Pb ratios at given 206Pb/204Pb ratios. Laser ablation–inductively coupled plasma–mass spectrometry and secondary ion mass spectrometry U-Pb dating on synmagmatic zircons yield ages of 77 ± 2–81 ± 1 Ma in the Late Cretaceous for their emplacement. Relict zircons have two groups of U-Pb ages in the late Mesozoic and the late Paleozoic, respectively. The whole-rock Sr-Nd isotopes in the granitoids are quite similar to those of Late Cretaceous mafic rocks in the Gangdese batholith. In addition, both synmagmatic zircons and relict zircons with Late Cretaceous U-Pb ages exhibit almost the same Hf-O isotope compositions to those of the slightly earlier mafic rocks. All these observations indicate that the granitoids were mainly derived from partial melting of the juvenile mafic arc crust. Therefore, reworking of the juvenile mafic arc crust is the mechanism for the origin of isotopically depleted granitoids in southern Tibet. It is this process that leads to differentiation of the juvenile mafic arc crust toward the felsic lithology in the continental arc. In this regard, the granitoids with depleted radiogenic isotope compositions do not necessarily contribute to the crustal growth at convergent plate boundaries.
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Cochelin, Bryan, Dominique Chardon, Yoann Denèle, Charles Gumiaux, and Benjamin Le Bayon. "Vertical strain partitioning in hot Variscan crust: Syn-convergence escape of the Pyrenees in the Iberian-Armorican syntax." Bulletin de la Société géologique de France 188, no. 6 (2017): 39. http://dx.doi.org/10.1051/bsgf/2017206.
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A new structural map of the Paleozoic crust of the Pyrenees based on an extensive compilation and new kinematic data allows for the evaluation of the mechanical coupling between the upper and lower crust of the abnormally hot foreland of the Variscan orogen of SW Europe. We document partitioning between coeval lower crustal lateral flow and upper crustal thickening between 310 and 290 Ma under an overall dextral transpressive regime. Partitioning also involved syn-convergence transtensional gneiss domes emplacement during this period. Late orogen-normal shortening of the domes and strain localization in steep crustal-scale transpressive shear zones reflects increasing coupling between the lower crust and the upper crust. The combination of dextral transpression and eastward flow in the Pyrenees results from the shortening and lateral escape of a hot buoyant crust along the inner northern limb of the closing Cantabrian orocline at the core of the Iberian-Armorican arc between ca. 305 and 295 Ma. Delamination or thermal erosion of the lithosphere enhanced orocline closure and explains (1) the switch from crust- to mantle-derived magmatism in the Iberian-Armorican arc and (2) the abnormally hot and soft character of the Pyrenean crust that escaped the closing syntax.
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Petterson, M. G. "The plutonic crust of Kohistan and volcanic crust of Kohistan–Ladakh, north Pakistan/India: lessons learned for deep and shallow arc processes." Geological Society, London, Special Publications 483, no. 1 (July 30, 2018): 123–64. http://dx.doi.org/10.1144/sp483.4.
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AbstractThe Kohistan–Ladakh terrane, northern Pakistan/India, offers a unique insight into whole-arc processes. This research review presents summaries of fundamental crustal genesis and evolution models. Earlier work focused on arc sequence definition. Later work focused on holistic petrogenesis. A new model emerges of an unusually thick (c. 55 km) arc with a c. 30 km-thick batholith. Volatile-rich, hornblende ± garnet ± sediment assimilation-controlled magmatism is predominant. The thick batholith has a complementary mafic–ultramafic residue. Kohistan crustal SiO2 contents are estimated at >56%. The new-Kohistan, silicic-crust model contrasts with previous lower SiO2 estimates (c. 51% SiO2 crust) and modern arcs that imply <35 km crustal thicknesses and arc batholith thicknesses of c. 7 km. A synthetic overview of Kohistan–Ladakh volcanic rocks presents a model of an older, cleaved/deformed Cretaceous volcanic system at least 800 km across strike. The Jaglot–Chalt–Dras–Shyok volcanics exhibit predominant tholeiitic-calc-alkaline signatures, with a range of arc-related facies/tectonic settings. A younger, post-collisional, Tertiary silicic volcanic system (the Shamran–Dir–Dras-2–Khardung volcanics) lie unconformably upon Cretaceous basement, and erupted within an intra-continental tectonic setting. Kohistan–Ladakh tectonic model controversies remain. In essence, isotope-focused researchers prefer later (Tertiary) collisions, whilst structural field-geology-orientated researchers prefer an older (Cretaceous) age for the Northern/Shyok Suture.
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Nathwani, Chetan L., Matthew A. Loader, Jamie J. Wilkinson, Yannick Buret, Robert H. Sievwright, and Pete Hollings. "Multi-stage arc magma evolution recorded by apatite in volcanic rocks." Geology 48, no. 4 (January 17, 2020): 323–27. http://dx.doi.org/10.1130/g46998.1.
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Abstract Protracted magma storage in the deep crust is a key stage in the formation of evolved, hydrous arc magmas that can result in explosive volcanism and the formation of economically valuable magmatic-hydrothermal ore deposits. High magmatic water content in the deep crust results in extensive amphibole ± garnet fractionation and the suppression of plagioclase crystallization as recorded by elevated Sr/Y ratios and high Eu (high Eu/Eu*) in the melt. Here, we use a novel approach to track the petrogenesis of arc magmas using apatite trace element chemistry in volcanic formations from the Cenozoic arc of central Chile. These rocks formed in a magmatic cycle that culminated in high-Sr/Y magmatism and porphyry ore deposit formation in the Miocene. We use Sr/Y, Eu/Eu*, and Mg in apatite to track discrete stages of arc magma evolution. We apply fractional crystallization modeling to show that early-crystallizing apatite can inherit a high-Sr/Y and high-Eu/Eu* melt chemistry signature that is predetermined by amphibole-dominated fractional crystallization in the lower crust. Our modeling shows that crystallization of the in situ host-rock mineral assemblage in the shallow crust causes competition for trace elements in the melt that leads to apatite compositions diverging from bulk-magma chemistry. Understanding this decoupling behavior is important for the use of apatite as an indicator of metallogenic fertility in arcs and for interpretation of provenance in detrital studies.
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Bédard, Jean H. "Parental magmas of Grenville Province massif-type anorthosites, and conjectures about why massif anorthosites are restricted to the Proterozoic." Earth and Environmental Science Transactions of the Royal Society of Edinburgh 100, no. 1-2 (March 2009): 77–103. http://dx.doi.org/10.1017/s1755691009016016.
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ABSTRACTTrace element inversion modelling of Grenvillean anorthosite massifs and associated rocks yield NMORB-normalised trace element profiles enriched in highly incompatible elements; commonly with negative Nb and Th anomalies. Model melts can be divided into subtypes that cannot be linked through fractional crystallisation processes. Most model melts are depleted in the heavy rare-earth elements and can be explained by partial melting of arc basaltic sources (5–60 melting ) with garnet-bearing residues. Some of the model melts have flat NMORB-normalised profiles (for rare-earth elements), have high compatible element contents, and might have been derived from mantle fertilised by arc magmatism, followed by low-pressure fractional crystallisation. Intermediate Ce/Yb types may represent mixtures of these end-members, or less probably, variations in the crustal source composition and residual assemblage. The active tectonic context now favoured for the Grenville Province appears to be inconsistent with plume or thermal insulation models. The heat source for crustal and mantle melting could record either post-orogenic thermal relaxation of a tectonically-thickened arc crust, or basaltic underplating caused by delamination of a mantle root or subduction slab beneath this arc crust. In this context, pre-Proterozoic anorthosites may be lacking, because prior to ca. 2·5 Ga, the crust may have been too weak to be thickened tectonically. The absence of post-Proterozoic anorthosites may be due to the secular decrease in radiogenic heating and cooling of the mantle and crust.
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Jolivet, Laurent, Armel Menant, Vincent Roche, Laetitia Le Pourhiet, Agnès Maillard, Romain Augier, Damien Do Couto, Christian Gorini, Isabelle Thinon, and Albane Canva. "Transfer zones in Mediterranean back-arc regions and tear faults." BSGF - Earth Sciences Bulletin 192 (2021): 11. http://dx.doi.org/10.1051/bsgf/2021006.
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Slab tearing induces localized deformations in the overriding plates of subduction zones and transfer zones accommodating differential retreat in back-arc regions. Because the space available for retreating slabs is limited in the Mediterranean realm, slab tearing during retreat has been a major ingredient of the evolution of this region since the end of the Eocene. The association of detailed seismic tomographic models and extensive field observations makes the Mediterranean an ideal natural laboratory to study these transfer zones. We review in this paper the various structures in back-arc regions differential retreat from the Alboran Sea to the Aegean-Anatolian region and discuss them with the help of 3D numerical models to better understand the partitioning of deformation between high-angle and low-angle faults, as well as the 3-D kinematics of deformation in the middle and lower crusts. Simple, archetypal, crustal-scale strike-slip faults are in fact rare in these contexts above slab tears. Transfer zones are in general instead wide deformation zones, from several tens to several hundred kilometers. A partitioning of deformation is observed between the upper and the lower crust with low-angle extensional shear zones at depth and complex association of transtensional basins at the surface. In the Western Mediterranean, between the Gulf of Lion and the Valencia basin, transtensional strike-slip faults are associated with syn-rift basins and lower crustal domes elongated in the direction of retreat (a-type domes), associated with massive magmatic intrusions in the lower crust and volcanism at the surface. On the northern side of the Alboran Sea, wide E-W trending strike-slip zones in the brittle field show partitioned thrusting and strike-slip faulting in the external zones of the Betics, and E-W trending metamorphic core complexes in the internal zones, parallel to the main retreat direction with a transition in time from ductile to brittle deformation. On the opposite, the southern margin of the Alboran Sea shows short en-échelon strike-slip faults. Deep structures are not known there. In the Aegean-Anatolian region, two main tear faults with different degrees of maturity are observed. Western Anatolia (Menderes Massif) and the Eastern Aegean Sea evolved above a major left-lateral tear in the Hellenic slab. In the crust, the differential retreat was accommodated mostly by low-angle shear zones with a constant direction of stretching and the formation of a-type high-temperature domes exhumed from the middle and lower crust. These low-angle shear zones evolve through time from ductile to brittle. On the opposite side of the Aegean region, the Corinth and Volos Rift as well as the Kephalonia fault offshore, accommodate the formation of a dextral tear fault. Here, only the brittle crust can be observed, but seismological data suggest low-angle shear zones at depth below the rifts. We discuss the rare occurrence of pure strike-slip faults in these contexts and propose that the high heat flow above the retreating slabs and more especially above slab tears favors a ductile behavior with distributed deformation of the crust and the formation of low-angle shear zones and high-temperature domes. While retreat proceeds, aided by tears, true strike-slip fault system may localize and propagate toward the retreating trench, ultimately leading to the formation of new plate boundary, as shown by the example of the North Anatolian Fault.
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Sharkov, E. V. "THE ORIGIN AND STRUCTURE OF THE LOWER CRUST OF OCEANS AND BACK-ARC SEAS: EVIDENCE FROM THE MARKOV DEEP (MID-ATLANTIC RIDGE) AND THE VOIKAR OPHIOLITE ASSOCIATION (POLAR URALS)." Geodynamics & Tectonophysics 10, no. 1 (March 23, 2019): 101–21. http://dx.doi.org/10.5800/gt-2019-10-1-0406.
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The Markov Deep (the axial part of the slow-spreading Mid-Atlantic Ridge, 6°N, Sierra Leone oceanic core complex) and the Paleozoic Voikar ophiolite association (Polar Urals) formed in the back-arc sea conditions. In both cases, the lower crust of a close structure was formed on the basements composed ofdepleted peridotites of the ancient lithospheric mantle. The available data show that the composition of the lower crust of the oceans and back-arc seas is dominated by layeredmafic-ultramafic intrusions originating from the MORB melts, and suggest a similar asthenospheric source of magmas. Sills and dykes formed from other magma sources represent the second structural element of the lower oceanic crust: in case of the ocean, mainly ferrogabbroids originating from specific melts with the OIB involvement, and, in case of the back-sea sea, gabbro-norites of the supra-subduction calc-alkaline series. In both cases, the upper crust originates frombasaltic flows that occurred later and are associated with new episodes in the tectonic development. According to [Sharkov,2012], the development of slow-spreading ridges takes place in discrete impulses and non-simultaneously along their entire length. Furthermore, oceanic core complexes (OCC) in their axial parts are the ridge segments, where spreading is resumed. At the OCC stage, newly formed basalt melts move upwards from the magma generation zone into fractures (dykes) through the lithospheric mantle, and the thickness of the lower crust is built up by sills. As spreading develops in this area, the crust becomes thicker from below due to underplating in form of large layered intrusions. The newly formed restites, in their turn, cause an increase in the lithospheric mantle thickness from below. Apparently, the lower crust formed in the back-arc seas according to a similar scenario, although complicated by the processes taking place in the subduction zone.
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Karlstrom, Leif, Josef Dufek, and Michael Manga. "Magma chamber stability in arc and continental crust." Journal of Volcanology and Geothermal Research 190, no. 3-4 (February 2010): 249–70. http://dx.doi.org/10.1016/j.jvolgeores.2009.10.003.
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Yanagi, Takeru, and Katsuyuki Yamashita. "Genesis of continental crust under island arc conditions." Lithos 33, no. 1-3 (October 1994): 209–23. http://dx.doi.org/10.1016/0024-4937(94)90061-2.
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Tenzer, Robert, and Peter Vajda. "Global maps of the step-wise topography corrected and crustal components stripped geoids using the CRUST 2.0 model." Contributions to Geophysics and Geodesy 39, no. 1 (January 1, 2009): 1–17. http://dx.doi.org/10.2478/v10126-009-0001-9.
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Global maps of the step-wise topography corrected and crustal components stripped geoids using the CRUST 2.0 modelWe compile global maps of the step-wise topography corrected and crustal components stripped geoids based on the geopotential model EGM'08 complete to spherical harmonic degree 180 and the CRUST 2.0 global crustal model. The spectral resolution complete to degree 180 is used to compute the primary indirect bathymetric stripping and topographic effects on the geoid, while degree 90 for the primary indirect ice stripping effect. The primary indirect stripping effects of the soft and hard sediments, and the upper, middle and lower consolidated crust components are forward modeled in spatial form using the 2 × 2 arc-deg discrete data of the CRUST 2.0 model. The ocean, ice, sediment and consolidated crust density contrasts are defined relative to the adopted reference crustal density of 2670 kg/m3. Finally we compute and apply the primary indirect stripping effect of the density contrast (relative to the mantle) of the reference crust. The constant value of -520 kg/m3is adopted for this density contrast relative to the mantle. All data are evaluated on a 1 × 1 arc-deg geographical grid. The complete crust-stripped geoidal undulations, globally having a range of approximately 1.5 km, contain the gravitational signal coming from the global mantle lithosphere (upper mantle) morphology and density composition, and from the sub-lithospheric density heterogeneities. Large errors in the complete crust-stripped geoid are expected due to uncertainties of the CRUST 2.0 model, i.e., due to deviations of the CRUST 2.0 model density from the real earth's crustal density and due to the Moho-boundary uncertainties.
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Sylvester, Paul J., Kodjo Attoh, and Klaus J. Schulz. "Tectonic setting of late Archean bimodal volcanism in the Michipicoten (Wawa) greenstone belt, Ontario." Canadian Journal of Earth Sciences 24, no. 6 (June 1, 1987): 1120–34. http://dx.doi.org/10.1139/e87-109.
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The tectono-stratigraphic relationships, depositional environments, rock associations, and major- and trace-element compositions of the late Archean (2744–2696 Ma) bimodal basalt–rhyolite volcanic rocks of the Michipicoten (Wawa) greenstone belt, Ontario, are compatible with an origin along a convergent plate margin that varied laterally from an immature island arc built on oceanic crust to a more mature arc underlain by continental crust. This environment is similar to that of the Cenozoic Taupo–Kermadec–Tonga volcanic zone. Michipicoten basaltic rocks, most of which are proximal deposits compositionally similar ([La/Yb]n = 0.63–1.18) to modern oceanic island-arc tholeiites, are interpreted as having formed along the largely submerged island arc. Voluminous Michipicoten rhyolitic pyroclastic rocks ([La/Yb]n = 4.3–18.7, Ybn = 5.7–15.9) probably erupted subaerially from the continental arc, with distal facies deposited subaqueously on the adjacent oceanic island arc and proximal facies deposited in subaerial and shallow subaqueous environments on, or along the flanks of, the continental arc. The compositional similarity between the lower (2744 Ma) and upper (2696 Ma) volcanic sequences of the belt suggests that this island- and continental-arc configuration existed for at least 45 Ma. The Michipicoten belt may be a remnant of a larger, laterally heterogeneous volcanic terrane that also included the Abitibi greenstone belt.
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Morris, Rebecca A., Susan M. DeBari, Cathy Busby, Sarah Medynski, and Brian R. Jicha. "Building Arc Crust: Plutonic to Volcanic Connections in an Extensional Oceanic Arc, the Southern Alisitos Arc, Baja California." Journal of Petrology 60, no. 6 (May 22, 2019): 1195–228. http://dx.doi.org/10.1093/petrology/egz029.
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Abstract The ∼50 km long Rosario segment of the Cretaceous Alisitos oceanic arc terrane provides undeformed three-dimensional exposures of the upper 7 km of an oceanic extensional arc, where crustal generation processes are recorded in both the volcanic and underlying plutonic rocks. These exceptional exposures allow for the study of the physical and chemical links between the rock units and help constrain the differentiation processes active during the growth and evolution of arc crust. This study focuses on the southern third of the Rosario segment, previously referred to as the southern volcano-bounded basin, and its plutonic underpinnings. Upper crustal rocks in the Rosario segment consist of a 3–5 km thick volcanic–volcaniclastic section with hypabyssal intrusions. Plutons intrude these units at various levels along-strike, but at each intrusive contact the transition is complete over a distance of <150 m, where stoped volcanic blocks are present. There is striking compositional overlap in whole-rock and mineral chemistry between the plutonic and volcanic units, suggesting a comagmatic source. Whole-rock geochemistry shows coherent trends in major and trace elements in mafic to intermediate compositions, but less coherent trends above 63 wt % SiO2. Units are predominantly low-K with flat rare earth element patterns, and show large ion lithophile element enrichment and high field strength element depletion. Initial Nd and Pb isotope ratios overlap for all units and imply no cratonic continental involvement. This agrees with low Sr/Y ratios of all rock types, indicative of thin, immature oceanic arc crust. Modeling results show that closed-system fractional crystallization drove crustal differentiation from mafic to intermediate compositions, but open-system processes likely occurred to produce some of the felsic compositions. Differentiation occurred in a two-step fractionation process. Step 1, from basaltic andesite to andesite, fractionated an anhydrous gabbroic cumulate (∼40% crystallization). Step 2, from andesite to rhyolite, fractionated a hydrous amphibole cumulate (∼65% crystallization, total), which is similar to what fluid dynamical models suggest for production of rhyolite (between 50–70% crystallization). Our results can be used as a reference model for differentiation processes relating to the growth of the middle and upper crust within active extensional arc systems. The Rosario segment plutonic rocks may be analogous to the low-velocity zone (Vp = 6·0–6·5 km s–1) imaged within the extensional Izu–Bonin arc. The chemistry of the plutonic and volcanic rocks is most similar to those of volcanic rocks in the Izu–Bonin active rift.
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Tatsumi, Y., N. Takahashi, S. Kodaira, and Y. Kaneda. "Arc evolution and continental crust formation at the Izu–Bonin–Mariana arc system." Geochimica et Cosmochimica Acta 70, no. 18 (August 2006): A638. http://dx.doi.org/10.1016/j.gca.2006.06.1188.
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Rivers, Toby, and David Corrigan. "Convergent margin on southeastern Laurentia during the Mesoproterozoic: tectonic implications." Canadian Journal of Earth Sciences 37, no. 2-3 (April 2, 2000): 359–83. http://dx.doi.org/10.1139/e99-067.
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A continental-margin magmatic arc is inferred to have existed on the southeastern (present coordinates) margin of Laurentia from Labrador to Texas from ~1500-1230 Ma, with part of the arc subsequently being incorporated into the 1190-990 Ma collisional Grenville Orogen. Outside the Grenville Province, where the arc is known as the Granite-Rhyolite Belt, it is undeformed, whereas within the Grenville Province it is deformed and metamorphosed. The arc comprises two igneous suites, an inboard, principally quartz monzonitic to granodioritic suite, and an outboard tonalitic to granodioritic suite. The quartz monzonite-granodiorite suite was largely derived from continental crust, whereas the tonalitic-granodiorite suite is calc-alkaline and has a juvenile isotopic signature. Available evidence from the Grenville Province suggests that the arc oscillated between extensional and compressional settings several times during the Mesoproterozoic. Back-arc deposits of several ages, that formed during relatively brief periods of extension, include (1) mafic dyke swarms subparallel to the arc; (2) continental sediments, bimodal volcanics and plateau basalts; (3) marine sediments and volcanics formed on stretched continental crust; and (4) ocean crust in a marginal basin. Closure of the back-arc basins occurred during the accretionary Pinwarian (~1495-1445 Ma) and Elzevirian (~1250-1190 Ma) orogenies, as well as during three pulses of crustal shortening associated with the 1190-990 Ma collisional Grenvillian Orogeny. During the Elzevirian Orogeny, closure of the Central Metasedimentary Belt marginal basin in the southeastern Grenville Province was marked by subduction-related magmatism as well as by imbrication of back-arc deposits. The presence of a continental-margin magmatic arc on southeastern Laurentia during the Mesoproterozoic implies that other coeval magmatism inboard from the arc took place in a back-arc setting. Such magmatism was widespread and chemically diverse and included large volume "anorogenic" anorthosite-mangerite-charnockite-granite (AMCG) complexes as well as small volume alkaline, quartz-saturated and -undersaturated "within-plate" granitoids. Recognition of the ~300 million year duration of the Mesoproterozoic convergent margin of southeastern Laurentia suggests that there may be useful parallels with the evolution of the Andes, which has been a convergent margin since the early Paleozoic.
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Condie, K. C. "Growth of continental crust: a balance between preservation and recycling." Mineralogical Magazine 78, no. 3 (June 2014): 623–37. http://dx.doi.org/10.1180/minmag.2014.078.3.11.
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AbstractOne of the major obstacles to our understanding of the growth of continental crust is that of estimating the balance between extraction rate of continental crust from the mantle and its recycling rate back into the mantle. As a first step it is important to learn more about how and when juvenile crust is preserved in orogens. The most abundant petrotectonic assemblage preserved in orogens (both collisional and accretionary) is the continental arc, whereas oceanic terranes (arcs, crust, mélange, Large Igneous Provinces, etc.) comprise <10%; the remainder comprises older, reworked crust. Most of the juvenile crust in orogens is found in continental arc assemblages. Our studies indicate that most juvenile crust preserved in orogens was produced during the ocean-basin closing stage and not during the collision. However, the duration of ocean-basin closing is not a major control on the fraction of juvenile crust preserved in orogens; regardless of the duration of subduction, the fraction of juvenile crust preserved reaches a maximum of ∼50%. Hafnium and Nd isotopic data indicate that reworking dominates in external orogens during supercontinent breakup, whereas during supercontinent assembly, external orogens change to retreating modes where greater amounts of juvenile crust are produced. The most remarkable feature of εNd (sedimentary rocks and granitoids) and εHf (detrital zircons) distributions through time is how well they agree with each other. The ratio of positive to negative εNd and eHf does not increase during supercontinent assembly (coincident with zircon age peaks), which suggests that supercontinent assembly is not accompanied by enhanced crustal production. Rather, the zircon age peaks probably result from enhanced preservation of juvenile crust. Valleys between zircon age peaks probably reflect recycling of continental crust into the mantle during supercontinent breakup. Hafnium isotopic data from zircons that have mantle sources, Nd isotopic data from detrital sedimentary rocks and granitoids and whole-rock Re depletion ages of mantle xenoliths collectively suggest that ≥70% of the continental crust was extracted from the mantle between 3500 and 2500 Ma.
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BERGER, JULIEN, RENAUD CABY, JEAN-PAUL LIÉGEOIS, JEAN-CLAUDE C. MERCIER, and DANIEL DEMAIFFE. "Dehydration, melting and related garnet growth in the deep root of the Amalaoulaou Neoproterozoic magmatic arc (Gourma, NE Mali)." Geological Magazine 146, no. 2 (September 17, 2008): 173–86. http://dx.doi.org/10.1017/s0016756808005499.
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AbstractThe Amalaoulaou Neoproterozoic island-arc massif belongs to the Gourma belt in Mali. The metagabbros and pyroxenites forming the main body of this arc root show the pervasive development of garnet. In the pyroxenites, the latter has grown by reaction between pyroxene and spinel during isobaric cooling. By contrast, in the metagabbros, garnet textures and relations to felsic veins exclude an origin through solid-state reactions only. It is proposed that garnet has grown following dehydration and localized melting of amphibole-bearing gabbros at the base of the arc. The plagioclase-saturated melts represented by anorthositic veins in the metagabbros and by trondhjemites in the upper part of the massif provide evidence for melting in the deep arc crust, which locally generated high-density garnet–clinopyroxene–rutile residues. Garnet growth and melting began around 850 °C at 10 kbar and the tonalitic melts were most probably generated around 1050 °C at P ≥ 10 kbar. This HT granulitic imprint can be related to arc maturation, leading to a P–T increase in the deep arc root and dehydration and/or dehydration-melting of amphibole-bearing gabbros. Observation of such features in the root of this Neoproterozoic island arc has important consequences, as it provides a link to models concerning the early generation of continental crust.
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Pollock, J. C., P. J. Sylvester, and S. M. Barr. "Lu–Hf zircon and Sm–Nd whole-rock isotope constraints on the extent of juvenile arc crust in Avalonia: examples from Newfoundland and Nova Scotia, Canada." Canadian Journal of Earth Sciences 52, no. 3 (March 2015): 161–81. http://dx.doi.org/10.1139/cjes-2014-0157.
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Avalonia, the largest accreted crustal block in the Appalachian orogen, consists of Neoproterozoic magmatic arc sequences that represent protracted and episodic subduction-related magmatism before deposition of an Ediacaran–Ordovician cover sequence including siliciclastic rocks. Zircon crystals were obtained from arc-related magmatic rocks and from clastic sedimentary sequences and analyzed in situ for their Hf-isotope composition. The majority of magmatic and detrital zircons are dominated by initial 176Hf/177Hf values that are more radiogenic than chondritic uniform reservoir (CHUR) with calculated crust formation Hf–TDM model ages that range from 0.84 to 1.30 Ga. These results suggest formation by partial melting of juvenile mantle in a Neoproterozoic continental arc. Some zircons have Hf–TDM model ages ca. 1.39–3.09 Ga with εHf values of –33.9 to –0.5 and more clearly indicate involvement of older lithosphere in their petrogenesis. Whole-rock Sm–Nd isotopic compositions from felsic volcanic rocks are characterized by positive initial εNd values with Mesoproterozoic depleted mantle model ages consistent with juvenile extraction. Results suggest a dominant mantle component with long-term light rare earth element (LREE) depletion mixed with an older crustal component with long-term LREE enrichment. The pattern of TDM model ages and variations in Lu–Hf and Sm–Nd isotopic character are compatible with a ca. 1.0–1.2 Ga igneous tectonomagmatic event that formed basement to Neoproterozoic magmatic arcs in Avalonia. The presence of evolved isotopic signatures, however, indicates that significant older Proterozoic crust is present locally beneath Avalonia, suggesting that Avalonia formed in a single Neoproterozoic arc system that generated juvenile mantle-derived crust, coupled with lesser anatectic reworking of significantly older crust.
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Whalen, Joseph B., Eric C. Syme, and Richard A. Stern. "Geochemical and Nd isotopic evolution of Paleoproterozoic arc-type granitoid magmatism in the Flin Flon Belt, Trans-Hudson orogen, Canada." Canadian Journal of Earth Sciences 36, no. 2 (February 1, 1999): 227–50. http://dx.doi.org/10.1139/e98-026.
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Granitoid magmatism spans three Flin Flon Belt evolutionary stages: (i) "evolved" arc (~1920 Ma) plus early juvenile arc (1904-1880 Ma) plutonism during intraoceanic arc-back-arc formation; (ii) early (1878-1860 Ma) and middle (1860-1844 Ma) successor arc plutonism following accretion and successor arc(s) development and; (iii) late (1843-1826 Ma) successor arc plutonism accompanying successor basin formation and waning arc magmatism. Amphibole-bearing mineralogy, metaluminous compositions, and igneous microgranitoid enclaves indicate derivation from infracrustal sources. Predominance of intermediate calc-alkaline compositions and negative Nb anomalies on normalized patterns over a 46-77 wt.% silica range indicate an arc setting. Basaltic end members indicate important contributions directly from the mantle. εNd(T) values are predominately in the range 0 to +4.3, reflecting mixing between depleted mantle melts and an Archean crustal component preserved in evolved arc plutons (-3.9 to -6). Temporal variations include the following: (i) early juvenile arc plutons are low K, high field strength element (HFSE) depleted, with relatively flat rare earth element (REE) patterns and negative Eu anomalies, indicative of low-pressure partial melting - fractionation in the mantle wedge, with residual pyroxene and plagioclase; (ii) early and middle successor arc plutonism is medium K, with steep REE patterns and no Eu anomalies, indicative of input from melting of basaltic sources (likely subducted back-arc oceanic crust) under high-pressure conditions with residual garnet and (or) amphibole and no plagioclase; (iii) late successor arc plutons are high K, more HFSE enriched, with both variable REE pattern slopes and Eu anomalies, indicative of a significant petrogenetic role of recycling of preexisting juvenile arc - accretionary complex crust.
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Petrishchevsky, A. M. "THE EARTH’S CRUST AND UPPER MANTLE OF THE EAST CHINA SEA (SEIMOTOMOGRAPHIC AND GRAVITY MODELS)." Tikhookeanskaya Geologiya 41, no. 5 (2022): 43–54. http://dx.doi.org/10.30911/0207-4028-2022-41-5-43-54.
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Gravity and seismic-tomography models of the tectonosphere detailing and specifying the structure of the crust and upper mantle of the East China Sea are considered. Crust of this region has considerably lower density extending into the subcrustal layer to a depth of 40–45 km. A transform-fault-related wide pull-apart zone of NW strike is identified on the western edge of the Pacific plate. In east regions of the East China Sea the lower layer of the Philippine oceanic lithosphere is underthrust beneath the Ryukyu arc and further ‒ beneath the viscous subcrustal layer of the continental shelf. In the western regions, the oceanic lithosphere is thrusted over the continental margin crust. Evidence of the central type structure of presumably plume origin is found in the layer below the asthenosphere on the western edge of the Philippine plate (a middle fragment of the Ryukyu arc).
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Lapierre, H., M. Tardy, C. Coulon, E. Ortiz Hernandez, J. L. Bourdier, J. Martínez Reyes, and C. Freydier. "Caractérisation, genèse et évolution géodynamique du terrain de Guerrero (Mexique occidental)." Canadian Journal of Earth Sciences 29, no. 11 (November 1, 1992): 2478–89. http://dx.doi.org/10.1139/e92-194.
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The Guerrero terrane (western Mexico) is composed of Late Jurassic – Early Cretaceous plutono-volcanic and volcano-sedimentary sequences of the Alisitos–Teloloapan arc that accreted to the North American craton at the end of the Early Cretaceous. The geodynamic evolution of the Guerrero terrane is that of the Alisitos–Teloloapan intraoceanic arc, partly built on continental crust and partly on oceanic crust. The growth of the arc was likely linked to the subduction of the Arperos and Olvidada basins fringing the North American borderland. The subduction was dipping west-south-west.The continent-based segment of the arc, which is presently exposed mainly in northwestern Mexico, is composed of aerial and submarine K-rich calc-alkaline basalts, andesites, and rhyolites and of siliceous pyroclastic rocks interbedded with Aptian–Albian bioclastic carbonates or red beds bearing dinosaurus foot prints. The calc-alkaline basalts and andesites show light rare earth elements enriched patterns and high concentrations in large ion lithophile elements. The siliceous andesites and rhyodacites display low contents in Y and heavy rare earth elements, uncommon for such calc-alkaline SiO2-saturated rocks. This depletion is likely linked to amphibole fractionation and to the presence of sphene and zircon, minerals known to concentrate the heavy rare earth elements.In contrast, the magmatic arc sequences built on oceanic crust, that crops out in central-southern areas of the Guerrero terrane, show an evolution with time. The activity of the arc began with depleted tholeiitic igneous rocks, followed first by mature tholeiitic basalts, then by calc-alkaline olivine basalts interbedded with micritic limestones and radiolarian oozes of Early Cretaceous age (Neocomian). At the end of the arc development, in Late Aptian–Albian, calc-alkaline pillow basalts and andesites erupted at the volcanic front whereas shoshonitic basalts emitted backwards. In the late Early Cretaceous, the arc drifted towards the north and collided with the craton. Arc tholeiites are characterized by flat rare earth element patterns or slightly depleted in light rare earth elements and by high εNd ratios. The calc-alkaline plutonic and volcanic rocks show light rare earth elements enriched patterns and their εNd ratios decrease with time. This decrease of the εNd ratios suggests that either the mantle source of the calc-alkaline rocks was contaminated by subducted terrigenous sediments derived from an old continental crust (North American craton) or that these calc-alkaline rocks derive from the partial melt of an oceanic island basalt source present in the mantle wedge. The shoshonitic features of the basalts are marked by the presence of sanidine in the groundmass and the high levels of K2O, Ba, and Sr of the unaltered rocks.
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Magni, Valentina, John Naliboff, Manel Prada, and Carmen Gaina. "Ridge Jumps and Mantle Exhumation in Back-Arc Basins." Geosciences 11, no. 11 (November 19, 2021): 475. http://dx.doi.org/10.3390/geosciences11110475.
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Back-arc basins in continental settings can develop into oceanic basins, when extension lasts long enough to break up the continental lithosphere and allow mantle melting that generates new oceanic crust. Often, the basement of these basins is not only composed of oceanic crust, but also of exhumed mantle, fragments of continental crust, intrusive magmatic bodies, and a complex mid-ocean ridge system characterised by distinct relocations of the spreading centre. To better understand the dynamics that lead to these characteristic structures in back-arc basins, we performed 2D numerical models of continental extension with asymmetric and time-dependent boundary conditions that simulate episodic trench retreat. We find that, in all models, episodic extension leads to rift and/or ridge jumps. In our parameter space, the length of the jump ranges between 1 and 65 km and the timing necessary to produce a new spreading ridge varies between 0.4 and 7 Myr. With the shortest duration of the first extensional phase, we observe a strong asymmetry in the margins of the basin, with the margin further from trench being characterised by outcropping lithospheric mantle and a long section of thinned continental crust. In other cases, ridge jump creates two consecutive oceanic basins, leaving a continental fragment and exhumed mantle in between the two basins. Finally, when the first extensional phase is long enough to form a well-developed oceanic basin (>35 km long), we observe a very short intra-oceanic ridge jump. Our models are able to reproduce many of the structures observed in back-arc basins today, showing that the transient nature of trench retreat that leads to episodes of fast and slow extension is the cause of ridge jumps, mantle exhumation, and continental fragments formation.
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Nahodilová, Radmila, Pavlína Hasalová, Pavla Štípská, Karel Schulmann, Prokop Závada, Jitka Míková, Andrew Kylander-Clark, and Petra Maierová. "Exhumation of subducted continental crust along the arc region." Gondwana Research 80 (April 2020): 157–87. http://dx.doi.org/10.1016/j.gr.2019.10.011.
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Müntener, Othmar, and Peter Ulmer. "Arc crust formation and differentiation constrained by experimental petrology." American Journal of Science 318, no. 1 (January 2018): 64–89. http://dx.doi.org/10.2475/01.2018.04.
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Straub, Susanne M., Arturo Gomez-Tuena, Finlay M. Stuart, Georg F. Zellmer, Ramon Espinasa-Perena, Yue Cai, and Yoshiyuki Iizuka. "Formation of hybrid arc andesites beneath thick continental crust." Earth and Planetary Science Letters 303, no. 3-4 (March 2011): 337–47. http://dx.doi.org/10.1016/j.epsl.2011.01.013.
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Rudnev, S. N., A. S. Gibsher, and D. V. Semenova. "Vendian Island-Arc Intrusive Magmatism of the Lake Zone of Western Mongolia (Geological, Geochronological, and Petrochemical Data)." Russian Geology and Geophysics 62, no. 6 (June 1, 2021): 619–32. http://dx.doi.org/10.2113/rgg20194153.
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Abstract —Based on new geochronological data on gabbroid and plagiogranitoid associations (Tavan-Hayrhan, East Bayan Tsagaan, Bayan Tsagaan Uul, Tungalag, Three Hills, and Shutkhuin massifs) located among the Vendian island-arc volcanic complexes of the Lake Zone of Western Mongolia, an independent stage of Vendian island-arc intrusive magmatism (560–542 Ma) is substantiated. Geochronological ages determined by xenogenic zircon from Vendian gabbroids and granitoids (716–559 Ma) indicate a wide time interval of their formation and different natures of the sources. Several such sources are assumed. The source of the first type is rocks of the late Riphean oceanic crust of the Paleoasian Ocean, on which the Vendian island arc of the Lake Zone formed later. This is evidenced by the presence of xenogenic zircon with the ages of ~716, 658–642, 613–611 Ma. The source of the second (probably main) type is rocks of the Vendian island-arc crust of the Lake Zone. This is indicated by the presence of xenogenic zircon with ages of 583–559 Ma, observed in all studied Vendian intrusive associations.
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d'Lemos, R. S., and M. Brown. "Sm–Nd isotope characteristics of late Cadomian granite magmatism in northern France and the Channel Islands." Geological Magazine 130, no. 6 (November 1993): 797–804. http://dx.doi.org/10.1017/s0016756800023165.
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AbstractSm–Nd isotopic studies of granites within the late Precambrian, Cadomian, orogenic belt of the North Armorican Massif (northwestern France) and Channel Islands reveal differences between arc-related granite magmatism in outboard terranes and intracrustal granite magmatism in inboard terranes. Late Cadomian (c. 570 Ma), arc-related granitoids exhibit a range of εnd( - 2 to - 6) and Nd model ages (TDM1.0–1.3 Ga) reflecting variable contamination between late Precambrian mantle derived magmas and ancient (c. 2.0 Ga?) continental crust. The contamination did not involve exposed granitic Icartian basement to anygreat degree, a more likely contaminant being unexposed lower crust of intermediate to acidic granulitic composition, or early Cadomian plutons which were themselves contaminated by lower crust. Voluminous granites of the Mancellian region (c. 550–540 Ma) share common isotopic characteristics (εNd-4 to -7, TDM1.5–1.7 Ga) with migmatites and anatectic granites produced by partial melting of metasedimentary sequences within the St Malo region consistent with a common source.
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Zhang, Jingbo, Rui Wang, Jun Hong, Ming Tang, and Di-Cheng Zhu. "Nb-Ta systematics of Kohistan and Gangdese arc lower crust: Implications for continental crust formation." Ore Geology Reviews 133 (June 2021): 104131. http://dx.doi.org/10.1016/j.oregeorev.2021.104131.
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Ahumada, Ma Florencia, Brígida Castro de Machuca, Patricia Alvarado, Jean-Baptiste Ammirati, and María Gimena López. "Modelo petrofísico del borde oriental de las sierras de Valle Fértil- La Huerta, Argentina a partir de datos sísmicos y petrológicos." Revista Mexicana de Ciencias Geológicas 34, no. 1 (April 1, 2017): 1. http://dx.doi.org/10.22201/cgeo.20072902e.2017.1.411.
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This paper is a contribution to the knowledge of the crustal structure of the eastern flank of the Valle Fértil - La Huerta ranges (Western Sierras Pampeanas, San Juan, Argentina) at 31°S, in the Andean foreland region, where the Nazca plate is subducting horizontally at about 100 km depth. A 1D velocity model was constrained, combining petrographic and seismological observations from analysis of 19 igneous and metaigneous plutonic rocks belonging to the Famatinian (Ordovician) magmatic arc, which make up most of the crystalline basement of these ranges. Granitoid lithologies predominate in the northern region whereas mafic lithologies are more common to the south. The seismological analysis consisted of modeling teleseismic receiver functions near three seismological stations: LUNA, MAJA and CHUC, in places where those rocks are dominant. Thus, P and S seismic-wave velocities (Vp and Vs) and Poisson´s coefficient (ν), among other elastic parameters, were obtained. The seismic velocity model indicates an overthickened crust with an average thickness between 55 and 60 km, which matches with global average values (~41km); this agrees well with the hypothesis of partial eclogitization in the lower crust. The presence of two seismic velocity discontinuities at mid-crustal levels (12 and 28 km depths), likely associated to décollements, might be related to the accretion of the Cuyania terrane to the Pampia terrane. We obtained low P seismic-wave velocities (Vp ~5.8 km/s), Vp/Vs ratio (~1.70) in upper crust levels consistent with granitoid lithologies, as well as high P seismic-wave velocities (Vp ~6.76 km/s), Vp/Vs ratio (~1.78) in lower crust levels; these figures match with mafic lithologies of a more dense (~3.00 g/cm3), lower crust with respect to other back-arc Andean regions. Also, these values are consistent with the existence of mafic rocks composed of olivine, ortho- and clinopyroxene, which constitute the root of the Famatinian magmatic arc. These results indicate high-grade metamorphic conditions and depths corresponding to geophysical properties of middle to lower crust and correlate with the hypothesis of a dehydrated, cool and magnesium-enriched mantle located in the region between the subducted Nazca slab and the bottom of the Cuyania terrain crust.
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Hanson, Ann E. H., Stacia M. Gordon, Kyle T. Ashley, Robert B. Miller, and Elizabeth Langdon-Lassagne. "Multiple sediment incorporation events in a continental magmatic arc: Insight from the metasedimentary rocks of the northern North Cascades, Washington (USA)." Geosphere 18, no. 1 (December 22, 2021): 298–326. http://dx.doi.org/10.1130/ges02425.1.
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Abstract The rheology and composition of arc crust and the overall evolution of continental magmatic arcs can be affected by sediment incorporation events. The exhumed Cretaceous–Eocene North Cascades arc exposes abundant metasedimentary rocks that were incorporated into the arc during multiple events. This study uses field relationships, detrital zircon geochronology, bulk rock geochemistry, geothermometry, and quartz-in-garnet geobarometry to distinguish approximate contacts and emplacement depths for different metasedimentary units to better understand their protolith incorporation history and impact on the arc. The Skagit Gneiss Complex is one of the main deep crustal units of the North Cascades arc. It includes metasedimentary rocks with distinct detrital zircon signatures: Proterozoic–Cretaceous (Group 1) or Triassic–Cretaceous (Group 2) zircon populations. Both metasedimentary groups achieved near-peak metamorphic conditions of 640–800 °C and 5.5–7.9 kbar; several Group 2 samples reveal the higher pressures. A third group of metasedimentary rocks, which was previously interpreted as metamorphosed equivalents of backarc sediments (Group 3), exhibited unimodal Triassic or bimodal Late Jurassic–Early Cretaceous detrital zircon signatures and achieved near-peak conditions of 570–700 °C and 8.7–10.5 kbar. The combined field and analytical data indicate that protoliths of Group 1 and Group 2 metasedimentary rocks were successively deposited in a forearc basin and underthrusted into the arc as a relatively coherent body. Group 3 backarc sediments were incorporated into the arc along a transpressional step-over zone. The incorporation of both forearc and backarc sediments was likely facilitated by arc magmatism that weakened arc crust in combination with regional transpression.
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Sato, Takeshi, Tetsuo No, Ryuta Arai, Seiichi Miura, and Shuichi Kodaira. "Transition from continental rift to back-arc basin in the southern Japan Sea deduced from seismic velocity structures." Geophysical Journal International 221, no. 1 (January 9, 2020): 722–39. http://dx.doi.org/10.1093/gji/ggaa006.
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SUMMARY We obtain the crustal structure from active-source seismic surveys using ocean bottom seismographs and seismic shots to elucidate the evolutionary process from continental rifting to the backarc basin opening in the Yamato Basin and Oki Trough in the southern Japan Sea. Results show that the crust changes from approximately 14–15 km thick in the basin (the southern Yamato Basin) to 16.5–17 km in the margin of the basin (the southwestern edge of the Yamato Basin). The P-wave velocity distribution in the crust of the southern Yamato Basin is missing a typical continental upper crust with P-wave velocities of 5.4–6.0 km s–1, and is thought be a thicker oceanic crust formed by a backarc basin opening. By contrast, the crust of the southwestern edge of the Yamato Basin might have been formed by continental rifting because there is an unit with P-wave velocities of 5.4–6.0 km s–1 and with a gentle velocity gradients, corresponding to the continental upper crust in this area. This variation might reflect differences in mantle properties from continental rifting to backarc basin opening of the Yamato Basin. Because the Oki Trough has a crustal thickness of 17–19 km and having a unit with P-wave velocities of 5.4–6.0 km s–1, corresponding to the continental upper crust with a high-velocity lower crust, we infer that this trough was formed by continental rifting with magmatic intrusion or underplating. These crustal variations might reflect transitional stages from continental rifting to backarc basin opening in the southern Japan Sea.
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Saltus, Richard W., and Travis L. Hudson. "There is more Wrangellia — magnetic characterization of southern Alaska crust." Canadian Journal of Earth Sciences 59, no. 4 (April 2022): 243–57. http://dx.doi.org/10.1139/cjes-2020-0209.
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In southern Alaska, Wrangellia-type magnetic crustal character extends from the Talkeetna Mountains southwest through the Alaska Range to the Bristol Bay region. Magnetic data analyses in the Talkeetna Mountains showed that there are mid-crustal differences in the magnetic properties of Wrangellia and the Peninsular terrane. After converting total field magnetic anomaly data to magnetic potential, we applied Fourier filtering techniques to remove magnetic responses from deep and shallow sources. The resulting mid-crustal magnetic characterization delineates the regional magnetic potential domains that correspond to the Wrangellia and Peninsular terranes throughout southern Alaska. These magnetic potential domains show that Wrangellia-type crust extends southwest to the Iliamna Lake region and that it overlaps the mapped Peninsular terrane. Upon reconsidering geologic ties between Wrangellia, Peninsular, and Alexander terranes, we conclude that Peninsular terrane is part of what we here call Western Wrangellia. Western Wrangellia contains the Lower Jurassic Talkeetna volcanic arc and is similar to Wrangellia of the Vancouver Island area, Canada (Southern Wrangellia), which contains the Lower Jurassic Bonanza volcanic arc. Others have previously made this correlation and proposed that the Talkeetna arc-bearing part of southern Alaska was displaced from the Bonanza arc-bearing part of Canada. We generally agree and propose that about 1000 km of dextral displacement along ancestral Border Ranges fault segments and other faults of south-central Alaska separated Western Wrangellia from Southern Wrangellia. We think this displacement was mostly in the Late Jurassic and earliest Cretaceous, perhaps between about 160 and 130 Ma.
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Kay, Suzanne Mahlburg, Brian R. Jicha, Gary L. Citron, Robert W. Kay, Ashley K. Tibbetts, and Tiffany A. Rivera. "The Calc-Alkaline Hidden Bay and Kagalaska Plutons and the Construction of the Central Aleutian Oceanic Arc Crust." Journal of Petrology 60, no. 2 (December 24, 2018): 393–439. http://dx.doi.org/10.1093/petrology/egy119.
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Abstract Calc-alkaline plutons are the major crustal building blocks of continental margins, but are rarely exposed in oceanic island arcs. Two of the best examples are the ∼10 km wide Hidden Bay and Kagalaska plutons that intrude Eocene mafic volcanic–sedimentary rocks on Adak and Kagalaska islands in the central Aleutian arc. Twenty new Ar/Ar and U/Pb ages, coupled with published ages, show that the Hidden Bay pluton was intruded in multiple stages from ∼34·6 to 30·9 Ma, whereas the Kagalaska pluton was intruded at ∼14 Ma. The plutons largely consist of medium- to high-K2O hornblende-bearing cumulate diorite (53–55 wt % SiO2) and hornblende–biotite granodiorite (57–64 wt %), with lesser amounts of gabbro (50–52 wt % SiO2), leucogranodiorite (67–69 wt % SiO2) and aplite (76–77 wt % SiO2) that can generally be linked to each other by crystal fractionation. The compositions of these plutons are generally similar to those of continental plutons, except for more oceanic-like large ion lithophile element and isotopic signatures (87Sr/86Sr = 0·703–0·7033; ɛNd = 9·4–7·7) that reflect oceanic- rather than continental-type crustal contaminants. Chemical similarities between the Hidden Bay homogeneous gabbros and high-Al basalts in Adak Pleistocene-Holocene volcanoes indicate little temporal evolution in the general character of the mantle-derived basalts. Rather than a unique arc setting and distinctive magmas, formation of the Aleutian calc-alkaline plutons seems to require a sufficient crustal thickness (∼37 km) and a high enough water content to stabilize pargasitic hornblende amphibole in a relatively closed magma system that favors increasing K, Ti and H2O at the end of a magmatic cycle. This termination of magmatism coincides with a northward migration of the magmatic front that is inferred to be associated with fore-arc subduction erosion. In accord with Adak region crustal architecture based on seismic data, crystallization models for the plutons suggest that mantle-generated hydrous arc basalts fractionated olivine and clinopyroxene in the lower crust to form high-Al basaltic composition magmas that rose into the mid-crust, where gabbro and diorite crystallized to form the magmas that buoyantly rose into the upper crust and crystallized to form the volumetrically dominant granodiorite (58–63 wt % SiO2). The most important temporal changes in chemistry can be explained by fore-arc crust incorporated into the mantle wedge by fore-arc subduction erosion creating ‘adakitic’ signatures at times of northward arc migration and a change to a more continental subducted sediment component at the time of Plio-Pleistocene glaciation.