Journal articles on the topic 'Alkali-rich felsic magmas'
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1
Marr, R. A. "Alkali Zirconosilicate Speciation in Halogen-Rich, Felsic, Peralkaline Magmas." Mineralogical Magazine 58A, no. 2 (1994): 559–60. http://dx.doi.org/10.1180/minmag.1994.58a.2.28.
2
PIMENTEL, MÁRCIO M., ELTON L. DANTAS, REINHARDT A. FUCK, and RICHARD A. ARMSTRONG. "Shrimp and conventional U-Pb age, Sm-Nd isotopic characteristics and tectonic significance of the K-rich Itapuranga suite in Goiás, Central Brazil." Anais da Academia Brasileira de Ciências 75, no. 1 (March 2003): 97–108. http://dx.doi.org/10.1590/s0001-37652003000100011.
Abstract:
The Itapuranga alkali granite and Uruana quartz syenite are large K-rich EW-elongated intrusions, in the central part of the Neoproterozoic Brasília Belt, central Brazil. They are associated with Pireneus lineaments, which cut the regional NNW-SSE structures of the southern part of the belt. SHRIMP and conventional U-Pb data for the Itapuranga and Uruana intrusions indicate crystallization ages of 624 ± 10 Ma and 618 ± 4 Ma, respectively. Three zircon cores from the Itapuranga granite yielded U-Pb ages between 1.79 and 1.49 Ga. Sm-Nd T DM ages for both intrusions are 1.44 Ga and epsilonNd(T) values are -5.1 and -5.7, suggesting the input of material derived from older (Paleo- to Mesoproterozoic) sialic crust in the origin of the parental magmas. Magma mixing structures indicate co-existence of mafic and felsic end-members. The felsic end-member of the intrusions is dominantly represented by crust-derived melts, formed in response to the invasion of Paleo/Mesoproterozoic sialic crust by alkali-rich mafic magmas at ca. 620 Ma. These intrusions are roughly contemporaneous with, or perhaps slightly younger than, the peak of regional metamorphism in the southern Brasília Belt. Their emplacement along the Pireneus lineament suggest a syn-tectonic origin for them, most probably in transtensional settings along these faults.
3
Stolz, A. J. "Fluid activity in the lower crust and upper mantle: mineralogical evidence bearing on the origin of amphibole and scapolite in ultramafic and mafic granulite xenoliths." Mineralogical Magazine 51, no. 363 (December 1987): 719–32. http://dx.doi.org/10.1180/minmag.1987.051.363.13.
Abstract:
AbstractXenoliths in an olivine nephelinite from the McBride Province, North Queensland, include Cr-diopside lherzolites, spinel and garnet websterites, felsic, 2-pyroxene and garnet granulites, and hornblendites. The spinel and garnet websterites are interpreted as crystal segregations from olivine basalt or alkali olivine basalt magma at ∼ 12 kbar followed by isobaric cooling (to approximately 900–1000°C) and subsolidus reequilibration. Garnet and 2-pyroxene granulites are mineralogically and texturally distinct and are considered to represent relatively large degrees of crystallization of basaltic magmas at comparable or slightly lower pressures (8–12 kbar). Mafic and ultramafic xenoliths have been modified to varying degrees following the relatively recent influx of a H2O- and CO2-bearing fluid. Variable amounts of amphibole and mica developed in response to the introduced fluid and it is argued that some hornblendites are the end-products of this process acting on spinel websterites. Felsic and 2-pyroxene granulite xenoliths display only minor evidence of increased PH2O. Mineralogical and textural evidence indicates high-sulphur Ca-rich scapolite in several garnet granulites did not form in response to the increased fluid activities. It is proposed the scapolite was a primary cumulate phase precipitated from alkali basaltic magma under elevated fo2 and fso2 conditions.
4
Bédard, Jean H. J., Don M. Francis, and John Ludden. "Petrology and pyroxene chemistry of Monteregian dykes: the origin of concentric zoning and green cores in clinopyroxenes from alkali basalts and lamprophyres." Canadian Journal of Earth Sciences 25, no. 12 (December 1, 1988): 2041–58. http://dx.doi.org/10.1139/e88-190.
Abstract:
The Mesozoic Monteregian alkaline province of southern Quebec includes mafic alnöite, monchiquite, basanite, camptonite, and alkali basalt dykes. Most carry phenocrysts of clinopyroxene that generally zone towards Ti–AlIV–Fe–Mn-rich and Mg–AlVI–Cr-poor rims. The zoning can best be explained through polybaric crystallization and differentiation during ascent from the upper mantle. In intermediate and leucocratic dykes, clinopyroxene AlIV–Ti contents decrease with the Mg/(Mg + ΣFe) ratio, probably reflecting concurrent fractionation of a Ti-rich phase. Pyroxene phenocrysts in Monteregian mafic dykes commonly have green clinopyroxene cores that are richer in Na and Fe and poorer in Mg and Cr than the enclosing titansalite phenocrysts. Some cores are euhedral and sector zoned, implying crystallization from a melt more evolved than their present hosts. The high AlVI contents of these cores imply high pressures of crystallization. The abundance of crustal xenoliths and evolved pyroxene cores indicates that the host magmas hybridized with felsic melts, cumulates, or metasomatites within the crust or an anomalously Fe–Na-rich upper mantle. This implies that the host dykes are not primary magmas but hybrids. Consequently, dyke chemistry cannot simply be inverted to determine the composition and mineralogy of the mantle source.
5
Nekvasil, Hanna, and William Carroll. "Experimental constraints on the compositional evolution of crustal magmas." Earth and Environmental Science Transactions of the Royal Society of Edinburgh 87, no. 1-2 (1996): 139–46. http://dx.doi.org/10.1017/s0263593300006556.
Abstract:
ABSTRACT:Recent water-undersaturated phase equilibrium data on the subsystems of the granite-H2O system have provided important new constraints on the topology of the cotectic surfaces and hence on the compositional evolution of felsic magmas. The effect of water on phase relations can be deduced from a comparison of anhydrous and H2O-saturated data or from data obtained in the presence of a CO2-bearing fluid. However, although new experimental evidence indicates that the silica enrichment of evolving H2O-undersaturated, H2O-unbuffered melts during the co-precipitation of quartz and feldspar is as previously thought for orthoclase-rich compositions, it suggests that such a trend is considerably less for Ab-rich compositions. For water-poor trachytic melts, the newly recognised strong destabilisation of the sanidine melt component relative to the anorthite melt component with increasing water content indicates that the co-precipitation of two feldspars will result in saturation of the melt with ternary alkali feldspar at an earlier stage (i.e. higher melt anorthite content) than previously thought. This, in turn, implies that the melt differentiation path will have a greater component of anorthite depletion during the equilibrium co-precipitation of ternary feldspars and that the melt will remain in the peritectic region of the two feldspar plus liquid surface over a greater interval of crystallisation, thereby enhancing the possibility that the resoption of plagioclase during the early stages of equilibrium with alkali feldspar may go to completion. Comparison of CO2-free and CO2-bearing haplogranitic phase equilibrium data suggests that CO2 may be playing an independent part in the modification of phase equilibria and may induce a significant destabilisation of the orthoclase melt component.
6
Rodríguez-García, Gabriel, and Jose Gilberto Bermúdez-Cordero. "Petrography, geochemistry and age of Cerro Frontino Gabro." Boletín de Ciencias de la Tierra, no. 38 (July 1, 2015): 25–40. http://dx.doi.org/10.15446/rbct.n38.46053.
Abstract:
The Gabro de Cerro Frontino was emplaced in the Cañasgordas Block, located in the Northern Segment of the Colombian Western Cordillera. It corresponds to a pluton composed of at least three magmatic pulses, emplaced during a short period of time. Gabbros and diorites are more common in the unit than clinopiroxenites, monzodiorites and monzonites. These rocks are composed of calcic to intermediate plagioclase, augite-egerine type clinopyroxene and biotite; olivine and flogopite may be present in some mafic rocks and alkali feldspar and quartz may be present in some felsic rocks. Sphene, magnetite and apatite are common accessory minerals. The silica content in the rocks varies between 37.08% and 54.4%, with constant values of MnO (0.1% 0.4%), impoverishment of Fe2O3, MgO, CaO, TiO2 and P2O5 as SiO2 increases, and enrichment of K2O, Na2O and Al2O3 as SiO2 increases. The basic and ultrabasic rocks fall in the sub-alkaline series, the rest of the samples fall in the medium to K-rich calc-alkaline series and in the shoshonitic series. The Gabro de Cerro Frontinocorresponds to magmas impoverished on heavy rare earth elements with respect to light rare earth elements, which suggests the contribution of a subduction component in the magma genesis. The LILE (Sr, K, Rb, Pb, Ba) are enriched with respect to the HFSE values that are relatively flat and impoverished; the unit also exhibits a negative anomaly of Nb with respect to Th and Ce, being a magmatic arc the environment of generation. The ages obtained in biotite using the Ar-Ar method fall between 9.87±0.18 Ma and 11.44±0.36 Ma, Middle to upper Miocene (Tortonian-Serravallian), similar to age of other plutons that are part of the Botón Arc.
7
Khedr, Mohamed Zaki, Gehad M. Saleh, Khaled M. Abdelfadil, Eiichi Takazawa, Kamal Abdelrahman, Akihiro Tamura, and Shaimaa Ali El-Shafei. "The Geology and Mineral Chemistry of Beryl Mineralization, South Eastern Desert, Egypt: A Deeper Insight into Genesis and Distribution." Minerals 14, no. 5 (April 28, 2024): 465. http://dx.doi.org/10.3390/min14050465.
Abstract:
Beryl mineralization in the Nugrus-Sikait domain in the South Eastern Desert (SED) of Egypt occurs as disseminated crystals in granitic pegmatite and quartz, as well as pegmatite veins crosscutting mélange schist and ophiolitic rocks. When granitic pegmatite comes into contact with the ophiolitic rocks, phlogopite and amphibole schists are formed due to K metasomatism. The ophiolitic mélange is intruded by leucogranite and related pegmatite along the NNW to NW Nugrus shear zone. Beryl samples have been collected from Um Sleimat, Madinat Nugrus, Wadi Abu Rusheid, and Wadi Sikait. Major oxides and in situ trace and rare earth elements (REEs) of beryl and associated minerals were analyzed through EPMA and LA-ICP-MS, respectively. The investigated beryl, based on its color and chemical compositions, can be classified into the two following types: pegmatitic beryl (type I) and schist-related beryl (type II). The former is colorless to pale green, and is mainly restricted in pegmatite veins; it is poor in Cr2O3 (up to 0.03 wt%) and MgO (Nil). The latter, deep green in color, is rich in Cr2O3 (up to 0.27 wt%) and MgO (up to 2.71 wt%), and occurs within quartz veins, phlogopite schists, and tremolite schists. The abundant beryl mineralization in phlogopite schists and their related quartz veins suggests that granite and associated pegmatite are the source rocks for the Be-bearing fluids that migrate along the NW-SE trending deep-seated tectonic zone, such as the Nugrus shear zone. Therefore, the formation of beryl in schists is attributed to the interaction of granitic/pegmatitic-derived Be-bearing fluids with serpentinite and gabbro interlayered with mélange schists. Variations in the trace and REE contents of both beryl types (I and II) indicate their two-stage formation from different compositions of Be-rich fluids, where light REEs, Zr, Nb, Ba, and Th decrease from type I beryl to type II. These two phases of beryl could be attributed to the magmatic/hydrothermal fluids associated with the pegmatite emplacement. The early phase of the late-stage magmatic-derived fluids was closely related to magma evolution and pegmatite formation, forming euhedral type I beryl. The late phase of pegmatite-derived fluids was mixed with serpentinite/schist-derived fluids that cause high V and Cr content in type II beryl. The composition of parent magmas of felsic rocks, the high degree of magma fractionation or the late stage melts, fluid compositions (rich in Be, Li, Cs, Rb, K), and alkali metasomatism, as well as the linear NW-SE trending deep-seated shear zone, are all factors possibly influencing beryl mineralization in the SED of Egypt.
8
Archibald, D. B., S. M. Barr, J. B. Murphy, C. E. White, T. G. MacHattie, E. A. Escarraga, M. A. Hamilton, and C. R. M. McFarlane. "Field relationships, petrology, age, and tectonic setting of the Late Cambrian–Ordovician West Barneys River Plutonic Suite, southern Antigonish Highlands, Nova Scotia, Canada." Canadian Journal of Earth Sciences 50, no. 7 (July 2013): 727–45. http://dx.doi.org/10.1139/cjes-2012-0158.
Abstract:
The West Barneys River Plutonic Suite consists of gabbro, syenite-monzonite, alkali-feldspar syenite to quartz alkali-feldspar syenite, and alkali-feldspar granite outcropping in an area of ∼100 km2 in the southern Antigonish Highlands. Magma mixing and mingling textures indicate a comagmatic relationship between some of the mafic and intermediate–felsic lithologies. However, nine U–Pb (zircon) ages, three by thermal ionization mass spectrometry (TIMS) and six by laser-ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS), from the West Barneys River suite and the lithologically similar Cape Porcupine Complex located 60 km to the east range from ca. 495 to 460 Ma, indicating that emplacement occurred over a significant span of time. Intermediate to felsic rocks consist mainly of perthitic K-feldspar and variable amounts of quartz; interstitial granophyre is present in some samples, consistent with shallow emplacement. Mafic phases are Fe-rich amphibole and clinopyroxene, and in some units, fayalite. Intermediate and felsic samples have chemical characteristics of within-plate ferroan A-type granitoid rocks. Gabbroic rocks consist of plagioclase (oligoclase–labradorite) and augite/diopside with less abundant orthopyroxene, olivine, biotite, and ilmenite/magnetite. Their chemical compositions are transitional from tholeiitic to alkalic and characteristic of continental within-plate mafic rocks. The εNd values are similar in gabbroic, syenitic, and granitic samples, ranging between 0.9 and 4.9, consistent with a co-genetic origin for the mafic and intermediate/felsic components of the suite, and derivation from Avalonian subcontinental lithospheric mantle in an extensional environment.
9
El-Dokouny, Hanaa A., Nasser M. Mahdy, Hany H. El Hadek, Mabrouk Sami, Rainer Abart, Mohamed S. Ahmed, Tehseen Zafar, and Ioan V. Sanislav. "Origin of Amphibole-Biotite-Fluorite-Rich Enclaves from Gabal El-Ineigi Fluorite-Bearing Granite, Central Eastern Desert of Egypt: Insights into Fluoride–Calcium and Silicate Liquid Immiscibility." Minerals 13, no. 5 (May 13, 2023): 670. http://dx.doi.org/10.3390/min13050670.
Abstract:
Gabal El-Ineigi fluorite-bearing rare-metal granite with A-type affinity, located in the Central Eastern Desert of Egypt, is distinguished by its abundance of large fluorite-quartz veins and mafic enclaves. Plagioclase (labradorite to oligoclase), Mg-rich biotite, and Mg-rich hornblende are the main components of mafic enclaves, with significant amounts of fluorite as essential phases, and titanite and Fe-Ti oxides (Nb-free rutile and ilmenite-rutile solid solution) as the main accessories. These enclaves are monzodioritic in composition, Si-poor, and highly enriched in Ca, Fe, Mg, and F compared to the host alkali feldspar F-poor Si-rich granites. Given the conflicting evidence for a restitic, xenolithic, magma mixing/mingling, cumulate, or bimodal origin for these enclaves, we propose that the mafic enclaves and felsic host granites are two conjugate liquids, with contrasting compositions, of a single parental melt. This is inferred by the normalized REE patterns that are similar. As a result, liquid immiscibility is proposed as a probable explanation for this mafic–felsic rock association. These enclaves can be interpreted as transient melt phases between pure silicate and calcium-fluoride melts that are preserved from the early stages of separation before evolving into a pure fluoride (Ca-F) melt during magma evolution. Due to element partitioning related to melt unmixing, the enclaves are preferentially enriched in Ca, F, Li, Y, and REE and depleted in HFSE (such as Zr, U, Th, Ta, Nb, Hf, and Ga) in comparison to the host granites. Furthermore, mafic enclaves exhibit W-type tetrad effects, while host granites exhibit M-type tetrad effects, implying that the REE partitioning, caused by liquid immiscibility, is complementary.
10
Blancher, S. B., P. D'Arco, M. Fonteilles, and M. L. Pascal. "Evolution of nepheline from mafic to highly differentiated members of the alkaline series: the Messum complex, Namibia." Mineralogical Magazine 74, no. 3 (June 2010): 415–32. http://dx.doi.org/10.1180/minmag.2010.074.3.415.
Abstract:
AbstractThe change in chemical composition trend of magmatic nepheline through magma evolution has been characterized from the alkaline series of the Messum complex in which nepheline occurs in a succession of different mineral parageneses from mafic-rich (theralites) to strongly evolved felsic-rich rock types (nepheline syenites). The nepheline compositions are dependent on those of coexisting feldspar(s). They record an evolution parallel to that of the melt schematized according to experimental phase diagrams, from initially Ca-rich compositions in equilibrium with calcic plagioclase towards increasingly Ca-poor, Na-rich and Si-rich compositions. The K contents show a maximum that corresponds to the appearance of alkali feldspar in the parageneses. This evolution is qualitatively preserved in spite of the low-T Na/K re-equilibration typical of plutonic nephelines. Although a slight increase in the silica content of nepheline is consistent with the experimentally defined magmatic trend, several high-silica nephelines from the Messum rocks as well as from other reported occurrences, cannot be reconciled with the experimental data. The nepheline solid-solution model available suggests that such ‘abnormal’ compositions might be related to different crystallization mechanisms between natural nephelines and some synthetic analogues.
11
Reich, Rebekka, Michael A. W. Marks, Thomas Wenzel, and Gregor Markl. "Pseudoleucite syenites at Loch Borralan, Scotland: Petrology and a genetic model." Canadian Mineralogist 58, no. 5 (September 1, 2020): 597–623. http://dx.doi.org/10.3749/canmin.2000019.
Abstract:
ABSTRACT The alkaline Loch Borralan intrusion (Assynt Region, NW Highlands of Scotland) consists of a composite arrangement of several ultramafic to felsic plutonic rock bodies which were emplaced around 430 Ma into the Moine Thrust Zone during the Caledonian Orogeny. Some of the Loch Borralan rocks are ultrapotassic and contain pseudoleucite, i.e., a pseudomorph of alkali feldspar and nepheline after leucite. In total, 25 samples have been investigated, representing garnet-bearing pseudoleucite syenites and accompanying rock types such as nepheline-garnet-bearing syenites, alkali feldspar syenites, an amphibole syenite, a biotite-clinopyroxene syenite, and calcite-bearing glimmerites. Pseudoleucite is always associated with garnet, biotite, orthoclase, and minor clinopyroxene and titanite. Mineral chemical data indicate rather primitive magma compositions with no major differences between the various investigated main rock units. The abundant occurrence of up to 2 cm large, mostly euhedral pseudoleucite crystals and petrological phase considerations suggest that magmatic leucite physically separated from its host magma as a flotation cumulate. Based on our data and a comparison with previous field-based and experimental work, K-rich basanitic to tephriphonolitic melts that originated from a K-enriched mantle source may be parental to these rocks. The high liquidus temperatures at low pressures (e.g., ∼1100 °C at 1 bar PH2O) required to crystallize leucite could have resulted from the ascent of successive melt batches in a composite intrusion. Later melt batches would increase the temperature in earlier, already partially cooled batches, causing an increase in temperature and a decrease in pressure during ascent. The subsequent decomposition of leucite to pseudoleucite is interpreted to result from either dry breakdown or autometasomatism, i.e., involvement of late-magmatic fluids.
12
Uddin, Naseer, M. Ishaq Kakar, Umar Farooq, Muhammad Panezai, Mukhtiar Ghani, and Nisar Ahmed. "PETROLOGY OF THE CRUSTAL PLUTONIC ROCKS OF NAWEOBA BLOCK, ZHOB OPHIOLITE, BALOCHISTAN, PAKISTAN." Earth Sciences Pakistan 5, no. 1 (March 18, 2021): 26–32. http://dx.doi.org/10.26480/esp.01.2021.26.32.
Abstract:
Zhob Ophiolite complex is composed of three detached blocks named Omzha, Ali Khanzai and Naweoba blocks. The crustal plutonic section of the Naweoba block is mapped and divided into gabbro and granite. Based on petrographical studies, the gabbros fell in the domain of gabbro, gabbronorite, and hornblende gabbro while granitic rocks fell in the vicinity of quartz-rich granitic rocks, granodiorite, plagiogranite and tonalite. Gabbroic rocks cover the maximum area of the crustal plutonic section and are usually medium-grained while at many places the grain size is quite large to be seen with naked eyes. Minerlogically gabbroic rocks consist of orthopyroxene, clinopyroxene, amphibole and plagioclase. These rocks maybe the fragments of main crustal plutonic section of the Zhob ophiolite. The granitic rocks having mafic minerals dominted in the eastern portion, while the felsic minerals dominted ones are in the west. The eastern side of the granitic body is compact and massive compared to western portion which is quite altered and shattered. Granitic rocks are composed of plagioclase, alkali feldspar and quartz where rutile and Cr-spinel exist in trace amounts. The gabbros of Naweoba block may have formed in a magma chamber as a result of fractional crystallization. While the granites maybe a late magmatic differentiate from the same magma chamber. The close correlation of gabbroic and granitic rocks of Naweoba block with Muslim Bagh, Khanozai and Bela ophiolites suggests their formation in supra subduction zone setting.
13
Barnes, Calvin G., and Kevin Werts. "Magma defrosting: evidence from plutonic rocks." Journal of Petrology, October 24, 2022. http://dx.doi.org/10.1093/petrology/egac112.
Abstract:
Abstract The concept of defrosting, heating and partially melting a crystal-rich, weakly mobile or immobile magma (magmatic mush), has gained wide support from volcanological studies. This process has been called on as a means to promote eruption of crystal-rich (monotonous) dacites and also to permit extraction of felsic magmas, which may accumulate in the upper crust and erupt as crystal-poor rhyolite or trachyte. Most studies of the latter type of defrosting call on a conjugate plutonic part of the system from which the felsic melt was extracted. Although petrographic and geochemical features of defrosting are well described for eruptive rocks, few studies have investigated petrographic and geochemical features of plutons that indicate defrosting. The Jurassic Ashland pluton is a tilted intrusive complex exposed in southern Oregon and northern California, USA. The central part of the pluton, quartz monzodiorite (QMD), displays petrographic features (crystal fragmentation, widespread resorption, local concentrations of refractory minerals) characteristic of partial melting. This unit is intruded, and overlain by biotite granite, which displays plagioclase cores identical in composition to plagioclase in the underlying QMD, resorbed titanite cores in plagioclase, and local subhedral to anhedral cores of alkali feldspar in poikilitic alkali feldspar crystals. Hornblende and biotite in the biotite granite display enrichments in Ti and Nb relative to these minerals in QMD, negating fractional crystallization as a petrogenetic process. Instead, these high Ti and Nb contents reflect significant partial melting of titanite in the QMD, thereby releasing high field strength elements to the defrosted biotite granite melt. The biotite granite magmas are thus interpreted as the melt-rich products of defrosting caused by intrusion of mafic–intermediate magmas into the QMD magma column.
14
Khedr, Mohamed Zaki, Saif M. Abo Khashaba, N. H. El-Shibiny, Reda A. El-Arafy, Eiichi Takazawa, Mokhles K. Azer, and Richard M. Palin. "Remote sensing techniques and geochemical constraints on the formation of the Wadi El-Hima mineralized granites, Egypt: new insights into the genesis and accumulation of garnets." International Journal of Earth Sciences, August 20, 2022. http://dx.doi.org/10.1007/s00531-022-02237-7.
Abstract:
AbstractThe Wadi El-Hima Neoproterozoic I- and A-type granites in the Southern Eastern Desert of Egypt are rich in garnets (up to 30 vol%) and are cut by NW–SE strike-slip faults, as confirmed from structure lineament extraction maps. These mineralized granites and garnet mineralization zones can be successfully discriminated using remote sensing techniques. Spectral angle mapper and matched filtering techniques are highly effective for mapping garnet-rich zones and show that the highest garnet concentrations occur along the intrusive contact zone of NW–SE striking faults. El-Hima granites have high SiO2 (73.5–75.1 wt%), Al2O3 (13.4–15.3 wt%) and total alkali (6.7–8.7 wt%) contents, suggesting that they were sourced from peraluminous (A/CNK > 1) parental magmas. Garnet-bearing trondhjemites are metasomatic in origin and formed after I-type tonalite-granodiorites, which originated in a volcanic arc tectonic setting. Garnet-rich syenogranites and alkali-feldspar granites are both post-collisional A-type granites: the syenogranites formed from peraluminous magmas generated by partial melting of lower crustal tonalite and metasedimentary protoliths during lithospheric delamination, and the alkali-feldspar granites crystallized from highly fractionated, felsic and alkali-rich peraluminous magmas in the upper crust. Garnets in El-Hima mineralized granites occur in three forms: (1) subhedral disseminated crystals, (2) vein-type crystals, and (3) aggregated subhedral crystals, reflecting different mechanisms of accumulation. All are dominantly almandine in composition (Alm76Sps10 Prp7Grs6Adr1) and have high average concentrations of heavy rare earth elements (HREE) (ΣHREE = 1636 ppm), Y = (3394 ppm), Zn (325 ppm), Li (39.17 ppm) and Ga (34.94 ppm). Garnet REE patterns show strong negative Eu anomalies with HREE enriched relative to LREE, indicating a magmatic origin. These magmatic garnets are late-stage crystallization products of Al-rich hydrous magmas, and formed at low temperature (680–730 °C) and pressure (2.1–2.93 kbar) conditions in the upper continental crust. Peculiar garnet concentrations in syenogranites near and along contact zones with alkali feldspar granites are related to peraluminous parent hydrous magma compositions. These garnets formed by in situ crystallization from A-type granite melts, alongside accumulation of residual garnets left behind after partial melting of the host garnet-rich granites along the intrusive contact. Magmatic-fluid flow along the NW–SE striking fault of Najd system enhanced garnet accumulation in melts, which formed clots and veins of garnet.
15
Guo, Jia, Kai Wu, Reimar Seltmann, Rongqing Zhang, Mingxing Ling, Congying Li, and Weidong Sun. "Unraveling the link between mantle upwelling and formation of Sn-bearing granitic rocks in the world-class Dachang tin district, South China." GSA Bulletin, July 21, 2021. http://dx.doi.org/10.1130/b35492.1.
Abstract:
Increasing evidence shows that the mantle contributes (directly or indirectly) to Sn-bearing granites worldwide. However, the specific role of mantle in the formation of tin granites and related mineralization remains poorly understood. In the world-class Dachang district, South China, tin mineralization is related to the Longxianggai equigranular/porphyritic biotite granites and tin orebodies are cut by granite porphyry dykes hosting mafic microgranular enclaves (MMEs). A combination of zircon U-Pb dating and Hf-O isotopes, mineral chemistry, and whole-rock elemental and Sr-Nd isotopic compositions—for granitic rocks and MMEs, is employed to constrain the petrogenesis and to unravel the link between tin fertility and mantle upwelling. Laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) zircon U-Pb dating indicates that the biotite granites were emplaced at ca. 93 Ma, and the granite porphyry dykes and MMEs were formed at ca. 86 Ma. The biotite granites are silica- and alkali-enriched with A/CNK ratios of 1.04−1.36, and exhibit elevated concentrations of Li, F, P, Rb, Cs, Ta, Sn, W, and U, showing affinities with highly fractionated S-type granites. Whole-rock geochemical and Nd isotopic (εNd(t) = −10.0 to −7.8) data, and in situ zircon Hf-O (εHf(t) = −9.9 to −3.9, δ18O = 6.2−8.9‰) isotopes indicate that the biotite granites were formed by partial melting of metasedimentary rocks at relatively high temperatures (≥782 °C), possibly with minor input of mantle material. Likewise, the post-ore granite porphyry dykes have similar chemical and mineralogical characteristics as fractionated S-type granites. Zircon Hf-O isotopes (εHf(t) = −9.0 to −4.9, δ18O = 6.5−8.2‰) and whole-rock geochemical data suggest they were derived from a similar source as the biotite granites, whereas elevated εNd(t) values of −5.0 to −3.3 for granite porphyry dykes relative to biotite granites reveal an increasing mantle input. Distinct εNd(t) (−0.4 and −0.3) and zircon Hf-O (εHf(t) = 1.5−5.0, δ18O = 6.5−7.2‰) isotopes of the MMEs, suggest that the mafic melt could be sourced from the asthenospheric mantle, contaminated by subcontinental lithospheric mantle/continental crust during magma ascent, and hybridized by felsic melt at emplacement-level. The magmatic sequence in the Dachang district is indicative of an extensional tectonic setting where mantle-derived magmas are predicted to migrate to shallower crustal levels as the crust progressively becomes thinner and hotter. High-temperature partial melting of mature metasedimentary crust triggered by heat input from the upwelled mantle, may contribute to biotite breakdown, which is important for concentrating tin in melts. Fractional crystallization of initially Sn-rich felsic melts under reduced conditions makes further tin enrichment and produces Sn-bearing granites (the Longxianggai pluton). Prolonged mantle upwelling results in distinct magma mixing and the formation of granite porphyry dykes and MMEs. These dykes are highly fractionated with elevated Sn and W contents, which show great potential to form hydrothermal Sn-W mineralization.
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Bédard, Jean H., Valentin R. Troll, Frances M. Deegan, Christian Tegner, Benoit M. Saumur, Carol A. Evenchick, Stephen E. Grasby, and Keith Dewing. "High Arctic Large Igneous Province Alkaline Rocks in Canada: Evidence for Multiple Mantle Components." Journal of Petrology, June 9, 2021. http://dx.doi.org/10.1093/petrology/egab042.
Abstract:
Abstract The Cretaceous High Arctic Large Igneous Province (HALIP) in Canada, although dominated by tholeiites (135-90 Ma), contains two main groups of alkaline igneous rocks. The older alkaline rocks (∼96 Ma) scatter around major fault and basement structures. They are represented by the newly-defined Fulmar Suite alkaline basalt dykes and sills, and include Hassel Formation volcanics. The younger alkaline group is represented by the Wootton Intrusive Complex (92.2-92.7 Ma), and the Audhild Bay Suite (83-73 Ma); both emplaced near the northern coast of Ellesmere Island. Fulmar Suite rocks resemble EM-type ocean island basalts (OIB) and most show limited crustal contamination. The Fulmar Suite shows increases of P2O5 at near-constant Ba-K-Zr-Ti that are nearly orthogonal to predicted fractionation- or melting-related variations; which we interpret as the result of melting composite mantle sources containing a regionally widespread apatite-bearing enriched component (P1). Low-P2O5 Fulmar Suite variants overlap compositionally with enriched HALIP tholeiites, and fall on common garnet lherzolite trace element melting trajectories, suggesting variable degrees of melting of a geochemically similar source. High-P2O5 Hassel Formation basalts are unusual among Fulmar rocks, because they are strongly contaminated with depleted lower crust; and because they involve a high-P2O5-Ba-Eu mantle component (P2), similar to that seen in alkali basalt dykes from Greenland. The P2 component may have contained Ba-Eu-rich hawthorneite and/or carbonate minerals as well as apatite, and may typify parts of the Greenlandic sub-continental lithospheric mantle (SCLM). Mafic alkaline Audhild Bay Suite (ABS) rocks are volcanic and hypabyssal basanites, alkaline basalts and trachy-andesites, and resemble HIMU ocean island basalts in having high Nb, low Zr/Nb and low 87Sr/86Sri. These mafic alkaline rocks are associated with felsic alkaline lavas and syenitic intrusions, but crustally-derived rhyodacites and rhyolites also exist. The Wootton Intrusive Complex (WIC) contains geochemically similar plutonic rocks (alkali gabbros, diorites and anatectic granites), and may represent a more deeply eroded, slightly older equivalent of the ABS. Low-P2O5 ABS and WIC alkaline mafic rocks have flat heavy rare-earth (HREE) profiles suggesting shallow mantle melting; whereas High-P2O5 variants have steep HREE profiles indicating deeper separation from garnet-bearing residues. Some High-P2O5 mafic ABS rocks seem to contain the P1 and P2 components identified in Fulmar-Hassel rocks, whereas other samples trend towards possible High-P2O5+Zr (PZr) and High-P2O5+K2O (PK) components. We argue that the strongly alkaline northern Ellesmere Island magmas sampled mineralogically heterogeneous veins or metasomes in Greenlandic-type SCLM, which contained trace phases like apatite, carbonates, hawthorneite, zircon, mica or richterite. The geographically more widespread apatite-bearing component (P1), could have formed part of a heterogeneous plume or upwelling mantle current that also generated HALIP tholeiites when melted more extensively, but may also have resided in the SCLM as relics of older events. Rare HALIP alkaline rocks with high K-Rb-U-Th fall on mixing paths implying strong local contamination from either Sverdrup Basin sedimentary rocks or granitic upper crust. However, the scarcity of potassic alkaline HALIP facies, together with the other trace element and isotopic signatures, provide little support for an ubiquitous fossil sedimentary subduction zone component in the HALIP mantle source.
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Tornos, Fernando, John M. Hanchar, Matthew Steele-MacInnis, Elena Crespo, Vadim S. Kamenetsky, and Cesar Casquet. "Formation of magnetite-(apatite) systems by crystallizing ultrabasic iron-rich melts and slag separation." Mineralium Deposita, September 2, 2023. http://dx.doi.org/10.1007/s00126-023-01203-w.
Abstract:
AbstractMagnetite-(apatite) ore deposits are interpreted as being formed by the crystallization of iron-rich ultrabasic melts, dominantly generated by the interaction of silicate melts with oxidized P-F-SO4-bearing sedimentary rocks. This hypothesis is supported by geologic evidence, experimental studies, numerical modeling, stable and radiogenic isotope geochemistry, mineralogy, and melt- and mineral-inclusion data. Assimilation of crustal rocks during ascent promotes separation from a silicate magma of Fe-rich, Si-Al-poor melts with low solidus temperatures and viscosities, allowing coalescence, migration, and emplacement at deep to subaerial crustal environments. When the iron-rich melt attains neutral buoyancy, fractional crystallization leads to melt immiscibility similar to that observed in industrial blast furnaces, which promotes separation of massive magnetite ore overlain by different types of “slag” containing actinolite or diopside ± phosphates ± magnetite ± feldspar ± anhydrite ± scapolite, commonly enriched in high field strength elements. The mineralogy and morphology of this iron-depleted cap strongly depend on the depth of emplacement and composition of the iron-rich magma. Most of these systems exhibit high oxygen fugacity, which inhibits the precipitation of significant sulfide mineralization. The initially high fO2 of these systems also promotes the formation of low-Ti (< 1 wt%) magnetite: Ti acts as an incompatible component and is enriched in the iron-poor caps and in the hydrothermal aureole. High fluid-phase pressures produced during massive crystallization of magnetite from the melt further facilitate the exsolution of magmatic-hydrothermal fluids responsible for the formation of aureoles of alkali-calcic-iron alteration with hydrothermal replacement-style iron mineralization. On the whole, these systems are dramatically different from the magmatic-hydrothermal systems related to intermediate to felsic igneous rocks; they are more akin to carbonatite and other ultramafic rocks.
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Dumańska-Słowik, M., T. Powolny, and G. Nguyen Khac. "Mineralogy and geochemistry of nepheline syenite from the Bang Phuc massif of the alkaline Cho Don complex in north-eastern Vietnam – implications for magma evolution and fluid-rock interactions." Journal of Petrology, June 22, 2023. http://dx.doi.org/10.1093/petrology/egad042.
Abstract:
Abstract The alkaline Cho Don complex in NE Vietnam comprises several mafic-felsic suites related to the widespread magmatism developed during the early-Permian–late-Triassic. The contribution explores the petrogenesis of nepheline syenite from the Bang Phuc massif and its petrogenetic relationship with cogenetic scapolite-rich gabbro. The nepheline syenite formed through fractional crystallization of pristine mantle-derived basaltic melt modified by subduction-related components (chiefly sediment-derived melts), as shown by e.g. low Ba/Th and high Th/Nb ratios of the rocks. The transition from gabbro to syenite follows a within-plate enrichment trend (e.g. increasing Ta/Yb, Nb/Yb, and Th/Yb ratios) that might reflect switch from post-orogenic to intra-plate regimes, accompanied by subduction–collision–extension events related to the Indosinian Orogeny. Furthermore, magma evolution involved the progressive contribution of asthenospheric-derived melts that resulted in the appearance of OIB-like signatures (e.g. high Nb/La ratios) in the nepheline syenite. Fractional crystallization of fluorapatite and mafic phases, as well as assimilation of carbonate wall rocks ultimately led to the decrease of LREE contents and/or modification of Zr/Hf ratios. Magmatic phases of the nepheline syenite include nepheline, sodalite, oligoclase, orthoclase, and annite, as well as accessory fluorapatite, fluorite, and minor amounts of zircon and metamict allanite-Ce. The nepheline equilibrated at temperatures ranging between 850-700°C, which reflects protracted residence at a higher temperature. Later, it has been locally altered to cancrinite, dawsonite, and natrolite via CO2- and alkali-rich fluid influx. The fluid-rock interactions were also manifested by the presence of chessboard-twinned albite and coarsening of braid-perthite into patch-perthite, as well as recrystallization of primary orthoclase into microcline. The orthoclase→microcline conversion, albeit fairly indiscrete under a polarizing microscope and confirmed by Raman micro-spectroscopy, is followed by the change of cathodoluminescence colours, i.e. from light-blue (activated by Ti4+ and/or Al-O--Al centres) in orthoclase towards brownish and/or greenish (activated by Mn2+ and structural defects) in microcline.