Journal Article10.1007/S00410-020-01697-X
The geochemical differentiation of S-type pegmatites: constraints from major–trace element and Li–B isotopic composition of muscovite and tourmaline
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TL;DR: In this article, the geochemical and isotopic properties of pegmatites have been analyzed using the major and trace element and Li-B isotopic composition of muscovite and tourmaline from internally zoned and un-zoned S-type pegmatite.
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Abstract: Pegmatites often exhibit regional as well as internal zonation. In this study, we use major and trace element and Li–B isotopic composition of muscovite and tourmaline from internally zoned and un-zoned S-type pegmatites and their host granites to characterize the geochemical and isotopic fractionation associated with their formation. The internally zoned pegmatites comprise three distinct textural zones, namely wall, intermediate and core. Muscovite and tourmaline occur in all the zones of these pegmatites. The trace element concentrations/ratios of muscovite from the three zones form well-defined differentiation trends marked by enrichment of incompatible elements such as Rb, Cs, Sr, B, Zn, Nb, Ta, P and the depletion of Ni, Co, V, Sc, Ti, Ba from the wall zone, through the intermediate zone to the core zone. This is suggestive of a strong role of fractional crystallization in producing the compositional diversity in the internally zoned pegmatites. Alkali element ratios such K/Rb and K/Cs in muscovite exhibit near exponential decline from the wall to the core zone which is suggestive of Rayleigh-type fractional crystallization. Fractional crystallization modelling reveals that the formation of the wall zone requires < 69% crystallization, the intermediate zone 85–95% and the core zone ca. 99% crystallization, leading to extreme enrichment of Rb and Cs and other incompatible elements. Muscovites and tourmaline from the un-zoned pegmatites display similar compositional trends as the internally zoned ones, but with a significant compositional gap between the host granite and the pegmatite. Lithium isotopic composition of muscovites and B-isotopic composition of tourmalines become progressively lighter from wall to core zone of the internally zoned pegmatite and from the granite to the pegmatite for the un-zoned pegmatites. This is suggestive of an important role of vapour exsolution in the formation of the pegmatites. Taken together, the geochemical and isotopic trends in the pegmatites can be explained by Rayleigh fractional crystallization operating in tandem with vapour exsolution.
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Citations
Lithium isotope behavior during magmatic differentiation and fluid exsolution in the Jiajika granite–pegmatite deposit, Sichuan, China
Huijuan Zhang,Shihong Tian,Denghong Wang,Xianfang Li,Tao Liu,Yujie Zhang,Xiaofang Fu,Xuefeng Hao,Kejun Hou,Yue Zhao,Yan Qin +10 more
TL;DR: Li isotopic compositions of two-mica granites, Li-rich and Li-poor pegmatites, and muscovites in the Jiajika pegmatite-type Li deposit were analyzed in this article.
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Lithium pegmatite of anatectic origin - A case study from the Austroalpine Unit Pegmatite Province (Eastern European Alps): geological data and geochemical model
Tanja Knoll,Benjamin Huet,Ralf Schuster,Heinrich Mali,Theodoros Ntaflos,Christoph Hauzenberger +5 more
TL;DR: In this article , a case study from the Eastern European Alps is presented, showing the continuous evolution from melt generation in staurolite bearing micaschist to spodumene (LiAlSi2O6) bearing pegmatite.
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Nature of the mineralizing fluids in the Balda and Motiya W-prospects, western India: Constraints from chemical and B-isotope composition of tourmaline
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I- and S-type granites in the Lachlan Fold Belt
Bruce W. Chappell,A. J. R. White +1 more
Abstract: Granites and related volcanic rocks of the Lachlan Fold Belt can be grouped into suites using chemical and petrographic data. The distinctive characteristics of suites reflect source-rock features. The first-order subdivision within the suites is between those derived from igneous and from sedimentary source rocks, the I- and S-types. Differences between the two types of source rocks and their derived granites are due to the sedimentary source material having been previously weathered at the Earth's surface. Chemically, the S-type granites are lower in Na, Ca, Sr and Fe3+/Fe2+, and higher in Cr and Ni. As a consequence, the S-types are always peraluminous and contain Al-rich minerals. A little over 50% of the I-type granites are metaluminous and these more mafic rocks contain hornblende. In the absence of associated mafic rocks, the more felsic and slightly peraluminous I-type granites may be difficult to distinguish from felsic S-type granites. This overlap in composition is to be expected and results from the restricted chemical composition of the lowest temperature felsic melts. The compositions of more mafic I- and S-type granites diverge, as a result of the incorporation of more mafic components from the source, either as restite or a component of higher temperature melt. There is no overlap in composition between the most mafic I- and S-type granites, whose compositions are closest to those of their respective source rocks. Likewise, the enclaves present in the more mafic granites have compositions reflecting those of their host rocks, and probably in most cases, the source rocks.S-type granites have higher δ18O values and more evolved Sr and Nd isotopic compositions, although the radiogenic isotope compositions overlap with I-types. Although the isotopic compositions lie close to a mixing curve, it is thought that the amount of mixing in the source rocks was restricted, and occurred prior to partial melting. I-type granites are thought to have been derived from deep crust formed by underplating and thus are infracrustal, in contrast to the supracrustal S-type source rocks.Crystallisation of feldspars from felsic granite melts leads to distinctive changes in the trace element compositions of more evolved I- and S-type granites. Most notably, P increases in abundance with fractionation of crystals from the more strongly peraluminous S-type felsic melts, while it decreases in abundance in the analogous, but weakly peraluminous, I-type melts.
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The classification of granitic pegmatites revisited
Petr Černý,T. Scott Ercit +1 more
TL;DR: In this paper, the classification of granitic pegmatites is approached from two directions, based on but broadened and refined from earlier works by Ginsburg and Cerný.
The application of trace elements to the petrogenesis of igneous rocks of granitic composition
TL;DR: In this article, the effects of these minerals on the rare earth elements (REE) patterns of granitic melts during partial melting or differentiation are exaggerated as compared to basaltic systems, making detection of residual phases easier.
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