莫桑比克Namacotche Li-Cs-Ta花岗伟晶岩群中的电气石:晶体化学和成因

Q4 Earth and Planetary Sciences
C. L. Leal Gomes, A. Neiva
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引用次数: 0

摘要

通过对Namacotche LCT长晶岩中三代电气石的野外工作、后向散射电子图像和详细的显微分析,可以区分岩浆电气石和热液电气石。第1代产于伟晶岩外中间带。它由带状晶体组成,其振荡内核为铁辉石和铁辉石,外核为铁辉石,边缘为富铁氟辉石。在伟晶岩内部的中间地带有不分带的富铁氟榴辉石晶体。所有晶体均由富a (Al, Li, B)伟晶岩熔体分馏而得。然而,分带晶体的边缘和一些非分带晶体的组成显示了热液流体的证据,因为它们在分馏趋势之外。第2代分带氟榴辉石晶体来自伟晶岩内部的中间带。它们的内核是粉色的,边缘是绿色的。边缘具有较高的YFe2+、Na、F含量YFe2+/(YFe2++Licalc)值和较低的Si、YAl、Licalc。而x位空缺含量、x位空缺/(Na+ x位空缺)值均大于核心。两个带都是热液区。边缘长满了杂草。第3代氟榴辉石宝石晶体产于剪切角砾岩爆破和碎屑中,基质为沸石。它们主要依赖于低温(280 ~ 150℃)的富流体热液环境。旋回a中的一部分可能是由于第1代岩浆碧玺晶体与晚期化学不平衡流体反应,从剪切的伟晶岩内外中间带中溶蚀而成,然后在低温热液中生长。从第3代a旋回到b旋回,再到c旋回的演化表明,在伟晶岩热液结晶后期,热液反应流体发生分馏,Li富、Fe2+贫。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Tourmalines in the Namacotche Li-Cs-Ta granitic pegmatite group, Mozambique: crystal chemistry and origin
The field work, backscattered electron images and detailed microanalyses of three generations of tourmaline from the Namacotche LCT pegmatites allows de distinction between the compositional magmatic and hydrothermal tourmalines. The generation 1 occurs in the outer intermediate zone of the pegmatite. It consists of zoned crystals with an oscillatory inner core of foitite and schorl, an outer core of schorl and an Fe-rich fluor-elbaite rim. Unzoned Fe-rich fluor-elbaite crystals occur in the inner intermediate zone of the pegmatite. All the crystals are derived by fractionation of a (Al, Li, B)-rich pegmatite melt. However, the rim of zoned crystals and some compositions of unzoned crystals show evidence of hydrothermal fluids, as they plot outside the fractionation trends. The zoned fluor-elbaite crystals of the generation 2 are from the inner intermediate zone of the pegmatite. They have a pink core and a green rim. The rim has higher YFe2+, Na, F contents YFe2+/(YFe2++Licalc.) value and lower Si, YAl, Licalc. and X-site vacancy contents, X-vacancy/(Na+X-vacancy) value than the core. Both zones are hydrothermal. The rim is an overgrowth. The fluor-elbaite gemmy crystals of the generation 3 occur in sheared breccia blasts and clasts with a cookeite matrix. They depend mainly on the fluid-rich hydrothermal environment of low temperatures (280-150ºC). Some from the cycle a may result from the dissolution of magmatic tourmaline crystals of the generation 1 from the sheared outer and inner intermediate zones of the pegmatite due to reaction with late fluids in chemical disequilibrium, followed by growth of tourmaline with low temperature hydrothermal fluids. The evolution from the cycle a to the cycle b and to the cycle c of the generation 3 implies that the hydrothermal reacting fluids were undergoing fractionation and becoming richer in Li and poorer in Fe2+ during the late hydrothermal crystallization of the pegmatites.
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Cadernos do Laboratorio Xeoloxico de Laxe
Cadernos do Laboratorio Xeoloxico de Laxe Earth and Planetary Sciences-Earth and Planetary Sciences (all)
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