Multiple origins of clinopyroxenes in alkali basaltic rocks
Multiple origins of clinopyroxenes in alkali basaltic rocks
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DOI：
10.1016/0024-4937(79)90043-4
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发表时间：
1979-04
期刊：
影响因子：
3.5
通讯作者：
S. Wass
中科院分区：
文献类型：
--
作者：
S. Wass
来自澳大利亚东部各省和中央高原的个别碱性玄武岩流中单斜辉石的地球化学和结构数据可用于区分这些单斜辉石的四种不同来源模式（三种在高压下）。来自这两个省的许多单一流量含有三种甚至四种来源的单斜辉石。稀有流含有单斜辉石核心和两个不同代单斜辉石的过度生长。每个过度生长物在成分上都类似于出现在捕虏体中的单斜辉石或作为同一主熔岩中的离散晶体。这种边缘关系提供了宿主岩浆经历了高压结晶的证据，并证实了一些捕虏体和巨型晶体是同源的。随着压力的降低，单斜辉石化学性质的主要变化是 Allv/Alvi 比率的增加，Ti 和 Al 的原子比例随 Si 的减少而线性增加，以及辉石的 Ti:(100 Mg/(Mg + Σ Fe)) 比率增加。 Al2O2wt, % 是一个不可靠的潜在地理气压指标；有必要考虑 Al 的四面体和八面体位点占用情况（Allv/Alvlratio）。推测某些玄武岩成分发生以单斜辉石为主的高压分馏，导致 SiO2 贫化、碱富集以及宿主岩浆的 Mg/(Mg + Fe2+) 值降低。
Geochemical and textural data on clinopyroxenes in individual alkali basaltic flows from provinces in eastern Australia and the Massif Central can be used to differentiate four different modes of origin (three at high pressure) for these clinopyroxenes. Many single flows from the two provinces contain clinopyroxenes of three, or even four, of these origins. Rare flows contain core clinopyroxene with overgrowths of clinopyroxenes of two distinct generation. Each of the overgrowth is compositionally analogous to clinopyroxenes occurring in xenoliths or as discrete crystals in the same host lava. Such rimming relationships provide evidence that the host magma has undergone high pressure crystallisation and confirm that some xenoliths and megacrysts are cognate. With decreasing pressure the major changes in clinopyroxene chemistry are an increase in the ratio Allv/Alvi, a linear increase of atomic proportions of Ti and Al with decreasing Si, and an increase in the Ti: (100 Mg/(Mg + Σ Fe)) ratio of the pyroxenes. Al2O2wt, % is an unreliable potential geobarometric indicator; consideration of tetrahedral and octahedral site occupancies by Al (Allv/Alvlratio) is necessary. High-pressure fractionation dominated by clinopyroxene is postulated for some basaltic-composition, resulting in SiO2depletion, alkali enrichment and decrease in the Mg/(Mg + Fe2+) value of the host magma.