Myrmekite as a marker between preaqueous and postaqueous phase saturation in granitic systems
Myrmekite as a marker between preaqueous and postaqueous phase saturation in granitic systems
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镁铁矿作为花岗岩系统中水前相和水后相饱和度的标记
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发表时间：
1979
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通讯作者：
M. Hibbard
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作者：
M. Hibbard
一个nonadjuvive，nonexsolution模式myrmekite增长是基于结构关系的沙泉斑状花岗闪长岩，中西部内华达州。结晶序列分为（1）前水相饱和阶段，以斜长石的主要生长（带状）为特征，石英和钾长石（斑晶），和（2）后期阶段饱和的特点是myrmekite，最终自形生长的斜长石和石英，和最终增长的钾长石斑晶和大部分的钾长石的基质，包括一些晶体冰长石的习性特征。myrmekite结果从微压淬火过程中分离的水相结晶的进展。在斜长石上出现的myrmekite作为叶状单位，延伸到钾长石中，是由于斜长石从熔体中生长的局部延续，同时排出富含钾长石成分的富水流体，从而沉淀出奥长石（myrmekite的基本成分）。晚期钾长石从富水流体中结晶出来，充填在镁铝榴石周围。硅镁石中的石英代表了硅不能从淬火的熔体中扩散出来，主要是按照二元共晶结晶的原理以蠕虫状出现。沙泉myrmekite模型进行测试，评估其发生在细晶伟晶岩系统，花岗质片麻岩，并在热液次生钾长石环境。除岩浆热液环境外，其他类型的岩浆岩均为后岩浆岩，如果考虑结晶时的构造环境，则其他类型的岩浆岩均为岩浆成因。
A nonreplacive, nonexsolution model of myrmekite growth is based on textural relationships in the Sand Springs porphyritic granodiorite, west-central Nevada. A sequence of crystallization is divided into (1) a preaqueous-phase saturation stage, characterized by major growth of plagioclase (zoned)., quartz, and K-feldspar (phenocrysts), and (2) a postaqueous-phase saturation stage characterized by myrmekite, final euhedral growth of plagioclase and quartz, and final growth of K-feldspar phenocrysts and most K-feldspar of the matrix, including some crystals with adularia-habit characteristics. Myrmekite results from micropressure quenching during the separation of an aqueous phase as crystallization progresses. The occurrence of myrmekite as lobate units on plagioclase, extending into K-feldspar, results from precipitation of oligoclase (the basic ingredient of myrmekite) as local continuations of plagioclase growth from a melt that simultaneously expels an aqueous-rich fluid enriched in K-feldspar component. Late K-feldspar crystallizes from the aqueous-rich fluid, filling in around the myrmekite. Quartz in myrmekite represents the inability of silica to diffuse from the quenched melt and occurs as vermicules chiefly in accord with the principles of binary eutectic crystallization. The Sand Springs myrmekite model is tested by evaluating its occurrences in aplite-pegmatite systems, in granitic gneisses, and in the hydrothermal secondary K-feldspar environment. Myrmekite commonly occurs in all but the hydrothermal environment, which is postmyrmekite, and a fundamentally magmatic origin can be reasoned for the other rock types if the tectonic environment during crystallization is also considered.