Microstructures of melt-bearing regional metamorphic rocks
Microstructures of melt-bearing regional metamorphic rocks
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DOI：
10.1130/2011.1207(01
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发表时间：
2011-02
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通讯作者：
R. Vernon
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作者：
R. Vernon
区域混合岩中以前存在长英质熔体的最可靠的显微构造标准是细脉中的三种矿物(石英、钾长石和苏打斜长石)聚集体。其他几个标准可能是可靠的，即：(1)长石(从液体中沉淀)或包晶矿物(例如，石榴石、堇青石、斜辉石、钾长石)的正面体晶体包裹在长石“原粘粒”中；(2)长石(从液体中沉淀)或包晶矿物(例如，石榴石、堇青石、斜方辉石、钾长石)在具有丰富包裹体的相同矿物的残余颗粒上的无包裹体的自面体过度生长；(3)排列的、真面体的长石晶体；(4)在钾长石中的简单孪晶；(5)一粒长石和/或石英(推测已假象以前的熔体)与两粒其他矿物相遇的地方，60°的二面角；(6)石英、钾长石或钠长石的锥形体积，特别是在被推测为熔融时残留的颗粒包围的地方；(7)推断的原熔体的细脉(现在的矿物假象包括石英、钾长石或钠长石中的一种，尽管不太常见，涉及这些矿物中的两到三种)沿晶界或推断的粒内裂隙；(9)比围岩中斜长石颗粒更钠的斜长石细脉；(10)具有振荡分带的斜长石；(11)在原生颗粒之间的斑块或细脉中生长的石英和碱性长石的微粒状互生；(12)可由包晶颗粒和冷却熔体之间的反应解释的辛性交代聚集体；以及(13)从其中提取出亮素体的黑素体斑块和层。然而，所有这些标准都必须谨慎解读。其他一些拟议的标准是值得商榷的，例如：(1)随机矿物分布；(2)颗粒尺寸增大；(3)间隙颗粒；(4)被石英、钾长石或钠长石区域包围的推断反应矿物颗粒的腐蚀遗迹；(5)突出到矿物颗粒中；(6)没药石叶；和(7)斜长石核上出现的具有恒定碱性成分的斜长石边缘，其钙含量更高和/或成分不同。
The most reliable microstructural criterion for the former presence of felsic melt in regional migmatites is a three-mineral (quartz, K-feldspar, and sodic plagioclase) aggregate in veinlets. Several other criteria are potentially reliable, namely: (1) euhedral crystals of feldspar (precipitated from liquid) or peritectic minerals (e.g., garnet, cordierite, orthopyroxene, K-feldspar) lining felsic “protoleucosomes”; (2) inclusionfree euhedral overgrowths of feldspar (precipitated from liquid) or peritectic minerals (e.g., garnet, cordierite, orthopyroxene, K-feldspar) on residual grains of the same minerals with abundant inclusions in the mesosome; (3) aligned, euhedral feldspar crystals; (4) simple twinning in K-feldspar; (5) dihedral angles of 60° subtended where a grain of feldspar and/or quartz (inferred to have pseudomorphed former melt) meets two grains of other minerals; (6) cuspate volumes of quartz, K-feldspar or sodic plagioclase, especially where surrounded by grains inferred to have been residual during melting; (7) veinlets of inferred former melt (now mineral pseudomorphs consisting of one of quartz, K-feldspar or sodic plagioclase, preferably, though less commonly, involving two or three of these minerals) along grain boundaries or along inferred former intragranular fractures; (8) biotite pseudomorphed by feldspar; (9) veinlets of plagioclase that is more sodic than plagioclase grains in the adjacent rock; (10) plagioclase with oscillatory zoning; (11) microgranophyric intergrowths of quartz and alkali feldspar in patches or veinlets between primary grains; (12) symplectic replacement aggregates that can be explained by reactions between peritectic grains and cooling melt; and (13) melanosome patches and layers, from which leucosome has been extracted. However, all these criteria must be interpreted with care. Some other proposed criteria are questionable, for example: (1) random mineral distributions; (2) grain-size increase; (3) interstitial grains; (4) corroded relics of inferred reactant mineral grains surrounded by areas of quartz, K-feldspar, or sodic plagioclase; (5) projections into a mineral grain; (6) lobes of myrmekite; and (7) plagioclase rims with a constant sodic composition occurring on plagioclase cores that are more calcic and/or of variable composition.