Microstructures of melt-bearing regional metamorphic rocks

Abstract

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.