Schlieren formation in diatexite migmatite: examples from the St Malo migmatite terrane, France

摘要: Schlieren are trains of platy or blocky minerals, typically the ferromagnesian minerals and accessory phases, that occur in granites melt-rich migmatites, such as diatexites. They have been considered as: (1) unmelted residue from xenoliths source region; (2) mineral accumulations formed during magma flow; (3) compositional layering; (4) sites melt loss. In order to help identify schlieren-forming processes diatexites at St Malo, differences size, shape, orientation, distribution composition biotite schlieren their hosts investigated. Small grains much less abundant than hosts. generally larger, greater aspect ratios have, except with low (< 10%) contents, a stronger shape preferred orientation host biotite. The ranges similar, but defines tighter, sharper peaks on composition-frequency plots. Hosts show magmatic textures imbricated (tiled), unstrained plagioclase. Some only (tiled biotite, no crystal-plastic strain features), many indicating submagmatic subsolidus deformation (e.g. kinked grains) these most extensive evidence for recrystallization. Magmas Malo initially contained significant fraction residual plagioclase crystals; smaller were separated larger crystals flow. Since was also major, early crystallizing phase, plagioclase-rich domains developed rapidly reached rigid percolation threshold first, forcing further flow be concentrated into narrowing zones where had accumulated, hence increasing shear degree domains. result grain contacts tiling inertia-regime. Final amalgamation aggregates involved volume loss trapped between expelled after biotite-rich layers.