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Deformation of Incompletely Crystallized Magma Systems: Granitic Gneisses and their Tectonic Implications

1987; University of Chicago Press; Volume: 95; Issue: 4 Linguagem: Inglês

10.1086/629148

1537-5269

M. J. Hibbard,

Geological formations and processes

Certain textures in two-feldspar calc-alkaline granitic gneisses have been identified as indicators of the presence of intercrystalline melt during deformation of crystal-melt mushes. Fluid relocation or "fracting" is to be expected in an incompletely crystallized magmatic system as deviatoric stresses are applied to it. Relocation of late-stage fluids capable of crystallizing K-feldspar, quartz, microaplite, and myrmekite to low-pressure zones such as the pressure shadow region around earlier crystals occurs within some developing mylonitic gneiss fabrics and results in a distinctive assemblage of textures. Strain in these rocks is expressed by various brittle and ductile features, and by fluid relocation textures. The petrogenesis of these particular gneisses is therefore both magmatic and metamorphic, contrasting with gneiss-forming processes that had no participating magmatic component. Migmatitic complexes evolving by partial anatexis of non-magmatic rocks in a tectonically active environment are not expected to contain relict early magmatic textures, contrasting with most of the gneisses described in this report that have formed from what was initially a completely melted system. Identification of fluid relocation textures in granitic gneisses is proposed here as a step in resolving the question of timing of magmatism and metamorphism in continental crystalline complexes.