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Title
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CTP-16-2015: Quartz rich Gniess with garnets
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Description
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Sample CTP-16-2015 is a metamorphic rock which displays foliation and compositional banding in hand sample. Banding alternates between large white crystals that appear to be quartz, and black crystals with cleavage (possibly biotite, amphibole, or pyroxenes) that can not be identified in hand sample with confidence. The rock shows a distinct gneissose structure. Pea-sized garnet grains are visible in hand sample.
Minerals in thin section, particularly high birefringence minerals, are heavily fractures and hard to identify. The fracturing has damaged cleavage patterns in these high birefringence minerals. Isotropic almandine is easily identified throughout the thin section. The large white crystals are identified as quartz, with low relief, fracture patterns, low birefringence, and irregularly shaped crystals. The black grains are mainly biotite crystals, identified by a dark brown color, one-perfect cleavage, parallel extinction, and pleochroism. Other black crystals with a high birefringence minerals are hard to identify, although several grains with clear 120/60 degree cleavage and 90 degree cleavage patterns can be found, indicating that these minerals are amphiboles and pyroxenes respectively. Several grains of calcic plagioclase can be found in thin section, showing distinctive grid twinning textures. Small muscovite grains, with very high birefringence, can be seen in between several of the compositional banding structures.
Distinctive compositional banding and the association of garnets and biotite indicates that this is a gneiss. The presence of biotite means that this rock can not be of a very high grade, for biotite and garnet are only are common index minerals in the lower end of the gneiss range. Therefore, this rock has a medium-high/high metamorphic grade. The protolith for this rock was likely a shale, rich in Mg, Fe, and Si, that when metamorphose, formed coarse grains of biotite and quartz.
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Subject
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Metamorphic Petrology
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Creator
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Emma Sosa
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