Microstructures

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Syntectonic Porphyroblasts

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72. Syntectonic muscovite - Large muscovite porphyroblasts like this one from the Catalina metamorphic core complex, Arizona, are common in mylonitic granitic rocks. Muscovite grains, which may be original or new reaction products, are rapidly reoriented so that their (001) planes are at a low angle to the foliation, and the grain is tilted up in the direction of shear (here left lateral). In this position, the muscovite grains grow larger, sometimes exceeding the original, undeformed grain size of the rock. Often these so-called 'fish' are internally undeformed. In this example, the large, central grain shows very minor lattice bending at lower right and some microfaulting at upper left. Note the planar truncation of (001) planes around the margin of this grain, suggestive of solution mass transfer. Deformation was at uppermost greenschist facies

FOV 1.5 mm, Nicols Crossed. Section 20 µm thick.

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Syntectonic Porphyroblasts	Click image to enlarge
72. Syntectonic muscovite - Large muscovite porphyroblasts like this one from the Catalina metamorphic core complex, Arizona, are common in mylonitic granitic rocks. Muscovite grains, which may be original or new reaction products, are rapidly reoriented so that their (001) planes are at a low angle to the foliation, and the grain is tilted up in the direction of shear (here left lateral). In this position, the muscovite grains grow larger, sometimes exceeding the original, undeformed grain size of the rock. Often these so-called 'fish' are internally undeformed. In this example, the large, central grain shows very minor lattice bending at lower right and some microfaulting at upper left. Note the planar truncation of (001) planes around the margin of this grain, suggestive of solution mass transfer. Deformation was at uppermost greenschist facies FOV 1.5 mm, Nicols Crossed. Section 20 µm thick. Click here to view Flash animation in a new window.