[S-MP46_28PO1] Deformed rocks, Metamorphic rocks and Tectonics

[SMP46-P14] Deformation microstructures of a Kamila amphibolite mylonite and their formative temperatures

*Tomoyuki ARAI¹, Kyuichi KANAGAWA¹, Takashi YOSHINO² (1.Graduate School of Science, Chiba University, 2.Institute for Study of the Earth's Interior, Okayama University)

The Kohistan complex and the Kamila amphibolite belt in the northern Pakistan are considered to represent a Cretaceous island arc crust and a part of its lower crust, respectively. Here we report deformation microstructures of a Kamila amphibolite mylonite sample and their formative temperatures. The amphibolite mylonite sample studied is composed of 100 μm to 1 mm thick alternating layers of hornblende + pyroxene, plagioclase, and hornblende + plagioclase + quartz, intercalating a 3 mm thick layer of garnet + quartz + plagioclase. Composite planar fabrics of a top-to-south sense of shear are developed in this sample; C plane defined by compositional layering (= foliation), S plane defined by lenticular domains of plagioclase aggregate clockwise oblique to the C plane, and C′ plane anticlockwise oblique to the C plane. Hornblende + pyroxene layers contain pyroxene porphyroclasts of grain sizes ≈200 μm scattered in matrix mainly composed of hornblende grains with grain sizes ≈30 μm. Hornblende exhibits a strong crystallographic preferred orientation with (100) and [001] subparallel to foliation and lineation, respectively. Orthopyroxene porphyroclasts are elongated subparallel to foliation, and are accompanied by asymmetric tails mainly composed of hornblende indicating a top-to-south sense of shear. In addition, pyroxene porphyroclasts are surrounded by fine-grained (≈10 μm) hornblende and quartz, suggesting a breakdown reaction of pyroxenes (orthopyroxene + clinopyroxene + H2O = hornblende + quartz), which is a retrograde reaction from granulite facies to amphibolite facies. Plagioclase layers are composed of dynamically recrystallized plagioclase grains of An_47-54 in composition. Lenticular domains of plagioclase are likely porphyroclasts in origin. Plagioclase grains are polygonal in shape, and weakly aligned clockwise oblique to foliation, which also suggests a top-to-south sense of shear. Plagioclase exhibits a distinct crystallographic preferred orientation with {131} and <1-12> clockwise oblique to foliation and lineation, respectively by ≈20 degrees. But {131} and <1-12> are aligned subparallel to the S plane, suggesting the dominance of {131}<1-12> during the dynamic recrystallization of plagioclase. We applied three pyroxene geothermometers to the chemical compositions of orthopyroxene and clinopyroxene porphyroclasts, which yielded temperatures around 850 degrees C. We also applied a hornblende-plagioclase geothermometer to the average chemical compositions of hornblende and plagioclase in hornblende + plagioclase + quartz layers, and obtained a temperature of ≈610 degrees C. Thus, the amphibolite mylonite studied likely experienced a peak metamorphism of granulite facies at ≈850 degrees C, and subsequently a retrograde metamorphism of amphibolite facies at ≈610 degrees C, during which it was sheared by top-to-south thrusting.