A jadeite–quartz–glaucophane rock from Karangsambung, central Java, Indonesia |
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| Authors: | K Miyazaki J Sopaheluwakan I Zulkarnain & K Wakita |
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| Institution: | Geological Survey of Japan, 1-1-3 Higashi, Tsukuba, Ibaraki 305, Japan,;Research and Development Center for Geotechnology, Jl. Cisitu, 21/154D, Bandung, 40135 Indonesia |
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| Abstract: | High-pressure metamorphic rocks are exposed in Karangsambung area of central Java, Indonesia. They form part of a Cretaceous subduction complex (Luk–Ulo Complex) with fault-bounded slices of shale, sandstone, chert, basalt, limestone, conglomerate and ultrabasic rocks. The most abundant metamorphic rock type are pelitic schists, which have yielded late Early Cretaceous K–Ar ages. Small amounts of eclogite, glaucophane rock, garnet–amphibolite and jadeite–quartz–glaucophane rock occur as tectonic blocks in sheared serpentinite. Using the jadeite–garnet–glaucophane–phengite–quartz equilibrium, peak pressure and temperature of the jadeite–quartz–glaucophane rock are P = 22 ± 2 kbar and T = 530 ± 40 °C. The estimated P–T conditions indicate that the rock was subducted to ca 80 km depth, and that the overall geothermal gradient was ～ 7.0 °C/km. This rock type is interpreted to have been generated by the metamorphism of cold oceanic lithosphere subducted to upper mantle depths. The exhumation from the upper mantle to lower or middle crustal depths can be explained by buoyancy forces. The tectonic block is interpreted to be combined with the quartz–mica schists at lower or middle crustal depths. |
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| Keywords: | Cretaceous subduction complex geothermal gradient high-pressure metamorphic rocks Indonesia Karangsambung P–T conditions tectonics |
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