40Ar/39Ar geochronology and mineral chemistry of white micas from Sadegh ـ Abad augen orthogneisses, North Shahrekord


Faculty of Natural Resources and Earth Sciences, Shahrekord University, Iran.


The orthogneisses from the North Shahrekord metamorphic complex (NSMC), as a part of Pan ـ African basement in Iran, are situated within the central part of Sanandaj–Sirjan Zone. The rocks occur as small to medium bodies and blocks with amphibolites. They contain the mineral assemblage including quartz, K ـ feldspar (perthite and antiperthite), biotite, plagioclase, white mica, and chlorite, that white micas and biotites form a mylonitic foliation.  The electron microprobe analyses on white micas display that the chemical composition of the minerals is phengitic muscovite.  The 40Ar/39Ar dating on the micas from the NSMC orthogneisses yields the average age of 162.8 ± 0.9 Ma (the late of Middle Jurassic, Callovian stage), representing the period of cooling through the closure temperature for phengitic white mica. Considering the chemical compositions of phengitic muscovite and mineral assemblages, and microstructures, we suppose that the age reflects the time of mylonitization and amphibolite facies overprinting, after peak high ـ pressure metamorphism. Now, the new findings apply to represent a model for the exhumation rate and the thermal history of the orthogneisses from the NSMC, as a part of Zagros orogeny.


Main Subjects

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