Document Type : Original Research Paper
Author
Department of Civil Engineering, Shahid Rajaee Teacher Training University, Tehran, Iran
Abstract
The Kashmar granitoid (42.5 Ma) occurs in NE Central Iran Plate (CIP). It mainly includes felsic metaluminous (ASI ≤ 1) I–type granite and granodiorite plutons. Microprobe analyses show that the Kashmar amphiboles are low in Ti and (Na+K)A contents (all < 0.5 apfu), indicating magnesio–hornblende, a distinct mafic mineral of low–temperature I–type granites. Also, the content of Al2O3 is low, suggesting low–pressure crystallization. The Mg* ratio is high (0.60–0.75) and the AlVI is extremely low (< 0.1 apfu), but Fe3+ is much higher than AlVI, features confirming low–pressure and low–temperature conditions. Utilizing the modern thermo–barometers, the pressures of ≤ 3 kb and average temperature of 655 oC were calculated for Kashmar amphiboles. The attributed log fO2 values are negative, ranging from –16.59 to –19.40 and plotting above the QFM stability. Results of this study propose a thermal boundary of ~700 oC between felsic (~600–700 oC) and mafic (~700–800 oC) low–temperature I–type granites, and reinforce the modern granite subdivision.
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