Garnet-Biotite Chemistry for Thermometry of Staurolite Schist from South of Mashhad, NE Iran

Abstract

Mashhad Metamorphic Complex (MMC) with metamorphic, granitic intrusions and ophiolite units is situated in the suture zone of Paleotethys in NE Iran. Triassic staurolite-garnet schist of Khalaj represents the highest grade of metamorphism in MMC. In this study EPMA analyses of garnet and biotite used to determine the maximum temperature for regional metamorphism. The pattern of zoning in garnet also used to describe the exhumation history of the area during metamorphic events. Maximum temperature for staurolite garnet schist of MMC calculated with garnet-biotite thermometer is ~618°C. The quantitative analysis of major elements along core to rim profiles obtained for garnet porphyroblast show the presence of growth zoning. Mg and Fe increase slightly outwards, while Mn decreases from core to the rim. Preserved garnet growth zoning in temperature above 600°C, indicates that staurolite-garnet schists of MMC on the margin of Paleotethys in NE Iran were rapidly heated and cooled. However, growth zoning is not well preserved in the rim of garnet grain. It could be an evidence for the presence of later retrograde metamorphic phase which influenced regional metamorphic schists in MMC.