Tectonic implications of P-T paths derived for garnet-bearing felsic gneisses from the Dabie and Sulu ultrahigh pressure terranes, east-central China

Abstract

Felsic gneisses usually represent the vast majority of rocks in high-pressure (HP) and ultrahigh-pressure (UHP) terranes, but the derivation of metamorphic pressure-temperature (P-T) paths for these rocks have been rarely undertaken. We studied a felsic paragneiss with the mineral assemblage of plagioclase, quartz, phengite, biotite, garnet, epidote, K-feldspar, and rutile from the Chinese Continental Scientific Drilling main hole (depth: 775 m), located in the Sulu UHP terrane, eastern China, by analyzing minerals with the electron microprobe and computing P-T pseudosections with PERPLE_X. Garnet includes phengite, feldspar, quartz, and rutile and retains a Ca-rich core (XCa = 0.42). Towards the rim Ca contents decrease (XCa = 0.36) and Fe and Mg contents increase. This zoning points to a prograde P-T path at pressures lower than 1.3 GPa. In combination with Zr-in-rutile thermometry and garnet volume isopleths in the P-T pseudosection, peak P-T conditions of ca. 1.2 GPa and 610 °C were constrained which are below the wet granite solidus. K-feldspar formation at the expense of phengite indicates a nearly isothermal decompression accompanied by late fluid infiltration. A similar P-T path was additionally derived for a published felsic orthogneiss from the Dabie UHP terrane using P-T pseudosections. The prograde P-T paths obtained in this study imply that not all felsic gneisses from the Dabie and Sulu UHP terranes underwent UHP metamorphism. This is explained by tectonic juxtaposition of various felsic gneisses from different depths in an exhumation channel.