Petrology of low-temperature, ultrahigh-pressure marbles and interlayered coesite eclogites near Sanqingge, Sulu terrane, eastern China

摘要

Marbles and interlayered coesite-bearing eclogites near the village of Sanqingge in the Sulu ultrahigh-pressure (UHP) terrane of eastern China were studied to estimate their P-T evolution. Using gamet, omphacite and phengite as geothermobarometers, the coesite eclogites are calculated to have experienced P-T conditions of 3.4—3.7 GPa and approx 600 deg C (stage I), followed by decompression and a slight temperature decrease to 2.7—3.2 GPa and 520-560 deg C respectively (stage II) and later to 2.6—2.8 GPa and approx 500 deg C (stage III). No water influx affected the eclogites until reaching amphibolite facies conditions of 0.5—1.3 Gpa and 595—685 deg C (stage IV). As we interpret the occasional appearance of calcite with magnesite relics in the core as a reaction of UHP dolomite and magnesite with Ca-rich fluids at stage IV to form CaCO_3, the calculated pressure for stage I could be the maximum pressure experienced by these rocks. Thus, the crustal material of the Sanqingge quarry, originally sedimentary carbonates (now marbles) and interstratified basic tuffs (now eclogites), has been buried to a depth of >= 120 km at approx 600 deg C. This burial occurred in a subduction setting along a very low geotherm of 5—6°C/km. The exhumation possibly occurred in the environment of a subduction channel.