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

¹ The Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China
² Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstr. 18, D-70174 Stuttgart, Germany

^* E-mail: yfzhu{at}pku.edu.cn

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 garnet, omphacite and phengite as geothermobarometers, the coesite eclogites are calculated to have experienced P-T conditions of 3.4-3.7 GPa and 600°C (stage I), followed by decompression and a slight temperature decrease to 2.7-3.2 GPa and 520-560°C respectively (stage II) and later to 2.6-2.8 GPa and 500°C (stage III). No water influx affected the eclogites until reaching amphibolite facies conditions of 0.5-1.3 Gpa and 595-685°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 CaCO₃, 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 600°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.

KEYWORDS: coesite, eclogite, marble, magnesite, Sulu UHP terrane