中国大陆科学钻探预先导孔(CCSD-PP2)退变榴辉岩中碳硅石的发现及其地质意义

Discovery of moissanite in retrogressive eclogite from the Pre-pilot Hole of the Chinese Continental Scientific Drilling Project (CCSD-PP2) and its geological implication.

镜下鉴定和拉幔光谱测试结果表明中国大陆科学钻探工程预先导孔(CCSD-PP2)退变榴辉岩中石榴石内存在除绿辉石、磷灰石、锆石、金红石、磷灰石、石英、菱镁矿和磁铁矿等矿物包裹体外,还存在一种具有重要温压指示意义的碳硅石矿物包裹体。碳硅石包裹体拉幔光谱峰值稳定,主峰变化于786～789cm^(-1)之间,次峰为966～977cm^(-1),两组弱峰峰值分别为769～781cm^(-1)和915～918cm^(-1)。寄主矿物-石榴石成分的反环带特征及矿物包裹体组合表明CCSD-PP2中退变榴辉岩是在高温高压条件下形成的榴辉岩,经降温降压退变质作用形成的,碳硅石等超高压矿物是在榴辉岩峰期变质作用期间形成的。根据碳硅石形成于极度还原环境、压力大于6.0GPa,温度在1000℃以上的特点并结合前人的矿物温压计算结果分析认为苏鲁超高压变质带中的部分榴辉岩是扬子板块俯冲至200km以下的上地幔下部,经超高压变质作用、形成碳硅石等矿物后快速折返的产物。

The thin-sections for retrogressive eclogite are observed by Microscope and analyzed by Raman spectrum from the Pre-pilot Hole of the Chinese Continental Scientific Drilling Project (CCSD-PP2). A series of the inclusions within garnet are recognized including moissanit, omphacite, apatite, zircon, rutile, coesite, quartz, magnesite, and magnetite, among which the moissanit is firstly found in retrogressive eclogite, and is of important implication to temperature and pressure. The Raman shifts of moissanites are typical and stable, the main shifts, secondary ones, and another two weaker ones are 786 similar to 789 cm(-1), 966 similar to 977cm(-1), and 769 similar to 781 cm(-1) and 915 similar to 918 cm(-1), respectively. The compositional zoning and mineral assemblages in garnets show that the retrogressive eclogite in CCSD-PP2 were from retrogressive metamorphism of rutile eclogite which formed in ultrahigh pressure and temperature, and the moissanite formed at the peak stage metamormism. According to the formed conditions of moissanite, that is extremely low redox state and T > 1000 degrees C, P > 6.0GPa. and the temperature and pressure condition for eclogite calculated by mineralogical thermometer and barometer, speculate that a few eclogite in the Sulu ultrahigh pressure metamorphic belt was the result of subduction of Yangzi plate and rapid exhumation after captured moissanite, and the depth of subduction is deeper than 200 km, which equivalents to the low part of the upper mantle.