苏鲁三清阁榴辉岩中柯石英的发现及其地质意义探讨

Discovery of Coesite in Sanqinge Eclogite, Sulu UHP Terrane, E China, and Its Geological Significance

在苏鲁三清阁多硅白云母榴辉岩中发现了柯石英包体,该榴辉岩与超高压含菱镁矿大理岩共生。石榴石中存在的柯石英残晶和多晶石英假相包体表明榴辉岩的变质压力超过2.8GPa。运用石榴石–单斜辉石Fe2+–Mg温度计和多硅白云母压力计估算结果表明,矿物核部的平衡压力为3.1～3.5GPa(T=650～689℃),边部的平衡压力为2.9～3.3GPa(T=652～671℃)。与石榴石伴生的绿辉石普遍发育石英出溶结构。在磷灰石中观察到金属硫化物出溶体。磷灰石富含OH,F,Cl和S等挥发份,这些挥发性组分的深循环对地壳和地幔之间的相互作用研究具有重要意义。通过磷灰石,苏鲁—大别俯冲带能够将大量(～80000Mt)OH,F和Cl携带到地幔深处。

Here we report the discovery of coesite in phengite eclogite at Sanqinge, Ganyu county, Sulu UHP terrane, east China. These eclogites coexist with the UHP magnesite-bearing marble. The coesite relics and polycrystalline quartz after coesite are observed within garnet grains with radial fractures. This finding indicates that the Sanqinge phengite eclogites underwent metamorphism at pressure of 〉2.8 GPa. Metamorphic pressure for the mineral assemblages of cores correspond to 3.1-3.5 GPa （ T= 650-689℃ ） and their rim assemblages correspond to 2.9-3.3 GPa （ T = 652-671℃ ） by using garnet-omphacite Fe^2＋-Mg geothermometer and phengite baromemter. Quartz exsolution and sulfide exsolution textures occur in omphacites and apatites, respectively. Apatite contains abundant volatiles such as OH, F, Cl, and S, and its subduction to deep mantle is of great significance for the cycle of these volatiles. Based on our approximate estimation, the mass of OH, F and Cl subducted into deep mantle via Sulu- Dabie subduction zone was in the order of 80 000 Mt.