中国东部大别山超高压变质杂岩中的石英硬玉岩带

Quartz Jadeitite in Ultrahigh Pressure Metamorphic Belt in the Dabie Mountains,Eastern China

大别山的石英硬玉岩是大别山超高压变质杂岩中的重要成员，与大理岩和榴辉岩紧密共生，呈大小不等的构造透镜体产出在云母斜长片麻岩和含硬玉片麻岩中，分布在长约４０ｋｍ，宽约１ｋｍ的带内。透镜体中心常为花岗变晶结构，边部有不同程度退变并面理化，向外围逐渐变为含硬玉片麻岩。岩石的主要矿物组成为硬玉、石英、石榴石、金红石。退变的石英硬玉岩中还有钠长石、霓石、霓辉石、榍石等。硬玉和石榴石中都有柯石英包体。硬玉的Ｊｄ端元组分为８１．２５％～９０．２７％。恢复的石英硬玉岩的原岩为硬砂岩，与大理岩伴生的榴辉岩的原岩为泥灰岩。因此，石英硬玉岩与共生的大理岩和榴辉岩都属于榴辉岩相变质的表壳岩系，它的成带分布、其中有柯石英的产出，进一步证明大陆地壳能够俯冲到１００ｋｍ左右深度并迅速折返地壳后使其中的高压标志保存完好。

The quartz jadeitite is the important member of the ultrahigh pressure metamorphic complex in the Dabie mountains and closely associated with marble and eclogite. Quartz jadeitite occurs as rock blocks or lenses with various dimension and associated with marble and eclogite in the host micaplagioclase gneiss and some jadeite bearing gneiss, ranging in a belt trending E W with length of 40km and width of 1 km . The quartz jadeitite blocks or lense show granoblastic texture in the centre and foliation at the margin. It is always keeping in fresh in the centre and gradually retrograded approaching to the margin of a block and finally becomes jadeite bearing gneiss. The major mineral component of quartz jadeitite are: jadeite, quartz, garnet and rutile, with secondary minerals such as albite, aegirine, acmite augite and titanite in the retrograded samples. Coesite inclusions occur in both garnet and jadeite in fresh quartz jadeitite. The end member of jadeite is calculated to be 81 25～90 27%. Geochemical features indicate that the protolith of these quartz jadeitite to be greywacke, and the protolith of the associated eclogite in marble is reported to be marl. Therefore, the association of quartz jadeitite, marble and eclogite should be supercrust rocks sequence. The zonal distribution and the occurrence of coesite in jadeite reconfirmed that the continental crust can be subducted to great depth (around 100km)and then exhumed rapidly with its ultra,high pressure signature fairly preserved.