胶东莱西地区高压基性麻粒岩的变质作用

Metamorphism of the High Pressure Basic Granulites in Laixi, Eastern Shandong, China

胶东地区莱西－莱阳－栖霞一带晚太古代花岗片麻岩中，出露了相当规模的高压基性麻粒岩和超镁铁质岩组合，曾被认为它们是与苏鲁－大别超高压碰撞造山作用有关的一套岩石组合。高压基性麻粒岩中可识别出三期变质矿物组合：早期为Ｇｒｔ（核部）＋Ｃｐｘ＋Ｐｌ；中期为Ｇｒｔ（边部）＋Ｃｐｘ＋Ｏｐｘ＋Ｐｌ＋Ａｍｐ＋Ｑ＋Ｍｔ；晚期为Ｃｐｘ＋Ｐｌ＋Ａｍｐ＋Ｑ＋Ｍｔ。早期的石榴石含钙铝榴石和镁铝榴石组分较高，单斜辉石含Ａｌ高。应用矿物温压计和ＴＷＱ矿物反应热力学程序进行计算得到：高压麻粒岩早期矿物组合的变质作用温度约在８４０℃～８６０℃，变质压力大于１．４ＧＰａ；中期的变质作用条件为７２０℃～７８０℃、０．８ＧＰａ～１．２ＧＰａ；晚期的变质作用条件为６００℃～８００℃、０．５ＧＰａ～０．７ＧＰａ。高压基性麻粒岩具有降压退变质结构，高压麻粒岩相变质之后经历了一个近等温减压的过程，后来又经历了减温减压过程的变质作用演化，显示出顺时针的ＰＴ轨迹。矿物－全岩Ｓｍ－Ｎｄ等时线年龄为１７５２Ｍａ，指示出受吕梁运动的强烈影响，与华北克拉通大部分地区热历史吻合。可以认为这一高压基性麻粒岩和超镁铁质岩组合属于华北克拉通早前寒武纪基底的又一下地壳的岩石组合?

A high pressure basic granulite ultramafics rock association occurs in late Archaean gneisses in Laixi Laiyang Qixia area, eastern Shandong. The rock belt had once been considered another high pressure belt in Shandong peninsula except for Sulu UHP belt. In the high P granulite, three episodes of metamorphic mineral assemblages have been identified. The mineral assemblage of early metamorphic episode is Grt(core)+Cpx+Pl;the second is Grt(rim)+Cpx+Opx+Pl+Amp+Q+Mt; and the third is Cpx+Pl+Amp+Q+Mt. In the first assemblage, garnet porphyroblast is rich in grossular and pyrope, and clinopyroxene is rich. Using mineral themobarometers and TWQ program of Berman (1991), the P T condition of the three episodes of metamorphism are evaluated: for the first episode, T =840℃￣860℃, P >1 4GPa;for the second: T =720℃￣780℃, P =0 8GPa ￣1 2GPa; and for the third: T =600℃￣800℃, P =0 5GPa￣0 7GPa. The high pressure basic granulites from the eastern Shandong have undergone three episodes of metamorphism. From the reaction texture, mineral chemistry and PT estimation, we conclude the clockwise metamorphic PT path, that is the isothermal decompression at first and subsequent pressure temperature decreasing. The mineral whole rock Sm Nd isochron age of the granulites is 1752Ma,indicating the late thermal event which the high pressure granulite subjected to. It is concluded that the high pressure granulite and ultramafic rock association represents the lower continental crust of the North China craton during early Precambrian.