内蒙古大青山造山带含假蓝宝石尖晶石片麻岩的成因网格及动力学

Dynamics and genetic grids of sapphirine-bearing spinel gneiss in Daqing Mountain orogen zone, Inner Mongolia

内蒙古大青山造山带中存在着一类特殊的岩石类型——极度富铝贫硅的含假蓝宝石尖晶石片麻岩。含假蓝宝石尖晶石片麻岩的岩石学和矿物学的研究表明 ,变晶序列为 Sap+Sil+Gt+Pl+Bi(早期 )→ Sp+Gt+Sil+Pl+Perth+Bi(峰期 )→ Cord+Coru+Mt+Pl+Mic+Bi(峰后 )。利用 Gibbs相律和 Schreinemakers分析方法对该岩系的矿物演化历史进行了理论推导 ,结果表明其与实际矿物的演变历史完全吻合 ,均显示为顺时针的 PT演化路径 ,矿物的演化历史表明 ,该岩系经历了早期的迅速增压 ,峰期的近等压升温和峰期后的降温降压三个演化阶段 ,这样的 PT演化史显示了大青山造山带的碰撞造山历史。该类片麻岩的典型矿物组合 Sap+Gt+Sp+Bi+Sil+Pl表明 ,其曾经历了温度下限为 85 0℃、压力大约为 0 .9GPa的高温高压动力学环境 。

A special kind of rock which is sapphirine-bearing spinel gneiss from Daqing Mountain orogen zone, Inner Mongolia has been reported. This kind of rock is characterised by abundant Al-rich minerals and its chemical composition is alumina-rich and silica-undersatuated. The study of metamorphic minerals and crystalloblastic sequences indicates that evolution of crystalloblastic sequences in typical mineral assemblages of sapphirine-bearing spinel gneiss is Sap+Sil+Gt+Pl+Bi→Sp+Gt+Sil+Pl+Perth+Bi→Cord+Coru+Mt+Pl+Mic+Bi. By means of genetic grids,the crystalloblastic sequence of the main minerals assemblages in rock is inferred theoretically. The result is well consistent to the actual assemblages. According to crystalloblastic sequence and characteristics of assemblages formation of different stages of mineral development in rock, a clockwise P-T-t path is set up by studying genetic grids. The model of metamorphism is interpreted as follows: in the early stage of orogenics, the crust was thickened rapidly leading to pressure increase to peak and then crust was heated near isodar and temperature reached to peak condition, at last. Crust was lifted up and P-T fell down. In the course of oregenics sapphirine-bearing spinel gneiss wes formed in orogen root zone. The dynamic conditions are P>0.85GPa, T>850℃.