阿尔泰基性麻粒岩的发现:来自矿物学的证据

Discovery of mafic granulites from the AItay orogenic belt:evidence from mineralogical study.

新疆北部阿尔泰造山带富蕴乌恰沟一带首次发现代表中、下地壳深变质作用的基性麻粒岩详细的野外和室内研究表明阿尔泰基性麻粒岩主要由粗粒和细粒的麻粒岩组成(SiO_2含量为47.83～53.57％)。两者的矿物组合相近,为斜方辉石+单斜辉石+斜长石+角闪石+黑云母角闪石和黑云母成分富镁,可能与岩石的成分富镁有关。基性麻粒岩的岩石地球化学成分显示其富Mg/(Mg+Fe^(2+))和亏损U,Th,K及Rh,而且其原岩为钙碱性玄武岩,是岛弧构造环境下形成的产物,矿物化学特征表明斜方辉石中Mg/(Mg+Fe^(2+))介于0.65～0.71之间,为紫苏辉石,具高MgO+FeO,低Al_2O_3和Ca/(Ca+Mg+Fe),而单斜辉石具低TiO_2和Al_2O_3的特点,说明它们皆为变质成因,部分角闪石和褐色黑云母富钛,指示它们为高温下形成的产物。这些特征证实了上述岩石为基性麻粒岩。角闪石和黑云母氟和氯含量较低以及水逸度较高,暗示其流体成分氟和氯在变质作用过程中不起主导作用。根据麻粒岩中矿物共生组合、期次和矿物温压计估算,麻粒岩形成的峰期和退变质温压条件分别为750～780℃,0.6～0.7GPa和590～620℃,0.23～0.37GPa,前者属高温中压条件。阿尔泰地区基性麻粒岩的发现为进一步研究阿尔泰地区深部下地壳的组成和结构以及深化阿尔泰地区。

The Altay mafic granulite (AMG), representing metamorphic product of granulite-facies in the lower crust, was first found in the Fuyun Wuqiagou, Altay orogenic belt through detailed field survey and mineral-chemical study. The AMG consists of coarse-grained and fine-grained granulites with SiO2 contents of 47.83% similar to 53.57%, both of which have similar mineral assemblage of orthopyroxene, clinopyroxene, plagioclase, amphibole and brown biotite. Its mineral compositions of amphiboles and biotites are rich in Mg/(Mg + Fe2+), probably having relationship with whole-rock chemistry of the rocks. Mineral chemistry addresses that orthopyroxenes have higher Mg/(Mg + Fe2+) of 0.65 similar to 0.71, belonging to hypersthene, and having higher MgO + FeO contents, and lower Al2O3 contents and Ca/(Ca+Fe2+ + Mg) ratios. Clinopyroxenes have lower TiO2 and Al2O3. These chemical features indicate that orthopyroxenes and clinopyroxenes in the rocks have metamorphic genetics. Biotites and some amphiboles are Ti-enriched, replying products formed under high-T metamorphic conditions, and even granulite-facies conditions. Based on mineral paragenesis, mineral generation and mineral geothermobaromemter, the P-T conditions of peak and retrograde metamorphism of the AMG are 750 similar to 780degreesC, 0.6 similar to 0.7 GPa and 590 similar to 620degreesC, 0.23 similar to 0.37 GPa, respectively, and the former one implies the crustal depth of ca. 20 similar to 25 km. The discovery of the AMG has an important significance on further study of formation and structure of deeper crust in the Altay area and continental geodynamic mechanism in North Xinjiang, China and even Central Asia.