The Petrological Study Of The Precambrian Metamorphic Basement In The Central Tianshan, Xinjiang Province

The Tianshan orogenic belt is located at the central Xinjiang Province, and tectonically among the Kazakhstan Plate, Tarim Plate and Siberian Plates. Because of the unique evolution process and the rich mineral resources, the Tianshan orogen has become an important area of the geologic study in the past hundred years. Petrological studies of the Precambrian metamorphic basement of the Tianshan orogen was made in this paper; the obtained results can provide some constrains on the compositions, characteristics and the formation and evolution history of the early continental crust.This work and available studies show that the Precambrian metamorphic basement of the Tianshan orogen can be divided into the northern Proterozoic (Baluntai) and southern Archean (Tiemenguan and Bositeng lake) basements. Studies of this paper indicate that the northern Proterozoic metamorphic basement consists of the interlayered schist, gneiss and amphibolite, and with minor marble, and has been subjected to the early stage of amphibolite-facies metamorphism and the late-stage of greenschist-facies metamorphism. The southern Archean metamorphic basement mainly compose by the gneiss, schist, amphibolite, with minor calcic-silicate, marble and granulite, and experienced three stages of metamorphism, including the peak granulite-facies, and the retrograde amphibolite- and greenschist-facies metamorphism. The garnet-orthopyroxene granulite reported firstly in the southern-central Tianshan orogen is a key of revealing composition and evolution of the Precambrian metamorphic basement. In addition, the Archean basement has been reworked by the extensively multi-stages of migmatization and deformation.Detailed mineral chemistry studies show that the amphiboles from various rocks are calcic-amphibole, including magnesio-hornblende, Tschermakite, Tschermakitic -hornblende and so on. Garnets from the various rocks show distinctly compositional changes. FeO-rich garnets occur in the mica schist, and CaO-rich garnets in amphibolite, and MnO-poor garnets in the granulite. In general, plagioclases in the amphibolite have relatively low contents of CaO, and belong to oligoclase and andesine. However, plagioclases in the granulite have higher contents of CaO, and belong to labradorite and bytownite. This research also indicates that the mineral chemical compositions are not only controlled by the bulk-rock chemical compositions and the metamorphic temperature and pressure conditions, but also by the local inequilibrium metamorphic reactions and the replaced primary minerals.