Petrology,Geochemistry And Geological Significance Of The Indosinian Tianhu Rock Mass In The East Tianshan Mountains

The Tianhu rock mass occurs near Tianhu Station of Lan Xin Railway station,Xinjiang,about 140 km away from Hami City.The rock mass is situated in the Mid-Tianshan uplift zone in the eastern part of the Tianshan,who belongs to Central Asian Orogenic Belt,and tectonically between the Xingxing Xia-Kuruktag fault and the Azik Kuduk-Weiya fault in the southern margin of the Paleozoic tectonic belt in the East Tianshan Mountains.The rock mass is near-equiaxed shape,about 14 km long and 12 km wide.The exposed area is about 130 km2.Its near-equiaxed shape and complex lithology of multiple intrusions are of great significance to the study of tectono-magmatic activities in this area and the tectonic evolution in the southern part of the Central Asian orogenic belt.The age of rock body formation,source area of parent magma,petrogenesis and tectonic environment are not clear yet.In this paper,the petrographic,geochemical and chronological characteristics of the Tianhu rock mass are studied in depth/intensively in order to solve the problems of the origin,formation age and diagenetic tectonic environment of the rock mass,at the same time the source area and formation mechanism of the parent magma are also discussed.Tianhu pluton is a typical circular plutonic rock mass,which intrudes into the schist and gneiss of the Mesoproterozoic Tianhu Group on both sides of the East and west,it also intrudes into the Hongliuhe Group of the Lower Permian on the South side.Its contact surface is mostly outward-dipping and the occurrence of cutting gneiss is obvious.This is a a typical variable intrusion contact relationship,and there is no developed foliation in the rock mass.The rock mass is composed of six intrusive bodies,which are divided into three lithofacies units according to the principle of petrological lineage unit division of granite.From its outside to inside,the rock mass is divided into three lithofacies units:medium-grained biotite monzonitic granite unit P2T~1,medium-fine-grained granodiorite unit P2T~2and porphyry diorite unit P2T~3.These three units are closely related in time and distributed in space as concentric zones nested with each other.The Zircon U-Pb age of monzonitic granite and porphyritic diorite in Tianhu intrusion are 245~258Ma and 230~250Ma respectively,which indicates that they are products of the same period and emplaced in the Middle Triassic.Based on the previous research results,it is considered that the Tianhu intrusion was formed in the early Indosinian period,during which the proliferative orogeny of the Central Asian orogenic belt has ended,and this period of magmatic activity represents the magmatic event developed in the intraplate tectonic evolution stage after the end of the orogeny.Its magmatic activity is closely related to the subduction,compression and subsequent collision of the Paleo-Tethyan Ocean.It is the product of the transition from the Central Asian tectonic system to the Tethyan system,which is an important symbol of Indosinian granites in the East Tianshan Mountains.