Petrogenesis Of Tebai Basic-ultrabasic Pluton In The Beidashan Area In The Southern Margin Of The Alax Block And It's Tectonic Significance

The Alxa block is located between the North China Plate,the Central Asian Orogenic Belt,the Tarim Plate and the Qilian Orogenic Belt.It is an important region for studying the Paleo-Asian Ocean and Central Asian Orogenic Belt.The Tebai basic-ultrabasic pluton is located in the west part of the Beidashan uplift belt on the southern margin of the Alxa block,It is a dike extending nearly east-west,with a length of⁴ km and exposed area less than 0.2km².The pluton is mainly composed of serpentinized peridotite,pyroxene peridotite,metamorphic gabbro and hornblende gabbro.Minerals are mostly oriented and altered because of late extrusion structure and hydrothermal alteration.This study was financially supported by the National Natural Science Foundation of China?41672064?.In this paper,petrology,mineralogy,and geochemistry are used to study the formation age,geochemical characteristics of Tebai pluton,so as to discuss the source area properties and tectonic environment.The main achievements are as follows:?1?Chromite from peridotite with high contents of Cr₂O₃ and FeO^T belongs to high Cr chromite,similar to the chromite in the Luobusa ophiolite in Tibet.However,they have very low TiO₂,even lower than the detection limit.According to the crystallization temperature and pressure of chromite,it is concluded that,worship of the peridotite chromite formed in the temperature is 1297?,pressure is 2.0 GPa,depth of 62.11 km of the environment.Its formation may be related to the subduction zone environment of the glass melt/rock reaction about Ann.?2?Tebai pluton is strongly altered and deformed.Most of olivine grains were altered into serpentine in peridotite,with obvious orientation in gabbros.The rock has a high contents of MgO and TFe₂O₃,and low contents of SiO₂,Al₂O₃,CaO,Na₂O and K₂O.The total concentration of rare earth elements was very low(3.77×10^-6),with an enrichment of LREE and depletion of HREE.The chondrite-normalized rare earth element patterns are U-shaped,the trace elements are relatively enriched with large ion lithophilic elements,such as Rb,Ba,Sr and U,and relative depletion of high field strength elements,such as Nb and Th.Peridotite has typical mantle pieces characteristics,and partial melting formed bojite and gabbro.?3?Zircon LA-ICP-MS U-Pb isotopes dating shows that the age of the bojite is 437.5±1.7Ma,the age of the diorite in peridotite was 447.7±4.1Ma,and the age of the gabbros in peridotite were 446.4±2.4Ma,which are consistent with the ductile thrust deformation structure of the upper silurian in the region.Combined with the regional geological background and the petrogeochemical characteristics of the Tebai pluton,this paper suggests that the age of the bojite may indicate that the Tebai pluton was formed in the late Ordovician.Tebai pluton is derived from the depleted mantle with a high degree of partial melting in the SSZ environment,and its formation is related to the subduction of the Paleo-Asian Ocean in the late Ordovician.