Petrogenesis and Tectonic Implications of A-Type Granitoids in the Xingxingxia-Hongliujing Area,Eastern Tianshan
Time of terminal accretion of the Central Asian Orogenic Belt (CAOB) is still controversial,and A-type granite in the southern margin of the CAOB can provide critical constraints on the termination of collision. However,the A-type granite in the southern margin of eastern Tianshan remains insufficiently studied. Three kinds of newly identified A-type granitoids,monzogranite,quartz diorite and syenogranite,were reported in this paper and zircon LA-ICP-MS U-Pb dating,trace elements as well as petrogeochemical analysis were conducted. All these granitoids are characterized by high SiO2 (71.32％-76.35％),alkali (Na2O+K2O=5.67％-9.18％),being metaluminous (molar A/CNK=0.85-0.98) and ferroan (FeOT/MgO=5.52-21.28). The granitoids are enriched in Pb,Hf,Zr,Sm,but depleted in P,Ti,Nb,with high Ga value (18.2×10-6-31.1×10-6) and Ga/Al>2.6. In addition,these granitoids have ΣREE of 77.67×10-6-271.82×10-6,LREE/HREE of 2.82-16.26,(La/Yb)N of 1.80-21.89. Eu anomalies are obviously positive (δEu=0.16-0.84) for both Monzogranite and syenogranite but weakly negative or positive (δEu=0.85-1.20) for the quartz diorite. Monzogranite and syenogranite rather than quartz diorite experienced obvious crystalline fractionation of plagioclase. Monzogranite associated with syenogranite and quartz diorite was also subjected to fractional crystalization of zircon,while the independent one experienced separation of sphene and apatite. Zircon U-Pb dating of monzogranite yields crystallization ages of (273.3±1.9) Ma and (273.4±3.4) Ma. These geochemical features indicate that the primary magma of these A-type granitoids most likely originated from the Xingxingxia complex with minor mantle component,and underwent a fractional crystallization. There exist two distinguished ranges of 307-284 Ma and 273 Ma for A-type granitoids in the Tianshan Orogen,and the elder formed in partial extension environment during the compression period while the younger were emplaced in post-collision background. The eastern section of the southern CAOB had been in post-collision environment in～273 Ma,and final collision between the eastern section of CAOB and the Tarim Plate should be earlier than 273 Ma.
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