Genesis and Tectonic Significance of the Chahawusu Monzonitic Granite in the Western Section of the Alataw Mountain, Xinjiang

The Chahawusu pluton, located in the Alataw mountains of west Tianshan, belongs to the Devonian Carboniferous back-arc basin tectonically.The pluton is mainly composed of gray white medium-grained monzonitic granite and gray white medium-fine grained two-mica monzonitic granite. The geochemical data shows that the rock has a high content of SiO₂, Al₂O₃, and alkali, A/CNK=1.08~1.14, belonging to the peraluminum high potassium calcium-alkaline granite series; the light rare earth elements are relatively enriched and the heavy rare earth elements are relatively depleted.The rare earth distribution mode is a right-leaning type enriched with light rare earth elements, with an obvious negative Eu anomalies;large ion stonephilic elements (LILE) Rb, Th, U, K, Hf are strongly enriched and high field strength elements (HFSE) Ba, Nb, Ta, Sr, P, Ti are depleted. The low I_Sr value (0.701 5~0.702 6) shows that the Chahawusu monzonitic granite is a low-strontium high-differentiation S-type granite. It has the characteristics of the A-type granite, and it is the product of feldspar separation and crystallization. The high positive ε_Nd(t) value (+0.37~+1.18), high content of Th and Ta, and Th/Ta ratio (6.12~8.54) in Monzonitic granite indicate that the magma source area is a long-term depleted mantle, affected by the assimilation and contamination of ancient crustal materials.All samples of the structural discriminant diagrams are in the junction of the volcanic arc granite or the syn-collision granite, even more closer to the syn-collision granite area. Chahawusu intrusion is in 313 Ma. Combined with the regional geological background, the authors believe that the Yili microplate has already met with the Junggar microplate before 310 Ma, and the closure of North Tianshan Ocean Basin was earlier than the end of the Late Carboniferous. After 310 Ma, the Alataw mountains area gradually entered the post-collision extension stage.Therefore,the Chahawusu monzonitic formed in a transitional environment from co-collision compression to post-collision extensional, being the post-collision granite. It is the result of mantle-derived basaltic magma underplating and the ACF process, the tectonic background being the active continental margin.