关键词: 铝质A2型花岗岩, 锆石U-Pb年代学, 早二叠世, 后碰撞, 西准噶尔

Abstract: The Late Carboniferous-Late Permian intrusive rocks are widely distributed in the central West Junggar, and the tectonic settings in which these rocks were formed are of great significance to reveal the tectonic evolution of the West Junggar during the Late Paleozoic. In this paper, the granitic porphyry from the Baibuxie-River that intruded into the Upper Carboniferous Molaoba Formation in the northern Barleik Mountains is studied to provide further constrains for the tectonic evolution of central West Junggar. We present the results of petrology, U-Pb dating of zircons, whole-rock major and trace elements and Sr-Nd isotopes of the granitic porphyry to investigate the petrogenesis and tectonic setting. The LA-ICP-MS zircon U-Pb age of the granite porphyry from the Baibuxi 4

e River is 276±5 Ma, indicating that it was formed in the Early Permian. The granitic porphyry is characterized by high SiO₂ (76.72%~79.25%), total alkali (Na2O+K2O=5.53%~7.06%) contents, and low magnesium values (Mg#=8~14). Its A/CNK values range from 1.07 to 1.40,which belong to peraluminous and subalkaline granite series. The chondrite-normalized REE distribution patterns exhibit a right-inclined “seagull” shape ((La/Yb) N =3.22~4.34), with a significant negative Eu anomaly (δEu=0.09~0.10). It is enriched in LILEs such as Rb and K, strongly depleted in Sr, P and Ti, moderately depleted in Ba, Nb and Ta elements, and has high FeOT/MgO (10.65~19.32) and 10000Ga/Al (3.61~4.41) values and Zr+Nb+Ce+Y concentrations (859.7×10-6~1054.8×10-6), typical of aluminum A2 type granite. The granite porphyry exhibits low initial 87Sr/86Sr ratios of 0.70482~0.70535 and high positive εNd(t) values of +6.3~+7.3. Based on the above-mentioned geochemical characteristics, we suggest that it was likely formed by partial melting of juvenile crust, followed by high degrees of fractional crystallization and minor crustal contamination under a post-collisional extension regime. Based on the comprehensive analysis of the regional data of coeval intrusive rocks, volcanic rocks, and sedimentary rocks and the fact that there are no post-Early Carboniferous ophiolitic mélange and subduction-related metamorphic rocks in the whole West Junggar, it can be inferred that there was no subduction of oceanic crust since the Early Carboniferous and the Junggar Ocean might have been closed at the early Late Carboniferous.

Key words: aluminous A2-type granite, zircon U-Pb geochronology, Early Permian, post-collision, West Junggar