新疆西准噶尔白布谢河花岗斑岩锆石U-Pb年龄、地球化学特征及其构造意义

Zircon U-Pb Age and Geochemistry of the Granitic Porphyry from the Baibuxie River of the West Junggar,Xinjiang,and Its Tectonic Significance

西准噶尔中部广泛发育晚石炭世—晚二叠世侵入岩,其产出构造背景对于揭示该区晚古生代构造演化具有重要意义。文章以巴尔鲁克山北侧侵入于上石炭统莫老坝组中的白布谢河花岗斑岩为研究对象,通过岩相学、锆石U-Pb年代学、全岩主—微量元素和Sr-Nd同位素分析,据此探讨其岩石成因和构造背景,以期为进一步深入研究西准噶尔中部构造演化提供可靠的地质资料。白布谢河花岗斑岩的LA-ICP-MS锆石U-Pb年龄为276±5 Ma,表明其形成时代为早二叠世。该花岗斑岩具有高SiO2(76.72%~79.25%),全碱(Na_(2)O+K_(2)O)为5.53%~7.06%,低镁值(Mg^(#)=8~14)的特征,A/CNK值为1.07~1.40,为准铝质—过铝质的亚碱性系列花岗岩;稀土元素配分曲线表现为右倾“海鸥型”((La/Yb)N=3.22~4.34),具有明显的Eu的负异常(δEu=0.09~0.10),富集大离子亲石元素Rb和K,强烈亏损Sr、P及Ti,中等亏损Ba、Nb及Ta元素;其FeO^(T)/MgO(10.65~19.32),10000Ga/Al(3.61~4.41)值和高场强元素组合(Zr+Nb+Ce+Y)(859.7×10^(-6)~1054.8×10^(-6))含量高,属于典型的铝质A_(2)型花岗岩。该花岗斑岩具有低的初始^(87)Sr/^(86)Sr比值(0.70482~0.70535),高的正ε_(Nd)(t)值(+6.3~+7.3)。综合上述元素和同位素地球化学特征,推测该岩体极可能是后碰撞伸展环境下,由新生地壳部分熔融后再经高度分异演化并伴有少量壳源物质混染的产物。结合区域已发表的近同期侵入岩、火山岩和沉积岩资料,以及区内缺乏早石炭世之后的蛇绿混杂岩和俯冲相关变质岩的事实,推测西准噶尔中部可能不存在晚于早石炭世的洋壳俯冲消减作用,暗示在晚石炭世早期准噶尔洋已基本闭合。

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(Na_(2)O+K_(2)O=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 FeO^(T)/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 A_(2) type granite.The granite porphyry exhibits low initial ^(87)Sr/^(86)Sr ratios of 0.70482~0.70535 and high positive ε_(Nd)(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.