北天山西段晚石炭世—早二叠世花岗岩成因及其构造意义

Petrogenesis and tectonic implication of Late Carboniferous to Early Permian granites from the western Chinese North Tianshan belt

北天山增生杂岩带是北天山洋最终闭合的产物,对其中的地质体进行详细研究是揭示北天山洋演化过程的关键。北天山增生杂岩带内存在一些花岗质侵入体,然而目前为止,对这些岩体的形成时代及成因研究十分有限。本文对北天山西段的萨尔萨拉岩体和艾鲁逊岩体进行了锆石U-Pb年代学、地球化学和锆石Lu-Hf同位素分析,并探讨其成因及大地构造意义。萨尔萨拉岩体和艾鲁逊岩体主要岩石类型均为二长花岗岩,锆石U-Pb年龄分别为303 Ma和298 Ma,为晚石炭世—早二叠世侵入体。两个岩体的锆石均具有非常高的ε;(t)值,分别为9.7~12.1(平均为11.2)和12.1~15.0(平均为13.2),相应的两阶段模式年龄为696~547 Ma和541~357 Ma,表明其源区为新生的地壳物质。两个岩体的样品均具有高硅、富碱、富钾特征,为弱过铝质高钾钙碱性花岗岩,其较高的CaO/Na_(2)O比值(>0.3)和Al_(2)O_(3)/(MgO+FeO_(2));值暗示其可能源于变质杂砂岩的部分熔融,北天山增生楔中的复理石砂岩为可能的源区岩石。结合区域资料,本文认为萨尔萨拉岩体和艾鲁逊岩体形成于同碰撞向后碰撞转换阶段,限定了北天山洋在晚石炭世末闭合。

The North Tianshan belt was formed during closure of the North Tianshan Ocean. Detailed study on the geological unit within the North Tianshan belt is crucial to reveal tectonic process of the North Tianshan Ocean. There are several granitic plutons intruding into the North Tianshan belt, but detailed studies including geochemistry and geochronology are limited. In this paper, we conducted zircon U-Pb dating, geochemistry and zircon Lu-Hf isotope analyses on the Saersala and the Ailuxun plutons in the western part of the North Tianshan belt. These data were used to discuss the petrogenesis and geological setting of the plutons. The Saersala and the Ailuxun plutons are both monzogranites. Their zircon U-Pb ages are 303 Ma and 298 Ma. Zircons from the two plutons all display high positive ε;(t) values, which are 9.7 to 12.1(average 11.2) for the Saersala pluton and 12.1 to 15.0(average 13.2) for the Ailuxun pluton. Their two stage model ages are 696~547 Ma and 541~357 Ma, respectively, reflecting juvenile magma sources for the studied plutons. Samples from the two plutons all have high SiO_(2)and total alkaline contents, and belong to weakly peraluminous high-K calc-alkaline granites. They have high CaO/Na;O values and Al_(2)O_(3)/(MgO+FeO_(2));values, indicating magma sources mainly of meta-greywackes. The flysch sandstones in the North Tianshan belt are potential source rocks for the studied monzogranites. Combine with other geological evidences, we suggest that the Saersala and the Ailuxun plutons were formed during the transition stage from collision to post-collision. The North Tianshan Ocean closed during the end of the Late Carboniferous.