新疆西南天山低压高温变质带深熔时代及其地质意义

西南天山木扎尔特地区低压高温变质带主要出露片麻岩类、斜长角闪岩类和麻粒岩类三种岩石类型,由于受到深熔事件的影响在其内部形成了规模不一的长英质脉体,然而对这些深熔长英质脉体的时代还缺乏准确的限定。本文通过详细的岩相学、锆石阴极发光图像研究,采用LA-ICP-MS技术对3件长英质脉体不同锆石微区进行了U-Pb定年,进而探讨西南天山低压高温变质带的形成时间。长英质脉体中的新生锆石具有弱的振荡环带,低的Th/U比值,锆石形态学和内部结构也表明锆石结晶于与深熔作用有关的熔体中,长英质脉体获得的深熔时代为276.5±2.0Ma、272.0±1.7Ma、265.5±1.5Ma,其年龄范围代表了木扎尔特地区低压高温变质带的形成时间可能为早中二叠世,该年龄与区域上西南天山造山带广泛出现的碰撞后岩浆事件的时代相一致,表明早二叠世西南天山造山带已进入造山后伸展减薄的后碰撞造山演化阶段。同时获得一组集中分布的深熔长英质脉体421.8±3.2Ma的继承锆石年龄,该年龄与寄主岩石夕线石榴黑云斜长片麻岩一组主要的年龄峰值一致,表明长英质脉体可能来源于夕线石榴黑云斜长片麻岩的部分熔融作用,对应于南天山志留纪晚期的岩浆事件。

Age and geological significance of anatexis from the low-pressure and high-temperature metamorphic belt, southwestern Tianshan

In the area of Muzhaerte, southwestern Tianshan, three types of rocks are exposed in the low-pressure and high-temperature belt, including gneisses, amphibolites and granulites. Due to the effect of anatexis, various sizes of felsic leucosomes appear in the rocks. However, the age of the anatexis forming the felsic leucosomes has not been well constrained. In this paper, petrography and zircon cathodoluminescence image were studied in detail. Different zircon micro areas in three felsic leucosome samples were selected for LA-ICP-MS zircon U-Pb dating to constrain the time of low-pressure and high-temperature belt in the southwestern Tianshan. New growth zircons in the felsic leucosome show planar or weakly oscillatory zonings and low Th/U ratios. The morphological and internal structural features of zircons suggest that their growths were associated with partial melting. The age of anatexis from the felsic leucosome is 276.5±2.0Ma, 272.0±1.7Ma and 265.5±1.5Ma, representing the formation age of low-pressure and high-temperature belt. The Early to Middle Permian ages correspond to the widespread post-collisional magmatism events in the southwestern Tianshan orogen. Our studies suggest that the southwestern Tianshan orogen have got into post-collision orogenic evolution stage of lithospheric thinning and crustal extension in the Early Permian. On the other hand, we also got a group of inheritable zircons ages of 421.8±3.2Ma in the anatectic felsic leucosome. This age consists with one of the major peak ages in the host rock sillimanite-garnet-biotite-plagioclase gneiss, and it shows that felsic leucosome could come from partial melting of the sillimanite-garnet-biotite-plagioclase gneiss, corresponding to the magma events of the Late Silurian in South Tianshan.