Variation of crustal thickness in central west Junggar orogenic belt: insight into its Late Palaeozoic tectonic evolution

The variation of crustal thickness of an orogenic belt through time will provide critical insights into the switch of tectonic setting. The geochemistry of subduction and collision-related magmatism is indicative of the crustal thickness. Late Palaeozoic magmatic products are prevalent in central west Junggar orogenic belt (C-WJOB), and can be employed to determine the temporal changes in crustal thickness as well as the tectonic evolution. Here, we constrain the crustal thickness and discuss the tectonic evolution of C-WJOB through by integrating Sr/Y and La/Yb ratios, zircon U-Pb ages and Hf isotopic compositions of the Late Palaeozoic igneous rocks. Our results show that the crust of C-WJOB was much thinner (20–25 km) than normal continental crust (average at 33 km) in the Early Carboniferous, and began to thicken from 350 Ma to 330 Ma at a slow rate. Crustal thickening was caused by the addition of subduction-related magma sourced from depleted-mantle but with contributions of subducted sediments. The crust started to thicken significantly at ~330 Ma and reached to a maximum thickness of ~40 km between 322 Ma and 310 Ma. The timing of maximum thickness corresponded with the emplacement of calc-alkaline to high-K calc-alkaline I-type dioritic magmas as well as adakitic magma. These magmas were generated by melting of both depleted mantle and juvenile lower crust, which was probably triggered by underplated mantle-derived basaltic magma. Lastly, crustal thinning happened between 310 Ma and 295 Ma due to regional post-collisional extension, although voluminous granitic magmatism occurred. The granitic magmas were formed by remelting of the juvenile lower crust, with a few contribution of mantle material. This study on the variation of crustal thickness in the Late Palaeozoic C-WJOB provides insight into how integrated geochemical and isotopic data can elucidate the complex histories of orogenic belts.

造山带地壳厚度随时间的变化，可为构造背景的转换提供关键启示。与俯冲及碰撞相关的岩浆作用地球化学特征，可指示地壳厚度。晚古生代岩浆产物广泛发育于准噶尔中西部造山带（central west Junggar orogenic belt, C-WJOB），可用于厘定地壳厚度的时间变化与构造演化过程。本次研究通过整合晚古生代火成岩的Sr/Y、La/Yb比值、锆石U-Pb年龄以及Hf同位素组成，限定了准噶尔中西部造山带的地壳厚度，并探讨了其构造演化历程。研究结果显示，早石炭世时期准噶尔中西部造山带的地壳厚度（20~25 km）远低于正常大陆地壳的平均厚度（33 km），并于350 Ma至330 Ma期间以缓慢速率开始增厚。此次地壳增厚源于俯冲相关岩浆的添加作用：该岩浆起源于亏损地幔，但受到俯冲沉积物的物质贡献。地壳在约330 Ma时开始显著增厚，并于322 Ma至310 Ma期间达到约40 km的最大厚度。地壳厚度达到峰值的时间，与钙碱性-高钾钙碱性I型闪长质岩浆以及埃达克质岩浆的侵位时间相吻合。此类岩浆由亏损地幔与新生下地壳的熔融共同形成，其熔融作用可能由底侵的幔源玄武质岩浆所触发。最后，尽管该时期发育大量花岗质岩浆作用，但由于区域碰撞后伸展作用，地壳于310 Ma至295 Ma期间发生减薄。该花岗质岩浆由新生下地壳的重熔形成，仅伴随少量幔源物质的贡献。本次对准噶尔中西部造山带晚古生代地壳厚度变化的研究，阐明了如何通过整合地球化学与同位素数据来解析造山带复杂的演化历史。