Ordovician to Triassic episodic growth of the Dananhu arc, Eastern Tianshan (NW China)

The evolution and closure timing of the Paleo-Asian Ocean are controversial in the Eastern Tianshan orogen, southern Altaids. The Dananhu arc of the Eastern Tianshan was involved in the long-lived evolution and terminal tectonic events during the formation of the Altaids, and its magmatic rocks are the faithful recorder. To constrain the subduction history of the Paleo-Asian Ocean, we review the tectonic of the Dananhu arc, by summarizing the geochemical and geochronological data of granites and volcanic rocks and mineral deposits, which will shed light on the accretionary history, closure of the Paleo-Asian Ocean, and the continental growth in Altaids. Zircon U-Pb ages of the granitic rocks across the middle of the Dannanhu arc range from 453 Ma to 227 Ma and demonstrate overall southward younging through four main areas: the Kalatage belt (ca. 453–299 Ma), the Keizer belt (ca. 401–268 Ma), the Tuwu belt (360–302 Ma), and the Kanguer subduction complex (ca. 288–227 Ma). These granitic rocks are calc-alkaline arc-related and adakitic granites that are enriched in Si and K. They increase in Th/Yb and decrease in Cr and Ce/Pb southwards over time. The consistent geochemical variations indicate progressive maturity of the arc crust. Their whole-rock εNd(t) and zircon εHf(t) values coordinated with the progressive change in the crustal thickness estimated by the granites indicate a multi-phase switching of subduction. The Ordovician to Permian granitic rocks in the arc have stable high whole-rock εNd(t) values (ca. +5 – +10), and zircon εHf(t) values (ca. +7-+16), which significantly shifted to +2 – +4 and +4 – +12 during ca. 243–227 Ma. These changing relations accompanied a sudden crustal thickening, suggesting that continental crustal material was involved in the petrogenesis of these rocks during the convergence among the Dananhu arc, the Harlik and the Yamansu-central Tianshan continental arc.

阿尔泰造山带南部的东天山造山带（Eastern Tianshan orogen）中，古亚洲洋（Paleo-Asian Ocean）的演化与闭合时限迄今尚存争议。东天山的大南湖弧（Dananhu arc）卷入了阿尔泰造山带（Altaids）形成过程中的长期演化与末期构造事件，其岩浆岩便是该过程的忠实记录载体。为约束古亚洲洋的俯冲演化历史，本文通过系统总结大南湖弧花岗岩、火山岩及矿床的地球化学与年代学数据，对其构造特征开展综述，以期为阿尔泰造山带的增生演化、古亚洲洋闭合以及大陆生长提供科学启示。
横穿大南湖弧中部的花岗岩类岩石的锆石U-Pb年龄（zircon U-Pb ages）介于453 Ma至227 Ma之间，并呈现出整体向南变新的时空分布规律，可划分为四个主要构造单元：卡拉塔格带（约453–299 Ma）、克孜尔带（约401–268 Ma）、土屋带（360–302 Ma）以及康古尔俯冲杂岩（约288–227 Ma）。
该类花岗岩类岩石属于钙碱性弧相关埃达克质花岗岩（calc-alkaline arc-related and adakitic granites），富硅富钾。随着向南的空间序列与演化时间推移，其Th/Yb比值逐渐升高，而Cr与Ce/Pb比值则逐步降低。这种一致的地球化学变化指示弧地壳逐渐趋于成熟。
全岩εNd(t)与锆石εHf(t)值，结合通过花岗岩估算得到的地壳厚度的渐进变化特征，表明俯冲作用经历了多期次转换过程。
该弧内奥陶纪至二叠纪的花岗岩类岩石具有稳定的高全岩εNd(t)值（约+5至+10）与锆石εHf(t)值（约+7至+16），但在约243–227 Ma期间，这两个同位素比值分别显著偏移至+2至+4与+4至+12。
这种同位素变化关系伴随地壳的突然增厚，指示在大南湖弧、哈尔里克（Harlik）与雅满苏-中天山大陆弧（Yamansu-central Tianshan continental arc）发生汇聚的过程中，陆壳物质参与了该阶段岩浆岩石的成因作用。