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 ε_Nd<i>(t)</i> and zircon ε_Hf<i>(t)</i> 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 ε_Nd<i>(t)</i> values <i>(ca</i>. +5 – +10), and zircon ε_Hf<i>(t)</i> values (ca. +7-+16), which significantly shifted to +2 – +4 and +4 <i>–</i> +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（Altaids）南部的东天山造山带（Eastern Tianshan orogen）中，古亚洲洋（Paleo-Asian Ocean）的演化与闭合时限长期存在争议。东天山的大南湖弧（Dananhu arc）卷入了阿尔泰造山带形成过程中的长期演化与末期构造事件，其岩浆岩是该过程的忠实记录者。为约束古亚洲洋的俯冲历史，本文综述了大南湖弧的构造演化，通过总结花岗岩、火山岩及矿床的地球化学与年代学数据，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启示。横穿大南湖弧中部的花岗岩类岩石锆石U-Pb年龄范围为453 Ma至227 Ma，且在四个主要区域整体呈现向南年龄变新的趋势：卡拉塔格带（Kalatage belt，约453–299 Ma）、克孜尔带（Keizer belt，约401–268 Ma）、土屋带（Tuwu belt，360–302 Ma）以及康古尔俯冲杂岩（Kanguer subduction complex，约288–227 Ma）。此类花岗岩类岩石均为钙碱性弧相关埃达克质花岗岩，富硅、富钾；其Th/Yb比值随时间向南逐渐升高，而Cr与Ce/Pb比值则向南逐渐降低，一致的地球化学变化指示弧地壳的渐进式成熟。全岩ε_Nd(t)与锆石ε_Hf(t)值，结合由花岗岩估算得到的地壳厚度的渐进变化，表明俯冲作用存在多期次的转换。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ε_Nd(t)值（约+5至+10）与锆石ε_Hf(t)值（约+7至+16），而在约243–227 Ma期间，这两个值分别显著偏移至+2至+4与+4至+12。这种变化伴随地壳厚度的突然增厚，表明在大南湖弧、哈尔里克（Harlik）与雅满苏-中央天山大陆弧汇聚过程中，大陆地壳物质参与了此类岩石的成因作用。