Early Paleozoic tectono–magmatic evolution in the South Altun orogenic belt, northwest China: Insights from zircon U–Pb geochronology, Hf isotope and geochemistry of the granitoids

Precise timing of granitoids and constraints of their magma sources are critical to understanding the subduction–collision tectonic evolution of the Altun orogenic belt (AOB), a vital part of the Qinghai-Tibet plateau in northwestern China. We provide in-situ zircon U–Pb age, Hf isotopic composition and whole-rock geochemistry of the Yusupualeke granitic pluton in the South Altun orogenic belt (SAOB), an integral unit of the AOB, to determine the tectono-magmatic evolution of the SAOB at early Palaeozoic. The Yusupualeke granitic pluton comprises the medium-coarse grained porphyritic monzogranite and medium-fine grained granodiorite. The monzogranite sample yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 476.8 ± 3.6 Ma (MSWD = 0.59), while the granodiorite yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 453.2 ± 4.7 Ma (MSWD = 0.013). Zircon U–Pb ages suggest that the subduction–collision process of the SAOB remained from the middle stage of the early Palaeozoic to the end. Both of the studied granitoids belong to metaluminous to weakly peraluminous series and show typical I-type granite characteristics with depletions in Ba, Nb, Sr, P, and Ti, and enrichments in light rare earth elements (LREEs), Rb, Th, K, and slightly negative Eu anomalies. Based on the geochronological data and regional geological background, we believe that the porphyritic monzogranites were formed during the northward subduction process of the South Altun ocean (SAO), while the granodiorites were formed during the tectonic regime transition stage. Besides, combing the previous research achievements with our newly obtained data, we put forward a new division and geodynamic model of the early Palaeozoic tectono-magmatic evolution for the South Altun orogenic belt.

精准限定花岗岩类的形成时代及其岩浆源区属性，对于理解中国西北青藏高原重要组成部分——阿尔金造山带（Altun orogenic belt, AOB）的俯冲-碰撞构造演化具有关键意义。本次研究针对作为阿尔金造山带完整组成单元的南阿尔金造山带（South Altun orogenic belt, SAOB）内的玉苏普阿勒克花岗岩体，开展了原位锆石U-Pb年龄、Hf同位素组成以及全岩地球化学测试分析，以期阐明南阿尔金造山带早古生代的构造-岩浆演化过程。该玉苏普阿勒克花岗岩体由中粗粒斑状二长花岗岩与中细粒花岗闪长岩两个岩相单元构成。其中，二长花岗岩样品的加权平均²⁰⁶Pb/²³⁸U年龄为476.8±3.6 Ma（平均标准权重偏差mean square of weighted deviates, MSWD=0.59），花岗闪长岩样品的加权平均²⁰⁶Pb/²³⁸U年龄为453.2±4.7 Ma（MSWD=0.013）。锆石U-Pb定年结果显示，南阿尔金造山带的俯冲-碰撞作用贯穿早古生代中期至末期。本次研究的两类花岗岩均为准铝至弱过铝系列，具备典型的I型花岗岩地球化学特征：亏损Ba、Nb、Sr、P及Ti，富集轻稀土元素（light rare earth elements, LREEs）、Rb、Th、K，且呈现微弱的负Eu异常。结合年代学数据与区域地质背景，研究认为斑状二长花岗岩形成于南阿尔金洋（South Altun ocean, SAO）北向俯冲阶段，而花岗闪长岩则形成于构造体制转换阶段。此外，结合前人研究成果与本次新获取的测试数据，本研究为南阿尔金造山带建立了全新的早古生代构造-岩浆演化划分方案与地球动力学模型。