Petrogenesis and tectonic implications of Early Paleozoic granitoids in the Qiaerlong district of the West Kunlun orogenic belt: constraints from petrology, geochronology, and Sr-Nd-Hf isotope geochemistry

The West Kunlun orogenic belt (WKOB) is a complex orogenic belt that preserves the evidence of Early Palaeozoic and Late Palaeozoic orogenic events, with the former resulting from the closure of the Proto-Tethys Ocean. To better reveal the relationships between Early Palaeozoic granitoids in the WKOB and the tectonic evolution of the Proto-Tethys Ocean, we analysed the rocks of Akedala pluton, which is exposed within the Qiaerlong basin, and compared the results with contemporaneous magmatic rocks elsewhere in the WKOB. Integrated petrology, geochronology, and Sr–Nd–Hf isotope analyses of the rocks revealed the following: (1) The Akedala pluton is mainly composed of metaluminous–peraluminous I-type quartz monzonite and monzogranite; U–Pb dating of zircon indicates that quartz monzonite and monzogranite components is emplaced at 473–479 Ma and 462 Ma, respectively. (2) Both of them were enriched in large-ion lithophile elements and light rare earth elements and depleted in high field strength elements with negative Nb, Ta, Zr, P and Ti anomalies, indicating that they formed in a subduction zone environment. (3) Quartz monzonites were enriched in MgO concentrations (>3.10 wt.%), and show variable whole rock Sr-Nd isotopic ratios ((87Sr/86Sr)i = 0.7042–0.7058, εNd(t) = – 0.4 to +1.1), and had positive zircon εHf(t) (+0.0 to +6.8) values, which suggests they are derived from a mixed source of juvenile crust and older lower crust. (4) Monzogranites were characterized by high Sr (461–804 ppm) and low Y (2.00–3.68 ppm) and Yb (0.19–0.31 ppm) contents with high Sr/Y (164–325) and (La/Yb)N (12.6–46.7) ratios, indicating an adakitic affinity; they had relatively homogeneous whole rock (87Sr/86Sr)i (0.7044–0.7045) and positive εNd(t) (+2.0 to +2.9) and positive zircon εHf(t) (+1.1 to +4.3) values with low MgO, Cr, Ni and Nb contexts, suggesting that they were mainly generated from partial melting of thickened lower crust. Comparative analysis of these results with other Early Palaeozoic arc magmas in the WKOB shows that the Akedala pluton was formed due to the southward subduction of the Proto-Tethys Ocean. Divergent bidirectional subduction of the Proto-Tethys Ocean was initiated in the Early Cambrian, continued through the Middle Ordovician, and ended in the Early Silurian, due to terminal collision between the Tarim block and the West Kunlun terrane, which created the Early Palaeozoic orogenic belt.

西昆仑造山带（West Kunlun orogenic belt, WKOB）是一套复杂的造山带，保存了早古生代与晚古生代造山作用的地质证据，其中早古生代造山事件由原特提斯洋（Proto-Tethys Ocean）的闭合所触发。
为深入揭示西昆仑造山带内早古生代花岗岩类（granitoids）与原特提斯洋构造演化之间的内在联系，我们对恰尔隆盆地内出露的阿克达拉岩体（Akedala pluton）开展了综合岩石学、地质年代学以及锶-钕-铪（Sr–Nd–Hf）同位素分析，并将研究结果与西昆仑造山带内其他同期岩浆岩进行对比。
通过整合岩石学、地质年代学与Sr-Nd-Hf同位素分析，我们得到以下认识：（1）阿克达拉岩体主要由准铝质-过铝质I型石英二长岩与二长花岗岩组成；锆石U-Pb定年结果显示，石英二长岩与二长花岗岩的侵位时代分别为473~479 Ma与462 Ma。（2）两类岩石均富集大离子亲石元素与轻稀土元素，亏损高场强元素，且表现出Nb、Ta、Zr、P、Ti的负异常，指示其形成于俯冲带构造环境。（3）石英二长岩具有较高的MgO含量（>3.10 wt.%），全岩Sr-Nd同位素比值范围为(⁸⁷Sr/⁸⁶Sr)i = 0.7042~0.7058，εNd(t)值为-0.4~+1.1，同时锆石εHf(t)值为正值（+0.0~+6.8），表明其源区为新生地壳与古老下地壳的混合源区。（4）二长花岗岩具有高Sr（461~804 ppm）、低Y（2.00~3.68 ppm）与Yb（0.19~0.31 ppm）含量，且Sr/Y比值（164~325）与(La/Yb)N比值（12.6~46.7）较高，表现出埃达克质（adakitic）亲和性；其全岩(⁸⁷Sr/⁸⁶Sr)i比值相对均一（0.7044~0.7045），εNd(t)值为正值（+2.0~+2.9），锆石εHf(t)值亦为正值（+1.1~+4.3），且MgO、Cr、Ni与Nb含量较低，指示其主要由加厚下地壳的部分熔融作用形成。
将本次研究结果与西昆仑造山带内其他早古生代弧岩浆岩进行对比分析后可知，阿克达拉岩体形成于原特提斯洋向南俯冲的构造背景。原特提斯洋的双向发散俯冲始于早寒武世，持续至中奥陶世，并于早志留世随塔里木地块（Tarim block）与西昆仑地体（West Kunlun terrane）最终碰撞而终结，进而形成了早古生代造山带。