Petrogenesis and geodynamic setting of the Triassic granitoid plutons in West Qinling, China: insights from LA-ICP-MS zircon U–Pb ages, Lu–Hf isotope signatures and geochemical characteristics of the Zhongchuan pluton

West Qinling is one of the most important parts of the Qinling orogenic belt and includes acidic–intermediate plutons and many types of ore deposits. In this article, we collected geochemical and geochronological data for the Triassic granitoid plutons of West Qinling and found that nearly all plutons share the similar features with the Zhongchuan pluton. We present new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb ages, major and trace element geochemistry, and zircon Hf isotope systematics for the granites of the Zhongchuan pluton to elucidate the evolution of granitoid plutons in West Qinling during the Triassic. LA-ICP-MS zircon U–Pb dating indicates that the Xujiaba and Guandigou units formed at 220.1 ± 1.2 and 215.9 ± 0.85 Ma, respectively, reflecting the time of the Late Triassic. The rocks of the Zhongchuan pluton are metaluminous to weakly peraluminous and have a high-K calc-alkaline to shoshonite series with high SiO₂ (63.59–76.22%) and low P₂O₅ (0–0.2%) concentrations, a high K₂O/Na₂O ratio (1.18–17.92), a high differentiation index (78.45–93.04) and a medium A/CNK ratio (0.98–1.69). The zircon Hf isotope dating indicates that the Xujiaba and Guandigou units have an inhomogeneous <i>ε</i>_Hf(<i>t</i>) (−4.425 to 1.067 for Xujiaba and −4.920 to 2.042 for Guandigou) and two-stage Hf model ages (1123–1531 Ma for Xujiaba and 1115–2342 Ma for Guandigou). The geochemical and isotopic data imply that the granites of each unit share the same origin. They probably derived from the partial melt of metagreywackes and then mixed with the mantle-derived magma. Based on the regional geological history, petrographic characteristics and new geochemical and isotopic data of the Zhongchuan pluton, we suggest that the Triassic magma was derived from the partial melts of metagreywackes and was influenced by the mantle-derived melt during the collision of the Yangtze and Qinling plates.

西秦岭是秦岭造山带（Qinling orogenic belt）的核心组成部分之一，区内广泛发育中酸性岩体与多种类型的矿床。本文收集了西秦岭三叠纪花岗岩类岩体的地球化学与年代学数据，发现几乎所有岩体均与中川岩体（Zhongchuan pluton）具有相似的特征。为阐明西秦岭三叠纪花岗岩类岩体的演化过程，本文提供了中川岩体花岗岩的全新激光剥蚀电感耦合等离子体质谱法（laser ablation inductively coupled plasma mass spectrometry, LA-ICP-MS）锆石U-Pb年龄、主量与微量元素地球化学数据以及锆石Hf同位素体系数据。LA-ICP-MS锆石U-Pb定年结果显示，许家坝单元（Xujiaba）和官地沟单元（Guandigou）的形成年龄分别为220.1±1.2 Ma和215.9±0.85 Ma，指示晚三叠世的成岩时限。中川岩体的岩石为准铝质至弱过铝质，属于高钾钙碱性-钾玄岩系列，具有高二氧化硅（SiO₂，63.59~76.22%）、低五氧化二磷（P₂O₅，0~0.2%）含量，较高的K₂O/Na₂O比值（1.18~17.92）、较高的分异指数（78.45~93.04）以及中等的A/CNK比值（0.98~1.69）。锆石Hf同位素数据表明，许家坝单元和官地沟单元的ε_Hf(t)值分别为-4.425~1.067和-4.920~2.042，两阶段Hf模式年龄分别为1123~1531 Ma（许家坝单元）和1115~2342 Ma（官地沟单元）。地球化学与同位素数据显示，两个单元的花岗岩具有相同的成因源区，其物质可能来源于变杂砂岩的部分熔融，并随后混入了幔源岩浆。结合区域地质演化历史、岩石学特征以及本次获得的中川岩体地球化学与同位素数据，本文认为三叠纪岩浆起源于变杂砂岩的部分熔融，且在扬子板块与秦岭板块碰撞过程中受到了幔源熔体的改造。