Triassic granites in the West Qinling Orogen, China: implications for the Early Mesozoic tectonic evolution of the Paleo-Tethys ocean

The Western Qinling Orogen is marked by secular Triassic magmatism, a thorough understanding of the mechanism of which could provide critical insights into the reconstruction of the Palaeo-Tethys tectono-magmatic activity. Here, we present new geochemical, geochronological and Lu-Hf isotopic data for three batholiths and review regional data. These batholiths have similar rock assemblages (mainly granodiorite and monzogranite), which are characterized by dominantly plagioclase, quartz, and K-feldspar, subsequently amphibole and biotite. Zircon U-Pb data reveal that they represent the long-duration magmatism during the Triassic (ca. 251–223 Ma). Mineralogical characteristics and geochemical affinities manifest that these rocks in the Zeku district can be explicitly delineated as high-K, calc-alkaline, weakly fractionated I-type granites. They yield significantly negative εHf(t) values ranging from −12.81 to −1.26 with old two-stage mantle depleted model ages between 2054 and 1319 Ma. In tandem with Th/Nb (0.98), Th/La (0.39) and La/Nb (2.40) ratios and moderate pressure conditions (ca. 7–10 kbar), the studied granites were derived from partial melting of the Middle Paleoproterozoic to Middle Mesoproterozoic lower crust source region with minor mafic hydrous magma addition. Our new understandings, in conjunction with the temporal-spatial distribution characteristics of magmatism in the West Qinling Orogen, as well as the regional tectonic evolution, suggest that the superimposed orogeny evolved from the northward subduction of the Palaeo-Tethys Ocean (264–225 Ma) through syn-collision (225–215 Ma) to post-collision (beginning at ca.215 Ma) between the North China Craton and South China Block. In this scenario, at the convergent continental margin, the oceanic plate may have undergone melting producing the melts parental to the Zeku granites.

西秦岭造山带以持续的三叠纪岩浆作用为显著特征，深入理解其形成机制，可为重塑古特提斯构造-岩浆活动提供关键依据。本文报道了3个岩基的全新地球化学、年代学及Lu-Hf同位素数据，并梳理了区域相关数据。这些岩基具有相似的岩石组合，以花岗闪长岩和二长花岗岩为主，矿物组成以斜长石、石英、钾长石占主导，其次为角闪石与黑云母。锆石U-Pb定年数据显示，其形成于三叠纪（约251~223 Ma），代表了长期持续的岩浆作用。矿物学特征与地球化学属性表明，泽库地区的此类花岗岩可明确归为高钾钙碱性、弱分异I型花岗岩。其εHf(t)值显著为负，范围介于-12.81至-1.26之间，两阶段亏损地幔模式年龄为2054~1319 Ma，相对较老。结合Th/Nb（0.98）、Th/La（0.39）与La/Nb（2.40）比值以及中等压力条件（约7~10 kbar），指示研究区花岗岩起源于古元古代中期至中元古代中期的下地壳源区的部分熔融，并伴随少量含水镁铁质岩浆的加入。结合西秦岭造山带岩浆作用的时空分布特征与区域构造演化历史，本次研究认为该造山带的叠加造山过程经历了三个阶段：古特提斯洋向北俯冲阶段（264~225 Ma）、华北克拉通与华南地块之间的同碰撞阶段（225~215 Ma），以及后碰撞阶段（约215 Ma开始）。在此构造背景下，汇聚大陆边缘的洋壳发生熔融，其熔体即为泽库花岗岩的母岩浆。