Late Triassic magmatic rocks in the southern East Kunlun Orogenic Belt, northern Tibetan Plateau: Petrogenesis and tectonic implications

The East Kunlun Orogenic Belt (E-KOB) documents successive subduction and accretion processes of the Paleo-Tethyan Ocean. However, the tectonic regime of the E-KOB at the late orogenic stage remains ambiguous. This study presents new results of whole-rock geochemistry and zircon U-Pb-Hf isotopes of the Xidatan pluton in the southern E-KOB. Detailed investigation reveals that the pluton is composed of quartz monzodiorite, host monzogranite, and igneous microgranular enclaves (IMEs). Zircon U-Pb dating results suggest that they are coeval magmatic rocks emplaced at ca. 206–203 Ma. Geochemical studies reveal that the quartz monzodiorite and IMEs share a common magma source, evidenced by consistent compositions of major elements, congruently enriched LREEs and LILEs, and consistently moderately depleted HFSEs and HREEs. Enriched to slightly juvenile zircon Hf isotopes (ε_Hf(t) = −4.24 to +0.49) indicate that the primitive magmas of the quartz monzodiorite and IMEs involve the addition of mantle-derived magmas. The quartz monzodiorite and IMEs have identical ages, and consistent isotope features with the host monzogranite, together with the observation of ingestion of xenocrysts from host felsic rocks into the IMEs indicating that they were produced through the mixing of the mantle-derived mafic magmas and host felsic magmas. In contrast, The host monzogranite has moderately high SiO₂ and Na₂O+K₂O, but low Fe₂O₃^T and MgO contents. It shows crust-like trace element patterns such as strong enrichments in LREEs and LILEs, but remarkable depletions in HREEs. Along with enriched zircon Hf isotopes (ε_Hf(t) = −3.51 to −0.25), the host monzogranite was most likely to have originated from partial melting of lower crustal mafic rocks induced by underplated mantle-derived magmas. Considering abundant Late Triassic igneous rocks across the E-KOB, these new data provide further evidence that the Late Triassic magmatism in the E-KOB may be related to the delamination of thickened orogenic root under an extensional setting.

东昆仑造山带（East Kunlun Orogenic Belt, E-KOB）记录了古特提斯洋（Paleo-Tethyan Ocean）持续的俯冲与增生过程，但该造山带造山晚期的构造体制仍存在争议。本研究报道了东昆仑南部西大滩岩体的全岩地球化学与锆石U-Pb-Hf同位素最新研究结果。详细的岩相学调查显示，该岩体由石英二长闪长岩、寄主二长花岗岩以及火成微粒包体（igneous microgranular enclaves, IMEs）组成。锆石U-Pb定年结果表明，这些岩石均为同时期形成的岩浆岩，侵位时代约为206~203 Ma。地球化学研究揭示，石英二长闪长岩与火成微粒包体具有一致的主量元素组成，轻稀土元素（Light Rare Earth Elements, LREEs）与大离子亲石元素（Large Ion Lithophile Elements, LILEs）均显著富集，高场强元素（High Field Strength Elements, HFSEs）与重稀土元素（Heavy Rare Earth Elements, HREEs）则中等亏损，指示二者具有共同的岩浆源区。富集至略具新生特征的锆石Hf同位素组成（ε_Hf(t) = −4.24 ~ +0.49）表明，石英二长闪长岩与火成微粒包体的原始岩浆包含地幔源岩浆的贡献。石英二长闪长岩与火成微粒包体年龄一致，且与寄主二长花岗岩具有相似的同位素特征，加之观察到寄主长英质岩的捕虏晶被包裹于火成微粒包体中，指示二者是地幔源基性岩浆与寄主长英质岩浆混合作用的产物。
相比之下，寄主二长花岗岩具有中等偏高的SiO₂与Na₂O+K₂O含量，但Fe₂O₃^T与MgO含量较低。其微量元素配分模式具有典型的地壳源特征：轻稀土与大离子亲石元素强烈富集，而重稀土元素显著亏损。结合其富集的锆石Hf同位素组成（ε_Hf(t) = −3.51 ~ −0.25），寄主二长花岗岩最可能起源于底侵地幔源岩浆诱发的下地壳基性岩部分熔融。
考虑到东昆仑造山带广泛发育晚三叠世火成岩，本次新获得的数据进一步证明，东昆仑造山带晚三叠世岩浆作用可能与伸展背景下加厚造山根的拆沉作用相关。