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。然而，东昆仑造山带造山晚期的构造体制仍不明确。本研究针对东昆仑造山带南部的西大滩岩体（Xidatan pluton）开展了全岩地球化学与锆石U-Pb-Hf同位素分析，获取了全新的测试数据。详细的岩相学调查显示，该岩体由石英二长闪长岩（quartz monzodiorite）、寄主二长花岗岩（host monzogranite）以及火成显微粒状包体（igneous microgranular enclaves, IMEs）组成。锆石U-Pb定年结果表明，这些岩石均为同期形成的岩浆岩，侵位时代约为206~203 Ma。地球化学研究显示，石英二长闪长岩与火成显微粒状包体具有共同的岩浆源区，证据包括二者主量元素组成一致，轻稀土元素（light rare earth elements, LREEs）与大离子亲石元素（large ion lithophile elements, LILEs）均同步富集，高场强元素（high field strength elements, HFSEs）与重稀土元素（heavy rare earth elements, HREEs）则均呈中等程度亏损。锆石Hf同位素（ε_Hf(t)）值为-4.24~+0.49，整体富集且兼具轻微的新生幔源特征，指示石英二长闪长岩与火成显微粒状包体的原始岩浆均混入了幔源岩浆组分。石英二长闪长岩与火成显微粒状包体不仅形成时代一致，且与寄主二长花岗岩具有相似的同位素组成特征；此外，研究观察到火成显微粒状包体中捕获了寄主长英质岩石的捕虏晶（xenocrysts），这表明二者是由幔源基性岩浆与寄主长英质岩浆混合形成的。与之相反，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。该岩石具有典型的地壳型微量元素配分模式，表现为轻稀土元素与大离子亲石元素强烈富集，而重稀土元素显著亏损。结合其锆石Hf同位素（ε_Hf(t)）值为-3.51~-0.25且整体富集的特征，寄主二长花岗岩最可能是由底侵幔源岩浆诱发下地壳基性岩部分熔融形成的。考虑到东昆仑造山带广泛发育晚三叠世（Late Triassic）火成岩，本次获取的新数据进一步证明，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（delamination of thickened orogenic root）密切相关。