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

ABSTRACT

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.

KEYWORDS:

• East Kunlun Orogenic Belt
• monzogranite
• igneous microgranular enclaves
• geochemistry
• petrogenesis

Acknowledgments

We would particularly like to thank the Editor-in-Chief Dr. Robert J. Stern for expeditious editorial handling, and Dr. Fuhao Xiong and an anonymous referee for their thoughtful and constructive reviews that greatly improved the earlier version of the manuscript. Financial support for this study was jointly provided by the National Natural Science Foundation of China (Grant Nos. 41930217, 42022016, 42172239), the Youth Innovation Team of Shaanxi Universities, and the MOST Special Fund from the State Key Laboratory of Continental Dynamics, Northwest University. Xiang Ren wishes to acknowledge support from the OeAD Scholarship financed by the Austrian Federal Ministry of Education, Science and Research during his visit to the University of Graz (grant No. MPC-2022-02168).

Disclosure Statement

There are no potential conflicts of interest to declare.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2023.2169962

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [41930217,42022016,42172239].