This study focuses on quartz monzonite and diorite plutons in Dachaigou, East Kunlun, examining their formation, age, petrogenesis, magmatic source, and tectonic setting. Through comprehensive field observations, zircon geochronology, geochemistry, and Hf isotope analysis, new insights into the Paleozoic tectonic evolution of the proto-Tethys Ocean in the East Kunlun orogenic belt are provided. Zircon U–Pb dating reveals that quartz monzonite and diorite formed at 417.4 ± 2.1 and 404.6 ± 1.8 Ma, respectively. Geochemical analysis shows that the A/CNK value of quartz monzonite ranges from 0.98 to 1.07, with the Pb content proportional to SiO2, which aligns with the characteristics of I-type granite. The εHf(t) values for quartz monzonite and diorite are −0.84 to 2.2 and −13.22 to 2.04, respectively. Considering the formation age, geochemical characteristics, and regional tectonic evolution, it is concluded that quartz monzonite has a crust-derived origin, whereas diorite has a crust–mantle mixing origin. Both were formed in a post-collision extensional tectonic setting resulting from slab break-off, implying the Proto-Tethys Ocean East Kunlun entered a post-collisional extension stage in the Late Silurian–Early Devonian.

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