The Petrogenesis of Early Permian Granodiorites in the Northern Segment of the Changning-Menglian Suture Zone, Western Yunnan, and Their Tectonic Implications

The Changning-Menglian suture zone, as the remnant of the main Paleo-Tethyan oceanic basin in its southern segment, lacks direct magmatic evidence constraining the timing of subduction initiation in its northern segment. The petrogenesis and tectonic setting of the newly discovered Early Permian (~280 Ma) Wayao granodiorite in the northern segment remain unclear, hindering our understanding of the timing of subduction initiation and processes of the Paleo-Tethyan Ocean in the Changning-Menglian suture zone. This study presents systematic petrographic, zircon U-Pb geochronological, whole-rock major and trace element geochemical, and Sr-Nd-Hf isotopic analyses on the newly discovered Early Permian granodiorite in the Wayao area, northern segment of the Changning-Menglian suture zone, western Yunnan. Zircon U-Pb dating yields a crystallization age of ca. 280 Ma, confirming its emplacement during the Early Permian. The petrogeochemical characteristics indicate that it belongs to the metaluminous, calc-alkaline series of I-type granite. It is enriched in large-ion lithophile elements (LILEs; e.g., Rb, Th, U, La, Pb) and depleted in high-field-strength elements (HFSEs; e.g., Ba, Nb, Sr, Ti), exhibiting a pronounced negative Eu anomaly. Whole-rock Sr-Nd isotopes (εNd(t) = −5.6–−6.1) and zircon Hf isotopes (εHf(t) = −1.34–−10.01) suggest that the magma was predominantly derived from the partial melting of ancient crustal material (primarily metamorphosed basic rocks, such as amphibolite), with a minor addition of mantle-derived components (magma mixing). Combined with petrogeochemical discriminant diagrams (e.g., Sr/Y vs. Y, Rb vs. Yb + Ta) and the regional geological context, this granodiorite is interpreted to have formed in an active continental margin tectonic setting associated with the eastward subduction of the Paleo-Tethys Ocean (represented by the Changning-Menglian Ocean). This discovery fills the gap in the record of Early Permian subduction-related magmatic rocks in the northern segment of the Changning-Menglian suture zone. It provides crucial petrological evidence constraining that the eastward subduction and consumption of the northern Paleo-Tethys Ocean had already commenced by the Early Permian.