Geochemistry, Geochronology, and Tectonic Implications of the Hornblende Gabbro from the Yeba Formation in the Dazi Area, Southern Lhasa Terrane

This study investigates hornblende gabbros and phyllites from the Yeba Formation in the South Lhasa Terrane, the southernmost unit of the Eurasian continent, through petrographic, geochemical, zircon U–Pb, and 40Ar-39Ar analyses, complemented by Hf isotope data. Zircon U–Pb dating of the hornblende gabbros yields an age of 165 ± 1.3 Ma, while phyllite samples record ages of 173 ± 1.0 Ma and 168 ± 1.5 Ma, representing the crystallization ages of their parent rocks, indicating Middle Jurassic magmatism. Muscovite 40Ar-39Ar plateau ages of 78.47 ± 1.38 Ma and 80.07 ± 1.19 Ma reflect Late Cretaceous (~79 Ma) metamorphism. Geochemically, the hornblende gabbros belong to the medium-K calc-alkaline series, enriched in LILEs (e.g., Rb, Ba, U, and K) and depleted in HFSEs (e.g., Nb, Ta, P, Zr, Hf, and Ti), with pronounced negative P and Ti anomalies that are typical of arc magmas. Structural discrimination diagrams place all samples within calc-alkaline continental and volcanic arc fields, indicating formation in a subduction-related arc setting. Integrating these results with regional data suggests that Yeba Formation magmatism (192–162 Ma) corresponds to the mature island arc stage of Neo-Tethys subduction beneath the Gangdese margin. The ~79 Ma metamorphic event reflects deformation linked to the northward subduction of the Neo-Tethyan Ocean and crustal thickening in the southern Lhasa Terrane. This study provides direct metamorphic ages that constrain the evolution of the back-arc basin and refines prior interpretations that substituted metamorphic timing with magmatic crystallization ages.