Geochronology and Geochemistry of Konglong Miocene Potassic–Ultrapotassic Rocks, Central Lhasa Terrane: Constraints on the Magmatic Source, Evolution and Tectonics

ABSTRACT Potassic–ultrapotassic volcanic rocks are the products of post‐collisional magmatism in the Tibetan Plateau and can reveal the deep lithospheric processes and subduction dynamics of the Tethys oceanic crust and Indian continent. To better understand the genesis and tectonic setting of potassic magmatic rocks, this study presents zircon U–Pb ages, Sr–Nd–Pb and Hf isotope data, and whole‐rock major and trace element data for the Konglong potassic–ultrapotassic volcanic rocks in the central Lhasa terrane. The Konglong volcanic rocks mainly consist of low‐silica trachyte and high‐silica rhyolite. Zircon U–Pb dating reveals that the trachyte formed at 21.0 ± 0.1 Ma, while the rhyolite formed at 20.6 ± 0.1 Ma. Both rock types are characterised by relatively high potassium contents and are K‐rich (K 2 O = 5.81%–8.52%; K 2 O/Na 2 O > 1). These rocks are strongly enriched in large‐ion lithophile elements (e.g., Rb, Th, U, and K) and light rare earth elements (LREEs), with noticeable depletions in high‐field‐strength elements (e.g., Nb, Ta, and Ti) and heavy rare earth elements (HREEs) can be observed in these samples. The rhyolite contains comparatively low concentrations of Ba, Sr, and δEu, and the trachyte features glomeroporphyritic aggregates. The isotopic signature shows relatively high radiogenic 87 Sr/ 86 Sr (t) ratios (0.71–0.72), low εNd (t) values (−11.1 to −9.8) and zircon εHf (t) values (−12.2 to −10.0) and more uniform radiogenic Pb isotope ratios ( 206 Pb/ 204 Pb (t) = 18.79–18.95, 207 Pb/ 204 Pb (t) = 15.76–15.81, 208 Pb/ 204 Pb (t) = 39.51–40.00). These findings suggest that the enriched lithospheric mantle source region in the Konglong area has experienced two episodes of metasomatism. During the oceanic slab subduction stage, the enrichment was primarily derived from oceanic crustal sediments, whereas during the Indian continental crust subduction, it was dominated by melts and fluids produced by metamorphism of the Indian crust. During the Miocene, delamination of the lithospheric mantle slab led to asthenospheric mantle flow upwelling, with high heat flow triggering partial melting of the enriched lithospheric mantle and mixing with magma of Lhasa crustal origin. The mixed magma underwent differentiation through crystal‐melt separation, with the trachyte representing a crystal‐enriched and melt‐depleted cumulate and the rhyolite representing the resultant high‐silica, crystal‐poor melt.