Zircon U–Pb Geochronology, Geochemistry, and Sr–Nd–Pb–Hf Isotopes of the Qula Syenogranite in the Central Lhasa Terrane, Tibet, Western China: Petrogenesis and Tectonic Implications

The Early Cretaceous post-collisional magmatic evolution of the central Lhasa Terrane remains hotly debated, and the Qula syenogranite provides new constraints on crust–mantle interaction. Whole-rock major and trace element analyses, zircon LA-ICP-MS U–Pb dating, and Sr–Nd–Pb–Hf isotopic analyses were carried out on seven samples from this newly recognized pluton to decipher its petrogenesis and tectonic setting. This pluton exhibits typical geochemical signatures of a weakly peraluminous, ferroan A2-type granite, including elevated SiO2 and total alkalis, low Al2O3 and CaO contents, high FeOT/(FeOT + MgO) (0.86–0.91) and 10,000 × Ga/Al (2.9–3.1), prominent negative Eu anomalies, chondrite-normalized rare earth element patterns with a seagull shape, enrichment in Rb, Th, U, Zr, and Hf, and depletion in Ba, Sr, P, Nb, and Ti. Zircon U–Pb dating yields a weighted mean crystallization age of 105.6 ± 0.46 Ma (MSWD = 1.16, n = 24), corresponding to the late Early Cretaceous. Positive zircon εHf(t) (+4.4 to +5.8), moderately negative whole-rock εNd(t) values (−5.3 to−3.4), and radiogenic Pb isotopic compositions together indicate that the parental magma most likely originated from the partial melting of juvenile lower crust, with contributions from both ancient basement and mantle-derived melts. Slab break-off of the southward-subducted Bangong–Nujiang lithosphere and consequent asthenospheric upwelling following the Qiangtang–Lhasa collision provide the tectonic scenario most consistent with these data, and the magma subsequently underwent extensive fractional crystallization dominated by K-feldspar, plagioclase, and biotite to form the Qula A2-type syenogranite.