Petrogenesis of the early Cretaceous Badui intrusion: Implications for the tectonic significance of the northern Lhasa terrane, Tibet

Abstract

The northern Lhasa terrane in Tibet is widely developed with Cretaceous magmatic rocks, but their petrogenesis and tectonic setting are still controversial. This study focuses on the Badui pluton in the Luolong area of Tibet and conducts a series of analyses on these granitic rocks, including petrology, geochemistry, and zircon U-Pb geochronology. The Badui pluton mainly consists of biotite monzogranite and diorite porphyry veins with zircon U-Pb ages of 131.7 and 120.1 Ma, respectively, showing that they both formed in the Early Cretaceous. The biotite monzogranite has SiO₂ and total alkali (Na₂O+K₂O) contents of 70.02-71.50 and 6.60-8.06%, respectively, and a Rittmann index (σ) of 1.53-2.40, indicating a metaluminous to weakly peraluminous I-type granite affinity. The diorite porphyry has SiO₂ and total alkali (Na₂O+K₂O) contents of 58.78-60.85 and 5.49-6.57%, respectively. The rare earth and trace element compositions indicate that both the biotite monzogranite and diorite porphyry are enriched in light rare earth elements and large ion lithophile elements, however, depleted in heavy rare earth elements and high field strength elements such as Nb, Ta, and Ti. Together with the previous studies, formation of the Early Cretaceous Badui pluton is a response for the southward subduction of the Bangong-Nujiang Ocean. The diorite porphyry has high Sr, low Y characteristics and more depleted Hf isotopic composition, suggesting that subduction sediments were added during the partial melting of subducted oceanic crust, while the biotite monzogranite has opposite characteristics, suggesting that it was derived from partial melting of ancient lower crustal material.