Transition from non-adakitic rocks to adakitic rocks in the southern Lhasa subterrane: implications for progressive crustal thickening and tectonic evolution during the Early Cenozoic

Continental collision belts are commonly characterized by substantial granitoid formations and significant crustal thickening. However, the relationship between the record of granitoid magmatism and the change of continental crust thickness remains equivocal. To better understand this complex relationship, this study integrated new zircon U-Pb-Hf-O isotopes, whole-rock elements and Sr-Nd-Pb isotopic data of the Early Eocene collision-related monzonite, quartz-monzonite and granite in the Namling area of the southern Lhasa subterrane, South Tibet, China. These magmatic rocks are divided into two distinct episodes with contrasting geochemical characteristics. The early-episode quartz-monzonites and granites with zircon U-Pb ages of 51.4–52.3 Ma have high SiO₂ (64.7–72.31 wt%) and K₂O (3.69–5.68 wt%), low MgO (0.29–1.68 wt%) and Sr (151–367 ppm) contents, resulting in low Sr/Y (4.79–15.52) and (La/Yb)_N (6.52–11.08) ratios. Compared with the early-episode granitoids, the late-episode monzonites and quartz-monzonites yield younger zircon U-Pb ages of 47.5–47.7 Ma and display lower SiO₂ (62.34–63.62 wt%) and K₂O (3.09–4.02 wt%), higher MgO (2.11–2.40 wt%) and Sr (551–620 ppm) contents, and higher Sr/Y (29.75–57.84) and (La/Yb)_N (8.49–13.50) ratios, indicative of adakitic rocks. Although the two episodes of granitoids show distinct major and trace element contents, they are both characterized by positive bulk-rock Ɛ_Nd(t) (+0.09 to + 1.95; +2.14 to + 2.63) and zircon Ɛ_Hf(t) (+3.8 to + 8.5; +8.3 to + 11.5), high Pb isotopic ratios (²⁰⁷Pb/²⁰⁴Pb = 15.624–15.673 and 15.636–15.682, ²⁰⁸Pb/²⁰⁴Pb = 38.753–39.112 and 38.810–39.113), as well as intermediate δ¹⁸O ratios (+5.94 to + 6.91 ‰; +5.01 to + 5.93 ‰). These geochemical features suggest that these Namling granitoids most likely originated from the Lhasa juvenile mafic lower crust. However, the early-episode quartz-monzonites and granites may also include some ancient Indian crustal materials that caused their low Ɛ_Nd(t) (0.09–1.95) and Ɛ_Hf(t) (3.8–8.5) values and high δ¹⁸O (5.94–6.91 ‰) isotopic values. Temporal changes in Sr/Y, (La/Yb)_N, Dy/Yb, and Ho/Yb ratios of the Early Cenozoic granitoid rocks (SiO₂ ≥ 56 wt%) in the southern Lhasa subterrane indicate that the continental crust underwent progressive thickening, which was caused by basaltic magmas underplating and Indian indentation. The formation of the two episodes of the Early Eocene granitoids was most likely triggered by the break-off of the subducted Neo-Tethyan oceanic slab.