Petrogenesis of late Cretaceous high Ba–Sr granodiorites, SE Lhasa block, China: implications for the reworking of juvenile crust and continental growth

Abstract

The high Ba–Sr rocks can provide significant clues about the evolution of the continent lithosphere, but their petrogenesis remains controversial. Identifying the Late Cretaceous high Ba–Sr granodiorites in the SE Lhasa Block could potentially provide valuable insights into the continent evolution of the Qinghai-Tibet Plateau. Zircon U–Pb ages suggest that the granodiorites were emplaced at 87.32 ± 0.43 Ma. Geochemically, the high Ba–Sr granodiorites are characterized by elevated K₂O + Na₂O contents (8.18–8.73 wt%) and K₂O/Na₂O ratios (0.99–1.25, mostly > 1), and belong to high-K calc-alkaline to shoshonitic series. The Yonglaga granodiorites show notably high Sr (653–783 ppm) and Ba (1346–1531 ppm) contents, plus high Sr/Y (30.92–38.18) and (La/Yb)_N (27.7–34.7) ratios, but low Y (20.0–22.8 ppm) and Yb (1.92–2.19 ppm) contents with absence of negative Eu anomalies (δEu = 0.83–0.88), all similar to typical high Ba–Sr granitoids. The variable zircon εHf(t) values of − 4.58 to + 12.97, elevated initial ⁸⁷Sr/⁸⁶Sr isotopic ratios of 0.707254 to 0.707322 and low εNd(t) values of − 2.8 to − 3.6 with decoupling from the Hf system suggest that a metasomatized mantle source included significant recycled ancient materials. The occurrence of such high Ba–Sr intrusions indicates previous contributions of metasomatized mantle-derived juvenile material to the continents, which imply the growth of continental crust during the Late Cretaceous in the SE Lhasa. Together with regional data, we infer that the underplated mafic magma provides a significant amount of heat, which leads to partial melting of the juvenile crust. The melting of the metasomatized mantle could produce a juvenile mafic lower crust, from which the high Ba–Sr granitoids were derived from reworking of previous mafic crust during the Late Cretaceous (ca. 100–80 Ma) in the SE Lhasa.