Early Mesozoic magmatism records the tectonic evolution from syn- to post-collisional setting in the Central Lhasa subterrane, Tibet

Abstract

The Early Mesozoic (ca. 240–185 Ma) magmatic rocks in the central Lhasa subterrane (CL) provide crucial insight into the geodynamics of the Early Mesozoic orogenic assembly in the Lhasa Terrane. Here we present an integrated geochemical and geochronological study combining new and published data of the Early Mesozoic magmatism in the CL to reveal the geodynamic processes and magmatic responses involved in this orogeny. The here studied A-type granodiorites from the Luoza batholith display zircon U-Pb ages of 199 Ma, and zircon saturation temperatures of 833–857 °C. In addition, zircons have highly enriched radiogenic Hf isotope compositions (εHf(t) = −20.1 to −4.3). The Luoza granodiorite samples have high (87Sr/86Sr)t ratios between 0.7174 and 0.7209, and negative εNd(t) values between −9.5 and − 9.1. We propose the granodiorites were generated via partial melting of ancient basement rocks with minor inputs of basaltic melts (~10%). The here studied monzogranites and syenogranites from the Menba batholith are normal calc-alkaline and highly fractionated I-type granites that yield zircon U-Pb ages from 197 to 184 Ma and 187 Ma, respectively. The zircon grains of both rock types have relatively higher εHf(t) values (−13.8 to +0.5) than those of the Luoza granodiorites. The Menba monzogranites have (87Sr/86Sr)t ratios between 0.7062 and 0.7081, and εNd(t) values between −6.9 and − 3.4. The calculated εNd(t) values for the Menba syenogranites range from −4.5 to −4.0. The isotope data set indicates that the Menba granitoids were generated via mixing of ancient crustal melts and mantle-derived magmas (~50%). The high-temperature and low-pressure conditions indicated by the Early Jurassic Luoza A-type granitoids are geodynamically linked to a post-collisional setting. This evidence, in combination with Early Mesozoic magmatism identified within the CL, clearly displays a transitional conversion from ca. 240–210 Ma S-type-dominated to ca. 210–185 Ma I-type-dominated magmatism. This distinct change in magmatism reflects the geodynamic change from a syn-collisional to a post-collisional setting, involving the breakoff of the Sumdo Paleo-Tethys oceanic slab at ca. 210 Ma. The Early Mesozoic magmatic belt within the CL may record the orogenic assembly between the central and southern Lhasa subterranes, which represents the important evidence for closure of the Sumdo Paleo-Tethys Ocean. Consequently, the Early Mesozoic magmatic belt within the CL reveals that the Sumdo Paleo-Tethys Ocean extended at least from the Luoza area, central Lhasa Terrane, to the easternmost end of the Lhasa Terrane.