Late Devonian-Early Carboniferous magmatism in the Lhasa terrane and its tectonic implications: Evidences from detrital zircons in the Nyingchi Complex

Abstract

The Late Paleozoic tectonic evolution of the Lhasa terrane remains poorly understood due to the paucity of the Late Paleozoic magmatic rocks exposed at the surface. Detrital zircons in the sedimentary rocks can provide a record of magmatic rocks that have been eroded. Here we report detrital zircon U-Pb ages, trace-element and Hf isotopic data of metasedimentary rocks from the Nyingchi Complex in the eastern Himalayan syntaxis. Detrital zircons from the metasedimentary rocks yield major age populations of 330–364Ma, 490–800Ma, 1000–1200Ma, and 1500–1800Ma. The weighted mean ages of the youngest three detrital zircons indicate Carboniferous (~330Ma) depositional age for their sedimentary protoliths. Provenance analysis indicates that the sedimentary detritus was sourced from the Lhasa terrane itself. The presence of abundant 330–364Ma detrital zircons indicates that the Lhasa terrane was characterized by Late Devonian-Early Carboniferous magmatism. The trace-element compositions of the 330–364Ma detrital zircons indicate that their magmatic host rocks mainly include mafic rocks and granitoids, and minor carbonatite. Some mafic host rocks probably formed in rift-related tectonic setting, and the others formed in arc-related tectonic settings. The granitic host rocks were S-type granites. The 330–391Ma zircons have negative εHf(t) values (−19.3 to −2.5), suggesting that their magmatic host rocks resulted from partial melting of the enriched mantle or ancient crustal materials. Combined with previous studies, we propose that the Late Devonian-Early Carboniferous magmatic rocks in the Lhasa terrane probably formed in an arc-back-arc system which resulted from the southward subduction of the Paleo-Tethys oceanic crust. The back-arc basin developed as the Sumdo Paleo-Tethys ocean, which began to shrink as oceanic crust subducted northwards underneath the North Lhasa terrane during the Late Carboniferous-Permian and finally closed during the Triassic.