Tectonic transition from Paleo- to Neo-Tethyan Ocean in Tangjia-Sumdo area, Southern Tibet: Constraints from Early Jurassic magmatism

Abstract

The history Paleo-Tethyan Ocean basin in Tangjia-Sumdo area of Lhasa block has not been investigated in detail, particularly the tectonic evolution after its closure. To better understand this issue, we investigated U-Pb zircon chronology, zircon Hf isotopes, and whole rock geochemistry of Early Jurassic granitic rocks in Tangjia-Sumdo area on Lhasa Block. Three types of samples were obtained in this study, diorites (199.8 ± 1.1 Ma), monzogranites (194.3 ± 1.1 Ma and 200.5 ± 1.2 Ma), and granites (184.5 ± 2.1 Ma, 185.5 ± 4.0 Ma and 185.6 ± 2.1 Ma), which can be divided into two stages according to their eras. The whole rock geochemical characteristics show that all the samples are I-type granitoids. The geochemical analyses show that the first stage of magmatism was derived from partial melting of crustal material mixed with mantle material, and the second stage of magmatism was derived from partial melting of the crust. Based on the zircon Hf isotopic characteristics (the calculated εHf(t) values of the diorite are − 16.7 to − 7.7, monzogranites are + 1.8 to + 5.2, granites are + 1.6 to + 9.9), we conclude that diorites were derived from the ancient continental crust, and the other two groups of samples were derived from juvenile crust. Based on a comparison with previous research results and regional tectonic development, we suggest that the first stage of magmatism was the result of slab break-off of subducting Tangjia-Sumdo Paleo-Tethyan oceanic crust. The second stage of magmatism was the result of northward subduction of Neo-Tethyan Ocean, and it may represent tectonic transition from Paleo- to Neo- Tethys Ocean in Early Jurassic in Lhasa Block.