The Early Jurassic sedimentary records in the Central Lhasa Terrane, southern Tibet: Implications for the subduction and accretion of the Tethys Ocean

The properties of the Middle Triassic to Early Jurassic basin in the Central Lhasa terrane are highly important for defining the relationship between the Zhikong-Sumdo Ocean and the Yarlung-Tsangpo Ocean. The Jialapu Formation in the Mailonggang-Xionglai Basin in the Central Lhasa terrane was studied utilizing petrology, zircon U–Pb chronology, and chromium spinel electron probe analysis on the basis of geological investigations in the field. The Jialapu Formation is a set of volcanic-sedimentary strata that are mainly composed of medium- to coarse-grained clastic rocks, with andesite and dacitic tuff interlayers. The U-Pb age spectrum of detrital zircon shows a primary age peak of 220–180 Ma and secondary age peaks of 650–500 Ma and 1250–1050 Ma. After weighted averaging, the maximum depositional age (MDA) of the Jialapu Formation was 184 ± 1 Ma, indicating that the Jialapu Formation was deposited mainly in the Early Jurassic. The provenance analysis revealed that the clastic materials of the Jialapu Formation were derived mainly from accretionary arcs in the Zhikong-Soumdo accretionary complex, and partly from the Gangdese basement and coetaneous magmatic arcs in the southern margin. On the basis of the results of the regional geological survey, the Lower Jurassic Jialapu Formation exhibits a transitional property from the wedge-top basin to the back-arc basin, which is a basin resulting from the continuous subduction-accretion of the Zhikong-Sumdo Ocean and Yarlung-Tsangpo Ocean.