Geochemistry And Petrogenesis Of Sandstones And Their Basaltic Interlayers Of Shexing Formation From Linzhou Basin, South Tibet

The collision between India and Asia was the most important tectonic event in the evolution of the Tibetan Plateau, which was recorded in the Late Cretaceous to Paleogene sedimentary and magmatic rocks in the southern Lhasa terrane. The Late Cretaceous Shexing Formation strata and later eruption of the Linzizong volcanic succession ( ca. 65 similar to 45Ma) is the best candidate to show the collisional processes. This study provides new petrological, geochemical and zircon U-Pb geochronological data on the sandstones and basaltic interlayers collected from the highest level of Shexing Formation in the Linzhou basin in southern Tibet. The materials of the Shexing greywacke were from the felsic magmatic source region. Hf isotope implies that the majority of the sediments came from the central Lhasa terrane to the north of the basin, and others from the Gangdese arc batholiths to the south of the basin. The youngest age of the detrital zircons in Shexing sandstone constrains the deposition were later than 98 Ma in Linzhou basin. The interlayers close to the top of the Shexing Formation are basalts and basaltic andesites, with enriched light rare earth elements ( REE) , depleted heavy REE, weak negative Eu anomaly, strongly enriched large ionic lithophile elements ( LILEs, such as Ba, Th, U, Pb) , and depleted high-field strength elements ( HFSEs, such as Nb, Ta). These basalts and basaltic andesites are high potassium calc-alkaline basalt series, with the affinity of the typical Andes-type basalts from continental arc. The detrital zircons from these interlayers define their eruption to be later than 110Ma. Our results suggest that the Linzhou basin is the back-arc tectonic setting, which located between the Gangdese arc and the central Lhasa terrane during the Late Cretaceous. The Neo-Tethys Ocean subducted beneath the Gangdese arc and the backarc system and triggered the mantle partial melting to produce the eruption of the basalts and basaltic andesites after 98 similar to 110Ma. The interval of 33Myr between the upper most Shexing Formation (98 Ma) and the Linzizong volcanics (65 Ma) , may represent the uplift and erosion processes of the Gangdese arc before the collision of India and Asia.