Back-arc basin evolution in the southern Lhasa sub-terrane, southern Tibet: Constraints from U-Pb ages and in-situ Lu-Hf isotopes of detrital zircons

Sedimentary rocks in the Gangdese magmatic arc document a long but poorly constrained series of tectonic events that led to basin formation. We investigate one of these events by using detrital zircon grains from Mesozoic sedimentary rocks from the Linbuzong Formation located within the Gangdese magmatic arc. Our aims are to determine the provenance of the rocks in this back-arc basin and to document the paleo-topographical uplift of the Gangdese region in southern Tibet. Detrital zircons from the Linbuzong Formation, located in the middle-eastern segment of the southern Lhasa sub-terrane, have ages in four dominant populations: 140-300 Ma, 480-700 Ma, 1000-1250 Ma, and 1400-1700 Ma. The youngest detrital zircon cluster has a weighted age of ca. 149 Ma which, in conjunction with previously published data, indicates that sedimentary rocks in the Linbuzong Formation were deposited after 149 Ma. The youngest detrital zircon grains have low Hf-176/Hf-177 isotopic ratios and negative (epsilon Hf)(t) values, similar to igneous zircons from the central Lhasa sub-terrane which is considered to be the predominant source for sediments. However, few zircon grains from the youngest age cluster have high Hf-176/Hf-177 ratios with significantly positive (epsilon Hf)(t) values, suggesting derivation of some sediments from the southern Lhasa sub-terrane, primarily the Gangdese magmatic arc. Combined with the evidence for two conspicuous age populations (480-700 Ma and 1000-1250 Ma) and the Hf isotopic characteristics of the Linbuzong Formation, we propose that the Carboniferous-Permian metasedimentary rocks from the central Lhasa subterrane serve as the main source of sediments for the Linbuzong Formation. We conclude that the central Lhasa sub-terrane was exhumed to the surface and attained a high relief, whereas the Gangdese magmatic arc had a relatively low relief (near sea-level) during the Late Jurassic to the Early Cretaceous.