Late Cretaceous High-Sr/Y Granitic Rocks in the Eastern Tibetan Plateau: Crustal Thickening During Continental Collision

High-Sr/Y granitic rocks,characterized by intermediate–high SiO2 contents(>56 wt.%),high La/Yb ratios(>20),and low Y(<18 ppm)and Yb(<1.9 ppm)contents,have drawn much attention from geologists in the past decades.They were originally named adakites and defined as granitic magma in convergent plate margins formed by partial melting of subducted young(<25 Ma),hot,and hydrated oceanic slab.Recent studies suggest that these rocks can also be formed in a variety of non-subduction-related tectonic settings through different petrogenetic processes including partial melting of thickened lower continental crust,providing important constraints on crustal growth and evolution throughout the Earth’s history.The Tibetan plateau,generally considered as an archetype of collisional orogen,was formed by the successive accretion of different tectonic terranes to the southern margin of the Eurasia plates since the Early Paleozoic.Geologically,the Tibetan plateau comprises several W–E trending terranes(the Kunlun,North Qiangtang,South Qiangtang,and Lhasa terranes)and northernmost India(the Himalaya region).These terranes have undergone significant crustal thickening during the continental collision events.The crustal thickness of the Tibetan plateau is asymmetric,being thickest under the Lhasa terrane and thinning more gradually towards the north than to the south.From south to north,the crustal thickness increases from^50 km in the Himalaya region,to^70–80 km under the Lhasa terrane,and decreases to^65 km under the North and South Qiangtang terranes and^55 km beneath the Kunlun terrane.The present crustal thickness of the Tibetan plateau has generally been attributed to the Cenozoic collision between the Indian and Eurasian plates and the subsequent intracontinental convergences since^55 Ma.However,doubts were raised about this interpretion in the past few years.For example,recent studies indicated the crust of the Lhasa terrane had been significantly thickened during ca.140–130 Ma in response to the subduction of the Neo-Tethyan Ocean,although the Cretaceous marine strata across the Lhasa terrane demonstrates that it was thin enough to be within the reach of sea level during at least the early Late Cretacous.Whether the South Qiangtang terrane underwent crustal thickening before Cenozoic or not remains an unanswered question.In this study,new zircon U–Pb geochronologic and Hfisotopic,and whole-rock geochemical data of the Late Cretaceous high-Sr/Y granitoids from the eastern South Qiangtang terrane are presented.LA–ICP–MS zircon U–Pb dating produced ages(92.0±1.1 Ma,88.8±0.7 Ma,and 91.5±0.6 Ma)of formation for the Leiwuqi granitoids of the South Qiangtang terrane,Tibetan Plateau,indicating that they were formed during the Late Cretaceous.The granitoids have low A/CNK ratios(0.94–1.06;<1.10)and P2O5 contents(0.02–0.12 wt.%),geochemically similar to I-type granitoids.They are characterized by high Sr(350–938 ppm,most>400 ppm),low Y(5.94–9.67 ppm),with high Sr/Y(36–119,most>40)and(La/Yb)N ratios(67–394).They exhibit negative zirconεHf(t)values(–9.8 to–1.1)and low MgO(0.32–0.69 wt.%),Cr(2.07–12.17 ppm),and Ni(1.10–5.94 ppm)contents.These features suggest that the granitoids probably were derived from partial melting of thickened lower continental crust(>50 km).The crust of the South Qiangtang terrane(at least its eastern part)may have been significantly thickened before the Late Cretaceous.