Tectonic shortening and crustal thickening in subduction zones: Evidence from Middle-Late Jurassic magmatism in Southern Qiangtang, China

We report new zircon U-Pb geochronologic and Hf-isotopic data, and whole-rock major and trace element data, for the Labuco granitoids of the southern margin of the Southern Qiangtang terrane (SQ), Tibet. Five intermediate-felsic samples yielded zircon U-Pb ages of 169-156 Ma, making the Labuco granitoids contemporaneous with Middle-Late Jurassic magmatism in the SQ. The granitoids exhibit a range of zircon fHf(t) values from -7.3 to -0.6. The samples from Labuco can be divided into low-Sr/Y granitoids (LSG) and high-Sr/Y granitoids (HSG). The LSG are normal calc-alkaline I-type granitoids, characterized by varying major and trace element contents indicative of partial melting of ancient mafic lower crust. The HSG are characterized by high Sr/Y ratios (64-75), (La/Yb) N (chondrite-normalized) ratios of 57-76, and (Gd/Yb) N ratios of 6.6-8.9. These signatures indicate that the HSG were derived by partial melting of garnet-bearing thickened lower crust. The across-arc variation in magma geochemistry in the SQ was caused by tectonic shortening and crustal thickening, which occurred as a result of the northward subduction of the Bangong-Nujiang Ocean lithosphere during the Middle-Late Jurassic. These results have important implications for our understanding of tectonic shortening, crustal thickening, and the geometry of modern and ancient subduction zones. (C) 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.