Petrogenesis of early Late Cretaceous Asa-intrusive rocks in central Tibet, western China: post-collisional partial melting of thickened lower crust

The timing and mechanisms of collapse of the middle and western segments of the Lhasa-Qiangtang orogenic belt in central Tibet are poorly constrained. Here, we report whole-rock geochemical, and zircon U-Pb age and Hf-isotopic data for the Asa-intrusive rocks located at the southern edge of the northern Lhasa subterrane. The Asa-intrusive rocks include the Namujeler granite porphyry (NGP), the Gernicamdro granodiorite porphyry (GGDP), and the Neeze granodiorite porphyry (NGDP). The NGP and GGDP have adakitic geochemical characteristics, such as high Sr (317-511 ppm), Sr/Y (57.8-96.8), and (La/Yb)(N) (13.1-16.0), and low Y (4.68-5.49 ppm) and heavy rare-earth element contents (e.g., 0.44 <= Yb <= 0.57 ppm). In situ zircon U-Pb dating of three samples yielded Late Cretaceous ages (NGP = 88.7 Ma; GGDP = 89.7 Ma; NGDP = 90.1 Ma). Zircon epsilon Hf(t) values vary over a wide range (NGP = - 11.5 to + 9.3; GGDP = + 4.6 to + 7.6; NGDP = - 21.2 to + 7.6). Our results suggest that the Asa adakitic rocks (NGP and GGDP) are most likely generated by partial melting of thickened mafic lower crust under a garnet-bearing amphibolite facies. The presence of the Asa adakitic rocks indicates that the crust beneath the Lhasa-Qiangtang collision zone had experienced thickening and the crustal thickness remains quite large (> 40 km) at ca.90 Ma. On the basis of evidence from the Asa-intrusive suite and coeval igneous rocks, along with some stratigraphic and tectonic constraints, we proposed that the middle and western segments of the Lhasa-Qiangtang orogenic belt was collapsed by lithospheric delamination during the early Late Cretaceous (ca. 94-82 Ma), and the thickened lithospheric keel did not delaminate as a wholly, it delaminated piece by piece. From east to west, the time of the lithospheric delamination is getting younger.