Petrogenesis And Geological Significance Of Late Cretaceous Acid Intrusive Rocks In Asa,Tibet

Here we report zircon U–Pb age,and Hf isotopic and whole-rock geochemical data for the Asa intrusive rocks located at the southern edge of the northern Gangdise subterrane.The Asa intrusive rocks include the Namujeler granite porphyry,the Gernicamdro granodiorite porphyry,and the Neeze granodiorite porphyry.In situ zircon U–Pb dating results of three samples are as follow:88.67±0.56Ma,89.67±0.57Ma and90.1±1.0Ma.The corresponding values of Zircon?_Hf?t?are respectively:–11.5 to+9.3,+4.6 to+7.6 and–21.2 to+7.6).The Namujeler granite porphyry and Gernicamdro granodiorite porphyry have adakitic geochemical characteristics,such as high Sr?317⁵11 ppm?,Sr/Y?57.8⁹6.8?,?La/Yb?_N?13.1¹6.0?,and low Y?4.68⁵.49 ppm?,heavy rare earth element contents?e.g.,0.44?Yb?0.57 ppm?,and no pronounced Eu anomaliy??Eu=0.87¹.03?.The Asa Neeze granodiorite porphyry is not adakitic rocks due to its lower Sr?231³43 ppm?and?La/Yb?_N ratios?8.4⁹.2?,and a weak negative Eu anomalies??Eu=0.76⁰.79?.Our studies on regional tectonic background,geochemistry and Hf isotope data suggest that the Asa adakitic rocks are most likely generated by partial melting of thickened mafic lower crust under a garnet-bearing amphibolite facies?⁴0km?.The Namujeler granite porphyry is most likely generated by partial melting of a combined thickened mafic lower crust?including juvenile crust beneath the northern Gangdise subterrane and ancient basement beneath the central Gangdise subterrane?under a garnet-bearing amphibolite facies.The Gernicamdro granodiorite porphyry can be generated by the partial melting of northern Gangdise subterrane thickened juvenile lower crust under a garnet-bearing amphibolite facies.The Neeze granodiorite porphyry can be formed by partial melting of the combination of juvenile crust?beneath the northern Gangdise subterrane?and ancient crust?beneath the central Gangdise subterrane?at a shallower depth?³0 km and with plagioclase as a residual phase?.The presence of the Asa adakitic rocks indicates that the crust beneath the central–northern Gangdise subterrane had experienced thickening and the crustal thickness beneath the Gangdise-Qiangtang collision zone remains quite large?>40 km?at ca.90Ma.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 Gangdise-Qiangtang orogenic belt was collapsed by lithospheric delamination during the early Late Cretaceous?ca.94⁸2 Ma?,and the thickened lithospheric keel delaminated piece by piece.From east to west,the time of the lithospheric delamination is getting younger.This unrooting process led to the gradual collapse of the Gangdise-Qiangtang orogenic belt from east to west,and gradually entered into the post-collision extended environment between 81 and 75 Ma.