The Petrogenesis And Geological Significance Of Adakitic Rock In Qunrang,southern Tibetan: Constraints From Zircon U-Pb Geochronology And Geochemistry

The Late Oligocene-Miocene adakitic rocks are widespread in the Lhasa block, while these rocks in the Tethyan Himalaya and the Yarlung Zangbo Suture zone are sporadically reported. Special-temporal connection between them remains ambiguous. Here we report zircon U-Pb ages and geochemical data of the monzonite porphyry dikes at Qunrang, closely south of Yarlung Zangbo Suture Zone,the research of them would provide constraints for tectonic and magmatic activities.These rocks are acid(Si O2=66-68.3 wt.%), potassic(K2O/Na2O=1.0-1.38), high aluminum(Al2O3=15.85-16.20 wt.%), low magnesium(Mg O=0.44-1.27 wt.%), low abundances of compatible element(Cr=14.81-24.62 ppm, Ni=8.79-12.28 ppm) and low Y and HREE(Y=12.23-17.18 ppm, Yb=0.78-1.05 ppm) coupled with high Sr(822.2-1386 ppm), Sr/Y and(La/Yb)N ratios(Sr/Y=64.56-81.30,(La/Yb)N=95.1-138.4), absence of a negative Eu anomaly, similar to the geochemical characteristics of typical adakites.These data suggest that they were derived from partial melting of thickened lower continental crust consisting mainly of garnet-amphibolite or amphibolite. Zircon U-Pb data indicate that their ages are of 10.02 ± 0.33 Ma, coeval with other adakitic rocks in the Tethyan Himalaya and Lhasa block. Synthesized with regional geological data, we propose that activities of South Tibetan Detachment System(STDS) would probably trigger the decompression and dehydration of lower residual thickened crust and finally generated the east-west directed Qunrang adakites.The slide of STDS resulted in decompression and dehydration of Asian lower crust, and finally generated adakitic rocks. The petrogenesis of Late Oligocene-Miocene adakitic rocks in southern Tibet was considered as the result of lithospheric thinning beneath the southern Tibet.However, in the article we first proposed the Qunrang adakitic rocks were induced by the activities of South Tibetan Detachment System, which could not only give a new explanations about adakitic rocks,but also provide important constraint about the time limit of STDS.