Petrogenesis And Tectonic Significance Of Zheduoshan Miocene Granite,Western Sichuan

Large amount of granitoid intrusions are exposed in the western Sichuan,eastern Tibet plateau.Previous studies mainly focused on discussion of regional chronological frameworks and tectonic settings.Detailed petrology and geochemistry evidence of these rocks for interpreting the source regions,petrogenesis,magmatic processes,and tectonic implications are limited.The Zheduoshan batholith exposed along the Xianshuihe fault zone is the only Cenozoic large granitoid batholith that has been identified in this area.It is a key area for studying the petrogenesis and geodynamic setting in the region.Therefore,detailed studies of petrogenesis,magmatic source area and tectonic implication of Zheduoshan batholith are present in this dissertation.The composition of the Zheduoshan batholith is very complex,among which Miocene granitoid are widely distributed,mainly exposured in the middle and northwest part.This study reports zircon U-Pb age and Hf isotope,whole-rock major and trace element date of nine monzogranites from Zheduoshan batholith.Two groups of granitoids are identified including normal calc-alkaline S-type monzogranites(Group 1,ca.18 Ma)and highly fractionated S-type monzogranites(Group 2,ca.13 Ma).Group 1 samples are weakly peraluminous,(A/CNK = 1.0-1.1),high-K calc-alkaline with SiO2 contents of 72.96-73.95 wt.%.These rocks are enriched in Rb,Th,and U,depleted in Ba,Nb,Ta,Sr and P,with negative Eu anomalies,and display low heavy rare earth elements(HREEs)and Y abundances,and high[La/Yb]N ratios(147-324).Group 2 samples are characterized by high SiO2 contents(74.44-76.28 wt.%)and differentiation index(DI = 90.34-91.87),and significantly Ba,Nb,Ta,Sr,P anomaly.Group 1 samples show negative zircon ?Hf(t)values(-9.55 to-6.08),and most likely derived from partial melting of Mesoproterozoic metamorphic sandstones under high temperature conditions.Group 2 samples have higher zircon ?Hf(t)values of-7.19 to-1.32,which is higher than those of Group 1 samples.They,however,have similar source composition(metagreywack).This could be account for the isotopic herterogeneous of the crust.The differences in major and trace elements of Group 2 samples can be interpreted as suffering from extensive fractional crystallization.The Miocene Zheduoshan rocks were generated from high temperature and pressure condition,indicated from the geochemical characteristics,temperature and pressure calculations.Combined with these data,the depths of about ca.35 km was estimated for the source region of the rocks.Based on the geophysical simulation data,the approximate range of the low-velocity high-conductivity layer in the Zheduoshan area of the Tibetan Plateau is at a depth of 15?50 km,which is in good agreement with the depth that we estimate.Therefore,the results of this paper provide new petrological evidence for the exist of the low-velocity high-conductivity layer of the Zheduoshan area,western Sichuan.This also indicates that,at least since the Miocene,the melting state of the middle-lower crust in the eastern Tibet Plateau is very available for the generation of the crustal channel flow.