Chronology And Geochemistry Of Late Cretaceous Granites In Southern Lhasa Terrane

There have been large area distribution of Mesozoic magmatic rocks in the southern Lhasa block.The discussion of the petrogenesis and source information of these magmatic rocks and tectonic setting,geodynamics background is still going on.This paper reports the results of chronology,Hf isotopes and bulk-rock geochemistry of the medium-acidic intrusive rocks that intruded into the Yeba Group volcanic rocks in Cotsu-Songka area in the southern Lhasa block.Combined with the research results of the magmatism of southern Lhasa block,this paper also presents investigation of the petrogenesis of the granitoid rocks and the regional magma-tectonic evolution from Mesozoic to Cenozoic in the southern Lhasa block.The data of U-Pb chronology shows that these granitoid rocks intruded in the Late Cretaceous(90.3~98.7Ma).The Hf isotope data shows that ?Hf(t)values from +9.4~+15.7 with a small range of variety.TDM=135~380Ma,TDMC=150~557Ma.The bulk-rock Nd isotope data shows the similar result with the Hf isotope.The ?Nd(t)values from +3.6~+4.3.The Nd isotope mantle model age belongs to Neoproterozoic-Early Paleozoic(476~728Ma).These isotope results indecates that these granitoid rocks are from the juvenile crust.The granitoid rocks are alkali-rich and they belongs to high potassium cal-alkaline-potassium area.The A/CNK=0.66~1.03 which means they are metaluminous – peraluminous.The Mg O=0.25~3.82%(Mg#=24.33~40.68)which means they are lack of Mg.They are rich in LREE and depleted in HREE.[La/Yb]N=8.0~25.6 and ?Eu=0.57~1.10.They are enriched in LILE and depleted in HFSE.These granitoid rocks are mainly derived from the partial melting of the juvenilemafic crust during the subtraction process of Neo-Tethys.However,the granitoid rocks of the east and west area of southern Lhasa block have remarkable character of ancient crust.These granitoid rocks have adakitic affinity and they tend to derive from the patial melting of the thickened lower-crust under the tectonic setting of Neo-Tethys subduction.