Petrogenesis Of Early Cretaceous Magmatism In Coqen And Contraining On Compositional Structure Of Lithosphere

Our predecessors have focused on ancient basement of central Lhasa subterrane for long time,but most data has been reported from central and eastern part of central Lhasa subterrane.There is lack of directly petrological support from eastern part of central Lhasa subterrane,leading debate about Early Cretaceous lithosphere architecture of central Lhasa subterrane.This study reports the zircon LA-ICPMS U–Pb age and Hf isotopic data,whole-rock major and trace element composition data,and Sr-Nd isotopic data for granites,enclaves and dioritic dyke from Nixiong Batholith and volcanic rocks from Zenong Group.The dyke is 0.5 to 1 m wide emplacing into the granitoids.The quartz dioritic prophyrite yields zircon U–Pb age of 113.9±2 Ma,the granitoids and dioritic enclave are dated at 113.1±2 Ma and 113.9±1 Ma,which indicates that these rocks were contemporaneous.The granitoids have zircon?_Hf(t)of–6.8 to–1.2,matching the T_DM2 of 1.14 to 1.46 Ga.The dioritic porphyrites are characterized by low SiO₂(53.58 to 55.57 wt.%),high Mg O(6.3 to8.4 wt.%)and Mg#(64 to 70).Besides,they have high compatible elements(Cr=412–672ppm,Ni=106–188 ppm),as high as primitive magma(Ni=300 ppm,Cr=300–500 ppm).These rocks yielded heterogeneous zircon?_Hf(t)of–5 to+3.7,with T_DM2 of 0.71 to 0.96 Ga.The samples have high initial(⁸⁷Sr/⁸⁶Sr)_i(0.710 to 0.714)and the whole-rock?_Nd(t)are–1.2 to–4.1,corresponding to the T_DM of 1.14 to 1.35 Ga.The dioritic enclave have similar geochemical character to the granitoids,while it have lower SiO₂(62.33 wt.%).The enclave yielded different isotope composition from the host rocks,which is more depleted(?_Hf(t)=–1.5–+5.6,T_DM=0.71–0.95 Ga,(⁸⁷Sr/⁸⁶Sr)_i=0.707,?_Nd(t)=–3.7).Our geochemical data indicate that the dioritic porphyrites were derived from partial melting of ancient lithospheric mantle,the host granitoids were derived from partial melting of ancient lower crust and the dioritic enclave was the product of magma mixing between mafic magma from asthenospheric mantle and crust-derived melts.The crust represented by granitoids and lithospheritic mantle represented by dioritic porphyrites are both ancient,indicating that the crust and underlain lithospheric mantle are coupled in central Lhasa.Besides,the coupled crust and mantle indicate the central Lhasa Terrane was a micro-continental block in late Early Cretaceous.The volcanic rocks from Coqin Zenong Group give zircon U–Pb ages of 130 Ma to 110Ma.These rocks are separated into two groups of highly fractionated group and moderately fractionated group.The samples from highly fractionated group is characterized by high content of SiO₂(>70 wt.%)and high differentiation index(>80),while samples from moderately fractionated group have lower content of SiO₂(<70 wt.%)and lower differentiation index(<80).The samples of 130 Ma to 120 Ma are more enriched in iron and depleted in magnesium compared with samples of 110 Ma,which may be result of fractionation of biotite and amphibole.They have similar isotope composition with(⁸⁷Sr/⁸⁶Sr)_ifrom 0.7073 to 0.7082 and?_Nd(t)from–8.5 to–5.9,indicating they origin from partial melting lower crust and then experienced fractionation of essential minerals and auxiliary minerals including titanite,ziocon and monazite.The samples of 110 Ma are more enriched in magnesium and depleted in iron,which indicates fractionation of amphibole.These samples have more depleted isotopic composition and they have initial ⁸⁷Sr/⁸⁶Sr of0.7073 to 0.7082 and?_Nd(t)of–8.5 to–5.9,indicating the input of mantle derived material.The highlyed fractionated samples have experienced fractionated of essential minerals.All these characters demonstrate the crust of central Lhasa subterrane is very mature.