Origin of Granodiorite and Mafic Microgranular Enclave in Saizhisi,Qinghai Province:Zircon U-Pb Geochronological,Geochemical and Sr-Nd-Hf Isotopic Constraints

Cui Jiawei; Zheng Youye; Sun Xiang; Wu Song; Gao Shunbao; Tian Liming; Sun Junyi; Yang Chao

doi:10.3799/dqkx.2016.515

Abstract：The Qilian orogenic belt is divided into the North,Central and South Qilian belts.The Saizhisi granodiorite is located be-tween Central and South Qilian.In order to study the petrogensis and dynamic background of Saizhisi rocks,zircon U-Pb chronology, Hf isotope,whole rock geochemistry and Sr-Nd isotopes are reported for granodiorite and mafic microgranular enclaves (MMEs)in this area.The LA-ICP-MS analyses on zircons from both the Saizhisi granodiorite and MMEs yielded similar ages at 446.1 ±1.3 Ma and 446.0±1.0 Ma,respectively.The MMEs contain needle-like apatite,and they are characterized by lower SiO2 contents and higher Na2 O/K2 O ratios.Their REEs are similar to those of Saizhisi granodiorite and ∑REE are lower than those of the host granodiorite;The 86 Sr/87 Sr of MMEs=0.706 4-0.706 7,theεNd (t )of MMEs=-7.38 to -7.97.The elemental,U-Pb dating and Sr-Nd isotope suggest that they might have been formed by mixing of mafice and felsic member magmas.The Saizhisi granodiorite is metaluminous to weakly peraluminous with SiO2 =66.45%-68.12%,Na2 O/K2 O=0.80 -0.97,and A/CNK=0.91 -1.03.The granodiorite is en-riched in LILEs and depleted in HFSE,enriched in LREE contents and depleted in HREE contents with negative Euanomaly.The 86 Sr/87 Sr(0.709 3-0.709 5)andεNd (t )(-7.3794- -7.9674 )of Saizhisi granodiorite is similar to those of I-type granite of the Qil-ian Orogen belt.TheεHf (t )of Saizhisi granodiorite=1.7-6.8,T DM2 =995 -1 750 Ma.It is concluded that the Saizhisi granodiorite might have been formed by mixing of mafice and felsic member magmas.Combined with the regional tectonic evolution,it is suggested that the Saizhisi granodiorite was formed in a subduction environment and mostly generated by the mantle which was formed by the lower crust intruded by the upwelling mantle.