Geochemical Characteristic And Origination Of Wang Anzheng Rock Mass In Laiyuan In Hebei Province

The Wang Anzhen rock mass in Hebei province is a typical granite rock mass of those formed by magmatic activities in the northern Taihang Mountains in Mesozoic,and has draw great attention of geologists.Investigating the magmatic origin and active stages in this area will provide us guiding insights on the whole Mesozoic magmatic activities in the north Taihang Mountains.Although a great deal of work has been done in this area,its geodynamic background and rock formation are still in dispute.Based on previous works,in this paper we perform rock lithofacies microscopic observation,geochemical analysis and LA-ICP-MS zircon chronology studies on five combinations of groups of intrusive rocks in Wang Anzhen rock mass,that is,adamellite,granodiorite,granite porphyry,biotite adamellite,quartz monzonite,respectively.The results are summarized as follows:(1)According to the microscopic observation,granite porphyry and quartz monzonite have porphyritic texture,and we speculate that there may exist a rapid cooling process of these two groups of rocks after invading.(2)According to the rock geochemistry analysis,the main intrusive rocks in Wang Anzhen are alkaline and high-K calc-alkaline series,both of which are rich potassium rock series.The adamellite,granodiorite and granite porphyry are peraluminous,and biotite monzonitic and quartz monzonite prevail aluminum.The rocks evolves from being peraluminous to aluminum in the whole process.In the illustration of the genesis,the adamellite,granodiorite,granite porphyry show as granite of type A,while biotite monzonitic and quartz monzonite fall in the granite area of type I.(3)In the intrusive rock of Wang Anzhen body,adamellite shows negative?Eu anomaly,is relatively high with Th,Rb,P,Ti,and relatively low with Ba,Sr,Ta,Nb;granodiorite shows negative?Eu anomaly,is relatively high with Th,Rb,P,Ti,Hf,and relatively low with Ba,Sr and Ce,Nb,Y;Granite porphyry shows ?Eu negative anomaly,is relatively high with Th,Rb,P,Ti,and relatively low with Ba,Sr and Ce,Nb,Y.Biotite monzonitic granite and quartz monzonite show weak negative ?Eu anomaly,are relatively high with Ba,Sr,La,Ce,Nd,Y,Sm,and relatively low with Ta,Nb,U,Rb,P,Ti.8Eu is either negative anomaly or weak negative anomaly,and the depletion of Sr shows the crystallization of plagioclase.The depletion of Ba is related to the crystallization of potash feldspar,and the depletion of P indicates that the crystallization of apatite.In short,the evolution of the granite in this area has experienced crystal fractionation of amphibole,plagioclase,k-feldspar and apatite.Depleted Ta and Nd indicate characteristic of continental crust.The high Ba and Sr,and low Rb show characteristic of mantle material.The geochemical characteristics of Wang Anzhen intrusive rock show that the magma mainly originates from crustal remelting,and mantle source materials are involved.(4)Combining the results of the zircon U-Pb chronology studies and geochemical characteristics,it is shown that in this area there are two periods magma intrusion activities which are denoted as the early and the late,respectively.Early magmatic activity should be in 135 Ma-137 Ma and thus belong to the late Jurassic,while the late magmatic activity should be in 129 Ma-131 Ma and thus belong to the early cretaceous.The intrusive sequence of Wang Anzhen is adamellite-granodiorite-biotite-monzonitic granite-quartz monzonite.