Geochronology, geochemistry and geological significance of Late Triassic magmatism in Qingchengzi orefield, Liaoning
辽宁青城子铅锌多金属矿田晚三叠世岩浆岩年代学、地球化学及地质意义

The Qingchengzi orefield which lies in geotectogene of Liaodong rift is an important polymetallic mineral district in northern China for its clustering several large Pb-Zn,Ag and Au deposits.A phase of Late Triassic magmatism,represented by Shuangdinggou biotite monzogranite and Xinling granite,is intimately related to polymetallic mineralization.LA-ICPMS zircon U-Pb age of 224.2±1.2Ma was obtained for the Shuangdinggou intrusion.Geochemical data for the Shuangdinggou intrusion reveal SiO2 content of 69.07%～71.31%,while K2O accounts for 3.53%～5.22% and Na2O for 3.87%～4.14%,indicating it belongs to high K calc-alkaline series.Al2O3 content ranges between 12.46%～14.48% and A/CNK<1 suggest metaluminous feature.Trace element geochemistry of the biotite monzogranite displays high total REE content and demonstrate strong fractionation between light and heavy REE elements (La/Yb) N=35.43～79.01,LREE/HREE=23.09～35.10] and minor negative Eu anomalies(δEu=0.68～0.97) .The biotite monzogranite also shows LILE(such as Rb,Th,K,Pb) enrichment and HFSE(including Nb,Ta,P,Ti) depletion,with unusual high Ba and Sr,low Y and Yb abundances manefesting adakitic geochemical signature.Based on above-stated petrological and geochemical data it is inferred that the biotite monzogranitic magma might be derived from the partial melting of a thickened lower crust with prominent garnet and rutile retained in the residual assemblages,and magma mixing is probably involved based on the unusually high Nb/Ta ratio(18.4～21.2) .It is suggested that slab break-off during the process of Yangtze Craton and North China Craton continental deep subduction in Late Triassic may be responsible for the ore-related magmatism.