Characteristics And Metallogenic Potential Of Ershiyizhan Intrusion In Upper Heilongjiang Basin

Ershiyizhan intrusion is located in the Upper Heilongjiang Basin in the northeast of the Great Xing'an Mountains,Ershiyizhan area in the north of Shibazhan Township,Tahe County.After many years of exploration and research,the porphyry copper-gold deposits have been found in the intrusion,however,no major breakthrough has been made in the metallogenic system and magmatic evolution.In this paper we present zircon U-Pb dating,Hf isotopic,major and trace elements,and Sr-Nd-Pb isotopic data and mineralogy analysis for Mesozoic intrusive rocks in the Ershiyizhan intrusion,in the northeastern part of Upper Heilongjiang Basin.With these data,we aim to explain the petrogenesis,magma evolution and tectonic setting of the Ershiyizhan intrusion.?1?The magmatism of Ershiyizhan intrusion can be subdevided into Late Triassic-Early Jurassic for granite,quartz-syenite,monzogranite?185.7-206.9Ma?and Early Cretaceous for quartz-monzonite porphyry and quartz-monzonite?127.6-133.2Ma?.?2?The Mesozoic magmatic rocks in Ershiyizhan intrusion are enriched in Si O₂and Al₂O₃,A/CNK more than 1.1,belonging to peraluminous high-K calc-alkaline series and shoshonite series.In addition,they are enriched in large ion lithophile elements?Rb?Th?K?Pb?,and depleted in high field strength elements?Nb?Ta?Ti?P?,suggesting a island-arc environment or active continental marginal environment.The normalized curve of rare earth element chondrite shows the enrichment of light right rare earth element and the depletion of heavy right rare earth element,with a slight negative Eu anomaly,?Eu is 0.62-0.90,LREE/HREE is9.48-14.81,?La/Yb?_Nis 11.57-23.22,indicating that the magma has undergone strong fractionation and crystallization.?3?The zircon Hf isotopes and whole-rock Sr-Nd-Pb isotopes parameters show the primary magams were generated by the partial melting of the lower crust materials and with the contribution of the mantle materials.Based on these data,combined with the regional tectonic evolution history and geochemical characteristics of the Ershiyizhan intrusion,we conclude that the Late Triassic-Early Jurassic magmatism of Ershiyizhan intrusion occured under the tectonic setting of the Mongolian-Okhotsk Ocean southward subduction to the bottom of the Erguna block.The Early Cretaceous magmatism occured in an extensional environment relate to the delamination of the thickened lower crust after the closure of Mongolian-Okhotsk Ocean.?4?The chemical composition study on biotite and amphibole show that the biotites in Early Jurassic granite and quartz-syenite are Fe-biotite with enrichment in TFe O and poor in Mg O,the magma crystallization condition of the early Jurassic granite and quartz-syenite were high temperature,relatively lower oxygen fugacity,but deep emplacement,the formation conditions are not consistent with the porphyry Cu-Au deposit.The biotites in the Early Cretaceous quartz-monzonite and quartz-monzonite porphyry are Mg-biotite and the amphiboles belong to Mg-amphibole,the calculation show that the early Cretaceous quartz-monzonite and quartz-monzonite porphyry in Ershiyizhan intrusions formed under high temperature,high oxygen fugacity and water content,shallow emplacement,furthermore the magma mainly derived from partial melting of crustal materials,and part of mantle materials is involved,with huge potential of forming great porphyry Cu-Au deposit.