Geochemical Characteristics And Genesis Of Volcanic Rocks Of The Tamulangou Fomation In Keyihe Area,Great Xing’an Range,NE China

The Mesozoic volcanic rocks in the Great Xing’an Range have always been the focus area of research.Many different understandings have been put forward on the study of its genetic mechanism and geological tectonic environment,but most of them put subvolcanic rocks of different periods under the same tectonic environment,while the differences of magmatism and dynamic evolution process of different periods,different lithologies and different regions are not discussed enough.Tamulangou Formation is the basaltic-Andesite stratum with the lowest Mesozoic volcanic strata found so far in the Great Xing’an Range area.Previous studies on its genesis have not formed a unified understanding,and the depth of magma source area,temperature and pressure conditions and other aspects of the estimation of less,no in-depth discussion on magma dynamic evolution and volcanic eruption mechanism.In this paper,the petrography,petrogeochemistry,mineral geochemistry,zircon U-Pb chronology and Sr-Nd isotopic tracer of the volcanic rocks of Tamulangou Formation in Keyihe area,Great Xing’an Range are studied in order to provide reference for the petrogenesis of the Tamulangou Formation.The Tamulangou Formation in Keyihe area is mainly a set of continental intermediate-basic volcanic lava.According to rock specimens,thin section identification and the characteristics of rock principal elements,the lithology is andesite,Alveolar amygdaloidal andesite and basaltic andesite,belonging to the quasi-aluminum high potassium calc-alkaline series.The characteristics of trace elements in the rocks show that the volcanic rock samples have high rare earth content,obvious fractional distillation of heavy and heavy rare earth,LREE enrichment,HREE depletion,and weak negative Eu anomaly.Large ionic lithophile elements such as Rb and Ba are enriched,while HFSE such as Nb,Ta and Ti are depleted.It is suggested that magma may originate from the enriched mantle and be mixed with crustal materials.(⁸⁷Sr/⁸⁶Sr)_ivalues range from 0.704941 to 0.705017 andε_Nd（t）values range from 1.4 to 1.7,indicating that the Sr-Nd isotopes are enriched in the mantle,suggesting that their formation may be related to subduction of oceanic crust.LA-ICP-MS zircon U-Pb dating of Tamulangou Formation volcanic rocks in Keyihe area shows that the weighted average age is 165.8±1.5Ma and the age is Middle Jurassic.The electron probe test of plagioclase and pyroxene is carried out for the main minerals in volcanic rocks.Plagioclase is mainly andesine,while pyroxene is all augite.According to the estimation of the temperature and pressure conditions of mineral formation,the crystallization temperature of pyroxene is 1196～1224℃,the formation pressure is 6.4～9.2kbar,and the formation depth is 21.2～30.4km.The crystallization temperature of plagioclase phenocrysts is 1154～1170℃,forming pressure is9.4～14.1kbar,and forming depth is 30.9～46.5km.The formation temperature of plagioclase matrix is estimated to be 1118℃.The magma eruption temperature is about1112℃.The results of temperature and pressure calculation show that the magma in this area has obvious deep source characteristics,and there may be magmatic recharge in the process of magma evolution.Based on the experimental data and previous studies,the petrogenesis of the volcanic rocks in Tamulangou Formation is discussed.The formation age of Tamulangou Formation in Keyihe area is the Middle Jurassic.The magma originated from the enriched mantle metasomatized by subduction fluid and was lightly contaminated by crustal materials.In the process of magma evolution,the process of magma recharge may occur,that is,more basic magma injection,and the mechanism of magma eruption may be related to magma mixing.The volcanic rocks of Tamulangou Formation were formed in the tectonic setting of the active continental margin,which may be related to the subduction of the Nenjiang Ocean remnant basin to the Erguna Massif under the superimposed influence of the closure of the Mongolia-Okhotsk Ocean and the subduction of the Paleo-Pacific plate in the Middle Jurassic.