Early Cretaceous Volcanic Rocks In Shanghulin Basin Of Northwestern Great Xing'an Range: Geochemistry Characteristics And Petrogenesis

The Shanghulin Basin in Erguna region of the NW Great Xing'an Range,tectonically located in the southeastern margin of the Mongol–Okhotsk Suture,is widely covered by Mesozoic volcanic rocks,where is an important part of the largest Mesozoic volcanic belt in China.This paper elucidates the forming age,geochemical characteristics,petrogenesis and tectonic setting of the Early Cretaceous volcanic rocks from the Shanghulin Basin,based on detailed studies on their petrography,geochronology,petrogeochemistry and zircon Hf isotope analysis.Zircon U-Pb dating indicates that the volcanic rocks within the Shanghulin Basin produced in the late Early Cretaceous,among which intermediate–basic volcanic rocks formed in 123 Ma,and the acid volcanic rocks formed in 124–114 Ma.The intermediate–basic volcanic rocks have silicon content of 44.55–56.64 wt.%,total alkali content(Na2O + K2O)of 44.55–56.64 wt.%,low Mg#(0.23–0.41)values and high ? REE of 162.81 × 10-6–255.48 × 10-6.The samples exhibit LREEs enrichment,HREEs depletion [(La/Yb)N = 9.01–19.55],and a weak negative Eu anomalies with ?Eu=0.71–1.03.Besides,the samples are enriched in large ion lithophile elements(LILEs;e.g.,Rb,Ba,Th,U,K and Sr),depleted in high field strength elements(HFSEs;e.g.,Nb,Ta,and Ti),and are relatively depleted in Yb and Lu.The acid volcanic rocks have silicon content of 67.89–80.95 wt.%,total alkali content(Na2O + K2O)of 4.51–10.91 wt.%,low Mg#(0.07–0.32)values,and A/CNK values of 1.03–2.31.The ?REE,between 66.85 × 10-6–216.26 × 10-6,is lower than that of the intermediate–basic volcanic rocks,and the LREEs and HREEs show a certain degree of fractionations.The samples have pronounced negative Eu anomalies(?Eu=0.2–0.67),an(La/Yb)N = 6.55–13.28.Different with the intermediate–basic volcanic rocks,the acid volcanic rocks have pronounced low Ba,Sr,P and Ti contents,and pronounced high Rb,Th,U and K contents,with Nb and Ta relatively depleted.The ?Hf(t)values is +2.6 to +8.3 and corresponding two-stage model ages(TDM2)is 655–1014 Ma.This model ages are similar to the Hf isotope characteristics of the Phanerozoic granite of the Xing'an–Mongolia Orogenic Belt,indicating that the nature of magma source of the samples is probably identical to the Phanerozoic granite.Both the field geological phenomena and geochemistry characteristics indicate that there is no genetic relationship between the coeval intermediate–basic and acid volcanic rocks,namely,the acid volcanic rocks are not products formed by magmatic differentiation of intermediate–basic volcanic rocks.The intermediate–basic volcanic rocks have not experienced significant contamination of crust materials,the magma derived from the enriched lithospheric mantle formed by metasomatism of fluid in subduction zone,and underwent the evolutionary process dominated by fractional crystallization.While,the magma source of the acid volcanic rocks is crustal materials accreted from depleted mantle in the Neoproterozoic.The magma shows similar characteristics to the I type granite,which has low Sr and Yb content,low forming pressure and shallow source area,and the mantle derived intermediate–basic magma provides heat source for its melt.Combined regional coupled geological data and temporal-spatial distribution regularities of volcanic rocks in NE China and adjacent areas,the data above reveal the late Early Cretaceous volcanic rocks in the Erguna region produce in an extensional environment,which is related to the post-orogenic extension of the Mongol–Okhotsk ocean.