| The northern part of the greater Hinggan mountains in Heilongjiang province is located in the eastern part of the middle part of the Xingmeng orogenic belt and the eastern part of the greater Hinggan mountains block in Erguna.The cretaceous period was a period of large-scale igneous rock formation in the northern part of the region.Due to the background of complex tectonic evolution,the relationship between the genesis of rocks in this region and the tectonic evolution background is still controversial.In this paper,a detailed research was carried out on the petrology,geochemical characteristics and the geochronology of the Early Cretaceous granites?including granite porphyry,monzonite and granodiorite?,in the Luoguhe area,Mohe County,Heilongjiang Province.The tectonic setting and magma origin have also been discussed.U-Pb dating of zircon reveals that there are three kind granites at 143±1Ma,126±1Ma and 141±1Ma,respectively,which were all in the Early Cretaceous.The SiO2 content of all samples ranges from 64.19%to 71.36%,the Al2O3 content ranges from 14.52%to16.02%,the total alkali content?K2O+Na2O?ranges from 7.46%to 8.50%,the Na2O content ranges from 4.23%to 5.14%,Mg#value ranges from 64 to 69,and the aluminum saturation index ranges from 0.92 to 1.01.The monzonite and granodiorite have the characteristics of high Sr and low Y and Yb(Sr=643×10-6-1070×10-6,Y=4.78×10-6-9.61×10-6,Yb=0.42×10-6-0.86×10-6),the Sr/Y ratio is relatively high?100-136?,which is similar to the adakite.A lot of diorite enclaves have been observed in the monzonite and granodiorite,with SiO2 content ranging from 55.36%to 54.89%,the total alkali content?K2O+Na2O?ranging from 7.72%to 7.81%,Mg#value ranging from 63 to 71,and the aluminum saturation index ranging from 0.67 to 0.82.Besides,two kind of host rocks have relatively high content of SiO2,TiO2,Al2O3,FeOT,MgO,P2O5and CaO,and all samples show a good linear relationship in the Hack diagram.The total content of rare earth in the two host rocks is close,and both are enriched in light rare earth.Compared with the two host rocks,diorite enclaves have similar trace element contents,and all show the negative anomaly of Nb,Ta,Ti.The granites in the studied area have typical characteristics of the I-type granite.Combined with the genetic analysis of adakite,the granite is related to the mixing between the acidic magma originating from the crust and the basic magma originating from the mantle.Under the background of regional extensional structures,underplating of the mantle-derived magma triggered the partial melting of thickened lower crust,which leads to the characteristics of magma mixing in the granites. |
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