Petrogenesis And Geological Significance Of Late-Mesozoic Granitoids In North Qinling Tectonic Belt

In recent years,the late Mesozoic granites and volcanic rocks have been reported in the western part of the North Qinling tectonic belt and even in the West Qinling area.Their scale,petrogenesis,source nature and tectonic system are directly related to the dynamic background of the evolution of the Qinling since the late Mesozoic.Therefore,the Cuihuashan pluton in the North Qinling tectonic belt and the Muhuguan pluton in the southern margin of the North China Block are selected as the research objects to study their petrogenesis and formation background.The Cuihuashan and Muhuguan granitic plutons show quite different petrogenetic types,geochemical characteristics and source compositions.The former intuded in the North Qinling tectonic belt,displaya emplacement in-situ or semi in-situ,the coarse-grained biotite monzogranite of Cuihuashan pluton have the ages from 152 Ma to144 Ma,and of the medium and fine-grained biotite monzogranite have the age of 114 Ma,indicates consistently that the late Mesozoic Yanshanian magmatism occurred in both western part of North Qinling and West Qinling besides as well as the Xiaoqinling area in the southern margin of North China block,which is characterized by large variation of major elements,with a rich alkali,higher sodium,weak peraluminous,weak or medium negative Eu anomaly and a geochemical feature similar to the adakitic rocks with high Sr and low Yb.It's ?_Nd(t)is from-5.65 to-3.07,and T_DM2 from 1399.72 Ma to 1189.79 Ma,which is formed by the partial melting from rocks of the Qinling group due to the heating of radioactive element decay under the thickening crust.The Muhuguan granite ploton is intruded into the both of North Qinling tectonic belt and southern margin of North China block,which shows magmatic ectopic emplacement.Some dioritic enclaves can be found and diabase dikes intruded in Muhuguan granite ploton,implying the influence of mantle-derived magmatism during its formation.the monzogranite of Muhuguan pluton has the ages of 151?150 Ma,and the dioritic enclave in the host manzogranite give a forming age of 150 Ma.In addition,the granodiorite yield a little oder age of 159 Ma.Muhuguan granitoides are characterized by of the higher silicon,low aluminum,rich potassium,low Mg#,medium negative Eu anomaly and belong to I-type granite.Their ?_Nd(t)=-10.44?-4.84,and T_DM2=1788.20?1334.45 Ma,which is the result from the crust-mantle magma mixing between the old continental crust and a small amount of new mantle in Taihua Group of the southern margin in the North China block under the background of subduction of the ancient Pacific plate to Eurasia or lithospheric thinning of North China Craton.Consequently,both Muhuguan and Cuihuashan granitic plutons were formed in completely different tectonic background,and late Mesozoic magmatism triged by the westward subduction of the paleo Pacific plate and the destruction of the North China Craton was roughly reached to the Muhuguan pluton in Lantian County.