Formation age,geochemical characteristics and genesis of Biqigenao granitic complex in northeastern Damao Banner,Inner Mongolia
The Biqigenao granitic complex in northeastern Darhan—Maoming'an Union Banner(Damao Banner)is composed of two phases of intrusions,the lithology of which is granodiorite and monzogranite,the former being the main lithology of the granitic complex.Granodiorite is mainly composed of plagioclase(An=14～30),quartz,alkali feldspar and amphibole,with mafic microgranular enclave inside;The monzogranite is mainly composed of quartz,alkali feldspar and plagioclase(An=12～23).Zircon U-Pb dating shows that the granitic complex was formed in the Late Permian,and its formation age is 259 Ma.The main granodiorite with SiO2=66.37％～68.70％is aluminous—weakly peraluminous(A/CNK=0.99～1.01),which is characterized by enrichment of light rare earth and large ion lithophile elements,depletion of heavy rare earth and high field strength elements,belongs to Ⅰ-type granite;mafic microgranular enclave is rounded or elongated flat lenticular,and the age is consistent with the granodiorite,which is the product of magma mixing;Monzogranite is high silica granite(SiO2=71.51％～73.09％),alkali-rich(Na2O+K2O=7.91％～8.61％)and weakly peraluminous to strongly peraluminous(A/CNK=1.07～1.11),is a differentiated Ⅰ-type granite.The εH f(t)of Ziron distribution range is-3.0～3.4,and the corresponding Hf isotope two-stage model age is 1473～1057 Ma.It has relatively low(87Sr/86Sr)i value(0.704829～0.705327),ε Nd(t)value is-5.85～-8.86,and the corresponding Nd two-stage model age is 1.69～1.45 Ga.Based on the regional research data,it is considered that the Biqigenao granitic complex is formed by mixing the depleted mantle component with the partially melted ancient crustal material induced by it,and belongs to the post-collisional granite.
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