EARLY PALEOZOIC ADAKITIC ROCKS IN MOHE AREA AT THE NORTHERN END OF THE DA HINGGAN MOUNTAINS AND THEIR GEOLOGICAL SIGNIFICANCE

Abstract

Early Paleozoic adakitic rocks recently discovered in Mohe area, northern Da Hinggan Mountains, are mainly composed of monzogranite and granodiorite. These rocks have high SiO 2, and Na 2O (Na 2O/K 2O > 1), and low contents of HREE and high Sr/Y ratio (> 40). They are characterized by LREE-enrichment and weak Eu anomaly (δEu = 0.71∼1.1). Compared with the adakites generated by subducted slab-melting, the Mohe rocks display higher SiO 2 and K 2O contents, and can be classified as high-K calcalkaline series rocks. These features make them resemble Type II adakites, which may have originated by partial melting of the thickened lower crust. Occurrence of the adakitic rocks in Mohe area suggests that the crust in southeastern Ergun Massif has been thickened by magmatic underplating during the Paleozoic. On the other hand, porphyry copper and gold deposits related to the adakitic rocks in this area can be a new target for ore-prospecting.