GEOCHEMICAL CHARACTERISTICS AND GEOLOGICAL SIGNIFICANCE OF INTERMEDIATE AND INTERMEDIATE-ACID VOLCANIC ROCKS IN THE NORTHERN SECTOR OF THE DA HINGGAN VOLCANIC ERUPTION ZONE

Petrological and geochemical studies of intermediate-acid volcanic rocks of the Late Jurassic Jixiangfeng Formation in the northern sector of the Da Hinggan Mountains show that there exist high-Sr and low-Y type volcanic rocks in the area, which are so-called adakitic rocks in the literature.The authors think that the magmas of the intermediate-acid volcanic rocks were derived from basaltic magmas of the lower crust and are mixed magmas produced by the increase of geothermal gradients due to diapiric injection of basaltic magmas and partial melting of intermediate-basic metamorphic igneous rocks in the low er crust, and that they resulted from a series of tectonomagmatic activities that took place when the closing of the Mongolian-Okhotsk sea trough and continent-continent collision caused crustal thickening and brought about the formation of the Hinggan-Mongolian orogenic belt in the process of oblique subduction of the Paleo-Pacific plate beneath the Siberian plate.The tectonic setting in which they formed was affected and constrained by the Paleo-Asiatic tectonic domain in the setting of the peri-Pacific domain, rather than only resulted from the activity of the peri-Pacific tectonic mobile belt.The acid volcanic rocks of the Early Cretaceous Shangkuli Formation belong to continental crust anatexis type ones (S-type volcanic rocks) and may have formed by compression and migration of the Indian-Australia plate toward the NNE and delamination of the lower crust caused by the lithospheric extension in eastern China.