The Middle Devonian Bimodal Association of Volcanic Rocks in the Northern Area of East Junggar, Xinjiang

The Middle Devonian volcanic rocks in the northern area of East Junggar, located between the Ertix and Ulungur rivers of northern Xinjiang, may be divided into basic and acid ones. It is evident that a compositional gap exists between the two groups so that the volcanic rocks are not in line with a calc-alkaline series because the intermediate rocks are absent in the area. The fact shows that the volcanic rocks are a typical bimodal association. The formation of the bimodal association of volcanic rocks in the area was closely related to continental rifting or continental extension in the Middle Devonian. In such a tectonic setting, magmas were first produced by partial melting of the mantle. Where crustal thinning was greater, the magmas ascended and erupted on the surface directly so that the basic volcanic rocks formed, but olivine and / or partial pyroxene fractionation occurred in the magmas during their ascent through the thinning crust. On the other hand, where crustal thinning was less, ascending mantle-derived magmas reached the lower crust and accumulated there, resulting in partial melting of the lower crust and thus giving rise to the contaminated magma which was consolidated as acid volcanic rocks on the surface.