Opening and Termination History of the Heilongjiang Ocean: Implications for the Evolution of the Jiamusi and Songliao blocks, NE China

Abstract:

The occurrence of blueschists in the Zhangguangcailing Orogen has brought much attention to researchers, which has been considered to be generated from the subduction between the Songliao and Jiamusi blocks in NE China. Controversies have been long surrounded the timing and origin of this orogen, especially the relationship with Songliao and Jiamusi blocks. In this study, we attempt to address these issues by an integrated study on the Zhangguangcailing Orogen.

Petrographic and geochemical results indicate that the protoliths of the blueschists in Zhangguangcailing Orogen can be subdivided into tholeiitic and alkaline groups. The tholeiitic protolith formed around 275 Ma and they were considered to be formed in a rifting environment. This continental rifting further developed into an ocean between the Jiamusi and Songliao blocks called the Heilongjiang Ocean, in which some oceanic islands were developed, as evidenced by the discovery of alkali rocks with OIB affinities and protolithic ages of 186 Ma – 142 Ma.

In regarding to the timing and process for the termination of the Heilongjiang Ocean, we propose that the oceanic subduction took place in the Jurassic, resulting in the formation of an active continental margin at the Zhangguangcailing Orogen, as evidenced by exhumation of subduction-related I type granitoids during 163 – 190 Ma and the emergence of a series of ~ 188 Ma andesitic mafic rocks. Moreover, the new mineral Ar-Ar dating from the mafic rocks indicates that the blueschist-faceis metamorphism took place around 158 – 180 Ma, supporting the Jurassic subduction of the Heilongjiang Ocean. It is also proposed that the termination of the Heilongjiang Ocean is in close association with the Paleo-Pacific regime.