Geochemistry and geochronology of Heilongjiang complex : implications for the evolution of the Jiamusi and Songliao blocks, NE China

Abstract

The Heilongjiang Complex represents a remnant of an oceanic plate suturing between the Jiamusi and Songliao blocks in NE China. Controversy has long surrounded the timing and origin of this complex, as well as its relationships with the Songliao and Jiamusi blocks. The presence of the high-pressure mafic and pelitic rocks in the Heilongjiang Complex is crucial to these controversial issues. This project attempts to address these issues by an integrated study including field study followed by geochronological and geochemical investigations on the Heilongjiang Complex and associated granitoids between the Jiamusi and Songliao blocks. The lithological association and geochemical data indicate that the mafic protoliths of the Heilongjiang complex could be divided into tholeiitic and alkali basalts. One tholeiitic sample yield protolithic ages of 275±2 Ma and contain populations of older inherited zircons up to 1200 Ma, which, together with their geochemical features, providing a possible link to a continental rifting. In combination with previous studies, a continental rifting zone is considered to have developed between the Jiamusi and Songliao blocks, and its crust thinning progressively, the rifting gave rise to the generation of shallow lithospheric mantle-derived mafic rocks at 256 Ma. The further development of this rift zone led to the formation of an ocean domain 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 ± 1.1 M, 162 ± 3.9 Ma and 142 ± 1 Ma. However, the timing and process for the termination of the Heilongjiang Ocean are still unclear. 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 Ma-190 Ma and the emergence of a series of ~ 188 Ma andesitic mafic rocks with arc-related features within the Heilongjiang Complex. Moreover, new mineral Ar-Ar dating from mafic samples indicates that the blueschist-facies metamorphism took places around 158-178 Ma, supporting the Jurassic subduction of the Heilongjiang Ocean. The termination of this subduction/collision between the Jiamusi and Songliao blocks took place around Early Cretaceous (140-130 Ma), as evidenced by the recognition of blueschists generated on an oceanic island setting with a protolithic age of 141.8±1 Ma, consistent with the early Cretaceous within-plated granitoids (120-130 Ma) in the Zhangguangcailing Orogen. Supporting evidence for this model also comes from meta-sedimentary rocks, of which the youngest concordant zircon ages are around 170 Ma, indicating that the latest sedimentary sequences were deposited at some time after 170 Ma, consistent with the idea that the ocean between the Jiamusi and Songliao blocks may not have closed until the Early Cretaceous. (451 words)