GEOCHEMICAL AND GEOCHRONOLOGICAL CONSTRAINS ON BLUESCHISTS FROM THE HEILONGJIANG COMPLEX AND ITS TECTONIC IMPLICATION

The Heilongjiang Complex in Northeast China is featured by well-exposed blueschists, in association with granitic gneisses, marbles, mica shists, greenschists, quartzites and meta-ultramafic rocks. It has long been suggested that this complex has recorded a paleo-ocean between the Jiamusi and Songliao blocks. However, the lifespan and the tectonic evolution of the ocean remain highly debated. In this study, we provide new constraints on this issue by carrying out an integrated geochronological and geochemical study on the petrogenesis of blueschists in the complex.

Petrographic and major and trace elemental results indicate that the protoliths of the blueschists in Yilan area can be subdivided into tholeiitic and alkaline groups. Magmatic zircons from a tholeiitic blueschist sample yielded a mean ²⁰⁶Pb/²³⁸U age of 275 ± 2.1 Ma, interpreted as its protolithic age. The presence of large amounts of old inherited zircons (up to 1200 Ma), together with the geochemical features of blueschists, indicates that the tholeiitic basalts were generated in a continental rift setting, which may be related to the breakup of Gondwana. We suggest that the further development of the rift led to the formation of an ocean separated the Jiamusi Block from the Songliao Block, in which some oceanic islands developed. These oceanic islands could be represented by the alkaline basaltic protoliths of the Yilan blueschists, as supported by our geochemical data. Magmatic zircons from an alkaline basaltic blueschist sample yielded a mean ²⁰⁶Pb/²³⁸U age of of 141 Ma, indicating that the ocean had not been closed by ~141 Ma. Our results do not seemingly support previous models that considered the closure of the ocean at 180-170 Ma.