Abstract:

The Mudanjiang blueschists are the typical outcrop of the Heilongjiang high pressure metamorphic rocks in NE China. The Mudanjiang blueschists occur as tectonic lenses and/or tectonic fragments which mainly distributed in the area of Yiziquan and Modaoshi along the Mudanjiang fault. The protolith of the blueschists is the subalkali-basalt according to their geochemical characteristics. Both the REE patterns and spidergrams of trace elements for these basalts are similar to that of the typical E-MORB.This study together with the studies from regional geology,petrology,geochemistry, and geo-chronology, suggests that the protolith of Mudanjiang blueschists at Yiziquan and Modaoshi is mainly dismembered E-MORB affinities formed in an initial spreading mid-ocean ridge setting. The authors suggest that the Heilongjiang blueschists are of ophiolitic nature and represent a high-pressure metamorphic zone formed due to the westward subduction of Jiamusi block and finally collided with Songnen block. The presently preserved blueschists represent the remaining part of large-scale accretionary complex overthrusted above the Jiamusi block during the Mesozoic. The results, together with our recently published SHRIMP zircon and Ar-Ar dating age, suggest that the final closure between the Jiamusi and Songliao blocks took place during the Late Triassic to Early Jurassic as a result of westward subduction of Pacific Ocean.

Key words: accretionary complex, blueschists, geochemical characteristics, high-pressure metamorphic belt, Mudanjiang;metamorphic rocks