Zircon SHRIMP U-Pb dating of the amphibolite from "Heilongjiang Group" and the granite in Mudanjiang area, NE China, and its geological significance

Lithology and geochemistry of rocks in Mudanjiang area indicate that the so-called "Heilongjiang Group" contains remnants of oceanic crust, representing formations developed in an ancient active continental margin setting. The Mashan Group in this area is the basement of the Jiamusi Massif, and the "Heilongjiang Group" represents the orogenic sequence, including Alpine-type Iherzolite and amphibolite schist that has a geochemical affinity of MORB. SHRIMP zircon U-Pb dating shows that the amphibolite schist yielded a magmatic age of 777 +/- 18 Ma, which was reworked by metamorphism at 437 +/- 7 Ma; a syn-collsional granite emplaced to the south of "Heilongjiang" Group at 461 6 Ma. We propose that the "Heilongjiang" Group indicates a paleo-collisional zone and the collisional timing of the Mudanjiang collisional zone is about Ordovician-Early Silurian according to the Ca. 440 Ma extensive metamorphism and the occurrence of the syn-collisional granite magmatism.