Geochronology and geochemistry of the Heilongjiang Complex and the granitoids from the Lesser Xing'an-Zhangguangcai Range: Implications for the late Paleozoic-Mesozoic tectonics of eastern NE China

Abstract

An integrated study on two subduction-related units (the Heilongjiang Complex and magmatic rocks in the Lesser Xing'an-Zhangguangcai Range) was carried out to explore the Paleozoic to early Mesozoic tectonic framework of the eastern Northeast China, with a focus on the Jiamusi block. The Heilongjiang Complex, which is considered as the suture between the Jiamusi and Songliao blocks, is composed of blueschist, amphibolite and ultramafic rocks. The blueschist from the Yilan area shows geochemical features similar to OIB and has a 206Pb/238U age of 288±2Ma, which is interpreted as the crystallization age of protolith formed in an ocean island setting. In contrast, amphibolite from the Xiachengzi area shows geochemical features similar to N-MORB. Zircons in this amphibolite show magmatic character and yield a 206Pb/238U age of 248±4Ma, which is interpreted as the protolith age. Calcic amphiboles in the amphibolite give a 40Ar/39Ar plateau age of 195±3Ma, which is interpreted as the metamorphic age of the Heilongjiang Complex. Magmatic rocks in the Lesser Xing'an-Zhangguangcai Range mostly belong to I-type granites that were formed in an active continental margin setting. Zircon U-Pb ages of these granites range from ~200Ma to ~174Ma with a westward younging trend shown as two groups of emplacing ages of ~200Ma in the east and ~180Ma in the west. Our new data support an early occurrence (likely since the early Permian) of a large ocean (namely the Mudanjiang Ocean) in the area, whose subduction resulted in the metamorphic unit (the Heilongjiang Complex) and the magmatic unit (the Lesser Xing'an-Zhangguangcai Range) in the Jurassic.