High-pressure metamorphism in the Mudanjiang area, southern Jiamusi massif: Petrological and geochronological characteristics of the Heilongjiang complex, China

The Heilongjiang complex exposed in the Mudanjiang area, southern Jiamusi massif, is composed predominately of metabasites, pelitic schists, marbles and meta-siliceous rocks that were tectonically interleaved mélange similar to the series of ophiolite, representing the rock associations of the accretionary complex in the southwestern margin of Jiamusi massif. The geothermobarometer estimations of the peak mineral assemblage (Pg+Gln+Phn+Ep) which are included in porphyroblastic albite and epidote show that the p-T condition of the peak metamorphic stage was T=320-480℃ and p=800-1600 MPa, indicating typical epidote-blueschist facies metamorphism. In addition, 40Ar/39Ar plateau age of 164.9±0.5Ma is dated from a pelitic schist in the Heilongjiang complex. Due to the analogous temperature in K-Ar system of phengite (350-430℃) to the peak metamorphism condition mentioned above, the 40Ar/39Ar plateau age may be taken as the timing of high-pressure metamorphism, which represents the widespread metamorphism event in the Heilongjiang complex at the end of the Middle Jurassic.