⁴⁰Ar/³⁹Ar ages of the high-P/T metamorphic rocks of the Heilongjiang Complex in the Jiamusi Massif, northeastern China

Abstract

The Heilongjiang Complex in northeastern China is composed mainly of alternating blueschists and pelitic schists which underwent high-P/T type metamorphism of the epidote blueschist facies. Phengites in the pelitic schists from two areas in the Heilongjiang Complex yielded ⁴⁰Ar/³⁹Ar plateau ages of 165 ± 0.8 Ma (Mudanjiang area) and 171 ± 0.7 Ma (Yilan area). Phengites in blueschists from the Yilan area give ⁴⁰Ar/³⁹Ar isochron ages of 145 ± 0.9 Ma and 146 ± 1.3 Ma. The previously reported peak metamorphic temperature of 350-450 °C for the Heilongjiang Complex is close to the Ar retention temperatures of white mica, suggesting that the ages reported here closely date the metamorphic culmination in this complex. These ages indicate that a widespread high-P/T metamorphism occurred in the Heilongjiang Complex at 165-171 Ma. Subsequent to this period, a similar type of metamorphism continued at least until 145-146 Ma in the Yilian area.