U-Pb and 40Ar/ 39Ar age constrains on protolith and high-P/T type metamorphism of the Heilongjiang Complex in the Jiamusi Massif, NE China

Abstract

The Heilongjiang Complex is mainly composed of high-P/T type metamorphic rocks of blueschists and pelitic schists, and is distributed along the western margin of the Jiamusi Massif in NE China. LA-ICP-MS U-Pb ages of detrital zircons in pelitic schist (LG1-3) from the Mudanjiang district give a weighted mean age of 250.6 ± 2.3 Ma (MSWD = 2.2). Pelitic schist from the Yilan district (09YL10-1) contains 193-348 Ma and 392-561 Ma detrital zircons, with minor amounts of 783-987 Ma zircons. The weighted mean ages of the youngest detrital zircon age group are 230.6 ± 3.5 Ma (MSWD = 1.2; LG1-3) and 199.1 ± 3.1 Ma (MSWD = 1.0; 09YL10-1), and these constrain the maximum depositional age of the protoliths of the Heilongjiang pelitic schists. Phengites from pelitic schists in the Yilan (422YQ-3) and Luobei districts (408HB-1) yielded 40Ar/ 39Ar plateau ages of 179.9 ± 0.8 Ma and 164.7 ± 0.2 Ma, respectively. A 40Ar/ 39Ar phengite age of 189.8 ± 0.8 Ma as total gas age (apparent age distribution from 183 Ma to 196 Ma) was also obtained for Yilan garnet-barroisite schist (423YJ-1). Reliable geochronological data suggest that a paleo-ocean located between the Jiamusi Massif and the Songnen Massif to the west was still present at least up to 199-231 Ma, and subduction-related high-P/T type metamorphism occurred during the Jurassic at 145-184 Ma.