Metamorphic evolution of high-grade granulite-facies rocks of the Mashan Complex, Liumao area, eastern Heilongjiang Province, China: Evidence from zircon U–Pb geochronology, geochemistry and phase equilibria modelling

Abstract

The nature and age of the Mashan Complex (or Mashan Group) in northeast China is key for determining the Precambrian geological evolution and origins of the Jiamusi Massif. Here we present a detailed investigation of high-grade granulite-facies rocks of the Mashan Complex from the Liumao area, Heilongjiang Province, China. This includes zircon U–Pb dating, petrological and geochemical analysis, geothermobarometry, phase equilibria modelling, and a P–T path. The metamorphic rocks consist mainly of two groups: pelitic and felsic granulites; amphibolite-facies Sil–Grt–Bi, Sil–Grt, and Grt–Bi gneisses. The protoliths of the metamorphic rocks were clay rocks and sandstones, which were derived dominantly from silicic rocks in a continental island arc setting. In the pelitic granulites, four metamorphic assemblages (M1–M4) were recognised. Three metamorphic assemblages (M2–M4) were identified in the felsic granulites. Geothermobarometric data and phase equilibria modelling indicate that the M2, M3, and M4 assemblages in the pelitic granulite were metamorphosed at 9.2 kbar/845–865 °C, 4.7–5.1 kbar/820–865 °C, and 4.0–4.8 kbar/660–720 °C, respectively. These features imply a collisional orogenic process with a clockwise P–T path with nearly isothermal decompression segments. Similar results were obtained for the felsic granulite, with the metamorphic P–T conditions of the peak assemblage (M2) evaluated as 9.0–9.2 kbar/845–870 °C. The similar P–T paths indicate that the rocks underwent an evolutionary process that started with crustal thickening (M2), followed by rapid crustal uplift (M3), and then final cooling (M4). Based on our new zircon U–Pb age data, the peak of granulite-facies metamorphism in the Liumao area of the Jiamusi Massif occurred at ca. 530 Ma and final cooling occurred at ca. 490 Ma.