The metamorphic characteristics of metapelites of the Mashan Group in Mashan area, eastern Heilongjiang Province, China: Constraint on the crustal evolution of the Jiamusi Massif

Abstract

Mashan Complex in the southeastern margin of the Jiamusi Massif has traditionally been interpreted to be the oldest stratigraphic sequence in eastern NE China. Here we investigated the Mashan Complex in the Ximashan area and present lots of new LA–ICP–MS zircon U–Pb, petrological and mineralogical chemical data, in addition with phase equilibria modeling, to constrain the timing of metamorphism, P–T conditions and the P–T path of the Mashan Complex, and furthermore to deduce the crustal evolution of the Jiamusi Massif. The protoliths of the metapelites from the Mashan Complex in the Ximashan area are clay rocks and sandstones, whose provenance is dominantly felsic rocks which formed from a continental island arc. Three metamorphic generation assemblages (M2–M4) are recognized in the studied samples: a peak granulite facies stage, a post-peak near-isothermal decompression stage, and a late near-isobaric cooling stage, which together define a clockwise P–T path. It reveals important information related to the collision orogeny, subsequent uplift, and post-collision process. Zircon U–Pb dating of two metapelitic samples yielded metamorphic ages of the older group varying from 535 ± 6 to 480 ± 5 Ma, which can be perfectly correlated with the age range (501 ± 6 to 498 ± 6 Ma) of magmatic activities recorded by the gneissic granite sample. The data presented here, together with those published, allow us to preliminarily infer that the peak of the granulite-facies metamorphism in the Ximashan area of the Jiamusi Massif might have occurred at 530–500 Ma. The younger of the metamorphic ages (476–453 Ma) should be regarded as the age of late stage. Therefore, a collision orogeny regime is favored to interpret the metamorphic evolution of metapelites of the Mashan Group in the Mashan area.