Geochronology and geochemistry of the Carboniferous magmatism in the northern Great Xing’an Range, NE China: Constraints on the timing of amalgamation of Xing’an and Songnen blocks

Abstract

The Carboniferous period marks an important transition from accretion to collision in the Great Xing’an Range, NE China. In this study we present new data to provide a better constraint on the magmatic process in the studied area during this period and its implications for the regional tectonic evolution in the eastern Central Asia Orogenic Belt. The new age data we report consist of zircon SIMS U–Pb ages of 306–315Ma and zircon LA-ICP-MS U–Pb ages of 331Ma from basic intrusive rocks and 329.4Ma from granites in the Erguna-Xing’an block. Zircons are euhedral–subhedral in shape, display straight rhythmic stripes and oscillatory zoning in CL images with Th/U ratios of 0.20–3.44, which indicate that the zircons are of igneous origin. The basic intrusive rocks show SiO2 contents ranging from 47.75% to 53.80%, Mg# values (49–71), Al2O3 contents (14.35–17.76), and belong to the medium and high-k calc-alkaline series. Moreover, they are enriched in LREE and some LILEs as well as HFSEs, and depleted in Nb, Ta and Ti. Furthermore, they also show lower Sm/Th (0.70–4.38) and higher Th/Y (0.06–0.35) ratios, and variable Th/Zr (0.02–0.06), Ba/Th (29–249), Ba/La (9–20) and Ba/Nb (12–43) ratios. The geochemical data suggest that these rocks were likely derived from partial melting of mantle source with minor spinel metasomatized by slab-derived fluids. Integrating with regional geological data from the northern Great Xing’an Range, we conclude, therefore, that the Carboniferous intrusive rocks formed through the partial melting of a metasomatized subcontinental lithospheric in a subduction to post-collisional extensional setting triggered by the upwelling of the asthenosphere generated by slab break-off of the subducted oceanic crust between Erguna-Xing’an and Songnen block, which had amalgamated at no later than the end of Early Carboniferous.