Permian volcanism in the Mongolian orogenic zone, northeast China: Geochemistry, magma sources and petrogenesis

Lower Permian volcanism was the first magmatic activity to occur after the collision events in the Mongolian orogenic zone, east China. The Permian volcanic rocks are therefore a key to understanding the dynamics of the unified continental lithosphere. The volcanic rocks consist of basic and intermediate rocks. The intermediate rocks with high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7051 to 0.7052) and low ε_Nd values (-0.73 to -3.57) generally overlie the basic rocks in the field. The basic rocks have relatively low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7034 to 0.7051) and high ε_Nd values (2.72 to -0.10). Two parallel Rb-Sr isochrons give almost the same age, about 270 Ma. One consists of the basic rocks giving an initial isochron ⁸⁷Sr/⁸⁶Sr ratio of 0.7035. The other consists of the intermediate rocks and one sample of basalt, which give an initial isochron ⁸⁷Sr/⁸⁶Sr value of 0.7051. The strong correlations between SiO₂ and other major elements suggest that fractional crystallization played an important role in the magmatic processes. However, fractional crystallization cannot explain the geochemistry of most incompatible trace elements and Sr-Nd isotope characteristics. The positive correlation between Th/Nb and (La/Sm)_N ratios demonstrates the direct relation between the enrichment of the light rare earth elements and the contamination of continental sediments. The high contents of large ion lithosphere elements (LILE) in the Permian volcanic rocks may suggest an additional 'crust + fluid' component, especially in the intermediate rocks, which are highly enriched in Ba (>400 ppm) relative to the basic rocks (< 200 ppm). We propose that the subduction slab dropped into depleted mantle and released fluid, which induced the mantle metasomatism and LILE enrichment. The metasomatized mantle partially melted and formed the 'primary' magma. This primary magma assimilated with the Proterozoic biotite-quartz schist during its rise, and finally formed the Permian volcanic rocks. Magma assimilated with the Proterozoic biotite-quartz schist in small amounts could have produced the basic rocks, while assimilation of larger amounts of magma (because of longer assimilation time) would generate intermediate rocks.