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
εNd 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 εNd 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.
