Permian-Triassic adakitic igneous activity at Northern Mongolia: Implication for Permian-Triassic subduction system at the Siberian continental margin

The geochemistry of the Permian-Triassic large-scale igneous rock body of northern Mongolia is a key factor in understanding the subduction-related magmatism at the margin of "Siberian continent." Several studies have been done in the Permian-Triassic igneous body; however, its detailed magmagenesis and tectonic significance remain unclear. This paper investigates the geochemistry of the Upper Permian andesites (Bugat/Baruunburen Formation) and the Late Permian-Middle Triassic plutonic rocks (Selenge plutonic rock complex) of the Permian-Triassic igneous body, and intermediate dike intruding into them, and discusses the Late Permian-Middle Triassic magmatism of the Siberian continental margin. These rocks show a linear distribution on the variation diagram. They are therefore likely to be derived from a single magmatic source. The rocks, characterized by low K2O/Na2O, high Sr/Y, high La/Yb, and high Sr/La ratios are adakitic rocks of basaltic slab-melt origin. The samples are enriched in Cr and Ni and have a high Mg# compared with the typical slab-melt. This is likely due to an interaction between the slab-melt and the overlying mantle peridotite during its ascent. The Nb/Ta variation of the samples may point crustal contamination to the magma. The paleolatitude of the Bugat/Baruunburen Formation is calculated to be 37.1 degrees N based on thermal remanent magnetization. Therefore, the Late Permian-Middle Triassic large-scale adakitic igneous activity had taken place in the volcanic arc along the Siberian continental margin in the mid-latitudes of the Northern Hemisphere. The geochemical characteristics of the intermediate dike are almost the same as those of the Bugat/Baruunburen Formation and the Selenge plutonic rock complex, indicating that adakitic igneous activity continued after the Early Triassic.