The chronology, geochemistry of the Early Permian granodiorite in Langshan area, Inner Mongolia and its tectonic setting

There exists a large area of Late Paleozoic magmatic rocks on the western side of Langshan Mountain in Inner Mongolia, whose ages are concentrated on Early Carboniferous to Late Permian. The magmatic rock assemblages of different ages are of great significance for understanding the Late Paleozoic tectonic background of Langshan area. Based on petrology, petrography, geochemistry and Hf isotopes, this study mainly focused on the granodiorites in Chagannaihuduge zone of Langshan area. LA-ICPMS U-Pb dating on two granodiorite samples yielded ages of(299±1)Ma and(293±2)Ma respectively. The mafic minerals in granodiorties are dominated by hornblende and biotite. The geochemical data reveal that the granodiorites are of calc-alkaline nature characterized by enrichment of Na₂O (3.45%-4.96%), high Na₂O/K₂O value (1.33-2.52) and show good negative correlation between P₂O₅ and SiO₂, which is similar to the characteristics of I-type granites. The Hf isotopic signature for granodiorites and their behavior of elemental geochemical characteristics together indicate that their co-magmatic origin mainly derived from Palaeoproterozoic-Mesoproterozoic continental crust and subordinately from mantle-derived magma. The granodiorites show similar patterns on the chondrite-normalized REE patterns, and display relatively high concentration of light rare earth elements (LREEs) but low content of heavy rare earth elements (HREEs) with minor negative Eu anomalies. The overall chemical similarities of these granodiorites on the primitive mantle-normalized variation diagrams display affinity to arc signature. By combining the large scale regional exposures of diorite+quartz diorite+granodiorite rock assemblage and basalt+basaltic andesite+andesite+anganite in Dashizhai Formation, the authors hold that Langshan area was under the tectonic setting of continental margin arc during Early Permian, similar to that of central-eastern part of NNC.