Two Triassic magmatic activities in Binbalechagan area of Dong Ujimqin Banner, Inner Mongolia: Geochronologic record, petrogenesis and tectonic settings

Triassic intrusive rocks (TIRs) are widespread in Binbalechagan area of Dong Ujimqin Banner in Inner Mongolia, and hence this area has advantage of studying Early Paleozoic evolution of central-east segment of Central Asia Orogenic Belt (CAOB). Based on regional geological survey, the authors recognized two Triassic magmatic activities by evidence from geochronology:mid-Triassic adamellites (with isotope ages of 238-230 Ma) and late-Triassic syenogranites (with isotope ages from 223 to 215 Ma). The former rocks whose geochemical features are between I-type and A-type granites are characterized by magnesia biotite, high silicon, rich K, peraluminum and calc-alkaline nature, abundant light rare earth elements and large ion lithophile elements, and significant negative Eu and Ba anomalies. However, the latter rocks belong to A-type granites and are characterized by high silicon, enriched alkali, abundant K and, weak peraluminum nature. Otherwise, total REE in the latter is lower, and negative anomaly degrees of Eu and Ba in the latter are more obvious than those of the former. These TIRs probably originated from partial melting of the lower crust which consisted of newly derived mantle. In combination with regional information, it can be concluded that the TIRS was formed after the closure of the Paleo-Asian Ocean. In mid-Triassic, the study area probably was in post-orogenic background, and hence this area was mainly in an intraplate tectonic setting in late Triassic.