Zircon U-Pb dating and petrogenesis of the Early Jurassic rhyolite in Badaguan area, northern Da Hinggan Mountains

The newly-discovered Early Jurassic rhyolite in Badaguan area of Da Hinggan Mountains is of great significance in investigating the tectonic evolution history of Northeast China. The rocks are mainly composed of rhyolites, with a small amount of dacitic and rhyolitic tuffs. LA-ICPMS shows the rhyolites were formed in Early Jurassic, and their formation ages range from 185 Ma to 190 Ma. Geochemical studies show that the rhyolites are characterized by rich silicon and alkalis, poor calcium and magnesium and high FeO_T/MgO ratios. Hence the rhyolites belong to high-potassium calc-alkaline rocks. All samples have the similar characteristics of REE patterns, which are characterized by relatively poor total REE content (∑REE=103.80×10^-6-194.31×10^-6), significant fractionation of HREE and REE(La/Yb)_N=7.12-10.22) and moderate Eu negative anomalies (δEu=0.34-0.74). The rhyolites are characterized by enrichment of Rb, Th, U, K and LREE, strong depletion of Ba, Sr and HFSE (e.g., Nb, Ta, P and Ti), and relative loss of Nb, Ta. The relatively low Ga/Al ratios indicate that the rhyolites are Ⅰ-type granites of high differentiation. The discrimination diagrams of (Y+Nb)-Rb and (Yb+Ta)-Rb show that the rhyolites were formed on the active continental margin. The comprehensive studies show that the Badaguan area was located on an active continental margin before Early Jurassic, which was affected by the subduction of Mongol-Okhotsk plate towards metamorphic rocks in the Erguna Block.