GEOCHEMICAL CHARACTERISTICS AND LASER ⁴⁰Ar/³⁹Ar DATING OF THE VOLCANICS FROM THE MANITU FORMATION OF TUQUAN BASIN IN CENTRAL DAXINGANLING

The volcanic rocks of Manitu Formation in the Tuquan Basin of Central Daxinganling region comprise a series of acid, acid-intermediate and intermediate rocks, namely, trachyandesite, andesite, dacite and rhyolite, with age of 138.0±3.0 Ma by Laser ⁴⁰Ar/³⁹Ar dating, identical to the early Early-Cretaceous. The geochemical characteristics of these rocks display a typical high-K calc-alkaline series with an average ΣREE of 117×10^-6, 136×10^-6 and 131×10^-6, average (La/Yb)_N of 6.38, 7.62 and 6.27, and average δEu of 0.88, 0.85 and 0.40 for the intermediate, acid-intermediate and acid rocks, respectively. All the rocks share the similar geochemical characteristics of significant enrichment in Rb, Ba, Th, U, K (LILE) and LREE, depletion in Nb, Ta, P and Ti (HSFE), indicating a crustal magmatic genesis. The chorology, geochemistry, magmatic genesis and tectonic background of the Manitu Formation largely distributed in Daxinganling have a remarkable variation by different areas on the ground of a great number of available information. Therefore, the volcanic rocks of the Manitu Formation in this study are most likely to be derived from partial melting of a thickened lower crust as a result of collision setting of Mongol-Okhotsk orogenic belt.