LASER ⁴⁰Ar/³⁹Ar DATING AND GEOCHEMISTRY OF THE HIGH-Mg# VOLCANIC ROCKS FROM THE TUQUAN BASIN IN CENTRAL DAXINGANLING

A detailed study is carried out on petrology, laser ⁴⁰Ar/³⁹Ar dating and geochemistry of the high-Mg# volcanic rocks from Tuquan Basin in Central Daxinganling region on the ground of their genesis, magmatic origin and geological implication. It is concluded that the volcanic rocks comprise a series of the acid-medium rocks (namely, andesite and dacite) with the age of 136.0±2.0 Ma by the method of laser ⁴⁰Ar/³⁹Ar dating, identical to early Early-Cretaceous. Geochemically, these rocks show a close affinity to adakitic rocks, with SiO₂≥57.50%, high A1₂O₃ (≥16.66%), MgO<3.31%, Mg#>45, low Y(averagely 17.53×10^-6, <18×10^-6) and Yb(averagely 1.84×10^-6, <1.90×10^-6), high Sr(>534×10^-6) and Ba (789×10^-6-925×10^-6), Sr/Y=22.71-73.54, Y/Yb=8.32-11.32, enriched LREE, depleted HREE and without significant Eu anomaly (average Eu/Eu*=0.94). The features listed above imply that the original magma for the studied rocks is most likely resulted from partial melting of the thickened lower crust and in turn contaminated by the basaltic magma. Combined with the tectonic evolution characteristics in this region in Mesozoic, the volcanics from the Tuquan Basin should be derived from partial melting of the thickened lower crust as the result of the collision of Mongol-Okhotsk (Paleo-Pacific) orogenic belt.