关键词: 大兴安岭南部, 满克头鄂博组, 白音高老组, 火山岩, 岩石成因

Abstract: The Baiyinshana basin is an important Mesozoic uranium-producing volcanic basin in the southern Great Xing’an Range, where several volcanic-type uranium occurrences have been discovered; However, there is a lack of research on the petrogenesis of the volcanic rocks and the tectonic background of their formation. In this paper, zircon U-Pb geochronology, geochemistry, and Hf isotope analyses were conducted on the Mesozoic volcanic rocks of the Manketouebo and Baiyingaolao Formations in the Baiyinshana basin, as well as on the granitic porphyries that intrude them. The dating results show that the ages of rhyolites from the Manketouebo Formation, the Baiyingaolao Formation, and the granite porphyry are 154.3-152.3, 137.7-129.8 and 129.6-129.3 Ma, respectively. Those rocks are characterized by high SiO2, moderate alkali, low CaO and MgO, and are strongly peraluminous. They are enriched in rare earth elements (REEs) and large-ion lithophile elements (LILEs), but depletion in heavy rare earth elements (HREEs) and high-field-strength elements (HFSEs). combined with theIR low P2O5 contents (0.01%-0.04%), 10 000Ga/Al ratios (1.90-2.79), and low zircon saturation temperatures (averaging 785.2 ℃), these features suggest that they belong to I-type granites. The εHf (t) values of the rocks range from 6.1 to 7.9, 5.5 to 9.8 and 6.4 to 9.5, and their TDM2 ages range from 812 to 700, 834 to 562 and 774 to 579 Ma, respectively. These data indicate that the magma was derived from crustal material that originated from the Neoproterozoic depleted mantle. Based on regional tectonic evolution and tectonic discrimination diagrams, it is determined that the study area was in an extensional tectonic environment during the Late Juraassic to Early Cretaceous. This extension may be related to lithospheric extension following the closure of the Mongol-Okhotsk Ocean in the Late Jurassic. Furthermore, the Early Cretaceous was influenced by the superimposition of the Mongol-Okhotsk and the Paleo-Pacific tectonic domains.

Key words: southern Great Xing’an Range, Manketouebo Formation, Baiyingaolao Formation, volcanic rocks, petrogenesis