LA-ICP-MS U-Pb zircon, columbite-tantalite and Ar-40-Ar-39 muscovite age constraints for the rare-element pegmatite dykes in the Altai orogenic belt, NW China

摘要

The Chinese Altai is renowned for its rich rare-element resources. Nine representative rare-element (REL) pegmatites were dated using LA-ICP-MS and Ar-40-Ar-39 methods. The columbite grains yield a weighted mean Pb-206/U-238 age of 239.6 +/- 3.8 Ma for the Dakalasu (Be-Nb-Ta) pegmatite and concordia U-Pb ages of 258.1 +/- 3.1 Ma and 262.3 +/- 2.5 Ma for the Xiaokalasu (Li-Nb-Ta) pegmatite. The zircons display a weighted mean Pb-206/U-238 age of 198.5 +/- 2.5 Ma for the Husite (Be) pegmatite and concordia U-Pb ages of 194.3 +/- 1.6 Ma and 248.2 +/- 2.2 Ma for the Qunkuer (Be) and Taerlang (barren) pegmatites. The muscovite Ar-40-Ar-39 dating gives plateau ages of 286.4 +/- 1.6 Ma, 297.0 +/- 2.6 Ma, 265.2 +/- 1.5 Ma, 178.8 +/- 1.0 Ma, 162.2 +/- 0.9 Ma, 237.7 +/- 1.3 Ma, 237.4 +/- 1.2 Ma and 231.9 +/- 1.2 Ma for the Talate (Li-Be-Nb-Ta), Baicheng (Nb-Ta), Kangmunagong (barren), Husite (Be), Qunkuer (Be-Nb-Ta), Xiaokalasu (Li-Nb-Ta), Weizigou (Be) and Taerlang (barren) pegmatites, respectively. These new ages coupled with previous geochronological work suggest that the REL pegmatites in the Chinese Altai formed during early Permain - Late Jurassic time. The REL pegmatites located in the Central Altaishan terrane are younger than those in the Qiongkuer-Abagong terrane, showing a correlation with the coeval and adjacent granites. The formation of the REL pegmatites and these granites indicates frequent and strong magmatic activity in the post-orogenic and anorogenic setting. The spatial and temporal distribution of pegmatites and granites reveals a magmatism path from the SE (of age early-middle Permian), to the NW (middle Permian - Middle Triassic) and finally to the central part (Middle Triassic - Jurassic) of the Chinese Altai.