Cretaceous-Paleogene basaltic rocks of the Tuyon basin, NW China and the Kyrgyz Tian Shan: the trace of a small plume

The Tuyon basin, NW China, and the central portion of the Kyrgyz Tian Shan preserve a volumetrically small series of basaltic extrusive and intrusive units emplaced primarily in Mesozoic-Paleogene sedimentary rocks. Four units from the Tuyon basin yield 40Ar/39Ar ages of 67, 59, 46 and between similar to 110 and 120 Ma. Major and trace element analysis combined with Sr, Nd and Pb isotopic data suggest that these units were derived from apparently ocean island basalts (OIB)-like mantle melts, which have characteristics close to asthenospheric PREMA, mixed with an enriched component in the continental mantle lithosphere or emplaced with some crustal contamination. Two basalts from Kyrgyzstan were also dated by 40Ar/39Ar as similar to 54 Ma; these results agree with previous K-Ar analysis which show that most Kyrgyz basalts were emplaced during the Eocene. Major element data from the Kyrgyz basalts (Knauf et al., 1980; Fortuna et al., 1994) show strong similarities with the Tuyon data, suggesting that the more extensive Tuyon data can be taken as representative of the entire basaltic suite. There is little geological data to support either Late Cretaceous-Paleogene rifting or the presence of a large plume beneath the region. Instead, these magmas point to the presence of a small thermally anomalous region in the mantle beneath the central Tian Shan which might best be described as a small plume which probably rooted at a shallow level in the asthenosphere. (C) 2000 Elsevier Science B.V. All rights reserved.