Genesis and Geological Significance of Late Ordovician Nb-rich Basalts from Tanjianshan Group in Saishitengshan Mountain, Northern Margin of Qaidam Tectonic belt

The crystallization age of meta–basalts from Tanjianshan Group in Saishitengshan mountain, in the western part of the northern margin of Qaidam tectonic belt, was 444±4 Ma, which has the characteristics of rich Na₂O, poor K₂O, high TiO₂, Nb, and low LILE/HFSE and HREE/HFSE ratios. The chondrite–normalized REE distribution curve shows a slightly right–leaning distribution pattern with relatively enriched LREE and flat HREE. On the primitive mantle–normalized trace element diagrams, Nb and Ta show weak positive anomalies, which is consistent with the geochemical characteristics of Nb–rich basalts. The comprehensive analysis shows that the magma sources of Nb–rich basalt in Saishiteng Mountain maybe the spinel–phase lherzolite, which is the product of the mixture of upwelling asthenosphere mantle and depleted mantle wedge at the edge of the back–arc basin near the island arc side caused by the steep angleroll–back of the subduction oceanic slab. It indicates that the western part of the northern margin of Qaidam basin was under the stage of back–arc extension during the late Ordovician, and the continental collision had not begun. Combined with the existing regional data, it is considered that the Tanjianshan Group in the northern margin of Qaidam Basin were the volcanic–sedimentary products of different periods and different tectonic settings in the process of ocean–continent transition from Late Cambrian to Early–middle Silurian,and had experienced the whole subduction process from oceanic subduction to continental collision. Due to tectonic mélange, various types of rocks were occurred in the northern margin of Qaidam basin.