ABSTRACT
Late Cenozoic volcanic activity in the Udokan ridge is related to reactivation of the Kodar-Udokan and Chukchudu weak zones. The former locates in the area of the Baikal Rift System (BRS) overlapping the Proterozoic foredeep in the Aldan shield. The latter joins the Stanovoy suture which belongs to the Olekma-Stanovaya Orogenic System (OSOS) reactivated due to collision of the Bonin and Honshu arcs. Diverse volcanic pulses in the weak zones indicate independent tectonic activity in the BRS and OSOS. Trace element abundances in lavas from both zones have been studied by ICP-MS technique. The initial 14 Ma olivine melaleucetite magmas in the Kodar-Udokan zone were inferred to represent phlogopite-bearing mantle source depleted in Pb and Y (Ce/Pb = 20; Y/Ho = 22). Alkali olivine basalts and basanites erupted between 3.2 and 2.4 Ma exhibited hybrids of two or three end-members including, beside the Pb-Y depleted component, those of the lower crust and similar to the primitive mantle. In the Chukchudu zone, the 4.0–3.5 Ma basanites were dominated by the OIB-like component. During the last 1.8 Ma, the low-Y material melted. It is suggested that lavas of the Kodar-Udokan zone represent the continental mantle which underlain the Aldan shield in the Archean, but partly or completely modified by the Proterozoic processes of deep tectono-thermal reactivation. Lavas of the Chukchudu zone exhibit lithospheric mantle material formed during deep Phanerozoic convection and/or mantle material of the Baikal-Vitim terrane subducted beneath the Archean crust of the Aldan shield in the Paleozoic.
