Isotopic, geochemical, and geochronological constraints on the origin and evolution of Cenozoic volcanism, Baikal Rift Zone, Siberia

Abstract

The Cenozoic Baikal Rift Zone, one of the world's major continental rifts, is expressed as an approximately 1800 km-long southwest-northeast oriented system situated along a suture between the Archean Siberian craton and younger Paleozoic fold belts. Oligocene to Recent age volcanic rocks and associated mantle xenoliths are distributed in three major regions of the rift zone: the Udokan volcanic field in the northeast, the Vitim volcanic field -200 km southeast of Lake Baikal, and a broad zone of volcanism to the southwest of Lake Baikal. This large-scale study investigates the geochemical evolution of the Baikal Rift Zone volcanism using major element, trace element, and isotopic (Sr, Nd, and Pb) geochemistry, as well as 40Ar/39Ar geochronology. In addition, a detailed isotopic study of megacrysts provides insight into the relationship between megacrysts and host lavas and the nature of the lithospheric mantle. Results of this study suggest that the source region for Baikal Rift Zone volcanic rocks is remarkably homogeneous considering the large region and time span over which these rocks erupted, and that crustal contamination has not been a significant controlling factor. The Tunka Basin, Oka Plateau, and Tuva volcanic fields in the western Baikal Rift Zone, and the Vitim volcanic field southeast of Lake Baikal, have a fairly restricted compositional range from basaltic andesite, alkaline basalt and hawaiite, to basanite and mugearite. In the Udokan volcanic field in the northeast, a more evolved series of rocks ranging to trachytic compositions erupted across the suture between the Archean craton and Paleozoic fold belts. Major and trace elements indicate that fractional crystallization of pyroxene, olivine, and plagioclase produced the observed range in compositions. The trace element and isotopic compositions of Baikal Rift Zone volcanic rocks are comparable to those observed in ocean island basalts. The Sr, Nd, and Pb isotope systematics can best be explained by mixing between the end-member mantle components DMM ( typical of the depleted source of MORB mantle) and EMI (enriched mantle type 1). Megacrysts of clinopyroxene, amphibole, garnet, biotite, and anorthoclase mirror the isotope systematics of the host volcanic rocks, and suggest that the megacrysts were derived from a melt closely related to their host rocks. Furthermore, megacrysts of clinopyroxene, as well as Cr-diopside from spinel lherzolites, have an unradiogenic Pb isotopic composition that may be representative of the EMI-like composition of the subcontinental lithospheric mantle.