Composition and sources of basalts in the Late Paleozoic rift system of Central Asia: Geochemical and isotopic data

The Late Paleozoic rift system of Central Asia developed from the Late Carboniferous throughout the Late Permian, i.e., over a time period from 300 to 250 Ma. Differing in age, individual rift zones of this system are composed of petrographically similar basaltic and bimodal basalt-comendite-pantellerite associations. The basalts of zones of different ages and structural settings display many analogous compositional features. All of them affiliate with the alkaline K-Na petrochemical series, and variations in their major-element chemistry are accounted for by fractionation processes. The most significant differences between the rocks of distinct rift zones are their isotopic signatures. For the rocks of the Gobi-Tien Shan zone, these are -15 < ε_Sr < +9 and +8 > ε_Nd > +2. The analogous values for the rocks of the Gobi-Altai and northern Mongolia zones are -3 < ε_Sr < +18 and +6 > ε_Nd > 0, +2 < ε_Sr < +12 and +1.6 > ε_Nd > -0.5, respectively. These variations are related to the involvement of two isotopic sources in the melting processes. One of them was the depleted upper mantle, and the other was the mantle of the EM-II type enriched in radiogenic Sr. The latter source generally dominated over the former and made a more significant contribution to the composition of the mantle plume responsible for the origin of the rift system.