Devonian Gabbroids of the NW Rudny Altai as Indicators of the Lithospheric Mantle Contribution to Petrogenesis and Ore Formation

The paper reports new petrogeochemical, Sm–Nd isotopic, and U–Pb age data on the basic intrusive rocks of the NW Rudny Altai, which are simultaneous to the rift-related bimodal basalt–rhyolite volcanism on the Devonian continental margin of the Siberian continent. The gabbroids belong to the low-Ti, high-Mg tholeiitic series, which is partially differentiated and do not form a continuous series with Devonian granitoids. The rocks have the low concentrations of incompatible elements, Nb/YРМ <1, with a strong Nb depletion relative to LREEs and Th (Nb/Nb* = 0.2–0.3), combined with La/Smn >1.5, Hf/Hf* ~0.9 and εNd (T) from +2.9 to +4. The geochemical characteristics of the rocks are intermediate between basalts of continental-margin island arcs and back-arc basins. U–Pb zircon isotopic dating (LA-ICP-MS) of the gabbroids yields ~383 Ma. Trace-element variations suggest the formation of primary basic magmas from a slightly depleted mantle that was metasomatically modified by subduction fluids/melts and/or by plume melts long before the generation of basic magmas. The Nd isotopic composition of the gabbroids is consistent with the evolution of the Precambrian oceanic lithospheric mantle, which was incorporated into the folded framing of the Siberian continent during the Early Paleozoic accretionary stage. The obtained results confirm the hypothesis that the ore sources of the Rudny Altai base-metal deposits are related to the sub-continental lithospheric mantle. According to the geodynamic scenarios, the Devonian basic magmas were formed in an extensional setting during reactivation of the marginal–continental lithosphere, which, according to our interpretation, corresponds to the riftogenic (initial) stage of the development of the West Pacific-type back-arc basins.