Nd-Sr Isotopic Study of Magmatic Rocks and Ar-40/Ar-39 Dating of the Mafic Dike of the Proterozoic Ulan-Sar'dag Ophiolite Melange (Southern Siberia, East Sayan, Middle Belt, Russia)

We report the first radiogenic Nd-Sr isotope data in the magmatic rocks island-arc ophiolite assemblage from the middle branch of the East Sayan ophiolite complexes to better constrain geodynamic processes in this segment of the CAOB in southern central Siberia. The magmatic rocks belong to the following geochemical types: (1) Ensimatic island-arc boninites; (2) island-arc assemblage; (3) enriched basalts of mid-ocean ridges; and (4) oceanic island-like basalts. The boninites have a positive value epsilon Nd (T), which is generated from a depleted mantle source (N-MORB). The island-arc assemblage has negative or slightly positive epsilon Nd (T) and was formed from an enriched mantle source due to the subduction of terrigenous rocks. The source of the terrigenous material was most likely the rocks of the Archean TTG (Trondhjemite Tonalite Granodiorite) complex of the Gargan block. Isotopic ratios for E-MOR and OIB-like basalts are characterized by positive or slightly negative values of epsilon Nd (T). The mafic dike, which crosscut ophiolite rocks, corresponds to OIB-like basalts. The values of epsilon Nd (T), measured Sr-87/Sr-86 and I (Sr), in the mafic dike correspond to the EM I mantle source. The E-MOR and OIB-like basalts appear to be formed in late-stage asthenospheric mantle melting via the decompression melting processes. The obtained isotope geochemical data for the E-MOR and OIB-like basalts probably indicate the mixing of island-arc melts with asthenospheric melts. We undertook Ar-40/Ar-39 dating of the mafic dike, which crosscut the ophiolite unit. The mafic dike has a whole-rock Ar-40/Ar-39 weighted mean plateau age of 799 +/- 11 Ma. The dating constrains the minimum age of the ophiolite and island-arc magmatism in the region.