Late Neoprotherozoic Granitoid Magmatism of the Baikal-Muya Fold Belt, Ophiolite and Post-Ophiolite Plagiogranites

Abstract

Three different-age series of granitoid veins and dikes of the Baikal–Muya fold belt were studied. Two of them, plagiogranites of the ophiolite complex and the first postophiolitic plagiogranites, are associated with the suprasubduction ophiolites of the eastern Baikal–Muya belt. The third series is represented by hypabyssal tonalite–plagiogranite–leucogranite complex of the Kichera zone in the western Baikal–Muya belt. The composition and isotope-geochemical characteristics (eNd(T) = –0.9; –1.3) of the plagiogranite veins no more than 60 cm thick, and eNd(T) values (–1.8…+ 0.2) of host layered leucocratic gabbros in the Sredne-Mamakan ophiolitic complex are consistent with the previously established suprasubduction nature of the ophiolite association. Tonalites and plagiogranites of the dyke system of the post-ophiolitic magmatic series intersect the dunite–pyroxenite–gabbro banded series of the Sredne-Mamakan ophiolites of the eastern Baikal–Muya fold zone. High Sr/Y ratios and low concentrations of Y and heavy REE indicate that these granitoids are ascribed to the adakite series. LA-ICP-MS study of zircon from post-ophiolitic plagiogranites yields the crystallization age of 629 ± 5 Ma. Sm–Nd isotope-geochemical characteristics of plagiogranitoids (eNd(T) = +2.5; +4.0) in combination with geochemical data confirm their origin during partial melting of a mafic protolith corresponding to the Neoproterozoic oceanic crust. The adakitic granitoids in the Kichera zone of the western Baikal–Muya belt belong to the tonalite–leucogranite differentiated series of the hypabyssal complex, which has no a direct spatial relationship with unambiguous ophiolite associations. The chemical composition and Sm–Nd isotope-geochemical characteristics of these rocks (eNd(T) = +3.2…+7.1) indicate the heterogeneity of the predominantly juvenile island-arc or oceanic Neoproterozoic crust, which experienced partial melting at 595 ± 5 Ma.