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Petrology of mafic lavas within the Onega plateau, central Karelia: evidence for 2.0 Ga plume-related continental crustal growth in the Baltic Shield

Abstract:

The Onega plateau consistitues part of a vast continental flood basalt province in the SE Baltic Shield. It consists of Jatulian-Ludikovian submarine volcanic, volcaniclastic and sedimentary sequences attaining in places 4.5 km in thickness. The parental magmas of the lavas contained ~ 10% MgO and were derived from melts generated in the garnet stability field at depths 80-100 km. The Sm-Nd mineral and Pb-Pb whole-rock isochron ages of 1975 ± 24 and 1980 ± 57 Ma for the upper part of the plateau and a SHRIMP U-Pb zircon age of 1976 ± 9 Ma for its lower part imply the formation of the entire sequence within a short time span. These ages coincide with those of picrites in the Pechenga-Imandra belt (the Kola Pininsula) and komatiites and basalts in the Karasjok-Kittila belt (Norway and Finnmark). Togehte with lithostratigraphic, chemical and isotope evidence, these ages suggest the derivation of the three provinces from a single large (~600 000 km2 of the Baltic Shield and represent a major contribution of juvenile material to the existing continental crust at 2.0 Ga. The uppermost Onega plateau lavas have high (Nb/Th)N = 1.4-2.4, (Nb/La)N = 1.1-1.3, positive εNd(T) of + 3.2 and unradiogenic Pb-isotope composition (μ1 = 8.57), comparable with those of modern oceanic plume-derived magmas (oceanic flood basalt and ocean island basalt). these parameters are regarded as source characteristics. the lower sequences have (Nb/Th)N = 0.58-1.2, (Nb/La)N = 0.52-0.88 and εNd(T) = -2.6. They have experienced mixing with 10-30% of continental crust and resemble contaminated lavas from other continental flood basalt provinces. the estimated Nb/U ratios of 53 ± 4 in the uncontaminated rocks are similar to those found in the modern mantle (α47) suggesting that by 2.0 Ga a volume of continental crust similar to the present-day value already existed.