PETROLOGY AND GEOCHEMISTRY OF CRUSTALLY CONTAMINATED KOMATIITIC BASALTS FROM THE VETRENY BELT, SOUTHEASTERN BALTIC SHIELD: EVIDENCE FOR AN EARLY PROTEROZOIC MANTLE PLUME BENEATH RIFTED ARCHEAN CONTINENTAL LITHOSPHERE

Abstract

New isotope and trace element data are presented for komatiitic basalts and a related peridotite Vinela Dike from the large Vetreny Belt in the southeastern Baltic Shield. The MgO contents of the erupted and intruded magmas are inferred to increase from 13 to 17% towards the center of the belt, which implies the respective increase in liquidus temperatures from 1370 to 1440°C. The elevated liquidus temperatures suggest that the source of the komatiite magmas had a substantially higher potential temperature (1630°C) than the ambient mantle (1480°C) and are regarded as evidence for the existence of a mantle plume underlying the region at ∼2.45 Ga. Parental magmas to the lavas and the Vinela Dike were shown to have komatiite composition and were derived from a long-term LREE-depleted mantle source with ϵNd(T) of ca. +2.6. The evolution of these magmas en route to the surface was mainly controlled by 4–15% contamination with older felsic crustal rocks, which resulted in substantial changes in incompatible trace element and isotope ratios. The obtained SmNd internal isochron ages of 2449 ± 35 and 2410 ± 34 Ma for the lavas, 2430 ± 174 Ma for the Vinela Dike, and a whole-rock PbPb age of 2424 ± 178 Ma for the lavas together with a UPb zircon age of 2437 ± 3 Ma are identical to the reported UPb zircon and baddeleyite ages for numerous mafic-ultramafic layered intrusions in central and northern Karelia. From their chemical and isotope similarities, it is likely that these rocks had allied parental magmas. These magmas may have been emplaced in a continental rift setting during the interaction of a mantle plume and continental crust. Impinging of a plume head beneath the continental lithosphere resulted in its thinning, stretching, and rifting but failed to open a new ocean. This extensive magmatic event was responsible for a substantial contribution of early Proterozoic juvenile material to the Archean continental crust in the Baltic Shield.