2.2 ga mafic sills in the central part of the Karelian Сraton: U-Pb dating of zircons and geochemistry of gabbro-dolerites in the lake Bolshozero area

Abstract

New age and compositional data for mafic igneous intrusions in the Bolshozero area, western Karelia, are reported. In this area, Jatulian gabbro-dolerites form several bodies that cut through the host Archean and Paleoproterozoic supracrustal and granitoid rocks. Paleoproterozoic mafic igneous rocks vary in composition from melanogabbr to leucocratic gabbro-dolerites and gabbro-pegmatites. Such variations are probably caused by in situ fractional crystallization of the mafic melt, and point to a gently sloping primary orientation of the bodies. The gabbro-dolerites were altered under greenschist –epidote-amphibolite facies, which caused the replacement of primary magmatic minerals by amphibole, epidote, and titanite. According to the results of U-Pb SIMS (SHRIMP II, VSEGEI) dating of zircons extracted from the gabbro-pegmatites, the age of their magmatic crystallization is considered to be 2225 ± 20 Ma. This dating supports the assumption about the Jatulian age of magmatic crystallization of gabbro-dolerites in Bolshozero area. In their chemical composition, the gabbro-dolerites of Bolshozero area show a subalkaline series affinity, with the exception of gabbro-pegmatites, which are strongly enriched in Na2O, LILE, and LREE. Trace element distribution, LREE enrichment and Nb depletion suggest that crustal contamination has contributed significantly to the evolution of the primary melts. This assumption is supported also by the Nd isotopic composition data (εNd2200 = –0.3). The analysis of the available geochemical and age data and comparison with mafic igneous rocks of similar age located in the western part of the Karelian Craton suggest that mafic sills in Bolshozero area belong to the large igneous province aged ca. 2.22 Ga, which marks the termination of the Paleoproterozoic endogenic lull.