Mesoproterozoic orangeites (Kimberlites II) of West Karelia: Mineralogy, geochemistry, and Sr-Nd isotope composition

Abstract

Mineralogical and petrological-geochemical features of the Mesoproterozoic (1.23–1.20 Ga) alkaline ultrabasic rocks from the Kostomuksha-Taloveis (Russia) and Lentiira-Kuhmo (Finland) areas, West Karelia, have been studied. In terms of mineralogy and geochemistry, these rocks more resemble group II kimberlites of South Africa (orangeites) than olivine lamproites or ultramafic lamprophyres. On the basis of phenocryst composition, the studied orangeites are divided into three types: Cpx-Phl-Ol, Phl-Ol, and Phl-Carb orangeites. The Cpx-Phl-Ol orangeites from the Kostomuksha cluster clearly differ from analogous rocks from the Lentiira cluster. The composition of Phl-Ol orangeites is indicative of derivation by intense fractional crystallization; Cpx-Phl-Ol orangeites from the Kostomuksha area display evidence of intense lithosphere assimilation. The Phl-Carb orangeites from the Taloveis cluster and Cpx-Ol orangeites from the Lentiira cluster most closely approximate primary melts. The Kostomuksha orangeites are characterized by lowto moderate-radiogenic (87Sr/86Sr)1220 ratio varying from 0.7038 to 0.7067. The Phl-Carb orangeites of Taloveis have less radiogenic Nd isotope composition (ɛNd from −11 to −12) as compared to the Cpx-Phl-Ol and Phl-Ol orangeites of Kostomuksha (ɛNd from −6.9 to −9.4). The Cpx-Phl-Ol orangeites from Lentiira contain fresh olivine. By morphology and composition, there are three olivine generations: (1) large rounded, usually zoned crystals with Fo 92 core, 0.33–0.37 wt % NiO, and 0.03–0.04 wt% CaO, which are interpreted as xenocrysts from depleted peridotites; (2) anhedral rounded zoned olivines of intermediate size with Fo 82–83 cores, 0.03–0.05 wt % CaO, 0.12–0.17 wt % NiO, and up to 0.40 wt % MnO. These olivines were entrapped by orangeite melt and presumably represent a cumulate of basaltic melts or were derived from metasomatized peridotites; (3) fine euhedral olivines and xenocryst rims corresponding to Fo 88–89 with 0.10–0.42 wt % CaO, 0.14–0.35 wt % NiO, and up to 0.07–0.21 wt % MnO; their origin was presumably related to the crystallization from kimberlite melt. The calculation of $$f_{O_2 }$$ of kimberlite melt during crystallization of perovskites using Nb-Fe perovskite oxyba-rometer showed that Cpx-Phl-Ol orangeites of Kostomuksha and orangeites of Lentiira crystallized at similar oxygen fugacities corresponding to ΔNNO from −3.3 to −1.1 and from −3.3 to −0.9, respectively. The Sm-Nd and Rb-Sr isotope study provided evidence for the contribution from ancient enriched source in the genesis of the orangeites. It was proposed that their mantle source was formed in two stages: (1) metasomatic reworking of previously depleted lithospheric source at the Karelian Craton base during Paleoproterozoic orogenic events 2.1–2.0 Ga ago; (2) extension-related generation of orangeite melts 1.27–1.20 Ga ago.