Olivine and Clinopyroxene Phenocrysts as a Proxy for the Origin and Crustal Evolution of Primary Mantle Melts: a Case Study of 2.40 Ga Mafic Sills in the Kola–Norwegian Terrane, Northern Fennoscandia

Abstract

New petrographic, geochemical, and isotopic (Sr, Nd, and δ18О) data on olivine and pyroxene phenocrysts provide constraints on the composition and crustal evolution of primary melts of Paleoproterozoic (2.40 Ga) picrodoleritic sills in the northwest Kola province, Fennoscandian Shield. The picrodolerites form differentiated sills with S-shaped compositional profiles. Their chilled margins comprise porphyritic picrodolerite (upper margin) and olivine gabbronorite (bottom) with olivine and clinopyroxene phenocrysts. Analysis of the available data allows us to recognize three main stages in the crystallization of mineral assemblages. The central parts of large (up to 2 mm) olivine phenocrysts (Ol–1–C) crystallized at the early stage. This olivine (Mg# 85–92) is enriched in Ni (from 2845 to 3419 ppm), has stable Ni/Mg ratio, low Ti, Mn and Co concentrations, and contains tiny (up to 10 μm) diopside–spinel dendritic lamella that probably originated due to the exsolution from high Ca- and Cr- primary magmatic olivine. All these features of Ol–1–C are typical of olivine from primitive picritic and komatiitic magmas (De Hoog et al., 2010; Asafov et al., 2018). Ol–1–C contains large (up to 0.25 mm) crystalline inclusions of high-Al enstatite (Mg# 80–88) and clinopyroxene (Mg# 82–90), occasionally in association with Ti-pargasite and chromian spinel (60.4 wt.% Al2O3). These inclusions are regarded as microxenoliths of wall rock that were captured by primary melt at depths more than 30 km and preserved due to the conservation in magmatic olivine. The second stage was responsible for the crystallization of Ol–1 rim (Ol–1–R), small (up to 0.3 mm) olivine (Ol–2, Mg# 76–85) grains, and central parts of large (up to 1.5 mm) clinopyroxene (Cpx–C) phenocrysts in the mid-crustal transitional magma chamber (at a depth of 15–20 km) at 1160–1350°C. At the third stage, Cpx–C phenocrysts were overgrown by low-Mg rims (Mg# 70–72) similar in composition to the groundmass clinopyroxene from chilled picrodolerite and gabbro-dolerite in the central parts of the sills. This stage likely completed the evolution of picrodoleritic magma and occurred in the upper crust at a depth of about 5 km. All stages of picrodoleritic magma crystallization were accompanied by contamination. Primary melts were contaminated by upper mantle and/or lower crust as recognized from xenocrystic inclusions in Ol–1–C. The second contamination stage is supported by the negative values of eNd(2.40) = –1.1 in clinopyroxene phenocrysts. At the third stage, contamination likely occurred in the upper crust when ascending melts filled gentle fractures. This caused vertical whole-rock Nd heterogeneity in the sills (Erofeeva et al., 2019), and difference in Nd isotopic composition of clinopyroxene phenocrysts and doleritic groundmass. It was also recognized that residual evolved melts are enriched in radiogenic strontium but have neodymium isotopic composition similar to other samples. It could be explained by the interaction of the melts with fluid formed via decomposition of biotite from surrounding gneisses under the effect of high-temperature melts.