Pyroxenes of the Khibiny alkaline pluton, Kola Peninsula

Abstract

Seven pyroxene varieties were identified in nepheline syenites and foidolites of the Khibiny pluton: enstatite, ferrosilite, diopside, hedenbergite, augite, aegirine-augite, and aegirine. Enstatite and augite are typical of alkaline and ultramafic rocks of dike series. Ferrosilite was found in country quartzitic hornfels. Diopside is a rock-forming mineral in alkaline and ultramafic rocks, alkali gabbroids, hornfels in xenoliths of volcanic and sedimentary rocks and foyaite, melteigite-urtite that assimilate them, and certain hydrothermal pegmatite veins. Hedenbergite was noted in hornfels from xenoliths of volcanic and sedimentary rocks and in a hydrothermal pegmatite vein at Mount Eveslogchorr. Aegirine-augite is the predominant pyroxene in all types of nepheline syenites, phonolites and tinguaites, foidolites, alkaline and ultramafic rocks of dike series, fenitized wall rocks surrounding the pluton, and xenoliths of Devonian volcanic and sedimentary rocks. Aegirine is an abundant primary or, more often, secondary mineral in nepheline syenites, foidolites, and hydrothermal pegmatite veins. It occurs as separate crystals, outer zones of diopside and aegirine-augite crystals, and homoaxial pseudomorphs after Na-Ca amphiboles. Microprobe analyses of 265 pyroxenes samples allowed us to distinguish ten principal trends of isomorphic replacement and corresponding typomorphic features of pyroxenes. Compositional variations in clinopyroxenes along the sampled 35-km profile from the margin of the Khibiny pluton to its center confirm the symmetric zoning of the foyaite pluton relative to semicircular faults of the Minor Arc and the Main (Central) Ring marked by Devonian volcanic and sedimentary rocks, foidolites, and related metasomatic rocks (rischorrite, albitite, and aegirinite). Changes in the composition of pyroxenes are explained mainly by the redistribution of elements between coexisting minerals of foyaites in the process of their intense differentiation under the effect of foidolite melts that have intruded into the circular fault zones. ton: the augite cores are surrounded by an outer zone of aegirine-augite and/or aegirine. Rare grains of Cr-bearing augite (Ca 0.75-0.79 Na 0.08-0.18 Mg 0.00-0.09 ) 1.00 were found in a spinel peridotite nodule from a volcanic pipe in urtite of Mount Namuaiv (Arzamastsev et al., 1988). In carbonated monchiquite cement of the same volcanic pipe, augite

Rare Earth Mineralization in Igneous Rocks: Sources and Processes

“…Carbonatites with appreciable REE mineralization have been reported at Khibiny and Vuoriyarvi (Kola, Russia), Ozerny and Arshan (Siberia, Russia), Fen (Norway), Khanneshin (Afghanistan), Amba Dongar (India), Barra Bussen and Sakharov (1972), Epshtein et al (1972), and Yakovenchuk et al (2005) . 4), some containing very high levels of REEs and readily amenable to processing (TABLE 1 in Chakhmouradian and Wall 2012).…”

Self-Organization of the Khibiny Alkaline Massif (Kola Peninsula, Russia)

“…The clinopyroxenes of the Khibiny massif, predominating in the majority of rocks, are represented by diopside, hedenbergite, augite, aegirine-augite and aegirine (Yakovenchuk et al, 2005;Yakovenchuk et al, 2008). Diopside is a rock-forming mineral of alkaliultrabasic rocks, alkali-feldspar trachytes, melteigite-urtite, metamorphosed to hornfels volcanogenic-sedimentary rocks of basalt composition and their host foyaite.…”

“…5 and 6). Silica content in nepheline changes according temperature of rock forming (Yakovenchuk et al, 2010c): 4.3 a.p.f.u in hornfels and foyaite (about 900 °C), 4.2 a.p.f.u in foidolite (about 500 °C), 4.1 a.p.f.u in rischorrite and apatite-nepheline rock (about 400 °C), and 4.0 a.p.f.u in pegmatite-hydrothermal veins (about 200 °C). The chemical composition of nepheline in foyaite samples picked along the A-B-C-D-E-F profile (see Fig.…”

Rock-Forming feldspars of the Khibiny alkaline pluton, Kola Peninsula, Russia

“…In the light of the original and published data (Ivanyuk et al, 1996(Ivanyuk et al, , 2006Goryainov et al, 1998Goryainov et al, , 2007Yakovenchuk et al, 2005Yakovenchuk et al, , 2008Konopleva et al, 2008), the formation of the Khibiny pluton comprises the following sequence of events:…”

“…Most of these rocks are composed of alkali feld spars, nepheline, and clinopyroxene with striking typomorphism (Borutsky, 1988(Borutsky, , 2005Yakovenchuk et al, 2005Yakovenchuk et al, , 2008. Microcline and orthoclase per thite are the major minerals of foyaite and alkali syen ite of the Khibiny pluton; perthite free granular albite is sporadic.…”