Vol 32, No 3 (2024)

Petrographic, mineralogical, geochemical, isotope-geochemical studies of granophyres and host ferrogabbro, quartz ferromontsogabbro, quartz montsodiorites, and quartz monzonites in the Mesoproterozoic Valaam sill in the Ladoga Graben on the Karelian craton have been carried out. The sill is poorly layered: ferrogabbros are common in the lower part of the sill, the middle part consists of quartz gabbro-monzonites and quartz monzonites, granophyres form a network of veins mainly in the upper part of the sill. Geochemical features of ferrogabbro, iron-rich composition of olivine and pyroxene, low Ca composition of plagioclase indicate evolution along the Fenner trend. Granophyres have petro- and geochemical characteristics of anorogenic alkaline granites, are characterised by negative Eu/Eu* = = 0.15–0.49 and REE distribution similar to those of granophyres of layered intrusives. All rocks of the sill are characterised by a similar isotopic composition of Sr (⁸⁷Sr/⁸⁶Sr)_T = 0.7043–0.7066, and ε_Nd values ranging from –9.6 to –11.2. Model calculations show that fractional crystallisation can lead the initial ferrogabbro melt into immiscibility. Ilmenite-magnetite-silicate microstructures have been identified in ferrogabbro and ferromontzogabbro from the sill; similar microstructures in layered intrusives are considered evidence for immiscibility of Fe-enriched and Si-enriched liquids (Holness et al., 2011; Dong et al., 2013). The segregation of the high-silica melt may have occurred in a crustal chamber at around 350 MPa and 960^oC; the sill formation at around 70 MPa injected magma in the form of a crystalline mush through which acidic melt migrated. This melt underwent fractional crystallisation and reacted with host minerals. At the level of sill formation, it crystallised under supercooling into granophyre aggregates. The example of the Valaam sill shows that after fractionation according to the classical Fenner trend reaches the final composition – ferrogabbro, its continuation with a conjugate decrease in SiO₂ and Fe contents can be associated with incomplete separation and mixing of iron-rich melts and separated acidic melt. Such a mechanism can be realised during the formation of the mafic part of AMCG-type massifs.

The paper reports the study of geochemistry, mineral-phase assemblages of rocks of the Ary-Bulak ongonite massif, compositions of major, minor and accessory minerals (quartz, feldspars, topaz, zinnwaldite, prosopite, rare Ca–Al-fluorides, W-ixiolite, columbite, zircon, cassiterite, and fluocerite), fluoride–calcium (F-Ca) phase, and fluorite formed from it. The rock-forming minerals of porphyritic ongonites are quartz, albite and sanidine, and minor minerals are topaz and zinnwaldite. The ongonitic matrix is composed of a quartz–sanidine–albite assemblage with micron-sized needle-shaped topaz crystals. In transitional porphyritic rocks and in the endocontact aphyric zone, the interstices between matrix minerals are filled with a F-Ca phase formed from a F-Ca (fluoritic) stoichiometric melt. Fluoride–silicate liquid immiscibility in ongonitic magma and fluid-magmatic processes led to the redistribution of REE, Y, and many trace elements between melts, fluids, minerals and a contrasting change in mineral-phase assemblages in the rocks. This is associated with the appearance of M-type (T_{1 La–Nd}, T_{4 Er–Lu}) and W-type (T_{3 Gd–Ho}) tetrad effects in the chondrite-normalized REE patterns of rocks. Degassing of magmatic fluids through the endocontact aphyric zone was accompanied by the crystallization of Sr-bearing prosopite and hydrous Ca–Al-fluorides. Aphyric rocks, compared to porphyritic ongonites and porphyritic transitional rocks, are enriched in H₂O, Sr, Ba, Rb, Sn, W, Ta, Be, Zr, Hf, Sb, As, Sc, but contain less Li, Pb, Zn, Y and REE. During the effect of magmatic fluids on rocks enriched in Ca and F, especially in the endocontact aphyric zone, albite was partially or completely replaced by the F-Ca phase and kaolinite, and the F-Ca phase recrystallized into aggregates of micron-sized grains of stoichiometric fluorite without trace elements. Rb-Cs mica also crystallized in the rim of zinnwaldite laths, the zones of which maximally enriched in rubidium with the cation relation Rb > K > Cs may be a new mineral. The geochemistry of the rocks, the features of their mineral-phase assemblages, the compositional evolution of the minerals and the F-Ca phase are a consequence of the formation of the Ary-Bulak massif from ongonitic magma during a fluid-magmatic process complicated by fluoride–silicate liquid immiscibility with the participation of fluoritic and other fluoride melts, as well as magmatic fluids of P-Q and the first types.

A novel petrogenetic scheme is discussed for the formation of a melilite leucite clinopyroxenite body from an alkaline–ultrabasic paralava in the Purtovino area. Its protolith was likely a mixture of Upper Permian sedimentary rocks (aleurolite, marl, among others). Degassing, evaporation, and thermal (contact) metamorphism have significantly influenced the petrogenesis to produce a wide diversity of species present in mineral associations. The crystallization of paralava in a shallow setting was accompanied by an intense degassing and vesiculation of the melt, causing locally high porosity in the rock. An elevated degree of oxidation of the initial melt and progressive growth of fO₂ were likely related to the H₂ loss during the vesiculation and dissociation of H₂O. Consequently, ferrian magnesiochromite (Mchr) and chromian spinel (Fe³⁺-enriched) were the early phases to crystallize; they were followed by members of the magnesioferrite–magnetite series. In situ melting of quartz-bearing and carbonate–clay rocks led to the development of domains of peralkaline felsic glass that surround partially resorbed quartz grains. Numerous grains of wollastonite and rare larnite formed during contact pyrometamorphism. The alkalis increased progressively during crystallization, with a notable enrichment in Na (up to 0.30 apfu) in the akermanite–gehlenite series. The formation of leucite following melilite is indicated. Euhedral grains of Cpx display concentric cryptic zonation, with a zone of extreme Mg enrichment due to a local deficit in Fe²⁺. As consequences of the continuing rise in fO₂, esseneite crystallized in the rim of zoned clinopyroxene. Two schemes of coupled substitution account for the composition of Cpx grains analyzed in various textural relationships: Mg²⁺ + Si⁴⁺ → (Fe³⁺ + Al³⁺) and (Ti⁴⁺ + Al³⁺) + (Na + + K)+ → 2Mg²⁺ + Si⁴⁺. The pre-existing grains of olivine (associated with Mchr) were likely replaced completely by sepiolite–palygorskite associated with brownmillerite and its probable Fe³⁺-dominant counterpart, srebrodolskite. The investigated layer of alkaline microclinopyroxenite is unique in the Russian Plate, and a search is thus required to recognize other pyrogenic products. Also, further research is required to evaluate the contents and volumes of coal (or other sources of hydrocarbons) that could cause spontaneous and long-lasting combustion to form the considerable volume of paralava recognized in the Purtovino area.