Fenner trend and the role of fractional crystallisation and ferrobasaltic magma immiscibility in granophyre petrogenesis: the case of the mesoproterozoic Valaam sill in the Ladoga graben, Karelia

Abstract

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, andεNdvalues 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 960oC; 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 SiO2and 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.