Quartz gabbronorites of the kem-ludy islands, belomorian mobile belt: Crystallization and metamorphic evolution
V.M. Grigorieva1, A.L. Perchuk1, 2, V.M. Kozlovsky3, N.G. Zinovieva1
1Lomonosov Moscow State University, Geological Faculty, Moscow, Russia
2Korzhinskii Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka, Russia
3Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences, Moscow, Russia
Keywords: quartz gabbroic rocks, geothermobarometry, thermodynamic modeling, Belomorian mobile belt, Precambrian.
Abstract
Quartz gabbroic rocks are generally considered as a rare variety of mafic plutonic rocks. Their occurrences have been reported, among other settings, in Precambrian gneiss complexes. This study presents the first detailed petrological investigation of quartz gabbronorites from two small intrusive bodies hosted within the gneissic sequence of the Kem-Ludy area of the Archean-Paleoproterozoic Belomorian Mobile Belt (BMB), Karelia. The studied quartz gabbronorites exhibit variable degrees of metamorphic overprint, ranging from low to complete. In two samples, primary magmatic minerals (clinopyroxene, orthopyroxene, plagioclase, and quartz) are preserved in significant amounts, while metamorphic alteration is locally developed. In contrast, garnet amphibolite from the marginal zone of the quartz gabbronorite body is completely devoid of primary magmatic minerals; only inclusions of relic clinopyroxene within garnet are observed. Geothermobarometric data and phase equilibrium modelling indicate that crystallization of the quartz gabbronorites occurred at temperatures of approximately 1100 °C and pressures of ~0.7 GPa, followed by subsequent subisobaric cooling to ~850°С. These estimates suggest that the rocks formed under lower- to mid-crustal conditions, either through fractional crystallization or in its absence. The results imply that the distribution of quartz gabbronorites within the Belomorian Mobile Belt may be significantly underestimated.
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