Reconstructing the magmatic crystallization conditions and alteration processes of quartz dolerites of the Kremennaya mountain stock (Mountain Crimea)

Abstract

Introduction. The 60 m wide quartz dolerite stock, opened by a quarry, is heavily alternated by secondary processes, which makes it difficult to study.Aim. Reconstruction of primary magmatic structures and determination of conditions conductive to their crystallization and late alterations.Materials and methods. In order to assess the conditions of magmatic crystallization and massive late alterations, thin sections of selected samples were studied and chemical analysis of rocks was performed.Results. The stock was established to have a primary double structure with a wide centre and a narrow annular border (1 m). This is due to the contraction and reintroduction of a melt along the edge of the stock. The melt inside the ring is adiabatically heated to 1175° and the water pressure drops to 0.5 kbar, as compared to the centre (900° and 2 kbar). Therefore, the centre and edge have a different primary composition with plagioclase (An50), qartz, magnetite and glass (5%) in the centre and early plagioclase, quartz, pyroxene, plagioclase microlites, miarolitic cavities and glass (30–40%) at the border. Final hardening and new contraction lead to the formation of radial cracks in the ring. The mechanical work, spent on their formation, sharply reduces the level of internal energy, while the expansion of cracks leads both to adiabatic cooling of the solution in the border zone of the stock and massive low-temperature metasomatosis (260–132°C). The reactions proceed diffusionally when the solution is stagnant with the massive formation of pseudomorphoses. Due to a decrease in the volume of reacting solid phases and an increase in the total porosity under the conditions of dominant decompression, part of the silica is removed upwards. The centre is cooled slowly with the preservation of relics and their successive alteration from acid to alkaline ones. Initial acid reactions are provided by the complete dissociation of a weak carbonic acid, followed by strong acid and final alkaline reactions provided by chlorine and incomplete dissociation of carbonic acid, respectively.