Petrogenetic peculiarities of Fe-Ti oxide minerals in the processes of crystallization and evolution of late Cretaceous volcanic complexes of the Lesser Caucasus

Abstract

Drawing from the determined differences between iron-titanium oxide minerals, we analyzed the conditions of crystallization and evolution of late-Cretaceous magmatic complexes of the Lesser Caucasus. It was found that the rocks of basalt-andesibasalt complex, which correspond to the early substage (upper Coniacian-lower Santonian) of late-Cretaceous volcanism in the Qazakh, Agjakand, Agdara depressions, have crystallized in the conditions of relatively highly- thermobaric crystallization of titanomagnetite, poorly differentiated and evolutionized according to the Fenner trend. In the second substage of volcanism, due to decrease in permeability of the Earth’s crust, the elevation of the remaining magma to the upper horizons was hindered. Therefore, within the Qazakh depression, shallow intermediate sites of crystallization developed where moderately titaniferous magnetite crystallized with the participation of oxidized fluids earlier than hornblende, pyroxene and plagioclase. Thus, the remaining magma evolutionized its composition through Bowen’s reaction series. In the Agjakand and Agdara depressions, change of previous expansion to compaction was the cause of hindering of partly fractioned portion of the magma. The latter thermally interacted with the above-embedded maghemite, hematite and in a number of cases magnetite. In the Khojavand depression, rocks of trachibasalt- trachiodolerite complex, which characterize the late substage of the Santonian volcanism, contain moderately titanium magnetites and maghemites. In the second substage of volcano-plutonism, rocks of tephrite-teshenite complex developed. There, accompanied by oxidized fluids, highly-clayey titanomagnetite crystallized before chrome-diopside and salite. However, the ulvospinel titanomagnetite in teshenites, having associated with barkevikite and kaersutite, crystallized at a relatively higher temperature. Within the Senonian volcanites of the Azykh depression, along with the moderately-titanium magnetite, chromic titanomagnetite and rarely chromite was determined. Similar mineralogical diversities are also characteristic for the Gochas depression.