Еволюція мезозойських перидотитів угольського офіолітового комплексу (мармароська скельна зона, внутрішні українські Карпати)

Abstract

The paper presents the results of the study of peridotites of the Ugolka ophiolite complex of the Inner Ukrainian Carpathians. The work is based on the data of prior research and original materials of geological observations from the middle course of the Tereblya River, and on new laboratory analyzes of peridotites. Middle Triassic – Lower Cretaceous (?) Peridotites of the Ugolka ophiolite complex form olistolites in the Lower Cretaceous olistostrome-conglomerates of Soimul formation. Petrographic data make it possible to classify disparate olistolites of the ophiolite complex, and peridotites in particular, into two types of ophiolite associations, which were formed in different conditions according to indicator minerals (including spinelides). Results of the study of spinelites form two groups that correlate with two equilibrium mineral associations: first – olivine + spinel + orthorhombic pyroxene + monoclinic pyroxene; and second – amphibole + talc. Larger isometric or xenomorphic individuals of chromium spinels are found in harzburgites and lherzolites. Smaller chromium spinel individuals form elongated and elliptical grains, stretched along the structure of the rock, and are found in the interstices of pyroxene and olivine crystals. X-ray microprobe analysis revealed that the composition of chromium spinel grains of the first group contains more magnesium and less chromium. chromium spinels of the second group are smaller, and contain less magnesium and more chromium. On the binary diagram Cr#–Mg# chromium spinels from the peridotites of the Ugolka complex are concentrated in two areas: abyssal peridotites and suprasubduction peridotites. chromium spinel grains from the first group crystallized at a low degree of partial melting (Dmelt up to 14 %), which is typical for abyssal peridotites. chromium spinels of the second group crystallized at a higher degree of melting (Dmelt up to 30 %), which is characteristic of peridotites of suprasubduction zones. On the correlation diagram of the average chromium spinel and the total spreading speed, the chromium spinel compositions of the first group of the Ugolka complex corresponds to the peridotites of ultra-slow spreading zones. Small grains from the second group of chromium spinels of the Ugolka complex are associated with peridotites of fast spreading zones. The temperature of metamorphism of rocks of the first mineral association is estimated by spinel and olivine-spinel geothermometers, and is contained within range of 900–1 290 ºС, at pressure aprox. 15 kbar. The temperature of metamorphism of rocks forming the second mineral association, I estimated at 430–450 °C, at pressure of 4.0–4.5 kbar. The study of peridotites of the Ugolka complex is informative for reconstruction of the geological evolution of the original sediments and determining the stages of formation of thrust-nappe structures.