MINERALOGICAL AND PETROGRAPHIC FEATURES OF VOLCANIC ROCKS FROM THE VYHORLAT-HUTYNSKE VOLCANIC RIDGE (IN THE OUTSKIRTS OF THE KOLCHYNO AND KLENOVETS VILLAGES, TRANSCARPATHIA)

Abstract

Neogenic volcanic rocks of the Transcarpathian trough are represented by two formations: rhyolite-ignimbritic (Miocene) and andesite-basaltic (Miocene–Pliocene). During Pliocene, in the central core of the Panonskyi Middle Massif an external volcanic arch has been formed – powerful eruptions of lavas and pyroclastic material of the basic and intermediate composition have been occurred. Today it is a chain of tectonovolcanic structures of the Vyhorlat-Hutynske ridge, which have been controlled by Zakarpatskyi, Oasky and Prypanonskyi deep faults. We studied andesite-basaltic lava rocks of the Neogene age (Upper Sarmatian–Panon, absolute age – 10.3–10.7 million years), which are exposed in the abandoned quarry on the right bank of the Vyznytsia River between the villages of Kolchyno and Klenovets (Mukachevо district). The rocks are dark gray to black, have a porphyry structure and a massive texture, which sometimes turns into striped (black strips enriched with iron oxides and hydroxides), contain xenoliths of sedimentary rocks up to 4 cm in size. Amazing forms of columnar jointing, which have been formed due to the peculiarities of lava flows movement, are characteristic for andesite-basalts. The columns are usually hexagonal; their size in the width is 30–50 cm. To the top of the exposed strata, columnar jointing gradually passes into the slab parting. We studied the mineralogical and petrographic features of exposed volcanic rocks, the chemical and microelement composition of their rock forming and accessory minerals, the conditions for the formation of volcanites. The groundmass of andesito-basalts has a hyalopilitic structure and is composed by laths of plagioclase, grains of pyroxene and volcanic glass. Plagioclase prevails among the impregnation; its composition in phenocrysts corresponds to the labradorite and bytownite, and in the groundmass of the rocks – to labradorite and andesine. Dark-coloured minerals in phenocrysts are represented by orthorhombic pyroxenes (hypersthene, ferrohypersthene) and clinopyroxenes (diopside, augite). Accessory apatite and ilmenite make no more than 5 % of the rock. Using the pyroxene geothermometer of D. Lindsley, it was determined that the orthorhombic pyroxenes were the first to crystallize in the rock, the crystallization temperature was 1 100– 1 000 °C. As the temperature dropped to 900–700 °C, clinopyroxenes crystallized. The calculations carried out according to L. Perchuk's method showed that the temperature interval of andesite- basalts formation was 1 070–1 050 °C.