Composition of basalts from the pre-Jurassic basement of Western Siberia (West-Tarkosalinskaya area,yamalo-nenets autonomous district)

Abstract

This paper presents the first results of study of mineralogical, petrological and geochemical composition of volcanites from pre-Jurassic basement of the West-Siberian megabasin near the West-Tarkosalinsk oil and gas deposit. West-Tarkosalinsk area is located on the territory of Purovsky region of Yamalo-Nenets Autonomous District of Tyumen region. Gubkinsky and Tarko-Sale are the towns nearest to the deposit. Volcanites of the well West-Tarkosalinsk 905 from the depth of 4362-4397 m have a small amount of dolerites present in the upper part of the section, which change to porphyritic basalts with depth. By chemical composition, the studied volcanites belong to moderateand high calc-alkaline basalts and andesibasalts. The rocks have undergone post-magmatic transformation (formation of albite, replacement of volcanic glass with chlorite, development of the secondary carbonate, prehnite, titanite and pumpellyite) under the conditions of prehnite-pumpellyite facies (bottoms of the greenschist facies of metamorphism). Volcanites contain high concentration of (g / t) Ti (up to 8673), V (up to 275), Mn (up to 1304), Zn (up to 95), Sr (up to 231), Zr (up to 162), Ba (up to 397), Ce (up to 41). The content of rare earth elements in rocks is about 59-116 g / t. The trend of distribution of rare earth elements normalized to chondrite for basalts is characterized by a predominance of light lanthanide elements over heavy lanthanide elements and either a lack of (W-Tar 905 / 4396.56), or the presence of a weak negative Eu anomaly. E-MORB basalts have a similar distribution of rare earth elements, but at a lower content of light lanthanide elements. According to petrological and geochemical characteristics, the studied volcanites are similar to the basalts of Koltogorsky-Urengoysky rift of the West-Siberian megabasin.