Age and composition of plagiogneisses from the Yangiyugan, Artic area: the first evidence of Precambrian blocks existing in the basement of the West Siberian Platform

Abstract

So far, Precambrian formations were not found in oil gas fields of the Western Siberian Platform (i.e., in central, western, and northern parts), where they were assumed to be. This article analyzes data on the mineral composition, geochemistry, and age of plagiogneisses from the Yangiyugan oil exploration area (Arctic part of Western Siberia, 80 km to the southeast of Salekhard). Rocks underlying this basement were naturally formed aggregates and are composed almost entirely of quartz and oligoclase, with the presence of muscovite and biotite, amphiboles (edenite, ferroedenite, and ferropargasite), clinozoisite–epidote, almandine garnet, clinochlore, microcline, magnetite, titanomagnetite, rutile, and fluorapatite. The mineral association tends to change with depth and plagiogneisses formed from trondhjemites (or leucocratic plagiogranites) under conditions of increasing temperature and amphibolites–facies metamorphism. First, trondhjemites occurred in the Ediacaran period about 566 ± 3 Ma (the dating of zircons by SHRIMP-II). The Sm-Nd isochron age of 691 ± 58 Ma also indicates Neoproterozoic origin for the gneiss protolith. It turned out that metamorphism of trondhjemite and its transformation into plagiogneiss took place in the early Ordovician (486 ± 4 Ma). The most recent tectonic–thermal events (U-Pb of 329 ± 9 and Rb-Sr of 292 ± 23 Ma) in the area are apparently associated with collisional processes and diaphthoresis. The metamorphic complexes of the studied area are the deep segments of a middle Paleozoic island arc system or, more likely, several island arcs. These segments were brought to the upper crust by rifting and graben formation about 250 million years ago. The isotope readings and geochronologic data in this article are the first direct evidence on the presence of Precambrian blocks in the pre-Jurassic oil-gas-bearing basement of the Western Siberian Platform.