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Neoproterozoic flood basalts of the upper beds of the Volhynian Series (East European Craton)

Białowolska A., Bakun-Czubarow N., Fedoryshyn Y.

Geological Quarterly

2002

Vol. 46, No 1

37-58

The effusive rocks of the Ratno Beds of the Volhynian Series known from the western slope of the Ukrainian Shield are represented by lower Vendian flood basalts whose normative composition is that of quartz tholeiites. These are plagioclase-pyroxene basalts displaying intergranular, intersertal, doleritic, ophitic and amygdaloidal textures; they range from aphanitic to medium-grained and contain about 7 vol. % of palagonite - an altered glass with a high iron and considerable magnesium content. The range in composition of plagioclases (andesine-bytownite) and clinopyroxenes (augite-ferropigeonite) suggests that the Ratno Beds basalts formed by fractional crystallisation of a parent magma. Residual magma underwent liquation, producing a separate acid glass (69-73 wt. % of SiO2) phase within a basic one considerably poorer in SiO2 but rich in iron and magnesium. The Ratno Beds basalts are relatively rich in silica, iron, titanium and vanadium as well as in REE and LREE in particular but poor in Ni, Co and Cr. Normative composition, geochemical characteristics and tectonic position suggest classification as continental quartz tholeiites. Hydrothermal solutions are responsible for rich native copper mineralisation in basalts of certain parts of Volhynia (Ivance and Policy). The Vendian volcanism of the Volhynian Series lithologically correlated with the Sławatycze Series of eastern Poland, can be related to continental rifting accompanying the breakup of Rodinia, with crustal fractures mainly running concordantly with the suture zone between Fennoscandia and Sarmatia, thus almost perpendicular to the Tornquist rift; other fracture trends may also have controlled Vendian volcanism.

Neoproterozoic flood basalts of Zabolottya and Babino Beds of the volcanogenic Volhynian Series and Polesie Series dolerites in the western margin of the East European Craton

Bakun-Czubarow N., Białowolska A., Fedorysin J.

Acta Geologica Polonica

2002

Vol. 52, nr 4

481-496

In the Volhynia region, on the western slope of the Ukrainian Shield, occurs a large complex of Lower Neoproterozoic III (Lower Vendian) effusive-tuffogenic rocks, referred to the Volhynian Series. This series belongs to a large province of flood basalts extending over an area of ca 140 000 km^2 in the western part of the East European Craton. The series is underlain by the terrigenous rocks of the Polesie Series, with volcanic ashes and dolerites in its upper part. The effusive-tuffogenic rocks of both series, together with hypabyssal dolerites, represent the Volhynian Trap Formation. Within the lower - Zabollotya and Babino Beds of the Volhynian Series there have been found alkali basalts and olivine tholeiites, whereas the basalts of the upper, Ratno Beds are quartz tholeiites exclusively. All the basalts are mainly within plate continental basalts. The diversity of basalt composition within the Volhynian Series suggests that in the Early Vendian, during the process of formation of parent magmas for basalts of particular beds, the mantle material beneath Volhynia has been melted to various degrees. Distinct variability can also be observed in basaltic pyroxene composition (particularly in the Babino and Ratno Beds) as well as in variations of REE distribution patterns in the studied basalts. Most likely, the strongly differentiatedpyroxenes were not in a state of equilibrium with the magma. The native copper mineralization of basalts from the Ratno, Babino and Zabolottya Beds is described. Higher concentration of Au and Ag in the rocks with a high content of copper supports the hypothesis that the mineralizing hydrothermal solutions originated in a common igneous source. The Vendian magmatic activity in the study area was associated with the last stages of breakup of the Rodinia supercontinent.