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The Stratigraphic Boundary Between the Lower and Upper Lopian (Upper Archean) in Central Karelian Greenstone Structures

The Upper Archean Vedlozero-Segozero greenstone belt lies in Central Karelia.It extends approximately north-south from Lake Segozero to Lake Vedlozero for about 300 km and comprises the Hautavaara, Koikary, Palaselga, Semch, Sovdozero and some other structures.
Within the Lopian supracrustals of Karelia there exists a marked intra-Lopian boundary. New U-Pb zirkon age Geochronological studies have shown the radiological age of the intra-Lopian boundary to be 2850-2860 Ma. Stratigraphically, the boundary is seen as a sharp change in sedimentation pattern, the appearance of crusts of weathering, basal polymictic conglomerates and a terrigeneous rock unit. A section of Koykary and Hautavaara structures of Vedlozero-Segozero greenstone belt, extending on 400 kms and including in self, except named, yet a number of structures (Semch, Sovdozero, Palaselga, Oster) is a reference section of Upper Archean (lopian) formations of Central Kareliya [1]
The 4- to 5 km-thick Lower Lopian unit (reference section of Koikary, Hautavaara structure) consists of komatiitic , basaltic and andesitic volcanics with complementary basic-ultrabasic intrusions, subvolcanic andesite and dacite bodies, and chemical-pyroclastic sediments. Isotopic dating of andesite-dacite volcanics from Hautavaara structure are from 2995 + 20 up to 2854 + 14 Ma (U-Pb, zircon) [2]
The 1.5-to 2.0-km-thick Upper Lopian is dominated by terrigeneous rocks. Acid volcanic products with intrusive magmatism ranging from diorites to granites occur in some sections. The sedimentary rocks consist of inequigranular sandstones, graphitic siltstones, silicites, and dolomites. The isotope oxygen composition of sealing wax dolomites and grey dolomites indicates of their sedimentary genesis[3]. The occurence of red dolomites and silicites suggests the presence of free oxygen in the upper Archaean depositional basin. The top border of lopian is accepted in stratigraphic scale of 2700 + 100 Ma. The border between the top and the bottom of lopian is carried out on boundary 2850 Ma [4]] In stratigraphic investigations it is difficult to use crusts of weathering, locally preserved in the present erosion section, as a boundary marker. It is hard to select basal polymictic conglomerates among many lithotypes of conglomerates and tuff conglomerates. Neither is it possible to use volcanics when correlating sections in the case of multi-stage cyclic eruptions and a variety of volcanic facies, especially with lateral variations.
Therefore, the terrigenous sedimentary unit is proposed as the boundary marker. It varies in thickness from a few to 150 m. In the Hautavaara structure this from bottom to top of polymictic conglomerates, graywacke, quartz sandstones, inequigranular quarz-plagioclase sandstones, graded sand-argillite sequences with intraformational conglomerate lenses, arkoses. In the Koikary structure, at the base of the marker bed there occur crust of weathering and scour products represented by chloritites and siltstone graywacke. These are overlain by volcanic-terrigenous graywacke, quartz gravelstones and sandstones. The total thickness of the marker unit is 50-70 m. The occurrence of quartz sandstones in the section and the presence of well-rounded quartz clasts in chemical silicites and scoured tuffs suggest that there was a crust of weathering a top on the granitoid basement blocks in the source area and that this crust existed simultaneously with a crust of chemical weathering a top of the komatiites. The boundary thus described can be traced at the bottom of the terrigenous rocks that rest on the eroded surface of the Lower Lopian strata. Quartz sandstone beds most common whereas polymictic conglomerates and weathering crusts are only developed locally. The terrigeneous pack offered as marker of border between the bottom and the top of lopian expands correlational opportunities of lopian sections in local structures.
1. M.B.Raevskaya, V.Y.Gorkovets, A.I.Svetova.,Precambrian stratigraphy of Karelia.Reference section of Upper Archean deposits,1992, Petrozavodsk, KRC RAS
2. A.I.Tugarinov, E.V.Bibikova. Geokhronologia Baltijskogo shchita po dannym circonometrii. M. 1980, 130 p.
3. Metallogenic evolution of Archean greenstone belts in Karelia.Part 1, 1993, St.Petersburg, Nauka.
4. V.A.Glebovitsky, V.M.Shemjakin, Stratigraficheskaya shcala rannego dokembria Sveko-Karelskogo regiona. V knige Glavneishie rubezhi geologicheskoy evolutsii Zemli v dokembrii. 1995, St.Petersburg. p.42-43

Последние изменения: 16 июля 2006