Литосфера (Aug 2017)
Sc, Cr, Co, Ni, Th and rare earth elements systematics in black shales of Murav’inskaya and Velsovskaya formations (Lyapin-Kutim anticlinorium, the Northern Urals): to the provenance reconstructions
Abstract
The features of the Sc, Cr, Co, Ni, Th and rare earth elements distribution in the black shales of the Riphean Murav’inskaya and Velsovskaya formations in southern part of the Lyapin-Kutim anticlinorium (Northern Ural) are analyzed. The contents of the listed elements in 20 samples of black shales are determined by the ICP-MS method at the IGG UB RAS (Ekaterinburg). Samples of the shales of the Murav’insky formation are selected from the outcrops along the river Vels, below the mouth of Pos’mak River, as well as along Pravaya Rassokha River. The selection was made from outcrops more or less evenly distributed over the studied area. The shales of the Velsovskaya formation were studied from the drilling holes in the basin of Sur’ya Vagranskaya River, as well as in outcrops along the Sibiryakovskaya Martayka and Pravaya Rassokha rivers. In the Sc-Th/Sc diagram, the data points of the black shales of the Murav’inskaya and Velsovskaya formations are fairly compact located between the points of the average composition of the Archean and Proterozoic granites, on the one hand, and tonalite-trondhjemite-granite (TTG) associations of the same age. Approximately the same distribution of the data points of the black shales of these formations can be seen in the La/Sc-Th/Co and La/Sm-Sc/Th diagrams. The average value of ΣРЗЭ in black shales of the Murav’inskaya formation is 316 ± 90 ppm, in the shales of the Velsovskaya Formation it is noticeably lower - 200 ± 47 ppm. The black shales of the Murav’inskaya Formation are substantially enriched with light lanthanides (La/Yb)Nmean = 40, and noticeably depleted by heavy lanthanides, (Gd/Yb)Nmean = 5.4. For the black shales of the Velsovsky formation (La/Yb)Nmean = 22, (Gd/Yb)Nmean = 3.1. This ones and a number of the other data make it possible to conclude that the complexes of rock-sources of fine aluminosiliciclastics for black shales of Murav’inskaya and Velsovskaya formations had a rather specific composition. On the basis of a comparison of the geochemical features of the black shales of the southern part of the Lyapin-Kutim anticlinorium and potential sources of fine aluminosiliciclastics, the conclusion is based that such rock complexes could be, from one side, the Archean and/or Proterozoic TTG associations and granites of the same age of the Eastern European platform, and, from another side, certain local rocks with high Cr contents and significant differentiation of REE, similar in composition to the products of kimberlite magmatism.
Keywords
