Литосфера (May 2020)
Murzinka massif in the Middle Urals as an example of an interformational granite pluton: magmatic sources, geochemical zonation, peculiarities of formation
Abstract
Murzinka massif constitutes an interformational sheet-like body reaching up 6 km in length steeply dipping to the east. Proterozoic metamorphic rocks of predominantly granulite facies (p = 5–6 kbar, T = 750–800℃) occur at the base of the massif, with Silurian-Devonian volcanic-sedimentary rocks metamorphosed in the epidote-amphibolite facies occurring in its roof. The petrogenic elements were determined at the Laboratory for Physicochemical Research Methods of the Zava ritsky Institute of Geology and Geochemistry, UB RAS. The content of trace elements was determined at the laboratories of the University of Granada in Spain and at the Institute of Geology and Geochemistry using the ICP-MS method. In the eastern direction, the rocks change their composition from predominantly basic to granitoid as they approach the massif. The gneisses of the granitoid composition experienced a high degree of melting; the anatectic melt formed the western part of the Murzinka massif. The granites form three complexes: 1) Yuzhakovo – veins of biotite orthoclase antiperthite gra nites varying in K2O content in the metamorphic rocks of the base of the massif; 2) Vatikha – biotite orthoclase antiperthite granites making up the western part of the murzinka massif; 3) Murzinka – two-mica predominantly microcline granites occurring in the eastern part of the massif. Vatikha and Murzinka granites have the same isotopic age (about 255 Ma). A clear geochemical zonation is revealed in the massif: from west to east (from the base to the roof), the contents of Rb, Li, Nb, Ta increase in the granites of the Vatikha and Murzinka complexes. In the same direction, the ratios K/Rb, Zr/Hf, Nb/Ta decrease, as well as the contents of Ba and Sr. Naturally, the compositions of such rock-forming minerals as plagioclase and biotite also change. The isotopic characteristics of the granites of the Vatikha (Sri = 0.70868–0.70923 and εNd255 from –8.9 to –11.9) and Murzinka (Sri = 0.70419–0.70549, εNd255 from –2.6 to +2.3) complexes suggest that the substratum of the former was represented by Proterozoic granite-gneisses, whereas the rocks of the newly formed crust, possibly similar to the Silurian-Devonian volcanogenic-sedimentary rocks, which are at contact with the Murzinka granites, served as the substrate for the latter.
