Литосфера (Oct 2018)
Murzinka massive at the middle urals as an example of the interformational granite pluton: magmatic sources, geochemical zonality, peculiarities of formation
Abstract
Murzinka massif is a sheet-like interformational body steeply deeping to the East with length about 6 km. Proterozoic metamorphic rocks of the predominantly granulite facies ( P = 5-6 kbar, T = 750-800°C) occur at the base of massif, and volcanic-sedimentary Silurian-Devonian rocks metamorphosed in the epidote-amphibolite facies - in the roof of it. Analyzes of rocks are made in the Institute of Geology and Geochemistry. A.N. Zavaritsky (Ekaterinburg, Russia) by standard methods. Petrogen elements were determined on the X-ray fluorescence spectrometers CPM-18, CPM-25, VRA-30 and the rare elements - on the ICP-MS mass spectrometer ELAN-9000 company Perkin Elmer. In the eastern direction the rocks lying in the base of the massif change their composition from predominantly basic to granitic. The gneisses of granitoid composition underwent a high degree of melting, and theirs anatectic melt formed the western part of Murzinka massif. The granites form three complexes: yuzhakovsk - vein of biotite orthoclase antiperthite granites, varying in K2O content, in the metamorphic rocks of the base of the massif, the vatikha - biotite orthoclase antiperthite granites in western part of the murzinka massif, and the murzinka s.s. - two-mica predominantly microcline granites 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 the west to the east (from the base to the roof), the contents of Rb, Li, Nb, Ta grow 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 content of Ba and Sr. Accordingly, the compositions of such rock-forming minerals as plagioclase and biotite also change. The isotope 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 the Proterozoic granite-gneisses, and of the second - the rocks of the newly formed crust, possibly similar to the Silurian-Devonian volcanogenic-sedimentary rocks, which contact with the murzinka granites at the west.
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