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Russian Geology and Geophysics

2004 year, number 2


T.V. Donskaya, E.V. Sklyarov, D.P. Gladkochub, A.M. Mazukabzov, and E.P. Vasil'ev†
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
Keywords: Inverted metamorphism, structural-metamorphic evolution, PT conditions, southeastern Sayans
Pages: 175-192


Detailed structural and petrological studies of the Shutkhulai metamorphic complex have shown inverse metamorphic zoning and growth of paleotemperatures up the crustal section within a given structure, namely, in the region of transition between gneisses, granite gneisses, migmatites of the upper slab, and metasedimentary deposits of the Oka series (the lower slab). Petrographic study of rocks of the Shutkhulai metamorphic complex, analysis of compositions of rock-forming minerals, and calculation of pressure and temperature of metamorphism, along with structural observations, suggest that structurally the complex is a combination of zones with different parameters of metamorphism not typical of the classic metamorphic zoning: a high-pressure zone (6.2-8.5 kbar) at lower temperatures and a low-pressure zone (3.5-4.2 kbar) at higher temperatures. On the basis of petrochemical study of the rocks making up the Shutkhulai complex, it was concluded that the whole volcanosedimentary unit was subjected to metamorphic transformation.
A qualitative model has been proposed to describe how the Shutkhulai metamorphic complex evolved during a continuous metamorphic process. It is supposed that at the initial stage of its evolution the deep-seated "hot" rocks were thrust over hypsometrically higher "cold" deposits, and the thrust slab exerted the maximum effect on the rocks of the underlying slab. Throughout this stage, metamorphism had an action on the most remote rocks of the transition zone, in which the maximum pressures have been recorded. At subsequent stages of metamorphic evolution, unloading of the overburden caused a drop in pressure in the upper slab and in the adjacent part of the transition zone. The model suggests that the inverted metamorphic isograds persist owing to a high rate of tectonic and metamorphic processes at which the geothermal gradients did not re-establish equlibrium.