Минералогия и условия образования метабазитов офиолитовой ассоциации северного склона Кузнецкого Aлатау

The ophiolites of the northern slope of the Kuznetsk-Alatau Ridge are the most representative fragments of the sub-oceanic crust in the region. In this article, we present the results of a study of the metabasites from ophiolite paragenesis of the Severnaya, Zelenaya, Barkhatnaya mountains and Semenovsky massif. The selected samples from the gabbroid and subvolcanic complexes were studied in microsections, and the compositions of the main rock-forming minerals (plagioclases, amphiboles) were investigated in detail on an electron microscope equipped with micro-dispersion analyzer. It was established that feldspars are represented by a wide spectrum of composition: from albite to bitovnite; not less common amphiboles are also characterized by a wide diapason: edenite, ferro-edenit, ferropargasit, magneziogastingsit, magnesian, ferrous and actinolite hornblendes. Chlorite, which observed in all samples, represents ripidolit and piknochlorite. Accessories are apatite, ilmenite, sphene and, in rare cases, rutile. The data obtained by the phase relationship, and changing chemical composition of essential minerals (feldspars, amphiboles) allow suppose several steps of the transforming investigated metamorphic rock because signs of both progressive and retrograde metamorphism are found. The behavior of accessory minerals also expresses the variability of the thermodynamic parameters of metamorphism. This is confirmed by the traditional geothermobarometry methods (using the plagioclase-amphibole mineral pair). Observed retrograde metamorphic processes reflect the movement of ophiolite mass in the upper layers of the crust, and this makes it possible to connect metamorphism with obduction of ophiolites. For most of the investigated regions the early stage of ophiolite obduction in terms of location the “Riphean” oceanic island arc can be associated with episode of epidote-amphibolite facies of regional metamorphism at (T = 500-700°C and P = 2-6 kbar). The collision of Kuznetsk-Altai island arc to the structures of the Central Asia and Siberian Craton superterrane in Late Cambrian-Ordovician was controlled by vertical movement along submeridional tectonic discontinuities and resulted the metamorphism at low temperatures (350-510°C) and high pressure (6-9 kbar). The completion of these processes is characterized by a decrease in pressure (2-6 kbar) at the same temperatures. The result of these events is the formation of serpentinite melange on the Semenovsky massif.