The structure of the basal horizon of the rhyolite thick and its origin (Northern Caucasus, the Upper Chegem)

Abstract

Most researchers refer the powerful thickness of the rhyolite of the Upper Cheghem volcano-tectonic depression to “ignimbrite”, i. e. soils formed by sintering or welding particles in ash streams. However, the features of the detailed structure of the entire thickness and in particular the basal level of the black vitrophyre of the base of the powerful thickness of the rhyolite suggest an outpouring of fluidized magma in a state of “self-organizing criticality”, rather than sintering the ash material. Special attention is given to the enrichment of basal layers with xenoliths of varying degrees of smoothness and sharp contacts of black vitrophyre with other types of rhyolite in the thickness consisting of numerous “streams” or “units” having a thin columnar separation, passing from one “stream” to another and the boundaries of “streams” are not expressed. The nature of the structure, placement and contacts of the layer of black vitrophyre does not allow us to accept this assumption, as, indeed, the ignimbrite nature of most of the rhyolite thickness, if we mean by “ignimbrite” a sintered or even welded pyroclastic material. It is possible to allow its formation as a result of the outflow (exactly the outflow, rather than explosion) of the fluidized supply to the surface from a number of centers, which was accompanied by a simultaneous deflection of the substrate, which contained an increased amount of water, so that the glassy rhyolite were exfoliated, which is observed in some places. It can be noted that if a fluid with a temperature above the critical point arises in the magmatic focus at some level, it is capable of fluctuations, of “self-organizing criticality”. A layer of black glassy rhyolite is developed in the basement; its thickness is variable but can reach up to 20-30 m some places. These black vitrophyre sometimes directly overlie the granite substrate. In some places, spherules (rather spheres of regular shape - spheres, usually located in the upper part of black rhyolite or in their basement where the layer of vitrophyre is located) are connected with black glassy rhyolite. In the Elbrus volcanic region, the similar spheroidal jointing in acid black vitrophyre does not occur, as, indeed, in black glassy rocks.