Rhyolite-perlite transition zone in the acid volcanoes of the Eastern Rhodopes Paleogene volcanic area (SE Bulgaria)

Abstract

In the siliceous volcanoes, the transition between rhyolites and perlite is carried out through a zone with the following morphology: i) rhyolite balls (spheruloids with spherulitic or felsitic texture) among the perlites; ii) alternation of subparallel perlite and rhyolite bands with felsitic texture; iii) a combination of the above two morphologies – rhyolite bands with spheruloids adhering to them; iv) alternation of finger-like wedge-shaped perlite and rhyolite wedges. The rhyolite spheruloids have Na composition (they are composed of tridymite and Na-K feldspar), while the perlites including them, as well as all rhyolites in the Eastern Rhodopes, are potassic. This chemistry, as well as many morphological features, show their immiscibility origin, which has been recently proven also experimentally (Kotel’nikov et al., 2016, 2019). Complex spheruloids are also observed, which consist of two types of spheruloids with different distribu¬tion of alkalis in them: small, in which Na>K and large spheruloids, which include small ones and in which potassium predominates. It is assumed that they are a product of a two-step immiscibility – in the first step large drops and matrix are formed, and in the second the large drops decompose into small drops (enriched with Na) and a new matrix (enriched with potassium), which is the building element of the large drops. The drops crystallize as spherulites, while the matrix solidifies as perlite. The spheruloids are products of nucleation and growth mechanism, i.e. they are formed in the fields between the binodal and spinodal curves. The other type of transition – the alternation of perlite and rhyolite bands is assumed to be formed by the spinodal decomposition, i.e. in the field limited by the spinodal curves.