Petrology of the metamorphic basement of the Tisza Block at the Jánoshalma High, S Hungary

Abstract

The Variscan metamorphic basement of the Great Hungarian Plain (Tisza Block, E Hungary) consists of deeply buried crystalline highs comprising several types of gneiss and metabasic rocks. A common gneiss type is an orthogneiss variety that can be followed across the basement from Jánoshalma northeastward within the Tisza Block. We believe the Jánoshalma High to be a uniform orthogneiss block of peraluminous composition; this is suggested by the presence of idiomorphic accessory phases (apatite needles, zircon), polygonal feldspar texture, occurrence of rock bodies of exotic origin, xenoliths (amphibolite, eclogite) and xenocrysts (feldspar megacrysts, amphibole, garnet) of varying composition and metamorphic evolution. The peculiar features of the orthogneiss body are indicative of processes that may have taken place in an ancient Alaskan-type orogenic belt (subduction-accretionary complex). In these situations, because of the continuous ample sediment supply, the accretionary prism retreats oceanward, forming structures of increasing steepness with depth, accreting against the continental margin. Thereafter, this causes oceanward retreat of the subduction trench as well. For this reason mantle-derived magmas do not penetrate the continental margin but the accretionary prism itself, resulting in the melting of its material. In the case of the Jánoshalma High, following the emplacement of the onetime granodioritic body at shallower crustal levels, it metamorphosed via to a two-stage overprint. An early high-temperature (HT) and a succeeding medium-temperature (MT) event formed orthogneiss from the ancient intrusive rock. This is the first detailed investigation of the Jánoshalma high that has been published.