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 Janoshalma northeastward within the Tisza Block. We believe the Janoshalma 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, fo...