International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

ECLOGITE AND GARNET AMPHIBOLITE PEBBLES FROM MIOCENE CONGLOMERATES (PANNONIAN BASIN, HUNGARY): IMPLICATIONS FOR THE VARISCAN METAMORPHIC EVOLUTION OF THE TISZA MEGAUNIT

Pages: 355 - 366

Authors: PETER HORVATH, GABOR KOVACS, GYORGY SZAKMANY

Abstract: Eclogite and garnetiferous amphibolite pebbles were found in Miocene conglomerates directly overlying the Variscan crystalline basement of the Tisza Megaunit in the Pannonian Basin of Hungary. Peak metamorphic conditions were in the medium temperature part of the eclogite facies (600–650 °C, 13–15 kbar) with garnet, omphacitic pyroxene, quartz and rutile as primary assemblage. Breakdown from peak P-T conditions resulted in symplectitic intergrowth of clinopyroxene, plagioclase and quartz. Continuous retrogression involving garnet caused formation of the matrix assemblage of clinopyroxene, plagioclase, amphibole, ilmenite and quartz. Amphibole-plagioclase porphyroblasts formed during the last stage of retrogression. Amphibole-bearing symplectites record P-T conditions of 500–570 °C and 8–12 kbar in garnetiferous amphibolite, but no pyroxene was detected there. K-Ar geochronological data on amphibole show 348±13 Ma cooling ages which is nearly 100 Ma younger than previously suggested. Final uplift to surface conditions occurred in Miocene times causing the first appearance of exotic metabasite pebbles in clastic sedimentary rocks.

Keywords: Variscan Orogeny, Pannonian Basin, Tisza Megaunit, thermobarometry, garnetiferous amphibolite, eclogite

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