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  • Author or Editor: Gyöngyi Lelkes-Felvári x
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The results of seven new 40 Ar/ 39 Ar analyses and two K/Ar analyses of mineral separates of metamorphic rocks from the Transdanubian part (west of the Danube River) of the Tisza Mega-Unit is presented, and a review of the geochronological data available in the literature is given. Investigated rocks come from boreholes and include amphibolite facies rocks, mainly containing garnet, staurolite, kyanite, andalusite and sillimanite as index minerals. The investigated area covers 120 × 85 km in Transdanubia, including all tectonic units distinguished by previous authors. The 40 Ar/ 39 Ar spectra of the muscovites yielded fairly well-established plateau ages ranging from 307 Ma-312 Ma in all units. Similar muscovite Ar/Ar cooling ages of ca 310 Ma were obtained from other parts of Tisza Mega-unit east of the Danube, e.g. from the kyanite-sillimanite-bearing metapelites of the Villány-Bihar Subunit (Great Plain; Lelkes-Felvári et al. 2003).

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U/Pb data on single zircons from acidic volcanics are presented from the Kékkút-4 borehole, Balaton Highland, Transdanubian Central Range, Pelso Mega-Unit. Generally accepted as Lower Permian in age, they were also considered - due to some metamorphic features - to belong to the metamorphic, Lower Paleozoic basement. Clear, elongated crystals of these thick, acidic volcanics are regarded as co-genetic magmatic products, supplying a U/Pb age of 291.4±4.7 Ma (Lower Permian), representing the first radiometric magmatic age-dating of Upper Paleozoic acidic volcanism in this tectonic unit.

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Ultramylonites and margarite-bearing quartz-feldspar S-C mylonites, containing amphibolite lenses with symplectitic texture, were encountered in a borehole (Bajánsenye-B-M-I) close to the west of the Transdanubian Central Range Unit. These rocks demonstrate a ductile, horizontal extensional shear zone attaining a thickness of 300 m. Microstructural data, mineral parageneses and mineral chemistry of these rocks indicate a multistage metamorphic evolution, which is consistent with that of the Koralm-Pohorje basement. The youngest mylonitic event (Early Tertiary) took place in the Bajánsenye mylonites at 430-450oC (greenschist-facies); it rejuvenated coarse-grained muscovite crystals of eo-Alpine age (Early-Middle Cretaceous). The radiometric data presented in this paper demonstrate for the first time an important Early Tertiary tectonic zone in this area.

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The Dorozsma Complex (DC) is defined and its main lithologies are described. The complex makes up the bulk of the Algyõ basement high, representing an outlier of a Cretaceous nappe system in the southernmost part of the Tisza Mega-unit (Great Hungarian Plain). The DC exhibits a polymetamorphic history characterized by a Permian, low-pressure amphibolite facies metamorphism, overprinted by a pressure-dominated eo-Alpine, amphibolite facies metamorphism, accompanied by penetrative mylonitization. "Carboniferous breccias" described by earlier authors and covering the metamorphic rocks sporadically are redefined as tectonized, mostly cataclastic rocks. The uppermost unit of the basement below the Neogene sediments is a nappe composed of Triassic clastic and carbonate sediments. Petrographic similarities of the DC with several units of the Koralpe-Wölz nappe system in the Eastern Alps indicate that both units were formed within a continuous belt in the Cretaceous.

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Core samples of metamorphic rocks from the Sarkadkeresztúr Basement High (Békés-Codru Unit, Tisza Mega-unit) were examined, consisting mainly of two-feldspar augengneiss, garnet-andalusite-staurolite-bearing micaschist and paragneiss. The protolith of the plagioclase-microcline-augengneiss was found to be S-type granite (syenogranite) of unknown age. According to the major element analyses the protoliths of the studied metagranitoids of this basement can be classified as a syenogranite of metaluminous and peraluminous character displaying subalkaline affinity and a calcic granitic composition. The REE patterns are characterized by a slight enrichment of the light REEs and negative Eu and Yb anomalies. On the basis of discrimination diagrams for major and trace elements, the studied rocks were formed in a syn-collisional (continent-continent collision zone) tectonic environment. The low-pressure amphibolite facies metamorphism, which affected this basement, can be assigned to the Variscan.

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