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The polymetamorphic evolution path of metapelites characteristic of the Algyo-Ferencszállás Metamorphic Complex of the Békés-Codru Unit (SE Hungary) is presented on the basis of microstructural and mineral paragenetic features, mineral chemical data and geothermobarometric calculations. The first (Variscan?) metamorphic event recorded by the Ca-rich part of zoned garnets from the Ferencszállás area is poorly defined, yielding conditions of 520-560 oC and 820-1010 MPa. This event was followed by a new tectonometamorphic phase that began with the formation of garnet, kyanite, muscovite, plagioclase and biotite. This younger (Alpine) event is marked by the decomposition of staurolite and biotite, and an increase in temperature and pressure. Peak metamorphic conditions reached c. 650 oC and 900 MPa in the Ferencszállás and Újszentiván areas. This P-T peak was followed by continuous decrease in temperature and pressure to c. 500-560 oC and 400-530 MPa. The P-T path calculations from the progressively zoned garnets of the Algyo area show an increase in temperature and pressure similar to the Ferencszállás area – but in shallower crustal depth – and yield 650-680 oC and 500-600 MPa for the metamorphic peak. This study, in combination with recently published geochronological data, presents the first record of Alpine amphibolite facies metamorphic overprint of the basement of the Tisza Unit, which was considered as not affected by medium-grade metamorphism younger than Variscan until now.

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Acta Geologica Hungarica
György Bárdossy
Péter Árkai

Bárdossy György-Fodor János: Evulation of Uncertainties and Risk in Geology (New Mathemathical Approaches for their Handling; Lavrenchuk, Vladimir: Scandium in Bauxit and Clay

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K-Ar age data of illite-K-white mica-rich, <2 µm grain size fraction samples were determined on various lithotypes of the Paleozoic-Mesozoic, very low- to low-grade metamorphic complex, the Jurassic ophiolitic mélange and the Cretaceous-Paleocene sedimentary sequence of Mt. Medvednica, Croatia. K-Ar ages of the high-temperature anchizonal-epizonal Paleozoic-Mesozoic complex scatter around ca. 110 Ma for slates, phyllites and marbles with phyllite intercalations, while they are significantly younger (ca. 80 Ma) for metavolcanoclastic rocks that are devoid of detrital K-white mica. Beside the Cretaceous (Alpine) K-Ar radiometric age data, no evidence of a possibly older, Variscan metamorphic event was detected. In the Jurassic ophiolitic mélange and the Cretaceous-Paleocene sequence the obtained mixed isotopic age data do not provide reliable estimates for the age of the diagenetic alterations. In the Paleozoic and Mesozoic formations from the Bükk, Szendrõ and Uppony Mts. (NE Hungary) and in the Paleozoic rocks series from the Internal Dinarides, similar Alpine (Cretaceous) K-Ar ages were determined. These data may suggest metamorphic processes related to the subduction of the Dinaridic oceanic crust beneath Eurasia accompanied by compressional crustal thickening.

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New mineral paragenetic, illite Kübler index, chlorite “crystallinity”, apparent crystallite thickness, lattice strain, and K-white mica geobarometric data proved that the Eoalpine (Paleozoic-Mesozoic) metamorphic complex was affected by medium-pressure, high-temperature anchizonal regional metamorphism, whereas the Jurassic ophiolitic mélange and the Late Cretaceous-Paleocene sedimentary sequence of Mt. Medvednica were diagenetically altered. Mineral chemical investigations carried out on phyllosilicate flakes found in various microstructural positions revealed complete mineral chemical homogenization of chlorite and K-white mica of selected slate samples from the Eoalpine (Paleozoic-Mesozoic) metamorphic complex. One possible explanation of this feature is an Alpine (Cretaceous) regional metamorphic event with polyphase deformational history. Variscan low-temperature metamorphism overprinted by an Alpine (Cretaceous) event, with temperatures at least as high as those of the Variscan one, may be an alternative, although more complicated explanation. However, no isotope geochronological evidence supports this assumption. At present only one metamorphic event can be detected. Its physical conditions were ca. 350-400 °C on the basis of illite Kübler index and chlorite Al(IV) empirical thermometers and 3-4 kbar using K-white mica b cell dimension measurements.

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Metamorphic mineral assemblages in low-temperature metaclastic rocks often contain paragonite and/or its precursor metastable phase (mixed K-Na-white mica). Relationships between the bulk rock major element chemistries and the formation of paragonite at seven localities from Central and SE-Europe were studied, comparing the bulk chemical characteristics with mineral assemblage, mineral chemical and metamorphic petrological data. Considerable overlaps between the projection fields of bulk chemistries of the Pg-free and Pg-bearing metaclastic rocks indicate significant differences between the actual (as analyzed) and effective bulk chemical compositions. Where inherited, clastic, inert phases/constituents were excluded, it was found that a decrease in Na/(Na+Al*) and in K/(K+Al*) ratios of rocks favors the formation and occurrence of Pg and its precursor phases (Al* denotes here the atomic quantity of aluminum in feldspars, white micas and “pure” hydrous or anhydrous aluminosilicates). In contrast to earlier suggestions, enrichment in Na and/or an increase in Na/K ratio by themselves do not lead to formation of paragonite. Bulk rock chemistries favorable to formation of paragonite and its precursor phases are characterized by enrichment in Al and depletion in Na, K, Ca (and also, Mg and Fe2+). Such bulk rock chemistries are characteristic of chemically “mature” (strongly weathered) source rocks of the pelites and may also be formed by synand post-sedimentary magmatism-related hydrothermal (leaching) activity. What part of the whole rock is active in determining the effective bulk chemistry was investigated by textural examination of diagenetic and anchizone-grade samples. It is hypothesized that although solid phases act as local sources and sinks, transport of elements such as Na through the grain boundaries have much larger communication distances. Sodium-rich white micas nucleate heterogeneously using existing phyllosilicates as templates and are distributed widely on the thin section scale. The results of modeling by THERMOCALC suggest that paragonite preferably forms at higher pressures in low-T metapelites. The stability fields of Pg-bearing assemblages increase, the Pg-in reaction line is shifted towards lower pressures, while the stability field of the Chl-Ms-Ab-Qtz assemblage decreases and is shifted towards higher temperatures with increasing Al* content and decreasing Na/(Na+Al*) and K/(K+Al*) ratios.

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