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Together with U–Th–Pb dating, thermobaric analyses and Sm–Nd garnet dating, tight constraints on the metamorphic evolution are obtained. Monazite-Allanite reactions, chemical information and textural observations provide information on the prograde evolution in metapelites (e.g. 1) were also analyzed using U–Th–Pb dating of monazite. Şimleu Silvaniei, Ţicău and Preluca Inselbergs) and from the Rodna Mts. Samples from the Transylvanian inselbergs (i.e. Alpine) metamorphism, samples from the Apuseni Mountains (Mts.) were investigated using U–Th–Pb dating of monazites, Sm–Nd garnet dating and thermobarometric analyses. Thus, to evaluate timing and metamorphic conditions of post-Variscan (i.e.
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However, correlations of the metamorphic and structural evolution between the ALCAPA, Tisza and Dacia mega-units are still problematic because of (1) poly-metamorphic pre-Alpine and Alpine metamorphic overprint under similar P–T conditions and (2) extensive Cenozoic cover of the Pannonian and Transylvanian basins fill hiding bedrocks (cf. 2013) challenged this interpretation, emphasising the importance of Alpine overprint. 2002 Munteanu and Tatu 2003 Balintoni et al. Cretaceous-age) metamorphic evolution of the Tisza and Dacia mega-units as being retrogressive, taking place under low-grade conditions, while medium-grade conditions are exclusively attributed to pre-Variscan and Variscan events (e.g. However, some authors referred and still refer to the Alpine (i.e. The Alpine prograde medium-grade overprint is pronounced in the contact zone between the Tisza and Dacia mega-units and forms a continuous belt with the Cretaceous metamorphic imprint in the Eastern Alps, when back-rotated to its original position during the Cretaceous.ĭuring the last few years, significant advances were made in understanding the tectonometamorphic evolution of the Alps-Carpathians-Dinarides orogenic system during the Cretaceous, based on new information (e.g. Together with other studies from the basement of the Pannonian basin, this implies that the Dacia Mega-Unit and parts of the Tisza Mega-Unit experienced a medium-grade metamorphic overprint and synkinematic garnet-growth during late Early Cretaceous times. Exclusively mid-Cretaceous monazite ages from the inselbergs and the Rebra-Unit of the Rodna Mountains, allow extending the Alpine prograde overprint across the Transylvanian basin. Petrographic observations of prograde allanite breakdown reactions, Sm–Nd garnet analyses and thermobaric estimates (500–550 ☌/5–8 kbar) from parts of the Tisza and Dacia mega-units constrain medium-grade conditions during Early Cretaceous times. Early Cretaceous ages from rims of chemically zoned grains and from monazite inclusions in garnet, biotite and staurolite represent newly formed metamorphic grains that crystallized on the prograde path during Alpine metamorphism. Permian to Early Triassic monazite ages correlate with the intrusion of granitic melts and pegmatites. Relic monazite in the latter is often replaced by rhabdophane and/or surrounded by allanite coronas. Pre-Variscan and Variscan ages were obtained from the core of zoned monazite grains and from samples that apparently escaped Alpine overprinting. Electron-microprobe U-Th–Pb dating of monazite in samples from the Apuseni Mountains, the Rodna Mountains, as well as the Şimleu Silvaniei, Ticău and Preluca inselbergs revealed a complex pattern of Alpine and pre-Alpine age clusters. However, a mid-Cretaceous Sm–Nd garnet age (103.6 ± 1.8 Ma) from the Apuseni Mountains is at odds with this interpretation. Polyphase medium-grade metamorphism observed in mineral assemblages from the crystalline basement is often correlated with Variscan and pre-Variscan events.
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The Tisza and Dacia mega-units constitute a central part of the Alps-Carpathians-Dinarides orogenic system.