Mid-Cretaceous Alpine metamorphism of the Tisza Mega-Unit (Inner Carpathians) constrained by illite K–Ar dating and mineral metamorphic indicators

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Apuseni Mts. represent the biggest exhumed area of two major tectonic blocks of the Inner Carpathians: the Tisza and Dacia Mega-Units, with the Biharia Nappe System (NS) thrusted over the Codru NS, which in turn is thrusted over the Bihor Unit. Alpine metamorphism was previously detected in the Bihor Unit and the Biharia NS, but not in the Codru NS. This study is aimed at explaining this difference, using deformed Permian volcanic rocks from the Codru NS. Illite/mica is the dominant metamorphic mineral in the deformed felsic rocks, while chlorite dominates in the deformed mafic rocks. Illite/mica is a mixture of 1 M and 2M1 polytypes (also 1M d in one sample), perfectly non-swelling, and with the 0.31–0.35 Δ° 2θ range of the standardized Kübler index. Electron probe microanalysis (EPMA) documented two varieties of illite/mica: illite/phengite and illite/muscovite. Combined mineral metamorphic indicators (illite/mica characteristics, presence of epidote, and the absence of metamorphic biotite) restrict the maximum metamorphic temperatures to the 250–310 °C range, explaining the lack of Alpine ages in the earlier studies: temperatures below resetting of the old coarse-grained micas, which were used for dating. Our new K–Ar data constrain the cooling ages of illite/mica in the Codru NS to 117—112 ± 3.2 Ma in the Dieva and Moma nappes, and 102 ± 3 Ma in the Arieseni nappe. These ages correspond to the denudation during the eastward advancing of the thrust wedge during the “Austrian” tectonic phase.