| Resum: |
The Hercynian metamorphic event is poorly characterized in internal zone complexes of the Betic Cordillera (Spain), as it has been, to a great extent, overprinted by the mineral assemblages formed during the Alpine event. Identification of the signals of the Hercynian episode is easier in series largely unaffected by the Alpine event, such as Intermediate units between the Maláguide and the Alpujárride Complexes, which consist of a set of thrust slices. With the aim of characterizing the Hercynian paragenesis, a detailed comparison of the mineral assemblages of Paleozoic and overlying Triassic sequences, unaffected by the pre-Alpine event, was carried out. Mineral assemblages were characterized by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, electron microprobe, and transmission-analytical electron microscopy. A rapid increase in the illite crystallinity values was observed at the Triassic_Paleozoic transition in the upper tectonic slices. In addition, the diagenetic to anchizonal dickite-, sudoite-, and pyrophyllite-bearing assemblages, characterizing the Triassic rocks, contrast with Paleozoic assemblages consisting of white K-mica ± paragonite + chlorite + mica-chlorite and chloritevermiculite mixed layers + garnet, suggesting that this assemblage corresponds to the Hercynian metamorphic event. This assemblage records temperatures on the order of 400C and an intermediate pressure regime. Paleozoic rocks contain, in addition, tobelite, which comprises some of the detrital grains and strongly masks the illite crystallinity values. Tobelite has been identified only in the upper thrust slices, suggesting that changes in the detrital input is primarily responsible for the disappearance of tobelite at the transition from the Maláguide to the Alpujárride domain. Tobelite appears finely intergrown with white Kmica and its origin is uncertain. It could have been inherited as tobelite, but a more likely hypothesis is that the intergrowths of white K-mica and tobelite were formed at low temperature from an NH-bearing mica precursor. |
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