Low-grade retrogression of a high-temperature metamorphic core complex: Naxos, Cyclades, Greece

Retrogressive deformation and metamorphism are often reported from the main low-angle shear zones and detachments of metamorphic core complexes, but their importance is not sufficiently emphasized for the footwall interior. In order to contribute to a better understanding of exhumation-related retrogression processes within and at the top of metamorphic core complexes, an integrated detailed microstructural, textural, 40Ar/39Ar geochronological, and thermobarometric study on the Naxos metamorphic core complex within the Aegean Sea is presented that provides a new perspective on low-grade retrogression during exhumation through shallow ductile levels. We found variable retrogressive deformation within the Naxos metamorphic core complex, which even pervasively affected significant portions of the migmatite-grade metamorphic core and remnant high-pressure areas of the metamorphic core complex, where retrogression led to pervasive formation of new fabrics within greenschist-facies metamorphic conditions during brittle-ductile transition. Within a continuum of retrogression, 40Ar/39Ar white mica dating allowed us to deduce three retrogressive ages at 16.52 ± 0.39 Ma (within the Naxos metamorphic core complex), 12.6 ± 0.28 Ma (Moutsounas detachment shear zone on the eastern boundary of the metamorphic core complex), and 10.43 ± 0.44 Ma to 8.40 ± 0.76 Ma (last ductile activity along the Naxos-Paros shear zone to the north of the metamorphic core complex). A further stage of retrogression at 12−11 Ma occurred along distinct low-angle normal faults within the middle Miocene Naxos Granite. Retrogressive microstructures, low-temperature calcite fabrics in marbles, and chloritization in metapelites (at temperatures of ∼350−130 °C) in the metamorphic core complex core resulted mainly from late-stage E-W shortening and folding. Late-stage flow of hydrous fluids resulted in resetting of fabrics and enhancement of ductile deformation. The middle−late Miocene retrogression events are also reflected by a similarly aged tectonic collapse basin in the hanging-wall unit above the detachment. The wide temporal range of retrogression within the Naxos metamorphic core complex coincides in age with retrogressive deformation within other metamorphic core complexes of the Aegean Sea. We interpret the long temporal range of retrogression to reflect outward, southwestward retreat of the subduction and sequential activation of major detachment zones

First Limnological Characterization of the Tropical Crater Lake Amparihibe in the Makira Protected Area, Madagascar. eco.mont (Journal on Protected Mountain Areas Research)|eco.mont Vol. 1 No. 1|

The newly established Makira Protected Area and its environs in Northeast Madagascar host roughly 50% of the entire Malagasy floral biodiversity, but very little was known about the two freshwater lakes found within the park. Lake Amparihibe was explored for the first time in November 2007. According to preliminary 40Ar/39Ar dating, the volcanic crater formed as far back as 25.3 million years ago, but no information is available about when the crater started to fill with water. The protected crater lake has a maximum depth of 28 m and was anoxic beyond a depth of 15 m. During the single sampling occasion, steep gradients in temperature, oxygen, conductivity and pH revealed a stable stratification. Several phyto- and zooplankton taxa showed distinct depth-specific abundance maxima along the steep physico-chemical gradients. The majority of plankton organisms have a cosmopolitan (tropical and temperate) distribution, however more taxonomic research is necessary before definitive conclusions can be drawn. Exposure of a multi-mesh gill net together with visual inspection by snorkeling yielded no presence of fish. Despite the full protection status of the park, illegal introduction of alien fish species seems a realistic threat to this pristine Malagasy lake

Pre-Variscan geological events in the Austrian part of the Bohemian Massif deduced from U-Pb zircon ages

In an attempt to elucidate the pre-Variscan evolution history of the various geological units in the Austrian part of the Bohemian Massif, we have analysed zircons from 12 rocks (mainly orthogneisses) by means of SHRIMP, conventional multi-grain and single-grain U-Pb isotope-dilution/mass-spectrometry. Two of the orthogneisses studied represent Cadomian metagranitoids that formed at ca. 610 Ma (Spitz gneiss) and ca. 580 Ma(Bittesch gneiss). A metagranite from the Thaya batholith also gave a Cadomian zircon age (567-5 Ma). Traces of Neoproterozoic zircon growth were also identified in several other samples, underlining the great importance ofthe Cadomian orogeny for the evolution of crust in the southern Bohemian Massif. However, important magmatic events also occurred in the Early Palaeozoic. A sample of the Gfhl gneiss was recognised as a488-6 Ma-old granite. A tonalite gneiss from the realm of the South Bohemian batholith was dated at 456-3 Ma, and zircon cores in a Moldanubian metagranitic granulite gave similar ages of 440-450 Ma. This Ordovician phase of magmatism in the Moldanubian unit is tentatively interpreted as related to the rifting and drift of South Armorica from the African Gondwana margin. The oldest inherited zircons, in a migmatite from the South Bohemian batholith, yielded an age of ca. 2.6 Ga, and many zircon cores in both Moravian and Moldanubian metagranitoid rocks gave ages around 2.0 Ga. However, rocks from the Moldanubian unit show a striking lack of zircon ages between 1.8 and 1.0 Ga, reflecting an ancestry from Armorica and the North African part of Gondwana, respectively, whereas the Moravian Bittesch gneiss contains many inherited zircons with Mesoproterozoic and Early Palaeoproterozoic ages of ca. 1.2, 1.5 and 1.65?1.8 Ga, indicating a derivation from the South American part of Gondwan