The timing of high-pressure metamorphism on Andros Island, Greece: evidence for both Cretaceous and Eocene blueschist-facies events?
Final Report Abstract
This study corroborates interpretations suggesting that the Makrotantalon Unit on Andros represents a tectonic slice with Pelagonian affinity in the nappe stack of the Cycladic Blueschist Unit. Previously reported Cretaceous 40Ar–39Ar dates of a garnet-glaucophane schist from the Makrotantalon Unit could not be reproduced by Rb–Sr geochronology, but this is not an indication of contamination with excess Ar. Instead, the newly dated samples record disturbance of the Rb–Sr isotope system by partial recrystallization. Subsets of these phengite populations, representing the smaller grain size fractions, yielded low-precision dates ranging from c. 21 to c. 15 Ma that document deformation-related resetting and recrystallization of a presumably Cretaceous white mica population. Although these Miocene dates cannot be linked with blueschist-facies metamorphism, they provide time constraints on the formation of shear zones that overprinted the original thrust contact during exhumation. The geological relevance of a Cretaceous high-pressure event is confirmed by a Rb–Sr date of c. 121 Ma for an epidote-glaucophane schist collected further away from the tectonic contact. The occurrence of a second blueschist-facies event in the Eocene is verified by Rb–Sr dates of two epidote-glaucophane schists (c. 44 Ma and c. 40 Ma) that can unambiguously be assigned to the Makrotantalon Unit. This study is not providing high-precision geochronology, but rather shows that a disturbed data set can still make sense in the regional chronological framework. The new apparent ages are consistent with existing Ar–Ar and Rb–Sr data of similar rocks from the study area and are considered to place geologically meaningful constraints on the time of two distinct blueschist-facies events that occurred c. 60–70 Ma apart. The Cape Steno mélange in southern Andros is different to the HP/LT mélanges of the CBU on the neighboring islands of Syros and Tinos. The combination of Jurassic metaophiolites and HP/LT metamorphism is unusual and a unique feature. Judging from field observations, geochronological and bulk rock geochemical data, we consider a relationship to the Makrotantalon Unit of NW Andros to be very likely.
Publications
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The Jurassic meta-ophiolitic rocks of Cape Steno, Andros, Greece: a high-pressure/low-temperature mélange with Pelagonian affinity in the Cycladic Blueschist Unit?. International Journal of Earth Sciences, 111(3), 949-968.
Höhn, Melina; Bröcker, Michael & Berndt, Jasper
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The timing of blueschist-facies metamorphism in the Makrotantalon Unit on Andros Island, Greece: Cretaceous and Eocene high-pressure/low-temperature events?. Geological Magazine, 159(8), 1437-1453.
Bröcker, Michael; Scherer, Erik E.; Xypolias, Paris & Höhn, Melina
