The geological significance of ⁴⁰Ar/³⁹Ar and Rb–Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re)crystallization during exhumation?

Abstract

The Attic‐Cycladic crystalline belt in the central Aegean region records a complex structural and metamorphic evolution that documents Cenozoic subduction zone processes and exhumation. A prerequisite to develop an improved tectono‐metamorphic understanding of this area is dating of distinct P–T–D stages. To evaluate the geological significance of phengite ages of variably overprinted rocks, 40Ar/39Ar and Rb–Sr analyses were undertaken on transitional blueschist–greenschist and greenschist facies samples from the islands of Syros and Sifnos. White mica geochronology indicates a large age variability (40Ar/39Ar: 41–27 Ma; Rb–Sr: 34–20 Ma). Petrologically similar samples have either experienced greenschist facies overprinting at different times or variations in ages record variable degrees of greenschist facies retrogression and incomplete resetting of isotopic systematics. The 40Ar/39Ar and Rb–Sr data for metamorphic rocks from both islands record only minor, localized evidence for Miocene ages (c. 21 Ma) that are well documented elsewhere in the Cyclades and interpreted to result from retrogression of high‐pressure mineral assemblages during lower pressure metamorphism. Field and textural evidence suggests that heterogeneous overprinting may be due to a lack of permeability and/or limited availability of fluids in some bulk compositions and that retrogression was more or less parallel to lithological layering and/or foliation as a result of, possibly deformation‐enhanced, channelized fluid ingress. Published and new 40Ar/39Ar and Rb–Sr data for both islands indicate apparent age variations that can be broadly linked to mineral assemblages documenting transitional blueschist‐to‐greenschist‐ and/or greenschist facies metamorphism. The data do not record the timing of peak HP metamorphism, but may accurately record continuous (partial) resetting of isotopic systematics and/or (re)crystallization of white mica during exhumation and greenschist facies retrogression. The form of 40Ar/39Ar phengite age spectra are complex with the lowest temperature steps yielding Middle to Late Miocene ages. The youngest Rb–Sr ages suggest maximum ages of 20.6 ± 0.8 Ma (Syros) and 22.5 ± 0.6 Ma (Sifnos) for the timing of greenschist facies overprinting. The results of this study further accentuate the challenges of interpreting isotopic data for white mica from polymetamorphic terranes, particularly when mixing of populations and/or incomplete resetting of isotopic systematics occurs during exhumation. These data capture the full range of isotopic age variations in retrogressed HP rocks documented in previous isotopic studies, and can be interpreted in terms of the geodynamic evolution of the Aegean.

Geochronological challenges posed by continuously developing tectonometamorphic systems: insights from Rb–Sr mica ages from the Cycladic Blueschist Belt, Syros (Greece)

“…The extent to which the wide range of mineral ages found in previous studies can be interpreted in terms of specific crystallization events was recently discussed by Bröcker et al . (). They concluded that actual ages of crystallization were masked by some combination of partial resetting of older ages and mixing of different generations of mica.…”

Pulsed dehydration and garnet growth during subduction revealed by zoned garnet geochronology and thermodynamic modeling, Sifnos, Greece

“…The second metamorphic stage consisted of Miocene greenschist to amphibolite facies overprinting of the earlier metamorphism in the southern part of the island (Fig. 1), dated at 24-18 Ma using K-Ar, Rb-Sr, and Ar-Ar on white micas (Altherr et al, 1979;Wijbrans et al, 1990;Bröcker et al, 2013). This latter phase of metamorphism occurred during exhumation and infiltration of an ascending fluid phase (Breeding et al, 2003).…”

The geological signature of a slab tear below the Aegean

“…(Altherr et al, 1982;Henjes-Kunst et al, 1988;Wijbrans and McDougall, 1988;Schliestedt et al, 1994;Keay et al, 2001;Hejl et al, 2002;Parra et al, 2002;Ring et al, 2003;Bröcker et al, 2004;Jolivet et al, 2004a;Kumerics et al, 2005;Brichau et al, 2006;Bröcker and Franz, 2006;Duchêne et al, 2006;Martin et al, 2006;Brichau et al, 2007;Skarpelis et al, 2008;Baltatzis et al, 2009;Iglseder et al, 2009;Liati et al, 2009;Seward et al, 2009;Bolhar et al, 2010;Brichau et al, 2010;Denèle et al, 2011;Ring et al, 2011;Schneider et al, 2011;Grasemann et al, 2012;Bargnesi et al, 2013;Berger et al, 2013;Bröcker et al, 2013;Soukis and Stockli, 2013;Beaudoin et al, 2014;Huet et al, 2014;Laurent et al, 2015) A C C E P T E D M A N U S C R I P T…”

“…The deepest parts of the metamorphic pile of Syros and Sifnos display younger ages for the ductile exhumation (see Bröcker et al, 2013). The kinematics of the main exhumation below the Vari detachment is mostly top-to-the east (Trotet et al, 2001a;2001b) and it precedes the Aegean extension.…”