Petrochronology of polygenetic white micas (Naxos, Greece)

Naxos in the Greek Cyclades preserves a type example of polymetamorphism. The southern and northern parts of the island record different Tertiary P–T histories between Eocene and Miocene times, including a blueschist facies event, one or more amphibolite/greenschist facies overprint(s) and contact metamorphism. Age attributions for these events are inconsistent in the literature. Here, we propose a new approach that combines electron probe microanalyzer (EPMA) characterization of the white mica (WM) with 39Ar‐40Ar–Rb‐Sr multichronometry. Textural–petrographic–compositional observations reveal that the polygenetic WM consists of five different generations: pre‐Eocene relicts, paragonite, high‐Si phengite, low‐Si phengite and muscovite. EPMA mapping of four WM samples, previously analysed by Rb‐Sr, reveals major element compositions heterogeneous down to the μm scale. Each WM consists of chemically distinct generations, documenting submicron‐scale retrogression of high‐pressure (HP) phengite grains to muscovite. Four WM samples from a N‐S traverse across the island were analysed by 39Ar‐40Ar stepheating, comparing coarse and fine sieve size fractions to obtain overdetermined K‐Ar systematics. Fine sieve fractions are richer in Cl than coarse ones. Linear arrays in Cl/K‐age isotope correlation diagrams show two predominant WM generations (one Cl‐poor at ca. 38 Ma and one Cl‐rich at