White mica ⁴⁰Ar/³⁹Ar age spectra and the timing of multiple episodes of high-Pmetamorphic mineral growth in the Cycladic eclogite-blueschist belt, Syros, Aegean Sea, Greece

Abstract

New geochronology from Syros in the Cycladic eclogite-blueschist belt, Aegean Sea, Greece, shows that ⁴⁰Ar/³⁹Ar geochronology consistently dates microstructural events in metamorphic rocks. We demonstrate that the age spectra depend on microstructure in a predictable and systematic way. Ages can be inferred by applying the method of asymptotes and limits to data from the step-heating experiments. The results are consistent with previously published estimates for the timing of a sequence of distinct and discrete episodes of high-P metamorphic mineral growth observed regionally across this belt. Arrhenius plots from these experiments imply that phengitic white mica is highly retentive of argon, and therefore (if these data can be extrapolated to the natural environment) the ages can be interpreted as recording the timing of episodic deformation and metamorphism. Porphyroblastic growth begins: (i) for omphacite-jadeite-eclogite facies parageneses at c. 53 Ma; and (ii) for garnet-glaucophane facies parageneses at c. 47 Ma. The Kini Shear Zone started as an extensional post-epidote-albite-transitional-blueschist facies shear zone that had completed operation by c. 31 Ma. The scatter in ages is due to the effect of deformation, recrystallization and multiple growth events in shear zones that continued operating for 3-6 million years from the start of each episode.