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

Is metamorphism and its causative tectonics best viewed as a series of punctuated events or as a continuum? This question is addressed through examination of the timing of exhumation of the Cycladic Blueschist Belt (CBB). The cause of scatter beyond analytical error in Rb-Sr geochronology was investigated using a suite of 39 phengite samples. Rb-Sr ages have been measured on phengite microsamples drilled from specific microstructures in thin sections of calcschists and metabasites from the CBB on Syros. The majority are from samples that have well-preserved blueschist facies mineral assemblages with limited greenschist facies overprint. The peak metamorphic temperatures involved are below the closure temperature for white mica so that crystallization ages are expected to be preserved. This is supported by the coexistence of different ages in microstructures of different relative age; in one sample phengite from the dominant extensional blueschist facies fabric preserves an age of 35Ma while post-tectonic mica, millimetres away, has an age of 26Ma. The results suggest that micro-sampling techniques linked to detailed microstructural analysis are critical to understanding the timing and duration of deformation in tectonometamorphic systems. North of the Serpentinite Belt in northern Syros, phengite Rb-Sr ages are generally between 53 and 46Ma, comparable to previous dates from this area. South of the Serpentinite Belt phengite in blueschist facies assemblages associated with extensional fabrics linked to exhumation have ages that range from 42Ma down to c.30Ma indicating that extensional deformation while still under blueschist facies conditions continued until 30Ma. No age measurements on samples with unambiguous evidence of deformation under greenschist facies conditions were made; two rocks with greenschist facies assemblages gave phengite ages that overlap with the younger blueschist samples, suggesting blueschist facies phengite is preserved in these rocks. Two samples yielded ages below 27Ma; one is from a post-tectonic microstructure, the other from a greenschist in which the fabric developed during earlier blueschist facies conditions. These ages are consistent with previous evidence of greenschist facies conditions from c.25Ma onwards. The data are consistent with a model of deformation that is continuous on a regional scale.