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

R A Cliff,J E Dixon,C E Bond,R W H Butler

doi:10.1111/jmg.12228

Abstract

AbstractIs 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 35 Ma while post‐tectonic mica, millimetres away, has an age of 26 Ma. 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 46 Ma, 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 42 Ma down to c. 30 Ma indicating that extensional deformation while still under blueschist facies conditions continued until 30 Ma. 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 27 Ma; 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. 25 Ma onwards. The data are consistent with a model of deformation that is continuous on a regional scale.

Highlights

Considerable progress has been made in improving the analytical quality of geochronological data from metamorphic belts and the range of ages determined commonly greatly exceeds the analytical uncertainties
The amounts of Sr ranged from a few hundred picograms up to hundreds of nanograms; for the phengite the lowest amounts involved significant blank corrections and this produces an important increase in uncertainty in 87Sr/86Sr but it leads to strong correlation with the 87Rb/86Sr ratio so that the effect on the age uncertainty is demagnified
The temperatures involved along the P–T–t paths followed by the Cycladic Blueschist Belt (CBB) remained below those required for significant diffusive equilibration of white mica as discussed above

Summary

Introduction

Considerable progress has been made in improving the analytical quality of geochronological data from metamorphic belts and the range of ages determined commonly greatly exceeds the analytical uncertainties. Often the data are evaluated in the framework of an episodic model and supposed excess scatter in ages is attributed to poorly understood ‘geological error’ (e.g. incomplete resetting of the isotope system). This begs the question; is the geological history of an orogenic belt best described as a series of distinct, discrete ‘events’ or as an evolving continuum of processes? Temperature, pressure and stress variations are undoubtedly continuous but the response of individual rocks may be discontinuous as strain migrates within a rock pile and crystallization of minerals may be further limited by kinetic considerations such as fluid access.

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