Tectonic Activity Recorded by Multiple Episodes of Zircon Growth: an Ion Microprobe (SHRIMP) Study of the Geological Evolution of the Cyclades, Greece

Abstract

To interpret the significance of multiple episodes of zircon growth, the processes controlling zircon growth must be distinguished. There is currently little agreement concerning the origin and significance of metamorphic zircon and how it can be related to pressure-temperature-time ( P T t ) paths (cf. Fraser et al., 1997; Roberts and Finger, 1997). New zircon growth has been attributed to a number of different processes: (1) melt crystallisation; (2) net-transfer reactions; (3) hydrothermal precipitation; or (4) solid-state recrystallisation/replacement processes. Each of these mechanisms has different implications for the interpretation of zircon ages. As zircon remains closed to isotopic exchange at temperatures in excess of those commonly experienced during either metamorphism or magmatism, zircon ages will generally record the time of growth rather than the time of cooling below its closure temperature. While dating of different layers of zircon can constrain the time of zircon growth it will not shed light on the zircon formation mechanism. To help interpret zircon ages in terms of tectonic processes it is necessary to study zircon morphology, internal structure and chemistry and to try and relate this to the textural relations and chemistry of the zircon host rock. Regional setting. The tectonic evolution of the Cyclades during the Alpine orogeny includes polyphase metamorphism, deformation, fluid infiltration, anatexis and shearing. Hence the Cyclades provides a natural laboratory to assess the behaviour of zircon developed in response to different mechanisms. The Cyclades consist of a Hercynian orthogneiss basement, in unconformable contact with a variegated series of Mesozoic marbles, schists and metavolcanics (Drift et al., 1978). During the Alpine orogeny these rocks experienced an Eocene high pressure metamorphism (M1) overprinted by OligoMiocene greenschist-amphibolite facies medium pressure metamorphism (M2). They form the lower plate to a sequence of metamorphic core complexes exposed in the Aegean (Lister et al., 1984). The upper plate is comprised of ophiolites, Upper Cretaceous/Permo-Triassic limestones and granitoids that have experienced Cretaceous high temperature, low pressure metamorphism but have not experienced any Alpine metamorphism (Reinecke et al., 1982), along with unmetamorphosed sediments and volcanics. Methods. Zircons were separated from rock units that had experienced Alpine orogenesis from the islands of Naxos, Ios and Sifnos. They were imaged using cathodoluminescence techniques to identify internal zircon structures and choose potential target sites for analysis. Layers that could clearly be identified as new growth, potentially related to metamorphism, were preferentially analysed. U, Th and Pb/U in the sample zircons relative to those in the SL13 reference zircon were measured using the sensitive high resolution ion microprobe (SHRIMP). The main advantage in using SHRIMP for U-Pb dating, as opposed to conventional isotope dilution techniques, is that it allows the analysis of = 30 pm areas within mineral grains and thus the measurement of ages for different growth zones. To ensure no 'mixed ages' occurred due to spatial overlap of more than one growth zone by the probe, all mounts were imaged after analysis to check the actual pit location. Assessment of the age data-sets for the presence of discrete age populations was carried out both by construction of age probability distributions and by a mixture-modelling procedure. Zircon characteristics. The identification of multiple layers of metamorphic zircon growth is made according to zircon morphology, internal structure and chemistry. In general, magmatic zircon growth is characterised by the development of oscillatory zoning while metamorphic zircon is generally unzoned, displays low luminescence and has variable, but generally low Th/U ratios. However, zircons produced by small degrees of partial melting can appear as unzoned, low luminescent overgrowths