ZIRCON RECRYSTALLIZATION IN GRENVILLIAN ECLOGITES - EVIDENCE FROM A CL AND SHRIMP STUDY

Zircon separates from eclogites in the High Pressure belt of the Grenville Province were examined with an optical microscope and a cathodoluminescence detector attached to an SEM in order to characterize their form and internal zonation. Optical imaging revealed that the majority of grains had rounded terminations, were variably resorbed, and had length:width ratios of 2:1 or less, compatible with growth in a metamorphic environment. Many of the zircons contained fluid inclusions, both larger inclusions showing evidence for necking and/or trains of smaller secondaries, implying the existence of a fluid during metamorphism. CL imaging of the zircons revealed a range of zoning features that were qualititively classified into primary (i.e., igneous), modified primary, and secondary (i.e., metamorphic) types. Possible primary growth features, such as oscillatory zoning, are very rare in the sample suite. Modified primary zoning, in which oscillatory zones are broader and more diffuse than in primary zoning, occurs in a subset of grains. The majority of zircons shows evidence for various types of secondary zoning. Some grains display metamorphic growth zoning (e.g., radial sector and 'fir-tree' types), features that are associated with growth under high temperature metamorphic conditions. Other grains, which commonly have resorbed margins, display zoning that is unrelated to the crystal margins and must have undergone intra-crystalline diffusion or partial recrystallization. SHRIMP analyses of such grains show that U, Th and Pb concentrations very dramatically across short distances, suggesting that these elements were remobilised during metamorphism. In addition, reverse discordance is common. The presence of secondary fluid inclusions and the inference of trace-element remobilization implies that ion-probe analysis sites within these zircons may not have behaved as closed systems, and hence that interpretation of their ages, and by extension the timing of eclogite-facies metamorphism, is not straightforward.