INTERPRETING METAMORPHIC HISTORIES PRESERVED IN KYANITE

Cathodoluminescence (CL) images reveal zoning patterns in kyanite that can be used to decipher complex metamorphic histories of metapelitic rocks. In this study, we aim to evaluate if kyanite can preserve a record of multiple metamorphic events, including high- to ultrahigh-pressure (UHP) conditions. We examined three kyanite-bearing metapelites from diamond-bearing localities within the Rhodope Metamorphic Complex in eastern Greece. One of the samples originates from the lower tectonic unit; the other two samples originate from the upper tectonic unit. The prograde pressure-temperature-time histories of these units are similar, but the upper unit remained ca. 100°C hotter than the lower unit. Evidence of UHP metamorphism was largely overprinted during exhumation through granulite facies.

Zoning patterns observed in CL images and grain morphologies vary among the samples. Kyanite grains from the lower unit are typically mm to cm in scale and encompass a broad range of CL intensities; patterns are interpreted as both sector and oscillatory zoning. All grains contain a CL-bright domain with rounded margins that cross-cuts the zoning patterns within the core of the grain. Evidence of dissolution is only apparent in the outer 1–2 mm of some grains. Kyanite grains from the upper unit are small (< cm in diameter) and exhibit sector, oscillatory, and planar zoning patterns. The planar pattern is only observed within the upper unit and is characterized as micrometer-scale variations in CL intensity without a systemic orientation, resembling broken glass. As with the lower unit, these zoning patterns are cross-cut by a CL-bright domain with lobate boundaries. Some grains show minimal evidence of dissolution whereas others are almost entirely dissolved.

These data demonstrate that kyanite is a useful tool to identify discrete metamorphic events. The presence of the CL-bright domain in all samples suggests that this domain formed as these samples were metamorphosed at granulite facies, likely at mid-crust conditions (7–8 kbar). Differences in the zoning patterns observed in the upper and lower units may be attributed to local bulk chemistry or the conditions of prograde metamorphism or exhumation to the mid-crust. Work is in progress to resolve the significance of the planar zoning pattern observed in the upper unit.