KINETIC IMPLICATIONS OF ARAGONITE/CALCITE TRANSFORMATION FOR MIOCENE EXHUMATION OF HIGH-PRESSURE ROCKS FROM WESTERN CRETE

The preservation of metamorphic aragonite provides an important constraint on the pressures and temperatures of metamorphism in convergent margin settings. The island of Crete, positioned as the fore-arc high of the modern Hellenic subduction zone, exposes some of the world's youngest aragonite-bearing blueschist rocks. There, the high-pressure, low-temperature (0.8 to 1.0 GPa, 300 to 400 °C) rocks of the Plattenkalk and Phyllite-Quartzite (PQ) units are exposed in the footwall of the Miocene Cretan detachment. Previous estimates for peak metamorphic conditions are based on the paragenesis involving carpholite and chloritoid in the metapelites of the PQ unit. Published P-T paths for these rocks cross the aragonite/calcite boundary at around 350 °C during decompression. Experimental work on the kinetics of the aragonite/calcite transformation indicates that at such a high temperature complete retrogression of aragonite to calcite should occur. However, carbonaceous horizons within the PQ contain aragonite, thus conflicting with the previous thermobarometry.

There are several potential solutions to this problem. One possibility is that previous work overestimated the peak metamorphic conditions, and the rocks of the PQ unit passed into the calcite stability field at temperatures sufficiently low to allow the preservation of aragonite. This may result from the poorly known thermodynamic properties of carpholite or potentially unwarranted assumptions about the activity of water. A second possibility is that the rocks underwent a counter-clockwise P-T path with early heating and compression followed by cooling below 200°C before decompressing into the stability field of calcite. Alternatively, rapid cooling may have allowed aragonite to persist metastably during decompression at higher temperatures. If rocks of the PQ unit were cooled quickly enough, aragonite could survive even if it entered the calcite stability field at high temperatures. New apatite fission-track and zircon (U-Th)/He ages are obtained to constrain the rate of cooling of the PQ unit. These data will limit the range of temperatures through which aragonite could be preserved and remain consistent with the thermochronology.