16 Petrologic Insights on Modern and Ancient Plate Margins II: The Volcanic, Plutonic, and Metamorphic Records

Sunday, 18 March 2018
1:30 PM - 5:30 PM
University Amphitheater

DETERMINING THE P/T PATH OF METAMORPHOSED PHYLLITES USING CHLORITOID-CHLORITE-PHEGITE AND OTHER BLUESCHIST FACIES ASSEMBLAGES IN PELOPONNESUS, GREECE

Paper number: 16-3
Program: Theme Sessions
Time: 2:10 PM – 2:30 PM

Authors

Abstract

This study looks closely at Metamorphosed Phyllites from Peloponnesus, Greece that formed as a result of the subduction of the African plate under the Eurasian plate that has been in convergence since the late Mesozoic. Samples from the Phyllite-Quartzite (PQ) unit show evidence of blueschist facies metamorphism at high-pressure, low-temperature (HP-LT). A single sample contains abundant glaucophane, coexisting with quartz and phengite, evidence for HP-LT conditions. Other phyllites contain the general assemblage Qtz+Phg+Mg-Ctd+Chl+Ttn±Ab±Pa. Samples were analyzed using the scanning electron microscope to determine the mineral and chemical compositions. Standards based quantitative analyses were performed on chloritoid, chlorite, phengite, epidote and glaucophane in seven thin sections and used to investigate the thermobarometric conditions under which they formed.

Chlorites are either chlinochlore or chamosite, with XMg of ranging from 0.38 to 0.75. In general, XMg of chloritoid increases with that of chlorite, and ranges from 0.08 to 0.25. For samples containing both chlorite and chloritoid, we assess the temperature of the phyllites using the Mg-Fe exchange thermometer of Vidal et al. (1999). Calculated KDs suggesting a range of temperatures from 350 to 530 ˚C. The Si content of phengite in the seven analyzed samples ranges from 3.08 to 3.42, averaging 3.26, and showing minimal within sample variation. In the absence of K-feldspar, this information provides a minimum pressure of ~10 kbar at 450 ˚C. This is significantly lower pressures than the 17 kbar found by Theye and Seidel (1991) as we have not found Si contents as high as those authors. In addition, the influence of the pyrophyllite component on calculated celadonite activity should be re-evaluated.

If the HP-LT PQ blueschists experienced maximum pressures much lower than previously thought, models for subduction in the Aegean would not require nearly the rate and scale of exhumation proposed by other workers relying on these estimates. This study will continue to address the data to attain a better understanding of the P/T path of the phyllites.

Geological Society of America Abstracts with Programs. Vol. 50, No. 2

doi: 10.1130/abs/2018NE-310515

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