Constraining the exhumation history of high-pressure subduction zone rocks : insights from the Cycladic islands, Greece and the application of novel thermobarometry techniques

Cisneros, Miguel
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The mechanisms that lead to exhumation of high-pressure low-temperature (HP-LT) metamorphic rocks have remained enigmatic since the recognition of the petrologic significance of blueschist facies mineral assemblages. Much of our understanding about these mechanisms stems from constraints of the pressure-temperature history of exhumed metamorphic rocks. In this work I describe the results of four projects that focus on testing new thermobarometers that can be applied to metamorphic rocks, among other rock types, and then apply multiple integrative techniques to constrain the pressure-temperature-fluid histories of rocks from Syros and Tinos, Greece. Elastic thermobarometry is a rapidly expanding technique that can be used to constrain pressures and temperatures from inclusion-host mineral pairs. I present a compilation of new inclusion-host mineral pairs that can be used to constrain PT conditions, and an adaptive script that can be used for calculating entrapment pressure or temperature conditions from residual inclusion pressures. I further complete a case-study of the quartz-in-epidote barometer, because of its great abundance in metamorphic rock types and its potential utility for understanding geologic systems that currently lack barometers. Results from heating measurements of quartz inclusions in epidote from Alpine metamorphic rocks indicate two promising conclusions: 1) calculated entrapment pressures show good agreement with previously constrained pressure conditions, indicating that further investigation of the barometer is warranted, and 2) the quartz-in-epidote barometer may not require temperature-dependent entrapment pressure corrections.

The Cycladic Blueschist Unit on both Syros and Tinos, Greece, preserves exhumed, HP subduction zone rocks that have an enigmatic history after having reached peak metamorphic conditions. I present a combination of thermobarometry and mineral chemistry techniques that include further application of the quartz-in-epidote barometer, oxygen isotope thermometry, and fluid chemistry as deduced from oxygen, hydrogen, and carbon isotopes, to better understand the protracted exhumation history of metamorphic belts. Results from PT-fluid data from select outcrops that record changing kinematics, indicate that retrograde metamorphic rocks from Syros Greece underwent cooling during decompression, and interaction with slab derived fluids during exhumation. Lower pressure samples from Tinos Greece and hydrogen isotopes possibly show evidence of interaction with meteoric derived fluids during later stages of exhumation along a low-angle detachment.