Insight Into the Early Exhumation of the Cycladic Blueschist Unit, Syros, Greece: Combined Application of Zoned Garnet Geochronology, Thermodynamic Modeling, and Quartz Elastic Barometry

Constraining conditions and mechanisms of the early stages of exhumation from within subduction zones is challenging. Although pressure, temperature, and age can be inferred from the exhumed rock record, it is generally difficult to derive each of these parameters from any single rock, thus demanding assumptions that diverse data from multiple samples can be safely combined into a single pressure‐temperature‐time (P‐T‐t) path that might then be used to infer tectonic context and mechanisms of exhumation. Here, we present new thermobarometric and geochronologic information preserved in a single sample from Syros, Greece, to deduce the conditions and rates of the earliest phase of exhumation as a part of the well‐preserved high‐pressure metamorphic rocks of the Cycladic Blueschist Unit (CBU). The sample studied here is a garnet‐bearing, quartz‐mica schist that records two distinct metamorphic events. Results from thermodynamic models and quartz‐in‐garnet elastic geobarometry show that metamorphic garnet cores formed as P‐T conditions evolved from ∼485°C and 2.2 GPa to 530°C and 2.0 GPa, and that garnet rims formed as conditions evolved from ∼560°C and 2.1 GPa to ∼550°C and 1.6 GPa. Sm‐Nd geochronology on garnet cores and rims yields ages of 45.3 ± 1.0 and 40.5 ± 1.9 Ma, respectively, thus indicating a 4.8 ± 2.1 Myr growth span. Given the decompression path calculated based on garnet core and rim P‐T estimates, we conclude that the distinct phases of garnet growth preserve evidence of the initial exhumation of portions of the CBU. Plain Language Summary Constraining the evolution of rocks formed at great depths (>60 km) and their subsequent returned to Earth's surface is a challenging endeavor, because information regarding the pressure, temperature, and timing of formation is typically elucidated through the study of multiple samples and thus through the comparison of different datasets. Here, we present pressure, temperature, and age data that were derived by analyzing garnet crystals within a single sample from a well‐known paleo‐subduction zone locate in Syros, Greece. Our results show that the sample records two metamorphic events that occurred as a result of initial ascent to the surface from depths of ∼66 to 48 km. Key Points Metamorphic garnets record two stages of growth during HP/LT metamorphism and exhumation of the Cycladic Blueschist Unit (CBU) on Syros, Greece Garnet growth spanned 4.8 ± 2.1 Myr including decompression from 2.1 to 1.6 GPa Garnet