EPISODIC GARNET-GROWTH METAMORPHISM DURING AEGEAN SUBDUCTION AND SYN-OROGENIC EXHUMATION

Garnet geochronology, when combined with phase equilibria modeling and garnet trace element zonation, permits construction of the pressure-temperature (P-T) history of Aegean subducted lithologies. The island of Sifnos is comprised of the Cycladic Blueschist Unit (CBU), which represents part of a subduction-related accretionary complex in the Aegean Sea. The CBU in northern Sifnos is well preserved, lithologically heterogeneous, and is regarded as a structurally coherent sequence that essentially experienced the same metamorphic evolution, consisting of peak P-T conditions of ~2.2 GPa and ~560˚C.

High precision Sm-Nd garnet geochronology was performed on several of these lithologies, generating multi-point isochron ages on both bulk garnet separates and distinct, microsampled growth zones of larger (cm-sized) porphyroblasts. This chronology reveals a metamorphic history involving slow initiation of garnet growth at 53.4 ± 2.6 Ma, followed by a period of rapid growth between 46.95 ± 0.61 Ma and 44.96 ± 0.53 Ma. Individual samples from this growth pulse exhibit entire porphyroblast growth durations spanning just tens to hundreds of thousands of years. Microdrilled garnet rims from some samples, along with bulk ages from garnets in which Sm is heavily concentrated in crystal rims, provided younger ages between 42.5 ± 3.0 Ma and 42.9 ± 1.7 Ma. Petrographic observations along with phase equilibria modeling suggests renewed growth of garnet in these samples at pressures of 1.0-1.6 GPa and 500-540˚C, during the early stages of (relatively isothermal) exhumation. Fluid inclusion microthermometry further corroborates this observation of a lower P re-equilibration event.

This study shows that integrating accurate estimates of the pressure and temperature conditions of garnet growth with a precise chronology is critical, especially in instances wherein polyphase mineral growth occurs over such an overall short (< 10 Myr) time interval. Further, utilizing this large isotopic dataset, in combination with the characterization of garnet trace element zonation and phase equilibria modeling, provides a detailed P-T-t evolution of the CBU and helps elucidate the potential mechanisms for rapid mineral growth at high pressure and the short timescales of early exhumation.