Rapid, paced metamorphism of blueschists (Syros, Greece) from laser-based zoned Lu-Hf garnet chronology and LA-ICPMS trace element mapping

Abstract

Unravelling the timing and rate of subduction-zone metamorphism and H2O release requires linking time to the compositional changes of petrogenetic indicator minerals in blueschists and eclogites. Garnet is a key mineral in this regard, as it forms by, and records, H2O-releasing mineral reactions that can be linked to pressure and temperature conditions. These can be placed in time using chronology. Bulk-or multi-grain ages are the norm in garnet chronology. While these ages provide important and precise time constraints on reactions across both facies, the duration, rates and style of growth—crucial parameters in the petrological and geochemical evolution of subducting oceanic crust—remain unconstrained. Domain dating, i.e., dating of individual growth zones, is necessary to investigate these aspects, but is generally impeded in “common”-sized garnet grains due to sample size requirements. To overcome these limitations, we use a low-loss micro-sampling by laser cutting combined with Lu-Hf chronology. We used this approach to precisely date multiple growth zones in a 1-cm garnet grain from glaucophane-bearing micaceous schist of the Cycladic Blueschist Unit (CBU), Syros Island, Greece. The analysis was combined with major- and trace-element mapping (EPMA, LA-ICPMS) to investigate garnet petrogenesis. Garnet core and mantle zones are chemically comparable and identical in age within 0.1 Myr precision (2σ), indicating an initial growth pulse at 51.8 ± 0.1 Ma (MSWD = 1.13). The two rim zones, which are chemically distinct, were resolvably younger at 51.3 ± 0.2 Ma (MSWD = 0.67). Integrated with compositional data, these age constraints provide a time-resolved record of extremely fast, pulsed garnet growth and fluid-release reactions, such as chloritoid breakdown. These reactions can be considered near-instantaneous on subduction time scales, indicating that re-equilibration far outpaces changes in pressure and temperature conditions. The analysed sample is associated with the upper-subunit of the CBU with oceanic affinities, which does not share the last part of the prograde history with the rest of the CBU, i.e., the margin sub-unit. We interpret our sample as a sliver that became attached to the base of the ophiolitic sub-unit and was thrusted onto the lower margin unit at high-pressure. Beyond providing insights into the timing of subduction-zone processes, our new protocol for zoned garnet Lu-Hf geochronology of “common-sized” garnet opens new possibilities for constraining the causes and rates of garnet growth and the pace of tectonic processes in general.