P–T Evolution of the Cyclades Blueschist Unit: Constraints on the Evolution of a Nascent Subduction System From Zr‐In‐Rutile (ZiR) and Quartz‐In‐Garnet (QuiG) Thermobarometry

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 25; no. 4
• Main Authors: Spear, Frank S., Wolfe, Oliver M., Thomas, Jay B., Hubbard, Julia E., Castro, Adrian E., Cheney, John T.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01-04-2024; Wiley
• Subjects: CBU; Chlorite; Cyclades blueschists; Earthquakes; Evolution; Garnet; Ios; Island arcs; Islands; Metamorphism; Minimum temperatures; Outcrops; P–T paths; Quartz; QuiG barometry; Rock; Rocks; Rutile; Seismic activity; Seismicity; Sifnos; Subduction; Subduction zones; Syros; Thermometry; Volcanism; ZiR thermometry; Greece; Syros; Sifnos
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2023GC011121

New results that employ Zr‐in‐rutile thermometry (ZiR) and quartz‐inclusion‐in‐garnet (QuiG) barometry constrain the P–T conditions of garnet formation in blueschists and eclogites from the island of Syros, Greece. QuiG barometry reveals that garnet from different regions across the island formed at pressures ranging from 1.1 to 1.8 GPa and ZiR thermometry on rutile inclusions in garnet constrains the minimum temperature of garnet formation to have been 475–550°C. Most importantly, there is no systematic difference in the conditions of garnet formation from different regions across the island and these results are nearly identical to those obtained from the islands of Sifnos and Ios, Greece. A model is proposed whereby the rocks from all three islands were initially metamorphosed along a relatively shallow geotherm of around 11°C/km to a depth of around 45 km and were then subjected to metamorphism along a geotherm of around 7–8°C/km, which could have been caused by either an increase in the dip of the subduction zone or an increase in the rate of subduction. Garnet formed along this steeper geotherm was accompanied by the release of significant H2O from the breakdown of chlorite over a duration of 1 Ma or less based on thermal and diffusion modeling. It is concluded that rocks from Syros, Sifnos and Ios all followed a similar, roughly counter‐clockwise prograde P–T path and that the present outcrop configuration is largely due to a complex exhumation history.
Plain Language Summary
The metamorphism of rocks in subduction zones releases large quantities of H2O, which ultimately helps flux melting in the overlying mantle leading to explosive island arc volcanism and provides a trigger for large earthquakes. One of the dominant processes that produce large amounts of H2O is the formation of the mineral garnet. Here, we present results that constrain the pressure and temperature conditions for the formation of garnet from three islands in the Greek Cyclades: Syros, Sifnos, and Ios. Our results indicate that garnet formed in different rocks on all three islands in a similar subduction channel along a trajectory where the pressure increase was relatively rapid and the temperature remained nearly constant. The duration of garnet formation in the entire suite of samples is estimated to have occurred in 1 million years or less, during which time significant quantities of fluid are inferred to have been released and which most likely had a major impact on volcanism and seismicity in the region while subduction was active.
Key Points
Blueschist and eclogite assemblages do not necessarily reflect equilibrium crystallization
Exhumation following garnet growth must occur within 1 Ma to preserve compositional zoning
The prograde subduction P–T path in the Cyclades is concave upward with initial shallow subduction followed by near isothermal loading