The Cycladic Blueschist Unit on Tinos, Greece: Cold NE Subduction and SW Directed Extrusion of the Cycladic Continental Margin Under the Tsiknias Ophiolite. Issue 9 (22nd September 2020)

• Record Type: Journal Article
• Authors: Lamont, Thomas N.; Searle, Michael P.; Gopon, Phillip; Roberts, Nick M. W.; Wade, Jon; Palin, Richard M.; Waters, David J.
• Is Part Of: Tectonics. Volume 39:Issue 9(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-09-22
• Subjects: lawsonite -- petrological modeling -- Cycladic Blueschist Unit -- extrusion tectonics -- Tinos; Geology, Structural -- Periodicals; 551.8
• DOI: 10.1029/2019TC005890
• Languages: English
• ISSNs: 0278-7407

Abstract

Abstract: High pressure‐low temperature ( HP‐LT ) metamorphic rocks structurally beneath the Tsiknias Ophiolite make up the interior of Tinos Island, Greece, but their relationship with the overlying ophiolite is poorly understood. Here, new field observations are integrated with petrological modeling of eclogite and blueschists to provide new insight into their tectonothermal evolution. Pseudomorphed lawsonite‐, garnet‐, and glaucophane‐bearing schists exposed at the highest structural levels of Tinos (Kionnia and Pyrgos Subunits) reached ~22–26 kbar and 490–520°C under water‐saturated conditions, whereas pseudomorphed lawsonite‐ and aegirine‐omphacite bearing eclogite reached ~20–23 kbar and 530–570°C. These rocks are separated from rocks at deeper structural levels (Sostis Subunit) by a top‐to‐SW thrust. The Sostis Subunit records P‐T conditions of ~18.5 kbar and 480–510°C and is overprinted by pervasive top‐to‐NE shearing that developed during exhumation from (M1 ) blueschist to (M2 ) greenschist facies conditions of ~7.3 ± 0.7 kbar and 536 ± 16°C. These P‐T‐D relationships suggest that the Cycladic Blueschist Unit represents a discrete series of tectonometamorphic subunits that each experienced different tectonic and thermal histories. These subunits were buried to variable depths and sequentially extruded toward the SW from a NE dipping subduction zone. The difference in age and P‐T conditions between the HP‐LT rocks and the overlying metamorphic sole of the Tsiknias Ophiolite suggests that this NE dipping subduction zone was active between circa 74 and 46 Ma and cooled at a minimum rate of ~1.2–1.5°C/km/Myr prior to continent‐continent collision between Eurasia and Adria/Cyclades. Plain Language Summary: High pressure‐low temperature metamorphic rocks make up the interior of Tinos Island, Greece and underlie an ancient piece of oceanic crust and mantle (Tsiknias Ophiolite). The rocks that make up Tinos were once mudstones and limestones deposited on the NE margin of the Cycladic‐Adriatic continent and were deeply buried in a subduction zone. In this work, we investigate the structures and mineralogy of the Tinos high‐pressure rocks and find they were buried to variable depths (80–60 km) and reached temperatures of 500–550°C. These packages of rock were then emplaced against each other during their return journey toward the surface after subduction ended. We document evidence for circa 25 Myr (ca. 74 and 46 Ma) of subduction prior to continent‐continent collision (between Eurasia and Adria/Cyclades) on Tinos and that this records the closure of an ancient ocean to the NE of the Cyclades. Key Points: Blueschist and eclogite facies metamorphism on Tinos record peak P‐T conditions of 22–26 kbar, 490–520°C and 20–23 kbar, 530–570°C respectively HP‐LT rocks were sequentially underplated and extruded to the SW by a combination of top‐to‐SW thrusting and top‐to‐NE normal‐sensed shear HP metamorphism was associated with NE subduction of the Cycladic continental margin ~25 Myr after obduction of the Tsiknias Ophiolite