The Cycladic subduction zone from birth to death: Insights into the subduction cooling rate conundrum

Abstract

<p>The Attic-Cycladic Massif (ACM) preserves the entire evolution of a NE dipping subduction zone.  This includes the intra-oceanic subduction initiation associated with ophiolite obduction and formation of a metamorphic sole, to subduction-termination associated with burial and exhumation of the Cycladic continental margin to eclogite-blueschist facies conditions. The Tsiknias Ophiolite represents a piece of ca. 162 Ma Tethyan oceanic lower crust and mantle that was thrust towards the SW onto the ACM during a subduction initiation/ophiolite obduction event during the initial stages of oceanic closure. Beneath the Tsiknias Ophiolite lies a ~250 m thick sequence of amphibolites which represent the lower plate. These record an inverted metamorphic gradient at ca. 8.5 kbar reaching > 750°C at the top (associated with small-scale partial melting) and 600°C at the base and formed during high-grade metamorphism of ca. 190 Ma oceanic crust along the subduction zone interface beneath a major thrust fault (Tsiknias Thrust) under geothermal gradients of 30°C/km. U-Pb zircon dating of leucodioritic melt veins constrains the timing of metamorphism to ca. 74 Ma, which may correlate with the switch in the motion of the Nubian plate from transcurrent to convergent with respect to Eurasia. Highly deformed greenschist facies pelagic metasediments underlie the amphibolites suggesting an inverted lithological sequence. This can be explained by the zone of active thrusting propagating down structural level with ongoing subduction, such that underplated material became accreted to the base of the ophiolite. A Miocene aged greenschist-facies shear zone truncates the metamorphic sole rocks and metasediments, placing them directly against the Cycladic Blueschist Unit (CBU) associated with burial of the Cycladic continental margin down the same NE-dipping subduction zone some 25 Myr later. Lawsonite bearing eclogite and blueschist-facies rocks crop-out < 1 km structurally beneath the metamorphic sole and record <em>P-T</em> conditions of 23 kbar and 550°C at ca. 53-46 Ma. These rocks experienced variable retrogression through blueschist and then greenschist facies conditions. This retrogression was largely due to differential growth of lawsonite depending on bulk rock composition during prograde and peak metamorphic conditions causing some rocks to hold large quantities of water at peak conditions. Subsequent exhumation caused lawsonite to break down, hydrating the adjacent rocks and facilitating growth of secondary amphibole and epidote. These <em>P-T-t</em> conditions imply the CBU experienced geotherms of 6-7°C/km during peak metamorphism, which suggests the subduction zone cooled at an average rate of ca. 1.5°C/km/Myr between ca. 74 and ca. 53-46 Ma. This decrease in cooling rate raises two questions:  (1) is this cooling rate a result of thermal conduction due to the burial of cold old oceanic lithosphere following subduction initiation?, or (2) are the hot apparent geothermal gradients recorded in the metamorphic sole due to processes other than conduction from the overriding lithospheric mantle?. Our thermometry data from the Tsiknias metamorphic sole suggest that: (1) the maximum temperatures increase structurally upwards towards the Tsiknias Thrust, (2) peak metamorphic temperatures are superimposed on the structure, and (3) the length scale of heating is inconsistent with thermal conduction alone.</p>