The Cycladic Blueschist Unit on Tinos, Greece: Cold NE Subduction and SW Directed Extrusion of the Cycladic Continental Margin Under the Tsiknias Ophiolite

摘要

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 similar to 22-26 kbar and 490-520 degrees C under water-saturated conditions, whereas pseudomorphed lawsonite- and aegirine-omphacite bearing eclogite reached similar to 20-23 kbar and 530-570 degrees C. These rocks are separated from rocks at deeper structural levels (Sostis Subunit) by a top-to-SW thrust. The Sostis Subunit recordsP-Tconditions of similar to 18.5 kbar and 480-510 degrees C and is overprinted by pervasive top-to-NE shearing that developed during exhumation from (M-1) blueschist to (M-2) greenschist facies conditions of similar to 7.3 +/- 0.7 kbar and 536 +/- 16 degrees C. TheseP-T-Drelationships 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 andP-Tconditions between theHP-LTrocks 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 similar to 1.2-1.5 degrees C/km/Myr prior to continent-continent collision between Eurasia and Adria/Cyclades.