Disruption of a high-pressure unit during exhumation: petrology and geochronology of garnets within the Cycladic Blueschist Unit (Thera, Ios and Naxos islands, Greece)

Abstract

<p>Understanding the disruption of a tectonic nappe that experiences a subduction-related pressure temperature (P-T) loop is challenging. Thrust imbrications may disrupt single nappes during its subduction and/or exhumation which can be revealed by detailed petrological and geochronological work. Garnet commonly forms during subduction. It most likely hosts early prograde, peak high-pressure (HP) and subsequent metamorphic mineral inclusions making such assemblages a useful tool for detailed petrological and geochronological investigations. Multiple approaches were used to determine the detailed P-T loop of the Cycladic Blueschist Unit passive margin sequence (Greece) such as Zr-in-rutile thermometry coupled with quartz-in-garnet elastic barometry, average P-T and phase equilibrium thermodynamic modeling. U-Pb garnet and zircon geochronology age data were in addition determined to complement already existing age data.</p><p>The results of this approach reveal that the passive margin sequence in Thera (Santorini), Ios and Naxos was subducted as a coherent continental fragment at a subduction rate of ~2.1 km/My and a heating rate of ~12 °C/My. Prograde and peak HP metamorphism occurs at <em>c.</em> 50 and <em>c.</em> 40 Ma respectively.  Along Thera, Ios and Naxos, prograde and peak P-T condition increase from sub-blueschist to upper blueschist facies metamorphism. Subsequently, the sequence was disrupted by one or several thrust faults during its exhumation. The passive margin sequence of Naxos was thrust onto the Ios sequence during the Oligocene at <em>c.</em> 30 Ma. This imbrication is revealed by different exhumation rates of ~6 km/My for the passive margin sequence of Naxos and of ~3 km/My for the one of Ios. The passive margin sequence of Thera, Ios and the upper part of Naxos was exhumed to upper crustal levels, whereas the lower part of the Naxos passive margin sequence was exhumed to the lower crust leading to thermal relaxation of 9–96°C following tectonic accretion. This indicates that thermal relaxation following tectonic accretion in the Cyclades controlled the thermal evolution of the evolving Cycladic orogen during a tectonically quiet period before lithospheric extension.</p>