Chromian minerals and talc veins in the metasandstones of the Ochi nappe, southern Evia, Greece: The role of fluids in retrograde metamorphism during exhumation

Unusual minerals were formed during the retrograde metamorphism of high-pressure metamorphic rocks in the Ochi nappe, southern Evia, Greece, specifically widespread sub-mm-scale talc veins and chromian varieties of chlorite, phengite, biotite and allanite. Albite-quartz-calcite and calcite-quartz schists are cut by common 10–200 μm wide veins of talc, rimmed by Cr-chlorite with <0.6 apfu Cr. Hydraulic fracturing by fluids derived from dehydration of subducted chromite-rich serpentinite in the Ochi nappe created fractures in the schists where talc precipitated. Mineral chemistry and textures are related to regional metamorphic and tectonic phases, which included Eocene blueschist metamorphism followed by Oligocene syn-orogenic wedge extrusion. The overprint resulted in retrograde growth of albite porphyroblasts, emplacement of talc veins, and formation of phengite with <0.08 apfu Cr and later biotite with <0.3 apfu Cr. Late Oligocene shearing flattened many pre-existing minerals. Neogene exhumation within the North Cycladic detachment system produced decompression fractures that nucleated precipitation of allanite (some with <0.4 apfu Cr and > 0.8 apfu REE), apatite, titanite and zircon, reflecting the availability of halogen-rich fluids. The evolution of the retrograde metamorphic minerals assemblages can thus be tracked, with pervasive CrMg infiltration in the Oligocene, and more focussed Miocene metasomatism by halogen-rich fluids. [199 words].