Calcite needles in marbles on Syros (Cyclades, Greece) - indicators for an obscured aragonite foliation and static exhumation
Keiter, Mark; Tomaschek, Frank; Schmid-Beurmann, Peter; Ballhaus, Chris; Lons, Jurgen
In marble units on Syros Island (Cyclades, Greece) a strong crystallographicallypreferred orientation (CPO) of calcite can be observed, expressed by calcite needles aligned at a high angle to blueschist facies metamorphic fold axial planes. The CPO of calcite results from a retrograde topotactic recrystallization of aragonite to calcite during exhumation. Calcite c-axes grew at an angle of about 50-60° on c-axes of their host aragonite, and the aragonite was oriented with c-axes parallel to fold axes, lying in the fold axial plane of prograde isoclinal shear folds. Indirectly, a macroscopically not developed foliation, defined by aligned aragonite during Eocene peak metamorphism (~1.5 GPa, 500°C), can be deducted. The ubiquitous preservation of principally undeformed calcite needles in marbles across Syros suggests that exhumation of the rocks was not accompaniedby pervasive internal deformation at least since the aragonite- calcite transition at around I GPa.