Relation between the intensity of deformation and retrogression in blueschist metapelites of Tinos Island (Greece) evidenced by chlorite-mica local equilibria

The late greenschist deformation of the Cycladic Blueschists unit on Tinos Island evolves along a NE-SW gradient of non-coaxial strain across a major extensional shear zone. Deformation is more intense and the greenschist overprint is also more severe close to the shear zone that separates the Cycladic Blueschists from the upper ophiolitic unit in the NE of the island. The compositional variability of chlorite and K-white mica (KWM) in metapelites sampled along a NE-SW cross-section is characteristic of their P-T conditions of crystallization and is mainly characterized by variations of their Si, Al, Fe and Mg contents due to the Tschermak substitution (Si-IV(Fe, Mg)(VI)=(AlAlIV)-Al-VI). Multi-equilibrium P-T estimates based on the assumption of local equilibrium indicate that each sample recorded a part of the P-T history, which depends on the distance to the contact to the main shear zone, i.e. the intensity and the pattern of deformation. The combination of P-T estimates obtained from all the samples collected along the transect provides a continuous P-T path which is composed of two parts. It provides insights on the syn- and post-orogenic exhumation for some of the Cycladic Blueschists. In particular, we could distinguish two stages in the exhumation process. A first stage in the blueschist facies occurred along a cold P-T path during the construction of the Hellenides. The second exhumation stage occurred along a warmer P-T path during the formation of the Aegean sea. Between the two stages of exhumation, there is a thermal reequilibration of the crust and a partial thermal overprint of the blueschist parageneses. (C) 2002 Elsevier Science B.V. All rights reserved.