FLUID MASS TRANSFER OF TRACE ELEMENTS DURING RETROGRESSION OF A HIGH PRESSURE, LOW TEMPERATURE METAMORPHIC TERRANE, TINOS & SYROS ISLANDS, CYCLADES, GREECE

Retrograde, greenschist-facies alteration zones (selvages) surrounding fractures/veins in blueschists from the islands of Tinos and Syros, Greece, were investigated using geochemical mass balance analysis to assess the extent of elemental mass transfer during retrogression and exhumation of Cycladic high-pressure, low-temperature (HP-LT) rocks. Extensive Cretaceous-Eocene subduction of crustal rocks in the Aegean region resulted in the formation of the Attic-Cycladic Blueschist Belt (ACCB), a regional band of blueschist- and eclogite-facies metamorphic rocks. Subsequent Oligocene-Miocene exhumation of the ACCB was accompanied by layer- and contact-parallel, retrograde fluid infiltration that produced a regionally widespread, greenschist-facies overprint in the northern Cyclades. Major and trace element concentrations from two overprinted fracture/vein selvages in blueschists were obtained using XRF and ICPMS. One was collected adjacent to a 0.5 meter-wide, retrograde, quartz-calcite-albite vein on Tinos, and the other from a retrograde fracture zone on Syros. Excellent spatial control on protolith-selvage relations allowed for sampling at the cm-scale and bulk averaging of “altered” and “unaltered” samples to minimize intrinsic geochemical heterogeneities. Mass balance ratios of altered to unaltered rocks indicate that U and Pb, and to some extent Th, Cu, and S were added to the blueschist protoliths, whereas the bulk rock mass remained relatively constant during greenschist-facies overprinting. The observed metasomatic addition of U and Th to crustal rocks may concentrate heat-producing elements in the crust and significantly impact the thermal budget during exhumation of HP-LT terranes. In addition, recent work on Cycladic rocks from Tinos and Syros (Bröcker and Enders, 2001) reports unusual enrichments of traditionally fluid-immobile elements such as Ti, Zr, and the rare earth elements (REE) and postulates fluid-assisted mass transfer during metamorphism as a possible cause. Our data indicate that Ti, Zr, and the REEs remained relatively unchanged during greenschist-facies retrogression, thus any mass transfer of these elements would have been restricted to HP-LT metamorphism and did not occur during retrograde overprinting.