Blueschist preservation in a retrograded, high‐pressure, low‐temperature metamorphic terrane, Tinos, Greece: Implications for fluid flow paths in subduction zones

The preservation of high‐pressure, low‐temperature (HP‐LT) mineral assemblages adjacent to marble unit contacts on the Cycladic island of Tinos in Greece was investigated using a new type of digital outcrop mapping and numerical modeling of metamorphic fluid infiltration. Mineral assemblage distributions in a large blueschist outcrop, adjacent to the basal contact of a 150‐meter thick marble horizon, were mapped at centimeter‐scale resolution onto digital photographs using a belt‐worn computer and graphics editing software. Digital mapping reveals that while most HP‐LT rocks in the outcrop were pervasively retrograded to greenschist facies, the marble‐blueschist contact zone underwent an even more intense retrogression. Preservation of HP‐LT mineral assemblages was mainly restricted to a 10–15 meter zone (or enclave) adjacent to the intensely retrograded lithologic contact. The degree and distribution of the retrograde overprint suggests that pervasively infiltrating fluids were channelized into the marble‐blueschist contact and associated veins and flowed around the preserved HP‐LT enclave. Numerical modeling of Darcian flow, based on the field observations, suggests that near the marble horizon, deflections in fluid flow paths caused by flow channelization along the high‐permeability marble‐blueschist contact zone likely resulted in very large fluid fluxes along the lithologic contact and significantly smaller fluxes (as much as 8 times smaller than the input flux) within the narrow, low‐flux regions where HP‐LT minerals were preserved adjacent to the contact. Our results indicate that lithologic contacts are important conduits for metamorphic fluid flow in subduction zones. Channelization of retrograde fluids into these discrete flow conduits played a critical role in the preservation of HP‐LT assemblages.