EPSS Colloquium fall-2019

Serpentinized ultramafic rocks are found in most subduction-related metamorphic complexes as massive bodies, exotic blocks, or fine-grained matrix within shear zones. When present, serpentinites play a critical role in fluid-transport, chemical cycling, and rheologic boundaries within subduction zone complexes. However, their propensity for fluid alteration and deformation can lead to a complex chemical and physical history that is often difficult to interpret. Here we study the multi-stage history of the serpentinites associated with the subduction complex on Syros, Greece. We use major, trace element, and stable isotope geochemistry to investigate the melt and fluid-history of serpentinites, and magnetite (U-Th)/He thermochronometry and trace elements to constrain their low-temperature cooling history. On the island of Syros, Greece, serpentinites are associated with remarkably preserved blueschist-and-eclogite-facies rocks that experienced HP-LT subduction in the Eocene. Whole rock trace and major element geochemistry, and stable isotope (δD and δ18O) analyses on the serpentinites indicate that the precursor mantle rocks likely derived from a mid-ocean ridge or hyper-extended margin, and experienced low-temperature serpentinization by seawater, with minor overprinting by oxidized, sedimentary fluids during subduction. Magnetite (U-Th)/He thermochronometry of internal fragments from large grains within a chlorite schist and a serpentinite record Mid-Miocene exhumation-related cooling ages, whereas smaller grains from the serpentinite record mineral formation associated with Pliocene normal faulting. Magnetite trace elements record evolving fluid-chemistry during magnetite growth associated with blackwall alteration and more recent faulting. These results reveal evidence for multiple episodes of fluid-rock alteration from pre- to post-subduction, which has implications for the cooling history and local geochemical exchanges of this HP-LT terrane. Particularly, magnetite trace element and (U-Th)/He thermochronometry is a relatively new technique that can provide key timing constraints on fluid-rock interactions.