Melange a trois: Metamorphic controls on recycling and mass transfer within subduction zones

This dissertation comprises three studies that provide new constraints and theories for our understanding of mass-transfer within subduction zones. Within Chapter 1, we examine the geochemical zoning of metamorphic garnets produced by prograde metamorphism of Tethyan MORB within the western Italian Alps. Metamorphism peaked within the coesite-eclogite facies, and we use these garnets to investigate the record of mineral reactions occurring during subduction. We find that anomalous patterns of growth zoning in garnets are consistent with the decomposition of coexisting titanite + clinozoisite, a dehydration reaction that is a likely candidate to mobilize key tracers from the metamorphic system to volcanic arcs. Using existing and new geothermobarometric constraints, we are able to reconstruct the apparent depth-interval of this reaction during metamorphism; this new approach to the metamorphic record preserved in eclogites allows for a more complete view of point-to-point mass transfer occurring within the subduction system. Chapters 2 and 3 are a pair of studies bearing upon the geochemical evolution of the melange matrix of the Catalina Schist, California. Melange matrix formed through the synergistic effects of deformation and metasomatism to produce hybridized rock types from sediment, basalt, and peridotite protoliths. Elemental systematics are largely dominated by mechanical mixing; however, numerous elements are highly controlled by the stability relationships of key host minerals buffered by the evolving bulk composition of melange zones. Sr-Nd isotope systematics appear to be dominated by mechanical mixing trajectories. In contrast to these systems recording mechanical mixing, Pb and B isotope ratios do not preserve mechanical mixing arrays, despite elemental systematics consistent with mixing. B isotope ratios are consistent with existing models for fluid flow in the complex, where lower-grade melange is an appropriate source for fluids, while higher-grade samples preserve the signature of evolved fluids progressing towards the mantle wedge. Pb isotope ratios are largely indistinguishable as a function of metamorphism and are more radiogenic than all constrained compositions input to the Catalina subduction zone, suggesting fractionation of the U-Th-Pb system during metamorphism due to mineral reactions.