METAMORPHIC PROCESSES RECORDED IN DELAMINATED METABASALTS PRESERVED ON SOUTH ISLAND, NEW ZEALAND

Orogenic belts often provide essential information about the tectonic evolution of convergent margins throughout the world. Records of tectonism and accretion preserved in the boundaries and structural features of such terranes are often obscured or obliterated by metamorphism and deformation. The Otago Schist is part of an extensive belt of low grade metamorphosed Mesozoic sediments and basalts on the South Island of New Zealand. The tectonic setting and origin of the Chrystalls Beach Metabasalt Formation (CBMF) is not well-correlated with the surrounding terranes of the Otago schist. The Chrystalls Beach accretionary mélange is an excellent site to construct tectonic and metamorphic history and to compare it other terranes of the Otago Schist. CBMF includes an assemblage of pillow lava and breccia that ranges from well-preserved to extremely sheared and deformed. The assemblage also contains spectacular epidote veins with quartz and chlorite around pillow margins and in fractures that have not been closely mapped and sampled. With petrographic and geochemical analyses, we aim to differentiate among previously proposed interpretations of vein formation and to establish the sequence of alteration to emplacement events. Field observations and cross-cutting relations at various scales indicate that epidote veins likely predated metamorphism. Geochemical, petrographic, and isotopic data suggest that the vertical upflow zone of epidote veins was a result of seafloor hydrothermal processes at a mid-ocean ridge and is not related to subduction. The secondary minerals and associated fabrics demonstrate the degree of alteration by relatively low-temperature fluid-rock interactions. Seafloor hydrothermal alteration may have led to changes in crustal density and have implications for the preservation and emplacement of the delaminated mid-ocean ridge lithosphere in the CBMF. Interpretation of CBMF pillows and veins and their tectonic signatures will put their origin and alteration history into the context among the other terranes of the Otago Schist.