LARGE-SCALE FLOW OF SLAB-DERIVED FLUIDS IN AN ACCRETIONARY PRISM, OTAGO SCHIST, NEW ZEALAND

Regional quartz vein formation and the fluxes, flow paths, and sources of metamorphic fluids were investigated in the Mesozoic accretionary prism of New Zealand using a unique traverse sampling technique and a new type of chemical mass balance analysis of outcrops. Samples were collected at meter or sub-meter intervals along outcrop-length traverses and composited in order to remove the effects of local, bed-scale heterogeneities and obtain average chemical compositions of whole outcrops. Mass balance analysis used Zr as an immobile geochemical reference frame and as a monitor of sedimentary sorting processes. SiO₂-Zr systematics produced by sedimentary processes differ greatly from those caused by metasomatic mass transfer of silica, allowing evaluation of vein formation mechanisms. Relatively undeformed metasedimentary outcrops of low metamorphic grade (mostly prehnite-pumpellyite facies) are nearly unveined and characterized by sedimentary compositional trends. More deformed outcrops of higher metamorphic grade (mostly greenschist facies) contain 10 to 30 vol. % quartz veins. These outcrops underwent mass addition of externally-derived silica into quartz veins, accompanied by addition of Na and Ca, and removal of K, Ba, Rb, Th, U, and W. Average silica additions suggest a time-integrated fluid flux of ca.10⁴-10⁵ m³_(fluid)/m²_(rock) for fluids ascending through the prism. Dehydration of spilitized oceanic crust subducting beneath the prism is the most probable source for this large fluid flux and could also have caused the Na-Ca-K metasomatism. The W removed from deep levels of the prism may have been deposited in focused, retrograde Au-W-quartz veins at shallow levels by ascending fluids. The results strongly suggest that a large fraction of the fluid released during dehydration of a downgoing slab, at least to depths of 25-30 km, migrates upward into the overlying crust. The transfer of SiO₂ from subducting slabs into accretionary prisms is a plausible mechanism for long-term bulk silica-enrichment of the continents beyond that possible by magmatic differentiation.