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Elevated concentrations of certain large ion lithophile elements (LILE; e.g., Ba, K, Rb, Cs, Ca, Sr), U, and Pb in arc magmas relative to high field strength elements (HFSE; e.g., Ti, Th, Hf, Nb, Zr) are considered key indicators of fluid addition to arc magma source regions worldwide, but the fluid sources and processes of mass transfer are controversial. Dehydration of downgoing slabs releases fluids that can flow through and react with metamorphosed ultramafic-mafic rock packages in melange zones near slab-mantle interfaces. New geochemical data from Syros, Greece, reveal that these fluids preferentially leach LILEs, U, and Pb when they infiltrate and react with subducted metasedimentary rocks. Transfer of these LILE-, U-, and Pb-enriched fluids to the mantle wedge at subarc depths could directly trigger partial melting and generate magmas with elevated Ba/Th, Sr/Th, Pb/Th, and U/Th, as well as radiogenic Sr. Alternatively, if fluid transfer occurs at shallower depths (e.g., Syros), the metasomatized mantle could be carried deeper by wedge corner flow to ultimately undergo partial melting in subarc regions. Keywords: dehydration, metamorphism, subduction zones, arcs, magmas, chemical evolution.

U-Pb isotopic analyses were done using ion microprobe depth-profiling on zircon crystals from Dalradian rocks in Barrow's garnet zone, northeast Scotland, to determine the timing of metamorphic fluid infiltration and investigate fluid-zircon interactions. Zircon crystals collected from altered metasedimentary rock adjacent to a cross-cutting quartz vein and from relatively unaltered, continuously equivalent layers distal to the vein indicate that Paleozoic fluid influx caused isotopic and chemical alteration along zircon crystal surfaces and internal fractures. 'Ion drilling' into natural zircon crystal faces yielded U-Pb depth profiles with 50-100 nm spatial resolution that permitted U-Pb isotopic age analysis of thin grain rims affected by fluid-mediated growth/recrystallization at 500-550[degrees]C. We interpret the measurements to indicate that fluid influx at 462 [+ or -] 8.8 Ma caused the growth/recrystallization of new zircon along pitted crystal surfaces and fractures in Archean zircon grains. Our age estimate overlaps the accepted age range (ca. 467-464 Ma) for peak Barrovian garnet growth, confirming that metamorphism and fluid influx were contemporaneous. The new zircon was enriched in U, Th, and common Pb relative to host grains, suggesting that U, Th, and Pb were actively transported by upper greenschist-facies metamorphic fluids. We present the first field-based evidence that common Pb in zircon can serve as an indicator of metamorphic fluid infiltration. Archean ages for some detrital zircons suggest contributions from a Lewisian/Scourian provenance.

Regional quartz-vein formation and the fluxes, flow paths, and sources of metamorphic fluids were investigated in the Mesozoic accretionary prism of New Zealand by using a new chemical mass-balance analysis of outcrops. Samples were collected at meter or sub-meter intervals along outcrop-length traverses and combined to obtain average chemical compositions of whole outcrops. Mass-balance analysis used Zr as an immobile reference frame and as a monitor of sedimentary sorting processes. Si[O.sub.2]-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 10-30 vol% quartz veins. These outcrops underwent mass addition of externally derived silica into quartz veins, accompanied by addition of Na and removal of K and W. Average silica additions suggest a time-integrated fluid flux of ~[10.sup.4]-[10.sup.5] [m.sup.3.sub.(fluid)]/[m.sup.2.sub.(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-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. Transfer of Si[O.sub.2] 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. Keywords: fluid flow, quartz veins, accrctionary prism, Otago Schist, New Zealand, Au-W mineralization.