DATING METAMORPHIC FLUID INFILTRATION IN BARROW’S GARNET ZONE, SCOTLAND, USING ION-MICROPROBE DEPTH-PROFILE ANALYSIS OF ZIRCON

Depth-profile and polished-grain ion microprobe (SIMS) U-Pb analysis was done on zircons from upper-greenschist-facies Dalradian metasedimentary rocks in Barrow's classic garnet zone, northeast Scotland, to constrain the timing of metamorphic fluid infiltration and investigate fluid-zircon interactions during metamorphism. Zircons collected from altered metasedimentary rock adjacent to a cross-cutting quartz vein and from relatively unaltered, continuously-equivalent metasedimentary layers distal to the vein indicate that Paleozoic fluid infiltration resulted in sub-micron-scale isotopic and chemical alteration of zircon crystal surfaces and fractures within the grains. Depth-profile "drilling" into unpolished, natural zircon crystal surfaces generated a continuous array of discordant data and a precise fluid infiltration (lower intercept) age of 462 ± 8.8 Ma. Polished-grain zircon SIMS analyses support the age estimate and also document the first Archean zircon ages for the Dalradian, suggesting previously unreported sedimentary contributions from a Lewisian source terrane. Our fluid infiltration age overlaps the accepted age (~470 Ma) for peak Barrovian metamorphic garnet growth, confirming that fluid influx accompanied metamorphism in Barrow’s garnet zone. We contend that metamorphic fluids caused the growth/recrystallization of up to 1.3 mm of new zircon along pitted crystal surfaces and fractures in preexisting Archean-aged zircon grains. Newly grown/recrystallized zircon was enriched in U, Th, and common Pb relative to the host grains, indicating that U, Th, and Pb were mobile in metamorphic fluids. We present the first field-based evidence that common Pb in zircon can serve as an indicator of fluid infiltration and volatile-rich conditions during metamorphism. Fluid flux estimates and metasomatic alteration adjacent to quartz veins in northeast Scotland are markedly similar to those reported for veins in the accretionary prism of southern New Zealand, implying that oceanic crust subducting beneath Dalradian metasediments may have been an important source for infiltrating metamorphic fluids in Barrow's garnet zone.