OXYGEN-ISOTOPE RECORD OF FLUID INFILTRATION AND MASS-TRANSFER DURING REGIONAL METAMORPHISM OF PELITIC SCHIST, CONNECTICUT, USA

We present petrologic and oxygen isotopic evidence for the interaction of deep crustal fluids with kyanite zone pelitic schist during amphib olite facies metamorphism of the Wepawaug Schist, south-central Connec ticut. We focus on a sample of schist (sample MBW-1) cut by a 2-6 cm w ide quartz vein. The vein is surrounded by zones of wallrock alteratio n (selvages) that are rich in micas relative to quartz and feldspar, h ave low Si/Al and Na/Al, contain staurolite and kyanite, and vary in t hickness from about 1-5 cm. Staurolite and kyanite are rare or absent beyond the selvage margins. We have measured the delta(18)O of quartz, plagioclase, muscovite, garnet, kyanite, staurolite, garnet, and biot ite along several mm-scale resolution traverses across the quartz vein and the adjacent schist. Garnets in the selvages record core-to-rim i ncreases in delta(18)O of nearly 2 parts per thousand. Modeling of pro grade reaction histories indicates that this zonation requires the inf iltration of external fluids. Beyond the selvage margins, isotopic zon ation in garnet is about 0.8 parts per thousand from core-to-rim and i s consistent with prograde reaction with little or no infiltration. We suggest, therefore, that the selvages were zones of significant fluid infiltration and that the region now occupied by the quartz vein was the major fluid conduit. Earlier petrologic studies (Ague, 1994b) indi cated that quartz veins and adjacent selvages were conduits for major down-temperature how of H2O-rich fluids with time-integrated fluid flu xes of similar to 3 X 10(5) m(3) m(-2). Isotopic modeling of advective flow suggests that down-temperature fluxes of this magnitude would ha ve increased bulk delta(18)O by similar to 1 parts per thousand, consi stent with the isotopic record preserved by zoned selvage garnets. Qua rtz in veins surrounded by selvages from five other localities through out the amphibolite facies have delta(18)O that is statistically indis tinguishable from that of the bulk of the quartz in MBW-1. Thus, we co nclude that the amphibolite facies portion of the Wepawaug Schist was a zone of major, channelized outflow of metamorphic fluids down the re gional temperature gradient. During the latter stages of amphibolite f acies metamorphism subsequent to the bulk of vein and selvage formatio n, MBW-1 was infiltrated by isotopically light fluids that were probab ly derived from synmetamorphic igneous intrusions. This infiltration m odified the isotopic composition of plagioclase throughout the rock an d, therefore, we suggest that the infiltration was pervasive. Muscovit e retains its pre-infiltration isotopic composition, however, which su ggests short timescales of fluid-rock interaction on the order of 10(3 )-10(4) years. The total duration of flow may have been longer than th is because our calculations do not take episodic flow into account. Mo deling of possible isotopic shifts resulting from diffusion of oxygen isotopes between matrix phases during slow cooling indicates that MEW- 1 must have been dry for most of its retrograde cooling history.