Oxygen isotope record of fluid infiltration and mass transfer during regional metamorphism of pelitic schist, Connecticut, USA

Abstract

We present petrologic and oxygen isotopic evidence for the interaction of deep crustal fluids with kyanite zone pelitic schist during amphibolite facies metamorphism of the Wepawaug Schist, south-central Connecticut. We focus on a sample of schist (sample MBW-1) cut by a 2–6 cm wide quartz vein. The vein is surrounded by zones of wallrock alteration (selvages) that are rich in micas relative to quartz and feldspar, have low Si Al and Na Al , contain staurolite and kyanite, and vary in thickness from about 1–5 cm. Staurolite and kyanite are rare or absent beyond the selvage margins. We have measured the δ 18O of quartz, plagioclase, muscovite, garnet, kyanite, staurolite, garnet, and biotite along several mm-scale resolution traverses across the quartz vein and the adjacent schist. Garnets in the selvages record core-to-rim increases in δ 18O of nearly 2%. Modeling of prograde reaction histories indicates that this zonation requires the infiltration of external fluids. Beyond the selvage margins, isotopic zonation in garnet is about 0.8% from core-to-rim and is 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) indicated that quartz veins and adjacent selvages were conduits for major down-temperature flow of H 2O-rich fluids with time-integrated fluid fluxes of ∼3 × 10 5 m 3 m −2. Isotopic modeling of advective flow suggests that down-temperature fluxes of this magnitude would have increased bulk δ 18O by ∼1‰, consistent with the isotopic record preserved by zoned selvage garnets. Quartz in veins surrounded by selvages from five other localities throughout the amphibolite facies have δ 18O that is statistically indistinguishable from that of the bulk of the quartz in MBW-1. Thus, we conclude that the amphibolite facies portion of the Wepawaug Schist was a zone of major, channelized outflow of metamorphic fluids down the regional temperature gradient. During the latter stages of amphibolite facies metamorphism subsequent to the bulk of vein and selvage formation, MBW-1 was infiltrated by isotopically light fluids that were probably derived from synmetamorphic igneous intrusions. This infiltration modified the isotopic composition of plagioclase throughout the rock and, therefore, we suggest that the infiltration was pervasive. Muscovite retains its pre-infiltration isotopic composition, however, which suggests 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 this because our calculations do not take episodic flow into account. Modeling of possible isotopic shifts resulting from diffusion of oxygen isotopes between matrix phases during slow cooling indicates that MBW-1 must have been dry for most of its retrograde cooling history.