Oxygen isotope constraints on fluid infiltration associated with high-temperature-low-pressure metamorphism (Chugach metamorphic complex) within the Eocene Southern Alaska forearc

The Chugach metamorphic complex is a high-temperature-low-pressure metamorphic belt developed in the Eocene forearc of southern Alaska. A transect along Tana Glacier shows 1 8 O/ 1 6 O depletion of quartz with increasing metamorphic grade in both rock matrix and associated veins. These 1 8 O/ 1 6 O depletions are too large to result from closed system increase in metamorphic grade (temperature). Therefore, this indicates exchange of the metasedimentary rocks with an external reservoir of lower δ 1 8 O, hot aqueous fluid and/or felsic magma, at some stage(s) in the thermal and deformational history of the Chugach metamorphic complex. The concordant decrease in δ 1 8 O values of quartz from both rock matrix and associated veins indicates that the fluid infiltration occurred prior to or during formation of the quartz veins. In turn, this fluid infiltration must have preceded the deformation of the quartz veins. The metasediments of the Chugach metamorphic complex experienced at least two stages of fluid infiltration and associated 180/160 depletion. The first stage occurred prior to or during the second deformation (D 2 ), which was near the thermal peak for intermediate-grade zones (andalusite, staurolite). The second stage occurred after D 2 but prior to the third deformation (D 3 ) and before the thermal peak of metamorphism for the high-grade core (sillimanite zone schists and gneiss). It is unlikely that this pattern of 1 8 O/ 1 6 O0 depletion is the result of a single, geometrically simple fluid flow system operating at the scale of the exposed metamorphic section at Tana Glacier and at the peak of metamorphism. Evidence against such simple flow scenarios includes (1) structural evidence for multiple phases of deformation; (2) textural relationships among deformation fabrics, multiple generations of low-δ 1 8 O quartz veins, and metamorphic index minerals; and (3) constraints on the patterns of isotopic shifts in flow systems based on transport theory. Instead, evidence suggests that the δ 1 8 O-distance profile is more likely the result of more than one generation of fluid infiltration occurring at different times, places, and orientations during the prograde metamorphic and structural development of the Tana Glacier section. General oxygen isotope data sets such as those presented here for the Tana Glacier area and those previously published for other areas may not uniquely constrain the flow geometry responsible for the depletion patterns. These data sets also need to be integrated with structural and petrological studies in order to constrain the timing of fluid flow with respect to deformation and thermal history of the metamorphic belt. The general lowering of δ 1 8 O in the intermediate- to high-grade rocks of the Chugach metamorphic complex was caused by fluid infiltration (recorded by injection of quartz veins) prior to the thermal peak of metamorphism. These fluids were most likely exchanged with hot igneous intrusions. Thus, the detailed structural, thermal, and oxygen isotope record of these rocks indicates that magmatic heating is most likely responsible for this episode of high-temperature, low-pressure, Buchan metamorphism.