OXYGEN ISOTOPE CONSTRAINTS ON FLUID INFILTRATION ASSOCIATED WITH HIGH TEMPERATURE--LOW PRESSURE METAMORPHISM (CHUGACH METAMORPHIC COMPLEX), EOCENE SOUTHERN ALASKA FORE-ARC

The Chugach Metamorphic Complex (CMC) is a high-temperature/low-pressure metamorphic belt developed in the Eocene forearc of southern Alaska. A transect along Tana Glacier shows ¹⁸O/¹⁶O depletion of quartz (£ 4 permil) with increasing metamorphic grade in both rock matrix and associated veins. These ¹⁸O/¹⁶O depletions are too large to result from closed system effects of increasing temperature. Instead, these depletions indicate infiltration of an external reservoir of lower d¹⁸O, hot aqueous fluid at some stage(s) in the thermal and deformational history of the CMC. The concordant decrease in d¹⁸O 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. Textural relationships among deformation fabrics, multiple generations of low d¹⁸O quartz veins, and metamorphic index minerals indicate that the metasediments of the CMC experienced at least two stages of fluid infiltration and associated ¹⁸O/¹⁶O depletion. The first stage occurred prior to or during the second deformation (D₂) which was near the thermal peak for intermediate grade zones (andalusite, staurolite). The second stage occurred after D₂ but prior to the third deformation (D₃), and before the thermal peak of metamorphism for the high grade core of the CMC (sillimanite zone schists and gneiss). The combination of structural, petrologic, and isotopic evidence argue against the existence of a single, geometrically simple fluid flow system operating at the scale of the exposed metamorphic section at Tana Glacier, and at the thermal peak of metamorphism.

These infiltrating fluids were most likely exchanged with hot igneous intrusions emplaced into the CMC during its prograde development. 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.