LIMITED FLUID-ROCK INTERACTION AT MARBLE-GNEISS CONTACTS DURING CRETACEOUS GRANULITE-FACIES METAMORPHISM, SEWARD PENINSULA, ALASKA

Stable-isotope profiles show that flat-lying marble units acted as imp ermeable barriers to upward fluid flow in transitional amphibolite-gra nulite grade rocks of the Kigluaik Mountains, Seward Peninsula, Alaska . The degree of permeability is related to the composition of the marb le. The margin of a thick pure dolomite marble chemically reacted with underlying metasyenite (a(H2O) = 0.2) to form a 2 cm boundary layer o f calcite + forsterite by introduction of SiO2. No fluid penetrated pa st this reaction front, although the high temperature of metamorphism (approximately 800-degrees-C) allowed transport of carbon and oxygen i sotopes for an additional 2 cm by diffusion through the solid dolomite . A second marble with a higher silica content underwent more decarbon ation, which enhanced porosity and lead to a greater extent of isotope transport (2-3 m) in contact with quartzo-feldspathic gneiss below. A n estimate of total fluid flux across the bottom of this marble layer based on the shape of the isotope profile is 1 cm3/cm2 directed down, out of the marble. At two other marble-gneiss contacts steep isotopic gradients coincide with lithologic contacts, indicating very little cr oss-lithology fluid flow. The extent of diffusional transport of isoto pes in the marbles is limited and interpreted as indicating the transi ent presence of a pore fluid, generated by thermally driven devolatili zation reactions. No wholesale pervasive advection of C-O-H fluid occu rred across the thick, continuous, marble units near the exposed base of the Kigluaik Group section during the entire regional metamorphic c ycle. Activities of pore-fluid species were controlled by internal pro cesses. Movement of volatiles and stable-isotopes between contrasting rock-types was dominantly diffusive. Channelized fluid pathways throug h the marble units developed during uplift and cooling but were not pr esent during peak metamorphism. Heating of the section occurred by con duction, probably from an underlying magma source, and not by advectio n of a C-O-H fluid.