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Todd, C.S., 1992

Investigations on the role of fluid during granulite facies metamorphism, Kigluaik Mountains, Seward Peninsula, Alaska

Bibliographic Reference

Todd, C.S., 1992, Investigations on the role of fluid during granulite facies metamorphism, Kigluaik Mountains, Seward Peninsula, Alaska: University of Washington, Seattle, Ph.D. dissertation, 139 p.


Flat-lying thick marble units acted as impermeable barriers to upward fluid flow in amphibolite to granulite grade rocks of the Kigluaik Mountains, Seward Peninsula, Alaska. The degree of impermeability can be related to the composition of the marble. The margin of a thick, pure dolomite marble chemically reacted with underlying metasyenite to form a 2-cm-thick border of calcite + forsterite. No fluid penetrated past this reaction front. The high temperatures at which this process occurred (nearly 800 degrees C) allowed C and O isotopic interaction for an additional 2 cm by diffusion through the solid dolomite. A second marble with a higher silica content underwent more decarbonation, which enhanced porosity and led to a greater extent to isotopic interaction. An estimate of 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 exist, coincident with the lithologic contacts, indicating very little cross-lithology fluid flux. Fluid composition during metamorphism was locally controlled by lithology. At a contact where a homogeneous Hbl-bearing gneiss lies above a marble, Hbl in the gneiss reacted to form fine-grained granular Opx, Cpx, Pl, and Kfs within 60 cm of the marble. The Hbl-bearing and Hbl-free gneiss assemblages are related by Bt and Hbl dehydration reactions. These reactions were driven by a reduction in H2O activity due to dilution with CO2 from the marble. Water-activity calculations based on biotite dehydration equilibria in the Hbl-free gneiss indicate a trend from 0.14 to 0.08 toward the marble. Mineral assemblages confirm that the marble was a source of high XCO2 fluids. This example provides an indication of the limited role (60 cm) played by fluid movement in controlling mineral parageneses in this terrane. It is concluded that there was no pervasive infiltration of C-O-H fluid across the thick, continuous, marble units of the amphibolite to granulite grade Kigluaik Group during peak metamorphism. Movement of volatile species and isotopic interaction between rocks during peak metamorphism was dominated by diffusive processes. No evidence for channelized fluid pathways through the marble units during peak metamorphism has been found. Therefore, heating of the terrane occurred by conduction, not advection via a fluid. In granulite grade rocks of the Kigluaik Mountains, no peak metamorphic fluids survived as fluid inclusions. Retrograde CO2-rich fluids were trapped late in the metamorphic cycle and have variable density around 0.5 g/cm3. In amphibolite-grade rocks, aqueous fluid inclusions may represent samples of near-peak metamorphic fluids. Density of these inclusions varies from 0.9 to 0.8 g/cm3 with salinities less than 4 wt% NaCl equivalent.

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