Abstract

Calcareous argillites in the Upper Cambrian Big Horse Limestone Member of the Orr Formation, west-central Utah, have undergone contact metamorphism where they were intruded by the Jurassic Notch Peak stock. Metamorphism of the rocks resulted in nearly complete decarbonation, yet the mineral assemblages indicate that the fluid phase was nearly free of CO2, indicating interaction with a substantial volume of water. This study is designed to determine the volume of externally derived water with which the Big Horse Limestone Member interacted and the mode of interaction. The rocks in the aureole are characterized by three grades of metamorphism, separated by the diopside and the wol-lastonite isograds. Assemblages in incipiently metamorphosed samples are calcite + quartz + muscovite + biotite + chlorite + plagioclase. Diopside-zone samples contain calcite + quartz + diopside + tremolite + phlogopite + K-feldspar + plagioclase or a subset of this assemblage. Wollastonite-zone assemblages are typified by calcite + wollastonite + diopside + vesuvianite + grossular + K-feldspar + plagioclase. Conditions within the diopside zone, determined from calcite + dolomite temperatures of interbedded dolomitic marbles and from the phase equilibria in the system CaO-MgO-Al2O3-SiO2-KAlSi3O8-CO2-H2O, were about 434 °C and xCO2 = 0.024, at a pressure of 2 kbar. These are the inferred conditions for the invariant assemblage calcite + quartz + tremolite + diopside + phlogopite + K-feldspar, which occurs locally in the diopside zone. Conditions in the wollastonite zone appear to have been T > 450 °C, xCO2 ≤ 0.002. The amounts of volatiles released by the rocks were calculated by the mass balance protolith → metamorphic rock, balanced by appropriate amounts of H2O and CO2. Diopside-zone rocks evolved 45 ± 23 g of CO2 and 16 ± 7 g of H2O per kilogram of protolith. Wollastonite-zone rocks released 158 ± 45 g of CO2 and 10 ± 3 g of H2O per kilogram of protolith. The amounts of H2O required to maintain the equilibrium fluid compositions are 0.73 ± 0.36 kg of H2O per kilogram of rock in the diopside zone and 11.2 ± 5.0 kg of H2O per kilogram of rock in the wollastonite zone. This water/rock ratio in the wollastonite zone is about 25 times that indicated by stable-isotope studies on the same rocks. The T-xCO2 path of metamorphism of this rock was (1) at the diopside isograd, the conditions were near, but below, the invariant point calcite + quartz + tremolite + diopside + phlogopite + K-feldspar; (2) minor reaction buffered the conditions to those of the invariant point, 434 °C, xCO2 = 0.024, which represents the megascopically recognizable diopside isograd; (3) infiltration of 0.3 rock masses of H2O drove the reaction phlogopite + 3 calcite + 6 quartz = 3 diopside + K-feldspar + 3CO2 + H2O to completion; (4) the reaction tremolite + 3 calcite + 2 quartz -I- 5 diopside + 3CO2 + H2O occurred throughout the diopside zone until tremolite was consumed, when T = 464 °C and xCO2 = 0.04; and (5) infiltration of at least 13.3 rock masses of H2O at the wollastonite isograd was required to drive the reaction calcite + quartz = wollastonite + CO2 to completion at T = 464 °C, xCO2 = 0.004. Infiltration was abetted by the reduction in solid volume of 25%. The high water/rock ratio in the wollastonite zone combined with the low water/rock ratio in the diopside zone indicates that the high ratio does not represent the fluid flux, but rather the volume of the fluid reservoir with which the wollastonite-zone rocks equilibrated. Mass-balance calculations show that there was more than sufficient H2O in the pore space in the Notch Peak stock to have acted as the reservoir. The reaction front represented by the wollastonite isograd was the final outermost locus of effective mixing between the reservoir and the evolved fluid, and the position of the front was governed by the rates of diffusion of CO2 through the fluid and of heat transfer through the rock. Simple calculations of the temperature and fluid flux through the aureole, based on heat- and fluid-transport equations, indicate that heat transport on the margins of the stock occurred primarily by conduction, particularly outside the wollastonite zone, and that the major fluid flux through the aureole occurred within the first year of intrusion, long before the temperature had risen sufficiently for metamorphism to have occurred. Fluid fluxes through the aureole after 1000 yr, when significant metamorphism occurred, dropped to the order of 1 kg/(m2 yr), or less. This amount is sufficient to have interacted with the diopside-zone rocks, giving water/rock ratios (by mass) of the order 0.1. The fluid flux through the wollastonite zone is given by the rate of advancement of the isograd, which gives fluxes ranging from >42 kg/(m2 · yr) at 50 m from the contact to 0 kg/(m2 · yr) at the maximum distance of 410 m. The results of this study indicate that in those metamorphic terranes in which there is a change from rock-dominated to fluid-dominated systems across an isograd, the water/rock ratio in the fluid-dominated system represents not the fluid flux, but the mass of fluid in a reservoir with which the rocks equilibrated.

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