Abstract

The Pingree Park area in the northeast Front Range, Colorado, is underlain by Precambrian pelitic and semipelitic gneiss and schist that were metamorphosed about 1.75 b.y. ago, forming a distinct zonal pattern of minerals indicating an increase in grade of metamorphism to the west and north. Three isograds have been identified. The andalusite-sillimanite isograd is defined by the first occurrence of sillimanite in the prograde direction and is interpreted to mark the reaction of andalusite to sillimanite. A band as much as ∼1 km across contains both andalusite and sillimanite and represents a divariant band or reaction overstepping of 20 °C or less. The K-feldspar–sillimanite isograd separates microcline + sillimanite from prograde muscovite + quartz assemblages and is interpreted to represent the reaction (K, Na) muscovite + quartz = (K, Na) microcline + sillimanite + H2O. Despite the presence of significant amounts of sodium in both muscovite and microcline, the reaction shows no perceptible divariant band. There has been extensive reaction of microcline + sillimanite to retrograde muscovite + quartz. The migmatite-in isograd is defined by the first occurrence of migmatitic rocks in the prograde direction and is interpreted to represent the inception of partial melting. The isograds form an eastward-plunging set of synformal surface the south limb of which has a gentle dip and the north limb of which apparently has a much steeper dip.

Based on the chemical composition of the minerals and comparison with experimentally calibrated equilibria, the conditions of metamorphism at the K-feldspar–sillimanite isograd are estimated to have been PH2O ≅ Ptotal = 3 to 4 kbar, T = 650 ± 30 °C, fO2 ≅ 10−12 to 10−15 bars, and fHF ≅ 1/4 to 1 bar. The geothermal gradient at the peak of metamorphism was ∼60 °C/km.

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