A graphite-absent sequence of quartzite and schist on the north flank of the Picuris Range in north-central New Mexico provides a natural laboratory to test the thesis of Bruton and Helgeson that fluid pressure (Pf) is the effective pressure on solid phases during metamorphism. We have studied the Hondo Canyon and Section 8 areas of this range in detail. The presence of kyanite reacting to sillimanite in Ortega Formation quartzites and of andalusite reacting to sillimanite in neighboring Rinconada Formation schists may be explained by an effective pressure difference of 200 ±100 bars between the two units. Composition, redox, T, and Pr (rock pressure) can all be ruled out as controlling factors, leaving a difference in Pf as the most likely cause of the differences in the mineral assemblages.

The presence of chloritoid + kyanite in the Ortega quartzite and staurolite in the adjacent jacent Rinconada schists can best be explained by bulk compositional effects. The schists have a higher bulk ratio of Mg/(Mg + Fe) than the quartzites, which allows staurolite and biotite to be stable at lower temperatures relative to chloritoid and muscovite than in the quartzites. In the absence of chloritoid, staurolite has higher R2+ and lower H content than would be expected in the quartzites. The concurrent reduction of tetrahedral vacancies reduces the activity of staurolite relative to the reaction chloritoid + kyanite = staurolite + quartz + fluid and stabilizes staurolite in the Rinconada schists. This reaction has a steep P-T slope that permits but does not require differences in Pf between the two units at constant T.

A difference in Pf of 200 ± 100 bars between the two rock types can best be explained by a contrast of permeability between the quartzites and mica schists, because the preferred orientation of minerals and planar grain boundaries in the micaceous rocks favor greater and more rapid fluid flow. Pf builds up to values closer to lithostatic pressure in the quartzites than in the mica-rich rocks. In fluid-present systems during low- and mediumgrade metamorphism generally, the effective pressure on solid minerals appears to be fluid pressure rather than rock pressure.

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