Abstract

The Truchas Peaks region of northern New Mexico includes an apparent equilibrium occurrence of the Al2SiO5 triple point. Andalusite occurs in rocks at the southern end of the Truchas Peaks uplift. Kyanite is present along the eastern edge and sillimanite in the northern part. Kyanite, andalusite, and sillimanite coexist in the center of the area. Near the triple point zone, isograds are controlled by topography: kyanite occurs along ridges, kyanite–andalusite on hillsides and kyanite–andalusite–sillimanite in valleys. The distribution of minerals fits a model of near-horizontal isotherms and isobars, with pressures and temperatures increasing with depth and geothermal gradients increasing from north to south.

Where three aluminum silicates coexist, quartzite contains the assemblage chloritoid–staurolite–kyanite–andalusite–sillimanite–magnetite–hematite–quartz. Experimental data on the phase boundary Fe chloritoid + Al sihcate = Fe staurolite + quartz at the hematite–magnetite f(O2) buffer, corrected for minor elements, indicates temperature near 535°C. Gamet–biotite geothermometry gives a similar temperature. Pelitic schist contains cordierite–biotite–chlorite–kyanite–muscovite-quartz in the same area, and graphic and algebraic analysis suggests that this schist crystallized with X(H2O) less than 1. Comparison with experimental data, taking into account mineral compositions and estimated X(H2O), yields T = 540°C, P(total) = 4 kbar. Calculated P–T conditions are consistent with the position of the triple point according to Holdaway (1971).

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