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Present address: Williams, Department of Geology and Geography, University of Massachusetts, Amherst, Massachusetts 01003; Smith, Hall Southwest Water Consultants, 3303 Northland Drive, Suite 311, Austin, Texas 78731.

The Proterozoic metamorphic belt of northern and central New Mexico contains rocks of two distinctly different metamorphic grades, locally lying in direct contact. A large region exhibits coexisting kyanite, andalusite, and sillimanite. The triple-point assemblages resulted from peak metamorphic conditions falling near 500°C, 4 kb, and they occur across 75,000 km2 in 14 separate mountain ranges. Triple-point metamorphic grade was attained after north-verging folding and ductile thrusting but before south-verging mylonitic shearing. Prograde and retrograde P-T paths were nearly isobaric.

However, two mountain ranges preserve a different metamorphic history. Granitic and migmatitic gneiss in the Santa Fe Range formed at 650° to 750°C. Sillimanite–K-feldspar gneiss in the northern Taos Range records peak conditions as high as 700° to 800°C, 9 to 11 kbar. In the latter area, high-grade metamorphism was followed by south-verging shearing, southeast-verging folding, and decompression, on a retrograde P-T path that passed through 500°C, 4 kb. Deformation produced a thick mylonitic shear zone that appears in scattered outcrops across the entire mountain range. The shear zone separates gneiss from structurally overlying kyanite–andalusite–sillimanite quartzite. Mineral textures indicate that the shearing occurred during and after the thermal peak in the gneissic rocks, and during the thermal peak in the overlying triple-point rocks.

We suggest that the kyanite–andalusite–sillimanite rocks are separated from the rocks of higher metamorphic grade by a synmetamorphic ductile detachment fault represented by the thick mylonitic shear zone. The detachment separated an actively extending lower crust from an upper crust that was not being thinned. We infer that the detachment extends in the subsurface beneath the entire belt where rocks show kyanite–andalusite–sillimanite metamorphic grade. Lower-crustal extension, with heat carried upward toward the detachment, seems to have caused the triple-point metamorphism.

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