Abstract

In the Picuris Range near Taos, New Mexico, the Precambrian Ortega Quartzite and Rinconada Formation are folded together and share a common history. The Ortega contains chloritoid-Al silicate and essentially no staurolite, and the Rinconada contains staurolite-almandine–bearing assemblages with minor graphite. Al silicate–bearing Rinconada rocks locally contain Mn-rich garnet.

All the rocks of the area crystallized at temperatures a little above the Al silicate triple point, as indicated by the paragenetic sequence kyanite → andalusite → sillimanite. Analysis of the assemblages in relation to chloritoid, staurolite, and Al silicate stability data, taking into account the mineral compositions and the effect of graphite on fluid composition, shows that all the rocks crystallized at 532 ± 20 °C and about 3,700 b Ptot, but PH2O/Ptot was 0.88 or less in the Rinconada rocks and near 1 in the Ortega rocks. This small difference in PH2O appears to be the most important factor in explaining the juxtaposition of chloritoid and staurolite in the two units.

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