FLUID-TRANSPORT-LIMITED ANDALUSITE PORPHYROBLAST CRYSTALLIZATION IN THE RINCONADA FORMATION, PICURIS MOUNTAINS, NEW MEXICO
Porphyroblast crystallization is governed by the competition between nucleation and growth. Sluggish heat transport relative to chemical transport may restrict nucleation rate relative to growth rate, resulting in dramatically fewer larger crystals than is typical. Fluid flow is a necessary, although not sufficient condition for this to occur. A combination of detailed field mapping, petrography, X-ray fluorescence, electron microprobe analysis, scanning electron microscope, and cathodoluminescence methods suggest that the band of large andalusites experienced elevated fluid flow relative to the adjacent otherwise-similar rocks. Geothermometry reveals differing peak temperatures between the large-andalusite-bearing specimens and those adjacent as well. These results are consistent with a model in which the band of large andalusites experienced increased fluid flow, presumably moving along the boundary between the Rinconada Formation and Ortega Quartzite.