FLUID-TRANSPORT-LIMITED ANDALUSITE PORPHYROBLAST CRYSTALLIZATION IN THE RINCONADA FORMATION, PICURIS MOUNTAINS, NEW MEXICO

One-of-a-kind gigantic (up to ~30 cm) andalusite porphyroblasts can be found in a generally layer-parallel band at the base of the Rinconada Formation that lays conformably on the Ortega Quartzite in the Hondo Syncline located in the Picuris Mountains, New Mexico. The Picuris Mountains are one of several Precambrian cores uplifted during the Laramide time exposing supercrustal rocks metamorphosed to amphibolite facies by two orogenic episodes, the Yavapai orogeny 1800 – 1700 Ma, and Mazatzal orogeny 1700 – 1600 Ma, followed by subsequent plutonic episodes between 1500 – 1300 Ma. It is the latter plutonic emplacement in Northern New Mexico that generated porphyroblast growth on a regional scale, reflecting near-triple point conditions.

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.