MN-ANDALUSITE HORIZON IN THE ORTEGA FM. QUARTZITE: IMPLICATIONS FOR THE PROTEROZOIC TECTONIC HISTORY OF THE TUSAS MTS., NORTHERN NEW MEXICO

Thick (>1km), ultra-mature Paleoproterozoic quartzites that outcrop across the southwestern US provide important constraints on Proterozoic tectonics and Rodinia reconstruction. The quartzite contains a distinctive horizon characterized by Mn-bearing andalusite and trace-element-rich oxides. It occurs along cross beds, on cleavage planes, and in areas of high strain. Study of this horizon provides one tool for understanding the origin and tectonic history of quartzites.

In the Tusas Mts., NM, distinctive green Mn-andalusite occurs along cross-beds in the lower 10s of meters of the Ortega Fm. The distribution of Mn in andalusite porphyroblasts suggests an Mn source that was closely associated with the oxides. Multiple deformation fabrics are preserved (S1, S2, S3) within the porphyroblasts and evidence for two to three periods of Mn-andalusite growth, including high-Mn core regions, intermediate zones with euhedral faces, and low-Mn outer zones. High-Mn core regions preserve Mn-andalusite with up to 25-27 wt% Mn and a weakly developed S1 fabric. Syn to post-S2 Mn-andalusite is represented by the intermediate zones. Fibrous textures preserved as patterns in the birefringence of the porphyroblasts (x-pol) caused by slight Mn concentration variations perfectly mimic an S2 crenulation cleavage and, in some samples, define euhedral crystal faces. This period of growth represents the dominant phase of Mn- andalusite and defines the S2 cleavage in outcrop. The final period of Mn-andalusite growth is interpreted to be syn to post-S3 and is characterized by overgrowths on already existent porphyroblasts and/or a complete recrystalization of those porphyroblasts depending on the degree of S3 foliation development in the rock. The unique character of Mn-andalusite to preserve textures in an otherwise annealed quartzite allows for the identification of multiple generations of fabric development and mineral growth that is essential to the interpretation of the tectonic history of Proterozoic rocks across the southwestern US. In addition, the anomalous geochemical signature and characteristic mineral assemblage of the Mn-horizon can be used for quartzite correlation across the southwestern US and possibly beyond.