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Master of Science (M.S.)

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MN-Andalusite, Tusas Mountain Range, New Mexico


The Proterozoic syn-orogenic supra-crustal package exposed in the Tusas Mountain Range in northern New Mexico includes the anomalously thick, clean, ultramature Ortega Formation quartzite (Bauer and Williams, 1989). A unique Mn-bearing horizon spans the basal contact of the quartzite and contains Mn-andalusite, quartz, hematite, rutile, zircon, monazite, phyllosilicates, and locally kyanite and gahnite in intermittent aluminous, hematite-bearing layers, on the surface of cross-beds, concentrated in high strain zones, and on cleavage planes. Large-scale Mn zoning in Mn-andalusite results from a decrease in Mn content away from localized Mn highs (XMn of up to 0.76) that commonly correspond with the location of high-Mn core regions. High Mn core regions are included in Mn-andalusite, preserve some of the earliest fabrics, and are associated with sedimentary structures (i.e. cross beds and graded beds). Both hematite and rutile from the Mn horizon are rich in trace elements, and hematite contains zones of rutile mineralization that create patchy/mottled, wormy, cross-hatched, and/or banded textures within the hematite that could be linked with oxidation-exsolution textures in ilmenite and spinel. The Mn horizon may be a result of (1) the formation of an Fe-Mn oxyhydroxide crust through extensive lateritic weathering of the metavolcanic units and/or (2) the chemical deposition of Fe and Mn phases in an oxygen stratified enriched basin environment.

Mn-andalusite preserves subtle textures in the Mn distribution and interference colors of the birefringence that can be used to understand mineral replacement and microstructural relationships in otherwise an well-annealed quartzite. A feathery texture that resembles the vi crystal habit of a phyllosilicate, such as pyrophyllite, locally preserves crenulation cleavages. High-Mn halos around kyanite and localized areas of low Mn content in Mn-andalusite that mimic crystal shapes together represent areas of kyanite replacement.

The general tectonic histories preserved in the Mn-andalusite layer of the Hopewell Lake-Jawbone Mountain area, Quartzite Peak, and Kiowa Mountain of the Tusas Mountains include the growth of an M1 mineral assemblage in association with D1 (kyanite and/or pyrophyllite), the growth of an M2 assemblage post-D2 (kyanite and/or Mn-andalusite), and the growth of an M3 assemblage syn-D3 (a second generation of kyanite and/or Mn-andalusite). In the Hopewell Lake-Jawbone Mountain region, Mn-andalusite of the Hopewell anticline preserves phyllosilicate defined crenulation cleavages while the Jawbone syncline is dominated by a history of only kyanite growth suggesting a metamorphic divide between the anticline and syncline that could be a result of the movement along a late-stage D3 fault at or near Route 64. At Kiowa Mountain, high-Mn halos and low Mn regions suggest that Mn-andalusite replaced kyanite when changing P-T conditions during M2 lead to an Mn-andalusite-only M2 assemblage. Textures preserved in the Mn-distribution and interference colors of Mnandalusite are an essential tool for understanding the structural and metamorphic histories of the Mn horizon and the Ortega Formation quartzite.