Semester of Graduation

Summer 2024

Degree

Master of Science (MS)

Department

Geology and Geophyscis

Document Type

Thesis

Abstract

Tourmalinites and their associated tourmalines record significant information on both the host rock and fluid compositions in which they grew. This information is utilized to evaluate tourmalines preserved within a section of Paleoproterozoic crust, in a region where outcrops are poorly exposed. Recently recognized Paleoproterozoic tourmalinites from the Cerro Colorado, Tusas Mountains, New Mexico, provides a new avenue to elucidate a formational environment for this locality.

Field work, mineral chemistry, whole rock chemistry, and boron isotopic data were used to constrain the formational environment of the tourmalinites. Tourmalinites from the Cerro Colorado, Tusas Mountains, are structurally situated in a regional antiform and are hosted within a staurolite-garnet-biotite schistose host rock as highly deformed discontinuous and continuous stratabound layers. Tourmalinites contain the major assemblage tourmaline+quartz+plagioclase. Tourmaline is very fine grained with an average diameter of ≤10µm. Tourmaline compositions within the host rock and the tourmalinites are similar. Chemical analysis of tourmaline show little variation, with tourmalines classified as belonging to the alkali and hydroxyl groups. Both Mg-rich schorl and Fe-rich dravite species are present. The average formula is: (Na0.64□0.24Ca0.12)(Mg1.64Fe2+1.14Al0.13Ti0.09)Al6(BO3)3(Si5.91Al0.09)O18OH3(OH0.85F0.10O0.05). Minor chemical substitutions within tourmaline are given largely by the MgFe-1, with less contribution by FeAl-1, and (AlO)(Mg(OH))-1 exchange vectors. Tourmalinite trace element chemistry normalized to average continental crust are enriched in elements such as K, Sr, Ni, Sc, Cr, Zr, and Hf. Boron isotopic values for tourmaline within tourmalinites are consistent and range from -14.1 to -12.1 δ11B ±2‰.

Comparison of tourmaline chemistry, field relations, and associations with other tourmalinite forming environments suggests that tourmalinites were formed within a metasomatic environment. Garnet-biotite-plagioclase-sillimanite geothermobarometers suggests that tourmalines crystallized between 550-600oC and 3-4 kbar. Minor chemical variations and the lack of zoning suggests that tourmaline crystallized during a single event from continentally sourced B-bearing fluids from as suggested by the negative δ11B values. Local metasediments could react to provide the necessary Al required for tourmaline formation. The occurrence of tourmalinites within the region also may have preserved a highly localized infiltration of a B-rich fluid during the Paleoproterozoic.

Date

5-31-2024

Committee Chair

Dutrow, Barbara L.

DOI

https://doi.org/10.31390/gradschool_theses.5993

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