TOURMALINITE COMPOSITIONS AS INDICATORS OF FORMATIONAL ENVIRONMENT: AN EXAMPLE FROM THE TUSAS MOUNTAINS, NORTHERN NEW MEXICO, USA

Tourmalinites, rocks containing >15% tourmaline, may be useful for determining the environment of formation because the chemical signatures of tourmalines commonly reflect the host-rock environment. Tourmalinites require a substantial quantity of boron to form, suggesting that their formation is primarily from metamorphism of boron-rich units or from infiltration of preexisting rocks by boron-bearing fluids. In this study, tourmalines are used to evaluate the likely environment for tourmalinite formation in the southern Tusas Mountains, New Mexico, USA, where stratabound tourmalinites are hosted in sillimanite-bearing schists.

The tourmalinite has ~50% tourmaline, with the remainder comprised of quartz with minor feldspar, sillimanite, and biotite. The stratiform rock alternates between quartz- and tourmaline-rich layers. Tourmalines are granoblastic and locally poikiloblastic with quartz inclusions. Chemical compositions of tourmaline grains, obtained by Electron Microprobe, demonstrate a restricted range of compositions: Al (6.17-6.44 apfu), Fe (1.00-1.14 apfu), and Mg (1.65-1.75 apfu). The tourmalines belong to the alkali group and hydroxy-secondary groups and are classified as dravites with an average chemical formula of (Na_0.58□_0.26Ca_0.16)(Mg_1.70Fe²⁺_0.84Fe³⁺_0.21Al_0.17Ti_0.04V³⁺_0.01Cr³⁺_0.01)Al_6.0(Si_5.83Al_0.16)O₁₈(BO₃)₃(OH)₃(OH_0.67O_0.24F_0.09). The tourmalines have a minor oxy-component (0.167-0.299 apfu).

Based on the provenance diagram of Henry & Guidotti (1985 Am. Min.), tourmalines fall in the metapelitic or metapsammitic fields, suggesting that the host rocks during tourmalinite formation were likely aluminous clastic sediments. The lack of chemical zoning suggests that if fluids are responsible for the tourmalinites, they likely formed from a single B-bearing fluid event. Tourmalines from meta-evaporites tend to display compositional trends between oxy-dravite and povondraite. However, tourmalines from the Tusas tourmalinites have only a slight indication of that trend. Consequently, the tourmaline data appear to retain the metapelitic signature even with boron influx. These data show the power of tourmaline composition as an indicator of formational environments for tourmalinites.