Semester of Graduation

Summer 2026

Degree

Master of Science (MS)

Department

Geology and Geophysics

Document Type

Thesis

Abstract

Metapelites contain minerals sensitive to pressure, temperature, and bulk composition used to interpret metamorphic conditions. Tourmaline, a common and typically overlooked accessory mineral in metapelites, exhibits compositional zoning reflecting its chemical responsiveness to changes in its host environment. This study demonstrates that tourmaline zoning chronicles the major discontinuous metamorphic-mineral reactions in metapelites across a roughly isobaric terrain from garnet-to-sillimanite-zone conditions in West-Central Maine, USA. Through petrography, backscatter-electron imaging, and electron-probe microanalysis, tourmaline is texturally and chemically characterized. Using mineral-chemical trends, tourmaline zoning is correlated to matrix chemical reactions. Reaction temperatures are estimated using phase-equilibrium modelling and geothermometry.

Tourmalines belong to the schorl-dravite series. Garnet- and staurolite-zone tourmalines contain three compositionally distinct domains surrounding a detrital core. The earliest overgrowths, nucleating on the core, decrease in XMg (Mg/Mg+Fe) and Al. In the second and third overgrowths, this compositional trend is reversed, and tourmaline increases in XMg, Al, X-site vacancy. With increasing grade, compositional polarity in tourmaline diminishes and staurolite-zone tourmaline have homogeneous outer rims. Sillimanite-zone tourmalines are homogeneous, dravitic, and the most Al-rich. Systematic element partitioning show that matrix minerals and tourmaline have achieved chemical equilibrium such that tourmaline compositions reflect matrix reactions.

Overgrowth chemistry in garnet- and staurolite-zone tourmaline reflects the host rock’s metamorphic-mineral assemblage. Chemical discontinuities between overgrowths are interpreted with respect to metamorphic reactions in the matrix: (1) between the first and second overgrowths; and (2) between the second and third overgrowths. Each overgrowth reflects mineral assemblage changes facilitated by discontinuous and continuous reactions during prograde metamorphism at 3.2kbar. The first overgrowth likely grew under chlorite-zone conditions (< 400°C). In garnet-zone tourmaline, discontinuity (1) records the biotite-in reaction (430±50°C), 2Ms+8Chl=5Bt+6SiO2+8H2O. Discontinuities (2) in garnet-zone tourmaline, and (1) in staurolite-zone tourmaline, record the garnet-in reaction (460±50°C), 1.5Chl+2Ms+3SiO2=Grt+2Bt+3H2O. Discontinuity (2) in staurolite-zone tourmaline records the staurolite-in reaction (500±50°C), 2Grt+1.5Chl+3Ms=St+3Bt+8SiO2+4H2O. Sillimanite-zone tourmaline chemistry reflects growth >550°C, after staurolite breakdown. Geothermometry results and calculated reaction temperatures are consistent with previous work. This study is a first to pair specific chemical zoning with reactions among matrix minerals, thus underscoring tourmaline’s utility as a reliable chemical indicator of prograde mineral reactions in low-to-medium-grade metapelites.

Date

7-10-2026

Committee Chair

Dutrow, Barbara

LSU Acknowledgement

1

LSU Accessibility Acknowledgment

1

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