Tourmaline as a petrogenetic indicator mineral: an example from the staurolite-grade metapelites of NW Maine

Tourmaline, a mineral often overlooked in petrologic studies, is shown to be a useful petrogenetic indicator mineral. Despite the large number of possible substitutions in tourmaline, generalizations relating tourmaline composition and rock type are made. Based on available literature data, distinct regions are defined within Al-Fe(tot)-Mg and Ca-Fe(tot)-Mg diagrams for tourmaline from different rock types. As an example of the utility of tourmaline in petrologic studies, the accessory tourmaline from staurolite-grade pelitic schists from NW Maine is investigated. These tourmaline grains typically display three general styles of chemical zoning: (1) a lack of zoning, (2) a continuous core-to-rim zonation attributed to growth during progressive metamorphism, and (3) a zonation marked by a distinct discontinuity apparently representing detrital tourmaline grains surrounded by metamorphic tourmaline overgrowths. All of the samples in this study contain si-, Al-, and Ti-saturating phases (quartz, staurolite, and ilmenite, respectively). Consequently, the substitutional complexities of tourmaline in equilibrium with the matrix are minimized. Systematic element partitioning suggests that chemical equilibrium has been attained among the rims of the tourmaline grains and matrix minerals. Tourmaline rims have the highest Mg/Fe and Na./Ca ratios of any of the phases in the metapelites. The compositions of tourmaline cores that are presumed to be detrital in origin are optically and chemically distinct from their tourmaline overgrowths. In the Al-Fe(totl Mg and Ca-Fe(totfMg diagrams, the compositions of the detrital cores fall in the fields of rock types that are commonly found in the presumed source region for the pelitic sediments.