Abstract

Tourmaline is a ubiquitous accessory phase in rocks spanning a wide range of P–T–X conditions; owing to its compositional sensitivity to variations in such conditions, it is potentially an ideal indicator mineral. Here, we apply a variety of tools to reconstruct the P–T–X history of the Variscan metamorphic basement of the central Massif Central in France using tourmaline compositions and zoning. Tourmaline thermometry, using the temperature dependence of element partitioning among sectors in individual grains, indicates that these grains record and preserve the full metamorphic history of their host rocks, from nucleation at 350°C along the prograde path to a peak at 650°C, and down to 450°C on the retrograde path. Using major- and trace-element compositions, we are able to recognize specific reactions and changes in mineral assemblage and use these to constrain the prograde path of these rocks. Combining P–T constraints with experimentally determined partitioning of Na between fluids and tourmaline provides a record of Na concentrations in the metamorphic fluid for the entire metamorphic path. These results show that tourmaline can provide a fully quantitative record of the P–T–X conditions in its host environment during growth. This is especially valuable for the prograde history, where most of its growth takes place, and for which few other reliable sources of information are available. Tourmaline, therefore, has the potential to become the prime indicator mineral for P–T–X conditions in a wide variety of geological environments.

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