A Cautionary Tale About Not ‘Joining the Dots’ to Infer Anticlockwise P- T Paths

Abstract

Uncertainty surrounds the cause and interpretation of anticlockwise pressure-temperature ( $P$– $T$) paths in metamorphic terranes. Here, we focus on the viability of a commonly proposed mechanism—magmatic heat transfer during thickening—using a case study of the Roineabhal terrane in Scotland, where such an anticlockwise $P$– $T$ path has been proposed. Phase equilibria modelling of new samples, combined with previous $P$– $T$ estimates, provides evidence for regional kyanite-grade granulite-facies metamorphism, with an additional spatially localised region of ultrahigh temperature conditions adjacent to an anorthosite intrusion. The spatial geometry of the ultrahigh temperature samples, combined with scaling arguments and thermal modelling of these results, shows that the ultrahigh temperature metamorphism is contact in nature and should not be joined to the regional metamorphism to infer an anticlockwise $P$– $T$ path. Rather, the regional metamorphism features hairpin $P$– $T$ loops, overlain adjacent to the anorthosite by a short-lived, high-temperature excursion. Because metamorphic rocks typically yield a fragmentary record during fluctuating thermal conditions, due to requiring hydration to maintain equilibrium during down-temperature evolution, it is critical to assess in this manner the thermal viability of the range of $P$– $T$ paths that could connect the preserved assemblages. In general, intrusion radii of tens of kilometres, or repeated intrusions of smaller bodies in quick succession (e.g., < 10 kyr for a 1-km radius), would be required for true magmatically driven anticlockwise $P$– $T$ paths. Given the unlikely nature of these requirements in most tectonic settings, such anticlockwise $P$– $T$ paths are likely to be rarer than reported. For many scenarios, $P$– $T$ paths commonly interpreted as anticlockwise $P$– $T$ paths are instead likely to take the form described in this study.