The Diverse Habitats of Eclogite Formation: Insights From the Mesoproterozoic Glenelg Inlier, Scotland

Abstract

Eclogite-facies rocks provide important constraints on the behaviour of convergent plate boundaries and the geometries of tectonic reconstructions due to the high to ultrahigh pressure conditions at which they form. Many eclogite occurrences are documented near the suture zone of active collisional settings where they are interpreted to mark the approximate location of former ocean basin subduction. Such observations influence tectonic interpretations for older eclogites within more deeply eroded and/or less well-exposed terranes. The eclogitic Glenelg inlier in northwest Scotland is one such example, with c. 1 Ga eclogites having previously been interpreted as marking the trace of a Grenville-aged collisional suture zone that defines a third ‘arm’ to the Grenville orogen alongside well-defined sutures in North America and Scandinavia. Here we use a combination of geochronology, phase equilibrium modelling and accessory-phase thermometry to show that the eclogite-facies assemblages were produced at $\sim$18–19 kbar and 700°C–750°C from c. 1.1 to 1.0 Ga. Accounting for the foreland basin setting of equivalent-aged sedimentary rocks in the region and demonstrating the thermal viability of this setting, we show that eclogite formation occurred in deforming foreland crust adjacent to the Grenville orogen, in a setting broadly analogous to fault-bounded basement uplifts in the forelands of active deformation belts, such as the Himalayas and Andes. Our results demonstrate that eclogite-facies rocks can form in a greater range of tectonic settings than are sometimes considered, with implications for tectonic reconstructions of collisional zones. In this instance, our results remove the need for a third ‘arm’ of the Grenville orogen by placing Glenelg in a foreland setting, reconciling the absence of plentiful Grenville-aged metamorphic rocks in northwest Scotland, the sedimentological record and paleomagnetic data in the wider region.