Seeing through metamorphic overprints in Archean granulites: Combined high-resolution thermometry and phase equilibrium modeling of the Lewisian Complex, Scotland

Jon Wade,Christine Gopon,Jacob B Forshaw,David J Waters,Phillip Gopon

doi:10.2138/am-2022-8214ccby

Jon Wade, Christine Gopon + Show 3 more

Open Access

https://doi.org/10.2138/am-2022-8214ccby

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Abstract

Abstract The Lewisian Complex in northwest Scotland presents a record of the transition from the Neo-Archean to the Paleoproterozoic. However, this record is complicated by a long and varied history after peak metamorphism that has erased and/or partially reset much of the early history of the rocks. Such overprinting is a common feature of Archean granulites and poses a substantial problem when trying to understand the tectonic processes that were active prior to the onset of modern plate tectonics. By combining careful petrography with phase diagram modeling and a range of exchange thermometers we obtain the peak and retrograde temperature history of the Lewisian Complex from a single, well-preserved, representative sample of garnet-bearing mafic granulite. We present the application of high-resolution electron probe microanalysis (HR-EPMA) to characterize sub-micrometer orthopyroxene exsolution lamellae in clinopyroxene. We discuss ways to mitigate issues associated with HR-EPMA including surface contamination, beam drift, standards, and the need to correct for secondary fluorescence effects. The resulting compositions from our HR-EPMA analyses provide an independent measure of the retrograde temperature conditions and can also be used to back-calculate the compositions of clinopyroxene in the peak assemblage. We obtain peak metamorphic conditions for the Lewisian of &gt;11 kbar and &gt;1025 °C, and constrain subsequent metamorphic overprints to 850 °C (Grt-Cpx), 590 °C (Opx-Cpx), and 460 °C (Mag-Ilm). These peak and retrograde temperatures span the range of those found in the literature. Whereas recent phase equilibrium studies assume equilibrium among all preserved high-T minerals, this study considers microstructural and mineral-chemical evidence for corona formation that reflects post-peak decompression with partial equilibration at ~850 °C, as recognized in some earlier studies.

Highlights

The Lewisian Complex of NW Scotland comprises a sequence of Archean igneous rocks which have undergone a series of metamorphic events
Modern phase equilibrium modeling techniques (e.g., De Capitani and Brown, 1987; De Capitani and Petrakakis, 2010; Connolly, 1990; Connolly and Petrini, 2002; Powell et al, 1998; Holland and Powell, 2011; Green et al, 2016), have been applied to the Badcallian metamorphism (e.g., Johnson and White, 2011; White et al, 2017; Feisel et al, 2018; Forshaw et al, 2019), but this approach has been hampered by the large P-T field in which the peak mineral assemblage occurs, by domains of partial re-equilibration that exist within samples, and issues relating to partitioning of minor elements among the dominant minerals
Understanding the mineral relationships that are preserved in the breakdown microstructures is crucial if the aforementioned thermometers and barometers are to be applied correctly, and accurate P-T estimates are to be obtained for the different stages of metamorphism

Summary

Introduction

The Lewisian Complex of NW Scotland comprises a sequence of Archean igneous rocks which have undergone a series of metamorphic events. This study focusses on the central region, specially the mafic rocks of Scouriemore (Figure 1), that underwent (ultra)high‐temperature metamorphism during the Archean (Badcallian) regional metamorphic episode (Park, 1970; Sills and Rollinson, 1987). The scatter in P-T estimates is largely due to the one (or more) subsequent metamorphic events that have wholly erased or partially reset the peak mineral compositions (Barnicoat, 1983; Sills and Rollinson, 1987). Modern phase equilibrium modeling techniques (e.g., De Capitani and Brown, 1987; De Capitani and Petrakakis, 2010; Connolly, 1990; Connolly and Petrini, 2002; Powell et al, 1998; Holland and Powell, 2011; Green et al, 2016), have been applied to the Badcallian metamorphism (e.g., Johnson and White, 2011; White et al, 2017; Feisel et al, 2018; Forshaw et al, 2019), but this approach has been hampered by the large P-T field in which the peak mineral assemblage occurs, by domains of partial re-equilibration that exist within samples, and issues relating to partitioning of minor elements among the dominant minerals

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