TY - JOUR
T1 - Seeing through metamorphic overprints in Archean granulites
T2 - combined high resolution thermometry and phase equilibrium modeling of the Lewisian Complex, Scotland
AU - Gopon, Phillip
AU - Forshaw, Jacob
AU - Wade, Jon
AU - Waters, David
AU - Gopon, Christine
PY - 2021/8/25
Y1 - 2021/8/25
N2 - The Lewisian Complex in NW 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 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 > 11 kbar and > 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 c. 850°C, as recognized in some earlier studies.
AB - The Lewisian Complex in NW 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 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 > 11 kbar and > 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 c. 850°C, as recognized in some earlier studies.
U2 - 10.2138/am-2022-8214CCBY
DO - 10.2138/am-2022-8214CCBY
M3 - Article
SN - 0003-004X
JO - American mineralogist, The
JF - American mineralogist, The
ER -