Evolution of coronas in Norwegian anorthosites: re-evaluation based on crystal-chemistry and microstructures

Coarse grained garnet-pyroxene coronas in anorthositic gabbros of the Bergen and Sognefjord areas preserve relics of two stages of reaction during cooling from magmatic T: (1) ol + plag→cpx I+ opxI + spn; (2) cpxI + opxI + spn + plag→cpxII + opxII + gnt. Geothermobarometry on the products of reaction (1), using Fe³⁺ cpx and Al^IV opx values derived by X-ray structure refinement (XRef), yields T=900-950° C, P=10-11 kb. Even higher T, probably at lower P, is inferred for reaction (2); this is consistent with TEM observations of pigeonite lamellae in cpxI. Transmission electron microscopy (TEM) observations show abundant submicroscopic amphibole lamellae in cpxII, and XRef analyses of coexisting massive amphiboles suggest incomplete occupancy of the H site; both observations suggest low but finite {Mathematical expression} during granulite-facies metamorphism. TEM observations also show that later (retrograde) hydration, particularly of orthopyroxenes, is more extensive than suggested by petrographic studies. Late amphiboles have fully occupied H sites, indicating higher {Mathematical expression}. The integration of XRef and TEM studies with more conventional petrographic and electron microprobe (EMP) data, as in this study, gives a powerful tool for the study of metamorphic reactions.