Zoned feldspar records hydration-dehydration cycles in lower-crustal shear zones

Abstract

Metamorphic reactions provide crucial insights into the conditions that rocks experienced and significantly influence their rheological behaviour. However, some reactions are challenging to identify in the rock record. This study focuses on zonation and microstructures in plagioclase as indicators of metamorphic history, employing a case study from Lofoten, northern Norway, where various types of plagioclase microstructures are preserved together. We use a combination of electron microscopy techniques (electron backscatter diffraction, transmission electron microscopy, and electron microprobe analyses) complemented with local equilibrium thermodynamic modelling to examine the relationships between composition, microstructures, and underlying metamorphic processes, and put that in context with previous observations of metamorphically zoned plagioclase, often referred to as “complex feldspar”. Needle-shaped Ca-richer zones in plagioclase represent pseudomorphs after zoisite/clinozoisite, indicative of plagioclase breakdown following fluid infiltration at high pressure and subsequent reversal of the reaction. Polygonal plagioclase with complex zonations also originates from the hydration/dehydration cycle but has additionally undergone deformation during hydration. Minor changes in pressure and temperature at upper amphibolite-facies conditions can significantly affect the stability and microstructures of plagioclase and thereby the rheological behaviour of the lower crust.