The interaction between reaction and deformation: an experimental study using a biotite + plagioclase + quartz gneiss

Abstract

AbstractMetamorphic reactions commonly accompany ductile deformation of crustal rocks. We performed an experimental study to determine: (i) the effect of syn‐deformation reaction on strain weakening and localization, and (ii) the effect of crystal plastic deformation on reaction extent and distribution. Experiments were conducted on a fine‐grained gneiss (58 vol.% quartz, forming the interconnected matrix, 13 vol.% biotite, 28 vol.% plagioclase and 1 vol.% garnet/Fe‐Ti oxides). General shear experiments were performed at 745 and 800 °C, 1.5 GPa, two strain rates, and shear strain (γ) from 0.6 to 5, yielding three suites with initial phase strength contrast between the matrix quartz and weak biotite of 45×, 25× and 10×; hydrostatic experiments were performed on cores and powders at 750 and 800 °C and 1.5–2 GPa for the same times. At these conditions, biotite reacts with plagioclase and quartz to form garnet, K‐feldspar and water (no melt was observed). Greater reaction extent was observed in deformed samples than in equivalent hydrostatic samples, because of the increased surface area and internal strain energy. In all of the deformed samples, reaction contributes to strain weakening, due principally to a switch to grain boundary sliding in the fine‐grained reaction products. The degree to which syn‐deformational reaction causes strain weakening and localization in this polyphase aggregate depends on the phase strength contrast and how it evolves. In samples with low‐phase strength contrast, strain and reaction are homogeneously distributed; however, in samples with high‐phase strength contrast, ductile strain and reaction interact positively to produce a narrow ductile shear zone. Similar concentration of reaction is observed in some natural ductile shear zones.