Interactions between deformation and dissolution-precipitation reactions in plagioclase feldspar at greenschist facies

Abstract

Rocks often undergo deformation and metamorphism simultaneously. However, relatively little research has been carried out on the interactions between deformation, fluid influx and a recently identified type of metamorphic reaction, termed interface-coupled dissolution-precipitation. In this study, optical microscopy and electron backscatter diffraction (EBSD) were used to investigate the interactions between deformation and dissolution-precipitation reactions in feldspar from metagabbros deformed at mid-crustal conditions. Fracturing and fluid influx promoted two types of dissolution-precipitation reactions that worked in tandem to convert Ca-bearing plagioclase to pure albite. Conventional dissolution and precipitation, here defined as dissolution precipitation reactions involving a transport step, worked to heal fractures and produce fine-grained albite. In parts of original grains that were not fractured, interface-coupled dissolution-precipitation occurred to albitize mm-cm scale grain fragments. Because interface-coupled replacement displays fast kinetics, reaction fronts were not preserved, so coupled dissolution-precipitation was identified using the following microstructural criteria: a lack of preserved zoning, indicating a fast reaction mechanism; orientation inheritance during reaction indicating epitaxial nucleation/topotactic replacement; intracrystalline strain containing Burgers vectors that indicate distortion was not derived from crystal plasticity; intragranular microporosity and second phase inclusions which share orientations with twin and cleavage planes of their parent grains. These criteria could be applied to any system to identify interface-coupled replacement in the absence of preserved reaction fronts. Brittle fracturing and dissolution-precipitation resulted in an overall grain size reduction, and a transition from dominantly brittle to dominantly viscous deformation, producing mylonites at greenschist facies, and contributing to the development of a regional-scale shear zone.