TEXTURAL EVIDENCE FOR REACTION HARDENING BY FELDSPAR PRECIPITATION IN MYLONITES

Optical and SEM observation of two mylonites shows that the textures of these rocks are dominated by chemical/metamorphic processes rather than deformational/microstructural processes. A mylonite from the Moine thrust at Loch Eriboll, Scotland contains in descending abundance: quartz, plagioclase, microcline, and muscovite. Muscovite (~3%) defines a moderate foliation as anastomosing discontinuous flakes in a texture dominated by elongate framework silicates. These muscovite foliations truncate earlier foliations and microstructures involving microcline and other grains, suggesting the open-system replacement reaction: microcline + 2H⁺ = muscovite + 6 quartz + 2K⁺ (1). In support of this reaction, quartz, a reaction by-product is found locally in veins and pressure shadows. In other settings textures provide evidence for the operation of this reaction in reverse. Microcline completely surrounds and embays quartz, and partially replaces the frayed edges of muscovite. Thus the reaction is seen to be reversible, and the local K⁺/H⁺ activity ratio must have been locally buffered by this reaction, in a moderately closed system. Earlier mica folia locally contain randomly oriented, apparently brecciated flakes that suggest high strain rate slip events. K⁺ for H⁺ exchange on the newly exposed surfaces could have produced alkaline solutions that kept the local fluid in the microcline stability field.

A more mafic mylonite from the Norumbega fault zone (Fort Foster, Maine) contains amphibole, plagioclase, quartz, K-feldspar, biotite, sphene, and Fe-Ti oxides. Biotite and amphibole are replaced by plagioclase and orthoclase by reactions analogous to reaction (1). Brecciated zones of orthoclase and amphibole are cemented by plagioclase and locally microcline. Monomineralic bands of quartz and plagioclase define the foliation rather than contiguous biotite grains. As in the Moine mylonites, abundant replacement reactions involving the precipitation of feldspars define much of the texture. The stabilization of feldspars at the expense of phyllosilicates is strong evidence for reaction hardening of the fault zones. This strengthening apparently led to fast strain-rate slip events (muscovite breccias) in the Moine mylonites, and to abundant pseudotachylite along the Norumbega mylonites.