Metamorphic differentiation via enhanced dissolution along high permeability zones

Abstract

Metamorphic differentiation, resulting in segregated mineral bands, is commonly recorded in metamorphic rocks. Despite the ubiquitous nature of compositionally layered metamorphic rocks, the processes that are responsible for metamorphic differentiation receive very little attention. Here, detailed petrography, quantitative mineral chemistry and bulk rock analyses are applied to investigate compositional variations and assemblage microstructure. Furthermore, thermodynamic modelling is applied to provide additional constraints on the P-T-XH2O conditions of assemblage formation and mass transfer. The studied outcrop, located within the Bergen arcs of southwestern Norway, preserves the hydration of anorthositic granulite at amphibolite-facies conditions. The amphibolite-facies hydration is expressed as both a statically hydrated amphibolite and a shear zone lithology, defined by the interlayering of amphibolite with leucocratic domains. Within the granulite, quartz-lined fractures surrounded by amphibolite-facies alteration haloes represent relics of initial fluid infiltration associated with brittle failure. The fracture assemblage (quartz + plagioclase + zoisite + kyanite ± muscovite ± biotite) is identical to that occurring within leucocratic domains of the shear zone. Consequently, the compositional layering of the shear zone lithology is linked to fluid infiltration along localized zones of high permeability that result from fracturing. Mass-balance calculations indicate that quartz-lined fractures and compositional differentiation of the shear zone resulted from mass redistribution internal to the shear zone rather than partial melting or precipitation of minerals from externally derived fluid. The process of internal fractionation within the shear zone is driven by enhanced dissolution along highly permeable fracture planes resulting in the loss of MgO, Fetot and K2O from the leucocratic domains. Elements dissolved in the fluid are then transported and ultimately either precipitated in comparatively impermeable amphibolite domains or removed from the system resulting in an overall mass loss. The mass transfer causing metamorphic differentiation of the shear zone is the result of coupled reaction and diffusion under differential stress. The mechanisms of mass redistribution observed within this shear zone provides further insight into the processes that facilitate mass transfer in the Earth's crust.