Structural controls of metasomatism on a regional scale

Abstract

Examples of structurally controlled regional metasomatism span most metamorphic temperatures, pressures, and tectonic settings. Permeability is crucial, and may be intrinsic or the result of deformation processes such as micro-fracturing (e.g. grain boundary sliding), fracturing, faulting, foliation and shear zone development, and hydro-fracturing. Chromatographic theory has been developed to explain metasomatic zonation, and applies not only where mineral reaction and isotopic fronts formed normal to the fluid flow in a porous medium, but also where flow was parallel to fractures or lithological contacts. Metasomatic fronts may be sharp or broadened by diffusion and dispersion effects during fluid advection. In the upper crust, fluid advection involved in regional metasomatism, such as silicification, albitization, potassic alteration and dolomitization, is controlled mainly by intrinsic permeability, faults, fractures and microfractures in sedimentary basins, volcanic sequences and some granites. For skarn formation, fluid advection is controlled by interface-coupled dissolution-precipitation (“reaction-controlled permeability”) in addition to fractures and microfractures. Cross-layer diffusion in addition to layer-parallel advection is probably important in the formation of skarns along lithological contacts. Although subject to some debate, many workers have provided mineralogical and isotopic evidence for the existence of high time-integrated fluid fluxes and fluid flow directions in many thermal aureoles and regional metamorphic belts. Studies of quartz veins and vein selvages in metamorphic rocks have provided examples both of local derivation of veins and derivation of silica and metasomatic changes in the selvages during advection of external fluids. Also subject to debate is whether metamorphic fluids in the middle crust are solely derived from devolatilization reactions at depth or whether such processes as dilatancy pumping or syn-metamorphic intrusion can lead to lateral or downward fluid advection. Regional metasomatism occurs in many Proterozoic metamorphic belts, but the Mount Isa Inlier is probably the best natural laboratory regarding structural controls. It exhibits enormous strain heterogeneity at all scales that led to fluid channeling, diverse reactive rocks, episodic metamorphic and intrusive events spanning at least 250 Ma, and abundant sources of reactive saline and hypersaline fluids. Examples of widespread metasomatism in the Mount Isa Inlier include Na-Ca alteration associated with extensive breccia development in calcsilicate rocks, albitization of schists and metapsammitic rocks, and the formation in high-strain zones of tremolite pods, quartz-chlorite rocks, and unusual cordierite-rich rocks.