Fluid interaction and element mobility in the development of ultramylonites

Abstract

ABSTRACT Shear zones are often characterized by the presence of mylonitic fabrics. The textural development of such fabrics is enhanced by the presence of a fluid phase. In a single specimen of rock from the Brevard fault zone in North Carolina, we can demonstrate the development of ultramylonite domain by focused fluid flow. The ultramylonite interface retains a sharp textural and chemical discontinuity; this suggests that solute transport was dominantly parallel to the tectonic layering. Major chemical changes between the ultramylonite zone and the protolith include losses of Si(>2, Na 2 0, and K 2 0 and gain of CaO, FeO, and HjO in the high-strain domain. INTRODUCTION During the tectono-thermal evolution of mountain belts, zones of high strain are often recorded by the development of mylonites. Although mylonites and subcategories within mylonites (Sibson, 1977) can be generated by numerous well-documented physical processes, their chemi-cal and isotopic evolution is much more complex. Mechanical processes often associated with the development of high-strain zones include micro-fracturing, dilatancy, grain-boundary sliding, and dislocation glide. These properties are strongly dependent on the modal mineralogy (bulk compo-sition), heterogeneity of the rock, and the temperature and pressure con-ditions during deformation. Such variations in the mechanics of deforma-tion have led to the more common classification of shear zones as either ductile or brittle (Sibson, 1977; Ramsay, 1980). However, most shear zones are characterized by the availability of fluids that cause a drastic change in the style of deformation by altering the behavior and assem-blage of minerals (hydration reactions) and the rates of chemical and mechanical processes during progressive deformation. The presence of a fluid phase during deformation enhances solution transport, microfractur-ing, and recrystallization Suc.h chemically active zones generate mineral assemblages that record the metamorphic history of the mylonites, al-though it is not unusual to find minerals that persist through such an event. These relict minerals, if recognized, can identify equilibrium vs. nonequilibrium assemblages observed in shear zones. Clearly, the pres-ence of relict minerals would have a substantial impact on the trace-element and isotopic signature of the rock. To more carefully document the behavior of elements in a strain zone that has abundant fluid, we have chosen to study a transect across the Brevard fault zone, near Rosman, North Carolina. Within this tran-sect, one particular outcrop provided a single large specimen that over a distance of-25 cm showed the entire strain-gradient fabric from proto-mylonite to ultramylonite (see Sibson, 1977, for mylonite series) ob-served by us as discontinuous patches in the longer (15-km) transect. We report petrographic, textural, and chemical data from this block and pro-vide a physical model to explain these data.