Microstructural and chemical changes associated with cataclasis and frictional melting at shallow crustal levels: the cataclasite-pseudotachylyte connection

Abstract

In pelitic schists of the Nason terrane of Washington state, pseudotachylyte can be demonstrated to have developed subsequent to the formation of foliation-parallel cataclasite zones. Numerous microstructural lines of evidence point to a genetic link between pseudotachylyte and cataclasite, and chemical evidence indicates that cataclasis played an important role in controlling pseudotachylyte, chemistry. Besides optical and textural differences between the cataclasite and pseudotachylyte, which may occur together, there is unambiguous evidence of a melt origin for the pseudotachylyte, while such evidence within the cataclasite is absent. TEM evidence indicates that glass is present in at least some of the veins as demonstrated by a lack of electron diffraction. Microlites present in the glass lack dislocations. High-temperature microlites, of a mineralogy and composition not present in the host schist, occur in various habits which depend on the distance from the vein margin. Also present are assimilation textures, apparently quenched sulfide droplets and vesicles and amygdules. In a pseudotachylyte vein studied in detail, five microstructural domains occur. These include vein margin cataclasite, a transitional dark pseudotachylyte, vein-interior glass, opaque and sulfide-rich blebs and translucent haloes surrounding some of the blebs. The cryptocrystalline cataclasite matrix grades into the dark pseudotachylyte, which optically grades into the glass. After adjustment for silica, the host schist and cataclasite matrix are chemically very similar. The dark pseudotachylyte and glass, with the exception of Na 2O and CaO, are chemically quite similar to the cataclasite matrix. The main difference may be the result of the segregation of the opaque (plagioclase) blebs. Thus, the pseudotachylyte chemistry is strongly dependent upon the chemistry of the material comprising the cataclasite matrix, which in turn is dependent on mineral susceptibility to incorporation into the cataclasite matrix. Fluids infiltrated along the cataclasite zones prior to pseudotachylyte generation, as recorded by hydration reactions, sulfide veining, Na 2O loss and fluid inclusions. This demonstrates that pseudotachylytes can form under fluid-rich conditions, as opposed to the commonly assumed dry conditions.