EFFECTS OF LITHOLOGY, CATACLASIS AND MELTING ON THE COMPOSITION OF FAULT-GENERATED PSEUDOTACHYLYTES IN LEWISIAN GNEISS, SCOTLAND

Petrological studies of fault-generated pseudotachylytes, formed during coseismic slip, need to consider cataclastic processes as well as rapid, closed-system melting and freezing in order to account for the observed similarities and the variations between the chemical compositions of pseudotachylytes and their host-rocks. Whole-rock analyses of pseudotachylytes and their gneissic host-rocks from the Lewisian Complex of northwest Scotland indicate that the composition of the pseudotachylyte is sensitive not only to the bulk host-rock composition but also to its mineralogy and fabric. In anisotropic (e.g., gneissic) rocks, selective cataclastic and melting effects may both contribute to determining the whole-rock composition of pseudotachylytes. Electron microprobe analyses of pseudotachylyte matrices indicate more rapid decomposition and fusion of hydrous ferromagnesian phases, such as biotite and amphibole, and to a lesser extent feldspar relative to quartz. The pseudotachylytes of this study appear to have formed neither by ultracataclasis alone nor by classical equilibrium partial melting or total fusion, but through the interaction of selective cataclastic and melting processes controlled by the lithological layering and mineralogical heterogeneity in their host-rocks. This interpretation may be widely applicable to pseudotachylytes generated by faulting in foliated host-rocks.