Abstract
Pseudotachylites are generally considered to be produced by flash melting of the most fusible minerals, with compositions often dominated by biotite mica. We present phase fraction calculations for 237 pseudotachylite analyses from 28 localities spanning a range of host rock compositions from granites to peridotites. Pseudotachylite matrix compositions cannot be reproduced by a simple linear mixture of the minerals in their host rocks and commonly show a potassium deficit in pseudotachylites which contain significant contributions from biotite or muscovite. This is strongly indicative of peritectic melting of mica under fluid absent conditions. Occasionally, a negative contribution from an aluminosilicate phase is required but with a positive contribution from potassium feldspar. This is most consistent with peritectic melting of micas under fluid present conditions. The present data therefore suggest that, while fluid absent flash melting might be the most common mode of formation of pseudotachylite, in some instances pseudotachylites can be produced in the presence of a free fluid. Potassium mass balance of pseudotachylites might therefore be a diagnostic indicator of fluid absent conditions in their source regions during seismogenic rupture. We give some examples of observational evidence of peritectic liquidus phases in pseudotachylites and suggest that the significance of these phases might have been overlooked in previous studies.