A CLOSER LOOK AT PSEUDOTACHYLYTE FORMATION: MELT-GENERATING REACTIONS REVEALED BY OPTICAL AND ELECTRON MICROSCOPY

The formation of pseudotachylyte (PST) has puzzled geologists for decades. A consensus now exists that they correspond to a silicate melt produced by frictional heating accompanying slip along a brittle fault surface. Spray (1991) proposed general processes for PST generation that include: (1) the mineral with the lowest melting T limits the T achieved along the slip surface; (2) melt lubrication of the slip surface can terminate the seismogenic process. High-resolution optical and electron microscopy of PST may reveal key grain-scale mineralogical and petrologic processes related to melt generation. This study explores the fine-scale features of PST generated in the Homestake shear zone, Sawatch Range, central Colorado. At Homestake, the host rock is a high-grade, fine-grained paragneiss (Kfs + Plag + Qtz + Bt + Ms + Sil + Cordierite). In outcrop, PST is typically black, aphanitic and occurs as: (1) thin (~1 mm) PST generation zones; (2) mm to cm-scale injection veins branching from generation zones; (3) or cm-scale reservoirs in dilatant fractures in unstrained gneiss. Unmelted wall rock clasts are observed suspended within the matrix, and consist of monomineralic quartz (Qtz), plagioclase (Plag), or lithic fragments of Qtz + Plag + Kfs + Sil. Mica is present in clasts only as rare inclusions in Plag. Within wall rock in contact with the PST, microscopic features include the presence of flow banding around asperities, dynamic recrystallization of Qtz, Plag, and Kfs including grain-size reduction, undulose extinction, and grain plucking. There is also an asymmetry in generation zones whereby deformation microstructures in the wall rock occur only on one side of the PST. The general lack of mica in PST; embayment of biotite back into the host rock along otherwise flat slip faces; and color banding (compositional variation) within PST directly around biotite in the wall rock, indicate that biotite breakdown is the likely trigger for melt generation. Additional detailed SEM and chemical analysis will further examine the role of biotite and feldspar in the melting process.