NANOSCALE OBSERVATIONS OF CRYPTOCRYSTALLINE MATRIX IN AN AMYGDULE-BEARING PSEUDOTACHYLYTE

Pseudotachylyte is created by frictional melting along faults during seismogenic rupture and is characterized by a glassy or cryptocrystalline matrix containing dispersed, partially melted survivor clasts derived from the wall rock. The centers of thicker, cm-scale veins can incorporate 1-100 µm microlites, amygdules, or both. Numerous studies have characterized survivor clasts and microlites using optical petrography and SEM BSE imaging, but the nature of the cryptocrystalline matrix in most pseudotachylytes is not well known as it is typically below the resolution of those methods. This study examines the nanoscale mineralogy and microstructure of pseudotachylyte from the Ikertôq shear zone which is a frontal structure of the Paleoproterozoic Nagssugtoqidian Orogen in western Greenland. Using Virginia Tech’s NanoEarth facilities, four 80-nm-thick, electron-transparent matrixes were prepared using focused ion beam (FIB) in situ liftout and ion milling, followed by TEM analysis. Micron-scale imaging and EDS spectroscopy were performed on thin sections using EPMA and FE-SEM. At the 10–100 µm scale, Fe-oxide-rimmed amygdules filled with calcite, dolomite, quartz, K-spar, or barite are embedded in a recrystallized matrix composed of quartz, calcite, and zeolite in the center of some veins. In one 38-mm-thick vein bounded by tonalite gneiss and amphibolite, TEM analysis revealed the cryptocrystalline matrix to be felted mats of 10–50-nm-wide phyllosilicates that are likely phengitic white mica. Locally, the phyllosilicates are interspersed with 100’s-nm-scale quartz and albite, and 40 nm Fe and Ti oxides as dendritic chains and individual crystals. By comparison with optical and BSE images of larger crystals, the quartz and albite are interpreted as nanoscale survivor clasts, and the Fe and Ti oxides are interpreted as nanoscale microlites. The most interesting microstructural observation was the presence of 20–50 nm, V-shaped arrays interpreted as possible ripplocations in a phyllosilicate mineral. Ripplocations are atomic-scale lattice defects recording c-axis-parallel strain, first observed in geologic materials in 2019 (Aslin et al., Nat. Comm.). This observation might suggest strain buildup in the pseudotachylyte matrix during quench cooling and contraction normal to vein walls.