MICRO- TO NANOSCALE CONSTRAINTS ON THE TIMING AND CONDITIONS ASSOCIATED WITH THE FORMATION LITHOSPHERIC-SCALE CONDUCTIVITY ANOMALIES IN ANHYDROUS CRUSTAL ROCKS

In tectonically active regions, domains of enhanced electrical conduction are commonly observed in the mid and lower crust. Their cause is typically attributed to melts, or fluids likely derived from metamorphic dehydration and/or magmatism. However, the longevity of these fluid systems should be geologically short (<few million years). Intriguingly in tectonically stable ancient terranes worldwide, deep conductivity anomalies are found that are unlikely to be the result of free-phase fluids, such as brines or melts, simply because the active tectonic drivers for fluid generation area absent. The origin of these conductivity anomalies is a long-standing mystery and has been the focus of much speculation since they were first observed. Here we use correlative electron backscatter diffraction (EBSD), laser ablation split stream (LASS) geochronology and atom probe microscopy to constrain the age of deformation and formation of an interlinked network of carbon network within a monazite from a major electrically-conductive, crustal detachment; the Woodroffe Thrust of central Australia. The exposed Woodroofe thrust is an Ediacaran (~550 Ma) aged structure that preserves high-grade (750 °C/14kbar) assemblages that rework the ultrahigh temperatures (1000 °C/8 kbar) Mesoproterozoic Musgrave Province. The data show the presence of 5nm thick films of carbon and sodium within interconnected dislocation arrays associated with low-angle grain boundaries. The association of carbon and sodium within the boundaries suggest that the source of carbon was the decrepitation of NaCl and CO₂ bearing metamorphic fluid inclusions (the residual fluids common in high-grade metamorphic rocks) during shear deformation of the host rock. LASS geochronology constrains the timing of the deformation enhanced recrystallization and neoblastic monazite growth to be coincident with the formation of the Woodroofe thrust.