Significance of pseudotachylite vein systems, Giles basic/uItrabasic complex, Tomkinson Ranges, western Musgrave Block, central Australia

Pseudotachylite vein-breccia networks and pseudotachylites intrafoliated with mylonites occur pervasively in the Tomkinson Ranges, western Musgrave Block, central Australia, about 50 km south and constituting part of the hanging wall of the Woodroffe thrust. The pseudotachylites are almost exclusively confined to gabbro, anorthosite and dolerite, and are rarely seen in basic granulites, felsic granulites and granitic gneiss. Pseudotachylite is ubiquitous in steeply tilted, deformed and mylonite-intersected sectors of the Giles Complex (Hinckley Range, Kalka, Michael Hills) but was not detected in the mildly tilted and little deformed western sectors of the Giles Complex (Blackstone Range, Cavenagh Range, Jameson Range), suggesting that fusion events concentrated in deformed relatively deep crustal levels. Two principal modes of occurrence of pseudotachylite are recognised : 1, vein-breccia networks superimposed on older lithological contacts and associated with brittle fracture systems; 2, penetrative pseudotachylite laminae interleaved with mylonite along shear zones. It is inferred that friction fusion events triggered by seismic faulting have affected intermediate crustal levels where mylonite shears separate brittle fracture domains. Contemporaneous development of pseudotachylite in each domain may be suggested by the lack of observed intersecting relationships between the two types of pseudotachylite vein systems. Alternatively, the mylonite-related pseudotachylites may have formed in quasi-plastic deep crustal zones. Comparisons between the chemistry of pseudotachylites and bulk host rock composition suggest a limited degree of selective fusion, increasing the silica levels and lowering the Mgvalues and Cr levels in the melt. Alternatively, these variations may have been brought about by fluid phase activity. Evidence for a high temperature melt origin of the pseudotachylite includes finer-grained margins, resorbed microclasts, micron-scale subhedral crystal texture of the pseudotachylite and distinct chemistry of microphenocrysts compared with host rock mineral composition. Laser-Raman spectroscopy has shown that the material is mostly crystalline but also suggests the occurrence of minor glass components in the pseudotachylite. High-Al and high-K pyroxenes from the pseudotachylite suggest seismic overpressures of the order of 30 kb, or metastable disequilibrium quench crystallisation of the pseudotachylite melt.