LITHOLOGICAL HETEROGENEITY AND FABRIC INTENSITY IN GRANULITES, ARUNTA BLOCK, CENTRAL AUSTRALIA: EVIDENCE FOR STRAIN PARTITIONING

An investigation of the Mt. Hay area, central Australia, illustrates lithological heterogeneity and strain heterogeneity in granulite facies deformation. Mt. Hay granulites are part of the Proterozoic Strangways Metamorphic Complex (~8 kb, ~800°C), and are mafic granulites thought to be common rock types in the lower continental crust. Structural work was concentrated on Capricorn ridge, in which retrogression associated with uplift occurs only locally.

We have produced a geological map that is focused on documenting the scales of lithological heterogeneity, rather than defining deformational episodes. The four main compositional domains are anorthositic, pyroxenitic, plagioclase-pyroxenitic, and interlayered quartzofeldspathic and pyroxenitic. Foliation is predominantly defined by cm-scale compositional domains. Ultramylonite also occurs in 1-5m thick bands parallel to compositional layering. Foliations strike WNW-ESE and dip steeply SSE. Lineations, defined by mineral stretching and alignment, plunge steeply SE.

We correlate fabric intensity to strain intensity, and interpret the textures in the granulites as blastomylonitic. At all scales, compositional domains exhibit the same relative fabric intensities (quartzofeldspathic > pyroxenitic > anorthositic), indicating that deformation is preferentially accommodated in the quartzofeldspathic domains. Interstitial quartz in the anorthositic blastomylonite shows weakly developed lattice preferred orientation (LPO) and quartz pods in the ultramylonite show strong LPO, both indicating a component of dislocation creep during deformation. Plagioclase in the anorthositic blastomylonite shows no LPO, consistent with grain size sensitive creep and/or grain-boundary sliding. Shape preferred orientations of minerals are weakest in the anorthositic and strongest in the quartzofeldspathic domains. Thus, we conclude preliminarily that strain in the lower crust exposed in Mt. Hay is partitioned on a variety of scales, preferentially accommodated by dislocation creep in the quartz-rich domains.