Metamorphic and metasomatic evolution of the Snake Creek Anticline, Eastern Succession, Mt Isa Inlier

The Snake Creek Anticline is a regional D-2 structure, with the overturning of the western limb and the curvature of the axial trace from north to east interpreted as an effect of D-3, which produced folds and crenulations with shallowly dipping axial surfaces. Low-pressure/high-temperature metamorphism which affected the Mesoproterozoic Soldiers Cap Group reached its peak with the growth of sillimanite during intense foliation development during D-2. However, microstructural studies have revealed multiple porphyroblast growth stages, namely: (i) pre- to early D-1, (ii) late D-1 to early D-2, (iii) late to post-D-2, (iv) syn-D-3 and (v) syn-D-4, The kyanlte/andalusite, sillimanite, and sillimanite/K-feldspar isograds are D-2-related, whereas the garnet and andalusite/staurolite are composite isograds representing multiple growth events. To explain the occurrence of kyanite partially replaced by andalusite, and the replacement of early growth andalusite by staurolite + muscovite, it is suggested that the pressure-temperature-deformation path included a fall in temperature after D-1, followed by a rise in both temperature and pressure by the end of D-2. Quartz veining occurs throughout the evolution of the rocks, but the most notable events are: (i) an early (pre-/syn-D-2) phase of albitite veining, (ii) syn-D-2 quam (+ staurolite + andalusite + kyanite) veining, and (iii) late (syn-/post-D-5) albitite veining. Preliminary fluid-inclusion studies show that hypersaline fluids were dominant during at least some of this history. The Snake Creek Anticline and the Cloncurry Fault provided structural controls for the infiltration of the metasomatic fluids, inferred to be initially of igneous origin. The fluids may have been important in the transfer of heat during D-2.