The geology and petrology of the Adamsfield Ultramafic Complex,Tasmania

The Adamsfield Ultramafic Complex is one of a dozen Tasmanian ultramafic-mafic and ophiolite complexes emplaced during Cambrian time in the Tasman Geosyncline.The Adamsfield complex is composed of partlyserpentinized dunites, olivine orthopyroxenites and orthopyroxenites. Rocks are commonly layered and alternately rich in olivines (Fo_93–84) and orthopy roxenes (En_94–87). Spinels are a minor but widely disseminated phase. Orthopyroxenes and spinels are poor in Al₂O₃ and TiO₂. Clinopyroxenes are rare, plagioclase or garnet have not been found.Nominal equilibration temperatures calculated from coexisting mineral assemblages range from quasi-magmatic values (1200±100 °C) for little-deformed rocks down to subsolidus values (950 °C) for deformed and reacted assemblages. Olivine kink band orientations imply that deformation also took place at lower temperatures (<800 °C) but mineral compositions apparently failed to react further.Adamsfield mineral assemblages probably crystallized originally at low pressures from highly magnesian, titania-poor tholeiitic or andesitic magmas. Fine-grained igneous rocks from the Tasmanian ultramafic-mafic and ophiolite complexes include highmagnesia andesites of appropriate compositions and comprise a distinctive compositional group termed the Low-titania Ophiolite Association, poor in TiO₂ (<0.5 wt%), P₂O₅ (<0.1 wt%) and Zr, and rich in MgO, Ni, and Cr.