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Journal of the Geological Society

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A carbonatitic diatreme from Umberatana, South Australia

Department of Geology, University of Newcastle, New South Wales 2308, Australia

The breccia body at Umberatana, previously mapped as a diapir, intrudes Late Proterozoic metasediments and envelops Palaeozoic alkaline to peralkaline granites. Lineament-controlled emplacement led to limited structural disruption of the adjacent country rock and resulted in a phlogopite-rich breccia which contains albitite fragments, metasomatized Late Proterozoic clasts, altered alkaline basalts and dismembered metasomatized pelites of the granite thermal aureole. Breccia formation occurred after peralkaline granite emplacement (418 ± 2 Ma) and is thus unrelated to the Cambro-Ordovician Delamerian Orogeny. The breccia body is re-interpreted as a diatreme which resulted from multiple, possibly CO₂-rich, halogen-bearing pulses of mantle-derived fluids rich in Ca, Mg,Ti,Fe³⁺,P, Sr, Na and Ce. Fluid contributions from a sedimentary source cannot be excluded, because the fluids traversed Late Proterozoic metasediments. The close association of alkaline to peralkaline igneous lithologies and mantle-derived fluids is related to the deep fracturing of the lithosphere and mantle-degassing.