Evolution of four individual lamproite pipes, Ellendale volcanic field (Western Australia)

The volcanology and geochemistry of two olivine lamproite (Ellendale 4 and 9) and two leucite lamproite pipes (Mt North and 81-Mile Vent) of the Ellendale Volcanic Field (Western Australia) have been studied in detail. It is suggested that the pipes resulted from phreatomagmatic activity, when rising magma hit a near-surface aquifer in the friable Grant Group sandstone. The resultant tuff deposits consequently contain a high portion of detrital quartz grains in the ash matrix. The vast majority of the tuffs within the craters of Ellendale 4 and 9 are of mass flow origin. In contrast, the tuffs deposited against the crater walls of Mt North and 81-Mile Vent are mainly of base surge origin, but show an increase of mass flows towards the top. A decline in phreatomagmatic activity is marked by a period of intense mass flow deposition. In all four pipes investigated, lava fountaining took place during the transition from explosive to following extrusive activity. The lava lakes of the two olivine lamproite pipes show a petrographic zonation, with a central zone characterized by macroscopically visible poikilitic phlogopite and a marginal zone free of it. This zonation is mainly attributed to in-situ cooling. The chemical variations indicate that considerable olivine fractionation took place during the later magmatic phase at Ellendale 4. Systematic variations in the incompatible trace element pattern of Ellendale 9 are consistent with small variations in the degree of partial melting in the source region. The models derived provide a possible explanation of the systematic stratigraphic variations in diamond grade observed at Ellendale 4 and 9. At the leucite lamproite pipes the final magmatic phase extruded as lava domes. Here, the petrographic zonation in fine grained outer units and medium grained centers is due to variations in phenocryst content and size. Production from a shallow layered magma reservoir may be inferred, where the first produced fine-grained units reflect the top topmost, phenocryst-poor parts of the reservoir. Variations in the major element abundances at both pipes may be modeled in terms of separation of the early produced units from the later, phenocryst-rich units by crystal fractionation. However, the decoupling of major and trace elements observed in the Mt North lamproites indicates a role for processes other than simple crystal fractionation, e.g. phlogopite entrainment, magma mixing.