Petrology of the Campbell Island Volcanics, Southwest Pacific Ocean |
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| Authors: | Paul A Morris |
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| Abstract: | The disposition and petrology of a fractionated alkali olivine basalt—peralkaline rhyolite suite from subantarctic Campbell Island are discussed. These rocks (Campbell Island Volcanics: new name) are flows and high-level intrusions derived from two centres of igneous activity. Their age is Upper Miocene and they evolved over a period of 5 Ma. A gabbro intrusion pre-dates volcanism by 5 Ma. The ages of the flows and high-level intrusions cannot be separated, although the intrusions are chemically distinct as they contain all the intermediate members of the suite (mugearite and benmoreite). Similar La/Ce and Zr/Nb ratios for flows and high-level intrusions suggest a co-magmatic origin. Chemical variations indicate that the suite resulted from low-pressure mafic then felsic-dominated fractional crystallisation, which is substantiated for intermediate members of the suite by least-squares and Rayleigh fractionation modelling. One flow of alkali olivine basalt clearly pre-dates other volcanic rocks, and is thus regarded as being genetically unrelated.Although chemically similar to alkali olivine basalt and hawaiite, variations in the mineral chemistry and modal mineralogy of gabbro indicates a prolonged period of in-situ fractionation and re-equilibration. |
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