Geochemical evolution of the Dunedin Volcano, as exposed at North Head, East Otago

Robertson, David Iain, 1967-

North Head provides one of the most complete sections through the Dunedin Volcano. The initial, first and second eruptive phases are represented; the initial by a trachyte, the first by a series of basalts, trachybasalts and basanites capped by the Kaiwekite, and the second by a series of tephrites, basanites, trachy-basalts, and phonolites. Dominated by feldspar phenocrysts, diopside, olivine and titanomagnetite are also present, along with minor amounts of amphibole. The feldspar becomes more alkalic in nature as the rocks become more differentiated. Diopside compositions remain constant, while olivine and the titanomagnetites become marginally richer in iron. The chemistry of the minerals reflects the differentiation of the source, with CaO, MgO, and Ti02 contents decreasing, while Na20 and K20 contents increase. Whole rock element data again reflect the progressive differentiation of the lavas. Major elements which decrease are FeO, MgO, Ti02, P20 5, MnO and CaO, while Na20, K20 and Si02 increase. Similarly the trace incompatible elements increasing with increasing differentiation, while the compatible trace element concentrations decrease. Rare earth element patterns are slightly enriched in the light elements relative to the heavy elements. Both the trace element and REE patterns conform to alkali ocean island basalt patterns. Trace element ratios suggest that these lavas are within plate alkali basalts. Modeling of the crystal fractionation trends suggested that plagioclase, titanomagnetite, kaersutite and diopside are responsible for the observed trends. However the Harker plots suggest that olivine will also play some part. This, and the fact that the trace elements require considerably less fractionation than that suggested by the least square model, lead to the conclusion that crystal fractionation is not the only differentiation mechanism which operated during the life of the Dunedin Volcano.

viii, 139 p. : ill. (some col.) ; 30 cm.

1997Robertson