The geology and geochemistry of the north-eastern sector of Lyttelton volcano, Banks Peninsula, New Zealand. (1989)
AuthorsAltaye, Eliasshow all
Miocene volcanic activity constructed the Lyttelton composite cone 11 -10 Ma ago. The Lyttelton volcano which forms the north western half of Banks Peninsula represents a significant volume of mafic volcanic rocks together with some of felsic and minor intermediate composition. In addition to these, the volcano is characterized by pyroclastic deposits (lahars and lithic-crystal tuffs). Lyttelton lavas are intruded by numerous radial dikes and also by a variety of lava domes, sills and plugs. The volcanism was mainly Hawaiian in style, with some Vulcanian and occasional Strombolian styles of activity. Within this composite volcano, two major phase of volcanic activity are recognized. These are the main phase (the older) and late phase (younger) Lyttelton volcanics defined on the basis of field relationships, petrography and geochemistry. The late phase volcanics are designated formally as the Mt Pleasant Formation. The main and late phase Lyttelton volcanics range from mafic to felsic rocks compositions. The dikes range from basalt to trachyte and intruded the volcano during the main and late phase of volcanic activity. Sills and intrusions have felsic compositions. The major valleys and the lahar deposits represent periods of degradation of the active cone. Both the main and late phase (Mt Pleasant Formation) Lyttelton volcanics are alkaline tending transitional in geochemical affinity. The alkaline, sodic series Lyttelton rocks are members of the alkali olivine basalt association and this designation is consistent with mineralogy. Some intermediate and felsic Lyttelton rocks are subalkaline and potassic in composition, but they are classified as alkaline olivine basalt associations on the basis of their mineralogy. There are geochemical distinctions in major oxides, trace -elements and normative mineralogy between the main and late (Mt Pleasant Formation) Lyttelton rocks. The petrogenesis of the main and late Lyttelton volcanics mafic lavas is best explained by low pressure crystal fractionation of the observed phenocryst phases. The intermediate and felsic rocks are derived by similar processes with minor crustal contamination. Tectonically, Lyttelton volcanics represent “within plate” alkaline mafic volcanism.