Cinder cones of the Lyttelton Volcano, Banks Peninsula, New Zealand

Cinder cones are common on Banks Peninsula and this study concentrates on eleven sites situated on the Lyttelton Volcano. The study focuses on the structure, petrology and geochemistry of cinder cones and discusses their eruptive history. Lyttelton volcanism began in the Miocene (10.8 Ma) with early (L1) activity centred near "Head of the Bay". Constant replenishment of the main chamber caused L1 lavas to be dominantly basaltic with limited differentiation. The development of a large cone resulted in distension which caused the development of horizontally fed radial dikes. Initial dike activity was low and lavas were dominantly fed from the central conduit with little flank activity. As L1 grew though, dikes became more common and so did cinder cone activity on the flanks. Cinder cones would have been fed by volatile rich magmas quickly reaching the surface via dikes and hence erupting explosively. The increase in dikes during L1 also caused the increasing development of more evolved magmas that fed lavas. Then during a quiescent period there was large-scale collapse of the eastern flank, followed by erosion and weathering of the L1 volcanic edifice during which activity shifted north-east to "Charteris Bay". Renewed (L2) activity after the quiescent period gave rise to more evolved lavas due to the large increase in dike activity, caused by the large amounts of stress acting on the edifice. Hence cinder cone activity on the flanks was also very common. Cinder cone samples are dominated by phenocrysts of plagioclase, clinopyroxene, olivine and Ti-magnetite and show a wide range of mineral textures. This suggests that crystals have grown under a variety of temperatures and pressures and/or been derived from magmas of different compositions. Geochemically the cones are part of an alkaline basalt-hawaiite-mugearite- benmoreite-trachyte association. The deposits in this study are all inferred to have formed by Strombolian and Hawaiian type eruptions due to their small dispersal areas. These eruptions formed cinder cones whose morphology depended on numerous factors, all of which contributed to their overall development: In this study the deposits have been classified into four facies based on welding, dip direction, block and bomb size, colour and the presence or absence of some clasts. The Vent Facies consists of densely welded grey/black fused spatter and represents vent and early formed products. Vesicularity is 80% flattened clasts with deposits inferred to be welded due to their flattened nature. Spatter flows are common in the facies due to the rapid accumulation of spatter and its flowage. The Medial Facies consist of crudely bedded bomb, block and lapilli beds. Clasts are undeformed and non-welded. The Distal Facies consist of lapilli, ash, and crystals. Rare bombs occasionally occur and the deposits are generally well sorted and bedded. Overall, throughout each of the facies, clast size and welding decreases with increasing distance from the vent area