Geochemical investigations of the alkalic rocks of the Dunedin volcano, East Otago, New Zealand

The Dunedin Volcano, of Mid - Late Miocene age, is acomplex alkalic volcano which was active over a period of about 3 m.y. (13-10 m.y.). Activity commenced with the eruption of basaltic flows and tuffs in the central Otago Harbour depression. The earliest tuffs are interbedded with marine sands and calcareous sands indicating that the initial activity was submarine. Pillow structures and 'bomb sags' in the lower units of the volcanic massif are additional evidence that the initial activity was submarine. The initial basaltic activity was followed by extensive emplacement of quartz normative trachyte flows and tuffs in the central area, and a small emergent cone of trachytic composition was built. The formation of the early trachyte cone was followed by extensive, subaerial, shield-building eruptions of flows ranging in composition from basalt through intermediate rock types to phonolite and feldspathoidal trachyte. These were erupted from vents within the central depression, and the last significant activity in the central area resulted in the emplacement of coarse breccias which occupy vents on an axis through Port Chalmers, Portobello and Sandymount. The central activity was followed by a period of eruption controlled largely by activity from non-central fissures probably related to basement fractures. The conspicuous topographic highs of the Otago Peninsula and the Flagstaff-Mihiwaka ridges were built. Activity in the Dunedin Volcano was terminated about 10 m.y. ago with the emplacement of nepheline benmoreite lava domes at Mt.Cargill. The volcano has a complicated history. Activity from various vents (of the order of 40 have been recognized) over a period of 3 million years, during which the volcanics were being actively eroded, has resulted in stratigraphic complexities which are irresolvable on-a regional scale. Correlation of units over distances in excess of a few km are invalid, and the regional stratigraphy established by Benson (MS, 1968) is rejected. Although syn- and post-volcanic faults can be recognized in the Dunedin Volcano, structural complexities are very rarely observed. Relationships between the various volcanic units result from ·the interaction of flows, coulees, and intrusions erupted onto an actively eroding volcanic topography. in the volcano occurs the overburden during intrusion and the underlying sediments in the central depression. Volcanism in East Otago is intimately associated with extensive block faulting. Although the latest movements appear reverse there is strong evidence that faults in East Otago were normal during Cretaceous and, possibly, Early Tertiary time. A number of endogenous lava domes have been identified in the Dunedin Volcano, especially amongst the products of the later phases of the volcano's history. Most are composed of phonolite, but other examples consist of mugearite, hawaiite, benmoreite, trachyandesite, and nepheline benmoreite. Commonly the domes have been emplaced in cinder cones developed on the flanks of the volcano. […] The strontium isotopic chemistry and in particular the REE chemistry of the quartz normative trachytes require that special circumstances be invoked for the origin of these oversaturated melts. Two suggestions are made here. Either the trachytes derive :at the lower crust by partial melting of a quartz free parent, or they represent the end product in a long process of crystal fractionation in an isolated crustal magma chamber. The former model raises problems regarding the nature of the parental material, and the latter requires some form of preferential feldspar contamination to account for the REE pattern and other peculiarities of the chemistry