Port Chalmers Breccia and adjacent early flows of the Dunedin volcanic complex at Port Chalmers

Abstract

Abstract At least 230 m of basalt flows underlie trachytes of the Initial Eruptive Phase of Benson at Port Chalmers in the eroded core of the Dunedin volcano. The basalts may be equivalent to the lowest basaltic tuffs in the nearby Blanket Bay and Waipuna Bay sedimentary inliers, and thus are probably the oldest of the Dunedin volcanics. The flows abut the Port Chalmers Breccia which consists largely of fragments of preexisting volcanics filling a former vent. Explosive gas eruptions in the vent produced a thorough intermingling of schist and subvolcanic material derived from shallow crustal depths, with fragments of disintegrated vent intrusions and material eroded from the vent walls. Subvolcanic fragments found in the upper levels of the breccia range from gabbroic to syenitic rocks of varying undersaturation. Nepheline syenites with sodalite, katophorite, and eucolite indicate an agpaitic differentiation trend with enrichment in Na, Cl, F, and H2O. These, and the adjacent quartz-bearing trachyte flows, are the most highly evolved of the Dunedin igneous rocks, and represent trends to extreme undersaturation and slight oversaturation in silica respectively within a single alkaline volcanic province.