Abstract
Monogenetic volcanic fields and maar-diatreme volcanos present significant risks to cities and human life around the world. Further research is required to better understand monogenetic volcanic fields and maar-diatreme eruptive processes so their volcanic hazards can be managed. This master’s thesis is a physical volcanology and petrographic analysis study of four Miocene alkaline and ultramafic lamprophyre diatremes in the Alpine Dike Swarm, in North Otago and South Westland, South Island, New Zealand. Country rock breccias, pyroclastic rocks and coherent lamprophyre from lower diatreme and root zone deposits from four diatreme remnants Moeraki River, Mt Alta, Niger Peak and Lake Wanaka Diatreme, were mapped and analysed in detail through outcrop scale field mapping, and remote mapping on Google Earth and ArcGIS, and petrographic microscope. Interpretations of the various diatreme deposits and possible processes that formed them are proposed. The formation of Mt Alta Diatreme is also discussed as example of diatreme emplacement.
Moeraki River Diatreme was an outlier in this study, emplaced in the far north of the field, with no known in-situ remnants, UML and carbonatite composition, and more abundant mantle peridotite xenoliths. The Moeraki River debris likely represent the remnants of a diatreme root zone, but evidence for a xenolith-clogged dike is also presented. The source of the debris has been narrowed down to a location high above the main debris. If confirmed, diatreme remnants from Moeraki River debris indicate that the ADS was a previously more extensive maar-diatreme field.
Mt Alta Diatreme was studied in detail, and a variety of different lower diatreme and root zone deposits are examined. Mt Alta Diatreme consists of various diatreme breccia types, formed by phreatomagmatic explosions deep in the diatreme conduit which caused temporary evacuated spaces that infilled with rock falls variable amounts of lamprophyre. The country rock breccias are intruded by a complex dike-sill network with interconnected, branching dikes and sills, formed in dike-sill emplacement processes, and usual amorphous, peperitic inclusions which formed in formed as apophyses which branched off from the main lamprophyre dikes and intruded into wet, unconsolidated diatreme breccia where it produced blocky and fluidal peperitic boundaries with chilled margins. Mt Alta also contains previously unreported juvenile-rich lapilli tuff breccias which are interpreted as spatter deposits which formed in vapour explosion bubble bursts when lamprophyre magma interacted with groundwater within the diatreme. Lapilli country rock breccias are interrupted as lateral debris jet deposits which formed parallel to the sill during sill propagation. Very unusual, large (10 cm across) alkali feldspar megacryst xenocrysts were also observed. An approximate sequence of events is also proposed for the emplacement of Mt Alta Diatreme.
Niger Peak Diatreme is a lower diatreme-root zone consisting of schist breccias and alkaline lamprophyre dikes which formed by a combination of fracturing of in-situ country rock and collapsed country rock/rock fall into a temporality evacuated space created by phreatomagmatic explosions and intrusion of coherent lamprophyre dikes. The depth and the proportion of coherent lamprophyre at Niger Peak Diatreme have been re-evaluated.
Lake Wanaka Diatreme consists of diatreme breccias, formed by intra-diatreme rockfall and phreatomagmatic explosions, matrix-rich, juvenile-rich and country rock-rich lapilli tuff and tuff breccia, formed by lava fountaining, bubble bursts and debris jets, schist megablocks formed by country rock slabbing, and a large lamprophyre sill, formed by non-explosive sill emplacement.
It is hoped that the findings of this study will help answer several key questions about intraplate volcanism in New Zealand and lower diatreme-root zone processes and provide greater context to monogenetic volcanic fields, maar-diatreme volcanoes and phreatomagmatism.