| Summary: | This thesis is an account of an investigation into the structure, petrology and mineralogy of a small Tertiary intrusive centre in the Vididalur-Vatnsdalur area near the north coast of Iceland. The area concerned lies in the Tertiary area west of the neovolcanic zone. In upper Tertiary times the extrusion of flood basalts in the area studied was locally interrupted by the building of a central volcano characterized by distinctive basalts, andesites and pyroclastics, some of which interfingered with contemporaneously extruded flood basalts. The first episode of this central volcanic activity, of which only the top is seen in the area studied, was subsequently buried by the transgressing flood basalts, and slow downsagging occurred in parts of the area along the prevailing north to north-northeast fracture system. A second central volcanic episode produced basalt, andesite and rhyolite extrusions and some pyroclastics in the northeastern part of the area following the flood transgression. Injection of thin basic dykes proceeded parallel to the two phases of central activity and continued during a final episode in which thin pale grey basalts similar to the Lower Pleistocene to Recent flows elsewhere in Iceland were extruded on to the irregular central volcano land surface. Two phases of intrusive activity proceeded parallel to the extrusive activity, the older More deeply eroded First Phase products being emplaced in the time interval during which the older flood basalts buried the newly extruded earlier central volcano lavas. A consecutive Second Phase of intrusive activity proceeded simultaneously with the second central volcanic episode and its less deeply eroded products show many similarities to the contemporaneous extrusions. These First and Second Phase intrusions are probably the upper apophyses of larger bodies concealed at depth. The intrusions which form the main part of the study are concentrated about an intrusive/extrusive centre in northern Vididalsfjall, and consist of coarse- and fine-grained ...
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