Experimental and petrological investigations of some magmatic phenomena

Abstract

This thesis is devoted to a detailed investigation of a piorite sill of Tertiary age intruded into Torridonian strata on the island of Soay, south Skye. It is devided into four parts (1) studies on the petrography of the picrate, (2) the thermal metamorphism of the enclosing Torridonian strata, (3) and (4) a consideration of the genetic problems of the piorite and the metamorphosed and partially fused Torridonian especially in the light of experimental data on silicate systems related in composition thereto. The first section gives in detail the mineralogy and petrography of one of the piorite sills of Harker's "later peridotite" group of intrusions and particular attention is devoted to the question of the habit of the olivine phase in its textural setting through the sill in the light of Bowen's reinterpretation of the condition of intrusion of the Skye later peridotites. Part 2 of the thesis deals with the metamorphism induced in the associated Torridonian sediments. This metamorphism culminated in partial fusion of the sediments. The account of these phenomena, including the hybridism with the picrate is well set out and an analysis provided of a fused xenolith within the sill. In Part 3 an attempt is made in the light of experimental studies on a related silicate system to evaluate the liquidus of a piorite of the composition analysed. The discussion concerns the studies of Ricker on the Ca2Sio4-Mg2Sio4-Fe2Sio4 plane in the quatemary system and the application of the results, with, it must be admitted, considerable extrapolation to the related piorite. It is concluded that these studies indicate that the liquidus of the piorite would be at considerably lower temperatures than suggested by earlier phase equilibrium work. A beginning with some experimental work by the candidate in the picrate itself in the presence of water vapour is reported but no liquidus determinations were recorded. The temperature deduced for the liquidus is roughly placed around 1500oc. With still lower values, if volatiles are allowed for. This temperatures is still very high, at least 300oc. higher than that known for the liquidus of a calcalkali type of basalt. The discussion of the fusion of the Torridonian assemblage in relation to the experimental data on the granite system (Ab-Or-SiO2-H2O) is well presented and probably provided a real picture of the processes involved. The final part of the thesis is devoted to the petrogenesis. Included here are discussions on the significance of the variations in the shape and size of olivine crystals, on the interpretation of porphyritic texture in relation to the olivine phase and the bearing of these phenomena on the question of in situ crystallization of olivine. The evidence considered unfavourable to the hypothesis of intrusion of a basaltic liquid containing a high proportion of olivine crystals is marshalled but not all of the argument can be regarded as cogent evidence. The candidate has however brought forward good evidence from his own detailed studies that there are good grounds for extending the composition of true magmatic liquids beyond the limits given by Bowen. (Abstract shortened by ProQuest.)