Abstract
THE PAPERS PRESENTED can be divided into two broad subject areas: the study of pyroxenes in plutonic rocks as a means of tracing the history of magma input and crystallization within a magma chamber; and the study of relict pyroxenes in altered volcanic rocks as a means of 'seeing through' the effects of chemical alteration and thus of identifying any important magmatic characteristics.
The first was covered by studies on the following three plutonic bodies: Fongen–Hyllingen complex in Norway (Wilson et al), the Bushveld complex (Buchanan) and the Freetown complex, Sierra Leone (Wells & Bowles). Two important points of caution emerged from these studies. Buchanan examined the relative merits of using focused and defocused beams to probe pyroxenes and found that focused beams only gave analyses representative of the pyroxene as a whole provided exsolution lamellae were below a critical size. Wells & Bowles questioned the generally-held assumption that all layered textures in intrusive bodies have an igneous origin by providing convincing evidence of pyroxenes which grew by a late-stage autometamorphic process.
The implications of pyroxene studies on metabasalts was discussed by Nisbet & Pearce and by Rowbotham & Bevins. This type of application is still in its infancy but it was apparent that it has the potential to provide useful information on the magmatic character of rocks too altered for conventional studies. To be fully effective, more information is needed on the relationship between pyroxene composition and the temperature, composition and cooling history of its host magma. Nevertheless,
