Norman A. Lyttle

The petrography and chemistry of basic volcanic rocks from the Olympic Peninsula, Washington are described, and 26 new chemical analyses are presented. Petrographically and chemically the majority of these rocks can be regarded as basalts or metamorphosed basalts, and not spilites as has been previously assumed. Changes in chemistry are discussed and it is shown that the major changes involve hydration, oxidation and carbonitisation. Normative calculations indicate that the large majority of rocks are tholeiitic, although some are alkali basalts.

Projections of rock compositions in relation to phase boundaries at different pressures within the normative and modified CMAS tetrahedra have enabled three petrogenetic models of the Olympic Peninsula basalts to be established, which involve partial melting of mantle peridotite at low, intermediate and high pressures. Partial melting of peridotite at intermediate or high pressures, followed by olivine fractionation from the magmas during their ascent to the surface, can account for the observed compositions of Olympic Peninsula basalts.

The Olympic Peninsula basalts, and basalts of similar age found in the Coast Ranges of western Washington and Oregon, are interpreted as the products of island arc volcanism associated with the subduction of oceanic lithosphere off Washington and Oregon during the early Cenozoic era. Problems associated with magma genesis in island arcs are discussed, and some consideration is given to the spatial and temporal aspects of magma genesis in island arcs.

Problems concerned with the nomenclature and origins of spilites are reviewed, and conclusions are made regarding the most likely processes which cause spilitisation. Projections of basalts, spilites and Olympic Peninsula basalts onto the "spilite plane" diopside-clinochlore-feldspar within the CMAS tetrahedron are discussed, and conclusions are made regarding the nature of the olympic Peninsula volcanic rocks, and on the relationship between spilites and basalts in general.

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Pages:228
Supervisor: Barrie Clarke