Precambrian amphibolites and basic granulites of the south coast of Western Australia

Abstract

Field relations and chemical variation trends suggest that amphibolite and basic granulite bands occurring throughout the Precambrian gneissic terrain of the south coast of Western Australia were derived from basic igneous rocks. It is not known whether the original rocks were mainly intrusive, extrusive, or pyroclastic, but occasional discordant contacts indicate that intrusive types are certainly represented.

The chemical affinities of the original magmas are not entirely clear. Present compositions suggest dominantly subalkaline affinities, although a significant proportion of the analised samples show some alkaline characteristics. Data for supposedly immobile elements (Ti, P, Zr, Nb, Y) show possible similarities to recent island‐arc and ocean‐floor basalts, but these are not sufficiently strong to permit any firm conclusions regarding the original tectonic environment of eruption or intrusion. This absence of clear chemical similarities of the amphibolites and basic granulites to basalts of any particular modern tectonic environment may suggest chemical modification during possible low‐temperature alteration (e.g. ocean‐floor or burial meta‐morphism) or subsequent high‐grade regional metamorphism and anatexis, or the inapplicability of currently recognised tectonomagmatic principles to the Precambrian.