Archaean ultramafic, mafic and associated rocks, Mt Monger, Western Australia

Abstract

The rock succession at Mt Monger, Western Australia, represents a type of Archaean volcanic ‘cycle’. The oldest rocks are a felsic-volcanic and volcanogenic-sediment association, terminated by an unconformity, representing the last stage of an earlier ‘cycle’. The new ‘cycle’ begins with a distinctive suite of ultramafic lenses and high-Mg basalts which is overlain by tholeiitic basalts before returning to felsic volcanism and sedimentation which terminates the ‘cycle’. Despite low-grade metamorphism it has been possible to reconstruct, from the well-preserved primary textures, much of the original igneous history of the rock associations and to examine their possible relationships. Discrete ultramafic lenses are thought to result from high-level injection or extrusion of an olivine crystal-liquid mush. The close spatial association, similarity of texture and overlap in composition between the lenses and high-Mg basalts suggest a genetic relationship. Unusual textures indicating rapid crystallisation are ubiquitous in the high-Mg basalts and ultramafic lenses but are not found in the overlying tholeiitic basalts. Numerous small layered sills in the sequence, showing ‘Stillwater-type’ differentiation, are also derived from a parent magma rich in magnesium. This recently recognised magnesium-rich suite of rocks appears to be common in Archaean sequences, but finds no close parallel in younger successions. Terminology that equates this type of succession with younger ‘ophiolite’ belts thus seems unjustified.