Abstract

The markedly bimodal early Archean volcanic suite exposed near the Ruth Well Ni-Cu prospect, West Pilbara, shows the intimate interfingering and interlayering of spinilex-textured komatiite (MgO = 30-20%) and komatiitic basalt (MgO = 14-10%), implying closely spaced and/or closely timed eruptions of these two different extrusive products. The favored hypothesis involves derivation of basalt from komatiite in transient magma chambers from which only the lower density basalt can be erupted; parental komatiite is erupted directly from depth during periods of higher regional stress or in areas lacking magma chambers.Although metamorphic alteration of these magma products renders compositional constraints less firm, derivation of basalt from komatiite, in this particular case by fractional crystallization, is supported by data for MgO, Ni, Ti, and Zr. Rare pyroclastic ultramafic rocks are not part of this sequence but may represent catastrophic mixing of parental and daughter products.

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