A discrete, roughly circular, steep‐sided magnetic anomaly outlines a buried trachyandesite plug at Gearless Well in the Murchison Province of the Yilgarn Block. Agglomeratic texture indicates high level emplacement and probable venting. The mildly alkaline, silica‐saturated, heterogeneous trachyandesite is markedly alumina‐deficient and enriched in large ion lithophile elements and, to a lesser extent, high field strength elements. A seven‐point whole rock linear alignment on the Rb/Sr isochron diagram of samples from one drill hole is interpreted as a two‐component mixing line of meaningless age. Samples from another drill hole indicate the presence of a third component. Two two‐point biotite‐whole‐rock measurements yield similar ages which pool to give 2188 ± 11 Ma, the estimated time of emplacement of the trachyandesite plug. Samples defining the seven‐point line were produced by small to moderate amounts of partial melting of LIL‐enriched (large ion lithophile elements) upper mantle, and the line is interpreted as representing a mixture of depleted and fertile mantle material. The group of samples reflecting the third component is believed to be derived by a larger degree of partial melting of the same LIL‐enriched upper mantle source. The scatter observed in this group is probably due to interaction of the volatile and fluid‐charged magma with wall‐rock during ascent through the crust.