AN ARCHEAN CALC-ALKALINE LAMPROPHYRE SUITE, NORTHEASTERN YILGARN BLOCK, WESTERN-AUSTRALIA

Abundant lamprophyre minor intrusions, spatially associated with mafic sills and dykes in an Archean greenstone belt, exhibit a variety of u nusual forms ranging from sigmoid filling of tension gashes, through L -shaped or U-shaped bodies, to star-like pods. Gradation between, and mutual intrusion of, lamprophyres and other mafic bodies was observed in the field. The lamprophyres exhibit geochemical anomalies similar t o those of nearby gabbroic sills and dykes, and relict igneous pyroxen es and amphiboles in lamprophyres have compositions similar to those i n associated mafic rocks. All mafic rocks, including lamprophyres, exh ibit the sub-greenschist-grade metamorphism typical of the surrounding greenstones and field relations imply that an the mafic igneous rocks are of similar age. These data suggest that the lamprophyres and near by mafic rocks were derived from common parents. The lamprophyric magm a may have developed from tholeiitic to calc-alkaline mafic magma affe cted by local influx of volatiles at crustal depths.