Mineralogy and geochemistry of Proterozoic shoshonitic lamprophyres from the Tennant Creek Inlier, northern territory

Abstract

Post‐mineralisation shoshonitic lamprophyres intrude Warramunga Group metasediments in the Tennant Creek Inlier. Samples from drillcore are mostly phlogopite‐bearing and best classified as minettes. The suite ranges from olivine (now talc or serpentine) + Cr‐rich spinel‐bearing varieties with high Mg, clearly of mantle derivation, through clinopyroxene‐bearing rocks to more fractionated types with modal quartz and low Mg and Ni. The suite is strongly enriched in most incompatible elements (Rb, Zr, Th) but Ba is highly variable. Although indistinguishable in petrography and major element abundances, the lamprophyres are divisible into two distinct groups on the basis of trace‐element geochemistry. Group 1 (Zr/Nb > 20) is more variable, with generally higher Sr, Ba, LREE and lower U, Th, HREE, when compared with Group 2 (Zr/Nb < 12). The two groups are substantially spatially distinct, with the Group 2 rocks mostly restricted to a small part of the southeastern outcrop area of the Warramunga Group. The trac...