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 trace-element contents of Group 2 rocks may have been buffered by residual phlogopite in the source, suggesting more abundant modal phlogopite and more intense mantle metasomatism compared to the source of the Group 1 rocks. The incompatible element variation represents source inhomogeneity, probably reflecting differences in the degree and nature of mantle metasomatism prior to melting. Pronounced negative mantle-normalised Nb, Sr and Ti anomalies and other major and trace element characteristics are typical of high-K plate-margin (shoshonitic) volcanism and suggest that the source included a major crustal component. The presence of a significant crustal component in the mantle source region at the time of generation of the lamprophyres has important implications for the Proterozoic history of the Tennant Creek Inlier. The tectonic setting was almost certainly intraplate at 1660 Ma, but the subcontinental lithosphere had apparently previously been modified by addition of a significant crustal component. Show more