The geochemistry, petrogenesis and age of an unusual alkaline intrusion in the western Pilbara craton, Western Australia

The recently discovered Yanyare dykes are located approximately 80 km southwest of Karratha in the north-western Pilbara craton, Western Australia. The dykes intrude intercalated sedimentary and volcanic units of the PalaeoProterozoic Fortescue Group. The Yanyare-02 dyke has an inequigranular texture and is comprised of intensely altered olivine macrocrysts set in a matrix of phlogopite (55%), sanidine (20%) and apatite (5%). Replacement of phlogopite by rutile and chlorite is common. Sanidine occurs as euhedral grains and poikilitic plates enclosing apatite and is variably altered to muscovite and calcite. Aside from fluorapatite, no accessory minerals thought to characterise lamproites were observed. Yanyare-02 is classified as an olivine-phlogopite sanidine lamproite. Average initial (1.72 Ga) ratios for Yanyare-02 are: ⁸⁷Sr/⁸⁶Sr_i = 0.708; ¹⁴³Nd/¹⁴⁴Nd_i = 0.510; and ¹⁷⁶Hf/¹⁷⁷Hf_i = 0.281 (n = 6). Trace element data reveal extreme LREE enrichment, with La_N/Lu_N ≈ 100. The petrogenesis of Yanyare-02 is proposed to have involved melting of depleted lithosphere veined with ancient, enriched material of MARID composition. ⁴⁰Ar/³⁹Ar laser step-heating analyses of phlogopite grains produced a weighted mean age of 1724 ± 8 Ma (2σ). Rb-Sr analyses of whole-rock and phlogopite separates yielded an 8-point isochron age of 1681.6 ± 8.3 Ma (95% CL). An age of ca. 1.7 Ga makes Yanyare-02 the oldest lamproite known.