Major and trace element variations in oxide and titanate minerals in the West Kimberley lamproites, Western Australia

Abstract

New data are presented for groundmass chromian spinel, perovskite, ilmenite, and K-Ti-Ba-rich phases from the Miocene olivine and leucite lamproites of the West Kimberley region. The spinels range from early Ti-Al-Mg chromite through Ti-Mg chromite to Ti-chromite and, in Ellendale 4 and 9, Ti-Cr magnetite and Ti-magnetite. Most crystallized at 850–1220 °C and fO2 ~ MW + 1–2 log units except for Ellendale 4 and 9 spinels which underwent marked late oxidation at ~650–750 °C with fO2 increasing sharply to ~FMQ + 2–3 log units. Perovskite is ubiquitous in the olivine lamproites and the Walgidee Hills (WH) lamproite. Compositional features of the perovskite are a wide range in Cr, and high Sr, Nb, Th, and LREE contents with highly fractionated REE patterns (La/YbCN ~ 750–3000). Perovskite from WH defines an evolutionary trend of enrichment in Na, Sr, Y, Nb, U and REE, and depletion in Cr, Fe, and Th with magma fractionation. Late crystallizing WH perovskite shows a decrease in LREE due to relative depletion of LREE in residual magma by extended crystallization of perovskite (and apatite). Priderite ((K,Ba)(Ti,Fe3+)8O16) has low Mg and V, and a range in Cr contents which decrease with magma evolution. Jeppeite ((K,Ba)2(Ti,Fe)6O13), has higher Sr and Nb content than priderite. Both contain low Y and REEs. Wadeite (K2ZrSi3O9), a ubiquitous groundmass phase, has high Sc, Rb and Hf contents, and strongly LREE-depleted REE patterns with positive Ce anomalies. Noonkanbahite, a late crystallizing phase in WH, has low Cr and Ni, and high Sr, Nb and Y contents. REE patterns for noonkanbahite display high HREE, depleted MREE, enriched La-Ce-Pr, and a positive Eu anomaly.