The diamond-bearing ultrapotassic (lamproitic) rocks of the West Kimberley region, Western Australia.

Abstract

These lamproitic rocks are of Miocene age (20 m.y.) and comprise more than 100 separate pipes, plugs, sills and rare dykes. The recently-discovered 'kimberlitic' rocks - olivine lamproites - grade petrographically through leucite-bearing olivine-diopside lamproite to the better-known leucite lamproites with phlogopite, diopside and potassic richterite as the dominant ferromagnesian phase. The lamproites range from ultrabasic (20-29% MgO) to basic (= or <5%) compositions: all have very high K2O contents (4-12%) and high K2O/Al2O3 (average 1.2) and K2O/Na2O (typically >10) ratios. All are saturated to oversaturated in silica, and SiO2 and Al2O3 contents increase with decreasing MgO content and Mg/(Mg + Fe) ratio. The suite is characterized by very high contents of F, Ba, Rb, Sr, Pb, Th, U, Ti, Zr, Nb and LREE, and very low concentrations of CaO, CO2 and Sc. REE patterns are highly fractionated and enriched in LREE at 500-2000X chondritic abundances with very low abundances of HREE (4-6X chondritic). The Rb/Sr and 87Sr/86Sr ratios are high (0.3-0.4, 0.711-0.720) and 143Nd/144Nd ratios very low (epsilonNd -7 to -15), indicating derivation from an ancient, LILE-enriched mantle source.-J.M.H. Show more