ORIGIN AND SOURCE EVOLUTION OF THE LEUCITE HILLS LAMPROITES: EVIDENCE FROM SR-ND-PB-O ISOTOPIC COMPOSITIONS

Abstract

Whole-rock major and trace element and O, Sr, Nd and Pb isotopic data are reported for 3·0–0·89 Ma lamproites from the Leucite Hills, Wyoming, USA. The two main groups of lamproites, madupitic lamproites and phlogopite lamproites, are geochemically distinct and cannot be related to one another by either fractional crystallization or crustal contamination. It seems likely that the geochemical differences between these two rock types are related to variations in source mineralogy and depth of partial melting. The high Mg-number and large ion lithophile element abundances and negative ɛNd values of the lamproites indicate a mantle source that has experienced stages of both depletion and enrichment. The negative Nb, Ta and Ti anomalies in mantle-normalized trace element diagrams and low time-integrated U/Pb, Rb/Sr and Sm/Nd ratios of both lamproite groups and other Cenozoic igneous rocks from the Wyoming Archean Province indicate an ancient metasomatic enrichment (>1·0 Ga) of the mantle source associated with the subduction of carbonate-bearing sediments. Other chemical characteristics of the Leucite Hills lamproites, especially their high K2O and volatile contents, are attributed to more recent metasomatism (<100 Ma) involving influx from upwelling mantle during back-arc extension or plume activity.