Origin and source evolution of the Leucite Hills lamproites; evidencefrom Sr-Nd-Pb-O isotopic compositions

Abstract

Whole-rock major and trace element and O, Sr, Nd and Pb isotopic dataare reported for 3.0-0.89 Ma lamproites from the Leucite Hills,Wyoming, USA. The two main groups of lamproites, madupitic lamproitesand phlogopite lamproites, are geochemically distinct and cannot berelated to one another by either fractional crystallization or crustalcontamination. It seems likely that the geochemical differencesbetween these two rock types are related to variations in sourcemineralogy and depth of partial melting. The high Mg-number and largeion lithophile element abundances and negative epsilon Nd values ofthe lamproites indicate a mantle source that has experienced stages ofboth depletion and enrichment. The negative Nb, Ta and Ti anomalies inmantle-normalized trace element diagrams and low time-integrated U/Pb,Rb/Sr and Sm/Nd ratios of both lamproite groups and other Cenozoicigneous rocks from the Wyoming Archean Province indicate an ancientmetasomatic enrichment (>1.0 Ga) of the mantle source associatedwith the subduction of carbonate-bearing sediments. Other chemicalcharacteristics of the Leucite Hills lamproites, especially their highK (sub 2) O and volatile contents, are attributed to more recentmetasomatism (<100 Ma) involving influx from upwelling mantleduring back-arc extension or plume activity.

25 samples from this reference