GEOCHEMISTRY OF CRUSTAL XENOLITHS FROM THE HATCHER MESA LAMPROITE, WYOMING, USA: INSIGHTS INTO THE COMPOSITION OF THE DEEP CRUST AND UPPER MANTLE BENEATH THE WYOMING CRATON

摘要

Crustal xenoliths entrained in the Hatcher Mesa lamproite, Leucite Hills, in Wyoming, consist of ultramafic, mafic and felsic varieties. Geothermometric calculations based on two-pyroxene thermometry indicate equilibrium temperatures of approx 800 + - 50 deg C. Petrographic observations, along with whole-rock and mineral compositions, indicate that the xenoliths were generated by crystallization of mantle-derived mafic magmas during the Archean; these xenoliths were subsequently affected by high-grade metamorphism. The geochemical characteristics of the xenoliths, such as negative Nb-Ta-Ti and positive Pb anomalies, enrichment in the LILE (large-ion lithophile elements), and the Pb isotopic ratios point to parental magmas generated in an arc-like setting, supporting those models that involve accretion of the thinned, southern part of the Wyoming craton. Although the host lamproite and the xenoliths from the lower crust are not related genetically, their parental magmas originated from the underlying Wyoming mantle. It seems likely that the melting events that generated the protolith for the xenoliths contributed to the formation of a refractory mantle source under Wyoming. Such a modified, depleted source is considered essential for the petrogenesis of lamproites, including those occurring in the Leucite Hills.