Isotope geochemistry, petrology, and source evaluation of the Leucite Hills lamproites, Wyoming.

摘要

Major, trace element, Sr, Nd and Pb isotopic data are reported for the Leucite Hills lamproites and their lower crustal xenoliths. The new chemical data indicate that the two main groups of lamproites, phlogopite lamproites and madupitic lamproites are derived from primary magmas. Analysis of separated mineral grains show that the transitional madupitic lamproites can be included with the madupitic lamproites while the olivine-bearing phlogopite lamproites are similar to the phlogopite lamproites. The data from both types of lamproites form two distinct grouping in many isotopic and trace element diagrams.; The texture, mineralogy, and whole rock compositions indicate that the lower crustal xenoliths from the Leucite Hills lamproites are meta-igneous granulites. Estimates of pressure and temperature suggest that they formed at 4–9 kb, and about 800°C. The chemical and isotopic compositions of the xenoliths indicate that the protoliths of the granulites crystallized from a fractionated, mantle-derived melt. Based on Pb-Pb and Sm-Nd pseudoisochrons, and Nd model ages, the basic parental magma was probably either derived from a CHUR-like reservoir or a depleted mantle source metasomatised by subducted materials of continental crust origin.; The model favoured for the origin of the source of the Leucite Hills lamproites involves Archean depletion followed by a subsequent late-Archean enrichment event. Geochemical evidence such as No and At depletion and the low U/Pb ratio of the lamproites, as well as the correlation of the data from the lower crustal xenoliths and the lamproites on the Sr-Nd and Pb-Pb isotopic diagrams favor Archean metasomatism of the mantle source by subduction of continental crust materials. Some geochemical features of the Leucite Hills lamproites (e.g., high LILE and K₂O contents) are attributed to a recent metasomatic activity. This recent enrichment may be related to a plume activity which may also have induced melting in the source.