Petrology of Lamproites from Smoky Butte, Montana

Hyalo-armalcolite-phlogopite lamproites and sanidine-phlogopite lamproites occurring at Smoky Butte, Montana are rocks formed from rapidly quenched, high temperature, uncontaminated lamproitic magma. Petrographic variations are attributable to different cooling histories of several batches of compositionally identical magma. Compared with other occurrences of lamproite, the rocks are unusually rich in TiO2 and are characterized by the presence of abundant armalcolite and the most TiO2-rich phlogopites yet found in this paragenesis. Compositional data are given for titanian phlogopite, olivine, diopside, titanian potassian richterite, armalcolite, sanidine, analcite, and glass. The mafic minerals are Al-deficient and exhibit very little compositional variation. Original leucite has been pseudomorphed by sanidine or analcite. The latter mineral was probably formed at the same time that the glass lost K, and gained Na, during alteration by groundwater. All of the lamproites are strongly enriched in Ta, Hf, and the light REE (La /Yb = 162–280), and have high MgO and Cr contents. Mineralogical, geochemical, and previously published isotopic data are combined in developing a petrogenetic model which suggests that these lamproites were derived from an ancient (2.5 Ga) doubly metasomatized harzburgitic source, and that they represent relatively primitive lamproites which were intruded at near-liquidus temperatures.