Geochemistry, mineralogy and petrology of the Eocene potassic magmatism from the Milk River area, southern Alberta, and Sweet Grass Hills, northern Montana

Abstract

New whole-rock geochemical, quantitative mineralogical and electron-microprobe data are presented from lamproite, olivine minettes and mafic trachyte of the Milk River area, southern Alberta, and latites, trachytes and high-K rhyolite of the Sweet Grass Hills, northern Montana. These rocks comprise dikes, small plugs and a pipe complex associated with mainly syenite and hornblende monzonite plutons that core several buttes in the Sweet Grass Hills and mark the northern extent of the Cenozoic Montana Alkaline Province. Minerals restricted to peralkaline lamproite at the 49th Parallel locality include Ti-rich phlogopite-annite with Fe3+ in tetrahedral coordination, Na-poor sanidine, Ba-rich and Cl-poor fluorapatite, K-rich titano-ferronyböite/ferro-obertiite, LREE-rich strontian fluorapatite (SrO=17.5-25.2wt.%, Ce2O3=2.9-3.3wt.%), ilmenite, strontian barite, barian celestine, and unidentified Zr-Ti phase (TiO2=76.7-88.0wt.%, ZrO2=2.1-7.2wt.%) perhaps replacing primary Zr-Ti silicates. The strontian fluorapatite has the highest SrO content thus far reported in the literature for apatites from lamproites. Olivine minettes and mafic trachyte of the Milk River area contain augite, Al-diopside, Ti-rich magnetite, sanidine with higher Na, hyalophane, Cl-bearing fluorapatite with negligible Ba, ferroan magnesite phenocrysts, and primary calcite, all of which are notably absent in the 49th Parallel lamproite. Mica displays distinct core-to-rim zoning trends in these rocks, characterized by increasing FeOt and TiO2 at decreasing Al2O3 in lamproite and increasing FeOt and TiO2 at increasing Al2O3 in minettes and trachytes. The 49th Parallel lamproite has higher Ti, K, P, Ba, Rb, Sr, Nb, Th, U, Zr, Hf and REE concentrations, higher K/Al, (Ce/Yb)CN, (La/Sm)CN, (Gd/Yb)CN, Rb/Cs, Ce/Pb, Th/Ta, Th/Yb, and lower Ba/La, Ba/Th and Nb/U relative to minettes of the Milk River area. Major-element, least-squares mass balance and trace-element models call for the origin of latites, trachytes and high-K rhyolite in the Sweet Grass Hills complex by two-stage fractional crystallization of primitive olivine minette magmas concurrent with negligible, if any, assimilation of crustal materials. Oscillatory and reverse zoning in phenocrysts and hybrid phenocryst assemblages record recharge of more evolved magma chambers with primitive minette magmas. The calculated evolutionary models are inconsistent with the compositional variations displayed by the olivine minettes and lamproite of the Milk River area. These rocks represent distinct low-degree partial melts of sub-lithospheric mantle enriched in K and volatiles by the recent (<100Ma) plume activity or convective upwelling of the upper mantle above subducted Farallon slab, which underwent variable assimilation of the ancient, LREE-Ba-Sr enriched and HFSE depleted Wyoming-Medicine Hat lithosphere during the post-Laramide extension. The olivine minettes derived from more oxidized, carbonated region of the mantle source, compared to the more reduced source of the 49th Parallel lamproite. The extremely negative εNd values, unradiogenic Pb and moderately radiogenic Sr isotopic compositions, correlated with LREE, Ba enrichment and relative Nb, Ta, Ti and other HFSE depletions of the Cenozoic K-rich rocks of the MAP, may reflect carbonatite metasomatism of the lithospheric mantle associated with the widespread Mesoproterozoic (1.5-1.4Ga) anorogenic magmatism and intracontinental rifting related to mantle upwelling and plume activity beneath a thick Precambrian supercontinent. © 2012.