Geochemistry of Precambrian diabase dikes from Wyoming

Major and trace element concentrations in contact zones of 27 Precambrian diabase dikes from the Beartooth, Wind River, Bighorn, Owl Creek, and Laramie ranges in Wyoming indicate, in general, that the dikes are continental tholeiites. Relative to most young continental tholeiites, however, the Wyoming diabases are depleted in Sr relative to Rb, K, Ca, and usually, Ba and enriched in Rb relative to K and, sometimes, Ba; the Laramie dikes are also depleted in Ba relative to K. The Wyoming diabases (together with similar basaltic rocks from Antarctica and Tasmania and, perhaps, submarine tholeiites) define a fractionation trend in which Sr is rapidly depleted relative to Rb and K and enriched at a smaller rate relative to Ca (the Sr-depletion trend) as compared to the trend exhibited by most basalts (the normal-Sr trend). The existence of chilled contacts on the dikes and the lack of a systematic correlation between element distribution and secondary mineral content suggest that with respect to the trace elements investigated, secondary minerals in the dikes (primarily hornblende and chlorite) grew under essentially closed-system conditions (probably during deuteric alteration). Of the models considered plausible to explain the relative Sr-depletion and Rb-enrichment in the Wyoming diabases, that of extensive plagioclase crystallization from Ca-Al-rich, primary tholeiitic magmas are favored by existing data. If such a model is valid, the distribution of tholeiites exhibiting a Sr-depletion trend may offer an indirect method of mapping the distribution of Ca-Al-rich ultramafic source areas in the upper mantle. The Ba-depletion relative to K and Sr in the Laramie dikes is most readily explained in terms of a localized Ba-depleted upper-mantle source area.