Lithology, chemistry, age, and origin of the Proterozoic Cardenas Basalt, Grand Canyon, Arizona

Abstract

Abstract The Cardenas Basalt, a 300-m-thick flow sequence, occurs in the middle of the 4000-m-thick Proterozoic sedimentary sequence in the Grand Canyon. It consists of a 100-m-thick lower member composed of about six, poorly exposed, coarsely ophitic pahoehoe flows of olivine basalt. In contrast, the upper member, 200 m thick, comprises four to six, resistant, aphyric, intersertal to intergranular flows that range from quartz tholeiite to tholeiitic andesite (icelandite). Petrographically, mafic intrusions exposed throughout the Inner Gorge of the Grand Canyon are similar to the lower member. Newly acquired RbSr isotopic data, in combination with those previously available, define a well-constrained isochron (10 points) at 1103±66 Ma. This date is concordant with a RbSr date from one of the mafic intrusions, supporting the conclusion that the Grand Canyon mafic magmatic episode occurred ∼ 1.1 Ga. The magma that fed both the intrusions and the lower member appears to have been principally derived from a slightly enriched mantle source. Subsequently, this magma underwent ∼ 20% fractionation of olivine and, possibly, minor assimilation of continental crust. Differences in SiO2, A12O3, TiO2, and P2O5 contents between the lower member and intrusions, apparently reflect heterogeneities in the mantle source, or different degrees of melting and/or crustal assimilation. The quartz-tholeiite magma of the upper member apparently resulted from more-extensive fractionation and crustal contamination of a mantle-derived magma. After its formation, this magma underwent further fractionation to produce, tholeiitic andesite (icelandite).