NON-MAGMATIC PERALKALINE SODIC METASOMATISM, NORTHWEST ARIZONA

Peralkaline sodic alteration is well known with peralkaline magmatic systems such as carbonatites and some syenites yet is essentially unreported in other settings. Likely caused by hydrothermal circulation of alkaline lake fluids, distinctive peralkaline sodic metasomatism, characterized by the presence of hydrothermal riebeckite and aegirine, impacted over 20 km² of the weakly peraluminous Miocene Wilson Ridge pluton in northwest Arizona. Mapping and remote sensing across >50 km² show that alteration is concentrated in the western, possibly upper parts of the tilted Wilson Ridge pluton. Where studied, the pluton primarily comprises biotite-hornblende granodiorite with subordinate diorite, aplites, and late biotite-bearing rhyolitic dikes. Over 20 km² contain by volume approximately 1/3 peralkaline metasomatism consisting of magnesio-riebeckite veinlets and breccias enveloped in albite-magnesio-riebeckite replacement of the igneous K-feldspar, plagioclase and biotite. Intense zones contain albite + aegirine-augite ± magnesio-riebeckite which are often accompanied by quartz dissolution. Alteration, brecciation, and veining postdate all but the late rhyolite; the latter is affected only by a widely pervasive albite-magnesio-riebeckite flooding. Whole rock geochemistry shows that although Na, Ca and K, and to a lesser extent Fe, are mobile, high-field-strength elements (Ti, Zr, REE, Nb) are little changed. Thus, despite the mineralogical similarities to alteration linked with peralkaline magmas, an alternative, distinctive origin is implied by the unusually large area of alteration, the absence of peralkaline igneous rocks, the lack of minor element enrichments characteristic of fenites, and – importantly – a distinctive, non-magmatic Sr isotopic signature. In contrast to typical sodic(-calcic) alteration which is found in many geologic settings, and which is consistent with near-neutral NaCl-rich fluids, alteration at Wilson Ridge requires a distinctly alkaline fluid. Given that the evidence seems to negate a peralkaline magmatic source, an evaporitic Na-CO3-SO4-Cl source similar to modern and ancient alkaline lakes seems most likely, perhaps contributed from contemporaneous evaporitic sediments of the nearby Horse Spring formation which have similarly elevated Sr isotopic ratios.