GEOCHEMICAL EVALUATION AND IGNEOUS PETROGENESIS OF HYDROTHERMALLY ALTERED EVOLVED RHYOLITES, SOUTHERN WAH WAH MOUNTAINS, UTAH

The Red Beryl and Tetons rhyolite units of the Blawn Formation of the southern Wah Wah Mountains, Utah, are both highly evolved topaz-bearing rhyolites that contain elevated concentrations of incompatible trace elements when compared to common I-type granites and rhyolites, including the rare earth elements (REE), F, Be, Li, and U. The two portions of the Blawn Formation are also variably evolved and have undergone different degrees of hydrothermal alteration. They are also geochemically classified as A-type rhyolites and are therefore thought to have been generated in an anorogenic tectonic setting not associated with subduction-related magmatism.

The Red Beryl rhyolite samples are all classified as high-Si, peraluminous, A-type rhyolites and the deposit shows a progression from lower, more evolved, more F-, Li-, and REE-enriched compositions to upper, less evolved, less REE- and trace element-enriched compositions. The later erupted Red Beryl rhyolite unit is heavily argillically altered, suggesting that hydrothermal alteration may have stripped this later unit of much of its trace element contents. The later Red Beryl rhyolites have also undergone higher degree hydrothermal alteration than the other units in the study area, as evidenced by the presence of narrow siliceous veins, the alteration of feldspar phenocrysts to clay minerals, the presence of significantly positive Ce anomalies in chondrite-normalized REE diagrams, and the presence of secondary minerals such as red beryl, fluorite, and topaz. The Tetons rhyolite samples are similarly all classified as high-Si, peraluminous, A-type rhyolites but are largely unaltered and the unit contains interlayered flows of HREE-, F-, Li-, and U-enriched high-Si rhyolite and LREE-enriched dacite.

Several petrogenetic models have been proposed to explain the petrogenesis of A-type, F-rich, topaz-bearing rhyolites. Our favored petrogenetic model for the formation of the Blawn Formation rhyolites is that their parental melts were derived through low-degree partial melting (~3-15 %) of a granulite source rock before these melts underwent extensive (>70 %) fractional crystallization of a predominantly sanidine and plagioclase assemblage, generating the major and trace element geochemical signatures that are preserved in these rocks.