Tales of tertiary tuffs in central Utah
De Vries, Rimmer D.
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Volcanic rocks from the Goldens Ranch Formation, the Moroni Formation and the East Tintic Volcanic district of central Utah have important lithological, compositional and mineralogical distinctions which complicate their direct correlation, contrary to previous studies. New mineral and chemical data shows that the tuffs from the Goldens Ranch Formation are not correlative with the tuffs of the Moroni Formation, nor are they related by fractionation from the same parent composition for they have different elemental abundances at a given silica content. The Moroni tuffs are possible differentiation products of the Salt Creek dike, located several miles northwest of the thickest and most densely welded tuffs of the Moroni Formation. From within the Goldens Ranch Formation there are two distinct pyroclastic suites: (1) the Chicken Creek Tuff (CCT) of Juab, Sage and Dog valleys, and (2) the tuffs outcropping at the Painted Rocks Recreation Area (PR). Mineralogically, the CCT is richer in phenocrysts, including dusty plagioclase, but lack sphene which is common in the PR tuffs. Chemically, the CCT is distinct from the PR tuffs, containing different elemental compositions at a given silica content. The PR tuffs include two main tuff units, a lower quartz-rich tuff and a massive upper tuff, both of which are chemically similar and contain similar minerals. The lower PR tuff is quartz-rich and is mineralogically and chemically similar to the Fernow Tuff of the southern East Tintic Mountains. Existing K/Ar dates, taken from the literature, show that the Moroni Formation is older than the volcanics of the Goldens Ranch Formation and the East Tintics. Within the Goldens Ranch Formation, the PR tuffs slightly predate the CCT. However, the CCT and units of the East Tintics are essentially the same age. The Packard Latite of the East Tintic Mountains is stratigraphically and chemically correlative with the Fernow Tuff and both have been considered as caldera-forming deposits. Based on chemistry, mineralogy and age, the PR tuffs are considered correlative to these caldera-forming deposits. Based on stratigraphy and chemistry, the CCT is considered correlative with the post-caldera Copperopolis Latite of the East Tintic Mountains. However, these correlations are tentative pending the stratigraphic relationship between the CCT and the Fernow. The Fernow is stratigraphically above a mafic tuff which is chemically similar to both the CCT and the Copperopolis Latite, and thus confuses the scenario.