MULTI-STAGE INTRUSION AND DIFFERENTIATION: PETROLOGIC DEVELOPMENT OF THE LITTLE COTTONWOOD STOCK, UTAH

The Little Cottonwood Stock (LCS) is an Oligocene, calc-alkaline granitic intrusion in Utah’s Wasatch Mountains. The stock contains two smaller mineralized zones, the Red Pine Porphyry (RP) and the White Pine intrusion (WP) (Sharp, 1958), and also hosts various compositions of crosscutting dikes, including aplite and rhyolite dikes. Our study is to determine whether the RP, WP, and rhyolite in the LCS result from further differentiation of the LCS host magma during cooling, or if they represent different intrusions.

Petrographic evidence shows clear differences: the WP, unlike the LCS, has no hornblende or visible titanite in hand sample, but does contain trace amounts of microscopic titanite, as well as uranothorite, and distinctive elongated “soda straw” zircons. The RP contains significant amounts of pyrite, large quartz phenocrysts, and hardly any modal mafic minerals.

The WP has been subjected to quartz-sericite-pyrite alteration, making it difficult to discriminate between differentiation and alteration trends. Enrichment in Al is evidence of alteration, and decline of Ca with silica could be due to alteration or magmatic differentiation. However, the WP is enriched in immobile Ti and P and follows a seemingly distinct differentiation trend from the LCS. It has U-Pb zircon ages of ~31-29 Ma, younger than the LCS. The WP probably developed from a separate intrusion(s) than the LCS.

The RP is the most silicic of the three major intrusive bodies, and is the most differentiated, based on depletion in Fe, Ca, and Mg, and negative Sr anomalies. It also is chemically altered and has large amounts of pyrite. The RP has a younger U-Pb zircon age of ~27 Ma.

The rhyolite and aplite are significantly more silica-rich than the host WP-RP and LCS, respectively. Silica variation diagrams for Ti, Fe, Mg, P, and LREEs show distinct depletions as compatible elements. Y, Nb, and Ga in the rhyolites are enriched compared to the other intrusions, suggesting they are separate magmas.

This reveals a history of influx of new magmas and differentiation of older magmas: intrusion and differentiation of the LCS, intrusion and differentiation of the WP, intrusion of the RP, and possibly differentiation of the RP and LCS to produce the rhyolite dikes and aplite dikes, respectively, or intrusion of a separate magma to form the RP.