THE GRANITE PEAK MOUNTAIN INTRUSION: AN A-TYPE GRANITE(?) OF LATE JURASSIC AGE IN WESTERN UTAH

The Granite Peak Mountain intrusion of western Utah has been hypothesized to be a Proterozoic A-type granite because it intrudes medium-grade metamorphic rocks and is enriched in Be. However, new U-Pb zircon ages acquired by laser ablation ICP-MS show that the composite intrusion is Late Jurassic in age (150 Ma), a time of rare plutonism in the Basin and Range province. In contrast, the more common Middle Jurassic plutons are typically calc-alkalic with strong subduction-zone affinities. Geochemically and mineralogically, the Granite Peak pluton is also enigmatic; it ranges from hornblende monzonite to muscovite granite. Abundant tourmaline-beryl-muscovite pegmatites intrude the complex. The most evolved granites are ferroan, high-K, and enriched in Ga and high-field-strength-elements like typical A-type granitoids and have very low magnetic susceptibilities. However, unlike most A-types, the granites are distinctly peraluminous and muscovite-rich and the monzonites are magnesian and titanite-bearing. Moreover, the high concentrations of P and the compositions of biotite in the granite are more similar to those in strongly peraluminous (S-type) granites. The intrusion is strongly differentiated with progressive depletions of typically compatible elements like Sr, Ba, Eu, Ti, V, and P, but Y, U, Th, LREE and HREE also decline with evolution. The trends suggest significant fractionation of REE-U-Th-Y-rich phases such as monazite, xenotime, zircon, and titanite. Sr-Nd-Pb isotopic ratios show that the intrusion is similar to other Jurassic plutons in the north-central Great Basin. Notably, it is not like strongly peraluminous S-type granites in the region, but bears a similarity to Neogene A-type magmas. These paradoxical characteristics imply that the Granite Peak Mountain intrusion evolved from an A-type magma that assimilated metasedimentary wall rocks taking on some of the characteristics of S-type magmas. The parental magmas may be part of a short-lived episode of anorogenic magmatism related to termination of subduction along the western margin of North America and to the emplacement of the Independence dike swarm into the transtensional boundary in eastern California.