Chapter 16: Cretaceous magmatism, metamorphism, and metallogeny in the east-central Great Basin

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Abstract
Compilation of published and new data on the distribution, timing, and composition of igneous rocks, ore deposits, and metamorphic rocks in the east-central Great Basin reveals systematic relations among Cretaceous magmatism, mineralization, metamorphism, and deformation. Magmatic compositions change with time from Early Cretaceous (∼125 to 100 Ma) quartz diorite, monzonite, and quartz monzonite, to mid-Cretaceous metaluminous (± hornblende) granodiorite and granite, and ultimately to Late Cretaceous (∼90 to 70 Ma) strongly peraluminous (two-mica) granite and granodiorite. Sr, O, and Nd isotopic data indicate a concomitant increasing crustal component, with a significant (meta)sedimentary component in the later plutons. Style and type of mineralization closely correlate with intrusion compositions. Porphyry Cu and Cu skarn deposits associated with the early plutons give way to porphyry Mo-Cu, polymetallic W skarn, and base-metal replacement mineralization associated with the metaluminous granodiorites and granites. F-rich, lithophile-element skarns and greisen mineralization are characteristic of the Late Cretaceous two-mica granites. Ar-Ar and U-Pb studies in the Snake and Ruby ranges and clustering of metamorphic K-Ar dates elsewhere are compatible with a Late Cretaceous (90 to 70 Ma) metamorphic culmination. Major tectonic subsidence in the foreland basin in central Utah, reflecting Sevier thrusting, began at about the same time (∼90 Ma). These observations are interpreted in the following way: increased deep (conductive) heat input accompanied renewal of magmatism, leading to increasing incorporation of crustal components with time in subduction-related magmas. Metallogeny followed the systematic changes in magma compositions. The metamorphic (thermal) culmination occurred simultaneously with maximum incorporation of crustal components in the magmas. Progressive thermal weakening of the crust was enhanced by wholesale assimilation or anatexis at this stage; thus major thrusting and the metamorphic culmination were synchronous with emplacement of two-mica granites. Simple one-dimensional thermal models are consistent with this scenario. Alternative interpretations that depend on crustal thickening as the primary mechanism are inconsistent with thermal considerations and the geologic record, although thickening probably played some role. Similarities between the Cretaceous history in the east-central Great Basin and other areas in the western United States suggest analogous processes elsewhere, modified by local factors. Magmatic and metallogenic differences among the Cretaceous, Jurassic, and Tertiary periods in the eastern Great Basin, and between the Great Basin Cretaceous events and collisional orogens may reflect differences in the thermal evolution and magmatic fluxes.

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