Trace-element and Sr and Nd isotopic geochemistry of Cretaceous bentonites in Wyoming and South Dakota tracks magmatic processes during eastward migration of Farallon arc plutons

Abstract

Bentonite beds, which are clay deposits produced by the submarine alteration of volcanic tephra, preserve millions of years of volcanic products linked to magmatic systems for which records are otherwise lost through erosion and alteration. Cretaceous strata from the Bighorn Basin, Wyoming, and southwestern South Dakota contain bentonites that originated from arc magmatism produced by subduction of the Farallon plate. We analyzed the bulk major- and trace-element geochemistry, and the 87Sr/86Sr (n = 87) and 143Nd/144Nd (n = 26) isotopic compositions of individual bentonite beds from these areas spanning 40 m.y. of volcanism to recover signals of magmatic processes and to attempt to trace bentonite geochemical and isotopic signatures to contemporaneous Cordilleran plutonic rocks. Using multiple immobile elements (e.g., Zr, TiO2, Nb, Ta, and rare earth elements), distinct temporal trends show variations in the effects of mineral fractionation and changes in crustal thickness. Bentonite Sr and Nd isotopic compositions allow ash beds to be correlated with specific batholithic complexes in Idaho and western Montana. With this data set, we observed the following: (1) The volcanic arc migrated across the 0.706 isopleth between 115 and 105 Ma; (2) between 105 and 95 Ma, magmatism stalled in central Idaho and was supported through significant MASH (mixing-assimilation-storage-homogenization) processing; (3) by 85 Ma, a shallowing subduction angle resulted in the eastward migration of the volcanic front into western Montana while volcanism in Idaho diminished; and (4) around 75 Ma, evidence of Idaho volcanism is lost. Montana plutonism continued with significant assimilation of radiogenic basement and regional centers of local magma emplacement (i.e., Pioneer batholith).

Tracking 40 Million Years of Migrating Magmatism across the Idaho Batholith Using Zircon U-Pb Ages and Hf Isotopes from Cretaceous Bentonites

“…In the other three formations, the Mowry, Frontier, and Pierre, we collected multiple bentonite beds and merged zircons from each bed into combined groups. Sampled zircons (see [11] for detailed bentonite information; stratigraphy and sampling locations in Figure 2) were analyzed via laser ablation witha Photon Machines Analyte G2 Excimer laser (λ = 193 nm; pulse width = 8 ns) for 10 s ablation bursts, using a He carrier gas on a 40 µm target (Figure 3). The U-Pb data were acquired using a Thermo Element2 HR ICPMS with a dual mode secondary electron multiplier at the Arizona LaserChron Center.…”

“…Using a two-stage depleted mantle model age (Hf DMM ) for each analysis (from [11]), an age range for mantle extraction of the source rock can be determined that is dependent on the 177 Lu/ 176 Hf ratio of the continental crust used. While this measurement is not fully quantitative and is based on several assumptions (e.g., 176 Lu/ 177 Hf), it can provide firstorder constraints on minimum and maximum separation ages, as well as the magnitude of mantle versus basement contribution to the overall Hf signature.…”

“…The Idaho batholith and related plutons extending into western Montana represent the most inboard occurrence of Farallon-related plutonism, which lasted from the Middle Cretaceous to the Eocene [9]. Idaho plutonism began in western Idaho primarily in ac- [11] for more detailed locations). Larger map shows simplified locations of the Idaho batholith, including the Southern Atlanta lobe (SAL), Northern Atlanta lobe (NAL) and the Bitterroot lobe (BR), in addition to the Boulder batholith (BB); the 0.706 Sr isopleth, the isotopic boundary separating Laurentia from exotic and accreted terranes now exposed in the Blue Mountains.…”

“…The proportion of ages that are in accord with the stratigraphic eruptive age and that are older in each bentonite zircon population changes through time, recording differences in the tectonic setting of each set of ash eruptions as Cretaceous magmatism migrated spatially [4] across the western margin of Laurentia during Farallon plate subduction. These differences also reflect changes in magmatic processes related to crustal thickness and the depth of magma generation [4]: the late Albian and early Cenomanian, when Sr isotopes in the bentonites were homogeneous and their whole-rock trace element compositions record magma generation in a MASH zone in the thickest crust [11], is precisely when bentonite beds contain the largest proportion (40%) of autocrystic zircons.…”

“…The whole-rock geochemistry and Sr and Nd isotopic compositions of these bentonites track changes in crustal thickness and the migration of the magmatic front across Idaho into Montana. By combining variations in crustal thickness and Nd depleted mantle model ages, magma source terranes have also been identified [11].…”