Abstract
Cretaceous strata preserved in Wyoming contain numerous large bentonite deposits formed from the felsic ash of volcanic eruptions, mainly derived from Idaho batholith magmatism. These bentonites preserve a near-continuous 40 m.y. chronology of volcanism and their whole-rock and mineral chemistry has been used to document igneous processes and reconstruct the history of Idaho magmatism as emplacement migrated across the Laurentian margin. Using LA-ICP-MS, we analyzed the U-Pb ages and Hf isotopic compositions of nearly 700 zircon grains from 44 bentonite beds from the Bighorn Basin, Wyoming. Zircon populations contain magmatic autocrysts and antecrysts which can be linked to the main pulses of the Idaho batholith and xenocrysts ranging from approx. 250 Ma to 1.84 Ga from country rocks and basement source terranes. Initial εHf compositions of Phanerozoic zircons are diverse, with compositions ranging from −26 to nearly +12. Based on temporal trends in zircon ages and geochemistry, four distinct periods of plutonic emplacement are recognized during the Mid- to Late Cretaceous that follow plutonic emplacement across the Laurentian suture zone in western Idaho and into western Montana with the onset of Farallon slab shallowing. Our data demonstrate the utility of using zircons in preserved tephra to track the regional-scale evolution of convergent margins related to terrane accretion and the spatial migration of magmatism related to changes in subduction dynamics.
Highlights
Bentonites, volcanic ash beds devitrified and transformed into clay, are found globally and range in age from near-modern to Precambrian [1]
Spot analyses on individual grains yielded a wide range of ages, from Cretaceous to Paleoproterozoic and more than half of the analyzed grains (23) have concordant 206Pb/238U and 207Pb/235U ages (Figure 4A), which is here defined as a concordance of 95 to 105%
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
Summary
Bentonites, volcanic ash beds (airfall tuffs) devitrified and transformed into clay, are found globally and range in age from near-modern to Precambrian [1]. Airfall tuffs can be deposited over vast areas, including areas distant from the interiors of orogenic belts where the effects of thermal overprinting are absent. These distally deposited ash beds have the potential to serve as a robust source of geochronologic and geochemical data that track the evolution of their magmatic source terranes. Prior studies typically have sampled bentonites that formed during a limited time interval of only a few million years and so long-term changes in tectonic setting and the magmatic process that led to eruption of tephra have not been considered. We have analyzed a series of bentonites beds that span 40 million years of eruption
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