There is a nearly continuous record of magmatism through the Late Cretaceous-early Paleogene in Idaho and adjacent areas of Oregon and Montana, including the various phases of the Idaho batholith. We ...suggest that much of this magmatic record, however, has been obscured by subsequent tectonic deformation, erosion, and magmatic disruption and cannibalization, the latter of which can be tracked by zircon inheritance. Specifically, a mid-Cretaceous magmatic arc was significantly deformed by the western Idaho shear zone and intruded by the 83-67 Ma Atlanta peraluminous suite of the Idaho batholith. The northern part of the Atlanta peraluminous suite was, in turn, intruded by the 65-55 Ma Bitterroot lobe of the Idaho batholith. Consequently, the present age distribution of magmatism is strongly biased toward the youngest phases of plutonism; much of the older phases were destroyed by tectonic, magmatic, and erosional processes. The destruction of granitic batholiths may characterize Cordilleran-style orogens worldwide, which can lead to significant underestimates of magmatic fluxes.
The western Idaho shear zone is a major, lithospheric-scale structure separating accreted terranes of the Blue Mountains from continental North America. We document the occurrence of the western ...Idaho shear zone in West Mountain, west-central Idaho. Rocks deformed by the western Idaho shear zone at West Mountain are dominantly orthogneisses, although exposures on West Mountain containing screens of metamorphosed sedimentary rocks are also present. Steeply E-dipping, N-NNE-oriented foliations and downdip lineations characterize the fabric in the orthogneisses, consistent with dextral transpressional kinematics. The foliation orientation changes from 005° to 024° from the northern to the southern part of the field area, and this is interpreted to reflect a primary along-strike variation in the orientation of the western Idaho shear zone. The westernmost unit in West Mountain (Four Bit Creek tonalite) has a U-Pb zircon age of 101 ± 3.0 Ma, yet it is only weakly deformed. We interpret this unit to have been emplaced pretectonically, thus constraining the initiation of the western Idaho shear zone. The youngest unit at West Mountain is the undeformed Rat Creek granite (88.2 ± 3.3 Ma). U-Pb analyses of zircons from orthogneisses at West Mountain span ages of 111-91 Ma, indicating both precursory and continuous magmatism coeval with western Idaho shear zone deformation. Two Lu-Hf garnet isochron ages, 97.3 ± 0.7 Ma and 99.5 ± 1.4 Ma, are interpreted to indicate peak metamorphism during western Idaho shear zone deformation. Geochemical analyses suggest that the westernmost exposed orthogneiss units are dominantly derived from continental material in West Mountain, and yet there is also evidence for a component of accreted terrane rocks at depth east of the western Idaho shear zone.
We conducted a (U-Th)/He zircon thermochronology study of the southern part of the Idaho batholith (central Idaho, USA) to constrain cooling through ∼200 °C and exhumation of the batholith. Samples ...were collected adjacent to the Idaho-Oregon (IDOR) seismic transect and at localities where U-Pb zircon, geochemical, and fabric analyses were conducted. The rocks affected by the western Idaho shear zone and associated border zone suite of the batholith cooled through the closure temperature for He in zircon prior to ca. 60 Ma, before or during emplacement of the voluminous Atlanta lobe. In contrast, the Atlanta lobe (Atlanta peraluminous suite) records a relatively constant cooling rate, in which the (U-Th)/He zircon ages are systematically ∼30 m.y. younger than the U-Pb zircon ages. We interpret this data to reflect post-magmatic isobaric cooling with little or no unroofing. The only deviation from a smooth regional cooling pattern occurs near Sawtooth Valley, where samples from the Sawtooth Range on the west side of the valley show distinctly younger ages than those from the White Cloud Peaks to the east. We interpret this difference to reflect recent cooling and exhumation associated with extensional deformation. The regionally consistent pattern of cooling and hence exhumation indicates that the current exposure level of the Idaho batholith was <5 km deep (assuming a geothermal gradient of 40 °C/km) at 50 Ma during the initiation of Challis magmatism. Our data are consistent with the existence of a crustal plateau during formation of the Atlanta lobe of the Idaho batholith.
Study of igneous and sedimentary rocks that may be representative of the chemical and isotopic characteristics of portions of the upper continental crust (UCC) has provided important insights to the ...origin and growth of the continents, as well as the large-scale chemical evolution of the silicate Earth. For example, changes in the major and trace element compositions, as well as long-lived radiogenic isotope systematics of (meta)sedimentary rocks through time have led to the conclusion that at least some portions of the Archean UCC were enriched in mafic-ultramafic components compared to UCC with younger provenance (Taylor and McLennan, 1985; Condie, 1993; Gaschnig et al., 2016; Chen et al., 2016; Tang et al., 2016; Garçon et al., 2017). Short-lived radiogenic isotope systems are an additional means of tracing the contributions of diverse mantle-derived components to the UCC. Tungsten-182 anomalies have been observed in both ancient and modern rocks. Although these anomalies were ultimately created by processes that occurred while 182Hf was extant during the first ~60 Ma of Solar System history, the causes of their incorporation and preservation in the rock record, as well as the frequency and distribution of rocks in the UCC with isotopic anomalies remain poorly understood. Here, μ182W values (deviations in 182W/184W from laboratory reference standard in parts per million) are reported for previously well-characterized, fine-grained glacial diamictites deposited between ~3.0 and ~0.3 Ga. Glacial diamictites deposited during the Mesoarchean as part of the Witwatersrand Supergroup in southern Africa are characterized by an average μ182W value of −12.5 ± 5.0 (2SD), yet a diamictite of similar age from the spatially associated Pongola Supergroup (Mozaan) is not isotopically anomalous. The isotopically anomalous diamictites are also characterized by comparatively high Ni and highly siderophile element (HSE) abundances, indicative of significant contributions from ultramafic, most likely komatiitic components. No resolvable 182W anomalies were found in glacial diamictites with lower Ni and HSE, deposited after 2.3 Ga. This in turn suggests that W isotopic anomalies in Archean UCC, at least in part, reflect contributions from deep mantle upwellings that produced some of the komatiites. The new results provide further evidence that the isotopic composition of W in the Archean crust was highly variable.
•Some Archean South African glacial diamictites are characterized by negative μ182W values as low as −14 ± 4.5.•Samples deposited after the Paleoproterozoic show no anomalous 182W signatures.•μ182W values broadly correlate with Ni and HSE abundances, indicative of significant amounts of ultramafic components.
We present an integrated study of the postcollisional (post-Late Jurassic) history of the Blue Mountains province (Oregon and Idaho, USA) using constraints from Cretaceous igneous and sedimentary ...rocks. The Blue Mountains province consists of the Wallowa and Olds Ferry arcs, separated by forearc accretionary material of the Baker terrane. Four plutons (Lookout Mountain, Pedro Mountain, Amelia, Tureman Ranch) intrude along or near the Connor Creek fault, which separates the Izee and Baker terranes. High-precision U-Pb zircon ages indicate 129.4-123.8 Ma crystallization ages and exhibit a north-northeast-younging trend of the magmatism. The 40Ar/39Ar analyses on biotite and hornblende indicate very rapid (<1 m.y.) cooling below biotite closure temperature (∼350 °C) for the plutons. The (U-Th)/He zircon analyses were done on a series of regional plutons, including the Lookout Mountain and Tureman Ranch plutons, and indicate a middle Cretaceous age of cooling through ∼200 °C. Sr, Nd, and Pb isotope geochemistry on the four studied plutons confirms that the Izee terrane is on Olds Ferry terrane basement. We also present data from detrital zircons from Late Cretaceous sedimentary rocks at Dixie Butte, Oregon. These detrital zircons record only Paleozoic-Mesozoic ages with only juvenile Hf isotopic compositions, indicating derivation from juvenile accreted terrane lithosphere. Although the Blue Mountains province is juxtaposed against cratonic North America along the western Idaho shear zone, it shows trends in magmatism, cooling, and sediment deposition that differ from the adjacent part of North America and are consistent with a more southern position for terranes of this province at the time of their accretion. We therefore propose a tectonic history involving moderate northward translation of the Blue Mountains province along the western Idaho shear zone in the middle Cretaceous.
The EarthScope IDOR (Idaho-Oregon) project was designed to investigate the formation of the steep accretionary boundary associated with the Salmon River suture zone and western Idaho shear zone ...(WISZ), in western Idaho and eastern Oregon (western USA). The research was designed to test three hypotheses concerning the present geometry of the boundary: (1) a crustal detachment; (2) a lithospheric detachment; and (3) a curved original geometry for the WISZ. The data presented in this volume indicate that none of these original three models is correct. Rather, the WISZ continues vertically downward through the entire crust, its steep orientation consistent with a significant component of strike-slip and/or transpressional tectonism. Moreover, the western Idaho shear zone is extremely well preserved despite abundant subsequent magmatism (Idaho batholith, Challis event, Columbia River Basalt Group) and tectonism (Cretaceous contraction, Eocene extension, Miocene-present extension). The ten research papers in this volume report on various aspects of this history. Four papers address the kinematics and tectonic history of rocks spatially associated with the WISZ, including its along-strike northward continuation. Two papers address the tectonic history of the accreted terranes, including geochemical arguments for different lithospheres sampled during magmatic episodes. Three papers document deformation, exhumation, and emplacement rates in the Idaho batholith. The final paper constrains the modern crustal architecture along a transect from the accreted terranes in the west to cratonic North America in the east. Together, these studies constrain the tectonic history of a subvertical tectonic boundary in the North American Cordillera.
Twenty-four composite samples of the fine-grained matrix of glacial diamictites deposited from the Mesoarchaean to Palaeozoic have been analysed for their silicon isotope composition and used to ...establish, for the first time, the long-term secular Si isotope record of the compositional evolution of upper continental crust (UCC). Diamictites with Archaean and Palaeoproterozoic Nd model ages show greater silicon isotope heterogeneity than those with younger model ages (irrespective of depositional age). We attribute the anomalously light Si isotope compositions of some diamictites with Archaean model ages to the presence of glacially milled banded iron formation (BIF), substantiated by the high iron content and Ge/Si in these samples. We infer that relatively heavy Si isotope signatures in some Palaeoproterozoic diamictites (all of which have Archaean Nd model ages) are due to contribution from tonalite-trondhjemite-granodiorites (TTGs), evidenced by the abundance of TTG clasts. By the Neoproterozoic (with model ages ranging from 2.3 to 1.8 Ga), diamictite Si isotope compositions exhibit a range comparable to modern UCC. This reduced variability through time is interpreted as reflecting the decreasing importance of BIF and TTG in post-Archaean continental crust. The secular evolution of Si isotopes in the diamictites offers an independent test of models for the emergence of stable cratons and the onset of horizontal mobile-lid tectonism. The early Archaean UCC was heterogeneous and incorporated significant amounts of isotopically light BIF, but following the late Archaean stabilisation of cratons, coupled with the oxygenation of the atmosphere that led to the reduced neoformation of BIF and diminishing quantities of TTGs, the UCC became increasingly homogeneous. This homogenisation likely occurred via reworking of preexisting crust, as evidenced by Archaean Nd model ages recorded in younger diamictites.
•Ancient glacial diamictites record a secular trend in the average silicon isotope composition of the upper continental crust.•2.90-2.30 Ga diamictites are heterogeneous for δ30Si compared to modern crust, due to BIF and TTG in their source.•Neoproterozoic and Palaeozoic diamictites approximate the δ30Si of modern upper continental crust.•Secular diamictite Si isotope trends support reworking of cratons that progressively homogenised Si isotopes in the crust.•Diamictite Si isotopes align with models supporting lithosphere stabilisation and crustal reworking after ∼3.0 Ga.
The Idaho batholith of the North American Cordillera is a large and long-lived silicic intrusive center. We studied fabrics at the regional scale within the different intrusive suites of the Idaho ...batholith, using microstructural characterization, anisotropy of magnetic susceptibility measurements, and shape preferred orientation analyses. Each studied outcrop was collocated with existing U-Pb zircon geochronology and ongoing (U-Th)/He zircon thermochronology results. Three new 40Ar/39Ar biotite ages, collocated with the existing U-Pb zircon ages, constrain the cooling rates within the batholith. The presence of dominantly magmatic microstructures allows us to interpret the results relative to the U-Pb zircon ages and observe spatial and temporal patterns of fabric development. The early (pre-80 Ma) intrusive suites exhibit solid-state microstructures that show a consistent orientation only in the western part of the batholith. Fabrics in the 83-67 Ma Atlanta peraluminous suite are weak and inconsistent in orientation, despite emplacement during regional contraction. We hypothesize that the lack of consistently oriented fabrics in the Atlanta lobe results from either: (1) topographic effects that caused local extensional/neutral strain environments in the upper parts of a crustal plateau; or (2) emplacement in thin, horizontal magmatic sheets. In contrast, fabrics within the 66-53 Ma Bitterroot peraluminous suite are well developed and consistently oriented (NW-striking; NE-dipping), recording localized contraction during magmatism.
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