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Combined Hf-O isotopic analyses of zircons from tuffs and lavas within the Sierra Madre Occidental (SMO) silicic large igneous province are probes of petrogenetic processes in the lower and upper crust. Existing petrogenetic and tectonomagmatic models diverge, having either emphasized significant crustal reworking of hydrated continental lithosphere in an arc above the retreating Farallon slab or significant input of juvenile mantle melts through a slab window into an actively stretching continental lithosphere. New isotopic data are remarkably uniform within and between erupted units across the spatial and temporal extent of the SMO, consistent with homogeneous melt production and evolution. Isotopic values are consistent with enriched mantle magmas (80%) that assimilated Proterozoic paragneisses (~20%) from the lower crust. δ18Ozircon values are consistent with fractionation of mafic magma and not with assimilation of hydrothermally altered upper crust, suggesting that the silicic magmas evolved at depth. Isotopic data agree with previous interpretations where voluminous juvenile melts entered the lithosphere during the transition from a continental arc experiencing slab rollback (Late Eocene) to the arrival of a subducting slab window (Oligocene and Early Miocene) and failure of the upper plate leading to the opening of the Gulf of California (Late Miocene). An anomalously large heat flux and extension of the upper plate allow for the sustained fractionation of the voluminous SMO magmas and assimilation of the lower crust.

The Mesoproterozoic MARA terrane of western South America is a composite igneous-metamorphic complex that is important for Paleozoic paleogeographic reconstructions and the relative positions of Laurentia and Gondwana. The magmatic and detrital records of the MARA terrane are consistent with a Laurentian origin; however, the metamorphic and deformation records lack sufficient detail to constrain the correlation of units within the MARA terrane and the timing and mechanisms of accretion to the Gondwana margin. Combined regional mapping, metamorphic petrology, and garnet and monazite geochronology from the Sierra de Maz of northwest Argentina suggest that the region preserves four distinct litho-tectonic units of varying age and metamorphic conditions that are separated by middle- to lower-crustal ductile shear zones. The Zaino and Maz Complexes preserve Barrovian metamorphism and ages that are distinct from other units within the region. The Zaino and Maz Complexes both record metamorphism ca. 430–410 Ma and show no evidence of the regional Famatinian orogeny (ca. 490–455 Ma). In addition, the Maz Complex records an earlier granulite facies event at ca. 1.2 Ga. The Taco and Ramaditas Complexes, in contrast, experienced medium- and low-pressure upper amphibolite to granulite facies metamorphism, respectively, between ca. 470–460 Ma and were later deformed at ca. 440–420 Ma. The Maz shear zone that bounds the Zaino and Maz Complexes records sinistral oblique to sinistral deformation between ca. 430–410 Ma. The data suggest that at least some units in the MARA terrane were accreted by translation, and the Gondwana margin of northwest Argentina transitioned from a dominantly convergent margin to a highly oblique margin in the Silurian.

The Stibnite-Yellow Pine district contains the largest antimony resource in the United States, as well as significant gold, and is a historic producer of tungsten. Application of in situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) direct dating of scheelite from two Au-Sb-W ore deposits, Yellow Pine and Hangar Flats, yielded an older group of U-Pb ages in the range of 60.0 ± 2.8 to 57.0 ± 1.1 Ma and a younger U-Pb age for scheelite intergrown with stibnite of 47.4 ± 1.1 Ma. These in situ analyses were calibrated by isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U-Pb lower intercept ages of two coarsely crystalline scheelite samples that yielded ages of 57.52 ± 0.22 and 56.62 ± 0.16 Ma. Scheelite of the latter age is of sufficient quality to serve as a primary reference material for LA-ICP-MS scheelite U-Pb geochronology. The group of older U-Pb scheelite ages agrees with 40Ar/39Ar ages of 56.9 ± 1.2 to 56.38 ± 0.54 Ma on adularia from Yellow Pine and Hangar Flats, whereas the younger U-Pb scheelite age is similar to an 40Ar/39Ar age of 46.00 ± 0.40 Ma on adularia from an epithermal gold-silver deposit in the adjacent Thunder Mountain caldera. Our results indicate that the main stage of tungsten mineralization occurred at ca. 57 Ma, whereas the main stage of antimony mineralization occurred at ca. 47 Ma—thereby providing first-time age constraints for antimony and tungsten mineralization in the Stibnite-Yellow Pine district.

We addressed fundamental questions about the lithology, age, structure, and thermal evolution of the deep crust of the retroarc hinterland of the North American Cordilleran orogen through systematic investigation of zircons from Cretaceous plutons in the Snake Range and Kern Mountains of east-central Nevada. Geochronological (U-Pb) and geochemical (trace element, O and Hf isotopes) characterization of pre- and synmagmatic growth domains of zircons, coupled with traditional petrologic methods (petrography, field relationships, and whole-rock major-element, trace-element, and Sr-Nd and Pb isotope geochemistry), fingerprinted temporal variations in crustal contributions to magmatism. The samples are typical felsic, peraluminous Cordilleran interior granitoids that formed between 102 ± 2 Ma and 71 ± 1 Ma (95% confidence). Over the entire time span of magmatism, 87Sr/86Srinitial, εNd(t), 208Pb/204Pb, and εHf(t) exhibit incrementally more “crustal” ratios. The oldest and youngest samples, respectively, predate and postdate all published timing constraints of Cretaceous peak metamorphism in the region and exhibit the least and most radiogenic whole-rock isotopic results in the study (87Sr/86Srinitial = 0.7071 vs. 0.7222; εNd(t) = −3.4 vs. −18.8; 208Pb/204Pb = 38.8 vs. 40.1). Accordingly, the least intrasample variability of εHf(t), δ18OZrc, and trace-element ratios in magmatic zircon domains is also observed in these oldest and youngest samples, whereas greater intrasample variability is observed in intermediate-age samples that intruded during peak metamorphism. The geochemistry of zircon growth in the intermediate-age samples suggests assimilation of partially molten metasedimentary crust led to increased heterogeneity in their magma chemistry. Interaction of magmas with distinctive crust types is indicated by contrasts between four categories of inherited zircon observed in the studied intrusions: (1) detrital zircon with typical magmatic trace-element ratios; (2) zircon derived from high-grade 1.8–1.6 Ga basement; (3) zircon with anomalously low δ18O of uncertain origin, derived from 1.7/2.45 Ga basement (or detritus derived thereof); and (4) zircon from variably evolved Jurassic–Early Cretaceous deep-seated intrusions. The progression of zircon inheritance patterns, correlated with evolving geochemical signatures, in Late Cretaceous granitic plutons is best explained by early, relatively primitive intrusions and their penecontemporaneously metamorphosed country rock having been tectonically transported cratonward and superposed on older basement, from which the later, more-evolved Tungstonia pluton was generated. This juxtaposition consequentially implies tectonic transport of synorogenic plutonic rocks occurred in the Cordilleran hinterland during the Sevier orogeny as a result of the interplay of retroarc magmatism and convergent margin tectonism.

Garnet Sm–Nd and Lu–Hf isotopic data are increasingly used in tandem to characterize tectono-metamorphic events and their duration because the two chronometers may yield a progression from older Lu–Hf to younger Sm–Nd ages. In this contribution, we utilize garnet compositional zoning and monazite U–Pb ages to evaluate explanations for differences in garnet Sm–Nd and Lu–Hf ages for low- to mid-amphibolite facies rocks from the Picuris and Tusas Mountains, New Mexico. Ten Sm–Nd ages, eight Lu–Hf garnet ages (6 previously published), and four matrix monazite U–Pb dates span 1422–1370 Ma, 1456–1399 Ma, and 1377–1356 Ma, respectively. The weighted means of garnet ages are 1398 ± 11 Ma for Sm–Nd (N = 10/10) and 1402 ± 4 Ma for Lu–Hf (N = 6/8); however, Sm–Nd and Lu–Hf ages from three samples differ by 29, 34, and 55 my with differences between 2σ age envelopes of 12, 9, and 38 my., respectively. In these samples, primary Lu and Sm zoning in garnet, diffusion modification of zoning, or disequilibrium is sufficient to cause older Lu–Hf ages and younger Sm–Nd ages. Alternatively, two of the three Lu–Hf ages, c. 1450 Ma, may reflect the onset of a prolonged regional event or an earlier thermal event, and the three c. 1370 Ma garnet Sm–Nd ages may reflect diffusional re-equilibration. These three c. 1370 Ma ages are indistinguishable from the monazite ages, and garnet have trace element zoning consistent with diffusional alteration of growth zoning in Sm and Nd but not Lu and Hf. Garnet from the other seven samples have prograde zoning with little to no evidence for diffusional relaxation of zoning in these elements. We conclude that the majority of garnet preserve primary compositions and indistinguishable garnet ages of c. 1400 Ma, indicating a c. 22 my garnet growth history based on weighted mean. The two oldest c. 1450 Lu–Hf and three youngest c. 1370 Sm–Nd garnet ages require an earlier thermal history and modification of garnet composition, respectively. Results of this study redefine the youngest extent of amphibolite facies metamorphism during the Picuris orogeny to at least c. 1370 Ma.

The Acasta Gneiss Complex (AGC) of the Northwest Territories, Canada, contains some of the oldest evolved terrestrial crust and has a sustained record of more than a billion years of magmatism (>4.0–2.9 Ga). These rocks provide an opportunity to investigate the nature of continental crust formation on the early Earth. Because complexities in zircons and bulk rocks are characteristic of the early Earth record—including the AGC—strategies are needed to extract accurate and meaningful age and isotopic information. In order to evaluate the accuracy of the early Earth Hf isotope record, we examined AGC zircons from a range of lithologies using paired chemical abrasion isotope dilution U-Pb age and solution Lu-Hf isotope analysis and compared these with previous results obtained using laser ablation split-stream (LASS) analysis. We describe an approach whereby LASS is used to identify rocks with the least complex zircons, and, when appropriate, solution methods are then used to refine the age and Hf isotopic composition to the highest precision. This two-pronged analytical approach results in a more robust determination of the age and Hf isotopic record of complex rocks and zircons and allows identification of complexity in the Hf isotopic record that would not be apparent by solution analysis alone, thereby refining the record of magmatic evolution on the early Earth. Despite the better precision, solution techniques are unsuitable for rocks with complexly zoned zircons. In particular, zircons from the two oldest AGC rocks in this study have positive initial eHf values when determined by solution analysis (+2.8 and +4.9 at 4.0 and 3.9 Ga, respectively) but have negative eHf values when determined by LASS (−3.4 and −4.7, respectively). This is attributable to the presence of later radiogenic overgrowths on the zircon grains which are incorporated in solution analyses but can be avoided using LASS. This provides important clarity to the AGC Hf isotope record. In total, all but one of the AGC rocks we analyzed, including the oldest samples, have negative eHf values and indicate derivation from an enriched reservoir; none of these samples—in contrast to whole rock Nd isotope compositions—have sufficiently positive eHf values to indicate their derivation from a depleted mantle reservoir. Despite the presence of ancient AGC crust, there is no record of corresponding mantle depletion. This implies that extraction of Hadean crust in this region did not happen in sufficient volume to result in widespread mantle depletion in the AGC source by the Eoarchean. Our results underscore the importance of identifying different components in ancient zircons—and the rocks that contain them—and accurately determining the age and isotopic composition of those components. This is critically important for clarifying the record of the formation of enriched crust and development of the depleted mantle in Earth’s early history.

The timing of deformation and deposition within syntectonic basins provides critical information for understanding the evolution of strain in mountain belts. In the U.S. Cordillera, contractional deformation was partitioned between the Sevier thrust belt in Utah and several structural provinces in the hinterland in Nevada. One hinterland province, the Central Nevada thrust belt (CNTB), accommodated up to ∼15 km of shortening; however, in most places, this deformation can only be bracketed between Permian and Eocene. Cretaceous deposits of the Newark Canyon Formation (NCF), which are sparsely exposed along the length of the CNTB, offer the opportunity to constrain deformation timing. Here, we present mapping and U-Pb zircon geochronology from the NCF in the Diamond Mountains, which demonstrate deposition of the NCF during proximal CNTB deformation. Deposition of the basal NCF member was under way no earlier than ca. 114 Ma, a tuff in the middle part of the section was deposited at ca. 103 Ma, and the youngest member was deposited no earlier than ca. 99 Ma. Intraformational angular unconformities and abrupt along- and across-strike thickness changes indicate that NCF deposition was related to growth of an east-vergent fault-propagation fold. Clast compositions define unroofing of upper Paleozoic sedimentary rocks, which we interpret as the progressive erosion of an anticline ∼10 km to the west. CNTB deformation was contemporaneous with shortening in the Sevier thrust belt, which defines middle Cretaceous strain partitioning between frontal and interior components of the Cordillera. Strain partitioning may have been promoted by renewed underthrusting during a period of high-flux magmatism.

Studies of the high-pressure (HP) As Sifah eclogites in the NE Saih Hatat window, Oman, have used different combinations of radiometric dating results (Ar/Ar, Sm-Nd vs. U-Pb, Rb-Sr) to interpret disparate tectonic models for the timing, geometry, and cause of continental subduction – including its relationship to the Samail Ophiolite. To determine the timing of continental subduction, we coupled petrochronological analyses of major (garnet) and accessory phases (zircon, rutile) from the highest-grade metamorphic rocks (As Sifah eclogites) in the Saih Hatat. Early Permian (283.8±0.7 Ma) tuffaceous zircon cores are consistent with earlier interpretations that the As Sifah rocks were sourced from a distal portion of the Arabian continental margin. Data from a range of bulk compositions and metamorphic assemblages consistently suggest a single metamorphic event, with garnet growth from ∼81–77 Ma – though with slight, consistent offsets in the timing of metamorphic (re)crystallization between different lithologies. These new data confirm previous interpretations for continental HP metamorphism in a single NE-dipping subduction zone beneath the obducted Samail Ophiolite; there is no robust evidence for a ∼110 Ma event or for SW-dipping subduction beneath the Arabian plate. Combined with other constraints, our data suggest that the As Sifah unit was subducted at rates similar to other small continental HP bodies, followed by two stages of cooling and exhumation – likely associated with the dragging to mantle depths by a mafic root, an initial phase of rapid exhumation from mantle depths, and a lengthy residence (≤40 My) in the lower to middle crust.

International audience; Anatectic migmatites flow in response to tectonic or gravitational forces: from micro-scale deformation of crystals and associated displacement of melt to regional flow of partiallymolten crust. Significant parameters for understanding migmatite systems include driving forces, duration, rate, magnitude, and trajectory of flow. The evolution of fabrics derived from migmatite, in addition to P-T-time data, informs the geometry and kinematics of crustal flow; e.g., during formation of migmatite domes. Geologic and numerical modeling studies indicate that partially-molten crust may flow rapidly (cms/yr), particularly in extension/transtension systems in which upper-crust normal faulting solicits low-viscosity deep (near-Moho) material. The result of rapid flow from deep to shallow is formation of hot migmatite bodies that record low-P (LP)/ high-T (HT) conditions of emplacement, which may be tens of km less than the source depth.In eclogite inclusions in the Montagne Noire migmatite dome, French Massif Central (FMC), mineral and rock composition (bulk, mineral major/trace elements; O isotopes in zircon and garnet; bulk Sm-Nd, Lu-Hf, Rb-Sr), textures (inclusions, symplectites), extent of zircon recrystallization, and deformation fabrics (flattening/constriction at eclogite conditions) allow reconstruction of the flow paths of eclogite and therefore the provenance and flow history of the host migmatite, which no longer records this information. For example, eclogite in the core of the dome is isotopically similar to eclogite and other HP mafic rocks exposed in other FMC domes, whereas eclogite at the dome-margin is distinct. This is consistent with interpretation of zircon textures (highly recrystallized in dome-core eclogite; minimal HP zircon growth in dome-margin) and other observations consistent with the interpretation that dome-core eclogite experienced protracted HT metamorphism and deformation during flow in the deep crust, whereas dome-margin eclogite was more locally-sourced. Variation of paths in different domains of a migmatite complex shows how partially-molten crust flows laterally and vertically on a regional scale and informs early (HP-HT) and later (LP-HT) geotherms during orogenic evolution.

Polyphase structural mapping and mineral age dating across the Salmon River suture zone in west-central Idaho (Riggins region; ∼45°30′N, ∼117°W–116°W) support a late Mesozoic history of penetrative deformation, dynamothermal metamorphism, and intermittent magmatism in response to right-oblique oceanic-continental plate convergence (Farallon–North America). High-strain linear-planar tectonite fabrics are recorded along an unbroken ∼48 km west-to-east transect extending from the Snake River (Wallowa intra-oceanic arc terrane; eastern Blue Mountains Province) over the northern Seven Devils Mountains into the lower Salmon River Canyon (ancestral North America; western Laurentia). Given the temporally overlapping nature (ca. 145–90 Ma) of east-west contraction in the Sevier fold-and-thrust belt (northern Utah–southeast Idaho–southwest Montana segment), we propose that long-term terrane accretion and margin-parallel northward translation in the Cordilleran hinterland (∼41°N–46°N latitude; modern coordinates) drove mid- to upper-crustal shortening >250 km eastward into the foreland region (∼115°W–113°W). During accretion and translation, the progressive transfer of arc assemblages from subducting (Farallon) to structurally overriding (North American) plates was accommodated by displacement along a shallow westward-dipping basal décollement system underlying the Cordilleran orogen. In this context, large-magnitude horizontal shortening of passive continental margin strata was balanced by the addition of buoyant oceanic crust—late Paleozoic to Mesozoic Blue Mountains Province—to the leading edge of western Laurentia. Consistent with orogenic float modeling (mass conservation, balance, and displacement compatibility), diffuse dextral-transpressional deformation across the accretionary boundary (Salmon River suture: Cordilleran hinterland) was kinematically linked to eastward-propagating structures on the continental interior (Sevier thrust belt; Cordilleran foreland). As an alternative to noncollisional convergent margin orogenesis, we propose a collision-related tectonic origin and contractional evolution for central portions of the Sevier belt. Our timing of terrane accretion supports correlation of the Wallowa terrane with Wrangellia (composite arc/plateau assemblage) and implies diachronous south-to-north suturing and basin closure between Idaho and Alaska.

New metamorphic pressure-temperature (P-T) paths and Lu-Hf garnet ages reveal a temporal correlation between Middle to Late Jurassic retroarc underthrusting and arc magmatism in southwestern North America. P-T paths were determined for 12 garnet porphyroblasts from six samples from the Chloride Cliff area of the Funeral Mountains in southeastern California. The composite path shows a pressure increase from 4.2 to 6.5 kbar as temperature increased from 550 to 575 °C, followed by a pressure decrease to 5.1 kbar during a further increase in temperature to 590 °C. Lu-Hf garnet ages from a pelitic schist (167.3 ± 0.7 Ma) and a garnet amphibolite (165.1 ± 9.2 Ma) place these P-T paths in the Middle Jurassic. We interpret the near-isothermal pressure increase portion of the P-T path to have developed during thrust-related burial, similar to lower grade rocks at Indian Pass, 8 km to the southeast, where garnet P-T paths show a pressure increase dated by the Lu-Hf method at 158.2 ± 2.6 Ma. We interpret the pressure decrease portion of the composite P-T path from the Chloride Cliff area to reflect exhumation contemporaneous with cooling in the Indian Pass area documented from muscovite 40Ar/39Ar step-heating ages of 152.6 ± 1.4 and 146 ± 1.1 Ma. The conditions and timing of metamorphism determined for the Indian Pass and Chloride Cliff areas, and isogradic surfaces that cut across stratigraphy, support the interpretation that the strata were dipping moderately NW during metamorphism, parallel to the thrust ramp that buried the rocks. Burial likely resulted from top-SE motion along the Funeral thrust, which was later reactivated as a low-angle normal fault with opposite motion to become the currently exposed Boundary Canyon detachment that was responsible for Miocene and possibly older exhumation. The part of the burial history captured by garnet growth occurred ∼6 m.y. before the 161 Ma peak of high-flux magmatism in the arc. Burial was contemporaneous with metamorphic ages from the western Sierra Nevada metamorphic belt, with the possible timing of accretion of arc terranes in northern California, and with the initiation of Franciscan subduction. Burial ages are also similar in timing with generally E-W crustal shortening in the retroarc that produced the East Sierra thrust system, the Luning-Fencemaker fold and thrust belt, the possible early history of the Central Nevada thrust belt, and the western thrusts of the southern Sevier belt. The timing of tectonic burial documented in this study and of high-flux magmatism in the arc supports the interpretation that the development of a coherent arc-trench system in the Early Jurassic resulted in the underthrusting of melt-fertile material beneath the arc along west- to northwest-dipping faults such as the Funeral thrust in the Jurassic, which penetrated the basement to the west as well as the roots of the magmatic arc, leading to increased magmatism.

The ratio of the halogens F and Cl in apatite is highly sensitive to changes in the composition of an evolving silicate melt or aqueous fluid. For this reason apatite chemistry is widely used as a monitor of halogen behavior in magmatic-hydrothermal systems. Apatite is an ubiquitous mineral in iron oxide – apatite (IOA) mineral deposits, where P and volatiles such as F, Cl, H2O and S play a major role in ore genesis. In this study, we present a combination of textural, micro-analytical and isotopic data for apatite from three Andean IOA deposits (Carmen, Fresia and Mariela) of Early Cretaceous age from the Coastal Cordillera of northern Chile. Apatite textures and compositions show evidence of post-crystallization alteration. Apatite is predominantly zoned with respect to Cl and F, showing a decoupled geochemical behavior between these two elements. Overall, four types of apatite or domains were identified in the analyzed grains based on the XCl-apatite/XF-apatite and XCl-apatite/XOH-apatite ratios determined using the atomic proportions of F, Cl and OH. F-apatite is characterized by a XCl-apatite/XF-apatite 25 and a XCl-apatite/XOH-apatite > 4.5 and up to 3000; Cl-OH-F-apatite has a XCl-apatite/XF-apatite = 0.15–8 and a XCl-apatite/XOH-apatite = 0.25–5.45; and Cl-OH-apatite shows a XCl-apatite/XF-apatite = 8–75 and a XCl-apatite/XOH-apatite = 0.5–3. Carmen apatite is mostly F-apatite, but shows Cl-OH compositions restricted to rims and fractures, whereas apatite from Mariela is dominantly Cl-apatite and Cl-OH-apatite. Apatite from Fresia have variable compositions between Cl-OH-F, Cl-OH- and Cl-apatite, where Cl- and Cl-OH-apatites are characterized by an enrichment of S, Na, Sr and Fe relative to F-apatite. Most notably, S and Na correlate with Cl. In Carmen and Fresia, Cl-OH-apatite is slightly depleted in LREEs, Th and U, a finding consistent with micro-textural evidence of metasomatic processes including coupled dissolution-reprecipitation and formation of secondary monazite and xenotime inclusions. In contrast, apatite from Mariela exhibits no depletion in LREEs and displays a homogeneous distribution of Th and U between the different apatite types, with no monazite and xenotime inclusions. The textural types of apatite identified in this study, coupled with the halogen and trace element composition of apatite, are consistent with modification of primary, F-apatite by interaction with hydrothermal fluids, which led to the formation of Cl-OH-apatite and Cl-apatite. In addition, the initial 87Sr/86Sr ratios and eNd values of apatite (0.7038 to 0.7050 and −0.3 to +6.5, respectively), calculated considering a 130 Ma age, are consistent with a magmatic origin for the primary F-apatite with minimal or no crustal contribution. Thus, textural, geochemical, and isotopic results of apatite support a magmatic-hydrothermal origin for these Andean IOA deposits with variable degrees of metasomatic overprint as evidenced by the formation of Cl-OH and Cl-apatite.

Chronological data from hard structures have been instrumental in reconstructing information about the past across numerous disciplines. Isotopic and trace elemental chronologies from the depositional layers of speleothems, corals, bivalve shells, fish otoliths and other structures are routinely used to reconstruct climate, growth, temperature, geological, archeological and migratory histories. Recent in situ analytical advances have revolutionized the use of these structures. This is particularly true of fish, in which detailed origin, life-history, and migration history can be reconstructed from their otoliths. Specifically, improvements in laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) have allowed increases in temporal resolution, precision, and sample throughput. Many studies now combine multiple chemical and isotopic tracers, taking advantage of multivariate statistical methods and multiple trace-elements and isotope systems to glean further information from individual samples. This paper describes a novel laser ablation split-stream (LASS) methodology which allows simultaneous collection of the Sr isotope composition (87Sr/86Sr) and trace-elemental data from chronologically deposited carbonate samples. The study investigates the accuracy and precision of varying laser spot sizes on a marine shell standard and fish otoliths using LASS and presents a comparison to traditional “single stream methods” using pre-existing otolith data on the same samples. Our results indicate that LASS techniques can be used to provide accurate and precise data at the same laser spot sizes as previous otolith studies, thereby doubling analytical throughput, while also providing improved spatially and temporally-matched data reduction using newly developed features for the Iolite data reduction platform.

One of the fundamental tenets of geochemistry is that the Earth’s crust has been extracted from the mantle creating a crustal reservoir enriched—and a mantle depleted—in incompatible elements. The Hf-Nd isotope record has long been used to help understand the timing of this process. Increasingly, however, it has become apparent that these two isotope records do not agree for Earth’s oldest rocks. Hf isotopes of zircon from juvenile, nominally mantle-derived rocks throughout the Eoarchean have broadly chondritic initial isotope compositions and indicate large-scale development of the depleted mantle reservoir started no earlier than ~ 3.8 Ga. In contrast, the long-lived Sm-Nd isotope record shows large variation in Nd isotope compositions. Most notably, Paleo- and Eoarchean terranes with chondritic initial Hf isotope compositions have significantly radiogenic Nd isotope compositions indicative of the development of a widespread depleted mantle reservoir very early in Earth’s history which, by extension, requires extraction of significant volumes of enriched crust. These two isotope systems, therefore, indicate two fundamentally different scenarios for the early Earth and has been called the Hf-Nd paradox. However, an important unresolved question remains: Do these records represent primary isotopic signatures or have they been altered by subsequent thermomagmatic processes? We have been able to provide clarity in the Hf isotope record by analyzing zircon from Eo- and Paleoarchean magmatic rocks by determining its U-Pb crystallization age and linking this to its corresponding Hf isotope composition. We can do this unambiguously—even in complex polymetamorphic gneisses—with the laser ablation split stream (LASS) technique whereby we determine U-Pb age and Hf isotope composition simultaneously in a single zircon volume. The existing Nd isotope data, in contrast, are all from bulk-rock analyses. These analyses are potentially problematic in old, polymetamorphic rocks because of the inability to link the measured isotopic composition to a specific age. In addition, the REE budget in these rocks is hosted by accessory phases that can be easily mobilized during later metamorphic and magmatic events. We can now use the LASS approach in REE rich phases (e.g., monazite, titanite, allanite, apatite) to determine U-Pb age and Nd isotope composition in a single analytical volume. New Nd isotope data from the Acasta Gneiss Complex (Fisher et al., EPSL, 2020) show that REE-rich accessory phases are not in isotopic equilibrium with their bulk rock compositions and clearly demonstrate mobilization after initial magmatic crystallization. This post-magmatic open-system behavior may well explain the disagreement in the Hf-Nd isotope record in high-grade polymetamorphic terranes like Acasta. In less complicated, lower-grade rocks, such as in the Pilbara terrane, these REE-rich phases yield consistent U-Pb and Sm-Nd age and isotope compositions indicating that the Nd isotope system in these rocks has remained closed since formation. Of particular note, in the Pilbara samples, the Hf and Nd isotope systems have consistent, broadly chondritic, initial Hf and Nd isotope compositions. In these less-complicated samples, where the Sm-Nd isotope system has remained closed, the Hf and Nd isotope systems agree and there is no Hf-Nd paradox.

[Extract]: Granite generation is a fundamental process for the growth and evolution of Earth’s continental crust. I-type granitic rocks, nominally derived from infracrustal sources, are the most common granite type and are voluminously emplaced in convergent margin settings. A puzzling feature is that many I-type granites show isotopic evidence for reworking of older supracrustal material, in conflict with the I-type designation. How the supracrustal component was incorporated by I-type magmas is a matter of deduction, particularly given difficulties in recognizing the putative infracrustal source region in the exposed geology. We report a case study of I-type granitic magma generation by hybridization between metasedimentary-derived partial melt and intercalated mafic granulite units during extraction of silicic magma from the lower crust in the Hidaka Metamorphic Belt (HMB), Japan (Hammerli et al., 2018). Isotopic data (Nd, Hf, O) obtained by microanalysis of accessory minerals in former melt networks (leucosomes) suggest that hybridization operates on a (sub-) grain scale, where repeated injections of externally derived melt attempt to approach local equilibrium with the host mafic granulites during transfer through complex melt pathways (see our figure 3; and also Hasalova et al. [2011] and references therein).

The construction of mountain belts by continent-continent collision is a fundamental phase of the supercontinent cycle and an inevitable consequence of subduction-driven plate tectonics on Earth. Yet, quantitative estimation of the rate and duration of prograde orogenic metamorphism in high-grade collisional belts has proven elusive to constrain. Differences in the chemical partitioning and diffusive behavior of elements associated with the 147Sm-143Nd and 176Lu-176Hf isotopic systems in metamorphic garnet can provide valuable insights on the time scales over which this mineral grows and cools in regionally metamorphosed terranes. Here, we present new Sm-Nd and Lu-Hf isotopic results from multiple garnet and whole-rock aliquots from a granulite-grade metasedimentary rock from the Proterozoic Putumayo Orogen of South America. Using a combination of internal isochron dating by these two decay systems, combined with apatite U-Pb dating and pressure-temperature-time (P-T-t) constraints from mineral equilibria and chemical diffusion geospeedometry, we provide first-order insights into the time scales of a Proterozoic continent-continent collisional metamorphic event in Amazonia associated with its incorporation to the Supercontinent Rodinia. Our results indicate that these metasediments attained peak granulite-grade metamorphic conditions of nearly 0.62 GPa and 680 °C at approximately 1035 Ma, and started to cool slowly, presumably during exhumation, at a rate of ∼5 K/Myr. Simple diffusion scaling relationships and numerical modeling of Sm-Nd and Lu-Hf in garnet indicate that, while Sm and Nd were both fully re-equilibrated at peak temperatures and therefore date cooling, Lu and Hf underwent diffusive decoupling due to incomplete Hf re-equilibration. By modeling the impact of this differential re-equilibration in the development of the Sm-Nd and Lu-Hf isochrons, we constraint the duration of the prograde metamorphic path and argue that garnet growth in this sample took place over millions of years—approximately 25 Myr according to our preferred interpretation. The determined apatite U-Pb date vs. grain-size relationship cannot be reconciled with the thermal history described above when modeled using the existing experimental diffusion data, and instead suggest that much slower Pb diffusivities in this mineral compared to those predicted by the existing experimental data may have inhibited its resetting. We note that the inferred temperature-time history for the metamorphic loop of this segment of the Putumayo basement is comparable to that proposed for the metamorphism and exhumation of metasediments in the India-Asia collision zone, suggesting that similar tectonic processes and rates likely operated during the Mesoproterozoic Putumayo orogeny.

The upper Columbia River and associated valley systems are highly contaminated with metal wastes from nearby smelting operations in Trail, British Columbia, Canada (Teck smelter), and to a lesser extent, Northport, Washington, USA (Le Roi smelter). Previous studies have investigated depositional patterns of airborne emissions from these smelters, and documented the Teck smelter as the primary metal contamination source. However, there is limited research directed at whether these contaminants are bioavailable to aquatic organisms. This study investigates whether smelter derived contaminants are bioavailable to freshwater zooplankton. Trace metal (Zn, Cd, As, Sb, Pb and Hg) concentrations and Pb isotope compositions of zooplankton and sediment were measured in lakes ranging from 17 to 144 km downwind of the Teck smelter. Pb isotopic compositions of historic ores used by both smelters are uniquely less radiogenic than local geologic formations, so when zooplankton assimilate substantial amounts of smelter derived metals their compositions deviate from local baseline compositions toward ore compositions. Sediment metal concentrations and Pb isotope compositions in sediment follow significant (p 0.001) negative exponential and sigmoidal patterns, respectively, as distance from the Teck smelting operation increases. Zooplankton As, Cd, and Sb contents were related to distance from the Teck smelter (p 0.05), and zooplankton Pb isotope compositions suggest As, Cd, Sb and Pb from historic and current smelter emissions are biologically available to zooplankton. Zooplankton from lakes within 86 km of the Teck facility display isotopic evidence that legacy ore pollution is biologically available for assimilation. However, without water column data our study is unable to determine if legacy contaminants are remobilized from lake sediments, or erosional pathways from the watershed.

Southern West Greenland contains some of the best-studied and best-preserved magmatic Eoarchean rocks on Earth, and these provide an excellent vantage point from which to view long-standing questions regarding the growth of the earliest continental crust. In order to address the questions surrounding early crustal growth and complementary mantle depletion, we present Laser Ablation Split Stream (LASS) analyses of the U–Pb and Hf isotope compositions of zircon from eleven samples of the least-altered meta-igneous rocks from the Itsaq (Amitsoq) Gneisses of the Isukasia and Nuuk regions of southern West Greenland. This analytical technique allows a less ambiguous approach to determining the age and Hf isotope composition of complicated zircon. Results corroborate previous findings that Eoarchean zircon from the Itsaq Gneiss (∼3.85 Ga to ∼3.63 Ga) were derived from a broadly chondritic source. In contrast to the Sm–Nd whole rock isotope record for southern West Greenland, the zircon Lu–Hf isotope record provides no evidence for early mantle depletion, nor does it suggest the presence of crust older than ∼3.85 Ga in Greenland. Utilizing LASS U–Pb and Hf data from the Greenland zircons studied here, we demonstrate the importance of focusing on the magmatic (rather than detrital) zircon record to more confidently understand early crustal growth and mantle depletion. We compare the Greenland Hf isotope data with other Eoarchean magmatic complexes such as the Acasta Gneiss Complex, Nuvvuagittuq greenstone belt, and the gneissic complexes of southern Africa, and all lack zircons with suprachondritic Hf isotope compositions. In total, these data suggest only a very modest volume of crust was produced during (or survived from) the Hadean and earliest Eoarchean. There remains no record of planet-scale early Earth mantle depletion in the Hf isotope record prior to 3.8 Ga.

We present coupled Lu–Hf and Sm–Nd dates from multiple growth zones within a single large garnet porphyroblast in an eclogite from the Huwan shear zone in the Hong'an orogen, central China. The eclogite sample contains large garnet porphyroblasts up to several centimetres in diameter and a second major population of smaller, millimetre-sized garnet grains. Elemental compositions and mineral inclusions in garnet suggest that the large garnet crystal formed during an early episode of metamorphism, with the abundant small-sized garnets in the matrix growing during a late metamorphic period contemporaneous with the overgrowth of the rim of the large garnet crystal. Ten coupled Lu–Hf and Sm–Nd dates determined from five micro-sawed garnet sections of a ~ 2 cm garnet coherently decrease from core to rim. Lu–Hf dates from each section are consistently older than the corresponding Sm–Nd dates. Petrographic and chemical observations of the eclogite show that three generations of garnet growth occurred, with the latter two generations coincident with growth of the rim of the large garnet, and that of the smaller matrix garnets. Five micro-sawed growth zones were analysed for Lu–Hf and Sm–Nd isotopes. The calculated Lu–Hf dates from the mega garnet record garnet growth initiation at about 397 Ma, during prograde to peak metamorphic conditions of the first orogenic episode, lasting about 11 Ma. Sm–Nd garnet dates are systematically younger than corresponding Lu–Hf dates, with the Sm–Nd garnet rim date > 25 Ma younger than the Lu–Hf date. We suggest that the Sm–Nd dates represent a combination of 1) growth and simultaneous diffusion at elevated temperatures during the first metamorphic episode, 2) diffusive age resetting as a result of continued elevated temperatures during residence at mid-crustal depths and/or slow cooling between the two orogenic episodes, and 3) mixing of growth generations. U–Pb dates on three zircon inclusions from the garnet separates range from 386 to 351 Ma, possibly suggesting zircon crystallization during exhumation and cooling. This study highlights that coupled microsampling Lu–Hf and Sm–Nd chronology on a single garnet (down to ~ cm in diameter) can help decipher the timescales of burial and heating (for Lu–Hf) and cooling from elevated temperatures (for Sm–Nd) from a single orogenic cycle, and that zoned Lu–Hf and Sm–Nd garnet can be utilized as a geo (thermo) -chronometer. Lastly, these Lu–Hf garnet, Sm–Nd garnet and U–Pb zircon dates confirm an early high-pressure metamorphic episode during the Devonian before the final convergence of the South China and North China Blocks in the Triassic.

Initial Hf and Nd isotope compositions of Earth's oldest rocks provide essential information on the differentiation of the Earth into enriched crustal and depleted mantle reservoirs in its early history. The majority of Eo-Paleoarchean rocks worldwide, however, have isotope compositions that appear to be decoupled: initial Hf isotope compositions, determined on zircon, are broadly chondritic with little variation; initial Nd isotopes on bulk rocks, in contrast are highly variable with both supra- and sub-chondritic compositions. Most of these studies are from polymetamorphic terranes where the potential for disturbance of the isotope system is high. This is particularly true for the Sm-Nd system where more easily altered REE-rich accessory phases are the major repositories for these elements. In order to better understand crust-mantle evolution during the Archean—and to address the issue of Hf and Nd isotope decoupling—we examine a suite of well-preserved Paleoarchean granites from the Pilbara Craton. Our approach integrates the initial Hf isotope composition and U-Pb ages of zircon, the initial Nd isotope compositions of titanite and apatite, and U-Pb ages of titanite by laser ablation split stream (LASS) analysis. The zircon and titanite U-Pb data yield crystallization ages of 3.47 to 3.28 Ga, in good agreement with the combined apatite-titanite-WR Sm-Nd isochron ages of each sample, demonstrating that both the U-Pb and Sm-Nd systems have not been modified since igneous crystallization. The initial Hf isotope compositions of zircon from all samples are broadly chondritic with eHf(i) values of −0.3 to +0.8, in agreement with the bulk-rock Hf. The initial Nd isotope compositions of the titanite and apatite are also broadly chondritic (eNd(i) titanite, −1.0 – +2.0; apatite, −0.6 – +0.9) and agree with the Nd isotope composition of the bulk-rock (eNd(i) = +0.2 to +1.2) and the initial 143Nd/144Nd ratios determined from the titanite-apatite-WR isochrons (eNd(i) −0.9 to +1.3). From these data, we make two fundamental observations. First, the granites in this study were derived from a source that was chondritic with respect to both Hf and Nd isotopes from 3.47 to 3.28 Ga; neither system supports the presence of either a strongly depleted mantle or enriched crustal source. Second, the Lu-Hf and Sm-Nd isotope systems in the Pilbara samples are in full agreement. This stands in stark contrast to the record of rocks from Eo-Paleoarchean terranes of higher metamorphic grade, where the Hf and Nd isotope compositions have been “decoupled”. This further underscores the importance of recognizing potential effects of high-grade metamorphism on the Sm-Nd bulk-rock record. The integrated age-isotope approach taken here illustrates a way to assess the integrity of bulk-rock Nd isotope data through examination of the Sm-Nd isotope systematics of the LREE-rich accessory minerals in rocks.

The chemistry of the major reservoirs in the silicate Earth reflects a long history of differentiation into- and interaction between- incompatible element enriched crust and depleted mantle. Evidence from 142Nd variability in ancient mantle-derived rocks imply such differentiation began very early in Earth's history, while the short-lived 146Sm (half-life 103 Ma) was extant, but the size and composition of primitive reservoirs in the Hadean and early Archean is debated. Therefore, pivotal to the study of Earth evolution is the recognition of well-preserved ancient magmatic rocks that can provide robust constraints on source compositions to test geodynamic models. We present the first combined petrological, geochemical and isotope (146-147Sm-142-143Nd and 176Lu–176Hf) study of basaltic rocks from the East Pilbara Terrane, Pilbara Craton, Western Australia. The basaltic pillow lavas, from the ca. 3470 Ma Mount Ada Basalt of the Pilbara Supergroup, include quartz-phyric quartz-normative tholeiites, plagioclase-phyric olivine-normative tholeiites and clinopyroxene spinifex textured olivine-normative tholeiites and include samples that classify as siliceous high Mg basalts. Their major and trace-element compositions suggest that the basalts are high-degree partial melts (20–30%) produced at shallow mantle depths of 0.5–1 GPa, which is consistent with formation of both quartz-normative and olivine-normative parental magmas. Isotopic data from this sample suite defines Sm–Nd and Lu–Hf isochrons with dates of 3484 ± 113 Ma and 3463 ± 50 Ma, respectively. The samples yield coupled initial eNd = +1.1 and eHf = +2.1, and μ142Nd values indistinguishable from the modern mantle. The uniform initial eNd and eHf in samples from this study and other Mount Ada Basalt basaltic samples across four greenstone belts indicates that this portion of the Paleoarchean mantle was well mixed at the scale of melt generation and evolved with slightly suprachondritic Sm/Nd and Lu/Hf. Absence of 142Nd variations suggests the portion of the Paleoarchean Pilbara mantle sampled by these basalts retains no signature of Hadean Sm/Nd fractionation.

Periodic loss of the lower lithosphere into the convecting mantle due to gravitational instability is postulated to be a major mechanism for lithosphere recycling in orogenic zones, but unequivocal petrologic evidence of this process is elusive. The Granatifera Tuff, located in the Mercaderes–Rio Mayo area of the southern Colombian Andes, contains a wide variety of crustal and mantle xenoliths. Here we focus on the thermobarometry and Lu–Hf isotope systematics of crustal garnet clinopyroxenite xenoliths, the results of which offer the first evidence of recent, and likely active, crustal foundering in the Northern Volcanic Zone of the Andean arc. We find that most of these xenoliths equilibrated between 60–80 km depths, ∼7–27 km below the seismically determined Moho in this region, and that at least one crustal garnet clinopyroxenite re-equilibrated at depths exceeding 95 km. A second garnet clinopyroxenite equilibrated at ∼150 km depths, and is either foundered lithospheric material or the product of reaction between peridotite and a mobile component (either silicic melt or fluids) at >4 GPa. All of the investigated garnet clinopyroxenites are negatively buoyant relative to the upper mantle asthenosphere. The presence of minor amounts of secondary amphibole and orthopyroxene, coupled with the lack of major-element retrograde zonation in primary phases within these xenoliths, indicates that these rocks were rapidly transported to, and briefly resided at, shallow depths before eruption. Lu–Hf ages from two garnet clinopyroxenites and one garnet–clinopyroxene hornblendite are

We report Hf isotope data for 3467 Ma igneous zircons from the Owens Gully Diorite of the Pilbara Craton. These zircons, designated OG1 or OGC, have simple, well-defined U-Pb age systematics. Measurements obtained by single crystal dissolution yield a moderate range in 176Hf/177Hf (mean 0.280633 ± 34) and 176Lu/177Hf (mean 0.00119) that translate into initial (3467 Ma) 176Hf/177Hf values that are equivalent within measurement uncertainty (mean 0.280554 ± 7, eHf = +0.6 ± 0.2, 2s). Laser ablation analysis yielded clustered initial 176Hf/177Hf ratios (mean 0.280560 ± 20, 2s) indistinguishable from those obtained by solution analysis. The antiquity and moderately high 176Lu/177Hf of these zircons result in significant corrections for radiogenic ingrowth of 176Hf (to 6 eHf units). Depth profiling by concurrent Pb-Hf isotope determination reveals trends to lower 207Pb/206Pb in several crystals, although these are not accompanied by shifts in 176Hf/177Hf. We conclude that zircons from the Owens Gully Diorite are homogeneous for Hf isotopes at the scale of sampling and within the uncertainty limits of the measurement techniques employed. Zircon OG1/OGC would appear to be an ideal quality monitor for laser ablation Hf isotope determination in ancient zircons, and for laser ablation techniques where Hf isotope and age information are determined concurrently or simultaneously.

Rock clasts entrained in glacial deposits sourced from the continental interior of Antarctica provide an innovative means to determine the age and composition of ice-covered crust. Zircon U-Pb ages from a suite of granitoid clasts collected in glacial catchments draining central East Antarctica through the Transantarctic Mountains show that crust in this region was formed by a series of magmatic events at ∼2.01, 1.88–1.85, ∼1.79, ∼1.57, 1.50–1.41, and 1.20–1.06 Ga. The dominant granitoid populations are ca. 1.85, 1.45 and 1.20–1.06 Ga. None of these igneous ages are known from limited outcrop in the region. In addition to defining a previously unrecognized geologic history, zircon O and Hf isotopic compositions from this suite have: (1) mantle-like δ18O signatures (4.0–4.5‰) and near-chondritic Hf-isotope compositions (eHf ∼ +1.5) for granitoids of ∼2.0 Ga age; (2) mostly crustal δ18O (6.0–8.5‰) and variable Hf-isotope compositions (eHf = −6 to +5) in rocks with ages of ∼1.88–1.85, ∼1.79 and ∼1.57 Ga, in which the ∼1.88–1.79 Ga granitoids require involvement of older crust; (3) mostly juvenile isotopic signatures with low, mantle-like δ18O (∼4–5‰) and radiogenic Hf-isotope signatures (eHf = +6 to +10) in rocks of 1.50–1.41 Ga age, with some showing crustal sources or evidence of alteration; and (4) mixed crustal and mantle δ18O signatures (6.0–7.5‰) and radiogenic Hf isotopes (eHf = +3 to +4) in rocks of ∼1.2 Ga age. Together, these age and isotopic data indicate the presence in cratonic East Antarctica of a large, composite igneous province that formed through a punctuated sequence of relatively juvenile Proterozoic magmatic events. Further, they provide direct support for geological correlation of crust in East Antarctica with both the Gawler Craton of present-day Australia and Proterozoic provinces in western Laurentia. Prominent clast ages of ∼2.0, 1.85, 1.57 and 1.45 Ga, together with sediment source linkages, provide evidence for the temporal and spatial association of these cratonic elements in the Columbia supercontinent. Abundant ∼1.2–1.1 Ga igneous and metamorphic clasts may sample crust underlying the Gamburtsev Subglacial Mountains, indicating the presence of a Mesoproterozoic orogenic belt in the interior of East Antarctica that formed during final assembly of Rodinia.

The Mora Formation (Narcea Group) is one of the oldest Precambrian supracrustal successions in northern Spain. Here, we use U–Pb and in situ Hf isotope analysis on detrital zircon to determine its age and provenance. The youngest U–Pb dates constrain the maximum depositional age of the Mora Formation at 565 ± 11 Ma. Results indicate: (1) a dominant Ediacaran zircon population (33%; 565–633 Ma, Cadomian) within a spectrum of Neoproterozoic ages (40%; 636–996 Ma); and (2) smaller Mesoproterozoic (5%; 1004–1240 Ma), Palaeoproterozoic (11%; 1890–2476 Ma) and Archean (11%; 2519–3550 Ma) populations. Results here do not point to one specific cratonic source area; instead, detritus may have been derived from the West African craton and Amazonia, or even the concealed Iberian basement. The lack of 1.3–1.8 Ga grains suggests exclusion of the Sahara Craton as a major source, but this is not certain. This mixed composition favours a complex source history with reworking of detritus across terrane/craton boundaries. Hafnium isotope compositions indicate a range of crustal and juvenile sources, with initial eHf values between −15.8 and 11.1, and Hf model ages from 0.8 to 3.7 Ga. For Neoproterozoic zircons (80%), juvenile components (eHf(i) ~+10) may be related to Rodinia fragmentation and the onset of an active margin setting leading to the Cadomian orogeny. Palaeoproterozoic to Paleoarchean grains (20%) all have negative eHf values and Meso- to Eoarchean Hf model ages. This indicates an early (Archean) sialic crustal component for northwestern Gondwana.

U-Pb dating of detrital zircons was undertaken in the matrix component of the Swartleikrans Bed (Bloupoort Formation, Gifberg Group), located in the Vredendal Outlier of western South Africa. Results from this study indicate detrital zircons were derived from both local and regional (or distal) sources. The deposit is reportedly Neoproterozoic in age, but convincing age constraints are absent. These meta-diamictitic rocks were interpreted as glacial and are correlated with other diamictites in SW Gondwana, in the wake of the snowball Earth hypothesis. While sedimentological data is interpreted as glacial, geochemical proxies (Mo, Cr, U/Th, Ce*, V) point to an oxygenated depositional environment. Thus, if the deposit was related to a snowball Earth glacial event, then deposition must have been distal to the ice sheets. Nearly 50% of the dated detrital zircons (n = 68) have crystallization ages that cannot be correlated with known sources in the locally exposed basement. These predominantly Mesoproterozoic grains were either derived from distal sources or are reworked from unknown, now eroded, sedimentary rocks. Only 14% of all concordant zircon have an Early Mesoproterozoic to Late Palaeoproterozoic age, and a single grain relates to an older Paleoproterozoic event. Results are similar to other glacial deposits in the region (further north, in Namibia). In total, sedimentological observations and detrital zircon age populations point to a large catchment area, characteristic of glacial events of regional significance. However, direct regional correlation is still not possible using maximum depositional ages indicated by detrital zircons.

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.