Wyoming Province Research Articles

In situ Serpentinization and Hydrous Fluid Metasomatism in Spinel Dunite Xenoliths from the Bearpaw Mountains, Montana, USA

The Bearpaw Mountains, Montana, USA, form part of an Eocene high-K magmatic province on the northern margin of the Wyoming craton. Spinel dunite xenoliths from the Bearpaw Mountains contain evidence of interaction with a hydrous fluid with a subduction-related signature. These xenoliths fall into three categories. In Category 1, the rock has been subjected to moderate in situ serpentinization with antigorite appearing along olivine grain boundaries and forming reaction pockets through the breakdown of pyroxene, amphibole and spinel. In Category 2, newly formed clinopyroxene, amphibole, mica and calcite overprint and partially replace the antigorite in reaction pockets. In Category 3, metasomatic fibrous orthopyroxene appears at the expense of olivine in veins and in reaction pockets and sometimes forms spherulitic or radiating aggregates. The xenoliths have high B, Cl and H2O contents, and also whole-rock and mineral trace element patterns characteristic of subduction zones. It is suggested that the source of the hydrous fluid may have been part of the subducting slab of the Farallon plate, which collided with the craton keel ~55 Myr ago. Spinel dunite xenoliths may have originated from the topmost part of the mantle lithosphere below the abnormally thick crust of the Wyoming craton. Hydrous fluids rising through the mantle lithosphere, but unable to penetrate the thick mafic-underplated crust, may have accumulated beneath the Moho, causing this topmost part of the mantle to become hydrated and initiating serpentinization. Subsequent heating halted serpentinization and was accompanied by the growth of new minerals, which may have been partially due to deserpentinization but was also the result of metasomatism by a silica-rich hydrous fluid.

Thermal structure beneath the Snake River Plain: Implications for the Yellowstone hotspot

Basaltic magmatism associated with the Yellowstone hotspot has been widely attributed to upwelling of a mantle plume, yet the temporal and spatial distribution of these magmas and their compositional characteristics are distinctive from oceanic hotspot magmatism. Fundamental questions concern the influence of continental cratonic lithosphere in producing the differences, and the extent to which upper plate processes contribute to magma production. To better understand scenarios of melt generation, P– T conditions are estimated for segregation of primitive Snake River Plain (SRP) basalts from the mantle. Combined with analysis of trace element and seismic constraints, we conclude from this that (1) melt production was concentrated at depths between roughly 70–100 km, (2) mantle temperature was only slightly higher than ambient conditions with a maximum potential temperature of 1450 °C, and (3) the mantle source was relatively fertile (Mg# < 90). These results suggest that the seismically imaged plume below Yellowstone is significantly cooler than upwellings beneath Hawaii, Iceland and many other oceanic “hotspots”. Our findings, in combination with other geochemical and geodynamic considerations, are permissive of magma generation within the ancient lithospheric mantle keel associated with the Wyoming craton. Plume contributions, while not excluded, involve physical and geochemical implications that suggest they are subordinate.

The influence of plume head–lithosphere interaction on magmatism associated with the Yellowstone hotspot track

Although commonly attributed to a mantle plume, time-transgressive magmatism of the Snake River Plain–Yellowstone (SRPY) province differs in important ways from that associated with typical oceanic hotspots. A fundamental question concerns the relative contributions of lithosphere vs. upwelling sub-lithospheric mantle to formation of SRPY basaltic magmas. Specifically, association of this province with initially thick and cold Archean lithosphere (Wyoming craton) poses a problem in that this lid will hinder and possibly prevent melting of rising plume material. Assuming an anhydrous peridotite mantle, melting can only occur if (1) the lid can be substantially thinned over geologically reasonable time and/or (2) the upwelling material is exceptionally warm, or (3) the lid was suitably thin to begin with. Petrologic modeling indicates that SRPY primitive basalts last segregated from mantle at conditions (< 1500 °C and ∼ 100 km depth; Leeman, W.P., Schutt, D.L., Hughes, S.S., 2009. Thermal structure beneath the Snake River Plain: implications for the Yellowstone hotspot. J. Volcanol. Geotherm. Res.) only slightly warmer than MORB-source mantle and significantly cooler than sources of many oceanic hotspot magmas. In this study, geodynamic models were developed to evaluate lithospheric thinning processes. The motivation for the modeling is the observation that if the lithosphere is initially more than 200 km thick – typical of many cratons – then thinning by at least a factor of two is required to allow decompression melting of an ascending plume, assuming low volatile content and high excess temperature (potential temperature > 1500 °C). Fully dynamic models were applied to investigate the extent and rate of lithosphere thinning assuming an initial structure representative of the Wyoming craton. We find that thermal erosion by plume impingement alone appears incapable of providing the required lithospheric thinning. Alternative models (e.g., low-angle Laramide subduction, lithospheric delamination) also conflict with geochemical evidence that SRPY basalts contain a dominant contribution of old, isotopically evolved mantle material — presumably derived from subcontinental lithospheric mantle (SCLM). We conclude that SCLM is likely to be preserved, that the thick SCLM lid prevents substantial melting of rising plume material (tomographically imaged), and that SRPY basalts are predominantly derived by melting of lithospheric mantle.

Timing and duration of Palaeoproterozoic events producing ore-bearing layered intrusions of the Baltic Shield: metallogenic, petrological and geodynamic implications

Abstract There are two 300–500 km long belts of Palaeoproterozoic layered intrusions in the Baltic (Fennoscandian) Shield; the Northern (Kola) Belt and the Southern (Fenno-Karelian) Belt. New U–Pb (TIMS) ages and radiogenic isotopic (Nd–Sr–He) data have been determined for mafic-ultramafic Cu–Ni–Ti–Cr and PGE-bearing layered intrusions of the Kola Belt. U–Pb zircon and baddeleyite data from gabbronorite and anorthosite bodies of the Fedorovo-Pansky, Monchepluton, Main Ridge (Monchetundra and Chunatundra) and Mt Generalskaya intrusions, and from gabbronorite intrusions and dykes associated with the Imandra lopolith, yield ages from c . 2.52–2.39 Ga. The age range of 130 Ma recorded in the Kola Belt samples, associated with at least four intrusive phases (three PGE-bearing and one barren), is significantly greater than that for intrusions of the Southern (Fenno-Karelian) Belt which clusters at 2.44 Ga. Nd isotopic values for the Kola Belt range from −1.1 to −2.4 and indicate an enriched mantle ‘EM-1 type’ reservoir for these layered intrusions. Initial Sr isotopic data for the Kola intrusions are radiogenic relative to bulk mantle, with I Sr values from 0.703 to 0.704, but geochemical data and 4 He/ 3 He isotopic ratios of various minerals record a significant contribution from a mantle source rather than simply crustal melting. The geological and geochronological data indicate that in the eastern part of the Baltic Shield, mafic‐ultramafic intrusive magmatism was active over a protracted period and was related to plume magmatism associated with continental breakup that also involved the Superior and Wyoming provinces.

Petrogenesis and tectonic implications of paleoproterozoic mafic rocks in the Black Hills, South Dakota

Four suites of mafic to intermediate intrusions and flows that occur in the Precambrian core of the Black Hills were analyzed for major and trace elements to establish tectonic settings of magmatism during the late Archean and early Proterozoic history of the region. Most of the mafic rocks were later metamorphosed to amphibolites during the Black Hills orogeny, the collision of the Wyoming and Superior cratons which began at ∼1760 Ma. Compositions of the mafic suites define at least two distinct tectonic settings of magmatism between 2480 and 1880 Ma. The 2.48 Ga Blue Draw intrusion, located on the eastern edge of the Precambrian core, is differentiated with compositions ranging from gabbro to granodiorite. It is a calcalkaline intrusion with relatively high SiO 2 from 51.8 to 60.5 wt.% and K 2O mostly >1 wt.%. REE patterns are slightly concave upward and show LREE enrichment. The intrusion is also enriched in Ba, Rb, Th, and Zr. These and other geochemical discriminants are typical of modern calcalkaline basalts. The intrusion's calcalkaline character is similar to the neighboring, slightly older 2.56 Ga Little Elk granite-gneiss. These two intrusions support the presence of late Archean/early Proterozoic arc magmatism on the eastern margin of the Wyoming craton. The three other mafic suites comprising metamorphosed flows and sills occur within a sequence of deep marine sediments, including black shales, limestones, iron formations, and cherts. U–Pb zircon ages suggest their emplacement between 2.01 and 1.88 Ga. Mafic rocks in the eastern half of the Precambrian core in the Mt. Rushmore and Pactola Dam quadrangles and near Bear Mountain on the western margin of the Precambrian core have slightly depleted to enriched tholeiitic compositions with SiO 2 ∼50 wt.% and K 2O generally <0.5 wt.%. They have flat to slightly light-enriched REE patterns. The chemical characteristics of these two suites point to E-MORB-type magmatism that characterizes present-day, mantle plume-related magmatism at oceanic spreading centers. A mantle plume source is especially evident in the composition of mafic rocks in the Rochford district in the north-central part of the Precambrian core. These mafic rocks have LREE-enriched patterns. They have the highest TiO 2 concentrations, typically >2.5 wt.%, and the lowest SiO 2 concentrations, typically <50 wt.%. They also have the highest Nb/Yb and Ta/Yb ratios and are relatively depleted in Y. With the Bear Mountain and Rushmore–Pactola suites, the Rochford suite provides evidence for rifting of the crust over a mantle plume that has produced a submarine spreading center. This plume-related magmatism in the Black Hills was apparently coeval with 2.01 Ga basic magmatism in southeastern Wyoming and together these locations may indicate an extended presence of a mantle plume under the eastern Wyoming craton. The plume may have led to the break-up of the previously proposed paleocontinent Kenorland.

134 COMBINATION OF ELECTROCHEMICAL AND PHOTODYNAMIC THERAPY

Deposition of the Huronian Supergroup, dominantly fluvial clastic sediments with local basal volcanic accumulations, began at ∼2.49 Ga in a broad trough that extended far south of the present Lake Huron area atop a large Archean continent that was assembled at ∼2.7 Ga and broken up at ∼2.15 Ga after intrusion of widespread gabbro sheets and dykes including Nipissing diabase. The Wyoming Province, which contains sequences similar in age and stratigraphy to the distinctive Huronian Supergroup, may be a fragment of this continent; the Superior Province is a much larger fragment. Tectonic controls of deposition of the Huronian and its paleoplacer uranium deposits, and of the pre-2.2 Ga deformation of these strata, were probably unrelated to processes that controlled deposition of the Animikie and Marquette Range sequences in the Lake Superior region at ∼2.0 Ga. The Huronian, Animikie, and Marquette Range sequences were deformed during the ∼1.85 Ga Penokean orogeny which occurred when an assembly of island arc terranes collided with the Superior block. The Sudbury structure, including the Whitewater Group and its contained sedex deposits and the Sudbury igneous complex and its NiCuPGE ores, were also formed at ∼1.85 Ga.The Elliot Lake pyritic paleoplacer uranium ores, myriad lenses of radioactive pyritic gravels, were deposited by early Huronian fluvial systems. Other endogenic Huronian mineral deposits include disseminated NiCu sulphides in gabbros, Cu in red beds high in the Huronian succession, and silica in quartz arenites. Intrusions of Nipissing gabbro near Sudbury contain minor NiCu mineralization. These intrusions also caused recycling of metals from Archean and Huronian rocks to form concentrations of Cu, Au, Ag, Fe and W in veins and other epigenetic deposits. Most important of these are the unconformity-related Ag veins of the Cobalt area which had particularly complex, still undeciphered, histories that probably included pre-Huronian weathering.Metallogenic patterns in the Early Proterozoic rocks were controlled by a complex interplay of factors including the distribution of mineral deposits in the Archean basement, tectonically controlled variations in Huronian deposition and deformation, and oxyatmoversion, the transition from anoxic to oxygenic atmospheric conditions that occurred during deposition of the Huronian Supergroup.

GEOCHEMISTRY OF CRUSTAL XENOLITHS FROM THE HATCHER MESA LAMPROITE, WYOMING, USA: INSIGHTS INTO THE COMPOSITION OF THE DEEP CRUST AND UPPER MANTLE BENEATH THE WYOMING CRATON

Crustal xenoliths entrained in the Hatcher Mesa lamproite, Leucite Hills, in Wyoming, consist of ultramafic, mafic and felsic varieties. Geothermometric calculations based on two-pyroxene thermometry indicate equilibrium temperatures of ~800 ± 50°C. Petrographic observations, along with whole-rock and mineral compositions, indicate that the xenoliths were generated by crystallization of mantle-derived mafic magmas during the Archean; these xenoliths were subsequently affected by high-grade metamorphism. The geochemical characteristics of the xenoliths, such as negative Nb–Ta–Ti and positive Pb anomalies, enrichment in the LILE (large-ion lithophile elements), and the Pb isotopic ratios point to parental magmas generated in an arc-like setting, supporting those models that involve accretion of the thinned, southern part of the Wyoming craton. Although the host lamproite and the xenoliths from the lower crust are not related genetically, their parental magmas originated from the underlying Wyoming mantle. It seems likely that the melting events that generated the protolith for the xenoliths contributed to the formation of a refractory mantle source under Wyoming. Such a modified, depleted source is considered essential for the petrogenesis of lamproites, including those occurring in the Leucite Hills.

Imaging Yellowstone plume‐lithosphere interactions from inversion of ballistic and diffusive Rayleigh wave dispersion and crustal thickness data

Diffusive and ballistic Rayleigh wave dispersion data from three PASSCAL seismic deployments are combined with crustal thickness constraints from receiver function analysis to produce a high‐resolution shear velocity image of the Yellowstone hot spot track crust and uppermost mantle. This synoptic image shows the following crustal features: the eastern Snake River Plain (ESRP) high‐velocity midcrustal layer, low‐velocity lower crust beneath the 4.0–6.6 Ma Heise caldera field, high‐velocity lower crust beneath the &lt;2.1 Ma Yellowstone Calderas, and low‐velocity upper crustal volume beneath the &lt;2.1 Ma Yellowstone caldera fields. The low‐velocity lower crust beneath the 4.0–6.6 Ma Heise caldera field is found to extend outward 50–80 km from the ESRP margins, consistent with outflow of the magmatically heated and thickened ESRP lower crust. In addition, the lack of 10 km of crustal thickening of the ESRP crust, associated with the estimated 10 km of magmatic thickening, requires that the ESRP lower crust has flowed outward in a complex fashion governed by preexisting lower crustal strength heterogeneity. Within the northern Wyoming province, the so‐called 7.x km/s lower crustal magmatic layer is found to extend westward to the N‐S oriented pre‐Cambrian rift margin. The high‐velocity, hence high‐density, 7.x layer beneath the &lt;2.1 Ma Yellowstone caldera fields has apparently inhibited heating of the subcaldera lower crust and instead magmatic heat and fluids are exchanged with the country rock above 13 km depth. The narrow 80 km diameter plume imaged by body wave tomograms, after being sheared to horizontal by plate drift, is manifest as a very low velocity (3.9 km/s) layer that is only about 110 km wide. The ESRP mantle lithosphere has been thinned to about 28 km thickness by the plume's transport of heat and magma upward, lateral advection of the lower lithosphere by plume shear, and ongoing lithospheric dilatation.

Structural and tectonic evolution of the Cherokee Ridge arch, south-central Wyoming: Implications for recurring strike-slip along the Cheyenne Belt suture zone

The Cherokee Ridge arch is an east–west-trending structure that separates the Washakie Basin of south-central Wyoming from the Sand Wash Basin of northwestern Colorado. It is characterized by a distinct east–west-trending lineament on Landsat images and a zone of northwest–southeast-striking, en echelon, high-angle faults that cut Upper Cretaceous to Miocene sedimentary rocks. The lineament is interpreted to be the surface expression of the Cheyenne belt, a buried suture zone that separates Proterozoic crust of the Colorado province from Archean crust of the Wyoming province. Results of this study indicate that the surface and near-surface structures along the Cherokee Ridge arch are the result of periodic strike-slip motion along the Cheyenne belt since Late Cretaceous time and possibly before. Previous structural and paleomagnetic studies support the conclusion that left-lateral transpressional motion occurred during the Late Cretaceous and early Tertiary as the area was under the contractional tectonic regime of the Laramide orogeny. The left-lateral shear zone is interpreted to be a lateral ramp accommodating west-directed thrusting that culminated in the formation of the Rock Springs uplift as the Colorado Plateau rotated in a clockwise direction at the end of the Laramide orogeny. A significant amount of vertical reverse displacement would also have occurred at this time along the south-dipping shear zone, giving the arch its present-day structural relief. This interpretation is also consistent with uplift of the Uinta Mountains during the Eocene. Right-lateral transtensional motion is interpreted to have taken place after the Miocene as the Colorado Plateau rotated to the west relative to the more stable Wyoming province during Tertiary extensional events. Northwest translation of the Colorado Plateau during Cenozoic extension is also supported by structural and paleomagnetic studies. Most surface structures observed along the arch are related to this most recent period of wrenching.

Tracking the Archean-Proterozoic suture zone in the northeastern Great Basin, Nevada and Utah

It is important to know whether major mining districts in north-central Nevada are underlain by crust of the Archean Wyoming craton, known to contain major orogenic gold deposits or, alternatively, by accreted crust of the Paleoproterozoic Mojave province. Determining the location and orientation of the Archean-Proterozoic suture zone between these provinces is also important because it may influence subsequent patterns of sedimentation, deformation, magmatism, and hydrothermal activity. The suture zone is exposed in northeastern Utah and southwestern Wyoming and exhibits a southwest strike. In the Great Basin, the suture zone strike is poorly constrained because it is largely concealed below a Neoproterozoic–Paleozoic miogeocline and Cenozoic basin fill. Two-dimensional resistivity modeling of three regional north-south magnetotelluric sounding profiles in western Utah, north-central Nevada, and northeastern Nevada, and one east-west profile in northeastern Nevada, reveals a deeply penetrating (>10 km depth), broad (tens of kilometers) conductor (1–20 ohm-meters) that may be the Archean-Pro-terozoic suture zone, which formed during Early Proterozoic rifting of the continent and subsequent Proterozoic accretion. This major crustal conductor changes strike direction from southwest in Utah to northwest in eastern Nevada, where it broadens to ∼100 km width that correlates with early Paleozoic rifting of the continent. Our results suggest that the major gold belts may be over-isolated blocks of Archean crust, so Phanerozoic mineral deposits in this region may be produced, at least in part, from recycled Archean gold. Future mineral exploration to the east may yield large gold tonnages.

Yellowstone plume–continental lithosphere interaction beneath the Snake River Plain

The Snake River Plain represents 17 m.y. of volcanic activity that took place as the North American continent migrated over a relatively fixed magma source, or hotspot. The identification of a clear seismic image of a plume beneath Yellowstone is compelling evidence that the Miocene to recent volcanism associated with the Columbia Plateau, Oregon High Lava Plains, Snake River Plain, Northern Nevada Rift and Yellowstone Plateau represents a single magmatic system related to a mantle plume. A remaining enigma is, why do radiogenic isotope signatures from basalts erupted over the Mesozoic–Paleozoic accreted terrains suggest a plume source while basalts erupted across the Proterozoic–Archean craton margin indicate an ancient subcontinental mantle lithosphere source? We show that ancient cratonic lithosphere like that of the Wyoming province superimposes its inherent isotopic composition on sublithospheric plume and/or asthenospheric melts. The results show that Yellowstone plume could have a radiogenic isotope composition similar to the mantle source of the early Columbia River Basalt Group and that the plume source composition has persisted to the present day.

Trace element and isotopic characterization of Neoarchean and Paleoproterozoic iron formations in the Black Hills (South Dakota, USA): Assessment of chemical change during 2.9–1.9 Ga deposition bracketing the 2.4–2.2 Ga first rise of atmospheric oxygen

Banded iron formations (BIFs) were deposited in the Black Hills, South Dakota, prior to and after the first main oxygenation of the atmosphere at ∼2.4–2.2 Ga. The oldest, Neoarchean horizon (Nemo Iron Formation) is an oxide-facies, detritus-free BIF deposited at ∼2.9–2.6 Ga, in association with mafic phyllite. These BIFs show rare earth and yttrium elemental (REY) signatures typical of ambient seawater, including Pr/Yb < 1 and positive La, Eu, and Y anomalies in Post-Archean-Australian-Shale (PAAS)-normalized REE diagrams, but they lack clear Ce anomalies. Deposition of an Early Paleoproterozoic horizon (Benchmark Iron Formation) is constrained to the interval between 2.56 and 2.48 Ga. These BIFs also show the REE distribution patterns typical of ambient seawater. Benchmark BIFs also exhibit slightly negative Ce anomalies, but BIFs included within the younger, 2.10–2.02 Ga Estes metaconglomerate exhibit mostly positive Ce anomalies, which are probably the result of oxidative supergene remobilization of Ce during transport. Mixed, silicate–carbonate–sulfide–oxide–facies BIFs with slightly elevated detrital inputs were deposited in Middle Paleoproterozoic time, between 2.02 and 1.97 Ga (Homestake Iron Formation and equivalents) and between 1.89 and 1.87 Ga (Rochford Iron Formation). These BIFs are characterized by variably fractionated, PAAS-normalized REY patterns, which include: Pr/Yb ratios of 0.2–4.1; absence of La anomalies; and presence of positive Eu, positive and negative Ce, and mostly negative Y anomalies. Sm–Nd isotopes from individual microbands of the Neoarchean BIF define Eo- to Mesoarchean, Nd T DM ages of 3.6–2.9 Ga, which are generally compatible with crustal residence ages inferred from elsewhere in the Wyoming craton (i.e., in the 2.72–2.67 Ga Atlantic City BIF, Wind River Range). Model ages of microbands from the Early and Middle Paleoproterozoic BIFs are highly variable, ranging from 3.8 to 2.15 Ga. The youngest components in the Neoarchean and Middle Paleoproterozoic BIFs, inferred from the model ages, correspond to associated, mafic metatuffs and metabasalts, which yield ∼2.9 and ∼2.15 Ga model ages, respectively. Further, the oldest (i.e., ≤3.9–3.6 Ga) components indicate that Eo- to Mesoarchean crust was exposed during the entire BIF depositional time frame, between ∼2.9 and 1.88 Ga. Pb isotopes distinguish the Neoarchean from Early and Middle Paleoproterozoic BIFs. Whereas all BIFs reflect the generally high- μ ( 238U/ 204Pb) values typical of the Wyoming craton, thorogenic-versus-uranogenic Pb-isotope data arrays constrain sources of Pb in the various iron formations, particularly those deposited in Early and Middle Paleoproterozoic time. Early Paleoproterozoic BIFs exhibit low 208Pb/ 204Pb relative to 206Pb/ 204Pb ratios, implying a highly U/Th-fractionated source. This source is compatible with the peraluminous, ∼2.60 Ga Bear Mountain granite exposed in the W-central Black Hills, which itself was derived from even older crust. In contrast, the Middle Paleoproterozoic BIFs define a Pb-isotopic array that is compatible with a moderately fractionated source resembling the metaluminous, ∼2.56 Ga Little Elk granite exposed in the NE Black Hills. The Neoarchean BIFs are generally less radiogenic than their younger counterparts, and plot near the intersection of the two Paleoproterozoic arrays on the thorogenic diagram. Although the ∼2.9–1.9 Ga Black Hills BIFs bracket the worldwide, 2.4–2.0 Ga BIF depositional gap, which curiously coincides with the coupled, first atmospheric–biospheric oxygenation and subsequent, positive δ 13C excursion, we are unable to delineate major changes in BIF geochemical compositions that might reflect concomitant changes in atmospheric oxygenation. However, slightly negative Ce anomalies recorded in the 2.56–2.48 Ga, oxide–facies BIFs, and an overall trend of decreasing, positive Eu anomalies between ∼2.9 and 1.9 Ga among Black Hills BIFs generally (in agreement with the trend observed worldwide), might argue for the onset of bottom-water oxidation at the Archean-Proterozoic boundary and a decrease of high-temperature hydrothermal activity over the worldwide, BIF depositional gap. In the Black Hills, the 2.4–2.0 Ga depositional gap reflects either a ∼400 Myr absence of seawater between incipient, intracratonic rifting of supercraton Superia (or supercontinent Kenorland) at ∼2.5–2.4 Ga and its final breakup at ∼2.1 Ga, a ∼400 Myr missing rock record across a mapped unconformity, or both.

Research Focus: Crust formation in the western United States

Between about 1.8–1.6 Ga, an accretionary belt ~1300 km wide was added to the southern margin of the Archean Wyoming craton. In the prevailing arc-accretion model (Condie, 1982; Karlstrom and Bowring, 1988; 1993), most of this material is considered to be mantle-derived juvenile crust, and therefore represents newly formed crust (at 1.8–1.6 Ga). The interesting and highly provocative paper by Bickford and Hill in this issue of Geology (p. 167–170) challenges the view that crustal growth in southern Laurentia primarily involved accretion of juvenile arc rocks to an Archean core. Although the Bickford and Hill paper focuses specifi cally on Paleoproterozoic crustal evolution of southern Laurentia, it has relevance to mechanisms and rates of crustal formation and growth throughout geologic time. Bickford and Hill present an alternative interpretation for Paleoprotero zoic crustal evolution in which they suggest that signifi cant volumes of older (ca. 1.85 Ga Trans-Hudson/Penokean) crust may be present in southern Laurentia, and that rifting of this crust produced the widespread bimodal volcanic sequences present in the southwestern United States (see also Hill and Bickford, 2001). The principal lines of evidence cited by Bickford and Hill to support their interpretations are the presence of pre-1800 Ma inherited zircons and Nd and Pb isotopic signatures suggestive of older crust, and the abundant rhyolite-dominated bimodal volcanic sequences present in southern Laurentia. In their model, rifting of older crust resulted in basaltic magmatism that provided a heat source for the partial melting of Trans-Hudson/Penokean crust to form the rhyolites. The basaltic component of these bimodal sequences would represent juvenile additions to the crust. The rifting and arc-accretion models are not mutually exclusive.

In suspect terrane? Provenance of the late Archean Phantom Lake metamorphic suite, Sierra Madre, Wyoming

The 2.68 Ga Phantom Lake metamorphic suite of the Sierra Madre is a volcanogenic, volcaniclastic, and siliciclastic sequence that may have been deposited on or near the margin of the Wyoming Province or, alternatively, it may represent part of an exotic block accreted onto the southern margin of the Wyoming Province. The metamorphosed supracrustal rocks of the Phantom Lake metamorphic suite, along with quartzofeldspathic gneisses and granitoids of similar age, have light rare-earth element (LREE) – enriched REE patterns with little to no Eu-anomaly. These patterns are comparable to those of modern oceanic arc rocks and sediments. Both supracrustal and metaigneous rocks have radiogenic initial εNd from +4.5 to –2.5 and Nd crustal residence ages between 2.7 and 3.0 Ga. It is proposed that these juvenile rocks were part of an intra-oceanic arc system formed beyond the influence of detritus from the Wyoming Province and subsequently were accreted onto the southern Wyoming Province following intrusion of granitic gneisses in the Sierra Madre at ca. 2.64 Ga. The younger 2.43 Ga Baggot Rocks granite has less radiogenic εNd of –3.9 suggesting that the rocks of the Sierra Madre had accreted to the Wyoming Province by 2.43 Ga. The supracrustal sequences at South Pass, Bradley Peak, and the Rattlesnake Hills have similar, radiogenic initial Nd isotope compositions. Together with the Phantom Lake metamorphic suite, they represent juvenile additions to existing continental crust and provide evidence that lateral accretion of oceanic terranes was an important process of late Archean crustal growth in the Wyoming Province.

Archean crustal growth by lateral accretion of juvenile supracrustal belts in the south-central Wyoming Province

Neoarchean supracrustal sequences in the south-central Wyoming Province are exposed as relatively small belts in Laramide uplifts. Some sequences are composed of materials derived mainly from pre-existing Wyoming province crust, but others are dominated by juvenile components. The latter include the Miners Delight Formation in the Wind River Range, the Rattlesnake Hills Group in the Granite Mountains, and the Bradley Peak succession in the Seminoe Mountains. U–Pb zircon dates from interbedded metavolcanic rocks suggest that these supracrustal belts are of at least two different ages: ca. 2.67 and ca. 2.72 Ga. We identify a time of contractional deformation and accretion of some of these supracrustal packages to the southern Wyoming Province at ~2.65–2.63 Ga. Magmatism is nearly synchronous with deformation. Some granitoids intrude the Wyoming Province basement, as well as the juvenile rocks thrust onto this basement; these have Nd isotopic compositions indicating that these plutons assimilated some old continental basement during ascent. Plutons intruding the supracrustal rocks located farther from the margin do not show this continental influence. The time scale and geologic processes of deposition, contractional deformation, and plutonism appear analogous to Phanerozoic examples of oceanic terrane accretion, such as formed the Klamath Mountains Province of California and Oregon. We conclude that a major episode of Neoarchean crustal growth involved both the lateral accretion of juvenile terranes and the intrusion of arc magmas formed from mantle-derived and (or) juvenile crustal sources and was driven by geologic processes very similar to modern plate tectonics.

The Wyoming Province: a distinctive Archean craton in Laurentian North America

The Wyoming Province is a distinctive Archean craton in the northwestern United States that can be subdivided into three subprovinces, namely, from oldest to youngest, the Montana metasedimentary province, the Beartooth–Bighorn magmatic zone, and the Southern accreted terranes. Archean rocks of the Montana metasedimentary province and the Beartooth–Bighorn magmatic zone are characterized by (1) their antiquity (rock ages to 3.5 Ga, detrital zircon ages up to 4.0 Ga, and Nd model ages exceeding 4.0 Ga); (2) a distinctly enriched 207Pb/204Pb isotopic signature, which suggests that this part of the province was not produced by the amalgamation of exotic terranes; and (3) a distinctively thick (15–20 km), mafic lower crust. The Montana metasedimentary province and Beartooth–Bighorn magmatic zone were cratonized by about 3.0–2.8 Ga. Crustal growth occurred via continental-arc magmatism and terrane accretion in the Southern accreted terranes along the southern margin of the province at 2.68–2.50 Ga. By the end of the Archean, the three subprovinces were joined as part of what is now the Wyoming Province. Subsequent to amalgamation of the Wyoming crust to Laurentia at ca. 1.8–1.9 Ga, Paleoproterozoic crust (1.7–2.4 Ga) was juxtaposed along the southern and western boundaries of the province. Subsequent tectonism and magmatism in the Wyoming region are concentrated in the areas underlain by these Proterozoic mobile belts.

Archean geochronological framework of the Bighorn Mountains, Wyoming

The Bighorn Mountains of the central Wyoming Province expose a large tract of Archean crust that has been tectonically inactive and at relatively high crustal levels since ~2.7 Ga. Seven sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon and titanite age determinations on samples of the main lithologic units provide a geochronological framework for the evolution of this area. The oldest, precisely dated magmatic event occurred at 2950 ± 5 Ma, when diorite to granite dykes and sills intruded an older gneiss complex exposed in the central and southern Bighorn Mountains. Rocks as old as 3.25 Ga may be present in this gneissic basement, as indicated by the oldest dates obtained on areas of zircon grains that are interpreted as inherited cores. A tonalitic gneiss was intruded into the gneiss complex at 2886 ± 5 Ma. Deformation of the central and southern gneisses preceded the intrusion of the Bighorn batholith, a tonalitic to granitic intrusion that occupies the northern portion of the uplift. This composite batholith was intruded over the period 2.86–2.84 Ga. Ca. 3.0–2.8 Ga crust is also present in the Beartooth Mountains, the Washakie block of the northeastern Wind River Range, the Owl Creek Mountains, and the northern Granite Mountains, but late Archean deformation and plutonism has obscured much of the earlier history in the southern portion of this area. The entire area, referred to as the Beartooth–Bighorn Magmatic Zone, has been undeformed since 2.6 Ga. Proterozoic extension was focused in those parts of the Wyoming Province outside of this domain.

The Teton – Wind River domain: a 2.68–2.67 Ga active margin in the western Wyoming Province

The Archean rocks in western Wyoming, including the Teton Range, the northern Wind River Range, and the western Owl Creek Mountains, preserve a record of a 2.68–2.67 Ga orogenic belt that has many of the hallmarks of modern plate tectonics. A 2683 Ma tholeiitic dike swarm is undeformed and unmetamorphosed in the western Owl Creek Mountains. In the Wind River Range, these dikes have been deformed and metamorphosed during thrusting along the west- to southwest-directed Mount Helen structural belt, which was active at the time that the 2.67 Ga Bridger batholith was emplaced. In the northern Teton Range, the Moose Basin gneiss, which contains relict granulite-facies assemblages, appears to have been thrust upon the amphibolite-grade layered gneiss. The syntectonic Webb Canyon orthogneiss was intruded into the thrust at or before 2673 Ma. We interpret these relations, along with isotopic data indicating that the layered gneiss in the Teton Range consists of juvenile components, to indicate that the western Wyoming Province was the site of active margin tectonics at 2.68–2.67 Ga. This involved a magmatic arc in the present Wind River Range and back-arc spreading in the Owl Creek Mountains. The immature, juvenile layered gneiss in the Teton Range probably represents an accretionary prism or fore-arc basin onto which high-pressure rocks containing a mature sedimentary sequence were thrust at 2.67 Ga. Although it may be questioned as to when modern-style plate tectonics began in other cratons, it was certainly operating in the Wyoming Province by 2.67 Ga.

Tectonic histories of the Paleo- to Mesoarchean Sacawee block and Neoarchean Oregon Trail structural belt of the south-central Wyoming Province

The Sacawee block is a narrow belt of Paleo- to Mesoarchean crust that extends for ~70 km across the northern Granite Mountains. It is composed of the ~3.3 Ga Sacawee orthogneiss, additional calc-alkalic and tonalitic orthogneisses, and the ~2.86 Ga Barlow Gap Group. The Sacawee block basement is characterized by negative εNdvalues and Paleoarchean Nd crustal residence model ages. A broad east–west-trending zone of Neoarchean high strain, which is part of the Oregon Trail structural belt, transects the Sacawee block and was studied at two locations, the Beulah Belle Lake area and West Sage Hen Rocks. U–Pb analyses of magmatic zircon from a sheared amphibolite within the high-strain zone of the Beulah Belle Lake area constrain the age of the Neoarchean deformation to be later than 2688 ± 5 Ma. At West Sage Hen Rocks, metamorphic zircons in a sheared amphibolite provide a direct date on the shear zone of 2649 ± 2.8 Ma. These data, combined with similar ages of deformation from two other shear zones, are interpreted to suggest that the Neoarchean Oregon Trail structural belt is a pervasive feature of the Sacawee block and may represent a deformation front related to accretion. Multiple east–west-trending shear zones within the Sacawee block are evidence for tectonic modification of the crust between ~2.65 and 2.63 Ga and horizontal convergence analogous to modern plate tectonics processes. The Sacawee block is either a rare exposure of ancient basement typical of that which originally underlay much of the Wyoming Province or it is an exotic block that was accreted to the core of the Wyoming Province in Neoarchean time.

The tonalite–trondhjemite–granodiorite (TTG) to granodiorite–granite (GG) transition in the late Archean plutonic rocks of the central Wyoming Province

The 2.95–2.82 Ga quartzofeldspathic gneisses and granitoids in the Bighorn, western Owl Creek, and northeastern Wind River uplifts in the central Wyoming Province include low-K tonalite–trondhjemite–granodiorite (TTG) and high-K granodiorite–granite (GG) rocks. Both types of granitoids were intruded contemporaneously, although TTGs are more abundant in the older gneisses. The TTG suite consists of calcic to marginally calc-alkalic rocks that straddle the boundaries between metaluminous and peraluminous and between ferroan and magnesian compositions. Rare-earth element (REE) patterns of these rocks may be highly fractionated with low heavy rare-earth element (HREE) contents and modest to absent Eu anomalies but may also be less strongly HREE depleted. These rocks do not represent first-generation continental crust: most have unradiogenic Nd and radiogenic 207Pb/204Pb isotopic compositions that require the incorporation of isotopically evolved sources. The GG suite has compositions that are transitional between Archean TTG and modern, continental margin calc-alkalic rocks. The GG suite is characterized by higher alkali contents relative to CaO than the TTG suite and higher K/Na ratios but exhibits a similar range in REE patterns. The Nd, Sr, and Pb isotopic compositions of the GG suite are slightly less variable but lie within the range of those of the TTG suite. We interpret them as having a source similar to that of the TTG, perhaps forming by partial melting of preexisting TTG. The shift from TTG-dominated to GG-dominated continental crust was a gradual transition that took place over several hundred million years. Clearly subduction-related calc-alkalic magmatism is not recognized in the Wyoming Province prior to 2.67 Ga.