Wyoming Province Research Articles

The reappearance of the Huronian in Wyoming: rifting and drifting of ancient continents

Striking stratigraphic and sedimentological similarities between the Early Proterozoic Huronian Supergroup of the Canadian Shield and the Snowy Pass Supergroup of Wyoming suggest that they were deposited in a single, broad, epicratonic basin developed atop a large Archean continent that included the Superior and Wyoming geological provinces. Breakup of the continent after the 2.2 Ga intrusion of widespread gabbro sheets and dykes resulted in the separation of the Archean Superior and Wyoming cratons and their Early Proterozoic covers. These crustal fragments were subsequently reassembled during Early Proterozoic (~1.85 Ga) orogenesis, the end result being the present 2000 km separation of the Huronian and Snowy Pass supergroups and their Archean basements.

Lead isotopic evidence for the origin of Paleo- and Mesoproterozoic rocks of the Colorado Province, U.S.A.

Lead isotopic ratios of K-feldspars and whole-rocks from 1.7- and 1.4-Ga plutonic rocks of the Colorado Province are relatively non-radiogenic for 207 Pb 204 Pb , plotting below the average crust model curve of Stacey and Kramers (1975), indicating that the terrane was derived primarily from juvenile, mantle material. Slightly more radiogenic ratios in the northern part of the terrane, near the Archean Wyoming Province, suggest minor inclusion of an older component. The data from 1.7-Ga plutons plot in a broad field suggesting two episodes of re-equilibration with whole-rock Pb, probably related to heating events in the Mesoproterozoic (1.4 Ga) and Cretaceous (70 Ma). Possible differences in calculated whole-rock Th U , coupled with slight Pb isotopic variations, along the north-south transect suggest either a terrane boundary through central Colorado (near Salida and Gunnison), or fundamental differences in source rocks (metasedimentary vs. metavolcanic). UPb analyses of multigrain splits of detrital zircons from quartzites throughout the Colorado Province have failed to identify Archean detritus. The oldest 207Pb 206 Pb ages found (in two samples of quartzite from northern Colorado) are about 2.0 Ga (perhaps derived from rocks of the Trans-Hudson orogen), in contrast to 2.75-Ga detrital zircon in a Paleoproterozoic quartzite from the southern part of the Wyoming Province. While we are not yet able to discern if these ages are true provenance ages or mixtures of Archean and Paleoproterozoic components, the absence of easily recognizable Archean zircons supports other isotopic data and a conclusion that most of the Paleoproterozoic crust of the Colorado Province was ultimately derived from a juvenile (at 1.8 Ga) mantle reservoir.

Age and composition of Archean crystalline rocks from the southern Madison Range, Montana: Implications for crustal evolution in the Wyoming craton

The southern Madison Range of southwestern Montana contains two distinct Precambrian lithologic assemblages: (1) a complex of tonalitic to granitic gneisses that has been thrust over (2) a medium-grade metasupracrustal sequence dominated by pelitic schist. Crystallization ages for the protolith of a granodioritic gneiss that intruded the metasupracrustal sequence ([approximately]2.6 Ga)-along with an intercalated meta-andesite ([approximately]2.7 Ga) confirm the sequence as Archean. Chemical (major and trace element), isotopic (Rb-Sr, Sm-Nd, Pb-Pb), and geochronologic (U-Pb zircon) data for selected components of the gneiss complex indicate two groups of gneisses: an older, tonalitic to trondhjemitic group ([approximately]3.3 Ga) and a younger, mostly granitic group ([approximately]2.7 Ga). Both groups of gneisses exhibit the radiogenic Pb and nonradiogenic Nd isotopic signature characteristic of Middle and Late Archean rocks from throughout the Wyoming province. The older gneisses, in particular, appear to be compositionally, isotopically, and chronologically comparable to other Middle Archean gneisses from the northern part of the province (for example, Beartooth Mountains). The Late Archean gneisses, however, exhibit some distinct differences relative to their temporal counterparts, including (1) trace-element patterns that are more suggestive of crustal melts than subduction activity and (2) higher initial Sr isotopic ratios that suggest more involvement of older crustmore » in their petrogenesis. These comparisons suggest that the juxtaposition of Late Archean terranes in the northern Wyoming province was the result, at least in part, of intracratonic processes. 41 refs., 6 figs., 2 tabs.« less

Nd isotopic evidence for the antiquity of the Wyoming province

Sm-Nd isotopic data on Late Archean age metasedimentary rocks from around the Wyoming province yield ϵ Nd values at 2.6 Ga from -12.9 to +3.3, which correspond to depleted mantle model ages of 2.7 to 3.8 Ga. These results indicate that crust of a wide variety of ages was exposed and eroding in Late Archean time. The predominance of Middle Archean model ages suggests that continental crust was extensive in the Wyoming province by this time. No resolvable correlation exists between initial ϵ Nd value and geographic location or depositional environment of the metasedimentary rock samples; thus, there is no indication that any part of the province was established at a time different from the rest. The Nd isotopic characteristics of Archean rocks from the Wyoming province contrast with those of the Superior province, confirming that, although their Proterozoic evolution has been parallel, the two cratons did not share a common Archean history.

Trans-Hudson orogen and Williston basin in Montana and North Dakota: New COCORP deep-profiling results

COCORP (Consortium for Continental Reflection Profiling) deep reflection profiling across the Williston basin and underlying Trans-Hudson orogen shows the following. (1)There is no evidence for a precursor rift basin beneath the axis of the Williston basin. With the exception of small-scale structures (e.g., Nesson and Cedar Creek anticlines), the basement surface beneath the Williston basin is smooth and has a dish-shaped geometry. (2) Crustal penetrating reflections within the eastern Trans-Hudson orogen dip eastward, in apparent contradiction to the conventional that the Superior province underthrusts the eastern side of the orogen. (3) Basement reflections beneath the center of the orogen define a broad crustal-scale antiform, the axis of which coincides approximately with the depocenter of the overlying Williston basin. Midcrustal reflections defining the crest and western flank of this antiform also coincide generally with the location and shape of the source of the North American Central Plains conductivity anomaly. (4) Crustal penetrating reflections beneath the western side of the orogen dip westward, supporting the current view that subduction polarity was to the west beneath the Wyoming province. (5) The eastern edge of the Wyoming province is marked by east-dipping upper-crustal reflections and subhorizontal lower-crustal reflections that are truncated by the west-dipping reflectivity of the Trans-Hudson orogen.

Thick-skinned deformation of the Archean Wyoming province during Proterozoic arc-continent collision

Two major mylonite zones are exposed in the southeastern Archean Wyoming province: the Cheyenne belt, which marks the boundary between the Wyoming and Colorado provinces, and the Laramie Peak shear zone, across which Archean rocks have been uplifted differentially at least 10 km. Between the two mylonite zones is a 60-100-km-wide belt of Archean migmatitic gneiss, which has been intruded by diabase dikes and peridotites and contains enclaves of high-grade metasedimentary rocks. We interpret this belt of high-grade rocks as a tectonically reactivated block of Archean crust, uplifted during the development of the Cheyenne belt collisional zone at ca. 1.8 Ga. This block does not exhibit a decreasing metamorphic gradient away from the mylonite zone, but is at uniformly high grade. Thus, rather than a thick-skinned thrust-ramp model for uplift, we invoke vertical uplift along a high-angle reverse fault without rotation. This style of uplift may be a tectonic response to transpression, underplating, or tectonic interfingering at depth during collisional orogeny, and it may be a more common feature of basement uplifts within foreland fold-and-thrust belts than has been previously recognized.

Proterozoic mineral deposits near plate margins of the archean wyoming province, USA

Abstract Proterozoic mineral deposits located near the margin of the Archean Wyoming Province can be interpreted in a plate tectonic context. The southern margin of the Archean Wyoming Province is exposed in southeastern Wyoming where Early Proterozoic island areas have been sutured to an Early Proterozoic passive margin developed during rifting of the Archean Wyoming craton. Mineral deposits include uranium-thorium-gold bearing quartz pebble conglomerate deposited during rifting, stratiform sulphide in black shale of the foredeep basin, anomalous platinum group element values in layered complexes of the island arcs, and a variety of mineral deposits that formed during closure of the arcs and passive margin. The eastern margin of the Wyoming Province is probably located under Phanerozoic cover just east of the Black Hills of South Dakota. The Black Hills is a segment of the Early Proterozoic Trans-Hudson Orogen which is interpreted here as an area exhibiting evidence of opening and closing of an intracratonic basin. In the Black Hills, domes underlain by Archean rocks are overlain by an Early Proterozoic clastic successions that preserves evidence of rifting. These clastic successions are succeeded by marine quartzite which is, in turn, overlain by a eugeosynclinal succession containing iron formation, graywacke, and a variety of volcanic units. The basal Early Proterozoic clastic succession contains uranium and gold-bearing quartz pebble conglomerate formed during rifting. The eugeosynclinal succession contains the Homestake iron formation which is host to the Early Proterozoic Homestake type gold deposits. Geologic and geochemical evidence can be marshalled to support either an epigenetic or syngenetic origin for the Homestake-type gold deposits. If the gold deposits are syngenetic they may have formed in a foredeep basin developed during closure of the intracratonic basin.

Early proterozoic tectonics and metallogeny of the lake huron region of the Canadian shield

Abstract 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.

Suppression of dominant topographic overprints in gravity data by adaptive filtering: Southern Wyoming province

In areas of the world with a protracted tectonic history, the gravity field may be dominated by anomalies due to individual structural features developed during the latest tectonic event. These individual tectonic features often have a topographic expression. Adaptive filtering techniques can be used to suppress the gravity effects of such features. Iterative space domain techniques are particularly well suited to these types of problems. One such technique, the stochastic gradient algorithm, allows easy updating of filter coefficients at each spatial step through the data. This approach eliminates windowing and spectral estimation problems associated with cross‐spectral admittance schemes. In this study, adaptive filtering of Bouguer gravity and topography in the southern Wyoming Province was successful in suppressing gravity signatures of Laramide structural features. The regional gravity trends due to isostatic effects were significantly absent from the residual anomaly map. A large, continuous, positive gravity anomaly stretching from the southern Wind River Range to the northern end of the Laramie Range was another outstanding feature observed in the residual anomaly. This positive anomaly is about the same magnitude, size and shape as anomalies due to greenstone belts observed in other Archean cratonal areas. Scattered occurrences of greenstone lithologies are known throughout the area of the anomaly, but a continuous feature is absent in outcrop. Due to the unusual structural history of the area, it is possible that such a continuous belt does exist, but was buried at shallow crustal levels by Laramide thrusting, or later collapse of a Laramide uplift due to a change in the regional stress field.

The nature of Archean terrane boundaries: an example from the northern Wyoming Province

The Archean northern Wyoming Province can be subdivided into two geologically distinct terranes, the Beartooth-Bighorn magmatic terrane (BBMT) and the Montana metasedimentary terrane (MMT). The BBMT is characterized by voluminous Late Archean (2.90-2.74 Ga) magmatic rocks (primarily tonalite, trondhjemite, and granite); metasedimentary rocks are preserved only as small, rare enclaves in this magmatic terrane. The magmatic rocks typically have geochemical and isotopic signatures that suggest petrogenesis in a continental magmatic arc environment. The MMT, as exposed in the northern Gallatin and Madison Ranges, is dominated by Middle Archean trondhjemitic gneisses (3.2–3.0 Ga); metasedimentary rocks, however, are significantly more abundant than in the BBMT. Each terrane has experienced a separate and distinct geologic history since at least 3.6 Ga ago based on differences in metamorphic and structural styles, composition of magmatic and metasupracrustal rocks, and isotopic ages; consequently, these may be described as discrete terranes in the Cordilleran sense. Nonetheless, highly radiogenic and distinctive Pb-Pb isotopic signatures in rocks of all ages in both terranes indicate that the two terranes share a significant aspect of their history. This suggests that these two Early to Middle Archean crustal blocks, that initially evolved as part of a larger crustal province, experienced different geologic histories from at least 3.6 Ga until their juxtaposition in the Late Archean (between 2.75 to 2.55 Ga ago). Consequently, the boundary between the BBMT and MMT appears to separate terranes that are not likely to be exotic in the sense of their Phanerozoic counterparts. Other Archean provinces do appear to contain crustal blocks with different isotopic signatures (e.g. West Greenland, India, South Africa). The use of the term exotic, therefore, must be cautious in situations where geographic indicators such as paleontologic and/or paleomagnetic data are not available. In these cases, isotopic signatures are one of the most useful features for assessing overall genetic relations amongst geologically distinct terranes.

Comment and Reply on "Identification of 2.0 to 2.4 Ga Nd model age crustal material in the Cheyenne belt, southeastern Wyoming: Implications for Proterozoic accretionary tectonics at the southern margin of the Wyoming craton"

Comment and Reply on "Identification of 2.0 to 2.4 Ga Nd model age crustal material in the Cheyenne belt, southeastern Wyoming: Implications for Proterozoic accretionary tectonics at the southern margin of the Wyoming craton"

3.96 Ga zircons from an Archean quartzite, Beartooth Mountains, Montana

U-Pb isotopic systematics of detrital zircons incorporated in a middle Archean quartzite from the Beartooth Mountains, Montana, were investigated with the SHRIMP ion microprobe. These new data reveal an extended and previously unrecognized record of crustal evolution for the northern Wyoming province. Seventy-eight analyses of 67 grains yielded a range of {sup 207}Pb/{sup 206}Pb ages from 2.69 to 3.96 Ga. Concordant analyses from 43 separate grains defined a maximum age for the deposition of the quartzite of 3.30 Ga; other provenance ages extend to 3.96 Ga. Ages of 3.30 Ga indicate that older crustal components with ages up to 3.96 Ga, or detritus from them, were also in the provenance of this quartzite. This older age is equivalent to that of the oldest known rock from the Acasta gneisses of the Slave province and is exceeded only by the > 4.0 Ga age of detrital zircons of the Yilgarnmore » block of Western Australia. These data support an increased probability for the survival of sialic crust created before the cessation of the late bombardment at 3.8 to 3.9 Ga.« less

Neodymium, strontium, and lead isotopes in the Maloin Ranch Pluton, Wyoming: Implications for the origin of evolved rocks at anorthosite margins

Neodymium, strontium, and lead isotopic data are used in this study to investigate the origin of chemically evolved rocks in the Maloin Ranch Pluton, a composite body that borders and intrudes the Laramie Anorthosite. The range of rock types present is similar to that found at the margins of Proterozoic anorthosites throughout the world. In the Maloin Ranch Pluton, these include ferrodiorite at the base of the intrusion, overlain progressively by fine-grained monzonite, monzosyenite, and porphyritic granite. Biotite gabbro and fine-grained granitic dikes are present locally at various levels of this sequence. The origin of the evolved rocks and their possible relation to associated anorthositic bodies has been much debated. In the Maloin Ranch Pluton, each rock type has distinct isotopic characteristics which, together with trace-element data previously reported, suggest different source characteristics for each member. Strontium and neodymium isotopic data for Maloin Ranch ferrodiorite and Laramie anorthositic rocks show considerable overlap, consistent with a comagmatic relation. Biotite gabbro is chemically and isotopically the most primitive rock type in the Maloin Ranch Pluton. The data suggest that biotite gabbro has a mantle source, but has undergone extensive fractionation in the crust. Our results suggest that the remainder (and bulk) of the intrusion formed by partial melting of the lower crust due to the emplacement of the Laramie Anorthosite. Maloin Ranch fine-grained monzonite is chemically more evolved than the underlying ferrodiorite (i.e., lower Mg #, higher silica and alkalis), yet has more primitive isotopic characteristics (lower I Sr ; higher epsilon Nd). Formation of fine-grained monzonite by in situ fractionation of ferrodiorite is precluded by both the isotopic data and by mass balance. The isotopic data do not rule out formation of fine-grained monzonite by fractionation or contamination of ferrodiorite at depth, but this model requires that the ferrodiorite is isotopically heterogeneous and that very large volumes of ferrodiorite were present. Alternatively, trace-element and isotopic characteristics of the fine-grained monzonite are explained by partial melting of mantle-derived lower crust, added to the margin of the Archean Wyoming craton at about 1.8 Ga. Neodymium, strontium, and lead isotope data for Maloin Ranch monzosyenite and porphyritic granite also suggest a lower crustal source. These data indicate a > 1.8 Ga source component for the Maloin Ranch monzosyenites, but do not require input of Archean crust from the Wyoming province to the north.

Geochemistry of igneous rocks from the Crazy Mountains, Montana, and tectonic models for the Montana Alkalic Province

Tectonic models have postulated that subduction of a flat‐lying oceanic plate was intimately linked with mid‐Eocene magmatism in the northwestern United States and have assumed that this linkage is expressed in the geochemical characteristics of igneous rocks. Geochemical data from the Crazy Mountains in south central Montana can be interpreted as indicating the presence of a subduction‐related chemical component. Because radiogenic isotopic data for these rocks restrict the major chemical events in their mantle source to the Late Cretaceous‐Early Tertiary (50–100 Ma) and the mid‐Proterozoic (1.6–1.8 Ga), subduction during one or both of these periods is the most plausible explanation for the origin of the arc‐like geochemical features. Assessment of geologic and tectonic constraints, however, indicates that Late Cretaceous‐Early Tertiary subduction cannot explain the regional distribution, the contemporaneity, and the compositional range of the Eocene igneous rocks. The Eocene magmatic event does not show interpretable age progressions, as most subduction‐related models suggest. There is considerable geographic overlap in the chemical characteristics of the Eocene igneous rocks, so that chemical zoning cannot be demonstrated south of the U.S.‐Canada border. The zone of igneous activity was diffuse and discontinuous, unlike most subduction‐related magmatic arcs. The presence of thick lithospheric mantle beneath the Archean Wyoming craton presents difficulties for models that involve subduction of a flat‐lying slab. The lithospheric mantle probably controlled the depth of subduction and probably forced the slab to depths greater than those of plausible source regions for subalkalic, mantle‐derived basalts. Flat slab subduction models cannot account for the thermal inputs required for extensive magmatism. Subduction‐related models fail to explain the CO2‐rich character of alkalic magmatism in central Montana. The arc‐like geochemical patterns in the Crazy Mountains samples could result either from subduction unrelated to Late Cretaceous‐Early Tertiary events, or from mantle geochemical processes that are unrelated to subduction. If geochemical processes unrelated to subduction are involved, trace element discrimination diagrams are misleading and are inadequate criteria for interpreting the tectonic environment. The evidence for mid‐Proterozoic chemical modification of the mantle beneath the Wyoming craton suggests that the arc‐like character could be inherited from subduction events between 1.6 and 1.9 Ga, but a specific mid‐Proterozoic subduction geometry cannot be identified. Alternative tectonic models for Eocene magmatism can be built around plausible heating or decompression mechanisms for regional magma generation. The advection of a thermal anomaly by mantle upwelling, or regional uplift related to the thickening‐flexure‐rebound cycle in the Cordillera to the west could have triggered the regional Eocene magmatic event.

Identification of 2.0 to 2.4 Ga Nd model age crustal material in the Cheyenne belt, southeastern Wyoming: Implications for Proterozoic accretionary tectonics at the southern margin of the Wyoming craton

A detailed survey was conducted of the Nd-isotope compositions of Precambrian metamorphic rocks both within and adjacent to the Cheyenne belt, a metamorphosed crustal segment in southeastern Wyoming that defines the Proterozoic suture zone between the Archean Wyoming craton and accreted Proterozoic island arc(s). The results of the study indicate that Proterozoic miogeoclinal sedimentary rocks exposed north of the Cheyenne belt and deposited at the southern margin of the Wyoming craton contain only Archean crust-derived material and no detritus from Proterozoic island arcs approaching from the south. Similarly, Proterozoic crust exposed south of the Cheyenne belt contains no crustal material derived from Archean sources. However, within the Cheyenne belt, paragneiss of the Barber Lake block has 2.0 to 2.4 Ga Nd model ages and is interpreted as metamorphosed intermixed sedimentary detritus from both Archean and Proterozoic sources. The paragneiss is thought to represent remnants of oceanic trench or foredeep sediments deposited in close proximity to both the Archean craton and the Proterozoic island arcs, similar to sedimentary rocks with 2.0 to 2.4 Ga Nd model ages exposed in the Penokean orogen in Wisconsin. The overall lack of 2.0 to 2.4 Ga modal age crustal material in the vicinity of the Cheyenne belt may reflect the fact that much of this crustal material was overthrust onto the Wyoming craton during accretion and then eroded during postorogenic rebound of the continental margin.

New geochronologic limits on the depositional age of the Hurwitz Group, Trans-Hudson hinterland, Canada

The tectonic evolution of the Hurwitz Group, a widespread succession of predominantly metasedimentary rock up to 7 km thick in the Hearne province of the Canadian Shield, has not been fully evaluated due to a lack of absolute geochronologic data. The Hurwitz Group has been interpreted previously as either a passive-margin foredeep sequence associated with the 1.9-1.77 Ga Trans-Hudson orogen, or correlative with the Snowy Pass and 2.45 Ga Huronian Supergroups, located on the Wyoming and Superior cratons, respectively. U-Pb baddeleyite-zircon analyses from a gabbro sill yield a minimum age of 2094 +26/-17 Ma for the lower parts of the Hurwitz Group, which could be as old as ca. 2400 Ma, depending on the time of metamorphism. It is clear that the mafic magmatism and sedimentation documented in the lower Hurwitz Group are not linked temporally to the Trans-Hudson orogen, but rather are related to an earlier episode of crustal destabilization and extension, possibly the opening of the Manikewan Sea.

Evidence for Proterozoic mylonitization in the northwestern Wyoming province

The 3-km-thick Madison mylonite zone encompasses the contact between the Cherry Creek Metamorphic Suite and the pre-Cherry Creek metamorphic complex in the southern Madison Range of southwest Montana. In the shear zone, greenschist- and epidote-amphibolite-facies assemblages overprint earlier amphibolite- and granulite-facies assemblages. Large-scale metasomatic conversion of granitic gneiss to mylonite gneiss increased siderophile (Fe, Mn, Mg, P, and Ca) elements and decreased K, Al, and Si in the mylonite gneiss, suggesting extensive fluid flux in the Madison mylonite zone. Southeast-dipping foliations adjacent to the Madison mylonite zone are rotated about their northeastern strike through the vertical to northwest-dipping orientations, indicating southeast-directed thrusting of the pre-Cherry Creek metamorphic complex over the Cherry Creek Metamorphic Suite. Complete stratigraphic continuity of Cherry Creek units within the mylonite zone shows the predominance of ductile shearing. Ductile thrusting is also indicated by asymmetric fabrics, stretching lineations, finite strain, and metamorphic gradients within the shear zone. If the foliations were passively rotated during simple shear, a minimum of 10 km of distributed ductile slip occurred in the Madison mylonite zone. 40Ar/39Ar thermochronology of hornblende and muscovite adjacent to the Madison mylonite zone records a Late Archean (2.5 Ga) cooling event. Within the shear zone, a discordant 40Ar/39Ar muscovite spectrum is consistent with initial cooling to argon closure at about 1.9 Ga with subsequent disturbance at about 1.6 Ga. Hornblendes from the Madison mylonite zone show highly discordant argon-loss spectra with temperature steps starting at 1.8 Ga and ending at 2.5 Ga. These argon age spectra indicate Early Proterozoic heating and argon loss in the Madison mylonite zone, resetting and subsequent retrogressing of muscovite but only partial resetting of hornblende due to its higher argon closure temperature. This study strongly suggests Early Proterozoic (1.80-1.90 Ga and possibly later) thrusting in the Madison mylonite zone, which may be correlative with retrograde metamorphism and Early Proterozoic isotopic resetting northwest of the Madison Range. The Madison mylonite zone is probably a foreland thrust zone on the margin of a major compressional orogen of Early Proterozoic age that reworked the Archean basement of the northwestern Wyoming province.

Structural analysis of a ductile-brittle Precambrian shear zone in the Sierra Madre, Wyoming: western extension of the Cheyenne belt?

Abstract Existing plate tectonic models for the Cheyenne belt, a Proterozoic suture at the southern margin of the Archean Wyoming craton, are based largely on data from the Medicine Bow Mountains, southern Wyoming. This paper tests the hypothesis that a major structural and geochronological discontinuity in the Sierra Madre of southern Wyoming is the western extension of the Cheyenne belt. The Sierra Madre shear zone consists of a cataclastic western segment and a mylonitic eastern segment that comprises several mylonite zones. The eastern segment records early ductile thrusting (ca. 1750 Ma) and later dextral strike-slip faulting as revealed by macrostructural, microstructural, and quartz fabric analysis. Structures associated with these events are kinematically compatible and coeval with structures within the suture zone in the Medicine Bow Mountains. The correlation of shear zones in the two ranges is supported further by lithologic similarity of distinctive rocks north of and within the shear zones and the presence of unusual quartz fabrics documenting 〈 c 〉 slip in synkinematic granites in both ranges. Isolated exposures of mylonite along the dominantly cataclastic western segment suggest that the mylonitic shear zone was formerly more extensive. The western segment of the Sierra Madre shear zone is a single zone of intense cataclasis that truncated and dismembered the earlier ductile shear zone. Kinematic analysis within the cataclastic zone suggests that it formed during north-directed thrusting. East-striking, south-dipping thrust faults and associated overturned-to-the-north recumbent folds are developed in supracrustal rocks directly north of the cataclastic fault zone. This spatial association of north-vergent structures and their apparent kinematic compatibility suggests that they are genetically related. Two major northwest-striking, cataclastic, dextral strike-slip in the eastern Sierra Madre appear to merge with the cataclastic thrust fault. These cataclastic faults are interpreted as parts of a north-vergent, thrust-tear system that cut across the original suture zone as rocks in the upper plate of the thrust were transported to the north. It is not known whether this deformational event occurred in response to continued, post-suture convergence to the south or if it represents a separate and distinct Precambrian event. The prominent bend in the Sierra Madre shear zone may be an oroclinal flexure produced as a consequence of northward translation along the cataclastic thrust system.

Geochemistry and origin of Archean granites from the Black Hills, South Dakota

The Little Elk Granite (2549 Ma) and granite at Bear Mountain (BMG) (~2.5 Ga) of the Black Hills formed as a result of a collisional event along the eastern margin of the Wyoming Province during the late Archean. Geochemical modelling and Nd isotopic data indicate that the Little Elk Granite was generated by the partial melting of a slightly enriched (εNd = −1.07 to −3.69) granodioritic source that had a crustal residence time of at least 190 Ma. The medium-grained to pegmatitic, peraluminous, leucocratic BMG was produced by melting a long-lived (>600 Ma), compositionally variable, enriched (εNd = −7.6 to −12.3) crustal source. This produced a volatile-rich, rare-earth-element-poor magma that experienced crystal–melt–volatile fractionation, which resulted in a lithologically complex granite.The production of volatile-rich granites, such as the BMG and the younger Harney Peak Granite (1715 Ma), is a function of the depositional and post-depositional tectonic environment of the sedimentary source rock. These environments control protolith composition and the occurrence of dehydration and melting reactions that are necessary for the generation of these volatile-rich leucocratic granites. These types of granites are commonly related to former continental–continental accretionary boundaries, and therefore their occurrence may be used as signatures of ancient continental suture zones.

A Geochemical Study of Magmatism across a Major Terrane Boundary: Sr and Nd Isotopes in Proterozoic Granitoids of the Southern Laramie Range, Wyoming

Proterozoic granitoids in the Laramie Range of southeastern Wyoming crop out on both sides of an inferred suture between the Archean Wyoming craton and Proterozoic terranes to the south. These rocks offer an unusual opportunity to test the various hypotheses for the geometry of the suture. No difference in major or trace element composition is observed across the suture, nor is there a large difference in isotopic composition. Nd and Sr isotopic data indicate that only a small proportion of Archean material has contributed to this granitoid suite, even in two samples that intrude Archean gneiss. The isotopic data therefore require a dominantly Proterozoic lower crust both beneath the Proterozoic rocks and beneath the margin of the Archean block. This observation is inconsistent with models in which the suture between the Archean and Proterozoic blocks dips to the south. This inconsistency can be reconciled by magmatic underplating or mechanical underthrusting of Proterozoic crust beneath the Archean block during terrane accretion or post-accretion strike-slip faulting.