Archean geochronological framework of the Bighorn Mountains, Wyoming

2006 ◽  
Vol 43 (10) ◽  
pp. 1399-1418 ◽  
Author(s):  
Carol D Frost ◽  
C Mark Fanning
Keyword(s):  
Ion Microprobe ◽  
Entire Area ◽  
Wind River Range ◽  
Gneiss Complex ◽  
Granitic Intrusion ◽  
Northern Portion ◽  
Tonalitic Gneiss ◽  
Archean Crust ◽  
Wyoming Province ◽  
Bighorn Mountains

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.

Geologic Map of the Park Reservoir Quadrangle, Sheridan County, Wyoming

2020 ◽  
Vol 57 (4) ◽  
pp. 375-388
Author(s):  
Ryan Bessen ◽  
Jennifer Gifford ◽  
Zack Ledbetter ◽  
Sean McGuire ◽  
Kyle True ◽  
...  
Keyword(s):  
Structural Features ◽  
Geologic Mapping ◽  
Rock Types ◽  
Basement Rocks ◽  
Geologic Maps ◽  
Geologic Map ◽  
Tear Fault ◽  
Mapping Techniques ◽  
Wyoming Province ◽  
Bighorn Mountains

This project involved the construction of a detailed geologic map of the Park Reservoir, Wyoming 7.5-Minute Quadrangle (Scale 1:24,000). The Quadrangle occurs entirely in the Bighorn National Forest, which is a popular recreation site for thousands of people each year. This research advances the scientific understanding of the geology of the Bighorn Mountains and the Archean geology of the Wyoming Province. Traditional geologic mapping techniques were used in concert with isotopic age determinations. Our goal was to further subdivide the various phases of the 2.8–3.0 Ga Archean rocks based on their rock types, age, and structural features. This research supports the broader efforts of the Wyoming State Geological Survey to complete 1:24,000 scale geologic maps of the state. The northern part of the Bighorn Mountains is composed of the Bighorn batholith, a composite complex of intrusive bodies that were emplaced between 2.96–2.87 Ga. Our mapping of the Park Reservoir Quadrangle has revealed the presence of five different Archean quartzofeldspathic units, two sets of amphibolite and diabase dikes, a small occurrence of the Cambrian Flathead Sandstone, two Quaternary tills, and Quaternary alluvium. The Archean rock units range in age from ca. 2.96–2.75 Ga, the oldest of which are the most ancient rocks yet reported in the Bighorn batholith. All the Archean rocks have subtle but apparent planar fabric elements, which are variable in orientation and are interpreted to represent magmatic flow during emplacement. The Granite Ridge tear fault, which is the northern boundary of the Piney Creek thrust block, is mapped into the Archean core as a mylonite zone. This relationship indicates that the bounding faults of the Piney Creek thrust block were controlled by weak zones within the Precambrian basement rocks.

The Precambrian geochronology of Rajasthan and Bundelkhand, northern India

1970 ◽  
Vol 7 (1) ◽  
pp. 91-110 ◽  
Author(s):  
A. R. Crawford
Keyword(s):  
Field Studies ◽  
New Age ◽  
Peninsular India ◽  
Northern India ◽  
Gneiss Complex ◽  
Granitic Intrusion ◽  
Age Determinations

Many new age determinations are reported for the Precambrian of Rajasthan and Bundelkhand in northern peninsular India. All are by Rb–Sr and mostly from total-rock analyses. They show that the oldest rocks in the area are undated sediments intruded by the Bundelkhand and Berach Granites, dated at about 2550 m.y. The overlying Aravalli System was intruded by granites dated at between 1900 and 2100 m.y., and is succeeded by the Delhi System, which was intruded by granite dated at 1650 m.y. Other granitic intrusion at 950–1000 m.y. was followed by repeated pegmatitic intrusion. The Banded Gneiss Complex of Rajasthan contains components of ages varying from at least 2000 m.y. to less than 1000 m.y. Nepheline-syenites at Kishangarh have an age of 1490 ± 150 m.y., but a biotite in an inclusion gives 970 m.y., which is the age of the Newania carbonatite.These determinations show that the Precambrian sequence in Rajasthan is much older than previously suggested. They confirm the antiquity of the Bundelkhand–Berach craton suggested by field studies, denying its derivation from Aravalli System rocks by granitization.

Comparative petrology of gneissic rocks in southwestern Newfoundland

1992 ◽  
Vol 29 (12) ◽  
pp. 2663-2676 ◽  
Author(s):  
J. Victor Owen
Keyword(s):  
Confining Pressure ◽  
Metamorphic Grade ◽  
High Grade ◽  
Grade Metamorphism ◽  
Metasedimentary Rocks ◽  
Gneiss Complex ◽  
High Grade Metamorphism ◽  
Tonalitic Gneiss ◽  
Silicate Layers ◽  
Bulk Chemistry

In southwestern Newfoundland, pelitic migmatites of the Meelpaeg Subzone of the Gander Zone are separated by faults and plutons from metasedimentary rocks of the Port-aux-Basques gneiss complex (PBGC). The PBGC is a polymetamorphic sequence of amphibolite-facies, pelitic, semipelitic, and psammitic rocks (and associated metabasic dykes). Maximum metamorphic grade surpassed the first sillimanite (i.e., staurolite-consuming) isograd. Metamorphic conditions approached 650–700 °C at Pmax approximately 6.5–8.5 kbar (1 kbar = 100 MPa).The Meelpaeg gneisses also include sillimanite-grade, two-mica rocks, but they lack the Barrovian mineralogy (e.g., kyanite, staurolite, rutile) characterizing parts of the PBGC. The Meelpaeg rocks attained temperatures similar to those of the PBGC, but confining pressure was substantially lower (approx. 4 kbar), indicating uplift from relatively shallow structural levels.Both groups of paragneisses also differ in some aspects of their bulk chemistry (notably CaO/K2O ratios) and their lithologic associations. The Meelpaeg metapelites are less calcic and relatively potassic (mean CaO/K2O = 0.32) compared with their counterparts in the PBGC (mean CaO/K2O = 1.12), but both groups of rocks have similar bulk Fet/(Fet + Mg) ratios (mean XFe ≈ 0.75). In contrast with the PBGC, which contains abundant metabasites and thin coticule-like (garnet + quartz) seams, the Meelpaeg metapelites are associated with biotite + garnet "tonalitic" gneiss and, despite their relatively lime-poor composition, calc-silicate layers and pods.In terms of contrasting lithologic associations and bulk chemistry, paragneiss of the PBGC is distinct from gneissic rocks in the Meelpaeg Subzone. This underscores difficulties in relating rocks in the Port-aux-Basques area to well-established lithotectonic entities elsewhere in Newfoundland. Despite apparent differences in their protoliths and contrasts in metamorphic pressure, available U–Pb data suggest that high-grade metamorphism in both areas occurred during the middle Silurian.

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

2006 ◽  
Vol 43 (10) ◽  
pp. 1489-1510 ◽  
Author(s):  
B Ronald Frost ◽  
Carol D Frost ◽  
Mary Cornia ◽  
Kevin R Chamberlain ◽  
Robert Kirkwood
Keyword(s):  
Plate Tectonics ◽  
Accretionary Prism ◽  
Granulite Facies ◽  
Isotopic Data ◽  
Active Margin ◽  
Magmatic Arc ◽  
Wind River Range ◽  
Teton Range ◽  
Wyoming Province ◽  
Back Arc

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.

scholarly journals Tectonostratigraphic terranes within Archaean gneiss complexes: examples from Western Australia and southern West Greenland

1991 ◽  
Vol 39 ◽  
pp. 199-211
Author(s):  
Allen P. Nutman
Keyword(s):  
Western Australia ◽  
Field Work ◽  
Ion Microprobe ◽  
Late Archaean ◽  
West Greenland ◽  
Tectonic Processes ◽  
Gneiss Complex ◽  
Northwestern Margin ◽  
Geological Boundaries ◽  
Orogenic Belts

New field work and isotopic data show that the Godthabsfjord region of West Greenland consists of a collage of tectonostratigraphic terranes, which evolved separately prior to tectonic juxtaposition in the late Archaean. In Western Australia the Narryer Gneiss Complex, which lies on the northwestern margin of the Yilgarn Craton, is, unlike the Godthabsfjord region, very poorly exposed (less than 1 % ). In consequence it is impossible to follow geological boundaries in this complex, and instead the complex has been studied by a very extensive use of within-grain zircon U-Pb geochronology on the ion microprobe SHRIMP. The zircon geochronology suggests that the Narryer Gneiss Complex also consists of several discrete terranes of early to mid Archaean gneisses. In both the Godthabsfjord region and the Narryer Gneiss Complex, late Archaean juxtaposition of terranes was accompanied by intrusion of crustally­derived granites, deformation, and amphibolite facies metamorphism. Thus some Archaean high grade gneiss complexes consist of terranes that underwent independent evolution until they were brought together at a later time. In this respect their anatomy resembles post-Archaean orogenic belts that formed as a consequence of plate tectonic processes.

Evidence for an Early Archean component in the Middle to Late Archean gneisses of the Wind River Range, west-central Wyoming: conventional and ion microprobe U-Pb data

1989 ◽  
Vol 101 (2) ◽  
pp. 198-206 ◽  
Author(s):  
John N. Aleinikoff ◽  
Ian S. Williams ◽  
William Compston ◽  
John S. Stuckless ◽  
Ronald G. Worl
Keyword(s):  
Ion Microprobe ◽  
Wind River Range ◽  
West Central

scholarly journals Palaeoproterozoic age of a basement gneiss complex in the Charcot Land tectonic window, East Greenland Caledonides

2004 ◽  
Vol 6 ◽  
pp. 57-66 ◽  
Author(s):  
Kristine Thrane
Keyword(s):  
Structural Characteristics ◽  
Crystalline Basement ◽  
Ion Microprobe ◽  
Sr Isotope ◽  
East Greenland ◽  
Gneiss Complex ◽  
Form Part ◽  
Thrust Sheets ◽  
Tectonic Window ◽  
Basement Gneiss

The Charcot Land tectonic window exposes crystalline basement gneisses, which form part of the foreland of the East Greenland Caledonides. These gneisses were previously believed to be Archaean in age, on the basis of imprecise K-Ar analyses carried out in the early 1980s on hornblende from amphibolitic bands and inconclusive Rb/Sr isotope data. New U-Pb singlezircon ion microprobe analyses on the gneisses of the window yield upper intercept ages of 1916 ± 21 and 1928 ± 11 Ma, and are interpreted to represent the age of crystallisation of the igneous protolith. The foreland gneisses of the Charcot Land window are similar in age to parts of the allochthonous gneiss complexes of structurally overlying thrust sheets, but the two terranes have different lithological and structural characteristics. No Archaean rocks have been identified with certainty in any of the East Greenland Caledonian foreland windows.

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