Grampian and late Grenville events recorded by mineral geochronology near a basement–cover contact in north Mayo, Ireland
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The Palaeoproterozoic to early Neoproterozoic Annagh Gneiss Complex structurally underlies the Dalradian sequence in north Mayo, Ireland, and has been proposed as the depositional basement to Dalradian metasediments. The Annagh Gneiss Complex was deformed, metamorphosed and migmatized during the Grenville Orogeny and later reworked under amphibolite-facies conditions. This paper focuses on the timing of the post-Grenville events and particularly on the possible presence of post-Grenville, pre-Grampian deformation that could be attributed to the Knoydartian Orogeny. Seven U–Pb titanite analyses from Annagh Gneiss Complex gneisses have a weighted mean 207 Pb/ 206 Pb age of 963 ± 8 Ma, which dates cooling after the main Grenville metamorphism. Locally, a later phase of titanite growth at 943 ± 8 Ma post-dates the last phase of Grenville deformation. The weak discordance of the titanite data suggests that post-Grenville events had little effect on the U–Pb system in titanite. If the discordance was caused by a tectonic event, this is likely to have occurred during the early Ordovician Grampian Orogeny rather than in the Neoproterozoic. Within the Annagh Gneiss Complex, cross-cutting metadolerites provide a structural marker allowing post-Grenville deformation to be distinguished. In contrast, correlative metadolerites cutting the adjacent Dalradian metasediments share all Grampian deformation events affecting their host. Ar–Ar hornblende ages from the post-Grenville metadolerites indicate that reworking of the Annagh Gneiss Complex and the first episodes of Dalradian deformation occurred during the Grampian Orogeny in this part of Ireland. One sample yields a 475 ± 4 Ma Ar–Ar plateau age, which is interpreted to date Grampian deformation. Younger Ar–Ar hornblende and Rb–Sr mica ages record post-Grampian cooling. Neither field nor isotopic evidence for the Knoydartian Orogeny has been found in this part of Ireland.Keywords:
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Association Round Table 555 high/low or shallow/deep.Special facies types include stromatoporoid and evaporite, both supratidal and deep.Facies and fabrics vary considerably throughout the sequence, both interformationally and intraformationally.Mudstones, wackestones, and packstones are most common although grainstones and boundstones also occur.Within textural constraints, each fabric contains their respective amounts of skeletal and nonskeletal allochems.Because of frequent and sharp facies changes, it is important to discriminate among different facies that superficially have similar fabrics.Examples are deep/shallow evaporites, or supratidal/subtidal oolites and pisolites.Peloidal wackestones/grainstones, skeletal wackestones/packstones, and mottled mudstones are the prevalent fabric types.Significant sedimentary structures include burrows, flat pebble interclasts, desiccation cracks, bird's-eye structures, and collapse breccias.Porosity types common to all, except for the Ashern and Bakken, are intercrystal, interparticle, moldic, vuggy, and breccia.Significant porosity in the Ashern and Bakken formations is from fractures.
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U-Pb measurements on zircons separated from Precambrian basement rocks from Oklahoma yield the following: (1) the Spavinaw Granite, which appears in outcrops in Mayes County, northeastern Oklahoma, is 1,370 ± 20 m.y. old; (2) the ages of rocks exposed in the eastern Arbuckle Mountains in southeastern Oklahoma are: Tishomingo Granite, 1,374 ± 15 m.y.; Troy Granite, 1,399 ± 95 m.y.; and Blue River Gneiss, 1,396 ± 40 m.y. The granophyric Spavinaw Granite is associated with rhyolitic volcanic rocks and was clearly emplaced in the shallow crust; rocks of similar age and petrography occur in the subsurface in southeastern Kansas. The rocks exposed in the eastern Arbuckles, although of similar age, have textures suggesting emplacement at deeper crustal levels.
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The Proterozoic Baldwin gneiss in the central Transverse Ranges of southern California, a part of the Mojave crustal province, is composed of quartzofeldspathic gneiss and schist, augen and granitic gneiss, trondhjemite gneiss, and minor quartzite, amphibolite, metagabbro, and metapyroxenite. Sensitive high resolution ion microprobe (SHRIMP) data indicate that augen and granitic gneisses comprise a magmatic arc intrusive suite emplaced between 1783 ± 12 and 1675 ± 19 Ma, adjacent to or through thinned Archean crust. High U/Th rims on zircons in most samples suggest an early metamorphic event at ∼1741 Ma, but peak amphibolite facies metamorphism and penetrative, west vergent deformation occurred after 1675 Ma. The Baldwin gneiss is part of a regional allochthon emplaced by west vergent deformation over a Proterozoic shelf‐slope sequence (Joshua Tree terrane). We hypothesize that emplacement of this regional allochthon occurred during a late Early or Middle Proterozoic arc‐continent collision along the western margin of Laurentia.
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Research Article| February 01, 1977 Reconnaissance geochronology of the crystalline basement rocks of the Coastal Cordillera of southern Peru E. J. COBBING; E. J. COBBING 1Institute of Geological Sciences, 154 Clerkenwell Road, London EC1R 5DU, England Search for other works by this author on: GSW Google Scholar J. M. OZARD; J. M. OZARD 2Defence Research Establishment Pacific, Fleetmail Office, Victoria, B.C., Canada Search for other works by this author on: GSW Google Scholar N. J. SNELLING N. J. SNELLING 3Institute of Geological Sciences, 64/78 Gray's Inn Road, London WC1X 8NG, England Search for other works by this author on: GSW Google Scholar Author and Article Information E. J. COBBING 1Institute of Geological Sciences, 154 Clerkenwell Road, London EC1R 5DU, England J. M. OZARD 2Defence Research Establishment Pacific, Fleetmail Office, Victoria, B.C., Canada N. J. SNELLING 3Institute of Geological Sciences, 64/78 Gray's Inn Road, London WC1X 8NG, England Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1977) 88 (2): 241–246. https://doi.org/10.1130/0016-7606(1977)88<241:RGOTCB>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation E. J. COBBING, J. M. OZARD, N. J. SNELLING; Reconnaissance geochronology of the crystalline basement rocks of the Coastal Cordillera of southern Peru. GSA Bulletin 1977;; 88 (2): 241–246. doi: https://doi.org/10.1130/0016-7606(1977)88<241:RGOTCB>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Granulite-facies gneiss of the Arequipa massif extensively developed along the Coastal Cordillera of southern Peru gives a whole-rock isochron age of 1,811 ± 39 m.y. (λ 87Rb = 1.47 × 10−11yr−1) and an initial 87Sr/86Sr ratio of 0.7086 ± 0.0009. Migmatitic granite within the gneiss is probably of late Precambrian or early Paleozoic age. Mineral ages, both K-Ar and Rb-Sr, from the gneisses and granites are disturbed. Potassium feldspar from the gneiss appears to have been reset by the migmatitization event, whereas mica and potassium feldspar from the migmatitic granite appear to reflect established tectonic-thermal events of Late Devonian and Late Triassic time.In both metamorphic grade and age pattern, these rocks are similar to those of the Trans-Amazonian nucleus of the Guianas, Brazil, Uruguay, and the Buenos Aires province of Argentina. We suggest that the Arequipa massif has always been an integral part of the Trans-Amazonian nucleus and that the Andean belt is ensialic, at least in Peru. The manner in which the structural trends in the Arequipa massif strike into the Pacific Ocean leads to speculation concerning the evolution of the Pacific. Simple rifting, tectonic erosion, and major transcurrent faulting are considered as possible mechanisms to explain the truncation. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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Precambrian basement rocks exposed within tectonic windows in the North American Cordillera help to define the Precambrian crustal structure of western North America and possible reconstructions of the Late Proterozoic supercontinent Rodinia. New geologic mapping and U–Pb dating in the infrastructure of the Priest River metamorphic complex, northern Idaho, documents the first Archean basement (2651 ± 20 Ma) north of the Snake River Plain in the North American Cordillera. The Archean rocks are exposed in the core of an antiform and mantled by a metaquartzite that may represent the nonconformity between basement and the overlying Hauser Lake gneiss, which is correlated with the Prichard Formation of the Belt Supergroup. A structurally higher sheet of augen gneiss interleaved with the Hauser Lake gneiss yields a U–Pb zircon crystallization age somewhat greater than 1577 Ma. The slivers of augen gneiss were tectonically interleaved with the surrounding Hauser Lake gneiss near the base of the Spokane dome mylonite zone, which arches across this part of the Priest River complex. We conclude that the Spokane dome mylonite zone lies above the Archean basement–cover contact and that it was, in part, equivalent to the basal decollement of the Rocky Mountain fold and thrust belt. New U–Pb dates on metamorphic monazite and xenotime reveal peak metamorphism at ca. 72 Ma, compatible with movement along the Spokane dome mylonite zone at that time. The Archean basement could be interpreted as the western extension of the Hearne province, or a new Archean basement terrane separated from the Hearne province by an Early Proterozoic suture. The unique assemblage of 2.65 Ga basement, ~1.58 Ga felsic intrusive rocks, and the Middle Proterozoic Belt Supergroup can be used as a piercing point for the identification of the conjugate margin to Laurentia. Our new dating supports previous correlations of Australia’s Gawler craton (2.55–2.65 Ga) and its 1590 Ma plutons with the Priest River complex basement gneisses. The Priest River complex basement may be a piece of eastern Australia stranded during rifting of the supercontinent Rodina in the Late
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