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RIZZOTTO, GILMAR J., LÉO A. HARTMANN, JOÃO O. S. SANTOS i NEAL J. MCNAUGHTON. "Tectonic evolution of the southern margin of the Amazonian craton in the late Mesoproterozoic based on field relationships and zircon U-Pb geochronology". Anais da Academia Brasileira de Ciências 86, nr 1 (marzec 2014): 57–84. http://dx.doi.org/10.1590/0001-37652014104212.
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New U-Pb zircon geochronological data integrated with field relationships and an airborne geophysical survey suggest that the Nova Brasilândia and Aguapeí belts are part of the same monocyclic, metaigneous and metasedimentary belt formed in the late Mesoproterozoic (1150 Ma-1110 Ma). This geological history is very similar to the within-plate origin of the Sunsás belt, in eastern Bolivia. Thus, we propose that the Nova Brasilândia, Aguapeí and Sunsás belts represent a unique geotectonic unit (here termed the Western Amazon belt) that became amalgamated at the end of the Mesoproterozoic and originated through the reactivation of a paleo-suture (Guaporé suture zone) in an intracontinental rift environment. Therefore, its geological history involves a short, complete Wilson cycle of ca. 40 Ma. Globally, this tectonic evolution may be related with the final breakup of the supercontinent Columbia. Mafic rocks and trondhjemites in the northernmost portion of the belt yielded U-Pb zircon ages ca. 1110 Ma, which dates the high-grade metamorphism and the closure of the rift. This indicates that the breakup of supercontinent Columbia was followed in short sequence by the assembly of supercontinent Rodinia at ca. 1.1-1.0 Ga and that the Western Amazon belt was formed during the accretion of the Arequipa-Antofalla basement to the Amazonian craton.
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MAZUR, STANISŁAW, ALFRED KRÖNER, JACEK SZCZEPAŃSKI, KRZYSZTOF TURNIAK, PAVEL HANŽL, ROSTISTLAV MELICHAR, NICKOLAY V. RODIONOV, ILYA PADERIN i SERGEY A. SERGEEV. "Single zircon U–Pb ages and geochemistry of granitoid gneisses from SW Poland: evidence for an Avalonian affinity of the Brunian microcontinent". Geological Magazine 147, nr 4 (15.01.2010): 508–26. http://dx.doi.org/10.1017/s001675680999080x.
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AbstractSeven granitoid gneisses from the contact zone between the eastern margin of the Variscan belt and the Brunian microcontinent in SW Poland have been dated by ion-microprobe and207Pb/206Pb single zircon evaporation methods. The zircons define two age groups for the gneiss protoliths: (1) late Neoproterozoicc.576–560 Ma and (2) early Palaeozoicc.488–503 Ma granites. The granitoid gneisses belonging to the basement of the Brunian microcontinent contain abundant Mesoproterozoic to latest Palaeoproterozoic inherited material in the range of 1200–1750 Ma. The gneisses of the Variscan crustal domain lack Mesoproterozoic inherited zircon cores. Trace element geochemistry of Proterozoic gneisses reveals features resembling either volcanic arc or post-collisional granites. The studied rocks are geochemically similar to other Proterozoic orthogneisses derived from the basement of the Brunian microcontinent. Gneisses with early Palaeozoic protolith ages are geochemically comparable to granitoid gneisses widespread in the adjacent Sudetic part of the Bohemian Massif and are considered characteristic of peri-Gondwanan crust. Our data prove the dissimilarity between the Brunia plate and the westerly terranes of the Variscan belt. The occurrence of granitic gneisses with late Neoproterozoic protolith ages and widespread Mesoproterozoic inheritance in our dated samples support an East Avalonian affinity for the Brunian microcontinent. In contrast, the abundance of gneisses derived from an early Palaeozoic granitic protolith and devoid of Mesoproterozoic zircon cores supports the Armorican affinity of the Variscan domain bordering on the Brunia plate from the west. Structural evidence shows that the eastern segment of the Variscan belt is juxtaposed against the Brunian microcontinent along a N–S-trending tectonic contact, possibly equivalent to the Rheic suture.
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Naganjaneyulu, K., i M. Santosh. "The Central India Tectonic Zone: A geophysical perspective on continental amalgamation along a Mesoproterozoic suture". Gondwana Research 18, nr 4 (listopad 2010): 547–64. http://dx.doi.org/10.1016/j.gr.2010.02.017.
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Hynes, Andrew, i Toby Rivers. "Protracted continental collision — evidence from the Grenville OrogenThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent." Canadian Journal of Earth Sciences 47, nr 5 (maj 2010): 591–620. http://dx.doi.org/10.1139/e10-003.
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The Grenville Orogen in North America is interpreted to have resulted from collision between Laurentia and another continent, probably Amazonia, at ca. 1100 Ma. The exposed segment of the orogen was derived largely from reworked Archean to Paleoproterozoic Laurentian crust, products of a long-lived Mesoproterozoic continental-margin arc and associated back arc, and remnants of one or more accreted mid-Mesoproterozoic island-arc terranes. A potential suture, preserved in Grenvillian inliers of the southeastern USA, may separate rocks of Laurentian and Amazonian affinities. The Grenvillian Orogeny lasted more than 100 million years. Much of the interior Grenville Province, with peak metamorphism at ca. 1090–1020 Ma, consists of uppermost amphibolite- to granulite-facies rocks metamorphosed at depths of ca. 30 km, but areas of lower crustal, eclogite-facies nappes metamorphosed at 50–60 km depth also occur and an orogenic lid that largely escaped Grenvillian metamorphism is preserved locally. Overall, deformation and regional metamorphism migrated sequentially to the northwest into the Laurentian craton, with the youngest contractional structures in the northwestern part of the orogen at ca. 1000–980 Ma. The North American lithospheric root extends across part of the Grenville Orogen, where it may have been produced by depletion of sub-continental lithospheric mantle beneath the long-lived Laurentian-margin Mesoproterozoic subduction zone. Both the Grenville Orogen and the Himalaya–Tibet Orogen have northern margins characterized by long-lived subduction before continental collision and protracted convergence following collision. Both exhibit cratonward-propagating thrusting. In the Himalaya–Tibet Orogen, however, the pre-collisional Eurasian-margin arc is high in the structural stack, whereas in the Grenville Orogen, the pre-collisional continental-margin arc is low in the structural stack. We interpret this difference as due to subduction reversal in the Grenville case shortly before collision, so that the continental-margin arc became the lower plate during the ensuing orogeny. The structurally low position of the warm, extended Laurentian crust probably contributed significantly to the ductility of lower and mid-crustal Grenvillian rocks.
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Shellnutt, J. G. "The enigmatic continental crust of North-Central Africa: Saharan Metacraton or Central Sahara Shield?" South African Journal of Geology 124, nr 2 (1.06.2021): 383–90. http://dx.doi.org/10.25131/sajg.124.0047.
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Abstract The continental crust of North-Central Africa between the Tuareg and Arabian-Nubian shields and south to the Central African Orogenic Belt is enigmatic due to the few bedrock exposures especially within the central region. The current understanding, based on a review of geochronology and isotope geochemistry, is that the central Sahara region is a large, coherent craton that was ‘highly remobilized’ during the Late Neoproterozoic amalgamation of Gondwana and referred to as the Saharan Metacraton. However, new data from the Guéra, Ouaddaï, and Mayo Kebbi massifs and the Lake Fitri inlier of Chad suggest that it may be a composite terrane of older cratonic blocks or microcontinents with intervening Mesoproterozoic to Neoproterozoic domains and referred to as the ‘Central Sahara Shield’. It is postulated that the older crust and juvenile crust were sutured together along a Pan-Gondwana collisional belt (Central Sahara Belt) that bisects the central Sahara region. The ‘Central Sahara Shield’ hypothesis suggests the Chad Lineament, a narrow arcuate gravity anomaly within central Chad, could be a collisional belt suture zone and that it may explain the existence of the relatively juvenile crust that typifies southern and eastern Chad. The new data improves upon the existing knowledge and challenges the lithotectonic paradigm of the Saharan Metacraton. Further investigations are required to fully characterize the crust of the central Sahara region and to test the contrasting hypotheses.
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Tohver, E., B. A. van der Pluijm, K. Mezger, J. E. Scandolara i E. J. Essene. "Two stage tectonic history of the SW Amazon craton in the late Mesoproterozoic: identifying a cryptic suture zone". Precambrian Research 137, nr 1-2 (kwiecień 2005): 35–59. http://dx.doi.org/10.1016/j.precamres.2005.01.002.
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Murphy, J. Brendan, i Gabriel Gutiérrez-Alonso. "The origin of the Variscan upper allochthons in the Ortegal Complex, northwestern Iberia: Sm–Nd isotopic constraints on the closure of the Rheic Ocean". Canadian Journal of Earth Sciences 45, nr 6 (czerwiec 2008): 651–68. http://dx.doi.org/10.1139/e08-019.
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Northwestern Iberia preserves a stack of allochthons in which the vestiges of a suture zone generated during the Variscan orogeny by the late Paleozoic collision between Laurussia and Gondwana are exposed. Lower allochthons contain Ordovician ophiolite (known as the Lower Ophiolite), and are structurally overlain by Devonian ophiolite (Upper Ophiolite), which are in turn structurally overlain by allochthons containing high-grade metamorphic rocks with continental affinities and Late Cambrian – Early Ordovician protolith ages (known as the Upper Units). Geochemical and Sm–Nd isotopic data from the Upper Ophiolite and the structurally overlying Upper Units exposed in the Ortegal Complex of Galicia show that these allochthons are derived from a variety of mantle and crustal sources and indicate that the suture zone juxtaposes a variety of oceanic assemblages. The general isotopic characteristics of each assemblage are similar to allochthons in other Variscan complexes in NW Iberia suggesting that the allochthons are each derived from a common source and may be regionally extensive. One of the bodies mapped within the Upper Ophiolite (Purrido amphibolite) is a composite body that, in addition to recently identified Mesoproterozoic mafic rocks, is characterized by a juvenile signature at ca. 395 Ma that was chemically modified from coeval intra-oceanic subduction. The very high ϵNd of this Late Devonian ophiolite is typical of several penecontemporaneous ophiolites within the Variscan orogen including the Lizard Complex (Britain) and the Massif Central (France), suggesting derivation from a regionally extensive anomalous mantle characterized by time-integrated depletion in Nd relative to Sm. Paleozoic mafic rocks in the Upper Units have ϵNd values typical of Paleozoic mafic rocks in Avalonia, which are thought to have been derived from subcontinental lithospheric mantle (SCLM) that was enriched at ca. 1.0 Ga. They exhibit elevated Th/Yb and Ce/Yb relative to Ta/Yb suggesting that their composition has been contaminated by subduction zone components, although the age of this contamination is unclear. Felsic rocks in the Upper Units were derived by melting of Mesoproterozoic or older (West African?) crust. These data, when combined with other geologic constraints, including the outboard position of the Upper Units relative to the ophiolite, support the hypothesis that the Upper Units collectively represent a crustal fragment that drifted from Gondwana during the formation of the Rheic Ocean, was transferred to Laurussia in Silurian or early Devonian times, and was subsequently thrust over the Gondwanan margin during the closure of the Rheic Ocean and the Variscan orogenesis.
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Ershova, Victoria, Andrei Prokopiev, Daniel Stockli, Daria Zbukova i Anton Shmanyak. "Provenance and Stratigraphy of the Upper Carboniferous—Lower Permian Strata of October Revolution Island (Severnaya Zemlya Archipelago): Implications for Geological History of the Russian High Arctic". Minerals 12, nr 10 (20.10.2022): 1325. http://dx.doi.org/10.3390/min12101325.
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Small depressions across the north-eastern part of October Revolution Island (Severnaya Zemlya archipelago, Kara terrane) are filled with continental terrigenous rocks, dated as Upper Carboniferous–Lower Permian in age based on palynological data. These rocks overlie Ordovician volcaniclastic rocks above a prominent angular unconformity. U-Pb dating of detrital zircons from the Late Carboniferous–Lower Permian rocks reveals that most grains are Ordovician in age, ranging between 475–455 Ma. A subordinate population of Silurian detrital zircons is also present, contributing up to 15% of the dated population, while Precambrian grains mainly yield Neo-Mesoproterozoic ages and do not form prominent peaks. The combined U-Pb and (U-Th)/He ages indicate that most zircon (U-Th)/He ages were reset and average at ca. 317 Ma, suggesting ~6–7 km of Late Carboniferous uplift within the provenance area. This provenance area, mainly comprising Ordovician magmatic and volcanic rocks, was located close to the study area based on the coarse-grained nature of Late Carboniferous–Lower Permian rocks of north-eastern October Revolution Island. Therefore, we propose that Late Paleozoic tectonism significantly affected both the southern margin of the Kara terrane, as previously supposed, and also its north-eastern part. We propose that the Late Paleozoic Uralian suture zone continued to the north-eastern October Revolution Island and was responsible for the significant tectonic uplift of the studied region. This suture zone is now hidden beneath the younger Arctic basins.
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DOSTAL, J., J. D. KEPPIE, M. A. HAMILTON, E. M. AARAB, J. P. LEFORT i J. B. MURPHY. "Crustal xenoliths in Triassic lamprophyre dykes in western Morocco: tectonic implications for the Rheic Ocean suture". Geological Magazine 142, nr 2 (marzec 2005): 159–72. http://dx.doi.org/10.1017/s0016756805000440.
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Dykes of calc-alkaline lamprophyre cutting granite of the Hercynian Jebilet Massif of the Moroccan Meseta (western Morocco) contain crustal xenoliths. The xenoliths range in composition from mafic (cognate cumulates) and upper crustal granitic rocks through gneisses to middle crustal felsic granulites. SHRIMP U–Th–Pb zircon analyses of these rocks indicate that the dykes were likely intruded during Middle Triassic times (∼235 Ma), whereas the xenoliths contain zircons with concordant Carboniferous–Early Permian, Neoproterozoic and Palaeoproterozoic ages (280–328 Ma, c. 540–615 Ma, 700 Ma and ∼2000 Ma). The 280–328 Ma ages appear to record synchronous intrusive and high-grade (up to granulite facies) Variscan metamorphic events, suggesting that high-grade metamorphism may have facilitated the S-type granitic magmatism. On the other hand, the ∼540–615 Ma, 700 Ma and 2000 Ma ages correspond with Pan-African and Eburnian orogenic events recorded in the West African Craton. In a Triassic reconstruction, Morocco is juxtaposed against Nova Scotia (Canada), and some have proposed that the basement of the easternmost terrane (Meguma terrane) is a piece of the West African craton. However, lower crustal xenoliths from Devonian dykes (∼370 Ma) cutting the Meguma terrane have yielded Late Devonian, Neo- and Mesoproterozoic ages (378 Ma, 575–629 Ma, ∼880–1050 Ma and ∼1530 Ma). The presence of ∼1 Ga ages suggests that the basement of the Meguma terrane is Avalonian rather than West African, implying that in a Pangean reconstruction, the Rheic Ocean suture between NW Africa and Maritime Canada coincides with the Atlantic Ocean.
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McMechan, M. E., K. G. Root, P. S. Simony i D. R. M. Pattison. "Nailed to the craton: Stratigraphic continuity across the southeastern Canadian Cordillera with tectonic implications for ribbon continent models". Geology 49, nr 1 (18.09.2020): 101–5. http://dx.doi.org/10.1130/g48060.1.
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Abstract Cambrian and Upper Devonian to Mississippian strata can be confidently traced westward, without strike-slip offset, from the autochthonous section above North American basement into the southeastern Canadian Cordillera, and are thus “nailed” to the craton. These strata are in turn stratigraphically pinned to older (Mesoproterozoic Belt-Purcell Supergroup, Neoproterozoic Windermere Supergroup, and Ediacaran), intermediate-aged (Ordovician–Silurian), and younger (Permian to Middle Jurassic) strata found only in the mountains, thus linking them to the adjacent autochthonous craton. The overlapping distribution of linking successions, regionally traceable unique stratigraphic horizons in the Belt-Purcell and Windermere Supergroups, and across-strike stratigraphic features show that the entire Cariboo, northern Selkirk, Purcell, and Rocky Mountains are directly tied to the adjacent North American craton without discernible strike-slip or oblique displacement, or substantial purely convergent plate-scale (>400 km) horizontal displacement. They link the entire width of the Belt-Purcell and Windermere basins in the southeastern Canadian Cordillera to the adjacent craton and show that any proposed Cretaceous ribbon continent suture, with its thousands of kilometers of proposed displacement, cannot run through the southeastern Canadian Cordillera.
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Chatterjee, Nilanjan, A. C. Mazumdar, A. Bhattacharya i R. R. Saikia. "Mesoproterozoic granulites of the Shillong–Meghalaya Plateau: Evidence of westward continuation of the Prydz Bay Pan-African suture into Northeastern India". Precambrian Research 152, nr 1-2 (styczeń 2007): 1–26. http://dx.doi.org/10.1016/j.precamres.2006.08.011.
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Peck, William H., i Michael S. Smith. "Cordierite-gedrite rocks from the Central Metasedimentary Belt boundary thrust zone (Grenville Province, Ontario): Mesoproterozoic metavolcanic rocks with affinities to the Composite Arc Belt". Canadian Journal of Earth Sciences 42, nr 10 (1.10.2005): 1815–28. http://dx.doi.org/10.1139/e05-071.
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Cordieritegedrite rocks in the southern Grenville Province occur near the base of the Central Metasedimentary Belt boundary thrust zone, interpreted by some as a crustal suture between the 1.291.24 Ga Composite Arc Belt and >1.4 Ga rocks of Laurentia. Major and trace-element compositions of these rocks are consistent with volcanic protoliths that range in composition from basalt to dacite. These cordieritegedrite rocks have low CaO (average 1.2 wt.%) and major element and oxygen-isotope ratios suggestive of hydrothermal alteration before metamorphism. Rare-earth element (REE) compositions also indicate igneous protoliths, although some REE patterns have been modified by local melt extraction. The trace-element compositions of cordieritegedrite rocks, and neodymium-isotope systematics, are similar to those of metavolcanic rocks in the Composite Arc Belt and are consistent with the extension of the Composite Arc Belt to the base of the boundary thrust zone.
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Rizzotto, Gilmar José, João Orestes S. Santos, Léo Afraneo Hartmann, Eric Tohver, Márcio Martins Pimentel i Neal J. McNaughton. "The Mesoproterozoic Guaporé suture in the SW Amazonian Craton: Geotectonic implications based on field geology, zircon geochronology and Nd–Sr isotope geochemistry". Journal of South American Earth Sciences 48 (grudzień 2013): 271–95. http://dx.doi.org/10.1016/j.jsames.2013.10.001.
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Cutts, J. A., A. Zagorevski, V. McNicoll i S. D. Carr. "Tectono-stratigraphic setting of the Moreton’s Harbour Group and its implications for the evolution of the Laurentian margin: Notre Dame Bay, Newfoundland1This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology." Canadian Journal of Earth Sciences 49, nr 1 (styczeń 2012): 111–27. http://dx.doi.org/10.1139/e11-040.
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The Moreton’s Harbour Group lies along the Red Indian Line, the fundamental Iapetus suture that separates rocks of peri-Laurentian affinity with rocks of peri-Gondwanan affinity in the Newfoundland Appalachians. Characterization of age and environment of formation of the Moreton’s Harbour Group is an important constraint on evolution of the Laurentian margin during Ordovician closure of Iapetus Ocean and associated marginal basins. The Moreton’s Harbour Group comprises a fault-bounded ophiolitic sequence of layered gabbro, sheeted diabase, pillow basalt, and felsic intrusive rocks. It is offset by high-angle shear zones that were contemporaneous with a 477.4 ± 0.4 Ma syn-tectonic and syn-magmatic suite of trondhjemite and tonalite. Trace element data from the felsic suite indicate formation in a supra-subduction zone setting, although isotopic data from the felsic intrusive rocks (εNd (–5.02) to (–10.53), Tdm 1200–1800 Ma) indicate a significant amount of contamination from Mesoproterozoic or older continental crust. The age and tectonic setting of the Moreton’s Harbour Group suggest that it is the northernmost extent of the ca. 480 Ma Annieopsquotch Ophiolite Belt. We present a model in which the Moreton’s Harbour Group formed in response to propagation of the Annieopsquotch Ophiolite Belt spreading centre into the Dashwoods microcontinent. This ridge propagation model supports the formation of the Annieopsquotch Ophiolite Belt immediately outboard of Dashwoods and explains its rapid accretion to the composite Laurentian margin.
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Hughes, K. Stephen, James P. Hibbard, Jeffrey C. Pollock, David J. Lewis i Brent V. Miller. "Detrital Zircon Geochronology Across the Chopawamsic Fault, Western Piedmont of North-Central Virginia: Implications for the Main Iapetan Suture in the Southern Appalachian Orogen". Geoscience Canada 41, nr 4 (3.12.2014): 503. http://dx.doi.org/10.12789/geocanj.2014.41.052.
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The Chopawamsic fault potentially represents the main Iapetan suture, previously unidentified in the southern extent of the Appalachian orogen. The fault trends through the north-central portion of the western Piedmont of Virginia and separates the composite metaclastic Potomac terrane, commonly interpreted to be of Laurentian affinity, from the Chopawamsic terrane, the remains of a Middle Ordovician volcanic arc of uncertain crustal affinity. To gain insight on the first-order orogenic significance of the Chopawamsic fault, we report the results of LA–ICP–MS U–Pb analyses of 1,289 detrital zircons from 13 metasedimentary rock samples collected from both sides of the fault. The near exclusivity of Middle Ordovician zircon grains (ca. 470 – 460 Ma) in four sampled metasedimentary rocks of the Chopawamsic Formation likely represents the detrital recycling of syndepositional Chopawamsic volcanic rocks. A subset of Cambrian and older grains hint at one or more additional, older sources. Samples from the Potomac terrane include mostly Mesoproterozoic zircon grains and these results are consistent with previous interpretations that the metaclastic rocks are Laurentian-derived. The youngest zircons (ca. 550 – 500 Ma) and the age of cross-cutting plutons indicate that at least some parts of the Potomac terrane are Late Cambrian – Early Ordovician. The results imply temporally discrete and geographically isolated sedimentary systems during deposition of sedimentary rocks in the Chopawamsic and Potomac terranes. Metasedimentary rocks near Storck, Virginia, previously identified as a successor basin, contain detrital zircon populations that indicate they are actually peri-Gondwanan derived metasedimentary rocks unrelated to a successor basin system; their geographic position between the Laurentian-derived Potomac terrane and the Chopawamsic terrane suggests a peri-Gondwanan affinity for the Chopawamsic arc and geographic separation of the Chopawamsic and Potomac terranes in the Middle Ordovician. Consequently, we tentatively support the hypothesis that the Chopawamsic fault system represents the main Iapetan suture in the southern Appalachian orogen. Most detrital zircons from samples of the Arvonia successor basin crystallized in the Ordovician—Silurian or Mesoproterozoic. These data suggest that the Arvonia basin was deposited in the latest Ordovician to Early Silurian only after the Late Ordovician accretion of the Chopawamsic arc to Laurentia. SOMMAIRELa faille de Chopawamsic représente peut-être la principale suture japétienne, non-reconnue dans prolongement sud de l’orogène des Appalaches. La faille traverse la portion nord du centre du piedmont ouest de Virginie et sépare le terrane métaclastique de Potomac, d’affinité laurentienne pensait-on, du terrane de Chopawamsic, vestige d’un arc volcanique de l’Ordovicien moyen d’affinité crustale incertain. Afin de mettre en lumière la signification orogénique première de la faille de Chopawamsic, nous présentons les résultats d’analyses U-Pb par ICP–MS par AL sur 1 289 zircons détritiques provenant de 13 échantillons de roches métasédimentaires prélevés de chaque côté de la faille. L’existence quasi-exclusive de grains de zircon de l’Ordovicien moyen (env. 470 – 460 Ma) dans quatre roches métasédimentaires de la Formation de Chopawamsic représente vraisemblablement le recyclage détritique des roches volcaniques synsédimentaires de Chopawamsic. Un sous-ensemble de grains cambriens et plus anciens, évoque l’existence d’une ou plusieurs sources plus anciennes additionnelles. Les échantillons du terrane de Potomac renferment principalement des grains de zircon du Mésoprotérozoïque, ce qui correspond avec les interprétations antérieures voulant que les roches métaclastiques soient d’origine laurentienne. Les zircons les plus jeunes (env. 550 – 500 Ma) ainsi que l’âge des plutons qui recoupe l’encaissant indiquent qu’au moins certaines parties du terrane de Potomac sont de la fin du Cambrien ou du début de l’Ordovicien. Les résultats impliquent l’existence de systèmes sédimentaires distincts au cours du temps, et isolés géographiquement durant le dépôt des roches sédimentaires dans les terranes de Chopawamsic et de Potomac. Les roches métasédimentaires près de Storck en Virginie, jadis interprétées comme bassin successeur, renferment des populations de zircons détritiques qui indiquent qu’ils proviennent en fait de roches métasédimentaires péri-gondwaniennes sans rapport avec un système de bassin successeur; leur localisation géographique entre le terrane de Potomac issu des Laurentides et le terrane de Chopawamsic porte à penser que l’arc de Chopawamsic est d’affinité péri-gondwanienne, et que les terranes de Chopawamsic et de Potomac à l’Ordovicien moyen étaient séparés géographiquement. En conséquence il nous semble justifié de proposer que le système de faille de Chopawamsic représente la principale suture japétienne dans le sud de l’orogène des Appalaches. La plupart des zircons détritiques des échantillons du bassin successeur d’Arvonia ont cristallisés entre l’Ordovicien et le Silurien ou au Mésoprotérozoïque. Ces données suggèrent que le bassin d’Arvonia s’est rempli de la fin entre l’Ordovicien et le début du Silurien, seulement après l’accrétion de l’arc de Chopawamsic à la Laurentie, à la fin de l’Ordovicien.
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de Wit, M. J., S. Bowring, R. Buchwaldt, F. Ö. Dudas, D. MacPhee, G. Tagne-Kamga, N. Dunn, A. M. Salet i D. Nambatingar. "Geochemical reconnaissance of the Guéra and Ouaddaï Massifs in Chad: evolution of Proterozoic crust in the Central Sahara Shield". South African Journal of Geology 124, nr 2 (1.06.2021): 353–82. http://dx.doi.org/10.25131/sajg.124.0048.
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Abstract In 1964, W.Q. Kennedy suggested that the crust of Saharan Africa is different from the rest of Africa. To date, the geologic evolution of this region remains obscure because the age and composition of crystalline basement are unknown across large sectors of the Sahara. Most of Africa comprises Archaean cratons surrounded by Palaeo- to Mesoproterozoic orogenic belts, which together constitute Africa’s three major shields (the Southern, Central and West African Shields), finally assembled along belts of Pan-African rocks. By contrast, central Saharan Africa (5.3x106 km2), an area just over half the size of Europe, is considered either as a Neoproterozoic region constructed of relatively juvenile crust (0.5 to 1.0 Ga), or as an older (North African) shield that was reactivated and re-stabilized during that time, a period commonly referred to as “Pan African”. Here, using U-Pb zircon age determinations and Nd isotopic data, we show that remote areas in Chad, part of the undated Darfur Plateau stretching across ¾ million km2 of the central Sahara, comprise an extensive Neoproterozoic crystalline basement of pre-tectonic gabbro-tonalite-granodiorite and predominantly post-tectonic alkali feldspar granites and syenites that intruded between ca. 550 to 1050 Ma. This basement is flanked along its western margin by a Neoproterozoic continental calc-alkaline magmatic arc coupled to a cryptic suture zone that can be traced for ~2400 km from Tibesti through western Darfur into Cameroon. We refer to this as the Central Saharan Belt. This, in a Gondwana framework, is part of a greater arc structure, which we here term the Great Central Gondwana Arc (GCGA). Inherited zircons and Nd isotopic ratios indicate the Neoproterozoic magmas in the central Sahara were predominantly derived from Mesoproterozoic continental lithosphere. Regional deformation between 613 to 623 Ma marks the onset of late alkaline granite magmatism that was widespread across a much larger area of North Africa until about 550 Ma. During this magmatism, the region was exhumed and eroded, leaving a regional peneplain on which early Palaeozoic (Lower-Middle Cambrian) siliciclastic sediments were subsequently deposited, as part of a thick and widespread cover that stretched across much of North Africa and the Arabian Peninsula. Detrital zircons in these cover sequences provide evidence that a substantial volume of detritus was derived from the central Sahara region, because these sequences include ‘Kibaran-age’ zircons (ca. 1000 Ma) for which a source terrain has hitherto been lacking. We propose that, in preference to calling the central Sahara a “ghost” or “meta” craton, it should be called the Central Sahara Shield.
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Malobela, T., B. Mapani, M. Harris, D. H. Cornell, A. Karlsson, A. K. Jonsson, C. Lundell i M. Kristoffersen. "Age and geological context of the Barby Formation, a key volcanic unit in the Mesoproterozoic Sinclair Supergroup of southern Namibia". South African Journal of Geology 122, nr 4 (1.12.2019): 519–40. http://dx.doi.org/10.25131/sajg.122.0038.
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Abstract Volcanic and sedimentary rocks of the Sinclair Supergroup occur in the Konkiep Terrane of Southern Namibia. Three volcanic and sedimentary cycles are recognised. In this work we describe and date volcanic rocks of the Barby Formation, a key unit in the Sinclair area. The coeval Spes Bona Syenite and the Tiras Granite Gneiss are also described and dated. The rock types in the Barby Formation are rhyolites, basaltic trachyandesites, trachybasalts and trachydacites as well as volcanoclastic rocks. The rocks are largely undeformed and partly altered by deuteric and contact metamorphic processes but not regionally metamorphosed. Our samples represent both the calc-alkaline and alkaline trends documented in previous work. U-Pb ion probe and laser ablation inductively coupled plasma (LA-ICP) multicollector mass spectrometer Lu-Hf microbeam analyses were made of zircon and baddeleyite grains from four samples. A felsic tuff sample from the base of the Barby Formation has a 207Pb/206Pb zircon age of 1214 ± 5 Ma (2σ). A rhomb porphyry sample from the top of an 8.5 km-thick stratigraphic section gives a 207Pb/206Pb baddeleyite age of 1217 ± 2 Ma. The Spes Bona Syenite which intrudes the top of the Barby Formation has a 207Pb/206Pb baddeleyite age of 1217 ± 3 Ma and an indistinguishable LA-ICP collision cell mass spectrometer Rb-Sr biotite isochron age of 1238 ± 20 Ma, showing that there was no >350°C regional metamorphic event. Multi-element diagrams for the calc-alkaline samples show a dominant signature of reworked crust which is superimposed on a possible subduction signature. However the alkaline samples contain clear subduction signatures which are not seen in the underlying 1.37 Ga Kumbis rhyolite. The Barby Formation samples and coeval Spes Bona Syenite have Lu-Hf crustal residence ages between 1682 and 1873 Ma, suggesting that both of these units formed from a mixture of juvenile mantle-derived and older crustal material. The Barby Formation is considered to have originated due to a subduction event which took place during the assembly of the Rodinia supercontinent. The duration of the Barby magmatic episode is constrained to a maximum 9 m.y. period between 1219 and 1210 Ma, and during this period the Konkiep Terrane was an active continental margin. The 1204 ± 9 Ma Tiras Granite Gneiss is slightly younger than the Barby Formation and intruded across the Lord Hills Shear Zone, which is the suture between the hardly metamorphosed Konkiep Terrane and the highly metamorphosed Grunau Terrane of the Namaqua-Natal Province. Its intrusion reflects the end of subduction-related volcanism, due to the collision of Namaqua terranes with the Konkiep Terrane.
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Willner, Arne P., Axel Gerdes, Hans-Joachim Massonne, Cees R. Van Staal i Alexandre Zagorevski. "Crustal Evolution of the Northeast Laurentian Margin and the Peri-Gondwanan Microcontinent Ganderia Prior to and During Closure of the Iapetus Ocean: Detrital Zircon U–Pb and Hf Isotope Evidence from Newfoundland". Geoscience Canada 41, nr 3 (29.08.2014): 345. http://dx.doi.org/10.12789/geocanj.2014.41.046.
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Detrital zircon populations in sedimentary rocks from the Laurentian margin and the accreted microcontinent Ganderia on both sides of the main Iapetus suture (Red Indian Line) in central Newfoundland have been studied by combined U–Pb and Lu–Hf isotope analyses. Variation in εHf(t) values with age of zircon populations of distal provenance (>900 Ma) reflect the crustal evolution within the source continents: in zircon derived from Laurentia, episodes of juvenile magma production in the source could be detected at 1.00 – 1.65 and 2.55 – 3.00 Ga, and mixing of juvenile and recycled crust in continental magmatic arcs occurred at 0.95 – 1.40, 1.45 – 1.60, 1.65 – 2.05 and 2.55 – 2.75 Ga. These ages are consistent with the crustal history of northeastern Laurentia. Similarly, zircon of distal provenance from Ganderia reveals times of juvenile magma production in the source at 0.70 – 0.90, 1.40 – 1.75, 1.85 – 2.40 and 2.7 – 3.5 Ga, and episodes of mixing juvenile and recycled crust at 0.95 – 1.35, 1.45 – 1.60, 1.70 – 2.15 and 2.6 – 2.8 Ga. These data reflect the crustal evolution in the present northern part of Amazonia, its likely source craton. The evolution of magmatic arcs at the margins of both continents can be studied in a similar way using detrital zircon having a proximal provenance (<900 Ma). In contrast to the Laurentian margin, Ganderia is characterized by development of Neoproterozoic – Cambrian continental arcs (ca. 500 – 670 Ma) that were built on the margin of Gondwana. εHf(t) values indicate recycling of Neo- and Mesoproterozoic crust. During and following accretion of the various elements of Ganderia to Laurentia, the syn-tectonic Late Ordovician to Silurian sedimentary rocks deposited on the upper plate (composite Laurentia) continued showing only detritus derived from Laurentia. These sedimentary rocks contain detrital zircon from Iapetan juvenile, continental and successor arcs that were active between ca. 440 and 550 Ma, and from continuing magmatic activity until 423 Ma. Arrival of the first Laurentian detritus at the outermost part of Ganderia indicates that the Iapetus ocean was closed at ca. 452 Ma. The magmatic arcs along the former Laurentian margin in Newfoundland evolved differently. In the northwestern part, εHf(t) values point to recycling of Mesoproterozoic and Paleoproterozoic crust. In the southwest, εHf(t) values indicate addition of juvenile crust, recycling of Mesoproterozoic crust and mixing with juvenile magma. SOMMAIRELes populations de zircons détritiques des roches sédimentaires issus de la marge laurentienne et du microcontinent d’accrétion de Ganderia, des deux côtés de la principale suture Iapetus (linéation de Red Indian) dans le centre de Terre-Neuve, ont été étudiés par analyses combinées U–Pb et Lu–Hf. Les variations des valeurs εHf(t) en fonction de l’âge des populations de zircons distaux (>900 Ma) reflètent l’évolution de la croûte des continents sources : les zircons de Laurentie ont permis de détecter des épisodes magmatiques juvéniles dans la source entre 1,00 - 1,5, et 2,55 - 3,00 Ga, ainsi que des épisodes de mélange de croûte juvénile avec des croûtes d’arcs magmatiques continentaux recyclés entre 0,95 – 1,40, 1,45 – 1,60, 1,65 – 2,05, et 2,55 – 2,75 Ga. Ces datations correspondent bien à l’histoire de la croûte de la portion nord-est de la Laurentie. De même, le zircon distal de Ganderia révèle des épisodes de production de magmas juvéniles dans la source entre 0,70 - 0,90, 1,40 - 1,75, 1,85 - 2,40, et 2,7 - 3,5 Ga, ainsi que des épisodes de mélanges de matériaux juvéniles et de croûtes recyclés entre 0,95 - 1,35, 1,45 - 1,60, 1,70 - 2,15, et 2,6 - 2,8 Ga. Ces données reflètent l’évolution de la croûte dans la portion nord actuelle de l’Amazonie, son craton source probable. L’évolution des arcs magmatiques à la marge de ces deux continents peuvent être étudiées de la même manière en utilisant le zircon détritique proximal (<900 Ma). Contrairement à la marge laurentienne, celle de Ganderia est caractérisée par le développement d’arcs continentaux Néoprotéozoïque-Cambrien (env. 500 – 670 Ma) qui se sont constitués à la marge du Gondvana. Les valeurs de εHf(t) indiquent un recyclage de la croûte au Néoprotérozoïque et au Mésoprotérozoïque. Durant et après l’accrétion des divers éléments de Ganderia et de la Laurentie, les roches sédimentaires syntectoniques de la fin de l’Ordovicien et du Silurien qui se sont déposées sur la portion supérieure de la plaque (Laurentie composite) ne montrent toujours que des débris provenant de la Laurentie. Ces roches sédimentaires renferment des zircons détritiques juvéniles iapétiques, et d’arcs continentaux et d’arcs subséquents, qui ont été actifs entreentre (env. 440 et 550 Ma) et d’une activité magmatique continue jusqu’à 423 Ma. L’apport des premiers débris à la marge extrême de Ganderia indique que l’océan s’est fermée il y a env. 452 Ma. Les arcs magmatiques le long de l'ancienne marge laurentienne à Terre-Neuve ont évolué différemment. Dans la portion nord-ouest, les valeurs de εHf(t) indiquent un recyclage de la croûte au Mésoprotérozoïque et au Paléoprotérozoïque. Dans la portion sud-ouest, les valeurs de εHf(t) indiquent l’ajout d’une croûte juvénile, un recyclage de la croûte mésoprotérozoïque et un mélange avec un magma juvénile.
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Tucker, Naomi M., Martin Hand i Chris Clark. "The Bunger Hills: 60 years of geological and geophysical research". Antarctic Science 32, nr 2 (27.02.2020): 85–106. http://dx.doi.org/10.1017/s0954102019000403.
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AbstractCorrelation of Rodinian and Gondwanan crustal domains relies on a thorough knowledge of those vestiges preserved today. The Bunger Hills hold a critical place in East Antarctica, recording the Mesoproterozoic assembly of Australo-Antarctica in Rodinia and the Neoproterozoic–Cambrian amalgamation of Indo- and Australo-Antarctica in Gondwana. It is situated in a region of disputed overlap between the different components of Rodinia and Gondwana, where there is little consensus on the location of sutures in this region and thus often speculative geological interpretations. The Bunger Hills therefore provide an opportunity to better understand the tectonic setting and palaeogeography during the assembly of these supercontinents. Recent work has confirmed that the Bunger Hills are one of few rare outcrops in Wilkes Land, East Antarctica that can be directly correlated with the broader Musgrave–Albany–Fraser–Wilkes Orogen (MAFWO). Whilst other constituent terranes of the MAFWO have been intensely studied, our geological knowledge of the Bunger Hills was comparatively limited until recently. In light of recent geological and geophysical developments, this contribution serves as an updated and concise standalone reference for the present state of knowledge of the Neoarchean–Cambrian evolution of the Bunger Hills region.
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Golonka, Jan, Aleksandra Gawęda, Anna Waśkowska, David Chew, Krzysztof Szopa i Foteini Drakou. "Tracing Pre-Mesozoic Tectonic Sutures in the Crystalline Basement of the Protocarpathians: Evidence from the Exotic Blocks from Subsilesian Nappe, Outer Western Carpathians, Poland". Minerals 11, nr 6 (27.05.2021): 571. http://dx.doi.org/10.3390/min11060571.
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Pre-Mesozoic exotic crystalline blocks within the Outer Carpathian flysch have potential to unravel the nature of their eroded basement source(s) and to reconstruct the Paleozoic–Precambrian history of the Protocarpathians. Strongly tectonized Campanian–Maastrichtian grey marls in the Subsilesian Nappe of the Outer Western Carpathians in Poland contain a variety of different lithology types, including granitoids and andesites. Petrological investigations coupled with zircon and apatite U-Pb dating were performed on crystalline (subvolcanic) exotic blocks from a locality in the Subsilesian Nappe. U-Pb zircon dating yields magmatic crystallization ages of c. 293 Ma for the microgranitoid and c. 310 Ma for the andesite block, with inherited zircon cores yielding Archean, Paleoproterozoic, Mesoproterozoic and Cadomian ages. Whole rock trace element and Nd isotope data imply that the melt source was composed of a significant Neoproterozoic crustal component in both the microgranite and andesite. The Late Carboniferous–Permian magmatic activity likely continues outside the Carpathian Belt and can be linked to a Late Paleozoic transtensional zone, which is a continuation of the Lubliniec–Kraków Zone that extends under the Carpathians to Moesia. This Late Paleozoic transtensional zone was probably reactivated during the Late Cretaceous under a transpressional regime within the Żegocina tectonic zone, which caused the uplift of the Subsilesian Ridge and intensive erosion.
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Hall, Jeremy, Keith E. Louden, Thomas Funck i Sharon Deemer. "Geophysical characteristics of the continental crust along the Lithoprobe Eastern Canadian Shield Onshore–Offshore Transect (ECSOOT): a review". Canadian Journal of Earth Sciences 39, nr 5 (1.05.2002): 569–87. http://dx.doi.org/10.1139/e02-005.
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The Eastern Canadian Shield OnshoreOffshore Transect (ECSOOT) of the Lithoprobe program included 1200 km of normal-incidence seismic profiles and seven wide-angle seismic profiles across Archean and Proterozoic rocks of Labrador, northern Quebec, and the surrounding marine areas. Archean crust is 3344 km thick. P-wave velocity increases downwards from 6.0 to 6.9 km/s. There is moderate crustal reflectivity, but the reflection Moho is unclear. Archean crust that stabilized in the Proterozoic is similar except for greater reflectivity and a well-defined Moho. Proterozoic crust has similar or greater thickness, variable lower crustal velocities, and strong crustal reflectivity. Geodynamic processes of Paleoproterozoic growth of the Canadian Shield are similar to those observed in modern collisional orogens. The suturing of the Archean Core Zone and Superior provinces involved whole-crustal shearing (top to west) in the Core Zone, linked to thin-skinned deformation in the New Quebec Orogen. The Torngat Orogen sutures the Nain Province to the Core Zone and reveals a crustal root, in which Moho descends to 55 km. It formed by transpression and survived because of the lack of postorogenic heating. Accretion of the Makkovik Province to the Nain Province involves delamination at the Moho and distributed strain in the juvenile arcs. Delamination within the lower crust characterizes the accretion of Labradorian crust in the southeastern Grenville Province. Thinning of the crust northwards across the Grenville Front is accentuated by Mesozoic extension that reactivates Proterozoic shear zones. The intrusion of the Mesoproterozoic Nain Plutonic Suite is attributed to a mantle plume ponding at the base of the crust.
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LI, QIUGEN, SHUWEN LIU, ZONGQI WANG, QUANREN YAN, ZHAOJIE GUO, ZHICHENG ZHANG, HAIFEI ZHENG, CHUNFA JIANG, TAO WANG i ZHUYIN CHU. "Geochemical constraints on the petrogenesis of the Proterozoic granitoid gneisses from the eastern segment of the Central Tianshan Tectonic Zone, northwestern China". Geological Magazine 144, nr 2 (19.01.2007): 305–17. http://dx.doi.org/10.1017/s0016756806002688.
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The Tianshan orogen is divided into the Northern, Central and Southern Tianshan tectonic zones by the northern and southern sutures on both sides of the Central Tianshan Tectonic Zone. The eastern segment of the Central Tianshan Tectonic Zone is characterized by the presence of numerous Precambrian metamorphic blocks and is unconformably overlain by Ordovician–Silurian and late Palaeozoic strata. The Precambrian Kumishi and Pargantag metamorphic blocks are the largest older blocks in the eastern segment of the Central Tianshan Tectonic Zone, consisting mainly of metamorphic granitoids and sedimentary rocks in greenschist to amphibolite facies. There are two major lithological assemblages of the metamorphic granitoids: (1) quartz dioritic gneisses, and (2) granodioritic–monzogranitic gneisses with a minor amount of tonalitic and syenogranitic gneisses in both the Kumishi and Pargantag blocks. The quartz dioritic gneisses are characterized by low Sr/Ce (<5.3) and Sr/Y (<28), relatively high Mg no. (51.0–57.0), K2O (2.65–4.04 wt %) contents and εNd(t) values (−2.37–5.84), and negative Nb and Zr–Hf anomalies, as well as relatively flat chondrite-normalized REE patterns with slightly negative Eu anomalies, suggesting that the quartz dioritic gneisses were derived from partial melting of a depleted mantle source enriched by fluids and sedimentary melts from the subducted slab. However, most of granitic gneiss samples display high K2O contents, low Al2O3/(FeO* + MgO + TiO2) values, and relatively flat chondrite-normalized REE patterns with intensively negative Eu anomalies. Integrated low εNd(t) values and older TDM model ages suggest that crustal materials played a significant role in the petrogenesis of these granitoid gneisses and that they were mainly derived from the partial melting of calc-alkaline mafic to intermediate rocks in the crust. Also, variations in geochemical features between the Kumishi–Gangou and Pargantag regions, such as Zr and Hf, may reflect geographic variability in the development of coeval granitic magmas. Tectonic discrimination for granitoid, using trace elements, together with Nd isotopic data, demonstrates that these granitoid gneisses in the eastern segment of the Central Tianshan Tectonic Zone formed in a continental margin arc during late Mesoproterozoic times.
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Maldonado, Roberto, Luigi Solari, Peter Schaaf i Bodo Weber. "A Mesoproterozoic to Jurassic history of continental eclogites from the Guatemala Suture Zone–implications for a peri-Amazonian ancestry". Gondwana Research, kwiecień 2023. http://dx.doi.org/10.1016/j.gr.2023.03.027.
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Pham, Minh, Hieu, Kenta Kawaguchi, Anh i Phuc. "Geochemistry, zircon U-Pb geochronology and Sr-Nd-Hf isotopic composition of the Cha Val plutonic rocks in central Vietnam: Implications for Permian-Triassic Paleo-Tethys subduction-related magmatism". Vietnam Journal of Earth Sciences, 23.12.2021. http://dx.doi.org/10.15625/2615-9783/16842.
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together with abundant Permian-Triassic magmatic rocks. This magmatic complex provides important information to reconstruct the tectonic evolution of the Indochina block and surrounding areas. The Cha Val plutonic rocks mainly comprise diorite, quartz diorite, and granodiorite. Geochemically, they are metaluminous with low A/CNK (0.49 to 1.16 with an average of 0.85), medium to high K, low to medium SiO2, and Na2O/K2O>1. Trace and rare earth element compositions display enrichment in Cs, U, Pb, and Nd, but depletion in Ba, Nb, Ta, P, Eu, and Ti, similar to those of continental arc-related magmas. Rock-forming minerals of the Cha Val plutonic rocks are characterized by abundant hornblende. All observed petrographical and geochemical characteristics suggest that the Cha Val plutonic rocks are typical for I-type affinity generated from a subduction regime. LA-ICP-MS U-Pb zircon analyses of three representative samples yielded their crystallization ages between 258.0 Ma and 248.9 Ma, temporally coeval with Late Permian-Early Triassic magmatism previously reported in the Truong Son belt. The (87Sr/86Sr)i ratios (0.7081 to 0.7244), negative whole-rock εNd(t) values (-4.5 to -2.9), zircon εHf(t) values (-1.04 to 2.71), and whole-rock Nd and zircon Hf model ages (TDM2) (1394 Ma to 1111 Ma) indicate that the Cha Val plutonic rocks are derived from melting of Mesoproterozoic crustal materials with a minor contribution of mantle-derived melt. Together with other Permian-Triassic magmatic complexes along the Song Ma suture zone and the Truong Son Belt, the Cha Val plutonic rocks are a representative of magmatism associated with the subduction-collision that amalgamated the South China and Indochina blocks after the closure of a branch of Paleo-Tethys along the Song Ma suture zone during the Late Permian-Early Triassic Indosinian orogeny.
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Ringwood, Mary F., Roberta L. Rudnick i Andrew R. C. Kylander-Clark. "Metasediments from the lower crust reveal the history of the Picuris orogeny, southwest USA". Geological Society of America Bulletin, 20.04.2023. http://dx.doi.org/10.1130/b36836.1.
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Petrologic and geochronologic data for metapelitic lower crustal xenoliths from New Mexico (USA) and Chihuahua (Mexico) states provide evidence for both a magmatic and collisional component to the enigmatic Mesoproterozoic Picuris orogeny. These garnet-sillimanite-bearing metapelites are found within the southern Rio Grande rift at Kilbourne Hole and Potrillo Maar in southern New Mexico and northern Chihuahua. Geothermobarometry and rutile with Quaternary U-Pb dates indicate equilibration in the local lower crust, which is actively undergoing ultra-high temperature (UHT) metamorphism (Cipar et al., 2020). The samples contain older detrital zircons dating back to the Paleoproterozoic, marking their deposition at the surface. Coupled zircon U-Pb dates and trace-element ratios (e.g., Gd/Yb) show a clear transition from oscillatory-zoned, low-Gd/Yb detrital magmatic zircon to featureless, high-Gd/Yb metamorphic zircon between 1500 and 1400 Ma, marking the transition from subduction to collision during this period. Metamorphic zircon and monazite grew in two major intervals. The first, between ca. 1450 and 1350 Ma, documents the journey of the sediments to depth within the orogen and provides evidence of extended Mesoproterozoic metamorphism in the region. The second corresponds with UHT metamorphism that commenced at ca. 32 Ma and is associated with the Rio Grande rift. Whereas nearly all garnets are homogeneous in both major and trace elements, a single garnet from one sample has a core defined by abundant quartz and acicular sillimanite inclusions. The core and rim of this garnet is homogeneous in major and most trace elements, but the rim is enriched in the slowest diffusing elements, Zr and Hf, which likely indicates rim growth at higher temperatures. We interpret the garnet core to have grown at the time of emplacement of the sediments into the lower crust. Because this occurred in the sillimanite stability field and because the metamorphic zircon and monazite all have negative Eu anomalies, indicating their equilibration with feldspar (stable at depths of <45 km), we conclude that the sediments were not emplaced via subduction and/or relamination of forearc sediments, but were instead metamorphosed under warmer, shallower conditions in an orogenic setting. Collectively, the data point to a collisional orogen during the inferred timing of the Picuris orogeny. These samples may therefore define the location of the Picuris suture zone, a key feature of this orogenic event.
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Nguyen Kim, Hoang, Anh Nong Thi Quynh, Minh Pham, Hieu Pham Trung i Thao Nguyen Thanh. "Geochemistry and zircon U-Pb geochronology of the Dak Krong plutonic rocks in the Kontum Massif (central Vietnam) and their petrogenetic implications". Vietnam Journal of Earth Sciences, 9.06.2023. http://dx.doi.org/10.15625/2615-9783/18411.
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Dak Krong plutonic rocks are found in the Kontum Massif along the N-S-oriented Po Ko River and mainly within the distributive area of the Ben Giang-Que Son granitic bodies. The Hai Van complex crosscuts the Dak-Krong rocks. They are predominantly composed of plagioclase (35-47%), quartz (29-30%), K-feldspar (20-28%), and minor biotite (3-4%). Geochemically, they are characterized by meta- to peraluminous and high-K affinities and straddle I- and S-type granite fields. Zircon U-Pb dating results yielded two main magmatic stages (ca. 258 Ma and ca. 245 Ma) spanning two phases of magmatism presumably accepted as being associated with the Paleo-Tethys Ocean evolution: latest subduction to syn-collisional phases. The Hf isotope data from zircon with eHf(t) ranging between -6.4 and -0.5 indicates a crustal signature. From the results of eHf(t) values along with zircon Hf model ages (TDM2) ranging from 1165-1497 Ma, it is presumable that the Dak Krong plutonic rocks are the product of the partial melting of Mesoproterozoic crustal materials with a negligible contribution of mantle materials. Together with other Permian-Triassic magmatic complexes throughout the Kontum Massif (e.g., Ben Giang-Que Son, Hai Van, and Van Canh complexes) and other plutonic further to the north along the Truong Son Belt and the Song Ma suture zone, the Dak Krong plutonic rocks represent magmatism generated by the amalgamation of Indochina and South China during the Late Permian-Early Triassic, referred to as Indosinian orogeny.
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Costa, Antônio Gilberto, Carlos Alberto Rosiére, Luciano Melo Moreira i Daniele Piuzana. "CARACTERIZAÇÃO GEOTECTÔNICA DO SETOR SETENTRIONAL DO CINTURÃO RIBEIRA: EVIDÊNCIA DE ACRESÇÃO NEOPROTEROZÓICA NO LESTE DE MINAS GERAIS, BRASIL." Geonomos, 1.12.1995. http://dx.doi.org/10.18285/geonomos.v3i2.209.
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The early neoproterozoic evolution of eastern Minas Gerais is characterized by a history of a continental margin activity, including the accretion of suspect terranes. The Manhuaçu Terrane is one of those and is represented by a granitic continental plutonic arc and terrigeneous metasediments that reflect a continental margin. A metasedimentary gneiss belt at this margin with shallow to deep marine clastic lithologies as well as metavulcanic and metaplutonic mafic rocks was interpreted as an extensive tectonic segment with suspect development in a back-arc setting. Fragments of a volcanic arc are identified and interpreted as an evidence for a probable island-arc domain. In the studied region the Juiz de Fora and Paraiba do Sul Complexes are domains of these terranes. The distribution of magmatism in the studied region shows from west to east three different suites : 1) tholeiitic, 2) a medium to high-K cal-alkalic and 3) a high-K calc-alcalic/shoshonitic(?) magmatism which attests the evolution of early magmatism arcs (volcanic and plutonic) initially relates to ocean-plate subduction, followed by a continent-continent collision. Petrological, structural and geochemical data of mesoproterozoic/neoproterozoic metamorphic and magmatic suites of rocks are in agreement with the development of an accretionary orogeny. After a vulcanic-(island-arc) and a back-arc basin formation (by rifting of a continental margin with no spreading) in connection with eastward subduction, the island arc was accreted to a continental margin. Further subduction beneath it and a mechanism of flattening of the slab during the subduction process was responsible for the development of as granitic continental plutonic-arc (Andean-type batholith) eastwardly in a compressional setting. Continental plates became sutured and all intervening oceanic lithosphere was subducted beneath one of the converging masses. This resulted in the accretion of the Manhuaçu Microplate to the São Francisco Craton Domain. Plates continued to converge and the inversion of subduction polarity occurs resulting in a new subduction system (westward subduction) to the back of the Manhuaçu Microplate, in a easternward of the Espírito Santo state, with the establishment of a new magmatic arc of late neoproterozoic to eopaleozoic age. In this paper, only the probable early neoproterozoic volcanic (tholeiitic magmatism transitional between N-type MORB and island are basalts) and the plutonic (early continental calc-alkalic magmatism) arcs characterized by rocks with a very particular geochemistry and the back-arc basin setting will be discussed. Two alternative hypotheses to explain the evolution of these terranes may be postulated: 1) an island arc orogeny related to a westward subduction followed by a cordillerean type orogeny. With the advance of the island-arc and continent with offshore sediments, in different plates, a collision between these domains occurred. The old subduction zone was replaced by another one, eastward directed and the development of a continental magmatic arc occurred. Continental plates became sutured and all intervening oceanic lithosphere was subducted beneath one of the converging masses. Against this model are the presence of metavulcanic basic and intermediate rocks intercalated with marine and continental margin; 2) an extensional ensialic setting (aborted marginal basin) in the continental margin formed during the opening of a rapidly subsiding basin, with moderately rapid mantle upwelling, pronounced thinning of the continental crust and slight crustal contaminanton, without formation of oceanic crust or an island-arc may explain the association of basic metavulcanic rocks with marine and continental metasediments. Here, the low potassium contents of a few studied metavulcanic basic rocks and theire oceanic tholeiitic affinity are not well in agreement with this setting where continental basalts (flood basalts), or rocks ( basalts or basaltics andesites) belong to the high-K calc-alkaline series are expected to occur. Nevertheless, our evidences are not unequivocal enough for theire disapprovals.