Indentor-escape, delamination and orogenic collapse of the ca. 600-500 Ma East African/Antarctic Orogen in Mozambique and Dronning Maud Land (East Antarctica)

The East African/Antarctic Orogen (EAAO) is one of the largest orogenic belts on the planet, resulting from the collision of various parts of East and West- Protogondwana between ca. 600 and 550 Ma. The central and southern parts of the orogen are typified by high-grade rocks, representing the overprinted margins of the various colliding continental blocks. New fieldwork and geochronology in northern Mozambique reveals a protracted polyphase Ediacaran/Cambrian deformation history. New age constraints reveal high-grade metamorphism at 600-550 Ma, overprinting and older basement

Tracing the Sveconorwegian orogen into the Caledonides of West Norway: Geochronological and isotopic studies on magmatism and migmatization

The Sveconorwegian orogen represents a branch of Grenville-age (~1250–950 Ma) orogenic belts that formed during the construction of the supercontinent Rodinia. This study traces the Sveconorwegian records from its type-area in the Baltic Shield of South Norway into basement windows underneath Caledonian nappes, by combining zircon U–Pb geochronology and Hf–O isotopes. Samples along a N-S trending transect reveal multiple magmatic episodes during Gothian (ca. 1650 Ma), Telemarkian (ca. 1500 Ma) and Sveconorwegian (1050–1020 Ma vs. 980–930 Ma) orogenesis as well as Sveconorwegian migmatization (1050–950 Ma). Newly documented 1050–1020 Ma magmatism and migmatization extend the Sirdal Magmatic Belt to a 300 km-long, NNW-SSE trending crustal domain, with the northern boundary corresponding to the gradual transition from Telemarkian to Gothian crust. These Precambrian crustal heterogeneities largely controlled the development of Caledonian shear zones. The ca. 1050–1040 Ma granitic and mafic magmas show similar isotopic signatures with slightly negative or positive εHf(t) and moderate δ18O values (6–7‰), which indicates that crustal reworking was more dominant than juvenile inputs during their genesis. The generation of leucosomes and leucogranites at ca. 1030–1020 Ma, which have a more evolved Hf isotopic composition, probably reflects an even higher degree of remelting of older crust. The Hf–O isotopic patterns show that Sveconorwegian magmas differ from typical arc magmas by lower involvement of sedimentary components and juvenile material. This makes the 1050–930 Ma magmatism incompatible with a long-term subduction setting. The ca. 1650–1500 Ma samples, in contrast, generally have juvenile Hf isotopic compositions associated with varying δ18O values of 4.5–9‰, consistent with subduction-accretion processes involving significant sedimentary recycling. This accretionary margin was most likely transformed into the Sveconorwegian orogen through collisional interactions of Baltica, Laurentia and Amazonia in the context of Rodinia amalgamation.publishedVersio

A geochronological review of magmatism along the external margin of Columbia and in the Grenville-age orogens forming the core of Rodinia

A total of 4344 magmatic U-Pb ages in the range 2300 to 800 Ma have been compiled from the Great Proterozoic Accretionary Orogen along the margin of the Columbia / Nuna supercontinent and from the subsequent Grenvillian collisional orogens forming the core of Rodinia. The age data are derived from Laurentia (North America and Greenland, n = 1212), Baltica (NE Europe, n = 1922), Amazonia (central South America, n = 625), Kalahari (southern Africa and Dronning Maud Land in East Antarctica, n = 386), and western Australia (n = 199). Laurentia, Baltica, and Amazonia (and possibly other cratons) most likely formed a ca. 10 000-km-long external active continental margin of Columbia from its assembly at ca. 1800 Ma until its dispersal at ca. 1260 Ma, after which all cratons studied were involved in the Rodinia-forming Grenvillian orogeny. However, the magmatic record is not smooth and even but highly irregular, with marked peaks and troughs, both for individual cratons and the combined data set. Magmatic peaks typically range in duration from a few tens of million years up to around hundred million years, with intervening troughs of comparable length. Some magmatic peaks are observed on multiple cratons, either by coincidence or because of paleogeographic proximity and common tectonic setting, while others are not. The best overall correlation, 0.617, is observed between Baltica and Amazonia, consistent with (but not definitive proof of) their being close neighbours in a SAMBA-like configuration at least in Columbia, and perhaps having shared the same peri-Columbian subduction system for a considerable time. Correlation factors between Laurentia and Baltica, or Laurentia and Amazonia, are below 0.14. Comparison between the Grenville Province in northeastern Laurentia and the Sveconorwegian Province in southwestern Fennoscandia (Baltica) shows some striking similarities, especially in the Mesoproterozoic, but also exhibits differences in the timing of events, especially during the final Grenville-Sveconorwegian collision, when the Sveconorwegian evolution seems to lag behind by some tens of million years. Between the other cratons, the evolution before and during the final Grenvillian collision is also largely diachronous. After 900 Ma, magmatic activity had ceased in all areas investigated, attesting to the position of most of them within the stable interior of Rodinia.publishedVersio

Spectrum and Inoculum Size Effect of a Rapid Antigen Detection Test for Group A Streptococcus in Children with Pharyngitis

BACKGROUND: The stability of the accuracy of a diagnostic test is critical to whether clinicians can rely on its result. We aimed to assess whether the performance of a rapid antigen detection test (RADT) for group A streptococcus (GAS) is affected by the clinical spectrum and/or bacterial inoculum size. METHODS: Throat swabs were collected from 785 children with pharyngitis in an office-based, prospective, multicenter study (2009-2010). We analysed the effect of clinical spectrum (i.e., the McIsaac score and its components) and inoculum size (light or heavy GAS growth) on the accuracy (sensitivity, specificity, likelihood ratios and predictive values) of a RADT, with laboratory throat culture as the reference test. We also evaluated the accuracy of a McIsaac-score-based decision rule. RESULTS: GAS prevalence was 36% (95CI: 33%-40%). The inoculum was heavy for 85% of cases (81%-89%). We found a significant spectrum effect on sensitivity, specificity, likelihood ratios and positive predictive value (p<0.05) but not negative predictive value, which was stable at about 92%. RADT sensitivity was greater for children with heavy than light inoculum (95% vs. 40%, p<0.001). After stratification by inoculum size, the spectrum effect on RADT sensitivity was significant only in patients with light inoculum, on univariate and multivariate analysis. The McIsaac-score-based decision rule had 99% (97%-100%) sensitivity and 52% (48%-57%) specificity. CONCLUSIONS: Variations in RADT sensitivity only occur in patients with light inocula. Because the spectrum effect does not affect the negative predictive value of the test, clinicians who want to rule out GAS can rely on negative RADT results regardless of clinical features if they accept that about 10% of children with negative RADT results will have a positive throat culture. However, such a policy is more acceptable in populations with very low incidence of complications of GAS infection

Geochemistry of Sveconorwegian augen gneisses from SW Norway at the amphibolite-granulite facies transition

The augen gneisses of Rogaland-Vest-Agder were emplaced as amphibole-biotite granodiorites rich in K-feldspar phenocrysts. They were probably pre- or syntectonically emplaced and were metamorphosed under lower amphibole to granulite facies conditions after their emplacement. Three metamorphic zones defined by two isograds (Cpx-in and Opx-in) can be established in the augen gneisses using ferromagnesian minerals: zone 1 = Bt ± Am zone; zone 2 = Bt ± Am ± Cpx zone and zone 3, close to the Rogaland anorthosite complex, in granulite facies = Bt ± Am ± Cpx ± Opx zone. Pyroxene forming reactions occur in two stages in the augen gneisses (first producting Cpx and then Opx); amphibole appears to be an important reactant in the pyroxene-forming reactions while biotite is not. -from AuthorSCOPUS: NotDefined.jinfo:eu-repo/semantics/publishe

Origine magmatique et évolution métamorphique de la série des gneiss oeillées de Norvège méridionale: études pétrologique, géochimique et isotopique

Doctorat en Sciencesinfo:eu-repo/semantics/nonPublishe

Coupled delamination and indentor-escape tectonics in the southern part of the c. 650-500 Ma East African/Antarctic Orogen

The East African/Antarctic Orogen (EAAO) is one of the largest orogenic belts on the planet, resulting from the collision of various parts of East and West- Protogondwana between 620 and 550 Ma. The central and southern parts of the orogen are typified by high-grade rocks, representing the overprinted margins of the various colliding continental blocks. The southern third of this Himalayan-type orogen can be interpreted in terms of a lateral tectonic escape model, similar to the situation presently developing in SE-Asia. One of the escape-related shear zones of the EAAO is exposed as the approximately 20 km wide Heimefront transpression zone in western Dronning Maud Land (Antarctica). During Gondwana break-up, the southern part of the EAAO broke up into a number of microplates (Falkland, Ellsworth-Haag and Filchner blocks). These microplates probably represent shear zone-bound blocks, which were segmented by tectonic translation during lateral tectonic extrusion. The southern part of the EAAO is also typified by large volumes of late-tectonic A2-type granitoids that intruded at c. 530-490 Ma, and can constitute up to 50% of the exposed basement. They are likely the consequence of delamination of the orogenic root and the subsequent influx of hot asthenospheric mantle during tectonic escape. The intrusion of these voluminous melts into the lower crust was accompanied by orogenic collapse. The A2-type magmatism appears to terminate along the Lurio Belt in northern Mozambique. Therefore, the Lurio Belt could represent an accommodation zone, separating an area to the south in which the orogen underwent delamination of the orogenic root, and an area to the north, where the orogenic keel is still present. Erosional unroofing of the northern EAAO is documented by the remnants of originally extensive areas covered by Cambro- Ordovician molasse-type clastic sedimentary rocks throughout North Africa and Arabia, testifying to the size of this megaorogen. Whilst the EAAO molasse in the north covers almost the entire North African platform, probably resulting from a long lasting high standing mountain range (no delaminated root), the molasse deposits of the southern EAAO are comparatively smaller, possibly resulting from the rapid and mechanical thinning of the orogen in the south (delaminated root)

Geochemical signature of the Egersund basaltic dyke swarm, SW Norway, in the context of late-Neoproterozoic opening of the Iapetus Ocean

SCOPUS: ar.jinfo:eu-repo/semantics/publishe

U-Pb monazite ages in amphibolite- to granulite-facies orthogneiss reflect hydrous mineral breakdown reactions: Sveconorwegian Province of SW Norway

In the Rogaland-Vest Agder terrain of the Sveconorwegian Province of SW Norway, two main Sveconorwegian metamorphic phases are reported: a phase of regional metamorphism linked to erogenic thickening (M1) and a phase of low-pressure thermal metamorphism associated with the intrusion of the 931 ± 2 Ma anorthosite-charnockite Rogaland igneous complex (M2). Phase M1 reached granulite facies to the west of the terrane and M2 culminated locally at 800-850°C with the formation of dry osumilite-bearing mineral associations. Monazite and titanite U-Pb geochronology was conducted on 17 amphibolite- to granulite-facies orthogneiss samples, mainly from a suite of 1050 +2/-8 Ma calc-alkaline augen gneisses, the Feda suite. In these rocks, prograde negatively discordant monazite crystallized during breakdown of allanite and titanite in upper amphibolite facies at 1012-1006 Ma. In the Feda suite and other charnockitic gneisses, concordant to slightly discordant monazite at 1024-997 Ma probably reflects breakdown of biotite during granulite-facies M1 metamorphism. A spread of monazite ages down to 970 Ma in biotite ± hornblende samples possibly corresponds to the waning stage of this first event. In the Feda suite, a well defined monazite growth episode at 930-925 Ma in the amphibolite-facies domain corresponds to major clinopyroxene formation at the expense of hornblende during M2. Growth or resetting of monazite was extremely limited during this phase in the granulite-facies domain, up to the direct vicinity of the anorthosite complex. The M2 event was shortly followed by cooling through ca. 610°C as indicated by tightly grouped U-Pb ages of accessory titanite and titanite relict inclusions at 918 ± 2 Ma over the entire region. A last generation of U-poor monazite formed during regional cooling below 610°C, in hornblende-rich samples at 912-904 Ma. This study suggests: (1) that monazite formed during the prograde path of high-grade metamorphism may be preserved; (2) that monazite ages reflect primary or secondary growth of monazite linked to metamorphic reactions involving redistribution of REEs and Th, and/ or fluid mobilisation; (3) that the U-Pb system in monazite is not affected by thermal events up to 800-850°C, provided that conditions were dry during metamorphism. © Springer-Verlag 1998.SCOPUS: ar.jinfo:eu-repo/semantics/publishe