Metamorphisme et evolution geothermobarometrique d'un segment crustal charrie en zone profonde : Le Rouergue cristallin (Massif central - France)

SIGLECNRS T 59299 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Autochtonous greenstone belts of Madagascar : Implication for the archean tectonics in the frame of the greater darwhar craton

International audienceThe new geochronologic, petrologic and structural data acquired during the mapping project of the Programme de Gouvernance des Ressources Minérales de Madagascar, completed by academic works, lead to the re-definition of eight geological domains which make up the metamorphic Malagasy basement. The Center of the Island only consists of the Antananarivo and Antongil/Masora domain respectively. The Paleo- Mesoarchean Antongil/Masora domain (3.32-3.15 Ga) is interpreted as a fragment of the wider Dharwar craton of India. Its adjacent Neoarchean Antananarivo domain (2.7-2.5 Ga) is well know because it comprises a critical component of mafic-ultramafic gneisses which crop out as five N-striking belts interpreted as Archaean metamorphosed greenstone belts referred hereafter as “Tsaratanana Complex”. ,In the conventional view, the Tsaratanana complex is interpreted as a single allochthonous unit thrust over granitoid gneisses of the Antananarivo block during the Panafrican orogeny. Two aspects of this notion are controversial: (i) the age of the allochthon’s emplacement and (ii) the very nature of the allochthon itselfWe present new structural data and U-Pb isotopic ages arguing that neither the structural evolution nor the age constraints of the identified geological events are consistent with an allochtonous model of nappe tectonics. On the contrary, the “Tsaratanana Complex” is likely an autochthonous domain or was juxtaposed with the Antananarivo Domain in the late Archean / Early Palaeoproterozoic times. In this way, the Antananarivo Domain and the associated “Tsaratanana Complex” does not differ from the Eastern Dharwar Craton. Consequently, the reliability of the “Betsimisaraka suture” is discussed and we propose that Neoarchean rocks of the Eastern Dharwar Craton and the Antananarivo domain were symmetrically disposed around the Mesoarchaean nucleus formed by the Western Dharwar Craton and the Antongil/masora Domain as earlier as Neoarchean times

Zircon geochronology and Sm–Nd isotopic study: Further constraints for the Archean and Paleoproterozoic geodynamical evolution of the southeastern Guiana Shield, north of Amazonian Craton, Brazil

International audienceThe eastern part of the Guiana Shield, northern Amazonian Craton, in South America, represents a large orogenic belt developed during the Transamazonian orogenic cycle (2.26–1.95 Ga), which consists of extensive areas of Paleoproterozoic crust and two major Archean terranes: the Imataca Block, in Venezuela, and the here defined Amapá Block, in the north of Brazil.Pb-evaporation on zircon and Sm–Nd on whole rock dating were provided on magmatic and metamorphic units from southwestern Amapá Block, in the Jari Domain, defining its long-lived evolution, marked by several stages of crustal accretion and crustal reworking. Magmatic activity occurred mainly at the Meso-Neoarchean transition (2.80–2.79 Ga) and during the Neoarchean (2.66–2.60 Ga). The main period of crust formation occurred during a protracted episode at the end of Paleoarchean and along the whole Mesoarchean (3.26–2.83 Ga). Conversely, crustal reworking processes have dominated in Neoarchean times. During the Transamazonian orogenic cycle, the main geodynamic processes were related to reworking of older Archean crust, with minor juvenile accretion at about 2.3 Ga, during an early orogenic phase. Transamazonian magmatism consisted of syn- to late-orogenic granitic pulses, which were dated at 2.22 Ga, 2.18 Ga and 2.05–2.03 Ga. Most of the εNd values and TDM model ages (2.52–2.45 Ga) indicate an origin of the Paleoproterozoic granites by mixing of juvenile Paleoproterozoic magmas with Archean components.The Archean Amapá Block is limited in at southwest by the Carecuru Domain, a granitoid-greenstone terrane that had a geodynamic evolution mainly during the Paleoproterozoic, related to the Transamazonian orogenic cycle. In this latter domain, a widespread calc-alkaline magmatism occurred at 2.19–2.18 Ga and at 2.15–2.14 Ga, and granitic magmatism was dated at 2.10 Ga. Crustal accretion was recognized at about 2.28 Ga, in agreement with the predominantly Rhyacian crust-forming pattern of the eastern Guiana Shield. Nevertheless, TDM model ages (2.50–2.38 Ga), preferentially interpreted as mixed ages, and εNd < 0, point to some participation of Archean components in the source of the Paleoproterozoic rocks. In addition, the Carecuru Domain contains an oval-shaped Archean granulitic nucleus, named Paru Domain. In this domain, Neoarchean magmatism at about 2.60 Ga was produced by reworking of Mesoarchean crust, as registered in the Amapá Block. Crustal accretion events and calc-alkaline magmatism are recognized at 2.32 Ga and at 2.15 Ga, respectively, as well as charnockitic magmatism at 2.07 Ga.The lithological association and the available isotopic data registered in the Carecuru Domain suggests a geodynamic evolution model based on the development of a magmatic arc system during the Transamazonian orogenic cycle, which was accreted to the southwestern border of the Archean Amapá Block

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Identification of sources of potential fields with the continuous wavelet transform: Complex wavelets and application to aeromagnetic profiles in French Guiana

International audienceA continuous wavelet technique has been recently introduced to analyze potential fields data. First, we summarize the theory, which primarily consists of interpreting potential fields via the properties of the upward continued derivative field. Using complex wavelets to analyze magnetic data gives an inverse scheme to find the depth and homogeneity degree of local homogeneous sources and the inclination of their magnetization vector. This is analytically applied on several local and extended synthetic magnetic sources. The application to other potential fields is also discussed. Then, profiles crossing dikes and faults are extracted from the recent high-resolution aeromagnetic survey of French Guiana and analyzed using complex one dimensional wavelets. Maps of estimated depth to sources and their magnetization inclination and homogeneity degree are proposed for a region between Cayenne and Kourou

Relationships between lower and upper crust tectonic during doming: the mylonitic southern edge of the Velay metamorphic core complex (Cévennes-French Massif Central)

International audiencePetrological, structural, and thermochronological analyses have been focused on the Mylonitic Metamorphic Vellave Zone (MMVZ), which wraps around the southern edge of the Velay metamorphic core complex dome. Mineral-stretching lineations in the Sil-Bt zone are everywhere perpendicular to the dome edge, the strong mylonitic foliation displays top-to-the south normal shearing, and preferential quartz orientations show high-temperature prismatic deformation interpreted as pure shear patterns. The Sil-Bt zone reflects the dome dynamics during its forced emplacement and uplift. From the contact aureole of the Borne monzogranite, through greenschist facies, and until the And-Crd-Bt zone of the MMVZ, the series are affected by constant syn-metamorphic top-to-the-northeast shearing, acting since at least 320 Ma and reflecting upper crust tectonics during doming: the detachment system. A constriction-dominated deformation regime characterizes the And-Crd-Bt zone, together with the fact that thermochronometers (micas Ar/Ar, U-Th/Pb monazite) are set at 310 Ma, reflecting fast cooling and uplift high in the crust, consequently, the MMVZ is a mylonite that represents the interaction of dome dynamics and the tectonic environment of the upper crust. The Velay dome was forcibly emplaced in the country rocks, and has intruded its own detachment, showing the relationship between the detachment and the dynamic of the dome on its southern edge

A geological synthesis of the Precambrian shield in Madagascar

International audiencevailable U-Pb geochronology of the Precambrian shield of Madagascar is summarized and integrated into a synthesis of the region's geological history. The shield is described in terms of six geodynamic domains, from northeast to southwest, the Bemarivo, Antongil-Masora, Antananarivo, Ikalamavony, Androyan-Anosyan, and Vohibory domains. Each domain is defined by distinctive suites of metaigneous rocks and metasedimentary groups, and a unique history of Archean (similar to 2.5 Ga) and Proterozoic (similar to 1.0 Ga, similar to 0.80 Ga, and similar to 0.55 Ga) reworking. Superimposed within and across these domains are scores of Neoproterozoic granitic stocks and batholiths as well as kilometer long zones of steeply dipping, highly strained rocks that record the effects of Gondwana's amalgamation and shortening in latest Neoproterozoic time (0.560-0.520 Ga).The present-day shield of Madagascar is best viewed as part of the Greater Dharwar Craton, of Archean age, to which three exotic terranes were added in Proterozoic time. The domains in Madagascar representing the Greater Dharwar Craton include the Antongil-Masora domain, a fragment of the Western Dharwar of India, and the Neoarchean Antananarivo domain (with its Tsaratanana Complex) which is broadly analogous to the Eastern Dharwar of India. In its reconstructed position, the Greater Dharwar Craton consists of a central nucleus of Paleo-Mesoarchean age (>3.1 Ga), the combined Western Dharwar and Antongil-Masora domain, flanked by mostly juvenile "granite-greenstone belts" of Neoarchean age (2.70-2.56 Ga). The age of the accretionary event that formed this craton is approximately 2.5-2.45 Ga. The three domains in Madagascar exotic to the Greater Dharwar Craton are the Androyan-Anosyan, Vohibory, and Bemarivo. The basement to the Androyan-Anosyan domain is a continental terrane of Paleoproterozoic age (2.0-1.78 Ga) that was accreted to the southern margin (present-day direction) of the Greater Dharwar Craton in pre-Stratherian time (>1.6 Ga), and rejuvenated at 1.03-0.93 Ga with the creation of the Ikalamavony domain. The Vohibory domain, an oceanic terrane of Neoproterozoic age was accreted to the Androyan-Anosyan domain in Cryogenian time (similar to 0.63-0.60 Ga). The Bemarivo domain of north Madagascar is a terrane of Cryogenian igneous rocks, with a cryptic Paleoproterozoic basement, that was accreted to the Greater Dharwar Craton in latest Ediacaran to earliest Cambrian time (0.53-0.51 Ga)

Paleoproterozoic (2155–1970Ma) evolution of the Guiana Shield (Transamazonian event) in the light of new paleomagnetic data from French Guiana

International audienceWe present a comprehensive paleomagnetic study on Paleoproterozoic (2173–2060 Ma) plutonic and metamorphic rocks from French Guiana, representative of the full range of the main Transamazonian tectonothermal steps. Twenty-seven groups of directions and poles were obtained from combination of 102 sites (613 samples) based on age constraint, similar lithology and/or geographical proximity. Paleomagnetic results show variations between rocks of different ages which are supposed to be characteristic of magnetizations acquired during uplift and cooling of successive plutonic pulses and metamorphic phases. This is also reinforced by positive field tests (baked contact and reversal tests). Recent U/Pb and Pb/Pb on zircon and complementary 40Ar/39Ar on amphibole and biotite allow questioning the problem of magnetic ages relative to rock formation ages. Estimated magnetic ages, based on amphibole dating as a proxy, enable us to construct a Guiana Shield apparent polar wander path for the 2155–1970 Ma period. It is also possible to present paleolatidudinal evolution and continental drift rates related to specific Transamazonian tectonic regimes.French Guiana and probably the Guiana Shield were located at the Equator from ca. 2155 to 2130 Ma during the Meso-Rhyacian D1 magmatic accretion phase, related to subduction of Eorhyacian oceanic crust. After closure of the Eorhyacian Ocean and collision of West African and Amazonian plates, the Guiana Shield moved. The first evolution towards 60° latitude, occurs after 2080 Ma, during the Neorhyacian D2a post collisional sinistral transcurrent phase. During the Late Rhyacian D2b phase, up to 2050 Ma, the Guiana Shield reaches the pole and starts to move to lower latitudes on an opposite meridian. By the Orosirian D2c phase, from ca. 2050 to 1970 Ma, the Guiana Shield reaches the Equator.Based on the amphibole 40Ar/39Ar dates, we estimate the continental drift between 12 and 16 cm/y for the Meso to Late Rhyacian period followed by a lower rate between 9 and 14 cm/y up to Orosirian time. This study highlights rock ages and magnetic ages are prerequisite to any continental reconstruction especially when it is shown continental drift is important for a 100–200 Ma time period. Our results confirm the possibility of APWP construction on Paleoproterozoic plutonic rocks but suggest improvement will rely on the combination with multidisciplinary approaches such as structural geology and multi-method radiometric datin

Archean evolution of the Leo Rise and its Eburnean reworking

Recent geological mapping in southeastern Guinea, supported by zircon dating, has called into question traditional understanding concerning the evolution of the Leo Rise. Gneiss dated at about 3540 Ma appears to constitute the earliest evidence for continental accretion within the Leo Rise. The existence of a Leonian depositional cycle at about 3000 Ma is confirmed, marked by volcanic and sedimentary rocks that can be correlated with the Loko Group in Sierra Leone. The span of ages (3244-3050 Ma) suggests that the Leonian cycle comprises different episodes whose respective chronology is as yet uncertain. Clearly distinct from the Leonian cycle, the Liberian cycle (∼2900-2800 Ma) is represented in Guinea by granite and migmatite (∼2910-2800 Ma), reflecting remobilization of the ancient Archean basement and deformation of the Leonian rocks; no deposition is associated with this cycle. After the Liberian, the Nimba and Simandou successions, containing Liberian detrital zircons, are assigned to the Birimian (∼2200-2000 Ma). Finally, Eburnean tectonism caused intense deformation of the Archean craton, accompanied by high-grade metamorphism and the intrusion of granite and syenite with ages between 2080 and 2020 Ma. The evolution of the Kénéma-Man domain, attributed to the cumulated effect of the Leonian and Liberian cycles, is thus in part Eburnean. We can suppose, therefore, that the NNE-SSW-trending structures attributed to the Liberian in Sierra Leone are, in fact, Eburnean. The Kambui Supergroup, also affected by this tectonism, should thus be assigned to the Birimian rather than the Liberian, which would explain its similarities with the Nimba and Simandou successions