Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Metasedimentary rocks in the transition zone between the Pernambuco-Alagoas Domain and the Sergipano Belt (southern Borborema Province, NE Brazil) were studied in order to place constraints on the geological evolution of this portion of West Gondwana. High-grade metamorphic conditions are recorded by the garnet-sillimanite assemblage and common anatexis in pelitic paragneisses. Two samples dated by LA-ICP-MS in the north yielded a predominance of early Neoproterozoic detrital zircons, with age peaks between 990 Ma and 827 Ma, with the youngest grain suggesting deposition after 670 Ma. Low Th/U zircons (Th/U < 0.1) in these samples display a significant spread in ages, from 691 to 568 Ma. One sample in the south is dominated by a homogenous population of Paleoproterozoic grains, with a 2200 Ma peak. The southernmost sample also contains Paleoproterozoic zircons but the most abundant population is Neoproterozoic, and characterized by age peaks at c. 670, 647 and 623 Ma. These results show that deposition of sediments in the southern PEAL Domain and in the northern Sergipano Belt occurred in the Late Neoproterozoic. The differing age spectra between samples are correlated with potential source rocks in the study area or in its proximity, reflecting variable input from local sources. The data are interpreted to indicate that during the course of an extensional event at c. 673-647 Ma, Tonian granitic intrusions and synextensional metamorphic rocks were unroofed and eroded to provide zircons for sediments deposited in the north, whereas Paleoproterozoic and synextensional magmatic rocks were the main sources for sediments in the south. Peak metamorphic conditions and contractional deformation are constrained to c. 630-600 Ma.
ISSN: 0895-9811

hal-01468011
https://hal.archives-ouvertes.fr/hal-01468011
DOI : 10.1016/j.jsames.2016.09.010

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Detrital zircons in metasedimentary rocks attributed to three different complexes in the Central and Pernambuco-Alagoas domains of the Borborema Province, northeastern Brazil, were dated by LA-ICP-MS. Two samples from each of the following complexes were analyzed: Sertania and Surubim, in the Central Domain, and Rio Una, in the Pernambuco-Alagoas Domain. One sample of the Sertania Complex yielded only Paleoproterozoic zircons, whereas Neoproterozoic zircons predominate in the other, with the youngest zircon indicating deposition after 642 Ma. One of the samples of the Surubim Complex yielded, again, only Paleoproterozoic concordant zircon ages. In the other sample, although Paleoproterozoic zircons predominate, ages vary from the Mesoarchean to the Neoproterozoic. The age of the youngest zircon in this latter sample indicates deposition after 850 Ma and an overgrowth on an Archean grain yielded an age of 623 +/- 6 Ma (2 sigma), interpreted as the age of peak metamorphism. The two samples of the Rio Una Complex yielded predominantly detrital zircons with ages in the ranges 2.2-2.1 Ga and 1.7-1.6 Ga. In this complex, the youngest detrital zircon suggests deposition after 917 Ma and metamorphic zircons indicate peak metamorphic conditions between 600 Ma and 617 Ma.;The results of this study, together with available data from the literature, allow the following conclusions: (a) The Sertania Complex is a Neoproterozoic sequence, and not Paleoproterozoic, as previously thought; (b) The similarity in lithological associations, detrital zircon populations and C isotope signature between the Sertania and Surubim complexes suggest that they belong to the same lithostratigraphic unit; (c) Although the Rio Una Complex can be older than the Sertania/Surubim Complex, the lack of zircons younger than 900 Ma can also result from insufficient sampling or absence of young rocks alongside the drainage system that collected detritus to this complex: (d) The regional metamorphism related to the Brasiliano Orogeny attained its climax at 630-600 Ma; (e) The age spectra of detrital zircons require their derivation from erosion of rocks formed not only during known geological events in the Borborema Province and in the nearby Sao Francisco/Congo Craton, but also from the Amazonian Craton, suggesting accumulation in an intracratonic setting within a preexisting large continent: (f) The short timespan between deposition and orogenic deformation may explain the overall medium- to high-temperature metamorphism due to maintenance, in the subsequent compressional event, of the elevated geothermal gradients produced during broad-scale lithosphere extension.
ISSN: 0301-9268

hal-00443856
https://hal.archives-ouvertes.fr/hal-00443856
DOI : 10.1016/j.precamres.2009.09.009

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Terrane accretion and intraplate deformation are both common processes occurring during the assembly of large continental masses. The relative roles of these mechanisms are at the heart of a debate concerning the tectonic evolution of the Borborema Province, northeastern Brazil. In the Central subprovince, four terranes or domains have been proposed. In the Alto Moxotó domain, paragneiss of the Sertânia Complex were considered to record a distinct structural and geochronological evolution compared to those in the adjacent Rio Capibaribe domain, in the East. Here, we report the results of a geological study conducted in the type area of the Sertânia Complex. Kinematic criteria and metamorphic mineral assemblages indicate that the main foliation record top-to-the-W tectonic transport under upper amphibolite facies conditions (650–700 °C). The main foliation was affected by overturned folds and upright to inclined folds; the latter two formed coevally with the development of transcurrent shear zones. Samples of mafic orthogneiss and paragneiss tectonically intercalated in the same outcrop were dated by LA-ICP-MS. The orthogneiss sample yields a crystallization age of 1978 ± 8 Ma. Detrital zircons in the paragneiss sample show a multimodal pattern with a well-defined age peak at c. 700 Ma, showing that the paragneiss is younger than 700 Ma and so the main foliation is also younger than this age. In turn, these findings suggest that the occurrence of exclusively Paleoproterozoic zircons in three other dated samples of paragneiss cannot be used to accurately define the deposition age of the detritus. The structural and geochronological characteristics of the Sertânia Complex are similar to those of the Surubim Complex in the Rio Capibaribe domain. Together with previous results showing that the basement of the Alto Moxotó and Rio Capibaribe domains is also similar, we interpret these sequences as having been deposited on a once contiguous tract of continental crust. An accretionary terrane model is thus not required to account for the tectonic evolution of this portion of the Borborema Province. Instead, a model of intracontinental deformation of a preexistent continent is fully supported by the data and also by the lack of evidence for oceanic subduction.
ISSN: 0301-9268

hal-01622825
https://hal.archives-ouvertes.fr/hal-01622825
DOI : 10.1016/j.precamres.2017.07.006

Éditeur(s) : HAL CCSD Taylor & Francis
Résumé : Modern geochronology has moved beyond the acquisition of dates: the goal is to understand the significance of these numbers for the geodynamic evolution of Earth at all scales. The coupling of the laser-ablation microprobe (LAM) to inductively coupled plasma mass spectrometers (ICPMS, multicollector (MC)-ICPMS) has revolutionised geochronology and geochemistry over the last 10 years. These systems enable the rapid and precise in situ analysis of trace-element patterns and isotopic systems, while adding information related to microstructural context and major-element composition. The integration of these multiple sources of data is crucial in constraining the origin of the sample and the processes leading to its formation, so that we can understand the meaning of a date in terms of geological events. LAM-ICPMS measurement of U-Pb ages and trace-element patterns in zircon, coupled with LAM-MC-ICPMS analysis of Hf isotopes in the same grains, gives new insights into the processes of magma genesis. Applied to detrital zircons from modern drainages or sedimentary rocks (the TerraneChron approach), it becomes a powerful tool to investigate problems of crustal evolution on scales ranging from single terranes to continents. The in situ analysis (LAM-MC-ICPMS) of Re-Os systematics in single grains of sulfides in mantle-derived peridotites has demonstrated that most mantle rocks contain several generations of Os-bearing sulfides; whole-rock analyses are mixtures reflecting multiple melting and metasomatic events in the lithospheric mantle. These deep-seated events are commonly mirrored in the crust; Os model-age spectra from xenolith suites show age 'peaks' that correspond to the ages of thermal/tectonic events in the overlying crust, suggesting strong linkages between crust and mantle. Integrated studies of the timing and nature of crustal and mantle events, using these techniques, will be important for understanding the large-scale dynamics of the Earth.
ISSN: 0812-0099

hal-00411328
https://hal.archives-ouvertes.fr/hal-00411328
DOI : 10.1080/08120090802097450

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Gold mineralisation in the Yatela Main gold mine is hosted in a saprolitic residuum situated above Birimian supracrustal rocks, and at depth. The supracrustal rocks comprise metamorphosed calcitic and dolomitic marbles that were intruded by diorite (2106 ± 10 Ma, 207Pb/206Pb), and sandstone-siltstone-shale sequences (youngest detrital zircon population dated at 2139 ± 6 Ma). In-situ gold-sulphide mineralisation is associated with hydrothermal activity synchronous to emplacement of the diorite and forms a sub-economic resource; however, the overlying saprolitic residuum hosts economic gold mineralisation in friable lateritized palaeosols and aeolian sands (loess).Samples of saprolitic residuum were studied to investigate the morphology and composition of gold grains as a proxy for distance from source (and possible exploration vector) because the deposit hosts both angular and detrital gold suggesting both proximal and distal sources. U–Pb geochronology of detrital zircons also indicated a proximal and distal source, with the age spectra giving Archaean (2.83–3.28 Ga), and Palaeoproterozoic (1.95–2.20 Ga) to Neoproterozoic (1.1–1.8 Ga) zircons in the Yatela depocentre. The 1.1–1.8 Ga age spectrum restricts the maximum age for the first deposition of the sedimentary units in the Neoproterozoic, or during early deposition in the Taoudeni Basin. Models for formation of the residuum include distal and proximal sources for detritus into the depocentre, however, it is more likely that material was sourced locally and included recycled material. The creation of a deep laterite weathering profile and supergene enrichment of the residuum probably took place during the mid-Cretaceous-early Tertiary.
ISSN: 0899-5362

hal-01277679
https://hal.archives-ouvertes.fr/hal-01277679
DOI : 10.1016/j.jafrearsci.2015.07.017

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Dahomey belt in Togo and Benin is an important segment of the larger West Gondwana Orogen. Here, we review the geodynamic evolution of the Dahomey belt and discuss new U–Pb and Lu–Hf zircon data in light of similar data previously acquired on the geologically related Northern Borborema Province, in NE Brazil. Eighteen samples from different tectonic settings and regions within the belt were collected for zircon isotopic investigation. Passive margin deposits of the Atacora Structural Unit and lower units of the Volta Basin have detrital zircon signatures compatible with the flanking West Africa Craton. The arc-related magmatism resulted from the east-dipping subduction of the Goiás-Pharusian oceanic lithosphere and is represented by a variety of granitoids emplaced in the Benino-Nigerian Shield between 670 and 610 Ma. These granitoids were mainly sourced from crustal reservoirs with subordinate juvenile input. Detrital zircon ages from syn-orogenic deposits in Benino-Nigerian Shield suggest that arc development could have started as early as 780 Ma. The main period of melting in the internal part of the belt, the Benino-Nigerian Shield, is related to crustal thickening and occurs only ca. 30 m.y. after initiation of the continental collision, marked by the ca. 610 Ma ultra-high pressure (UHP) metamorphism recorded at Lato Hills. Foreland development represented by the upper units of the Volta basin developed soon after continental collision and persisted with the development of the west-verging thrust front synchronously with the main period of crustal melting due to collision at ca. 580 Ma. The subvertical Transbrasiliano Lineament in South America, that corresponds to the Kandi Lineament in Africa, provides a present-day fit between NW Africa and NE Brazil. Restoration of the movement of the Transbrasiliano-Kandi Lineament (strike-slip plate boundary) places the Dahomey belt and Borborema Province (NE Brazil) along the same section of the West Gondwana Orogen. This configuration would explain some of the misfits previously discussed in the literature and aligns the UHP eclogites in Togo and NE Brazil.
ISSN: 0301-9268

hal-01358584
https://hal.archives-ouvertes.fr/hal-01358584
DOI : 10.1016/j.precamres.2016.01.032

Éditeur(s) : HAL CCSD Elsevier
Résumé : Integrated field, LA-MC-ICP-MS geochronology and geochemical data from the Novo Oriente Group have been investigated in order to determine their provenance and possible tectonic setting. This group in the southwestern portion of the Ceara Central Domain, is a well-preserved metavolcanosedimentary sequence, in part exhalative, composed of two distinct formations. The proximal coastal Bonsucesso Formation comprises mainly quartzite and minor basic metavolcanic rocks and is in gradational contact with the distal Caranbas Formation which constitutes a metapelitic-volcanic-carbonate sequence, including metabasic rocks with pillow structure and sheared serpentinized ultrabasic rocks. The youngest detrital zircon found in the quartzite of the Bonsucesso Formation yielded an age of ca. 2.1 Ga and well-defined peaks around 2.2.2.3-2.4 and 2.5 Ga. In addition, zircons from a related metabasalt yielded an upper intercept age of 2083 +/- 28 Ma. We interpret the zircons found in this metabasalt as inherited from the host quartzite. This fact is supported by depleted-mantle Nd model ages (T-DM age) from correlated metabasic and metaultrabasic rocks from other places in the Novo Oriente sequence, which have yielded ages between 1.36 and 1.69 Ga, that is younger than the zircon age obtained in the metabasalt extruded over the Bonsucesso quartzite. Geochemical data of the metasedimentary rocks from the Caraubas Formation indicates a provenance composed of a mixture of felsic and intermediate sources typical of old stable cratons and older continental roots of active tectonic settings. The rocks of the Crania (Medio Coreau Domain) and Bacaja Complexes (southeastern Amazonian Craton) exposed to the west of the Novo Oriente Group, and largely covered by the Phanerozoic Parnaiba intracratonic basin, are the most likely source rocks. Since the precise age of the Novo Oriente Group is not yet well constrained, two distinct settings may be proposed: (1) it may represent a more evolved basin developed from extensional processes during the Mesoproterozoic (ca. 1.5-1.3 Ga); and (2) it could constitute part of the passive margin systems developed during the break-up of the Rodinia crown copyright supercontinent (ca. 0.95-0.8 Ga), associated with a pre-Brasiliano/Pan-African ocean (e.g. Pharusian Ocean). Crown.
ISSN: 1342-937X

hal-00512825
https://hal.archives-ouvertes.fr/hal-00512825
DOI : 10.1016/j.gr.2010.02.001