Catherine Guerrot

Proterozoic tectonism of the Arabian Shield

Abstract New field and analytical work, together with a new aeromagnetic map and a geologic, structural, geochemical, and geochronologic synthesis and reappraisal, offer a new view of the anatomy and geologic history of the Arabian Shield. Although Early Proterozoic rocks have been found in the eastern part of the Shield, the main geologic evolution of the Shield is limited to a period ranging from 900 to 530 Ma that led to the formation, amalgamation, and final cratonization of several tectonostratigraphic terranes. The pre-Panafrican structures (>690 Ma), which are difficult to decipher due to younger deformation, were essentially the result of the formation, amalgamation, and accretion of these terranes. The Panafrican tectonism (690–590 Ma) was marked by the formation of the Nabitah Belt and peripheral ranges punctuated by gneiss domes. Various sedimentary formations contemporaneous with this tectonism represent foreland or intracontinental molasse basins. After the Panafrican tectonism, widespread extension (590–530 Ma) brought about crustal thinning that generated bimodal magmatism and significant dike swarms; associated volcanics form the Shammar group. A marine transgression, associated with passive-margin-type structures with tilted blocks, marked the end of the thinning. The platform facies produced by this transgression correspond to part of the Jibalah Formation. Other basins formed as deep continental pull-apart basins along transform faults. This updated view of the Arabian Proterozoic geodynamic evolution provides a framework for reviewing the associated mineralizing events, and places them in a new chronology and structural history.

Late Eburnean granitization and tectonics along the western and northwestern margin of the Archean Kenema-Man domain (Guinea, West African Craton)

Abstract Recent mapping of the eastern part of Guinea has revealed a vast plutonic belt that rims the margin of the Archean Kenema–Man craton and that is made up of a variety of granitic rocks (granodiorite, biotite granite, monzogranite, two-mica granite), granodiorite, with common clinopyroxene, being the most abundant. Single-zircon dating by stepwise Pb-evaporation carried out on characteristic rock types (granodiorite, monzogranite, two-mica granite) reveals their age to be late Eburnean (c. 2090–2070 Ma), and indicates that their emplacement occurred within a relatively short time interval (⩽20 Ma). The granodiorite, biotite granite and monzogranite of the plutonic belt all present a calc-alkaline and highly potassic nature and show numerous analogies with the Neogene calc-alkaline rocks of the central Andes; there is evidence to suggest strong contamination from the Archean crust. These analogies make it possible to envisage the emplacement of the plutonic belt directly above a subduction zone. The occurrence of peraluminous two-mica granite at the northern extension of the plutonic belt in the Siguiri Basin may suggest that the convergence here was associated with the local melting of metasedimentary rocks in the deep part of the basin. Contemporaneously with its emplacement, the late Eburnean plutonic belt accommodated regional convergence by major WNW–ESE sinistral movements along the northwestern margin of the Archean block. Horizontal shortening towards the WSW is envisaged at the scale of eastern Guinea in relation with these kinematics. The late Eburnean plutonic belt of eastern Guinea is a key element of the Archean/Proterozoic transition zone in the southern part of West Africa. Evidence along the western and northwestern border of the Archean Kenema–Man domain leads us to interpret this transition zone as an active margin, along which strike–slip tectonism accommodated regional convergence towards the end of the Eburnean cycle.

Sources of salinity and boron in the Gaza strip: Natural contaminant flow in the southern Mediterranean coastal aquifer

isotopic ( 87 Sr/ 86 Sr, d 11 B, d 18 O, d 2 H, and d 34 SSO4) compositions of groundwater from the southern Mediterranean coastal aquifer (Israel) and the Gaza Strip in order to elucidate the origin of salinity and boron contamination. The original salinity in the eastern part of the aquifer is derived from discharge of saline groundwater from the adjacent Avedat aquitard (Na/Cl 1, high B/Cl, SO4/Cl, and HCO3, 87 Sr/ 86 Sr� 0.7083; d 11 B� 48%), although the d 18 O-d 2 H slope is identical to that of the Avedat aquitard. The geochemical data suggest that dissolution of pedogenic carbonate and gypsum minerals in the overlying loessial sequence generated the Ca-rich solution that triggered base exchange reactions and produced Na- and B-rich groundwater. The geochemical data show that most of the salinization process in the Gaza Strip is derived from the lateral flow of the Na-rich saline groundwater, superimposed with seawater intrusion and anthropogenic nitrate pollution. The methodology of identification of multiple salinity sources can be used to establish a long-term management plan for the Gaza Strip and can also be implemented to understand complex salinization processes in other similarly stressed coastal aquifers.

ASSESSMENT OF THE PRECISION AND ACCURACY OF LEAD-ISOTOPE RATIOS MEASURED BY TIMS FOR GEOCHEMICAL APPLICATIONS : EXAMPLE OF MASSIVE SULPHIDE DEPOSITS ( RIO TINTO, SPAIN)

The true reproducibility of lead-isotope measurements by thermal-ionization mass-spectrometer (TIMS) was assessed on both the international standard (NBS 982) and sulphide samples from the South Iberian Pyrite Belt (SIPB). Lead-isotope analyses were made on 21 pyrite and galena samples from Rio Tinto sulphide orebodies in the Spanish part of the SIPB. Independent lead-isotope analyses were made in the UM2 laboratory (Montpellier), using a VG Sector 54 mass spectrometer, and at BRGM (Orleans) using a Finnigan Mat 262 mass spectrometer. Internal precision and reproducibility of the isotope measurements were calculated for lead-isotope standards (NBS 982), and for pyrite and galena samples. The reproducibility (2σ) is 0.12% for 206Pb/204Pb, 0.16% for 207Pb/204Pb and 0.22% for 208Pb/204Pb while the internal precision is on average 0.01% for each isotope ratio (2σm). This indicates that the usually adopted errors of between 0.10% and 0.25% are appropriate for geological samples. TIMS has been the most common method for measuring U/Pb ratios by isotopic dilution, but since a few years ICP–MS has also been used for such determinations. We thus checked the reproducibility of U/Pb-ratio measurements by ICP–MS, in order to apply this method to sulphide samples. Independent measurements (2 to 7) for each of 20 analysed samples showed that the measured average reproducibility for U/Pb ratios is better than 5%. This method is thus suitable for determination of U/Pb ratios in sulphide samples and most other geological materials. Lead analyses for the Rio Tinto deposit were made of the pyritic and orebodies, and the stringers. This deposit, one of the biggest massive sulphides in the world, is remarkably homogeneous from a lead-isotope viewpoint, and no difference can be seen between pyritic and polymetallic orebodies. The isotope composition of the deposit can be considered as the average composition of the South Iberian crust during the Devonian–Early Carboniferous period of crustal fusion.

Chemical and strontium isotope characterization of rainwater in France: influence of sources and hydrogeochemical implications

Strontium isotope ratios and Ca2+, Na+, K+, Mg2+, Cl−, , and Sr2+ concentrations were measured in rainwater samples collected in four stations in France (Brest, Dax, Orleans and Clermont-Ferrand) over a period of 1 year. Each sample represented a monthly series of rain events. The chemical composition and the 87Sr/86Sr ratios of the rainwater samples varied considerably. Using Na concentrations as an indicator of marine origin, the proportion of marine and crustal elements was estimated from elemental ratios. Strontium isotopes were used to characterize the different sources using data from the four stations and the literature. Such sources include sea salts, crustal sources (carbonates, silicates and volcanic rocks) and anthropogenic sources (fertilizers, automobile exhausts, incinerators and urban heating).

Origins and processes of groundwater salinization in the urban coastal aquifers of Recife (Pernambuco, Brazil): A multi-isotope approach

In the coastal multilayer aquifer system of a highly urbanized southern city (Recife, Brazil), where groundwaters are affected by salinization, a multi-isotope approach (Sr, B, O, H) was used to investigate the sources and processes of salinization. The high diversity of the geological bodies, built since the Atlantic opening during the Cretaceous, highly constrains the heterogeneity of the groundwater chemistry, e.g. Sr isotope ratios, and needs to be integrated to explain the salinization processes and groundwater pathways. A paleoseawater intrusion, most probably the 120 kyB.P. Pleistocene marine transgression, and cationic exchange are clearly evidenced in the most salinized parts of the Cabo and Beberibe aquifers. All (87)Sr/(86)Sr values are above the past and present-day seawater signatures, meaning that the Sr isotopic signature is altered due to additional Sr inputs from dilution with different freshwaters, and water-rock interactions. Only the Cabo aquifer presents a well-delimitated area of Na-HCO3 water typical of a freshening process. The two deep aquifers also display a broad range of B concentrations and B isotope ratios with values among the highest known to date (63-68.5‰). This suggests multiple sources and processes affecting B behavior, among which mixing with saline water, B sorption on clays and mixing with wastewater. The highly fractionated B isotopic values were explained by infiltration of relatively salty water with B interacting with clays, pointing out the major role played by (palaeo)-channels for the deep Beberibe aquifer recharge. Based on an increase of salinity at the end of the dry season, a present-day seawater intrusion is identified in the surficial Boa Viagem aquifer. Our conceptual model presents a comprehensive understanding of the major groundwater salinization pathways and processes, and should be of benefit for other southern Atlantic coastal aquifers to better address groundwater management issues.

First evidence of early Birimian (2.21 Ga) volcanic activity in Upper Guinea: the volcanics and associated rocks of the Niani suite

Abstract During the 1:200,000-scale mapping of northeastern Guinea, a suite of volcanic rocks comprising porphyritic lavas, pyroclastics and pyroclastic breccias, was identified in the Niani area in the northeast of the Siguiri Basin. The texture of the rocks, the abundance of pyroclastic deposits and the diversity of lapilli they contain all point to explosive eruptive mechanisms. A Pb–Pb zircon evaporation age of 2212±6 Ma obtained from a sample of rhyodacite has yielded the first evidence of early Birimian volcanic activity in Guinea. Most of the volcanic rocks of the Niani volcanic suite may be defined as intermediate to acidic medium-K calc-alkaline igneous rocks. Some of them show HREE fractionation comparable to the one of high-Al/low-Yb TTGs and recent adakites. It seems probable that these lavas were emplaced in relation with the subduction of a young and hot oceanic plate.

On the non-existence of a Cadomian basement in southern France (Pyrenees, Montagne Noire): implications for the significance of the pre-Variscan (pre-Upper Ordovician) series

The deepest Hercynian metamorphic terrains in the Pyrenees and in the nearby Montagne Noire are made up of medium-grade orthogneisses and micaschists, and of high-grade, often granulitic, paragneisses. The existence of a granitic-metamorphic Cadomian basement and of its sedimentary Lower Paleozoic cover was advocated from the following main arguments: (i) a supposed unconformity of the Lower Cambrian Canaveilles Group (the lower part of the Paleozoic series) upon both granitic and metamorphic complexes; (ii) a ca . 580 Ma U-Pb age for the metagranitic Canigou gneisses. A SE to NW transgression of the Cambrian cover and huge Variscan recumbent (“penninic”) folds completed this classical model. However, recent U-Pb dating provided a ca . 474 Ma, early Ordovician (Arenigian) age for the me-tagranites, whereas the Vendian age (581 ± 10 Ma) of the base of the Canaveilles Group was confirmed [Cocherie et al. , 2005]. In fact, these granites are laccoliths intruded at different levels of the Vendian-Lower Cambrian series. So the Cadomian granitic basement model must be discarded. In a new model, developed in the Pyrenees and which applies to the Montagne Noire where the orthogneisses appear to be Lower Ordovician intrusives too, there are neither transgression of the Paleozoic nor very large Hercynian recumbent folds. The pre-Variscan (pre-Upper Ordovician) series must be divided in two groups: (i) at the top, the Jujols Group, mainly early to late Cambrian, that belongs to a Cambrian-Ordovician sedimentary and magmatic cycle ; the early Ordovician granites pertain to this cycle; (ii) at the base, the Canaveilles Group of the Pyrenees and the la Salvetat-St-Pons series of the Montagne Noire, Vendian (to earliest Cambrian?), are similar to the Upper Alcudian series of Central Iberia. The Canaveilles Group is a shale-greywacke series with rhyodacitic volcanics, thick carbonates, black shales, etc. The newly defined olistostromic and carbonated, up to 150 m thick Tregura Formation forms the base of the Jujols Group, which rests more or less conformably on the Canaveilles Group. The high-grade paragneisses which in some massifs underlie the Canaveilles and Jujols low- to medium grade metasediments are now considered to be an equivalent of the Canaveilles Group with a higher Variscan metamorphic grade; they are not derived from metamorphic Precambrian rocks. So, there is no visible Cadomian metamorphic (or even sedimentary) basement in the Pyrenees. However, because of its age, the Canaveilles Group belongs to the end of the Cadomian cycle and was deposited in a subsident basin, probably a back-arc basin which developed in the Cadomian, active-transform N-Gondwanian margin of this time. The presence of Cadomian-Panafrican ( ca. 600 Ma) zircon cores in early Ordovician granites and Vendian volcanics implies the anatexis of a thick (> 15 km?) syn-Cadomian series, to be compared to the very thick Lower Alcudian series of Central Iberia, which underlies the Upper Alcudian series. Nd isotopic compositions of Neoproterozoic and Cambrian-Ordovician sediments and magmatites, as elsewhere in Europe, yield Paleoproterozoic ( ca. 2 Ga) model-ages. From the very rare occurrences of rocks of this age in W-Europe, it can be envisionned that the thick Pyrenean Cadomian series lies on a Paleoproterozoic metamorphic basement. But, if such a basement does exist, it must be “hidden”, as well as the lower part of the Neoproterozoic series, in the Variscan restitic granulites of the present (Variscan) lower crust. So a large part of the pre-Variscan crust was made of volcano-sedimentary Cadomian series, explaining the “fertile” characteristics of this crust which has been able to produce the voluminous Lower Ordovician and, later, Upper Carboniferous granitoids.

Middle Carboniferous intracontinental subduction in the Outer Zone of the Variscan Belt (Montagne Noire Axial Zone, French Massif Central): multimethod geochronological approach of polyphase metamorphism

Abstract In the migmatitic dome of the Montagne Noire Axial Zone (Variscan French Massif Central), mafic eclogites yield zircon and rutile U–Pb SHRIMP and secondary-ion mass spectrometry (SIMS) ages of c. 315–308 Ma. These ‘young’ dates, obtained in two different laboratories, do not comply with the geological constraints available for the study area that suggest an older age of the high-pressure–low-temperature (M1) metamorphism. Nevertheless, the Sm–Nd age of the same rock at c. 358 Ma appears in better agreement with the geological constraints, and therefore might reflect the age of the high-pressure (HP) event. Similar 357–352 Ma monazite U–Th–Pbtot ages are obtained from relict grains in the Axial Zone kinzigites that represent restites enclosed in migmatites. Furthermore, monazite grains from biotite–garnet–staurolite micaschists from the dome envelope and Axial Zone kinzigites yield U–Th–Pbtot ages in the range 340–320 Ma. These dates are in good agreement with previously documented zircon and monazite ages from the migmatite and anatectic granites that represent a high-temperature–low-pressure (M2) event. The significance of the zircon and rutile ages in the eclogites is discussed in terms of hydrothermal circulations. A crustal-scale model considers: (1) a north-directed intracontinental subduction, responsible for the high-pressure–low-temperature (M1) metamorphism, coeval with kilometre-scale south-vergent recumbent folds in the Palaeozoic non-metamorphic cover, followed by (2) a high-temperature–low-pressure (M2) event, coeval with the development of the Axial Zone migmatitic dome, and upright folding in the Palaeozoic non-metamorphic series.

Chronology, petrogenesis and heat sources for successive Carboniferous magmatic events in the Southern–Central Variscan Vosges Mts (NE France)

Plutonic bodies of the Central and Southern Vosges Mts can be assigned to two major early Carboniferous magmatic events: a Visean Mg–K event (c. 345 and 340–336 Ma) and a younger S-type event (329–322 Ma). New petrological, geochemical and Sr–Nd isotopic data highlight the existence of two groups of Mg–K intrusions that might be related to the nature of their primary magma sources; that is, CHUR-like and enriched mantle, which interacted with juvenile and mature crustal material, respectively. The differences between these two groups are explained by a geodynamic scenario involving deep subduction and relamination of the Saxothuringian continental crust under the Moldanubian continent. The relaminated radiogenic Saxothuringian material is thought to have been responsible for dehydration melting of both subducted crust and underlying metasomatized mantle, thereby generating the Mg–K magma subsequently emplaced at middle crustal depth. During their ascent, the mafic magmas interacted with crustally derived felsic melts. Significantly later (c. 10–15 myr) a widespread mid-crustal anatexis occurred, generating voluminous granite intrusions from mixed crustal sources (paragneisses and/or immature felsic–intermediate metaigneous rocks mixed with Mg–K plutons). The principal heat source for such a major melting event is related to the presence of Mg–K plutons rich in heat-producing elements, which were responsible, after the time lag specified, for a temperature increase at mid-crustal levels by in situ radiogenic heat production. The current study underlines the importance of deep continental crust subduction and relamination for the magmatism and development of collisional orogens. Supplementary material: Analytical methods and data, and supplementary figures, are available at http://www.geolsoc.org.uk/SUP18795.