Deep inside a neoproterozoic intra-oceanic arc: growth,differentiation and exhumation of the Amalaoulaou complex (Gourma,Mali)

We show here that the Amalaoulaou complex, in the Pan-African belt of West Africa (Gourma, Mali), corresponds to the lower and middle sections of a Neoproterozoic intra-oceanic arc. This complex records a 90–130-Ma-long evolution of magmatic inputs and differentiation above a subducting oceanic slab. Early c. 793 Ma-old metagabbros crystallised at lower crustal or uppermost mantle depths (25–30 km) and have geochemical characteristic of high-alumina basalts extracted from a depleted mantle source slightly enriched by slab-derived sedimentary components ((La/Sm)_N < 1; ε_Nd: +5.4–6.2; ⁸⁷Sr/⁸⁶Sr: 0.7027–0.7029). In response to crustal thickening, these mafic rocks were recrystallised into garnet-granulites (850–1,000°C; 10–12 kbar) and subject to local dehydration–melting reactions, forming trondhjemititic leucosomes with garnet–clinopyroxene–rutile residues. Slightly after the granulitic event, the arc root was subject to strong HT shearing during partial exhumation (detachment faults/rifting or thrusting), coeval with the emplacement of spinel- and garnet-pyroxenite dykes crystallised from a high-Mg andesitic parental magma. Quartz and hornblende-gabbros (700–660 Ma) with composition typical of hydrous volcanic rocks from mature arcs ((La/Sm)_N: 0.9–1.8; ε_Nd: +4.6 to +5.2; ⁸⁷Sr/⁸⁶Sr: 0.7028–0.7031) were subsequently emplaced at mid-arc crust levels (~15 km). Trace element and isotopic data indicate that magmas tapped a depleted mantle source significantly more enriched in oceanic sedimentary components (0.2%). Exhumation occurred either in two stages (700–660 and 623 Ma) or in one stage (623 Ma) with a final exhumation of the arc root along cold P-T path (550°C, 6–9 kbar; epidote–amphibolite and greenschist facies conditions) during the main Pan-African collision event (620–580 Ma). The composition of magmas forming the Cryogenian Amalaoulaou arc and the processes leading to intra-arc differentiation are strikingly comparable to those observed in the deep section of exposed Mezosoic oceanic arcs, namely the Kohistan and Talkeetna complex. This evolution of the Amalaoulaou oceanic arc and its accretion towards the West African craton belong to the life and closure of the Pharusian Ocean that eventually led to the formation of the Greater Gondwana supercontinent, a similar story having occurred on the other side of the Sahara with the Mozambique Ocean.     

Hydrothermal Dolomite in the Upper Sinian (Upper Proterozoic) Dengying Formation,East Sichuan Basin,China

Hydrothermal Dolomite （HTD） is present in the Upper Sinian （Upper Proterozoic） Dengying Formation, east Sichuan Basin, China. The strata are comprised by primary dolomite. The HTD has various textures, including zebra dolomite, subhorizontal sheet-like cavities filled by saddle dolomite and breccias cemented by saddle dolomites as well occur as a fill of veins and fractures. Also co-occur MVT type lead-zinc ores in the study area. The δ13C and δ18O isotopes of HTD in the Upper Sinian Dengying Formation are lighter than those of the host rocks, while STSr/86Sr is higher. The apparent difference in carbon, oxygen and strontium isotopes, especially the large difference in S7Sr/S6Sr isotopes ratio indicate crystallization from hot basinal and/or hydrothermal fluids. Saddle dolomite was precipitated at temperatures of 270-320℃. The diagenetic parasequences of mineral assemblage deposited in the Dengying Formation are： （1） dolomite host rock →sphalerite-galena-barite-fluorite; （2） dolomite host rock →saddle dolomite →quartz; （3） dolomite host rock →saddle dolomite→bitumen; （4） dolomite host rock →saddle dolomite →barite. The mean chemical composition of the host dolomite matrix and HTD didn＇t change much during hydrothermal process. The fluids forming the HTDs in the Dengying Formation were mixtures of freshwater from the unconformity at the top of Sinian, fluids from diagenetic compaction and hydrocarbon generation ＆ expulsion from the Lower Cambrian Niutitang Formation mudstones or the Doushantuo Formation silty mudstones, and hydrothermal fluids from the basement. The hydrocarbon reservoirs associated with the HTD were mostly controlled by the basement faults and fractures and karsting processes at the unconformity separating Sinian and Cambrian strata. The hydrocarbon storage spaces of HTD included dissolved cavities and intercrystalline pores. Dissolution cavities are extensive at the top of Dengying Formation, up to about 46m below the unconformity between Sinian an     

Paleomagnetism of Middle Proterozoic mafic intrusions and Upper Proterozoic (Nankoweap) red beds from the Lower Grand Canyon Supergroup, Arizona

Paleomagnetic data from lavas and dikes of the Unkar igneous suite (16 sites) and sedimentary rocks of the Nankoweap Formation (7 sites), Grand Canyon Supergroup (GCSG), Arizona, provide two primary paleomagnetic poles for Laurentia for the latest Middle Proterozoic (ca. 1090 Ma) at 32°N, 185°E (dp=6.8°, DM=9.3°) and early Late Proterozoic (ca. 850–900 Ma) at 10°S, 163°E (dp=3.5°, DM=7.0°). A new ⁴⁰Ar/³⁹Ar age determination from an Unkar dike gives an interpreted intrusion age of about 1090 Ma, similar to previously reported geochronologic data for the Cardenas Basalts and associated intrusions. The paleomagnetic data show no evidence of any younger, middle Late Proterozoic tectonothermal event such as has been revealed in previous geochronologic studies of the Unkar igneous suite. The pole position for the Unkar Group Cardenas Basalts and related intrusions is in good agreement with other ca. 1100 Ma paleomagnetic poles from the Keweenawan midcontinent rift deposits and other SW Laurentia diabase intrusions. The close agreement in age and position of the Unkar intrusion (UI) pole with poles derived from rift related rocks from elsewhere in Laurentia indicates that mafic magmatism was essentially synchronous and widespread throughout Laurentia at ca. 1100 Ma, suggesting a large-scale continental magmatic event. The pole position for the Nankoweap Formation, which plots south of the Unkar mafic rocks, is consistent with a younger age of deposition, at about 900 to 850 Ma, than had previously been proposed. Consequently, the inferred 200 Ma difference in age between the Cardenas Basalts and overlying Nankoweap Formation provides evidence for a third major unconformity within the Grand Canyon sequence.     

Crystal-chemical peculiarities of globular layer silicates of the glauconite-illite composition (Upper Proterozoic, Northern Siberia)

The paper presents an overview of crystal-chemical peculiarities of the previously studied globular dioctahedral 2 : 1 layer silicates of the glauconite-illite compositions from the Upper Proterozoic sections in northern Siberia (Anabar and Olenek uplifts). Lithomineralogical peculiarities of the glauconite-bearing rocks are discussed. Geochronological data on some samples are given. Monomineral fractions of grains were studied with the modern chemical and physical methods (X-ray diffraction, oblique-texture electron diffraction (OTED), scanning electron microscopy, IR and Mossbauer spectroscopy, classical chemical and microprobe analyses, and others). Low-charge dioctahedral 2 : 1 layer silicates were classified with consideration of the IMA NC and AIPEA NC recommendations (Rieder et al., 1998; Guggenheim et al., 2006). This classification showed that the studied globular Al- and Fe-bearing varieties include a continuous isomorphic glauconite-illite series. Intermediate layer silicates beyond the IMA NC and AIPEA NC classification are assigned to Al-glauconites (Fe-illites). True illites should be supplemented with adjectives ??globular?? or ??platy,?? because green globules and the fine-dispersed Al-bearing clay minerals are traditionally recognized as ??glauconite?? and ??illite,?? respectively.     

Fuelwood and desertification: Sahel orthodoxies discussed on the basis of field data from the Gourma region in Mali

Until recently orthodoxies on fuelwood and desertification have formed the cornerstones of mainstream views in research and development related to natural resources in the Sahel. Based on an empirical study of the fuelwood situation in the Gourma region in Mali, this article throws a critical light on these viewpoints. There is no relationship between deforestation and domestic fuelwood consumption in the Gourma. The fuelwood used comes from dry wood collected from dead trees. Collection distances are, however, getting longer; so people need to use more time for the collection of wood, or more of their limited incomes have to be spent on buying fuelwood. Therefore, the fuelwood problem in the Gourma is of a social and economic rather than an ecological character. Sahel orthodoxies, which are manifested in the national Malian plan to fight desertification, are used by the Water and Forests Service to justify their policy of harassment towards the rural poor. These orthodox beliefs about environmental issues in drylands are generalizations which tend to gloss over important regional diversities. The data presented should therefore not be used to create new orthodoxies. They should rather be regarded as a warning not to use generalizations on local realities about which there is no reliable information.     

An unusual diagenetic structure in the Precambrian banded iron formation (BIF) of Orissa,India, and its interpretation

An unusual type of late diagenetic tectonic and compaction structure simulating boudinage phenomena is described and documented from the Precambrian banded iron formation (BIF) of Orissa, India. The structure was seemingly initiated by the development of tension cracks in the hydroplastic stage followed by rotation and imbrication of the segments of the iron (magnetite) bands. The tension cracks were subsequently filled up by finely crystalline diagenetic quartz veins.     

Late Proterozoic periglacial aeolian deposits on the West African Platform, Taoudeni Basin, western Mali

The Late Proterozoic Bakoye 3 Formation is a predominantly aeolian unit deposited in the glacially influenced cratonic Taoudeni Basin of western Africa. The Bakoye 3 can be divided into five distal units, two proximal units, and a local upper massive sandstone. The basal Unit 1 shows a complex interfingering of aeolian and subaqueous structures, and is interpreted as the precursor of the overlying erg sequences. Unit 2 consists of compound, trough cosets of aeolian cross-strata dominated by grain-flow strata. The unit is interpreted to represent draas with superimposed, small, crescentic dunes. A super bounding surface marks the termination and planation of the erg. Unit 3 is distinguished from the underlying Unit 2 by its larger, overall simple sets of trough cross-strata, interpreted to represent simple, large, crescentic dunes. Unit 4 occurs only locally in laterally discontinuous, large troughs. In one case the trough is filled by small sets of tabular cross-strata dominated by grain-flow deposits. At another section, wedges of coarse-grained wind-ripple strata fill the trough. Unit 4 may represent remnants of ergs or, more likely, local deposition in depressions. The depressions, in the latter scenario, formed with the development of a second super surface that truncates Unit 3. Unit 5 consists of very large sets of wind-ripple cross-strata with less common sets of grain-flow deposits. These deposits are believed to represent enormous dunes with large plinths and subordinate slip face development. A third super surface separates Unit 5 from overlying marine deposits. Together, Units 1–5 represent the core of the ergs in a distal position relative to adjacent upland source areas. Proximally, aeolian deposits are simple, smaller, trough sets interpreted as moderate sized crescentic dunes. Coarse-grained braided stream deposits are prominent. Locally, the top of the Bakoye 3 is marked by channelized mass-flow deposits containing aeolian blocks, and is believed to have resulted from iceberg grounding. An overall environment for the Bakoye 3 is one of uplands marked by ice sheets, with outwash plains extending distally to aeolian ergs. Super surfaces, all marked by polygonal fractures and coarsegrained sediment, represent periods of erg termination that may be linked to glacial-fluvial-aeolian cycles.     

Prospection géoélectrique pour l'étude de l'aquifère thermal des calcaires récifaux, Hmeïma–Boujabeur (Centre ouest de la Tunisie)

The Hmeïma–Boujabeur zone is considered as an important geothermal province for Tunisia. The reef limestone, characterized by intense fracturation and important karstification, confers them the property to be a regional thermal aquifer. A geophysical study based on two complementary geoelectrical methods, the Electrical Sounding (ES) and Magneto-Telluric Sounding (MTS), turned out to be very efficient to provide a complete electrical image of the underground until about 1-km depth. The synthetic approach integrating all data allowed us to delimit the favourable area for thermal water exploitation. To cite this article: M. Gouasmia et al., C. R. Geoscience 338 (2006).     

First U-Pb dates for detrital zircons from deposits of Serebryanka Group (Upper Proterozoic,Middle Urals)

The results of application of the LA-ICP-MS method for analysis of U-Pb ages of detrital zircons from Serebryanka Group (Middle Urals) has been presented for the first time. It has been found that the detrital zircon population in tillite-like conglomerates of the Tany Formation is represented nearly in equal shares by grains with Neoarchean and Paleoproterozoic U-Pb ages. This allows the basement crystalline rocks from the eastern part of the East European Craton to be considered as the principal source of aluminosilicate clastic material in the beginning of the Serebryanka. The population of detrival zircons in sandstones from the Kernos Formation consists of Meso- and Neoarchean (approximately 15%), Paleoproterozoic (∼60%), and Mesoproterozoic (∼26%) zircon grains. Comparison of the obtained data with the materials on detrital zircons from Tonian and Ediacarian sandstones in the southern Urals has shown that the principal role in formation of Tonian and Ediacaran sedimentary sequences was played by the products of Mid- and Late Paleoproterozoic crystalline rock erosion in the craton socle. In addition to this, accumulation of the Serebryanka Group took place with visible participation of more ancient—Early Paleoproterozoic, Neo- and Mesoarchean—rock associations on the paleo-water catchment areas. The end of the Serebryanka period differed from its beginning by the additional appearance of Mesoproterozoic complexes in alimentation zones; for comparison, the role played by these complexes in formation of Ectasian-Ediacarian deposits of the adjacent southern Urals segment is insufficient, judging by the currently available data. All these facts may indicate that there are significant differences in the spatiotemporal distribution of clastic material sources for Upper Precambrian deposits in the West Megazone of the southern and middle Urals.     

Magnetic fabrics and rock magnetism of Archaean and Proterozoic dike swarms in the southern São Francisco Craton, Brazil

Magnetic fabric and rock magnetism studies were performed on three mafic dike swarms (total of 38 dikes) from the southernmost part of the São Francisco Craton (SFC) (Minas Gerais State, SE Brazil). They cut Archaean granite–gneiss–migmatite and paleoprototerozoic terranes. These swarms are classified as basic–noritic (Sm–Nd age  2.65 Ga), basic (Rb–Sr age  1.87 Ga) and metamorphic (Rb–Sr age  1.87 Ga) suites, in which the second is the most important. Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). In most sites magnetic susceptibility is dominantly carried by ferromagnetic minerals, however, in some sites the paramagnetic contribution exceeds 70% of bulk susceptibility. Mainly coarse to fine-grained Ti-poor titanomagnetite up to pure magnetite carry the magnetic fabrics.Three primary AMS fabrics are recognized which are all coaxial with the AARM fabric. Normal AMS fabric is dominant in the basic suite (16 of 20 analyzed dikes) and occurs in 4 and 3 dikes from the basic–noritic and metamorphic suites, respectively. This fabric is interpreted as a result of magma flow in which the analysis of K_max inclination permitted to infer that the majority of dikes were fed by inclined flows (30° < K_max < 60°), although 44% of dikes from the basic suite were fed by horizontal or sub-horizontal flows (K_max < 30°). Intermediate AMS fabric was found in 50% of dikes from the basic–noritic and metamorphic suites, but in only 2 dikes from the basic suite. It is interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike strike. Inverse AMS fabric is a minority (2 dikes from each suite). The parallelism between AMS and AARM tensors for dikes with abnormal fabrics suggests a primary origin for them. Gyroremanent magnetization (GRM) effect was negligible for the majority of dikes, but it was found in two dikes from the basic suite with normal AMS fabric.Magnetic fabrics recognized for the three studied swarms do not depend on magnetic mineralogy, geochemical composition, dike strikes, nor the age of the swarms since the same magnetic minerals and magnetic fabric types are found in dikes from all suites. Inclined and horizontal flows allow us to infer the relative position of at least three magma sources (or magma chambers) from which the dikes were fed.     

Évolution récente et actuelle de la côte basque française (analyse et perspectives)

A recent multidisciplinary study, comprising existing data analysis and geomorphological and geological fieldwork, has helped clarify marine erosion processes of Pyrenean formations along the Basque Coast. An important discovery was made during our study of an alterite of regional extent, which calls into question recent and present evolution of the Basque Coast and the processes leading to coastline retreat. Modelling these factors has made it possible to propose a predictive scenario for future coastal evolution. To cite this article: A. Genna et al., C. R. Geoscience 337 (2005).     

Genesis and diagenetic evolution of Habo formation,Kachchh, Gujarat

The Kachchh Basin is a pericratonic rift basin situated at the western margin of the Indian plate. The Habo Dome embodies an important exposure of Bathonian to Kimmergian sediments among the Kachchh Mainland exposures. Based on vertical facies transitions, facies associations were documented: mixed shallow marine (Facies association 1), shoreface and lagoon deposits (Facies association II) and subtidal innershelf below fair weather wave base (Facies association III). The documented facies associations reflect that Habo Dome sediments deposited in a variety of environments from shallow marine to fluvio-deltaic and were strongly influenced by fluctuation of relative sea level. The dominance of floating grains and point contacts in the sandstone indicate that detrital grains do not show much pressure effects as a result of either shallow burial or early cementation. The sandstones were cemented by iron oxide, carbonate and silica in order of abundance. Three types of cements, blocky, rim and fibrous cement occur in the studied limestone representing phreatic, fresh water phreatic and deep burial diagenetic stages. Neomorphism and micritization are common. Both primary and secondary porosity exists in these sediments. Different graphs of porosity versus depth suggest a depth of burial in the range of 615–769 m.     

The Upper Jurassic (Kimmeridgian) fluvial–aeolian systems of the southern Neuquén Basin, Argentina

The Kimmeridgian Quebrada del Sapo Formation in the southernmost Neuquén Basin in Argentina represents a succession up to 40 m thick of coarse- to fine-grained fluvial deposits overlain by aeolian deposits. These fluvial–aeolian deposits reflect a significant palaeogeographic change in the basin and are related to a major, tectonically enhanced, relative sea-level fall. The fluvial section is dominated by braided-channel, fine-grained ephemeral, and sheetflood deposits. Aeolian facies are dominated by dune deposits, with minor sandsheet and interdune units. Changes in the nature of both fluvial and aeolian sedimentation within the studied area suggest a regional variability of accommodation/sediment supply conditions. The regional changes of the aeolian succession likely reflect different relative positions within a major erg. In the upwind margin of the erg, a shallow water table promoted water-lain sedimentation in interdune areas, whereas in the central parts of the erg, dry sediment accumulation took place above the regional water-table level. The vertical transition observed in the Quebrada del Sapo Formation, from fluvial to aeolian deposits, may be the result of a local climatic change to drier conditions due to the development of a climatic barrier imposed by growth of a magmatic arc to the west. Alternatively, the vertical transition could be related to a lowering of the water table associated with the compartmentalization of the basin during a period of low sea level.     

河北兴隆中、上元古界旋回层序初步研究

以岩相及相序为基础,不同级别旋回层序及其有序叠加形式为研究核心,在进行１／５万区域地质调查的同时,对河北兴隆中、上元古界旋回层序特征进行了初步研究。将厚近万米、延续时间达１０亿年左右的地层,划分为４个一级超层序、１７个二级大层序、６２个三级层序。三级层序的划分和识别建立在米级旋回层序及其有序叠加形式的基础之上。旋回性沉积记录的研究,为中、上元古界“系”的划分和建立提供了基本资料,一级超层序与银河年旋回存在成因联系。     

A barred-basin marine evaporite in the Upper Proterozoic of the Amadeus Basin, central Australia

The Ringwood evaporite is part of the 900 m.y. old Bitter Springs Formation, a warm-water shallow-marine sequence of stromatolitic dolomite and limestone, microfossiliferous chert, red beds, quartzite, and evaporites. The evaporite at Ringwood comprises two parts: (i) a lower 127 m characterized by brecciated pyritic bituminous dolomite, together with smaller amounts of dolomite-gypsum breccia, friable chloritic dololutite, coarsely crystalline anhydrite, and satin-spar gypsum; and (ii) an upper 133 m which is similar except that bituminous dolomite forms only one bed, and the characteristic rock-type is dolomite-gypsum breccia. The evaporite is overlain by limestone breccia and massive stromatolitic limestone, interpreted as an algal reef. Gypsum is secondary after anhydrite, and the ratio of gypsum to anhydrite increases upwards. The evaporite shows none of the features of a sabkha or desiccated deep ocean basin deposit, and instead is interpreted as the filling of a barred basin which was cut off from the ocean by growth of an algal barrier reef. As circulation became restricted, bituminous dolomite deposited in the lagoon behind the reef, together with pyrite from the destruction by anaerobic bacteria of algal debris derived from the reef. With continued evaporation, brine concentration increased and gypsum precipitated. Occasional dust storms contributed wind-blown clay to the deposit. The barrier reef transgressed diachronously across the evaporite lagoon, and was eventually drowned when normal marine conditions became established. Burial of the evaporite to about 7000 m beneath the succeeding sediments of the Amadeus Basin converted gypsum to anhydrite, and formed chlorite by reaction of clay with dolomite. Late Palaeozoic tectonism folded and brecciated the rocks, and was followed by erosion which eventually exposed the evaporite to ingress of meteoric water. Hydration of anhydrite to gypsum ensued, the reaction becoming less complete with increasing depth from the ground surface.     

Geochronologic (ReOs) and fluid-chemical constraints on the formation of the Mesoproterozoic-hosted Nanisivik ZnPb deposit,Nunavut, Canada: Evidence for early diagenetic,low-temperature conditions of formation

The age and origin of the past-producing Nanisivik carbonate-hosted ZnPb deposit in Nunavut, Canada, have been controversial for decades. Various direct and indirect dating methods have produced results ranging from Mesoproterozoic to Ordovician in age, and previous studies of the mineralising fluids have suggested that the fluids were anomalously hot (> 150 °C). This study combines ReOs (pyrite) geochronology, in-situ sulphur isotope analysis, and fluid inclusion analysis to refine both the timing of mineralisation and the nature of mineralising fluids. ReOs pyrite analysis shows that the Nanisivik deposit formed ca. 1.1 Ga, broadly similar to the depositional age of the host rock and with the Grenville orogeny, making it one of few known Precambrian carbonate-hosted ZnPb deposits. In-situ sulphur isotope measurements from Nanisivik show a narrow δ³⁴S range of 27.54 ± 0.72, very similar to what has been reported before in bulk sample analyses. New fluid inclusion data show that the mineralising fluids were ~ 100 °C, which is not anomalous in the context of carbonate-hosted base-metal deposits. The fluids exhibit no significant spatial variation in homogenisation temperature in the 2-km-long ‘upper lens’ of the ore deposit, but recrystallisation and modification of fluid inclusions took place in the immediate vicinity of the cross-cutting ~ 720 Ma “mine dyke”. The deposit is broadly inferred to have formed during late Mesoproterozoic assembly of supercontinent Rodinia, when regional hydrostatic head developed under the influence of far-field stresses originating in the developing Grenville orogen. The Nanisivik deposit remains anomalous only in its age; most other aspects of this ore deposit are now shown to be quite typical for carbonate-hosted ore deposits.     

Les structures de la couverture Néoprotérozoïque terminal et Paléozoïque de la région de Tata, Anti-Atlas centre-occidental, Maroc: déformation polyphasée, ou interactions socle/couverture pendant l'orogenèse hercynienne?

The western Anti-Atlas was formed by a Precambrian basement in the core of anticlines, surrounded by a Neoproterozoic and Palæozoic cover. The structural study of the Tata regional rocks shows a heterogeneous deformation, characterised especially by two types of folds in two orthogonal directions: north-south to north-northeast-south-southwest-trending and east-west-trending.The north-south structures are present in all of the Palæozoic cover and belong to the major Variscan compression of Late Carboniferous age by a comparison of the other domains of the western Anti-Atlas. Alternatively, east-west folding is assigned only to the lower part of the cover and consists of a ductile heterogeneous deformation, especially marked at the basement-cover interface. These folds are associated with a subhorizontal cleavage, indicating a southern vergence of the structures. A discussion of the age and the tectonic style of these structures is proposed, as well as their significance within the Variscan belt along the northern margin of the West African Craton.     

Pétrologie et géomorphologie dans la région de Kango, Gabon: un grand dyke ultramafique-mafique archéen

The mineral assessment programme of Gabon, carried out during the 1980s, showed a 2870 Ma old igneous mafic-ultramafic complex in the Kango area that was first considered to be made up of five separate intrusions associated with Cr---Ni geochemical anomalies. Following the study of the regional relief, the surface materials and the petrography, a heavily incised zone has been selected where outcrops were abundant enough to reasonably define the structure of the complex. Therefore, where observed, the igneous body looks like a subvertical 1–2 km wide dyke composed of an early unit of cumulus feldspathic bronzitites and norites and a late gabbroic unit showing a chilled border against the Archean gneisses. The strike variation of gneisses foliation shows a right-lateral fault of the N40° fracture responsible for the emplacement of the dyke. The mafic-ultramafic complex presents some alkaline late-magmatic and cataclastic effects only along the N170° fractures that affect the dyke by a left-lateral fault. The very same petrography of the other mafic-ultramafic igneous bodies and their relationship with the fractures of the gneisses basement make it possible to interpret the different intrusions as a great dyke some 100 km long within Gabon and possibly extending into equatorial Guinea.     

Evaporative systems and diagenetic patterns in the Calatayud Basin (Miocene,central Spain)

This paper concerns the evaporite units, depositional systems, cyclicity, diagenetic products and anhydritization patterns of the Calatayud Basin (nonmarine, Miocene, central Spain). In outcrop, the sulphate minerals of these shallow lacustrine evaporites consist of primary and secondary gypsum, the latter originating from the replacement of anhydrite and glauberite. In the evaporative systems of this basin, gypsiferous marshes of low salinity can be distinguished from central, saline lakes of higher salinity. In the gypsiferous marsh facies, the dominant, massive, bioturbated gypsum was partly replaced by synsedimentary chert nodules and siliceous crusts. In the saline lake facies, either cycles of gypsiferous lutite‐laminated gypsarenite or irregular alternations of laminated gypsum, nodular and banded glauberite, thenardite and nodular anhydrite precipitated. Early replacement of part of the glauberite by anhydrite also occurred. Episodes of subaerial exposure are represented by: (1) pedogenic carbonates (with nodular magnesite) and gypsiferous crusts composed of poikilitic crystals; and (2) nodular anhydrite, which formed in a sabkha. Additionally, meganodular anhydrite occurs, which presumably precipitated from ascending, highly saline solutions. The timing of anhydritization was mainly controlled by the salinity of the pore solutions, and occurred from the onset of deposition to moderate burial. Locally, a thick (>200 m) sequence of gypsum cycles developed, which was probably controlled by climatic variation. A trend of upward‐decreasing salinity is deduced from the base to the top of the evaporite succession.     

Magnetic fabrics and rock magnetism of Proterozoic dike swarm from the southern São Francisco Craton, Minas Gerais State, Brazil

Magnetic fabric and rock magnetism studies were performed on 32 mafic dikes of a Proterozoic dike swarm from the southern São Francisco Craton (SFC; Minas Gerais State, SE Brazil). Magnetic anisotropies were determined by applying anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of remanent magnetization (ARM). The latter was performed imposing both anhysteretic (total (AAR) and partial pAAR)) and isothermal remanence magnetizations (AIRM). Partial anhysteretic remanence anisotropy was performed based on remanent coercivity spectra from a pilot specimen of each site. In most sites, AMS is dominantly carried by ferromagnetic minerals, however, in some sites, the paramagnetic contribution exceeds 70% of bulk susceptibility. Rock magnetism and thin section analysis allow classifying the dikes as non-hydrothermalized and hydrothermalized. Magnetic measurement shows that the mean magnetic susceptibility is usually lower than 5×10⁻³ (SI). Ti-poor titanomagnetites up to pure magnetite pseudo-single-domain (PSD) grain sizes carry the majority of magnetic fabrics for non-hydrothermalized dikes whereas coarse to fine grained Ti-poor titanomagnetites carry the majority of magnetic fabrics for hydrothermalized dikes.Three primary AMS fabrics are recognized which are coaxial with ARM fabric, except for two dikes, from both non-hydrothermalized and hydrothermalized dikes. Normal AMS fabric surprisingly is not dominant (31%). The parallelism between AMS, pAAR_0–30, pAAR_30–60 and pAAR_60–90 fabrics in the hydrothermalized dikes indicates that magnetic grains formed due to late-stage crystallization or to remobilization of iron oxides due to hydrothermal alteration after dike emplacement have acquired a mimetic fabric coaxial with the primary fabric given by coarse-grained early crystallized Ti-poor titanomagnetites. This fabric is interpreted as magma flow in which the analysis of K_max inclination permitted the inference that the dikes were fed by horizontal or subhorizontal fluxes (K_max<30°). Intermediate AMS fabric is the most important (41%) in the investigated swarm. It is interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike strike. ARM determinations for these sites also remained intermediate except for two dikes. In one of them, AIRM fabric resulted in normal AMS fabric while for the other AAR fabric resulted in inverse AMS fabric. A combination of AMS and ARM fabrics suggest that magmatic fabric for both dikes were overprinted by some late local event, probably related to Brasiliano orogenic processes after dike emplacement. InverseInverse AMS fabric is a minority (four dikes). ARM determinations also remained inverse suggesting a primary origin for inverse AMS fabric.