Development of igneous layering during growth of pluton: The Tarçouate Laccolith (Morocco)

We discuss the significance of igneous layering with respect to pluton growth processes. The case study is the Tarçouate Laccolith (Morocco), whose core consists of modally layered hornblende granodiorites with high amount of monzodioritic enclaves, contrasting with peripheral, non-layered biotite granodiorites with low amount of enclaves. Rhythmic layering, with modal grading, cross-stratification and trough layering is associated with monzodioritic layers and wraps around mafic enclaves. Its steep dips ≥ 45° result from tilting that occurred above solidus conditions, as indicated by sub-vertical and synmagmatic granite, aplite and monzodiorite dykes cutting across the layering.The systematic association of igneous layering with mafic enclaves in calc-alkaline plutons suggests that layering originates from recurrent injection of mafic magma. Viscosity calculations suggest that the physicochemical properties of magma alone cannot account for the presence of layering in the central hornblende granodiorite and its coeval absence in the peripheral biotite granodiorite of the Tarçouate Laccolith. Intermittent pulses of hot mafic magma into crystallizing granodiorite likely produced thermal perturbations able to trigger local convection, formation of mafic enclaves and development of igneous layering through protracted crystallization.     

Contribution to a geodynamic reconstruction of the Anti-Atlas (Morocco) during Pan-African times with the emphasis on inversion tectonics and metallogenic activity at the Precambrian–Cambrian transition

New geochronological analyses (U–Pb SIMS zircon ages) have yielded ages of 552 ± 5 Ma for the Bou Madine rhyolitic dome (Ougnat, eastern Anti-Atlas), 543 ± 9 Ma for the Tachkakacht rhyolitic dyke (Saghro–Imiter, eastern Anti-Atlas), and 531 ± 5 Ma for the Aghbar trachytic sill (Bou Azzer, central Anti-Atlas). Inherited zircon cores from the Aghbar trachytic sill and from the Bou Madine rhyolitic dome have been shown to be of Statherian age (ca. 1600–1800 Ma) and Palæoproterozoic (>2100 Ma) age, respectively, suggesting that a significantly older protolith underlies the Pan-African rocks in the Central and Eastern Anti-Atlas. Granodiorites and rhyolites from the Saghro–Imiter area have similar low ⁸⁷Sr/⁸⁶Sr (0.702–0.706) and ¹⁴³Nd/¹⁴⁴Nd (0.5116–0.5119) initial ratios, suggesting a mixture of mantle and lower crust sources. This can also be inferred from the low ¹⁸⁷Os/¹⁸⁸Os ratios obtained on pyrite crystals from the rhyolites.A recently published lithostratigraphic framework has been combined with these new geochemical and geochronological data, and those from the literature to produce a new reconstruction of the complex orogenic front that developed at the northern edge of the Eburnian West African craton during Pan-African times. Three Neoproterozoic magmatic series can be distinguished in the Anti-Atlas belt, i.e., high-K calc-alkaline granites, high-K calc-alkaline to shoshonitic rhyolites and andesites, and alkaline-shoshonitic trachytes and syenites, which have been dated at 595–570, 570–545 and 530 Ma, respectively.The accretion of the Pan-African Anti-Atlas belt to the West African super continent (WAC) was a four-stage event, involving extension, subduction, moderate collision and extension. The calc-alkaline magmatism of the subduction stage was associated with large-scale base metal and gold mineralisation. Metallogenic activity was greatest during the final extensional stage, at the Precambrian–Cambrian boundary. It is characterised by world-class precious metal deposits, base–metal porphyry and SEDEX-type occurrences.     

Mechanisms and duration of non-tectonically assisted magma emplacement in the upper crust: The Black Mesa pluton, Henry Mountains, Utah

A new study of Black Mesa pluton (Henry Mountains, Colorado Plateau, Utah, USA) indicates that it is a classic example of a small upper-crustal pluton assembled over a few years by incremental amalgamation of discrete magma pulses. The results of our petrostructural study of the pluton interior allow us to constrain the geometry, kinematics and timing of the processes. The symmetric internal fabric is interpreted as an evidence for a feeding by below and not laterally. The observed rotation of the lineation, from WNW–ESE on the very top to NNE–SSW below, lead us to propose that the fabric at the base of the pluton is a record of magma infilling process, and the fabric at the very top is a record of the strain due to the relative movement between magma and wallrocks. A consequence is that except at the contact between pluton and wallrocks (top and margins), the stretching direction, recorded by the lineation, is not parallel to the flow direction of the magma i.e. displacement. The Black Mesa pluton is a sheeted laccolith on its western edge and a bysmalith on its eastern edge. This E–W asymmetry in pluton geometry/construction and the symmetrical internal fabric indicates that the apparently different west and east growth histories could have occurred simultaneously. Our field data indicate pluton growth through an asymmetric vertical stacking of sill-like horizontal magma sheets.One-dimensional thermal models of the pluton provide maximum limits on the duration of its growth. We have constrained the number, the thickness, and the frequency of magma pulses with our structural observations, including: (1) the emplacement of the pluton by under-accretion of successive magma pulses, (2) the absence of solid-state deformation textures at internal contacts, and (3) the apparent absence of significant recrystallization in the wallrocks. Our results suggest that the emplacement of the Black Mesa pluton was an extremely rapid event, with a maximum duration on the order of 100 years, which requires a minimum vertical displacement rate of the wallrocks immediately above the pluton greater than 2 m/yr. Finally, our data show that the rates of plutonic and volcanic processes could be similar, a significant result for interpretation of magma transfer in arc systems.     

The polymetallic Au–Ag-bearing veins of Bou Madine (Jbel Ougnat, eastern Anti-Atlas, Morocco): tectonic control and evolution of a Neoproterozoic epithermal deposit

The Bou Madine ore deposit is located SW of Jbel Ougnat, the easternmost inlier of the Anti-Atlas Pan-African belt in Morocco. The host rocks are high-K calc-alkaline volcanic rocks, that are part of the Neoproterozoic Tamerzaga-Timrachine Formation (TTF, lower PIII). The TTF consists of ignimbrites of rhyolitic to dacitic compositions, andesite flows and hypovolcanic bodies (andesite dykes and rhyolite chonoliths) emplaced along N160°E tension gashes associated with a regional N30°E sinistral fault system. The mineralization is related to a high enthalpy geothermal system, eventually evolving into a low temperature epithermal system. A regional propylitisation (T around 260 °C) overprinted the TTF rocks prior to the emplacement of the mineralization. There were two main hydrothermal stages. During the first stage, massive veins with pyrite, arsenopyrite and minor pyrrhotite and cassiterite were formed. The veins were emplaced along N160°E-trending en echelon joints related to N120°E dextral arrays. A quartz-sericite-pyrite alteration overprinted the propylites around the veins (“bleached haloes”), at temperatures up to 300–310 °C. The second stage of mineralization was coeval with dextral re-activation of the N160°E veins, in relation with a NE-ward shift of the shortening direction. First, polymetallic sulphides (sphalerite, chalcopyrite, stannite, galena) were deposited at temperatures 260 °C. Younger quartz veinlets contain arsenopyrite and minor micrometre-size sulphides and sulpho-salts, hosting the precious metals. This was the low temperature epithermal stage (≈150 °C), in relation with invading meteoric water.     

Age of the Wirgane granodiorite intrusions (Western High-Atlas, Morocco): New U–Pb constraints

U–Pb geochronological data, obtained on single zircon grains using the ion microprobe Cameca IMS 1270 (CRPG-CNRS, Nancy), indicate a Neoproterozoic age (625 ± 5 Ma) for the two intrusions of Wirgane granodiorite. This age is in variance with the previously published conclusions where the intrusions were supposed Variscan. Consequently, the Wirgane intrusions are interpreted as remnants of the Neoproterozoic basement of the Variscan belt. They are similar to the Anti-Atlas calc-alkaline intrusions of the collisional to post-collisional stages of the Pan-African orogeny. At that time, the Wirgane region was an area of continental growth to the north of the major South Atlasic Fault.     

Intrusion and differentiation of granitic magma at a high level in the crust: the Puscao pluton, Lima Province, Peru

Regression surface techniques have been used on partial whole-rockchemical analyses from the Puscao pluton to establish the three-dimensionalnature of the chemical variations within this body, which wasemplaced at a depth of between 4 km and 8 km within the crust.Block diagrams produced by the computer program are consistentwith the hypothesis that during the period preceding the totalconsolidation of the magma, the pluton has undergone in situdifferentiation to give a layered structure. Statistical analysis confirms that there is a large scale variationwithin the pluton and that local variation, such as might beexpected to result from the contamination of the Puscao magmaby assimilated volcanic material, did not contribute significantlyto the overall variation. Evidence is presented which suggests that the layering withinthe Puscao pluton was disturbed as a result of the emplacementof the much later Cañhas pluton, and that this disturbancehas the form of a simple doming centred upon the site of thepresent Cañas intrusion.     

Tectonometamorphic evolution of an intracontinental orogeny inferred from P–T–t–d paths of the metapelites from the Rehamna massif (Morocco)

New petrographic and microstructural observations, mineral equilibria modelling and U/Pb (monazite) geochronological studies were carried out to investigate the relationships between deformation and metamorphism across the Rehamna massif (Moroccan Variscan belt). In this area, typical Barrovian (muscovite to staurolite) zones developed in Cambrian to Carboniferous metasedimentary rocks that are distributed around a dome‐like structure. First assemblages are characterized by the presence of locally preserved andalusite, followed by prograde evolution culminating at 6 kbar and 620 °C in the structurally deepest staurolite zone rocks. This Barrovian sequence was subsequently uplifted to supracrustal levels, heterogeneously reworked at greenschist facies conditions, which was followed locally by static growth of andalusite, indicating heating to 2.5–4 kbar and 530–570 °C. The ²⁰⁶Pb/²³⁸U monazite age of 298.3 ± 4.1 Ma is interpreted as minimum age of peak metamorphic conditions, whereas the ages of 275.8 ± 1.7 Ma and 277.0 ± 1.1 Ma date decompression and heating at low pressure, in agreement with previous dating of Permian granitoids intruding the Rehamna massif. The prograde metamorphism occurred during thickening and associated horizontal flow in the deeper crust (S1 horizontal schistosity). The horizontally disposed metamorphic zones were subsequently uplifted by a regional scale antiform during ongoing N–S compression. The re‐heating of the massif follows late massive E–W shortening, refolding and retrograde shearing of all previous fabrics coevally with regionally important intrusions of Permian granitoids. We argue that metamorphic evolution of the Rehamna massif occurred several hundred kilometres from the convergent plate boundaries in the interior of continental Gondwanan plate. The tectonometamorphic history of the Rehamna massif is put into Palaeozoic plate tectonic perspective and Late Carboniferous reactivation of (Devonian)–Early Carboniferous basins formed during stretching of the north Gondwana margin and formation of the Palaeotethys Ocean. The inherited heat budget of these magma‐rich basins plays a role in the preferential location of this intracontinental orogen. It is shown that rapid transition from lithospheric stretching to compression is characterized by specific HT type of Barrovian metamorphism, which markedly differs from similar Barrovian sequences along Palaeozoic plate boundaries reported from Variscan Europe.     

Early Cretaceous (Valanginian – Hauterivian) belemnites from western Morocco: stratigraphy and palaeoecology

Recent field work in Lower Cretaceous successions of the Agadir-Essaouira region, western High Atlas, Morocco, have resulted in 489 belemnite guards collected bed-by-bed from the lower Valanginian – upper Hauterivian interval. These findings are described and discussed in their biostratigraphic, taxonomic, palaeoecologic and palaeobiogeographic context. The most common belemnite genera, Hibolithes, Vaunagites?, and Duvalia, are of Meditteranean affinities and closely related to Spanish, French, and Bulgarian faunas. Less common taxa include Pseudoduvalia and Berriasibelus. The genus Pseudobelus, however, common in southeastern France and southern Spain, is rare in the Moroccan sections. The palaeoecological interpretation of these observations result in the recognition of three belemnite assemblages, characterizing different marine settings: 1) A Hibolithes - Duvalia assemblage which is typical for inner to middle shelf settings. These are relatively shallow marine environments with a high input of siliciclastics. 2) A Duvalia - Hibolithes assemblage of the outer shelf setting. Duvalia spp. is the most common genus, Hibolithes spp. is less common, while Berriasibelus spp. and Pseudobelus spp. are rare. 3) A Duvalia - Pseudobelus assemblage of hemi-pelagic preference. Duvalia is most common, next is Pseudobelus.     

Olivine-rich units in the Fongen–Hyllingen Intrusion, Norway: implications for magma chamber processes

The Fongen–Hyllingen Intrusion (FHI) is considered to have crystallised from stratified magma residing in a bowl-shaped magma chamber. Seven olivine-rich units, representing the most primitive cumulates in the central part of the intrusion, are associated with compositional reversals and are interpreted as having formed at the lowest part of the magma chamber floor. Based on phase-relationships, the crystallisation order is explained in terms of magma mixing and fractional crystallisation. Repeated influxes of small volumes of dense, primitive magma at the base of the chamber had a major impact on the crystallising assemblage on the local floor and a decreasing effect towards the flanks of the chamber. This was due to the small volume of replenishing magma, the geometry of the chamber and the consequent restriction of magma mixing to the deepest part of the chamber where the new magma was emplaced. It is estimated that the chamber floor sloped as little as 1–2°, but this was sufficient to give widely different cumulate sequences near the bottom of the chamber and on the flanks.     

The Geysers-Cobb Mountain Magma System, California (Part 2): timescales of pluton emplacement and implications for its thermal history

Over 400 ion microprobe U-Pb isotopic ages measured for zircons extracted from 24 geothermal wells that penetrate the Geysers Plutonic complex (GPC) allow us to conclude that the entire known extent of the GPC crystallized during the early Pleistocene. Nine samples of the microgranite porphyry that forms the shallow cupola (100-1,500 m below sea-level, mbsl) of the GPC yield the oldest model U-Pb age (1.75 ± 0.01 Ma after correction for initial U series disequilibrium; errors 1σ). Twelve samples from the main intrusive phase (orthopyroxene-biotite granite) present at depths >1,250 mbsl define a crystallization age of 1.27 ± 0.01 Ma. This coincides with the age determined for a structurally and compositionally distinct body of granodiorite (1.25 ± 0.01 Ma; N = 5 samples) that is intruded over a similar depth range. Two petrographically distinct varieties of orthopyroxene-biotite granite yield ages of 1.46 ± 0.03 (GPC21-6000) and 1.16 ± 0.02 Ma (CA5636 74F 21; three samples). U-Pb zircon ages for dikes intruded in metagraywacke country-rocks overlap with those obtained from the main body of the GPC and include the youngest material identified (dike sample NEGU2 ST1-7700: 1.11 ± 0.03 Ma). Overall, the U-Pb results demonstrate that the main body of the GPC (∼300 km³) was emplaced and crystallized within the upper crust over a short time interval (0.2 Ma) that overlaps with zircon crystallization ages of overlying silicic volcanic units.     

Geology and paleomagnetism of El Potrero pluton, Baja California: Understanding criteria for timing of deformation and evidence of pluton tilt during batholith growth

The 102 Ma El Potrero pluton, in the western foothills of Sierra San Pedro Mártir, in north-central Baja California, was emplaced during a long period of contractional deformation bracketed between 132 and 85 Ma that affected this segment of the Peninsular Ranges Batholith. The pluton records regional and emplacement related deformation manifested by: (1) a solid-state fabric developed on its eastern contact, which is produced by eastward lateral pluton expansion; (2) cleavage triple point zones in the host-rock NW and SE of the pluton; (3) subhorizontal ductile shear zones indicative of top-to-the-east transport; (4) magmatic and tectonic foliations parallel to regional structural trends and regional shear zones; (5) variable axial ratios of microgranitoid enclaves close to pluton–wall rock contacts; (6) evidence of brittle-emplacement mechanisms in the western border of the pluton, which contrast with features indicating mainly ductile mechanisms toward the east; and, (7) markedly discordant paleomagnetic directions that suggest emplacement in an active tectonic setting. The overall mean for 9 accepted paleomagnetic sites is Dec = 34.6°, I = 25.7° (k = 88.3, α₉₅ = 5.5°), and is deviated  35° with respect to the reference cratonic direction. This magnetization is interpreted to indicate a combination of tilt due to initial drag during vertical diapiric ascent (or westward lateral-oblique expansion) of the adjacent San Pedro Mártir pluton and later rotation ( 15°) by Rosarito Fault activity in the southwest; this rotation may have occurred as eastward contraction acted to fill the space emptied by the ascending San Pedro Mártir pluton. The Rosarito fault may have tilted several plutons in the area (Sierra San Pedro Mártir, El Potrero, San José, and Encinosa). Magnetic susceptibility fabrics for 13 sites reflect mostly emplacement-related stress and regional stress. Paleomagnetic data and structural observations lead us to interpret the El Potrero pluton as a syntectonic pluton, emplaced within a regional shear zone delimited by the Main Mártir Thrust and the younger Rosarito Fault.     

U–Pb zircon and microfabric data of (meta) granitoids of western Cameroon: Constraints on the timing of pluton emplacement and deformation in the Pan-African belt of central Africa

Striking characteristics of the western Neoproterozoic belt of Cameroon (NFBC) are the large volume of granitoids and crustal-scale shear zones. New structural and geochronological data from this area are provided to put constraints on the tectonic evolution of this segment of the belt and to make further correlations between major shear zones exposed on both sides of the Atlantic Ocean.

Three different complexes have been identified in the study area: the migmatitic complex of Foumbot (MCF), the metagranitoid complex of Bangwa (BC), and the Batié pluton (BP). The MCF was intruded by the BC, while the BP cuts through the BC. U–Pb zircon dating of metaleucogranite and metagranodiorite of the BC yielded concordant to subconcordant ages of 638 ± 2 Ma and 637 ± 5 Ma, respectively. A concordant U–Pb zircon age of 602 ± 1.4 Ma has been obtained from porphyrogranite of the BP. These ages are interpreted as emplacement ages. Continuous deformation from magmatic to solid-state flow along the BP margins and the (sub)parallelism of the steep solid-state foliation in the BP margins with the foliation in the surrounding BC and MFC suggest synkinematic emplacement of the BP along crustal-scale NNE to ENE-trending strike–slip shear zones. Subhorizontal foliations in migmatitic-gneiss xenoliths found in the BC suggest that the major transcurrent motion was preceded by thrusting.

The new data confirm previous assumptions that the western NFBC is equivalent to parts of the Borborema province of Brazil. There are geochronological correlations between the studied (meta)granitoids and Brasiliano pre- to syn-transcurrent granitoids of the Borborema province.     

Timing of granite emplacement and cooling in the Songpan–Garzê Fold Belt (eastern Tibetan Plateau) with tectonic implications

New U–Pb and Rb–Sr geochronology on syn- and post-orogenic granites provide constraints on the timing of major tectonic events in the Songpan–Garzê fold belt, west Sichuan, China. The Ma Nai granite was probably syn-kinematic with the main deformation and yields an age of 197±6 Ma that is interpreted as an upper age limit of the Indosinian event. Zircons and apatites from the post-kinematic Rilonguan granite also yield Jurassic ages (195±6 and 181±4 Ma). The post-orogenic Markam massif gives two ages of 188±1 and 153±3 Ma. Both granites are undeformed and cut structures in the Triassic sedimentary rocks. These results demonstrate that the major deformation and décollement tectonics in the Songpan–Garzê fold belt occurred prior to the Early Jurassic. The wide range of ages obtained for post-kinematic granites (from Early Jurassic to Late Jurassic) suggests that, locally, magmatic activity persisted for a long time (at least 50 Ma) after the Indosinian compressional tectonism. No Tertiary ages have been obtained, suggesting that these granites were not affected strongly by the India–Asia collision.     

Sudbury Breccia (Canada): a product of the 1850 Ma Sudbury Event and host to footwall Cu–Ni–PGE deposits

The Sudbury Structure, formed by meteorite impact at 1850 Ma, consists of three major components: (1) the Sudbury Basin; (2) the Sudbury Igneous Complex, which surrounds the basin as an elliptical collar; and (3) breccia bodies in the footwall known as Sudbury Breccia. In general, the breccia consists of subrounded fragments set in a dark, fine-grained to aphanitic matrix. A comparison of the chemical composition of host rocks, clasts and matrices indicates that brecciation was essentially an in-situ process. Sudbury Breccia forms irregular-shaped bodies or dikes that range in size from mm to km scale. Contacts with the host rocks are commonly sharp. The aspect ratio of most clasts is approximately 2 with the long axes parallel to dike walls. The fractal dimension (Dr)=1.55. Although there appears to be some concentration of brecciation within concentric zones, small Sudbury Breccia bodies within and outside these zones have more or less random strikes and steep dips. Sudbury Breccia bodies near an embayment structure tend to be subparallel to the base of the Sudbury Igneous Complex. Sudbury Breccia occurs as much as 80 km from the outer margin of the Sudbury Igneous Complex. In an inner zone, 5 to 15 km wide, breccia comprises 5% of exposed bedrock with an increase in brecciation intensity in embayment structures. Sudbury Breccia may be classified into three types based on the nature of the matrix: clastic, pseudotachylite and microcrystalline. Clastic Sudbury Breccia, the dominant type in the Southern Province, is characterized by flow-surface structures. Possibly, a sudden rise in pore pressure caused explosive dilation and fragmentation, followed by fluidization and flowage into extension fractures. Pseudotachylite Sudbury Breccia, mainly confined to Archean rocks, apparently formed by comminution and frictional melting. Microcrystalline Sudbury Breccia formed as a result of the thermal metamorphism, of the North Range footwall, by the Sudbury Igneous Complex. This produced a zone, approximately 1.2 km wide, wherein the matrix of the breccia either recrystallized or, locally, melted. An overprint of regional metamorphism obliterated contact effects in the South Range footwall. The Ni–Cu–PGE magmatic sulphide deposits may be classified into four types based on structural setting: Sudbury Igneous Complex–footwall contact, footwall, offset, and sheared deposits. Sudbury Breccia is the main host for footwall deposits (e.g., McCreedy East, Victor, Lindsley). Sudbury Breccia locally hosts mineralization in radial (e.g., Parkin and Copper Cliff) and concentric (e.g., Frood–Stobie) offset dikes.     

An unsuccessful attempt at U/Th dating of soil calcretes from the Doukkâli area (western Morocco) and environmental implications

A large number of calcrete samples from topsoils of the Doukkâli area, western Morocco, were studied by U-disequilibrium series methods. The ²³⁰Th/²³⁴U ages are rather uniformly distributed between>350 and 2 ka BP. Homologous samples for occurrence and structure display quite different ages, the ²³⁰Th/²³⁴U ages are considered apparent. This is explained by repeated deposition of secondary calcite in the calcrete pores, which caused lowering of the original ages. However, the apparent ages may be considered minimum ages of the calcrete formation, indicating that calcium carbonate mobilization and deposition has been taking place repeatedly since >350 ka.

The age range obtained is quite comparable with that of calcretes from southern Spain, suggesting similar conditions for the calcrete formation in the two areas.     

Stratigraphy,age and petrography of the Beni Issef successions (External Rif; Morocco): Insights for the evolution of the Maghrebian Chain

In the Beni Issef Massif, nearly 30 km west of Chefchaouen (Morocco), the thickest post-nappe succession within the Rifian sector of the Maghrebian Chain seals the tectonic contact between the Intrarifian External Tanger and Loukkos Units, related to the Rifian External Domain. This succession is very important for the reconstruction of the deformation timing of the Rifian Maghrebids. The age of its base, in fact, is an important constraint for defining an upper boundary to the stacking of both the Intrarifian and Maghrebian Flysch Basin Units, because clasts fed by the Melloussa and Numidian Flysch Nappes are abundant in the conglomerate layers. Field and biostratigraphic analyses pointed out the presence of a Lower Beni Issef Fm, unconformable on the Intrarifian External Tanger and Loukkos Units, and an Upper Beni Issef Fm, unconformable on both the Intrarifian Units and the Lower Beni Issef Fm. The Lower Beni Issef Fm, 150 m thick, consists of lenticular conglomerates with huge blocks in a marly-clayey matrix, followed by marls and minor sandstones. It deposited in a siliciclastic platform, shows a fining upward trend and is affected by metre- to hectometre-sized, locally reversed, folds. Samples collected 45–50 m above the base of the formation resulted not older than Late Tortonian in age, but an older age for the base of the formation cannot be excluded. The Upper Beni Issef Fm, up to 550 m thick, starts with coarse conglomerates followed by medium- to coarse-grained well-bedded sandstones and by grey-blue marls and mudrocks. It indicates deposition in a channelized marine delta, with evolution towards pro-delta pelites, and shows sub-horizontal or gently dipping beds towards the east. Biostratigraphic data indicate a probable Messinian age for this formation. The composition of the arenites of both Lower Beni Issef and Upper Beni Issef Fms is quartzolithic and all samples show a notable content of monocrystalline well-rounded quartz and sedimentary lithic fragments. Detrital modes, all falling in the Quartzose Recycled and Transitional Recycled fields, suggest a provenance from recycling of sedimentary successions, easily recognizable in the Flysch Basin and External Units, mainly the Numidian Nappe sandstones. A Tortonian age of the Lower Beni Issef Fm would agree with the Late Serravallian age of the uppermost beds of the External Tanger Unit and indicate that the most probable age for the stacking of the Intrarifian Units falls in the Late Serravallian-Middle Tortonian time span. The Lower Beni Issef Fm was involved in a compressive tectonic phase testified by north-south striking folds. Later, probably during Messinian, the Upper Beni Issef Fm deposited in a younger intramontane basin, resting on both the Intrarifian Units and the Lower Beni Issef Fm. Successively, the Upper Beni Issef Fm was passively transported piggyback on top of the fold and thrust belt during later tectonic evolution of the Rifian Maghrebids. This tectonic evolution results quite similar to that recognized in the Tellian and Sicilian Maghrebids and also in the southern Apennines.     

Hydrothermal alteration, fluid flow and volume change in shear zones: the layered mafic–ultramafic Kettara intrusion (Jebilet Massif, Variscan belt, Morocco)

During emplacement and cooling, the layered mafic–ultramafic Kettara intrusion (Jebilet, Morocco) underwent coeval effects of deformation and pervasive fluid infiltration at the scale of the intrusion. In the zones not affected by deformation, primary minerals (olivine, plagioclase, clinopyroxene) were partially or totally altered into Ca‐amphibole, Mg‐chlorite and CaAl‐silicates. In the zones of active deformation (centimetre‐scale shear zones), focused fluid flow transformed the metacumulates (peridotites and leucogabbros) into ultramylonites where insoluble primary minerals (ilmenite, spinel and apatite) persist in a Ca‐amphibole‐rich matrix. Mass‐balance calculations indicate that shearing was accompanied by up to 200% volume gain; the ultramylonites being enriched in Si, Ca, Mg, and Fe, and depleted in Na and K. The gains in Ca and Mg and losses in Na and K are consistent with fluid flow in the direction of increasing temperature. When the intrusion had cooled to temperatures prevailing in the country rock (lower greenschist facies), deformation was still active along the shear zones. Intense intragranular fracturing in the shear zone walls and subsequent fluid infiltration allowed shear zones to thicken to metre‐scale shear zones with time. The inner parts of the shear zones were transformed into chlorite‐rich ultramylonites. In the shear zone walls, muscovite crystallized at the expense of Ca–Al silicates, while calcite and quartz were deposited in ‘en echelon’ veins. Mass‐balance calculations indicate that formation of the chlorite‐rich shear zones was accompanied by up to 60% volume loss near the centre of the shear zones; the ultramylonites being enriched in Fe and depleted in Si, Ca, Mg, Na and K while the shear zones walls are enriched in K and depleted in Ca and Si. The alteration observed in, and adjacent to the chlorite shear zones is consistent with an upward migrating regional fluid which flows laterally into the shear zone walls. Isotopic (Sr, O) signatures inferred for the fluid indicate it was deeply equilibrated with host lithologies.     

Rare earth element transport and fractionation in small streams of a mixed basaltic–granitic catchment basin (Massif Central, France)

We present dissolved load (< 0.45 μm) rare earth element (REE) patterns of small streams from a catchment basin in the Massif Central in order to characterize the individual fractionation stages for the dissolved REE from the source to the catchment outlet. The upper part of the catchment is located on a basalt plateau, followed downstream by deep and narrow valleys with granitic and orthogneissic bedrock. Stream water has basalt-like REE patterns on the basaltic plateau close to the source, followed by a continuous depletion in light REE (La-Sm, LREE) downstream. Strontium and neodymium isotope ratios of stream water demonstrate that the dissolved REE are essentially of basaltic origin, even in the lower, granitic and gneissic part of the catchment. Mixing with gneiss or granite derived REE thus cannot explain the observed evolution of the REE patterns. There seems also to be no link with the calculated speciation of the dissolved REE. In contrast, a correlation between saturation indexes for hematite and La/Yb ratios suggests that REE fractionation is mainly controlled by precipitation of Fe-oxide particles that preferentially remove LREE from solution.