Gérard Gleizes

Magnetic susceptibility of the Mont-Louis andorra ilmenite-type granite (Pyrenees): A new tool for the petrographic characterization and regional mapping of zoned granite plutons

We have observed remarkably consistent patterns of concentric zonation in the values of low-field magnetic susceptibility measurements over the Variscan age Mont-Louis Andorra granite pluton of the eastern Pyrenees of Andorra, Spain and France. This zonation is a function of the rock petrology since a close correlation is shown between the petrographic nature (as defined by chemical analysis) and the magnetic susceptibility magnitude of the rocks. It reflects the dominantly paramagnetic nature of the granite, carried by Fe-bearing silicates, and this is demonstrated by the fact that low-field susceptibilities of representative specimens are almost equal to (1) their high-field susceptibilities and (2) their calculated susceptibilities using Fe contents, assuming a null ferromagnetic contribution. We conclude that this technique accurately reflects the modal abundances of ferromagnesian phases in rocks of the ilmenite series and that it represents a powerful and efficient tool for the reconnaissance surveying of petrological variations in granitoid plutons.

Variscan dextral transpression in the French Pyrenees: new data from the Pic des Trois-Seigneurs granodiorite and its country rocks

Abstract New structural data from both the Trois-Seigneurs granodiorite and its country rock lead to new conclusions about the Variscan tectonic history of this part of the Pyrenees. Sedimentary rocks of the massif, that increase in metamorphic grade from north to south, are imprinted by a single major deformation responsible for both the tight, upright, E-W-trending folds with vertical to steeply N-dipping foliations and the gently W-plunging stretching lineations. Syntectonic emplacement of the Pic des Trois-Seigneurs granodiorite is demonstrated by the parallelism between structures of the pluton, as determined by magnetic fabric measurements, and the foliations and lineations imprinted in the country rocks. Large deformation of the granodiorite magma during emplacement is indicated by strong anisotropy and ubiquitous magmatic microstructures. The high thermal gradients of regional metamorphism, which is also syntectonic, are attributed to a large thermal aureole provided by deep-seated magma reservoirs. Shear-sense criteria in XZ thin sections of the metasedimentary country rocks indicate a dextral sense of the non-coaxial component of strain throughout the massif and regardless of metamorphic grade. The overall structure of the massif is concluded to have derived from a major shearing event that is equated with the D 2 main phase observed in the entire Variscan Pyrenees. As the E-W-oriented stretch and dextral shear cannot be separated from compressive structures generally considered to be characteristic of D 2 , this phase is concluded as representing regional transpression.

Magnetic susceptibility and AMS of the Bushveld alkaline granites, South Africa

The Bushveld Complex in South Africa includes one of the worlds largest anorogenic alkaline granite intrusions (66,000 km2). The granite forms a composite laccolith, of 350 × 250 km in area and about 2 km in thickness, which was emplaced at about 5 km depth into sediments overlying the Kaapvaal craton, at 2054 Ma. The Bushveld granite and its roof-rocks have long been mined for Sn, W and F. The Bushveld granites have high magnetic susceptibilities (Km from 1000 to 4000 μSI), and a quantitative model is presented, suggesting that susceptibility fabrics are primarily carried by ferromagnetic minerals. The measured AMS foliations coincide with observed subhorizontal mineral lineations and compositional layering. Magnetic lineation trends vary considerably within the horizontal plane. The existence of a weak planar fabric and, an almost absent linear component may reflect (a) laccolithic emplacement by roof uplift, causing flattening magmatic fabrics, or (b) emplacement of largely crystal-free magma crystallizing in-situ and developing horizontal compositional layering from thermal chemical diffusion fronts and gravity-driven mechanisms. Weak magnetic fabrics, like those identified in the Bushveld granites require specific sampling schemes and procedures, in addition to rigorous constraint of magnetic mineralogy and crystallization sequence.

The maladeta granite polydiapir, Spanish Pyrenees: A detailed magnetostructural study

Abstract The Maladeta granitic complex (Spanish Central Pyrenees) has been subjected to a detailed magnetostructural study, through the systematic measurement of magnetic susceptibility and anisotropy of magnetic susceptibility at 253 regularly spaced sampling sites. Due to the dominant paramagnetic property of the granites, a first-order correlation can be derived between the susceptibility magnitude and the bulk-rock iron content, hence the petrographic nature of the rock. The Maladeta complex is shown to be composed of four principal juxtaposed plutons, each one being normally zoned, i.e. becoming more basic towards the periphery. The largest pluton, namely the Colomers unit, is itself made of a number of sub-plutons due to distinct but more-or-less coeval batches of magmas. The various magnetic data, complemented by field and microstructural observations, allow us to propose that the Maladeta polydiapir probably exploited NE- and SE-trending openings of the brittle crust for its emplacement, that accompanied or just preceded a SW-verging compressive event. A later N-S compression marked by inverse dextral, and E-W-trending shears, is preferably attributed to late Variscan rather than to Alpine tectonics.

Oblique magmatic structures of two epizonal granite plutons, Hoggar, Algeria: late-orogenic emplacement in a transcurrent orogen

Abstract Two granite plutons in the Pan-African belt of Hoggar, Algeria, are studied using the anisotropy of magnetic susceptibility technique (AMS) at low-field, together with a study of microstructures. The Tesnou Complex is made of coalescent plutons at the margins of which dextral N-S-oriented shearing imprints solid-state textures. In the granite, microstructures and low magnetic anisotropy magnitudes indicate a magmatic-state weak deformation. Consistent NW-SE-trending sub-horizontal magnetic lineations, making an overall oblique pattern with respect to bordering shear faults, is attributed to low N-S-trending dextral shear strain magnitudes (γ

Syntectonic emplacement of the Maladeta granite (Pyrenees) deduced from relationships between Hercynian deformation and contact metamorphism

The relationships between Hercynian deformation and contact metamorphism in the aureole of the Aneto unit, a constituent pluton of the Maladeta granite complex (Spanish Pyrenees), provide new data relevant to the controversy between the syn- and post-tectonic models of emplacement for the Pyrenean granites. The deformation in the Aneto aureole is related to two superimposed Hercynian episodes. The first was a ‘top to the SW’ thrusting which induced SW-verging recumbent folds. The second, which was essentially compressive in the Aneto area, is represented by E–W-trending subvertical cleavage and folds with subhorizontal axes. This second episode can be assigned to the D2 main Hercynian tectonic event of the Pyrenees. The contact metamorphic assemblages grew during the D2 event and were deformed by it close to the contact. The conclusion that the pluton was emplaced during D2 is strengthened by the structural and kinematic similarities between the magmatic deformation in the southern rim of the pluton and the D2 deformation in the country rocks. This syntectonic timing is supported by new geochronological data that conflict with an existing Rb–Sr post-tectonic Permian whole-rock age for the Maladeta granite. More generally, in accordance with some very recent structural and U–Pb zircon studies, our results contradict the geodynamical models of the Pyrenees which relate the granite intrusions to a late extensional event.

Drainage and Emplacement of Magmas along an Inclined Transcurrent Shear Zone: Petrophysical Evidence from a Granite-Charnockite Pluton (Rahama, Nigeria)

A Pan-African granite-charnockite pluton of northeast Nigeria has been core-sampled at an average grid spacing of 2 km. Detailed petrographic and microstructural observations are combined with an AMS study of 172 stations. The continuum between charnockites and granites is revealed by their constituent minerals and the magnetic susceptibility map. The contemporaneity of emplacement of all the petrographic types is demonstrated by their similar mineral- and magnetic-fabrics. Inclined strike-slip shear zones were active during and slightly after the emplacement of this pluton. Earlier geochemical data indicate that magmas originated from several vertically juxtaposed lower-crust sources. The proposed model of emplacement implies that the shear zones were responsible for the migration, drainage and tapping of magmas from the source region. The inclined attitude of the shear zones would have acted as a roof-guide for the up-rising magmas. Finally, the transcurrent movement most likely enhanced the upward migration of magmas.

Gneiss domes and granite emplacement in an obliquely convergent regime: New interpretation of the Variscan Agly Massif (Eastern Pyrenees, France)

Like the metamorphic core complexes of the Basin and Range (United States), the gneiss domes of the west European Variscan range have been associated with large-scale extension. In particular, the development of the Agly Massif, a gneiss and micaschist dome in the eastern Pyrenees (France), has been related to N-S–directed, late Variscan or Cretaceous extension. However, new microstructural and kinematic investigations in the Agly Massif demonstrate that (i) there is no major detachment, (ii) the pervasive deformation associated with the early metamorphism indicates a southward vergence, and (iii) the numerous mylonitic bands observed at different levels of the section acted as gently dipping normal faults and display opposite shear senses on both northern and southern flank of the dome. Shearing on these bands caused a multi-kilometer–scale thinning distributed across the whole lithologic column. Two new U-Pb zircon analyses yielded an age of 317 ± 3 Ma for a deformed granite from the core of the dome, and an age of 307 ± 0.4 Ma for a deformed granite emplaced in the micaschist cover. This suggests that two phases of magmatism occurred in the Agly Massif, the first prior to doming and the second during doming and the emplacement of the main Pyrenean plutons associated with a dextral transpressive phase. Therefore, the Agly gneiss dome formed in a transpressive regime and not in a late Variscan or Cretaceous extensional regime related to the collapse of a previously thickened crust.

Magma emplacement and mafic–felsic magma hybridization: structural evidence from the Pan-African Negash pluton, Northern Ethiopia

Abstract The Negash pluton (50 km 2 ) consists of late Pan-African, high-K, calc-alkaline granitoids intruded into low-grade metavolcanics–metasediments. This almost circular massif consists of monzogranites, granodiorites, diorites–gabbrodiorites, and hybrid diorites. The anisotropy of magnetic susceptibility (AMS) method was used to determine internal structures of the pluton. The foliation trajectories are concentric and inward dipping. The lineation pattern displays an external zone characterised by horizontal concentrically oriented lineations and an internal zone with NW–SE oriented lineations. These petro-structural data clearly locate the feeder zone at the north-western tip of the pluton and indicate the subsequent expansion of the magmas towards the SE. The pluton is a result of in-situ assembly of four magma batches, which were forcefully injected into pre-existing foliated country rocks in relation to transpressional tectonic regime. Two types of mafic–felsic magma interactions are recognised: homogeneous and heterogeneous hybrid diorites at the north-western part, and mingled interfaces at the diorite–granodiorite contact zones mainly visible in the eastern and south-eastern parts. The in-situ mingling along diorite–granodiorite contacts was achieved at the level of emplacement during the injection of dioritic magma into the felsic magmas while the hybrid diorites are assumed to result from two-way conduit mixing and mingling during simultaneous rising of mafic and felsic magmas.

The Pan-African Toro Complex (northern Nigeria); magmatic interactions and structures in a bimodal intrusion

The Toro Complex is one of the Pan-African Older Granites of Nigeria, first described as a reversely zoned pluton made of a central dioritic mass surrounded by a broad granitic rim. It has been thoroughly reinvestigated both from the petrographic and structural points of view, with the help of systematic anisotropy of magnetic susceptibility (AMS) measurements. The granite main body is a hornblende–biotite porphyritic monzogranite characterized by an early submagmatic fabric displaying a concentric pattern of foliations and west plunging lineations (stage 1). This fabric is overprinted by a later one due to solid-state strain along north-south subvertical dextral shear zones (stage 2). In the vicinity of the diorite, an evengrained granite displays magmatic structures that are contemporaneous with this strike-slip event. The diorite–granite contact is a complex zone where field, petrographic and geochemical data enable recognition of the effects of mixing and mingling between a mafic and a felsic magma. Tonalites cropping out within this contact zone are interpreted as hybrid rocks. The reverse zonation of the diorite itself is also the result of some hybridization process. Magmatic interactions mainly resulted from in situ infiltration of granitic liquid into the dioritic mass. The detailed history of this bimodal intrusion began with the emplacement of the granitic magma acquiring a first stage fabric. Before full crystallization of the granitic core, intrusion of the dioritic magma permitted reheating of the granitic magma that then crystallized with specific structural characters. The second stage structures, whether characterized by magmatic fabric near the diorite or by solid-state strain features in north–south shear zones elsewhere in the granite, are related to late Pan-African dextral strike-slip tectonics in the basement of northern Nigeria. The bimodal Toro Complex is therefore considered as a late Pan-African syntectonic pluton.