Anne Nédélec

U-Pb, Single Zircon Pb-Evaporation, and Sm-Nd Isotopic Study of a Granulite Domain in SE Madagascar

Madagascar is a poorly known Precambrian domain at the eastern border of the Mozambique belt, strongly affected by the Pan-African orogeny. This large island represents a key part of Gondwanaland and the accurate dating of the major metamorphic event is critical for the reconstruction of that supercontinent. In the southeastern zone of the island, metamorphism has reached high-temperature and low-pressure conditions (850°C and 5 kbar) with moderate retrogression. A U-Pb conventional multigrain, single zircon stepwise Pb-evaporation and Sm-Nd geochronological study on meta-felsic rocks has shown that basement of at least lower Proterozoic age was strongly metamorphosed around 570-580 Ma. This tectometamorphic event nearly obliterated any pre-Pan-African zircon isotopic memory. The granulite-facies metamorphism is associated with the syn-metamorphic emplacement of the Anosyan charnock-ites and granites. A strong channelized fluid contribution at 545 Ma has produced the (re)crystallization of biotite-apatite lenses containing giant monazites. Finally, the emplacement of zircon-bearing calcite veins at 520 Ma represents the latest Pan-African event in this SE Madagascar domain. This precise determination of the Pan-African overprint in this region facilitates comparative discussion of the geodynamic evolution of Madagascar and surrounding areas at the end of the Proterozoic. There is good agreement between the single zircon Pb-evaporation results and conventional isotopie dating; nevertheless slightly younger apparent ages are obtained by the former method. This discrepancy, which may have some important implications, could be related to the discordancy of the analyzed zircon populations.

The Sturtian 'snowball' glaciation: fire and ice

Abstract The Sturtian ‘snowball’ glaciation (730 Ma) is contemporary with the dislocation of the Rodinia supercontinent. This dislocation is heralded and accompanied by intense magmatic events, including the onset of large basaltic provinces between 825 and 755 Ma. Among these magmatic events, the most important one is the onset of a Laurentian magmatic province at 780 Ma around a latitude of 30°N. The presence of these fresh basaltic provinces increases the weatherability of the continental surfaces, resulting in an enhanced consumption of atmospheric CO 2 through weathering, inducing a global long-term climatic cooling. Based on recent weathering laws for basaltic lithology and on climatic model results, we show that the weathering of a 6×10 6 km 2 basaltic province located within the equatorial region (where weathering of the province and consumption of CO 2 are boosted by optimal climatic conditions) is sufficient to trigger a snowball glaciation, assuming a pre-perturbation PCO 2 value of 280 ppmv. We show that the Laurentian magmatic province might be the main culprit for the initiation of the Sturtian ‘snowball’ glaciation, since the Laurentian magmatic province had drifted within the equatorial region by the time of the glaciation.

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.

A new insight into Pan-African tectonics in the East–West Gondwana collision zone by U–Pb zircon dating of granites from central Madagascar

Abstract The assembly of Gondwana was the result of a major collision orogen, the East African Orogen, between East and West Gondwana during Neoproterozoic times. Madagascar, which represents a fragment of East Gondwana, is located in a key area of this Pan-African orogen. Granites of unambiguous tectonic setting have been dated using the U–Pb zircon method in order to constrain the timing of orogenic events. The central part of Madagascar is characterized by syntectonic alkaline granitic sheets, referred to as “stratoid” granites. These are of both mantle and crustal derivation. Their U–Pb zircon ages are well defined between 627 and 633 Ma for both plutonic suites, regardless of either mainly mantle or crustally origin. It is not surprising that the crustally-derived suite contains inherited zircons in the 2.2–2.4 Ga range attesting to the existence of Lower Proterozoic crust in northern central Madagascar. The generation of huge amounts of granitic magma is regarded as the result of post-collision extension under a high heat flow regime. Therefore, an age between 700 and 650 Ma is inferred for the beginning of Gondwana assembly along the collision zone between central Madagascar and Kenya, i.e., in the central part of the East African Orogen. Following this, brittle fracturing of the stratoid granite series permitted the emplacement of the Ambatomiranty granitic dyke swarm at a minimum age of 560 Ma, in possible connection with a nearby shear belt. The strike–slip tectonic regime at ∼570–560 Ma is well known in southern Madagascar and in its Gondwana connections. This stage corresponds to intracontinental reworking and the final suturing of Gondwana.

Shape anisotropy versus magnetic interactions of magnetite grains: Experiments and application to AMS in granitic rocks

The magnetic fabric of ferromagnetic granitic rocks results from both the shape preferred orientation of individual magnetite grains and their distribution anisotropy through magnetic interactions between neighbouring grains. Measurement of the low-field magnetic anisotropy of single multi-domain magnetite grains shows a linear correlation between their magnetic anisotropy degree and their aspect ratio. Interactions between two elongated grains were studied experimentally using two types of grain arrangement: an “aligned” configuration and a “side-by-side” configuration. For a distance between the grain centers equal to approximately twice the average grain size, the magnetic susceptibility and its anisotropy are enhanced in both configurations, and the direction of kmax, the easiest magnetization axis, is stable in the “aligned” configuration, whereas it rotates toward an orthogonal direction in the “side-by-side” configuration. Depending on the distribution of the interacting magnetite grains, magnetic interactions may therefore either increase the whole-rock anisotropy magnitude, or reduce it as in the given example of the granitic rocks from Madagascar.

Structure and metamorphism of the granitic basement around Antananarivo: A key to the Pan-African history of central Madagascar and its Gondwana connections

The Precambrian basement of Madagascar acquired a polyphase imprint during the Pan-African orogeny. In northern central Madagascar, emplacement of stratoid alkaline granites at midcrustal depth (4–5 kbars) led to formation of a layered crust in a postcollisional extensional regime at 630 Ma (D1). Subsequently, the structures of the stratoid granites were rotated by the sinistral and transpressive E-W Antananarivo flexure (or virgation) zone (D2). East of Antananarivo the structures of the D1 layered crust and the D2 virgation are crosscut by the steeply dipping N-S foliations of the Angavo belt. Lineations gently plunging to the north attest that the Angavo belt is a major strike-slip shear zone that formed under low-pressure granulitic conditions (3 kbars, 790°C). The nearby porphyritic Carion granite was emplaced at the end of this period of N-S shearing (D3), which can thus be no younger than 530 Ma. Late-Pan-African (580–550 Ma) strike-slip motion along broadly N-S shear zones has been recognized elsewhere in Madagascar and in its Gondwana connections. Continuation of the Angavo belt as one of the high strain belts of the Arabian-Nubian Shield is discussed in the general framework of Gondwana assembly.

Paleomagnetism, geochronology and tectonic implications of the Cambrian-age Carion granite, Central Madagascar

Abstract The Carion granitic pluton in central Madagascar was intruded into warm continental crust following orogenic events related to the final amalgamation of Gondwana. U–Pb SHRIMP dating of the pluton yields an emplacement age of 532.1±5.2 Ma followed by relatively slow cooling as constrained by 40Ar/39Ar ages on hornblende and biotite. Four hornblende samples yielded a mean 40Ar/39Ar age of 512.7±1.3 Ma. A biotite sample yielded an age of 478.9±1.0 Ma. Paleomagnetic samples from the pluton and surrounding country rocks exhibit either SE-upwardly directed magnetizations (mean Dec=113°, Inc=−56°, k=106, α95=12°) or NW-downwardly directed magnetizations (mean Dec=270°, Inc=+64°, k=30, α95=11°) that pass a reversal test with a classification of ‘C’ and an angular difference of 14.4°. The ‘normal’ (negative inclinations) and ‘reverse’ (positive inclinations) directions also show a spatial bias within the pluton, suggesting a field transition from reverse to normal during cooling. The paleomagnetic pole calculated from the mean direction falls at 6.8°S, 001°E (dp=13°, dm=17°). Estimates of the blocking temperature for the magnetization are compared to the cooling history of the pluton and an age of 508.5±11.5 Ma is assigned to the pole. The Carion pole falls near similar-age poles from elsewhere in Gondwana, supporting the idea that the major orogenic events during Gondwana assembly were complete. A slight revision of the Gondwana apparent polar wander path (APWP) is proposed with rapid APW from 540 to 520 Ma; however, the proposed mechanisms to explain this rapid APW (including intertial-interchange true polar wander (TPW) or enhanced mantle driving forces) cannot fully explain all the data.

Stratoid granites of Madagascar: structure and position in the Panafrican orogeny

AbstractStratoid granites constitute a major feature of the Precambrien basement of Madagascar. A detailed structural study was carried out NNW of Antananarivo. New zircon isotopic data on a typical alkaline granite ascertain their Panafrican age (585 Ma). The sheets of granites metric to kilometric of thickness, are interlayered with migmatitic gneisses and amphibolites. Their internal structures, determined by anisotropy of magnetic susceptibility measurements, everywhere yield foliations gently dipping to the west, and lineations striking WSW-ENE. These structures were mostly acquired at the magmatic stage in the granites, in the country-rocks they resulted from high-temperature plastic deformation.The very constant structural pattern, interpreted in terms of shear deformation of a section of the crust, as well as the low P (P = 4 - 5 kb) - high T (T # 750°C) conditions, suggest that the emplacement of the stratoid granites was coeval with a late-orogenic stage in the Panafrican Mozambique belt, and po...

Granites : petrology, structure, geological setting, and metallogeny

1. What is a granite? 2. Origin of granitic magmas 3. Segregation of granitic melt 4. Genesis of hybrid granitoids: mingling and mixing 5. Transport of granitic magma 6. Emplacement and shape of granite plutons 7. Thermomechanical aspects in the country rocks around granite plutons 8. Crystallization of granitics magmas 9. Microstructures and fabrics of granites 10. Magnetic fabrics in granites 11. Zoning in granite plutons 12. Granites and plate tectonics 13. Precambrian granitic rocks 14. Granite metallogeny

Un nouvel exemple de magmatisme potassique à ultrapotassique : les syénites de l'Andringitra (Madagascar)

Abstract The Andringitra Range (Madagascar) is made of potassic to ultrapotassic syenites (K2O=8–11 weight%; K2O/Na2O=2–8). It is a new example of this rare type of magmatism, here related to the Late Precambrian Panafrican orogeny. In the Pic Boby area, the main rock type contains pyroxene (diopside) and amphibole (edenite), and is nearly silica-saturated. A subordinate type is an undersaturated diopside- and phlogopite-bearing syenite. Very high K, Ba, Rb, Sr and LREE contents, together with moderate to high Mg# (33–54), suggest a parental magma derived from the enriched (metasomatized) lithospheric mantle. Fractional crystallisation and crustal assimilation may be responsible for the most evolved compositions.