First paleomagnetic and 40Ar/39Ar study of Paleoproterozoic rocks from the French Guyana (Camopi and Oyapok rivers), northeastern Guyana Shield

In order to understand the Paleoproterozoic geographic evolution of the Guyana Shield, paleomagnetic and 40Ar/39Ar investigations were carried out on granitoids and volcano-sedimentary rocks from the Oyapok and Camopi rivers (French Guyana–Brazil frontier). Scanning electronic microscope, thermomagnetic and isothermomagnetic experiments show that magnetite is the main magnetic remanent carrier in most of the samples. The metavolcano-sedimentary rocks (Paramaca) show a weak magnetization and scattered magnetic directions. Therefore, no reliable magnetic component could be isolated from these samples. Samples taken from tonalite and meta-ultrabasite rocks yield a characteristic magnetic direction, carried by subautomorphous magnetite, that is well defined and distinct from that of the present Earth field and that of nearby Jurassic dikes. A virtual geomagnetic pole (VGP) deduced from this probable primary remanence was calculated, namely pole OYA, λ=28.0°S, φ=346.0°E, N=5, k=31.9 and A95=13.8°. Four 40Ar/39Ar ages, ranging from 2052 to 1973 Ma, were obtained from amphiboles and biotites of tonalite rocks, showing a relatively slow cooling rate of ca 4.8+2.6/−2.1°C Ma−1. The linear extrapolation of this cooling rate to the magnetite unblocking temperature (540 to 580°C) yields a magnetization age of 2036±14 Ma for pole OYA. Pole OYA differs significantly from available paleomagnetic results from Venezuela of the West Guyana Shield dated at 2000±10 Ma. This difference may indicate an important latitudinal movement of the Guyana Shield between 2036 and 2000 Ma with a velocity of 9±7 cm/year

Persistent paléosurfaces in the basement of French Massif Central: geodynamic implications.

National audienceThe siderolithic paleoweathering surfaces of the French Central Massif have been dated to the Late Jurassic/Early Cretaceous, contrasting with previously accepted Tertiary age and implying that the Massif has never hosted a thick sedimentary cover. This contradicts with former thermochronological results. Herein, we expose the arguments for and against the proposed geodynamic evolution of the French Massif Central constrained by paleomagnetic age determinations

Présentation du guide des curiosités géologiques de Guyane

Hervé Théveniaut met en lumière la richesse géologique de la Guyane. 24 sites patrimoniaux permettent de caractériser cette richesse

Archéologie et géologie : caractérisation du matériel lithique et chaîne opératoire

International audienc

Long lasting paleolandscapes stability of the French Massif Central during the Mesozoic.

International audienc

Long lasting paleolandscapes stability of the French Massif Central during the Mesozoïc.

National audienc

Paleomagnetic datings on "Siderolithic" paleoweathering profiles along French Massif Central.

International audienc

Paleomagnetic study of French Guyana Early Jurassic dolerites: hypothesis of a multistage magmatic event

International audienceA detailed paleomagnetic and anisotropy of magnetic susceptibility (AMS) study was carried out on 34 sites of Early Jurassic dolerite dykes from French Guyana, which formed during the initial opening of the Central Atlantic Ocean. Four types of AMS fabrics are recognized: (i) ‘Normal' fabric (21 dykes) defined by clustering of K1–K2 axes on the dyke plane whereas the K3 axis is nearly perpendicular to it. This fabric is interpreted as due to magma flow. The sub-horizontal inclination of the K1 axis permitted to suggest that the French Guyana dykes could be fed by horizontal magma fluxes from a distant magma source. (ii) ‘Reversal' fabric (8 dykes) is characterized by the K2–K3 plane close to the dyke plane and the K1 perpendicular to dyke orientation. Such fabric was attributed to the local shearing stress. (iii) ‘Intermediate' fabric (1 dyke) is defined by K1–K3 axes close to the dyke plane and K2 axis is perpendicular to this plane. It was interpreted as due to vertical compaction of a static magma column. (iiii) ‘Other' fabric (4 dykes) does not show any preferential orientation. Scanning electronic microscope and susceptibility versus temperature experiments show that minerals of the titanomagnetite family are main magnetic remanence carriers. Two magnetic components were isolated. Ages of magnetic remanences are estimated at 198.3±2.0 Ma to 192.3±1.5 Ma. Their virtual geomagnetic poles are calculated, Pole A: λ=73.2°N, φ=15.3°E, k=288.8, A95=3.4°, n=8, and Pole B: λ=81.6°N, φ=89.1°E, k=69.8, A95=4.2°, n=18. These two groups probably correspond to two distinct magmatic events which occurred in a short period. This hypothesis is consistent with published 40Ar–39Ar radiometric ages though with ‘mini plateau' spectra. These paleomagnetic results suggest the presence of magmatic pulses led to the construction of the Central Atlantic Magmatic Province in French Guyana during the Early Jurassic