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

摘要

A 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 K_1-K_2 axes on the dyke plane whereas the K_3 axis is nearly perpendicular to it. This fabric is interpreted as due to magma flow. The sub-horizontal inclination of the K_1 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 K_2-K_3 plane close to the dyke plane and the K_1 perpendicular to dyke orientation. Such fabric was attributed to the local shearing stress. (iii) 'Intermediate' fabric (1 dyke) is defined by K_1-K_3 axes close to the dyke plane and K_2 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, =15.3°E, K = 288.8, A_(95) = 3.4°, n = 8, Pole B: λ = 81.6°N, φ = 89.1°E, k = 69.8, A_(95) = 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 ~(40)Ar-~(39)Ar 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.