The Guyana and the West-African shield paleoproterozoic groupin : new paleomagnetic data from French Guyana and the Ivory-Coast.

International audienceThe aim of this study is to document the Palaeoproterozoic palaeomagnetic database for the Guiana and West African Shields in order to better understand the paleogeographic evolution of these two cratons. A total of 59 sites were sampled (33 in French Guiana and 26 in the Ivory Coast) in granites and metavolcanic rocks. Magnetic, petrographic and palaeomagnetic investigations were carried out on these rocks. Magnetic experiments and petrographic observations show that undeformed magnetite is the main magnetic remanent carrier in granites of French Guiana and both magnetite and haematite in rocks from the Ivory Coast. Both thermal and magnetic alternating-field demagnetizations were applied to the rocks. Four high-temperature magnetic remanent directions were isolated in French Guiana and the Ivory Coast. These directions are distinct from the present Earth's field and to the local Early Jurassic palaeomagnetic components. Reversal and contact tests were obtained for the collection from French Guiana. Based on these arguments and mineralogical investigations, we propose that the magnetic remanence represent a Palaeoproterozoic magnetization. Four virtual palaeomagnetic poles were calculated: GUI1, GUI2 for French Guiana; IC1 and IC2 for the Ivory Coast with their corresponding coordinates: GUI1: GUI1=-62°N, GUI1= 61°E, k= 18, A95= 10°, N= 15 ; GUI2: GUI2=-5°N, GUI2= 50°E, k= 26, A95= 18°, N= 5 ; IC1: IC1=-82°N, IC1= 292°N, k= 28, A95= 13°, N= 6 ; IC2: IC2=-25°N, IC2= 83°E, k= 11, A95= 16°, N= 9 . The magnetization age ranged from 2.04 to 1.97 Ga for the French Guiana poles (40Ar/39Ar) and between 2.10 to 2.00 Ga for the Ivory Coast poles (startigraphic ages). Combining these new palaeomagnetic poles and previously published data, two apparent polar wander paths were proposed for these two shields. The comparison of these two Palaeoproterozoic paths seems to indicate that the two cratons belonged to the same block at about 2.00 Ga but separated prior to 2.02 Ga. Although this hypothesis is supported by geological and tectonic observations in both shields, further palaeomagnetic, geochronological and petrographic constraints are needs between 2.04 and 2.10 Ga