CHARACTERIZATION OF MESOZOIC DIKES FROM MAURITANIA, WESTERN AFRICA AND IMPLICATIONS FOR THE COHERENCY OF THE AFRICAN CONTINENT

We report new rock magnetic data on Mesozoic basaltic dikes from Mauritania, Africa. These basalts are perhaps analogous to the Newark Supergroup basalts located on the eastern margin of North America; they may be part of the Central Atlantic Magmatic Province (CAMP). However, the exact timing and paleomagnetic signature of the dikes are not well constrained. To better understand the latter aspect, paleomagnetic samples were collected from fifteen sites; all samples were oriented in the field with a Brunton and Sun compass.

Detailed stepwise alternating field (AF) and thermal demagnetizations performed at the University of Rochester show linear decay to the origin after the removal of a low coercivity, low unblocking temperature overprint. Magnetic susceptibility measured using a KLY-3S Kappabridge reveal Curie points of 570-580°C, suggesting the presence of magnetite (Fe3O4). This is confirmed by the presence of a low temperature inflection point at ~ -153°C, which is characteristic of the cubic to monoclinic transition in magnetite known as the Verwey transition. Magnetic hysteresis loops measured with an Alternating Gradient Force Magnetometer (AGFM) show a range of domain states, with Mr/Ms values ranging from 0.05 to 0.33. The presence of nearly pure magnetite suggests high temperature oxidation upon initial cooling; we will test this hypothesis with SEM studies. Overall, we believe that the magnetic minerals in these rocks are high-resolution recorders of the Triassic-Jurassic magnetic field and thus can be used to study the coherency of the African continent during the rifting that formed the Atlantic Ocean.