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The Mesozoic era appears to have been a time of widespread magmatic activity that affected stable cratonal areas of western Africa. The exact timing and paleomagnetic signatures of many of the mafic rocks emplaced during the magmatic episode, however, are unknown. Defining ages and paleomagnetic directions of such rocks is important because the resulting data could place constraints on the coherency of the African continent during rifting that formed the Atlantic Ocean. Specifically, the cratons that compose the modern African continent probably moved (i.e. rotated) relative to each other during the Mesozoic era. This history is in turn important for understanding the evolution of related sedimentary basins and the petroleum reserves they may hold.

To make progress towards these eventual goals, our work focused on two field expeditions to remote areas of the west African Sahel and Sahara to collect samples of mafic intrusive rocks (dikes and sills) for rock magnetic, paleomagnetic and geochronological analyses. Our working hypothesis, based on the available stratigraphic information, is that all the rocks we have collected are part of the larger Central Atlantic Magmatic Province (CAMP), emplaced during a few million years near the Triassic-Jurassic boundary (approximately 200 million years ago).

Dike and sill outcrops in southeastern and northeastern Mauritania were sampled. Paleomagnetic samples were collected as drilled cores (using portable gasoline powered drills with diamond-tip bits) and oriented using Brunton (magnetic) and Sun compasses. Geochronological samples were collected as unoriented hand samples from each paleomagnetic site.

All subsequent rock magnetic and paleomagnetic studies were conducted in the paleomagnetic laboratories at the University of Rochester, as part of larger programs of undergraduate and graduate education integrated with the research.

Magnetic susceptibility data, collected with a KLY-4S Kappabridge (with low and high temperature capability) reveal Curie temperatures of 570-580 °C, consistent with the presence of end-member magnetite. Some data also show an inflection at approximately 120 K, typical of the cubic to monoclinic transition in magnetite known as the Verwey transition. The presence of nearly pure magnetite in some samples suggests high temperature oxidation upon initial cooling, creating small magnetite grains capable of acting as high-resolution recorders of the ancient magnetic field. Textures characteristic of high temperature oxidation were subsequently identified in reflected light microscope studies. Magnetic hysteresis data (measured using a Princeton Measurements Corporation Alternating Gradient Force Magnetometer), recorded as individual loops and first order reversal curve (FORC) diagrams, indicate a range of magnetic domain states (from single domain to multidomain), suggesting the need for careful demagnetization procedures to remove potential secondary viscous magnetizations.

Detailed stepwise alternating field and thermal demagnetization data (demagnetized with a Sapphire Instruments SI-4 Alternating Field Demagnetization Device or an ASC Model TD-48 thermal demagnetization device, with remanence directions measured using either a Geofyzika JR-5A high speed spinner magnetometer or a 2G 755R DC SQUID magnetometer) reveal linear decay to the origin of orthogonal vector plots after the removal of a low coercivity, low unblocking temperature, overprint.

Preliminary analyses of magnetic directions appear consistent with magnetic directions reported from other areas of northwestern Africa affected by CAMP magmatism. Confirmation of this interpretation, however, must await results of on-going paleomagnetic and geochronological analyses.