Palaeomagnetism of basic dykes of the Sfariates ridge

with Ian Fletcher, Birger Rasmussen, Albert Wilkinson (IMC), Emanetoullah Ould Limam (OMRG) and Sergei Pisarevsky

The Geological survey of Mauretania (OMRG), in conjunction with International Mineral Consultants (IMC) and the British Geological Survey, carried out detailed Au exploration and mapping work in the Sfariates Ridge area near the border between Mauretania and Spanish Sahara.

During fieldwork, several mafic dykes were sampled for palaeomagnetic studies, which have now been analysed. Many of the samples, which were collected from various dykes of varying exposure, width and mineralogy, either yielded unstable or inconsistent remamences. A series of sample sets, one set from a single large gabbro dyke (the Agâda dyke), and one from a series of north-south oriented anastomosing dykes (the Sfariat dykes), yielded consistent results, although the limited number of sample sites on each of these bodies only provide a preliminary estimate on a palaeopole.

Samples of the dykes were prepared for mineral separation, but they appeared to be very poorly mineralised in U-bearing dateable minerals. Polished sections have been prepared for one dyke of the Sfariat set and contained sub-10 micron zirconolite, a dateable U-bearing mineral phase that crystallised from the mafic melt. We conducted analysis on this mineral on polished sections in the Sensitive High Resolution Ion Microprobe (SHRIMP II) of the Perth Consortium at Curtin University of Technology. In order to be able to analyse such small mineral phases, the spatial resolution of the microprobe had to be pushed to its limits, and we managed to reduce the primary spot size to ~5-7 micron, resulting in a primary beam current of 0.2 nA. Despite this low primary beam intensity, sputtering efficiencies of Pb and U from zirconolite were high enough to result in adequate counts on the collector for meaningful results.

Secondary electron image of a zirconolite crystal that has been sampled twice using the SHRIMP II. The diameter of the sample spots is between 5 and 7 micron.

The data show relatively high amounts of non-radiogenic Pb because the analyses were conducted in situ, therefore allowing contribution to the secondary beam of matrix minerals (K-feldspar, plagioclase, clinopyroxen). At the small spatial resolution and small primary bean currents we employed, the ability to determine the exact incidence of the spot is seriously compromised. This, coupled with the relatively coarse step-range of the SHRIMP II sample stage, lead to a poor placement of the sample spot, in turn leading to the elevated common Pb amounts we observed.

Uncorrected (for common Pb) data do, however, plot along a linear array on the Pb-Pb isochron, indicating that non radiogenic Pb is derived from matrix phases alone, and was incorporated in these phases during crystallisation of the dyke.

The palaeomagnetic data, in combination with the emplacement ages for the dykes provide valuable, but preliminary, information for the position of the Reguibat schield and the West African craton at ca. 1.75 Ga.

False colour satellite image (ETM) of the Sfariat ridge, Mauretania