Palaeomagnetic investigation of Middle Devonian limestones of Algeria and the Gondwana reconstruction

Abstract

SUMMARY We have carried out a palaeomagnetic investigation of the upper middle Devonian marly limestone of the Hazzel Matti Formation outcropping at Meredoua in the northern Ahaggar in the Sahara Desert. the Givetian age of the formation is well constrained by palaeontological arguments. the formation has been deformed later than the Moscovian (upper middle Carboniferous). In the altered yellowish samples and in the heterogeneous samples made of mixed fresh and altered zones, several obviously secondary magnetization components are present. In the freshest bluegrey samples a SE down-dipping component can be isolated, particularly when a combined thermal and AF treatment is applied. This component, observed in 53 samples from three sites, passes the fold test of McFadden & Lowes (1981) and McFadden & Jones (1981) at the 95 per cent probability level. the mean direction for the three sites after tilt correction is: Ds= 118.9°, Is, = 34.2°, k = 378, α-95 = 6.3°, which corresponds to a south pole (MER) situated at Op=61.7°, λp= -16.2°, A95= 4.20 and places the site area under a palaeolatitude of -18.8°. However, the best grouping of the directions is achieved after 85 per cent of unfolding using a statistical method based on a bivariate extension of the Fisher's statistics (Legoff 1990; Legoff, Henry & Daly 1992). Such a tilt correction changes the pole position by 1.5°. As the α95 and K curves obtained from stepwise unfolding display a broad minimum and maximum value respectively, and because there was probably slight original syn-sedimentary dips, it is likely that 85 per cent of tilt correction is not significantly different from complete unfolding. The magnetic behaviour of the samples upon thermal and AF demagnetization, the evolution of the initial susceptibility upon thermal treatment, hysteresis cycles and investigation of the magnetic mineralogy through microscope examination and electron microprobe analyses lead to the following conclusion: greigite was probably a primary magnetic mineral formed during the diagenesis of the rock. However, its primary magnetization has been totally replaced by a VRM, except perhaps in rare samples in which the SE component can be found in the narrow range of 300-350°C. Magnetite is thought to carry the SE magnetization component and may have formed early during the diagenesis or soon after greigite. To consider the fold test negative would assign the MER pole a post-Moscovian age and place it in a situation inconsistent with the other poles of the West Gondwanian APWP, unless all the middle Carboniferous and probably lower Permian African poles are rejuvenated. We rather favour the interpretation of a positive fold test, which however does not constrain the magnetization to be of primary Givetian age; the SE component may as well have been acquired any time between the Givetian and at least the late Moscovian. Assuming that the MER pole has a Givetian age implies that the Rheic Ocean, which separated Laurussia from Gondwana in the early Palaeozoic, was almost closed in the middle Devonian, in agreement with palaeobiogeographic arguments; but it increases the discrepancy between the APWP of respectively West and East Gondwana. an intermediate lower Carboniferous age might be in better agreement with both African and Australian data. Clearly, however, more Gondwanian poles are needed to test the reliability of both West and East Gondwanian APWP, and check simultaneously the possibility of relative displacements between these two blocks during the Devono-Carboniferous time period.