A 1 200 km-long linear magmatic belt extends along the Mediterranean
coast the Maghreb from Eastern Tunisia to Morocco. This belt is mainly
composed of Langan calc-alkaline metaluminous to peraluminous granitoids
and associated andesites/cites Central and Eastern Algeria. In Tunisia
and Oranie/Western Morocco, calc alkaline activity started later (during
the Serravallian) and was followed by the placement of alkali basalts
and basanites since the Tortonian to the Pliocene and, in me places, the
Pleistocene. Available data on the tectonic setting, petrology, age and
biochemistry of this belt show that most of its striking features, e.g.
(1) very low magma production rate, subduction-related geochemical
imprint, extensive crustal contamination the calc-alkaline magmatism and
(2) progressive magmatic change from calc-alkaline alkaline, are
consistent with magma generation during a slab breakoff process as
proposed Carminati et al, in 1998. The magmatism associated with this
breakoff started in Central Eastern Algeria at 16 Ma, then propagated
eastwards and westwards, The upward of asthenospheric enriched
plume-type mantle through the tear in the downgoing first triggered
melting of the overlying lithospheric mantle which had been
metasomatised during a previous subduction period. Heat supply from this
uprising asthenosphere may have warmed up the continental crust and made
its involvement in assimilation processes easier. As the asthenosphere
ascended through the `window' in the slab, partial melting occurred at
the uprising boundary between asthenosphere and lithosphere, generating
basalts with transitional characteristics between those of calc-alkaline
and alkaline basalts. As the asthhenospheric upwelling proceeded
partial:melting then occurred in the sole asthenospheric mantle,
producing alkali basalts. (C) 2000 Academie des sciences Editions
scientifiques et medicales Elsevier SAS.