The Sidi El Hemissi Triassic “ophites” (Souk Ahras, NE Algeria): petrology, geochemistry, and petrogenesis

International audienceThe Sidi El Hemissi region, Souk-Ahras, is part of the Tellian Atlas where the Triassic sediments tectonically outcrop under the Tellian nappes of the Maghrebide chain. Mafic rocks, mainly gabbros and dolerites, called “ophites,” are interbedded as a ~ 200 × 30-m lenticular body within the Triassic gypsum-rich formation. These rocks show granular, microgranular, and microlithic textures and are composed of plagioclase, amphibole, pyroxene, and scarce olivine crystals. Albitization is the main alteration process, though chloritization, calcitization, and epidotization of ferromagnesian minerals can also be occasionally observed. The major, trace, and rare earth element studies show that these mafic rocks display relatively low P2O5 (less than 0.2 wt%) and moderate to low TiO2 contents (less than 2 wt%) and exhibit low-Ti continental tholeiitic basalt affinity. They are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) compared with high field strength elements (HFSE) and heavy rare earth elements (HREE). This, along with the observed weak Nb and Ce anomalies and the low-Ti contents, suggests an enriched mantle source for the generation of the magma, which likely underwent crustal contamination before emplacement within the Triassic sediments.The petrological and geochemical features of the Sidi El Hemissi ophites show many similarities with the basaltic rocks emplaced during the Late Triassic–Early Jurassic times, now cropping out in north-western Africa, south-western Europe, north-eastern, and south-eastern America. This large magmatic activity is believed to be related to the continental rifting associated with the early stages of the Pangea breakup