Étude pétrologique et géochronologique des pipes doléritiques de la Province Magmatique de l’Atlantique Central (CAMP) dans le bassin de Tindouf

Abstract

The presence of Mesozoic mafic rocks in southwestern Algeria has long been known. Recent studies indicate that this rock is mainly composed of tholeiitic dolerites emplaced at the end of Triassic, during the opening of the central Atlantic Ocean, 201 million years ago. These dolerites are part of a large magmatic province, called the Central Atlantic Magmatic Province (CAMP). Today, remnants of CAMP plumbing system include huge intrusive bodies (doleritic sills and dykes) and some lava flows preserved in Triassic basins. In the Tindouf Basin (Southwestern Algeria), CAMP rocks occur mainly as large sills and dykes especially in the northern flank of the basin. Doleritic pipes, that have never been studied so far, are spatially associated with these sills and dykes and thus supposed of CAMP origin. In this study, we present new field, petrographic, geochemical and geochronological data on the pipe, sills and dyke dolerites of the CAMP plumbing system of the northern flank of the Tindouf basin. The sills and dykes studied display homogeneous petrographic features, typical of CAMP tholeiites, whereas the doleritic pipes show more evolved characteristics with a hypovolcanic granophyric texture, containing alkali feldspars and quartz. Furthermore, we report for the first time the presence of abundant carbonatitic xenoliths in the pipe dolerites. Geochemically, all the investigated rocks are low-Ti tholeiites that can be subdivided into four groups, three of which correlate well with the intermediate, upper and recurrent units defined in the High Atlas of Morocco, whereas the fourth group, represented by the pipe dolerites, presents geochemical features that have never been reported before. However, their tholeiitic nature, their association in the field at the same geographic location with CAMP sills, and their trace element patterns, all indicate that pipe dolerites are of CAMP origin, representing the evolved products of Recurrent magmas. We performed 40Ar/39Ar biotite dating on four samples from the Tindouf basin (1 pipe, 1 sill of the recurrent unit and two hornfels in contact with doleritic sill). Plateau-ages of 193.7 ± 3.4 Ma, 197.6 ± 3.3 Ma, 202.5 ± 7.3 Ma and 199.8 ± 1.9 Ma was respectively obtained for the pipe, the recurrent sill and the two hornfels. Our study suggests that the small anticlines on the northern flank of the Tindouf Basin that host doleritic pipes in its cores can be explained as forced folds above igneous intrusions. Furthermore, we can explain the occurrence of carbonatitic xenoliths and sulphides in the pipe dolerites as the possible consequence of crustal assimilation of wall rocks by magma of the pipe conduits. Finally, the high CO2 and sulfur contents of the pipe dolerites, combined with the emplacement of large sills within Upper Devonian organic-rich shales of the Tindouf Basin, indicate a potentially significant emission of CO2 and sulfuric gases that may have contributed to climate change at the Triassic-Jurassic boundary.