Geochemistry and Petrogenesis of Lower Jurassic Mafic Rock Suites in the External Rif Belt, and Chemical Geodynamics of the Central Atlantic Magmatic Province (CAMP) in Northwest Morocco

Abstract

We present new field evidence, geochemical and isotopic data, and age constraints on Lower Jurassic mafic rock suites within a >200-km-long curvilinear belt in the Rif orogenic belt in northern Morocco and show that these rock assemblages formed as part of the Central Atlantic Magmatic Province (CAMP). The CAMP represents a large igneous province that straddles the edges of the modern peri-Atlantic continents. It developed ~200 Ma, following the initiation of the breakup of Pangea. Main magmatic rocks in the Rif External Zone include basaltic lavas, massive dolerite, and isotropic and cumulate gabbros, all intruded by dolerite and trondhjemite dikes and sills. Available U-Pb zircon ages from dolerite, gabbro, and trondhjemite dike rocks are 200±4, 196±4, and 192±Ma, respectively. Based on their geochemical affinities and isotopic compositions, the analyzed rocks define basalt-dolerite and gabbro-cumulate gabbro-trondhjemite groups. The basalt-dolerite group samples are subalkaline in nature and have low TiO2 contents, whereas the gabbro-cumulate gabbro-trondhjemite group samples are alkaline and display high TiO2 values. Most samples are tholeiitic in character and show large-ion lithophile and light rare earth element enrichment and high field strength element depletion compared with normal mid-ocean ridge basalt (MORB). Samples of both groups display low 143Nd/144Nd201 Ma (0.51182–0.51262) and high 87Sr/86Sri ratios with ɛNd values ranging from −1.51 to +4.85. The basalt-dolerite group rocks have enriched MORB compositions, compatible with the low-Ti CAMP suites, whereas the gabbro-cumulate gabbro-trondhjemite group rocks have oceanic island basalt compositions reminiscent of high-Ti CAMP suites in other continents. Magmas of the gabbro-cumulate gabbro-trondhjemite group underwent differentiation through tholeiitic fractionation. Magmas of the rocks of both groups included melt components, originated from partial melting of a previously subduction-modified subcontinental lithospheric mantle. Our results indicate that the Early Jurassic CAMP magmatism in northern Morocco marked a major episode of continental magmatism before the opening of the Maghrebian Tethys between Africa and Iberia in the latest Jurassic.