Rift-related magmatism of the Central Atlantic magmatic province in Algarve, Southern Portugal

Lower Jurassic volcanic-sedimentary successions of the Algarve Basin (∼ 198 Ma) are associated with the Central Atlantic Magmatic Province (CAMP), that was one aspect of Pangean intracontinental rifting. Volcanic products of the Algarve CAMP include subaerial lava flows, pyroclastic deposits and peperites, and contemporaneous sedimentation is dominated by mudstones and conglomerates, often containing volcanic fragments, intercalated in the volcanic sequence. These lithological characteristics and associations are compatible with a facies model typical of continental basaltic magmatism. Volcanic rocks are fractionated low-Ti tholeiites enriched in large ion lithophile elements relative to high field strength elements. The compositional spectrum (e.g., Ba/Nb-Zr/Nb) is consistent with derivation of primary magmas by partial melting of heterogeneous sources, initially continental lithospheric mantle progressively replaced by an asthenospheric source. Fractional crystallization processes occurred in magma chambers located at depths close to the crust/mantle boundary from density and geobarometric constraints, reflecting magma underplating during the initial stages of continental rifting. Sporadic occurrence of crustal xenoliths, widespread evidence for basement carbonate assimilation, as well as Sr ((87Sr/86Sr)0 > 0.7050) and oxygen (δ18O > + 7.40‰) isotope ratios that are higher than those typical of continental lithospheric mantle, indicate that crustal contamination also contributed to the observed geochemical variations. The Carnian-Norian (228.0 to 203.6 Ma) or older age estimated for the basal Silves Sandstones suggests that the inception of rift related sedimentation preceded volcanism by 20 to 30Ma. These results lead us to propose a passive rifting model for the Algarve sector of CAMP magmatism, the orientation of which was conditioned by strain localization along pre-existing lithospheric mechanical anisotropies. © 2007.