Agustina Ahijado

Cenozoic volcanism II: the Canary Islands

The Canarian archipelago comprises seven main volcanic islands and several islets that form a chain extending for c. 500 km across the eastern Atlantic, with its eastern edge only 100 km from the NW African coast. The islands have had a very long volcanic history, with formations over 20 million years old cropping out in the eastern Canaries. Thus all stages of the volcanic evolution of oceanic islands, including the submarine stage as well as the deep structure of the volcanoes, can be readily observed. Rainfall and vegetation cover are relatively low, with the exception of the island of La Palma, favouring both geological observation and rock preservation. Furthermore, the absence of surface water has promoted groundwater mining by means of up to 3000 km of subhorizontal tunnels (locally known as ‘galerias’). These galerias are especially numerous in Tenerife, La Palma and El Hierro, and allow the direct observation and sampling of the deep structure of the island volcanoes without requiring expensive and indirect geophysical methods.

Crustal contamination and fluid/rock interaction in the carbonatites of Fuerteventura (Canary Islands, Spain): A C, O, H isotope study

Abstract Fuerteventura—the second largest of the Canary Islands consists of Mesozoic sediments, submarine volcanic rocks, dike swarms and plutons of the Basal Complex, and younger subaerial basaltic and trachytic series. Carbonatites are found in two Basal Complex exposures: the Betancuria Massif in the central part of the island and the Esquinzo area in the north. δ 13 C values of the carbonatites increase progressively from south to north of the island. This phenomenon is attributed to different degrees of assimilation of sedimentary carbonate. Homogeneous, typically magmatic δ 18 O values for carbonatites which have preserved primary igneous textures and minerals suggest a well-mixed reservoir where changes in δ 13 C values result from the storage of carbonate magmas at different structural levels. The magma storage allowed assimilation of sediment to varying degrees before final emplacement of carbonatites. Shifts in δ 18 O towards more positive and negative values from presumed primary compositions are observed in the carbonatites. On the basis of the oxygen isotope compositions of calcite, mica and K-feldspar, and the hydrogen isotope compositions of micas, the changes in the δ 18 O values of the carbonatites can be related to fluid/rock interactions.

Contributions to the chronology of the Basal Complex of Fuerteventura, Canary Islands

Abstract K–Ar and Ar–Ar analyses of the Basal Complex of Fuerteventura confirm the early start of magmatism in the Late Cretaceous. Plateau ages of 63.1±0.8 Ma and 64.2±1.0 Ma have been obtained for the oldest syenite intrusions indicating the minimum age of these plutonics. The emplacement of carbonatites and syenites along ductile shear zones took place during a relatively short period of time between 23.2±1.0 and 22.1±0.9 Ma. This episode is synchronous all over the island. Younger plutons were intruded around 21.1±0.8 Ma ago and 20.7±0.9 Ma and are probably related to the building of Miocene subaerial edifices. Almost all the minerals and rocks of the Basal Complex show a clear effect of excess argon. Some was acquired during the hydrothermal stage but also some excess argon was trapped during the early crystallization stage of the magma. Moreover, only a part of the radiogenic Ar which had accumulated in pre-Miocene rocks was released during the Miocene magmatism.

Shear zones as a result of intraplate tectonics in oceanic crust: the example of the Basal complex of fuerteventura (Canary Islands)

Ductile shear zones affecting igneous rocks (pyroxenites, syenites, gabbros and carbonatites) belonging to the Basal Complex of the Island of Fuerteventura are described here. They develop typical mylonites with their cortege of structures and crystallographic fabrics. An evolution from brittle to ductile conditions has been recognized, and interpreted as being due to the ascent of alkaline magmas and related fluids and subsequent emplacement of weak rocks (carbonatites, syenites) along the shear zones. Local metamorphic changes favour the ductile behaviour via a reaction-enhanced softening mechanism. The shear zones are arranged in a nearly orthorhombic pattern. Kinematic criteria suggest an extensional tectonic scenario, with a bulk irrotational nonplane deformation. The long axis of the finite deformation ellipsoid is E-W and horizontal. The available data allow us to establish this deformation as being from the Late Oligocene to the Early Miocene. This is tentatively related to the plate-tectonics evolution and mantle anomalies in the northwestern corner of the African plate.

The dyke swarms of the Amanay Massif, Fuerteventura, Canary Islands (Spain) ☆

Abstract The sheeted dyke swarms of the Amanay Massif in Fuerteventura exhibit variable trends but generally strike N45°E (main) and N110° (subsidiary). On the basis of petrographic studies, the dykes are classified as ankaramitic basalts, trachybasalts (hawaiites) and lamprophyres (camptonites). This compositional variability is attributed to different extents of partial melting of the mantle and to differences in the degree of fractional crystallization. We present a detailed account of field characteristics, mineralogy and petrography of the sheeted dyke complex that intruded the sedimentary sequence, plutonic and submarine volcanic formations.

The submarine volcanic succession of the basal complex of Fuerteventura, Canary Islands: A model of submarine growth and emergence of tectonic volcanic islands

Three lithostratigraphic units have been distinguished in the volcanic succession of the basal complex of Fuerteventura Island. These units are, from bottom to top: the submarine volcanic group, the transitional volcanic group, and the subaerial volcanic group. These three groups record the submarine growth and emergence of the island. The volcanism is represented by ultra-alkaline and strongly alkaline igneous series. The igneous activity was due to the presence of an anomalous zone in the sublithospheric mantle, the low density of which also caused uplift of the Mesozoic oceanic crust. Two extensional phases and an intervening contractional phase developed coeval to the generation of the volcanic succession. The submarine volcanic group was deposited in the hanging wall basin of a large listric extensional detachment directed toward the SSW. The transitional volcanic group was syntectonic with respect to a late inversion of the listric detachment. Finally, the subaerial volcanic group resulted from a second episode of WNW extension. This study of the evolution of the basal complex of Fuerteventura serves as the basis for a tectonic model of submarine growth and emergence of volcanic islands.

Plume-related stable isotope compositions and fluid–rock interaction processes in the Basal Complex of La Palma, Canary Islands, Spain

Abstract Chemical and isotopic compositions of amphiboles, biotites, pyroxenes and feldspars from gabbros and basalts of La Palma, Canary Islands, were studied to determine primary, plume-related compositions and effects of late-stage water–rock interactions. All the studied amphiboles have Sr isotope ratios close to those typical for the mantle, excluding the possibility of significant seawater influence. The pyroxenes and amphiboles also have stable isotope compositions that are typical for mantle-derived phases, whereas biotites and feldspars show signs of interaction with meteoric water. On the basis of the oxygen isotopic compositions, the infiltrating meteoric water derived from precipitation at an approximate elevation of 3500 m above sea level, indicating that La Palma reached this height when the gabbro complexes were formed. The unaltered hydrogen and oxygen isotope compositions of amphiboles show a trend from normal mantle ranges to −90‰ and 5.1‰, respectively; these values are very close to compositions found in other Canary Island complexes by earlier studies, and support the theory that these compositions reflect a plume component originating from depth, rather than local phenomena.