Geochemistry of the dolerite dyke complex of the Alpujarride Complex (Betic Cordilleras, Spain): insights on the extensional collapse of the chain

 

The Betic Cordilleras (south of Spain) represent a collisional orogen disaggregated by extensional collapse in a continuous convergent setting between the Iberia and Africa plates during Miocene time. In this context, some of the nappes that conform the Alpujarride Complex of the Internal Zones of the chain (Los Reales nappe and Benamocarra Unit; Cuevas et al., 2001) are intruded by a dolerite dyke swarm of Oligocene age (Esteban et al., 2013) providing an excellent example for studying the products related to the extensional collapse.

Petrographically, the studied dykes display ophitic texture defined mainly by plagioclase and pyroxene. Despite visible alteration, the low loss on ignition values and the chemical index of alteration suggests minimal post-magmatic modification of chemical composition. Geochemically, the dykes are consistently classified as tholeiitic andesite-basalts. Chondrite C1-normalized patterns display gently sloping rare earth element (REE) profiles, with a slight enrichment in light REEs (LREEs), flat distribution of heavy REEs (HREEs) and minor negative or positive Eu anomaly. In N-MORB normalized patterns, they show significant enrichment in large-ion lithophile elements (LILEs) relative to high-field-strength elements (HFSEs) along with negative Nb anomaly. These facts denote a signature intermediate between that of N- and Transitional-MORB, with influences from continental crust indicative of a subduction-related tectonic environment. REE ratios further reveal some characteristic of the mantle source. Notably, low Sm/Yb and Tb/Yb, among others, indicate that the dykes likely originated from a spinel-bearing peridotite, that is, a garnet- and plagioclase-free mantle source. Also, inter-element relationships of Lu/Hf, La/Sm, La/Yb, Ba/La and Th/Th ratios imply that the lithospheric mantle was probably metasomatized by slab derived hydrous fluids rather than by sediment components. Tectonic discrimination diagrams, though sometimes controversial, point to the origin of the dykes in a context of back-arc basalts (BAB) or a transition zone between BAB and island arc tholeiites (IAT).

In conclusion, based on the available data, we infer that the dolerite dykes of the Alpujarride Complex, classified as tholeiitic basaltic andesites, originated from a depleted, spinel-bearing mantle source. This would have been metasomatized by fluids derived from the subducting slab during Alpine orogeny in a back-arc tectonic setting to produce the parental liquids with the observed N or Transitional-MORB compositions.

 

Cuevas, J., Navarro-Vilá, F. & Tubía, J.M (2001). Evolución estructural poliorogénica del Complejo Maláguide (Cordilleras Béticas). Boletín Geológico y Minero, 112, 47-58.

Esteban, J.J., Tubía, J.M., Cuevas, J., Seward, D., Larionov, A., Sergeev, S., Navarro-Vilá, F. (2013). Insights into extensional events in the Betic Cordilleras, southern Spain: New fission-track and U-Pb SHRIMP analyses. Tectonophysics, 603, 179-188.