The evolution of the magmatism in the external zones of the Betic Cordilleras during the mesozoic

Abstract

The Mesozoic igneous rocks from the External Zones of the Betic Cordilleras extend for some 300 km along the Subbetic Zone. They are poorly differentiated basic rocks which, altogether, correspond to a transitional series containing tholeiitic and sodium-alkaline terms. They crop out as small ophite stocks and dykes intruded into middle- and upper-Triassic rocks, or as submarine flows and sills interlayered with Jurassic materials.Geological and radiometric evidence points to an upper-Triassic-Liassic age for the ophite-generating magmatism, while the fissure volcanism began locally in the early-Liassic and extended throughout the Dogger. It reached its climax in the Tithonian and ceased abruptly in the lowermost Cretaceous.The magmas that generated the two groups of rocks originated within the mantle. During the ascent through a continental crust they were contaminated by deep-crust granitoid rocks and by the assimilation of pelites from the basement. The chemical composition and fractional-crystallization differentiation trends of the ophites belong to the tholeiitic series, while those of the middle-uppermost Jurassic magmas to the sodic-alkaline series. This magmatic evolution may be attributed to a geodynamic change from distensive in the Triassic to transtensive from the middle-Jurassic onwards.