Petrology and geotectonic development of the Western Ecuadorian Andes: the Basic Igneous Complex

Abstract

The Basic Igneous Complex (BIC) consists of several volcanic formations which form the Ecuadorian Western Cordillera and Coastal Cordillera. The rocks differ significantly from other Cordillerian successions and form an isolated tectnic block. The Upper Cretaceous to Lower Tertiary volcanic series of the Ecuadorian Basic Igneous Complex are of island-arc type according to their petrography and geochemistry. An outer tholeiitic island-arc series (parts of the Piñón Formation) can be distinguished from a central calc-alkalic series, depleted in potassium (Macuchi, Celica, and Silante Formations). The Chontal, Yunguilla, and Zapotillo Formations as well as the upper Cayo Formation (“Guayaquil cherts”) are interpreted to represent back-arc sediments and sediments of small intra-arc basins within a largely submarine island arc. The Cayo Formation consists almost entirely of volcaniclastic and epiclastic rocks deposited on the oceanward side of the island arc, which was situated close to the South American continent. Volcanic activity continued into early Eocene times as revealed by K/Ar-dating of the Cayo and Silante Formations (52 Ma). A small back-arc basin was filled with the sediments of the Yunguilla Formation. Similar to present-day volcanically active island arcs (e.g., Aleutians and Kuriles-Kamchatka) the BIC island arc was mostly underlain by oceanic crust, although parts of it built on continental crust (Celica Formation, southern Ecuador). In the Early Tertiary the island arc was accreted to the South American continent. Cessation of volcanic activity and accretion to the continent may be explained by increased drift velocities of the lithospheric plates accompanied by a change in the geometry and stress in the subduction zone.