Pre-collision Cretaceous–lower Tertiary volcanic rocks of western Ecuador from the coastal area (Piñón and San Lorenzo Formations) and Western Cordillera (Macuchi and Celica Formations) consist of three rock suites. The first suite includes tholeiites of ocean-floor affinities that have distinct light REE-depleted patterns and low La/Nb, La/Hf, and Th/Hf ratios. The island-arc tholeiites of the second suite have lower contents of Ti, Zr, Hf, and Nb. The third suite comprises calc-alkalic rocks, mainly continental-margin andesites and dacites. The continental volcanic-arc rocks crop out in an area east of the tholeiitic rocks. The boundary between the two areas is the major suture separating continental South America from accreted terrenes to the west.

A model is suggested for the geodynamic evolution of what is now western Ecuador during the Late Cretaceous. It includes two east-dipping subduction zones of Turonian to Santonian ages and the accretion of the oceanic island arc to the continent.

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