Oceanic plateau and island arcs of southwestern Ecuador:: their place in the geodynamic evolution of northwestern South America

Coastal Ecuador is made up of an oceanic igneous basement overlain by Upper Cretaceous to Lower Paleocene (approximate to 98-60 Ma) volcaniclastic and volcanic rocks of island-are affinities. The igneous basement, known as the Pi (n) over tilde on Formation, locally dated at 123 Ma, consists of olivine-free basalts and dolerites. Relative to N-MORB, both types of rocks exhibit high concentrations in Nb (0.3-10.75 ppm), Ta (0.03-0.67 ppm), Th (0.11-1.44 ppm), light and medium rare earth elements, and low Zr (22-105 ppm) and Hf (0.59-2.8 ppm) contents, thus showing oceanic plateau basalts affinities. Most of these oceanic plateau basalts tholeiites display rather homogeneous epsilon(Nd(T = 123 Ma)) ratios (similar to + 7), with the exception of two rocks with higher (+ 10) and lower (+ 4.5) epsilon(Nd(T =123 Ma)), respectively. All these basalts plot, with one exception, within the ocean island basalts field. Their (Sr-87/Sr-86)(i) ratios are highly variable (0.7032-0.7048), probably due to hydrothermal oceanic alteration or assimilation of altered oceanic crust. The rocks of the Pi (n) over tilde on Formation are geochemically similar to the oceanic plateau tholeiites from Nauru and Ontong Java Plateaus and to the Upper Cretaceous (92-88 Ma) Caribbean Oceanic Plateau lavas. The basalts and dolerites of the Upper Cretaceous-Lower Paleocene island arcs show calc-alkaline affinities. The epsilon(Nd) ratios (+6.1 to +7.1) of these are-rocks are very homogenous and fall within the range of intra-oceanic island-are lavas. The Upper Cretaceous-Lower Paleocene calc-alkaline and tholeiitic rocks from coastal Ecuador share similar high epsilon(Nd) ratios to Cretaceous intra-oceanic are rocks from north, central and South America and from the Greater Antilles. Since the Pi (n) over tilde on oceanic plateau tholeiites are locally overlain by early-late Cretaceous sediments (similar to 98-83 Ma) and yielded locally an Early Cretaceous age, they do not belong to the Late Cretaceous Caribbean Oceanic Plateau. The basement of coastal Ecuador is interpreted as an accreted fragment of an overthickened and buoyant oceanic plateau. The different tectonic units of coastal Ecuador cannot be easily correlated with those of western Colombia, excepted the Late Cretaceous San Lorenzo and Ricaurte island arcs. It is suggested that northwestern South America consists of longitudinally discontinuous terranes, built by repeated accretionary events and significant longitudinal displacement of these terranes. (C) 1999 Elsevier Science B.V. All rights reserved.