Geochronological, geochemical and isotopic characterisation of the Early Cretaceous Andean margin within Ecuador: The Alao Arc

Abstract

Tectonic models of the Early Cretaceous Andean margin in Ecuador are hindered by a paucity of accurate crystallisation ages of igneous rocks. The Alao Arc, situated on the western flank of the Cordillera Real in Ecuador, is mainly composed of submarine altered basalts to andesites, and the most deformed areas are frequently modified by greenschist metamorphism. To improve our knowledge of the evolution of the northern Andes prior to the accretion of the Caribbean Large Igneous Province we attempt to constrain the tectonic origin and crystallisation age of the arc's volcanic rocks, using whole-rock geochemistry and isotopic tracing, combined with 40Ar/39Ar geochronology. Major oxides and trace element abundances and Sr-Nd-Pb compositions show that the Alao Arc comprises two separate assemblages, namely, typical arc-like volcanic rocks and E-MORB-like basalts. These have been interpreted to have formed in a continental arc above thinned continental crust in an extensional setting, and the E-MORB-type basalts are considered to have formed in an intra-arc rift. As the Alao Arc's volcanic rocks are devoid of zircon and baddeleyite we attempted to date the time of their crystallisation using 40Ar/39Ar analyses of whole-rock groundmass and plagioclase. However, as the igneous rocks are modified by varying degrees of alteration with the formation of secondary K- and Ca-bearing phases, none of the ∼75 Ma to ∼16 Ma 40Ar/39Ar dates record the age of their crystallisation. The plateau and weighted mean 40Ar/39Ar date populations define peaks at ∼75–60 Ma and ∼ 45–30 Ma that coincide with previously recognised tectonic events that drove fluid circulation, modifying the Ar isotopic compositions. Based on the maximum depositional ages of intercalated sedimentary rocks, the degree of deformation, geochemical and isotopic trends of the Jurassic arc in Ecuador, and correlations with the Early Cretaceous Quebradagrande Arc of Colombia, we propose that the Alao Arc formed during ∼140–110 Ma.