Abstract

The coast and western Cordillera of Ecuador are made of accreted oceanic terranes. separated from the continental margin by a suture zone containing tectonic slices of mafic rocks. The western Cordillera contains three distinct magmatic units. Ultramafic and mafic cumulates from the suture zone (San Juan slice) represent likely the plutonic roots of oceanic plateau basalts. The mafic cumulates are LREE-depleted and Ta and Pb enriched (primitive mantle). Their Nd and Pb isotopic compositions suggest that they derived from an enriched OIB type mantle source. Pre-Coniacian arc-tholeiites present flat REE patterns, low Pb and Th contents, and high epsilon Nd (T = 100 Ma) (+7.5 to +7.9) which are indicative of their derivation from a mantle source. These arc-tholeiites developed likely in an intra-oceanic setting. The Eocene calc-alkaline lavas differ from the arc-tholeiites because they are LREE-enriched and have lower epsilon Nd (T = 50 Ma) ratios. Their high Pb and Th contents are probably related to crustal assimilation during the magmas ascent. Their Pb isotopic compositions support involvement of subducted pelagic sediments in their genesis. These lavas represent likely the remnants of a continental calc-alkaline magmatic arc. The continental-arc setting of the Eocene lavas demonstrates that these volcanic rocks postdate the accretion of the western Cordillera, upon which they rest unconformably. Therefore, the accretion of the western Cordillera may have occurred in late Palaeocene times, as for part of the oceanic terranes of coastal Ecuador. Nevertheless, the occurrence of a collisional event during late Santonian-early Campanian times is strongly suggested by: (i) the arrival of detrital quartz on oceanic series of the western Cordillera by Campanian-Maastrichtian times. (ii) a regional unconformity locally dated as early Campanian, (iii) the arc-jump observed on coastal Ecuador in Santonian or Campanian times, and finally (iv) a thermal event recognised in the eastern Cordillera around 85-80 Ma.

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