Petrography and geochemistry of magmatic units from the western Cordilleraof Ecuador (0 degrees 30 ' S): tectonic implications

The coast and western Cordillera of Ecuador are made of accreted oceanic te rranes, separated from the continental margin by a suture zone containing t ectonic slices of mafic rocks. The western Cordillera contains three distin ct magmatic units. Ultramafic and mafic cumulates from the suture zone (San Juan slice) repres ent likely the plutonic roots of oceanic plateau basalts. The mafic cumulat es OIB type mantle source. Pre-Coniacian arc-tholeiites present flat REE patterns, low Pb and Th conte nts, and high epsilon(Nd(T=100Ma)) (+7.5 to +7.9) which are indicative of t heir 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=500Ma)) ratios. Their high Pb and Th contents are probably related to crustal assimilation during the ma gmas ascent. Their Pb isotopic compositions support involvement of subducte d pelagic sediments in their genesis. These lavas represent likely the remn ants of a continental calc-alkaline magmatic are. The continental-are setting of the Eocene lavas demonstrates that these vol canic rocks postdate the accretion of. the western Cordiliera, upon which t hey rest unconformably. Therefore, the accretion of the western Cordillera may have occurred in late Palaeocene times, as for part of the oceanic terr anes of coastal Ecuador. Nevertheless, the occurrence of a collisional even t during late Santonian-early Campanian times is strongly suggested by : (i ) the arrival of detrital quartz on oceanic series of the western Cordiller a by Campanian-Maastrichtian times, (ii) a regional unconformity locally da ted as early Campanian, (iii) the are-jump observed on coastal Ecuador in S antonian or Campanian times, and finally (iv) a thermal event recognised da ted as early Campanian, (iii) the arc-jump in the eastern Cordillera around 85-80 Ma.