EVOLUTION OF THE NORTHERN PROTO-ANDEAN MARGIN IN ECUADOR, BASED ON NEW ZIRCON GEOCHRONOLOGY AND WHOLE ROCK GEOCHEMISTRY

The Andes at the western margin of South America is the locus of continued plate convergence from the Early Paleozoic to the present day. However the early evolution of the proto-Andean margin remains poorly understood and relies predominantly upon whole-rock Rb-Sr and K-Ar chronology.

We have used U-Pb zircon chronology together with major, trace and REE element geochemistry of igneous rocks along the strike of the Eastern Cordillera of Ecuador to construct a detailed chronologic framework, in order to identify tectonic regimes that shaped the northern proto-Andean margin.

The analyzed crystals are colorless - transparent ranging in size from 50 to 250 µm. In CL images, 95% of the zircons exhibit oscillatory zonation, characteristic of a magmatic origin. The samples exhibit two different age groups varying between 210-250 Ma and 170-180 Ma respectively.

The plutonic rocks constitute a metaluminous to peraluminous (A/CNK ~ 0.8-1.2), calc-alkaline suite. An unimodal and wide compositional range of the intrusives (49-78 wt. % SiO₂) is characteristic of I-type orogenic suites. Mantle-normalized trace element patterns reveal typical subduction-related signature. Chondrite-normalized REE patterns do not show significant HREE fractionation suggesting the absence of high-pressure residual mineralogy in the source and formation in a “normal thickness”, garnet-free crust. Slight Eu anomalies, lowering Sr contents, and concave-up REE patterns of samples dioritic in composition indicate a model involving fractionation of plagioclase, amphibole and pyroxene from a basaltic parent.

Our data demonstrate the existence of a composite continental margin dominated by two distinctive intrusive pulses. The first is related to the assembly and breakup of Gondwana (middle Carboniferous to Late Triassic), while the second is represented by volumetric plutons emplaced during the initiation of the modern Andean cycle in the Early Jurassic. These results are consistent with the idea that during the Andean orogenesis a single microcontinent collided with the South American continental margin in Early Cretaceous time as indicated by HP metamorphism at the western rim of the Eastern Cordillera. In contrast previous models suggested the amalgamation of several terranes in the Late Mesozoic or earlier in the region of Ecuador.