The Naranjal shear zone and its role in the evolution of the northwestern Ecuadorian

Abstract

The presence of mafic-ultramafic lithologies and several relative high values of Cr and Ni in drainage sediments samples collected by the PRODEMINCA project inside or close to the Naranjal Shear Zone (Ecuadorian Western Cordillera) suggest that along that structure occur lithologies carrying chromium-nickel-bearing minerals. The drainage sediments samples with relative high values of Cr and Ni and rock-specimens collected close to and inside the Naranjal Shear Zone contain chromian spinels, titanomagnetite and amphiboles. The presence of primary and detrital chromite or chromian spinels and titanomagnetite has been determined by reflected light microscopy. A round 20 detrital chromian spinels from the drainage sediments samples, 27 chromian spinels and 6 amphiboles phenocrysts from the rock-specimens have been analyzed using the Electron probe microanalyzer (EPMA) and Scanning electron microscope (SEM). In addition, the chromian spinel-bearing rock was analyzed using the X-ray fluorescence spectrometry to obtain its chemical composition. The chemical compositions of the chromium spinels from the drainage sediment samples and rock-specimens, chromian spinel-bearing rock and amphibole phenocrysts indicate that they are related to metamorphosed-ultramafic rocks forming part of a dismembered ophiolite complex. In this context, the chromian spinels-bearing ultramafic rocks exposed along the Naranjal Shear Zone are the best candidates to be part of the dismembered ophiolite complex emplaced to the West of Western Cordillera of Ecuador. We propose that the sequence carrying chromian spinels may represent the suture zone between two oceanic terranes, Pallatanga-Macuchi terrane in the East (Western Cordillera) and the Piñon terrane in the West (Coastal land). However, we do no excluded the possibility that represents the suture between an island arc complex of Cretaceous age and the Pallatanga, Piñon and Macuchi terranes.