Corte geológico transversal del arco Alao en la zona de Gualaceo, provincia Azuay

Abstract

The present work describes the rocks that outcrop along the El Descanso-Gualaceo-Rio Culebrillas section with a review of previously published geochemical and geochronological data from the Alao Arc. The study area is located 25 km east of Cuenca, in the western foothills of the Cordillera Real between the Cuenca-Macas road, in the Province of Azuay, Ecuador. Alao Arc consists of a forearc composed of a turbiditic sequence interlayered with volcanosediments (Maguazo Unit), a central part that presents basaltic to basaltic andesites breccias and lavas slightly metamorphosed with volcanoclastic sequences proximal to the arc (Alao-Paute Unit), and a backarc consisting of volcanosedimentary rocks metamorphosed from sub-greenschist to greenschist (El Pan Unit). The geochemical analyzes of Alao arc rocks indicate negative anomalies of Nb and Ti; In addition, these rocks are enriched in LREE, which is typical of volcanic arc rocks. The tectonic discrimination diagrams show a wide range of variety in the geochemical affinity of these rocks, associated with volcanic rocks that erupted through an attenuated continental crust. This Arc was probably formed in the Lower Cretaceous (120-140 Ma) through a continental crust thinned and subducted to the east; developed from the Late Jurassic to the Early Cretaceous due to the retreat of the trench to the Pacific by a roll-back phenomenon. The opening of the South Atlantic would have caused the cessation of the Arco Alao. Finally, the collision of the Caribbean large igneous province in the Upper Cretaceous gave rise to the reactivation of the pre-existing structures in a transpressive regime, deforming and metamorphosed the rocks of the Alao Arc to a greenschist facies of the Barrowian type.