Tracking ancient magmatism and Cenozoic topographic growth within the Northern Andes forearc: Constraints from detrital U-Pb zircon ages

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A comparison of detrital zircon ages from the Cenozoic forearc basins in SW Ecuador−NW Peru and those derived from the Amotapes Massif offers a singular opportunity to shed some light on the evolution of the NW Andean margin of South America. An extensive set of pre-Andean U-Pb zircon ages (older than Late Cretaceous) was obtained from 18 detrital samples coming from the Talara, Zorritos, Progreso, and Gulf of Guayaquil−Tumbes forearc basins, as well as from two metasedimentary rocks of the Amotapes Massif. U-Pb age analysis on detrital zircon grains allows a general source-to-sink scenario to be determined for this section of the Andes. These analyses give consistent insights into the early topographic growth during the modern Andean orogeny, the occurrence of proto-Andean magmatic periods in the western margin of Gondwana, and the evolution of the Amotapes Massif within the geodynamical setting of northwestern South America. Zircon ages at ca. 320 Ma and Neoproterozoic clusters for the Amotapes metasediments show significant similarities along the strike of the metamorphic belt, thus defining a common metasedimentary origin, and can be compared with previously dated rocks from the Eastern Cordillera of Peru. These strong similarities demonstrate the presence of a wide, polyphased metamorphic belt that includes the western parts of the Eastern Cordillera, the northern section of the Occidental Cordillera of Peru, and the Amotapes Massif. Detrital sediments of the forearc are marked by a major gap in Early Cretaceous−Jurassic ages and the minor presence of Ordovician (Famatinian orogeny) and Carboniferous−Permian (340−285 Ma) ages. The age spectrum is not compatible with sediment input from the eastern Andes and suggests that the external areas of the Andean chain (e.g., the Western Cordillera) were already uplifted prior to or immediately after forearc initiation during the Paleocene, thus representing a pervasive drainage divide throughout all of the Cenozoic. The Amotapes Massif or other terrains sharing the same age affinities (e.g., the Olmos terrane; Occidental Cordillera of Peru) are likely to represent the best potential sources for pre-Andean zircon grains in the forearc. Age clusters in both source and sink at ca. 600 Ma, 570−560 Ma, and 520 Ma most likely define magmatic periods and may represent first-order constraints for the difficult correlation between proto-Andean magmatic and orogenic periods to the north and south of the Huancabamba deflection. It is unlikely that the Neoproterozoic zircon grains came from the Pan African−Braziliano belts, thus suggesting the onset of significant magmatic activity in this section of the Andes at around 570−560 Ma and probably ca. 600 Ma. The 520 Ma cluster is compatible with the most prominent magmatic period in the southern Andes related to the end of the Pampean orogeny, thus suggesting the existence of a regional proto-Andean magmatic arc during Early Cambrian times.