Detrital-zircon geochronology and provenance of the El Oro Metamorphic Complex, Ecuador: Geodynamic implications for the evolution of the western Gondwana margin

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The El Oro Metamorphic Complex (EOMC) in SW Ecuador has been the subject of debate for several decades. While previous studies have focused on the metamorphic and deformation history of the complex to determine its geodynamic evolution, the pre-metamorphic history and its association to units in the north-central Andes remains poorly understood. Here we present a U-Pb detrital zircon provenance study to provide insights into the depositional history and the geodynamic setting of the EOMC. Our results imply that the southern portion of the EOMC (the Tahuín division) is composed of an older Palaeozoic (pre-Famatinian) sequence in the south (El Tigre unit; c. 525-510 Ma), and younger Palaeozoic (post-Famatinian) sediments in the central and northern parts of the Tahuín division (La Victoria and La Bocana units; c. 370-360 Ma). The provenance of the sedimentary sequences correlates with autochthonous post-Famatinian units (440–340 Ma) in the Eastern Cordillera of Peru and the southern Cordillera Real of Ecuador. Importantly, our data provide evidence for a genetic link between the northernmost (the Birón Complex) and the southern (La Victoria and La Bocana units) sequences of the EOMC. Post-Famatinian sediments were identified in the Birón Complex (Palenque Mélange division; c. 400-390 Ma) and are derived from the same sedimentary sources as the sediments of the southern EOMC. The depositional range of post-Famatinian sediments in the EOMC is therefore constrained to c. 400-360 Ma. Furthermore, we identified a Middle to Upper Triassic sedimentary sequence in the Birón Complex (Limón Playa unit; c. 240-215 Ma) that likely developed contemporaneously with c. 240–217 Ma syn-rift continental epiclastics of the Mitu Group in Peru. Combined, these new detrital zircon data demonstrate that the existing lithostratigraphic scheme is erroneous and that three temporally distinct sequences are present in the complex, with the oldest units in the south and the youngest in the north. Separation of the southern EOMC (the Tahuín division) from the autochthonous Gondwanan margin of the north-central Andes was likely due to a change in the regional subduction direction from SE to ENE at c. 140Ma, with the southern EOMC rotating in a clockwise direction into its present-day E–W orientation. This rotation also could have been a response to the first accretion of proto-Caribbean terranes, such as the Alao–Quebradagrande terrane at c. 120 Ma, with a maximum age for rotation constrained by the earliest sediments (Early (?) to Middle Albian, c. 113-105 Ma) arriving into the Celica-Lancones Basin, which unconformably overlie the Tahuín Group in the southern EOMC.