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Sedimentitas marinas del Neógeno en la bahía de Tumaco, Nariño
New Contributions to the Knowledge of the Chocó–Panamá Arc in Colombia, Including a New Segment South of Colombia
By Gilberto ZAPATA–GARCÍA and Gabriel RODRÍGUEZ–GARCÍA
Geological mapping, petrographic, lithogeochemical and geochronologic analyses show that the volcanic rocks and plutons that constitute the Chocó–Panamá Arc are segmented into three sectors: North (Santa Cecilia–La Equis Complex and Acandí Batholith); Central (Santa Cecilia–La Equis Complex and Mandé Batholith); and South (Timbiquí Formation and Napi Tonalite). In this study we include the South segment, which was not considered part of the Chocó–Panamá Arc in previous studies. This magmatism occurred during the Eocene (Ypresian Stage). The rocks of the Santa Cecilia–La Equis Complex and the Timbiquí Formation are constituted of basalts and andesites interbedded with pyroclastic facies of tuffs and agglomerates. These are subalkaline, and plot in the tholeiitic – normal calc–alkaline series. The Acandí Batholith presents two facies: the main facies is a tonalite–granodiorite–quartz diorite, and the secondary is a gabbro–diorite, located towards the edges of the intrusive. Both are metaluminous subalkaline, low potassium tholeiitic and medium K calc–alkaline. The Mandé Batholith is constituted by tonalite and quartz diorite, with porphyritic facies near the Santa Cecilia–La Equis Complex. The rocks are subalkaline within the tholeiitic series. The Chocó–Panamá Arc in the Central segment is limited by the Cenozoic sediments of the Atrato River, on the west, and by tectonic boundaries such as the Dabeiba–Pueblo Rico and the Amurrapá Faults, on the east, which separate this block from the accreted oceanic sequences with oceanic plateau geochemical affinities. The South segment is a tectonic block in contact with the Dagua Structural Complex, bound by the Las Aguilas Fault. According to the ages obtained in this study, and by other authors using K–Ar, Ar–Ar and U–Pb dating, the Chocó–Panama Arc magmatism in Colombia occurred between 56.5 Ma and 37 Ma.
Keywords: northwest South America, northwest Cordillera, lithogeochemistry, geochronology.