Contemporaneous Paleogene arc-magmatism within continental and accreted oceanic arc complexes in the northwestern Andes and Panama

Abstract

The basement rocks of the northwestern Andes are composed of accreted, fault bound, slivers of the Cretaceous Caribbean Large Igneous Province (CLIP). Previous geologic mapping, geochronological and geochemical data show that Paleogene arc-like units make up part of the accreted CLIP in SW Colombia (Timbiquí Complex), NW Colombia (Santa Cecilila La Equis Complex), and at the boundary between Colombia and Panama (Acandí Region). Similar Paleogene igneous units have been reported in Ecuador (Silante and Macuchí arc), and Panama (Chagres-Bayano Arc). These arc-like rocks are commonly interpreted to have formed as part of an intra-oceanic arc before accretion to the continental margin. Alternative explanations suggest that the Paleogene igneous rocks in Ecuador were formed within a continental magmatic arc. We compare Paleogene igneous rocks in the San Blas Complex (Chagres-Bayano Arc and Acandí Region), Santa Cecilia la Equis Complex, and the SW Complex (Timbiquí Complex, Silante and Macuchí arcs) using geochemical (major and trace elements), geochronological (zircon U-Pb) and isotopic (zircon Hf, and whole rock Pb, Sr, Nd isotopes) data. Zircon U-Pb analyses yield ages of 46–42 Ma for the Acandí Region, 59–51 Ma for the Santa Cecilia La Equis Complex, and 44–42 Ma for the Timbiquí Complex. Major and trace element data show low-K to calc-alkaline compositions with negative Nb anomaly and LREE/HREE enrichment typically attributed to arc-like signatures, for the three complexes. Pb, Sr, and Nd whole rock and zircon Lu-Hf isotopic data for the Santa Cecilia La Equis and the San Blas complexes, are consistent with a depleted mantle source mixed with Pacific sediments. However, the presence of inherited Proterozoic zircons, elevated 207Pb/204Pb ratios in some evolved igneous rocks, and crystallization under low pressure conditions, suggest assimilation of continental-derived sediments. These data indicate that the Santa Cecilia La Equis and the San Blas complexes likely originated in an intra-oceanic island arc setting, in a close proximity to the South American margin before accretion to the Andean margin. In the case of the SW Complex, the absence of Eu anomaly, slightly depleted HREE, as well as increased Sr/Y values with fractionation, indicate that these magmas formed in a moderately mature arc within a mid-shallow crust level. Moreover, radiogenic Pb and Sr isotopic values for the less evolved igneous rocks of the SW Complex evolving to less radiogenic values suggest involvement of continental sediments in the parental magma source, and assimilation of the mafic basement crust. This indicates that the Paleogene magmatic rocks of the SW Complex originated in a continental arc-like setting within previously accreted slivers of the CLIP.