Geochronological, geochemical and isotopic characterisation of the basement of the Chocó-Panamá Block in Colombia

Abstract

The Chocó-Panamá block is the land bridge that connects Central and South America and forms the trailing edge of the Caribbean Large Igneous Province (CLIP). New field observations are combined with new geochemical, isotopic and 40Ar/39Ar analyses of the mafic basement of the Chocó block within Colombia. These are integrated with new U/Pb ages of detrital zircons extracted from siliciclastic and carbonate rocks that unconformably overlie the mafic basement. The data are used to constrain the age, tectonic origin and evolution of the Chocó Block, which are used to improve tectonic models of the Caribbean Plate. The whole-rock chemical and isotopic compositions of the mafic lavas suggests they formed within an oceanic plateau, and are extremely similar to plateau units in Colombia, Ecuador, Panamá and Costa Rica. 40Ar/39Ar (groundmass) dates of the mafic basement are affected by early calcic and late potassic alteration, and none are accurate measurements of the time of crystallisation. A comparison of 40Ar(radiogenic), 39Ar(K) and 37Ar(Ca) reveals at least three argon reservoirs within the mafic groundmass, where at least two are a consequence of alteration. Our most accurate albeit imprecise estimate of the crystallisation age of the mafic basement is ≥86.0 ± 6.3 Ma, which is a plateau 40Ar/39Ar date from a groundmass aliquot which contains the lowest proportion of secondary Ar reservoirs. This age estimate is consistent with the few reliable crystallisation ages obtained for oceanic plateau rocks of the CLIP. U/Pb concordia dates of detrital zircons constrain the maximum depositional age of the overlying sedimentary rocks to ~28 Ma, supporting previous conclusions that the Chocó-Panamá block collided with South America during the Oligocene to Early Miocene. Detrital Precambrian – Triassic zircons in the older cover sequences are not present in younger sedimentary rocks due to surface uplift of the Serranía del Baudó, presumably due to collisional tectonics.