Late Precambrian to Early Mesozoic tectonic evolution of the Colombian Andes, based on new geochronological geochemical and isotopic data.

Abstract

⁴⁰Ar/³⁹Ar and U/Pb geochronology of the basement rocks in the Colombian Andes confirm the presence of the Grenvillian age high metamorphic grade belt . The Grenvillian, or locally known as Nickerie-Orinoquiense orogenic belt, is exposed within basements uplifts along the Eastern Cordillera of Colombia and the Sierra Nevada of Santa Marta in the Caribbean coast. Rare Earth element geochemistry and petrology indicate that the Nickerie-Orinoquiense basement rock's protoliths are dominantly of continental affinity, now consisting mainly of metapsammites, metavolcanics and metaplutonic rocks metamorphosed to granulite facies PT conditions. Nd crustal residence ages and U/Pb zircon data indicate variable involvement of 'older' Late Archean - Early Proterozoic components and 'younger' ~ 1.1 Ga additions, which were tectonically mixed during the Nickerie-Orinoquian collisional metamorphic episode. Low metamorphic grade rocks that overlie the Nickerie-Orinoquian basement are exposed along the Eastern Cordillera of Colombia at the Quetame-Floresta-Santander massifs, Périja Range and Merida Andes. A U/Pb zircon age obtained from a synkinematic pluton structurally concordant with the low metamorphic grade belt from the Santander Massif, yielded a 477 ± 16 Ma, indicating a Mid-Ordovician regional greenschist to amphibolite facies metamorphic event for these rocks. The latter is referred-to as the 'Caparonensis Orogeny' in the Venezuelan Andes. Rare Earth Element geochemistry and petrologic data indicate that the low metamorphic grade belt consists of a thick supracrustal sequence i.e. metapelitic-metapsammitic sequence with minor crosscutting mafic dikes. Additional trace element discrimination plots indicate that the Caparonensis synkinematic plutons are of continental arc affinity. ⁴⁰Ar/ ³⁹Ar geochronology, petrology and field observations in Santander Massif, indicate a widespread regional metamorphic overprint took place in Late Triassic-Early Jurassic time. This event was the result of a thermal welt associated with back-arc extension and concomitant intrusion of a high volume of calk-alkalic plutons. Deposition of a thick molassic sequence (2000-4000 m) followed, flanking the uplifted region. The lower Paleozoic metamorphic rocks were elevated from greenschist to sillimanite (locally kyanite) PT metamorphic conditions and the Mid-Upper Paleozoic sedimentary cover was locally metamorphosed from greenschist to lower PT metamorphic conditions, as a function of relative distance to the plutonic centers at time of metamorphism.