GEOCHEMISTRY OF JURASSIC VOLCANIC ROCKS OF THE NORTHERN ANDES: INSIGHTS FOR THE MESOZOIC EVOLUTION OF THE NORTHWESTERN CORNER OF THE SOUTH AMERICAN PLATE

The tectonic setting of Triassic and Jurassic limestones, red siliciclastic and volcaniclastic strata, and plutonic rocks in the northern Andes has been interpreted as: (1) a continental magmatic arc with a back-arc basin or (2) a rift related to the break-up of Pangea. This controversial interpretation is due to the lack of reliable ages, geochemical analyses, and uncertainties in paleogeographic reconstructions. The northern Andes are distinguished into a western belt that includes Triassic-Jurassic plutonic rocks and a thick volcaniclastic succession and an eastern belt that consists mostly of Jurassic and Lower Cretaceous continental sedimentary rocks. Rare-earth and trace element data as well as the isotopic composition of Sr, Nd, and Pb of rocks of the western belt were carried out to constrain the tectonic setting of these rocks.

Major and trace element analysis support previous interpretations of a calc-alkaline composition for Upper Triassic-Lower Jurassic volcaniclastic rocks. MORB-normalized spidergrams of these mafic rocks show peaks for K and Th and a marked trough for Nb, which is a typical signature for magmatic arcs. The Nd and Sr isotopic compositions fall on the continuation of the “mantle array” towards typical crustal compositions, thus indicating the addition of sediments derived from upper continental crustal rocks to the magma source. Moreover, the input of crustal material is also indicated by the Pb isotopic compositions, which are shifted towards the composition of enriched mantle reservoirs and marine sediments. Similarly, the high contents of K, Rb, Ba, and Pb indicate the involvement of crustal material in the magma source.

Our data support the hypothesis of a Mesozoic subduction-related magmatic arc, at least for the Late Triassic and Early Jurassic, which implies that the opening of the Proto-Caribbean Ocean started after arc magmatism ceased when the regional compressional setting have change to a rift-related setting.