Petrogenesis of the late Miocene Combia volcanic complex, northwestern Colombian Andes: Tectonic implication of short term and compositionally heterogeneous arc magmatism

摘要

During the Miocene, the evolution of the Northern Andes was influenced by the subduction of the young Oceanic Nazca Plate and the final collision of the Panama-Choco Block (PCB) with the continental margin of Colombia. This work presents an integrated field, geochemical, Hf-Nd-Sr isotopic and geochronological study of volcanic arc front rocks of the Combia volcanic complex in the northern segment of the Cauca-Valley (North of 5 degrees N) in order to reconstruct the petrologic and tectonic significance of a short life arc with a very heterogeneous geochemical signature. The Combia volcanic complex includes a series of basalts, andesites, pyroclastic rocks, porphyritic andesitic-dacitic domes, and dacitic to andesitic porphyries, with zircon U-Pb crystallization ages between 8.5 and 5.2 Ma. Negative anomalies of Nb and Ti, enrichment in Light Rare Earth Elements (LREE) and depletion in Heavy Rare Earth Elements (HREE) are characteristic of magma origin in a convergent margin setting. This magmatism shows different tholeiitic, talc-alkaline, and adakite-like geochemical signatures, which may be related to an initial adiabatic-controlled dry melting, followed by wet melting conditions from the hydrated mantle wedge. Variations in crustal thickness between 17 and 45 km estimated by the Ce/Y, La/Yb, and Sr/Y ratios are related to local thinning of a previously thickened continental crust Such pattern can be explained by strain partitioning in which crustal thinning may be a relatively local phenomenon. Arc magmatism in this segment of the margin apparently migrates eastward from the Western Cordillera to the Cauca-Valley between 11 and 9 Ma and was followed by a magmatic quiescence after similar to 5.2 Ma. This spatio-temporal history is consistent with continuous oblique convergence of the Nazca Plate and slab flattening beneath the continental margin until it achieves its modern tectonic configuration. (C) 2019 Elsevier B.V. All rights reserved.