Thermochronology and tectonics of the Mérida Andes and the Santander Massif, NW South America

Abstract

New apatite U–Pb and multiphase 40Ar/39Ar data constrain the high to medium temperature (~500°C–~300°C) thermal histories of igneous and metamorphic rocks exposed in the Mérida Andes of Venezuela, and new apatite and zircon fission track data constrain the ~500°C–~60°C thermal histories of pre-Jurassic igneous and metamorphic rocks of the adjacent Santander Massif of Colombia. Computed thermal history envelopes using apatite U–Pb dates and grain size information from an Early Palaeozoic granodiorite in the Mérida Andes suggest that it cooled from >500°C to <350°C between ~266Ma and ~225Ma. Late Permian to Triassic cooling is also recorded in Early Palaeozoic granitoids and metasedimentary rocks in the Mérida Andes by numerous new muscovite and biotite 40Ar/39Ar plateau dates spanning 257.1±1.0Ma to 205.1±0.8Ma. This episode of cooling is not recognised in the Santander Massif, where 40Ar/39Ar data suggest that some Early Palaeozoic rocks cooled below ~320°C in the Early Palaeozoic. However, most data from pre-Jurassic rocks reveal a regional heat pulse at ~200Ma during the intrusion of numerous shallow granitoids, resulting in temperatures in excess of ~520°C, obscuring late Palaeozoic histories.The generally accepted timing of amalgamation of Pangaea along the Ouachita–Marathon suture pre-dates Late Permian to Triassic cooling recorded in basement rocks of the Mérida Andes by >30Ma, and its effect on rocks preserved in north-western South America is unknown. We interpret late Permian to Triassic cooling in the Mérida Andes to be driven by exhumation. Previous studies have suggested that a short phase of shortening and anatexis is recorded at ~253Ma in the Maya Block, which may have been adjacent to the basement rocks of the Mérida Andes in the Late Permian. The coeval onset of exhumation in the Mérida Andes may be a result of increased coupling in the magmatic arc, which was located along the western margin of Pangaea. Triassic extension is documented in the Central Cordillera of Colombia and Ecuador between ~240Ma and ~215Ma, although extension at this time has not been clearly identified in the Mérida Andes or the Santander Massif. Permian to Triassic cooling is not recorded in the structurally isolated Caparo Block in the southern Mérida Andes, suggesting that it may have constituted a distinct fault block in the Triassic.New fission track data from the Santander Massif suggest that it started exhuming at ~40Ma during a period of accelerated convergence between the Nazca/Farallòn Plate and the western margin of South America. Exhumation in the Santander Massif occurred diachronously since ~18Ma in distinct fault blocks at rates of 0.5–1km/Ma, and may have been driven by east–west compression as a result of the indentation of the Panama–Chocó terrane to western Colombia.