Permo–Triassic metamorphism in the Mérida Andes, Venezuela: new insights from geochronology, O‑isotopes, and geothermobarometry

摘要

Although there is a consensus about Pangea assemblage in northwestern Gondwana spanning from the Late Carboniferous to the Early Permian, the tectonics of the Permo-Triassic period, including the onset of Pangea breakup, is still controversial. In this context, three regional tectonic features need to be considered: (1) The Ouachita-Marathon-Sonora suture, (2) east-dipping subduction of the Panthalassa oceanic crust beneath Gondwana, and (3) an extensional setting documented in Colombia, Ecuador, and Mexico suggesting Pangea breakup at ca. 240-220 Ma. A chemical, isotopic, and geochronological dataset is presented in this contribution to constrain the effects of these tectonic processes in the metamorphic basement of the Merida Andes in western Venezuela. U-Pb Secondary Ion Mass Spectrometry analyses on unpolished surfaces of zircons from orthogneisses yielded an average age of 251 +/- 4 Ma. The corresponding delta O-18 values suggest metamorphic recrystallization of zircon instead of high-T fluid interaction. Rb-Sr and Sm-Nd geochronology in white mica and garnet yielded ages of 234 +/- 3 Ma and 249 +/- 2 Ma, respectively, whereas a younger Rb-Sr date of 197 +/- 1 Ma was obtained from biotite. U-Pb and Sm-Nd ages constrain the latest Permian-earliest Triassic metamorphism in the Merida Andes at 251 +/- 4 Ma. Geothermobarometry data suggest amphibolite-facies peak metamorphic conditions at similar to 685 degrees C and similar to 6.0 kbar. Metamorphism might be related to post-orogenic collapse, after the collision of Gondwana and Laurentia to form Pangea. Rb-Sr ages suggest retrogression and cooling, possibly caused by thermal relaxation of a tectonically overthickened crust and onset of extensional setting, followed by Pangea breakup during the Late Triassic-Early Jurassic.