Permo-Triassic metamorphism in the Merida Andes, Venezuela: new insights from geochronology, O-isotopes, and geothermobarometry

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.

Automated summary

The study in the Merida Andes of Venezuela explores metamorphic processes occurring in the Late Permian to Early Triassic, potentially related to the formation and breakup of Pangea. The findings suggest peak metamorphism around 251 million years ago, indicating tectonic processes post-collision leading to retrogression and eventual extensional setting associated with the Pangea breakup during the Late Triassic to Early Jurassic.