PERMO-TRIASSIC METAMORPHISM IN THE MÉRIDA ANDES, VENEZUELA – FINAL PANGEA ASSEMBLAGE OR CIRCUM-SUPERCONTINENT SUBDUCTION? NEW INSIGHTS FROM GEOCHRONOLOGY, O-ISOTOPES, AND GEOTHERMOBAROMETRY

Although there is a consensus about Pangea assemblage spanning from the Late Carboniferous to the Early Permian in northwestern Gondwana, the tectonics of the Late Permian-Early Triassic period, including the onset of Pangea breakup, is still controversial. In this context, three regional tectonic features need to be considered: (1) east-dipping subduction of the Panthalassa oceanic crust beneath Gondwana, (2) the collision of Laurentia and Gondwana in the Ouachita-Marathon-Sonora suture, and (3) an extensional setting documented in Colombia and Ecuador suggesting Pangea breakup at ~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 Mérida Andes in western Venezuela. U-Pb secondary ion mass spectrometry analyses on unpolished surfaces of zircon grains from gneissic rocks yielded an average age of 251.1±4.0 Ma. The corresponding δ¹⁸O values suggest metamorphic recrystallization in zircon instead of High-T fluid interaction. Furthermore, Rb-Sr and Sm-Nd thermochronology in white mica and garnet yielded ages of 243.6±9.3 Ma and 249.3±1.8 Ma, respectively. Thus, the combination of three geochronometers involving different mineral phases allows constraining, for the first time, the Latest Permian-Earliest Triassic metamorphism in the Mérida Andes at 250±3.0 Ma. A clockwise P-T path and amphibolite-facies peak metamorphic conditions at ~680 °C and ~8.0 kbar are estimated via pseudosection modeling as well as geothermobarometry in distinct mineral assemblages of pelitic paragneiss and hornblende orthogneiss. Metamorphism was possibly triggered by post-orogenic gravitational collapse after the collision of Gondwana and Laurentia to form the Pangea supercontinent, affecting Early Palaeozoic basement rocks and Carboniferous-Permian sedimentary rocks in northwestern Gondwana. After the metamorphic peak, retrogression was perhaps driven by thermal relaxation of tectonically overthickened crust in the period between ~240 Ma and ~200 Ma, in accordance with Rb-Sr cooling ages of mica.