The Tahamí and Anacona Terranes of the Colombian Andes: Missing Links between the South American and Mexican Gondwana Margins.

New zircon trace-element concentration data and U-Pb geochronology on gneisses from Colombia's Central Cordillera at 6°N allow for the recognition of the distinct Anacona suspect terrane separate from the known Tahamí terrane. These blocks underwent disparate Paleozoic and Mesozoic evolutions involving anatexis, S-type granite crystallization, and metamorphism. Orthogneisses from the Tahamí terrane basement have yielded a 244 ± 2 Ma mean age (n p 15), and associated migmatitic paragneisses yielded a 237 ± 2 Ma mean age (n p 11). Zircon geochemistry and textures show that the orthogneiss age represents the time of crystallization of early melts in the orogenic cycle, whereas the paragneiss age represents the time of metamorphic recrystallization of the suite. In contrast, orthogneisses from the small Anacona terrane have yielded U-Pb ages of 479⁺¹⁵/_-11 Ma (median, n p 7) and 443 ± 8 Ma (mean, n p 8) in magmatic zircon rims. The main xenocrystic zircon populations are 1265-995 and 1510-1495 Ma (no Pan- African-Brasiliano signal). The above blocks experienced common histories with other known Paleozoic-Triassic peri- Gondwana terranes. The Tahamí terrane can be correlated with blocks now occupying southern Mexico (Chiapas Massif) and the northwestern Andes (Loja and Amotape) and perhaps the late Paleozoic-Triassic components of the Marañ ón complex in Peru. These areas underwent crustal reworking during Permo-Triassic transition from arc(?) magmatism to extension on the western margin of Pangea. In contrast, the Anacona terrane represents a portion of the Ordovician magmatic belt fringing Gondwana in the early Paleozoic. Potential correlatives include the Mixteca terrane in southern Mexico and the early Paleozoic component of the Marañ ón complex of Peru. The above correlations suggest that terranes in the Central Cordillera of Colombia, Central America, and southern Mexico may have occupied Gondwanan positions as far south as Ecuador and Peru. This southerly position constitutes a significant means in eliminating the problematic South America-Mexico overlap in Pangea reconstructions. [ABSTRACT FROM AUTHOR]