Abstract

U-Pb data obtained from granitoids and metamorphosed gabbros from the Cordillera Real and the El Oro metamorphic complex in Ecuador elucidate the pre-Cretaceous magmatic history and subsequent metamorphic evolution of the northern Andean basement. Two major tectonothermal episodes and later thermal overprinting have affected the region, obscuring the timing of plutonic and metamorphic events as constrained by K-Ar and Rb-Sr geochronology. U-Pb zircon and monazite ages show that S-type granitoids in the Cordillera Real and El Oro complex, and mafic intrusions in El Oro, were emplaced during the Triassic. The Piedras mafic intrusions, important components of the El Oro complex that were previously considered to be Precambrian, were intruded at 221 +18/−16 Ma. The S-type Marcabeli pluton and the Limon Playa intrusion yield ages of 227.5 ± 0.8 Ma and 200 ± 30 Ma, and in the Cordillera Real, the large S-type Tres Lagunas batholith was intruded at 227.3 ± 2.2 Ma. The Marcabeli and Tres Lagunas data show that S-type granite emplacement was contemporaneous in the two belts, supporting field and geochemical evidence that the Cordillera Real and El Oro complex were contiguous in the Triassic and then separated. The granitoids constrain the age of regional shearing, metamorphism, and anatexis (Moromoro tectonothermal event) related to Gondwana–North America separation. After the Moromoro event, subduction-related calc-alkaline granite batholiths intruded the Cordillera Real. Cessation of this magmatic episode is probably constrained by the 143 ± 1 Ma Azafran batholith. Metamorphic U-Pb ages are only recorded by 78–82 Ma monazite ages from the Limon Playa granite, which are similar to many reset K-Ar ages from elsewhere in both the Cordillera Real and El Oro complex.

Recycling of material originally derived from the Amazonian craton, together with younger Cambrian–Neoproterozoic crust, is suggested by single grain inherited zircon ages of 546 Ma, 2220 Ma, and 2876 Ma from the Marcabeli pluton, together with Neoproterozoic and Mesoproterozoic upper intercept ages for Tres Lagunas and Limon Playa zircons. The zircon data are supported by 1400–1600 Ma Nd-depleted mantle model ages (TDM) for both the Tres Lagunas granite and Paleozoic migmatitic gneisses from El Oro and Cordillera Real.

Plate tectonic reconstructions supported by paleontological and isotope data indicate that paleo-Mexico and the Colombian segment of the Andes were joined prior to the breakup of Pangea. Additional evidence supporting this scenario is given by the overlap of TDM ages for granitoids and metasedimentary rocks in the Acatlan complex of Mexico and the Cordillera Real and El Oro complex; these rocks serve as a suitable proxy for related lithologies in Colombia, the Nd TDM ages of which are unknown.

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