RYSTALLIZATION AGE and MAGMA SOURCES of the AntioqueÑo and OVEJAS BATHOLITHS, CENTRAL CORDILLERA, Colombia: EVIDENCE FROM COMBINED LA-ICP-MS U–PB DATING and HF-ISOTOPE ANALYSIS of ZIRCON GRAINS and WHOLE-ROCK GEOCHEMISTRY

The Antioqueño and Ovejas batholiths (AB-OB) comprise one of the largest, most continuous granitic suites in the Colombian Andes (>8000 km²). The crystallization age, magmatic sources, magmatic evolution, and post-crystallization cooling history are poorly defined for these plutons. Accurate constraints of these parameters are critical for understanding the Mesozoic-Cenozoic crustal evolution processes and the distribution of magmatic activity in the Northern Andes. We performed laser ablation-ICP-MS U-Pb dating and Hf-isotope analyses of magmatic zircons, combined with whole-rock elemental and radiogenic isotopic analyses (Pb, Nd) from six AB-OB samples. U-Pb zircon analyses give well-defined crystallization ages between ca. 77-71, Ma, suggesting relatively rapid incremental emplacement, near the Campanian-Maastrichtian transition. Published thermochronology indicates rapid post-crystallization cooling of ~800 to ~300°C from ~77 to 60 Ma, i.e., ~53 °C/Ma, which is consistent with shallow emplacement into cold, Paleozoic to Precambrian continental crust. Major elements show variations in composition from diorite to granite suggesting differentiation and possibly crustal contamination (medium- to high-K calc-alkaline composition). Pb and Nd isotope signatures indicate mixing of magma from mantle and crustal reservoirs or assimilation. εHf values ranging from -3 to +5 indicate magma sources were a mixture of subduction generated mantle magma and partial melts of older continental crust. Such signatures are consistent with other plutons in the Andean Northern Volcanic Zone (NVZ). Values for εNd and εHf are positively correlated considered also an indication of mixing of mantle and crustal components. Trace element ratios, REE patterns, and isotopes are consistent with a supra-subduction zone setting and show fractionation and assimilation typical of calc-alkaline continental margin magmatic arcs of the NVZ. These results are consistent with a recognized Mid Cretaceous pulse of magmatism in the Andes, which was associated with a period of high angle relatively rapid convergence.