EARLY MIOCENE MAGMATISM ALONG THE WESTERN CORDILLERA OF COLOMBIA SOUTH OF 4 DEGREES N: ARC INITIATION AFTER FARALLON PLATE BRAKE UP?

The Western Cordillera of Colombia is made up of allochthonous crustal blocks of oceanic affinity associated with the Caribbean Large Igneous Province. South of 4°N, the Western Cordillera is intruded by a series of poorly studied plutonic bodies that might record the initial-derived magmatism after the breakup of the Farallon Plate during Oligocene-Early Miocene times. Zircon U/Pb geochronology, geochemistry, and isotopic (zircon Hf and whole-rock Pb, Sr, Nd) data were used to study the origin of four of these plutons. These data show a magmatic pulse during Early Miocene with typical magmatic-arc trace elements patterns, ie. negative Nb and Ta anomalies, and high LREE/HREE. Isotopically, Pb, Sr, and Nd values are similar to those of the Caribbean Large Igneous Province and the Active Central American Arc. Hafnium isotopic values show a juvenile magma source (+7 to +12 εHf). These data suggest that the initial record of magmatism in the Western Cordillera started at ~22 Ma, after a period of magmatic quiescence, just a few million years after the Farallon Plate split up and formed the Nazca and Cocos plates, establishing the modern subduction setting for the Northern Andes.