NEOGENE MAGMATISM ALONG WESTERN CORDILLERA OF COLOMBIA AND ITS RELATIONSHIP WITH THE TECTONIC HISTORY OF THE NORTHERN ANDES

Colombian Western Cordillera is made up of allochthonous crustal blocks of oceanic affinity intruded by a series of Neogene plutonic bodies presumably related to the subduction of the Nazca plate beneath western Colombia. This magmatism is key to understanding tectonic events such as the accretion of the Panamá-Choco Block (PCB) to the North Andean Block (NAB) and changes in subduction-acretion dynamics at the margin. Zircon U/Pb geochronology, geochemistry, and isotopic (zircon Hf and whole rock Pb, Sr, Nd) data are used to characterize temporal and geochemical changes in magmatism. Magmatic pulses at ca. 22-17, 12-11, 9-7 Ma are recongnized and show 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. Middle and late Miocene plutons show high Sr/Y values (> 28), and Rb-Ta-Hf trace elements consistent with a collisional tectonic environment in northwestern Colombia. These data suggest partial melting at high-pressure conditions due to increase coupling between the subducting and overriding plates. Hafnium isotopic values show a variable range through time, juvenile magma sources (+7 to +12 εHf) from 22-20 Ma, mixed values (-28 to +12 εHf) from 20-10 Ma, and juvenile (+4 to +13 εHf) from 10-7 Ma. Our results suggest that magmatism ceased in northern Colombia at about 7 Ma probably as a result of subduction blockage during final accretion of the PCB to the NAB. In contrast, subduction was continuous through the Neogene in southwestern Colombia.