THE PALEOGENE INTRUSIVE OF THE SIERRA NEVADA DE SANTA MARTA, COLOMBIA: SHORT-LIVED MAGMATISM RELATED TO THE COLLISION OF THE CARIBBEAN PLATE WITH SOUTH AMERICA

The Santa Marta batholith (BSM) and the Buritaca, Latal and Toribio plutons, which outcrop in the NW Sierra Nevada de Santa Marta (SNSM), Colombia, have been considered in previous works as related to a magmatic arc. However, the fact that these intrusive bodies (1) were emplaced close to the continental margin within an alloctonous oceanic metamorphic terrane (Santa Marta belt), (2) are associated to short-term magmatism, and (3) are isolated from the main Andes, is hard to reconcile with models of subduction related arcs. New field work, geochemical and U-Pb age data allowed us to define the emplacement sequence, the cooling rates and to propose a petrogenetic model in relation to the Caribbean tectonics for these intrusives.

An early event was identified at ~64 Ma, which is characterized by the emplacement of small volumes of trondhjemitic magmas. A later event, between 58 and 49 Ma, is represented by granodioritic to tonalitic rocks, with bodies of amphibole-rich cumulates and abundant mafic enclaves. Within this second event, ages of 56-55 Ma were obtained for the major part of the BSM and for the Latal pluton, whereas the Buritaca and Toribio plutons yielded ages of 52-49 Ma.

The early trondhjemitic magmatism can be related to partial melting of amphibolite facies rocks of oceanic affinity, similar to those exposed in the Santa Marta belt. This event would mark the time when the Caribbean plate collided with the South American plate in the area. The second event is associated to flat subduction of the Caribbean plate edge beneath the South American plate under an abnormally high thermal regime, which resulted in intense dehydration, and possible melting, of the slab and may also have favored melting of the lower crust. The resulting hybrid magmas evolved by fractionated crystallization with significant amphibole segregation.

Rapid cooling rates (80 °C/Ma), defined by zircon (U-Pb) and hornblende, biotite and K-feldspar (Ar-Ar) ages, are probably related to syn-tectonic intrusion during uplift of the SNSM, caused by underplating of the Caribbean plate below South America.

The composition of rocks and the short-lived magmatism indicates a punctual event related to the collision of the Caribbean plate with the South American plate, but not to the development of a magmatic arc associated to a well established subduction system.