MIOCENE THOLEIITIC AND CALK-ALKALINE MAGMATISM FROM THE NORTHERN COLOMBIAN ANDES – IMPLICATIONS FOR MAGMA PETROGENESIS IN THE NORTHERN VOLCANIC ZONE

The Combia Formation in the northern Andes of Colombia comprises a unique occurrence of tholeiitic magmatism formed in an extensional basin setting. This volcanism is closely associated to calk–alkaline shallow intrusions. A review of existing and new geochemical and geochronological data suggests that both magma compositions coexisted at 12 to 6 Ma, but originated from different processes. Tholeiites formed via melting of a primitive mantle source, with a limited input of a sedimentary or continental contaminant. The calc–alkaline magmas are mainly adakitic and formed from fractionation of garnet and amphibole at high pressures from a hydrous melt from an enriched source. Petrographic and mineral chemistry data indicate that magmas underwent a complex history during ascent that includes: (1) crystallization of high-pressure phenocryst phases (Gnt1 and Amph1) at 900°C and 1200 Gpa in a mantle–derived melt, (2) stalling of differentiated magma at lower pressure conditions (Gnt2 + Amph2 + Plg) and (3) stalling at shallow conditions, where decompression occurred.

The tectonic setting that enabled the formation of these magmas is associated to the Caldas Tear, a slab window developed during subduction of the Nazca Plate beneath South America.