The Amagá basin between the Western and Central Cordilleras of the Northern Andes of Colombia host the Neogene volcanic and volcaniclastic Combia Formation. At this stage it is not clear how the formation of this unit is related to arc volcanism and which role the Nazca plate subduction beneath the western margin of South America plays. The timing, petrography and geochemical characteristics of Combia Formation rocks were studied in the western and eastern parts of the Amagá basin, in order to gain more information on the type of magma generation and volcanic activity that led to the deposition of the Combia Formation. Apatite and zircon fission-track dating largely confirm a 12-6 Ma age for the deposition of the Combia Formation. Petrographic and major element analyses show that mainly trachy-andesite ignimbrites with a calc-alkaline composition were deposited in the western Amagá basin, whereas the volcanic rocks of the eastern Amagá basin are lava flow and fallout deposits of basaltic andesites or of tholeiitic composition. Trace element and isotopic analyses show that slab dehydration and sediment melting were important in primary magma generation in the mantle wedge, but the primary magma was mixed with lower continental crustal melts, resulting in characteristic isotope signatures in the western and eastern Amagá basin. All this points to subduction driven arc volcanism with slab dehydration, sediment melting magma mixing.
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