Mid-Cretaceous extension-related magmatism in the eastern Colombian Andes

Cretaceous magmatism in the Eastern Cordillera of Colombia is related to lithospheric stretching during the late Early–early Late Cretaceous. The small amount of preserved igneous material is represented by small mafic intrusions. This study focuses on three localities, from east to west: Pajarito, Pacho, and Cáceres. The investigated igneous bodies are classified as gabbros, pyroxene–hornblende–gabbros, and pyroxene–hornblendites mainly composed of plagioclase, clinopyroxene, and/or amphibole. Although their timing of emplacement and geodynamic position seem similar, significant differences in their geochemical and petrological characteristics rule out simple models of melt genesis. Clinopyroxene and bulk chemistry indicate increasing alkalinity from west to east. Trace element concentrations point to melt sources that range from a slightly enriched mantle in the west to a highly enriched one in the east. In addition, the data reflect a decreasing degree of partial melting from west to east and the decreasing importance of residual garnet in the mantle source. Probable mantle metasomatism in the source region by slab-derived fluids, as displayed by high Ba/Nb and moderate Srn/Pn, is clear in the west and very slight to the east. Mantle metasomatism and melt generation probably are processes of different epochs. The lack of large volumes of igneous rocks and the absence of tectonically controlled unconformities in the investigated areas indicate that a mantle plume did not affect the regional tectonics and magmatism. We favor a model of rift-related magmatism in which melt composition is modified from east to west from a highly enriched to a less enriched mantle region, the latter metasomatized by fluids derived from an older subduction phase.