The Bolivar mafic-ultramafic complex, SW Colombia: the base of an obducted oceanic plateau

The Bolivar mafic-ultramafic complex (Valle Department) is a late Cretaceous banded sequence of dunites, Iherzolites, pyroxenites and gabbronorites. The gabbronontes, which can be layered or isotropic, are extensively amphibolitized and have suffered pervasive deformation and local migmatisation, indicating that these rocks were affected by a H2O-rich metasomatic event that eventually led to their partial fusion. The complex is cut by pegmatitic dykes, which vary petrographically according to their level of emplacement. In the lower levels plagioclase-hornblende dykes are found, whereas in the upper pans the dykes are rich in plagioclase, quartz, muscovite, and sericite. The dunites have been altered to serpentinite and form the host rock of a stockwork deposit of cryptocrystalline magnesite veins. Opal veins deposited after the magnesite veins occur in the upper levels of the deposit. The mechanism producing this type of deposit is commonly believed to involve the alteration of olivine (dunite) by CO2-rich fluids generated at depth. Interestingly, the worldʹs more important deposits of this type, located in a discontinuous belt through the Dinaric Alps and Hellenides have geological characteristics very similar to those of the Bolivar deposit. These characteristics are consistent with these deposits having formed during the process of obduction of anomalous (?plateau) ocean floor. It is proposed that the metasomatic event, the generation of the pegmatitic dykes, and the formation of the magnesite veins are all genetically related to an obduction process during which the igneous complex was accreted to the margin of Colombia. During obduction, geotherms beneath the overthrust lithospheric block rose, distilling the H2O and boron contained in the terrigenous sediments found in the upper (underthrust) pan of the block. The resulting fluids migrating through the overthrust block led to the formation of the migmatites, amphibolites and pegmatite dykes. A second generation of CO2-rich fluids, created in a similar fashion from calcareous sediments, may have been responsible for the magnesite mineralization.