Mafic pegmatites intruding oceanic plateau gabbros and ultramafic cumulates from Bolívar, Colombia: Evidence for a 'wet' mantle plume?

Abstract

The fault-bounded Bolívar Ultramafic Complex (BUC) on the eastern fringes of the Western Cordillera of Colombia was tectonically accreted onto the western coast of South America in the late Cretaceous-early Tertiary, along with pillow basalts of the Caribbean-Colombian Oceanic Plateau (CCOP). The complex consists of a lower sequence of ultramafic cumulates, successively overlain by layered and isotropic gabbroic rocks. The gabbros grade into, and are intruded by, mafic pegmatites that consist of large magnesiohornblende and plagioclase crystals. These pegmatites yield a weighted mean 40Ar-39Ar step-heating age of 90·5 ± 0·9 Ma and thus coincide with the timing of peak CCOP volcanism. The chemistry of the BUC is not consistent with a subduction-related origin. However, the similarity in Sr-Nd-Pb-Hf isotopes between the CCOP and the BUC, in conjunction with their indistinguishable ages, suggests that the BUC is an integral part of the plume-derived CCOP. The parental magmas of the Bolívar complex were probably hydrous picrites that underwent 20-30% crystallization. The residual magmas from this fractionation contained ∼7·0 wt % MgO and enough water to permit fractionation of magnesiohornblende (and bytownite) on the sidewalls of veins, forming hornblende-rich pegmatites. As the residual magmas migrated upwards through the complex, they became more evolved and, thus, fractionated less magnesiohornblende along with a greater proportion of more sodic plagioclase. At the highest preserved levels of the complex the pegmatites consist predominantly of andesine plagioclase with quartz and biotite. During crystallization of the pegmatites some of the residual magma was tapped or erupted and intruded at higher levels to form breccias and dykes that are exposed nearby at Vijes. Our calculations suggest that the BUC resulted from melting of a mantle plume source region that contained at least 400 ppm H2O. We propose that mantle plumes, as well as being heterogeneous in terms of radiogenic isotopes and trace elements, are also heterogeneous with regard to their water content, and so can have portions that are more hydrous than others. This has fundamental implications for mantle rheology and the environmental impact of Large Igneous Province eruptions. © Oxford University Press 2004; all rights reserved.