GEOCHEMISTRY AND ORIGIN OF ALKALINE BASALTS AND GABBROIC XENOLITHS FROM THE UTILA ISLAND (BAY ISLANDS, HONDURAS)

The island of Utila (Bay islands, Honduras), is situated in a back-arc environment, behind the Central American Arc. The Utila Volcanic Center (UVC) is Quaternary in age, based on geologic and stratigraphic relations with underlying carbonate rocks and is made of three distinct volcanic units: pyroclastic flows, main lava flow and vent facies (oldest to youngest). The UVC pyroclastics contain gabbroic inclusions up to 5 cm in size. The basalts are porphyritic extrusive rocks with phenocrysts composed of olivine, clinopyroxene, plagioclase and rare amphibole. Gabbros are composed of amphibole, plagioclase, clinopyroxene, olivine, and opaque minerals. Basalts are classified as alkaline basalts (trachybasalts) on the TAS diagram. The alkaline basalts plot into the field of Within Plate Basalts (WPB) on Zr-Zr/Y diagram. Primitive mantle normalized trace element patterns show enrichment in Ba, Nb, Ta and LREEs similar to ocean island basalts. The UVC lavas plot into the High Nb Basalts field on Nb-Nb/U, and MgO-Nb/La diagrams. Both basalts and gabbros Sr, Nd and Pb isotopes are distinct from OIB mantle sources in the Central American-Southern Caribbean region but similar to Pacific MORB. A combination of OIB-like trace element characteristics and Pacific MORB-like isotopic ratios suggest that the UVC basalts are derived from a source created by hybridization of a depleted sub-arc mantle wedge by silicic melts generated by melting of subducted basaltic slab. Later, tectonic movement along the Motagua Fault has possibly caused decompressional melting of the previously slab melt-metasomatized lithospheric mantle source.. These slab melts were probably derived from a detached slab beneath the Northern Honduras that are clearly identified on P-wave tomographic images of the Northern Central America arc.