ORIGIN OF IGNEOUS ROCKS IN THE CHAGRES RIVER BASIN (CRB), CENTRAL PANAMA, AND RELATIONSHIP TO CARRIBEAN MAFIC COMPLEXES

CRB igneous rocks comprise a suite of highly deformed basalts to andesites and gabbros to plagiogranites. Rare volcaniclastic breccias and sandstones are also observed. Primary structures and contacts are generally obscured tectonically or by tropical weathering. Most rocks are altered, particularly volcanics and volcaniclastic sediments. Ultramafic rocks, evidence for an oceanic basement/lithospheric mantle, are absent. Mafic intrusive lithologies, the most pristine rocks, comprise less-deformed domains. Geologically, the igneous suite of the CRB is interpreted as a highly tectonized submarine lava flow-dike complex, associated with overlying and intercalated breccias and sediments, all of which were intruded by plutonic rocks. Rare oxidized scoria provides evidence for subaerial or shallow marine emplacement. Geological reconstruction envisages a mostly submarine volcanic island complex containing abundant intrusions (magma chambers) with high rates of effusion. Geochemically, the CRB rocks are tholeiities. Mobile element contents (Cs, Ba, Rb, K, Sr) are highly variable. In some rocks, immobile trace elements are enriched relative to N-MORB. There is no trace element evidence to suggest a subduction zone setting or a typical enriched oceanic island source for the complex as a whole. The CRB suite likely belongs to the Cretaceous to Early Tertiary mafic complexes present across the Caribbean that have been accreted since Oligocene time to form the foundation of the Central American landbridge. Our data, together with that for other mafic terranes, support the existence of three mantle sources: (1) a dominant source transitional between MORB and E-MORB, (2) an OIB-type intraplate source, and, more rarely, (3) a primitive island arc source. The model of an origin from a developing oceanic plateau derived from the Galapagos plume is supported by these data. Early arc rocks probably formed during and after accretion of this oceanic plateau.