THE ECLOGITE-BEARING SERIES OF ISLA-MARGARITA, VENEZUELA - GEOCHEMISTRY OF METABASIC LITHOLOGIES IN THE LA-RINCONADA AND JUAN-GRIEGO GROUPS

XRF analysis for major and minor elements in 94 samples of the metabasic La Rinconada Group and in 28 samples of metabasic lithologies in the predominantly metasedimentary, overlying Juan Griego Group of Isla Margarita, Venezuela, has been carried out to compare the bulk compositions of the two eclogite-bearing metabasic suites. The two units differ in the age and geological setting of the protolith. The La Rinconada Group has a monotonous basaltic character, but shows irregular variations in bulk composition which are interpreted to reflect variations in the type of protolith, ranging from basaltic lava flow to gabbroic, cumulitic dyke. As a whole, the La Rinconada Group is related to a source of tholeiitic composition in a MORB-type volcanic-hypabyssal environment. The Juan Griego metabasic rocks are compositionally more homogeneous and uniformly basaltic in their major elements, again reflecting a tholeiitic source and a MORB-type environment. Despite their contrasting geological setting, the two suites of metabasic rocks can be inferred to have had a similar source and have probably developed in a passive, "Atlantic"-type continental margin setting. We infer that the La Rinconada Group represents early oceanic crust later covered by the siliceous, continent-derived clastics of the Juan Griego Group, in which the minor basaltic intrusions represent the last stages of waning magmatic activity. © 1990.