Petrology and Mineralogy of Eclogite and Garnet Amphibolite from Puerto Cabello, Venezuela

Occurrences of eclogite, eclogite-amphibolite, and garnet amphibolitefrom Puerto Cabello, Venezuela are described. Chemical analysesof thirteen rocks, eight garnets, six clinopyroxenes, and nineamphiboles are presented. Field evidence shows that the eclogitesand associated mafic rocks occur as sheets or lenses conformablewith pelitic and calcareous rocks and have been metamorphosedin situ. The associated country rocks have attained a metamorphicmineral assemblage typical of the epidote-amphibolite facies.The chemical composition of the eclogite and associated rocksis variable but is essentially that of tholeiitic basalt. Clino-pyroxenesfrom the eclogites are omphacites containing 34–51 moleper cent jadeite and the remainder largely diopside; coexistinggarnets contain approximately 50 mole per cent almandine, 24mole per cent pyrope, 25 mole per cent grossular+andradite,and 1 mole per cent spessartine. Determinations for the temperatureof metamorphism of these rocks give approximately 525±50°C. Determinations of total pressure of metamorphism, basedon the jadeite content of omphacite, on glaucophane II, kyanite,and calcite stability fields, and on stratigraphic reconstructionsgive a range of values between 5 and 10 kb with a most probablevalue near 7 kb. These data suggest a moderate geothermal gradientduring metamorphism of approximately 20 °C/km. An evaluationof the role of volatiles during metamorphism indicates thatthe original basalts were converted to eclogites in an environmentclosed to water and carbon dioxide. The field relationshipsat Puerto Cabello demonstrate the general instability of eclogite;the great majority of mafic rocks are amphibolites, garnet amphibolites,and eclogite-amphibolites. Eclogites, sensu stricto, are preservedonly in random ‘dry’ pockets within the stratigraphicsection containing the metabasalts. The transformation of basaltto eclogite to amphibolite is considered to have been a continuousparagenetic sequence during a single pseriod of metamorphism.