Portal de Periódicos da CAPES

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

1970; Oxford University Press; Volume: 11; Issue: 1 Linguagem: Inglês

10.1093/petrology/11.1.101

1460-2415

B. A. MORGAN,

Geological and Geophysical Studies Worldwide

Occurrences of eclogite, eclogite-amphibolite, and garnet amphibolite from Puerto Cabello, Venezuela are described. Chemical analyses of thirteen rocks, eight garnets, six clinopyroxenes, and nine amphiboles are presented. Field evidence shows that the eclogites and associated mafic rocks occur as sheets or lenses conformable with pelitic and calcareous rocks and have been metamorphosed in situ. The associated country rocks have attained a metamorphic mineral assemblage typical of the epidote-amphibolite facies. The chemical composition of the eclogite and associated rocks is variable but is essentially that of tholeiitic basalt. Clino-pyroxenes from the eclogites are omphacites containing 34–51 mole per cent jadeite and the remainder largely diopside; coexisting garnets contain approximately 50 mole per cent almandine, 24 mole per cent pyrope, 25 mole per cent grossular+andradite, and 1 mole per cent spessartine. Determinations for the temperature of metamorphism of these rocks give approximately 525±50 °C. Determinations of total pressure of metamorphism, based on the jadeite content of omphacite, on glaucophane II, kyanite, and calcite stability fields, and on stratigraphic reconstructions give a range of values between 5 and 10 kb with a most probable value near 7 kb. These data suggest a moderate geothermal gradient during metamorphism of approximately 20 °C/km. An evaluation of the role of volatiles during metamorphism indicates that the original basalts were converted to eclogites in an environment closed to water and carbon dioxide. The field relationships at 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 preserved only in random 'dry' pockets within the stratigraphic section containing the metabasalts. The transformation of basalt to eclogite to amphibolite is considered to have been a continuous paragenetic sequence during a single pseriod of metamorphism.