COMPARISON AND TECTONIC SIGNIFICANCE OF PAN-AFRICAN HP/UHP ECLOGITES FROM PERNAMBUCO AND MINAS GERAIS, BRAZIL

HP/UHP metamorphic rocks are relatively common within Phanerozoic collisional belts and record unique information about the early, deep history of orogens. Comparable Proterozoic examples are extremely rare, especially within the Pan-African belts. Neoproterozoic eclogites outcrop within collision-related nappes at the southern and northeastern margin of the Sao Francisco craton in southern Minas Gerais and central Pernambuco (Brazil), respectively. These occurrences are unique in the South American continent, and the geologic, tectonic, and petrologic similarities between the two are striking.

The Minas Gerais nappes consist of coarse-grained kyanite-garnet granulite with intercalations of impure quartzite, calc-silicate rocks, and boudins of mafic-ultramafic rocks, including eclogite. Early eclogite facies assemblages in these nappes contain coesite, indicative of abyssal burial and ultrahigh pressure metamorphism (c. 750° C, >28 kbar). The Pernambuco eclogites are also intercalated with ortho- and para-gneisses of continental origin, as well as thin ultramafic bodies. Field and structural observations suggest that their field disposition is nearly identical to that of the Minas Gerais rocks. The eclogites contain a high jadeite component (~ 50 mol%) omphacite and preliminary P-T data suggests T~ 650°C, P> 16 kbar for the peak stage. Most eclogite units display late-stage retrogression to amphibolite. Field observations of amphibolite boudins interpreted to be retrogressed eclogite suggest that the region of eclogite facies metamorphism may be more extensive than previously reported.

The only other occurrence of Pan-African UHP eclogite is a 630 Ma unit at the eastern margin of the West African craton in Mali. Temporal and geologic similarities between these three widespread occurrences may suggest correlative tectonic processes of collision and exhumation within Pan-African orogenic belts in the Neoproterozoic. Furthermore, upon consideration of thermobaric structure, orogen-scale architecture, and tectonic development of these nappes they may represent an ancient analogue of the Greater Himalayan Sequence of the Nepalese Himalaya.