The Bakhuis ultrahigh-temperature granulite belt: I. Petrological and geochronological evidence for a counterclockwise P-T path at 2.07-2.05 Ga.

The Bakhuis belt, one of the major granulite-facies domains in the Guiana Shield, consists of a core of banded rocks of the charnockite suite, metamorphosed and migmatized under granulite-facies conditions. Pelitic gneiss intercalations locally show sapphirine-quartz and orthopyroxene-sillimanite-quartz assemblages, with up to 10% Al2O3 in the orthopyroxene. These assemblages point to ultrahigh-temperature (UHT) metamorphism. P-T conditions are difficult to deduce, because the assemblages contain Fe3+ in sapphirine and sillimanite, and do not contain coeval garnet, but spinel, Feand Fe-Ti oxides instead. P-T conditions for the peak UHT metamorphism are estimated to have been 950 °C and 8.5-9 kb. An assemblage of impure corundum associated with quartz may also have formed during the UHT metamorphism. Unravelling the assemblages indicates a counterclockwise P-T path from an early cordierite-sillimanite assemblage via a subsequent sapphirine-quartz assemblage to the peak metamorphic assemblage orthopyroxene-sillimanitequartz. Retrogressive assemblages show an isobaric to near isobaric cooling path for the UHT occurrence. Single zircon Pb-evaporation and whole rock Sm-Nd dating were carried out on the Bakhuis granulites. Three zircons from a garnet-sillimanite-bearing gneiss yielded an age of 2072 ± 4 Ma, but a fourth grain provided increasing ages up to 2.10 Ga. An enderbitic granulite gave zircon ages in the range of 2.15-2.09 Ga. Zircons from a garnet-bearing gneiss defined an age of 2055 ± 3 Ma, while those from a garnet-bearing pegmatite layer gave ages ranging between 2085 Ma and 2058 Ma. A mylonitic orthopyroxene-bearing granite (a true charnockite) furnished a Pb-Pb zircon age of 2065 ± 2 Ma this is the first indication that the high-grade metamorphism was (at least locally) associated with melting and production of magmatic charnockite. Zircons from two samples of a discordant recrystallized basic dyke yielded ages of 2060 ± 4 Ma and 2056 ± 4 Ma, respectively. A discordant sheared pegmatite vein provided an age of 2059 ± 3 Ma. In conclusion, the 2072–2055 Ma ages are interpreted as the age of granulite metamorphism in the Bakhuis Mountains, and ages older than 2.07 Ga, and up to 2.15 Ga, are considered to reflect an inherited component from the early Transamazonian protoliths of the granulite. TDM model ages for the Bakhuis samples range from 2.40 to 2.19 Ga. These data, together with positive to slightly 175 GEOLOGIE DE LA FRANCE, N° 2-3-4, 2003 GEOLOGY OF FRANCE AND SURROUNDING AREAS, N° 2-3-4, 2003 The Bakhuis ultrahigh-temperature granulite belt (Suriname): I. petrological and geochronological evidence for a counterclockwise P-T path at 2.07-2.05 Ga La ceinture de granulites d’ultra-haute temperature des Monts Bakhuis (Suriname) : I. mise en evidence d’un chemin P-T anti-horaire a 2,07-2,05 Ga Geologie de la France, 2003, n° 2-3-4, 175-205, 12 fig., 6 tabl. Mots cles : Roche metamorphique, Facies granulite, Condition pression temperature, Datation, Pb-Pb, Sm-Nd, Zircon, Roche totale, Paleoproterozoique, Suriname, Bouclier guyanais.