LOOKING FOR THE EARLY ARCHEAN ROCKS IN SOUTH AMERICA: U-PB DATING AND HF ISOTOPES IN ZIRCONS FROM THE NORTH SAO FRANCISCO CRATON, BRAZIL

Rock fragments older than 3.0 Ga are rare worldwide. They occur generally as small isolated fragments or slivers within large cratons. In South America, the best candidates for the discovery of Paleoarchean rocks are the still poorly studied areas of the São Franscisco Craton. We performed Nd and Hf isotopic and U-Pb zircon geochronologic studies on orthogneiss samples collected in several outcrops from Petrolina, Pernambuco and Juazeiro, Bahia, in the northern portion of the São Francisco Craton. The northern limit of the SFC is rather irregular, displaying a series of bulges and hollows, where pre-Mesoproterozoic basement rocks and metasedimentary piles of the adjacent mobile belt are exposed, respectively. The basement comprises mainly granodiorite and tonalite and trondhjemite is rare. Sample CSF BROW is a gabbro-diorite xenolith found within grey gneiss collected in the Riacho da Vitória quarry. The analyzed rock shows two different zircon populations. The first is represented by large, prismatic and translucid zircon crystals that yield a concordant age around 3.5 Ga. Another population could be related to intrusive veins cross-cutting the mafic protolith and displays a upper intercept age of 2.56 Ga. Another sample comes from Riacho Uruais. The rock is a trondhjemite and displays a discordant age at around 3.2 Ga. Tree other samples collected close to Petrolina, along the BR 428 highway (samples CSF KM 11, 57 and 103) shows ages varying between 3.4 and 3.2 Ga and yield Hf TDM model at around 3.7 to 3.9 Ga. Similar pattern is displayed by Nd T_DM model ages range from 3.7 to 3.8 Ga, suggesting dominantly Early Archean sources for all analyzed samples of the Sobradinho block. Positive and negative End(t) values, calculated for crystallization ages obtained by zircon age determinations, demonstrate that juvenile and reworked crust are present in this Archean segment. The identification of early Paleoarchean rocks is a first in South America and encouraging for new research.