Ciro Teixeira Correia

RESUMO DESENVOLVIMENTO DE MÉTODO DE FRX POR DISCO DE VIDRO POR FUSÃO E COMPARAÇÃO COM A TÉCNICA DE PASTILHA DE PÓ PRENSADO NO INSTITUTO DE GEOCIENCIAS, UNIVERSIDADE DE SÃO PAULO

An X-ray fluorescence pressed powder pellet technique (PPP) currently in use at the X-Ray facility of the Institute de Geociencias, Sao Paulo University (IG-USP) has been extended to include additional elements and complemented by a full major and trace element calibration by fused glass disc (FGD) X-ray fluorescence. A total of 38 major and trace elements are available (F, Na, Mg, Al, Si, P, S, Cl, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Rb, Sr, Y, Zr, Nb, Mo, Sn, Sb, Ba, Pb, Bi, La, Ce, Nd, Th and U) with variable detection limits, but generally below 10 ppm for trace elements. Loss-on-ignitions determined by weight difference and totals provide extremely good control on data quality. A full analysis, including background, matrix correction and all relevant corrections can be achieved automatically in less than 60 minutes. Virtually any sample matrix can be accommodated. The data support the view that the fused disc and powder pellet techniques are complementary and together provide a definitive, rigorous XRF analysis. However, both techniques require considerable attention to details, with the glass disc technique prone to losses of F and S and increase detection limits for certain elements. The powder pellet technique requires fine micronising and caution when dealing with the light elements Si and Al. Additionally, the paper presents a new, previously unpublished experimentally determined Alpha coefficients for all matrix-corrected elements, which are based on the lithium metaborate system and contrast with the Philips theoretical alpha coefficients.

Petrology and geochemistry of the mafic dyke swarm of the Treinta Y Tres region, Northeast Uruguay

Abstract Transitional basalts of the Treinta y Tres region intrude late Proterozoic granites in northeast Uruguay. The age of the swarm is assumed to be close to 750 Ma. Major and trace geochemical diagrams indicate gabbro type fractionation, which is supported by MPR plots. In comparison to the Florida and Nico Perez suites, the Treinta y Tres swarm is enriched in Ti, P, Zr and Sr and depleted in LILE and REE. These features and the Sro and Ndo isotope values are discussed in terms of source heterogeneity and crustal contamination. The geochemical characteristics do not support crustal contamination. The Treinta y Tres suite is better explained by a source affected by two melting episodes. The dikes were originated from the latter melting, which occurred in a an isotopically heterogeneous source. Diagrams related to several incompatible elements ratios indicate that the mantle source of Uruguaian dikes and of continental basalts of Brazil have similar geochemical features since Early Proterozoic up to Mesozoic.

U-Pb zircon SHRIMP data from the Cana Brava layered complex: new constraints for the mafic-ultramafic intrusions of Northern Goias, Brazil

Abstract The Cana Brava Complex is the northernmost and least well known layered intrusion of a discontinuous belt of mafic-ultramafic massifs within the Brasilia Belt, which also comprises theNiquelândia and Barro Alto complexes. Available geochronological data from a range of techniques (K/Ar, Ar/Ar, Rb/Sr, Sm/Nd and U/Pb) provide a range of possible ages (time span from 3.9 Ga to 450 Ma), hence a precise and reliable age for the Cana Brava Complex is still lacking. Also, preliminary isotopic and geochemical data of the Cana Brava Complex suggest a significant crustal contamination, which could have affected bulk-rock Sr and Nd systematics resulting in meaningless age determinations. In this paper, we present new U-Pb SHRIMP zircon analyses from four samples of different units of the Cana Brava Complexwhich suggest that the intrusion occurred during the Neoproterozoic, between 800 and 780 Ma, i.e. at the same age ofNiquelândia. Discordant older 206Pb/238U ages are provided by inherited zircons, and match the age of the metamorphism of the embedding Palmeirópolis Sequence.

Os anfibolitos do complexo costeiro na região de São Sebastião, SP

Geochronological, petrographic and lithochemical data have been used to characterize the crustal evolution and the thermochronology of the Costeiro Complex, Sao Paulo State, Brazil. This part of the Ribeira Fold Belt is composed of paraderived rocks to the East, and orthoderived rocks to the West, which are organized along a ductile shear zone forming a positive flower structure whose axis runs ENE-WSW. The main amphibolite occurrences are found within predominant garnet-paragneisses along this axis. The age of crystallization of the intrusive basic magmas which originated the amphibolites was 580 Ma (U-Pb SHRIMP). They formed tabular intrusions or secondary magma chambers, that locally still preserve continental tholeiitic geochemical characteristics. The close proximity of the basic magmatism and the metamorphic peak is evident from the 570 Ma, U-Pb age determination of the overgrowths on detritic zircon crystals in the paragneisses. This suggests a tectonic environment where the basic magmas intruded a back-arc sedimentary basin established over continental crust. The source areas were probably rocks differentiated from the mantle between the Paleoproterozoic and Neoproterozoic.

Litoquímica dos granitóides da estrutura em flor de São Sebastião, São Paulo

The aim of this paper is to present lithochemical data from the granitoids found between Guaruja and Sao Sebastiao in a segment of the Costeiro Complex in Sao Paulo, State, Brazil. This part of the Ribeira Folded Belt is composed of para-derived rocks on the eastern side, and ortho-derived rocks on the western side of the belt, organized in a ductile shear zone forming a positive flower structure, whose axis runs ENE-WSW. Large boudins of amphibolite occur within predominant kinzigitic gneisses along this axis. The intrusive basic protoliths of the amphibolites have a 580 Ma (U-Pb SHRIMP) crystallization age. SiO2 contents of the granitoids vary between 68 and 74%, corresponding to monzogranites and granodiorites. Some outcrops display mafic enclaves of quartz-monzodiorites. The granitoids show calc-alkaline affinity while their mafic enclaves tend towards an alkaline affinity. The lower content in SiO2 coupled with coherent differences on some trace element concentrations (e.g. Zr-Hf) contribute to clearly distinguish the enclaves from their country rocks and to suggest intense chemical interaction among the materials during crystallization. The REE pattern, the depletion in Sr, P-Ti, Ta-Nb, and the distribution of the samples on the R1-R2 diagram suggest a syn-collision island arc environment for generation of these rocks.