Zorano Sérgio de Souza

Nd, Pb and Sr isotopes in the Identidade Belt, an Archaean greenstone belt of the Rio Maria region (Carajás Province, Brazil): implications for the Archaean geodynamic evolution of the Amazonian Craton

The Rio Maria granite–greenstone terrain (RMGGT), located in the Amazonian craton at about 250 km south of the Carajas Ridge, is a well preserved Mesoarchaean domain composed of metavolcanic rocks and several suites of granitoids ranging in age from ca. 2.96 to 2.87 Ga. The metavolcanics comprise a basal unit of komatiites and low-K tholeiites overlain by sodic metadacites. A Sm–Nd whole-rock reference isochron for the metabasalts, metagabbros (tholeiites) and metadacites gives an age of 3046±32 Ma (MSWD of 1.6, 1 σ ). Nd model ages ( T DM ) of the metadacites range between 3.04 and 3.11 Ga. The e Nd calculated at 3.0 Ga varies from +1.70 to +2.60 for the tholeiites and +0.30 to +3.22 for the metadacites. A Pb–Pb whole-rock linear array for the metabasalts yielded an anomalous older age of 3.41 Ga. The Sm–Nd age is coincident, within analytical error, with a Pb–Pb whole-rock errorchron age of 2944±88 Ma (1 σ ) obtained for the metadacites. Rb–Sr systematic for the metadacites yielded an isochron age that is ca. 400 Ma youngest. The isotopic, geochemical and geological data suggest that oceanic lithosphere was the probable source of the ultramafic and mafic rocks in such a way that partial melting of the depleted mantle generated komatiites and low-K tholeiites. Partial melting of oceanic crust transformed into garnet amphibolite or eclogite generated the parental magmas of the metadacites. Isotopic and trace element characteristics of both metavolcanic rocks and associated plutonics suggest intra-oceanic island arcs as the most probable tectonic setting of the RMGGT. Two main events of crustal accretion are recognised at about 3.04–2.96 Ga and 2.87 Ga. Geological correlation indicates that the oceanic island arcs were completely amalgamated at around 2.76 Ga and acted as sources for the overlying sediments of the Rio Fresco Group.

Pirometamorfismo ígneo na Bacia Potiguar, Nordeste do Brasil

In the Potiguar Basin (NE Brazil), cretaceous rocks (sandstones, siltstones, shales, limestones) are intruded by Paleogene to Neogene basic bodies. As a result, were formed buchites, pyrometamorphic rocks indicating very low pressures and very high temperatures. Field descriptions permitted distinguishing light buchites (LB) and dark buchites (DB), which were investigated throughout petrographic, electron microprobe and X-ray diffraction studies. LBs contain abundant clasts of quartz surrounded by radial tridymite needles, besides phenocrysts of sanidine and clinopyroxene included in a vitreous groundmass. DBs have mainly microcrystals of Fe-cordierite (sekaninaite), mullite, armalcolite, ilmenite and spinel, dispersed within a black cryptocrystalline matrix. Chemically, LBs are richer in SiO2 (~76.7%) and K2O (~5.7%) and poorer in Al2O3 (~12.8%) when compared to DBs (respectively ~51.5, ~0.2 and ~42.7%). Based on phase diagrams published in the literature, the habit of crystals (acicular, elongated sometimes hollow crystals) and the significant content of glassy material we consider that the liquid formed by melting at ~1100 - 1150oC of sedimentary material cooled quickly at very high temperatures and pressures below 1 kbar. The results obtained are relevant in petrological terms, and may also have economic implications since a large number of basic bodies intrude rocks with hydrocarbon reservoirs.

Petrografia e textura do pluton granitoide pós-colisional Serra da Macambira, Neoproterozoico da Província Borborema (Nordeste do Brasil)

O pluton Serra da Macambira faz parte do extenso magmatismo neoproterozoico que marca o final da orogenese Brasiliana/Pan-Africana na Provincia Borborema, sendo, aqui, objeto de caracterizacao geologica, petrografica e textural. Esta localizado no Estado do Rio Grande do Norte, encaixado em ortognaisses paleoproterozoicos (Complexo Caico) e metassupracrustais neoproterozoicas (Grupo Serido), na porcao setentrional da Provincia Borborema, Nordeste do Brasil. As rochas que o compoem foram classificadas na seguinte sequencia: enclaves de quartzo monzonito e biotita tonalito; monzogranitos porfiriticos; granitos equigranulares; diques tardios de granitos, microgranitos, pegmatitos, granofiros e aplitos. Relacoes de campo mostram que os granitos porfiriticos e enclaves microgranulares quartzo monzoniticos representam coexistencia de magmas granitico e dioritico ou gabroide, ambos precoces em relacao ao granito equigranular. A biotita e o mafico predominante nas rochas de todo o pluton. Sua cristalizacao em estagio magmatico precoce, juntamente com hornblenda, indica que os magmas progenitores eram subsaturados em agua (conteudos iniciais entre 2 e 4%). A zonacao composicional no plagioclasio pode refletir o predominio da cristalizacao fracionada, porem nao se pode descartar a influencia de processos de mistura de magma na composicao do liquido. Diques acidos tardios apresentam texturas granofirica e aplitica, caracterizando cristalizacao em condicoes hipabissais ou subita perda de volateis. Texturas igneas preservadas, ausencia ou fraca atuacao de eventos tectonicos, associacao de enclaves microgranulares intermediarios a maficos e a consistencia das amostras com o trend da serie magmatica monzonitica e/ou calcio-alcalina de alto K em diagramas de composicao modal sao encontradas em complexos magmaticos pos-colisionais a pos-orogenicos descritos na literatura.