Ian McReath

PbPb, RbSr, and KAr systematics of the Lagoa Real uranium province (south-central Bahia, Brazil) and the Espinhaço cycle (ca. 1.5-1.0 Ga)

Abstract Geochronologic investigations using PbPb, RbSr, and KAr systems have been carried out on the granite-orthogneiss complex of Lagoa Real, Bahia — one of Brazils most important uranium provinces. RbSr whole rock data on the basement complex yielded an isochron age of about 2700 Ma. The age corresponds to the peak of the Jequie orogenic cycle, when the regional crustal province was probably consolidated. PbPb data for five undeformed examples of the Sao Timoteo granite and seven gneissic samples defined a well-fitted isochron with an age of 1710 ± 100 Ma. The same undeformed samples furnished a similar RbSr whole rock isochron age of 1710 ± 45 Ma. These ages agree with published UPb (zircon) dates, confirming that the intrusion age was around 1700–1720 Ma. The magmas were probably formed largely by melting of crustal components, as shown by ( 87 Sr/ 86 Sr) i and μ 1 values. In complete contrast to the PbPb system, the RbSr system was seriously affected by later events in the orthogneisses. A RbSr isochron apparent age of about 1500 Ma, obtained in one locality, is repeated in other isolated RbSr ages, and also by a UPb (zircon) age obtained in albitites within the region. Other RbSr apparent ages of ca . 1200 and ca . 1000 Ma were obtained, but their real geologic significance remains uncertain. KAr ages in separated minerals fall within the interval 570-500 Ma and show the influence of the Brasiliano tectonothermal cycle.

The Santa Catarina Granulite Complex of Southern Brazil: A Review

Abstract The Santa Catarina granulite complex (SCGC) is mainly composed of intermediate to felsic orthogneisses of trondhjemitic, tonalitic and granodioritic compositions (TTG) intruded by subsidiary basic-ultrabasic rocks, and a small metasedimentary component. The ortho-derived rocks belong to different igneous suites. Nd model ages TDM of 2.7-2.8 Ga date the first major mantle-crust differentiation in the area, while ∼2.6 Ga Rb-Sr whole rock isochron ages in both depleted and undepleted granulites and Nd model ages of 2.4-2.3 Ga record subsequent magmatic events. Regional granulite-facies which reached ∼800°C at ca. 5-8 kb finished at ∼2.3 Ga according to U-Pb zircon and Sm-Nd mineral isochron ages, and was also followed by a regional amphibolite facies metamorphism at ∼2.0 Ga, recorded by Pb-Pb, Rb-Sr whole rock isochrons in rocks with little orthopyroxene and predominant amphibole. Tectonic stabilization was complete at the end of the Transamazonian Cycle at ∼1.9 Ga as indicated by K-Ar mineral ages, and apart from local reactivation along faults at the margins of the complex and granite intrusion, the SCGC was not affected by the Neoproterozoic Brasiliano Cycle, during which it acted as a microplate and was incorporated onto the eastern border of the Parana Craton. Isotopic and geochemical characteristics of Brasiliano crustal granites which intrude the complex suggest that it is not an adequate source for the melts, and thus the SCGC may be underlain by different Paleoproterozoic (2.0 Ga) crustal material.

Evolution of Brasiliano-age granitoid types in a shear-zone environment, Umarizal-Caraubas region, Rio Grande do Norte, northeast Brazil

A sequence of Brasiliano-age granitoid types is exposed in a small area near the cities of Umarizal and Caraubas in Rio Grande do Norte State, Northeast Brazil. Porphyritic K-alkali-calcic monzogranite is an important facies of the oldest Caraubas intrusion (RbSr whole rock isochron age of ca. 630 Ma), which suffered solid-state deformation due to movements on a major NE-trending shear zone. The intrusion of the Prado and part of the Quixaba bodies was probably controlled by the shear zone. These two bodies include mafic/intermediate rocks, some of which contain two pyroxenes, and have hybrid, partly alkaline and partly shoshonitic geochemical characteristics. Rock types and ages are similar to those of some Pan-African occurrences in southwestern Nigeria. The Tourao body, intruded at ca. 590 Ma, presents preferred mineral orientations which are probably largely magmatic, since little evidence is found for widespread solid-state deformation. On the other hand, its intrusion may have been facilitated by the presence of the shear-zone faults. The rocks form a monomodal felsic K-alkali-calcic suite. With the exception of the Quixaba body, all these earlier granitoids are magmatic epidote- and magnetite-bearing porphyritic monzogranites with trace element geochemical characteristics of modern syn-collisional granites. The latest intrusion at ca. 545 Ma is mainly represented by potassic quartz syenites and related rocks, some of which contain fayalite or ferrohypersthene. These rocks possess neither well developed mineral orientations of magmatic origin nor signs of solid-state deformation. They are mineralogically similar to, but younger than some of the “bauchites” of central Nigeria. Geochemical signatures are comparable with those of modern within-plate granites. All granitoids present high (87Sr/86Sr)i ratios which range from 0.708 to 0.712, and increase with decreasing age. Such ratios are compatible with important or dominant crustal contributions. On the other hand, the more mafic rocks are likely to have formed from enriched mantle.

The Atuba Complex, Southern South American Platform: Archean Components and Paleoproterozoic to Neoproterozoic Tectonothermal Events

Abstract The Atuba Complex consists of an Archean terrane, containing rocks differentiated from the mantle and incorporated into the continental crust between 3.1-2.7 Ga. This terrane was later reworked several times. The main metamorphism occurred in the Paleoproterozoic at about 2.1 Ga and can be related to the amalgamation of the Paleoproterozoic supercontinent “Atlantica”. Ages around 1.7 Ga may be related to the break-up of Atlantica. During the Mesoproterozoic, there was little activity in the Atuba Complex and also in the entire South American Platform. Isolated U-Pb zircon ages of 1.4 Ga and Sm-Nd model ages of about 1.2 Ga may be associated to the assembly of Rodinia. The evidence from the South American Platform indicates that the Rodinia rifting pulses started as early as the latest collisional events of the Grenville Cycle, instead of at ca. 0.7 Ga as commonly believed. Finally in the Neopropterozoic at 0.52-0.65 Ga (U-Pb, Rb-Sr, Sm-Nd and K-Ar dates) there were high-grade tectonothermal events associated with Gondwana assembly.

Paleoproterozoic greenstone-granite belts in Northern Brazil and the former Guyana Shield - West African Craton province

The mainly meta-volcano-sedimentary Vila Nova Group and associated granites constitute separate belts, which form part of a large paleoproterozoic (mainly rhyacian) province in the Guyana Shield of which northern Brazil forms a part, and the West African Craton. In Brazil the southwestern Serra do Ipitinga and Serra Tumucumaque-Serra do Navio belts have no obvious geometrical extensions in the Guyanas or Venezuela, and may represent deposits formed at penecontemporaneous passive continental margins and ocean floor spreading centres. To the Northeast the Serra Lombarda-Tartarugalzinho and Oiapoque belts are continuations of Guianese belts. In the former the igneous rocks have geochemical characteristics of suprasubduction environments. Belts in the Guyana Shield and West African Craton have many similar features. The megaprovince evolved in a number of stages, which may have started at about 2.3 Ga, and continued with diminished activity after 2.11 Ga. Both major juvenile additions and (possibly minor) reworking of earlier crust contributed to the growth of the province. The major transcurrent deformation, responsible for the present structure of the belts, probably occurred during the later stages of evolution of the province, but metamorphism and deformation are registered even in the oldest rocks.

In situ isotopic analyses of U and Pb in zircon by remotely operated SHRIMP II, and Hf by LA-ICP-MS: an example of dating and genetic evolution of zircon by 176Hf/177Hf from the Ita quarry in the Atuba Complex, SE Brazil

Remotely-operated SHRIMP dating of zircon is an interesting alternative for dating of zircon crystals. Although it does not represent any technical progress of the geochronological method using the U-Pb system in zircon it is a very useful and cheap facility. The procedure was first used for mass spectrometric analyses involving two international laboratories in Sao Paulo, Brazil and Beijing, China. It was applied to samples of three gneiss-migmatitic rocks from the Ita quarry in the Atuba Complex (located between the Luis Alves and the Apiai Domain) to test previous controversial hypotheses about its evolution. The presence of important archean and paleoproterozoic components in the complex is confirmed by analyses of zircon found in probably neoproterozoic leucosomes. Diorite intrusion also occurred during the neoproterozoic, associated with the 0.6Ga continental collisions involved in the assembly of Gondwana. The determination of Hf isotope ratios by LA-ICP/MS represents a new option for checking the relative importance of mantle (eHf >; 0) and crustal contributions (eHf < 0) during the growth of the zircon crystals. While the archean component in the complex was derived from the mantle (eHf +1.5 to +8.7) the paleoproterozoic component had a crustal contribution (eHf-9.1 to -10.1).

Cambro-Ordovician Magmatism in SE Brazil: U-Pb and Rb-Sr Ages, Combined with Sr and Nd Isotopic Data of Charnockitic Rocks from the Varzea Alegre Complex

Abstract In the central-southern region of Espirito Santo State, southeastern Brazil, several granitoids with a variable composition intruded high-grade metamorphic rocks, in the northern segment of the Ribeira fold belt. A close relationship between hydrous and anhydrous facies is present in some of the plutons, including the Varzea Alegre Igneous Complex, which has an inner domain formed by gabbro, diorite and granite, and an irregular outer ring of charnockitic rocks. These green megaporphyritic charnockites have primary anhydrous mineral assemblage, I-type and metaluminous character, and high-K calc-alkaline signature. U-Pb zircon single crystal ages obtained by TIMS indicate crystallization at about 500 Ma, similar to other late tectonic plutons of this part of the Ribeira belt. Sr and Nd isotopic ratios ranging from 0.7078 to 0.7096 and 0.5114 to 0.5116 respectively, are interpreted to be indicative of a hybrid origin from crustal and mantle-derived magmas. A binary diagram using Sr isotope ratios also demonstrates that the genesis of the charnockites probably included both magma mixing and fractional crystallization processes.

The Umarizal Igneous Association, Borborema Province, NE Brazil: Implications for the Genesis of A-Type Granites

Abstract The Umarizal igneous association (NE Brazil) consists of three A-type plutons, intruded over an interval of about 15 Ma, during the post-collisional phase of the Brasiliano orogeny. All the plutons have Sr isotopic compositions which indicate important or dominant crustal contributions. Crystallization of the Umarizal sill, formed of fayalite quartz syenite and syenogranite, and of the Lagoa stocks, formed by mangerite and granite, commenced under water-undersaturated conditions at pressures around 700-800 MPa and temperatures around 900°C, and continued during magma rise with the crystallization of hornblende at about 480-570 MPa. Crystallization of the Acao stock, which contains a rapakivi-like facies, commenced at similar temperatures, and hornblende appeared at slightly less than 800°C and around 500 MPa. Different f O2 conditions controlled the compositions of the ferromagnesian phases and the nature of the oxide mineral assemblage. Simple fractional crystallization models from homogeneous parent magmas are insufficient to explain the chemical variation of the rocks suites.

Brasiliano-age granitoids in the Sergipana Fold Belt, NE Brazil: the Coronel João Sá Pluton

Abstract The Coronel Joao Sa pluton is a zoned Brasiliano-age, late- to post-tectonic intrusion in the Macurure schist domain of the Sergipana Foldbelt, Northeast Brazil. Biotite-hornblende granodiorite predominates, and flattened ultramafic to intermediate enclaves are present. Igneous orientations are present but fabrics produced by strong solid-state flattening are absent. Rb-Sr whole rock isochrons including enclaves and their host rocks yielded a probable intrusion age of 614 Ma which dates the waning phase of the dominant deformation which produced the F2 foliation of the Sergipana belt and is close to ages reported for late tectonic granites in other Brasiliano domains of Northeast Brazil. More radiogenic Sr (ISr(614) = 0.71008 ± 52) is present in some felsic rocks, while less radiogenic Sr (ISr(614) = 0.70814 ± 18) is mainly found in the more mafic core of the main body. Sr isotopic equilibrium between most enclaves and their host rocks was attained. Textural evidence, especially in the enclaves, shows that magma mixing occurred, but late biotite formation in the enclaves caused modifications of their chemical compositions. The geological, petrographical and isotopic evidence shows that the pluton was probably fed by a complex feeder system into the crust under extensional conditions. Variable upper crustal contamination was probably responsible for the isotopic heterogeneity of Sr.

Extensional and colisional magmatic records in the Apiai Terrane, south-southeastern Brazil: integration of geochronological U-Pb zircon ages

O principal objetivo deste trabalho e apresentar uma sintese dos dados geocronologicos disponiveis para as sequencias metavulcano-sedimentares e nucleos de embasamento que ocorrem na porcao sul do Terreno Apiai, sul-sudeste do Brasil. Os dados obtidos, principalmente na ultima decada, tem modificado substancialmente o cenario tectonico do sul-sudeste brasileiro, identificando a presenca de bacias extensionais (rifts continentais) com magmatismo e sedimentacao associada do final do Paleoproterozoico (1790 - 1750 Ma) e do Mesoproterozoico (1600 - 1450 Ma). O desenvolvimento desses processos parecem ter se iniciado no final do Paleoproterozoico (Nucleos Betara, Perau e Apiai Mirim), evoluindo para bacias mais amplas durante o Mesoproterozoico com a deposicao das sequencias metavulcanossedimentares Betara, Perau, Votuverava e Agua Clara. Padrao distinto e observado para a Sequencia Itaiacoca, que ocupa a porcao setentrional do Terreno Apiai. O estudo geologico-geocronologico caracterizou a existencia de dois conjuntos litologicos temporalmente distintos, o primeiro representado por uma associacao plataformal metacarbonatica, que inclui rochas metabasicas de natureza toleitica/subalcalina, com idades minimas de deposicao relacionadas ao final do Mesoproterozoico/inicio do Neoproterozoico (Sequencia Itaiacoca: 1.030-900 Ma). O segundo conjunto e representado principalmente por rochas metapsamiticas com metavulcânicas associadas, incluindo traquitos ultrapotassicos, depositados na transicao Criogeniano/Ediacarano (Sequencia Abapa: 645-628 Ma). O presente cenario sugere historias tectono-sedimentares distintas para o Terreno Apiai, refletindo uma evolucao policiclica. Durante o Mesoproterozoico predominaram condicoes relativamente estaveis com a deposicao de grande parte das sequencias metavulcanossedimentares (Lajeado, Agua Clara, Betara, Perau, Votuverava), num contexto de margem passiva, sucedidos pela deposicao da Sequencia Itaiacoca, associada a regimes extensionais do Toniano. No Ediacarano prevaleceram condicoes instaveis, de margem ativa, caracterizadas por restrita sedimentacao (Sequencias Abapa, Antinha e Iporanga) e colocacao dos batolitos graniticos Tres Corregos, Cunhaporanga e Agudos Grandes, representantes de arcos magmaticos.