Jayme Alfredo Dexheimer Leite

SHRIMP U/Pb zircon geochronology of Neoproterozoic juvenile and crustal-reworked terranes in southernmost Brazil

U/Pb zircon ages of six rock units from southernmost Brazil were obtained by means of SHRIMP II techniques. The studied rocks occur on either side of the Neoproterozoic contact between a juvenile accreted terrane (the Vila Nova Terrane) and a crustal-reworked belt (the Dom Feliciano Belt). This contact is located in the western portion of the exposed Precambrian rocks in the state of Rio Grande do Sul, Brazil. Contrasting patterns of U/Pb ages are shown by zircons hosted in metamorphosed basic and granitic rocks of the Cerro Mantiqueiras region and in the post-tectonic Lavras Granite, both located in the Vila Nova Terrane and in the syntectonic crust-derived Cacapava Granite in the Dom Feliciano Belt. Contrasting isotopic characteristics also are supported by Nd and Sr isotopes. For the Vila Nova Terrane, Nd model ages are no older than 1.3 Ga, eNd(740 Ma) values are between +2 and +6 and 87Sr/86Sr ratios are low, ∼0.704 (Babinski et al., 1995; Leite, 1997). In the Dom Feliciano Belt, in contrast, Nd mode...

Deepest exposed crust of Brazil-SHRIMP establishes three events

The deepest exposed crust of Brazil is in the western portion of the exposed Precambrian shield of southernmost Rio Grande do Sul State and comprises a bimodal sequence of mafic garnet granulites and metatrondhjemites, intercalated with smaller volumes of metamorphosed pyroxenites, anorthosites, sillimanite gneisses, and banded iron formation. As determined by zircon U/Pb sensitive high-resolution ion microprobe (SHRIMP), this granulite terrain accreted from the mantle at the end of the Archean (ca. 2.55 Ga) and was deformed in high-pressure granulite facies metamorphic conditions in the Paleoproterozoic (ca. 2.02 Ga). A younger event, possibly ca. 900 Ma, caused local amphibolite facies retrogression in the complex. Zircons from a metabasalt and a metatrondhjemite show external morphologies typical of high-grade metamorphic rocks, whereas backscattered electron images reveal internal oscillatory zoning of their igneous precursors. Spot ages from zircon agree well with previous model Nd and Sm/Nd mineral isochron ages.

Zircon U-Pb SHRIMP dating of Palaeoproterozoic Isla Mala granitic magmatism in the Rio de la Plata Craton, Uruguay

Abstract An important event of K-rich granitic magmatism occurred at ca. 2.07 Ga in the Rio de la Plata Craton, Uruguay, as determined by U-Pb zircon SHRIMP geochronology of two Isla Mala Suite granitic rocks. Previous indications of Archaean ages are not confirmed. The absence of recrystallization or new zircon growth suggests that the investigated region of the craton remained unaffected by major Meso- or Neoproterozoic orogenies, although less intense deformation may have occurred and remained unregistered in the zircons.

SHRIMP U–Pb age for the emplacement of the Santana Granite and reactivation of the Porto Alegre Suture, southern Brazil

SHRIMP U–Pb zircon ages are reported from a sample of the Santana Granite that represents the beginning of alkaline magmatic activity in the Neoproterozoic of the Sul-Rio-Grandense Shield, Brazil. This granite outcrops in the city of Porto Alegre and has a NE–SW elongated shape (20 km long and 5 km wide). It is a medium- to coarse-grained syenogranitic rock with subvertical magmatic foliation and subhorizontal lineation with NE–SW trends, overprinted by a weak solid-state fabric. Zircons of this rock with 206Pb/238U ages between ca. 575 and ca. 617 Ma are interpreted as having formed during magmatic crystallisation; the best fit for this age using five analyses provided a mean 206Pb/238U age of 600±10 Ma. This age is also interpreted as the age of the reactivation of the Porto Alegre Suture, a crustal discontinuity along which the Santana Granite was emplaced. Younger isotopic data (<575 Ma) for zircons from this sample are interpreted as due to Pb loss from metamictization. The 600-Ma age is consistent with previous results and marks a magmatic episode, probably related to an extensional tectonic episode that affected the NE of the Sul-Rio-Grandense Shield at the end of the Brasiliano Cycle.

Metamorphic evolution and U-Pb zircon SHRIMP geochronology of the Belizário ultramafic amphibolite, Encantadas Complex, southernmost Brazil

The integrated investigation of metamorphism and zircon U-Pb SHRIMP geochronology of the Belizario ultramafic amphibolite from southernmost Brazil leads to a better understanding of the processes involved in the generation of the Encantadas Complex. Magmatic evidence of the magnesian basalt or pyroxenite protolith is only preserved in cores of zircon crystals, which are dated at 2257 ± 12 Ma. Amphibolite facies metamorphism M1 formed voluminous hornblende in the investigated rock possibly at 1989 ± 21 Ma. This ultramafic rock was re-metamorphosed at 702 ± 21 Ma during a greenschist facies event M2; the assemblage actinolite + oligoclase + microcline + epidote + titanite + monazite formed by alteration of hornblende. The metamorphic events are probably related to the Encantadas Orogeny (2257 ± 12 Ma) and Camboriu Orogeny (∼ 1989 Ma) of the Trans-Amazonian Cycle, followed by an orogenic event (702 ± 21 Ma) of the Brasiliano Cycle. The intervening cratonic period (2000-700 Ma) corresponds to the existence of the Supercontinent Atlantica, known regionally as the Rio de la Plata Craton.

Geocronologia Pb/Pb de zircões detríticos e análise estratigráfica das coberturas sedimentares proterozóicas do Sudoeste do Cráton Amazônico

A analise estratigrafica e dados radiometricos obtidos pelo metodo Pb/Pb de evaporacao em zircoes detriticos das coberturas proterozoicas do sudoeste do Craton Amazonico conduziu ao reconhecimento de duas sequencias deposicionais principais limitadas por discordância regional atraves de toda a extensao deste segmento do Craton. A sequencia mais antiga (Grupo Beneficente) assenta sobre as vulcânicas Teles Pires na Bacia do Cachimbo atraves de discordância angular e erosiva. E constituida por uma unidade inferior de clasticos grosseiros aluviais (Unidade Terrigena) cujas principais fontes de zircoes detriticos situam-se nas vulcânicas acidas a intermediarias paleoproterozoicas da Sequencia Teles Pires sotoposta e no nucleo arqueano da Provincia Amazonia Central a leste. Este pacote basal foi acumulado em leques aluviais e rios arenosos de padrao entrelacado. As idades Pb/Pb obtidas em zircoes detriticos das facies conglomeraticas da base desta unidade indicaram a idade maxima de 1,74 Ga para o inicio da sua deposicao. Na borda sul da Bacia do Cachimbo esta unidade inferior e recoberta concordantemente por clasticos finos e carbonatos, acumulados em ambiente marinho raso epicontinental, com ondas e tempestades como processos deposicionais dominantes. Neste setor, os estratos do Grupo Beneficente mergulham 35°/180 e sao recobertos por uma sequencia de rochas siliciclasticas horizontalizadas, ricas em detritos dos sedimentos sotopostos e correlacionada a formacao Dardanelos. Contudo, a Sequencia Dardanelos aflora principalmente na Bacia Caiabis/Aripuana, a sul do Cachimbo e nas bacias proterozoicas de Rondonia, Mato Grosso e Bolivia (coberturas Guajara Mirim, Aguapei e Sunsas). Na Bacia Caiabis/Aripuana, consiste de areias e cascalhos aluviais com fontes dos clastos predominantemente sedimentares e depositados em sistemas de rios entrelacados, com frequentes intercalacoes de areias de dunas eolicas, sahbkas litorâneas e areias de mar raso (tempestitos), cujos dados de paleocorrentes indicam a existencia contemporânea de uma margem continental a oeste da regiao. A idade maxima do inicio de sua deposicao e indicada pelos zircoes detriticos mais jovens dos conglomerados basais, para os quais foi obtido o valor de 1,3 Ga. O Grupo Aguapei aflora extensivamente no sudoeste de Mato Grosso, noroeste de Mato Grosso do Sul e oriente boliviano e e composto por uma sequencia transgressiva-regressiva que preenche um rift intracontinental de complexa historia geologica. Os zircoes detriticos separados de seu conglomerado basal apontam uma idade mais jovem de 1,35 Ga, interpretada como a idade maxima para o inicio de sua deposicao. Os dados radiometricos e geologicos obtidos possibilitam o reconhecimento de no minimo, duas coberturas sedimentares proterozoicas principais, as sequencias deposicionais Beneficente e Dardanelos, que recobrem o sudoeste do Craton Amazonico, ambas resultando de estiramento crustal e rifteamento nos intervalos 1,7 - 1,3 Ga e 1,3 - 1,0 Ga, respectivamente.

1.76 Ga volcano-plutonism in the southwestern Amazonian craton, Aripuanã-MT, Brazil: tectono-stratigraphic implications from SHRIMP U–Pb zircon data and rock geochemistry

Abstract Paleoproterozoic felsic and mafic volcanic rocks, extrusive and hypabyssal quartz porphyry, and high-K, A-type epizonal granites dominate the Aripuana region in the southwestern Amazonian craton in the Mato Grosso state of Brazil. The extrusive and intrusive felsic rocks display similar major and trace elements abundances and equivalent SHRIMP U–Pb ages of 1762–1755 Ma; contacts between them in the field occur gradually. The felsic volcanic activity occurred predominantly as explosive subaqueous volcanism. Various features such as absence of plate collision evidences, lack of any sign of oceanic crust, lack of complete trends of magmatic differentiation, and bimodal characteristics suggest that the magmatism was intracratonic. The characteristically undeformed felsic igneous rocks were formed by extensional events, probably triggered by mantle activation in an intracratonic ‘anorogenic’ or post-collisional environment. Throughout the Amazonian craton there are wide areas displaying similar magmatism responsible for the generation of distinctive A-type granites. These A-type granites and associated volcanic rocks yield ages of ∼1.76 Ga, suggesting that they are genetically linked and that there was a unique tectonic mechanism of widespread action in this craton.

SHRIMP U-Pb zircon dating of Pedras Grandes Suite, southern Santa Catarina State, Brazil

Two major magmatic pulses of the granitic Florianopolis Batholith in Santa Catarina State, southern Brazil, occurred between 613 ± 5 Ma and 595 ± 5 Ma, during the Neoproterozoic Brasiliano Cycle. These ages were obtained by U-Pb isotopic determinations with the sensitive high mass-resolution ion microprobe on igneous zircons from Pedras Grandes Suite in Santa Catarina State. Euhedral zircons remained unaltered close to a fluorite vein deposited at 180 ◦ C or more. These ages suggest a northern limit for the Pedras Grandes Suite,