Moacir José Buenano Macambira

Petrology of the anorogenic, oxidised Jamon and Musa granites, Amazonian Craton: implications for the genesis of Proterozoic A-type granites

The 1.88 Ga Jamon and Musa granites are magnetite-bearing anorogenic, A-type granites of Paleoproterozoic age. They intrude the Archaean rocks of the Rio Maria Granite–Greenstone Terrain in the eastern part of the Amazonian Craton in northern Brazil. A suite of biotite±amphibole monzogranite to syenogranite, with associated dacite porphyry (DP) and granite porphyry (GP) dykes, dominates in these subalkaline granites that vary from metaluminous to peraluminous and show high FeO/(FeO+MgO) and K2O/Na2O. In spite of their broad geochemical similarities, the Jamon and Musa granites show some significant differences in their REE patterns and in the behaviour of Y. The Jamon granites are related by fractional crystallisation of plagioclase, potassium feldspar, quartz, biotite, magnetite±amphibole±apatite±ilmenite. Geochemical modelling and Nd isotopic data indicate that the Archaean granodiorites, trondhjemites and tonalites of the Rio Maria region are not the source of the Jamon Granite and associated dyke magmas. Archaean quartz diorites, differentiated from the mantle at least 1000 m.y. before the emplacement of the granites, have a composition adequate to generate DP and the hornblende–biotite monzogranite magmas by different degrees of partial melting. A larger extent of amphibole fractionation during the evolution of the Musa pluton can explain some of the observed differences between it and the Jamon pluton. The studied granites crystallised at relatively high fO2 and are anorogenic magnetite-series granites. In this aspect, as well as concerning geochemical characteristics, they display many affinities with the Proterozoic A-type granites of south-western United States. The Jamon and Musa granites differ from the anorthosite–mangerite–charnockite–rapakivi granite suites of north-eastern Canada and from the reduced rapakivi granites of the Fennoscandian Shield in several aspects, probably because of different magmatic sources.

Proterozoic anorogenic magmatism in the Central Amazonian Province, amazonian araton: Geochronological, petrological and geochemical aspects

SummaryIn the Central Amazonian Province, the anorogenic granites are older in the eastern block (1.88 Ga, U-Pb; 1.8 - 1.6 Ga Rb-Sr and K-Ar) and in the central block (1.75 -1.7 Ga, Rb-Sr) than in the western one (1.55 Ga, U-Pb). The country rocks are of Archaean age in the eastern block, and in the western block they are of Lower Proterozoic age (Trans-Amazonian Event). There is a minimum difference of 200 Ma between the last metamorphic event and the formation of the anorogenic granites. Metaluminous to peraluminous subsolvus granites are largely dominant but hypersolvus granites, sometimes peralkaline, as well as syenites and quartz-syenites also occur. Wiborgites and pyterlites are found only in the western block but rapakivi-like textures are described in the province as a whole. Mineralizations include large deposits of Sn, as well as small occurrences of F, Zr, REE, Y, and W. The granites are generally rich in Si, K, Fe, Zr, Ga, Nb, Y, and REE and show very high K/Na, Fe/(Fe + Mg) and Ga/Al2O3 ratios. They are geochemically similar to A-type and within-plate granites and more particularly to the Proterozoic rapakivi granites of the Fennoscandian shield and the metaluminous granites of the North American anorogenic province. Petrographic, geochemical and Sr isotopic data indicate crustal sources for the granite magmas. Differences in the sources should explain the contrast observed in some of the granites. The model of crustal anatexis induced by underplating or intrusion of mantle-derived basic magmas is preferred to explain the generation of the crustal granitic magmas.ZusammenfassungIn der zentralen Amazonas-Provinz sind die anorogenen Granite im östlichen (1.88 Mia, U-Pb; 1.8-1.6 Mia, Rb-Sr und K-Ar) und im zentralen Block (1.75-1.7 Mia,Rb-Sr) älter als im westlichen Block (1.55 Mia, U-Pb). Die Nebengesteine sind im östlichen Block archaiischen Alters, während sie im westlichen Block im unteren Proterozoikum (Transamazonas Event) gebildet worden sind. Dies ergibt eine minimale Altersdifferenz von 200 Mio zwischen dem letzten metamorphen Ereignis und der Intrusion der anorogenen Granite. Es dominieren metaluminöse bis peraluminöse Subsolvus-Granite, jedoch treten auch Hypersolvus-Granite, stellenweise Peralkaline, wie auch Syenite und Quarz-Syenite auf. Wiborgite und Pyterlite kommen lediglich im westlichen Block vor, Rapakivi-ähnliche Texturen werden jedoch aus der gesamten Provinz beschrieben. An Mineralisationen treten große Sn-Lagerstätten und kleinere Vorkommen von F, Zr, SEE, Y und W auf. Die Granite sind generell reich an Si, K, Fe, Ga, Nb, Y und SEE und zeigen sehr hohe K/Na, Fe/(Fe + Mg) und Ga/Al2O3 Verhältnisse. Sie zeigen geochemische Ähnlichkeiten mit A-Typ und Intraplatten-Graniten, besonders jedoch mit den proterozoischen Rapakivi Graniten des fennoskandischen Schildes und mit den metaluminösen Graniten der nordamerikanischen anorogenen Provinz. Petrographische, geochemische und Sr-Isotopendaten deuten krustale Ausgangsgesteine der granitischen Magmen an. Unterschiede im Ausgangsmaterial sollen die verschiedenen Granittypen erklären. Als Modell für die Entstehung krustaler granitischer Magmen wird krustale Anatexis, hervorgerufen durch “underplating” oder Intrusion von aus dem Mantel stammenden, basischen Magmen angenommen.

Comparison of single filament Pb evaporation/ionization zircon ages with conventional U-Pb results: examples from the Precambrian of Brazil

Abstract This paper documents a systematic and detailed study comparing the 207 Pb/ 206 Pb apparent ages obtained by single zircon Pb evaporation/ionization method, in rock samples previously dated by conventional U-Pb zircon dissolution procedures. The technique applied here uses a single rhenium center filament to evaporate, ionize and measure the Pb isotopic ratios, in a thermal ionization mass spectrometer, by direct evaporation/ionization of a single zircon grain. The single zircon Pb evaporation/ionization method was applied in igneous and metamorphic rocks of Archean to Neoproterozoic age. The 207 Pb/ 206 Pb obtained for the igneous rocks with little or no evidence of deformation are, within the error, similar to the published U-Pb ages and may be considered as good indicator of the crystallization age of these rocks. However, uncertainties on the 207 Pb/ 206 Pb ratios measured on zircons may constitute a limitation of the Pb evaporation/ionization method, particularly for the precise determination of the age of crystallization of Neoproterozoic rocks or younger. For granulite facies rocks, where a Pb loss discordancy has been imprinted upon the zircons, the 207 Pb/ 206 Pb ages obtained by Pb evaporation/ionization method must be considered as minimum ages. The results obtained here along with the relative simplicity of the analytical procedures used in the single zircon Pb evaporation/ionization method show great potential for the application of this technique in the reconnaissance evaluation of the ages of rocks units of Precambrian crust where a geochronological framework is not well established.

1.88 Ga Oxidized A‐Type Granites of the Rio Maria Region, Eastern Amazonian Craton, Brazil: Positively Anorogenic!

1.88 Ga A‐type granites in the Archean Rio Maria granite‐greenstone terrane of eastern Amazonian craton, Brazil, have much in common with the oxidized ∼1.4 Ga “anorogenic” granites of the western United States. Nd isotopic data on these granites and their Archean country rocks show that (1) the Rio Maria crust was differentiated from depleted mantle at 3.0 Ga, (2) the Paleoproterozoic granites were derived from deep parts of this crust, and (3) the Paleoproterozoic granites postdate their protolith by at least 1 Ga. No convergent processes are known to have affected the eastern Amazonian craton at ∼1.9 Ga and, therefore, there is little doubt as to the anorogenic origin of the oxidized A‐type granites of Rio Maria. Recognition of this genuinely anorogenic Proterozoic granite suite in Amazonia adds cymatogenic flavor to the current debate on the origin of the mid‐Proterozoic “anorogenic” granites of the western United States.

Geocronologia e evolução crustal da área do depósito de Cu-Au Gameleira, Província Mineral de Carajás (Pará), Brasil

The Gameleira Cu-Au deposit is hosted by rocks of the Igarape Pojuca Group, Itacaiunas Supergroup, Carajas Mineral Province, southeastern Amazonian Craton. This group is represented mainly by mafic metavolcanic rocks (MVR), amphibolite, biotite schist, banded iron formation and/or hydrothermalite and cut by intrusive mafic rocks (IMR) as well as by deformed Itacaiunas granites of Archean age (2.56 Ga) and Pojuca and Gameleira granites of Proterozoic age (1.87 and 1.58 Ga). Zircon crystals from a saprolite (2615 ± 10 Ma and 2683 ± 7 Ma) and from an IMR sample (2705 ± 2 Ma) are coeval with those from gabbros of the neighboring Aguas Claras deposit. Pb-Pb dating of whole rock samples and chalcopyrite from the MVR indicated ages of 2245 ± 29 Ma and 2419 ± 12 Ma, respectively, while leached chalcopyrite yielded ages of 2217 ± 19 Ma and 2180 ± 84 Ma. These ages are interpreted as due to partial resetting provoked by the Proterozoic granitic intrusions or by tectonic reactivation along the Carajas and Cinzento strike slip systems or alternatively, as due to total resetting provoked by the last one. The T DM ages between 3.12 and 3.33 Ga for the MVR and IMR, and the initial µNd (t) values of -0.89 to -3.26 suggest continental contribution from older crustal material and possibly magma generation in a continental rift or active continental margin environment.

Zircon Geochronology and Petrology of Alkaline-Potassic Syenites, Southwestern Serrinha Nucleus, East São Francisco Craton, Brazil

Abundant syenitic magmatism occurred on the São Francisco Cráton (Bahia State, Brazil) during the Paleoproterozoic period. Two syenitic stocks (Serra do Pintado and Morro das Agulhas-Bananas) from the eastern part of the craton have been dated at 2098 ± 2 Ma (U-Pb) and 2086 ± 17 to 2067 ± 22 Ma (Pb-Pb evaporation), respectively, suggesting that these are contemporaneous intrusions correlated with a post-Transamazonian episode of magmatism. In both of these stocks, hypersolvus alkali-feldspar syenites predominate. Subordinate hornblende-cumulates also occur, as well as dikes of alkali-feldspar syenite and alkali-granite. These syenitic plutons are alkaline, potassic, strongly enriched in LILE, and HFSE depleted. They also show moderate Al2O3, high La/Nb and Ba/La ratios, and low Ce/Pb ratios. Petrographic and geochemical data suggest that these syenitic rocks are the result of fractional crystallization from potassic mafic magmas. These data indicate that the magmas were produced during subduction, and that garnet was a stable constituent of the residue of partial melting, indicating a depth of at least 80 km. These potassic magmas did not crystallize plagioclase which, along with other chemical and mineralogical characteristics, indicates a lamprophyric affinity.

Uranium-lead dating method at the Pará-Iso isotope geology laboratory, UFPA, Belém - Brazil

Analytical procedures for U-Pb isotope dilution analyses at the Pará-Iso isotope geology laboratory of the Federal University of Pará (UFPA) are described in detail. The procedures are applied to zircon, titanite, rutile, apatite, columbite-tantalite and whole rock. Reagent preparation and chemical processing are done in clean-room conditions. Samples are dissolved using Teflon microcapsules in steel jacket Teflon Parr Instrument bomb or Teflon screw cap containers. U and Pb are separated using anion exchange AG 1 x 8 resin columns. Typical blanks for mineral sample amounts of 0.01 to 1.0 mg are less than 1 pg U and 20-30 pg Pb. Isotope analysis of the U and Pb from the same filament are carried out using a Finnigan MAT 262 mass-spectrometer in static and dynamic modes. The current analytical level is demonstrated on analyses of international standard zircon 91500 with three different 235U-205Pb and 235U-208Pb isotope tracers and whole rock standards. Results of analyses of two zircon samples are also presented.

Batólito Guaporeí: uma extensão do Complexo Granitoide Pensamiento em Mato Grosso, SW do Cráton Amazônico

O Batolito Guaporei e um corpo de aproximadamente 240 km 2 alongado segundo a direcao NW, localizado na regiao de Vila Bela da Santissima Trindade, estado de Mato Grosso. Situa-se nos domi- nios da Provincia Rondoniana-San Ignacio, no Terreno Paragua, na por- cao meridional do Craton Amazonico. E formado por monzogranitos e, subordinadamente, granodioritos, quartzo-monzonitos e sienogranitos, caracterizados por granulacao grossa e textura, em geral, porfiritica a porfi- roclastica. Possui biotita como mineral mafico primario, por vezes, associa- da a anfibolio, e encontra-se metamorfizado na facies xisto verde, exibindo estrutura milonitica, em estreitas zonas de cisalhamento. Evidencias geo- quimicas indicam que essas rochas derivam de um magma calcio-alcalino de alto potassio a shoshonitico, metaluminoso a levemente peraluminoso evoluido por cristalizacao fracionada associada a assimilacao crustal, possi- velmente gerado em ambiente de arco continental. Duas fases de deforma- cao relacionadas a Orogenia San Ignacio, caracterizadas pelo estiramento e alinhamento mineral evidenciadas pelas foliacoes S 1 e S 2 , foram identifica- das nestas rochas. Foi obtida pelo metodo de evaporacao de Pb em zircao uma idade de 1.314 ± 3 Ma, interpretada como idade de cristalizacao do corpo granitico. Dados Sm-Nd em rocha total indicam idade modelo T DM em torno de 1,7 Ga e valor negativo para e Nd (t = 1,3) (-14), corroborando a hipotese de envolvimento crustal na genese do magma. Os resultados ob- tidos apontam semelhancas entre essas rochas e aquelas de regiao adjacente em territorio boliviano, sugerindo que o Granito Guaporei representa uma extensao do Complexo Granitoide Pensamiento.

Field and petrographic data of 1.90 to 1.88 Ga I- and A-type granitoids from the central region of the Amazonian Craton, NE Amazonas State, Brazil

The SW Presidente Figueiredo district, which is located in the northeastern Amazonas State of the central Amazonian Craton, Brazil, consists of 1890 to 1898 Ma I-type granitoids (Terra Preta Granite, Agua Branca Suite), A-type hornblende-bearing syenogranites (Canoas Syenogranite, Mapuera Suite), felsic to intermediate volcanic rocks (Iricoume Group), and 1883 to 1889 Ma rapakivi granites (Sao Gabriel Granite, Mapuera Suite) and related rocks (quartz-gabbro-anorthosite and diorite), in addition to Castanhal quartz-monzonite, mylonites, and hornfels. The quartz-diorite facies of Terra Preta Granite were formed by mingling processes between a synplutonic quartz-gabbro dike and a hornblende granodiorite. Partially assimilated globules of Canoas hornblende-bearing syenogranite and their clear contacts with Terra Preta hornblende granodiorite suggest that Canoas Syenogranite is slightly younger than Terra Preta Granite. Canoas Syenogranite xenoliths inside Sao Gabriel Granite show that the granite is younger than the Canoas Syenogranite. New geologic and petrographic evidence improve the petrological understanding of these rocks and suggest that, in addition to fractional crystallization, assimilation and magma mingling played a role, at least at the local scale, in the evolution and compositional variation of the plutons. Such evidence is found in Terra Preta Granite mingled quartz-diorite, felsic material associated with the Canoas Syenogranite and in the intermediate microgranular enclaves, which exhibit primary biotite in hornblende-bearing rocks, plagioclase dissolution, corrosion of feldspars rims, alkali feldspar mantles, second apatite generation, and high xenocrystal contents in intermediate enclaves formed from the fragmentation of mafic intrusions. Petrographic analyses show that a deformational event recorded in the western part of the study area (with progressive deformation from E to W) is dated between the 1.90 Ga postcollisional magmatism and the intrusions of the Sao Gabriel Granite and related mafic/intermediate rocks (intra-plate). However, it is extremely necessary to obtain absolute ages for this metamorphic event.

Petrography, geochemistry and Sm-Nd isotopes of the granites from eastern of the Tapajós Domain, Pará state

O Dominio Tapajos, localizado na porcao sul do Craton Amazonico, e um dominio tectonico da Provincia Tapajos-Parima, um cinturao orogenico Paleoproterozoico, adjacente a uma crosta arqueana retrabalhada, a Provincia Amazonia Central. Esse dominio foi interpretado como resultado da acrecao de sucessivos arcos magmaticos, seguidos por um magmatismo pos-orogenico do tipo A, representado por granitos de ca. 1880 Ma da Suite Intrusiva Maloquinha. Este estudo foi realizado em quatro granitos dessa suite (Igarape Tabuleiro, Dalpaiz, Mamoal e Serra Alta) da porcao leste do Dominio Tapajos, assim como um granito tipo I (Igarape Salustiano), relacionado a Suite Intrusiva Parauari. Os granitos tipo A sao sienogranitos e monzogranitos, e subordinadamente feldspato alcalino granitos e quartzo sienitos. Essas rochas sao ferrosas, alcalicalcica a alcalinas e dominantemente peraluminosas, com anomalias negativas de Ba, Sr, P e Ti, e alto conteudo de elementos terras raras (ETR) com pronunciada anomalia negativa de Eu. Esse conjunto de caracteristicas e tipico de granitos tipo A. O Granito Igarape Salustiano apresenta monzogranitos e quartzo monzonitos magnesianos, calcicos a calcioalcalinos, alto K e dominantemente metaluminosos, com alto conteudo de Ba e Sr, e empobrecidos em high field strength elements (HFSE) e ETR pesados, caracteristicos de granitos do tipo I. A fonte do magma dos granitos tipo A e similar a de granitos pos-colisionais, enquanto os granitos tipo I mantem assinatura sincolisional. A maioria dos granitos estudados tem e Nd (-3,85 a -0,76) e idades modelo Nd T DM (2,22 a 2,46 Ga) compativeis com a crosta paleoproterozoica do Dominio Tapajos. Conclui-se que fonte crustal arqueana (e Nd de -5,01 e Nd T DM de 2,6 Ga) e local para os granitos tipo A.