Evandro Fernandes de Lima

Geology, geochemistry, and Pb–Pb zircon geochronology of the Paleoproterozoic magmatism of Vila Riozinho, Tapajós Gold Province, Amazonian craton, Brazil

Abstract Pb–Pb zircon geochronologic data define two different periods of intense igneous activity in the Vila Riozinho region, Tapajos Gold Province (TGP), south-central Amazonian craton. At ∼2.00–1.97 Ga, the Vila Riozinho volcanic sequence (2000±4 Ma, 1998±3 Ma) and the older Sao Jorge granite (1981±2 Ma, 1983±8 Ma) were formed. At ∼1.89–1.87 Ga, the younger Sao Jorge (1891±3 Ma), Jardim do Ouro (1880±3 Ma), and Maloquinha (1880±9 Ma) granites and the Moraes Almeida volcanic sequence (1890±6 Ma, 1881±4 Ma, 1875±4 Ma) were emplaced. Similar age intervals are registered throughout the TGP that is thus a little younger than the Paleoproterozoic terranes of the Maroni-Itacaiunas Province in the Guiana Shield. Geochemical and geochronologic data demonstrate that the Sao Jorge pluton is composed of two different granitoids, the older and younger Sao Jorge granites. These, as well as the Jardim do Ouro granite, are I-type and magnetite series. The Sao Jorge granites are high-K calc-alkaline and show significant geochemical variation, the Jardim do Ouro granite is more iron-rich and less oxidized. The Vila Riozinho Formation is calc-alkaline to shoshonitic and geochemically similar to the Sao Jorge granites. The Maloquinha granite and the Moraes Almeida Formation differ from the other studied rocks in petrographic and geochemical characteristics, have aluminous A-type affinities, and were probably derived by low-temperature crustal melting. The volcanic sequences of the studied area, formerly included into the Iriri Group of the Uatuma Supergroup, are divided into the Vila Riozinho and Moraes Almeida sequences. The presence of these two sequences in the same tectonic domain demonstrates the heterogeneity of the Uatuma Supergroup. The TGP registers accretionary processes related to the formation of the Atlantica supercontinent at ∼2.00 Ga. This was followed (at ∼1.88 Ga) by an intracontinental taphrogenic event that lasted throughout the Mesoproterozoic. The tectonic setting of the TGP was thus transitional between a subduction-related magmatic arc and a stable continental block undergoing extension.

Volcanic cycles and setting in the Neoproterozoic III to Ordovician Camaquã Basin succession in southern Brazil: characteristics of post-collisional magmatism

The Camaqua Basin comprises a volcano-sedimentary succession, located in southernmost Brazil, and represents a molasse basin formed at the post-collisional stage of the Brasiliano/Pan-African orogenic cycle in the Neoproterozoic III to Ordovician period. This basin is one of the most well-preserved ancient volcano-sedimentary sequences undeformed and unmetamorphic in the world, dominantly developed on a continental setting under subaerial conditions. It is composed of five major stratigraphic units, four of them with a distinct volcanic character from the bottom to the top, as: (1) Marica; (2) Bom Jardim; (3) Acampamento Velho; (4) Santa Barbara; and (5) Guaritas Allogroups. A concise sight of geochemical and isotopic rock data is presented, as well as stratigraphic correlation and description of rock structures and textures that lead to the identification of their genetic processes, the aim of this paper, indicating a relation with a coeval plutonism, and volcanism that evolved from high-K calc-alkaline to shoshonitic and ended with a silica-saturated sodic alkaline magmatism, with a crustal component represented by peraluminous granites. Volcanic deposits from bottom to top are made mostly of volcanogenic sedimentary deposits, succeeded by basic to intermediate lava and pyroclastic flows of shoshonitic affinity, followed by intermediate and acid lava flows and ignimbrites of sodic alkaline affinity. The last volcanic event is represented by basalt pahoehoe flows, probably of mildly alkaline sodic affinity.

The evolution of Neoproterozoic magmatism in Southernmost Brazil: shoshonitic, high-K tholeiitic and silica-saturated, sodic alkaline volcanism in post-collisional basins

The Neoproterozoic shoshonitic and mildly alkaline bimodal volcanism of Southernmost Brazil is represented by rock assemblages associated to sedimentary successions, deposited in strike-slip basins formed at the post-collisional stages of the Brasilian/Pan-African orogenic cycle. The best-preserved volcano sedimentary associations occur in the Camaquã and Campo Alegre Basins, respectively in the Sul-riograndense and Catarinense Shields and are outside the main shear belts or overlying the unaffected basement areas. These basins are characterized by alternation of volcanic cycles and siliciclastic sedimentation developed dominantly on a continental setting under subaerial conditions. This volcanism and the coeval plutonism evolved from high-K tholeiitic and calc-alkaline to shoshonitic and ended with a silica-saturated sodic alkaline magmatism, and its evolution were developed during at least 60 Ma. The compositional variation and evolution of post-collisional magmatism in southern Brazil are interpreted as the result mainly of melting of a heterogeneous mantle source, which includes garnet-phlogopite-bearing peridotites, veined-peridotites with abundant hydrated phases, such as amphibole, apatite and phlogopite, and eventually with the addition of an asthenospheric component. The subduction-related metasomatic character of post-collisional magmatism mantle sources in southern Brazil is put in evidence by Nb-negative anomalies and isotope features typical of EM1 sources.

Post-Collisional Alkaline Magmatism on the Taquarembó Plateau: A Well-Preserved Neoproterozoic–Cambrian Plutono-volcanic Association in Southern Brazil

The Taquarembo Plateau plutono-volcanic association (TPPVA), a magmatic association related to a silica-saturated alkaline series, represents a portion of the last episode of post-Brasiliano/Pan-African collisional magmatism in southern Brazil. It was preceded by a postcollisional high-K calc-alkaline and shoshonitic magmatism, which is more pronounced near the collisional belt. The TPPVA is a sequence of volcanic, volcaniclastic, and volcanogenic sedimentary deposits with hypabyssal associated rocks, lying on Paleoproterozoic granulites. Isotope data suggest that this alkaline postcollisional magmatism occurred over the period from 580 to 537 Ma. Two sequences of magmatic liquids, which evolved by mineral fractionation from low-Ti-P and high-Ti-P basaltic magmas, were identified. The former includes mildly alkaline silica-saturated basalts, metaluminous monzodioritic intrusions, and rhyolitic lavas, whereas the second includes hawaiites, mugearites, syenitic intrusions, and peralkaline to intermediate la...

Geochemistry of Palaeoproterozoic volcanic rocks of the Iricoumé Group, Pitinga Mining District, Amazonian craton, Brazil

The Iricoumé Group includes 1.88 Ga volcanic units of the Iricoumé–Mapuera volcano-plutonic association, part of the Uatumã magmatic series in the Guyana shield portion of the Amazonian craton. In the Pitinga Mining District, these rocks consist dominantly of felsic trachyte to rhyolite, associated with voluminous ignimbrite and minor ash-fall tuffs and surge deposits. Mafic rocks are present as basaltic clasts within volcanic breccias, and mostly as mafic microgranular enclaves in the associated Mapuera plutonic rocks. The felsic rocks have high contents of SiO2, FeOt, K2O, Rb, and alkalis; low TiO2, CaO, Sr, Ba, Nb, Ta, and Eu; and show metaluminous to weakly peraluminous bulk-rock compositions. They exhibit alkaline geochemical features, expressed by Na2O + K2O averages of 8.8 wt.%, FeOt/(FeOt + MgO) ≥ 0.8, and high Ga/Al ratios, compatible with A-type magmas. The studied samples plot in the field of within-plate or post-collisional rocks in a (Nb + Y) versus Rb diagram. Nb/Y ratios indicate that they are comparable to A2-type rocks which, allied with their high LREE/Nb ratios, suggest that they were produced from mantle sources modified by previous subduction in a post-collisional setting. Two compositional populations of Ca-amphibole, a Mg-rich (actinolite to Mg-hornblende) and a Fe-rich one (Fe-edenite to Fe-pargasite, Fe-hornblende and Fe-actinolite), characterize the Iricoumé Group volcanics. The Fe-rich amphiboles crystallized under lower fO2 and higher pressure conditions compared with the Mg-rich amphiboles, indicating different levels of crystallization or re-equilibration during ascent of the magmas. Zircons from rhyolites show trace-element compositions typical of magmatic crystals with high Th/U ratios, and REE patterns compatible with zircon-melt partition coefficients for silicic magma compositions. Their relatively lower zircon/rock partition coefficients are due to early apatite crystallization. Fractional crystallization mainly of plagioclase-hornblende and biotite-alkali feldspar with minor amounts of apatite explains the geochemical trends observed in the felsic Iricoumé volcanic rocks.

HIDROTERMALISMO NO COMPLEXO GRANÍTICO LAVRAS E VULCÂNICAS ASSOCIADAS, RS

The gold-sulphide mineralization in the Lavras do Sul region (RS) is associated with propilitization, sericitization, and silicification of host rocks. These hydrothermal alterations are evident in the Lavras Granite Complex: and volcanoclastic rocks. Episyenites and beresites have been identified among the hydrothermal products. Majors and traces elements distribution in granitic rocks have been extensively affected just where the hydrothermal alternation is extremely well-developed, reaching its most advanced stages. Loss on ignition, representing the amount of volatile elements, is the best indicator of hydrothermal alteration intensity, particularly in the propilitized and sericitized materials. In the granitic rocks, Au, Ag, and As are the best pathfinders for lithogeochemical exploration. K 2 O and Na 2 O show sharp decrease near the mineralized zones in the Merita area . REE had low mobility during the hydrothermal event identified in the Lavras Granite Complex: LREE depletion has been noticed only in deeply altered products as episyenites. REE have been determined in sericite, chlorite, and epidote concentrates. Even though subsurface data are not available, it is possible to suggest a porph yry copper model for this mineralization based upon the possibility of vein mineralization presently known, representing hydrothermal activity in the suprastructure of the main deposit.

Hybridisation of mafic microgranular enclaves in the Lavras Granite Complex, southern Brazil

Abstract Twenty-five mafic microgranular enclaves of the Lavras Granite Complex in southern Brazil were studied petrographically and geochemically to establish their origin and to investigate the processes involved in their differentiation. Mesoscopic and microscopic textures indicate that they are products of magma mingling between a basic end member of probable mildly alkaline affinity and host shoshonitic and alkaline granitic rocks. The hybridisation process involved at least the following mechanisms: (i) chemical diffusion of volatiles and very mobile elements such as K to the less polymerised liquids, leading to the crystallisation of hydrated mafic minerals; (ii) chemical diffusion of Ti and P to the less polymerised liquids, leading to titanite and apatite crystallisation; (iii) mechanical accretion in the basic magma of early crystallised host granite phases that promoted enrichment of their major constituents and of trace elements with high partition coefficients in these phases; (iv) chemical diffusion of elements such as Rb, Nb, Y, and Yb with high Kd in the major enclave phases, from host magma into the basic enclaves. These processes occurred simultaneously, probably before the dispersion of basic batch magma forming the mafic microgranular enclaves, and caused hybridisation and complex geochemical patterns. The patterns are very different from the linear trends predicted for near-equilibrium systems such as those of magma mixing or fractional crystallisation.

Paleoproterozoic (~1.88Ga) felsic volcanism of the Iricoumé Group in the Pitinga Mining District area, Amazonian Craton, Brazil: insights in ancient volcanic processes from field and petrologic data

O Grupo Iricoume corresponde ao mais expressivo vulcanismo Paleoproterozoico do Escudo das Guianas, craton Amazonico. As rochas vulcânicas sao coexistentes com os granitoides Mapuera, e pertencem ao magmatismo Uatuma. Possuem idades U-Pb em torno 1888 Ma, e assinaturas geoquimicas de magmas tipo-A. As vulcânicas do Iricoume consistem de traquitos a riolitos porfiriticos, associados a ignimbritos ricos em cristal e tufos co-ignimbriticos de queda e surge. A quantidade e a morfologia dos fenocristais podem ser utilizadas para distinguir lava (fluxo e domo) de unidades hipabissais. A morfologia dos cristais em ignimbritos permite a distincao entre unidades efusivas e ignimbritos, quando os piroclastos estao obliterados. Tufos co-ignimbriticos sao macicos e alguns exibem estratificacoes que sugerem deposicao por correntes de tracao. Temperaturas de cristalizacao de zircao e apatita variam de 799°C a 980°C, sao compativeis com temperaturas de liquidos tipo-A e podem ser interpretadas como temperatura liquidus minima. Estimativas de viscosidade para composicoes rioliticas e traquiticas fornecem valores proximos a de liquidos determinadas experimentalmente e ilustram curvas tipicas de decaimento exponencial, com a adicao de agua. O posicionamento das vulcânicas Iricoume e de parte dos granitoides Mapuera foi controlado por falhas anelares em ambiente intracratonico. Uma genese relacionada a ambiente de complexo de caldeiras pode ser assumida para a associacao vulcano-plutonica Iricoume-Mapuera no Distrito Mineiro de Pitinga.

The role of viscosity in the emplacement of high-temperature acidic flows of Serra Geral Formation in Torres Syncline (Rio Grande do Sul State, Brazil)

The acidic flows from Serra Geral Formation in Torres Syncline, Rio Grande do Sul, Brazil, are on the top of a volcanic sequence composed by a complex facies association of compound, simple and rubbly pahoehoebasic flows, acidic lava domes, and tabular acidic lava flows. The origin and emplacement conditions of the acidic volcanic rocks are discussed in this paper based on petrology, on calculated apatite saturation thermometry temperatures, and on estimated viscosity data. The liquidus temperatures for metaluminous rhyodacite to rhyolite samples are about 1,067.5 ± 25oC in average. The viscosity (η) values vary from 105 to 106 Pas for anhydrous conditions, suggesting the emplacement of high-temperature - low-viscosity lava flows and domes. The occurrence of acidic lava domes above simple pahoehoe flows as flow-banded vitrophyres was under low effusion rates, in spite of their high temperature and low viscosities, which are reflected in their small height. The emplacement of lava domes has continued until the eruption of rubbly pahoehoe flows and the geometry of these deposits rugged the relief. Presence of tabular acidic lava flows covering the landscape indicates that it was under high effusion rates conditions and such flows had well-insulated cooled surface crusts. The capacity to attain greater distances and overpass relief obstacles is explained not only by high effusion rates, but also by very low viscosities at the time of emplacement.

Vulcanismo félsico paleoproterozoico do Grupo Iricoumé, Domínio Erepecuru-Trombetas, Província Amazônia Central: dados de campo, caracterização petrográfica e geocronologia Pb-Pb em zircão

O Grupo Iricoume compreende rochas vulcânicas efusivas e piroclasticas, com texturas e estruturas bastante preservadas, que pertence a um extenso evento vulcano-plutonico que marcou a regiao central do Craton Amazonico durante o Orosiriano. Tais rochas estao expostas no noroeste do estado do Para, na porcao meridional do sudoeste do Dominio Erepecuru-Trombetas, sul do Escudo das Guianas. Estudos petrograficos permitiram distinguir um vulcanismo explosivo, predominante e representado por rochas piroclasticas (ignimbritos, reoignimbritos, tufo coignimbritico de queda e lapili-tufo relacionado a surge), e um efusivo, subordinado, representado por fluxos de lavas coerentes e rochas hipabissais (andesitos, lamprofiros espessartiticos e latitos). A maioria das rochas piroclasticas exibe feicoes diagnosticas da deposicao dos piroclastos sob altas temperaturas, sugerindo que as rochas vulcânicas estao provavelmente relacionadas a ambientes de geracao de caldeiras. As idades Pb-Pb de 1888 ± 2,5 e 1889 ± 2 Ma obtidas em zircao de ignimbritos traquidaciticos confirmam que a maioria das rochas estudadas pertence ao Grupo Iricoume. Por outro lado, a idade Pb-Pb de 1992 ± 3 Ma obtida em zircao de um andesito evidencia um episodio vulcânico efusivo orosiriano mais antigo, ja reconhecido, localmente, mais a sul, no Dominio Tapajos. Os dados obtidos demonstram a ampla extensao do vulcanismo Iricoume e rochas vulcânicas correlatas na porcao central do Craton Amazonico, e constituem argumentos favoraveis para associar esse episodio vulcânico e rochas magmaticas correlatas a uma silicic large igneous province (SLIP), como ja vem sendo descrito por alguns autores.