Volcanic rocks in Brazil through time and different tectonic settings

Journal of South American Earth Sciences 18 (2005) 233–235 www.elsevier.com/locate/jsames

Preface

Volcanic rocks in Brazil through time and different tectonic settings Volcanic activity is not significant in Recent geological times in Brazil. However, it was extremely relevant in past geological times, from the Archean to the Phanerozoic. The classical Mesozoic Parana´ flood basalt province and the Proterozoic intermediate to acid Uatuma˜ volcanic province of the Amazon craton are good examples of large volcanic provinces exposed dominantly in Brazilian territory, but extending also to neighboring countries. Moreover, basalts and komatiites, now transformed in greenstone belts, are well exposed in Archean and Paleoproterozoic terranes. Alkaline volcanic and hypabyssal rocks were abundant in Mesozoic and Cenozoic times, but also occurred in Precambrian times. From the economic point of view and mineral exploration perspective, understanding mafic and alkaline magmatism in the Brazilian continental platform, sedimentary basins, and associated oil and gas resources are topics of indisputable importance and relevance. Mineral resources related to Precambrian volcanic rocks include, among others, gold, copper, tin, and zinc deposits. Geochemical and isotopic characterization of the volcanic series has contributed to a better understanding of the tectonic and crustal evolution of the Brazilian platform. All mentioned aspects demonstrate the relevance of the study of volcanic rocks in Brazil and neighboring countries. With this in mind, Brazilian geoscientists decided to stimulate research addressing this theme of study and promote periodic events dedicated to volcanism and related geological settings. The first symposium was held in Gramado, Rio Grande do Sul, in 1999, and the second symposium in Bele´m, Para´, in November 2002. In both events, around 100 contributions were presented as invited talks, oral presentations, and poster sessions. This special issue of the Journal of South American Earth Sciences includes selected papers presented during the II Brazilian Symposium on Volcanism and Related Geologic Settings. The symposium was organized by the research group ‘Magmatism, crustal evolution, and metallogenesis of Caraja´s and adjacent provinces’ (PRONEX—Programa Nu´cleos de Exceleˆncia), coordinated by the Center of Geosciences of the Federal University of Para´. The northern branch of the Brazilian Geological Society, and IGCP-426 project ‘Granite Systems and Proterozoic Lithospheric Processes’ co-sponsored the symposium. 0895-9811/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2005.01.001

The scientific program of the II Symposium was thematically organized under the following broad headings: (1) Uatuma˜ and similar volcanic events and associated mineralization; (2) Metavolcanosedimentary sequences and associated mineralization; (3) Magmatism in Phanerozoic basins and its influence in oil deposits; (4) Alkaline volcanic rocks, lamprophyres, and kimberlites and related mineralization; (5) Textures and structures of volcanic rocks; and (6) Post-collisional volcanism related to the Brasiliano orogenic cycle. These subjects reflect partially an approach from the Amazon region, where the II Symposium was held, but they are also representative of the dominant research on volcanism in Brazil. Papers related to all these subjects are included in the present volume. Neoproterozoic volcanism and associated plutonism of southern Brazil related to the Braziliano orogenic cycle is discussed in two papers. Sommer et al. present the general geochemical characteristics of the shoshonitic and mildly alkaline volcanism of the Ramada Plateau. They put in evidence the similarities between the Ramada volcanism and associations of final stages of orogenic cycles and estimate that both the shoshonitic and high- and low-Ti alkaline magmatic rocks reflect melting of subductionmodified sources, while the high-Nb magmas show less influence of subduction-related metasomatism. A model involving slab break-off and asthenosphere upwelling is proposed to explain magma generation. Based mostly on Nd isotope data, Gastal et al. discuss magmatic processes and crustal evolution of the southern Brazilian shield, emphasizing Neoproterozoic granitoids formed during the Sa˜o Gabriel accretionary orogeny (900–700 Ma) and the Dom Feliciano collisional orogeny (660–550 Ma). Gastal and coworkers conclude that the Sa˜o Gabriel arc presents a depleted juvenile signature, whereas the Dom Feliciano belt evolved in an ensialic tectonic setting. Contribution of old crustal segments and three types of mantle components was evaluated. Influence of Neoarchean and Paleoproterozoic crusts was identified in the evolution of the Dom Feliciano belt. Two old enriched mantle components were identified in different kinds of rocks and tectonic domains. One of them was interpreted as related to Paleoproterozoic (2.3– 2.0 Ga) or older events, whereas the younger event probably occurred close to the end of the Paleoproterozoic. Subduction-related Brasiliano mantle should have contributed to the formation of some Neoproterozoic suites (605–580 Ma).

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Preface / Journal of South American Earth Sciences 18 (2005) 233–235

The contribution of Neoarchean crust increased toward the late igneous event (575–550 Ma). The controversial tectonic and crustal evolution of the Tapajo´s Gold Province of the Amazon craton is scrutinized by Lamara˜o et al., taking into account Nd data obtained from Paleoproterozoic magmatic rocks of Vila Riozinho area and the limited information available in the literature. 3Nd values are negative and do not favor a purely juvenile, mantle-derived origin for the magmatic series dominant in the studied area. TDM model ages are all Paleoproterozoic in age and a little older than the ages of rock crystallization, indicating a limited influence of older Archean crust in this province. The authors admit that the ca. 2.0 Ga, high-K to shoshonitic volcanic and plutonic rocks could represent a subduction-related magmatic arc, with magmas modified either through interaction with continental crust or by postto late-orogenic remelting of a slightly older Paleoproterozoic juvenile arc with minor Archean crust contribution. The diversified igneous rocks with ages of ca. 1.88 Ga are associated with the continental-scale event that affected the Amazon craton and was responsible for the breakup of the Paleoproterozoic continent formed at the end of the TransAmazonian orogeny. The authors emphasize the contrast in crustal evolution between the Xingu and Itaituba regions of the Amazon craton, which have a clear isotopic imprint of Archean crust, and the Tapajo´s province, where Archean crust influence is certainly limited. Finally, it is stressed that the boundary between Central Amazonia and Tapajo´s provinces is still poorly defined. Two papers are devoted to the evaluation of the origin and evolution of mineralization associated with metavolcanic-sedimentary sequences. Babinski et al. present geochronological and Pb isotope data on the metamafic dikes cutting the non-sulfide zinc-mineralized metasedimentary Vazante Group, Minas Gerais. They conclude that the metal-bearing fluids that affected the metamafic dikes could be the same responsible for galena formation in minor sulfide ore bodies of the Vazante deposit, suggesting a similar lead source for both non-sulfide and sulfide zinc deposits in the district. Deep-seated basin-derived metalbearing fluids associated with a long-lived Neoproterozoic hydrothermal system are estimated as this common source. Pe´rez-Aguilar et al. employed oxygen isotopes integrated with previous studies of the Kuroko-type hydrothermal system in the Mesoproterozoic Serra do Itaberaba Group (SE Brazil), looking for a better understanding of the alteration processes. Hydrothermal rocks were generated in large pre-metamorphic chloritic (CZ1) alteration halos, overprinted by restricted pre-metamorphic chloritic (CZ2), argillic and advanced argillic (marundite) alterations. They detected systematic variations in oxygen isotopic compositions, interpreted as evidence that the pre-existing isotope signatures of the hydrothermal systems were at least partially preserved in the Serra do Itaberaba hydrothermally altered rocks, overprinted by regional metamorphism. Genesis of CZ1 rocks is related to highly evolved hot sea

water and that of CZ2 rocks and marundite is related to evolved seawater-derived fluids with subordinate magmatic water component. The authors conclude that the studied rocks were related to a long-lived hydrothermal system, which makes possible the existence of associated base metal massive sulfide ore bodies, expanding the mineral potential of the Serra do Itaberaba Group and the Ribeira fold belt. Phanerozoic alkaline rocks are discussed in three papers of the Brası´lia University team that has been working on geochemical and petrological characterization of these rocks in central Brazil. Junqueira-Brod et al. present two papers on the kamafugite lavas and diatremes of the Late Cretaceous Goia´s Alkaline province. In the first paper, intensive variables are used to constrain the behavior of kamafugite magmas from the mantle source to their final emplacement in the upper crust. It is concluded that differentiation in both deep- and shallow-seated magma chambers occurred, with the deeper chambers being probably located in the upper crust and the shallow chambers in the unconformity between the Precambrian basement and the Phanerozoic sedimentary cover rocks. CO2 is considered as the most important volatile phase and crystal fractionation, magma mixing, and liquid immiscibility are invoked to explain the magmatic evolution of the studied kamafugites. The second paper is devoted to the textures and field aspects of kamafugite diatremes. These consist of a central breccia body, surrounded and overlain by lava flows and crosscut by dikes. Spheroidal juvenile fragments and irregularly shaped tuff pockets contained in the breccias are described in detail. Country rocks affected by thermal metamorphism developed columnar jointing and formed peperite-like mixtures. The authors conclude that there is no evidence of phreatomagmatic activity in the diatremes and the dominance of CO2 over H2O implies that accretionary lapilli and peperites are not exclusively associated with H2O-dominated processes. Finally, Ribeiro et al. discuss magma-fragmentation deposits in a carbonate– phosphate magma chamber of the Catala˜o I Complex in central Brazil. This complex is composed of ultramafic silicate rocks with subordinate carbonatite and associated phoscorite, nelsonite, and monazitite. Horizontal-layered alkaline rocks contain cylindrical to conical pipes filled with breccia. Mineralogy, dominated by gorceixite and ilmenite, and structures of the pipes were studied in detail. The authors conclude that fragmentation started inside the chamber. Rocks and structures point to an extreme case, where surge-like deposits formed within a conduit or even inside the magma chamber. Three papers with different approaches present magmatic and tectonic aspects related to the evolution of the Phanerozoic platform and coastal sedimentary basins of the Brazilian Atlantic coast. Guedes et al. identified Mesozoic and Tertiary magmatic pulses in the tholeiite and alkaline dike swarms from the onshore basement of the Santos Basin, employing K–Ar and 40Ar/39Ar data. The high TiO2 tholeiite rocks show contrasts in geochemistry

Preface / Journal of South American Earth Sciences 18 (2005) 233–235

and age (193–161 and 148–134 Ma, A and B dike groups, respectively). The alkaline rocks vary from basalts and trachytes to lamprophyres and have ages between 82 and 59 Ma. Group A dikes preceded the opening of the South Atlantic Ocean and are interpreted as coeval with the Karoo magmatism of South Africa. Group B dikes were directly related with the breakup of western Gondwana. The alkaline magmatism post-dates the opening of the Atlantic Ocean and is tentatively related with the upwelling of the Trindade Plume and the generation of Tertiary Brazilian platform basins. Thomaz Filho et al. focus on the evidence of lithosphere plate drift documented within the Campos Basin on the basis of hot spot volcanic tracks. They suggest a link between both the Poc¸os de Caldas-Cabo Frio alkaline rocks alignment and the Vito´ria-Trindade Chain with a single fixed hot spot and also postulate that the South American continent experienced a clockwise rotation during the Eocene, displacing the hot spot volcanic activity from Cabo Frio to the Vito´ria-Trindade Chain. The paper has economic implications for oil exploration insofar as the authors suggest that the northeast trend of the oil fields of the Campos Basin, containing, so far, the largest oil reserves in Brazil, is coincident with the path followed by the basin moving over the hot spot. Souza et al. studied the mafic to intermediate volcanic rocks associated with the Boa Vista Basin, a small Cenozoic basin found in northeastern Brazil. These rocks are medium-K, Fe-rich tholeiites related by low-pressure fractionation of olivine, plagioclase, magnetite, ilmenite, and apatite. Garnet-bearing metasomatised lherzolite was suggested as their magma source. Magmatic activity is related to continental rifting that reactivated deeprooted Precambrian shear zones. A geochemical study of the Mesozoic–Cenozoic calcalkaline magmatism in the South Shetland Arc of Antarctica is presented by Machado et al. Geochemical signature and phenocrysts observed in the basalt to andesite rocks suggest to the authors a source related to the subduction process and magma evolution from tholeiite to calc-alkaline series. They conclude that the magmas were derived from a lithosphere

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mantle source previously modified by subduction zone fluids and sediments.

Acknowledgements The II Brazilian Symposium on Volcanism received financial support from CENPES/Petrobras, IGCP 426 project (IUGS/UNESCO), Federal University of Para´ (UFPA), CAPES, Companhia Vale do Rio Doce (CVRD), and Secretariat of Culture of Para´ (SECULT). The following reviewers are thanked for their thoroughness and helpful comments and suggestions: Guillermo Alvarado, Costa Rica; Sylvia Arau´jo, Brası´lia; Calvin Barnes, Lubbock; Ray Cas, Victoria; Massimo Chiaradia, Leeds; Eric Christiansen, Provo; Marteen de Wit, Rondebosch; Ronald Fodor, Raleigh; Maria Helena Hollanda, Sa˜o Paulo; Valdecir Janasi, Sa˜o Paulo; Marta Mantovani, Sa˜o Paulo; Leila Marques, Sa˜o Paulo; Ian McReath, Sa˜o Paulo; Aroldo Misi, Salvador; Lauro Nardi, Porto Alegre; David Peate, Iowa City; Phillip Piccoli, College Park; Teal Riley, Cambridge; Arı´ Roisenberg, Porto Alegre; Krister Sundblad, Yliopisto; Eric Tohver, Ann Arbor; Maria Cristina Toledo, Sa˜o Paulo; Richard Tosdal, Vancouver; Horstpeter Ulbrich, Sa˜o Paulo; Netuno Villas, Bele´m; Jose´ Viramonte, Salta; Wilson Wildner, Porto Alegre; Martin Whitehouse, Stockholm. Reinhardt A. Fuck* Departamento de Geologia Geral e Aplicada, Instituto de Geocieˆncias, Universidade de Brası´lia, 70910-900 Brazil E-mail address: [email protected] Roberto Dall’Agnol Centro de Geocieˆncias, Universidade Federal do Para´, Caixa Postal 1611, Bele´m, Para´, Brazil Jorge S. Bettencourt Instituto de Geocieˆncias, Universidade de Sa˜o Paulo, 05508-080 Sa˜o Paulo, Brazil

* Corresponding author.