Jorge Silva Bettencourt

Mesoproterozoic tectonic correlations between eastern Laurentia and the western border of the Amazon Craton

Abstract A possible palaeogeographic link between the Grenville and Rondonia-Sunsas provinces is reinforced by the timing of thermometamorphic and plutonic events. Equivalent terms of regional geological time scales are: Ketilidian = Transamazonian; Palaeohelikian or Elsonian = Rondonian-San Ygnacio; Neohelikian or Grenvillian = Sunsas; Hadrynian = Brasiliano. Remobilization of the older crust is another feature common to both provinces in Helikian times. Felsic volcanics extruded 1650 million years ago in the eastern Grenville Province show 1.6 Ga analogues in the Roosevelt lava flows of the Amazon Craton. Also a recently defined 1470–1500 Ma Pinwarian event in the Grenville shows correlatives in the final stages of evolution of the Rio Negro-Juruena belt. Significant anorthosite-mangerite-charnokite-granite (AMCG) plutonism affected both regions during closely spaced periods of time. Two pulses of late potash-rich plutonism responsible for rapakivi intrusions from about 1.1 to 0.95 Ga are recognized in Grenvillia from Labrador to Texas and in the Rondonian-Sunsas Province of the Amazon Craton. Differences between the provinces include AMCG massifs of northeastern Laurentia whose equivalents in Rondonia are small layered mafic bodies, besides mangeritic/charnockitic intrusions and the large extensional and transtensional features of Sunsas age in the southwest Amazon sector. The AMCG plutonism roughly coeval with cratonic magmatism associated with transtensional features in the Amazon Craton allow two possible models to be postulated: (a) Amazonian transtensional features originated in response to collision with a left-lateral component, as along the Grenville front similar to the reactivation of the Asian continent due to the Himalayan push (diva basins and magmatism on the Sinian Platform); (b) major continental extensions related to the start of two Wilson cycles, one preceding the opening of a Grenvillian Ocean at around 1.4–1.3 Ga and, the other, due to the separation of Laurentia from Amazonia at the end of the Grenvillian cycle. Both have been related to extensive underplating.

Restored transect across the exhumed Grenville orogen of Laurentia and Amazonia, with implications for crustal architecture

New 40 Ar/ 39 Ar analyses from a transect across the major tectonic units of the southwest Amazon craton document the heterogeneous effects of the late Mesoproterozoic collision with the Grenville margin of North America. Basement rocks of the Amazon and adjacent Paragua cratons mostly preserve pre-Grenvillian ages (older than 1.3 Ga). Localized iso- topic age resetting at 1.18-1.12 Ga is caused by Grenvillian activation of widespread, sinistral strike-slip shear zones in the Amazon basement. In the Nova Brasilandia belt between these two cratons, new 40 Ar/ 39 Ar data record cooling through 920 Ma after the granulite facies deformation of this suture zone. Regional cooling rates calculated from compiled U/Pb, 40 Ar/ 39 Ar, and Rb/Sr thermochronologic data are used to establish post- Grenvillian exhumation patterns for the southwest Amazon and the North American belt. Paleodepths calculated for 1.0 Ga along a transect of the restored 1300-km-wide belt vary from uniformly deep levels (15-30 km) exposed in North America to shallower levels (5- 15 km) observed in the southwest Amazon. We interpret this difference as reflective of a change in tectonic architecture, i.e., thrust-dominated deformation in Laurentia versus strike-slip dominated deformation in the Amazon, with a commensurate variation in crust- al thickness. This interpretation explains the widespread preservation of both pre- Grenvillian ages and collisional ages from the Amazon craton, in contrast with the more homogeneous array of cooling ages from the North American Grenville Province marking the postorogenic extensional collapse of an overthickened crust. The asymmetrical oro- genic architecture from the reconstructed Grenville belt mirrors cross sections proposed for modern orogenic belts where deep-crustal rocks are not yet exposed.

Geological evolution of the basement rocks in the east-central part of the Rondônia Tin Province, SW Amazonian craton, Brazil: U–Pb and Sm–Nd isotopic constraints

On the basis of geologic, petrologic, and U–Pb geochronologic data the basement rocks in the east-central part of the Rondonia Tin Province (RTP, southwestern Amazonian craton) are grouped into five lithologic associations: (1) tonalitic gneiss (1.75 Ga); (2) enderbitic granulite (1.73 Ga); (3) paragneiss; (4) granitic and charnockitic augen gneisses (1.57–1.53 Ga); and (5) fine-grained granitic gneiss and charnockitic granulite (1.43–1.42 Ga). The first three are related to development of the Paleoproterozoic Rio Negro-Juruena Province and represent the oldest crust in the region. The tonalitic gneisses and enderbitic granulites show calc-alkaline affinities and Nd isotopic compositions (initial eNd=+0.1 to −1.5; TDM of 2.2–2.1 Ga) that suggest a continental arc margin setting for the original magmas. The paragneisses yield TDM values of 2.2–2.1 Ga suggesting that source material was primarily derived from the Ventuari-Tapajos and Rio Negro-Juruena crusts, but detrital zircon ages and an intrusive granitoid bracket deposition between 1.67 and 1.57 Ga. The granitic and charnockitic augen gneisses show predominantly A-type and within-plate granite affinities, but also some volcanic arc granite characteristics. The initial eNd values (+0.6 to +2.0) indicate mixing of magmas derived from depleted mantle and older crustal sources. These rocks are correlated to the 1.60–1.53 Ga Serra da Providencia intrusive suite that reflects inboard magmatism coeval with the Cachoeirinha orogen located to the southeast. The fine-grained granitic gneiss and charnockitic granulites represent the first record of widespread magmatism at 1.43–1.42 Ga in northern Rondonia. Their geochemical signatures and the slightly positive initial eNd values (+0.7 to +1.2) are very similar to those of the most evolved granites of the calc-alkaline Santa Helena batholith farther southeast. U–Pb monazite and Sm–Nd whole-rock-garnet ages demonstrate that a high-grade tectonometamorphic episode occurred in this region at 1.33–1.30 Ga. This episode attained upper-amphibolite conditions and is interpreted as the peak of the Rondonian-San Ignacio orogeny. The U–Pb and Sm–Nd data presented here and data published on rapakivi granites elsewhere indicate that the east-central part of the RTP is a poly-orogenic region characterized by successive episodes of magmatism, metamorphism, and deformation between 1.75 and 0.97 Ga.

Archean crustal sources for Paleoproterozoic tin-mineralized granites in the Carajas Province, SSE Para ´, Brazil: Pb /Pb geochronology and Nd isotope geochemistry

Whole-rock and K-feldspar Pb, single zircon Pb-evaporation, and Nd whole-rock isotopic data are presented for granites of the Velho Guilherme intrusive suite and the volcanic rocks of the UatumaGroup in the Carajas Mineral Province, southern Para ´, Brazil. Pb � /Pb zircon ages of 18679 /4 Ma, 18629 /16 Ma, and 18669 /3 Ma for the Antonio Vicente, Mocambo, and Rio Xingu granite massifs, respectively, show that the granites were emplaced at � /1870 Ma. Pb � /Pb whole-rock reference age of the Uatumavolcanic rocks, 18759 /158 Ma, is comparable within the limits of the error. 1870 Ma thus represents the time of an important magmatic event in the eastern part of the Amazonian craton. Nd model ages of the granites (3.0 � /3.2 Ga) and volcanic rocks (2.9 � /3.1 Ga) and their strongly negative initial oNd values indicate that the magmas were derived from Mesoarchean rocks with a long crustal residence time. A mantle contribution cannot be completely ruled out, however. # 2002 Elsevier Science B.V. All rights reserved.

Geoquímica e Assinaturas Nd-Sr do Complexo Granitoide Pensamiento, Província Rondoniana-San Ignacio, Pré-Cambriano de Bolívia Oriental: Caracterização Petrogenética de um Arco Magmático no Mesoproterozoico

O Complexo Granitoide Pensamiento (CGP) ocorre na porcao norte do Pre-Cambriano Boliviano, estando tectonicamente associado a evolucao da provincia Rondoniana-San Ignacio (1.55 - 1.30 Ga) do Craton Amazonico, constituido por uma provincia central de idade arqueana e provincias proterozoicas marginais. A evolucao proterozoica resulta do desenvolvimento de cinturoes acrescionarios sucessivamente mais jovens para sudoeste, a exemplo das orogenias Rondoniana-San Ignacio (1.37 - 1.32 Ga) e Sunsas (1.20 - 1.00 Ga). O CGP ocorre na parte setentrional do Pre-Cambriano Boliviano, ao norte do cinturao Sunsas, sendo constituido por granitos e termos subvulcânicos. Subordinadamente ocorrem sienitos, granodioritos, tonalitos, trondjemitos e dioritos. Em termos tectonicos, essas rochas sao classificadas em dois conjuntos: plutons sin a tardicinematicos e tardi a pos-cinematicos. Treze analises quimicas em rocha total para elementos maiores, tracos e ETR foram realizadas em rochas granitoides orogenicas do CGP. Diagramas de correlacao geoquimica indicam tendencia negativa entre os conteudos de MgO, Al2O3 and CaO em funcao do aumento de SiO2, sugerindo processos de cristalizacao fracionada na petrogenese das rochas investigadas. Em adicao os dados indicam uma composicao principalmente peraluminosa, subalcalina de alto K, compativel com ambiente de arco magmatico, para a geracao dos plutons estudados, corroborado pelo padrao de fracionamento dos ETRL/ETRP. Datacoes SHRIMP em zircao dos plutons La Junta e San Martin (sin a tardicinematicos; 1347 ± 21 e 1373 ± 20 Ma, respectivamente) em conjunto com idades modelo T DM entre 1,9 e 2,0 Ga e valores de eNd(1330) entre +1,8 e -4,3 sao semelhantes a resultados publicados em outros corpos coevos. Em adicao, os plutons Porvenir, San Cristobal e Piso Firme (tardi a pos-cinematicos) tem idades T DM modelo entre 1,6 e 1,7 Ga e valores de eNd(1330) positivos entre +2,7 e +1,5, o que sugere uma origem em arco magmatico intraoceânico. O pluton Diamantina (tardi a pos-cinematico; idade SHRIMP em zircao de 1340 ± 20 Ma) tem idades T DM modelo entre 1,6 e 1,9 Ga com valores de eNd(1330) entre +0,4 e -1,2. Isto corrobora a hipotese de significativa contribuicao de material juvenil mesoproterozoico durante a sua genese. Os resultados aqui obtidos interpretados em conjunto com os dados geologicos de unidades contemporâneas na contraparte brasileira reforcam a existencia de um arco magmatico juvenil mesoproterozoico que finalizou a evolucao acrescionaria da provincia Rondoniana-San Ignacio

Geologia e geocronologia da suíte metamórfica colorado e suas encaixantes, SE de Rondônia: implicações para a evolução mesoproterozóica do SW do cráton Amazônico

Combined geochronological (U-Pb, 40Ar/39Ar, and Rb-Sr) geological data help define an important tectonomagmatic event in the Colorado do Oeste and Cabixi regions, southeastern Rondonia, Brazil, and characterize the Colorado Metamorphic Suite (CMS), well represented by rock assemblages which experienced deformation and metamorphic recrystallization (upper-amphibolite facies). The suite is composed of: 1) porphyritic monzogranite associated with amphibolite (bimodal magmatism), 2) interlayered clastic and chemical metasedimentary rocks (sillimanite schists and iron formation), 3) muscovite-garnet leucogranite, and 4) mafic-ultramafic intrusive rocks. The CMS mafic rocks occur as undeformed isolated bodies of layered coarse-grained metagabbro, still preserving typical cumulate igneous texture. U-Pb zircon isotopic data for three fractions of sample RO-10 define a discordia with an upper intercept (crystallization) age of 1352 + 4/-3 Ma (MSWD = 0.18). The porphyritic ortogneisses (RO-15; monzogranite and amphibolite) yielded a Rb-Sr whole rock isochron age of 1360 + 45 Ma, and 87Sr/86Sr i = 0.7040 + 0.0012 (MSWD = 9.2). One sample of aplite that is subparallel to the main regional foliation has furnished an age of 1360 + 13 Ma. The 40Ar/39Ar data for muscovite from a anatectic leucogranite (RO-14) yielded plateau ages of 1312 + 3 Ma (grain 1), and 1303 + 2 Ma/1305 + 2 Ma (grain 2). Grain 3 presented an heterogeneous isotopic spectrum, the integrated age being 1289 + 2 Ma. Hornblende from two amphibolite samples yielded 40Ar/39Ar plateau ages of 1313 + 4 Ma; 1313 + 6 Ma and 1312 + 3 Ma (RO-18), and 1325 + 3 Ma; 1326 + 2 Ma; 1330 + 3 Ma (RO-19). The weighted-mean age is 1319 + 10 Ma, and is interpreted as the best estimated age for regional metamorphic cooling. These data suggest that southeastern Rondonia was affected by a tectonomagmatic event at ca. 1.36 - 1.32 Ga, predating the evolution of the Nova Brasilândia Terrane (1215 - 1110 Ma). However, the unequivocal characterization of an orogeny refered to the time interval 1.36 - 1.32 Ga is still uncertain due to scarcity of geochronological data in the region. Similar ages are reported for correlatable intrusive rocks which cut the polydeformed basement rocks of central Rondonia. Thus, these ages are compatible with available Mesoproterozoic tectonic models for the SW Amazonian Craton.

Granite-related ore deposits

This volume brings together a collection of papers that summarize current ideas and recent progress in the study of granite-related mineralization systems. They provide a combination of field, experimental and theoretical studies. Papers are grouped according to the main granite-related ore systems: granite-pegmatite, skarn and greisen-veins, porphyry, orogenic gold, intrusion-related, epithermal and porphyry-related gold and base metal, iron oxide–copper–gold (IOCG), and special case studies. The studies provide a broad spread in terms of both space and time, highlighting granite-related ore deposits from Europe (Russia, Sweden, Croatia and Turkey), the Middle East (Iran), Asia (Japan and China) and South America (Brazil and Argentina) and spanning rocks from Palaeoproterozoic to Miocene in age.

Granite-related ore deposits: an introduction

Abstract A symposium on Mineralization Associated with Granitic Magmatism was held within the framework of the 33rd IGC in Oslo, Norway, in August 2008. While our initial idea was to bring together field, experimental, and theoretical studies in order to review and summarize the current ideas and recent progress on granite-related mineralization systems, we were caught by surprise realizing that participants were inclined to focus more on ore deposits related to granitic magmatism. This spontaneous shift from granites, the major intended focus of the symposium, to mineralization associated with them, spawned the idea for a special issue on this theme and ultimately to the nine papers assembled here, chosen from about 60 scientific contributions at the symposium. Around twenty oral presentations were given and forty posters were presented at the meeting; the 60 papers were grouped according to the current main granite-related ore systems, as follows; granite-pegmatite, skarn and greisen-veins, porphyry, orogenic gold, intrusion-related, epithermal and porphyry-related gold and base metal, iron oxide–copper–gold (IOCG), and special case studies.

Datações U-Pb convencional versus SHRIMP do Maciço estanífero Santa Bárbara, Suíte Granitos últimos de Rondônia, Brasil

The Santa Barbara Granite Massif is part of the Younger Granites of Rondonia (998 - 974 Ma) and is included in the Rondonia Tin Province (SW Amazonian Craton). It comprises three highly fractionated metaluminous to peraluminous within-plate A-type granite units emplaced in older medium-grade metamorphic rocks. Sn-mineralization is closely associated with the late-stage unit. U-Pb monazite conventional dating of the early-stage Serra do Cicero facies and late-stage Serra Azul facies yielded ages of 993 ± 5 Ma and 989 ± 13 Ma, respectively. Conventional multigrain U-Pb isotope analyses of zircon demonstrate isotopic disturbance (discordance) and the preservation of inherited older zircons of several different ages and thus yield little about the ages of Sn-granite magmatism. SHRIMP U-Pb ages for the Santa Barbara facies association yielded a 207Pb/206Pb weighted-mean age of 978 ± 13 Ma. The textural complexity of the zircon crystals of the Santa Barbara facies association, the variable concentrations of U, Th and Pb, as well as the mixed inheritance of zircon populations are major obstacles to using conventional multigrain U-Pb isotopic analyses. Sm-Nd model ages and µNd (T) values reveal anomalous isotopic data, attesting to the complex isotopic behaviour within these highly fractionated granites. Thus, SHRIMP U-Pb zircon and conventional U-Pb monazite dating methods are the most appropriate to constrain the crystallization age of the Sn-bearing granite systems in the Rondonia Tin Province.

Investigações GPR nos distritos mineiros de Santa Bárbara e Bom Futuro: Província Estanífera de Rondônia

Investigacoes GPR - Ground Penetrating Radar foram realizadas nos distritos mineiros de Santa Barbara e Bom Futuro (Provincia Estanifera de Rondonia), visando averiguar a aplicabilidade do metodo para localizacao de depositos estaniferos primarios e secundarios, bem como otimizar os custos envolvidos na etapa de prospeccao. Os perfis de reflexao GPR foram adquiridos com antenas de 25, 50 e 100 MHz, o que permitiu investigar a subsuperficie ate 30 metros de profundidade. Em relacao aos depositos primarios das minas de Santa Barbara e de Bom Futuro, o metodo GPR permitiu identificar zonas com predominância de corpos de greisens. No deposito secundario aluvionar da mina de Bom Futuro, o metodo GPR permitiu localizar uma estrutura de paleocanal na base do sistema deposicional sedimentar. Os resultados obtidos apresentaram uma excelente concordância com as informacoes geologicas provenientes das sondagens, demonstrando essa nova aplicabilidade do metodo GPR para a exploracao mineral, na qual podera ser extendida para ambientes similares do Craton Amazonico. Alem disso, os custos envolvidos no processo exploratorio poderao ser otimizados, caso as investigacoes GPR sejam realizadas antes da execucao de sondagens.