Colombo C. G. Tassinari

A review of the geochronology of the Amazonian Craton: Tectonic implications

Abstract The available geochronological data for the Amazonian Craton permit delineation of its main age provinces and their respective tectonic development. Demarcation of the boundaries of each age province is primarily based on Rb-Sr whole-rock isochrons, supported by K-Ar determinations on minerals. Some U-Pb results on zircons, Pb-Pb whole-rock isochrons, and Sm-Nd model ages complement these data in key areas. At the center of the Amazonian Craton lies the Archean age, Central Amazonian Province bounded in its northern part by the Maroni-Itacaiunas mobile belt, which developed during early Proterozoic times. The western and southern margins of the Archean core are bordered by three tectonic provinces: (1) the Rio Negro-Juruena magmatic arc, (2) the Rondonian mobile belt, and (3) the Sunsas mobile belt. Each belt borders the western margin of the preceding belt, and each records a successively younger middle Proterozoic orogeny. Within the Central Amazonian Province, −2.7 Ga age determinations have been obtained from the Serra dos Carajas area. Although the Maroni-Itacaiunas belt includes several reworked Archean nuclei, rock units formed during the 2.2-1.9 Ga Trans-Amazonian Orogeny predominate. The isotopic evidence suggests that a mantle-derived component constituted a significant portion of the crust accreted at that time. The Rio Negro-Juruena Province is of Mid-Proterozoic age (1.75-1.50 Ga), and includes almost entirely mantle-derived material, juxtaposed with the pre-existent continental mass along a series of successive magmatic arcs. The Rondonian and Sunsas provinces both exhibit older sialic basements (>1.5 Ga). The former is characterized by important tectonomagmatic events in the 1.45-1.25 Ga interval, whereas the latter records tectonic reworking at 1.1-0.9 Ga. Finally, the Amazonian Craton is bordered to the east by the late Proterozoic Paraguai-Araguaia (or Araguaia-Tocantins) fold belt.

Geochronological review of the Precambrian in western Angola: links with Brazil

Abstract In this study the main features of the Precambrian of western Angola are presented as a way of outlining the position of the Congo Craton and coastal Pan-African fold belts. These were separated from the Sao Francisco Craton and Atlantic Brasiliano fold belts in Brazil by the opening of the South Atlantic. The main events and cycles that affected the Precambrian lithostratigraphical units in western Angola, can be correlated with similar events in the northern part of the Congo Craton and in northwestern Namibia. Using these correlation and geochronological studies in the Luanda-Malanje region, two tectono-metamorphic events (the Lunda-Cuango-Malanje and the Malanje-Andulo events) are defined and several Precambrian lithostratigraphical units are reinterpreted. The margin of the Congo Craton in northwestern Angola (Luanda-Malanje region) is located between the Eburnian porphyroblastic gneisses and the Pan-African migmatites. The southern margin of this craton alternates between the Angolan and Brazilian coasts; then, after a brief incursion into Namibia, this limit runs northeast through southeast Angola until it meets the Lufilian Arc.

Geochronological Systematics on Basement Rocks from the Río Negro-Juruena Province (Amazonian Craton) and Tectonic Implications

The Rio Negro-Juruena Province (RNJP) occupies a large portion of the western part of the Amazonian Craton and is a zone of complex granitization and migmatization. Regional metamorphism, in general, occurred in the upper amphibolite facies. The granites and gneisses of the RNJP yield Rb-Sr and Pb-Pb whole-rock isochron dates ranging from 1.8 Ga to 1.55 Ga, with initial 87Sr/86Sr ratios of ∼ 0.703 and a single-stage model μ1 value of ∼ 8.1. In order to improve the geochronological control, SHRIMP U-Pb zircon ages, conventional U-Pb zircon ages, and additional Pb-Pb whole-rock isochron ages were determined for samples of granitoids and gneisses from the Papuri-Uaupes and Guaviare-Orinoco rivers areas (northern part of the province) and Jamari-Machado rivers and Pontes de Lacerda areas (southern part). The granitoids from the northern part of the province yield conventional U-Pb zircon ages of 1709 ± 17 Ma and 1521 ± 31 Ma, and SHRIMP U-Pb concordant zircon results of 1800 ± 18 Ma. Samples of gneissic rocks...

The Beja Layered Gabbroic Sequence (Ossa-Morena Zone, Southern Portugal): geochronology and geodynamic implications

The Beja Igneous Complex (BIC) is a major geological feature in SW Iberian Variscides, consisting of three main units developed during different stages of the oblique collision between the Ossa Morena Zone (OMZ) upper plate and South Portuguese Zone (SPZ) lower plate, namely: (1) ca. 355 Ma to ca. 345 Ma Layered Gabbroic Sequence formed in early stages of collision magmatism; (2) the ca. 335-330 Ma to ca. 320 Ma Cuba-Alvito (gabbro-diorite) Complex formed throughout the late-collision magmatic event; and (3) the Baleizão Porphyry Complex corresponding to the period of post-collision magmatism, ca. 300 Ma. The new SHRIMP U-Pb age of 342±9 Ma reported here for amphibole-bearing pegmatite dykes cutting the layered gabbros is interpreted as dating the development of late fluid-rich melts in the Layered Gabbroic Sequence, synchronous with Fe-Cu-Co sulphide deposition. The close agreement between this data and available amphibole 40Ar/39Ar ages of BIC, Beja-Acebuches Ophiolite and other geological units of the OMZ southern border, may be taken as evidence for a moderate to rapid regional crustal uplift episode at ca. 340±5 Ma; this data, coupled with structural constraints, also allow to estimate the age for the transition between the D2a – D2b deformation phases of Variscan continental collision. A complex wedge system within the SW Iberian Variscides developed during this collision, involving the OMZ upper plate to the north and the SPZ passive margin in the lower plate. The Évora-Beja-Aracena Domain, located in the upper plate above the N-dipping subduction zone, is re-interpreted as a retro-wedge domain that was kinematically coupled to the SPZ pro-wedge and subduction system. Retro-wedge growth is linked to upper plate uplift (early collision) and a late-orogenic wedge thickening. The early stages of magmatism in the retro-wedge are related to asthenospheric mantle upwelling induced by the slab break-off. Regional LP-HT metamorphism and subsequent magmatic events in the retro-wedge domain were caused by long term high heat flow sustained by (1) mafic magma underplating, (2) stacking of high-heat producing upper-crustal lithologies, and reinforced (3) by (moderate to) rapid crustal uplifting. Mass advection and orogenic architecture were strongly affected by asymmetric removal towards the lower-part foreland basin and by transient mechanical properties of the wedge system associated with the anomalous thermal regime.

Absolute Dating of Permian Ash-Fall in the Rio Bonito Formation, Paraná Basin, Brazil

Abstract The Rio Bonito Formation in southern Parana basin contains a set of tonsteins interbedded with coal-seams. These tonsteins are composed mainly of kaolinite with zircon, apatite and beta-quartz paramorphs as accessory minerals, and were interpreted as volcanic ashes deposited by ash falls over pits protected by barrier islands in a barrier-lagoon system. A U-Pb dating of zircons in the tonstein A, which furnished an age of 267.1 ± 3.4 Ma (Early Permian) confirming previous age-dates based on palynology and correlating them with one of the main periods of volcanic activity in the Gondwana. The source of the pyroclastic material was attributed to the early Permian Choiyoi magmatic arc in Argentina, developed during the Sanrafaelic orogeny, and with a main peak of volcanic activity between 260 and 272 Ma.

The Late-Variscan fault network in central–northern Portugal (NW Iberia): a re-evaluation

Abstract The fault network in central–northern Portugal, especially the fault system with mean strike N25°, has been used to help deduce the late Palaeozoic dynamics of Western Europe. On the other hand, the N80° strike–slip fault system was recognized in previous works as Late-Variscan and left lateral, but was scarcely mapped and its importance neglected. This study shows that the N25° faults were dextral in the late stages of the Variscan Orogeny and sinistral during the Alpine Cycle due to pervasive reactivation. Fault rocks and intrusions are clearly different, according to age: mostly high temperature quartz infillings, but locally muscovite, tourmaline, and aplite dykes in Late-Variscan times, and low temperature cataclasites, fault gouges and Mesozoic mafic dykes in younger Alpine times. We dated dextral N45° segments of the N25° fault system because they are less prone to reactivation by the NNW–SSE Alpine compression, and can thus preserve the Variscan record. We used K–Ar in muscovite concentrates and obtained a minimum age of the analysed faults of ca. 312 Ma, which sets a lower limit to the so-called Late-Variscan wrench-faulting period. The present study shows that the N80° fault set is pervasive and sinistral in central–northern Portugal, and therefore, that it only admits one brittle dextral conjugate, the N25° fault system. Both were generated by a maximum compressive stress bearing between N50° and N55° in azimuth. We did not find evidence of a Variscan sinistral strike–slip movement in the N25° fault system, and therefore, this kinematics is believed to represent only the displacements accommodated during the Alpine Cycle.

THE RIO APA CRATON IN MATO GROSSO DO SUL (BRAZIL) AND NORTHERN PARAGUAY: GEOCHRONOLOGICAL EVOLUTION, CORRELATIONS AND TECTONIC IMPLICATIONS FOR RODINIA AND GONDWANA

The Rio Apa cratonic fragment crops out in Mato Grosso do Sul State of Brazil and in northeastern Paraguay. It comprises Paleo-Mesoproterozoic medium grade metamorphic rocks, intruded by granitic rocks, and is covered by the Neoproterozoic deposits of the Corumbá and Itapocumi Groups. Eastward it is bound by the southern portion of the Paraguay belt. In this work, more than 100 isotopic determinations, including U-Pb SHRIMP zircon ages, Rb-Sr and Sm-Nd whole-rock determinations, as well as K-Ar and Ar-Ar mineral ages, were reassessed in order to obtain a complete picture of its regional geological history. The tectonic evolution of the Rio Apa Craton starts with the formation of a series of magmatic arc complexes. The oldest U-Pb SHRIMP zircon age comes from a banded gneiss collected in the northern part of the region, with an age of 1950 ± 23 Ma. The large granitic intrusion of the Alumiador Batholith yielded a U-Pb zircon age of 1839 ± 33 Ma, and from the southeastern part of the area two orthogneisses gave zircon U-Pb ages of 1774 ± 26 Ma and 1721 ± 25 Ma. These may be coeval with the Alto Tererê metamorphic rocks of the northeastern corner, intruded in their turn by the Baía das Garças granitic rocks, one of them yielding a zircon U-Pb age of 1754 ± 49 Ma. The original magmatic protoliths of these rocks involved some crustal component, as indicated by the Sm-Nd TDM model ages, between 1.9 and 2.5 Ga. Regional Sr isotopic homogenization, associated with tectonic deformation and medium-grade metamorphism occurred at approximately 1670 Ma, as suggested by Rb-Sr whole rock reference isochrons. Finally, at 1300 Ma ago, the Ar work indicates that the Rio Apa Craton was affected by widespread regional heating, when the temperature probably exceeded 350°C. Geographic distribution, age and isotopic signature of the lithotectonic units suggest the existence of a major suture separating two different tectonic domains, juxtaposed at about 1670 Ma. From that time on, the unified Rio Apa continental block behaved as one coherent and stable tectonic unit. It correlates well with the SW corner of the Amazonian Craton, where the medium-grade rocks of the Juruena-Rio Negro tectonic province, with ages between 1600 and 1780 Ma, were reworked at about 1300 Ma. Looking at the largest scale, the Rio Apa Craton is probably attached to the larger Amazonian Craton, and the actual configuration of southwestern South America is possibly due to a complex arrangement of allochthonous blocks such as the Arequipa, Antofalla and Pampia, with different sizes, that may have originated as disrupted parts of either Laurentia or Amazonia, and were trapped during later collisions of these continental masses.

Middle Proterozoic vein-hosted gold deposits in the Pontes e Lacerda region, southwestern Amazonian craton, Brazil

Structural, geochemical, and isotope studies were carried out on the gold deposits of the Pontes e Lacerda region (Mato Grosso state, Brazil), where rocks of the Aguapei and Rondoniano mobile belts (southwestern Amazonian craton) occur. The orebodies are hosted in metavolcanic, gneiss-granite, quartzite, tonalite, and granite units. Tectonics involve oblique overthrusting (from northeast to southwest), which led to the formation of recumbent folds and thrusts (pathways for the mineralizing fluids), upright folds, and faults with dominant strike-slip component. These unconformities represent potential sites for mineralization. During geological mapping, it was observed that the orebodies consist of quartz, pyrite, and gold, and that the hydrothermal alteration zone contains quartz, sericite, pyrite (altered to limonite), and magnetite (altered to hematite). Chalcopyrite, galena, and sphalerite occur only in the Onca deposit. Chemical analysis of sulfides indicates high contents of Bi, Se, and Te in sulfide...

Geological and Isotopic Constraints on the Metallogenic Evolution of the Proterozoic Sediment-Hosted Pb-Zn (Ag) Deposits of Brazil

Abstract Integrated studies of seven Proterozoic sediment-hosted, Pb-Zn-Ag sulfide deposits of Brazil, permit the estimation of the age of the hosting sequence and the mineralization, the nature of the sulfur and metal sources, the temperature range of sulfide formation and the environment of deposition. These deposits can be classified into three groups, according to their ages. (a) Archean to Paleoproterozoic: the Boquira deposit, in Bahia state, consists of stratiform massive and disseminated sulfides hosted by parametamorphic sequences of grunnerite-cummingtonite+magnetite that represent a silicate facies of the Boquira Formation (BF). Lead isotope data of galena samples indicate a time span between 2.7 and 2.5 Ga for ore formation, in agreement with the stratigraphic position of the BF. The relatively heavy sulfur isotope compositions for the disseminated and stratiform sulfides (+8.3 to +12.8 ‰ CDT)suggest a sedimentary source for the sulfur. (b) Paleo to Mesoproterozoic: stratiform and stratabound sulfides in association with growth faults are present in the Canoas mine (Ribeira, in Parana state) and in the Caboclo mineralization (Bahia state). They are hosted by calcsilicates and amphibolites in the Canoas deposit, whereas in the Caboclo area the mineralization is associated with hydrothermally altered dolarenites at the base of the 1.2 Ga Caboclo Formation. The interpreted Pb-Pb age of the Canoas mineralization is coeval with the 1.7 Ga host rocks. Sulfur isotopic data for Canoas sulfides (+1.2 to +16 ‰ CDT) suggest a sea water source for the sulfur. The range between −21.1 and +8.8 ‰ CDT for the Caboclo sulfides could suggest the action of bacterial reduction of seawater sulfates, but this interpretation is not conclusive. (c) Neoproterozoic: stratiform and stratabound sulfide deposits formed during the complex diagenetic history of the host carbonate rocks from the Morro Agudo (Bambui Group), Irece and Nova Redencao (Una Group), yield heavy sulfur isotope values (+18.9 to +39.4 ‰ CDT). The uniform heavy isotope composition of the barites from these deposits (+25.1 to +40.9 ‰) reflect their origin from Neoproterozoic seawater sulfates. The late-stage, and most important, metallic concentrations represent sulfur scavenged from pre-existing sulfides or from direct reduction of evaporitic sulfate minerals. Lead isotope data from the Bambui Group suggest focused fluid circulation from diverse Proterozoic sediment sources, that probably was responsible for metal transport to the site of sulfide precipitation. (d) Late Proterozoic to Early Paleozoic: lead-zinc sulfides (+pyrite and chalcopyrite) of Santa Maria deposits, in Rio Grande do Sul, form the matrix of arkosic sandstones and conglomerates, and are closely associated with regional faults forming graben structures. Intermediate volcanic rocks are intercalated with the basal siliciclastic members. Lead isotope age of the mineralization (0.59 Ga) is coeval with the host rocks. Sulfur isotopic values between −3.6 and +4.1 are compatible with a deep source for the sulfur. Geological, petrographic and isotopic data of the deposits studied suggest that they were formed during periods of extensional tectonics. Growth faults or reactivated basement structures probably were responsible for localized circulation of metal-bearing fluids within the sedimentary sequences. Sulfides were formed by the reduction of sedimentary sulfates in most cases. Linear structures are important controls for sulfide concentration in these Proterozoic basins.

Microssonda Iônica de Alta Resolução e de Alta Sensibilidade (SHRIMP IIe/MC) do Instituto de Geociências da Universidade de São Paulo, Brasil: método analítico e primeiros resultados

This paper presents the characteristics of the high resolution secondary ion mass spectrometer coupled with an Ion Microprobe (SHRIMP IIe/MC), installed at the Institute of Geosciences of the University of Sao Paulo (IGc-USP), as well as the respective analytical procedures and the first results obtained with standard samples, making possible to conduct routine analysis of zircon samples. The standard of the Temora 2 zircon, with age of 416.78 Ma, was analyzed by the SHRIMP IIe/MC at the Institute of Geosciences of the University of Sao Paulo and yielded the age of 416.8 ± 3.8 Ma. Samples of the Archean zircon OG1 and the Neoproterozoic zircon Z6266, which yielded U-Pb ages of 3465.4 ± 0.6 Ma and of 559 ± 0.2 Ma, respectively, using Thermal Ionization Mass Spectrometry (TIMS), were analyzed by SHRIMP IIe/MC at the IG-USP and yielded U-Pb ages of 3462.6 ± 5.1 Ma and 561.0 ± 0.92 Ma, respectively, in Concordia diagrams. Therefore, these values and the literature values are very close. Both OG1 and Z6266 samples, due to the homogeneity in their composition and isotopic relationship, can be used as SHRIMP standard. These results indicate that data obtained using SHRIMP IIe at the IGc-USP are reliable and compatible with international standards. Dating of samples with ages ranging from Cenozoic to Archean, obtained using SHRIMP IIe/MC U-Pb geochronology on zircon crystals, is presented here to illustrate this point.