I. G. Pacca

The age of parana flood volcanism, rifting of gondwanaland, and the jurassic-cretaceous boundary.

The Paran�-Etendeka flood volcanic event produced ∼1.5 x 106 cubic kilometers of volcanic rocks, ranging from basalts to rhyolites, before the separation of South America and Africa during the Cretaceous period. New 40Ar/39Ar data combined with earlier paleomagnetic results indicate that Paran� flood volcanism in southern Brazil began at 133 � 1 million years ago and lasted less than 1 million years. The implied mean eruption rate on the order of 1.5 cubic kilometers per year is consistent with a mantle plume origin for the event and is comparable to eruption rates determined for other well-documented continental flood volcanic events. Paran� flood volcanism occurred before the initiation of sea floor spreading in the South Atlantic and was probably precipitated by uplift and weakening of the lithosphere by the Tristan da Cunha plume. The Parana event postdates most current estimates for the age of the faunal mass extinction associated with the Jurassic-Cretaceous boundary.

40Ar/39Ar dating of 1.0–1.1 Ga magnetizations from the Sa˜o Francisco and Kalahari cratons: tectonic implications for Pan-African and Brasiliano mobile belts

Paleomagnetic poles from 1.1-1.0 Ga dyke swarms in eastern Brazil (Sa˜o Francisco Craton) are compared with 1.0 Ga poles from granulites of the Namaqua Province in southern Africa (Kalahari Graton). The intrusive ages of dykes are estimated from40Ar/39Ar dating of outgassed biotites from baked country rocks. The age of magnetization for the granulites is derived by combining40Ar/39Ar hornblende and biotite dates. When restored to a Mesozoic pre-drift configuration the paleomagnetic poles are in crude spatial agreement but are temporally discordant. To satisfy both paleomagnetic and geochronologic constraints, a reconstruction involving separation of the Sa˜o Francisco and Kalahari cratons is required, indicating that the intervening Pan-African (Brasiliano) mobile belt may record a craton-craton collision.

Paleomagnetism of Middle Proterozoic (1.01 to 1.08 Ga) mafic dykes in southeastern Bahia State—São Francisco Craton, Brazil

Paleomagnetic results from profuse Middle-Late Proterozoic mafic dyke swarms in southeastern Bahia State (Sa˜o Francisco Craton) yield either an easterly direction with high upward inclination or a westerly direction with high downward inclination isolated during AF and/or thermal treatments. Thermal demagnetization behavior and thermochronologic and petrologic considerations indicate that these remanent components originated as primary TRMs. Four groups of directions were distinguished from dykes in spatially distinct areas: Ilheus normal polarity (D = 60.0°; I = −68.8°; α95 = 2.6°; N = 17) , Olivenca normal polarity (D = 82.4°; I = −71.0°; α95 = 5.1; N = 31), Itajudo Coloˆnia (D = 99.0; I = −71.9 ; α95 = 5.9°; N = 23) and Olivenca reversed polarity (D = 298.8°; I = 60.7°; α95 = 6.4°; N = 18), which yield paleomagnetic poles located at 100.4°E; 30.3°N (IN), 107.0°E; 16.1°N (ON), 111.0°E; 7.7°N (IC) and 280.2°E; 17.0°N (OR), respectively. These poles define an APW path for the Sa˜o Francisco Craton between the time interval 1.01–1.08 Ga which is characterized by at least two polarity intervals.

Time evolution of the South Atlantic Magnetic Anomaly

A Anomalia Magnetica do Atlântico Sul (SAMA) e uma das maiores anomalias do campo geomagnetico. A variacao secular da SAMA foi obtida e comparada com a evolucao de outrasanomalias usando modelos de campo por harmonicos esfericos para o periodo de 1590-2005. Uma analise dos dados de quatro observatorios da America do Sul mostra como esta anomalia de grande escala afetou suas medidas. Como a SAMA e uma anomalia de campo total baixo, o campo foi separado nas componentes nao-dipolar, quadrupolar e octupolar. A evolucao temporal das razoes dos campos nao-dipolar/total, quadrupolar/total e octupolar/total mostram valores elevados para o Atlântico Sul desde 1750. A evolucao da SAMA e comparada com a evolucao de outras grandes feicoes geomagneticas de superficie como os polos Norte e Sul e o Alto da Siberia, e sua comparacao mostra o equilibrio de intensidade entre estas anomalias em ambos os hemisferios. A analise dos campos nao-dipolares no periodo historico sugere que a SAMA e regida (i) pelo campo quadrupolar para a deriva, e (ii) pelos campos quadrupolar e octupolar para a intensidade e area de influencia. Alem disso, este estudo reforca a possibilidade de que a SAMA possa estar relacionada aos fluxos reversos no nucleo externo sob a regiao do Atlântico Sul.

The Anari and Tapirapuã Jurassic formations, western Brazil: paleomagnetism, geochemistry and geochronology

The Anari and Tapirapua formations are very similar from the point of view of paleomagnetic, geochronological and geochemical results. They date from around 197 Ma and the flows are mainly tholeiitic basalts with a low TiO2 and incompatible element content. The magnetic carriers in rocks from these two formations were highly oxidized titanomagnetites, maghemites and, probably, titanomaghemites. Paleomagnetic analysis has shown that the magnetizations are all normal in polarity and virtual geomagnetic poles obtained for both formations are indistinguishable at the 95% confidence level. The calculated mean for these poles is 250.3°E, 65.5°S (N = 15; A95 = 3.6°; KSC = 1578). This pole is compatible with Jurassic poles which have been determined for South America or transposed from Africa.

Late Cretaceous alkaline complexes, southeastern Brazil: Paleomagnetism and geochronology

Late Cretaceous alkaline rocks from Pocos de Caldas, Itatiaia, Passa Quatro and Sao Sebastiao Island were studied with the main purpose of improving the South American apparent polar wander path (APWP). RbSr age determinations for Passa Quatro and alkaline stocks from Sao Sebastiao Island are around 70 and 81 Ma respectively. The magnetic carriers in the rocks studied were maghemite and highly oxidized titanomagnetites. The paleomagnetic poles obtained in this work are 320.1°E, 83.2°S (N = 47, A95 = 2.7°, KSC = 2609.8) for Pocos de Caldas, 360.0°E, 79.5°S (N = 18, A95 = 5.7°, KSC = 607.4) for Passa Quatro and Itatiaia, and 331.9°E, 79.4°S (N = 18, A95 = 4.9°, KSC = 862.4) for Sao Sebastiao Island (the ISLAND set).

Paleomagnetic Constraints on the Rodinia Supercontinent: Implications for its Neoproterozoic Break-up and the Formation of Gondwana

An interpretation of available paleomagnetic data from the Laurentia, Congo-Sao Francisco, Kalahari, and Amazonia cratons favors the hypothesis that these units were juxtaposed in a supercontinent by 1000 Ma. This supercontinent is similar to Hoffmans (1991) Rodinia, except for the Kalahari craton, whose 1300 to 1000 Ma Namaqua-Natal mobile belt is now juxtaposed against the correlated 1300 to 1000 Ma Grenville belt in eastern Laurentia, Our model suggests that a continuous 1300 to 1000 Ma orogenic belt, formed by the Grenville, Sunsas, Kibaride-Irumide-Lurio, Namaqua-Natal, and Dronning Maud Land-Coats Land belts, represents the suture zone between the Amazonia, Congo-Sao Francisco, Kalahari-Grunehogna, and Laurentia blocks. The formation of western Gondwana (from our Rodinia supercontinent) may be accomplished by the closure of the large Mozambique Ocean and the more restricted Adamastor Ocean, combined with some counterclockwise rotation of the Congo-Sao Francisco craton. Rotation of the Congo-Sao Fra...

Paleomagnetic evidence for the evolution of Meso- to Neo-proterozoic glaciogenic rocks in central-eastern Brazil

Abstract Paleomagnetic studies on basic dikes in the eastern Sao Francisco Craton which have isotopic ages of 1.0–1.1 Ga, define an apparent polar wander path for South America over this time interval. The data indicate that the Sao Francisco Craton was at paleolatitudes between 40° and 65° at the time of emplacement of these dikes. Neo-Proterozoic sedimentary glaciogenic rocks, the Macaubas Group, Bebedouro Formation, Ibia Formation and Carandai Formation, crop out in central-eastern Brazil. An age of about 1.0 Ga has been proposed for these glacial deposits. Paleogeographical reconstructions of South America show a continental movement coherent with paleoenvironmental models proposed for the Macaubas Group and suggest that the glacial period may have occurred between 1.01 and 1.08 Ga.

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.

Paleomagnetism of Paleoproterozoic mafic dyke swarm from the Uauá region, northeastern São Francisco Craton, Brazil: tectonic implications

Abstract This paper presents paleomagnetic analyses of 146 samples collected from 45 mafic dykes (sites) and 18 samples from basement rocks in contact with dykes. Two groups of dykes are recognized: the first (metamorphic dykes-MD) is composed of amphibolites with distinct schistosity and the second group is formed of two main rock types; a BD (basic dykes) type which retains its original texture and mineralogy and a MBD (metamorphic basic dykes) type where the clinopyroxene is almost completely substituted by amphibole. The MBD dykes occur near shear zones which are associated with the metamorphism of these rocks (ages of 2003 ± 86 Ma and 1975 ± 2 Ma). Most K-Ar and 40 Ar 39 Ar mineral age determinations on the analysed dykes and basement rocks yielded dates in the interval 1.9-2.14 Ga. Rb Sr mineral isochrons on dykes from the BD type suggest two different generations of emplacement, at 2.38 Ga (minimum age) and 1.98 Ga. After AF and thermal demagnetization, a characteristic direction of magnetization (Dm = 17.1 °; Im = 69.8 °; N = 31; K = 40.6; α95 = 4.1 °) could be isolated for 29 metamorphosed and unmetamorphosed dykes, and basement rocks from two localities, which yielded a paleomagnetic pole situated at 331.4 °E; 23.8 °N (K = 16.8; α95 = 6.5 °). The paleomagnetic, petrographic, and geochronological data are interpreted to indicate that this characteristic component of magnetization is secondary. This component may have been acquired during the uplift and cooling of the Transamazonian cycle, at about 1.98-1.9 Ga ago. Tectonically, our data when compared with paleomagnetic poles of similar age from the Kalahari Craton are consistent with a model in which the Sao Francisco/Congo and Kalahari Cratons were part of a single stable continental mass in the late Paleoproterozoic.