Manoel S. D'Agrella-Filho

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.

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...

Palaeomagnetism of Precambrian rocks from Gabon, Congo craton, Africa

Palaeomagnetic results for 75 orientated hand samples (25 sampling sites) from the Ogooue Series amphibolites (27), Archean basement amphibolites and granulites (14) and Precambrian dolente dykes (34) are presented. Magnetization of many amphibolites showed unstable directions during alternating field (AF) and thermal demagnetization experiments and no directions of magnetization could be isolated. However, a characteristic direction could be isolated in a few amphibolite sites and for the granulites studied. The best group of site mean directions Dm=54°, Im=69° (κ=27, α95=18°, N=4) yielded a palaeomagnetic pole located at 44° E, 19° N (K=10, A95=30°). This pole is dose to a palaeomagnetic pole obtained for granulites from the Jequi`es complex (Sao Francisco craton) dated by the 40Ar/39Ar method as 2.0 Ga, suggesting that both areas belonged to the same tectonic unit at that time. The unmetamorphosed dolerite dykes (5) displayed multi-component behavior, but a common characteristic direction could be isolated for four dykes, Dm=262°, lm=49° (κ=13, α95=27°, N=4), which yielded a palaeomagnetic pole located at 72° E, 8° N (K=9; A95=33°). However, this pole is different from those obtained for dyke swarms in Brazil, which are supposedly of comparable age.

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.

Return to Rodinia? Moderate to high palaeolatitude of the São Francisco/Congo craton at 920 Ma

Abstract Moderate to high palaeolatitudes recorded in mafic dykes, exposed along the coast of Bahia, Brazil, are partly responsible for some interpretations that the São Francisco/Congo craton was separate from the low-latitude Rodinia supercontinent at about 1050 Ma. We report new palaeomagnetic data that replicate the previous results. However, we obtain substantially younger U–Pb baddeleyite ages from five dykes previously thought to be 1.02–1.01 Ga according to the 40Ar/39Ar method. Specifically, the so-called ‘A-normal’ remanence direction from Salvador is dated at 924.2±3.8 Ma, within error of the age for the ‘C’ remanence direction at 921.5±4.3 Ma. An ‘A-normal’ dyke at Ilhéus is dated at 926.1±4.6 Ma, and two ‘A-normal’ dykes at Olivença have indistinguishable ages with best estimate of emplacement at 918.2±6.7 Ma. We attribute the palaeomagnetic variance of the ‘A-normal’ and ‘C’ directions to lack of averaging of geomagnetic palaeosecular variation in some regions. Our results render previous 40Ar/39Ar ages from the dykes suspect, leaving late Mesoproterozoic palaeolatitudes of the São Francisco/Congo craton unconstrained. The combined ‘A-normal’ palaeomagnetic pole from coastal Bahia places the São Francisco/Congo craton in moderate to high palaeolatitudes at c. 920 Ma, allowing various possible positions of that block within Rodinia.

Paleomagnetism of the Amazonian Craton and its role in paleocontinents

In the last decade, the participation of the Amazonian Craton on Precambrian supercontinents has been clarified thanks to a wealth of new paleomagnetic data. Paleo to Mesoproterozoic paleomagnetic data favored that the Amazonian Craton joined the Columbia supercontinent at 1780 Ma ago, in a scenario that resembled the South AMerica and BAltica (SAMBA) configuration. Then, the mismatch of paleomagnetic poles within the Craton implied that either dextral transcurrent movements occurred between Guiana and Brazil-Central Shield after 1400 Ma or internal rotation movements of the Amazonia-West African block took place between 1780 and 1400 Ma. The presently available late-Mesoproterozoic paleomagnetic data are compatible with two different scenarios for the Amazonian Craton in the Rodinia supercontinent. The first one involves an oblique collision of the Amazonian Craton with Laurentia at 1200 Ma ago, starting at the present-day Texas location, followed by transcurrent movements, until the final collision of the Amazonian Craton with Baltica at ca. 1000 Ma. The second one requires drifting of the Amazonian Craton and Baltica away from the other components of Columbia after 1260 Ma, followed by clockwise rotation and collision of these blocks with Laurentia along Grenvillian Belt at 1000 Ma. Finally, although the time Amazonian Craton collided with the Central African block is yet very disputed, the few late Neoproterozoic/Cambrian paleomagnetic poles available for the Amazonian Craton, Laurentia and other West Gondwana blocks suggest that the Clymene Ocean separating these blocks has only closed at late Ediacaran to Cambrian times, after the Amazonian Craton rifted apart from Laurentia at ca. 570 Ma.

Paleomagnetic Study of the Juiz de Fora Complex, SE Brazil: Implications for Gondwana

The Juiz de Fora Complex is mainly composed of granulites, and granodioritic-migmatite gneisses and is a cratonic basement of the Ribeira belt. Paleomagnetic analysis on samples from 64 sites widely distributed along the Alem Paraiba dextral shear zone (SE Brazil, Rio de Janeiro State) yielded a northeastern, steep downward inclination direction (Dm=40.4°, Im=75.4, a95=6.0°, K=20.1) for 30 sites. The corresponding paleomagnetic pole (RB) is situated at 335.2°E; 0.6°S (a95=10.0°; K=7.9). Rock magnetism indicates that both (titano)magnetite and titanohematite are the main magnetic minerals responsible for this direction. Anisotropy of low-field magnetic susceptibility (AMS) measurements were used to correct the ChRM directions and consequently its corresponding paleomagnetic pole. This correction yielded a new mean ChRM (Dm = 2.9°, Im = 75.4°, a95 = 6.4°, K = 17.9) whose paleomagnetic pole RBc is located at 320.1°E, 4.2° N (a95=10.3°, K=7.5). Both mean ChRM and paleomagnetic pole obtained from uncorrected and corrected data are statistically different at the 95% confidence circle. Geological and geochronological data suggest that the age of the Juiz de Fora Complex pole is probably between 535–500 Ma, and paleomagnetic results permit further constraint on these ages to the interval 520–500 Ma by comparison with high quality paleomagnetic poles in the 560–500 Ma Gondwana APW path.