Papers by Ricardo Trindade
Journal of Geophysical Research: Solid Earth, 2015
Earth Planets and Space, 2009
Geomagnetic field variations at archeomagnetic timescales can be obtained from well-dated heated ... more Geomagnetic field variations at archeomagnetic timescales can be obtained from well-dated heated structures and archeological potsherds. Here, we present the first archeointensity results obtained on Portuguese ceramics (1550 to 1750 AD) collected at Brazilian archeological sites. The results are compared to those obtained from Western Europe and currently available geomagnetic field models. Continuous thermomagnetic and IRM acquisitions curves indicate that Ti-poor titanomagnetite is responsible for the remanence in these ceramic fragments. Five fragments (24 samples) out of twelve analyzed yielded reliable intensity estimates. The row archeointensity data were corrected for TRM anisotropy and cooling rate effect. The mean dipole moments are obtained for three different age intervals: 1550±30 AD, 1600±30 AD and 1750±50 AD. Mean intensities vary from 37.9±4.2 μT to 54.8±7.6 μT in agreement with the previously reported data for 1550 AD and 1750 AD. Relatively weaker, but still highly dispersed, values were obtained for 1600 AD ceramics.
Comptes Rendus Geoscience, 2007
At the end of the Neoproterozoic, the Earth may have experienced important environmental changes,... more At the end of the Neoproterozoic, the Earth may have experienced important environmental changes, with a transition between two supercontinents (from Rodinia to Gondwana), extensive glaciations with ice caps reaching the Equator and the beginning of metazoan diversification. In such a context, the palaeomagnetic record can be used to constrain both the palaeogeography and the palaeoclimate (palaeolatitudinal distribution of glacial deposits). Here we present an up-to-date geochronological and palaeomagnetic database for the Neoproterozoic glacial deposits, including poles recently obtained on ‘cap carbonates’ from China, Oman, and Amazonia. The database comprises ten poles (from eight different cratons), obtained directly on the glacial deposits or on the overlying ‘cap carbonate’, and two other palaeolatitudes derived from reference poles coeval to well-dated glacial units in the same craton. The occurrence of glacial deposition at low latitudes (<30 °) is attested by some good-quality poles, two of them well dated at ∼740 and ∼635 Ma. Based on these poles and on reference poles obtained on igneous rocks, tentative palaeogeographic reconstructions for ∼750, ∼620, and ∼580 Ma (ages for which the database has limited but still sufficient entries) were performed in order to investigate the tectonic context within which the glacial events were produced.À la fin du Néoprotérozoïque, la Terre a probablement été affectée par d’importants changements environnementaux, avec la transition entre deux supercontinents (du Rodinia au Gondwana), des glaciations sévères pendant lesquelles les calottes glaciaires atteignent la zone équatoriale, et le début de la diversification des métazoaires. Dans un tel contexte, les enregistrements paléomagnétiques peuvent être utilisés pour contraindre à la fois la paléogéographie et le paléoclimat (répartition paléolatitudinale des dépôts glaciaires). Nous présentons ici une base de données géochronologique et paléomagnétique pour les dépôts glaciaires néoprotérozoïques, incluant les pôles paléomagnétiques récemment acquis sur les capcarbonates de Chine, d’Oman et d’Amazonie. La base de données comporte dix pôles (de huit cratons différents), acquis directement sur les dépôts glaciaires ou sur les capcarbonates susjacents, ainsi que deux autres paléolatitudes, qui proviennent de pôles de référence contemporains des unités glaciaires bien datées, du même craton. L’existence de dépôts glaciaires aux basses latitudes (<30) est attestée par quelques pôles de bonne qualité, dont deux sont bien datés à 740 et 635 Ma. À partir de ces pôles, ainsi que des pôles de référence obtenus sur des roches ignées, des reconstitutions paléogéographiques pour ∼750,∼620 et ∼580 Ma (âges pour lesquels la base de données comporte un nombre de résultats limité, mais encore suffisant) sont proposées, dans le but de préciser le contexte tectonique dans lequel se sont produits les événements glaciaires.
Chemical Geology, 2007
The late Neoproterozoic Araras Group is exposed along the southern border of the Amazon craton an... more The late Neoproterozoic Araras Group is exposed along the southern border of the Amazon craton and at the northern Paraguay belt, central Brazil. It comprises carbonate rocks over 600 m thick overlying Marinoan glaciogenic diamictites, and includes the first welldocumented cap carbonate sequence in South America. δ 13 C values for Araras carbonates, interpreted as representative of the original seawater (δ 18 O N −10‰), exhibit a trend from strongly negative δ 13 C values around −9‰ at the base to enriched 13 C compositions towards the top of the succession reaching values above 0‰. The cap carbonate succession is composed of deep platform deposits of dolomite (Mirassol d'Oeste Formation) covered by limestone rich in crusts and cements (Lower Guia Formation). These rocks show δ 13 C values around −5‰ and anomalous sedimentary structures such as vertical tubes, megaripple bedding (tepee-like structure), and crystal fans of aragonite pseudomorphs. The anoxic deep-platform limestone succession (Guia Formation) that overlies the cap succession presents a homogeneous C-isotope curve, with δ 13 C values around −2.5 to −1‰. Upsection, values switch to positive (+0.1 to +0.3‰) in platform dolomites and shoreface to shallow-platform deposits (Serra do Quilombo Formation), but exhibit oscillations to slightly negative values in the overlying peritidal facies (Nobres Formation). 87 Sr/ 86 Sr values follow the general C-isotope trend, with values increasing upsection from 0.7074 to 0.7088. Striking shifts in δ 13 C and 87 Sr/ 86 Sr values are observed close to stratigraphic surfaces. The correlation between the isotope record and the relative sea-level curves supports the close association between paleoceanographic changes and paleoenvironmental evolution during the terminal Neoproterozoic on the Araras carbonate platform of Amazon craton.
Comptes Rendus Geoscience, 2007
A sedimentological and C–O isotopic study has been carried out in nine sections of the Sete Lagoa... more A sedimentological and C–O isotopic study has been carried out in nine sections of the Sete Lagoas Formation at its classical outcropping area, in the southern tip of the São Francisco craton (central Brazil), with the objective of refining its stratigraphic position within the Neoproterozoic. At the study area, the Neoproterozoic Sete Lagoas Formation comprises two shallowing-upward megacycles, corresponding to
Precambrian Research, 2003
The Holum granite (Vest-Agder, southernmost Norway) has been considered to be the oldest (980 Ma)... more The Holum granite (Vest-Agder, southernmost Norway) has been considered to be the oldest (980 Ma) undeformed ("posttectonic") pluton of a late-Sveconorwegian granitoid series that form two belts in southern Norway and SW Sweden. An integrated structural study of this granite pluton, based on a survey of its anisotropy of low-field magnetic susceptibility (AMS), complemented by microstructural observations, optical microscopy and rock magnetism investigations (including measurements of partial anhysteretic remanent magnetization and its anisotropy, pAARM), is presented here. The magnetic susceptibility and the AMS of the Holum granite are carried by large magnetite grains. Coaxiality between the magnetite-controlled AMS fabric and the rock fabric is verified, which allows the use of the former as a proxy for the latter. Microstructural evidence of a deformation continuum in the granite from magmatic to solid state, and homogeneity and continuity of the AMS fabrics across the pluton demonstrate that the Holum granite is not post-tectonic, but that it was emplaced in a tectonic strain field. A model of emplacement and deformation during the last Sveconorwegian regional folding phase evidenced in the country rocks of the pluton is proposed.
Imaging downward granitic magma transport • O. Bolle et al.
Terra Nova, 2007
Organic matter from the cap dolostones overlying the Marinoan-age glacigenic diamictites of the A... more Organic matter from the cap dolostones overlying the Marinoan-age glacigenic diamictites of the Araras Group, Amazon craton, has been studied to reconstruct the post-glacial ecosystem. Molecular fossils indicate that the post-Marinoan ecosystem was marked by an apparent decline in marine algal diversity. The proliferation of red algae may be explained by environmental changes, such as a massive nutrient input accompanying continental weathering after the ice thaw and a dimer light penetrating sea water due to the drowning of the platform. In addition, the presence of green sulphur bacteria indicates that sea water was stratified with an anoxic (possibly euxinic, i.e. sulphidic) layer at the water–sediment interface. Sulphur cycling probably occurred at the redox boundary as suggested by the recognition of active sulphate reduction. This observation supports a microbially induced model for the formation of the cap dolostones.
Geophysical Journal International, 2001
The fabric of fine-grained magnetite is isolated by partial remanence anisotropy (pAAR) measureme... more The fabric of fine-grained magnetite is isolated by partial remanence anisotropy (pAAR) measurements that combine cycles of anhysteretic remanence acquisition and alternating frequency (AF) tumbling demagnetization of the low-coercivity fraction. Since remanence intensities of the most coercive, fine-grained magnetite are very weak (rarely exceeding 10 per cent of the measured remanence in the studied samples), a methodological study is presented that defines optimal measurement schemes and examines the role of gyroremanence and background remanences due to hard remanence carriers. This procedure is applied to some `reddened' granites from Tana (Corsica) and Wichita (Oklahoma) and to the Tellnes ilmenite-rich norite (Norway). The pAAR fabrics for fine-grained magnetites are compared to the fabrics of the coarse-grained, primary magnetite grains, as shown by both the anisotropy of magnetic susceptibility (AMS) and the remanence anisotropy of the low-coercivity fraction. In Tellnes, fine-grained magnetite is believed to be of secondary origin; this is strongly suggested by the correlation between the pAAR fabrics and the orientations of sets of microfractures decorated by oxides. In the `reddened' granites, although no direct correlation between pAAR fabrics and microstructures could be evidenced, the comparison between unaltered specimens and hydrothermally altered ones reveals changes in magnetic properties and fabrics that are ascribable to this secondary geological process.
Geophysical Journal International, 2001
The fabric of fine-grained magnetite is isolated by partial remanence anisotropy (pAAR) measureme... more The fabric of fine-grained magnetite is isolated by partial remanence anisotropy (pAAR) measurements that combine cycles of anhysteretic remanence acquisition and alternating frequency (AF) tumbling demagnetization of the low-coercivity fraction. Since remanence intensities of the most coercive, fine-grained magnetite are very weak (rarely exceeding 10 per cent of the measured remanence in the studied samples), a methodological study is presented that defines optimal measurement schemes and examines the role of gyroremanence and background remanences due to hard remanence carriers. This procedure is applied to some ‘reddened’ granites from Tana (Corsica) and Wichita (Oklahoma) and to the Tellnes ilmenite-rich norite (Norway). The pAAR fabrics for fine-grained magnetites are compared to the fabrics of the coarse-grained, primary magnetite grains, as shown by both the anisotropy of magnetic susceptibility (AMS) and the remanence anisotropy of the low-coercivity fraction. In Tellnes, fine-grained magnetite is believed to be of secondary origin; this is strongly suggested by the correlation between the pAAR fabrics and the orientations of sets of microfractures decorated by oxides. In the ‘reddened’ granites, although no direct correlation between pAAR fabrics and microstructures could be evidenced, the comparison between unaltered specimens and hydrothermally altered ones reveals changes in magnetic properties and fabrics that are ascribable to this secondary geological process.
Terra Nova, 2007
The end of the Neoproterozoic is punctuated by glacial deposition, but the chronology of these de... more The end of the Neoproterozoic is punctuated by glacial deposition, but the chronology of these deposits is hindered presently by the paucity of geochronological data. Here, we present new radiometric dating for the basal Sete Lagoas cap carbonate deposits that overlie glacial units in the São Francisco craton. Six samples from aragonite-pseudomorph crystal-rich facies, showing pristine textures and constant 87Sr/86Sr ratios around 0.7075, yielded a Pb–Pb isochron age of 740 ± 22 Ma, which is interpreted as the depositional age for these remarkably preserved rocks. This age can be used to infer a low-to-moderate palaeolatitude of 20–30° for carbonate (and glacial) deposition. In addition, as it overlaps the ages obtained for the oldest Neoproterozoic glacial successions, our result reinforces the idea of a long-standing ‘Sturtian’ interval, suggesting that this event represents either different discrete glaciations or a protracted event encompassing almost 80 Ma.
Earth and Planetary Science Letters, 2006
Paleomagnetic analysis on 15 early Cambrian mafic dikes from Itabaiana (Paraíba State) yielded a ... more Paleomagnetic analysis on 15 early Cambrian mafic dikes from Itabaiana (Paraíba State) yielded a southern (northwestern) direction with steep upward (downward) inclination (Dm = 167.5°, Im = − 63.7°, α95 = 7.3°). AF and Thermal demagnetization, thermomagnetic curves, and hysteresis results suggest that this component is dominantly carried by fine-grained SD magnetite. The high stability of this component and positive baked contact tests on three dikes indicate it represents a primary thermoremanent magnetization. Ar–Ar analysis on whole-rock samples from two sites provides a strong constraint on the age of the Itabaiana paleomagnetic pole (134.6° E, 34.9° S; A95 = 7.3, K = 28) defined by plateau ages of 525 ± 5 and 526 ± 4 Ma. This pole completely satisfies six out of the seven quality criteria proposed by Van der Voo [R. Van der Voo, The reliability of paleomagnetic data, Tectonophysics 184 (1990) 1–9.] and permits a tight constraint on the Early Cambrian sector of the Gondwana apparent polar wander path. Paleogeographic reconstructions consistent with the available paleomagnetic and geological record show that Gondwana was sutured along three major orogenies, the Mozambique (Brasilano/Pan-African) Orogeny (800–650 Ma), the Kuunga Orogeny (570–530 Ma) and the Pampean–Araguaia Orogeny (540–520 Ma). We suggest that after rifting away from Laurentia at the end of the Neoproterozoic, opening the Iapetus ocean, the Amazonian craton and minor adjoining blocks, such as Rio Apa and Pampia, collided with the proto-Gondwana by Cambrian times at ca. 530–520 Ma. Unless for small adjustments, Gondwana was completely formed by 525 Ma whose paleogeography is defined by the Itabaiana pole.
Tectonophysics, 1999
Rock magnetism and magnetic fabric studies were performed in the Tourão granite (western part of ... more Rock magnetism and magnetic fabric studies were performed in the Tourão granite (western part of Seridó belt, NE Brazil). This pluton (350 km2) is composed of K-rich calc-alkalic monzogranites and was emplaced between a set of NE- and E-trending dextral strike–slip shear zones. Electronic and optical microscopy, thermomagnetic curves, hysteresis data and coercivity spectra revealed that magnetite (fine- and coarse-grained) is the unique ferromagnetic mineral in these rocks. Anisotropy of low-field magnetic susceptibility (AMS) was measured in 83 sites, and for many sites from central and southern parts of the intrusive body a large scatter was noticed. Anisotropy of partial anhysteretic remanence (pAAR) was determined for 29 sites in the 0–50 mT (pAAR0–50) and 19 sites in the 50–90 mT (pAAR50–90) windows to isolate the fabric contribution of the remanence bearing grains (magnetite). In most sites AMS is dominantly carried by magnetite, whereas in some sites the paramagnetic contribution exceeds 60% of the bulk susceptibility. The pAAR0–50 is assumed to be related to coarse-grained early crystallized magnetites, while the pAAR50–90 is interpreted as due to fine needle-shaped magnetites hosted in biotites.The combined analysis of these magnetic techniques permits interpretation in terms of fabric formation in the Tourão granite and the inference that at least two successive kinematic events were recorded in this intrusive body devoid of any solid-state deformation overprint. The AMS and pAAR0–50 fabrics for most of the sites correlate with the early NNE-trending magmatic fabric, whereas pAAR50–90 reveals a different fabric, probably related to late partitioning of magmatic straining along the E-trending shear zones. In the southern part of the body, a completely different pAAR0–50 fabric is correlated with the late intrusion of charnockite stocks.
Precambrian Research, 2004
New Pb–Pb ages, rock magnetic, and paleomagnetic data were obtained on Neoproterozoic carbonates ... more New Pb–Pb ages, rock magnetic, and paleomagnetic data were obtained on Neoproterozoic carbonates of the Salitre Formation which conformably overlie the glacial rocks of the Bebedouro Formation in undeformed areas of the Irecê basin, São Francisco craton. These data give new supporting evidence for a large-scale remagnetization and resetting of the U–Pb isotopic system at Cambrian times along the São Francisco and Irecê basins, which cover more than 300,000 km2 of the São Francisco craton surface. A – isochron age of 514±33 Ma was on undeformed, stromatolitic carbonates from the Salitre Formation. Although samples from other sites give almost non-radiogenic Pb signatures, they fall at the lower end of this line. Rock magnetic properties obtained for most of the sites are typical of remagnetized carbonates, and include: wasp-waisted hysteresis loops, contradictory Lowrie–Fuller and Cisowski tests, and anomalously high hysteresis ratios. Some samples, however, have presented only some of these magnetic characteristics. Four magnetic components (A, B, C, and D) were isolated from the Salitre carbonates. Three of them (A, B, and C) are comparable to previous results from the correlative Bambuı́ carbonates, which crop out ca. 600 km southward. The component C, carried by fine-grained SD-PSD magnetite, was identified at 23 sites. It directs steeply positive to the northeast, giving a paleomagnetic pole (SaC) at 33°N, 323°E (α95=4.0°, k=23.6). The SaC pole overlaps with that obtained from component C of Bambuı́ carbonates (BaC pole: 30°N, 321°E, α95=3.8°). The paleopoles from component B of Salitre (SaB) and Bambuı́ (BaB) also lie close but are statistically different. After rotation of South America to Africa, all these poles fall close to the ∼520 Ma segment of the Gondwana apparent polar wander path, suggesting that the resetting of the isotopic and magnetic systems in the Neoproterozoic carbonates of the São Francisco craton occurred at that time. The single polarity directions found along the whole sedimentary succession in both areas give additional support for rapid remagnetization events. We suggest that paleomagnetic data and most of the Pb–Pb isochron ages in the São Francisco carbonates recorded a regional-scale fluid migration event, in the aftermath of the Brasiliano (Panafrican) collision. This fluid-flow event may also have concurred for the genesis of the Pb–Zn mineralizations found across the basin.
Precambrian Research, 2004
Paleomagnetic results from unmetamorphosed Mesoproterozoic mafic dikes of Salvador, northern São ... more Paleomagnetic results from unmetamorphosed Mesoproterozoic mafic dikes of Salvador, northern São Francisco craton, yield either an easterly direction with high upward inclination (Dm=110.8°, Im=−79.3°, α95=8.4°, N=8, paleopole at 122.7°E, 6.4°N), or a westerly direction with high downward inclination (Dm=290.7°, Im=64.7°, α95=7.5°, N=7, paleopole at 282.6°E, 5.6°N). Additional paleomagnetic analyses on country rocks (granulites and amphibolitic dikes) yield paleomagnetic poles similar to those obtained in other metamorphic rocks from the São Francisco craton affected by the Transamazonian orogeny (2200–1900 Ma). Positive baked contact tests in two dikes, together with rock magnetic studies, supported by petrographic and radiometric analyses, indicate that the remanent directions isolated for the unmetamorphosed dikes are primary TRM’s. step-heating analyses on biotite from a granulite sampled ∼90 cm away from the dike’s contact yield an integrated age of 1021±8 Ma, whereas the plagioclase from the dike itself yields an integrated age of 1003±33 Ma, interpreted to be the time of the emplacement. Our data in conjunction with the available late Mesoproterozoic poles from the São Francisco craton allow to best constrain its APW path between 1080 and 1010 Ma.
Precambrian Research, 2008
New structural, geochronological and paleomagnetic data were obtained on dolerite dikes of the No... more New structural, geochronological and paleomagnetic data were obtained on dolerite dikes of the Nola region (Central African Republic) at the northern border of the Congo craton. In this region, metavolcanic successions were thrust southward onto the craton during the Panafrican orogenic events. Our structural data reveal at least two structural klippes south of the present-day limits of the Panafrican nappe suggesting that it has once covered the whole Nola region, promoting the pervasive hydrothermal greenschist metamorphism observed in the underlying cratonic basement and also in the intrusive dolerite dikes. Paleomagnetic measurements revealed a stable dual-polarity low-inclination magnetization component in nine dikes (47 samples), carried by pyrrhotite and magnetite. This component corresponds to a paleopole at 304.8°E and 61.8°S (dp = 5.4, dm = 10.7) graded at Q = 6. Both metamorphism and magnetic resetting were dated by the 40Ar/39Ar method on amphibole grains separated from the dikes at 571 ± 6 Ma. The Nola pole is the first well-dated paleomagnetic pole for the Congo craton between 580 and 550 Ma. It marks a sudden change in direction of the Congo craton apparent polar wander path at the waning stages of the Panafrican orogenic events.
Precambrian Research, 2011
Paleomagnetic, magnetic anisotropy data and 40Ar/39Ar ages are presented for high-grade metamorph... more Paleomagnetic, magnetic anisotropy data and 40Ar/39Ar ages are presented for high-grade metamorphic rocks of the Jequié block, São Francisco Craton. Jequié charnockites and enderbites gneisses from the eastern border of the block present northern and steep-downward directions, carried by Ti-poor titanomagnetite with high unblocking temperatures (550–600 °C). A mean direction for 12 sites of enderbites from the eastern sector yielded a magnetic component A (Dm = 47.0°, Im = 75.7°, α95 = 6.3°, K = 48.8) with a corresponding paleomagnetic pole at 339.6°E, 5.4°N (A95 = 11.2°). Sites sampled on other metamorphic rocks including granulite-facies, tonalites and dacites yield different magnetic components. Anisotropy of magnetic susceptibility measured for all sampling sites shows a high degree of anisotropy (P = 1.121–1.881), with NE- or NW-trending magnetic lineations and vertical magnetic foliations. These data were used to correct the mean site directions for all components. While the other components presented larger scatter after anisotropy correction, the component A shows a slightly tighter clustering of magnetic directions (Dm = 61.2°, Im = 76.5°, α95 = 5.4°, K = 66.2) giving a new, anisotropy corrected paleomagnetic pole at 342.1°E, −0.5°N (A95 = 9.6°). 40Ar–39Ar plateau ages of 2035 ± 4 Ma (hornblende) and 1876 ± 4 Ma (biotite) obtained for one of the samples with component A imply a very low cooling-rate of 1.4 °C/Ma for these rocks. Based on these ages and corrected unblocking temperatures of the magnetic component A, we argue that the characteristic magnetization of the Jequié metamorphic rocks was acquired by high temperature thermo-chemical processes during regional cooling of the adjacent Itabuna–Salvador–Curaçá belt between 2089 and 1985 Ma (pole age likely at 1.99 Ga). Comparison of this pole with available paleomagnetic poles from South America and Africa suggests that neither Atlantica nor Ur have ever existed, and dispersed continental fragments dominated the paleogeography at ca. 2.0 Ga ago.► First high-quality Paleoproterozoic paleomagnetic pole for the Congo-São Francisco craton. ► Paleomagnetic pole was corrected from the effects of magnetic anisotropy. ► Results provide a negative test to the Atlantica supercontinent at ca. 2.0 Ga.
International Geology Review, 1998
An interpretation of available paleomagnetic data from the Laurentia, Congo-Sao Francisco, Kalaha... more 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 Hoffman's (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 Francisco craton can explain the observed oblique convergence and wrench tectonics of Pan African-Brasiliano mobile belts that encircle this craton. The model is also consistent with the synchroneity between the Rodinia break-up and the assembly of Gondwana, as suggested by several authors (Laurentia began to separate from Rodinia at 625 Ma or later).
Earth and Planetary Science Letters, 2008
We apply a new diagenetic dating technique to determine the age of magnetization for Precambrian ... more We apply a new diagenetic dating technique to determine the age of magnetization for Precambrian sedimentary rocks in the SW Amazon craton. Two new paleomagnetic poles are reported from the rocks of the Aguapeí Gp.: red beds of the Fortuna Fm. (Plat = 59.8°N, Plon = 155.9°E, A95 = 9.5, K = 14, 18 sites, N/n 128/115, Q = 5) and the reverse-polarity mudstones of the overlying Vale da Promissão Formation (Plat = 49.5°N, Plon = 89.3°E, A95 = 12.5, K = 30, 6 sites, N/n = 94/80, Q = 4). The Fortuna Fm. magnetization is hosted by massive, interstitial hematite cement and constitutes a post-depositional remanence. The age of diagenesis of the red beds is well-constrained by the 1149 ± 7 Ma U Pb age of authigenic xenotime rims on detrital zircons determined by SHRIMP analysis. The magnetite-hosted remanence of the Vale da Promissão Fm. may be detrital in origin, but the age of deposition is poorly constrained. The reliable and precisely-dated Fortuna Fm. paleomagnetic pole fixes the paleogeographic position of the Amazon craton near the SE Appalachians portion of North America at 1.15 Ga. These data demonstrate a mobile Grenvillian link between these two cratons, and support the recent identification of Amazon crust in the Blue Ridge province region of North America.
Precambrian Research, 2008
In an attempt to improve our understanding of the Paleoproterozoic geodynamic evolution, a paleom... more In an attempt to improve our understanding of the Paleoproterozoic geodynamic evolution, a paleomagnetic study was performed on 10 sites of acid volcanic rocks of the Colider Suite, southwestern Amazonian Craton. These rocks have a well-dated zircon U–Pb mean age of 1789 ± 7 Ma. Alternating field and thermal demagnetization revealed northern (southern) directions with moderate to high upward (downward) inclinations. Rock magnetism experiments and magnetic mineralogy show that this characteristic magnetization is carried by Ti-poor magnetite or by hematite that replaces magnetite by late-magmatic deuteric alteration. Both magnetite and hematite carry the same characteristic component. The mean direction (Dm = 183.0°, Im = 53.5°, N = 10, α95 = 9.8°, K = 25.2) yielded a paleomagnetic pole located at 298.8°E, 63.3°S (α95 = 10.2°, K = 23.6), which is classified with a quality factor Q = 5. Paleogeographic reconstructions using this pole and other reliable Paleoproterozoic poles suggest that Laurentia, Baltica, North China Craton and Amazonian Craton were located in laterally contiguous positions forming a large continental mass at 1790 Ma ago. This is reinforced by geological evidence which support the existence of the supercontinent Columbia in Paleoproterozoic times.
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Papers by Ricardo Trindade