Click Here JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, B06204, doi:10.1029/2005JB004106, 2006 for Full Article Remagnetization in bituminous limestones of the Neoproterozoic Araras Group (Amazon craton): Hydrocarbon maturation, burial diagenesis, or both? E. Font,1,2 R. I. F. Trindade,1 and A. Nédélec2 Received 27 September 2005; revised 13 January 2006; accepted 9 March 2006; published 16 June 2006. [1] Neoproterozoic carbonates of the Araras Group exhibit two distinct magnetic components across the same carbonate succession in a cross-section between the Amazon craton and the Paraguay fold belt. Pink dolostones of the Mirassol d’Oeste Formation carry a dual polarity, primary component, whereas black bituminous limestones of the Guia Formation yield a secondary postfolding component. Magnetic signatures of the Guia limestones, such as high anhysteretic remanence magnetization/saturation isothermal remanence magnetization ratios, high-frequency-dependent magnetic susceptibility and contradictory Lowrie-Fuller and Cisowski tests, are typical of remagnetized carbonates. Unblocking temperatures suggest that the stable high-temperature remanence is carried by both pyrrhotite and magnetite for which an authigenic origin is suggested by scanning electron microscope observations. The different magnetic properties noted between dolostones with or without bitumen and between dolostones and limestones in the same metamorphic conditions lead to the hypothesis that the amount of hydrocarbon as well as the lithology influence nucleation of authigenic magnetic minerals in these rocks. Presence of magnetite pseudoframboids and euhedral iron sulphide crystals occurring in fracture and voids are in favor of a chemical remanence (CRM). The presence of pyrrhotite as one of the main carriers of CRM in these rocks, and its association with bitumen in fractures is probably related to an epigenetic enrichment of sulfur due to hydrocarbon seepage. However, hydrocarbon maturation solely could not explain the differences of the magnetic mineralogy observed in the craton and the fold belt. Enhanced magnetite formation in the thrust and fold belt is interpreted to be the result of higher temperatures leading to stronger diagenesis of clay minerals. Citation: Font, E., R. I. F. Trindade, and A. Nédélec (2006), Remagnetization in bituminous limestones of the Neoproterozoic Araras Group (Amazon craton): Hydrocarbon maturation, burial diagenesis, or both?, J. Geophys. Res., 111, B06204, doi:10.1029/2005JB004106. 1. Introduction [2] Carbonate rocks often show very stable magnetization but of secondary origin. Large-scale remagnetization in such rocks has been vastly documented for the Phanerozoic [e.g., McCabe and Elmore, 1989; Huang and Opdyke, 1996; Xu et al., 1998] and at a minor extent for the Neoproterozoic [D’Agrella-Filho et al., 2000; Trindade et al., 2003, 2004]. Remagnetized carbonates are recognized by typical magnetic properties [e.g., Jackson, 1990; Jackson et al., 1992, 1993; Channell and McCabe, 1994], and the nature of the remanence carriers themselves has been extensively documented by scanning electron microscopy studies [e.g., Suk et al., 1990, 1993; Sun and Jackson, 1994; 1 Departamento de Geofı́sica, Instituto de Astronomia, Geofı́sica e Ciências Atmosféricas, Universidade de São Paulo, São Paulo, Brazil. 2 Laboratoire des Mécanismes et Transferts en Géologie, UMR 5563, Observatoire Midi-Pyrenees, Université Paul-Sabatier, Toulouse, France. Copyright 2006 by the American Geophysical Union. 0148-0227/06/2005JB004106$09.00 Weil and Van der Voo, 2002]. Even though some earliest studies have suggested a thermoviscous resetting of the natural remanent magnetization (NRM) to explain the carbonate remagnetization [Horton et al., 1984; Kent, 1985], a chemical remanence magnetization (CRM) acquisition is presently recognized as the most suitable mechanism [e.g., McCabe and Elmore, 1989]. We note, however, that the cause of the CRM is still debatable. Questions also remain in respect to the source of the iron and to the role of external/internal fluids in the formation of the new magnetic minerals. [3] The most commonly invoked fluid-related agent of chemical remagnetization in carbonates is the orogenrelated fluids, believed to be capable of causing magnetite authigenesis along vast regions [Oliver, 1986; McCabe and Elmore, 1989]. Alternatively, magnetite formation may be linked to hydrocarbon migration and/or organic matter maturation [Elmore et al., 1987; McCabe and Elmore, 1989; Brothers et al., 1996; Banerjee et al., 1997; Blumstein et al., 2004]. Brothers et al. [1996], for example, have experimentally demonstrated that organic B06204 1 of 17 B06204 FONT ET AL.: REMAGNETIZATION IN GUIA LIMESTONES B06204 Figure 1. (a) Structural map of the Amazon craton and the northern Paraguay thrust and fold belt with localization of the Terconi quarry and outcrops from Cáceres region (squares), Mato Grosso state, Brazil. (b) Geological section A-B along a NW-SE transect from the Terconi quarry to Cuiabá city passing through the Cáceres region. (c) Illite crystallinity indices along the same section from Alvarenga [1990]. acids produced by maturation of organic matter can be responsible for magnetite formation and consequently for remagnetization. On the other hand, authigenic magnetite could also be linked to conversion of smectite to illite [Elmore et al., 1993; Hirt et al., 1993; Katz et al., 2000; Gill et al., 2002]. This reaction would furnish the iron needed for the new magnetic minerals. Recently, Moreau et al. [2005] have shown that sedimentary rocks can be remagnetized during burial diagenesis with fluids being produced in situ during burial. [4] In the Amazon craton, carbonates of the Neoproterozoic Araras Group exhibit two distinct magnetic components across the same carbonate succession [Trindade et al., 2003]. At the base of the Araras Group (Figure 1), just above diamictites of the Puga Formation, pink dolostones of the Mirassol d’Oeste Formation carry a dual polarity primary component, whose remanence is carried by detrital specular hematite [Font et al., 2005]. Upsection, grey to black bituminous limestones of the Guia Formation are found both in the cratonic area, above the Mirassol d’Oeste dolostones, and inside the Paraguay belt, in the Cáceres region. These limestones yield a secondary component confirmed by a negative fold test. Stratigraphy and sedimentology of these rocks are well established by Nogueira et al. [2003] and illite crystallinity index for both the craton and the fold belt are provided by Alvarenga [1990]. [5] The aim of this paper is to establish the nature of the magnetic carriers of the Guia limestones remanent magnetization and investigate the role of lithotype, temperature and presence of organic matter in influencing remagnetization. The contrasting lithology found between the Mirassol d’Oeste dolostones and the Guia limestones, the presence of bitumen in part of the stratigraphic section, and the temperature gradient toward the fold belt make this region an excellent natural laboratory to study remagnetization processes. In this way, we have conducted a detailed rock 2 of 17 FONT ET AL.: REMAGNETIZATION IN GUIA LIMESTONES B06204 B06204 Table 1. Site-Mean Paleomagnetic Directions of TR and TE Samplesa Lithology Locality Site Bedding Attitudes n/N Mean NRM, mA/m D, deg I, deg a95, deg k Calcareous Calcareous Calcareous Calcareous Calcareous Calcareous Calcareous Calcareous Calcareous Calcareous Calcareous Calcareous Calcareous Calcareous Dolostone Dolostone Dolostone Mean direction Cáceres Cáceres Cáceres Cáceres Cáceres Cáceres Cáceres Cáceres Cáceres Terconi Terconi Terconi Terconi Terconi Terconi Terconi Terconi TE-28 TE-27 TE-26 TE-25 TE-24 TE-23 TE-22 TE-21 TE-20 TE-19 TE-18 TE-16b TE-15 TE-14 TE-13 TR-7 TR-6 40/40SE 185/88W 14/84E 2/88E 192/90E 10/87E 185/86W 187/82W 10/88E 0/0 0/0 0/1 0/0 0/0 0/0 0/0 0/0 6/6 5/7 6/6 5/6 7/7 6/6 6/6 6/6 6/6 11/12 10/13 6/8 8/10 6/12 0/7 0/10 0/7 13/17 0.124 0.164 1.394 1.625 1.321 0.300 0.497 1.160 0.609 0.162 0.107 0.170 0.136 0.112 0.076 0.077 0.106 48.5 28.1 28.0 38.2 30.9 33.8 34.4 40.4 34.1 24.7 22.4 18.3 25.5 37.3 332.6 55.5 41.6 58.2 54.5 51.5 61.0 58.0 62.7 59.7 47.7 56.8 55.5 53.9 47.7 29.9 12.7 16.4 6.4 3.3 7.7 5.6 4.8 6.2 6.2 11.3 10.5 27.9 8.8 16.7 4.3 28.8 22.6 109.3 532.4 62.3 117.6 193.6 118.5 118.5 17.3 22.3 6.7 40.4 17.0 92.2 a Mean values of the natural remanence magnetization (NRM), declination (D), inclination (I), statistical value of a95, number of samples giving results (n) and total number of samples analyzed (N), dispersion parameter (k). b Rejected for mean direction. magnetic survey on Guia samples, at the craton and the fold belt, including a series of magnetic tests complemented by scanning electron microscope (SEM) observations and energy dispersive spectra (EDS) analyses. On the basis of these results, the mechanisms of remanence acquisition and the source of iron for authigenic magnetic minerals in these rocks are discussed. 2. Geological Setting and Sampling [6] The Neoproterozoic cap carbonates of the Araras Group are exposed on the southeast of the Amazon craton, where they overlie diamictites of the Puga Formation without significant hiatus [Nogueira et al., 2003]. The base of the Araras Group succession is represented by the dolostones of the Mirassol d’Oeste Formation and the limestones of the Guia Formation which are the focus of this work (Figure 1a). [7] The Mirassol d’Oeste Formation is represented by laminated pinkish dolomudstone principally constituted of planar to undulated microbialites with fenestral porosity and occurrence of tube-like structures in the intermediate part of the sequence. The top of the sequence changes to grey dolostones, fulfilled by hydrocarbon, with calcite cements and megaripples. Upsection, the Guia Formation is entirely constituted by bituminous limestones with terrigenous grains. In both grey dolostones and limestones, the hydrocarbon fulfills fractures, styloliths, primary and secondary porosity and dissolution cavities. In the underlying dolostones, bitumen is absent except for the immediately adjacent megaripple level suggesting that the Guia Formation is the source rock for the hydrocarbon. [8] Both Mirassol d’Oeste and Guia Formations crop out at the border of the Amazon craton, in the Terconi quarry, in nondeformed subhorizontal layers. Close to Cáceres, the Guia Formation crop out inside the deformed zone of the Paraguay belt (Figure 1b). At Terconi, the basal section of the Guia Formation crops out above the Mirassol d’Oeste dolostones, whereas in Cáceres, we found the top of the same unit, covered by the dolostones of the Serra do Quilombo Formation. This stratigraphic stacking is corroborated by facies analysis and chemostratigraphy [Nogueira et al., 2003, 2006]. [9] The Paraguay belt is a 1200 km long thrust and fold belt bordering the Amazon craton and the Rio de la Plata block (Figure 1). Normal to its strike, the belt comprises an undeformed sector at the border of the Amazon craton, and the external and internal zones [Alvarenga, 1990]. From the craton to the internal zone, deformation increases altogether with the schistosity and metamorphism. Accordingly, illite crystallinity indices provided by Alvarenga [1990] (Figure 1c) indicate an increase in maximum temperatures on the same direction. [10] Seventeen paleomagnetic sites were collected along more than 50 m of the carbonate succession. In the Terconi quarry, samples (T13 – T19) were collected approximately on meter-scale spacing, whereas in Cáceres, samples (T20 – T28) are spaced of a distance of 5 – 10 m. First paleomagnetic results from dolostones showed a dual-polarity magnetization with northward (southward) and negative (positive) magnetic directions that provided a 22 paleolatitude [Trindade et al., 2003]. Results obtained from the Guia limestones showed a northeast, positive (single polarity) component that failed a fold test [Trindade et al., 2003]. This component gives a paleomagnetic pole that overlaps those obtained for remagnetized carbonates from the São Francisco craton, whose magnetization acquisition was dated at around 520 Ma [D’Agrella-Filho et al., 2000; Trindade et al., 2004]. 3. Methods [11] A total of 135 oriented samples from both the bituminous upper dolostones of Mirassol d’Oeste Formation (24 samples) and the limestones of the Guia Formation (111 samples) were analyzed after stepwise thermal cleaning (Table 1). In order to reduce viscous overprints on the characteristic magnetization, samples were first stored in a 3 of 17 B06204 FONT ET AL.: REMAGNETIZATION IN GUIA LIMESTONES B06204 Figure 2. Paleomagnetic direction (declination Dec, inclination Inc) and magnetic properties (NRM, Kfd, SIRM, and DP) of Mirassol d’Oeste cap dolostones and bituminous Guia limestones [after Trindade et al., 2003; Font et al., 2005]. Saturation-induced remanent magnetization (SIRM) and the dispersion parameter (DP) are obtained by the LAP-GAP-SAP analysis of Kruiver et al. [2001]. Kfd is the frequency dependence susceptibility characterizing the superparamagnetic particles contribution [Jackson et al., 1992]. Shaded area indicates bituminous dolostones. low-field chamber for 3 months. Paleomagnetic data were obtained after alternating field (AF) and thermal treatments using 16 to 20 progressive steps. Measurements were performed in an automatic three-axis 2G-cryogenic magnetometer, housed in a magnetically shielded room (ambient field <1000 nT). The demagnetization data were plotted by Zijderveld [1967] diagrams, and principal component analysis [Kirschvink, 1980] was used to determine magnetic directions. Mean directions were calculated using Fisherian statistics [Fisher, 1953]. [12] The rock magnetic measurements consisted of isothermal remanence magnetization (IRM) and anhysteretic remanence magnetization (ARM) acquisition and demagnetization curves (Lowrie-Fuller and Cisowski tests [Johnson et al., 1975; Cisowski, 1981]), demagnetization of triorthogonal IRM acquisition [Lowrie, 1990] and measurements of frequency dependence on the magnetic susceptibility. The IRMs were induced using a SI-4 magnetizer system (Sapphire Instruments), while demagnetization and remanence measurements were performed with the 2G-cryogenic magnetometer coupled to a triorthogonal AF demagnetizer. For the Lowrie-Fuller and Cisowski tests, samples were first AF demagnetized at 200 mT and then acquired an ARM with a peak AF field of 160 mT and a biasing DC field of 0.1 mT. After AF demagnetization, samples acquired an IRM up to 160 mT and were subsequently stepwise demagnetized at fields up to 160 mT. The Lowrie test was performed after IRM acquisition at 1.4, 0.4 and 0.12 T fields along the z, y, and x axis, respectively. Samples were then thermally demagnetized up to 700C. Thermal demagnetization was performed using a Magnetic Measurements oven (peak temperature within ±5C). The frequency ependence of the magnetic susceptibility was obtained after three series of measurements with a Bartington MS2 dual-frequency (470 and 4700 Hz) alternating current bridge on whole rock specimens. [13] SEM observations and EDS analyses were performed on both carbon-coated and noncoated polished sections and rock fragments cut from the paleomagnetic samples, with SEMs Jeol JSM-T330A and Jeol JSM6360LV both coupled to EDS analyzers. 4. Paleomagnetism [14] The samples carry a natural remanent magnetization (NRM) ranging from 0.07 to 0.16 mA/m in the Terconi quarry and from 0.12 to 1.62 mA/m in Cáceres (Table 1 and Figure 2). Over the 111 samples collected in the bituminous limestones of the Guia Formation, 93 samples had given reliable magnetic directions (83% of the totality) after 4 of 17 B06204 FONT ET AL.: REMAGNETIZATION IN GUIA LIMESTONES Figure 3 5 of 17 B06204 B06204 FONT ET AL.: REMAGNETIZATION IN GUIA LIMESTONES Figure 4. Stereographic projections of posttilting samplebased mean magnetic directions in bituminous Guia limestones from Terconi and Cáceres. The stereographic projection of site-mean directions and the mean directions of the correspondent paleomagnetic pole are given on the left. thermal demagnetization (Table 1). Maximum angular deviation (MAD) values for principal components varies from 1.3 to 42.9 (in sample TE18.D3) with a mean MAD value of 10.88. No reliable data were obtained from the 24 samples collected in the bituminous dolostones of the Mirassol d’Oeste Formation where NRM intensities are the weakest (Table 1 and Figure 2). [15] Previous results obtained from the basal pink cap dolostones indicate a primary magnetization carried by detrital hematite [Trindade et al., 2003; Font et al., 2005] B06204 (Figure 2). Orthogonal plots show a univectorial component with unblocking temperatures between 600 and 670C for both polarities (Figure 3a). At the upper, bituminous level of dolostones, remanence rapidly reaches the noise level of our cryogenic magnetometer giving rise to chaotic Zijderveld diagrams (Figure 3b). Conversely, bituminous limestones found upsection show well-defined single- and multivectorial orthogonal plots after heating (Figure 3c). In most samples, a low-temperature component was isolated between 0 and 250C and is interpreted to be a viscous remanent magnetization (VRM). In diagrams of normalized intensity versus increasing demagnetization temperature, samples from Terconi quarry were completely demagnetized at 320 – 360C indicating pyrrhotite as the unique carrier of remanent magnetization. The large range of unblocking temperatures of 40C around 340C found in our samples suggest that iron sulphide did not behave systematically as ideal pyrrhotite during thermal treatment [Rochette et al., 1990; Zegers et al., 2003]. In contrast to Terconi samples, samples from the deformed zone of Cáceres have shown a second step at around 500C, suggesting mixture of pyrrhotite and magnetite (Figure 3c). Presence of magnetite in addition to pyrrhotite is probably responsible for the higher NRM intensities observed in these samples (Figure 2). Both pyrrhotite and magnetite carry the same magnetic component in Cáceres (Table 1 and Figure 4). [16] More than six samples of bituminous limestones were used to calculate the magnetic component of each paleomagnetic site. Magnetic directions (declination, inclination) and statistic values (a95 and k) for each site are given in Table 1. Site with a95 > 17 were dismissed. The stereographic projections of the sample-based mean magnetic directions for each site are shown in Figure 4. In all samples, a high-temperature component is isolated after the 250C heating step and cluster into the positive northeast sector of the stereograph (Figure 4). In sites where NRM intensities are weaker (Terconi quarry), mean values of a95 are approximately 15, whereas most sites from Cáceres, where NRM is higher, have a95 less than 8 (Table 1). Finally, based on site mean directions, a paleomagnetic pole is defined at +332.6/+29.9 with a95 = 4.3 and k = 92.2 (Table 1). 5. Rock Magnetism 5.1. IRM Acquisition Curves [ 17 ] Isothermal remanent magnetization acquisition curves were obtained at fields up to 160 mT for 8 representative samples of the Guia Formation (Figure 5a). IRM acquisition curves are smooth with concave shape and reach 90% of saturation at 160 mT which typifies low- to medium-coercivity remanence carriers. [18] IRM curves were then analyzed by cumulative logGauss functions (CLG) using the software developed by Kruiver et al. [2001] (Figure 5b). The maximum saturation field used here (160 mT) is much lower than the 1T field Figure 3. Examples of orthogonal and demagnetization intensity plots of samples from (a) Mirassol d’Oeste cap dolostones, (b) intermediary bituminous dolostones, and (c) bituminous Guia limestones (both Terconi and Cáceres region). Units of orthogonal plots are in mA/m. 6 of 17 B06204 FONT ET AL.: REMAGNETIZATION IN GUIA LIMESTONES B06204 Figure 5. (a) IRM acquisition curves of Guia limestones samples from Terconi and Cáceres, and (b) example of LAP-GAP-SAP analysis of sample TE20.B2 obtained using the software developed by Kruiver et al. [2001]. (c) Representation of the IRM parameters for limestones samples from Terconi (circle) and from Cáceres (triangle). SIRM is the calculated saturation isothermal remanent magnetization; DP is a dispersion parameter. originally used by these authors. However, hysteresis curves reveal that our samples saturate at fields below 200 mT (see below) thus justifying the use of these low inducing fields. The CLG analysis is based on the assumption that the IRM of each magnetic mineral conforms to a cumulative log Gaussian function of the magnetizing field and that the different curves combine linearly [Robertson and France, 1994]. Each magnetic carrier has characteristic values of saturation IRM (SIRM), B1=2 and dispersion parameter (DP). The B1=2 parameter (Hcr) represents the field at which half of the SIRM is reached and the DP parameter reflects the dispersion of the logarithmic distribution around B1=2 (corresponding to one standard deviation independent of concentration). For all samples, the CLG models were fitted with a single component for each rock-type suggesting only one magnetic carrier. Log B1=2 and calculated SIRM values are comparable for all samples and are bracketed between 48 and 90 mT and 20 and 200 mA/m, respectively, thus indicating low- to medium-coercivity magnetic carriers for these rocks. DP and SIRM values vary significantly and are higher in samples from the Cáceres region than Terconi samples (Figure 5c). SIRM values depend principally on the nature of the magnetic carriers, while DP variations could be due to several factors such as the homogeneity of the grain size distribution and/or to the crystallinity of the magnetic minerals into the sediments [e.g., Kruiver et al., 2003]. In Guia limestones, the different pattern of SIRMs observed between Terconi and Cáceres samples could be due to a different nature and/or a mixture of magnetic carriers. 5.2. Thermal Demagnetization of Triaxial IRM [19] The Lowrie test allows one to identify the magnetic minerals by simultaneously analyzing their unblocking temperatures and remanent coercivities [Lowrie, 1990]. This test was performed for the same samples of Guia limestones already used in the IRM analyses. The results of triaxial IRM demagnetization curves are represented together with 7 of 17 B06204 FONT ET AL.: REMAGNETIZATION IN GUIA LIMESTONES Figure 6 8 of 17 B06204 B06204 FONT ET AL.: REMAGNETIZATION IN GUIA LIMESTONES the total remanence demagnetization curves (the sum of the x, y, and z components) to highlight the unblocking temperatures of the remanence carriers (Figure 6). Coercivity intervals of 1.4– 0.4 T, 0.4 – 0.12 T and <0.12 T are referred to as hard (z), medium (y) and soft (x) fractions, respectively. [20] Thermal demagnetization of the induced IRMs gives rise to smooth and sigmoidal curves. The dominant carriers are distributed mainly in the soft and medium fraction for all samples while the hard fraction gives low readings sometimes close to the noise level of our cryogenic magnetometer (Figure 6). In all samples, medium and soft fractions demagnetized at temperatures of 280– 320C and at 580 – 600C indicating pyrrhotite and magnetite, respectively, as carriers of the isothermal remanent magnetization. Pyrrhotite is depicted in all samples in the soft and medium-coercivity fractions while magnetite is evident in the low-coercivity fractions for most samples (see TE14.B1). Furthermore, all the samples have a significant medium component persisting after heating to 400C which could also be attributed to magnetite. The total remanence curves (Figure 6) show that both pyrrhotite and magnetite are predominant in samples collected in the Terconi quarry while samples from the Cáceres region show magnetite as the principal carrier of the isothermal remanent magnetization. We note that the contribution of magnetite to the stable high-temperature magnetization observed on these rocks is also weak (Figures 3c and 6). 5.3. Lowrie-Fuller and Cisowski Tests [21] The modified Lowrie-Fuller test [Johnson et al., 1975] is based on the stability of IRM and ARM to the AF demagnetization. It was originally designed to distinguish between single-domain (SD) and multidomain (MD) magnetite grains, the ARM being harder than the IRM in SD grains. However, later measurements have shown that even large hydrothermal MD grains could show an SD-like behavior [e.g., Heider et al., 1992; Halgedahl, 1998]. At any rate, the test has been extensively used as a diagnostic of remagnetization in carbonate rocks [Channell and McCabe, 1994; Huang and Opdyke, 1996; D’Agrella-Filho et al., 2000]. IRM has shown to be harder than ARM in remagnetized carbonates, whereas in nonremagnetized ones both remanences have shown comparable stabilities against AF demagnetization. Remagnetized samples are also characterized by symmetrical IRM acquisition and demagnetisation curves in the Cisowski [1981] plot, suggesting a low degree of interaction of the fine magnetic carriers [Channell and McCabe, 1994; Huang and Opdyke, 1996]. These tests were applied to 11 samples of the Guia limestones of which 8 are given in Figure 7. [22] Differences in shape of ARM and IRM demagnetization curves are noted between the Terconi and Cáceres results. Samples collected in the Terconi quarry show either SD-like behavior (TE14.B1, TE16.A1 and TE18.C3) or B06204 comparable stabilities (TE13.E3). Samples collected in the Cáceres region systematically show the MD-like LowrieFuller behavior that typifies remagnetized carbonates (20.B2, 23.F2, 24.C2, 26A2). We suggest that the opposite results obtained in Terconi and Cáceres may reflect variations in the bimodal distribution of coercivities due to different proportions in the mixture of two magnetic carriers (namely pyrrhotite and magnetite). [23 ] Results of the Cisowski test give an R value (Figure 7) between 0.36 and 0.41 indicating low to moderate interactions of fine ferromagnetic particles. Approximation of the remanence coercive force (Hcr) given by the projection of R on the abscise axis, is comprised between 30 to 40 mT confirming that low- to intermediate-coercivity minerals are the dominant magnetic carriers. Values of R and Hcr are slightly superior in the Cáceres samples (R < 0.40) than in Terconi samples (R 0.40), confirming the different magnetic mineralogies of the Terconi and Cáceres localities (Figure 7). 5.4. Kfd and ARM/SIRM Ratio [24] Remagnetized carbonates have remarkable magnetic properties which can be used as proxies. For these rocks, characteristic values of frequency dependence of susceptibility and ARM/SIRM ratios have been reported: Kfd 5% (Kfd = [Khf-Klf]/Klf where Khf is high-frequency susceptibility and Klf is low-frequency susceptibility) and ARM/ SIRM > 10% [Jackson et al., 1992, 1993]. These values are linked to a very significant contribution of ultrafine particles to the bulk magnetic properties. Concerning the frequencydependent susceptibility, at high frequency (4700 Hz) the measurement time is short enough for these particles to behave as stable SD grains, while at low frequency (470 Hz) the measurement time is longer than the relaxation time and particles behave as superparamagnetic (SP). In our samples, Kfd values vary between 0.8 and 20% and are intimately related to the facies and the locality. Values of Guia limestones lie above the limits defined for remagnetized carbonates [Jackson et al., 1993], whereas pink dolostone values indicate absence of SP particles (Figure 8). In contrast to the basal dolostones, Terconi pink dolostones containing bitumen, at the top of the Mirassol d’Oeste Formation, show a strong contribution of SP particles. Also, the Guia limestones shows different results depending on the location: Cáceres samples have globally higher Kfd values than Terconi samples indicating a major contribution of SP particles at the fold belt. 6. Microscopy 6.1. Optical Microscopy [25] Samples are constituted by alternation of thin limemudstone and calcite laminations (Figures 9a and 9b). Bitumen is essentially localized in the intercrystalline porosity, in contact to calcite layers (Figure 9b). Bitumen is also present in stylolith, primary porosity (fenestral) and Figure 6. Examples of thermal demagnetization of triorthogonal IRM (soft 0.12T, medium 0.4T, hard 1.2T) in the Guia limestones from the Terconi quarry and Cáceres region. Samples contain predominantly pyrrhotite and magnetite. Their distinctive unblocking temperatures are better illustrated by the thermal demagnetization curves of the normalized resultant magnetization (IRM/SIRM) shown on the right. 9 of 17 B06204 FONT ET AL.: REMAGNETIZATION IN GUIA LIMESTONES B06204 Figure 7. Results of the Lowrie-Fuller and Cisowski test for the Guia limestones of Terconi quarry and Cáceres region. Parameter R is the crossing point of IRM acquisition and demagnetization curves. Hcr is obtained projecting R on the abscissa. voids (Figure 9b) and frequently associated with euhedral secondary dolomite [Nogueira et al., 2003]. 6.2. Scanning Electronic Microscopy [26] A SEM study was performed in order to determine the nature of the ferromagnetic minerals present in the rocks, and to infer their origin within the sediment. Such method has already proved its efficiency in linking the magnetic properties of the rock to the ferromagnetic minerals observed in the scanning electron microscope [Suk et al., 1990; Sun and Jackson, 1994; Xu et al., 1998]. [27] The bituminous dolostones are constituted by dolomicrite and dolosparite (Figure 9c, EDS 1) associated to zones rich in organic matter (Figure 9c, EDS 2). The latter contains abundant dolomite rhombohedra (Figure 9c, EDS 3). In 10 of 17 B06204 FONT ET AL.: REMAGNETIZATION IN GUIA LIMESTONES Figure 8. Frequency dependence susceptibility (Kfd) versus ARM/IRMmax ratio (%) for the Mirassol d’Oeste cap dolostones, the bituminous Guia limestones of Terconi and Cáceres, and for one sample from the intermediary bituminous dolostone layer of Terconi section. Typical values of Kfd and ARM/SIRM for remagnetized carbonates are noted [Jackson et al., 1992, 1993]. sample TE13, iron sulphides were depicted by EDS analysis of the bituminous dolomicrite (Figure 9c, EDS 4). [28] The Guia bituminous limestones from Terconi are constituted by a calcic and dolomitic matrix (Figure 10a). Iron-bearing minerals are essentially represented by iron sulphides, essentially pyrite and, at a minor content, pyrrhotite (Figure 10a, EDS 1). Iron sulphides are localized in the matrix or in contact with pockets containing organic matter (Figure 10a, EDS 2). In sample TE18, numerous iron sulphides are localized along a stratification plane where clay minerals dominate (Figure 10b, EDS 3). In this zone, Cu and Zn are also present and could be the result of mineralization. In the analysis of the matrix of sample TE18 (EDS 4), calcite and aluminosilicates coexist with barytine (Figure 10b). The distribution of iron-bearing minerals (principally iron sulfides, Figures 10c – 10f, EDS 5 – 6) into the matrix is not homogeneous. Instead, it appears to be concentrated in the organic matter-rich zones and in voids (Figure 10c). In sample TE18, a crust essentially composed of iron with less than 20 mm in thickness and 50 to 100 mm in length was observed (Figure 10d). The fragment appears to be internally structured with laminations and is very similar to those described by Belkaakoul and Aı̈ssaoui [1997] in Jurassic shallow water carbonates of the Paris Basin, also affected by extensive remagnetization. In the same sample, an octahedral coarse-grained (4 mm) magnetite was encountered (Figure 10e). The mineral seems to be included into the matrix and its habit suggests an authigenic origin. This kind of magnetite has already been observed in the Paleozoic remagnetized carbonates of North America [Sun and Jackson, 1994]. Iron sulphides are ubiquitous in samples from Terconi (and in minor extent in Cáceres) and are responsible for a major contribution of S in EDS analyses. They present a wide range of sizes, from less than 1 mm to more than 10 mm in diameter. The coarser population presents various morphologies, from euhedral to xenomorphic, whereas the smaller grains, generally associated with magnetite, are represented by cubic pyrite crystals. Distinction based on chemical composition (relative proportion of Fe and S) between pyrrhotite, greigite or pyrite, B06204 could not be checked with needed accuracy by EDS analyses. At any rate, since some amounts of magnetic pyrrhotite have been depicted in the rocks by the magnetic data, some of the sulphide grains observed in the thin section should correspond to this iron sulphide. For instance, some iron sulphides show the typical monoclinic structure of pyrrhotite (Figures 10f and 10g, EDS 6). [29] The Guia limestones from Cáceres are constituted by a (calcic and) dolomitic matrix associated to calcic concretions (Figure 11a, EDS 1) and organic matter-rich zones (pockets) (Figure 11a, EDS 2). The matrix of the rocks also contains organic matter and abundant iron oxides (Figure 11a, EDS 3). A detail of the calcic concretions shown in Figure 11a is illustrated by Figure 11b. Iron oxides (Figure 11b, EDS 4) are identified both in the concretions and in matrix. Dolomite (grey zones) and calcite (bright zones) coexist in the concretion (Figure 11b, EDS 5–6). In sample TE22, framboidal structures are observed in voids (Figure 11c) and show, after analysis by EDS, a major contribution of sulphur with minor counts of iron. Individual euhedral crystals are less than 1 mm in diameter and dominantly octahedral in shape. Framboids are well documented in remagnetized carbonates [Suk et al., 1990; Sun and Jackson, 1994], where SD to PSD magnetites were described. [30] All samples present some amounts of terrigenous elements as Mg, Si and Al and nonmagnetic minerals were also observed. EDS analyses has also identified the presence of rutile, titanite and pyrite and locally barytine. 7. Discussion [31] The Guia limestones have shown a negative fold test for their paleomagnetic directions. Accordingly, these rocks show typical magnetic signatures of remagnetized carbonates such as high ARM/SIRM ratio, high-frequency dependency of magnetic susceptibility and contradictory Lowrie-Fuller and Cisowski tests. Unblocking temperatures suggest that secondary remanence is carried sometimes by pyrrhotite and more frequently by a mixture of magnetite/ pyrrhotite, whose authigenic origin is suggested by SEM observations. Although some authors have proposed a thermoviscous origin for remagnetization in carbonates [e.g., Kent, 1985], the low burial temperatures (below 250C) and high unblocking temperatures obtained for the studied rocks preclude such possibility. A chemical remanent magnetization (CRM) is more suitable with the presence of magnetite framboids, iron sulphide crystals with well preserved faces and edges and their occurrence preferentially in voids or microcracks. However, different magnetic mineralogies are observed along the sedimentary succession (Figure 2) suggesting that several factors control the magnetic carrier production and the CRM remagnetization process. [32] Recently, Moreau et al. [2005] have shown that calcite content, organic matter and smectite to illite transformation could explain a CRM acquisition in marly sediments. Their experimental study reveals that magnetite is produced at temperatures as low as 150C, after heating in controlled atmosphere of inert gas. Hence the fluids generated by moderate heating would suffice for magnetite synthesis. Their experiments reinforce the idea already put forward by several authors that sedimentary rocks can be 11 of 17 B06204 FONT ET AL.: REMAGNETIZATION IN GUIA LIMESTONES B06204 Figure 9. Petrographic photographs of thin sections from bituminous limestones and dolostones: (a) bitumen fulfilling intercrystalline porosity (sample TE22) and (b) deformed bituminous sheet cutting original petrographic structures (sample TE24). Scanning electron microscopy photographs and EDS of iron-bearing minerals in rock fragment of (c) the bituminous dolostones (TE13, Terconi) showing bituminous pockets, euhedral dolomite, and numerous iron sulphides. remagnetized during burial diagenesis with fluids produced during burial and that both organic matter maturation and clay transformation appear to be key elements for such a remagnetization. The fact that a different magnetic mineralogy is observed in the studied localities of Terconi and Cáceres, where physical-chemical conditions were different, suggest that temperature, fluids and deformation did play an important role in producing the authigenic magnetic mineralogy of the Guia limestones and give us an insight into the role of these factors in the CRM acquisition mechanism in carbonate rocks. Below we discuss each factor separately. 7.1. Hydrocarbon Maturation [33] First, a strongly distinct magnetic mineralogy is observed between pink (Mirassol d’Oeste) dolostones carrying a primary dual-polarity magnetization and the bituminous dolostones devoid of remanent magnetization. Detrital hematite/magnetite are identified as carriers of the primary magnetization of the Mirassol d’Oeste dolostones [Font et al., 2005]. Detrital hematite was not depicted by magnetic tests nor observed in SEM in dolostones containing bitumen. Instead, iron sulphides are present in some amounts (compare SEM observations) as well as superparamagnetic particles, not observed but indicated by high Kfd values. The fact that both pink and bituminous dolostones are stratigraphically continuous and both crop out in the same nonmetamorphic context suggest that the presence of oil was the main factor contributing for the destruction of the original magnetic minerals (i.e., detrital hematite and magnetite). However, since no magnetic directions were retrieved by these rocks, this factor alone was not sufficient to produce a stable secondary magnetization. [34] In the bituminous Guia limestones higher NRM intensities are noted (Figure 2) and a stable magnetization is isolated. This secondary remanent magnetization is carried by magnetite and a mixture of pyrrhotite/magnetite. The great amount of iron sulphides and their association with bitumen in fractures and microcracks are in favor of a chemical precipitation probably related to an epigenetic enrichment of sulfur due to hydrocarbon maturation [Weaver et al., 2002; Otofuji et al., 2003; Zwing et al., 2005]. This suggests that when bitumen is associated to the limestone facies (as bituminous pockets or impregnating the carbonatic 12 of 17 B06204 FONT ET AL.: REMAGNETIZATION IN GUIA LIMESTONES Figure 10 13 of 17 B06204 B06204 FONT ET AL.: REMAGNETIZATION IN GUIA LIMESTONES matrix), it is responsible directly or indirectly for a CRM acquisition by magnetic carrier production. [35] An indirect evidence for a relationship between hydrocarbons and magnetic minerals in some carbonates is given by the presence of spherical magnetite supposed to be a product of microbial degradation [Elmore et al., 1987; Elmore and Leach, 1990]. Bacteria are remarkable in their metabolic diversity due to their ability to oxidize and reduce metals, which lead to the precipitation, transformation, or dissolution of magnetic minerals. Magnetotactic bacteria, which live in shallow to deep marine, form intracellular chains of small single domain magnetite grains called ‘‘magnetosomes’’ [e.g., Kirschvink and Chang, 1984; Chang et al., 1987]. Although there are no reports of magnetotactic bacteria living in hydrocarbon-related environments, at least some of the anaerobic varieties, for which the distribution and importance in natural environments are still unknown, could possibly live in hydrocarbon-related settings [Machel and Burton, 1991]. Lovely et al. [1987] discovered an organism (GS-15) that produced important quantities of ultrafine extracellular magnetite via the oxidation of acetate and concomitant reduction of Fe3+ and Fe2+ under laboratory conditions. Such organisms may thus produce free iron via oxidation of certain hydrocarbons in anaerobic diagenetic environments. Oxidation reaction of acetate is provided by Croal et al. [2004]: 2þ CH3 COO þ 8FeðOHÞ3 þ15Hþ ! 2HCO 3 þ 8Fe 20H2 O ð1Þ [36] Release of Fe2+ ions by microbial oxidation (equation (1)) is done from soluble iron-bearing minerals. Poorly soluble iron-bearing minerals such as magnetite or pyrite are not oxidized by these organisms, whereas the more soluble siderite and ferrous sulphides are. This gives a possible explanation for the correlation between authigenic magnetic mineral content and percentage of calcite (siderite?), noted in the present study and in the work of Moreau et al. [2005]. To oxide siderite and/or iron sulfides, reduced sulphur (as H2S and HS) must be present [Goldhaber and Reynolds, 1991]. In an organic-matter-rich environment, H2S could form by bacterial SO2 4 reduction, thermal release of H2S from organic matter and thermochemical SO2 4 reduction. Also, bacteria have the particularity to acidify the microenvironment around cell aggregates, facilitating the dissolution of ferrous sulfide (FeS) releasing available dissolved HS [Kappler and Newman, 2004], as described in the following equation [Schippers and Joergensen, 2002]: FeS þ Hþ $ Fe2þ þ HS ð2Þ [37] On the other hand, once dissolved Fe2+ is available, it reacts with the aqueous sulphur species to form B06204 diagenetic iron sulfides (greigite, pyrite, pyrrhotite and others). [38] Mineral precipitation generally is inorganic and occurred where the sulphide ion meet divalent ferrous ions, close to the surface of the organisms, or after migration of the ions. The best example of mineralization close to the surface of the bacteria are clusters of framboidal pyrite, such as those observed in the Guia limestone (Figure 10f), which can be considered as pyritized bacteria [Machel and Burton, 1991]. This would indicate an anaerobic environment influenced by microbial activity in these rocks and suggest that the iron and sulphur ions forming the magnetic carriers of the Guia limestone remanence could possibility be originated by hydrocarbon maturation via microbial activity, and precipitated authigenic minerals in the voids and microcraks of the rock. 7.2. Burial Diagenesis [39] In the Guia limestones, a different magnetic mineralogy is observed between sites from the Terconi quarry, where geological strata are nonmetamorphozed, and sites from the metamorphic thrust and fold belt region of Cáceres. Samples from Terconi quarry exhibit unblocking temperatures between 250 and 350C suggesting solely pyrrhotite as the remanence carrier. Samples from Cáceres exhibit a second step in NRM decay curves at around 550C suggesting a mixture of pyrrhotite and magnetite. Alternatively, this mixture could be present in both localities but the concentration of magnetic carriers must be higher in Cáceres. Both, pyrrhotite and magnetite carry the same remanence in these samples. Each group of samples is characterized by different responses in the Lowrie-Fuller test and distinct ranges of Kfd, DP, and SIRM values (Figures 2, 6, and 8). It suggests that, in addition to hydrocarbon maturation described above, another factor must be invoked to explain the different magnetic mineralogy observed in the Terconi quarry and in the Cáceres region. The transformation of clay minerals during burial diagenesis, in particular from smectite to illite, could be one of these potential factors [e.g., Hirt et al., 1993; Katz et al., 2000; Gill et al., 2002; Blumstein et al., 2004; Moreau et al., 2005]. As illustrated by the Figure 1, the two study sections (Terconi and Cáceres) are located in distinct structural zones where deformation, and consequently burial diagenesis, increase from the Terconi quarry, on the border of the Amazon craton, to the thrust and fold zone of the Paraguay belt, near Cáceres. Illite crystallinity data from this transect clearly show that both tectonic deformation and metamorphic temperature increase from the border of the craton to the internal zone of the fold belt (Figure 1). The dependence of the magnetic properties of the Guia limestones on the illite crystallinity indirectly suggest that part of the magnetite contributing to the stable high-temperature Figure 10. Scanning electron microscopy photographs and EDS spectra of iron-bearing minerals in rock fragment of the Guia limestones from Terconi. (a) Iron sulphides (i.e., pyrite and pyrrhotite) are disseminated in the matrix (Mg calcite) and sometimes associated to bituminous pockets (sample TE15). (b) Numerous pyrites are localized in clay-rich layers, and barytine is sometimes encountered in the calcic matrix (sample TE18). (c) Iron sulphides and oxides are localized in microcracks and fractures (sample TE18, Terconi). (d) Crust of pure iron stick on the matrix (sample TE18, Terconi). (e) Octahedral magnetite in the calcite matrix (sample TE18). (f) and (g) Monoclinic sulphur-bearing mineral (pyrrhotite?) (sample TE18). In EDS 5 and 6, O and C are not depicted because they range out of the limit of the Jeol JSM-T330A microscope. 14 of 17 FONT ET AL.: REMAGNETIZATION IN GUIA LIMESTONES B06204 B06204 Figure 11. Scanning electron microscopy photographs and EDS spectra of iron-bearing minerals in rock fragment of the Guia limestones from Cáceres. (a) Calcic concretions and bituminous pockets (sample TE26). (b) Detail of the calcic concretions shown in Figure 11a illustrating calcite (bright zones) containing dolomite crystals (grey zones) and an isolated iron oxide. (c) Typical framboids with presence of pyrite and magnetite (sample TE22, Cáceres). remanence of these rocks may come from clay minerals transformation. 8. Conclusions [40] The Guia limestones of the Araras Group (Mato Grosso, Brazil) show typical magnetic signatures of remagnetized carbonates such as high ARM/SIRM ratios, high-frequency-dependent magnetic susceptibility and contradictory Lowrie-Fuller and Cisowski tests. Unblocking temperatures suggest that the stable high-temperature rem- anence is carried by both pyrrhotite and a mixture of magnetite/pyrrhotite, whose authigenic origin in relation to hydrocarbon is indicated by SEM observations. The presence of magnetite framboids and euhedral iron sulphide crystals often located near or into fractures are in favor of a chemical remanent magnetization (CRM). Different magnetic properties between pink dolostones carrying a primary detrital remanent magnetization and bituminous dolostones devoid of remanent magnetization suggest that hydrocarbons are responsible for the attenuation of the NRM. In contrast, the Guia limestones have higher NRM intensities 15 of 17 B06204 FONT ET AL.: REMAGNETIZATION IN GUIA LIMESTONES and pyrrhotite±magnetite is the principal carrier of the magnetization. Sulfides associated with bitumen are probably related to an epigenetic enrichment of sulfur due to hydrocarbon seepage. Mixture of water and organic molecules with iron or sulphur bearing minerals could produce redox conditions favorable to the formation of iron sulphide and magnetite. However, hydrocarbon maturation cannot solely explain the difference in the magnetic mineralogy observed in the Terconi and Cáceres region and another factor is evocated to explain such differences. A higher amount of magnetite in the metamorphic fold belt (Cáceres) is interpreted to be the result of higher temperature conditions inducing clay minerals transformation and providing additional iron for authigenic magnetic minerals to form. [41] Acknowledgments. This research is part of the Ph.D. thesis of the senior author. It was supported by the Brazilian FAPESP (02/02762-0 and 05/53521-1), the French CNRS program ECLIPSE and the CAPESCOFECUB cooperation program (442/4). We thank M. D’Agrella Filho, A.C.R. Nogueira, and C. Ricomini, for their help in discussions and field work, and H. Sampaio, T. Aigouy, and P. de Parceval for assistance in microscopic observations. We appreciate the editorial comments by Randolph J. Enkin and the insightful reviews by Mike Jackson and Richard D. Elmore. References Alvarenga, C. J. S. (1990), Phénomènes sédimentaires, structuraux et circulation de fluides développés à la transition chaıcirc;ne-craton. Exemple de la chaı̂ne Paraguay d’âge protérozoı̈que supérieur, Mato Grosso, Brésil, thèse doct, 177 pp., Univ. Aix-Marseille III, Aix en Provence, France. Banerjee, S., R. D. Elmore, and M. H. Engel (1997), Chemical remagnetization and burial diagenesis: Testing the hypothesis in the Pennsylvanian Belden formation, Colorado, J. Geophys. Res., 102, 24,825 – 24,841. Blumstein, A. M., R. D. Elmore, M. H. Engel, C. Elliot, and A. Basu (2004), Paleomagnetic dating of burial diagenesis in Mississippian carbonates, Utah, J. Geophys. Res., 109, B04101, doi:10.1029/ 2003JB002698. Brothers, L. A., M. H. Engel, and R. D. Elmore (1996), The late diagenetic conversion of pyrite to magnetite by organically complexed ferric iron, Chem. Geol., 130, 1 – 14. Chang, S. R., J. L. Kirschvink, and J. F. Stolz (1987), Biogenic magnetite as a primary remanence carrier in limestones deposits, Phys. Earth Planet. Inter., 46, 289 – 303. Channell, J. E. T., and C. McCabe (1994), Comparison of magnetic hysteresis parameters of unremagnetized and remagnetized limestones, J. Geophys. Res., 99, 4613 – 4623. Cisowski, S. (1981), Interacting vs. non-interacting single domain behaviour in natural and synthetic samples, Phys. Earth Planet. Inter., 26, 56 – 62. Croal, L. R., A. J. Gralnick, D. Malasarn, and D. K. Newman (2004), The genetics of geochemistry, Annu. Rev. Genet., 38, 175 – 202. D’Agrella-Filho, M. S., M. Babinski, R. I. F. Trindade, W. R. Van Schmus, and M. Ernesto (2000), Simultaneous remagnetization and U-Pb isotope resetting in Neoproterozoic carbonates of the Sao Francisco craton, Brazil, Precambrian Res., 99, 179 – 196. Elmore, R. D., and M. C. Leach (1990), Remagnetization of the Rush Springs Formation, Cement, Oklahoma: Implications for dating hydrocarbons migration and aeromagnetic exploration, Geology, 18, 124 – 127. Elmore, R. D., M. H. Engel, L. Crawford, K. Nick, S. Imbus, and Z. Sofer (1987), Evidence for a relationship between hydrocarbons and authigenic magnetite, Nature, 325, 428 – 430. Elmore, R. D., D. London, D. Bagley, D. Fruit, and G. Guoqiu (1993), Remagnetization by basinal fluids: Testing the hypothesis in the Viola Limestone, southern Oklahoma, J. Geophys. Res., 98, 6237 – 6254. Fisher, R. A. (1953), Dispersion on a sphere, Proc. R. Soc. London, Ser. A, 217, 295 – 305. Font, E., R. I. F. Trindade, and A. Nédélec (2005), Detrital remnant magnetization in hematite-bearing Neoproterozoic Puga cap Dolostone, Amazon Craton: A rock magnetic and SEM study, Geophys. J. Int., 163, 1 – 10. Gill, J. D., R. D. Elmore, and M. H. Engel (2002), Chemical remagnetization and clay diagenesis: Testing the hypothesis in the Cretaceous sedimentary rocks of northwestern Montana, Phys. Chem. Earth, 27, 1131 – 1139. B06204 Goldhaber, M. B., and R. L. Reynolds (1991), Relations among hydrocarbon reservoirs, epigenetic sulfidization, and rock magnetization: Examples from the south Texas coastal plain, Geophysics, 56, 748 – 757. Halgedahl, S. (1998), Revisiting the Lowrie-Fuller test: Alternating field demagnetization characteristics of single-domain through multidomain glass-ceramic magnetite, Earth Planet. Sci. Lett., 160, 257 – 271. Heider, F. D., D. J. Dunlop, and H. C. Soffel (1992), Low-temperature and alternating field demagnetization of saturation remanence and thermoremanence in magnetite grains (0.037 mm to 5 mm), J. Geophys. Res., 97, 9371 – 9381. Hirt, A. M., A. Banin, and U. A. Gehring (1993), Thermal generation of ferromagnetic minerals from iron-enriched smectites, Geophys. J. Int., 115, 1161 – 1168. Horton, R. A., J. W. Geissman, and R. J. Tschauder (1984), Paleomagnetism and rock magnetism of the Mississippian Leadville (Carbonate) Formation and implications for the age of sub-regional dolomitization, Geophys. Res. Lett., 11, 649 – 652. Huang, K., and N. Opdyke (1996), Severe remagnetization revealed from Triassic platform carbonates near Guiyang, southwest China, Earth Planet. Sci. Lett., 143, 49 – 61. Jackson, M. (1990), Diagenetic sources of stable remanence in remagnetized paleozoic cratonic carbonates: A rock magnetic study, J. Geophys. Res., 95, 2753 – 2762. Jackson, M., W. W. Sun, and J. P. Craddock (1992), The rock magnetic fingerprint of chemical remagnetization in midcontinental Paleozoic carbonates, Geophys. Res. Lett., 19, 781 – 784. Jackson, M., P. Rochette, G. Filon, S. Banerjee, and J. Marvin (1993), Rock magnetism of remagnetized Paleozoic carbonates: Low-temperature behavior and susceptibility characteristics, J. Geophys. Res., 98, 6217 – 6225. Johnson, H. P., W. Lowrie, and D. V. Kent (1975), Stability of anhysteretic remanent magnetization in fine and coarse magnetite and maghemite, Geophys. J. R. Astron. Soc., 41, 1 – 10. Kappler, A., and D. K. Newman (2004), Formation of Fe (III)-minerals by Fe (II)-oxidizing photoautotrophic bacteria, Geochim. Cosmochim. Acta, 68, 1217 – 1226. Katz, B., R. D. Elmore, M. Cogoini, M. H. Engel, and S. Ferry (2000), Associations between burial diagenesis of smectite, chemical remagnetization, and magnetite authigenesis in the Vocontian trough, SE France, J. Geophys. Res., 105, 851 – 868. Kent, D. V. (1985), Thermoviscous remagnetization in some Appalachian limestones, Geophys. Res. Lett., 12, 805 – 808. Kirshvink, J. L. (1980), The least-squares line and plane and the analysis of paleomagnetic data, Geophys. J. R. Astron. Soc., 62, 699 – 718. Kirschvink, J. L., and S. R. Chang (1984), Ultrafine-grained magnetite in deep-sea sediments: Possible bacterial magnetofossils, Geology, 12, 559 – 562. Kruiver, P. P., M. J. Dekkers, and D. Heslop (2001), Quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetization, Earth Planet. Sci. Lett., 189, 269 – 276. Kruiver, P. P., C. G. Langereis, M. J. Dekkers, and W. Krijgsman (2003), Rock-magnetic properties of multicomponent natural remanent magnetisation in alluvial red beds (NE Spain), Geophys. J. Int., 153, 317 – 332. Lovely, D. R., J. F. Stolz, G. L. Nord, and E. J. P. Phillips (1987), Anaerobic production of magnetite by a dissimilatory iron-reducing organism, Nature, 330, 252 – 254. Lowrie, W. (1990), Identification of ferromagnetic minerals in a rock by coercivity and unblocking temperature properties, Geophys. Res. Lett., 17, 159 – 162. Machel, H. G., and E. A. Burton (1991), Chemical and microbial processes causing anomalous magnetization in environments affected by hydrocarbon seepage, Geophysics, 56, 598 – 605. McCabe, C., and D. R. Elmore (1989), The occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north America, Rev. Geophys., 27, 471 – 494. Moreau, M. G., M. Ader, and R. J. Enkin (2005), The magnetization of clay-rich rocks in sedimentary basins: Low-temperature experimental formation of magnetic carriers in natural samples, Earth Planet. Sci. Lett., 230, 193 – 210. Nogueira, A. C. R., C. Riccomini, A. N. Sial, C. A. V. Moura, and T. R. Fairchild (2003), Soft-sediment deformation at the base of the Neoproterozoic Puga cap carbonate (southwestern Amazon Craton, Brazil): Confirmation of rapid icehouse-greenhouse transition in snowball earth, Geology, 31, 613 – 616. Nogueira, A., C. Riccomini, A. Sial, C. Moura, R. Trindade, and T. Fairchild (2006), C and Sr isotope fluctuations and paleoceanographic changes in the late Neoproterozoic Araras carbonate platform, southern Amazon craton, Brazil, Chem. Geol., in press. 16 of 17 B06204 FONT ET AL.: REMAGNETIZATION IN GUIA LIMESTONES Oliver, J. (1986), Fluids expelled tectonically from orogenic belts: Their role in hydrocarbon migration and other geologic phenomena, Geology, 14, 99 – 102. Otofuji, Y., K. Takemoto, H. Zaman, Y. Nishimitsu, and Y. Wada (2003), Cenozoic remagnetization of the Paleozoic rocks in the Kitakami massif of northeast Japan, and its tectonic implications, Earth Planet. Sci. Lett., 210, 203 – 217. Robertson, D. J., and D. E. France (1994), Discrimination of remanencecarrying minerals in mixtures using isothermal remanent magnetisation acquisition curves, Phys. Earth Planet. Inter., 82, 223 – 234. Rochette, P., G. Fillion, J. L. Mattéi, and M. J. Dekkers (1990), Magnetic transition at 30 – 34 Kelvin in pyrrhotite: Insight into a widespread occurrence of this mineral in rocks, Earth Planet. Sci. Lett., 98, 319 – 328. Schippers, A., and B. B. Joergensen (2002), Biogeochemistry of pyrite and iron sulfide oxidation in marine sediments, Geochim. Cosmochim. Acta, 66, 85 – 92. Suk, D., D. R. Peacor, and R. Van der Voo (1990), Replacement of pyrite framboids by magnetite in limestone and implications for palaeomagnetism, Nature, 345, 611 – 613. Suk, D., R. Van der Voo, and D. R. Peacor (1993), Origin of magnetite responsible for remagnetization of early Palaeozoic limestones of New York State, J. Geophys. Res., 98, 419 – 434. Sun, W., and M. Jackson (1994), Scanning electron microscopy and rock magnetic studies of magnetic carriers in remagnetized early Paleozoic carbonates from Missouri, J. Geophys. Res., 99, 2935 – 2942. Trindade, R. I. F., E. Font, M. S. D’Agrella-Filho, A. C. R. Nogueira, and C. Riccomini (2003), Low-latitude and multiple geomagnetic reversals in the Neoproterozoic Puga cap carbonates, Terra Nova, 15, 441 – 446. Trindade, R. I. F., M. S. D’Agrella-Filho, M. Babinski, E. Font, and B. B. Brito Neves (2004), Paleomagnetism and geochronology of the Bebedouro cap carbonate: Evidence for continental-scale Cambrian remagnetization in the São Francisco craton, Brazil, Precambrian Res., 128, 83 – 103. B06204 Weaver, R., A. P. Roberts, and A. Barker (2002), A late diagenetic (synfolding) magnetization carried by pyrrhotite: Implications for paleomagnetic studies from magnetic iron sulphide-bearing sediments, Earth Planet. Sci. Lett., 200, 371 – 386. Weil, A. B., and R. Van der Voo (2002), Insights into the mechanism for orogen-related carbonate remagnetization from growth of authigenic Feoxide: A scanning electron microscopy and rock magnetic study of Devonian carbonates from northern Spain, J. Geophys. Res., 107(B4), 2063, doi:10.1029/2001JB000200. Xu, W., R. Van der Voo, and D. R. Peacor (1998), Electron microscopic and rock magnetic study of remagnetized Leadville carbonates, central Colorado, Tectonophysics, 296, 333 – 362. Zegers, T. E., M. J. Dekkers, and S. Bailly (2003), Late Carboniferous to Permian remagnetization of Devonian limestones in the Ardennes: Role of temperature, fluids, and deformation, J. Geophys. Res., 108(B7), 2357, doi:10.1029/2002JB002213. Zijderveld, J. D. A. (1967), A.C. demagnetization of rocks: Analysis of results, in Methods on Paleomagnetism, edited by D. W. Collinson, K. M. Creer, and S. K. Runcorn, pp. 245 – 286, Elsevier, New York. Zwing, A., J. Matzka, V. Bachtadse, and H. C. Soffel (2005), Rock magnetic properties of remagnetized Palaeozoic clastic and carbonate rocks from the NE Rhenish massif, Germany, Geophys. J. Int., 160, 477 – 486. E. Font and R. I. F. Trindade, Departamento de Geofı́sica, Instituto de Astronomia, Geofı́sica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão, 1226, 05508-090, São Paulo, Brazil. (font_eric@ hotmail.com) A. Nédélec, UMR 5563, LMTG, Université Paul-Sabatier, 14 avenue Edouard Belin, F-31400 Toulouse, France. ([email protected]) 17 of 17
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