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Magnetic fabric of a basaltic dyke swarm associated with Mesozoic rifting in northeastern Brazil
Journal of South American Earth Sciences 13 (2000) 179±189
Magnetic fabric of a basaltic dyke swarm associated with Mesozoic rifting in northeastern Brazil Carlos J. Archanjo a,*, Ricardo I. Trindade b, Jose Wilson P. Macedo c, Marcelus G. ArauÂjo a a
Instituto de GeocieÃncias, Universidade de SaÄo Paulo, rua do Lago 562 SaÄo Paulo SP, 05508-900, Brazil b Instituto AstronoÃmico e Geo®sico, Universidade de SaÄo Paulo, SaÄo Paulo SP, 01065-970, Brazil c DFTE/CCET, Universidade Federal do Rio Grande do Norte, Natal RGN, 59072-970, Brazil Accepted 1 November 1999
Abstract The anisotropy of magnetic susceptibility (AMS) has been studied in a 120 km long, Early Cretaceous tholeiitic dyke swarm emplaced during the early stages of rifting and opening of the equatorial Atlantic Ocean. The vertical dykes ®lled a set of Etrending fractures that cut the structural grain of the Precambrian basement of northeastern Brazil at a high angle. These strongly magnetic rocks contain pseudo-single domain, Ti-poor magnetite and secondary maghemite as revealed by thermomagnetic and hysteresis data. The contribution of the paramagnetic and the high coercivity antiferromagnetic fractions to the bulk susceptibility is less than 1.2%. The dykes generally show well-clustered AMS principal directions. The plunge of the magnetic lineation varies from nearly subvertical in the center of the swarm to horizontal in the west. The strike of the magnetic foliation is generally oblique to the dyke wall and exhibits a curved trend at the regional scale. This fabric pattern suggests that the magma source that fed the dykes was situated in the center of the swarm, which is presently below Tertiary sandstones. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Basaltic dykes are primary conduits of magmas formed in the mantle and emplaced into the crust by ®lling of fractures related to a extensional stress regime. Although it is obvious that the magma migrates upward, the ¯ow path may include lateral transport to great distances from the source area. The transition from horizontal to vertical magmatic ¯ow, as observed in the transcontinental MacKenzie dyke swarm of the Slave Province (Canada), occurs in the interval between 500 and 600 km from the locus and is thought to correspond to the outer boundary of an ancient mantle plume (Ernst and Baragar, 1992). Flow * Corresponding author. Tel.: +55-11-818-3994; fax: +55-11-8184258. E-mail address: [email protected] (C.J. Archanjo).
directions in dykes may be determined by anisotropy of magnetic susceptibility (AMS) measurements. AMS has been shown to be suciently sensitive to determine subtle fabrics in lava ¯ows with an anisotropy of only a few percent (Knight and Walker, 1988; CanÄoÂn-Tapia et al., 1996). Ma®c magmas ¯owing through a dyke will acquire a preferred orientation of titanomagnetite grains with the minimum axis (kmin, pole of magnetic foliation; kmax > kint > kmin) usually aligned perpendicular to the wall and the maximum axis (kmax, magnetic lineation) in the direction of the ¯ow. This ``normal'' or ``common'' con®guration can be found in rocks containing multidomain (MD) magnetite (Rochette et al., 1992). ``Normal'' AMS fabrics are far from being the only fabric type observed in dykes and lava ¯ows. In some studies, up to 50% of the samples show both anomalous (kmax perpendicular to the dyke wall) and intermediate (kint perpendicular to the dyke wall) arrangements (Knight and Walker, 1988; Rochette et
0895-9811/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 5 - 9 8 1 1 ( 0 0 ) 0 0 0 2 3 - 7
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al., 1992; Raposo and Ernesto, 1995; Raposo, 1997). Inverse fabrics may occur in rocks whose magnetic anisotropy is yielded by magnetite with a single domain (SD) structure. For such grains, the kmax direction is perpendicular to the grain elongation Ð i.e., the mineral lineation (Potter and Stephenson, 1988). Mixtures of primary MD particles and secondary SD particles may be responsible for the production of intermediate AMS type fabrics (Rochette et al., 1992). However, Raposo (1997) stressed that the intermediate-type fabrics of the ma®c swarms of Ponta Grossa and FlorianoÂpolis from southern Brazil are produced by pseudo-single domain (PSD)- to MDmagnetites. Apparently no SD-grains occur in such rocks, suggesting that those fabrics were formed during the emplacement and cooling of the ma®c magma. ``Intermediate'' arrangements can also result from superimposed fabrics of dierent generations on magma injection. The Early Cretaceous (145±125 Ma) Rio Ceara Mirim Dyke Swarm (CMDS) intruded the basement of the Borborema Province during rifting events responsible for the opening of the equatorial Atlantic Ocean (Sial et al., 1981; Almeida et al., 1988). This magmatism is contemporaneous with basalt and rhyolite ¯ows of the Northern Benue Trough (Maluski et al., 1995), the African counterpart of the Potiguar Rift. The tholeiitic dykes of the CMDS cut the structural grain of the Precambrian basement at a high angle, including mylonitic rocks formed by NNE-trending shear zones. Fault reactivation aecting the dykes, however, has been observed locally (Oliveira and Gomes, 1996). The dykes in®ll a set of fractures trending E-W near the southern border of the Potiguar Basin (Fig. 1). In the western part of the province, these dykes become
Fig. 1. Geological map of the Rio CearaÂ-Mirim Dyke Swarm and basaltic necks in northeastern Brazil (Almeida et al., 1988). Precambrian basement=dotted pattern. Mesozoic basins: P=Potiguar; I=Iguatu; RP=Rio do Peixe; A=Araripe. The inshore/oshore Potiguar rift after Matos (1992).
nearly parallel to the NE-trending Eocretaceous rift system of the Potiguar, Iguatu, and Araripe Basins. The aeromagnetic data available show that magnetic anomalies attributed to ma®c dykes generally occur as closely spaced 10 to 40 km long stripes with amplitudes that vary from 50 to 300 nT (Oliveira, 1992, 1994); a few exceed 100 km long. The longest magnetic anomalies strike NE-SW and are observed in the western part of the swarm. They are traced within the Precambrian basement as well as in the Potiguar Basin parallel to the fault system that delimits the rift (Oliveira, 1992). We report here the ®rst results of an AMS and magnetic mineralogy study of a set of basaltic dykes intrusive in the southern part of the CMDS along the border of the 600 m high Tertiary terrace of the Santana Plateau. These dykes were named ``sub-swarm IV'' by Bellieni et al. (1992) because of their alkaline anity and typical paleomagnetic directions when compared with other sub-swarms of the CMDS. 2. Geological setting The ma®c dykes extend for 120 km, forming individual bodies usually less than 500 m long. They are exposed generally as rounded blocks formed by typical onion-skin weathering. The contacts between the dykes and host rocks are not usually exposed, but the margins of the dykes are relatively well de®ned by the contrast between the sandy and dark argillaceous soils formed by the alteration of, respectively, felsic host rock and ma®c dyke. The eight dykes in the study area (D1 to D8 in Fig. 2) vary in width from a few centimeters to about 20 meters; trends vary from 0808 to 1108 in azimuth. The dykes cut foliated host rocks that trend from NW-SE in the west (D1 to D3) to NE-SW in the east (D6 to D8). The D4 and D5 dykes intrude an isotropic late Precambrian granite. Petrographically, the dykes are ®ne- to mediumgrained olivine basalts. In thin section, the grains form a mosaic of euhedral to subhedral calcic plagioclase, augite, and pigeonite. Olivine is an accessory mineral, and titanomagnetite is the main opaque mineral (Bellieni et al., 1992). Back-scattered electron (BSE) images of the basalts revealed that titanomagnetite grains occur as two textural types (Fig. 3): (i) euhedral to subhedral crystals (mean grain size, 100 mm) containing minute inclusions of silicates and, (ii) needle-like crystals up to 500 mm long and 2 to 8 mm wide, some showing skeletal growth morphologies. Dykes in the western part of the study area usually contain microphenocrysts of plagioclase, whereas those in the east are aphyric and ®ne grained. However, ®ne-grained and microporphyritic dykes may sometimes be found at the same site (e.g., D8) Ð both trending E-W and
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181
Fig. 2. Sampling locations (D1 to D8) of vertical ma®c dykes trending E-W and mean site AMS principal directions. Schmidt net, lower hemisphere. Kmax=lineation (full dot); Kmax/Kint plane=foliation (full line). Empty dot and dashed line=ill-de®ned lineation and foliation, respectively.
sharing the same orientation of AMS principal directions (discussed below). The contact of host gneissic rock and these dykes is exposed in a unweathered rock pavement of a river bed. Tertiary necks of olivine basalt (Sial et al., 1981) intrude the Precambrian basement, forming two parallel E-trending belts of elliptical to sub-circular bodies with a maximum diameter of 200 m (Fig. 2). Regionally the volcanic necks are aligned N-S, crossing the trend of the CMDS (Fig. 1).
3. Sampling and analytical methods
Fig. 3. BSE images of textural features of titanomagnetite from a basaltic dyke (site D7): A) Euhedral to subhedral and needle-like crystals (light grains), apparently without shape preferred orientation, enclosed in a silicate matrix (bar=500 mm); B) Detail of A showing nearly rounded silicate inclusions in euhedral grains and ``rods'' of titanomagnetite. Note the preferential growth of rods following the {111} spinel planes (bar=50 mm).
The samples came from the largest rock blocks, as fresh as possible and apparently in place irrespective of marginal or central zones of the dyke. For the only dyke in which the wall rock contacts are exposed (D8), the cores were taken at the margin and the central part of the both the porphyritic and the ®ne-grained dykes. Its chilled margins were avoided because of their vulnerability to deuteric alteration and their possibly dierent primary magnetic properties. Four to six cylindrical oriented cores 6 to 8 cm long and 2.5 cm in diameter were collected at each site using a portable gasoline-powered rock drill. At each site, the core spacing was from a meter to a few tens of meters. In the laboratory, each core was cut into 2.2 cm long samples, so that each measurement site yielded 8 to 12 oriented specimens. A total of 79 AMS determinations were made for the eight dykes. Low-®eld (3.8 10ÿ4 T; 920 Hz) anisotropy of magnetic susceptibility of each specimen was measured using a KLY-2 Kappabridge susceptometer at the LaboratoÂrio de Propriedades FõÂ sicas dos Materais GeoloÂgicos of the Federal University of Rio Grande do Norte. The magnitudes and orientations of the principal axes of the AMS ellipsoid were determined for each specimen of the site following a sequence of
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Table 1 AMS data for the sub-swarm IV, Rio CearaÂ-Mirim Dykes (D1-D8 in Fig. 2)a Station
k (10ÿ3 SI)
D1
38.43 38.84 39.58 37.90 39.37 39.40 39.43 33.92 37.12 37.28 36.82 36.07 53.09 54.52 56.92 57.45 56.73 53.39 51.09 55.94 55.54 66.53 59.60 56.30 38.32 41.17 42.96 44.22 47.00 42.49 45.54 46.33 47.90 52.60 47.91 49.59 40.87 42.40 48.58 47.26 58.65 48.29 52.85 44.41 44.54 43.32 49.87 43.01 48.94 43.93 50.18 48.54 43.79 40.70 51.61 45.50 46.88 34.27 46.47 42.07
D2
D3
D4
D5
D6
P% 1.9 1.2 2.0 1.9 1.7 2.4 2.0 1.8 2.0 1.9 1.7 1.8 2.8 4.0 3.8 4.7 4.5 3.6 4.2 4.7 4.7 3.2 2.9 3.3 1.6 1.5 1.4 1.1 1.3 1.3 1.2 1.3 1.8 2.2 2.6 2.6 1.9 1.2 2.4 2.5 2.3 2.8 2.5 3.5 4.4 2.6 2.7 2.5 3.4 3.4 3.0 2.8 2.6 2.3 4.9 4.5 5.4 2.6 2.7 2.9
T
kmax
kint
kmin
0.84 0.58 0.36 0.05 0.93 0.61 0.75 0.06 0.89 ÿ0.43 0.16 0.02 0.28 0.13 ÿ0.05 0.00 ÿ0.04 ÿ0.25 ÿ0.34 ÿ0.08 ÿ0.12 ÿ0.28 ÿ0.32 ÿ0.36 0.34 0.21 0.09 0.65 0.63 0.20 0.20 0.29 0.01 ÿ0.16 0.05 ÿ0.32 0.40 0.11 ÿ0.29 0.27 0.19 0.28 0.45 0.45 0.27 0.58 0.53 0.59 0.66 0.92 0.61 0.33 0.54 0.51 0.58 0.42 0.38 0.27 0.23 0.32
114/25 286/22 322/20 323/15 323/32 064/76 320/07 299/03 123/01 336/04 334/00 337/05 085/29 073/38 074/40 079/48 080/47 087/47 088/37 080/43 082/42 057/32 057/39 055/41 289/47 320/49 329/47 287/40 285/52 053/77 071/79 108/76 108/38 105/64 135/65 141/68 079/15 096/09 145/54 141/52 106/46 091/54 097/44 217/65 220/74 048/36 048/40 060/23 125/84 342/61 126/86 022/35 066/65 040/65 293/61 288/63 281/56 298/67 303/63 308/69
276/64 129/67 189/62 195/66 122/56 320/04 80/76 208/14 213/08 239/57 243/79 236/64 331/35 312/33 315/31 297/35 298/36 306/36 332/31 314/32 311/36 325/03 312/18 314/12 192/07 205/20 208/26 100/49 103/38 317/01 320/04 321/11 340/38 339/16 328/25 330/22 174/17 187/3 311/35 310/38 309/42 305/31 308/41 019/24 022/15 272/44 276/39 287/57 334/06 149/28 333/04 282/05 313/10 131/00 076/24 074/23 076/32 092/21 092/23 089/17
061/07 0190/8 059/19 058/18 227/09 229/14 228/12 041/76 024/82 069/33 064/11 069/25 204/41 196/34 200/35 192/19 193/19 200/20 214/38 202/30 199/27 230/58 203/46 210/46 096/42 100/34 100/32 194/04 194/01 227/13 229/10 230/07 224/30 243/20 235/05 239/03 309/67 297/80 046/07 044/05 208/11 205/16 203/16 112/07 113/05 157/24 162/27 159/22 244/03 242/06 243/02 190/24 219/23 221/25 173/15 170/14 173/12 186/09 187/12 183/13
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183
Table 1 (continued ) Station
D7
D8
SPP
k (10ÿ3 SI)
P%
T
kmax
kint
kmin
16.42 19.34 18.62 28.38 67.23 45.61 48.33 36.29 40.49 41.80 36.38 46.49 52.37 35.85 40.72 38.35 16.91 21.48 24.47 20.14 22.90 13.90 15.50 13.03 23.80 16.96 16.56 14.16 14.08
3.3 3.0 2.9 2.0 3.9 1.9 1.5 1.1 3.8 3.8 2.2 5.2 5.1 3.5 3.7 4.1 8.0 10.3 1.7 2.2 2.1 3.6 3.2 5.8 5.7 5.2 5.4 9.6 9.6
ÿ0.01 ÿ0.29 0.08 ÿ0.09 0.67 0.61 0.47 0.52 0.38 0.02 ÿ0.28 0.67 0.56 0.49 0.80 0.66 0.50 0.48 0.79 0.66 0.84 0.00 ÿ0.03 0.54 0.45 0.19 0.23 0.44 0.53
327/68 326/64 327/68 296/29 094/39 269/16 274/21 277/10 267/37 265/33 267/37 207/23 213/08 205/18 212/12 175/71 062/14 076/18 063/21 144/80 061/14 235/80 222/86 027/82 021/85 271/61 264/57 283/64 280/63
083/10 081/12 088/12 054/40 262/50 057/72 059/65 007/02 086/53 083/57 068/52 071/59 102/69 062/67 084/71 234/01 188/67 210/66 200/62 078/5 187/66 069/10 074/03 240/07 242/04 073/28 071/32 061/20 062/22
177/19 176/23 182/18 182/36 359/06 177/9 179/13 108/79 177/00 175/01 170/09 306/19 305/20 300/13 305/15 298/14 327/18 340/16 326/18 332/16 326/19 339/02 344/02 150/04 152/04 167/08 165/06 157/16 158/16
a Legend: k=magnitude of the magnetic susceptibility; P%=percentage of anisotropy (Graham, 1966); T=shape parameter; kmax, kint, kmin=azimuth and plunge (in degrees) of the principal directions of AMS ellipsoid (kmax > kint > kmin); ]=microporphyritic dyke at site D8; SPP=dykes from the SaÄo Paulo do Potengi dam.
15 susceptibility measurements along dierent orientations (Jelinek, 1978). The magnitudes and orientations of the average AMS ellipsoid of a given site are denoted maximum (Kmax=magnetic lineation), intermediate (Kint) and minimum (Kmin=pole of foliation) mean principal directions.
tional data were analyzed using both the maximum density orientation and the tensorial statistic of Jelinek (1978), utilizing an updated version of the program of Lienert (1991). The fabric is considered well de®ned
4. Low-®eld susceptibility and anisotropy The mean susceptibility of a specimen is the arithmetical average km=1/3 (kmax+kint+kmin) and P=100(kmaxÿkmin)/kmax yields the percentage of anisotropy (Graham, 1966). The dykes have susceptibilities in the range of 10ÿ2 < k (SI) < 0.8 10ÿ1 (Table 1 and Fig. 4). These values of susceptibility are comparable to those obtained in the dykes of the other CMDS sub-swarms and have previously been attributed to titanomagnetite (Bucker et al., 1986; Bellieni et al., 1992). Despite the high susceptibility magnitudes, the anisotropy degree of the ma®c dykes is relatively low (P < 4% in 84% of the specimens), which is typical for dykes. There is no correlation between P and and the magnetic susceptibiliy (Fig. 4). AMS direc-
Fig. 4. Anisotropy percentage degree (P%) versus susceptibility (k) of basaltic dykes. n=number of specimens.
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when the largest semi-angle of the 95% con®dence ellipse is less than 308, and poorly de®ned otherwise. The AMS data (Table 1) can be divided into three fabric subsets (Fig. 5), as summarized below: . Type I (D1 and D2): Both planar and weakly linear AMS ellipsoids are equally common. The degree of anisotropy is low, not reaching 5.0%. The foliations are NW-trending and the lineations plunge gently to moderately to the NW and E. . Type II (D3, D4, D5 and D6): The fabric is dominantly planar, although D5 has a linear distribution. As in the Type I sub-fabric, the degree of anisotropy is usually low (P < 5.5%). The foliations dip N and NE and lineations plunge steeply towards the Santana Plateau (D3, D4, and D6). Site D5 shows dispersed kmin axis yielding an ill-de®ned foliation, although the mean direction (kmin) keeps the same orientation of the nearest dykes (D4 and D6). In D3, the con®dence ellipse around the mean lineation is greater than 308 but the individual kmax directions plunge steeply. . Type III (D7 and D8): The fabric is typically planar and the degree of anisotropy locally increases up to 10%. The E- to NE-trending foliations dip steeply. Lineations are subhorizontal (D7) or are scattered in the plane of foliation (D8) (Fig. 5). The well
Fig. 5. Equal area projections (Schmidt net, lower hemisphere) of the axes of principal susceptibilities de®ning three sub-fabric types. Figures in parentheses are numbers of samples; e1 and e3 (lower right) are the semi-angle of the 95% con®dence ellipse around, respectively, Kmax and Kmin.
de®ned planar magnetic fabric of site D8 was obtained from specimens of two E-trending parallel dykes, one microporphyritic and the other ®ne grained. At this site, the orientation of the magnetic fabric is typically bimodal: the microporphyritic dyke has well clustered kmax axes plunging gently to SW, whereas in the ®ne-grained dyke the kmax axes plunge steeply to gently in the plane of magnetic foliation (Table 1). A strongly oblate magnetic ellipsoid (0.48 < T < 0.84) combined with the dispersion of kmax axes led us to consider that the linear fabric of site D8 has no signi®cance.
5. Rock magnetism The Curie/NeÂel temperatures of the magnetic mineralogy were investigated using a CS-2 apparatus which, coupled with the KLY-2 bridge, measures the suscepti-
Fig. 6. Representative plots of thermomagnetic curves on air of the specimen D1 (A) and D7 (B). The drop of the susceptibility around 5508C is attributed to Ti-poor magnetite. An additional in¯ection point at ca. 3508C (D7) suggests the presence of maghemite (see text).
C.J. Archanjo et al. / Journal of South American Earth Sciences 13 (2000) 179±189
bility of a rock fragment at variable temperature (Hrouda, 1994). A 0.3±0.5 cm3 sample of powdered basalt is submitted to a cycle of heating up to 6858C and cooling to room temperature; the susceptibility is measured regularly at steps of 38C. For at least one sample per site, we investigated the thermal change of the magnetic susceptibility during the heating-cooling cycle. Hysteresis loop and isothermal remanent magnetization (IRM) acquisition curves were obtained using a vibrating sample magnetometer (VSM-Molspin). In specimens D1, D2, D5, D6, and D8 the susceptibility increases with temperature (Hopkinson eect) before dropping abruptly between 5358C and 5708C (Fig. 6A). The susceptibility is mostly reversible during cooling. The IRM acquisition curves show a saturation ®eld of approximately 200 mT, indicating that the susceptibility essentially comes from Ti-poor magnetite. The shallow positive slope of the saturated, high-®eld, linear portion of the initial hysteresis loop shows that the paramagnetic silicates and the highly coercive fraction, like hematite, contribute less than 1.2 % to the total susceptibility (Table 2). The thermomagnetic curves of specimens D4 and D7 are rather dierent from those of other specimens. On heating, D4 and D7 (Fig. 6B) display a small decrease of susceptibility between 3208C and 4408C, followed by an increase before dropping at 5508C. The cooling path is not reversible. The wave-like form of the curve disappears down to 4808C and the total susceptibility is reduced by about 25% at room temperature compared to initial values. These data suggest that specimen D7 contains Ti-poor magnetite and ®negrained maghemite (De Boer and Dekkers, 1996). The transition at around 3508C is attributed to the inversion of maghemite to hematite on heating. Hysteresis loops were measured at room temperature by subjecting a sample to an external ®eld (H) which is ramped from 0 to 1 T, then to ÿ1 T, then back to 0 T.
185
Saturation magnetization (Ms) is the point at which magnetization no longer increases with increasing external ®eld. Saturation remanent magnetization (Mrs) is the magnetization remaining after the external ®eld has been reduced to zero. When the direction of the external ®eld is reversed and increased in the negative direction, the magnetization will decrease to zero. The negative applied ®eld at which the induced magnetization is zero is the coercivity (Hc). The back ®eld required to bring the remanent magnetization to zero is the coercivity of remanence (Hrs). Hysteresis parameters re¯ect the domain state of the magnetic minerals present in the sample, which in turn are a function of grain volume. Relative grain-size determinations, and thus domain state, can be made by calculating the ratios of hysteresis parameters, Mrs/Ms and Hrs/Hc (Table 2), which are usually displayed on a Day plot (Fig. 7). Single domain (SD) magnetite grains have grain-size in the range 0.03 to 0.1 mm with Mrs/ Ms=0.5 (Day et al., 1976; Dunlop, 1986). Pseudosingle domain (PSD) magnetite grains have grain-size in the range 0.1 to 10 mm, and multidomain (MD) particles have grain-size larger than 10 mm with Mrs/Ms < 0.1. The ratios Hcr/Hc and Mrs/Mr of the Rio Ceara Mirim dykes show that the ferromagnetic fraction falls principally in the range of pseudo-single domain (PSD) grains (Fig. 7). The Mrs/Mr ratio varies between 0.30 and 0.12, below the transition between SD and PSD grains. These results indicate that the observed anomalous fabrics are not due to SD particles. 6. Discussion 6.1. Source of susceptibility and magnetic anisotropy The magnetic susceptibility of the ma®c dykes is
Table 2 Hysteresis properties of samples from the northeastern Brazil basaltic dykesa Specimen
Hcr (mT)
Hc (mT)
Mrs (mA.m2)
Ms (mA.m2)
Hcr/Hc
Mrs/Ms
kmat (%)
D1 D2 D3 D4 D5 D6 D7A D7B D8A D8B D8C
31.79 30.52 33.36 27.13 29.57 23.05 39.92 27.47 25.98 24.48 25.31
16.71 14.35 13.40 11.94 11.69 10.22 21.62 12.36 6.71 8.42 9.21
193.77 212.00 164.00 201.96 218.08 203.87 647.83 3.93 41.83 206.77 96.90
793.14 1390.73 1083.01 1220.90 1328.47 1548.16 2438.85 18.32 361.71 1509.51 663.47
1.90 2.13 2.49 2.27 2.53 2.25 1.85 2.22 3.87 2.91 2.75
0.24 0.15 0.15 0.17 0.16 0.13 0.27 0.21 0.12 0.14 0.15
0.6 0.8 1.1 0.7 0.9 0.9 0.6 0.4 1.0 0.6 0.7
a
Legend: Hcr=remanent coercivity; Hc=coercivity; Mrs=remanent saturation magnetization; Ms=saturation magnetization (values corrected for the matrix susceptibility); kmat=matrix (paramagnetic and antiferromagnetic) susceptibility percent calculated in the high ®eld, saturated portion of the hysteresis loop.
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principally carried by Ti-poor, PSD-magnetites, as shown by their rock magnetic properties. Secondary alteration of the original Ti-Fe oxides is evidenced by the presence of maghemite (g-Fe2O3), a typical product of low temperature alteration of magnetite, which inverts to hematite at a higher temperature. The presence of maghemite in some samples, however, does not have noticeable in¯uence on the magnetic fabric. In site D7, where maghemitization aected the rock, the AMS principal axes are well clustered and the fabric is a typically one with kmin perpendicular to the dyke wall. Although pyroxene and amphibole dominate the ma®c mineralogy of the basalts, the susceptibility of these paramagnetic silicates does not reach 1.2% of the bulk rock susceptibility, as shown by the nearly ¯at high-®eld hysteresis curve. Thus, the anisotropy of susceptibility of the dykes must be due to ®ne-grained titanomagnetites, as suggested by the high coercivity of the ferrimagnetic fraction observed in the hysteresis loop. The magnetic fabric pattern of the Rio CearaÂ-Mirim sub-swarm IV is very well de®ned both on each site or on a regional scale, even with the feeble anisotropy Ð typically under 4%. The plunge of lineations increases from Type I to Type II sub-fabrics, whereas the transition between the Types II and III sub-fabrics is sharp (Fig. 2). The latter presents the highest degree of anisotropy and more oblate AMS ellipsoids. The lineations become subhorizontal westward and nearly vertical in the central part of the swarm. They point to a focal area towards the eastern sector of the Santana Plateau. The lineations of Type III sub-fabrics are
Fig. 7. Mrs/Ms versus Hcr/Hc (hysteresis ratios) for samples from the Rio CearaÂ-Mirim dykes (sub-swarm IV), with boundaries of single domain (SD), pseudo-single domain (PSD), and multidomain (MD) behavior for magnetite taken from Day et al. (1977) and Dunlop (1986).
either nearly horizontal (D7) or show a complex pattern (D8). In the latter case, six of the eight kmax axes are subhorizontal, yet the statistical Kmax mean is steep, nearly vertical (Fig. 5). The associated kmin show a bimodal distribution, suggesting that two distinct ¯ow regimes may have been combined. A close inspection of the data from site D8 shows that the microporphyritic dyke has kmax axes grouped in the SW direction, whereas the kmax axes of the ®ne-grained dyke are scattered along a great circle perpendicular to the foliation. This seems to support the interpretation that a signi®cant component of magma ¯ow to the eastern extremity of the swarm was subhorizontal, although multiple periods of injection might result in a locally complex ¯ow pattern. The foliation is typically oblique to the trend of the dykes, principally at extremities of the swarm (sites D1 and D8, Fig. 2). The origin of such an oblique magnetic foliation is not very well understood, although the consistent AMS fabric pattern observed along the swarm suggests that this pattern is a primary magmatic feature. Imbricated fabrics due to ¯ow gradients near the dyke walls (Knight and Walker, 1988) or intersection of fabrics due to back¯ow (Philpotts and Asher, 1994) may be the source of these oblique magnetic fabrics. Oblique type AMS fabrics were also produced by numerical models using uniformly distributed, noninteracting and low aspect ratio magnetic particles ¯owing in a steady-state viscous ¯uid (Dragoni et al., 1997). Models of progressive simple shear or combined progressive simple shear and pure shear ¯ow of a magma in which grains can rotate freely predict obliquities between shape preferred orientation of grains of dierent aspect ratio (Fernandez, 1987; Ildefonse et al., 1992; Jezek et al., 1994). The fabric intensity and orientation will depend on the aspect ratio of individual particles and, at later stages in the crystallization of the magma, the degree of interaction among the particles (Ildefonse et al., 1992). For non-interacting particles with aspect ratio varying from 1.2 to 4.0, grains with smaller aspect ratio will have rotated most. In other words, if the magnetic fabric of basalts were formed only by shape anisotropy of discrete nearly cubic titanomagnetites, the AMS should vary rapidly in intensity and orientation from a specimen to another along a dyke. The clustering of the AMS principal directions of a single dyke taken from specimens separated from each other by tens of meters argues against such a model. Hence, the anisotropy of distribution, which depends on the degree of interaction of magnetic particles with a pre-exising silicate fabric, has been invoked to explain the origin of the magnetic fabric in basalts (Hargraves et al., 1991; Stephenson, 1994; CanÄoÂn-Tapia et al., 1996). As Hargraves et al. (1991) have pointed out, even if perfectly spherical
C.J. Archanjo et al. / Journal of South American Earth Sciences 13 (2000) 179±189
magnetically isotropic particles are present in a rock, the rock will not be isotropic if the particles are distributed anisotropically and are close enough to interact magnetically. They carried out experiments on isotropic rocks (and synthetics), indicating that magnetic anisotropy would form by a pre-existing template of silicates which itself would be controlled by ¯ow when the rock was formed. The anisotropy of distribution may be enhanced by post-emplacement, sub-solidus deuteric alteration that exsolves ilmenite from titanomagnetites leaving a strongly magnetic Ti-poor magnetite. Below the Curie temperature, magnetic interactions between neighboring grains may have additionally increased or lowered the anisotropy degree, depending on the shape of particles and the aligned or spatial con®guration of the grains (Archanjo et al., 1995; GreÂgoire et al., 1995). The scattered values of the anisotropy degree (Fig. 4) are thought to be a consequence of such interactions. 6.2. Regional implications Chemical and isotopic data have shown that the CMDS was extracted from a LIL-enriched mantle source (Bellieni et al., 1992), probably related to partial melting in the early rifting and break-up stages of the Gondwana continent. The time span of around 20 Ma for this event suggests that the intrusion of dykes occurred by following successive pulses of ma®c magma from a melting zone (Almeida et al., 1988). A model for magma in®lling cracks in brittle upper crust from a deep source is shown in Fig. 8. The fan-like ¯ow line pattern would be produced as the magma moves away from its source area and spreads laterally. The available regional gravimetric map shows a 80 km long, elliptical NE-trending negative Bouguer anomaly situated at the northeastern part of the Santana Plateau (Moreira et al., 1990). If such an anomaly were related to the source of dykes, the gravimetric low would delineate the remnants of a still warm paleo-
Fig. 8. Model for the acquisition of ¯ow structures of the Rio CearaÂ-Mirim dykes (sub-swarm IV). The ``paleo'' source region, presently located under Tertiary sandstones of the Santana plateau, would feed a dyke set in®lling an anastomosing E-trending crack system. The lineation trajectory would form a fan-like pattern converging to the magmatic feed zone.
187
melting zone rooted in the mantle. The magnetic fabric of a set of ma®c dykes in the SaÄo Paulo do Potengi dam situated to the northeast of the sub-swarm IV (Fig. 2) seems to supports such a hypothesis. Eight cores from dykes of decimeter to meter width yielded a well-de®ned mean lineation (largest semi-angle of 95% ellipse around Kmax, e1=198) plunging 728 to the west (Fig. 9) Ð i.e., pointing to the Bouguer gravimetric low. The mean magnetic foliation (largest semi-angle of 95% ellipse around Kmin, e3=98) dips steeply to NNW and is slightly oblique to the E-trending dyke wall. 7. Conclusions The AMS of Mesozoic ma®c dykes from northeastern Brazil display a well-de®ned magnetic fabric, with magnetic lineation that plunges steeply in the center of the swarm and magnetic foliation mainly parallel to oblique to the dyke wall. The planes of magnetic foliation have a curved trajectory, trending EW in the center of the swarm but rotating clockwise to a NW direction near the western extremity and counterclockwise to a NE direction near the eastern extremity. This simple pattern is consistent with the upward vertical ¯ow near the center of the swarm and outward ¯ows near the extremities. The fabric patterns suggest that
Fig. 9. Equal area projection of AMS principal directions of basaltic dykes from the SaÄo Paulo do Potengi dam (see Fig. 2 for location); n=number of specimens; e1 and e3 are the semi-angle of the 95% con®dence ellipse around, respectively, Kmax and Kmin.
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the ``paleo'' melting zone that fed the dykes might have been situated below Tertiary sediments covering the Precambrian basement. The principal directions of magnetic anisotropy are controlled by PSD-, Ti-poor magnetite. Maghemite was detected in some rocks but does not have appreciable in¯uence on the magnetic fabric. Hysteresis parameters indicate that AMS of the dykes is controlled by highly coercive ®ne-grained, Tipoor magnetite but SD grains, which could give rise to secondary fabric types, were not detected. Acknowledgements This work was supported by Fundac° aÄo de Amparo a Pesquisa do Estado de SaÄo Paulo (FAPESP, Project No. 98/4420-2), Conselho Nacional de Pesquisa (CNPq), Instituto de GeocieÃncias of the Universidade de SaÄo Paulo, and Departamento de Geologia of the Universidade Federal do Rio Grande do Norte. William MacDonald, Alexander Cruden, and an anonymous reviewer are thanked for their critical comments that improved this paper. References Almeida, F.F.M., Carneiro, C.D.R., Machado, D.L., Dehira, L.K., 1988. Magmatismo PoÂs-PaleozoÂico no Nordeste Oriental do Brasil. Revista Brasileira de GeocieÃncias 18, 451±462. Archanjo, C.J., Launeau, P., Bouchez, J.L., 1995. Magnetic fabric vs. magnetite and biotite shape fabrics of the magnetite-bearing granite pluton of Gameleiras (northeast Brazil). Physics of the Earth and Planetary Interiors 89, 63±75. Bellieni, G., Macedo, M.H.F., Petrini, R., Piccirillo, E.M., Cavazzini, G., Comin-Chiaramonti, P., Ernesto, M., Macedo, J.W.P., Martins, G., Mel®, A.J., Pacca, I.G., De Min, A., 1992. Evidence of magmatic activity related to Middle Jurassic and Lower Cretaceous rifting from northeastern Brazil (CearaÂMirim): K/Ar age, paleomagnetism, petrology and Sr-Nd isotope characteristics. Chemical Geology 97, 9±32. Bucker, C., Schult, A., Bloch, W., Guerreiro, S.D.C., 1986. Rockmagnetism and paleomagnetism of an Early Cretaceous/ Late Jurassic dyke swarm in Rio Grande do Norte, Brazil. Journal of Geophysics 60, 129±135. CanÄoÂn-Tapia, E., Walker, G.P.L., Herrero-Bervera, E., 1996. The internal structure of lava ¯ows Ð Insights from AMS measurements I: Near-vent a'a. Journal of Volcanology and Geothermal Research 70, 21±36. Day, R., Fuller, M., Schmidt, V.A., 1976. Magnetic hysteresis properties of synthetic titanomagnetites. Journal of Geophysical Research 81, 873±880. De Boer, C.B., Dekkers, M.J., 1996. Grain-size dependence of the rock magnetic properties for a natural maghemite. Geophysical Research Letters 23, 2815±2818. Dragoni, M., Lanza, R., Tallarico, A., 1997. Magnetic anisotropy produced by magma ¯ow: Theoretical model and experimental data from Ferrar dolerite sills (Antarctica). Geophysical Journal International 128, 230±240. Dunlop, D.J., 1986. Hysteresis properties of magnetite and their dependence on particle size: a test of pseudo-single-domain remanence models. Journal of Geophysical Research 91, 9569± 9584.
Ernst, R.E., Baragar, W.R.A., 1992. Evidence from magnetic fabric for the ¯ow pattern of the magma in the Mackenzie giant radiating dyke swarm. Nature 356, 511±513. Fernandez, A., 1987. Preferred orientation developed by rigid markers in two-dimensional simple shear strain: A theoretical and experimental study. Tectonophysics 136, 151±158. Graham, J.W., 1966. Signi®cance of magnetic anisotropy in Appalachian sedimentary rocks. In: Steinhart, J.S., Smith, T.J. (Eds.), The Earth Beneath the Continents, American Geophysical Union Geophysical Monographs, 10, pp. 627±648. GreÂgoire, V., Saint Blanquat, M., NeÂdelec, A., Bouchez, J.L., 1995. Shape anisotropy versus magnetic interactions of magnetite grains: experiments and applications to AMS in granitic rocks. Geophysical Research Letters 22, 2765±2768. Hargraves, R.B., Johnson, D., Chan, C.Y., 1991. Distribution anisotropy: The cause of AMS in igneous rocks? Geophysical Research Letters 18, 2193±2196. Hrouda, F., 1994. A technique for the measurements of thermal changes of magnetic susceptibility of weakly magnetic rocks by CS-2 apparatus and KLY-2 Kappabridge. Geophysical Journal Interiors 118, 604±612. Ildefonse, B., Launeau, P., Fernandez, A., Bouchez, J.L., 1992. Eect of mechanical interaction on development of shape preferred orientations: A two-dimensional experimental approach. Journal of Structural Geology 14, 73±83. Jelinek, V., 1978. Statistical processing of anisotropy of magnetic susceptibility measured on group of specimens and its applications. Studies in Geophysics and Geodynamics 22, 50±62. Jezek, J., Melka, R., Schulmann, K., Venera, Z., 1994. The behavior of rigid triaxial ellipsoidal particles in viscous ¯ows: Modeling of fabric evolution in a multiparticle system. Tectonophysics 257, 203±220. Knight, M.D., Walker, G.P.L., 1988. Magma ¯ow directions in dykes of the Koolau Complex, Oahu, determined from magnetic fabric studies. Journal of Geophysical Research 93, 4301±4319. Lienert, B.R., 1991. Monte Carlo simulation of errors in the anisotropy of magnetic susceptibility: A second-rank symmetric tensor. Journal of Geophysical Research 96, 19539±19544. Matos, R.M.D., 1992. The northeast Brazilian rift system. Tectonics 11, 766±791. Maluski, H., Coulon, C., Popo, M., Baudin, P., 1995. 40Ar/39Ar chronology, petrology and geodynamic setting of Mesozoic to early Cenozoic magmatism from the Benue Trough, Nigeria. Journal of the Geological Society London 152, 311±326. Moreira, J.A.M., Baraud, R., Lins, F.A.L., Macedo, J.W.P., 1990. Contribuic° aÄo da gravimetria para o entendimento da sismicidade de JoaÄo CaÃmara. Anais, XXXVI Congresso Brasileiro de Geologia (Natal) 6, 2145±2153. Oliveira, D.C., 1992. O Papel do Enxame de Diques Rio Ceara Mirim na Evoluc° aÄo TectoÃnica do Nordeste Oriental (Brasil): Implicac° oÄes na Formac° aÄo do Rifte Potiguar. Unpublished Masters Thesis, Universidade Federal de Ouro Preto, 172 p. Oliveira, D.C., 1994. Novas considerac° oÄes sobre a evoluc° aÄo tectoÃnica do enxame de diques Rio Ceara Mirim Ð Nordeste Oriental do Brasil. R. Esc. Minas (Ouro Preto) 47, 223±227. Oliveira, D.C., Gomes, C.J.S., 1996. A expressaÄo de reativac° oÄes poÂsintrusaÄo do enxame de diques Rio CearaÂ-Mirim (MesozoÂico): Implicac° oÄes na evoluc° aÄo tectoÃnica da Bacia Potiguar. B. Geoci. PETROBRAS (Rio de Janeiro) 10, 121±145. Philpotts, A.R., Asher, P.M., 1994. Magmatic ¯ow-direction indicators in a giant diabase feeder dike, Connecticut. Geology 22, 363±366. Potter, D.K., Stephenson, A., 1988. Single-domain particles in rocks and magnetic fabric analysis. Geophysical Research Letters 15, 1097±1100. Raposo, M.I.B., 1997. Magnetic fabric and its signi®cance in the
C.J. Archanjo et al. / Journal of South American Earth Sciences 13 (2000) 179±189 FlorianoÂpolis dyke swarm, southern Brazil. Geophysical Journal International 131, 159±170. Rochette, P., Jackson, M., Aubourg, C., 1992. Rock magnetism and the interpretation of anisotropy of magnetic susceptibility. Reviews in Geophysics 30, 209±226. Sial, A.N., Long, L.E., Pessoa, D.A.R., Kawashita, K., 1981.
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Potassium-argon ages and strontium isotope geochemistry of Mesozoic and Tertiary basaltic rocks, northeastern Brazil. Academia Brasileira de CieÃncias 53, 115±122. Stephenson, A., 1994. Distribution anisotropy: Two simple models for magnetic lineation and foliation. Physics of the Earth and Planetary Interiors 82, 49±53.
Magnetic fabric of a basaltic dyke swarm associated with Mesozoic rifting in northeastern Brazil Carlos J. Archanjo a,*, Ricardo I. Trindade b, Jose Wilson P. Macedo c, Marcelus G. ArauÂjo a a
Instituto de GeocieÃncias, Universidade de SaÄo Paulo, rua do Lago 562 SaÄo Paulo SP, 05508-900, Brazil b Instituto AstronoÃmico e Geo®sico, Universidade de SaÄo Paulo, SaÄo Paulo SP, 01065-970, Brazil c DFTE/CCET, Universidade Federal do Rio Grande do Norte, Natal RGN, 59072-970, Brazil Accepted 1 November 1999
Abstract The anisotropy of magnetic susceptibility (AMS) has been studied in a 120 km long, Early Cretaceous tholeiitic dyke swarm emplaced during the early stages of rifting and opening of the equatorial Atlantic Ocean. The vertical dykes ®lled a set of Etrending fractures that cut the structural grain of the Precambrian basement of northeastern Brazil at a high angle. These strongly magnetic rocks contain pseudo-single domain, Ti-poor magnetite and secondary maghemite as revealed by thermomagnetic and hysteresis data. The contribution of the paramagnetic and the high coercivity antiferromagnetic fractions to the bulk susceptibility is less than 1.2%. The dykes generally show well-clustered AMS principal directions. The plunge of the magnetic lineation varies from nearly subvertical in the center of the swarm to horizontal in the west. The strike of the magnetic foliation is generally oblique to the dyke wall and exhibits a curved trend at the regional scale. This fabric pattern suggests that the magma source that fed the dykes was situated in the center of the swarm, which is presently below Tertiary sandstones. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Basaltic dykes are primary conduits of magmas formed in the mantle and emplaced into the crust by ®lling of fractures related to a extensional stress regime. Although it is obvious that the magma migrates upward, the ¯ow path may include lateral transport to great distances from the source area. The transition from horizontal to vertical magmatic ¯ow, as observed in the transcontinental MacKenzie dyke swarm of the Slave Province (Canada), occurs in the interval between 500 and 600 km from the locus and is thought to correspond to the outer boundary of an ancient mantle plume (Ernst and Baragar, 1992). Flow * Corresponding author. Tel.: +55-11-818-3994; fax: +55-11-8184258. E-mail address: [email protected] (C.J. Archanjo).
directions in dykes may be determined by anisotropy of magnetic susceptibility (AMS) measurements. AMS has been shown to be suciently sensitive to determine subtle fabrics in lava ¯ows with an anisotropy of only a few percent (Knight and Walker, 1988; CanÄoÂn-Tapia et al., 1996). Ma®c magmas ¯owing through a dyke will acquire a preferred orientation of titanomagnetite grains with the minimum axis (kmin, pole of magnetic foliation; kmax > kint > kmin) usually aligned perpendicular to the wall and the maximum axis (kmax, magnetic lineation) in the direction of the ¯ow. This ``normal'' or ``common'' con®guration can be found in rocks containing multidomain (MD) magnetite (Rochette et al., 1992). ``Normal'' AMS fabrics are far from being the only fabric type observed in dykes and lava ¯ows. In some studies, up to 50% of the samples show both anomalous (kmax perpendicular to the dyke wall) and intermediate (kint perpendicular to the dyke wall) arrangements (Knight and Walker, 1988; Rochette et
0895-9811/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 5 - 9 8 1 1 ( 0 0 ) 0 0 0 2 3 - 7
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al., 1992; Raposo and Ernesto, 1995; Raposo, 1997). Inverse fabrics may occur in rocks whose magnetic anisotropy is yielded by magnetite with a single domain (SD) structure. For such grains, the kmax direction is perpendicular to the grain elongation Ð i.e., the mineral lineation (Potter and Stephenson, 1988). Mixtures of primary MD particles and secondary SD particles may be responsible for the production of intermediate AMS type fabrics (Rochette et al., 1992). However, Raposo (1997) stressed that the intermediate-type fabrics of the ma®c swarms of Ponta Grossa and FlorianoÂpolis from southern Brazil are produced by pseudo-single domain (PSD)- to MDmagnetites. Apparently no SD-grains occur in such rocks, suggesting that those fabrics were formed during the emplacement and cooling of the ma®c magma. ``Intermediate'' arrangements can also result from superimposed fabrics of dierent generations on magma injection. The Early Cretaceous (145±125 Ma) Rio Ceara Mirim Dyke Swarm (CMDS) intruded the basement of the Borborema Province during rifting events responsible for the opening of the equatorial Atlantic Ocean (Sial et al., 1981; Almeida et al., 1988). This magmatism is contemporaneous with basalt and rhyolite ¯ows of the Northern Benue Trough (Maluski et al., 1995), the African counterpart of the Potiguar Rift. The tholeiitic dykes of the CMDS cut the structural grain of the Precambrian basement at a high angle, including mylonitic rocks formed by NNE-trending shear zones. Fault reactivation aecting the dykes, however, has been observed locally (Oliveira and Gomes, 1996). The dykes in®ll a set of fractures trending E-W near the southern border of the Potiguar Basin (Fig. 1). In the western part of the province, these dykes become
Fig. 1. Geological map of the Rio CearaÂ-Mirim Dyke Swarm and basaltic necks in northeastern Brazil (Almeida et al., 1988). Precambrian basement=dotted pattern. Mesozoic basins: P=Potiguar; I=Iguatu; RP=Rio do Peixe; A=Araripe. The inshore/oshore Potiguar rift after Matos (1992).
nearly parallel to the NE-trending Eocretaceous rift system of the Potiguar, Iguatu, and Araripe Basins. The aeromagnetic data available show that magnetic anomalies attributed to ma®c dykes generally occur as closely spaced 10 to 40 km long stripes with amplitudes that vary from 50 to 300 nT (Oliveira, 1992, 1994); a few exceed 100 km long. The longest magnetic anomalies strike NE-SW and are observed in the western part of the swarm. They are traced within the Precambrian basement as well as in the Potiguar Basin parallel to the fault system that delimits the rift (Oliveira, 1992). We report here the ®rst results of an AMS and magnetic mineralogy study of a set of basaltic dykes intrusive in the southern part of the CMDS along the border of the 600 m high Tertiary terrace of the Santana Plateau. These dykes were named ``sub-swarm IV'' by Bellieni et al. (1992) because of their alkaline anity and typical paleomagnetic directions when compared with other sub-swarms of the CMDS. 2. Geological setting The ma®c dykes extend for 120 km, forming individual bodies usually less than 500 m long. They are exposed generally as rounded blocks formed by typical onion-skin weathering. The contacts between the dykes and host rocks are not usually exposed, but the margins of the dykes are relatively well de®ned by the contrast between the sandy and dark argillaceous soils formed by the alteration of, respectively, felsic host rock and ma®c dyke. The eight dykes in the study area (D1 to D8 in Fig. 2) vary in width from a few centimeters to about 20 meters; trends vary from 0808 to 1108 in azimuth. The dykes cut foliated host rocks that trend from NW-SE in the west (D1 to D3) to NE-SW in the east (D6 to D8). The D4 and D5 dykes intrude an isotropic late Precambrian granite. Petrographically, the dykes are ®ne- to mediumgrained olivine basalts. In thin section, the grains form a mosaic of euhedral to subhedral calcic plagioclase, augite, and pigeonite. Olivine is an accessory mineral, and titanomagnetite is the main opaque mineral (Bellieni et al., 1992). Back-scattered electron (BSE) images of the basalts revealed that titanomagnetite grains occur as two textural types (Fig. 3): (i) euhedral to subhedral crystals (mean grain size, 100 mm) containing minute inclusions of silicates and, (ii) needle-like crystals up to 500 mm long and 2 to 8 mm wide, some showing skeletal growth morphologies. Dykes in the western part of the study area usually contain microphenocrysts of plagioclase, whereas those in the east are aphyric and ®ne grained. However, ®ne-grained and microporphyritic dykes may sometimes be found at the same site (e.g., D8) Ð both trending E-W and
C.J. Archanjo et al. / Journal of South American Earth Sciences 13 (2000) 179±189
181
Fig. 2. Sampling locations (D1 to D8) of vertical ma®c dykes trending E-W and mean site AMS principal directions. Schmidt net, lower hemisphere. Kmax=lineation (full dot); Kmax/Kint plane=foliation (full line). Empty dot and dashed line=ill-de®ned lineation and foliation, respectively.
sharing the same orientation of AMS principal directions (discussed below). The contact of host gneissic rock and these dykes is exposed in a unweathered rock pavement of a river bed. Tertiary necks of olivine basalt (Sial et al., 1981) intrude the Precambrian basement, forming two parallel E-trending belts of elliptical to sub-circular bodies with a maximum diameter of 200 m (Fig. 2). Regionally the volcanic necks are aligned N-S, crossing the trend of the CMDS (Fig. 1).
3. Sampling and analytical methods
Fig. 3. BSE images of textural features of titanomagnetite from a basaltic dyke (site D7): A) Euhedral to subhedral and needle-like crystals (light grains), apparently without shape preferred orientation, enclosed in a silicate matrix (bar=500 mm); B) Detail of A showing nearly rounded silicate inclusions in euhedral grains and ``rods'' of titanomagnetite. Note the preferential growth of rods following the {111} spinel planes (bar=50 mm).
The samples came from the largest rock blocks, as fresh as possible and apparently in place irrespective of marginal or central zones of the dyke. For the only dyke in which the wall rock contacts are exposed (D8), the cores were taken at the margin and the central part of the both the porphyritic and the ®ne-grained dykes. Its chilled margins were avoided because of their vulnerability to deuteric alteration and their possibly dierent primary magnetic properties. Four to six cylindrical oriented cores 6 to 8 cm long and 2.5 cm in diameter were collected at each site using a portable gasoline-powered rock drill. At each site, the core spacing was from a meter to a few tens of meters. In the laboratory, each core was cut into 2.2 cm long samples, so that each measurement site yielded 8 to 12 oriented specimens. A total of 79 AMS determinations were made for the eight dykes. Low-®eld (3.8 10ÿ4 T; 920 Hz) anisotropy of magnetic susceptibility of each specimen was measured using a KLY-2 Kappabridge susceptometer at the LaboratoÂrio de Propriedades FõÂ sicas dos Materais GeoloÂgicos of the Federal University of Rio Grande do Norte. The magnitudes and orientations of the principal axes of the AMS ellipsoid were determined for each specimen of the site following a sequence of
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Table 1 AMS data for the sub-swarm IV, Rio CearaÂ-Mirim Dykes (D1-D8 in Fig. 2)a Station
k (10ÿ3 SI)
D1
38.43 38.84 39.58 37.90 39.37 39.40 39.43 33.92 37.12 37.28 36.82 36.07 53.09 54.52 56.92 57.45 56.73 53.39 51.09 55.94 55.54 66.53 59.60 56.30 38.32 41.17 42.96 44.22 47.00 42.49 45.54 46.33 47.90 52.60 47.91 49.59 40.87 42.40 48.58 47.26 58.65 48.29 52.85 44.41 44.54 43.32 49.87 43.01 48.94 43.93 50.18 48.54 43.79 40.70 51.61 45.50 46.88 34.27 46.47 42.07
D2
D3
D4
D5
D6
P% 1.9 1.2 2.0 1.9 1.7 2.4 2.0 1.8 2.0 1.9 1.7 1.8 2.8 4.0 3.8 4.7 4.5 3.6 4.2 4.7 4.7 3.2 2.9 3.3 1.6 1.5 1.4 1.1 1.3 1.3 1.2 1.3 1.8 2.2 2.6 2.6 1.9 1.2 2.4 2.5 2.3 2.8 2.5 3.5 4.4 2.6 2.7 2.5 3.4 3.4 3.0 2.8 2.6 2.3 4.9 4.5 5.4 2.6 2.7 2.9
T
kmax
kint
kmin
0.84 0.58 0.36 0.05 0.93 0.61 0.75 0.06 0.89 ÿ0.43 0.16 0.02 0.28 0.13 ÿ0.05 0.00 ÿ0.04 ÿ0.25 ÿ0.34 ÿ0.08 ÿ0.12 ÿ0.28 ÿ0.32 ÿ0.36 0.34 0.21 0.09 0.65 0.63 0.20 0.20 0.29 0.01 ÿ0.16 0.05 ÿ0.32 0.40 0.11 ÿ0.29 0.27 0.19 0.28 0.45 0.45 0.27 0.58 0.53 0.59 0.66 0.92 0.61 0.33 0.54 0.51 0.58 0.42 0.38 0.27 0.23 0.32
114/25 286/22 322/20 323/15 323/32 064/76 320/07 299/03 123/01 336/04 334/00 337/05 085/29 073/38 074/40 079/48 080/47 087/47 088/37 080/43 082/42 057/32 057/39 055/41 289/47 320/49 329/47 287/40 285/52 053/77 071/79 108/76 108/38 105/64 135/65 141/68 079/15 096/09 145/54 141/52 106/46 091/54 097/44 217/65 220/74 048/36 048/40 060/23 125/84 342/61 126/86 022/35 066/65 040/65 293/61 288/63 281/56 298/67 303/63 308/69
276/64 129/67 189/62 195/66 122/56 320/04 80/76 208/14 213/08 239/57 243/79 236/64 331/35 312/33 315/31 297/35 298/36 306/36 332/31 314/32 311/36 325/03 312/18 314/12 192/07 205/20 208/26 100/49 103/38 317/01 320/04 321/11 340/38 339/16 328/25 330/22 174/17 187/3 311/35 310/38 309/42 305/31 308/41 019/24 022/15 272/44 276/39 287/57 334/06 149/28 333/04 282/05 313/10 131/00 076/24 074/23 076/32 092/21 092/23 089/17
061/07 0190/8 059/19 058/18 227/09 229/14 228/12 041/76 024/82 069/33 064/11 069/25 204/41 196/34 200/35 192/19 193/19 200/20 214/38 202/30 199/27 230/58 203/46 210/46 096/42 100/34 100/32 194/04 194/01 227/13 229/10 230/07 224/30 243/20 235/05 239/03 309/67 297/80 046/07 044/05 208/11 205/16 203/16 112/07 113/05 157/24 162/27 159/22 244/03 242/06 243/02 190/24 219/23 221/25 173/15 170/14 173/12 186/09 187/12 183/13
C.J. Archanjo et al. / Journal of South American Earth Sciences 13 (2000) 179±189
183
Table 1 (continued ) Station
D7
D8
SPP
k (10ÿ3 SI)
P%
T
kmax
kint
kmin
16.42 19.34 18.62 28.38 67.23 45.61 48.33 36.29 40.49 41.80 36.38 46.49 52.37 35.85 40.72 38.35 16.91 21.48 24.47 20.14 22.90 13.90 15.50 13.03 23.80 16.96 16.56 14.16 14.08
3.3 3.0 2.9 2.0 3.9 1.9 1.5 1.1 3.8 3.8 2.2 5.2 5.1 3.5 3.7 4.1 8.0 10.3 1.7 2.2 2.1 3.6 3.2 5.8 5.7 5.2 5.4 9.6 9.6
ÿ0.01 ÿ0.29 0.08 ÿ0.09 0.67 0.61 0.47 0.52 0.38 0.02 ÿ0.28 0.67 0.56 0.49 0.80 0.66 0.50 0.48 0.79 0.66 0.84 0.00 ÿ0.03 0.54 0.45 0.19 0.23 0.44 0.53
327/68 326/64 327/68 296/29 094/39 269/16 274/21 277/10 267/37 265/33 267/37 207/23 213/08 205/18 212/12 175/71 062/14 076/18 063/21 144/80 061/14 235/80 222/86 027/82 021/85 271/61 264/57 283/64 280/63
083/10 081/12 088/12 054/40 262/50 057/72 059/65 007/02 086/53 083/57 068/52 071/59 102/69 062/67 084/71 234/01 188/67 210/66 200/62 078/5 187/66 069/10 074/03 240/07 242/04 073/28 071/32 061/20 062/22
177/19 176/23 182/18 182/36 359/06 177/9 179/13 108/79 177/00 175/01 170/09 306/19 305/20 300/13 305/15 298/14 327/18 340/16 326/18 332/16 326/19 339/02 344/02 150/04 152/04 167/08 165/06 157/16 158/16
a Legend: k=magnitude of the magnetic susceptibility; P%=percentage of anisotropy (Graham, 1966); T=shape parameter; kmax, kint, kmin=azimuth and plunge (in degrees) of the principal directions of AMS ellipsoid (kmax > kint > kmin); ]=microporphyritic dyke at site D8; SPP=dykes from the SaÄo Paulo do Potengi dam.
15 susceptibility measurements along dierent orientations (Jelinek, 1978). The magnitudes and orientations of the average AMS ellipsoid of a given site are denoted maximum (Kmax=magnetic lineation), intermediate (Kint) and minimum (Kmin=pole of foliation) mean principal directions.
tional data were analyzed using both the maximum density orientation and the tensorial statistic of Jelinek (1978), utilizing an updated version of the program of Lienert (1991). The fabric is considered well de®ned
4. Low-®eld susceptibility and anisotropy The mean susceptibility of a specimen is the arithmetical average km=1/3 (kmax+kint+kmin) and P=100(kmaxÿkmin)/kmax yields the percentage of anisotropy (Graham, 1966). The dykes have susceptibilities in the range of 10ÿ2 < k (SI) < 0.8 10ÿ1 (Table 1 and Fig. 4). These values of susceptibility are comparable to those obtained in the dykes of the other CMDS sub-swarms and have previously been attributed to titanomagnetite (Bucker et al., 1986; Bellieni et al., 1992). Despite the high susceptibility magnitudes, the anisotropy degree of the ma®c dykes is relatively low (P < 4% in 84% of the specimens), which is typical for dykes. There is no correlation between P and and the magnetic susceptibiliy (Fig. 4). AMS direc-
Fig. 4. Anisotropy percentage degree (P%) versus susceptibility (k) of basaltic dykes. n=number of specimens.
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when the largest semi-angle of the 95% con®dence ellipse is less than 308, and poorly de®ned otherwise. The AMS data (Table 1) can be divided into three fabric subsets (Fig. 5), as summarized below: . Type I (D1 and D2): Both planar and weakly linear AMS ellipsoids are equally common. The degree of anisotropy is low, not reaching 5.0%. The foliations are NW-trending and the lineations plunge gently to moderately to the NW and E. . Type II (D3, D4, D5 and D6): The fabric is dominantly planar, although D5 has a linear distribution. As in the Type I sub-fabric, the degree of anisotropy is usually low (P < 5.5%). The foliations dip N and NE and lineations plunge steeply towards the Santana Plateau (D3, D4, and D6). Site D5 shows dispersed kmin axis yielding an ill-de®ned foliation, although the mean direction (kmin) keeps the same orientation of the nearest dykes (D4 and D6). In D3, the con®dence ellipse around the mean lineation is greater than 308 but the individual kmax directions plunge steeply. . Type III (D7 and D8): The fabric is typically planar and the degree of anisotropy locally increases up to 10%. The E- to NE-trending foliations dip steeply. Lineations are subhorizontal (D7) or are scattered in the plane of foliation (D8) (Fig. 5). The well
Fig. 5. Equal area projections (Schmidt net, lower hemisphere) of the axes of principal susceptibilities de®ning three sub-fabric types. Figures in parentheses are numbers of samples; e1 and e3 (lower right) are the semi-angle of the 95% con®dence ellipse around, respectively, Kmax and Kmin.
de®ned planar magnetic fabric of site D8 was obtained from specimens of two E-trending parallel dykes, one microporphyritic and the other ®ne grained. At this site, the orientation of the magnetic fabric is typically bimodal: the microporphyritic dyke has well clustered kmax axes plunging gently to SW, whereas in the ®ne-grained dyke the kmax axes plunge steeply to gently in the plane of magnetic foliation (Table 1). A strongly oblate magnetic ellipsoid (0.48 < T < 0.84) combined with the dispersion of kmax axes led us to consider that the linear fabric of site D8 has no signi®cance.
5. Rock magnetism The Curie/NeÂel temperatures of the magnetic mineralogy were investigated using a CS-2 apparatus which, coupled with the KLY-2 bridge, measures the suscepti-
Fig. 6. Representative plots of thermomagnetic curves on air of the specimen D1 (A) and D7 (B). The drop of the susceptibility around 5508C is attributed to Ti-poor magnetite. An additional in¯ection point at ca. 3508C (D7) suggests the presence of maghemite (see text).
C.J. Archanjo et al. / Journal of South American Earth Sciences 13 (2000) 179±189
bility of a rock fragment at variable temperature (Hrouda, 1994). A 0.3±0.5 cm3 sample of powdered basalt is submitted to a cycle of heating up to 6858C and cooling to room temperature; the susceptibility is measured regularly at steps of 38C. For at least one sample per site, we investigated the thermal change of the magnetic susceptibility during the heating-cooling cycle. Hysteresis loop and isothermal remanent magnetization (IRM) acquisition curves were obtained using a vibrating sample magnetometer (VSM-Molspin). In specimens D1, D2, D5, D6, and D8 the susceptibility increases with temperature (Hopkinson eect) before dropping abruptly between 5358C and 5708C (Fig. 6A). The susceptibility is mostly reversible during cooling. The IRM acquisition curves show a saturation ®eld of approximately 200 mT, indicating that the susceptibility essentially comes from Ti-poor magnetite. The shallow positive slope of the saturated, high-®eld, linear portion of the initial hysteresis loop shows that the paramagnetic silicates and the highly coercive fraction, like hematite, contribute less than 1.2 % to the total susceptibility (Table 2). The thermomagnetic curves of specimens D4 and D7 are rather dierent from those of other specimens. On heating, D4 and D7 (Fig. 6B) display a small decrease of susceptibility between 3208C and 4408C, followed by an increase before dropping at 5508C. The cooling path is not reversible. The wave-like form of the curve disappears down to 4808C and the total susceptibility is reduced by about 25% at room temperature compared to initial values. These data suggest that specimen D7 contains Ti-poor magnetite and ®negrained maghemite (De Boer and Dekkers, 1996). The transition at around 3508C is attributed to the inversion of maghemite to hematite on heating. Hysteresis loops were measured at room temperature by subjecting a sample to an external ®eld (H) which is ramped from 0 to 1 T, then to ÿ1 T, then back to 0 T.
185
Saturation magnetization (Ms) is the point at which magnetization no longer increases with increasing external ®eld. Saturation remanent magnetization (Mrs) is the magnetization remaining after the external ®eld has been reduced to zero. When the direction of the external ®eld is reversed and increased in the negative direction, the magnetization will decrease to zero. The negative applied ®eld at which the induced magnetization is zero is the coercivity (Hc). The back ®eld required to bring the remanent magnetization to zero is the coercivity of remanence (Hrs). Hysteresis parameters re¯ect the domain state of the magnetic minerals present in the sample, which in turn are a function of grain volume. Relative grain-size determinations, and thus domain state, can be made by calculating the ratios of hysteresis parameters, Mrs/Ms and Hrs/Hc (Table 2), which are usually displayed on a Day plot (Fig. 7). Single domain (SD) magnetite grains have grain-size in the range 0.03 to 0.1 mm with Mrs/ Ms=0.5 (Day et al., 1976; Dunlop, 1986). Pseudosingle domain (PSD) magnetite grains have grain-size in the range 0.1 to 10 mm, and multidomain (MD) particles have grain-size larger than 10 mm with Mrs/Ms < 0.1. The ratios Hcr/Hc and Mrs/Mr of the Rio Ceara Mirim dykes show that the ferromagnetic fraction falls principally in the range of pseudo-single domain (PSD) grains (Fig. 7). The Mrs/Mr ratio varies between 0.30 and 0.12, below the transition between SD and PSD grains. These results indicate that the observed anomalous fabrics are not due to SD particles. 6. Discussion 6.1. Source of susceptibility and magnetic anisotropy The magnetic susceptibility of the ma®c dykes is
Table 2 Hysteresis properties of samples from the northeastern Brazil basaltic dykesa Specimen
Hcr (mT)
Hc (mT)
Mrs (mA.m2)
Ms (mA.m2)
Hcr/Hc
Mrs/Ms
kmat (%)
D1 D2 D3 D4 D5 D6 D7A D7B D8A D8B D8C
31.79 30.52 33.36 27.13 29.57 23.05 39.92 27.47 25.98 24.48 25.31
16.71 14.35 13.40 11.94 11.69 10.22 21.62 12.36 6.71 8.42 9.21
193.77 212.00 164.00 201.96 218.08 203.87 647.83 3.93 41.83 206.77 96.90
793.14 1390.73 1083.01 1220.90 1328.47 1548.16 2438.85 18.32 361.71 1509.51 663.47
1.90 2.13 2.49 2.27 2.53 2.25 1.85 2.22 3.87 2.91 2.75
0.24 0.15 0.15 0.17 0.16 0.13 0.27 0.21 0.12 0.14 0.15
0.6 0.8 1.1 0.7 0.9 0.9 0.6 0.4 1.0 0.6 0.7
a
Legend: Hcr=remanent coercivity; Hc=coercivity; Mrs=remanent saturation magnetization; Ms=saturation magnetization (values corrected for the matrix susceptibility); kmat=matrix (paramagnetic and antiferromagnetic) susceptibility percent calculated in the high ®eld, saturated portion of the hysteresis loop.
186
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principally carried by Ti-poor, PSD-magnetites, as shown by their rock magnetic properties. Secondary alteration of the original Ti-Fe oxides is evidenced by the presence of maghemite (g-Fe2O3), a typical product of low temperature alteration of magnetite, which inverts to hematite at a higher temperature. The presence of maghemite in some samples, however, does not have noticeable in¯uence on the magnetic fabric. In site D7, where maghemitization aected the rock, the AMS principal axes are well clustered and the fabric is a typically one with kmin perpendicular to the dyke wall. Although pyroxene and amphibole dominate the ma®c mineralogy of the basalts, the susceptibility of these paramagnetic silicates does not reach 1.2% of the bulk rock susceptibility, as shown by the nearly ¯at high-®eld hysteresis curve. Thus, the anisotropy of susceptibility of the dykes must be due to ®ne-grained titanomagnetites, as suggested by the high coercivity of the ferrimagnetic fraction observed in the hysteresis loop. The magnetic fabric pattern of the Rio CearaÂ-Mirim sub-swarm IV is very well de®ned both on each site or on a regional scale, even with the feeble anisotropy Ð typically under 4%. The plunge of lineations increases from Type I to Type II sub-fabrics, whereas the transition between the Types II and III sub-fabrics is sharp (Fig. 2). The latter presents the highest degree of anisotropy and more oblate AMS ellipsoids. The lineations become subhorizontal westward and nearly vertical in the central part of the swarm. They point to a focal area towards the eastern sector of the Santana Plateau. The lineations of Type III sub-fabrics are
Fig. 7. Mrs/Ms versus Hcr/Hc (hysteresis ratios) for samples from the Rio CearaÂ-Mirim dykes (sub-swarm IV), with boundaries of single domain (SD), pseudo-single domain (PSD), and multidomain (MD) behavior for magnetite taken from Day et al. (1977) and Dunlop (1986).
either nearly horizontal (D7) or show a complex pattern (D8). In the latter case, six of the eight kmax axes are subhorizontal, yet the statistical Kmax mean is steep, nearly vertical (Fig. 5). The associated kmin show a bimodal distribution, suggesting that two distinct ¯ow regimes may have been combined. A close inspection of the data from site D8 shows that the microporphyritic dyke has kmax axes grouped in the SW direction, whereas the kmax axes of the ®ne-grained dyke are scattered along a great circle perpendicular to the foliation. This seems to support the interpretation that a signi®cant component of magma ¯ow to the eastern extremity of the swarm was subhorizontal, although multiple periods of injection might result in a locally complex ¯ow pattern. The foliation is typically oblique to the trend of the dykes, principally at extremities of the swarm (sites D1 and D8, Fig. 2). The origin of such an oblique magnetic foliation is not very well understood, although the consistent AMS fabric pattern observed along the swarm suggests that this pattern is a primary magmatic feature. Imbricated fabrics due to ¯ow gradients near the dyke walls (Knight and Walker, 1988) or intersection of fabrics due to back¯ow (Philpotts and Asher, 1994) may be the source of these oblique magnetic fabrics. Oblique type AMS fabrics were also produced by numerical models using uniformly distributed, noninteracting and low aspect ratio magnetic particles ¯owing in a steady-state viscous ¯uid (Dragoni et al., 1997). Models of progressive simple shear or combined progressive simple shear and pure shear ¯ow of a magma in which grains can rotate freely predict obliquities between shape preferred orientation of grains of dierent aspect ratio (Fernandez, 1987; Ildefonse et al., 1992; Jezek et al., 1994). The fabric intensity and orientation will depend on the aspect ratio of individual particles and, at later stages in the crystallization of the magma, the degree of interaction among the particles (Ildefonse et al., 1992). For non-interacting particles with aspect ratio varying from 1.2 to 4.0, grains with smaller aspect ratio will have rotated most. In other words, if the magnetic fabric of basalts were formed only by shape anisotropy of discrete nearly cubic titanomagnetites, the AMS should vary rapidly in intensity and orientation from a specimen to another along a dyke. The clustering of the AMS principal directions of a single dyke taken from specimens separated from each other by tens of meters argues against such a model. Hence, the anisotropy of distribution, which depends on the degree of interaction of magnetic particles with a pre-exising silicate fabric, has been invoked to explain the origin of the magnetic fabric in basalts (Hargraves et al., 1991; Stephenson, 1994; CanÄoÂn-Tapia et al., 1996). As Hargraves et al. (1991) have pointed out, even if perfectly spherical
C.J. Archanjo et al. / Journal of South American Earth Sciences 13 (2000) 179±189
magnetically isotropic particles are present in a rock, the rock will not be isotropic if the particles are distributed anisotropically and are close enough to interact magnetically. They carried out experiments on isotropic rocks (and synthetics), indicating that magnetic anisotropy would form by a pre-existing template of silicates which itself would be controlled by ¯ow when the rock was formed. The anisotropy of distribution may be enhanced by post-emplacement, sub-solidus deuteric alteration that exsolves ilmenite from titanomagnetites leaving a strongly magnetic Ti-poor magnetite. Below the Curie temperature, magnetic interactions between neighboring grains may have additionally increased or lowered the anisotropy degree, depending on the shape of particles and the aligned or spatial con®guration of the grains (Archanjo et al., 1995; GreÂgoire et al., 1995). The scattered values of the anisotropy degree (Fig. 4) are thought to be a consequence of such interactions. 6.2. Regional implications Chemical and isotopic data have shown that the CMDS was extracted from a LIL-enriched mantle source (Bellieni et al., 1992), probably related to partial melting in the early rifting and break-up stages of the Gondwana continent. The time span of around 20 Ma for this event suggests that the intrusion of dykes occurred by following successive pulses of ma®c magma from a melting zone (Almeida et al., 1988). A model for magma in®lling cracks in brittle upper crust from a deep source is shown in Fig. 8. The fan-like ¯ow line pattern would be produced as the magma moves away from its source area and spreads laterally. The available regional gravimetric map shows a 80 km long, elliptical NE-trending negative Bouguer anomaly situated at the northeastern part of the Santana Plateau (Moreira et al., 1990). If such an anomaly were related to the source of dykes, the gravimetric low would delineate the remnants of a still warm paleo-
Fig. 8. Model for the acquisition of ¯ow structures of the Rio CearaÂ-Mirim dykes (sub-swarm IV). The ``paleo'' source region, presently located under Tertiary sandstones of the Santana plateau, would feed a dyke set in®lling an anastomosing E-trending crack system. The lineation trajectory would form a fan-like pattern converging to the magmatic feed zone.
187
melting zone rooted in the mantle. The magnetic fabric of a set of ma®c dykes in the SaÄo Paulo do Potengi dam situated to the northeast of the sub-swarm IV (Fig. 2) seems to supports such a hypothesis. Eight cores from dykes of decimeter to meter width yielded a well-de®ned mean lineation (largest semi-angle of 95% ellipse around Kmax, e1=198) plunging 728 to the west (Fig. 9) Ð i.e., pointing to the Bouguer gravimetric low. The mean magnetic foliation (largest semi-angle of 95% ellipse around Kmin, e3=98) dips steeply to NNW and is slightly oblique to the E-trending dyke wall. 7. Conclusions The AMS of Mesozoic ma®c dykes from northeastern Brazil display a well-de®ned magnetic fabric, with magnetic lineation that plunges steeply in the center of the swarm and magnetic foliation mainly parallel to oblique to the dyke wall. The planes of magnetic foliation have a curved trajectory, trending EW in the center of the swarm but rotating clockwise to a NW direction near the western extremity and counterclockwise to a NE direction near the eastern extremity. This simple pattern is consistent with the upward vertical ¯ow near the center of the swarm and outward ¯ows near the extremities. The fabric patterns suggest that
Fig. 9. Equal area projection of AMS principal directions of basaltic dykes from the SaÄo Paulo do Potengi dam (see Fig. 2 for location); n=number of specimens; e1 and e3 are the semi-angle of the 95% con®dence ellipse around, respectively, Kmax and Kmin.
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the ``paleo'' melting zone that fed the dykes might have been situated below Tertiary sediments covering the Precambrian basement. The principal directions of magnetic anisotropy are controlled by PSD-, Ti-poor magnetite. Maghemite was detected in some rocks but does not have appreciable in¯uence on the magnetic fabric. Hysteresis parameters indicate that AMS of the dykes is controlled by highly coercive ®ne-grained, Tipoor magnetite but SD grains, which could give rise to secondary fabric types, were not detected. Acknowledgements This work was supported by Fundac° aÄo de Amparo a Pesquisa do Estado de SaÄo Paulo (FAPESP, Project No. 98/4420-2), Conselho Nacional de Pesquisa (CNPq), Instituto de GeocieÃncias of the Universidade de SaÄo Paulo, and Departamento de Geologia of the Universidade Federal do Rio Grande do Norte. William MacDonald, Alexander Cruden, and an anonymous reviewer are thanked for their critical comments that improved this paper. References Almeida, F.F.M., Carneiro, C.D.R., Machado, D.L., Dehira, L.K., 1988. Magmatismo PoÂs-PaleozoÂico no Nordeste Oriental do Brasil. Revista Brasileira de GeocieÃncias 18, 451±462. Archanjo, C.J., Launeau, P., Bouchez, J.L., 1995. Magnetic fabric vs. magnetite and biotite shape fabrics of the magnetite-bearing granite pluton of Gameleiras (northeast Brazil). Physics of the Earth and Planetary Interiors 89, 63±75. Bellieni, G., Macedo, M.H.F., Petrini, R., Piccirillo, E.M., Cavazzini, G., Comin-Chiaramonti, P., Ernesto, M., Macedo, J.W.P., Martins, G., Mel®, A.J., Pacca, I.G., De Min, A., 1992. Evidence of magmatic activity related to Middle Jurassic and Lower Cretaceous rifting from northeastern Brazil (CearaÂMirim): K/Ar age, paleomagnetism, petrology and Sr-Nd isotope characteristics. Chemical Geology 97, 9±32. Bucker, C., Schult, A., Bloch, W., Guerreiro, S.D.C., 1986. Rockmagnetism and paleomagnetism of an Early Cretaceous/ Late Jurassic dyke swarm in Rio Grande do Norte, Brazil. Journal of Geophysics 60, 129±135. CanÄoÂn-Tapia, E., Walker, G.P.L., Herrero-Bervera, E., 1996. The internal structure of lava ¯ows Ð Insights from AMS measurements I: Near-vent a'a. Journal of Volcanology and Geothermal Research 70, 21±36. Day, R., Fuller, M., Schmidt, V.A., 1976. Magnetic hysteresis properties of synthetic titanomagnetites. Journal of Geophysical Research 81, 873±880. De Boer, C.B., Dekkers, M.J., 1996. Grain-size dependence of the rock magnetic properties for a natural maghemite. Geophysical Research Letters 23, 2815±2818. Dragoni, M., Lanza, R., Tallarico, A., 1997. Magnetic anisotropy produced by magma ¯ow: Theoretical model and experimental data from Ferrar dolerite sills (Antarctica). Geophysical Journal International 128, 230±240. Dunlop, D.J., 1986. Hysteresis properties of magnetite and their dependence on particle size: a test of pseudo-single-domain remanence models. Journal of Geophysical Research 91, 9569± 9584.
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Potassium-argon ages and strontium isotope geochemistry of Mesozoic and Tertiary basaltic rocks, northeastern Brazil. Academia Brasileira de CieÃncias 53, 115±122. Stephenson, A., 1994. Distribution anisotropy: Two simple models for magnetic lineation and foliation. Physics of the Earth and Planetary Interiors 82, 49±53.