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Magnetic fabric and rock-magnetic character of the Mesozoic flood basalts of the Paraná Basin, Brazil
Journal of Geodynamics 28 (1999) 419±437
Magnetic fabric and rock-magnetic character of the Mesozoic ¯ood basalts of the Parana Basin, Brazil Endale Tamrat*, Marcia Ernesto Instituto AstonoÃmico e GeofõÂsico, Universidade de SaÄo Paulo, Rua do MataÄo 1226, CEP 05508-900, SaÄo Paulo, SP, Brazil Received 13 September 1998; received in revised form 23 January 1999; accepted 18 February 1999
Abstract We have studied the magnetic fabric of Mesozoic basaltic ¯ows from ®ve sequences of the Parana Magmatic Province (PMP), southern Brazil, to infer paleo¯ow direction and to locate possible magma feeders. A well de®ned orientation pattern, indicative of the ¯ow direction has been evidenced by the low ®eld anisotropy of magnetic susceptibility (AMS). One sequence from the southern part of the basin, JS (27 ¯ows, 513 specimens), shows maximum AMS ellipsoid trending approximately NW±SE. Two sequences from the west±central part of the basin, IC (13 ¯ows, 173 specimens), and PA (17 ¯ows, 324 specimens), trend E±W. Two sequences from the south-eastern part of the basin, CV (24 ¯ows, 436 specimens) and BV (20 ¯ows, 103 specimens) show maximum AMS ellipsoid trends approximately NE± SW. In all cases the minimum axes of the AMS ellipsoids are tightly grouped vertically or sub-vertically to the bedding with a relatively weak degree of anisotropy, indicative of the primary origin of the magnetic fabric. Rock-magnetic parameters of some representative samples, such as isothermal remanent magnetization (IRM), high ®eld hysteresis loops and thermomagnetic curves suggests that the dominant magnetic mineral is a pseudo-single to small multi-domain grain size of magnetite. These and other observations are consistent with the conclusions that ¯ows emanating from dierent sources may align their maximum susceptibility directions parallel to drainage that channel the ¯ow or they will re¯ect regional pre¯ow topographic structure and magma-source distributions. # 1999 Elsevier Science Ltd. All rights reserved.
* Corresponding author. Fax: +55-11-818-5034. E-mail address: [email protected] (E. Tamrat) 0264-3707/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 4 - 3 7 0 7 ( 9 9 ) 0 0 0 1 9 - 8
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1. Introduction Anisotropy of magnetic susceptibility (AMS) is a fast and non-destructive way of investigating rock fabrics, which are too subtle to be seen or measured in the ®eld. For this reason, it has been widely used to determine quantitative, three dimensional petrofabrics (MacDonald and Ellwood, 1987; Borradaile, 1988; Jackson and Tauxe, 1991; Rochette et al., 1992; Tarling and Hrouda, 1993). Its principle relies on a physical property of rocks that re¯ects the magnetic fabric of a geological material due to a preferred alignment of anisotropic magnetic mineral. The AMS of rocks depends on the intrinsic anisotropy of the individual grains, their shape preferred orientation and their distribution (Stacey, 1960; Hargraves et al., 1991; Rochette et al., 1992). Although AMS can arise from a variety of causes, in titanomagnetite bearing rocks, it has been most commonly adiscribed to shape anisotropy, which re¯ects the statistical alignment of elongated or planar magnetic grains within a rock. The AMS is a second-rank tensor ellipsoid given by the length and orientation of the three principal susceptibility axes (k1 > k2 > k3, maximum, intermediate and minimum, respectively). In general k1, the maximum magnetic susceptibility direction, corresponds to the mineral lineation, and k3, minimum magnetic susceptibility direction, is taken as the pole to the foliation. In several studies on pyroclastic ¯ows (Ellwood, 1982; Knight et al., 1986; Baer et al., 1997), dikes (Knight and Walker, 1988; Ernst and Baragar, 1992; Puranen et al., 1992; Staudigel et al., 1992; Raposo and Ernesto, 1995; Raposo, 1997), and lava ¯ows (Hrouda, 1982; MacDonald et al., 1992; CanÄoÂn-Tapia et al., 1994, 1995, 1996) AMS proved to be a reliable indicator of ¯ow direction. Studies on lava ¯ows show that the maximum axis of the magnetic susceptibility (k1) is commonly parallel to the ¯ow direction. Departures from this pattern have been usually attributed to the eects of convection patterns or local turbulence within the ¯ow boundaries (Khan, 1962) or to the occurrence of either a secondary fabric or the predominance of single-domain titanomagnetite grains (Rochette et al., 1992). Dragoni et al. (1997) using data from a Ferrar dolerite sill (Antarctica) showed that dierent patterns of AMS axes may still represent primary magnetic fabric, when the sill is modeled as a steady-state ¯ow of a Bingham ¯uid driven by a pressure gradient. Glen et al. (1997) recently reported AMS results from lava ¯ows of the Parana Magmatic Province (PMP), Southern Brazil, and from the Etendeka Province in Namibia. These two provinces were juxtaposed and formed a single province (ParanaÂ-Etendeka Igneous Province, PEIP) before the South Atlantic opening. Glen et al. (1997) found AMS directions trending mainly NE±SW, which allowed them to conclude that the magma ¯uxes in these areas were channeled along NE structures developed along the Brazilian and African continental margins, as a consequence of the rifting processes. Since the PEIP magmatism preceded the sea ¯oor spreading at the corresponding latitudes, by about 5 Myr (magmatism peak age of 0132 Ma), the authors also suggested that the rifting process started before the emplacement of the PEIP lava. However, their results are limited to lava cropping out at the southeastern most fringe of the PMP. Considering the wide areal extent of this province, and its varying chemical characteristics (e.g. Piccirillo et al., 1988), a more representative and systematic investigation of AMS directions of ¯ow sequences is needed in order to infer paleo¯ow directions within the
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Fig. 1. Generalized geological map of the Parana Magmatic Province (PMP) in Brazil, showing the location of the sampled sections.
422 E. Tamrat, M. Ernesto / Journal of Geodynamics 28 (1999) 419±437 Fig. 2. Pro®le of the ®ve PMP lava sections sampled in this study, showing sample locations, ¯ow units, magnetic polarity zones of Ernesto and Pacca (1988), and AMS principal directions of the k1 and k2 azimuths which characterize the mean ¯ow direction of the lava at each sampled individual ¯ow unit.
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423
PMP, and to constrain the possible location of magma feeders within the basin. In this paper we report results of magnetic fabric investigations from ®ve lava sequences widely distributed over the basin. Since the relationship between petrofabric and AMS fabric depends on the physical properties of the rock or the composition of rock matrix (diamagnetic and/or paramagnetic) and the nature of the present ferromagnetic mineral species, as well as the textural relationships between minerals (Borradaile, 1988; Rochette et al., 1992), a detailed study of the magnetic mineralogy is a requisite for kinematic interpretation of the AMS fabrics. For this reason, detailed rock-magnetic investigations were carried out by measuring various rock-magnetic parameters.
2. Geological setting The PMP represents one of the world's largest volumes of Mesozoic continental ¯ood basalt, covering an area of about 1.2 106 km2, mainly located in southern Brazil (Fig. 1). Lesser volumes of these lavas are also found in Uruguay, Paraguay and Argentina. On the eastern fringes of the basin, clis expose over one kilometer of lava ¯ows. Exposed sections generally lack overlying strata, so the original thickness of the volcanic sequence is unconstrained as a result of the unknown amount of material subsequently eroded. The maximum observed thickness is about 1700 m in a borehole drilled in the center of the basin (Almeida, 1986). Detailed geological, paleomagnetic and geochemical studies (Bellieni et al., 1986; Ernesto and Pacca, 1988; Piccirillo et al., 1988) suggest an average thickness of about 10±20 m for a single lava ¯ow. The Parana lava ¯ows (Serra Geral Formation, SGF) overlie the Botucatu Formation (Jurassic±Cretaceous), which is composed of typical aeolian sandstones representing the top of the Gondwana sequences. In the northern part of the basin the volcanic rocks are overlain by Upper Cretaceous sediments, mainly the Bauru Formation. Tholeiitic basalts and basaltic andesites are the dominant rock type (>90% by volume); however, in the upper part of the volcanic succession signi®cant quantities of silicic rocks composed of acid ¯ows (rhyolite and/ or rhyodacite) occur in the eastern and southwestern fringes of the formation (Mel® et al., 1988; Bellieni et al., 1986; Piccirillo et al., 1988). Associated with these lava ¯ows, sills and dyke swarms are also abundant, of which the most prominent is the Ponta Grossa swarm (Fig. 1). Tectonic lineaments such as the Rio Uruguai and the Rio Piquiri lineaments (Fig. 1), cut the basin from the eastern edge to the center. Based on geological and petrological data Piccirillo et al. (1988) suggested a subdivision of the PMP into three regions, according to the distribution and nature of the dierent volcanic rock types. In the southern Parana Basin (SPB; south of the Rio Uruguai lineament), the lower parts of the volcanic suite are composed of tholeiitic basalts and andesi-basalts of low titanium content (<2 wt%), while the upper portions are essentially represented by acid ¯ows of rhyodacites and rhyolites (Palmas type) (CV, BV and JS sequences in Fig. 2). The northern Parana Basin (NPB; north of the Rio Piquiri lineament) is dominated by tholeiites with high titanium content (>2 wt.%), although some porphyritic rhyodacite ¯ows (Chapeco acid rocks) in the southeastern-most areas are also found. In the central Parana Basin (CPB) the volcanic
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suites are similar to those occurring in the SPB and NPB, and may include both Palmas and Chapeco acid rocks which concentrate towards the eastern edge of the basin (Fig. 2). Recently, a large number of 40Ar/39Ar age determinations for the PEIP have become available (Renne et al., 1992, 1996) showing that the entire province erupted in a very narrow age interval (mainly 133±132 Ma), although minor volcanic activity of slightly older or younger ages are also documented (Turner et al., 1994). For the SPB the mean age is 13321 Ma while the CPB is slightly younger, with ages near 132 Ma (Renne et al., 1992).
3. Sampling and measurement procedures Samples were collected for integrated paleomagnetic and rock magnetic studies from various sections along road cuts. Details of sampling and description of the sampling areas were reported by Ernesto and Pacca (1988) and are shown in Figs. 1 and 2. Generally, at least three oriented blocks were taken from each outcrop and several standard paleomagnetic cylindrical specimens (2.2 cm in height) were drilled in the laboratory by using an electrical powered rock drill. Blocks were oriented in the ®eld using both magnetic and sun compass whenever possible. For this study we chose ®ve ¯ow sections located in dierent parts of the basin; two are from the southeastern (S. SebastiaÄo do Caõ -Carlos Barbosa (CV) and Bento Gonc° alves± VeranoÂpolis (BV)), one from southwestern (Jaguari±Santiago (JS)) and two from centraleastern areas of the basin (Iguac° u River±Cascavel, (IC) and Francisco BeltraÂo±Realeza-Iguac° u River (PA)). Location of these sections and other geologically important sections of the Parana Basin are localized in Fig. 1. Both IC and PA sections are from the CPB, and are essentially built of tholeiitic rocks (Fig. 2). The other sequences, CV, BV and JS are from the SPB, and consists of several acid ¯ows (up to 200 m) of Palmas type on the top and sometimes intercalated with basic ¯ows (Fig. 2 and Table 1). The stratigraphic columns and previous paleomagnetic polarity interpretation is shown in Fig. 2. Recognition of individual ¯ows was performed by Ernesto and Pacca (1988) and Ernesto et al. (1990) by means of ®eld observations, geochemical analysis and paleomagnetic results. Additional ¯ows are included in Fig. 2 and Table 1 (numbers followed by letters) after reinterpretation of the previous data to the light of the AMS results, as will be discussed later. More than ten standard cylindrical specimens of 2.2 cm in length were prepared from each site (¯ow) giving a total of 94 sites and 1872 specimens. Total low ®eld bulk magnetic susceptibility and AMS was measured before any demagnetization steps by means of a Sapphire SI-1 magnetic susceptibility instrument and a Minispin magnetometer (Molspin Ltd.) at the Paleomagnetic Laboratory of the Instituto Astronomico e Geo®sico (IAG), University of SaÄo Paulo (USP). Some weak susceptibility samples and one specimen from each ¯ow of a sequence was re-measured by using a KLY-3 Kappabridge susceptibility bridge (AGICO) at Instituto de GeocieÃncias, USP. No signi®cant dierences were observed between the two data sets, on the basis of either intensity and shape of susceptibility ellipsoids or directions of their axes. The magnetic mineralogy responsible for the AMS magnetic fabric was investigated by means of high ®eld hysteresis loops in a Vibrating Sample Magnetometer (VSM, Molspin Ltd.). In these tests, selected samples of a ¯ow (total 131 specimens) were crushed, small sub-
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425
Table 1 Mean of AMS parameters for individual ¯ows from the ®ve PMP sequences (site numbering is from bottom to top, based on Ernesto and Pacca (1988)) Site
n/Na D1b
I1 b a c
D3b
I3b ac
k 2 s (10ÿ2 SI)d Le
Iguac° u River±Cascavel (IC) (53836'W, 25819 'S) IC-1 7/14 6.3 5.3 42:13 107.1 63.8 16:05 3.724 2 0.9 IC-2 13/13 83.5 6.1 06:04 209.3 79.7 07:03 24.189 2 1.2 IC-3 11/13 74.0 6.2 13:03 179.5 68.0 04:03 22.425 2 2.2 IC-4 14/24 231 18.1 23:16 105.7 60.6 30:11 5.491 2 1.7 IC-5 18/25 268.8 13.7 10:05 148.2 64.4 07:05 27.257 2 8.9 IC-6 6/15 200 6.0 28:08 303.5 65.8 57:12 8.127 2 2.7 IC-7 31/41 81.1 6.3 13:05 186.9 67.7 07:04 32.137 2 3.4 IC-8 12/12 68.3 4.0 14:05 236.1 86.0 17:08 33.134 2 2.6 IC-9 12/17 184.5 7.9 14:04 67.1 73.3 12:03 20.268 2 6.7 IC-10a 15/20 315 22.4 27:04 164.7 64.6 07:05 19.895 2 4.1 IC-10b 11/16 85.7 5.6 35:09 283.5 84.1 27:13 38.609 2 2.6 IC-11 11/13 339.6 2.3 18:11 191.2 87.3 29:15 12.098 2 2.9 IC-12 12/18 84.6 18.1 13:04 258.6 71.8 10:07 17.634 2 4.7 Francisco BeltraÂo±Realeza-Iguac° u River (PA) (53828 'W, 25845 'S) PA-1 18/22 59.3 2.8 41:14 172.5 82.9 10:04 12.076 2 4.7 PA-2 14/14 93.6 27.9 11:05 243.6 58.5 08:03 36.415 2 6.5 PA-3 29/38 351.5 3.0 12:06 259.7 31.6 19:09 20.891 2 2.2 PA-4 12/17 70.7 16.2 21:05 275.5 72.3 07:03 36.999 2 2.9 PA-5 15/19 78.4 5.0 16:06 344.7 36.6 21:13 32.982 2 6.5 PA-6 11/14 52.6 0.9 20:16 143.5 46.3 42:19 72.23 2 11.8 PA-7 15/21 5.9 4.4 07:04 96.9 13.2 14:05 7.069 2 0.8 PA-8 10/16 276.2 6.9 28:20 28.3 7.9 31:19 5.438 2 3.1 PA-9 5/18 284.6 9.5 19:08 170.3 67.8 33:11 1.580 2 0.2 PA-10 26/30 92.5 4.7 05:02 330.5 81.2 02:02 41.025 2 2.5 PA-11 7/10 179.4 61.2 80:12 358.0 28.8 36:15 29.994 2 5.3 PA-12 30/30 259.6 7.3 07:02 21.8 76.4 09:02 32.827 2 7.3 PA-13 21/23 328.8 24.8 16:07 218.5 36.9 33:10 12.163 2 4.0 PA-14 34/43 358.2 5.5 22:03 212.7 83.3 03:02 37.410 2 4.1 PA-15 10/15 278.3 2.2 25:10 181.1 72.7 26:13 11.183 2 7.7 PA-16 44/58 87.7 3.2 23:03 184.0 62.7 04:04 38.478 2 9.4 PA-17 23/23 48.4 23.9 10:03 238.8 65.7 05:02 13.736 2 5.7 Jaguari±Santiago (JS) (54827 'W, 29811 'S) JS-0 21/25 120.7 12.2 10:03 259.1 73.9 04:03 25.668 2 2.7 JS-1 17/17 354.3 15.3 32:22 110.6 58.2 32:20 14.942 2 2.4 JS-2 19/22 353.6 6.6 22:09 86.2 22.0 14:09 13.913 2 1.1 JS-3 37/41 340.2 14.7 12:04 82.3 38.8 41:07 15.758 2 3.7 JS-4 30/30 182.9 14.0 12:04 8.9 75.9 05:04 23.196 2 2.2 JS-5a 12/20 309.2 38.9 04:02 137.6 50.8 03:02 24.740 2 1.9 JS-5b 13/16 333.5 4.3 23:03 68.8 53.6 05:04 15.380 2 1.4 JS-5c 28/28 301.4 32.3 13:04 97.6 55.4 14:04 37.271 2 7.8 JS-6 39/39 173.0 68.6 09:06 38.6 15.3 06:04 41.161 2 20.8 JS-7 9/11 221.9 2.0 33:03 0.3 87.4 05:03 25.580 2 1.2 JS-8 25/30 132.9 5.8 07:03 4.6 80.7 09:03 14.933 2 1.5 JS-9 12/22 96.8 0.1 31:05 6.5 72.7 09:06 20.964 2 7.5
Ff
Pf
A (%)g B (%)g Flow typeh
1.002 1.015 1.002 1.001 1.005 1.004 1.004 1.010 1.004 1.002 1.006 1.002 1.004
1.001 1.013 1.001 1.000 1.006 1.002 1.010 1.005 1.005 1.009 1.010 1.002 1.008
1.003 1.029 1.002 1.002 1.011 1.006 1.013 1.016 1.009 1.011 1.016 1.004 1.012
0.25 2.15 0.20 0.15 0.79 0.50 0.84 1.28 0.65 0.64 1.09 0.30 0.79
0.10 0.14 0.20 0.01 ÿ0.09 0.20 ÿ0.49 0.41 ÿ0.10 ÿ0.69 ÿ0.38 0.01 ÿ0.39
BR BR BR BR BR BR AB AB BR AB AB AB AB
1.000 1.004 1.003 1.003 1.010 1.007 1.004 1.004 1.005 1.003 1.001 1.006 1.002 1.001 1.002 1.002 1.004
1.002 1.011 1.002 1.014 1.003 1.005 1.001 1.001 1.003 1.008 1.008 1.006 1.001 1.010 1.001 1.010 1.009
1.003 1.015 1.004 1.016 1.013 1.013 1.005 1.004 1.007 1.012 1.008 1.013 1.003 1.011 1.003 1.012 1.014
0.15 0.94 0.35 0.94 1.14 0.99 0.45 0.40 0.60 0.74 0.45 0.94 0.25 0.59 0.25 0.69 0.89
ÿ0.30 ÿ0.68 0.20 ÿ0.98 0.70 0.11 0.30 0.40 0.30 ÿ0.59 ÿ0.59 ÿ0.09 0.10 ÿ0.89 0.10 ÿ0.79 ÿ0.58
AB TB TR BR TB TB BR BR TR BR BR br br BR AB AB BR
1.003 1.001 1.001 1.003 1.002 1.013 1.005 1.008 1.008 1.001 1.005 1.003
1.003 1.001 1.003 1.000 1.013 1.007 1.010 1.007 1.020 1.006 1.002 1.007
1.006 1.002 1.004 1.003 1.016 1.020 1.015 1.015 1.028 1.008 1.007 1.010
0.45 0.01 Sill 0.15 0.01 AB 0.25 ÿ0.20 TB 0.30 0.30 AB 0.89 ÿ1.18 AB 1.62 0.61 AB 0.99 ÿ0.48 AB 1.14 0.11 AB 1.76 ÿ1.14 A±TB 0.45 ÿ0.59 RHL 0.60 0.30 RHL 0.64 ÿ0.39 RHL (continued on next page)
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E. Tamrat, M. Ernesto / Journal of Geodynamics 28 (1999) 419±437
Table 1 (continued ) Site
n/Na D1b
I1 b a c
D3b
I3b ac
k 2 s (10ÿ2 SI)d Le
JS-10 9/13 130.3 13.8 15:08 228.8 31.3 20:11 2.283 2 0.1 JS-11 21/25 104.5 10.1 16:11 341.5 71.9 25:11 8.110 2 3.1 JS-12a 15/19 317.0 12.8 0.9:04 106.9 75.3 06:04 3.794 2 1.1 JS-12b 10/11 187.5 10.8 18:12 352.1 78.9 19:03 3.515 2 0.1 JS-13a 9/9 344.6 2.9 09:06 81.7 67.7 07:06 2.613 2 0.3 JS-13b 32/32 261.0 4.7 08:01 164.2 54.9 05:01 2.562 2 0.3 JS-14a 23/25 151.3 9.1 03:02 245.7 22.8 05:02 3.576 2 1.3 JS-14b 19/25 256.5 4.9 05:03 98.0 84.7 10:04 2.697 2 0.3 JS-15 13/20 145.7 0.3 13:04 237.4 81.0 08:04 3.824 2 0.3 JS-16 31/31 278.2 35.2 27:18 100.9 54.8 19:18 2.389 2 0.4 JS-17a 20/24 349.3 19.7 11:08 171.8 70.2 15:07 2.789 2 0.6 JS-17b 11/12 318.2 13.7 05:03 162.3 74.0 13:04 5.979 2 1.8 JS-18 19/21 349.5 2.7 08:06 246.4 78.5 18:06 2.264 2 0.4 JS-19 12/12 300.1 6.8 44:08 109.6 83.1 09:08 3.250 2 0.9 JS-20 7/8 318.9 13.6 18:03 132.9 76.3 14:03 4.589 2 0.6 S. SebastiaÄo do CaõÂ -Carlos Barbosa (CV) (51830 'W, 29826 'S) CV-1 22/25 60.0 14.1 08:07 151.5 5.9 35:07 4.059 2 0.9 CV-2 8/11 226.7 8.1 12:05 94.2 78.2 42:08 8.460 2 1.9 CV-3 51/51 236.7 4.5 06:04 341.7 72.9 33:06 6.093 2 3.3 CV-4 18/18 333.5 12.9 58:04 190.2 74.1 05:03 5.441 2 0.8 CV-5 10/14 240.8 10.6 05:01 149.7 6.0 10:02 21.063 2 1.3 CV-6 18/18 230.8 46.3 47:20 115.2 22.4 21:17 3.479 2 1.9 CV-7 9/9 78.7 1.3 02:01 169.5 30.8 01:01 15.979 2 1.0 CV-8 33/39 265.0 2.1 33:10 359.3 64.3 21:10 13.258 2 8.1 CV-9 8/9 56.0 32.8 23:11 297.1 36.9 14:12 8.671 2 0.9 CV-10 13/13 255.3 2.8 05:05 162.0 49.6 18:04 25.407 2 3.4 CV-11 19/19 61.9 0.9 03:01 153.1 52.8 07:01 39.601 2 4.0 CV-12 10/14 41.4 10.1 15:05 290.8 63.3 18:04 21.820 2 1.4 CV-13 14/20 50.3 12.9 10:03 276.1 71.9 05:05 31.537 2 1.7 CV-14 11/13 55.3 11.6 16:08 288.7 71.1 09:06 11.187 2 2.7 CV-15 10/14 144.3 20.9 24:16 250.1 35.4 37:19 12.276 2 11.1 CV-16 12/17 272.8 3.7 13:01 171.0 72.3 12:02 9.311 2 1.0 CV-17 7/9 139.4 4.2 12:08 230.3 12.3 50:07 0.844 2 0.05 CV-18 11/14 158.8 14.3 12:02 358.4 74.9 06:02 24.781 2 1.0 CV-19 17/26 318.9 5.5 12:05 58.1 59.0 19:07 22.652 2 8.9 CV-20 18/25 216.7 45.0 10:04 348.0 33.5 22:03 32.802 2 4.5 CV-21 29/40 357.2 4.5 24:03 169.3 85.4 04:03 22.524 2 2.7 CV-22 17/18 91.6 15.0 11:03 305.0 72.2 19:02 13.909 2 3.0 CV-23 35/35 53.9 4.5 13:08 149.1 48.8 23:08 20.138 2 9.5 CV-24 36/42 60.2 5.5 08:03 166.0 70.4 46:07 19.454 2 5.9 Bento Gonc° alves±VeranoÂpolis (BV) (51833 'W, 29807 'S) BV-1 3/3 113.7 53.6 ± 341.6 26.3 ± 25.706 2 0.4 5/6 74.2 55.7 43:32 166.6 1.6 47:26 48.699 2 6.7 BV-2 BV-3 4/6 101.7 47.9 35:13 203.3 10.3 40:11 31.358 2 4.3 BV-4 3/3 220.9 24.2 18:14 20.3 64.4 26:04 3.788 2 0.4 BV-5 4/4 78.9 17.2 25:04 285.3 70.9 16:07 15.784 2 1.4 BV-6 8/8 72.1 4.6 05:04 215.8 84.3 05:03 29.845 2 2.3
Ff
Pf
A (%)g B (%)g Flow typeh
1.007 1.004 1.005 1.003 1.008 1.006 1.008 1.005 1.002 1.002 1.004 1.009 1.003 1.001 1.003
1.004 1.003 1.011 1.014 1.015 1.009 1.007 1.003 1.004 1.003 1.001 1.004 1.003 1.005 1.009
1.012 1.007 1.016 1.017 1.022 1.015 1.016 1.009 1.006 1.005 1.005 1.014 1.006 1.006 1.012
0.94 0.55 1.04 0.99 1.47 1.04 1.18 0.69 0.40 0.35 0.45 1.14 0.45 0.35 0.74
0.20 0.10 ÿ0.58 ÿ1.07 ÿ0.57 ÿ0.28 0.01 0.10 ÿ0.20 ÿ0.10 0.30 0.40 0.01 ÿ0.40 ÿ0.59
RHL RHL RHD RHD RHL RHL RHL RHL RHL RHL RHL RHL RHL RHL RHL
1.003 1.002 1.004 1.000 1.009 1.001 1.012 1.001 1.001 1.011 1.007 1.005 1.007 1.003 1.002 1.010 1.007 1.003 1.012 1.005 1.001 1.008 1.002 1.007
1.001 1.001 1.000 1.010 1.005 1.002 1.002 1.001 1.002 1.004 1.003 1.011 1.009 1.004 1.003 1.011 1.001 1.009 1.007 1.002 1.005 1.004 1.002 1.001
1.004 1.003 1.004 1.010 1.014 1.002 1.014 1.003 1.003 1.015 1.011 1.016 1.016 1.007 1.006 1.021 1.008 1.012 1.020 1.008 1.006 1.012 1.004 1.007
0.35 0.25 0.40 0.50 1.14 0.15 1.28 0.20 0.20 1.28 0.89 1.04 1.13 0.50 0.40 1.52 0.74 0.74 1.57 0.65 0.35 0.99 0.30 0.70
0.20 0.10 0.40 ÿ0.99 0.40 0.01 0.99 ÿ0.10 ÿ0.10 0.70 0.30 ÿ0.58 ÿ0.18 ÿ0.10 ÿ0.20 ÿ0.08 0.60 ÿ0.59 0.41 0.20 ÿ0.40 0.40 0.01 0.70
TB TB TB TB TB TB TB TB TB TB TB AB AB TB TB TB AB AB AB AB AB RHD RHD RHD
1.007 1.004 1.004 1.007 1.006 1.011
1.005 1.002 1.008 1.016 1.026 1.009
1.012 1.006 1.012 1.023 1.032 1.020
0.94 0.50 0.79 1.47 1.85 1.52
0.20 0.20 ÿ0.39 ÿ0.86 ÿ1.91 0.22
AB AB AB AB AB TB
E. Tamrat, M. Ernesto / Journal of Geodynamics 28 (1999) 419±437
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Table 1 (continued ) Site
n/Na D1b
BV-7 2/4 100.1 BV-8 4/4 217.7 BV-9 5/5 61.7 BV-10 2/2 55.5 BV-11 4/5 193.2 BV-12 5/5 53.8 BV-13 5/8 224.9 BV-14 9/9 207.5 BV-15 4/5 356.4 BV-16 12/12 343.8 BV-17 4/4 58 BV-18 3/5 334.5 BV-20 7/8 233.7 BV-21 10/13 91.1
I1 b a c
D3b
I3b ac
k 2 s (10ÿ2 SI)d Le
Ff
Pf
A (%)g B (%)g Flow typeh
32.3 2.5 30.7 25.6 5.7 23.0 12.9 14.3 13.9 23.4 5.3 3.3 13.3 9.8
345.3 311.3 299.3 173.5 297.0 309.7 21.3 307.8 111.4 239.4 283.5 72.26 39.7 310.3
33.6 55.7 42.0 44.4 67.1 29.9 76.0 35.1 59.6 29.9 82.4 67.0 76.3 77.5
33.943 2 2.1 23.215 2 1.7 12.981 2 0.6 6.806 2 0.03 15.700 2 1.2 24.634 2 8.2 28.461 2 1.7 13.932 2 0.6 42.013 2 5.7 8.233 2 0.5 20.975 2 0.5 8.216 2 0.1 19.384 2 0.7 11.249 2 1.1
1.025 1.003 1.001 1.005 1.004 1.004 1.008 1.001 1.010 1.003 1.005 1.001 1.015 1.006
1.027 1.007 1.010 1.009 1.008 1.011 1.027 1.006 1.029 1.004 1.009 1.001 1.019 1.010
1.41 0.55 0.99 0.65 0.60 0.89 2.25 0.50 2.29 0.25 0.6 0.10 1.13 0.69
± 14:05 08:04 ± 23:19 10:01 09:02 18:10 29:13 50:05 13:07 ± 34:01 06:02
± 49:13 06:01 ± 23:10 30:02 09:07 11:07 40:19 07:06 16:05 ± 02:01 04:02
1.002 1.004 1.010 1.004 1.004 1.007 1.019 1.004 1.018 1.001 1.004 1.001 1.004 1.004
ÿ2.23 0.10 0.99 ÿ0.10 0.01 0.30 1.10 0.20 0.72 ÿ0.20 ÿ0.10 0.10 ÿ1.07 ÿ0.19
AB RHD TB TB TB T±AB TB TB TB T±AB T±AB T±AB RHD RHD
a
n/N = number of specimens used in statistical mean calculation/number of measured specimens. D1, I1; D3, I3, declination and inclination means corresponding to maximum and minimum susceptibility axes. c a is the region of con®dence calculated using the linear approximation of Lienert (1991). d k = mean bulk magnetic susceptibility and its standard deviation (s). e L (k1/k2)=lineation. f F (k2/k3)=foliation and P (k1/k3)=degree of anisotropy. g A(%) and B(%) shows degree of anisotropy and magnetic fabric based on calculation of CanÄoÂn-Tapia et al, (1994). h Flow types are characterized as AB=Andesi-basalt; TB=Tholeiitic-basalt; BR=Basic rocks; RHY=Rhyolite; RHD=Rhyodacite and TR=Transitional basalts. b
samples of mass between 0.6±0.9 g, and were submitted to ®elds up to 1 tesla (T). This peak ®eld was sucient to saturate both titanomagnetite and titanomaghemite grains of all sizes, but not ®ne-grained haematite. Values of coercive force (Hc), saturation magnetization (Ms), and saturation remanence (Mrs) were determined for each hysteresis loop, after corrections for high-®eld paramagnetic and diamagnetic slopes. Coercivity of remanence (Hcr) was determined from back-®eld measurements. Isothermal remanent magnetization (IRM) was imparted to some of the selected samples before determining the hysteresis loops. Thermomagnetic runs were performed in air on a horizontal translation Curie balance making use of a cyclic ®eld that varied from 0.1 to 0.8 T, depending on the magnetization intensity of each measured sample. During these measurements which took 50±60 min, the samples were heated up to 7008C and cooled down to room temperature.
4. Results 4.1. Anisotropy of magnetic susceptibility Useful parameters to describe the degree of anisotropy or the shape of the AMS ellipsoid
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Fig. 3. Plots of (a) degree of anisotropy P as a function of low-®eld bulk magnetic susceptibility km and (b) the magnetic fabric (B parameter) as a function of the degree of anisotropy (A parameter). The A and B parameters are de®ned by CanÄoÂn-Tapia (1994) as A = 100{1ÿ[k3+k2)/2k1]} and B = 100[(k3ÿ2k2)/k1+1].
(Tarling and Hrouda, 1993; CanÄoÂn-Tapia, 1994) such as lineation (L=k1/k2), foliation (F=k2/ k3), the degree of anisotropy (P=k1/k3) or A parameter and the shape parameter B are given in Table 1. Bulk susceptibility was calculated as the mean of the three principal susceptibilities km=[k1+k2+k3]/3, and varied from 8 10ÿ2 to 100 10ÿ2 SI in all measured sites (Fig. 3(a) and Table 1). Two distinct groups of km values have been seen on those sites composed of basic and acid lava ¯ows. The highest values of km came from basic ¯ows (usually >10ÿ1 SI) and the lowest values are from acid ¯ows (between 8 10ÿ2 and 10 10ÿ2 SI) as shown in Fig. 3(a) of the JS and BV sections. The general trend of high susceptibility values may suggest a very weak contribution of either paramagnetic or diamagnetic minerals to AMS ellipsoid. The degree of anisotropy (P ), which characterizes the shape of the susceptibility ellipsoid ranges from 1.001 (0.1%) to 1.032 (3.2%), with an average of 1% (Table 1 and Fig. 3a). There is no linear relationship between km and P (Fig. 3(a)), P and L or P and F (not shown here). The parameter P is always very low, as expected for igneous rocks with primary magnetic fabric (Hrouda, 1982). Plots of the B(%) parameter against the A(%) parameter for the ®ve sections are shown in Fig. 3(b). The A parameter measures the degree of anisotropy ranging from 0% (isotropic, sphere) to 100% (maximum anisotropy, line segment), while the B parameter divides magnetic fabrics into two broad ®elds ranging from purely foliated (B=ÿ100%, oblate ellipsoid) to
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purely lineated (B=100%, prolate ellipsoid) (CanÄoÂn-Tapia, 1994). From Fig. 3(b) it is clear that almost all the investigated sites are preferentially dominated by oblate ellipsoids of revolution or magnetic foliation (B is between ÿ2.5 and 1%, Fig. 3(b)). Small values of the A parameter are found in most sites (A < 2.5%, with a mode of <1% Fig. 3(b)). The principal directions of susceptibility for each ¯ow were examined by using an updated version of Lienert (1991) program, which uses the Hext±Jelinek statistical analysis (Hext, 1963; Jelinek, 1978). Representative examples of the principal susceptibility axes for single ¯ow units are given in lower-hemisphere equal-area plots (Fig. 4) and values of the mean susceptibilities
Fig. 4. Lower hemisphere equal are projection of the AMS principal axes of selected ¯ow units of (a) IC, (b) PA, (c) CV, (d) BV, (e) JS and (f) JS. Flow means and the corresponding regions of 95% con®dence are calculated by using Lienert's (1991) programs. Squares, triangles, and circles indicate site mean k1 (maximum), k2 (intermediate) and k2 (maximum) directions, respectively.
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are presented in Table 1. In most cases within-¯ow means show good consistency (Fig. 4(a)± (e)). Tight groupings of vertical and sub-vertical k3 directions and nearly horizontal k1 directions are characteristic of most ¯ows, due to the dominance of magnetic foliation as suggested above. The grouping of the mean principal susceptibility axes in most cases ranges from moderate to excellent according to the threshold criteria of CanÄoÂn-Tapia et al. (1994). In some cases, as shown in Table 1 for the BV section, the size of the con®dence regions are large due to the small number of samples despite the fact that directions are exceptionally well clustered. For some ¯ows two or three distinct and well clustered groupings of AMS directions are identi®ed (e.g. Figs. 4(e) and 4(f) and ¯ow numbers followed by letters in Table 1). This is usually found in thick ¯ows (10 m or more) originally identi®ed by Ernesto and Pacca (1988), of which oriented blocks were taken from dierent parts of the ¯ow (some times from outcrops on both sides of the road). Because all of these sub groups show well clustered AMS directions it is dicult to decide whether they are indeed part of the same ¯ow or rather they represent dierent ¯ow units. CanÄoÂn-Tapia et al. (1994, 1995, 1996, 1997) reported systematic changes in the orientation of AMS directions as a function of relative position within a ¯ow. These changes have been used to infer absolute ¯ow directions without making any prior assumptions. Unfortunately, the lack of good control on the relative position of the blocks in our case prevents us from further constraining ¯ow directions by using these changes. Instead, we assume that the horizontal k1 axis is parallel to ¯ow direction. The tight clusters of the principal susceptibility axes from each block, together with the sub vertical orientation of k3 axis justify this assumption as discussed by CanÄoÂn-Tapia et al. (1997). Therefore, all the units satisfying these threshold criteria had a ¯ow direction associated to each block separately, and identi®ed in Table 1 by the site numbers followed by letters. 4.2. Magnetic mineralogy During experiments of isothermal remanent magnetization acquisition (IRM) all samples showed similar behavior. A fast initial increase in the acquired IRM (Fig. 5(a)) is suggestive of magnetite as the dominant magnetic carriers, since magnetite saturates at around 200 mT. Occasionally, a small amount of high-coercivity magnetic minerals are also present, indicating a contribution of maghemite or a low concentration of haematite. Values of Hcr, were calculated from back ®eld IRM measurements, fall in the range of 30±50 mT (Fig. 5(a)). Representative curves of high ®eld hysteresis loops for two samples are given in Fig. 5b. The reversibility of the curves between 0.2±0.4 T indicates that the contribution from high coercivity magnetic mineral (hematite) to the magnetization of these rocks is negligible. At high ®elds, the slope of the curve is due to the paramagnetic contribution and can be related to phyllosilicates evidenced during ore microscopic observations (Ernesto and Pacca, 1988). After correction for the paramagnetic contribution, the ratio of the hysteresis parameters (Mrs/Ms Vs Hcr/Hc) was plotted on Day's plot (Day et al., 1977). These ratios are commonly regarded as the best indicators of magnetic domain states and variations in grain-size for magnetite and titanomagnetite, ranging from 0.5 for uniaxial single-domain (SD) grains down to <0.02 for true multi-domain (MD) magnetite (Dunlop, 1981). Increasing of the Ti content in titanomagnetite-bearing rocks raises the MD/PSD transition value to 0.1 (Dunlop, 1981).
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Fig. 5. (a) Representative curves of isothermal remanent magnetization (IRM) and back ®eld remanence acquisition (Hcr) for selected samples of each section. (b) Hysteresis loops behavior during high ®eld measurements (c) ratio of hysteresis parameters following Day et al. (1977). SD, PSD, SP and MD indicate respectively magnetic grain-size ®elds of single-domain, pseudo-single domain, superparamagnetic and multi-domain. (d) Thermomagnetic (J2±T) curves between room temperature and 7008C, at a heating rate of 608C minÿ1. Sample BV01 shows a single high Curie temperature close to 5808C and, JS20 a two phase behavior at around 4208C and 5808C. Forward (back) arrows indicate heating (cooling) cycle.
Results for the ®ve PMP sections investigated here fall within the range of pseudo-single (PSD) to small MD magnetites and/or titanomagnetites (Fig. 5(c)). During high ®eld thermomagnetic (Js±T) measurements, ranging from room temperatures to the maximum Curie temperature of hematite (6808C), two categories of behaviors were identi®ed. The ®rst group (about 90% of the measured samples) have single high Curie temperatures of around 5808C and reversible Js±T curves (e.g. BV-01 in Fig. 5(d)). The major decrease in magnetization intensity and the tendency for it to go to zero above 6008C suggest that a magnetic mineralogy is dominated by magnetite or Ti-poor titanomagnetite. The second group consists of samples from the upper part of the hand sample or block (8%), and are characterized by a two phase behavior of the decrease in magnetization intensity (e.g. JS-20 in
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Fig. 5(d)). A ®rst loss of magnetization at lower Curie temperature (between 3508±4208C) followed by another at around 5808C. The two phase behavior might be characteristic of the coexistence of unoxidized Ti-poor magnetite and oxidized Ti-poor magnetite which has undergone low-temperature oxidation to submicron maghemite (De Boer and Dekkers, 1996). This oxidation to a higher coercivity magnetic mineral is evidenced by the cooling curve which shows a ®nal magnetization intensity too small when compared with initial magnetization (JS20 in Fig. 5(d)). A lack of reversibility is observed in two samples of the IC sequence during cooling (not shown here). This might indicate that magnetite has formed either from transformation of initially paramagnetic minerals (phyllosilicates), or from inversion of an original magnetic mineral (titanomaghemite) into a stronger magnetic mineral (magnetite). In summary, the magnetic properties of the studied lava ¯ows are mainly controlled by magnetites that may have been slightly maghemitized, through low-temperature oxidation or weathering. Based on this evidence, the results of the AMS measurements are interpreted to re¯ect the statistical alignments of non-equant small to large grains of magnetite, and the contribution from other types of minerals is neglected.
5. Discussions 5.1. Nature of AMS The magnetic fabrics in ferrimagnetic dominated rocks may be attributed to the shape alignment of grains (shape anisotropy), the alignment of magnetic domains (domain anisotropy) or to the deformation of the crystal lattice (stress-induced anisotropy) (Stacey, 1960; Bhathal, 1971; Kapicka, 1983). Among these, shape anisotropy is the dominant component of AMS in magnetite-bearing rocks, which may be of primary or secondary origin. Theoretically it was shown that, for magnetite bearing rocks with primary magnetic fabric, the maximum susceptibility axis (k1) is parallel to the longest axis of elongated MD grains, whereas the minimum susceptibility axis (k3) is parallel to the long axis of uniaxial SD grains (e.g. Tarling and Hrouda, 1993). Stephenson et al. (1986) has experimentally veri®ed this theoretical prediction using synthetic samples. Therefore, before interpreting the AMS result of the PMP lava ¯ows it is necessary to investigate the origin and nature of the magnetic fabric in the studied sequences. Based on the rock-magnetic results, high values of the bulk magnetic susceptibilities (Fig. 3(a)), identical behavior of IRM acquisition curves, the magnetic hysteresis loops and thermomagnetic measurements (Figs. 5(a), 5(b) and 5(d)), we concluded that the AMS is associated with a magnetic mineralogy dominated by magnetite and/or titanomagnetite grains. The combination of the hysteresis parameters Mrs/Ms vs Hcr/Hc shows that all studied samples fall within a grain-size range of PSD to small MD (Fig. 5(c)). Other magnetically susceptible phases in lava ¯ows, notably the rock matrix (paramagnetic and diamagnetic) and maghemites can be ruled out as a signi®cant contributor of the AMS. Most paramagnetic minerals such as biotite and hornblende have magnetic susceptibilities much less than 10ÿ3 SI (Borradaile, 1988; Rochette et al., 1992), roughly about two orders of magnitude lower than those observed in the PMP lava ¯ows (Fig. 3(a)). The small amounts of maghemite present in some ¯ows may be
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due to weathering and only aected the upper most part of the ¯ows. Specimens showing such behavior are not included in the forthcoming interpretation of the AMS. PSD and MD grains of magnetite and/or Ti-poor titanomagnetite dominate the magnetic fabrics of the whole section studied here. Therefore the AMS ellipsoid is presumably associated with a primary shape anisotropy of PSD and MD grains of magnetite or/and titanomagnetite. 5.2. AMS and ¯ow direction Primary magnetic fabric associated with magnetite or titanomagnetite can be caused by stresses set up during emplacement or cooling of the lava. Such an orientation is generally developed along the ¯ow or elongation direction of the magnetic grains according to both theoretical and experimental modeling (e.g. Tarling and Hrouda, 1993; Stephenson et al., 1986). However, it seems ambiguous which susceptibility axis (i.e. k1 or k2) might represent the lava ¯ow direction. Khan (1962) showed that the mean intermediate (k2) susceptibility was roughly parallel to the ¯ow direction and both k1 and k3 are normal to the ¯ow. Hrouda (1982) reported a good agreement between the direction of the maximum axis of susceptibility and the ¯ow direction only in the intermediate and not in the frontal parts of the ¯ow. On lava ¯ows produced by a laminar ¯ow agreement between k1 and observed lineations, in a case where we could measure both, were reported by MacDonald et al. (1992). In a similar study of the Xitle lava ¯ows of Mexico, CanÄoÂn-Tapia et al. (1995) has found either the mean maximum or the mean intermediate susceptibilities pointing in the same direction as the geologically inferred ¯ow direction. CanÄoÂn-Tapia et al. (1995) reconciled these apparent contradictory results by considering the way lava ¯ows move along the slope of a pre-existing terrain as well as the resistance that may be encountered by the ¯ow at the front of the lobe due to the formation of a rigid crust or the accumulation of debris. This may cause subsidiary lobes to form in directions at an angle of up to 908 with the main lobe. In addition, when inferring lava ¯ow directions local and regional topography and magma characteristics have to be taken into consideration. As a result, one might expect ¯ow directions to vary signi®cantly from one locality to another, or to change with time at a given locality. PMP represents a ®ssure magmatism made up of ¯at lying lava ¯ows which dip gently (not more than 58) towards the central axis of the basin, although locally most of the ¯ows seems to lie horizontally. Geological features (dykes cutting the ¯ow sequences or circular structures) are not normally seen, making it dicult to locate possible magma sources. Accordingly, the observed k1 inclinations are shallow in the majority of the ¯ows, being less than 108 in acid ¯ows and less than 208 in basic ¯ows (Table 1). Therefore, it is possible to estimate the mean lava ¯ow axis of each sequence by using the k1 azimuths (k1 declination). To infer this we plot mean direction of both k1 and k3 axes of the principal susceptibility of each sequence using lower hemisphere equal area density plots (Figs. 6(a)±(c)). In constructing these plots, we used data for k1 inclinations less than 308 as reported in Fig. 2 and shown in Table 1. In some parts of the sections, the plunges of k1 show either a subvertical inclination or imbrication relationship dierent from the majority of the sites (e.g. PA-8, JS-5a, CV-6, BV-01 or site numbers with crosses in Table 1 and Fig. 2). In such situations an interchange of k1 and k2 axes might be expected due to the very low degree of anisotropy (Fig. 3(a)), although k2 axes
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Fig. 6. Density contours of maximum (k1) and minimum (k2) susceptibility directions of the mean eigenvectors of (a) IC and PA, (b) CV and BV, and (c) JS sections. Arrows in k1 density contour indicate mean site declination of the k1 (maximum) direction which characterizes the mean lava ¯ow direction of each section.
show ¯ow direction similar to the general trend of the lava ¯ows determined from the k1 axis (Fig. 2 and Fig. 4(e)). In such cases, therefore, we considered the k2 axes as representing the lava ¯ow direction and included in the data set shown in Table 1 (those sites marked with crosses) and in deriving Fig. 6. As shown in Fig. 6, we could observe three dierent lava ¯ow directions inside the studied sections of the PMP. The two central Parana sections, IC and PA, are characterized by dominantly E±W trends of the k1 azimuths (Fig. 6(a)). Two of the southern sections, BV and CV, which are located closer to the eastern margin, trend mainly NE±SW (Fig. 6b); the southwestern most section JS, on the contrary, trends NW±SE (Fig. 6(c)). The dierence in ¯ow directions in central and southern PMP suggests a regional pattern of magma ¯ow directions rather than an overall pattern expected if a unique magma feeder is considered. Amaral et al. (1966) suggested that the PMP ¯ows could have been fed from megatectonic structures represented by the Parana river that runs parallel to the basin axis (Fig. 1). This hypothesis is partially in accordance with the observed E±W ¯ow trend in IC and PA sections. Furthermore, the tectonic lineaments (Rio Uruguai and Rio Piquiri, Fig. 1) that limit the central PMP could have conditioned the lava ¯uxes if they underwent strike-slip
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movements during the emplacement of the ¯ows. However, the low degree of anisotropy that characterizes the Parana ¯ows, do not indicate a tectonically induced AMS. The divergent trends observed in southern PMP, on the western (JS) and eastern (BV and CV) sections, suggest a topographic control on magma ¯uxes which seem to run radially starting from a higher area in the south, and dipping towards the Rio Parana and the Atlantic margin. Shallow structures, like the one associated with a geoid high in southern Brazil (Molina and Ussami, 1998, this volume) could have existed due to the anomalous thermal conditions of the lithosphere, with a long history of thermal and/or mechanical uplift (Hegarty et al., 1996). In this case our AMS result for the southern PMP suggest magma feeder locations that are distinct from those related to the younger magmatic activities that gave rise to the Ponta Grossa and FlorianoÂpolis dikes which places the magma feeders on the southeastern Atlantic margin (Raposo and Ernesto, 1995; Raposo, 1997). Indeed the uplift of the margins in the southern area of PMP and the overall ¯ow dipping toward the central axis of the basin may have aected the observed k1 inclinations, and the plunging orientation (direction) could change. In this case, ¯ows in southern PMP could have been originated near the Rio ParanaÂ. However, this needs further investigation.
6. Conclusions From both the AMS and rock magnetic data presented here, we can conclude the following: . PSD and small MD magnetite and titanomagnetite grains are the major contributors for the magnetic anisotropy reported here. . Three dierent trends in k1-azimuth of the lava ¯ow directions are observed along the PMP: * E±W trends in two sections from the central western part of the basin (IC and PA) * NE±SW trends in two sections from the south eastern part of the basin (CV and BV) * NW±SE trends in one section from the southern part of the basin (JS). . These major AMS trends are in accordance with the topographic settings and fracture patterns observed inside the Parana Basin and in the crystalline basements. Previous AMS studies on dike swarms and basaltic sequences revealed similar trends in agreement with this study. Therefore, it seems reasonable to associate a paleo-¯ow direction with existing pre¯ow topography and crystalline basement structure during emplacement of magmas. Further AMS studies from the northern and western part of the PMP will help to envisage the dominance trends in lava ¯ow direction and in inferring possible magma feeder location inside the Parana Basin.
Acknowledgements We are grateful to J. Marins for his help in preparing samples, and to R. Siqueira for the aid with the laboratory routines. We thank B. MacDonald, J. Glen, P. Renne and an anonymous reviewer for their helpful and valuable comments on the ®rst version of the
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manuscript. This investigation was supported by the Brazilian Agency for Scienti®c Research (FAPESP).
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Hargraves, R.B., Johnson, D., Chan, C.Y., 1991. Distribution anisotropy: the cause of AMS in igneous rocks? Geophys. Res. Lett. 18, 2193±2196. Hext, G.R., 1963. The estimation of second-order tensors, with related tests and designs. Biometrika 50, 353±373. Hrouda, F., 1982. Magnetic anisotropy of rocks and its application in geology and geophysics. Geophys. Surveys 5, 37±82. Jackson, M.J., Tauxe, L., 1991. Anisotropy of magnetic susceptibility and remanence: developments in the characterization of tectonic, sedimentary, and igneous fabric. Rev. Geophysics 29, 371±376. Jelinek, V., 1978. Statistical processing of anisotropy of magnetic susceptibility measured on groups of specimens. Studia Geophys. Geodeti. 22, 50±63. Kapicka, A., 1983. Irreversible changes of anisotropy of the magnetic susceptibility of rocks due to uniaxial pressure. J. Geophysics 53, 144±148. Khan, M.A., 1962. Anisotropy of magnetic susceptibility of some igneous and metamorphic rocks. J. Geophys. Res. 67, 2873±2885. Knight, M.D., Walker, G.P.L., 1988. Magma ¯ow directions in dikes of the Koolau complex Oahu, determined from magnetic fabric studies. J. Geophys. Res. 93, 4301±4319. Knight, M.D., Walker, G.P.L., Ellwood, B.B., Diehl, J.F., 1986. Stratigraphy, paleomagnetic and magnetic fabric of the Toba tus: Constraints on the source and eruptive styles. J. Geophys. Res. 91, 10,355±10,382. Lienert, B.R., 1991. Monte-Carlo simulation of errors in the anisotropy of magnetic susceptibility, a second-rank symmetric tensor. J. Geophys. Res. 96, 19,539±19,544. MacDonald, W.D., Ellwood, B.B., 1987. Anisotropy of magnetic susceptibility: sedimentological, igneous, and structural-tectonic applications. Rev. Geophysics 25, 905±909. MacDonald, W.D., Palmer, H.C., Hayatsu, A., 1992. Egan range volcanic complex, Nevada: geochronology, paleomagnetism and magnetic fabrics. Phys. Earth Planet. Int. 74, 109±126. Mel®, A.J., Piccirillo, E.M., Nardy, A.J.R., 1988. Geological and magmatic aspects of the Parana Basin-An Introduction. In: Piccirillo, E.M., Mel®, A.J. (Eds.), The Mesozoic Flood Volcanism of the Parana Basin: Petrogenetic and Geophys. Aspects. IAG-USP Press, SaÄo Paulo, Brazil, pp. 1±13. Molina, E.C., Ussami, N., 1999. The geoid in southeastern Brazil and adjacent region: new constraints on density distribution and thermal states of the lithospher. J. Geodynamics 28 357±374. Piccirillo, E.M., Mel®, A.J., Comin-Chiaramonti, P., Bellieni, G., Ernesto, M., Marques, L.S., Nardy, A.J.R., Pacca, I.G., Roisemberg, A., Stolfa, D., 1988. Continental ¯ood volcanism from the Parana Basin (Brazil). In: MacDougal, J.D. (Ed.), Continental Flood Basalts. Kluwer Academic, Dordrecht, Germany, pp. 195±238. Puranen, R., Pekkarinen, L.J., Pesonen, L.J., 1992. Interpretation of magnetic fabrics in the early Proterozoic diabase dykes of Keuruu, central Finland. Phys. Earth Planet. Int. 72, 68±82. Raposo, M.I.B., Ernesto, M., 1995. Anisotropy of magnetic susceptibility in the Ponta Grossa dyke swarm (Brazil) and its relationship with magma ¯ow direction. Phys. Earth Planet. Int. 87, 183±196. Raposo, M.I.B., 1997. Magnetic fabric and its signi®cance in the FlorianoÂpolis dyke swarm, Southern Brazil. Geophys. J. Int. 131, 159±170. Renne, P.R., Ernesto, M., Pacca, I.G., Coe, R.S., Glen, J., PreÂvot, M., Perrin, M., 1992. Rapid eruption of the Parana ¯ood volcanics, rifting of southern Gondwanaland and the Jurassic-Cretaceous boundary. Science 258, 975±979. Renne, P.R., Deckart, K., Ernesto, M., FeÂraud, G., Piccirillo, E.M., 1996. Age of the Ponta Grossa dike swarm (Brazil), and implications to Parana ¯ood volcanism. Earth Planet. Sci. Lett. 144, 199±211. Rochette, P., Jackson, M.J., Aubourg, C., 1992. Rock magnetism and the interpretation of anisotropy of magnetic susceptibility. Rev. Geophys. 30, 209±226. Stacey, F.D., 1960. Magnetic anisotropy of igneous rock. J. Geophys. Res. 65, 2429±2442. Staudigel, H., Gee, J., Varga, R., 1992. Shallow intrusive directions of sheeted dikes in the Troodos ophiolite: anisotropy of magnetic susceptibility and structural data. Geology 20, 841±844. Stephenson, A., Sadikun, S., Potter, D.K., 1986. A theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals. Geophys. J. R. astr. Soc. 84, 185±200. Tarling, D., Hrouda, F., 1993. The Magnetic Anisotropy of Rocks. Chapman and Hall, London, p. 217. Turner, S., Regelous, M., Kelley, S., Hawkesworth, C., Mantovani, M., 1994. Magmatism and continental break-up in the South Atlantic: high precision 40Ar±39Ar geochronology. Earth Planet. Sci. Lett. 121, 333±348.
Magnetic fabric and rock-magnetic character of the Mesozoic ¯ood basalts of the Parana Basin, Brazil Endale Tamrat*, Marcia Ernesto Instituto AstonoÃmico e GeofõÂsico, Universidade de SaÄo Paulo, Rua do MataÄo 1226, CEP 05508-900, SaÄo Paulo, SP, Brazil Received 13 September 1998; received in revised form 23 January 1999; accepted 18 February 1999
Abstract We have studied the magnetic fabric of Mesozoic basaltic ¯ows from ®ve sequences of the Parana Magmatic Province (PMP), southern Brazil, to infer paleo¯ow direction and to locate possible magma feeders. A well de®ned orientation pattern, indicative of the ¯ow direction has been evidenced by the low ®eld anisotropy of magnetic susceptibility (AMS). One sequence from the southern part of the basin, JS (27 ¯ows, 513 specimens), shows maximum AMS ellipsoid trending approximately NW±SE. Two sequences from the west±central part of the basin, IC (13 ¯ows, 173 specimens), and PA (17 ¯ows, 324 specimens), trend E±W. Two sequences from the south-eastern part of the basin, CV (24 ¯ows, 436 specimens) and BV (20 ¯ows, 103 specimens) show maximum AMS ellipsoid trends approximately NE± SW. In all cases the minimum axes of the AMS ellipsoids are tightly grouped vertically or sub-vertically to the bedding with a relatively weak degree of anisotropy, indicative of the primary origin of the magnetic fabric. Rock-magnetic parameters of some representative samples, such as isothermal remanent magnetization (IRM), high ®eld hysteresis loops and thermomagnetic curves suggests that the dominant magnetic mineral is a pseudo-single to small multi-domain grain size of magnetite. These and other observations are consistent with the conclusions that ¯ows emanating from dierent sources may align their maximum susceptibility directions parallel to drainage that channel the ¯ow or they will re¯ect regional pre¯ow topographic structure and magma-source distributions. # 1999 Elsevier Science Ltd. All rights reserved.
* Corresponding author. Fax: +55-11-818-5034. E-mail address: [email protected] (E. Tamrat) 0264-3707/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 4 - 3 7 0 7 ( 9 9 ) 0 0 0 1 9 - 8
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1. Introduction Anisotropy of magnetic susceptibility (AMS) is a fast and non-destructive way of investigating rock fabrics, which are too subtle to be seen or measured in the ®eld. For this reason, it has been widely used to determine quantitative, three dimensional petrofabrics (MacDonald and Ellwood, 1987; Borradaile, 1988; Jackson and Tauxe, 1991; Rochette et al., 1992; Tarling and Hrouda, 1993). Its principle relies on a physical property of rocks that re¯ects the magnetic fabric of a geological material due to a preferred alignment of anisotropic magnetic mineral. The AMS of rocks depends on the intrinsic anisotropy of the individual grains, their shape preferred orientation and their distribution (Stacey, 1960; Hargraves et al., 1991; Rochette et al., 1992). Although AMS can arise from a variety of causes, in titanomagnetite bearing rocks, it has been most commonly adiscribed to shape anisotropy, which re¯ects the statistical alignment of elongated or planar magnetic grains within a rock. The AMS is a second-rank tensor ellipsoid given by the length and orientation of the three principal susceptibility axes (k1 > k2 > k3, maximum, intermediate and minimum, respectively). In general k1, the maximum magnetic susceptibility direction, corresponds to the mineral lineation, and k3, minimum magnetic susceptibility direction, is taken as the pole to the foliation. In several studies on pyroclastic ¯ows (Ellwood, 1982; Knight et al., 1986; Baer et al., 1997), dikes (Knight and Walker, 1988; Ernst and Baragar, 1992; Puranen et al., 1992; Staudigel et al., 1992; Raposo and Ernesto, 1995; Raposo, 1997), and lava ¯ows (Hrouda, 1982; MacDonald et al., 1992; CanÄoÂn-Tapia et al., 1994, 1995, 1996) AMS proved to be a reliable indicator of ¯ow direction. Studies on lava ¯ows show that the maximum axis of the magnetic susceptibility (k1) is commonly parallel to the ¯ow direction. Departures from this pattern have been usually attributed to the eects of convection patterns or local turbulence within the ¯ow boundaries (Khan, 1962) or to the occurrence of either a secondary fabric or the predominance of single-domain titanomagnetite grains (Rochette et al., 1992). Dragoni et al. (1997) using data from a Ferrar dolerite sill (Antarctica) showed that dierent patterns of AMS axes may still represent primary magnetic fabric, when the sill is modeled as a steady-state ¯ow of a Bingham ¯uid driven by a pressure gradient. Glen et al. (1997) recently reported AMS results from lava ¯ows of the Parana Magmatic Province (PMP), Southern Brazil, and from the Etendeka Province in Namibia. These two provinces were juxtaposed and formed a single province (ParanaÂ-Etendeka Igneous Province, PEIP) before the South Atlantic opening. Glen et al. (1997) found AMS directions trending mainly NE±SW, which allowed them to conclude that the magma ¯uxes in these areas were channeled along NE structures developed along the Brazilian and African continental margins, as a consequence of the rifting processes. Since the PEIP magmatism preceded the sea ¯oor spreading at the corresponding latitudes, by about 5 Myr (magmatism peak age of 0132 Ma), the authors also suggested that the rifting process started before the emplacement of the PEIP lava. However, their results are limited to lava cropping out at the southeastern most fringe of the PMP. Considering the wide areal extent of this province, and its varying chemical characteristics (e.g. Piccirillo et al., 1988), a more representative and systematic investigation of AMS directions of ¯ow sequences is needed in order to infer paleo¯ow directions within the
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421
Fig. 1. Generalized geological map of the Parana Magmatic Province (PMP) in Brazil, showing the location of the sampled sections.
422 E. Tamrat, M. Ernesto / Journal of Geodynamics 28 (1999) 419±437 Fig. 2. Pro®le of the ®ve PMP lava sections sampled in this study, showing sample locations, ¯ow units, magnetic polarity zones of Ernesto and Pacca (1988), and AMS principal directions of the k1 and k2 azimuths which characterize the mean ¯ow direction of the lava at each sampled individual ¯ow unit.
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423
PMP, and to constrain the possible location of magma feeders within the basin. In this paper we report results of magnetic fabric investigations from ®ve lava sequences widely distributed over the basin. Since the relationship between petrofabric and AMS fabric depends on the physical properties of the rock or the composition of rock matrix (diamagnetic and/or paramagnetic) and the nature of the present ferromagnetic mineral species, as well as the textural relationships between minerals (Borradaile, 1988; Rochette et al., 1992), a detailed study of the magnetic mineralogy is a requisite for kinematic interpretation of the AMS fabrics. For this reason, detailed rock-magnetic investigations were carried out by measuring various rock-magnetic parameters.
2. Geological setting The PMP represents one of the world's largest volumes of Mesozoic continental ¯ood basalt, covering an area of about 1.2 106 km2, mainly located in southern Brazil (Fig. 1). Lesser volumes of these lavas are also found in Uruguay, Paraguay and Argentina. On the eastern fringes of the basin, clis expose over one kilometer of lava ¯ows. Exposed sections generally lack overlying strata, so the original thickness of the volcanic sequence is unconstrained as a result of the unknown amount of material subsequently eroded. The maximum observed thickness is about 1700 m in a borehole drilled in the center of the basin (Almeida, 1986). Detailed geological, paleomagnetic and geochemical studies (Bellieni et al., 1986; Ernesto and Pacca, 1988; Piccirillo et al., 1988) suggest an average thickness of about 10±20 m for a single lava ¯ow. The Parana lava ¯ows (Serra Geral Formation, SGF) overlie the Botucatu Formation (Jurassic±Cretaceous), which is composed of typical aeolian sandstones representing the top of the Gondwana sequences. In the northern part of the basin the volcanic rocks are overlain by Upper Cretaceous sediments, mainly the Bauru Formation. Tholeiitic basalts and basaltic andesites are the dominant rock type (>90% by volume); however, in the upper part of the volcanic succession signi®cant quantities of silicic rocks composed of acid ¯ows (rhyolite and/ or rhyodacite) occur in the eastern and southwestern fringes of the formation (Mel® et al., 1988; Bellieni et al., 1986; Piccirillo et al., 1988). Associated with these lava ¯ows, sills and dyke swarms are also abundant, of which the most prominent is the Ponta Grossa swarm (Fig. 1). Tectonic lineaments such as the Rio Uruguai and the Rio Piquiri lineaments (Fig. 1), cut the basin from the eastern edge to the center. Based on geological and petrological data Piccirillo et al. (1988) suggested a subdivision of the PMP into three regions, according to the distribution and nature of the dierent volcanic rock types. In the southern Parana Basin (SPB; south of the Rio Uruguai lineament), the lower parts of the volcanic suite are composed of tholeiitic basalts and andesi-basalts of low titanium content (<2 wt%), while the upper portions are essentially represented by acid ¯ows of rhyodacites and rhyolites (Palmas type) (CV, BV and JS sequences in Fig. 2). The northern Parana Basin (NPB; north of the Rio Piquiri lineament) is dominated by tholeiites with high titanium content (>2 wt.%), although some porphyritic rhyodacite ¯ows (Chapeco acid rocks) in the southeastern-most areas are also found. In the central Parana Basin (CPB) the volcanic
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suites are similar to those occurring in the SPB and NPB, and may include both Palmas and Chapeco acid rocks which concentrate towards the eastern edge of the basin (Fig. 2). Recently, a large number of 40Ar/39Ar age determinations for the PEIP have become available (Renne et al., 1992, 1996) showing that the entire province erupted in a very narrow age interval (mainly 133±132 Ma), although minor volcanic activity of slightly older or younger ages are also documented (Turner et al., 1994). For the SPB the mean age is 13321 Ma while the CPB is slightly younger, with ages near 132 Ma (Renne et al., 1992).
3. Sampling and measurement procedures Samples were collected for integrated paleomagnetic and rock magnetic studies from various sections along road cuts. Details of sampling and description of the sampling areas were reported by Ernesto and Pacca (1988) and are shown in Figs. 1 and 2. Generally, at least three oriented blocks were taken from each outcrop and several standard paleomagnetic cylindrical specimens (2.2 cm in height) were drilled in the laboratory by using an electrical powered rock drill. Blocks were oriented in the ®eld using both magnetic and sun compass whenever possible. For this study we chose ®ve ¯ow sections located in dierent parts of the basin; two are from the southeastern (S. SebastiaÄo do Caõ -Carlos Barbosa (CV) and Bento Gonc° alves± VeranoÂpolis (BV)), one from southwestern (Jaguari±Santiago (JS)) and two from centraleastern areas of the basin (Iguac° u River±Cascavel, (IC) and Francisco BeltraÂo±Realeza-Iguac° u River (PA)). Location of these sections and other geologically important sections of the Parana Basin are localized in Fig. 1. Both IC and PA sections are from the CPB, and are essentially built of tholeiitic rocks (Fig. 2). The other sequences, CV, BV and JS are from the SPB, and consists of several acid ¯ows (up to 200 m) of Palmas type on the top and sometimes intercalated with basic ¯ows (Fig. 2 and Table 1). The stratigraphic columns and previous paleomagnetic polarity interpretation is shown in Fig. 2. Recognition of individual ¯ows was performed by Ernesto and Pacca (1988) and Ernesto et al. (1990) by means of ®eld observations, geochemical analysis and paleomagnetic results. Additional ¯ows are included in Fig. 2 and Table 1 (numbers followed by letters) after reinterpretation of the previous data to the light of the AMS results, as will be discussed later. More than ten standard cylindrical specimens of 2.2 cm in length were prepared from each site (¯ow) giving a total of 94 sites and 1872 specimens. Total low ®eld bulk magnetic susceptibility and AMS was measured before any demagnetization steps by means of a Sapphire SI-1 magnetic susceptibility instrument and a Minispin magnetometer (Molspin Ltd.) at the Paleomagnetic Laboratory of the Instituto Astronomico e Geo®sico (IAG), University of SaÄo Paulo (USP). Some weak susceptibility samples and one specimen from each ¯ow of a sequence was re-measured by using a KLY-3 Kappabridge susceptibility bridge (AGICO) at Instituto de GeocieÃncias, USP. No signi®cant dierences were observed between the two data sets, on the basis of either intensity and shape of susceptibility ellipsoids or directions of their axes. The magnetic mineralogy responsible for the AMS magnetic fabric was investigated by means of high ®eld hysteresis loops in a Vibrating Sample Magnetometer (VSM, Molspin Ltd.). In these tests, selected samples of a ¯ow (total 131 specimens) were crushed, small sub-
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425
Table 1 Mean of AMS parameters for individual ¯ows from the ®ve PMP sequences (site numbering is from bottom to top, based on Ernesto and Pacca (1988)) Site
n/Na D1b
I1 b a c
D3b
I3b ac
k 2 s (10ÿ2 SI)d Le
Iguac° u River±Cascavel (IC) (53836'W, 25819 'S) IC-1 7/14 6.3 5.3 42:13 107.1 63.8 16:05 3.724 2 0.9 IC-2 13/13 83.5 6.1 06:04 209.3 79.7 07:03 24.189 2 1.2 IC-3 11/13 74.0 6.2 13:03 179.5 68.0 04:03 22.425 2 2.2 IC-4 14/24 231 18.1 23:16 105.7 60.6 30:11 5.491 2 1.7 IC-5 18/25 268.8 13.7 10:05 148.2 64.4 07:05 27.257 2 8.9 IC-6 6/15 200 6.0 28:08 303.5 65.8 57:12 8.127 2 2.7 IC-7 31/41 81.1 6.3 13:05 186.9 67.7 07:04 32.137 2 3.4 IC-8 12/12 68.3 4.0 14:05 236.1 86.0 17:08 33.134 2 2.6 IC-9 12/17 184.5 7.9 14:04 67.1 73.3 12:03 20.268 2 6.7 IC-10a 15/20 315 22.4 27:04 164.7 64.6 07:05 19.895 2 4.1 IC-10b 11/16 85.7 5.6 35:09 283.5 84.1 27:13 38.609 2 2.6 IC-11 11/13 339.6 2.3 18:11 191.2 87.3 29:15 12.098 2 2.9 IC-12 12/18 84.6 18.1 13:04 258.6 71.8 10:07 17.634 2 4.7 Francisco BeltraÂo±Realeza-Iguac° u River (PA) (53828 'W, 25845 'S) PA-1 18/22 59.3 2.8 41:14 172.5 82.9 10:04 12.076 2 4.7 PA-2 14/14 93.6 27.9 11:05 243.6 58.5 08:03 36.415 2 6.5 PA-3 29/38 351.5 3.0 12:06 259.7 31.6 19:09 20.891 2 2.2 PA-4 12/17 70.7 16.2 21:05 275.5 72.3 07:03 36.999 2 2.9 PA-5 15/19 78.4 5.0 16:06 344.7 36.6 21:13 32.982 2 6.5 PA-6 11/14 52.6 0.9 20:16 143.5 46.3 42:19 72.23 2 11.8 PA-7 15/21 5.9 4.4 07:04 96.9 13.2 14:05 7.069 2 0.8 PA-8 10/16 276.2 6.9 28:20 28.3 7.9 31:19 5.438 2 3.1 PA-9 5/18 284.6 9.5 19:08 170.3 67.8 33:11 1.580 2 0.2 PA-10 26/30 92.5 4.7 05:02 330.5 81.2 02:02 41.025 2 2.5 PA-11 7/10 179.4 61.2 80:12 358.0 28.8 36:15 29.994 2 5.3 PA-12 30/30 259.6 7.3 07:02 21.8 76.4 09:02 32.827 2 7.3 PA-13 21/23 328.8 24.8 16:07 218.5 36.9 33:10 12.163 2 4.0 PA-14 34/43 358.2 5.5 22:03 212.7 83.3 03:02 37.410 2 4.1 PA-15 10/15 278.3 2.2 25:10 181.1 72.7 26:13 11.183 2 7.7 PA-16 44/58 87.7 3.2 23:03 184.0 62.7 04:04 38.478 2 9.4 PA-17 23/23 48.4 23.9 10:03 238.8 65.7 05:02 13.736 2 5.7 Jaguari±Santiago (JS) (54827 'W, 29811 'S) JS-0 21/25 120.7 12.2 10:03 259.1 73.9 04:03 25.668 2 2.7 JS-1 17/17 354.3 15.3 32:22 110.6 58.2 32:20 14.942 2 2.4 JS-2 19/22 353.6 6.6 22:09 86.2 22.0 14:09 13.913 2 1.1 JS-3 37/41 340.2 14.7 12:04 82.3 38.8 41:07 15.758 2 3.7 JS-4 30/30 182.9 14.0 12:04 8.9 75.9 05:04 23.196 2 2.2 JS-5a 12/20 309.2 38.9 04:02 137.6 50.8 03:02 24.740 2 1.9 JS-5b 13/16 333.5 4.3 23:03 68.8 53.6 05:04 15.380 2 1.4 JS-5c 28/28 301.4 32.3 13:04 97.6 55.4 14:04 37.271 2 7.8 JS-6 39/39 173.0 68.6 09:06 38.6 15.3 06:04 41.161 2 20.8 JS-7 9/11 221.9 2.0 33:03 0.3 87.4 05:03 25.580 2 1.2 JS-8 25/30 132.9 5.8 07:03 4.6 80.7 09:03 14.933 2 1.5 JS-9 12/22 96.8 0.1 31:05 6.5 72.7 09:06 20.964 2 7.5
Ff
Pf
A (%)g B (%)g Flow typeh
1.002 1.015 1.002 1.001 1.005 1.004 1.004 1.010 1.004 1.002 1.006 1.002 1.004
1.001 1.013 1.001 1.000 1.006 1.002 1.010 1.005 1.005 1.009 1.010 1.002 1.008
1.003 1.029 1.002 1.002 1.011 1.006 1.013 1.016 1.009 1.011 1.016 1.004 1.012
0.25 2.15 0.20 0.15 0.79 0.50 0.84 1.28 0.65 0.64 1.09 0.30 0.79
0.10 0.14 0.20 0.01 ÿ0.09 0.20 ÿ0.49 0.41 ÿ0.10 ÿ0.69 ÿ0.38 0.01 ÿ0.39
BR BR BR BR BR BR AB AB BR AB AB AB AB
1.000 1.004 1.003 1.003 1.010 1.007 1.004 1.004 1.005 1.003 1.001 1.006 1.002 1.001 1.002 1.002 1.004
1.002 1.011 1.002 1.014 1.003 1.005 1.001 1.001 1.003 1.008 1.008 1.006 1.001 1.010 1.001 1.010 1.009
1.003 1.015 1.004 1.016 1.013 1.013 1.005 1.004 1.007 1.012 1.008 1.013 1.003 1.011 1.003 1.012 1.014
0.15 0.94 0.35 0.94 1.14 0.99 0.45 0.40 0.60 0.74 0.45 0.94 0.25 0.59 0.25 0.69 0.89
ÿ0.30 ÿ0.68 0.20 ÿ0.98 0.70 0.11 0.30 0.40 0.30 ÿ0.59 ÿ0.59 ÿ0.09 0.10 ÿ0.89 0.10 ÿ0.79 ÿ0.58
AB TB TR BR TB TB BR BR TR BR BR br br BR AB AB BR
1.003 1.001 1.001 1.003 1.002 1.013 1.005 1.008 1.008 1.001 1.005 1.003
1.003 1.001 1.003 1.000 1.013 1.007 1.010 1.007 1.020 1.006 1.002 1.007
1.006 1.002 1.004 1.003 1.016 1.020 1.015 1.015 1.028 1.008 1.007 1.010
0.45 0.01 Sill 0.15 0.01 AB 0.25 ÿ0.20 TB 0.30 0.30 AB 0.89 ÿ1.18 AB 1.62 0.61 AB 0.99 ÿ0.48 AB 1.14 0.11 AB 1.76 ÿ1.14 A±TB 0.45 ÿ0.59 RHL 0.60 0.30 RHL 0.64 ÿ0.39 RHL (continued on next page)
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Table 1 (continued ) Site
n/Na D1b
I1 b a c
D3b
I3b ac
k 2 s (10ÿ2 SI)d Le
JS-10 9/13 130.3 13.8 15:08 228.8 31.3 20:11 2.283 2 0.1 JS-11 21/25 104.5 10.1 16:11 341.5 71.9 25:11 8.110 2 3.1 JS-12a 15/19 317.0 12.8 0.9:04 106.9 75.3 06:04 3.794 2 1.1 JS-12b 10/11 187.5 10.8 18:12 352.1 78.9 19:03 3.515 2 0.1 JS-13a 9/9 344.6 2.9 09:06 81.7 67.7 07:06 2.613 2 0.3 JS-13b 32/32 261.0 4.7 08:01 164.2 54.9 05:01 2.562 2 0.3 JS-14a 23/25 151.3 9.1 03:02 245.7 22.8 05:02 3.576 2 1.3 JS-14b 19/25 256.5 4.9 05:03 98.0 84.7 10:04 2.697 2 0.3 JS-15 13/20 145.7 0.3 13:04 237.4 81.0 08:04 3.824 2 0.3 JS-16 31/31 278.2 35.2 27:18 100.9 54.8 19:18 2.389 2 0.4 JS-17a 20/24 349.3 19.7 11:08 171.8 70.2 15:07 2.789 2 0.6 JS-17b 11/12 318.2 13.7 05:03 162.3 74.0 13:04 5.979 2 1.8 JS-18 19/21 349.5 2.7 08:06 246.4 78.5 18:06 2.264 2 0.4 JS-19 12/12 300.1 6.8 44:08 109.6 83.1 09:08 3.250 2 0.9 JS-20 7/8 318.9 13.6 18:03 132.9 76.3 14:03 4.589 2 0.6 S. SebastiaÄo do CaõÂ -Carlos Barbosa (CV) (51830 'W, 29826 'S) CV-1 22/25 60.0 14.1 08:07 151.5 5.9 35:07 4.059 2 0.9 CV-2 8/11 226.7 8.1 12:05 94.2 78.2 42:08 8.460 2 1.9 CV-3 51/51 236.7 4.5 06:04 341.7 72.9 33:06 6.093 2 3.3 CV-4 18/18 333.5 12.9 58:04 190.2 74.1 05:03 5.441 2 0.8 CV-5 10/14 240.8 10.6 05:01 149.7 6.0 10:02 21.063 2 1.3 CV-6 18/18 230.8 46.3 47:20 115.2 22.4 21:17 3.479 2 1.9 CV-7 9/9 78.7 1.3 02:01 169.5 30.8 01:01 15.979 2 1.0 CV-8 33/39 265.0 2.1 33:10 359.3 64.3 21:10 13.258 2 8.1 CV-9 8/9 56.0 32.8 23:11 297.1 36.9 14:12 8.671 2 0.9 CV-10 13/13 255.3 2.8 05:05 162.0 49.6 18:04 25.407 2 3.4 CV-11 19/19 61.9 0.9 03:01 153.1 52.8 07:01 39.601 2 4.0 CV-12 10/14 41.4 10.1 15:05 290.8 63.3 18:04 21.820 2 1.4 CV-13 14/20 50.3 12.9 10:03 276.1 71.9 05:05 31.537 2 1.7 CV-14 11/13 55.3 11.6 16:08 288.7 71.1 09:06 11.187 2 2.7 CV-15 10/14 144.3 20.9 24:16 250.1 35.4 37:19 12.276 2 11.1 CV-16 12/17 272.8 3.7 13:01 171.0 72.3 12:02 9.311 2 1.0 CV-17 7/9 139.4 4.2 12:08 230.3 12.3 50:07 0.844 2 0.05 CV-18 11/14 158.8 14.3 12:02 358.4 74.9 06:02 24.781 2 1.0 CV-19 17/26 318.9 5.5 12:05 58.1 59.0 19:07 22.652 2 8.9 CV-20 18/25 216.7 45.0 10:04 348.0 33.5 22:03 32.802 2 4.5 CV-21 29/40 357.2 4.5 24:03 169.3 85.4 04:03 22.524 2 2.7 CV-22 17/18 91.6 15.0 11:03 305.0 72.2 19:02 13.909 2 3.0 CV-23 35/35 53.9 4.5 13:08 149.1 48.8 23:08 20.138 2 9.5 CV-24 36/42 60.2 5.5 08:03 166.0 70.4 46:07 19.454 2 5.9 Bento Gonc° alves±VeranoÂpolis (BV) (51833 'W, 29807 'S) BV-1 3/3 113.7 53.6 ± 341.6 26.3 ± 25.706 2 0.4 5/6 74.2 55.7 43:32 166.6 1.6 47:26 48.699 2 6.7 BV-2 BV-3 4/6 101.7 47.9 35:13 203.3 10.3 40:11 31.358 2 4.3 BV-4 3/3 220.9 24.2 18:14 20.3 64.4 26:04 3.788 2 0.4 BV-5 4/4 78.9 17.2 25:04 285.3 70.9 16:07 15.784 2 1.4 BV-6 8/8 72.1 4.6 05:04 215.8 84.3 05:03 29.845 2 2.3
Ff
Pf
A (%)g B (%)g Flow typeh
1.007 1.004 1.005 1.003 1.008 1.006 1.008 1.005 1.002 1.002 1.004 1.009 1.003 1.001 1.003
1.004 1.003 1.011 1.014 1.015 1.009 1.007 1.003 1.004 1.003 1.001 1.004 1.003 1.005 1.009
1.012 1.007 1.016 1.017 1.022 1.015 1.016 1.009 1.006 1.005 1.005 1.014 1.006 1.006 1.012
0.94 0.55 1.04 0.99 1.47 1.04 1.18 0.69 0.40 0.35 0.45 1.14 0.45 0.35 0.74
0.20 0.10 ÿ0.58 ÿ1.07 ÿ0.57 ÿ0.28 0.01 0.10 ÿ0.20 ÿ0.10 0.30 0.40 0.01 ÿ0.40 ÿ0.59
RHL RHL RHD RHD RHL RHL RHL RHL RHL RHL RHL RHL RHL RHL RHL
1.003 1.002 1.004 1.000 1.009 1.001 1.012 1.001 1.001 1.011 1.007 1.005 1.007 1.003 1.002 1.010 1.007 1.003 1.012 1.005 1.001 1.008 1.002 1.007
1.001 1.001 1.000 1.010 1.005 1.002 1.002 1.001 1.002 1.004 1.003 1.011 1.009 1.004 1.003 1.011 1.001 1.009 1.007 1.002 1.005 1.004 1.002 1.001
1.004 1.003 1.004 1.010 1.014 1.002 1.014 1.003 1.003 1.015 1.011 1.016 1.016 1.007 1.006 1.021 1.008 1.012 1.020 1.008 1.006 1.012 1.004 1.007
0.35 0.25 0.40 0.50 1.14 0.15 1.28 0.20 0.20 1.28 0.89 1.04 1.13 0.50 0.40 1.52 0.74 0.74 1.57 0.65 0.35 0.99 0.30 0.70
0.20 0.10 0.40 ÿ0.99 0.40 0.01 0.99 ÿ0.10 ÿ0.10 0.70 0.30 ÿ0.58 ÿ0.18 ÿ0.10 ÿ0.20 ÿ0.08 0.60 ÿ0.59 0.41 0.20 ÿ0.40 0.40 0.01 0.70
TB TB TB TB TB TB TB TB TB TB TB AB AB TB TB TB AB AB AB AB AB RHD RHD RHD
1.007 1.004 1.004 1.007 1.006 1.011
1.005 1.002 1.008 1.016 1.026 1.009
1.012 1.006 1.012 1.023 1.032 1.020
0.94 0.50 0.79 1.47 1.85 1.52
0.20 0.20 ÿ0.39 ÿ0.86 ÿ1.91 0.22
AB AB AB AB AB TB
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Table 1 (continued ) Site
n/Na D1b
BV-7 2/4 100.1 BV-8 4/4 217.7 BV-9 5/5 61.7 BV-10 2/2 55.5 BV-11 4/5 193.2 BV-12 5/5 53.8 BV-13 5/8 224.9 BV-14 9/9 207.5 BV-15 4/5 356.4 BV-16 12/12 343.8 BV-17 4/4 58 BV-18 3/5 334.5 BV-20 7/8 233.7 BV-21 10/13 91.1
I1 b a c
D3b
I3b ac
k 2 s (10ÿ2 SI)d Le
Ff
Pf
A (%)g B (%)g Flow typeh
32.3 2.5 30.7 25.6 5.7 23.0 12.9 14.3 13.9 23.4 5.3 3.3 13.3 9.8
345.3 311.3 299.3 173.5 297.0 309.7 21.3 307.8 111.4 239.4 283.5 72.26 39.7 310.3
33.6 55.7 42.0 44.4 67.1 29.9 76.0 35.1 59.6 29.9 82.4 67.0 76.3 77.5
33.943 2 2.1 23.215 2 1.7 12.981 2 0.6 6.806 2 0.03 15.700 2 1.2 24.634 2 8.2 28.461 2 1.7 13.932 2 0.6 42.013 2 5.7 8.233 2 0.5 20.975 2 0.5 8.216 2 0.1 19.384 2 0.7 11.249 2 1.1
1.025 1.003 1.001 1.005 1.004 1.004 1.008 1.001 1.010 1.003 1.005 1.001 1.015 1.006
1.027 1.007 1.010 1.009 1.008 1.011 1.027 1.006 1.029 1.004 1.009 1.001 1.019 1.010
1.41 0.55 0.99 0.65 0.60 0.89 2.25 0.50 2.29 0.25 0.6 0.10 1.13 0.69
± 14:05 08:04 ± 23:19 10:01 09:02 18:10 29:13 50:05 13:07 ± 34:01 06:02
± 49:13 06:01 ± 23:10 30:02 09:07 11:07 40:19 07:06 16:05 ± 02:01 04:02
1.002 1.004 1.010 1.004 1.004 1.007 1.019 1.004 1.018 1.001 1.004 1.001 1.004 1.004
ÿ2.23 0.10 0.99 ÿ0.10 0.01 0.30 1.10 0.20 0.72 ÿ0.20 ÿ0.10 0.10 ÿ1.07 ÿ0.19
AB RHD TB TB TB T±AB TB TB TB T±AB T±AB T±AB RHD RHD
a
n/N = number of specimens used in statistical mean calculation/number of measured specimens. D1, I1; D3, I3, declination and inclination means corresponding to maximum and minimum susceptibility axes. c a is the region of con®dence calculated using the linear approximation of Lienert (1991). d k = mean bulk magnetic susceptibility and its standard deviation (s). e L (k1/k2)=lineation. f F (k2/k3)=foliation and P (k1/k3)=degree of anisotropy. g A(%) and B(%) shows degree of anisotropy and magnetic fabric based on calculation of CanÄoÂn-Tapia et al, (1994). h Flow types are characterized as AB=Andesi-basalt; TB=Tholeiitic-basalt; BR=Basic rocks; RHY=Rhyolite; RHD=Rhyodacite and TR=Transitional basalts. b
samples of mass between 0.6±0.9 g, and were submitted to ®elds up to 1 tesla (T). This peak ®eld was sucient to saturate both titanomagnetite and titanomaghemite grains of all sizes, but not ®ne-grained haematite. Values of coercive force (Hc), saturation magnetization (Ms), and saturation remanence (Mrs) were determined for each hysteresis loop, after corrections for high-®eld paramagnetic and diamagnetic slopes. Coercivity of remanence (Hcr) was determined from back-®eld measurements. Isothermal remanent magnetization (IRM) was imparted to some of the selected samples before determining the hysteresis loops. Thermomagnetic runs were performed in air on a horizontal translation Curie balance making use of a cyclic ®eld that varied from 0.1 to 0.8 T, depending on the magnetization intensity of each measured sample. During these measurements which took 50±60 min, the samples were heated up to 7008C and cooled down to room temperature.
4. Results 4.1. Anisotropy of magnetic susceptibility Useful parameters to describe the degree of anisotropy or the shape of the AMS ellipsoid
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Fig. 3. Plots of (a) degree of anisotropy P as a function of low-®eld bulk magnetic susceptibility km and (b) the magnetic fabric (B parameter) as a function of the degree of anisotropy (A parameter). The A and B parameters are de®ned by CanÄoÂn-Tapia (1994) as A = 100{1ÿ[k3+k2)/2k1]} and B = 100[(k3ÿ2k2)/k1+1].
(Tarling and Hrouda, 1993; CanÄoÂn-Tapia, 1994) such as lineation (L=k1/k2), foliation (F=k2/ k3), the degree of anisotropy (P=k1/k3) or A parameter and the shape parameter B are given in Table 1. Bulk susceptibility was calculated as the mean of the three principal susceptibilities km=[k1+k2+k3]/3, and varied from 8 10ÿ2 to 100 10ÿ2 SI in all measured sites (Fig. 3(a) and Table 1). Two distinct groups of km values have been seen on those sites composed of basic and acid lava ¯ows. The highest values of km came from basic ¯ows (usually >10ÿ1 SI) and the lowest values are from acid ¯ows (between 8 10ÿ2 and 10 10ÿ2 SI) as shown in Fig. 3(a) of the JS and BV sections. The general trend of high susceptibility values may suggest a very weak contribution of either paramagnetic or diamagnetic minerals to AMS ellipsoid. The degree of anisotropy (P ), which characterizes the shape of the susceptibility ellipsoid ranges from 1.001 (0.1%) to 1.032 (3.2%), with an average of 1% (Table 1 and Fig. 3a). There is no linear relationship between km and P (Fig. 3(a)), P and L or P and F (not shown here). The parameter P is always very low, as expected for igneous rocks with primary magnetic fabric (Hrouda, 1982). Plots of the B(%) parameter against the A(%) parameter for the ®ve sections are shown in Fig. 3(b). The A parameter measures the degree of anisotropy ranging from 0% (isotropic, sphere) to 100% (maximum anisotropy, line segment), while the B parameter divides magnetic fabrics into two broad ®elds ranging from purely foliated (B=ÿ100%, oblate ellipsoid) to
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purely lineated (B=100%, prolate ellipsoid) (CanÄoÂn-Tapia, 1994). From Fig. 3(b) it is clear that almost all the investigated sites are preferentially dominated by oblate ellipsoids of revolution or magnetic foliation (B is between ÿ2.5 and 1%, Fig. 3(b)). Small values of the A parameter are found in most sites (A < 2.5%, with a mode of <1% Fig. 3(b)). The principal directions of susceptibility for each ¯ow were examined by using an updated version of Lienert (1991) program, which uses the Hext±Jelinek statistical analysis (Hext, 1963; Jelinek, 1978). Representative examples of the principal susceptibility axes for single ¯ow units are given in lower-hemisphere equal-area plots (Fig. 4) and values of the mean susceptibilities
Fig. 4. Lower hemisphere equal are projection of the AMS principal axes of selected ¯ow units of (a) IC, (b) PA, (c) CV, (d) BV, (e) JS and (f) JS. Flow means and the corresponding regions of 95% con®dence are calculated by using Lienert's (1991) programs. Squares, triangles, and circles indicate site mean k1 (maximum), k2 (intermediate) and k2 (maximum) directions, respectively.
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are presented in Table 1. In most cases within-¯ow means show good consistency (Fig. 4(a)± (e)). Tight groupings of vertical and sub-vertical k3 directions and nearly horizontal k1 directions are characteristic of most ¯ows, due to the dominance of magnetic foliation as suggested above. The grouping of the mean principal susceptibility axes in most cases ranges from moderate to excellent according to the threshold criteria of CanÄoÂn-Tapia et al. (1994). In some cases, as shown in Table 1 for the BV section, the size of the con®dence regions are large due to the small number of samples despite the fact that directions are exceptionally well clustered. For some ¯ows two or three distinct and well clustered groupings of AMS directions are identi®ed (e.g. Figs. 4(e) and 4(f) and ¯ow numbers followed by letters in Table 1). This is usually found in thick ¯ows (10 m or more) originally identi®ed by Ernesto and Pacca (1988), of which oriented blocks were taken from dierent parts of the ¯ow (some times from outcrops on both sides of the road). Because all of these sub groups show well clustered AMS directions it is dicult to decide whether they are indeed part of the same ¯ow or rather they represent dierent ¯ow units. CanÄoÂn-Tapia et al. (1994, 1995, 1996, 1997) reported systematic changes in the orientation of AMS directions as a function of relative position within a ¯ow. These changes have been used to infer absolute ¯ow directions without making any prior assumptions. Unfortunately, the lack of good control on the relative position of the blocks in our case prevents us from further constraining ¯ow directions by using these changes. Instead, we assume that the horizontal k1 axis is parallel to ¯ow direction. The tight clusters of the principal susceptibility axes from each block, together with the sub vertical orientation of k3 axis justify this assumption as discussed by CanÄoÂn-Tapia et al. (1997). Therefore, all the units satisfying these threshold criteria had a ¯ow direction associated to each block separately, and identi®ed in Table 1 by the site numbers followed by letters. 4.2. Magnetic mineralogy During experiments of isothermal remanent magnetization acquisition (IRM) all samples showed similar behavior. A fast initial increase in the acquired IRM (Fig. 5(a)) is suggestive of magnetite as the dominant magnetic carriers, since magnetite saturates at around 200 mT. Occasionally, a small amount of high-coercivity magnetic minerals are also present, indicating a contribution of maghemite or a low concentration of haematite. Values of Hcr, were calculated from back ®eld IRM measurements, fall in the range of 30±50 mT (Fig. 5(a)). Representative curves of high ®eld hysteresis loops for two samples are given in Fig. 5b. The reversibility of the curves between 0.2±0.4 T indicates that the contribution from high coercivity magnetic mineral (hematite) to the magnetization of these rocks is negligible. At high ®elds, the slope of the curve is due to the paramagnetic contribution and can be related to phyllosilicates evidenced during ore microscopic observations (Ernesto and Pacca, 1988). After correction for the paramagnetic contribution, the ratio of the hysteresis parameters (Mrs/Ms Vs Hcr/Hc) was plotted on Day's plot (Day et al., 1977). These ratios are commonly regarded as the best indicators of magnetic domain states and variations in grain-size for magnetite and titanomagnetite, ranging from 0.5 for uniaxial single-domain (SD) grains down to <0.02 for true multi-domain (MD) magnetite (Dunlop, 1981). Increasing of the Ti content in titanomagnetite-bearing rocks raises the MD/PSD transition value to 0.1 (Dunlop, 1981).
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Fig. 5. (a) Representative curves of isothermal remanent magnetization (IRM) and back ®eld remanence acquisition (Hcr) for selected samples of each section. (b) Hysteresis loops behavior during high ®eld measurements (c) ratio of hysteresis parameters following Day et al. (1977). SD, PSD, SP and MD indicate respectively magnetic grain-size ®elds of single-domain, pseudo-single domain, superparamagnetic and multi-domain. (d) Thermomagnetic (J2±T) curves between room temperature and 7008C, at a heating rate of 608C minÿ1. Sample BV01 shows a single high Curie temperature close to 5808C and, JS20 a two phase behavior at around 4208C and 5808C. Forward (back) arrows indicate heating (cooling) cycle.
Results for the ®ve PMP sections investigated here fall within the range of pseudo-single (PSD) to small MD magnetites and/or titanomagnetites (Fig. 5(c)). During high ®eld thermomagnetic (Js±T) measurements, ranging from room temperatures to the maximum Curie temperature of hematite (6808C), two categories of behaviors were identi®ed. The ®rst group (about 90% of the measured samples) have single high Curie temperatures of around 5808C and reversible Js±T curves (e.g. BV-01 in Fig. 5(d)). The major decrease in magnetization intensity and the tendency for it to go to zero above 6008C suggest that a magnetic mineralogy is dominated by magnetite or Ti-poor titanomagnetite. The second group consists of samples from the upper part of the hand sample or block (8%), and are characterized by a two phase behavior of the decrease in magnetization intensity (e.g. JS-20 in
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Fig. 5(d)). A ®rst loss of magnetization at lower Curie temperature (between 3508±4208C) followed by another at around 5808C. The two phase behavior might be characteristic of the coexistence of unoxidized Ti-poor magnetite and oxidized Ti-poor magnetite which has undergone low-temperature oxidation to submicron maghemite (De Boer and Dekkers, 1996). This oxidation to a higher coercivity magnetic mineral is evidenced by the cooling curve which shows a ®nal magnetization intensity too small when compared with initial magnetization (JS20 in Fig. 5(d)). A lack of reversibility is observed in two samples of the IC sequence during cooling (not shown here). This might indicate that magnetite has formed either from transformation of initially paramagnetic minerals (phyllosilicates), or from inversion of an original magnetic mineral (titanomaghemite) into a stronger magnetic mineral (magnetite). In summary, the magnetic properties of the studied lava ¯ows are mainly controlled by magnetites that may have been slightly maghemitized, through low-temperature oxidation or weathering. Based on this evidence, the results of the AMS measurements are interpreted to re¯ect the statistical alignments of non-equant small to large grains of magnetite, and the contribution from other types of minerals is neglected.
5. Discussions 5.1. Nature of AMS The magnetic fabrics in ferrimagnetic dominated rocks may be attributed to the shape alignment of grains (shape anisotropy), the alignment of magnetic domains (domain anisotropy) or to the deformation of the crystal lattice (stress-induced anisotropy) (Stacey, 1960; Bhathal, 1971; Kapicka, 1983). Among these, shape anisotropy is the dominant component of AMS in magnetite-bearing rocks, which may be of primary or secondary origin. Theoretically it was shown that, for magnetite bearing rocks with primary magnetic fabric, the maximum susceptibility axis (k1) is parallel to the longest axis of elongated MD grains, whereas the minimum susceptibility axis (k3) is parallel to the long axis of uniaxial SD grains (e.g. Tarling and Hrouda, 1993). Stephenson et al. (1986) has experimentally veri®ed this theoretical prediction using synthetic samples. Therefore, before interpreting the AMS result of the PMP lava ¯ows it is necessary to investigate the origin and nature of the magnetic fabric in the studied sequences. Based on the rock-magnetic results, high values of the bulk magnetic susceptibilities (Fig. 3(a)), identical behavior of IRM acquisition curves, the magnetic hysteresis loops and thermomagnetic measurements (Figs. 5(a), 5(b) and 5(d)), we concluded that the AMS is associated with a magnetic mineralogy dominated by magnetite and/or titanomagnetite grains. The combination of the hysteresis parameters Mrs/Ms vs Hcr/Hc shows that all studied samples fall within a grain-size range of PSD to small MD (Fig. 5(c)). Other magnetically susceptible phases in lava ¯ows, notably the rock matrix (paramagnetic and diamagnetic) and maghemites can be ruled out as a signi®cant contributor of the AMS. Most paramagnetic minerals such as biotite and hornblende have magnetic susceptibilities much less than 10ÿ3 SI (Borradaile, 1988; Rochette et al., 1992), roughly about two orders of magnitude lower than those observed in the PMP lava ¯ows (Fig. 3(a)). The small amounts of maghemite present in some ¯ows may be
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due to weathering and only aected the upper most part of the ¯ows. Specimens showing such behavior are not included in the forthcoming interpretation of the AMS. PSD and MD grains of magnetite and/or Ti-poor titanomagnetite dominate the magnetic fabrics of the whole section studied here. Therefore the AMS ellipsoid is presumably associated with a primary shape anisotropy of PSD and MD grains of magnetite or/and titanomagnetite. 5.2. AMS and ¯ow direction Primary magnetic fabric associated with magnetite or titanomagnetite can be caused by stresses set up during emplacement or cooling of the lava. Such an orientation is generally developed along the ¯ow or elongation direction of the magnetic grains according to both theoretical and experimental modeling (e.g. Tarling and Hrouda, 1993; Stephenson et al., 1986). However, it seems ambiguous which susceptibility axis (i.e. k1 or k2) might represent the lava ¯ow direction. Khan (1962) showed that the mean intermediate (k2) susceptibility was roughly parallel to the ¯ow direction and both k1 and k3 are normal to the ¯ow. Hrouda (1982) reported a good agreement between the direction of the maximum axis of susceptibility and the ¯ow direction only in the intermediate and not in the frontal parts of the ¯ow. On lava ¯ows produced by a laminar ¯ow agreement between k1 and observed lineations, in a case where we could measure both, were reported by MacDonald et al. (1992). In a similar study of the Xitle lava ¯ows of Mexico, CanÄoÂn-Tapia et al. (1995) has found either the mean maximum or the mean intermediate susceptibilities pointing in the same direction as the geologically inferred ¯ow direction. CanÄoÂn-Tapia et al. (1995) reconciled these apparent contradictory results by considering the way lava ¯ows move along the slope of a pre-existing terrain as well as the resistance that may be encountered by the ¯ow at the front of the lobe due to the formation of a rigid crust or the accumulation of debris. This may cause subsidiary lobes to form in directions at an angle of up to 908 with the main lobe. In addition, when inferring lava ¯ow directions local and regional topography and magma characteristics have to be taken into consideration. As a result, one might expect ¯ow directions to vary signi®cantly from one locality to another, or to change with time at a given locality. PMP represents a ®ssure magmatism made up of ¯at lying lava ¯ows which dip gently (not more than 58) towards the central axis of the basin, although locally most of the ¯ows seems to lie horizontally. Geological features (dykes cutting the ¯ow sequences or circular structures) are not normally seen, making it dicult to locate possible magma sources. Accordingly, the observed k1 inclinations are shallow in the majority of the ¯ows, being less than 108 in acid ¯ows and less than 208 in basic ¯ows (Table 1). Therefore, it is possible to estimate the mean lava ¯ow axis of each sequence by using the k1 azimuths (k1 declination). To infer this we plot mean direction of both k1 and k3 axes of the principal susceptibility of each sequence using lower hemisphere equal area density plots (Figs. 6(a)±(c)). In constructing these plots, we used data for k1 inclinations less than 308 as reported in Fig. 2 and shown in Table 1. In some parts of the sections, the plunges of k1 show either a subvertical inclination or imbrication relationship dierent from the majority of the sites (e.g. PA-8, JS-5a, CV-6, BV-01 or site numbers with crosses in Table 1 and Fig. 2). In such situations an interchange of k1 and k2 axes might be expected due to the very low degree of anisotropy (Fig. 3(a)), although k2 axes
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Fig. 6. Density contours of maximum (k1) and minimum (k2) susceptibility directions of the mean eigenvectors of (a) IC and PA, (b) CV and BV, and (c) JS sections. Arrows in k1 density contour indicate mean site declination of the k1 (maximum) direction which characterizes the mean lava ¯ow direction of each section.
show ¯ow direction similar to the general trend of the lava ¯ows determined from the k1 axis (Fig. 2 and Fig. 4(e)). In such cases, therefore, we considered the k2 axes as representing the lava ¯ow direction and included in the data set shown in Table 1 (those sites marked with crosses) and in deriving Fig. 6. As shown in Fig. 6, we could observe three dierent lava ¯ow directions inside the studied sections of the PMP. The two central Parana sections, IC and PA, are characterized by dominantly E±W trends of the k1 azimuths (Fig. 6(a)). Two of the southern sections, BV and CV, which are located closer to the eastern margin, trend mainly NE±SW (Fig. 6b); the southwestern most section JS, on the contrary, trends NW±SE (Fig. 6(c)). The dierence in ¯ow directions in central and southern PMP suggests a regional pattern of magma ¯ow directions rather than an overall pattern expected if a unique magma feeder is considered. Amaral et al. (1966) suggested that the PMP ¯ows could have been fed from megatectonic structures represented by the Parana river that runs parallel to the basin axis (Fig. 1). This hypothesis is partially in accordance with the observed E±W ¯ow trend in IC and PA sections. Furthermore, the tectonic lineaments (Rio Uruguai and Rio Piquiri, Fig. 1) that limit the central PMP could have conditioned the lava ¯uxes if they underwent strike-slip
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movements during the emplacement of the ¯ows. However, the low degree of anisotropy that characterizes the Parana ¯ows, do not indicate a tectonically induced AMS. The divergent trends observed in southern PMP, on the western (JS) and eastern (BV and CV) sections, suggest a topographic control on magma ¯uxes which seem to run radially starting from a higher area in the south, and dipping towards the Rio Parana and the Atlantic margin. Shallow structures, like the one associated with a geoid high in southern Brazil (Molina and Ussami, 1998, this volume) could have existed due to the anomalous thermal conditions of the lithosphere, with a long history of thermal and/or mechanical uplift (Hegarty et al., 1996). In this case our AMS result for the southern PMP suggest magma feeder locations that are distinct from those related to the younger magmatic activities that gave rise to the Ponta Grossa and FlorianoÂpolis dikes which places the magma feeders on the southeastern Atlantic margin (Raposo and Ernesto, 1995; Raposo, 1997). Indeed the uplift of the margins in the southern area of PMP and the overall ¯ow dipping toward the central axis of the basin may have aected the observed k1 inclinations, and the plunging orientation (direction) could change. In this case, ¯ows in southern PMP could have been originated near the Rio ParanaÂ. However, this needs further investigation.
6. Conclusions From both the AMS and rock magnetic data presented here, we can conclude the following: . PSD and small MD magnetite and titanomagnetite grains are the major contributors for the magnetic anisotropy reported here. . Three dierent trends in k1-azimuth of the lava ¯ow directions are observed along the PMP: * E±W trends in two sections from the central western part of the basin (IC and PA) * NE±SW trends in two sections from the south eastern part of the basin (CV and BV) * NW±SE trends in one section from the southern part of the basin (JS). . These major AMS trends are in accordance with the topographic settings and fracture patterns observed inside the Parana Basin and in the crystalline basements. Previous AMS studies on dike swarms and basaltic sequences revealed similar trends in agreement with this study. Therefore, it seems reasonable to associate a paleo-¯ow direction with existing pre¯ow topography and crystalline basement structure during emplacement of magmas. Further AMS studies from the northern and western part of the PMP will help to envisage the dominance trends in lava ¯ow direction and in inferring possible magma feeder location inside the Parana Basin.
Acknowledgements We are grateful to J. Marins for his help in preparing samples, and to R. Siqueira for the aid with the laboratory routines. We thank B. MacDonald, J. Glen, P. Renne and an anonymous reviewer for their helpful and valuable comments on the ®rst version of the
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manuscript. This investigation was supported by the Brazilian Agency for Scienti®c Research (FAPESP).
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