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Tectonophysics 448 (2008) 115 – 124 www.elsevier.com/locate/tecto
Anisotropy of magnetic susceptibility studies in Tertiary ridge-parallel dykes (Iceland), Tertiary margin-normal Aishihik dykes (Yukon), and Proterozoic Kenora–Kabetogama composite dykes (Minnesota and Ontario) John P. Craddock a,⁎, Bryan C. Kennedy a , Avery L. Cook a , Melissa S. Pawlisch a , Stephen T. Johnston b , Mike Jackson c b
a Macalester College, 1600 Grand Avenue, St. Paul, MN 55105 USA Department of Earth and Ocean Sciences, University of Victoria, Victoria, B.C., Canada c Institute for Rock Magnetism, University of Minnesota, Minneapolis, MN 55445 USA
Received 25 May 2006; received in revised form 19 November 2007; accepted 22 November 2007 Available online 21 December 2007
Abstract Mafic dykes of different ages were collected from three different tectonic settings and analyzed using anisotropy of magnetic susceptibility (AMS) as a proxy for magmatic flow during intrusion. In Iceland, ridge-parallel basaltic dykes were sampled on each side of the active tectonic boundary. The dykes are b 10 m wide along a 1–2 km strike, and are the result of a single intrusion from 1–2 km deep magma chambers in oceanic crust. Thirteen samples were collected (7 N. American plate; 6 European) and 153 cores were analyzed by AMS and preserve a vertical Kmax orientation indicating vertical emplacement. The Eocene Aishihik dyke swarm intrudes the Yukon–Tanana terrane in the Yukon province, Canada over an area ~ 200 by 60 km. These dykes were intruded normal to the accretionary margin, are porphyritic andesites, and have an intermediate geochemical signature based on major and trace element analyses. Ten dykes were sampled and 111 cores analyzed using AMS, and the dykes preserve a vertical Kmax orientation, indicating intrusion was vertical through ~ 30 km of continental crust. The 2.06 Ga Kenora–Kabetogama dykes in northern Minnesota and western Ontario crosscut a variety of Archean terranes (thickness ~ 50 km) in a radiating pattern. The unmetamorphosed basaltic dykes are 1–120 m wide, 10–110 km in length, are vertical in orientation and can be grouped as either being single intrusion or multiple intrusion (composite) dykes. AMS data preserve a vertical Kmax orientation for the southerly locations (2 dykes, n = 53) and horizontal Kmax for the remainder to the northwest (15 dykes, n = 194). Maximum magnetic susceptibility axes (4 dykes, n = 92) for composite dykes are scattered and yield inconsistent flow directions with regard to the dyke margin. Almost all of our results are “normal” in that, the magnetic foliation (the plane containing Kmax and Kint, normal to Kmin) is parallel to the dyke planes, which gives us confidence that the magnetic lineations (i.e., Kmax orientations) are parallel to magmatic flow. © 2007 Elsevier B.V. All rights reserved. Keywords: Mafic dykes; Intrusion mechanisms; AMS; Tectonics
1. Introduction Intrusion of magmatic fluids into crustal rocks has always been a mechanical paradox contrasting hot weak fluids being forced into cold, stiff host rocks with minimal metamorphic alteration or deformation in the host. Field and petrographic observations in dyke swarms are often complex (forking ⁎ Corresponding author. E-mail address:
[email protected] (J.P. Craddock). 0040-1951/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2007.11.035
directions, cryptic layering in composite dykes, xenolith alignment, etc.; Philpotts and Asher, 1994) and indicate multiple intrusive flow directions. Knight and Walker (1988) and Ernst (1990) have pioneered the application of AMS methods to Proterozoic mafic dyke swarms (Ernst and Baragar, 1992; Ernst and Duncan, 1995) as a proxy for primary magmatic flow directions during dyke intrusion. AMS studies on dyke swarms in other tectonic settings have been completed: Hawaii (Knight and Walker, 1988); the Troodos ophiolite (Staudigel et al., 1992); Makhtesh Ramon, Israel (Baer, 1995)
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and the Independence dyke swarm, California (Dinter et al., 1996) and others (see Cañón-Tapia, 2004 for a recent review). A variety of complicating factors are now recognized, including the “inverse” AMS of single-domain magnetite (Rochette et al. 1992; Ferre, 2002; Potter and Stephenson, 1988), the “distribution anisotropy” of clusters of magnetic grains (Stephenson, 1994; Hargraves et al., 1991), non-flow-parallel grain alignment by viscous fluid flow (Cañón-Tapia and Chávez-Álvarez, 2004), and tectonic overprinting of primary flow fabric (Park et al., 1988). Aware of these caveats, our goal in this study was to use the AMS technique to document igneous flow fabrics in the Proterozoic Kenora–Kabetogama swarm in cratonic North America (Archean Superior province; ~ 50 km thick) and compare this with Eocene dykes in an accreted terrane (Yukon– Tanana terrane; ~ 30 km thick), and with ridge-parallel dykes in thin, active Iceland (30 km thick, with magma chambers at 1– 5 km depth) along the Mid-Atlantic ridge. 2. Methods 2.1. Magnetic instrumentation Oriented hand samples (Iceland, Aishihik, KK) or cores (KK, some composite dykes) were collected from the various field locations, and oriented cores (or cubes) were prepared for AMS analysis (Table 1; Tauxe et al., 1998). The “Roly-Poly” is a low-field AC magnetic susceptibility bridge with an automated sample handler for determining anisotropy of susceptibility at room temperature, and is housed at the Institute for Rock Magnetism at the University of Minnesota. An alternating current in the external “drive” coils produces an alternating magnetic field in the sample space with a frequency of 680 Hz and an amplitude of up to 1 mT. The induced magnetization of a sample is detected by a pair of “pickup” coils, with a sensitivity of 1.2E− 6 SI volume units. For anisotropy determination, a sample is rotated about three orthogonal axes, and susceptibility is measured at 1.8° intervals in each of the three measurement planes. The susceptibility tensor is computed by least squares from the resulting 600 directional measurements. Very high precision results from the large number of measurements; in most cases principal axis
Table 1 AMS results Sample
Dykes sampled
n=
Anisotropy (%)
Lineation Foliation
Iceland basaltic dykes Aishihik dykes, Yukon Kenora–Kabetogama dykes Single intrusion Composite intrusions
13 11
153 14.9 111 13.65
11.86 7.09
3.16 6.43
247 14.21 92 11.9
3.14 7.6
2.06 6.8
21 17 4
Explanation: anisotropy (Kmax − Kmin / Kmean); lineation (Kmax − Kint / Kmean); foliation (Kint − Kmin / Kmean) for the Roly-Poly instrument.
orientations are reproducible to within two degrees, and axial ratios to within about 1%. 2.2. X-ray fluorescence Eleven of the Aishihik Lake dyke samples were analyzed for major and trace element composition using X-ray fluorescence as there was no geochemical data on these intrusions. Samples were split with a vise wedge and only pieces lacking weathered or saw-marked edges were selected to be used in XRF analyses. Powders were prepared by further splitting the sample and reducing these pieces to a fine powder in a Spex 8510 Shatterbox. The use of pre-contaminated bowls (iron for trace element and tungsten for major element powders) reduced the chance of cross contamination between the samples. Pressed powder pellets were prepared for trace element analyses by mixing exactly 10 g of rock powder with 15 drops of 2% polyvinyl alcohol and pressing the mixture into pellets on a stainless steel mold under a pressure of 6 tons. Major elements were prepared by first heating approximately 10 g of each major element powder to 1000 °C to drive off all water. The amount of water lost in heating (loss on ignition) was recorded after samples had cooled and these numbers were taken into account in the reporting of element totals. Exactly 1 g of dried powder was mixed with 5 g lithium metaborate/tetraborate flux and 0.1 g NH3NO4. Once mixed, the powder was placed in a platinum crucible and 2 drops of HBr were added. The crucibles were heated on a Spex Fluxy until the material was molten, then poured into a platinum mold creating a homogenous glass disk. The pellets and beads were analyzed by a Phillips PW-2400 XRF. 3. Field relations and results 3.1. Iceland ridge-parallel swarm Iceland is one of the best-exposed and best-studied geological settings in the world, combining a mid-ocean ridge, a hotspot plume, active glaciation, and little vegetation covering the rocks which range from 16–0 Ma (Gudmundson and Kjartansson, 1996). The interplay of these geologic processes results in some very complex local geodynamics, as recorded by a variety of methods: seismicity and focal mechanism solutions, fault slip data, GPS and borehole strainmeter surveys, tiltmeter-elevation change and gravity surveys, and hydrofracture and overcoring measurements. Most recently, Linde et al. (1993; Mt. Hekla) and Stefansson et al. (1993; South Iceland Lowland project), have made significant advances in monitoring and predicting both volcanic eruptions and destructive seismic events, respectively. Dyke intrusion has been observed seismically (Einarsson and Brandisdottir, 1980) and studied extensively by Sigurdsson (1980), Gudmundsson and Brynjolfsson (1993) and Paquet et al. (2007). Thirteen single-intrusion basaltic dykes were sampled around Iceland and all but one (sample 4) strikes parallel to the ridge axis (Fig. 1; site data). The dykes are generally b1 m wide and can be traced a few tens of meters along strike. In ten
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Fig. 1. Digital elevation map of Iceland showing dyke locations and typical field exposure of a dyke. AMS results are plotted on lower hemisphere projections (Kmax — solid circles, Kmin — open circles) with representative vertical flow on the left (all sites except 4, 11 and 12) and dike-parallel horizontal flow (sites 11 and 12) and a curious result for the ridge-normal dike (site 4) on the right.
of thirteen dykes Kmax is sub-vertical and Kmin plots horizontally and normal to the plane of the dyke. The steep magnetic lineations suggest sub-vertical flow. Two ridgeparallel dykes (samples 11 and 12; horizontal Kmax and vertical Kmin) and one ridge-normal dike (#4; vertical Kmax and horizontal Kmin) exhibit anomalous fabrics, with Kmin and Kmax both within the plane of the dyke (see Rochette et al., 1992; Cañón-Tapia and Chávez-Álvarez, 2004). Sample anisotropy percentages are quite high, ranging from 5–34% for the Icelandic suite (153 cores), with an average lineation and foliation of 12.23% and 3.16%, respectively (Table 1). Thermomagnetic analysis (using a Kappabridge KLY-2 susceptibility bridge with a CS-2 furnace) shows that these
dykes have a bimodal magnetic mineralogy: a relatively low-Ti titanomagnetite Fe 3 − x Ti x O 4 with a Curie temperature Tc ~ 510 °C (TM10, x ~ 0.10) and a more Ti-rich composition with Tc ~ 180 °C (TM60, x ~ 0.60). Hysteresis measurements show that for both compositions the major domain state is pseudo-single domain (PSD): the ratio of saturation remanence to saturation magnetization (Mr/Ms) ~ 0.1. 3.2. Aishihik dyke swarm, Yukon, Canada The Yukon–Tanana terrane is sutured between the Coast Plutonic and Stikinia terranes bounded on the north by the Tintina dextral fault, on the south by the Denali dextral fault,
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and is composed of metamorphosed marine sediments of Devonian–Permian age crosscut by a variety of Mesozoic plutons (Johnston and Timmerman, 1993; Johnston and Erdmer, 1995). The dynamics of accretion of the various terranes is complex and debated (Jackson et al., 1991; Johnston et al., 1996) but it is assumed that the Aishihik dykes were intruded after accretion of the Yukon–Tanana terrane to another terrane (Stikinia?) or N. America in the Triassic. The porphyritic dykes crosscut a variety of rock types (Fig. 2B) including the Cretaceous–Paleocene Ruby Range batholith; the dykes probably fed and do not intrude the Eocene Mount Creedon volcanics, giving an age constraint. Within this terrane the ~ N–S Aishihik dyke swarm intrudes an area of 200 by 60 km (Fig. 2A). Dykes are generally b 4 m in width (Fig. 2B) and can be traced for a few hundreds of meters in places, and we sampled the southern 100 km of the swarm. Petrographically, the dykes are unaltered (some chlorite after olivine) porphyritic andesites (Fig. 2C). Major and trace element XRF analysis of the dykes, which have an aphanitic groundmass and sodic phenocrysts, indicates a consistent REE chemistry and a calcalkaline single source magma with a within-plate tectonic setting (Fig. 2E; Table 2; Cook, 2001). Ten dykes were sampled and 111 cores were analyzed by AMS; eight of the ten dykes preserve a vertical Kmax orientation within the plane of the dyke, suggestive of vertical intrusion (Fig. 2D; site data, Table 1). The remaining two dikes preserve complex Kmax and Kmin plots within the dyke plane. The average anisotropy is 13.65%, with average lineations of 7.09% and foliations of 6.43%. Multiple magnetic phases and domain states were identified in these dykes. Pyrrhotite was identified in some samples by a Curie point Tc ~ 325 °C and a low-temperature magnetic transition near 34 K (Dekkers et al., 1989; Rochette et al., 1990), with dominantly PSD–SD domain states indicated by the remanence ratio Mr/Ms ~ 0.4. In other samples, nearly pure multi-domain (MD) magnetite (Tc ~ 565 °C, Mr/Ms ~ 0.01) was the major susceptibility source. 3.3. Proterozoic Kenora–Kabetogama swarm The Kenora–Kabetogama (KK = Fort Frances = Marathon; swarm A25 and A26 in Ernst et al., 1996) mafic dyke swarm intrudes Archean granitic, metavolcanic, and metasedimentary rocks of the southern Archean Superior Province of the Canadian Shield, represented in northern Minnesota and southwestern Ontario by four lithotectonic terranes, the Wabigoon, Quetico, Wawa, Minnesota river valley (MRV; see Schmitz et al., 2006)) sub-provinces (Fig. 3). Recent reviews of the geology and evolution of these sub-provinces were given by Card (1990), Blackburn et al. (1991), Williams (1991), and Williams et al. (1991). The Wabigoon (northern) and Wawa (southern) sub-provinces are “granite–greenstone” terranes considered to represent accreted slices of continental platform, ocean floor, and volcanic arc rocks. They are juxtaposed with the intervening metasedimentary Quetico sub-province. These terranes are considered to have been accreted by 2600 Ma into the currently ENE-trending belts bounded by north-dipping thrust faults, and were of a thickness
of 40–50 km at the time of dyke intrusion. The Superior province has been tectonically stable for ~ 2 Ga as the KK dikes are not offset along any terrane boundaries. The outcrop extent of the KK dyke swarm is approximately 30,000 km2 (Southwick and Day, 1983). The western terminus of the swarm, like the Superior province boundary, is however obscured by a thick overburden of Phanerozoic sedimentary rocks and glacial drift. Samples of basalt and diabase were obtained from drill cores (at depths of a few hundred meters) that intersected four of the hundreds of strong NW-trending normal and reversely-polarized linear magnetic anomalies in northwestern Minnesota (Chandler, 1991). Considering these magnetic anomalies to be KK dykes, and exposures of mafic dykes in the Minnesota River valley, the total extent of the swarm approaches 100,000 km2 , ranking it among the largest dyke swarms in the world. At the southern boundary of the Superior Province in northeastern Minnesota (Fig. 3), the KK dykes are overlain by metasedimentary rocks of the Animikie Basin, thus constraining their intrusion to the Early Proterozoic. K–Ar ages for the dykes range from 2240 to 1520 Ma (Hanson and Malhotra, 1971) and probably record Ar loss and/or resetting during post-emplacement metamorphism. A Rb–Sr whole-rock isochron age of 2120 ± 67 Ma was determined from a composite dyke near Lake Kabetogama by Beck and Murthy (1982). A regression line fit to Sm–Nd whole-rock and mineral data yields an age of 2065 ± 120 Ma (Wirth et al., 1995; Wirth and Vervoort, 1995) and is within analytical error of preliminary U–Pb analyses of zircons from the southeastern (2075 ± 2 Ma; L. Heaman, pers. commun.) and southern (2.067 ± 2 Ma, Schmitz et al., 2006; TIMS zircon age, Franklin dykes) part of the swarm. Early major element geochemical work on the KK dyke swarm suggested the existence of two distinct groups of dykes: low-Ti/ high-Mg and high-Ti/low-Mg (Manzer, 1978). Southwick and Day (1983) studied the field geology of the swarm, as well as the detailed petrography and mineral chemistry of a composite dyke from Lake Kabetogama. In a study of the major and trace element characteristics of the swarm, Southwick and Halls (1987) proposed a common magmatic origin for all of the dykes, suggesting that they represent a cogenetic differentiation sequence from an evolving magma system that was tapped at two stages. Schmitz et al. (1995) have identified two sub-groups in the KK swarm based on incompatible trace element abundances, low-ITE (Zr b 100 ppm) and high-ITE (Zr N 120 ppm) groups, and both seem to have been intruded from an aesthospheric mantle-enriched source without crustal contamination. The detailed geology and structure of the swarm have been described by Southwick and Day (1983). In summary, the trends of dykes fan from N15°E in the eastern part of the swarm to N60°E in the south. Outcrops and aeromagnetic signatures of individual dykes can be traced along strike for over 200 km (Southwick and Halls, 1987). The dykes occupy a NW-trending vertical fracture set perpendicular to the ENEtrending Archean fabric, with evidence for some local strikeslip movement along some dyke margins (Craddock and Moshoian, 1995). Dips on the planar, sub-parallel dyke
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Fig. 2. Digital elevation map of southwest Yukon, Canada with terranes and bounding faults and the Aishihik dike swarm (A). B. Dike field photo. C. Photomicrograph showing aphanitic groundmass with plagioclase phenocrysts. D. Lower hemispheres stereoplot of AMS data (Kmax = closed circles, Kmin = open circles) that shows vertical flow (8 of 10 samples) E. Tectonic discrimination diagram (upper) and REE spider diagram for the Aishihik dykes.
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Table 2 Major and trace element geochemical data for the Aishihik dyke swarm Sample 5 SiO2 TiO2 Al2O3 FeO Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total La Ce Rb Ba Th U K Nb Sr Pb P Zr Ti Y Ga Cr Ni Co Sc V Zn
61.6 0.476 17.67 0 5.04 0.04 1.91 4.82 4.42 2.44 0.233 0.4608 99.11 12.1 29.1 61.1 1444.6 4.8 2.9 2020 3.7 853 5.9 102 118.1 285 13.1 18.8 9.5 9.8 17 10.3 92.4 21.5
Sample 8 54.49 0.762 14.89 0 7.89 0.13 7.95 8.11 3.49 1.66 0.249 0.9777 100.6 10.2 29 47.2 832.1 2.9 1.1 1380 5.4 602.8 4.1 109 103.7 457 19.1 16.8 403.7 100.6 34.1 37 124.5 45.6
Sample 9 53.35 1.694 16.55 0 9.56 0.15 4.64 8.58 2.8 1.35 0.459 1.6184 100.75 18.1 44.7 39.5 948 3.5 1.7 1120 4.6 603.1 8.3 201 151 1016 27 20 67.8 13.4 30.8 26 150.2 125
Sample 10
Sample 11
Sample 12
Sample 13
Sample 16
66.81 0.37 16 0 3.32 0.04 2.05 3.75 4.32 2.8 0.141 0.8687 100.47
54.77 0.79 17.17 0 8.24 0.09 3.89 6.21 3.71 2.6 0.263 1.8038 99.54
66.39 0.365 16.36 0 2.93 0.04 1.33 3.92 4.17 3.26 0.191 0.5065 99.46
64.43 0.418 15.71 0 4.06 0.05 2.6 4.12 4.2 2.61 0.146 0.7819 99.13
61.4 0.489 17.24 0 5.11 0.1 2.42 6.09 4.15 1.62 0.167 0.6061 99.39
11 26.5 75.3 1464.8 7.9 2.7 2320 4.7 658 7.9 61 109.8 222 10.1 16.2 86 29.9 13.2 10 66 27.4
12 31.6 97.7 1664.5 5.8 2.7 2160 2.8 1102.7 4.5 115 116.9 474 18.2 19.5 18.7 13.3 33 25.5 153.1 42.9
contacts are within 10° of vertical, indicating that the dykes have not rotated since intrusion. Dyke widths vary from less than 10 cm to greater than 120 m. Most dykes have distinct fine-grained margins at their contacts with the country rock. Inward from the dyke margins, textures vary from basalt through diabase; gabbroic textures are characteristic of dykes greater than 20 m in width. Dykes of greater width often have heterogeneous internal structures and are composed of symmetrical and rhythmic compositional layers that are truncated toward the cores of dykes. Most commonly these layers are sub-parallel to the outer dyke margin, although structures reminiscent of sedimentary cross bedding have been noted. These structures suggest that they were formed by multiple intrusions of magma. The discontinuity between layers is wholly compositional, without chilled margins or significant grain size variation (Schmitz et al., 1995). The absence of chilled margins indicates that there was little time for significant cooling between the emplacement of magma pulses. The KK dikes mostly contain pure magnetite (Tc ~ 580 °C) with a range of grain sizes/domain states (Mr/Ms from 0.1 to 0.35). Some pyrrhotite is also present in a few samples.
10.8 25.6 84.8 1979.6 8.6 4.4 2710 5.4 763.1 7.4 83 126.5 219 13.1 16.6 16 9.8 11 8.1 62.6 24
10.7 26.9 67.7 1498.2 7.3 3.5 2170 4.3 620.4 7 64 114.9 250 10.8 16.4 102.5 36.3 16.4 11.9 85.3 34
12 29.5 55.5 1148.6 4.5 1.9 1340 3.1 705.6 10.6 73 114.7 293 13.5 16.8 21.7 11.1 18.4 14.6 94 62.9
Sample 17 66 0.295 17.75 0 3.11 0.07 0.68 3.73 4.5 2.67 0.173 0.514 99.49 23.8 44.8 63.5 1920.7 6.9 4.1 2210 4.9 909.3 12.4 76 176.3 177 15.9 17.5 3.7 5.8 8.7 6 29.4 28.9
Sample 18
Sample 19
61.39 0.575 16.75 0 5.83 0.1 2.56 5.12 3.97 2.21 0.273 0.6945 99.47
71.37 0.225 15.24 0 1.99 0.01 0.57 2.02 4.42 3.06 0.099 0.6512 99.66
17.1 37.8 53.7 1580.7 5.7 3.2 1840 4.6 806.3 7 119 145.9 345 18.5 17.7 21.4 10.6 18.8 14.5 108.6 58.6
14.1 30.6 65.9 1555 7.8 3.5 2540 5.7 467.7 9.9 43 116.6 135 9.5 15.9 7.1 6.3 8.1 5 19.2 14.5
3.4. Single-intrusion dyke AMS Seventeen single-intrusion dykes were sampled (Fig. 3; site data) and 247 cubes were analyzed by AMS (Table 1). Kmax axes are all sub-horizontal within the plane of the dyke and Kmin axes are all dike-normal and sub-horizontal for samples collected north and west of the Penokean suture (Pawlisch et al., 1997). Different AMS ellipsoid orientations were obtained from samples collected along the southeast margin of the KK swarm near Virginia, MN (strike N10°W) and in the Minnesota River valley near Franklin, MN (strike N60°E), and the portion of the KK swarm nearest to the Penokean subduction margin. Closest to the margin, Kmax is vertical, while Kmin is horizontal and dike-normal (Fig. 3, white stars). Two dikes in the northeast (Fig. 3, black squares) preserve an intermediate AMS ellipsoid fabric: dike-parallel, horizontal Kmax and a sub-vertical Kmin. However, the dominant trend suggests a source area for the KK magmas in the south and east, feeding the propagating swarm to the north and west (subhorizontal flow northwestward of the margin). The impingement of the Penokean belt and Animikie basin, and younger Keweenaw rift limit the exposure of KK dyke presumably nearer the source to the southeast.
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Fig. 3. Aeromagnetic basemap of Minnesota (Chandler, 1996) with tectonic features identified. Kenora–Kabetogama dykes are found north and west of the red line and appear as linear white lines with their extensions in black off the map. Inset explains dyke AMS stereoplots. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.5. Composite dyke AMS Four composite dykes were sampled (n = 92) in great detail (Fig. 3), two from the center of the swarm in the Quetico terrane,
and two from the northern terminus near Kenora, Ontario in the northern Wabigoon terrane (Table 1). Each of the dykes are ~ 20 m wide and preserve multiple intrusions that are increasingly coarse and felsic toward their interiors. Discordant
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boundaries between different intrusive phases are common, as are cryptic layering zones (“cross beds”). AMS results for both dykes are clearly anomalous and cannot be interpreted in terms of flow orientations: Kmax and Kmin axes have a variety of trends and plunges. 4. Discussion Visual observation of dyke intrusion is unlikely to occur, but seismic tracking of dyke intrusion was monitored during the 1977–8 eruption of Krafla in Iceland (Brandsdottir and Einarsson, 1979; Sigurdsson, 1980). During this eruption, with measured caldera inflation–deflation cycles, seismic events were recorded and located along the leading edge of the advancing tabular sheet of magma which eventually reached the surface through a drill hole or fissure within the MAR axis. The eruptive source was at a depth of ~ 5 km and the dyke intrusion front was both upward from the source and northeastward within the ridge axis in the plane of the dyke (see also Paquet et al., 2007). AMS studies of flow fabrics in oceanic crust are limited to an ophiolitic suite and hotspotrelated dykes in Hawaii. The Troodos ophiolite preserves subhorizontal flow (Rochette et al., 1991; Staudigel et al., 1992). The Koolau dyke swarm on Oahu, Hawaii preserves an interpretable AMS fabric with flow directions in numerous orientations within a well-organized swarm (Knight and Walker, 1988). Our regional study of dykes in Iceland shows a clear vertical orientation of magnetic lineations, which may be interpreted as a proxy for vertical magmatic flow, perhaps expected along a ridge axis where the crust is ~ 30 km (see Allen et al., 2002) and basaltic magma reservoirs are 1–5 km in depth. Geoffrey et al. (2002) and Callot and Geoffrey (2004) report a mixed AMS flow result for Tertiary dikes in east Greenland, where the majority (23 of 75) dikes preserve a horizontal, southerly intrusion flow. “Inverse fabrics”, where Kmax is dike-normal are best explained either because of the inverse “single-domain effect”, or because the grain long axes are not preferentially aligned with flow in the dyke, or some combination of these (see Borradaile and Gauthier 2001; Archanjo et al., 2002; Ferre, 2002). Dyke swarms intruded in continental crust are numerous (Ernst et al., 1995) and tend to be more prevalent in host rocks that are supracrustal and of low metamorphic grade (as opposed to granulites; Buchan and Halls, 1990) and are increasingly studied by AMS techniques. Both radiating (Mackenzie, 1267 Ma, Ernst, 1990, Ernst and Baragar, 1992) and linear (Labrador, early Proterozoic, Cadman et al., 1992; Abitibi, 1140 Ma, Ernst, 1990; Botswana, 1800 Ma, Ernst and Duncan, 1995) dyke swarms of Precambrian age preserve a vertical AMS fabric near the source and a sub-horizontal fabric away from the source. Baer (1995) found good correlation between AMS ellipsoidal, petrofabric and dyke margin flow indicators in the radiating Cretaceous Makhtesh Ramon dykes, Israel, but overall complex local flow. The linear Jurassic Independence swarm preserves a sub-vertical AMS ellipsoid at an acute angle to the dyke walls suggesting a tectonic strain overprint during or after intrusion (Dinter et al., 1996). Our results for the Eocene
Aishihik dyke swarm provide an intermediate crustal example: vertical dyke intrusion in an accreted terrane with a thickness of ~ 30 km of an intermediate melt (andesitic), although we have no constraints on the depth of the magma chamber. The tectonic setting and age of these andesitic dikes are suggestive of a shallow magmatic system. The Kenora–Kabetogama dyke swarm is one of the largest and best-exposed radiating swarms with a presumed source area in southeastern Minnesota postulated by convergence of the swarm from the north and west. These are not equivalent to the metamorphosed dykes along strike in western Wisconsin and northern Michigan (Green et al., 1987; King, 1990; Beutel et al., 1995). The KK dykes intrude the MRV, Wawa, Quetico, and Wabigoon terranes of the Superior province (Chandler, 1991; Southwick and Chandler, 1996; Craddock and Magloughlin, 2005) which are north of, and proximal to, the Penokean suture and their extensions are buried by younger accretion to the south (Fig. 3). Our single-intrusion AMS fabrics suggest a magmatic source in the southeast near the Penokean margin (vertical flow) and that the dykes propagated horizontally outward to the west and north as part of a great swarm. The distance between the two southerly sites where vertical flow is preserved and the nearest dyke along strike where horizontal flow is measured by AMS, is approximately 150 km, similar to other continental dyke swarms. This relationship could be better resolved if the southern extent if the KK swarm was not buried. AMS as a proxy for magmatic flow in composite dykes of this swarm was inconclusive. In all three of the cases studied here, we find a minority of samples (3, 2, and 6 dikes in Iceland, the Yukon and KK swarms, respectively) where the magnetic foliation (the plane normal to Kmin) is oriented perpendicular to the dyke plane, rather than parallel. Models of alignment of elongate particles in the viscous fluid flow during emplacement all predict magnetic foliation coinciding with the dyke plane (e.g., Cañón-Tapia and Chávez-Álvarez, 2004). Viscous drag from the walls is expected to produce a cross-dyke velocity profile, with a dyke-parallel shear plane and dyke-normal velocity gradient. In the numerical simulations of Cañón-Tapia and Chávez-Álvarez (2004), involving a range of shear strains and grain shapes, a small proportion (b5%) of resulting AMS fabrics had Kmin oriented in the shear plane (type ‘i’ fabric), and in these cases Kmax remained parallel to flow. This may be the origin of the minority AMS fabrics that we observe, although the ubiquity of type ‘i’ orientations in our study compliments their rarity in theoretical models. 5. Conclusions Thin crust and shallow magmatic sources seem to favor the propagation of vertical dyke intrusions that may also be fissure eruptions, as is the case in Iceland. Magmatic systems that feed great dyke swarms seem to flow upward in a central locale, then horizontally for many hundreds of kilometers. The Kenora– Kabetogama great dyke swarm appears to be the result of horizontal flow from a single vertical source, although caution must be exercised, at least with interpreting AMS data in such
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