Proterozoic paleomagnetism in the Nipigon Embayment ofnorthern Ontario: Pillar Lake Lava, Waweig Troctolite and
Gunflint Formation tuffs
G.J. Borradailea,∗, R.S. Middletonb
a Geology & Physics Department, Lakehead University, 955 Oliver Road, Thunder Bay, Ont., Canada P7B 5E1b East-West Resources, 402-905 West Pender Street, Vancouver BC, Canada V6C 1L6
Received 29 August 2005; received in revised form 19 October 2005; accepted 25 October 2005
Abstract
By using unusual combinations of demagnetization techniques, Proterozoic paleomagnetic vectors and paleopoles are providedfor two recently discovered post-tectonic Proterozoic units near Armstrong, northern Ontario, and also for well-dated GunflintFormation, which by previous techniques yielded problematical paleomagnetic data. The first paleomagnetic data are provided alsofor the Seagull Pluton and Inspiration Sills. Characteristic remanent magnetizations (ChRM) for thePillar Lake Lavas indicatea Keweenawan age, more specifically∼1000–1040 Ma by comparison with the well-established APWP for the Late ProterozoicSuperior craton. Four combinations of demagnetization techniques yield declinations in the range 108–133◦ and inclinations in therange−65 to−70◦ (n = 100), which define paleopoles near 200 W/48 N corresponding to a location on the Keweenawan APWP
Table 1Some relevant published geochronology for northern Ontario and N. Minnesota
Age (Ma) Decay series Source
KeweenawanMichipicoten Volcanic 1087+13
−7 U/Pb Palmer and Davis, 1987Portage Lake Volcanic (MN) 1096± 1.8 U/Pb Paces and Miller, 1993Osler Volcanic Group 1097± 3.7 U/Pb Davis and Sutcliffe, 1985
1107+4−2
Agate Point Rhyolite 1098± 0.6 U/Pb Davis and Sutcliffe, 1985Duluth Gabbro (MN) 1099± 0.5 U/Pb Paces and Miller, 1993Nathan Layered Series (MN) 1107± 0.6 U/Pb Paces and Miller, 1993Coldwell plutons 1108± 1.0 U/Pb Heaman and Machado, 1992Osler Porphyry 1108± 0.5 U/Pb Davis and Sutcliffe, 1985Nipigon-Logan diabase sills (cf. Inspiration Sills) 1109+4
−3 U/Pb Davis and Sutcliffe, 1985; Paces and Miller, 1993.Seagull Pluton 1112± 2.4 U/Pb Hart and Whaley (2005)Sibley Fm. 1339± 33 Rb/Sr Franklin, 1980;Sutcliffe, 1991
(reset?)∼1537 cf. U/Pb site* below(dep’n?)
English Bay Felsic Complex* 1537+10−2 U/Pb Davis and Sutcliffe, 1985
AnimikianGunflint Formation 1878± 1.3 U/Pb Fralick et al., 2002
ArchaeanMetamorphism of basement (age of late Archean,
Shaded: intrusive (English Bay Complex includes extensive tuff beds); MN: Minnesota, north shore of Lake Superior.
∼2701± 3 Ma (Davis and Sutcliffe, 1985). The non-conformable and unconformable cover of Proterozoicrocks is not deformed and tilts gently(≤5◦) toward theMid-continental rift system (Figs. 1 and 2) althoughthis is difficult to verify for Proterozoic plutons. TheProterozoic cover primarily comprises diabase sills, tra-ditionally collectively grouped as the Nipigon Sills,despite some petrological and geochemical distinctions.In order of decreasing abundance, sedimentary rocks,intrusions and lavas are also well known (Fig. 1,Table 1).Whereas these rocks are devoid of metamorphic fab-rics, there is subtle evidence of metamorphism to sub-greenschist facies with isotropic mineral assemblagesboth in pre-Penokean Gunflint Formation (Easton, 2005)as well as in the unconformably overlying post-PenokeanSibley Group (Franklin et al., 1980). It is unknownwhether this is related to burial-metamorphism or dis-tal hydrothermal expulsion from the southerly Penokeanevent (1870–1835 Ma;Easton, 2000; Holm et al., 1998)or even from the Grenville Event (1000 Ma;Hyodoand Dunlop, 1989). Thus, it is appropriate for paleo-magnetists to heed the concerns ofHalls and Zhang(1995)andZhang and Halls (1995)for the metamorphicre-setting of paleomagnetic directions in older Protero-zoic dikes and those ofSchmidt and Williams (2003)for re-magnetization in the pre-Penokean Gunflint sed-
imentary rocks. Crystallization–re-magnetization haseven been suspected for the younger Proterozoic rocks(Keweenawan), especially where potassic alteration hasoccurred (Middleton et al., 2004). Elsewhere, paleo-magnetists are aware that ephemeral metamorphic flu-ids may substantially reset paleomagnetic signals, com-pletely erasing or obfuscating primary magnetic direc-tions of adjacent platform regions by crystallization re-magnetization (Elmore and McCabe, 1991; Lu et al.,1991; Sun et al., 1993; Sun and Jackson, 1994).
The Proterozoic rocks occupy the∼1000 Ma Mid-continental Rift region (Green, 1983) with two arms ofthe rift approximately parallel to the NW and NE shoresof Lake Superior. The Nipigon region, north of LakeSuperior is characterized mainly by Proterozoic dia-base sills. It was tentatively considered as an aulacogenalthough the absence of substantial synchronous volcan-ism makes this unlikely (Green, 1983; Franklin et al.,1980). The Proterozoic sills and sedimentary rocks northof Nipigon are now described with less bias as the Nip-igon Embayment.
Our goal is to provide the first paleomagnetic resultsof three non-metamorphosed, Proterozoic igneous rocksthat have not been studied before. The Pillar Lake Lavasand the Waweig Troctolite which are recent undateddiscoveries (MacDonald, 2004), and the Seagull Plu-
Fig. 1. Simplified Proterozoic geology of the Nipigon Embayment with Archean metamorphic basement undifferentiated (after the GeologicalSurvey of Ontario).
Fig. 2. (a) Geology of the Pillar Lake area afterMacDonald (2004)and related work byMacDonald et al. (2004, 2005). (b) Available partlyoverlapping aeromagnetic map and first-derivative of the terrain-corrected Bouguer gravity map revealing the subcrop ring structure hypothesizedfor the Waweig Troctolite (simplified from geological Survey of Canada maps).
ton which is a well-known platinum-target dated at∼1113 Ma. Furthermore, although the Gunflint Forma-tion (1878 Ma,Fralick et al., 2002) generally yieldspaleomagnetic signals reset in the Keweenawan time(∼1000 Ma) (Schmidt and Williams, 2003), we mayhave obtained primary magnetizations by focussing ontuffaceous layers and using a different demagnetizationstrategy.
1.1. The present study
From our paleomagnetic evidence from previouslyunstudied rocks, we shall calculate the coeval paleopolepositions and compare them with the existing, publishedapparent polar wander path (APWP). The APWPs forthe Proterozoic rocks of northern Ontario have pale-opoles ranging from 2500 Ma to∼900 Ma, dated mostlyby U–Pb geochronology. Thus, we may at least inter-polate relative ages for from the paleopoles we havecalculated for the previously unstudied rocks describedhere.
Recently, a previously overlooked volcanic sequence,the Pillar Lake Lavas, was mapped underlying Pro-terozoic diabase sills byMacDonald (2004), south ofArmstrong in northern Ontario (Figs. 1 and 2a). Wenoted that the Pillar Lake Lavas are mostly quite fresh,undeformed and non-metamorphosed in flows that arepillowed or massive, from a few metres to >10 m inthickness. From different sections, we sampled at least10 different flow horizons separated by inter-flow brec-c rag-m thei d ora s isa eree laval ase-m age.T geso ndS msm them( byd akedc s hasn mag-n rlyd con-t tc howa ders
their paleomagnetic signals unstable. Overall, bedrock ismore poorly exposed than in general in northern Ontariodue to thick glacial overburden, apart from the usualproblems of dense forest and poorly accessible lakeshorelines.
Another new rock type we have investigated is asteeply layered troctolitic–gabbro complex (Fig. 2a andb) bearing neither evidence of regional metamorphism(fresh olivine and pyroxene are present) nor of pene-trative deformation. Its magmatic layering dips∼65◦Swhere it underlies the Proterozoic sills and the Pillar lakeLava (Map;Fig. 2a). Reasonably inferred as Protero-zoic, it is tentatively termed the Waweig Ring Complex,after its prominent arcuate geophysical signature thatshows its extension (Fig. 2b) to the south of the sampledarea (Fig. 2a). The Pillar Lake Lavas dip <4◦SE in thisarea, as determined from structure contours (Fig. 2a).Diabase sills form prominent topographic scarps withdip-slopes, gently to the SE or E also but their structurecontours and topographic expression transgress those ofthe lavas, indicating locally, a slightly greater dip anddifferent strike (Fig. 2a) which implies that the sills maycut through the lavas. Dip corrections for paleomagneticmeasurements were made from the field measurementsat individual sites; the dip correction was in all cases #4◦.These dips express regional tilting toward the Nipigonembayment, a probable early expression of the Protero-zoic Mid-continental rift. Since any tilting of the base-ment in this region (for example, toward the∼1000 MaMid-continental rift) may not postdate the troctolite, a
irec-illar
a aretionubjecteti-low
uge,lls,anyye
rre-hneti-esent-ut
ions.e
ias and volcaniclastic sediment in which some fents still retain nearly glassy portions. However,
nter-flow sediment layers mostly contain weathereltered volcanic-clasts and their maximum thicknest least 1 m. This indicates that the lava flows wrupted at different times. Within the present area,
ies unconformably upon Archean metamorphosed bent and we attribute the lava to a Proterozoiche Proterozoic “Logan” diabase sills, with U–Pb af 1009–1020 Ma (Paces and Miller, 1993; Davis autcliffe, 1985) overlie and crosscut the lavas, frotructural map relations (Fig. 2a). Sills crop out a fewetres above the lavas and therefore posdate
Fig. 2a) although the contact is invariably concealediabase talus. Unambiguous evidence of chilled/bontacts between the dated sills and undated lavaot yet been observed. By chance, the characteristicetic direction (ChRM) of the lava and sills is similairected up to the SE, so that the paleomagnetic “
act” field test (e.g.,Everitt and Clegg, 1962) cannoorroborate their relative ages. Both sills and lavas slteration possibly of a potassic character that ren
dip-correction was not made for its paleomagnetic dtions. Absolute ages are neither yet available for the PLake Lavas nor for the Troctolite. Paleomagnetic datprovided for these rocks by a variety of demagnetizatechniques; in all cases, every specimen has been sto incremental thermal or alternating field demagnzation, usually preceded by two or three cycles oftemperature demagnetization.
The Nipigon embayment is characterized by a hvolume of diabase sills (Fig. 1, Sutcliffe, 1984, 19871991), generally collectively termed the Logan Sifor which paleomagnetism is well-documented in mstudies fromDuBois (1960) until most recently bMiddleton et al. (2004). Paradoxically, throughout thNipigon embayment, diabase sills commonly cospond to magnetic lows (Fig. 2b) despite their higsusceptibility because their natural remanent magzations are both strong and usually oppose the prearth’s field (Middleton et al., 2004). This hinders interpretation of magnetic maps in their vicinity withomeasurements of the sills’ remanent magnetizat(The sills’ Koeningsberger ratios [Q] show a rang
19≥ Q ≥1.2; n = 522 from borehole logs;n = 209 out-crops.)
Recent work distinguished two new lithological vari-ants within the mapped distribution of diabase sills,termed the Nipigon and Inspiration Sills on the basisof mineralogy or trace element geochemistry, respec-tively (MacDonald et al., 2004). The Inspiration sillswere reported from the vicinity of the Pillar Lake Lavas.There is as yet no reason to suspect age differences thatare tectonically important for these geochemically dis-tinct diabase sills (∼1010–1020 Ma;Paces and Miller,1993; MacDonald et al., 2005; Hart and Whaley, 2005).Moreover, we shall show that their characteristic mag-netization directions are not significantly different.
Below we present new paleomagnetic data for thePillar Lake Lava, Waweig Troctolite, and Inspirationdiabase sills (the local equivalent of the regional LoganSills). We also present new paleomagnetic data for someother rocks in the region that have been precisely dated;the Gunflint Formation (1878 Ma;Fralick et al., 2002)and the Seagull Pluton (1113 Ma,Hart and Whaley,2005).
1.2. Regional setting
The Proterozoic Animikie Group of northern Ontariocomprises the largely clastic Gunflint and Rove forma-tions that almost completely escaped the effects of thePenokean tectonothermal event to the south, for whichthe metamorphic climax occurred at 1870–1835 Ma
,dja-ctive
of
ec-der-thern-ul-
1870
itedt
ossi-
nicen-ircael-uffs
(Fig. 1) (Sutcliffe, 1991). A previous study yielded aRb–Sr age for the Sibley Group of∼1340± 33 Ma(Wanless and Loveridge, 1978) which may correspond tore-setting by burial or other atectonic metamorphism thatproduced isotropic, epi-greenschist facies assemblages(Fraser et al., 1978; Franklin et al., 1980).
The uppermost part of the Proterozoic Keweenawanis characterized by volcanic rocks and diabase sillsin northern Ontario with volcanism, constrained to1109–1086 Ma from U–Pb geochronology (Davis andSutcliffe, 1985; Green, 1983). Whilst completely non-deformed, the rocks show zeolite to sub-greenschistfacies regional alteration controlled by depth or strati-graphic position (Franklin et al., 1980; Fraser et al.,1978). As with the other Proterozoic Groups of northernOntario, the hydrothermal/burial metamorphism couldreset the paleomagnetic record.
Proterozoic metamorphism in northern Ontario andnorthern Minnesota may be attributed to burial meta-morphism or, for suitable ages, post-Penokean fluid-expulsion, post-Penokean plutonism or rift-marginhydrothermal effects. There is incontestable petro-graphic evidence for sub-greenschist metamorphism insuitably sensitive lithologies and there may also be evi-dence for regional alkaline metasomatism with local-ized hydrothermal foci (Easton, 2000; Puschner et al.,1999). Fluid-related chemical re-magnetizations mayproduce fine magnetite grains with robust magnetiza-tions that may be confused with characteristic (pri-mary?) magnetizations (ChRM) whereas their magnetic
terlas-
werein
dd
1;al.,
ionrelydoc-l.,
t-ntlye ofor-
evalton
gantheupe-
(Easton, 2000; Holm et al., 1998). In northern Ontariothe Animikie Group may occupy a back-arc basin acent to the subduction inferred to have been ato the south (Fralick et al., 2002). U–Pb ages fromzircons in reworked thin-bedded (10–80 cm) tuffsthe Gunflint Formation yield an age of 1878± 1.3 Ma(Fralick et al., 2002). Despite the absence of any ttonic petrofabrics the Gunflint Formation has ungone sub-greenschist facies metamorphism in norOntario (Easton, 2000; Fraser et al., 1978). Metamorphism may be attributable to burial and fluid expsion from the Penokean belt, sometime betweenand 1935 (Penokean metamorphism) and∼1537 Ma(when the Sibley Group was unconformably deposabove).Schmidt and Williams (2003)recognized thamost Gunflint paleomagnetic signals were reset, pbly in Keweenawan times (∼1010–1090 Ma).
The Sibley Group shows no evidence of tectodeformation, comprising clastic and chemical sedimtary rocks, including red-beds, probably deposited c1537+10
−2 Ma by association with the English Bay Fsic Complex, which is characterized by stratified t
recording is in reality synchronous with some lamineralization. Paleomagnetism provides some csic examples of secondary magnetizations thatinitially confused with original signals, especiallyburied red-bed sequences (Butler, 1992; McElhinny anMcFadden, 2000) or due to widespread fluid flow analkaline metasomatism (Elmore and McCabe, 199Lu et al., 1991; Sun and Jackson, 1994; Sun et1993). Fortunately, Proterozoic rocks in the regyield rather simple magnetization histories. Only rahas chemical-hydrothermal re-magnetization beenumented (Halls and Zhang, 1995; Middleton et a2004; Zhang and Halls, 1995). In particular, most posSibley Group paleomagnetic targets are dominaigneous with thermoremanent magnetizations frenoteworthy alteration or burial/hydrothermal metamphism. The English Bay felsic tuffs (1537 Ma,Fig. 1)most closely date Sibley sedimentation from covolcanic–stratigraphic relationships. The Seagull Plu(1113 Ma) cuts Sibley Group strata as do the “LoSills” (1009–1020 Ma). Logan Sills occupy most ofdepressed Archean surface to the north of Lake S
Fig. 3. Flow directions inferred from mineral alignments defined by anisotropy of (low field) magnetic susceptibility (AMS). (a) From base ofDiabase sill overlying and possibly cutting Pillar Lake Lava; NW–SE flow axes imbricated and steeper than the dip of the sill. (b) By comparison,AMS-defined flow axes for the Logan diabase sills in the northen part of the Nipigon Embayment. (c) Flow axes inferred for the Pillar Lake Lava;essentially E–W and nearly perpendicular to the N–S margin of the Nipigon Embayment (equal area stereoplots).
rior which is known as the Nipigon Embayment. Withinthe sills, mineral alignments have been determined byanisotropy of magnetic susceptibility (low-field AMS)as well as by anisotropy of anhysteretic remanence(AARM) (Borradaile and Henry, 1997; Borradaile and
Jackson, 2004; Middleton et al., 2004). This revealsflow axes in the sills that plunge gently toward therift and aligned nearly perpendicular to the rift margin(Fig. 3d). The Pillar Lake Lavas and Waweig Troc-tolite are recently identified new components of the
igneous history, also free of any paleomagnetically ormetamorphically significant modification. AMS-definedflow axes in the Pillar Lake Lava are subhorizontal andalmost perpendicular to the N–S boundary of the NipigonEmbayment (Fig. 3c and d). Diabase sills appear to cutthe Pillar Lake Lava, just south of Armstrong (Fig. 2a),thus suggesting they are pre-1009 Ma. Their magma flowaxes, defined by magnetic fabrics, dip very gently towardthe N–S boundary of the Nipigon Embayment (Fig. 3aand d). The Osler Volcanic Group (1110–1086 Ma) iscurrently considered the youngest expression of igneousactivity in this part of the Mid-continental rift.
1.3. Previous paleomagnetic research
In order of popularity, sills, other igneous rocks, andcertain stratified rocks have been the subject of inten-sive and careful Proterozoic paleomagnetism for almost40 years. Unfortunately, the only stratified rocks thatpreviously readily yielded paleomagnetic informationare the Late Keweenawan Osler and related VolcanicRocks (∼1090 Ma,Table 1). Sedimentary rocks are gen-erally disappointing due to their intransigence to paleo-magnetic study (see review inSchmidt and Williams,
2003) as well as to geochronological study. Advancesin geochronology and in demagnetization techniqueslend more credibility to paleomagnetic information ofthe last 20 years but do not do a disservice to the pio-neering paleomagnetic work (inter alia DuBois, 1960;Irving and McGlynn, 1981; Robertson and Fahrig, 1971;Robertson, 1973; Pesonen and Halls, 1979). That definedan apparent polar wander path (APWP) for later Pro-terozoic (“Keweenawan”) poles (∼1600 to∼900 Ma)(Fig. 4; path [A]) that has only been corroborated andrefined by further work (inter alia Palmer et al., 1981;Stupavsky and Symons, 1982, 1983; Halls and Pesonen,1982; Pesonen and Halls, 1983; Costanzo-Alverez andDunlop, 1988; Symons et al., 1994; Middleton et al.,2004).
Refinements to geochronology have been moreimportant than those to pole-positions. Success with ear-lier Proterozoic rocks (2500–1800 Ma) is more recent,e.g., Fig. 4 (path [B]; inter alia Buchan et al., 1993,1996, 2000; Halls and Davis, 2004; Bates and Halls,1991). Formerly, a more westerly APWP track was underconsideration (Irving and McGlynn, 1981), essentiallyfollowing a locus from Canada (∼2000 Ma) along theAmerican continent to the south Atlantic (1750 Ma) and
c rocks desone ) forborate kyd Hall 4; Middleton
lier Pro 6,
Fig. 4. Proterozoic apparent polar wander paths for ProterozoiMcGlynn, 1981; Robertson and Fahrig, 1971; Robertson, 1973; Plater Proterozoic (“Keweenawan”) poles (∼1600 to∼900 Ma). Corroand Symons, 1982, 1983; Halls and Pesonen, 1982; Pesonen anet al., 2004; Zhai and Halls, 1994; Halls and Heaman, 2000). [B] Ear2000; Halls and Davis, 2004; Bates and Halls, 1991).
of Northern Ontario, ages in Ma ([A]inter alia DuBois, 1960; Irving ann and Halls, 1979). These defined an apparent polar wander path (APWPd and refined by further work (inter alia Palmer et al., 1981; Stupavss, 1983; Costanzo-Alverez and Dunlop, 1988; Symons et al., 199terozoic rocks (2500–1800 Ma) (inter alia Buchan et al., 1993, 199
back to central Canada (∼1600 Ma). Although it con-tains valuable paleomagnetic information which couldbe re-validated by improved geochronology and struc-tural corrections, it is not usually cited now (also DavidSymons, 2005, personal communication). For example,at that time it was not known that some sites were notstrictly integral components of the Superior craton or thatsome were affected by Grenville Tectonism (∼1000 Ma)(Hyodo and Dunlop, 1989).
Older Proterozoic rocks, especially cratonic maficdikes, generally are more precisely dated poles althoughthe number of poles currently available is sparse (e.g.,Fig. 4path [B] due to Buchan et al.,op.cit). For the olderProterozoic, recent careful work dated the Gunflint For-mation at 1878 Ma (Fralick et al., 2002). We may havesucceeded in determining their primary ChRM althoughprevious paleomagnetic work mostly recognized mag-netic overprints similar to Keweenawan paleofield direc-tions (Schmidt and Williams, 2003).
2. Paleomagnetic procedures
Syngenetic paleopoles may only be defined if thecharacteristic magnetic remanence (ChRM) is success-fully isolated from secondary magnetizations and re-magnetizations. This requires careful experimentationwith demagnetization techniques which change fromone study to another and from one lithology to another(Schmidt, 1993; Borradaile et al., 2001, 2003, 2004).The greatest complication arises where parts of twom orc etiza-t oer-c ag-n ures TD).T ag-n f thei atingg s asa dM ana
tions agnet tudy,r f 21d tured ndT ol-o lter-n ents
of demagnetization and re-measurement are essentialsince cores from the same outcrop, even from the samehand specimen, may require different levels of demagne-tization to isolate the same ChRM direction. Moreover,in the absence of multiple demagnetization and multipleremanence-component measurements it is not possibleto define the several points necessary to define a linearcomponent on a vector plot (e.g.,Fig. 5b here) (Schmidt,1993; van der Voo, 1993, p. 47;Butler, 1992, Chapter5), by principal component analysis (PCA; Kirschvink,1980).
The initial measurement of a specimen yields the nat-ural remanent magnetization (NRM), a vector sum ofnumerous differently oriented magnetic vectors of differ-ent ages. Normally, only one of these, the characteristicremanence (ChRM) will be of geological significanceand it may also be the primary or co-genetic magne-tization for the rock. NRMs for the diabase sills andPilar Lake Lava are mostly rather simple since they com-prise just two differently oriented components that werereadily isolated from one another by our demagnetiza-tion procedures. Their viscous overprints and any weakspurious magnetizations were readily removed by lowtemperature demagnetization (LTD). LTD requires twoor three cycles of cooling below the Verwey transition(120–135 K) in liquid Nitrogen (77 K), within a magnetic
weretingingthe
d. In0ved
hefol-forovedouldnoup
h aityeldagne-Mree-
agnetic components of different age are partlyompletely erased by the same steps of demagnion. Such components have either overlapping civity spectra when subject to alternating field demetization (AF) or overlapping unblocking-temperatpectra when treated by thermal demagnetization (hose specimens require a careful choice of demetization technique(s) and cautious evaluation o
ncremental demagnetization data to avoid genereologically meaningless paleomagnetic directiondvised by several authors (inter alia Dinares-Turell ancClelland, 1991; Dunlop, 1979; Halls, 1979; Hoffmnd Day, 1978).
Above all, one must apply multiple demagnetizateps to every specimen. On average, about 10 demization treatments were used per specimen in this sanging from a minimum of 7 steps to a maximum oemagnetization steps. Our choice of low temperaemagnetization (LTD or cryogenic cycling), AF aD or their combination was tailored to suit each lithgy on the basis of pilot studies of the effects of aate demagnetization procedures. Multiple increm
-
shield. Different cores from the same specimenssubjected to thermal demagnetization (TD), alternafield demagnetization (AF) with and without precedLTD to determine the most effective means by whichcharacteristic remanence (ChRM) could be isolatesome cases, thermal demagnetization (150, 200, 30◦C)before AF demagnetization, more effectively remocomponents with a low unblocking temperature (TUB)without risking laboratory-heating alteration. All tresults reported here used LTD prior to TD, AF or TDlowed by AF. However, with the benefit of hindsight,some sedimentary specimens, LTD may have remhematite-borne high-coercivity components that chave been “primary” (Borradaile, 1994). For this reasosome tentative results we obtained from Sibley Grrocks may be quite spurious (Table 1).
Thermal demagnetization was performed witMMTD80 unit manufactured by John Shaw (Universof Liverpool Paleomagnetic Institute); alternating fidemagnetization used a Sapphire Instruments demtizer with a peak AF field of 200 mT and automatic ARcancellation. AF demagnetization used a static th
Fig. 5. Pillar lake Lava outcrops, south of Armstrong, northern Ontario. The lavas are fresh, show clear primary volcanic features and are non-metamorphosed and free of any penetrative tectonic deformation. (a) Some thin flow units, top surfaces marked. The flows are separated byvolcaniclastic debris and sedimentary rock. (b) Detail of a lava flow base and the underlying sedimentary rock.
axes (six-permutation) scheme and the SPIN05.EXEsoftware alerts the possibility of spurious gyroremanentmagnetization artefacts, according to the procedure ofStephenson (1983, 1993).
Remanences were measured before each step of mag-netization with a Molspin magnetometer; in the caseof weakly magnetized Proterozoic sediments (Rove,Gunflint and Sibley Formations) our AGICO automatic
spinner magnetometer was used due to its higher sensi-tivity. Thermal demagnetization used temperature stepsthat included 150, 200, 250, 300, 350, 375, 400, 425,450, 475, 500, 525, 550◦C, as appropriate. Alternat-ing field demagnetization used 15–18 steps from peakAF of 2.5 mT to a peak AF of 120 mT. Susceptibil-ity of specimens was monitored between stages ofTD and AF demagnetization for pilot groups of spec-
imens (q.v. Potter and Stephenson, 1990). This mon-itors the slight possibility that remanence vectors areinadvertently deflected by laboratory-induced changesin anisotropy.
Vector plots were viewed interactively as three-dimensional rotatable graphs to verify the linearity ofstable vector components visually, before determiningthe characteristic direction by principal component anal-ysis (Kirschvink, 1980). Each stable vector was definedby ≥4 remanence-measurement points on the vectordiagram. Our software (SPIN05.EXE) also producestraditional two-dimensional Cartesian plots (Zijderveld,1967; Dunlop, 1979) and provide various interpreta-tion tools as well as controlling automating the mea-surements made with the Molspin or AGICO spinnermagnetometer.
Approximately half the cores were standard 25 mmdiameter cores with a height 0.82× diameter, for whichthe Molspin and AGICO spinners are equipped to handle.However, since the remanences of all igneous specimenswere high (>0.2 A/m) we used smaller core 12.5 mmin diameter with the same diameter–height aspect ratio.This reduces the cost of laboratory drill-bits and drillingwas faster. To orient the smaller core on the rotatingstage of the Molspin magnetometer we manufactureda Vespel holder with a cubic exterior form, into which
the 0.5-in. core was locked with a nylon set-screw.This arrangement permits precise alignment in succes-sive spin-orientations on the Molspin-stage, without theerror associated with manual alignment of standard 1-in.cores. Cores were core-drilled precisely in geographiccoordinates, in the laboratory by Anne Hammond, fromoriented field-block specimens of 2–5 kg.
3. Paleomagnetic results
Principal paleomagnetic details of the characteristicvectors are presented inTable 2; the details of the sitesand lithologies follow. In each case, the mean vectorfor a sample of ChRMs was calculated. However, it issafer to quantify our confidence in this direction by deter-mining the 95% confidence region for the mean-vectoron the stereogram. All confidence-estimates require amodel for the distribution; the simplest and usually mostvalid in the case of ChRMs from non-deformed rockis that they cluster in a fashionsimilar to a spherical-Normal or von Mises distribution on the stereogram; thisis termed the Fisher Distribution, afterFisher (1953).The only reasonable physical alternative for a singlegeneration of magnetic vectors is that the Fisher-typedistribution is “distorted” to an ellipse on the surfaceof the sphere. The simplest elliptical dispersion fol-
Table 2Paleomagnetic summary of characteristic (ChRM or “primary”) vectors
clinatio
P.1.2.1.5
.5
.3
.7
K .6
46.8
L : increm ncrementalt 95% ple (alls DE test eesC illiTesladm thep rage s cates somed
ontinen
ppropr
n Declination In
illar Lake Lavaa (≥6 flows in outcrop;≥3 at drill-site: PSV?)Surface LTD + Therm 39 108.3 −70Surface LTD + AF 20 114.3 −65Borehole LTD + Thermb 31 112.7 −66Borehole LTD + AFb 10 133.2 −69
ama Hill Limestone 12 146.1 −77Dolomite (Sibley Gp.) 4 beds (spurious)Gunflint tuffsa (6 beds) 17 303
TD: low temperature demagnetization (two or three cycles); AFhermal demagnetization (6–12 steps);α95: radius (degrees) for theamples acceptably Fisherian by Hypothesis test byFisher et al., 1987;elcius); Hcr (mT): coercivity of remanence for the ChRM in maleopole; PSV?: possibility that number of flows too few to aveelay between eruptions;n: specimens.a Dip-corrected (dips≤ 4◦ towards Nipigon embayment or Mid-cb Possibilty of drilling induced remanence?c Plutons in Archean basement, not dip corrected for want of ad LTD, thermal to 300◦C then AF.
ental (12–20 steps) alternating field demagnetization; Therm: iconfidence region of the mean vector, for Fisher-distributed sam-statistic);TUB (◦C): unblocking temperature(s) for the ChRM (degr;, dp: principal radii of the elliptical, 95% confidence region for
ecular variation but presence of interflow sedimentary units indi
t-rift), includes sites dated byFralick et al. (2002).
lows theBingham (1964)distribution; causes includetectonic strain of vectors or paleofield-deflection bythe rock’s intrinsic magnetic anisotropy (Borradaile,2003).
Following standard statistical procedures (Fisher etal., 1987, p. 149;Mardia and Jupp, 2000, p. 109), we firstestablished that the distribution was non-uniform. Thetest uses the statisticAn; if larger than a certain criticaltabled value (at the 95% confidence level), one may rejectthe null hypothesis that the sample was drawn from a uni-form distribution. This simple statistic requires the vec-tors to be reported in terms of longitude and co-latitude;they must also be centred with their mean direction ver-tical, at the centre of the stereonet; these procedures arepart of our software algorithm (SPIN05.EXE). All sam-ples of vectors in our study passed this test; the ChRMdistributions were significantly non-uniform in allcases.
Next, we determined whether or not it was reason-able to expect the sample to follow a Fisher-distribution.Fisher et al. (1987)provide a non-parametric test usingthe statisticDE. Where DE > 1.094, one must rejectthe hypothesis that the sample was drawn from aFisher-distribution population (Fisher et al., 1987; TableA8). Fisher et al. show thatDE is derived from theKolmogorov–SmirnovD-statistic, which relies on themaximum difference between two distributions to ascer-tain whether or not they were drawn from the same popu-lation. Thus, it is logical thatD be defined as a differencebetween the observed distribution and an exponential-
cificisect
ribu-ionsdes)deredinto
ll thet dis-5%nceamthe
u-as
gionion
thisre-
ctors
are determined from the central 95% of each sample-distribution.
3.1. Pillar Lake Lava
The lava was sampled at three main groups of sites(Fig. 2a), the first of which was an inclined borehole(045/78) with at least 99% core-recovery. Since thelava shows horizontal flow structure in outcrop and atthe borehole site outcrop, the presence of the flow-lamination in retrieved drill-core, permitted its reorien-tation into geographic coordinates (Fig. 6a). The restora-tion was performed using our software; by restoring theflow-layering’s pole to vertical, the ChRM directionswere correspondingly restored to their in situ orienta-tions. At least three separate flow horizons were sampledfrom the bore, which reached the Archean basementafter 132 ft of drilling. Random samples of stainless-steeldrill-stem used by Magnusson Drilling had no detectableremanence but one may never exclude the possibilityof drilling-induced remanence (DIR;Jackson and Vander Voo, 1985; Middleton et al., 1984). Although DIRcomponents are commonly core-parallel, our drill-sitevectors are shallower rather than too steep although theangular dispersion is worrisome for one demagnetizationstrategy (seeFig. 6).
LTD followed by 16-step AF demagnetization of10 cores and LTD (Fig. 6b and c) followed by ther-mal demagnetization of 31 cores yielded the resultsshown in Fig. 6d. At this site, secondary magneti-
har-pro-igh
sell
irec-tlyxam-
ated
two-
m-nts,
utedochres
re-
based Fisher-type distribution, thus yielding the spetest-statistic for our purpose,DE. Essentially the testbased on the distribution of inclinations with respto the mean vector of the sample. Thus, the disttion of observations must be first centred, inclinatare then converted to their complements (co-latituand as apparent below, the co-latitudes must be orin ascending magnitude. The test is incorporatedour measurement and interpretation software. In acases shown subsequently, the distributions are notinguishable from the Fisher-distribution at the 9level. However, we also show the Bingham confideregion about the maximum Eigenvector of the Binghdistribution if there is any detectable anisotropy inorientation-distribution.
Although all samples follow the Fisher distribtion, some contain outliers. These are defineddirections that lie beyond the 95% confidence refor the sample (as opposed to the confidence regfor the mean-vector). Any directions beyondsample-confidence region were excluded from thecalculation of the mean-vector. Thus, the mean-ve
zations were very scattered and not especially cacteristic of Bruhnes’ epoch re-magnetization appriate to the present latitude. The ChRMs, with hunblocking temperatures (>300◦C) or high coercivitie(>30–35 mT) are all upwards seeking and quite wclustered. LTD + TD produces a steeper cluster of dtions than cores demagnetized by LTD + AF. Differentreated cores were adjacent sub-samples. Further eples of demagnetization are shown inFig. 7from surfacesamples.
The bulk of the Pillar Lake Lava samples is fromleast six different flow horizons, intermittently exposalong and to the east of the Armstrong Highway inmain sections (Fig. 2a). LTD followed by AF demagnetization (Fig. 8a), and LTD followed by TD (Fig. 8b) allproduce well-defined clusters of ChRM directions, doinantly upward to the ESE. Secondary or B-componesub-parallel to the present earth’s field, are attribto viscous magnetization overprints by Bruhnes’ epNormal polarity; they are only well defined in 24 coof the sample subject to LTD + TD (Fig. 8a). LTDprior to TD appears to effectively remove spurious
Fig. 6. Paleomagnetic information from the Pillar Lake Lava at the borehole site. (a) Method of re-orienting drill core to geographic coordinates; (b)typical demagnetization plots; (c) intensity decay plots of AF demagnetization; and (d) review of stable ChRM vectors. All stereograms are equalarea projection in this paper.
magnetizations and sharpen the turning points that definethe distal ends of stable vectors. A comparison of all thePillar Lake ChRMs is provided inFig. 8c, where theFisher confidence regions substantially overlap, exceptfor the borehole site sample subject to LTD + AF demag-netization treatment. The prime question of paleomag-netic data from a newly described formation is whetherthe ChRM directions, and their mean vector, representprimary magnetization. The similarity of direction fromdifferent lava flows provides an arguable test but the pres-ence of almost antiparallel vectors from a few specimens(Fig. 8a and b) tentatively indicates the possibility of apositive reversal-test although the data are too few to
satisfy a statistical test. Probably, the most convincingarguments for the primary nature of their ChRMs are thefreshness of the lavas and the similarity of the ChRMdirections, from different flows, to those reported fromKeweenawan rocks (upwards to the SE;Fig. 9a and b).Although weathered and eroded interflow sediment mayindicate a protracted and intermittent extrusion history,the number of flows would otherwise be inadequate tohave time-averaged secular variation (≥6 flows from out-crops,≥4 from the drill site).
It is relevant to consider the importance of the dif-ference between these different demagnetization trials.Are the mean vectors (r mean vectors for each ofr
Fig. 7. Further examples of thermal and AF demagnetization of Pillar Lake Lava, from surface exposures.
samples) sufficiently similar that we can consider allr samples to have been drawn from the same popula-tion? Paleomagnetists commonly use the approach ofMcFadden and McElhinny (2000, p. 113 and earlier)toinfer an answer to theinformal question, “are two meanvectors the same?” That approach ultimately comparesthe degree of overlap of confidence regions in a semi-quantitative manner and assumes that the population isFisherian. Whereas each of the four samples (Fig. 8c) areacceptably Fisherian at the 95% level followingFisheret al.’s test (1987), the differences between them maylead us to suspect that the population was not Fisherianand a non-parametric test is preferred to compare meanvectors.
Fisher et al. (1987)andMardia and Jupp (2000)offera rigorous algorithm to compare directions, which doesnot assume a distribution-model for them (i.e., it is non-parametric) and reduces to objective hypothesis-testing.Most of their statistical procedures are simplified bycentering one or more sample-means on the vertical
direction (decl/incl = 360/+90 = trigonometric longitudeand latitude 360/00). In the case of samples with mean-vectors on opposite hemispheres (e.g., reversed paleo-magnetic polarities), testing is facilitated by reflectingone sample so that both are distributed on the same hemi-sphere. They may then be centred for convenience (ornecessity) of further calculations. For the comparisonof r sample-mean vectors,Fisher et al. (1987, p. 202)andMardia and Jupp (2000)designed test statistics thatmay be derived from the directions of all samples. Theirtest statisticGr is calculated from all the data in thersamples; it is designed to follow the well-known and con-venient�2-distribution. (Different authors use slightlydifferent expressions forGr and thus different criticalvalues for the 95th percentile.) In the procedure that wefollowed, we were obliged to reject the hypothesis thatthe two samples are drawn from the same populationwith the same mean if the calculated value ofGr > 5.99.Our statistical decisions are summarized inFig. 8d forthe Pillar Lake Lavas; the Null Hypothesis H0 is that
Fig. 8. Stable ChRM vector directions from Pillar Lake Lava. (a) Three-step LTD followed by 12-step thermal demagnetization. (b) Three-step LTDfollowed by≥14 steps of AF demagnetization. (c) Stereographic comparison of Fisher means and confidence regions. (d) Statistical hypothesis testof Pillar Lake Lava ChRM mean directions; all sub-samples tested positive for Fisherian nature.
Fig. 9. Equal area stereoplots of ChRM directions. (a) Inspiration Sills (diabase overlying and possibly cutting Pillar Lake Lava); inset compareswith two traditional Logan sills∼30 km to south. (b) Waweig Troctolite, a possible igneous ring-complex mostly occurring as subcrop to the diabasesills). (c) Seagull Pluton, dated at 1113 Ma.
the mean vectors are “the same”. More accurately, thetest defines whether, at the 95% confidence level, wemustreject or not reject the hypothesis that the sampleswere drawn from the same population. For Natural Sci-ence data, the 95% confidence level is usually the mostreasonable compromise between a Type I Error (reject-ing a true hypothesis) and a Type II Error (accepting awrong hypothesis) (Borradaile, 2003). It is interesting tonote that hypothesis-testing is also somewhat dependentupon discipline. Sample-sizes commonly used to definemean vectors and paleopoles in paleomagnetic studies(n = 10–20) are even smaller than those used in clini-cal medical trials. Engineering-testing, quality controlstudies and physics studies traditionally demand largersamples and may tolerate more stringent hypothesis tests(e.g., at the 99% level) without risking a Type II Error.
It is comforting that the demagnetization treatmentsof outcrops specimens by LTD + TD and LTD + AF andof borehole specimens by LTD + TD all result in the def-inition of the same ChRM (and subsequently nearly thesame paleopole) (Fig. 8d). However, we must reject thehypothesis that borehole specimens subject to demag-netization by LTD + AF were drawn from the sample.Since adjacent cores subject to LTD + TD agree with allthe outcrop data, and since LTD + TD treatments pro-duce the same results as LTD + AF for the outcrops, themost reasonable explanation of this aberration is that thesample of borehole specimens subject to LTD + AF istoo small (n = 10 versusn = 20, 31 and 39 for the othertrials). It may also be compounded by an inaccurate cor-rection for core-re-orientation from the drill-site sample.To err on the side of caution paleopoles are calculated
separately for all four of these trials shown inFig. 8cand d.
3.2. Inspiration diabase sills
As with our previous experience with the Loganand Nipigon sills, demagnetization by LTD + AF andLTD + TD treatments yielded strong stable ChRMs,directed up to the ESE with two almost antiparallelChRM vectors from one part of one of the three sillswe sampled (Fig. 9a). The antiparallel vectors hint ata positive reversal test but the data are too few to sat-isfy a rigorous statistical evaluation. However, the ChRMdirections are similar to proven primary magnetizationdirections in nearby Logan sills (Middleton et al., 2004)and provide circumstantial evidence that these are pri-mary Keweenawan magnetizations. Other Keweenawansites with more numerous data generally indicate thatthe opposed primary vectors are not precisely antiparal-lel (inter alia Pesonen and Halls, 1983). Unfortunately, acontact has not yet been exposed between the sills and theLavas that might provide a paleomagnetic contact-test(Everitt and Clegg, 1962) or visual evidence of a bakedcontact or chilled margin that might assist in assigningrelative lithological ages or verifying a primary age forthis magnetization.
3.3. Waweig Troctolite
This poorly exposed body provided coarse-grainedf arge
blocks. Due to their coarse-grained nature and lack ofinternal structure they were much more difficult to ori-ent than any other lithologies in this study. LTD fol-lowed by thermal demagnetization was the most suc-cessful demagnetization strategy which yields a looseconcentration (Fisherk = 15.6; normally we only workwith samples for whichk > 20). Despite the weakerconcentration, it is symmetrical, Fisherian, and definesan acceptably small confidence region for the meanvector that is up to the NE (Fig. 9b). This is quitedifferent from the young Keweenawan SE-up direc-tions found in the younger Proterozoic rocks in thisregion (e.g., Inspiration and “Logan type” sills,Fig. 9a).Their paleopole position lies near 1550 Ma on theAPWP (Fig. 11b), reasonably close to the GunflintFormation hematite overprint found bySchmidt andWilliams (2003). However, the Waweig Troctolite’smean vector (Table 2) differs at the 95% level fromthat of Schmidt & Williams Gunflint hematite overprint(decl/incl/α95 = 70.3/−51.4◦/3.2◦).
3.4. Seagull Pluton
This non-metamorphosed pluton is currently a targetfor platinum exploration and opportunities were avail-able to extract oriented blocks from ten exposures thatprovided a satisfactory sample. Demagnetization by LTDfollowed by thermal demagnetization and completed byAF demagnetization was found to be the most satisfac-tory strategy, avoiding alteration that may occur with
F nt Form (b)t
resh igneous specimens by hand-sampling of l
ig. 10. Six reworked tuffaceous sedimentary beds in the Gunfliypical demagnetization plot.
high temperatures. The ChRMs cluster tightly (k = 45)
ation, dated at 1878 Ma (Fralick et al., 2002). (a) Equal area stereoplot;
Fig. 11. Comparison of paleopoles calculated from our stable ChRM vector orientations with published paleopoles for the Superior ProvinceProterozoic. (a) Early Proterozoic paleopoles mostly from cratonic dikes with high precision U–Pb ages (in Ma). The data (boxed) from Scmidtand Williams are for their hematite-borne ChRM; they also found a clear magnetite-borne Keweenawan overprint. (b) Our data compared withavailable paleopoles for younger Proterozoic rocks of the Superior craton; the APWP is widely corroborated in general form although conjecturalin detail. Geochronology of the APWP is in some parts less precise than for (a) and although the APWP is somewhat debatable in detail, it is widelycorroborated in general form.
though some outliers impose a slight elongation to theorientation distribution (Fig. 9c). The mean vector wasdetermined excluding the orientations that lie outsidethe 95% confidence zone for thesample. The meanvector is slightly different from that found in manyKeweenawan rocks, such as the Logan diabase and OslerVolcanics, although the pluton’s age (∼1120 Ma) issimilar.
3.5. Gunflint Formation
Along the north shore of Lake Superior, nearthe Minnesota–Ontario border, the mid-ProterozoicGunflint Formation crops out. The principal lithologiesare well-bedded dark shales with silty horizons andiron-formations. However, several pale grey reworkedvolcanic ash horizons are evident, as 10–80 cm thickbeds. We sampled these selectively for paleomagneticpurposes. The strata are quite free from tectonicdeformation, although syn-depositional slump foldsand convolute lamination are evident in some horizons.However, subtle sub-greeenschist facies metamorphicassemblages, without tectonic fabrics are summarizedfrom earlier studies (Easton, 2000). The metamorphismmay be attributed to distal effects, perhaps by fluidexpulsion, of the Penokean orogen (pre-1760 Ma) to thesouth (Easton, 2000). Elsewhere such distal orogenichydrothermal events have been known to cause chemicalre-magnetizations (Lu et al., 1991; Sun et al., 1993; Sunand Jackson, 1994).
s at1 ns( e-op tionsa ly re-m rseda ear∼ titeo ta usinga thiso inedp outc edF dt liffs dia-b notw risko gand l
be interpreted as a Logan-diabase re-magnetization butthe absence of any well-defined secondary thermal re-magnetization (e.g.,Fig. 10b) leads us to suspect thatit may be primary, which is reasonably close to datedpole positions for the Molson dykes (∼1880 Ma;Hallsand Heaman, 2000on track A, Fig. 4). On the alter-native track ([B] mainly due to Buchanop. cit.) theinterpolated could be as great as 2200 Ma; however, thisseems unlikely in view of the precision of the recentlydetermined depositional age (1878 Ma) for the tuffs thatyielded the paleopoles here. This may be explainedby the fact that poles along track [B] are sparse andinclude sites for which it would be very difficult to ver-ify the absence of tilting, especially near the GrenvilleFront.
4. Conclusions
Table 2summarizes pertinent paleomagnetic statis-tics for what we believe are primary magnetic vectors.Their corresponding paleopole positions are calculatedwith their locational uncertainty expressed by thedpand dm, defining 95% confidence regions (A95) (seeFigs. 10 and 11).
(1) The Gunflint Formation reworked volcanic ashesare the rocks of known greatest age for which weretrieved stable ChRMs. Their paleopole plots closeto that obtained by Buchan for 2000 Ma U–Pb agedikes (Fig. 11a). This is agreeably close toFralick et
ageint
mayole,han
isr a
tero-
reith
ter-
earsro-
. Ifer-onoserble
The ash horizons provide precise U–Pb age878.3± 1.3 Ma for essentially synchronous zircoFralick et al., 2002). A detailed and very careful palmagnetic study bySchmidt and Williams (2003)sam-led the Gunflint shales and associated iron formand concluded that these lithologies were extensiveagnetized by the Logan diabase sills, giving Revend Normal Keweenawan magnetizations falling n1100 Ma and near 1550 Ma on their APWP (hemaverprints,Schmidt and Williams, 2003, fig. 9). Thalso has been our previous experience. However,different demagnetization strategy and applying
nly to the tuffaceous horizons we may have obtarimary directions. They are stable vectors, withlear overprinting and they follow a tightly clusterisher distribution (Fig. 10a), plunging down towar
he NW. We selected six different tuff beds in cections where we could establish that the youngerase sills were not locally present (i.e., at leastithin 20 m stratigraphic thickness) to reduce thef sampling sites thermally re-magnetized by Loiabase sills. Our mean vector (Fig. 10a) may stil
al.’s (2002)U–Pb age of 1878 Ma. A reasonablerange for the thick and slowly deposited GunflFormation could be 1800–2000 Ma, so that wecomfortably accept the agreement of our paleopBuchan’s and the U–Pb ages obtained by BucandFralick et al. (2002).
(2) Apparently, the next younger rock we studiedthe Waweig Troctolite. Its paleopole plots neasparsely documented part of the younger Prozoic APWP perhaps at∼1600 Ma (Fig. 4, track [A])or ∼1400 Ma (Fig. 11b). Our best indications athat it might lie in that age range by association wmost Nipigon plutonism. However, there is an alnative possibility that it may lie near∼2200 Ma onBuchan’s APWP (Fig. 11a).
(3) The well-dated Seagull Pluton (1113 Ma) appto tightly define a paleopole on the younger Pterozoic APWP between 1200 and 1100 Mathis pluton had been tilted similarly to the ovlying diabase sills (1009 Ma), a tilt correctiwould displace the paleopole westwards, clto 1120 Ma. Unfortunately, there is no relia
structural information with which to perform thecorrection.
(4) The Inspiration sills’ paleopoles plot∼1120–1140 Ma on the younger Proterozoic APWP, sim-ilar to the Logan sills in the region (Fig. 11b).
(5) The Pillar Lake Lava paleopoles, derived from dif-ferent demagnetization strategies and subsurfaceand surface samples plot close to 1110–1120 Ma onthe APWP (Fig. 11b).
Our results for dated lithologies (Gunflint Tuffs andSeagull Pluton) concur with existing APWPs and theirgeochronology. Our new contribution is to tentativelypropose “APWP ages” for the Waweig Troctolite at∼1400–1600 Ma, whereas the Inspiration Sills and thePillar Lake Lava may be Keweenawan, in the range1100–1140 Ma. It is possible that paleomagnetic signalswere reset regionally by fluids expelled from very dis-tant tectonothermal events such as the Penokean Event(∼1850 Ma) (Schmidt and Williams, 2003), and north-east of Lake Superior even from the Grenville event(∼1000 Ma) (Hyodo and Dunlop, 1989). Studies in otherregions indicate that such re-magnetizations may beimposed at great distances from the focus of tectono-metamorphic activity (Lu et al., 1991; Sun et al., 1993;Sun and Jackson, 1994).
The first most important general lesson that we havelearned is thatSchmidt’s (1993)recommendations aboutthe selection of demagnetization strategy seemed tobe the most important guide in isolating geologically
ialse.g.,ofiza-eachques
ayess-lls ingh
,po-
en-,m-con-uch
iallytherper-ation
by using frozen CO2 (Borradaile, 1994) which doesapproach the low temperature transition of magnetite.For example, whereas our LTD-combinations appearto have erased the important Keweenawan (hematite)overprint recognized bySchmidt and Williams (2003)in the Gunflint Formation, by serendipity this appearsto have isolated a possibly primary magnetizationin the Gunflint Formation Tuff (and provisionally inthe Sibley Group). Comparing different demagnetiza-tion strategies on different pilot sub-samples is advis-able but need not be limited to the combinations weused (LTD + AF, LTD + TD, LTD + TD + AF). Chemi-cal demagnetization, demagnetization by exposure tomicrowave radiation, TD in an inert gas and simultane-ous TD–AF are some other lesser common techniques(Butler, 1992; Dunlop andOzdemir, 1997; van der Voo,1993).
The second contribution to general understanding ismore parochial. The Proterozoic cover and igneous rocksof the Superior Province of the Canadian Shield are“non-deformed” and “post-metamorphic” in the classi-cal sense of structural geology. Proterozoic strata arealmost horizontal, pristine igneous mineral assemblagesare mostly evident, in contrast to the heterogeneouslydeformed, high-grade metamorphosed Archean base-ment. Nevertheless, whereas the epi-greenschist faciesmetamorphism, mostly for Sibley and earlier rocks(Easton, 2000), is a “negligible” concern to classi-cal structural geology there is now clear evidence thatthese epi-metamorphic events have modified paleomag-
1;3;
ro-rec-ockdied1;
f reli-venro-
rs toruc-,
aterress
to
er-
meaningful vectors from formerly intractable matersuch as the Gunflint Formation and Sibley Group (Schmidt and Williams, 2003). However, our successpreceding AF or TD with low temperature demagnettion (LTD) should not be considered as a panacea;demagnetization technique or combination of technimay isolate different characteristic vectors, which mserve different research goals. Although LTD succfully erases remanence associated with domain wapolydomain magnetite by cycling their lattice throuthe Verwey transition (∼120 K, Dunlop andOzdemir1997) it also LTD erases hematite remanence comnents at its Morin transition (near 210 K but depdent on lattice substitutions and grain size;Borradaile1994). Moreover, the LTD removal of hematite coponents may not be systematically related to theirventional stability determined by other methods sas AF and thermal demagnetization (Borradaile, 1994).Caution is therefore recommended with LTD, especwhere remanence is partly carried by minerals othan magnetite; of course, the effects of low temature may be restricted to hematite demagnetiz
netic signatures significantly (Bates and Halls, 199Middleton et al., 2004; Schmidt and Williams, 200Zhang and Halls, 1995).
The third general observation is that within Protezoic rocks, especially igneous rocks, structural cortions are extremely difficult. In certain areas, fault-blrotation is determinable and has clearly been stuby some paleomagnetists (e.g.,Bates and Halls, 199Halls and Zhang, 1995; Symons et al., 1994) but theheterogeneity of basement structure and absence oable markers usually prevent the restoration (or ethe recognition) of tilting events that affected Protezoic units. Paradoxically, paleomagnetism appeabe a better detector of fault-block rotations than sttural features in the Archean basement (Bates and Halls1991). Even small undetermined tilts (<10◦) may pro-duce errors in paleopole position that are far grethan their 95% confidence regions, which only expwithin-site vector-dispersion (A95; Fig. 11), discredit-ing their paleopole locations on APWPs. Attemptsreconstruct the Rodinia supercontinent (e.g.,Weil et al.,1998) require comparison of APWPs from different t
ranes but that goal will continue to elude us if we cannoteven establish reliable APWPswithin Archean terranes,as cautioned by Buchan et al. (2001).
Acknowledgments
Catherine Borradaile is thanked for help with manydays of field sampling and help in the laboratory.Bjarne Almqvist performed thermal demagnetization ofsome samples and Cory Hercun performed some of theAF demagnetization. Anne Hammond provided excel-lent drill-core in the laboratory from our hand-sampledblocks. We are indebted to the Natural Sciences andEngineering Research Council of Canada for 29 years ofcontinuous support to our laboratory. We are especiallygrateful to Dr. Phil Schmidt (CSIRO, Australia) and Dr.David Symons (University of Windsor) for thoughtfuland helpful reviews.
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