Paleomagnetism, geochronology and tectonic implications of ...users.clas.ufl.edu/jmeert/carion.pdfUNCORRECTED PROOF 1 Paleomagnetism, geochronology and tectonic implications of the

UNCORRECTED PROOF

1 Paleomagnetism, geochronology and tectonic implications of the

2 Cambrian-age Carion granite, Central Madagascar

3 Joseph G. Meerta,b, , Anne Nedelecc, Chris Halld, Michael T.D. Wingatee,4 Michel Rakotondrazafyf

5 aDepartment of Earth Sciences, Indiana State University, Terre Haute, IN 47809, USA

6 bNorwegian Geological Survey, Leiv Eirikssons vei 39, Trondheim, Norway

7 cUniversite Paul Sabatier, UMR 5563 CNRS, 38 rue des 36-ponts, 31400 Toulouse, France

8 dDepartment of Geological Sciences, University of Michigan, 2563 CC Little Bldg., Ann Arbor, MI 48109, USA

9 eTectonics Special Research Centre, University of Western Australia, Perth, WA 6845, Australia

10 fDepartement des Sciences de la Terre, Faculte des Sciences, Universite d’Antananarivo, BP 906, Antananarivo, Madagascar1112 Received 1 March 2000; accepted 16 February 2001

13 Abstract

14 The Carion granitic pluton in central Madagascar was intruded into warm continental crust following orogenic events related

15 to the final amalgamation of Gondwana. U–Pb SHRIMP dating of the pluton yields an emplacement age of 532.1 ± 5.2 Ma

16 followed by relatively slow cooling as constrained by 40Ar/39Ar ages on hornblende and biotite. Four hornblende samples

17 yielded a mean 40Ar/39Ar age of 512.7 ± 1.3 Ma. A biotite sample yielded an age of 478.9 ± 1.0 Ma. Paleomagnetic samples

18 from the pluton and surrounding country rocks exhibit either SE-upwardly directed magnetizations (mean Dec = 113�,19 Inc =� 56�, k = 106, a95 = 12�) or NW-downwardly directed magnetizations (mean Dec = 270�, Inc= + 64�, k = 30, a95 = 11�)20 that pass a reversal test with a classification of ‘C’ and an angular difference of 14.4�. The ‘normal’ (negative inclinations) and

21 ‘reverse’ (positive inclinations) directions also show a spatial bias within the pluton, suggesting a field transition from reverse to

22 normal during cooling. The paleomagnetic pole calculated from the mean direction falls at 6.8�S, 001�E (dp = 13�, dm = 17�).23 Estimates of the blocking temperature for the magnetization are compared to the cooling history of the pluton and an age of

24 508.5 ± 11.5 Ma is assigned to the pole. The Carion pole falls near similar-age poles from elsewhere in Gondwana, supporting

25 the idea that the major orogenic events during Gondwana assembly were complete. A slight revision of the Gondwana apparent

26 polar wander path (APWP) is proposed with rapid APW from 540 to 520 Ma; however, the proposed mechanisms to explain

27 this rapid APW (including intertial-interchange true polar wander (TPW) or enhanced mantle driving forces) cannot fully

28 explain all the data. D 2001 Published by Elsevier Science B.V.29

30 Keywords: Madagascar; Cambrian; Gondwana; Geochronology; Paleomagnetism

31

321. Introduction

33The assembly of the Gondwana continent during

34the latest Neoproterozoic to earliest Cambrian (550–

35530 Ma) resulted from the amalgamation of a variety

0040-1951/01/$ - see front matter D 2001 Published by Elsevier Science B.V.

PII: S0040-1951 (01 )00163 -9

* Corresponding author. Norwegian Geological Survey, Leif

Eirikssons vei 39, N-7491 Trondheim, Norway.

E-mail address: [email protected] (J.G. Meert).

www.elsevier.com/locate/tecto

*

Tectonophysics 6454 (2001) xxx–xxx

UNCORRECTED PROOF

36 of cratonic nuclei during distinctive orogenic events

37 and followed the breakup of the earlier Rodinia super-

38 continent (Powell et al., 1993; Meert and Van der Voo,

39 1997; Meert, 1999a,b; Fig. 1a,b). The principal oro-

40genic event in the region of eastern Africa and

41Madagascar is the 650–800 Ma East Africa Orogen

42(Stern, 1994) involving the coalescence of arc terranes

43in the Arabian–Nubian shield and culminating with

44continent–continent collision in the region of Kenya–

45Tanzania. A younger (� 550 Ma) orogenic pulse is

46evident in the regions of Mozambique, southern India,

47Antarctica, Sri Lanka, western Australia and southern

48Madagascar and was given the name Kuunga Orog-

49eny (Meert and Van der Voo, 1997). Meert and Van

50der Voo (1997) considered the East Africa Orogeny

51and the Kuunga Orogeny as distinct orogenic pulses

52in the formation of Gondwana, although others (Yosh-

53ida, 1995) dispute the notion of a divided East

54Gondwana required by the model of Meert and Van

55der Voo (1997).

56While a detailed accounting of these orogenic

57events is, in many cases, only poorly known, recent

58work indicates that the tectonic history of Madagascar

59may help unravel details of the collisional events

60during East Africa–East Gondwana assembly (Tucker

61et al., 1999; Handke et al., 1999; Kroner et al., 1999).

62According to most reconstructions, the island was

63located at the periphery of Rodinia (Fig. 1a) at

64� 800 Ma and rifted from Rodinia, along with the

65rest of East Gondwana, sometime after 750 Ma,

66although the exact timing of that separation is the

67subject of considerable debate (Torsvik et al., in press;

68Powell et al., 1993; Wingate and Giddings, 2000).

69Handke et al. (1999) describe the widespread occur-

70rence of calc-alkaline magmatism in central Madagas-

71car during the interval from 780 to 800 Ma. This

72magmatism was attributed to the presence of an east-

73ward-dipping subduction zone (beneath Madagascar).

74Nedelec et al. (1995) and Paquette and Nedelec

75(1998) detailed the emplacement of a suite of alkaline

76‘stratoid’ granites in central Madagascar at � 630 Ma

77during a post-collisional extension phase of tectonism.

78As described below, the 630-Ma phase was followed

79by transcurrent tectonics in central and southern

80Madagascar during the interval from 570 to 530 Ma.

81This interval also corresponds to the timing of the

82main metamorphism (collision) in southernmost

83Madagascar, Sri Lanka, southern India and Antarctica

84(Meert, 1999a). We consider this interval of tectonism

85(570–530 Ma) to represent the final stages of colli-

86sion between the elements of East Gondwana and East

87Africa. Fitzsimons (in press) noted that at least two

Fig. 1. (a) Paleoreconstruction of the supercontinent Rodinia at 750

Ma just prior to breakup along the (present-day) western margin of

Laurentia. All continents are rotated to a Laurentian reference

frame. The position of Madagascar is assumed to lie at the periphery

of the supercontinent as part of a united East Gondwana. (b)

Paleoreconstruction of Gondwana at � 510 Ma based on data in

Meert and Van der Voo (1997) and Meert (1999a).

J.G. Meert et al. / Tectonophysics xx (2001) xxx–xxx2

UNCORRECTED PROOF

88 ‘Pan-African’ age orogenic belts cut across the East

89 Antarctic shield that juxtapose three distinct crustal

90 fragments formerly thought to comprise the East

91 Antarctic ‘Grenvillian Craton’. Both Fitzsimons (in

92 press) and Meert (1999a) noted that the idea of an

93 East–West Gondwana collision oversimplifies the

94 history of Gondwana assembly. Indeed, although the

95 notion of a united ‘‘West Gondwana’’ has been

96 abandoned, the idea of a united East Gondwana is

97 still largely sacrosanct. Previous geochronologic work

98 on the Carion granite (Kroner et al., 1999) suggested

99 an age of 556 ± 1.7 Ma and we sampled these rocks in

100 attempt to document the position of Madagascar

101 during the final stages of Gondwana assembly.

102 2. Geological setting

103 The Carion granitic massif is situated east of

104 Antananarivo. It is about 20 km in diameter; its

105 elliptical shape is deeply indented in the north-west

106 (Fig. 2a,b). The Carion massif has been emplaced near

107 the western border of the Angavo belt (Fig. 2a), a

108 major Pan-African structure that runs parallel to the

109 eastern coast of Madagascar (Windley et al., 1994). At

110 the latitude of Antananarivo, the Angavo belt is

111 characterized by north–south steep foliations and

112 shallowly plunging lineations acquired in low-pres-

113 sure granulitic conditions averaging 790 �C and 3.3

114 kb (Ralison and Nedelec, 1997; Ralison, 1998; Nede-

115 lec et al., 2000). Paquette and Nedelec (1998) pro-

116 posed that this major shear belt was active during the

117 570–550-Ma time interval, as were other major N–S

118 shear zones in southern Madagascar (Paquette et al.,

119 1994; Kroner et al., 1996). On the basis of microprobe

120 monazite ages, Martelat et al. (2000) assign a younger

121 age (between 550 and 530 Ma) to the transcurrent

122 tectonics.

123 Lautel (1953) first suggested the post-tectonic set-

124 ting of the Carion granite. The porphyritic nature of

125 the rocks helps to recognize steeply dipping magmatic

126 foliations and lineations. These steep lineations con-

127 trast with the gently dipping lineations of the nearby

128 Angavo belt (Nedelec et al., 2000). Incipient high-

129 temperature solid-state deformation features (elongate

130 quartz grains, sometimes with chessboard subgrain

131 patterns) can be seen in thin sections, but no proper

132 orthogneissification has been observed. Foliations of

133the country rocks come into parallelism with the

134massif contour everywhere, with the exception of

135the northern indentation, where they appear to be

136crosscut by the intrusion. The Carion massif did not

137develop any contact metamorphism in its high-grade

138country rocks. All these data support a post-tectonic

139setting, but also show that the time lag between the

140formation of the Angavo shear belt (and its associated

141granulitic metamorphism) and the emplacement of the

142Carion magmas must have been short. Therefore, we

143prefer to assign a late- to post-tectonic setting to this

144massif, rather than a strictly speaking post-tectonic

145setting. Taking into account the late- to post-tectonic

146setting deduced from the considerations developed

147above, no significant uplift would have occurred

148between the formation of the Angavo belt and the

149emplacement of the Carion magmas. The pressure

150calculations, corresponding to depths of 10–11 km,

151presented by Nedelec et al. (2000) are good estimates

152to infer the depth of emplacement of the Carion

153massif.

1543. Paleomagnetic studies

155

1563.1. Sampling

157We collected 90 samples from eight sites within the

158Carion pluton in central Madagascar (Fig. 2b). Addi-

159tional 29 samples were collected in the country rocks

160(migmatites and granites) immediately adjacent to the

161Carion massif. Samples were drilled in the field using

162a gasoline-powered drill and oriented using both

163magnetic and sun compasses. The samples were then

164cut into specimens at Indiana State University and

165both bulk susceptibility and anisotropy of magnetic

166susceptibility (AMS) were measured using a Sapphire

167Instruments SI-2B susceptibility meter. Individual

168samples were then stepwise-treated using thermal

169(60 min at each temperature level) or alternating field

170demagnetization, and a treatment sequence was

171selected that resulted in the clearest separation of

172individual vector components. Rock magnetic tests

173including isothermal remanence acquisition (IRM)

174and three-axis IRM demagnetization experiments

175were performed in order to identify the main magnetic

176carriers in the rocks. Mean susceptibilities ranged

177from 1.8� 10� 2 to 7.4� 10� 2 SI and natural rema-

J.G. Meert et al. / Tectonophysics xx (2001) xxx–xxx 3

UNCORRECTED PROOF


UNCORRECTED PROOF

178 nent magnetization (NRM) intensities from 100 mA/

179 m to 10 A/m.

180

181 3.2. Paleomagnetic results

182 Table 1 summarizes the mean paleomagnetic

183 results for the Carion massif. Individual samples

184 exhibited a variety of unblocking schemes (Figs. 3

185 and 4). Some (Fig. 3a,b) showed near univectorial

186 decay using either thermal or AF demagnetization.

187Others (Fig. 3c,d) showed multi-component decay

188before reaching stable high temperature or high coer-

189civity directions. The overprint directions in these

190samples are discussed below. Samples that were not

191included in the analysis exhibited unstable behavior

192during demagnetization with no well defined linear

193segments or great-circle trajectories. The primary

194component shows dual-polarity magnetization with a

195clear spatial bias within the pluton (Fig. 4). We

196operate under the assumption that Madagascar was

Fig. 2. (a) Regional geologic setting of the Carion pluton in central Madagascar. The Angavo shear zone formed at ca. 550 Ma and was

subsequently intruded by the post-tectonic to late syn-tectonic Carion pluton. (b) Sampling locations within the Carion pluton. The numbered

squares refer to paleomagnetic sampling sites.

Table 1

Paleomagnetic results

Site number Latitude,

longitude

n/N Polarity Mean

susceptibility

(� 10� 6 SI)

Dec/Inc k a95 VGP

latitude

VGP

longitude

dp, dm Magnetic

lineation

Magnetic

foliation

pole

Site 1 18�55.286S,47�42.561E

12/12 Mixed 53,968 127�/� 54� 60 6� 16.1�S 184�E 6�, 8� 120�/68� 265�/20�

Site 2 18�55.530S,47�48.888E

14/14 Reverse 51,066 286�/ + 65� 29 8� 3.2�S 006�E 11�, 13� 132�/29� 246�/33�

Site 3 18�59.865S,47�47.252E

11/19 Mixed 50,461 271�/ + 64� 44 7� 13�S 002�E 9�, 11� 81�/65� 313�/16�

Site 4 18�58.815S,47�44.993E

4/7 Reverse 42,027 239�/ + 38� 34 16� 34.8�S 331�E 11�, 19� Random Random

Site 5 18�58.165S,47�44.308E

10/10 Reverse 18,502 283�/ + 75� 30 8� 11.0�S 020�E 13�, 15� 96�/43� 353�/16�

Site 6 18�55.547S,47�42.228E

9/9 Mixed 74,739 263�/ + 62� 42 8� 18.0�S 358�E 10�, 12� 291�/25� 199�/44�

Site 7 18�56.100S,47�45.349E

8/10 Mixed 67,096 284�/ + 64� 40 9� 4.0�S 005�E 11�, 13� 348�/61� 276�/10�

Site 8 18�56.771S,47�45.349E

10/11 Reverse 63,965 296�/ + 67� 30 9� 1.2�N 012�E 12�, 15� 173�/37� 287�/30�

Site 9 18�53.488S,47�43.460E

15/20 Mixed 60,429 113�/� 54� 41 6� 6.9�S 178�E 6�, 9� 142�/56� 299�/42�

Site 10 18�53.736S,47�41.534E

7/7 Mixed 60,030 100�/� 57� 42 9� 4.0�N 177�E 10�, 14� 135�/67� 267�/15�

Mean (all) 10/10 Mixed 278�/ + 62� 31 9� 6.8�S 001�E 10�, 14� 130�/64� 281�/31�Normal polarity 3/3 Normal 113�/� 56� 106 12� 6.7�S 180�E 12�, 17�Reverse polarity 7/7 Reverse 270�/ + 64� 30 11� 13.4�S 002�E 14�, 18�Carion only 8/8 Mixed 278�/ + 62� 27 11� 6.8�S 001�E 13�, 17�VGP mean 10/10 Mixed 20 11� 6.5�S 002�E A95 = 11�

n= number of samples used in the study; N= total number of samples collected; Dec = declination; Inc = inclination; k = kappa precision

parameter; a95 = cone of 95% confidence about the mean direction; VGP latitude, longitude = virtual geomagnetic pole positions; dp, dm= 95%

confidence about the paleomagnetic pole in the colatitude direction (dp) and at a right angle to the colatitude direction (dm); lineation =magnetic

lineation (Kmax) from anisotropy of magnetic susceptibility studies (AMS); pole to magnetic foliation = pole to the magnetic foliation based on

AMS studies (Kmin).


UNCORRECTED PROOF

Fig. 3. (a) Thermal demagnetization of sample CAR-24a (site 4) exhibiting near uni-vectorial decay, and the ‘reverse’ direction (closed) circles in the stereoplot represent down (+)

inclinations. (b) Alternating field demagnetization of CAR-24b also exhibiting near uni-vectorial decay. (c) Thermal demagnetization of sample CAR-56 (site 9) near the margin of the

Carion pluton showing a two-component magnetization with a high ‘hard’ normal direction (open) circles in the stereonet indicate up (� ) inclinations and (d) thermal

demagnetization of sample CAR-70 (site 10) showing multi-component behavior along a great-circle trajectory trending from a low-temperature ‘normal’ direction toward a high

temperature ‘reverse’ direction.

J.G.Meert

etal./Tecto

nophysics

xx(2001)xxx–

xxx6

UNCORRECTED PROOF

Fig. 4. (a) Polarity distribution observed in the Carion massif. Note that N)R simply means that the normal (� ) directions are unblocked at

lower temperatures than the (+) reverse directions. The actual proposed field transition is R)N. (b) Samples near the margin of the pluton

show low-temperature ‘normal’ directions and high temperature ‘reverse’ directions and (c) samples from the interior of the pluton show

predominately reverse directions.


UNCORRECTED PROOF

197 located in the southern hemisphere at the time of

198 remanence acquisition and therefore (� ) inclinations

199 are ‘normal’ polarity and (+) inclinations represent

200 ‘reverse’ polarities. Sites located at the margin of the

201 pluton (including two sites in the country rock imme-

202 diately adjacent to the pluton) exhibited either (a)

203 present earth field (PEF) overprints and an E/SE

204 intermediate up direction (Fig. 3c, Table 1) with a

205 mean direction of Dec = 113�, Inc =� 56� (k = 106,

206 a95 = 12�) or (b) a trend from an E/SE up direction

207 toward a W/NW intermediate down direction (Fig.

208 3d). Interior sites in the pluton exhibited either single

209 component directions with a W/NW intermediate

210 down direction with a mean of Dec = 270�, Inc= + 64�211 (k = 30, a95 = 11�) or (b) trend from N/NW down

212 towards a W/NW down direction (Fig. 4c). We dis-

213 cuss the possible significance of these directional

214 patterns in light of the geochronologic results below.

215 The overall mean direction obtained by inverting the

216 normal polarity directions (Fig. 5) is Dec = 278�,217 Inc= + 62� (k = 31, a95 = 9�). This yields a south paleo

218 magnetic pole at 6.8�S, 001�E (dp = 10.5�, dm =

219 13.6�). Exclusion of the non-Carion sites yields an

220 identical mean direction of Dec = 278�, Inc= + 62�221 (k = 27, a95 = 11�) and therefore an identical pole

222 (Table 1). McFadden and McElhinny (1990) devel-

223oped a test for antipodal reversals using small data-

224sets. In order to apply the reversal test, we have

225assumed that the normal magnetization acquired in

226the nearby country rocks at site 9 is contemporaneous

227with the normal polarity zonation in the Carion

228pluton. The angle between the mean normal direction

229(Dec = 113�, Inc =� 56�, n = 3 sites) and the reverse

230direction (Dec = 270�, Inc= + 64�, n = 7 sites) is 14.4�.231The critical angle for the reversal test according to

232McFadden and McElhinny (1990) is 14.5� and, there-233fore, the Carion mean directions pass a reversals test

234with a classification of ‘C’.

235

2363.3. Rock magnetic tests

237We conducted several rock magnetic studies on the

238Carion rocks both to determine the magnetic carriers

239and to characterize magnetic fabrics within the rocks.

240Unblocking temperature spectra (Fig. 6a) and coer-

241civity spectra (Fig. 6b) show characteristics of titano-

242magnetite and magnetite along with some (relatively

243minor) hematite. Most thermal demagnetization

244curves show ‘hard-shoulder’ characteristics at temper-

245atures above 500 �C (Figs. 3c,d, 4c and 6a) and many

246samples that show distributed blocking temperatures

247(Figs. 3a, 4b and 6a) still show a fairly hard shoulder

248above 500 �C. Isothermal remanent magnetization

249studies (Fig. 6c) show either saturation in fields of

2500.3 T or near saturation in fields of 0.1–0.3 T

251followed by a small rise in intensity in increasing

252fields, suggesting the presence of hematite in some

253samples. Three-axis IRM demagnetization studies

254(Lowrie, 1990) show the presence of several magnetic

255carriers (Fig. 6d,e,f,g). The rapid decay in intensity of

256the intermediate and soft components at low temper-

257atures (Fig. 6e,f,g) indicate the presence of goethite or

258high-Ti magnetite and the predominant loss in inten-

259sity above 575 �C is indicative of magnetite. Although

260not clearly depicted in the three-axis IRM results in

261Fig. 6c, there is a small stable component isolated at

262temperatures above 600 �C (Fig. 6a) that we attribute

263to hematite in a few of the samples. We feel that the

264main carriers of the primary remanence in the Carion

265rocks are titanomagnetite and pure magnetite with a

266minor component of hematite.

267Anisotropy of magnetic susceptibility (AMS) stud-

268ies complement those of Nedelec et al. (2000), who

269noted steeply dipping magnetic lineations (mean 142�/Fig. 5. (a) Site mean directions and a95 errors for both normal and

reverse polarity.


UNCORRECTED PROOF

270 70�) and foliations (mean pole: 299�/16�). Our study271 (Table 1) yielded less steeply dipping lineations (mean

272 130�/54�) and slightly steeper foliations (mean pole:

273281�/31�), but there is an overall consistency between

274the two studies. The major deformation is in the

275nearby Angavo shear zone (Fig. 2a) and it shows a

Fig. 6. Rock magnetic behavior of the Carion samples during (a) thermal demagnetization, (b) alternating field demagnetization, (c) isothermal

remanence acquisition, (d–g) three-axis IRM demagnetization, following the methodology of Lowrie (1990). Applied fields were 1.3 T along

the Z-axis of the core, 0.4 T along the Y-axis and 0.1 T along the X-axis.


UNCORRECTED PROOF

276 shallowly northern magnetic lineation (mean 352�/277 19�) distinct from the Carion pluton (Nedelec et al.,

278 2000). As noted above, we consider the Carion pluton

279 to be post to late syn-tectonic with regard to the

280 shearing in the Angavo Belt.

281 4. Geochronology

282

283 4.1. Previous work

284 Vachette and Hottin (1974) obtained two Rb/Sr

285 isochron ages of 682 and 527 Ma on the Carion

286 granite. The older age is likely devoid of any geo-

287 logical significance, as it was calculated by bringing

288 together various non-cogenetic rocks in addition to the

289 typical Carion porphyritic granite. Kroner et al. (1999)

290 reported a U–Pb evaporation age of 556 ± 1.7 Ma for

291 the Carion pluton, but the geochemical and petrologic

292 description of the sample along with the sampling

293 location (corresponding to our site 9) places it outside

294 the archetypal Carion massif. Indeed, we suggest that

295 the 556 ± 1.7-Ma age (Kroner et al., 1999) dates the

296 timing of the Angavo shear belt and the formation/

297 deformation of the host rocks rather than the emplace-

298 ment age of the Carion pluton. A subsequent study

299(Kroner et al., 2000) reported a new Pb–Pb evapo-

300ration age for the Carion pluton of 538 ± 0.9 Ma. In

301order to fully constrain the age of the Carion pluton,

302we undertook U–Pb SHRIMP and 40Ar/39Ar studies

303from the Carion rocks.

304

3054.2. U–Pb SHRIMP geochronology

306Sample ME36 was collected from the main coarse-

307grained monzogranitic phase of the Carion pluton (site

3088, analytical details are given in Appendix A). Almost

309all analyses are concordant to within analytical uncer-

310tainty (Fig. 7; Table 2). Ratios of 206Pb*/238U (Pb*

311indicates radiogenic Pb) are well grouped, with 21 of

31222 analyses yielding a weighted mean of 0.08604

313± 0.00036 (X2 = 0.6), equivalent to an age of 532.1 ±

3142.1 Ma (1s). The excluded analysis (8.1 in Fig. 7)

315yields a significantly younger age of 505 Ma, suggest-

316ing that this zircon has lost a small amount of radio-

317genic Pb. The age of the zircons can also be estimated

318by another method, without explicit correction for

319common Pb. If the Pb is a mixture of common and

320radiogenic Pb, the analyses will lie (by amounts

321proportional to their common Pb contents) along a

322mixing line between initial Pb, at U/Pb = 0, and radio-

323genic Pb, on Concordia. A regression through the

Fig. 7. (a) SHRIMP dating results of individual zircons listed in Table 2. Filled symbols show data used to calculate the mean age of

532.1 ± 5.2 Ma.


UNCORRECTED PROOF

324 uncorrected data, anchored at initial Pb and excluding

325 analysis 8.1, yields a Concordia intercept of 532.1 ±

326 2.0 Ma (1s), identical to the 204-corrected result. The

327 best estimate of the crystallisation age of the Carion

328 granite is 532.1 ± 5.2 Ma (95% confidence interval,

329 including calibration uncertainty).

330

331 4.3. 40Ar/39Ar geochronology

332 Although U–Pb ages can be reasonably correlated

333 to the timing of primary remanence acquisition in

334 rapidly cooled igneous rocks, slowly cooled plutonic

335 rocks generally acquire partial thermal remanences

336 (pTRMs) over a wide range of temperatures during

337 the cooling of the pluton. In order to better constrain

338 the timing of magnetic remanence acquisition during

339 the slow cooling of the pluton, we separated horn-

340 blende and biotite for dating by the 40Ar/39Ar method.

341 Fig. 8 shows stepwise degassing results from the

342 samples of hornblende (CAR-2, 7) and biotite (CAR-

343 31; Table 3). Each sample was split into two fractions

344that were run separately in order to test for consis-

345tency. A plateau was calculated as an error weighted

346mean where MSWD<2 and over 50% of the 39Ar was

347represented in the overlapping age spectral segment.

348Fig. 8a,b shows stepwise degassing of hornblende

349from CAR-2. A small amount of excess argon is

350observed at low laser power, but the two splits show

351well defined plateaus of 511.7 ± 0.7 and 511.5 ± 1.1

352Ma (all 40Ar/39Ar age errors are 1s unless otherwise

353noted). Fig. 8c,d shows stepwise degassing of sample

354CAR-7. These splits also yielded well defined plateau

355ages of 513.3 ± 1.0 and 514.1 ± 1.0 Ma. The mean ± -

356standard deviation of the hornblende plateau ages is

357512.7 ± 1.3 Ma.

358Fig. 8e,f shows stepwise degassing results from the

359biotites removed from sample CAR-31. Splits from

360this sample showed a bit of variation. The first split

361(V9a) shows a well-defined plateau age of 478.9 ± 1.0

362Ma, whereas the second split (V9b) yields a slightly

363older total gas age of 486.5 ± 1.0 Ma and a fairly flat

364spectrum around 485 Ma. The difference in ages

Table 2

Ion microprobe U–Pb analytical data for ME36 zircons

Grain area 238U (ppm) Th/U f206 (%) 207Pb*/206Pb * 206Pb*/238U Apparent age (Ma)

( ± 1s) ( ± 1s) ( ± 1s)

1.1 282 0.64 0.444 0.05506 0.00230 0.08563 0.00134 529.6 7.9

2.1 69 1.87 0.017 0.06358 0.00223 0.08747 0.00271 540.6 16.1

3.1 34 1.49 0.017 0.06004 0.00460 0.08539 0.00338 528.2 20.1

4.1 40 1.26 0.082 0.05646 0.00891 0.08619 0.00357 533.0 21.2

5.1 191 0.89 0.017 0.05863 0.00121 0.08555 0.00169 529.2 10.0

6.1 400 0.42 0.017 0.05819 0.00129 0.08573 0.00134 530.3 7.9

7.1 208 0.90 0.017 0.05740 0.00152 0.08695 0.00131 537.4 7.8

8.1 138 0.83 0.130 0.05881 0.00228 0.08143 0.00183 504.7 10.9

9.1 501 0.67 0.017 0.05882 0.00068 0.08710 0.00123 538.4 7.3

10.1 54 1.22 0.275 0.05821 0.00602 0.08414 0.00228 520.8 13.6

11.1 210 0.94 0.033 0.05823 0.00225 0.08614 0.00199 532.7 11.8

12.1 69 1.31 0.018 0.05948 0.00275 0.08425 0.00881 521.5 52.4

13.1 218 0.86 0.017 0.05904 0.00126 0.08555 0.00194 529.1 11.5

14.1 172 2.16 0.145 0.05809 0.00237 0.08546 0.00162 528.6 9.6

15.1 107 1.86 0.091 0.05768 0.00292 0.08737 0.00173 539.9 10.3

16.1 253 0.81 0.078 0.05859 0.00188 0.08690 0.00163 537.2 9.7

17.1 189 1.47 0.017 0.05840 0.00106 0.08707 0.00134 538.2 7.9

18.1 118 1.26 0.018 0.05915 0.00196 0.08425 0.00165 521.4 9.8

19.1 80 1.20 0.015 0.05890 0.00607 0.08533 0.00270 527.9 16.1

20.1 129 0.79 0.519 0.05557 0.00255 0.08356 0.00243 517.3 14.5

21.1 266 0.56 0.143 0.05829 0.00208 0.08566 0.00113 529.8 6.7

22.1 328 0.64 0.040 0.05931 0.00109 0.08610 0.00099 532.5 5.9

Analyses are listed in the order in which they were conducted. f206 is the proportion of common 206Pb in total measured 206Pb. Apparent ages are

calculated from 206Pb*/238U ratios.


UNCORRECTED PROOF

Fig. 8. (a–d) Stepwise degassing spectra for hornblende samples (two splits per sample) CAR-2 and CAR-7 along with the corresponding Ca/K

spectra; (e– f) stepwise degassing spectra for biotite sample CAR-31a along with the corresponding Ca/K spectra. Data are listed in Table 3.


UNCORRECTED PROOF

365 obtained from these two splits of CAR-31 might be

366 attributed to the higher Ca-concentration in the second

367 run (V9b), suggesting that an additional phase (horn-

368 blende?) influenced the apparent ages in that sample.

369 Therefore, we adopt the 478.9 ± 1.0 Ma plateau from

370 split V9a as more representative of the biotite 40Ar/39

371 Ar age for the sample.

372

373 4.4. Cooling history and age of magnetization

374 One of the main objectives of this paleomagnetic

375 study is to determine the age of magnetization in the

376 Carion pluton. In order to constrain the age of magnet-

377 ization, it was necessary to determine the thermal

378 history of the pluton and its relationship to the block-

379 ing temperatures of the magnetic minerals. Although

380 there are close analogies between the closure temper-

381 ature (Tc) of isotopic systems and the blocking tem-

382 perature (Tb) of magnetic minerals, the comparison is

383 not straightforward (McDougall and Harrison, 1999;

384 Dodson and McClelland-Brown, 1985; Dunlop and

385 Hale, 1977; Pullaiah et al., 1975). Nevertheless,

386 several attempts have been made to relate the timing

387 of magnetic remanence with the thermal history of the

388 rocks with varying success (Costanzo-Alvarez and

389 Dunlop, 1998; Briden et al., 1993; Berger and York,

390 1979, 1981a,b; Buchan et al., 1977; Buchan and

391 Dunlop, 1976). Most of these studies were aimed

392 toward deciphering complex magnetic directions in

393 polymetamorphic terranes. Although the exact rela-

394 tionship between Tb and Tc is not rigorously defined,

395 knowledge of the thermal history of the Carion pluton

396 will provide some constraints on the age of magnetism

397detailed in this study. There are two major assump-

398tions that are made in the following analysis. The first

399is that the higher unblocking temperature magnet-

400ization in the rocks is a result of pTRM acquisition

401during the slow cooling of the pluton rather than some

402younger chemical or viscous process. The second is

403that the blocking temperatures (Tb) observed in the

404rocks are closely related to the closure temperature of

405the isotopic system (Tc) used to define the cooling

406history. In our analysis, we will argue that while each

407assumption introduces some error into the age, the

408spread of magnetic ages determined in the following

409analysis includes a reasonable estimate for the age of

410magnetization.

411Zircononium saturation temperatures and thermo-

412barometry indicate that the Carion magma intruded

413warm crust (� 780 �C) at a temperature between 844

414and 889 �C (calculated from data in Madison Raza-

415natseheno, 1998). The crystallization temperature for

416magmatic zircons is taken as approximately 850 ± 50

417�C and, therefore, the Carion magma had cooled

418below this temperature by 532.1 ± 5.2 Ma. The closure

419temperature for hornblende has been widely studied

420because of its utility in 40Ar/39Ar dating (McDougall

421and Harrison, 1999). Among the more controversial

422issues is the dependence of Tc on Mg#, where Mg# is

423defined as: Mg# =Mg/(Mg + Fe) (Harrison, 1981;

424Onstott and Peacock, 1987; Cosca and O’Nions,

4251994). Baldwin et al. (1990) investigated Fe-rich

426hornblendes and concluded that the results of Harrison

427(1981) were appropriate for hornblendes with Mg#

428less than 0.5. The Mg# of four samples distributed

429throughout the Carion pluton is 0.61 (calculated from

Table 340Ar/39Ar results

Sample Mineral Pkg Run 37Ar/39Ar Total gas

age (Ma)

Plateau

(MSWD)

%39Ar in

plateau

Plateau

age (Ma)

CAR-2a Hbl V7 A 3.845 522.25 ± 0.81 0.9 69.5 511.71 ± 0.74

CAR-2a Hbl V7 B 4.622 524.60 ± 1.08 1.61 58.9 511.53 ± 1.11

CAR-7b Hbl V8 A 3.421 518.86 ± 1.05 1.59 65.8 513.27 ± 1.04

CAR-7b Hbl V8 B 3.466 518.99 ± 1.03 1.53 85.9 514.07 ± 1.04

CAR-31ac Bio V9 A 0.00365 478.21 ± 1.00 1.53 85.8 478.91 ± 1.01

CAR-31ac Bio V9 B 0.1517 486.52 ± 1.01

All error estimates are 1s. Plateau ages were calculated as an error weighted mean where MSWD<2 and over 50% of the 39Ar was represented

in the overlapping age spectral segment.a J-value: 9.52708e� 3 ± 1.32794e� 5.b J-value: 9.50050e� 3 ± 2.00610e� 5.c J-value: 9.48803e� 3 ± 2.21569e� 5.


UNCORRECTED PROOF

430 data in Madison Razanatseheno, 1998) and the differ-

431 ence in age between the zircon U–Pb and 40Ar/39Ar

432 age of hornblende is � 20 Ma. Applying the diffusion

433 rate of Harrison (1981), the Tc for our hornblende

434 would lie between 450 and 500 �C. Dahl (1996)

435 applied an ionic porosity (Z) model to estimate Tc of

436 hornblende. This model depends on accurate measure-

437 ments of the Mg#, A-site occupancy, Al-Tschermack

438 substitution and Fe3 + . Microprobe analyses of horn-

439 blendes from the Carion pluton yielded ionic porosity

440 values of Z = 37.1 and Z = 37.6 (see Dahl, 1996 for

441 details; Madison Razanatseheno, 1998). Assuming a

442 cooling rate of � 18 �C/Ma, the Dahl (1996) model

443 yields a consistent result of Tc = 550 �C for our ede-

444 nitic hornblende. Therefore, we assign a Tc = 500 ± 50

445 �C to the hornblendes in our study.

446 The closure temperature for biotite also shows

447 significant variability as a function of composition

448 and cooling rate (see McDougall and Harrison, 1999).

449 Grove (1993) noted that argon diffusivities in biotite

450 are enhanced by replacement of Mg2 + by Fe2 + in the

451 lattice; lowered by incorporation of Al(VI) into the

452 octahedral sheet and significantly lowered by replace-

453 ment of the hydroxyl group by halogen group ele-

454 ments such as F� . Our biotites are characterized by a

455 lack of Al(VI), Fe# of 0.344 and F# of 0.333 (see

456 Madison Razanatseheno, 1998 for detailed geochem-

457 istry). These compositions will have a relatively small

458 influence on argon diffusivity and, collectively, we

459 consider that a reasonable closure temperature for

460 biotite in the Carion pluton is 350 ± 50 �C.461 The combined cooling curve is shown in Fig. 9.

462 The pluton was cooled from 850 �C at 532 ± 5 (U–Pb

463 zircon) Ma to 500 �C at 512.7 ± 1.3 Ma (40Ar/39Ar

464 hornblende), or approximately 18 �C/Ma slowing to

465 � 4 �C/Ma between 512 and 478 Ma. The slow

466 cooling is consistent with the idea that the Carion

467 pluton was intruded into already warm continental

468 crust (� 780 �C).469 Detailed theoretical thermal blocking–relaxation

470 time (Tb–t) relationships for pure magnetite and

471 hematite were developed by Pullaiah et al. (1975).

472 The approach was to extrapolate geologically reason-

473 able Tb for magnetic minerals from laboratory

474 unblocking temperatures (Tbl) and relaxation times

475 and vice versa. The characteristic remanence in most

476 Carion pluton samples is carried by magnetite with at

477 least some percentage of grains exhibiting ‘hard-

478shoulder’ behavior at temperatures above 500 �C479(Figs. 3c,d, 4c and 6a). Each sample was treated for

4801 h in the laboratory at a particular temperature step.

481Fig. 9 shows the range of Tb’s for the Carion pluton

482samples. The best estimate for the magnetization age

483is taken from the midpoint of the magnetic blocking

484temperature range ± the upper and lower age intercepts

485as determined from the isotopic studies. Although

486there are clearly many caveats to the above approach,

487we feel that the analysis provides a reasonable esti-

488mate for the age of the paleomagnetic pole of

489508.5 ± 11.5 Ma.

4905. Discussion

491The Carion pluton paleomagnetic data yields a pa-

492leomagnetic south pole at 6.8�S, 001�E (dp = 10.5�,

Fig. 9. Cooling history of the Carion pluton obtained from U–Pb

and 40Ar/39Ar isotopic ages. The possible range in geologically

reasonable magnetic blocking temperatures (Tb) are also shown

along with the intersection of those Tb with the cooling curve.

Collectively, they provide an estimate of the upper and lower

bounds of the magnetic age. Error bars reflect the uncertainty in age

and closure temperature of the isotopic systems as described in the

text.


UNCORRECTED PROOF

493 dm= 13.6�) with an age of 508.5 ± 11.5 Ma. This

494 Middle Cambrian-age pole is the oldest pole from the

495 island of Madagascar. According to most tectonic

496 models, the assembly of this part of Gondwana was

497complete by this time and, therefore, the pole should

498fall near similar-age poles from other regions of

499Gondwana. Fig. 10a shows the position of the Carion

500pole (after rotation to Africa) along with other poles

501from Gondwana that span the 550–500-Ma interval

502(Table 4). Several of the poles in Table 4 have been

503recalculated from Klootwijk (1980) by eliminating

504sites that had a95 errors greater than 20�. In most

505cases, the resulting pole did not differ significantly

506from the original pole. Fig. 10b shows a newly

507proposed apparent polar wander path (APWP) based

508on the poles listed in Table 4. The new path is very

509similar to that proposed by Meert and Van der Voo

510(1996) in its general trend; however, it differs from

511the longer length path proposed by Kirschvink et al.

512(1997) or the more tortuous path of Evans (1998).

513The difference in the paths is largely the result of

514conflicting interpretations regarding the significance

515of ‘outlier’ poles from Gondwana (e.g. poles 15 and

51623 of Table 4 and the Black Mountain pole of

517Ripperdan and Kirschvink, 1992). We have chosen

518the conservative approach by assuming that these

519more remote poles are indeed outliers and that the

520heavier concentration of poles along our proposed

521path adequately describes the motion of Gondwana

522during the 550–500-Ma interval.

523The paleomagnetic and geochronologic data from

524this study also suggests that the Earth’s magnetic field

525underwent at least one period of reversal during the

526acquisition of the remanence in these rocks. On the

527basis of the Tb spectra, we propose that the sequence

528was from reverse field (R) to normal field (N) as the

529reverse direction is found primarily in the higher Tb530magnetites (see Figs. 3 and 4).

531Our path indicates a period of rapid motion

532between � 540 and 520 Ma of � 1.94�/Ma (or

533� 21 cm/year) and a slightly lower rate of motion

534between 520 and 510 Ma of 1.48�/Ma (� 16 cm/

535year). These rates of apparent polar wander (or higher

536using the analysis of Kirschvink et al., 1997) invite

537speculation about a causative mechanism. True polar

538wander (TPW) is the most commonly cited explan-

539ation for periods of rapid APW (Van der Voo, 1994;

540Kirschvink et al., 1997; Evans, 1998). However,

541TPW is difficult to establish in the absence of data

542from other continents for the same period of time

543(Meert, 1999b). In fact, new data from Cambrian

544rocks in Scandanavia (Torsvik and Rehnstrom, in

Fig. 10. (a) The mean paleomagnetic pole for the Carion pluton

(white dot) along with poles from Gondwana in the age range of

550–500 Ma (pole numbers correspond to those in Table 4) and (b)

a possible apparent polar wander path (APWP) for Gondwana

during that same interval based on the poles and/or mean poles

listed in Table 4 (see text for complete discussion).


UNCORRECTED PROOF

545 press) suggest that Baltica experienced very little

546 APW and, therefore, an explanation that relies on

547 TPW to explain these high rates is less compelling.

548 Alternative explanations include rapid motion of

549 Gondwana away from sites of mantle upwelling or

550 toward sites of mantle downwellings (Gurnis and

551 Torsvik, 1994; Meert, 1999b). We cannot distinguish

552 on the basis of a limited dataset which of these

553 explanations offers a more reasonable hypothesis for

554 the observed rapid APW of Gondwana, but it may be

555 a combination of a smaller component of TPW along

556with enhanced motion away/toward thermal anoma-

557lies in the mantle.

558Fig. 11a–d shows a series of paleolatitudinal re-

559constructions for Gondwana during these time peri-

560ods. The rapid motion is easily visualized by using the

561West African craton (dark-shading) as a reference

562point within Gondwana. The motion between 540

563and 520 Ma for western Gondwana is poleward and

564is in good agreement with the occurrence of tillites

565presumably of this same age in the Taoudeni Basin

566(Mauritania; Bertrand-Sarfati et al., 1995).

Table 4

Paleomagnetic poles from Gondwana

Pole

number

Name of pole a95 Plat Plong Age Reference

1 Lower Arumbera SS 12.0� � 8.2� 338.8� 550.0 Kirschvink, 1978

2 Brachina Fm 16.0� � 7.4� 323.5� 550.0 McWilliams and

McElhinny, 1980

3 Bunyeroo 10.7� 28.2� 349.7� 550.0 McWilliams and

McElhinny, 1980

4 Sinyai Dolerite 5.0� � 28.4� 319.1� 547.0 Meert and Van

der Voo, 1996

5 U Arumbera SS 4.1� � 12.6� 337.6� 535.0 Kirschvink, 1978

6 Todd River 6.7� � 9.0� 336.5� 535.0 Kirschvink, 1978

7 Hawker Group-Aa 11.4� 25.5� 351.2� 525.0 Klootwijk, 1980

8 Hawker Group-Ba 21.2� 21.2� 348.3� 525.0 Klootwijk, 1980

9 Ntonya Ring Structurea 1.9� 27.5� 344.8� 522.0 Briden et al., 1993

10 Mean Sao Francisco Polea,b 12.9� 29.3� 355.2� 520.0 D’Agrella-Filho

et al., 2000

11 Sor Rondane 4.5� 10.6� 8.3� 515.0 Zijderveld, 1968

12 Aroona-Wirealpa-Ac 14.4� 11.1� 1.0� 510.0 Klootwijk, 1980

13 Aroona-Wirealpa-Bc 22.6� 15.2� 354.4� 510.0 Klootwijk, 1980

14 Tempe Fmc 5.0� 11.1� 355.0� 510.0 Klootwijk, 1980

15 Hugh River Shales 9.0� 61.8� 12.4� 510.0 Embleton, 1972

Carion Granite 11� 12.7� 359.7� 508.0 This study

16 Hudson Fmc 14.0� 21.0� 357.6� 508.0 Luck, 1972

17 Lake Frome-Ac 10.1� 18.2� 5.1� 505.0 Klootwijk, 1980

18 Lake Frome-Bc 27.7� 13.0� 1.0� 505.0 Klootwijk, 1980

19 Giles Creek Dolomite-Lc 32.6� 12.7� 0.2� 505.0 Klootwijk, 1980

20 Giles Creek-Uc 11.7� 9.6� 9.1� 505.0 Klootwijk, 1980

21 Illara Sandstonec 10.8� 15.8� 351.1� 505.0 Klootwijk, 1980

22 Deception Fm 6.5� 13.6� 351.7� 500.0 Klootwijk, 1980

23 Shannon Fm 10.6� 64.9� 1.8� 500.0 Klootwijk, 1980

24 Mt. Loke/Killer Ridge 8� 34.0� 1.6� 499.0 Grunow and

Encarnacion,

in press

All poles were rotated to African coordinates using the following euler poles: Australia 24.5, 112.3, � 56.3; East Antarctica: � 7.78, � 31.42,

+ 58; South America: 45.5, � 32.2, + 58.2; Madagascar: � 3.41, � 81.7, + 19.73 (Lawver and Scotese, 1987).

Pole number reference for Fig. 11; Plat = pole latitude (� S), Plong = pole longitude.a Mean of poles #7–10 is 25.9�N, 349.8�E, A95 = 6�.b This pole is a mean (� 520 Ma) taken from the BGB, BGC, SF and PQ poles in the cited reference.c Mean of poles #12–21 (excluding #15) is 14.2�N, 359.4�E, A95 = 4.2�.


UNCORRECTED PROOF

567 Finally, we note that one of the major concerns

568 regarding paleomagnetic results from a large igneous

569 pluton is the possibility of vertical axis or non-vertical

570 axis rotation. Our study is limited in addressing this

571 problem because our sampling comes from a single

572 pluton. Nevertheless, we note that theromobarometric

573 results from the Carion pluton and surrounding rocks

574 (Nedelec et al., 2000) are compatible with a relatively

575 simple uplift history following intrusion and that the

576 Carion pluton was emplaced in a post-tectonic to late

577 syn-tectonic setting. We also note the strong agree-

578 ment between our Carion pole and similar age poles

579 from elsewhere in Gondwana as evidence in support

580 of our claim that little, if any, significant tilting or

581rotation of the Carion pluton has occurred since its

582emplacement.

5836. Conclusions

584The Carion granitic pluton in central Madagascar

585was intruded into warm continental crust following

586orogenic events related to the final amalgamation of

587Gondwana. The late syn-tectonic to post-tectonic set-

588ting of the pluton is supported by the geochemistry,

589thermobarometry and magnetic fabric studies of the

590pluton and surrounding rocks. U–Pb SHRIMP dating

591of the pluton yields an emplacement age of 532.0 ± 5

Fig. 11. A series of paleolatitudinal reconstructions for Gondwana based on the paleomagnetic data in Table 4 and this study: (a) 540 Ma; (b)

520 Ma; (c) 510 Ma; (d) 475 Ma. The West African craton is shaded for easy reference.


UNCORRECTED PROOF

592 Ma followed by relatively slow-cooling documented

593 by 40Ar/39Ar ages on hornblende and biotite. A mean

594 40Ar/39Ar age calculated from four plateau ages on the

595 hornblende samples is 512.7 ± 1.3 Ma. A biotite sam-

596 ple yielded a plateau age of 478.9 ± 1.0 Ma. Paleo-

597 magnetic samples from the pluton and surrounding

598 country rocks exhibit either SE-upwardly directed

599 magnetizations (meanDec = 113�, Inc =� 56�, k = 106,600 a95 = 12�) or NW-downwardly directed magnetiza-

601 tions (mean Dec = 270�, Inc= + 64�, k = 30, a95 =

602 11�) that pass a reversal test with a classification of

603 ‘C’ and an angular difference of 14.4�. The ‘normal’

604 (negative inclinations) and ‘reverse’ (positive inclina-

605 tions) directions also show a spatial bias within the

606 pluton, suggesting a field transition from reverse to

607 normal during cooling. The paleomagnetic pole calcu-

608 lated from the mean direction (in the pluton) falls at

609 6.8�S, 001�E (dp = 13�, dm = 17�). Estimates of the

610 blocking temperature for the magnetization were

611 compared to the cooling history of the pluton and an

612 age of 508.5 ± 11.5 Ma is assigned to the pole. After

613 rotating the Carion pole to East Africa (12.7�N,614 359.7�E), it falls near similar-age poles from else-

615 where in Gondwana, supporting the idea that the

616 major orogenic events during Gondwana assembly

617 were complete by that time. A slight revision of the

618 Gondwana APWP is proposed with rapid APW from

619 540 to 520 Ma; however, the proposed mechanisms to

620 explain this rapid APW (including inertial-interchange

621 true polar wander or enhanced mantle driving forces)

622 cannot fully explain all the data. We suggest that,

623 perhaps, a combination of enhanced mantle driving

624 forces (either cold-downwellings or hot upwellings) in

625 combination with a smaller amount of TPW could

626 explain this rapid APW.

627 7. Uncited reference

628 Harrison and Watson, 1983

629 Acknowledgements

630 The authors wish to thank Chad Pullen for his help

631 in the field and assistance in the laboratory, Marcus

632 Johnson of the University of Michigan for running the

633 Ar/Ar samples, Les Lezards de Tana for logistical

634support in the field and some of the best camp

635cooking we have had in a long time and especially for

636protecting us from bad fady. Financial support was

637provided through the National Science Foundation

638grant EAR98-05306 and by a US–Norway Fulbright

639Fellowship to JGM. A.N. received financial support

640from INSU-Interieur de la Terre Program and CNRS-

641NSF program. Whole rock analyses were performed at

642the University of Bonn (Germany) with the cooper-

643ation of Prof. M. Raith. This is Tectonics Special

644Research Centre publication no. 125. The manuscript

645benefited from an early review by Elizabeth Eide and

646also through the reviews of Trond Torsvik, Wulf

647Gose, and editorial suggestions of Kip Hodges.

648Appendix A. SHRIMP and 40Ar/39Ar methodology

649

650A.1. SHRIMP

651Several thousand zircons were separated from 500

652g of rock; the least magnetic fraction was mounted for

653U–Pb analysis using the SHRIMP II ion microprobe

654at Curtin University of Technology, Perth, Australia.

655The zircons are colourless, subhedral to euhedral,

656range up to 500 mm in length, and have length to

657width ratios between 2:1 and 4:1. No internal struc-

658tures other than euhedral growth zoning are visible;

659some crystals contain irregular to acicular cavities or

660inclusions. The best areas of 22 zircons, free of any

661cracks or inclusions, were selected for analysis. U–

662Th–Pb ratios and absolute abundances were deter-

663mined relative to the University of Western Australia

664CZ3 standard zircon (206Pb/238U = 0.91432 (564 Ma);

665550 ppm 238U), using standard operating and data

666processing procedures described by Compston et al.

667(1984, 1992) and Claoue-Long et al. (1995). The

668proportion of common 206Pb to total 206Pb ( f206 in

669Table 2), estimated using measured 204Pb/206Pb, is

670sufficiently small to be insensitive to the choice of

671common Pb composition, and an average crustal

672composition appropriate to the age of the mineral

673was assumed. Concentrations of 238U range from 35

674to 500 ppm, and average of 180 ppm; Th/U ratios

675range from 0.4 to 2.2, with a mean of 1.0. Values for

676f206 range between 0.01% and 0.1% for 16 of 22

677analyses, and are between 0.1% and 0.5% for the re-

678maining six analyses.


UNCORRECTED PROOF

679680 A.2. 40Ar/39Ar methodology

681 The minerals were taken from the same samples

682 used for paleomagnetic studies using standard mineral

683 separation techniques. Specifically, we removed bio-

684 tite and hornblende from sample CAR-31 (site 5) and

685 hornblende from samples CAR-2 and CAR-7 (site 1).

686 The samples were then packaged and sent to the

687 irradiation facilities at the University of Michigan.

688 All argon age analyses were performed using a

689 VG1200S mass spectrometer equipped with a Daly

690 detector operated in analog mode. Samples were step-

691 heated using a Coherent INNOVA model 70 argon ion

692 laser with a nominal maximum output power of 5 W.

693 All analyses were performed with a fully automated

694 fusion system, with laser power being set under

695 computer control. Step-heating durations were 60 s,

696 and the total fusion plus gas clean-up time was 3 min.

697 Laser fusion system blanks were monitored frequently

698 (typically every five to six fractions) and were nor-

699 mally about 1�10� 13, 3� 10� 13, 3� 10� 14, 3�700 10� 14, and 5� 10� 12 ml STP for masses 36 through

701 40, respectively. The irradiation standard used was

702 hornblende MMHb-1 with an assumed K–Ar age of

703 520.4 Ma (Samson and Alexander, 1987). Each irra-

704 diation consisted of a set of samples and standards

705 sealed within evacuated fused silica tubes, and from

706 each tube, at least three analyses for each of five

707 packets of MMHb-1 were analyzed. The resulting

708 values of J were interpolated for each unknown

709 sample location. Error estimates for J include scatter

710 about the interpolation function. Mass discrimination

711 in the mass spectrometer was monitored daily by

712 analyzing an aliquot of atmospheric argon with a

713 volume of about 5� 10� 9 ml STP.

714 References

715 Baldwin, S.L., Harrison, T.M., FitzGerald, J.D., 1990. Diffusion of

716 40Ar in metamorphic hornblende. Contrib. Mineral. Petrol. 105,

717 691–703.

718 Berger, G.W., York, D., 1979. 40Ar/39Ar dating of multicomponent

719 magnetizations in the Archean Shelley Lake granite, northwest-

720 ern Ontario. Can. J. Earth Sci. 16, 1933–1941.

721 Berger, G.W., York, D., 1981a. Geothermometry from 40Ar/39Ar

722 dating experiments. Geochim. Cosmochim. Acta 45, 795–811.

723 Berger, G.W., York, D., 1981b. 40Ar/39Ar dating of the Thanet

724 gabbro, Ontario: looking through the Grenvillian metamorphic

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729dence for an early Cambrian glaciation. Geology 23, 133–136.

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731paleomagnetic pole: the case of the Ntonya ring structure, Ma-

732lawi. J. Geophys. Res. 98, 1743–1749.

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