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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: joe.meert@ngu.no (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
J.G. Meert et al. / Tectonophysics xx (2001) xxx–xxx4
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).
J.G. Meert et al. / Tectonophysics xx (2001) xxx–xxx 5
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.
J.G. Meert et al. / Tectonophysics xx (2001) xxx–xxx 7
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.
J.G. Meert et al. / Tectonophysics xx (2001) xxx–xxx8
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.
J.G. Meert et al. / Tectonophysics xx (2001) xxx–xxx 9
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.
J.G. Meert et al. / Tectonophysics xx (2001) xxx–xxx10
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.
J.G. Meert et al. / Tectonophysics xx (2001) xxx–xxx 11
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.
J.G. Meert et al. / Tectonophysics xx (2001) xxx–xxx12
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.
J.G. Meert et al. / Tectonophysics xx (2001) xxx–xxx 13
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.
J.G. Meert et al. / Tectonophysics xx (2001) xxx–xxx14
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).
J.G. Meert et al. / Tectonophysics xx (2001) xxx–xxx 15
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�.
J.G. Meert et al. / Tectonophysics xx (2001) xxx–xxx16
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.
J.G. Meert et al. / Tectonophysics xx (2001) xxx–xxx 17
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.
J.G. Meert et al. / Tectonophysics xx (2001) xxx–xxx18
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-
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