UNCORRECTED PROOF 1 Paleomagnetism, geochronology and tectonic implications of the 2 Cambrian-age Carion granite, Central Madagascar 3 Joseph G. Meert a,b, , Anne Ne ´de ´lec c , Chris Hall d , Michael T.D. Wingate e , 4 Michel Rakotondrazafy f 5 a Department of Earth Sciences, Indiana State University, Terre Haute, IN 47809, USA 6 b Norwegian Geological Survey, Leiv Eirikssons vei 39, Trondheim, Norway 7 c Universite ´ Paul Sabatier, UMR 5563 CNRS, 38 rue des 36-ponts, 31400 Toulouse, France 8 d Department of Geological Sciences, University of Michigan, 2563 CC Little Bldg., Ann Arbor, MI 48109, USA 9 e Tectonics Special Research Centre, University of Western Australia, Perth, WA 6845, Australia 10 f De ´partement des Sciences de la Terre, Faculte ´ des Sciences, Universite ´ d’Antananarivo, BP 906, Antananarivo, Madagascar 11 12 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 40 Ar/ 39 Ar ages on hornblende and biotite. Four hornblende samples 17 yielded a mean 40 Ar/ 39 Ar 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, a 95 = 12°) or NW-downwardly directed magnetizations (mean Dec = 270°, Inc= + 64°, k = 30, a 95 = 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 32 1. Introduction 33 The assembly of the Gondwana continent during 34 the latest Neoproterozoic to earliest Cambrian (550– 35 530 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
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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
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
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-
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
725veil and implications for paleomagnetism. Can. J. Earth Sci. 18,
726266–273.
727Bertrand-Sarfati, J., Moussine-Ouchkine, A., Ait Kaei Ahmed, A.,
7281995. First Ediacaran fauna found in western Africa and evi-
729dence for an early Cambrian glaciation. Geology 23, 133–136.
730Briden, J.C., McClelland, E., Rex, D.C., 1993. Proving the age of a
731paleomagnetic pole: the case of the Ntonya ring structure, Ma-
732lawi. J. Geophys. Res. 98, 1743–1749.
733Buchan, K.L., Dunlop, D., 1976. Paleomagnetism of the Halibur-