Detrital footprint of the Mozambique ocean: U–Pb SHRIMP and Pb evaporation zircon geochronology of...

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Tectonophysics 375 (2003) 77–99

Detrital footprint of the Mozambique ocean: U–Pb SHRIMP and

Pb evaporation zircon geochronology of metasedimentary

gneisses in eastern Madagascar

Alan S. Collinsa,*, Alfred Kronerb, Ian C.W. Fitzsimonsa, Theodore Razakamananac

aTectonics SRC, Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australiab Institut fur Geowissenschaften, Universitat Mainz, 55099 Mainz, Germany

cDepartement des Sciences de la Terre, Universite de Toliara, Toliara, Madagascar

Received 3 May 2002; received in revised form 11 December 2002; accepted 5 June 2003

Abstract

The southern East African Orogen is a collisional belt where the identification of major suture zones has proved elusive. In

this study, we apply U–Pb isotopic techniques to date detrital zircons from a key part of the East African Orogen, analyse their

possible source region and discuss how this information can help in unravelling the orogen.

U–Pb sensitive high-mass resolution ion microprobe (SHRIMP) and Pb evaporation analyses of detrital zircons from

metasedimentary rocks in eastern Madagascar reveal that: (1) the protoliths of many of these rocks were deposited between

f 800 and 550 Ma; and (2) these rocks are sourced from regions with rocks that date back to over 3400 Ma, with dominant age

populations of 3200–3000, f 2650, f 2500 and 800–700 Ma.

The Dharwar Craton of southern India is a potential source region for these sediments, as here rocks date back to over 3400

Ma and include abundant gneissic rocks with protoliths older than 3000 Ma, sedimentary rocks deposited at 3000–2600 Ma

and granitoids that crystallised at 2513–2552 Ma. The 800–700 Ma zircons could potentially be sourced from elsewhere in

India or from the Antananarivo Block of central Madagascar in the latter stages of closure of the Mozambique Ocean. The

region of East Africa adjacent to Madagascar in Gondwana reconstructions (the Tanzania craton) is rejected as a potential

source as there are no known rocks here older than 3000 Ma, and no detrital grains in our samples sourced from

Mesoproterozoic and early Neoproterozoic rocks that are common throughout central east Africa. In contrast, coeval sediments

200 km west, in the Itremo sheet of central Madagascar, have detrital zircon age profiles consistent with a central East African

source, suggesting that two late Neoproterozoic provenance fronts pass through east Madagascar at approximately the position

of the Betsimisaraka suture. These observations support an interpretation that the Betsimisaraka suture separates rocks that were

derived from different locations within, or at the margins of, the Mozambique Ocean basin and therefore, that the suture is the

site of subduction of a strand of Mozambique Ocean crust.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Madagascar; Neoproterozoic; U-Pb SHRIMP geochronology; Detrital zircons; Mozambique Ocean; Gondwana

0040-1951/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0040-1951(03)00334-2

* Corresponding author. Tel.: +61-89266-3127; fax: +61-89266-3153.

E-mail address: [email protected] (A.S. Collins).

A.S. Collins et al. / Tectonophysics 375 (2003) 77–9978

1. Introduction

Identifying oceanic suture zones in middle to

lower crustal rocks is not a straightforward task.

Intense ductile deformation and metamorphic over-

print commonly obscures the mix of highly deformed

oceanic and continental margin rock types that char-

acterise Phanerozoic suture zones (Dewey, 1977).

The Mozambique Ocean suture is a case in point

(Shackleton, 1996). The remains of this major Prote-

rozoic ocean lie within the East African Orogen

(EAO; Stern, 1994; Collins and Windley, 2002;

Meert, 2003; de Wit, 2003) that formed as a broadly

linear orogen during the amalgamation of Gondwana.

The change from greenschist dominated supra-crustal

rocks to amphibolite- and granulite-facies gneisses

from the northern Arabian–Nubian Shield to the

southern Mozambique Belt, in part, reflects an in-

crease in the degree of exhumation along the EAO.

This along-strike change is parallelled by an increase

in the uncertainty of the location of suture zones

within the orogen (Shackleton, 1996).

In addition to attempting to identify suture zones

by lithology alone, a number of isotopic tracer tech-

niques can be used to isolate rock units with similar

isotopic ratios indicating coeval formation, a shared

thermal history and/or a similar intrusion record. From

these data, the outboard extent of the bounding con-

tinents can be recognised, the derivation of exotic

tectonic units within the orogen constrained and

potential locations for sutures identified (e.g. Borg

and DePaolo, 1994; Stern, 2002). Similarly, the age

range of detrital grains holds considerable information

about the age of their source region and their deposi-

tional palaeogeography. Sediments derived from ei-

ther side of an oceanic basin will have distinct source

characteristics, and by delineating the locus of sedi-

ment derived from the one margin of the orogen a

provenance front can be drawn. By doing this for both

orogen margins, two provenance fronts are created,

between which lies an oceanic suture. Deformation

during closure of the ocean basin will potentially

imbricate and displace these provenance fronts, but

as long as the orogen developed as a dominantly

forward-propagating thrust pile these provenance

fronts should be broadly preserved.

In this paper, we present U and Pb isotope analyses

of zircons from metasedimentary rocks caught up in

the eastern EAO in Madagascar that restrict the

depositional age of their protoliths. These data are

compared with previously published detrital zircon

data from elsewhere in Madagascar, and neighbouring

regions of Gondwana, to investigate possible source

regions and constrain the location of a strand of the

Mozambique Ocean suture.

2. The tectonic framework of central east

Madagascar

Collins et al. (2000), Kroner et al. (2000) and

Collins and Windley (2002) divided central and north

Madagascar into five tectonic units (Fig. 1). All rocks

in a unit share a similar tectonic history and each unit is

separated from the other units either by a regionally

significant unconformity or by a shear zone. The five

tectonic units are summarised below:

1. The Antongil block, consisting of gneiss with

f 3200 Ma protoliths intruded by 2600–2500 Ma

granite, and largely unmetamorphosed since

f 2500 Ma (Tucker et al., 1999b; Collins et al.,

2001).

2. The Antananarivo block, which consists of gneiss

with 2500–2600 Ma protoliths interlayered with

820–740 Ma granitoids and gabbros, pervasively

deformed and metamorphosed to amphibolite and

granulite-facies conditions between 700 and 550

Ma (Tucker et al., 1999b; Kroner et al., 2000;

Collins et al., 2003a).

3. The Itremo sheet, containing Palaeoproterozoic to

early Neoproterozoic metasedimentary rocks (Cox

et al., 1998, 2000, 2001, in press) multiply

deformed and thrust over, and imbricated with,

the Antananarivo block (Moine, 1966; Tucker et

al., 2001b; Collins et al., 2003b). Metasedimentary

rocks of the Itremo sheet nonconformably overlie

orthogneiss (Cox et al., 1998) with a protolith that

was elsewhere in the Itremo sheet dated at 2511

+ 3/� 2 Ma (Tucker et al., 1999b). This is of

similar age to many of the protoliths of gneisses in

the Antananarivo block (Tucker et al., 1999b;

Kroner et al., 2000).

4. The Tsaratanana sheet, which is formed of 2700–

2500 Ma mafic gneiss with Mid–Late Archaean

Sm/Nd ages and zircon xenocrysts (Tucker et al.,

Fig. 1. Map of Madagascar showing the tectonic units of Collins et

al. (2000) and Collins and Windley (2002), based on original

mapping summarised by Besairie (1973) and the interpretations of

Hottin (1976). Asterisks mark sample sites: 20 =M99/20; 41 and

42 =M98/41 and M98/42; 50 =M98/50. a =Antananarivo, Masoa-

la =Masoala peninsula, Antongil = Bay of Antongil, Vd =Vondrozo

region. Subdivision of the Tsaratanana Sheet: Mae =Maevatanana

belt, Ad =Andriamena belt, Bf =Beforona belt, Ad =Androna belt.

Shear zones: If = Ifanadiana shear zone, B =Betsileo shear zone,

R =Ranotsara shear zone, A=Ampanihy shear zone, Be =Bekily

shear zone, Ts = Tranomaro shear zone.

A.S. Collins et al. / Tectonophysics 375 (2003) 77–99 79

1999b; Collins et al., 2001). This tectonic unit was

deformed and metamorphosed at f2500 Ma

(Goncalves et al., 2000) and cut by 800–760 Ma

gabbro intrusions. Later contractional deformation

continued until after 630 Ma (Collins et al., 2003a;

Goncalves et al., 2003).

5. The Bemarivo belt, consisting of SE–NW striking

metasedimentary rocks, granites and gneisses

overlain by contractionally deformed metavol-

canics. Young granulite-facies metamorphism in

the Bemarivo belt is dated as 510–520 Ma

(Tucker et al., 1999a). Northeast Madagascar has

been linked with the Seychelles and northwest

India because of the presence in each of these

areas of f 750 Ma volcanic and/or magmatic

rocks (Tucker et al., 1999a; Torsvik et al., 2001;

Ashwal et al., 2002). The Bemarivo belt crops out

only in the north of the island is not discussed

further here.

Central and northern Madagascar are separated

from southern Madagascar by the sinistral Ranotsara

shear zone (Fig. 1, Windley et al., 1994). This

natural boundary forms the southern limit of our

study area.

Many similarities exist between the geology of the

Antongil block and the western Dharwar craton in

India (Fig. 2). These include: (1) ortho- and para-

gneisses with protoliths that date back to over 3000 Ma

(Nutman et al., 1992; Peucat et al., 1993; Nutman et

al., 1996; Tucker et al., 1999b; Chadwick et al., 2000;

Collins et al., 2001); (2) granitoid intrusion between

2510 and 2550 Ma (Tucker et al., 1999b; Chadwick et

al., 2000; Jayananda et al., 2000; Paquette et al., 2003);

and (3) preservation of Archaean Rb/Sr whole-rock

and Nd model ages (Vachette and Hottin, 1971; Harris

et al., 1994; Bartlett et al., 1998; Tucker et al., 1999b).

In contrast, the Antananarivo Block and Itremo Sheet

preserve no known rocks older than 2600 Ma (Tucker

et al., 1999b; Kroner et al., 2000). Detrital zircons in

the metasedimentary rocks of the Itremo sheet have

age populations that differ markedly from the known

geology of the Dharwar Craton, which led Cox et al.

(1998, in press) to suggest an East African source for

these rocks.

These correlations imply a boundary zone be-

tween the Antananarivo block and the Antongil

block separating Archaean crustal blocks of different

Fig. 2. Reconstruction of a part of Gondwana (Lawver et al., 1998) showing the interpreted extent of the East African Orogen (after Collins and

Windley, 2002) and the range of publishedU–Pb zircon ages from the regions surroundingGondwananMadagascar (Paquette et al., 1993; Holzl et

al., 1994; Lenoir et al., 1994a,b; Kroner et al., 1996, 1997a,b; Ring et al., 1997; Teklay et al., 1998; Borg and Krogh, 1999; Tucker et al., 1999a,b;

Chadwick et al., 2000; Kroner et al., 2000; Moller et al., 2000; Sacchi et al., 2000; Collins et al., 2001; de Wit et al., 2001; Kroner et al., 2001;

Muhongo et al., 2001; Tucker et al., 2001a; Braun and Kriegsman, 2003; Paquette et al., 2003; Reddy et al., 2003; Cox et al., in press; Rivers and

Johnson, in press). All numbers refer to ages in Ma; italics refer to the timing of high-grade metamorphism. DML=Dronning–Maud Land,

LB=Lurio Belt, IrB = Irumide Belt, U–U=Usagaran–Ubende Belt, TC=Tanzanian Craton, KdB=Kibaran Belt,WDC=Western Domain of the

Dharwar Craton, EDC=Eastern Domain of the Dharwar Craton, P–C =Palghat–Cauvery shear zone system, EAO=East African Orogen.


origins. This boundary zone is today a highly

strained paragneiss belt with emerald mineralisation

and entrained podiform ultramafic–mafic bodies

(Besairie, 1970; Hottin, 1976). Contractional defor-

mation along this zone is interpreted to have oc-

curred between 630 and 527 Ma (Collins et al.,

2003a).

3. Analytical techniques

3.1. Sensitive high-mass resolution ion microprobe

(SHRIMP)

Zircons were separated from crushed rocks by

magnetic and methylene iodide liquid separation.

Fig. 3. Cathodoluminescence (CL) images of selected zircon grains. (a) M98/42—Multiple population of zircons showing common rounding of

form, truncation of luminescence bands and common thin, highly luminescent, rims. Numbers are SHRIMP spots and refer to analysis numbers

in Table 1 (M42.58 to M42.63). (b) M98/42—Sector-zoned igneous grain abraded, partially rounded and partially rimmed with thin bright

zircon. The circle marks the site of SHRIMP spot M42.05 and age refers to 204Pb-corrected 206Pb/207Pb age. (c) M98/42—Oscillatory-zoned

zircon grain abraded and rounded. The circle marks the site of SHRIMP spot M42.15, and age refers to the 204Pb-corrected 206Pb/207Pb age. (d)

M99/20—Rounded and abraded zircon with distinct core (spot M20.10) partially rimmed with broadly oscillatory-zoned zircon. (e) M99/20—

Rounded and abraded zircon grain with distinct oscillatory-zoned core (spot M20.08) rimmed with broad-banded zircon (spot M20.07). Bright

spots are SHRIMP sites, and ages refer to 204Pb-corrected 206Pb/207Pb ages. (f) Broken zircon grain with only slightly rounded crystal face

terminations. Distinct core and rim separated by thin bright CL band. In the top right (� ), rim zircon invades the core suggesting that the rim

did not form as a simple zircon overgrowth. Bright spots are SHRIMP sites M20.14 and M20.15, and ages refer to 204Pb-corrected 206Pb/238U

ages.


Table 1

U–Pb SHRIMP zircon results

Spot Concentration 232Th/ %com 206Pba/ Fb 207Pba/ Fb 207Pba/ Fb Ages %

(ppm)238

U206

Pb238

U235

U206

Pb 206Pba/ Fb 207Pba/ Fb 207Pba/ Fb

U Th 238U 235U 206Pb

disconc.

M99-20 kyanite +biotite paragneiss—Fenaorivo

M20.02 384 52 0.14 0.08 0.5425 0.0206 17.1232 0.6775 0.2296 0.0026 2796 91 2942 525 3044 18 9

M20.03 663 278 0.43 0.05 0.4437 0.0094 9.6743 0.9115 0.1586 0.0145 2396 58 2404 658 2436 156 3

M20.04 459 69 0.15 0.39 0.2648 0.0210 7.1874 0.6665 0.1998 0.0095 1502 109 2135 519 2800 79 85

M20.05 1069 52 0.05 0.03 0.2949 0.0050 6.5906 0.1338 0.1624 0.0018 1667 33 2058 127 2478 19 49

M20.06 198 117 0.61 � 0.07 0.5886 0.0432 21.6925 1.6059 0.2668 0.0026 2960 166 3170 973 3290 15 10

M20.07 169 55 0.34 0.09 0.5480 0.0159 18.0861 0.6175 0.2401 0.0043 2831 73 2994 488 3116 29 11

M20.08 681 509 0.77 � 0.05 0.5744 0.0032 21.6703 0.1475 0.2732 0.0011 2950 39 3169 140 3327 6 14

M20.09 135 76 0.58 0.00 0.4991 0.0057 12.4804 0.1758 0.1814 0.0015 2594 40 2641 164 2665 14 2

M20.10 268 77 0.30 � 0.02 0.6352 0.0050 26.0756 0.2343 0.2976 0.0013 3167 43 3349 214 3459 7 9

M20.11 122 23 0.20 1.06 0.1342 0.0024 1.0665 0.0408 0.0665 0.0019 798 17 737 41 516 115 � 36

M20.12 85 72 0.87 0.44 0.5069 0.0077 12.5711 0.2458 0.1838 0.0022 2631 45 2648 223 2651 26 0

M20.13 53 44 0.85 � 0.10 0.6285 0.0119 21.0244 0.4640 0.2417 0.0028 3110 58 3140 387 3137 18 0

M20.14 134 161 1.24 5.94 0.1183 0.0020 – – 0.0668 0.0015 723 15 – – – – –

M20.15 358 5 0.02 3.92 0.0842 0.0009 0.3086 0.0127 0.0596 0.0010 518 9 273 13 � 1548 664 � 397

M20.16 51 48 0.98 0.23 0.6595 0.0132 22.2569 0.5172 0.2468 0.0029 3243 62 3195 423 3151 19 � 3

M20.17 306 234 0.79 0.18 0.4397 0.0034 9.8300 0.0932 0.1637 0.0009 2360 34 2419 90 2478 10 5

M20.18 174 78 0.46 0.36 0.1373 0.0034 1.2095 0.0380 0.0669 0.0012 832 22 805 38 738 64 � 11

M20.19 111 168 1.56 0.22 0.1295 0.0020 1.1316 0.0330 0.0652 0.0016 786 16 768 33 722 132 � 8

M20.20 92 129 1.45 0.27 0.4801 0.0106 10.8909 0.2649 0.1669 0.0017 2515 54 2514 239 2503 18 � 1

M20.21 266 130 0.50 0.37 0.1358 0.0013 1.1904 0.0227 0.0667 0.0010 814 14 796 23 728 53 � 11

M20.22 2809 1052 0.39 1.94 0.0884 0.0003 0.7599 0.0086 0.0780 0.0007 553 8 574 9 685 55 25

M20.23 908 72 0.08 0.47 0.4204 0.0019 10.9921 0.0713 0.1936 0.0009 2263 32 2522 70 2739 10 21

M20.24 1062 36 0.03 0.34 0.3135 0.0020 7.1914 0.0863 0.1692 0.0017 1756 26 2135 84 2522 18 43

M20.26 1127 81 0.07 0.16 0.2238 0.0030 4.8047 0.1077 0.1569 0.0028 1309 23 1786 104 2410 31 85

M20.27 3976 1522 0.40 1.85 0.1147 0.0003 0.9889 0.0078 0.0777 0.0005 710 11 698 8 692 34 � 1

M20.28 706 160 0.23 0.09 0.1240 0.0014 1.1341 0.0167 0.0670 0.0006 756 14 770 17 816 21 8

M20.29 5445 2003 0.38 0.17 0.1390 0.0005 1.2616 0.0062 0.0673 0.0002 846 13 829 6 802 9 � 4

M98-42 garnet +biotite paragneiss—Ambodibonara emerald mine

M42.01 139 0 0.00 0.00 0.4625 0.0102 14.7053 0.3609 0.2306 0.0025 2443 43 2796 313 3056 17 25

M42.01b 342 170 0.51 0.03 0.6439 0.0183 22.0578 0.6317 0.2484 0.0009 3215 68 3186 497 3175 5 � 1

M42.02 351 217 0.64 0.01 0.6348 0.0128 21.2983 0.4414 0.2433 0.0012 3179 48 3152 371 3142 8 � 1

M42.03 140 69 0.51 � 0.01 0.6457 0.0140 21.9898 0.4896 0.2470 0.0013 3215 52 3183 405 3166 8 � 1

M42.04 142 32 0.23 � 0.01 0.6072 0.0138 20.3403 0.4808 0.2429 0.0015 3061 53 3108 399 3139 10 3

M42.05 172 61 0.36 0.02 0.5753 0.0127 19.0015 0.4371 0.2395 0.0015 2925 49 3042 368 3117 10 6

M42.06 381 160 0.43 0.01 0.6037 0.0124 20.6621 0.4324 0.2482 0.0010 3042 48 3123 365 3173 6 4

M42.07 380 192 0.52 0.00 0.6033 0.0124 20.7541 0.4351 0.2495 0.0010 3066 48 3127 367 3182 7 5

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(continued on next page)

M42.08 234 91 0.40 0.02 0.5610 0.0120 18.4823 0.4140 0.2389 0.0016 2865 47 3015 352 3113 11 8

M42.09 284 100 0.36 0.03 0.5869 0.0123 19.8003 0.4452 0.2447 0.0020 2985 48 3082 374 3151 13 6

M42.10 1024 14 0.01 2.46 0.5834 0.0116 18.7717 0.4622 0.2334 0.0034 2950 45 3030 386 3074 11 4

M42.11 624 269 0.45 0.00 0.5940 0.0120 19.9673 0.4224 0.2438 0.0015 3020 47 3090 358 3145 10 5

M42.12 163 62 0.39 0.06 0.5736 0.0128 19.1158 0.4397 0.2417 0.0014 2935 50 3048 370 3131 9 7

M42.13 340 66 0.20 0.00 0.5427 0.0112 17.7598 0.4099 0.2374 0.0024 2804 45 2977 349 3102 16 11

M42.14 270 126 0.48 0.00 0.6275 0.0132 21.2648 0.4721 0.2458 0.0017 3151 50 3151 393 3158 11 1

M42.15 359 238 0.69 0.02 0.6300 0.0130 21.2813 0.4463 0.2450 0.0010 3153 49 3151 375 3153 6 0

M42.16 315 184 0.60 0.05 0.6284 0.0131 21.2161 0.4498 0.2449 0.0010 3149 49 3149 377 3152 7 0

M42.17 293 54 0.19 0.00 0.5695 0.0119 19.1089 0.4087 0.2434 0.0011 2902 47 3047 348 3142 7 8

M42.18 342 126 0.38 0.11 0.5707 0.0121 19.7043 0.4345 0.2504 0.0016 2920 47 3077 366 3187 10 10

M42.19 382 197 0.53 0.00 0.5630 0.0116 18.5694 0.5556 0.2392 0.0052 2883 46 3020 449 3115 35 8

M42.20 482 184 0.39 0.01 0.6063 0.0122 20.3337 0.4255 0.2432 0.0014 3057 47 3107 360 3141 9 3

M42.21 345 187 0.56 0.00 0.5705 0.0121 18.9657 0.4469 0.2411 0.0025 2922 47 3040 375 3127 17 7

M42.22 174 69 0.41 0.00 0.5597 0.0129 18.6344 0.4483 0.2414 0.0016 2862 50 3023 376 3130 11 9

M42.23 254 109 0.44 0.02 0.6440 0.0137 21.8835 0.4753 0.2464 0.0011 3205 51 3179 395 3162 7 � 1

M42.24 241 102 0.44 0.00 0.6112 0.0131 20.7346 0.4891 0.2460 0.0025 3085 50 3126 404 3159 16 3

M42.25 300 140 0.48 0.00 0.6657 0.0139 22.6397 0.4882 0.2467 0.0014 3297 51 3212 404 3163 9 � 4

M42.26 409 259 0.65 0.01 0.6368 0.0133 21.6768 0.4578 0.2469 0.0009 3183 50 3169 383 3165 6 0

M42.27 255 139 0.56 0.01 0.6360 0.0135 21.2784 0.4976 0.2427 0.0024 3180 51 3151 410 3137 16 � 1

M42.28 257 117 0.47 0.00 0.6183 0.0130 20.9286 0.4791 0.2455 0.0022 3101 49 3135 397 3156 14 2

M42.29 236 102 0.45 0.01 0.6079 0.0134 21.0325 0.4730 0.2509 0.0012 3078 51 3140 393 3191 7 4

M42.30 234 27 0.12 0.00 0.5663 0.0121 19.1183 0.4814 0.2449 0.0033 2892 47 3048 399 3152 21 9

M42.31 154 37 0.25 1.71 0.4932 0.0113 16.6081 0.4441 0.2442 0.0034 2561 46 2912 373 3145 19 22

M42.32 342 107 0.32 0.03 0.6042 0.0128 20.3266 0.4389 0.2440 0.0010 3061 49 3107 369 3146 6 3

M42.33 326 195 0.62 0.00 0.6228 0.0130 21.1252 0.4479 0.2460 0.0010 3152 50 3144 376 3159 6 1

M42.34 456 339 0.77 0.03 0.6304 0.0128 21.4558 0.4422 0.2469 0.0009 3179 49 3159 372 3165 6 0

M42.35 177 75 0.44 0.07 0.6090 0.0141 20.5644 0.5203 0.2449 0.0025 3080 54 3118 425 3152 16 3

M42.36 304 126 0.43 0.00 0.6063 0.0127 20.6171 0.4449 0.2466 0.0013 3080 49 3121 374 3163 8 4

M42.37 406 230 0.58 0.01 0.6120 0.0128 20.8789 0.4412 0.2474 0.0009 3098 49 3133 371 3168 6 3

M42.38 491 237 0.50 3.09 0.4609 0.0094 14.6581 0.3954 0.2307 0.0041 2450 39 2793 338 3052 15 25

M42.39 315 134 0.44 0.14 0.6126 0.0128 20.9498 0.4460 0.2480 0.0011 3090 49 3136 375 3172 7 3

M42.40 403 178 0.46 0.01 0.5576 0.0117 18.3752 0.4043 0.2390 0.0016 2874 46 3010 345 3113 11 9

M42.41 292 88 0.31 0.07 0.5131 0.0111 16.8440 0.3783 0.2381 0.0014 2676 45 2926 326 3107 9 16

M42.42 286 108 0.39 0.00 0.6270 0.0136 21.2005 0.4671 0.2452 0.0010 3152 51 3148 389 3154 7 1

M42.43 384 274 0.74 0.01 0.6337 0.0130 21.4703 0.4489 0.2457 0.0009 3192 49 3160 376 3157 6 0

M42.44 275 125 0.47 0.00 0.5350 0.0116 17.7265 0.4340 0.2403 0.0028 2772 46 2975 366 3122 18 13

M42.45 187 68 0.38 0.00 0.6324 0.0139 21.4368 0.4837 0.2458 0.0013 3168 52 3159 401 3158 8 0

M42.46 273 160 0.60 0.04 0.6409 0.0135 21.9457 0.5048 0.2483 0.0023 3196 51 3181 415 3174 14 � 1

M42.47 124 51 0.43 0.04 0.6505 0.0159 21.8160 0.5582 0.2432 0.0018 3227 59 3176 450 3141 12 � 3

M42.48 234 54 0.24 0.01 0.5883 0.0125 19.8159 0.5312 0.2443 0.0040 2978 48 3082 433 3148 26 6

M42.49 308 114 0.38 0.00 0.5995 0.0128 20.3105 0.4429 0.2457 0.0010 3035 49 3106 372 3157 7 4

M42.50 243 117 0.50 0.00 0.6159 0.0132 20.9216 0.4843 0.2464 0.0022 3108 50 3135 401 3162 14 2

M42.51 280 89 0.33 0.02 0.6080 0.0128 20.5583 0.4436 0.2452 0.0011 3071 49 3118 373 3154 7 3

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Spot Concentration 232Th/ %com 206Pba/ Fb 207Pba/ Fb 207Pba/ Fb Ages %

(ppm)238

U206

Pb238

U235

U206

Pb 206Pba/ Fb 207Pba/ Fb 207Pba/ Fb

U Th 238U 235U 206Pb

disconc.

M42.52 1776 26 0.01 0.00 0.5299 0.0115 16.4765 0.3590 0.2255 0.0006 2745 47 2905 311 3021 4 10

M42.53 198 52 0.27 0.00 0.5733 0.0126 19.6584 0.4467 0.2487 0.0014 2924 49 3075 375 3177 9 9

M42.54 314 93 0.31 0.24 0.5422 0.0138 16.5068 0.4310 0.2208 0.0013 2809 46 2907 364 2987 10 7

M42.55 204 86 0.44 0.09 0.6329 0.0137 21.5376 0.4828 0.2468 0.0014 3160 52 3163 400 3164 9 0

M42.56 274 140 0.53 0.33 0.5858 0.0127 20.1198 0.4640 0.2491 0.0019 2978 49 3097 387 3179 12 7

M42.57 281 154 0.56 0.01 0.5937 0.0129 20.0590 0.4498 0.2450 0.0014 3013 50 3094 377 3153 9 5

M42.58 405 44 0.11 4.14 0.4718 0.0098 14.9130 0.4819 0.2292 0.0057 2471 40 2810 399 3041 23 22

M42.59 175 88 0.52 0.01 0.5925 0.0133 19.4474 0.5228 0.2380 0.0035 2998 51 3064 427 3107 24 4

M42.60 243 86 0.37 0.10 0.5829 0.0124 19.6233 0.4303 0.2442 0.0012 2970 48 3073 363 3147 8 6

M42.61 400 276 0.71 0.00 0.6043 0.0124 20.6019 0.4309 0.2473 0.0009 3061 48 3120 364 3167 6 4

M42.62 225 79 0.36 0.05 0.5699 0.0123 19.2173 0.4266 0.2445 0.0013 2919 48 3053 361 3150 8 8

M42.63 334 177 0.55 0.00 0.6222 0.0129 21.3852 0.4580 0.2493 0.0014 3120 49 3156 383 3180 9 2

M42.64 327 197 0.62 0.05 0.6136 0.0128 20.7889 0.4406 0.2457 0.0010 3098 49 3129 371 3157 7 2

M42.65 301 123 0.42 0.02 0.5962 0.0124 20.2508 0.4317 0.2463 0.0011 3026 48 3103 364 3161 7 5

M42.66 230 87 0.39 0.02 0.6332 0.0137 21.7045 0.4813 0.2486 0.0012 3169 51 3171 399 3176 8 0

Mad 98/50 Arkosic gneiss

Mad 98/50-1 174 224 1.28 0.03 0.6299 0.0057 21.5980 0.0021 0.2486 0.0006 3150 23 3166 9 3166 9 0

Mad 98/50-2 97 3 0.03 0.15 0.0893 0.0001 0.7214 0.0097 0.0585 0.0031 552 5 552 23 552 23 0

Mad 98/50-3 192 144 0.75 0.66 0.1340 0.0011 1.2251 0.0003 0.0663 0.0014 811 7 812 14 812 14 0

Mad 98/50-4 246 5 0.02 0.13 0.0890 0.0007 0.7184 0.0045 0.0585 0.0010 550 5 550 9 550 9 0

Samples run on the SHRIMP II based at the John de Laeter Centre for Excellence in Mass Spectrometry in Perth, Australia. Common Pb derived from measured 204Pb and assuming a

Broken Hill Pb isotopic ratio. % com 206Pb = percentage common 206Pb; % disconc. = percentage disconcordant.a Radiogenic Pb only.b All errors are absolute, 1r errors.

Table 1 (continued)

A.S.Collin

set

al./Tecto

nophysics

375(2003)77–99

84


Grains were handpicked and mounted in epoxy resin

discs that were coated with a thin film of gold that

produced a resistivity of 10–20 V across the disc.

The mounts were then imaged under cathodolumi-

nescence (CL). The resulting images (Fig. 3) high-

light distortions in the crystal lattice (Stevens Kalceff

et al., 2000) that are related to trace-element distri-

bution and/or radiation damage (Rubatto and Geba-

uer, 2000).

U–Pb isotopic data were collected on the SHRIMP

II based in the John de Laeter Centre of Mass Spec-

trometry, Perth, Western Australia. Sensitivity for Pb

isotopes in zircon was f 18 cps/ppm/nA, primary

beam current was 2.5–3.0 nA and mass resolution

was f 5000. Correction of measured isotopic ratios

for common Pb was based on the measured 204Pb in

each sample and often represented a < 1% correction to

the 206Pb counts (see ‘‘%com. 206Pb’’ in Table 1). The

common Pb, component, being small and largely sur-

face contaminant, was modelled on the composition of

Broken Hill ore Pb. As the number of analysed grains is

important in detrital studies in order to minimise the

chance of missing an important source component

Table 2

Single-grain Pb evaporation results

Sample

number

Zircon colour

and morphology

Grain

no.

Mass

scansa

Mad 98/41 light grey, long, 1 66

prismatic, idiomorphic 2 63

3 66

Mean of three grains as above, slightly 1–3 195

rounded ends 4 82

5 82

Mad 98/42 clear, oval to round 1 88

detrital 2 44

3 78

4 88

5 84

Mad 98/50 clear to milky grey, long 1 64

prismatic, ends well rounded 2 66

3 171

4 130

5 88

Zircon morphology and isotopic data from single-grain evaporation. Errors

internal standard at 0.000027 (2r).a Number of 207Pb/206Pb ratios evaluated for age assessment.b Observed mean ratio corrected for nonradiogenic Pb where necessar

(Dodson et al., 1988), five mass scans were undertaken

in each analysis instead of the more usual seven.

Pb/U isotopic ratios were corrected for instrumen-

tal inter-element discrimination using the observed

covariation between Pb+/U+ and UO+/U+ (Compag-

noni et al., 1977; Compston et al., 1984, 1992, 1995)

determined from interspersed analyses of the Perth

standard zircon CZ3. CZ3 is a single zircon megacryst

from Sri Lanka with an age of 564 Ma and a206Pb/238U ratio of 0.0914 (Nelson, 1997).

3.2. Pb–Pb single zircon evaporation

We used the method developed by Kober (1986)

involving repeated evaporation and deposition of Pb

from chemically untreated single zircons in a double-

filament arrangement (Kober, 1987). Our laboratory

procedures as well as comparisons with conventional

and ion-microprobe zircon dating are detailed in Kro-

ner and Todt (1988) and Kroner and Hegner (1998).

Isotopic measurements were carried out on a Finnigan-

MAT 261 mass spectrometer at the Max-Planck-Insti-

tut fur Chemie in Mainz.

Evaporation

temperature

(jC)

Mean 207Pb/206Pb ratiob

and 2r (mean) error

207Pb/206Pb age

and 2r (mean) error

1600 0.239317F 73 3115.4F 0.5

1599 0.239104F 57 3114.0F 0.4

1601 0.239082F 71 3113.9F 0.5

0.239169F 42 3114.4F 0.3

1599 0.246369F 68 3161.6F 0.4

1598 0.244126F 69 3147.1F 0.4

1599 0.232680F 59 3070.6F 0.4

1598 0.239123F 84 3114.1F 0.6

1599 0.239529F 45 3116.8F 0.3

1598 0.240323F 66 3122.1F 0.4

1599 0.240583F 57 3123.8F 0.4

1588 0.123261F 74 2004.0F 1.1

1600 0.125782F 88 2039.8F 1.2

1596 0.141430F 79 2244.8F 1.0

1600 0.143300F 78 2267.5F 0.9

1598 0.182994F 82 1680.4F 0.7

of combined mean ages (bold print) are based on reproducibility of

y. Errors based on uncertainties in counting statistics.


The calculated ages and uncertainties are based on

the means of all ratios evaluated and their 2r mean

errors. Mean ages and errors for several zircons from

the same sample are presented as weighted means of

the entire population. During the course of this study,

we repeatedly analysed fragments of large zircon

grains from the Palaborwa Carbonatite, South Africa.

These zircons, used as an internal standard, are euhe-

dral, colourless to slightly pink and completely homo-

geneous when examined under CL. Conventional U–

Pb analyses of six separate grain fragments from this

sample yielded a 206Pb/207Pb age of 2052.2F 0.8 Ma

(2r; Todt, unpublished data), while the mean206Pb/207Pb ratio for 18 grains, evaporated individual-

ly over a period of 12 months, is 0.126634F 0.000027

(2r error of the population), corresponding to an age of

Fig. 4. Ambodibonara emerald mine (S21j25V43.0U, E47j54V23.2U). (a) Osubparallel to the foliation of highly deformed paragneiss. B = Emerald-be

relationship between pegmatite and isoclinally folded paragneiss—the s

psammitic schist with kyanite–biotite schist partings folded into an isoclina

this outcrop, 2.5-cm diameter coin for scale. (d) Euhedral kyanite crystal al

2051.8F 0.4 Ma, identical to the U–Pb age. The

above error is considered the best estimate for the

reproducibility of our evaporation data and corre-

sponds approximately to the 2r (mean) error reported

for individual analyses in this study (Table 2). In the

case of combined data sets the 2r (mean) error may

become very low, and whenever this error was less

than the reproducibility of the internal standard, we

have used the latter value (that is, an assumed 2r error

of 0.000027).

The analytical data are presented in Table 2, and the206Pb/207Pb spectra are shown in histograms that per-

mit visual assessment of the data distribution from

which the ages are derived. The evaporation technique

provides only Pb isotopic ratios, and there is no a priori

way to determine whether a measured 206Pb/207Pb ratio

verview of the mine entrance showing lensoid pegmatite bodies (P)

aring biotitite body. Circled figures for scale. (b) Detail of intrusive

ource for samples M98/41 and M98/42. (c) Foliated and lineated

l fold, Fenaorivo beach. Sample M99/20 was taken from this rock at

ong the foliation plane at Fenaorivo beach, 7-mm wide pen for scale.


reflects a concordant age. Thus, all 206Pb/207Pb ages

determined by this method are necessarily minimum

ages. However, many studies have demonstrated that

there is a very strong likelihood that these data repre-

sent true zircon crystallisation ages when (1) the206Pb/207Pb ratio does not change with increasing

temperature of evaporation and/or (2) repeated analy-

ses of grains from the same sample at high evaporation

temperatures yield the same isotopic ratios within error.

Comparative studies by single grain evaporation, con-

ventional U–Pb dating, and ion-microprobe analysis

have shown this to be correct (Cocherie et al., 1992;

Jaeckel et al., 1997; Karabinos, 1997; Kroner et al.,

1997b).

4. Sample and zircon characteristics

Four samples were collected from the extensive, but

poorly exposed, paragneiss belt along the eastern coast

of Madagascar (Fig. 1). These samples were initially

Fig. 5. U–Pb concordia plot of zircons from sample M98/42. The data are li

1r level. Inset shows a probability density distribution of < 10% discordant

data are presented both as a Gaussian relative probability curve and as a hi

classified in the field and during thin-section analysis

as likely to be metasedimentary rocks by (1) their high

Al and Si and low Ca composition reflected in large

modal percentages of quartz, mica and garnet and the

presence of alumino-silicate polymorphs (Fig. 4d) (the

exception, the relatively high Ca, hornblende-bearing,

M98/50, is interpreted as having a calcareous sediment

component); and (2) the heterogeneity of the outcrop

with diverse rock types interlayered on a centimetre or

decimetre scale (Fig. 4). CL imaging of zircon grains

strengthened this interpretation for samples M98/20

and M98/42 as both these samples contain many

broken and rounded grains with truncated CL zones

that suggest erosion of the grain during sedimentary

transportation (Fig. 3). Many of these grains have rims

of either new zircon, or recrystallised zircon that are in-

terpreted to have formed during peak Late Neoproter-

ozoic metamorphism (Fig. 3a and c), demonstrating

that the grain morphology formed prior to metamor-

phism and was not the result of breakage during sample

preparation.

sted in Table 1. Concordia dates are in Ma; error ellipses are drawn at

analyses demonstrating a considerable spread in Archaean ages. The

stogram of number of analyses. y-Axis scale refers to the histogram.

onophysics 375 (2003) 77–99

4.1. M98/41

Sample M98/41 is a sillimanite + biotite pelitic

gneiss from the Ambodibonara emerald mine f 50

km SW of Mananjary (S21j25V43.0U, E47j54V23.2U;Fig. 4a). The sample comes from decimetre-scale

pelitic gneiss horizons interlayered with garnet + bio-

tite psammitic gneiss (sample M98/42 below) and

biotite-bearing quartzites. Cross-cutting, deformed

amphibolite/biotitite emerald-bearing dykes cut the

metasedimentary rocks and are themselves intruded

by lensoid pegmatities (Fig. 4b).

4.2. M98/42

Sample M98/42 also comes from the Ambodibo-

nara emerald mine and is a garnet + biotite psammitic

gneiss interlayered with sample M98/41. The zircon

grains range in size from 100 to 250 Am and cross-

sectional shape from subcircular to elongate subangu-

lar. Individual zircon grains have a variety of forms

and crystal faces are commonly rounded. Under CL,

A.S. Collins et al. / Tect88

Fig. 6. U–Pb concordia plot of zircons from sample M98/50. The data ar

drawn at 1r level.

many grains preserve sector and oscillatory zoning

characteristic of original formation by magmatic crys-

tallisation (Fig. 3a and b), some preserve a distinct core

and a number of grains are partially metamict and

fractured. A sub-10 Am rim of brightly luminescent

zircon mantles many grains (Fig. 3a–c) and truncates

preexisting luminescence bands (Fig. 3a and c). These

rims are too small to be analysed by the techniques

used here, but are interpreted to have formed during

the Late Neoproterozoic metamorphic event seen

throughout Madagascar (Paquette et al., 1993; Kroner

et al., 1996; Kroner et al., 2000; de Wit, 2003).

4.3. M98/50

Sample M98/50 is a millimetre-banded biotite +

hornblende gneiss from a road cutting on the RN25 2

km west of Kianjavato village (S21j22V38.2U,E47j51V25.1U). It is interlayered with K-feldspar +

sphene and diopside rich centimeter-scale bands that

are together interpreted as a calcareous supracrustal

rock package.

e listed in Table 1. Concordia is marked off in Ma; error boxes are

nophysics 375 (2003) 77–99 89

4.4. M99/20

M99-20 is from a 10-cm-thick psammitic layer

(Fig. 4c) within a thick succession of kyanite + bio-

tite (F staurolite) schists (Fig. 4d) exposed at

Fenaorivo beach (S17j23V24.4U, E49j25V35.1U).These schists are asymmetrically folded into isoclinal

Z-folds plunging gently to the southwest (Fig. 4c).

Zircons were scarce and morphologically varied,

ranging in form from circular to subangular in

cross-section (Fig. 3d–f), which is consistent with

mechanical erosion during transport. A diverse range

of patterns is seen under CL that suggests a similarly

diverse origin of the grains. Two subspherical grains

have a bright response with broad (20–50 Am) thick

bands that may have formed during a metamorphic

event. One of these grains contains a relic core with thin

oscillatory zones characteristic of an igneous origin

(Fig. 3e). Other grains preserve truncated oscillatory

zoning, re-entrant structures interpreted as metamicti-

zation fronts, and homogenous dark responses inter-

A.S. Collins et al. / Tecto

Fig. 7. U–Pb concordia plot of zircons from sample M99/20. The data are

at 1r level. Inset is a blow up of the < 900 Ma part of the plot.

preted as high U, largely metamict, grains. The di-

verse origin of the grains is fully consistent with an

interpretation of the grains being detrital. One broken

grain has a thick rim with indistinct CL bands, sepa-

rated from the core by a bright luminescence band

(Fig. 2f).

5. U–Pb SHRIMP results

5.1. M98/42

Sixty seven analyses of zircons from sample M98/

42 yielded a restricted 207Pb/206Pb age range between

2977 and 3198 Ma (Fig. 5; Table 1). A discordia line

through all the data has an upper intercept of 3180F 11

Ma and a lower intercept of 1104F 180 Ma (MSWD=

3.4). If this discordia line was an accurate representa-

tion of the data, it would imply that there was a single

age source for the zircons in this rock. However, we

note that the upper intercept age is older than most of

listed in Table 1. Concordia dates are in Ma; error ellipses are drawn

A.S. Collins et al. / Tectonoph90

the concordant analyses (Table 1) and that the lower in-

tercept corresponds to no known geological events in

Madagascar. A probability density distribution plot of

these analyses (Fig. 5, inset) highlight an old 207Pb/206Pb age population that has a weighted mean age of

3160F 3 Ma that is interpreted as representing the do-

minant source to the sedimentary protolith of this

paragneiss. The CL response of these grains suggests

that they were derived from an igneous source (Fig.

3a–c). Younger rocks have contributed to the sediment,

although these are of lesser modal importance (Fig. 5,

inset).

5.2. M98/50

M98/50 was analysed as a reconnaissance of addi-

tional metasedimentary rocks in east Madagascar. Four

concordant analyses were obtained from sample M98/

50 (Fig. 6; Table 1). Two detrital cores were analysed

that have 207Pb/206Pb ages of 3176F 4 Ma and

811F 7 Ma. A further two rims were analysed that

have a mean age of 551F 2 Ma.

Fig. 8. Probability density distribution plot of zircon results from M99/20

quoted for analyses older than 1.0 Ga, whereas 206Pb/238U ages are quote

5.3. M99/20

Zircons are scarce in M99/20. As a result, only 27

analyses could be obtained. This is considerably

below the minimum number of analyses needed to

ensure that all major sources have been sampled

(Dodson et al., 1988). However, the obtained analyses

still have interesting implications for the age of the

rock and the source region. Zircon analyses form a

diverse array of ages stretching back to 3459F 7 Ma

(Figs. 7 and 8). Four analyses, < 10% discordant,

were older than 3000 Ma with further populations at

2600 Ma, 2500 Ma and between 708 and 832 Ma

(Fig. 8). One younger 206Pb/238U core analysis

yielded an age of 553F 9 Ma. This analysis is 19%

discordant and the grain is highly metamict. These

data suggest that Archaean rocks form a significant

component of the source region. The best upper

constraint on the age of deposition is provided by

the youngest concordant core analysis, which came

out at 710F 11 Ma. One grain has a broad (f 30 Am)

rim that has a 206Pb/238U age of 518F 9 Ma around a

ysics 375 (2003) 77–99

. Note that due to the U–Pb decay systematics 207Pb/206Pb ages are

d for analyses younger than 1.0 Ga (Cawood and Nemchin, 2000).


723F 15 Ma core (Fig. 3f). Both core and rim plot to

the left of the concordia (Fig. 7) and have relatively

high common Pb values (Table 1). However, the

similarity between the 206Pb/238U age of the core

analysis and the 709–832 Ma age population suggest

that the discordance may be largely an artefact of the

common Pb correction, in which case, the rim age of

518F 9 Ma is of considerable importance. The grain

is broken along an angular fracture that because of a

lack of rounding may have formed during sample

preparation. Therefore, the broken nature of the grain

cannot be used as evidence that the rim formed before

deposition. Broad luminescence bands can be seen in

the rim and a brightly luminescent band separates the

core from the rim. In one edge of the grain, the dark

rim forms a bulge into the core (marked ‘‘� ’’ in Fig.

3f). These features are similar to those noted in

metamorphically recrystallised rims (Hoskin and

Black, 2000) as is the low Th/U ratio of 0.02 (Table

1, Maas et al., 1992). Therefore, we interpret this rim

Fig. 9. Histograms of the distribution of radiogenic Pb isotopes—(207Pb/2

Ambodibonara emerald mine, east Madagascar. (a) Spectrum for three grain

Spectra for grains from older sources. Mean ages are given with 2r error

as a solid-state recrystallisation of part of the original

detrital grain during Cambrian mid-amphibolite facies

metamorphism.

6. Pb–Pb evaporation results

6.1. M98/41

Evaporation of five zircons from M98/41 produced

three distinct Archaean 207Pb/206Pb ages (Fig. 9; Table

2). Aweighted mean of 195 ratios from Grains 1, 2 and

3 produced an age of 3114.4F0.4 Ma. Analyses from

these grains were combined because their similar Pb

isotopic ratios suggested an identical (within error) age

of formation. Eighty-two ratios from Grain 4 yielded a

mean age of 3147.1F0.4 Ma. Eighty-two ratios of

Grain 5 produced an age of 3161.6F 0.4 Ma. These

are all interpreted as minimum crystallisation ages of

detrital grains within the pelitic gneiss.

06Pb)*—derived from evaporation of detrital zircons from M98/41,

s derived from the same source and integrated from 195 ratios. (b, c)

.

Fig. 10. Histograms of the distribution of radiogenic Pb isotopes—(207Pb/206Pb)*—derived from evaporation of detrital zircons from M98/42,

Ambodibonara emerald mine, east Madagascar. The spectra plotted have been integrated from the number of ratios shown on each diagram.

Mean ages are given with 2r error.


6.2. M98/42

Four zircon grains were evaporated from sample

M98/42 and four distinct Archaean 207Pb/206Pb ages

were produced (Fig. 10; Table 2). Over 80 Pb isotope

ratios were analysed from each grain and the resulting

ages are 3070.6F 0.4, 3116.8F 0.3, 3122.1F 0.4 and

3123.8F 0.4 Ma. These ages lie within the spread of207Pb/206Pb ages obtained by SHRIMP analysis (see

above).

Table 3

Summary of the age constraints on each sample analysed in this

study

Sample Depositional age (Ma) with 1r errors

Max Min

M98/41 3114.04F 0.4 –

M98/42 2987F 10 –

M98/50 811F 7 551F 2

M99/20 710F 11 Ma 518F 9

7. Discussion

7.1. Age of the metasedimentary rock protoliths

The depositional ages of these paragneisses are

constrained by the ages of the youngest concordant

detrital core (710F 11Ma) and the oldest metamorphic

rim that formed in situ within the sedimentary rock

(551F 2 Ma). The limited outcrop in eastern Mada-

gascar means that age differences between the samples

may reflect real differences in the depositional and

metamorphic history of the samples. This is summar-

ised in Table 3, where the actual age constraints on each

sample are presented. A main conclusion of this study

is that in contrast to previous work that interpreted all

the metasedimentary rocks of eastern Madagascar as

Archaean (Besairie, 1967; Hottin, 1976; Tucker et al.,

1999b), we demonstrate that at least some of them are

of Neoproterozoic/Cambrian age.

nophysics 375 (2003) 77–99 93

7.2. Source characteristics

All analysed samples contain detrital zircons older

than 3000 Ma. The source to M99/20 contains zircons

that date back to f 3460 Ma. Significant populations

at 3000–3200, f 2650, f 2500, 800 and 700 Ma

mean that the source area must also contain rocks of

these ages (Fig. 11). The two detrital grains from

sample M98/50 match populations in M99/20 suggest-

ing the two samples had a similar source. M98/41 and

M98/42 produced results similar to each other and

show that they are derived from a source with a

restricted age range between 2977 and 3198 Ma.

7.3. Potential source regions

Age populations from detrital zircon studies can

only tell you the ages of some rocks in the source

region. They do not comprehensively sample all rocks

and different sedimentary rocks will be derived from

A.S. Collins et al. / Tecto

Fig. 11. Time–space cartoon showing the main events in the Dharwar Cra

Braun and Kriegsman, 2003). The Southern Granulite Terrane is separated f

that was interpreted by Collins and Windley (2002) as a continua

Betsimisaraka = samples analysed as part of this study, Itremo= Itremo G

Molo sequence are after Cox et al. (1998, 2000, 2001, in press).

vastly different drainage basin areas. In addition, the

potential source regions may not have been adequately

dated. All this makes the identification of source areas

by age populations alone, an activity fraught with

difficulties. In the discussion below we do not claim

to uniquely identify the exact region of continental

crust where the metasedimentary rocks described in

this paper were sourced. Instead, we discuss the most

likely areas based on palaeogeographic reconstruc-

tions of Gondwana, the geology of the region, and

the available geochronological database of the adja-

cent parts of Gondwana (Fig. 2).

Malagasy rocks with U–Pb ages older than 3 Ga

are rare and restricted to the Antongil Block in the

northeast of the island (Fig. 1). Elsewhere on the

island the oldest rock so far dated by U–Pb zircon

methods is a 2900F 17 Ma metagranite from the

poorly known Vondrozo region that may lie in a

southern continuation of the Betsimisaraka Suture

(Fig. 1, Collins et al., 2001). Rocks with U–Pb ages

ton and the Southern Granulite Terrane (after Chadwick et al., 2000;

rom the Dharwar Craton by the Palghat–Cauvery shear zone system

tion of the Betsimisaraka suture zone of eastern Madagascar.

roup, Molo =Molo sequence. Data from the Itremo Group and the


older than 2.6 Ga so far have not been found in either

the Antananarivo block or the Tsaratanana sheet

(Paquette and Nedelec, 1998; Kroner et al., 1999,

2000; Collins et al., 2001).

The Antongil Block shares many similarities with

the western Dharwar Craton of southern India and has

been interpreted as a part of this tectonic unit left

behind in Madagascar as Gondwana broke up

(Agrawal et al., 1992; Collins and Windley, 2002;

Raval and Veeraswamy, 2003). The basement of the

Western Domain of the Dharwar Craton (Chadwick et

al., 2000) is made of 3400–3200 Ma orthogneiss

(Peucat et al., 1993) and supracrustal rocks of the

Sargur Group, deposited between 3130 and 2960 Ma

(Nutman et al., 1992). These are overlain by later

supracrustal rocks of the Dharwar Group (Swami Nath

et al., 1976; Chadwick et al., 1989) that were deposited

between 3000 and 2600 Ma (Nutman et al., 1996) (Fig.

11). Rare 2550 Ma granitoid plutons cut the Western

Domain (Rodgers, 1988). The Eastern Domain of the

Dharwar Craton is dominated by the Dharwar Batholith

(Chadwick et al., 2000), which consists of 2513–2552

Ma granitoid plutons (Jayananda et al., 2000). Ages of

rocks in the Dharwar Craton correlate well with the age

population of detrital grains analysed in this study. This

observation when coupled with: (1) their palaeogeo-

graphic position within Gondwana (Lawver et al.,

1998; Reeves and de Wit, 2000; Reeves et al., 2002);

and (2) the lack of any Archaean–Phanerozoic struc-

ture between eastern Madagascar and the western

Dharwar Craton, provides strong evidence that the

Dharwar Craton was a source of the detrital zircons

described here.

Zircons from a Sargur Group quartzite of the

Western Domain of the Dharwar Craton yielded a

restricted age range of detrital zircons between 2930

and 3242 Ma (Nutman et al., 1992). This range is

extremely similar to the restricted 2977–3198 Ma

range of zircons from samples M98/41 and M98/42.

This age similarity, when combined with the large

distance between analysed metasedimentary gneisses

and the sporadic outcrop in eastern Madagascar,

means that it is possible that the Ambodibonara

metasedimentary gneiss protoliths lie on the western

edge of the Archaean Antongil block and are Mala-

gasy corollaries of the Sargur Group. Alternatively,

these rocks may well correlate with the other Neo-

proterozoic metasedimentary gneisses described in

this study, but are derived from an almost identical

source to that of the Sargur Group, or from the Sargur

Group itself.

Derivation from the Dharwar Craton explains the

age range of the Archaean zircons in the eastern

Malagasy metasedimentary gneisses, but it does not

explain the f 700–800 Ma grains in samples M99/20

and M98/50. Rocks of this age occur elsewhere in

southern India, but are restricted to the Eastern Ghats

(Krause et al., 2001), rare plutons in the Madurai

Block (Santosh and Drury, 1988) and the Madras

Granulite Belt (Miyazaki et al., 2000) that are not

dated as yet by U–Pb techniques. There are, however,

rocks of this age in the Seychelles microcontinent and

northwest India that may have formed a continental

margin arc to the north of the Dharwar Craton (Torsvik

et al., 2001; Tucker et al., 2001a; Ashwal et al., 2002).

It is conceivable that sediments from this region were

transported south in a linear continental margin basin.

It is also plausible that central Madagascar contributed

significant detritus to these sediments There are many

granitoid rocks in the Antananarivo Block and Itremo

Sheet of central Madagascar that crystallised between

740 and 825 Ma (Guerrot et al., 1993; Handke et al.,

1999; Kroner et al., 2000) Ma. In addition, the com-

mon 2500 Ma protoliths in the Antananarivo Block

may have contributed to the prominent 2500 Ma age

peak.

7.4. Regional implications

Detrital zircon U–Pb data from Proterozoic meta-

sedimentary rocks in the Itremo Sheet of central Mada-

gascar (Cox et al., 1998, 2000, 2001, in press) show

that: (1) two distinct sedimentary successions exist, the

f 1700–800 Ma Itremo Group and the 620–560 Ma

Molo sequence; (2) the U–Pb detrital age distribution

of the Itremo Group has distinct populations at 2670,

2500, 2135 and 1875Ma; and (3) the U–Pb detrital age

distribution of the Molo sequence has distinct popula-

tions at 1065, 950, 830 and 640 Ma.

The depositional age of the Molo sequence is

broadly equivalent to that of samples M99/20 and

M98/50 (Table 3) and is found f 200 km west of the

metasedimentary rocks described in this study (Figs. 1

and 12). They were both deposited before the intense

east–west contractional deformation resulting from the

closure of the Mozambique Ocean (Powell et al., 1993;

Fig. 12. Summary of approximate positions of a Neoproterozoic

‘‘Dharwar derivation’’ provenance front and the coeval ‘‘East

African derivation?’’ provenance front superimposed on a recon-

struction of part of the East African Orogen (EAO) in a Gondwana

reconstruction (after Collins and Windley, 2002). The form of the

‘‘East African derivation’’ provenance front is approximate and is

drawn parallel to the outcrop of the Itremo Group. Light grey

region =The EAO, juvenile Neoproterozoic rocks and/or region

affected by 750–500 Ma upr-amphibolite grade or higher meta-

morphism. Dark grey = present day water.


Tucker et al., 2001b; Collins et al., 2003a), yet they

have very different detrital zircon age populations.

The Dharwar Craton is likely to be a major source

for many of the zircons in the metasedimentary rocks of

eastern Madagascar described in this paper (see dis-

cussion above). In contrast, the lack of zircons older

than 2900 Ma in the Itremo Group and younger than

2500 Ma in the Dharwar Craton (Fig. 11) suggests that

the Dharwar craton is unlikely to be a source of either

the Itremo Group or the Molo sequence. Possible

locations of the source of these rocks include India

south of the Dharwar craton (the Southern Granulite

Terrane) and Sri Lanka where zircon ages corres-

ponding to all the main populations in the Itremo

Group and Molo sequence have been identified (Figs.

2 and 11, Holzl et al., 1994; Braun and Kriegsman,

2003). Proterozoic events in southern India, especially,

are poorly understood and many of the 2200–1800 Ma

zircons reported from here and Sri Lanka are detrital

grains that themselves have no known local source

(Braun and Kriegsman, 2003). A possible ultimate

origin for these grains and those from both the Itremo

Group and the Molo sequence is central East Africa

(Fig. 2), the region now encompassed by Kenya,

Tanzania, Zambia, Malawi and northern Mozambique

that lay adjacent to Madagascar in Gondwana (Lawver

et al., 1998; Reeves et al., 2002). In this region,

potential sources of the 2670 Ma (Tanzania Craton,

Borg and Krogh, 1999), 2135 Ma (slightly older that

the < 2100 Ma zircons reported from the Ubende Belt,

Lenoir et al., 1994b; Ring et al., 1997), 1875 Ma

(voluminous post-tectonic granites in the Usagaran

Belt, Reddy et al., 2003), 1065 Ma and 950 Ma

(Irumide Belt, Kroner et al., 1997b, 2001; de Waele

and Mapani, 2002; Rivers and Johnson, in press) and

640 Ma (granulite-grade metamorphism in Tanzania,

Moller et al., 2000) zircons have been reported from an

area of comparable size to many modern drainage

basins (Fig. 2). The remaining two age peaks at 2500

and 830 Ma are plausibly sourced locally from the

central Malagasy basement, which, at least, the Itremo

Group unconformably overlies (Cox et al., 1998).

East Africa lay adjacent to Madagascar in Gond-

wana, but the two are separated by a zone of juvenile

Neoproterozoic crust that has been interpreted as the

site of a strand of the Mozambique Ocean suture

(Collins and Windley, 2002; Stern, 2002). Collins

and Windley (2002) suggested that central Madagas-

car, southern India and Sri Lanka may represent a

broadly linear microcontinent that can be traced north

through Somalia and east Ethiopia to the Arabian

peninsula. The observations that East Africa and

central Madagascar may both have contributed to

the Molo sequence suggest that either (1) central

Madagascar had collided with East Africa prior to

deposition of the Molo sequence, or (2) any (ocean-

ic?) basin between the two was not wide enough to

inhibit sedimentary transport from both margins to

the Molo depocentre. The first interpretation supports

that of Paquette and Nedelec (1998), de Wit et al.

(2001) and de Wit (2003) who interpreted a phase of

pre-640 Ma collisional deformation in Madagascar

that Meert (2003) correlated with the earliest of his

two proposed eastern Gondwana orogenies (his 750–

620 Ma East African Orogeny). The second interpre-

tation is supported by the apparent African prove-

nance of the pre-800 Ma Itremo Group, suggesting

that source regions in central Madagascar did not

change much throughout the early and middle Neo-

proterozoic, and by the extensive crustal shortening


and granulite-grade metamorphism that occurred after

deposition of the Molo sequence (Moine, 1968; Cox

et al., 2001) suggesting the closure of a basin west of

Madagascar in latest Neoproterozoic/Cambrian times.

This late deformation is approximately coeval with

the timing of Meert’s (2003) 570–530 Ma Kuunga

Orogeny.

These observations imply that two Neoproterozoic

provenance fronts run through central east Madagas-

car. One provenance front formed sometime between

620 and 560 Ma and delineates the furthest east

occurrence of sediments derived from the same source

as the Molo sequence (with a possible East African

component). This front lies east of the Molo sequence

and probably east of the Itremo Group outcrop (Fig.

12). Similarly, a second provenance front developed

sometime between 721 and 549 Ma (at greatest error)

and marks the western extent of sediments containing

a component derived from the Dharwar craton. This

front must pass west of the samples described in this

study (Fig. 12). It is unknown whether these prove-

nance fronts meet, are thrust imbricated at their

mutual contact, or are separated by a region of

sediment derived from a third, as yet uncharacterised,

source.

The metasedimentary rocks analysed in this study

all come from the boundary zone between the Anta-

nanarivo and Antongil blocks (Fig. 1). This zone

forms an isotopic and lithological boundary (Hottin,

1976; Collins and Windley, 2002) and was interpreted

as an oceanic suture zone by Collins and Windley

(2002). They called this suture the Betsimisaraka

suture zone and projected it east into the Palghat–

Cauvery shear zone system that cuts off southern

India from the Dharwar Craton (Fig. 2). This study

supports the interpretation of the Betsimisaraka suture

marking the site of a strand of the Mozambique Ocean

by locating the margins of pre- to early syn-collisional

Neoproterozoic sedimentary rocks derived from dif-

ferent tectonic blocks (and possibly from either side of

the EAO) at approximately the site of the Betsimisar-

aka suture zone (Fig. 12).

Acknowledgements

Chris Powell had a love of fieldwork in Mada-

gascar, and the authors share great memories of Chris’

enthusiasm and at times bombastic drive in the wilds

of the Malagasy bush. He is sorely missed.

Bregje Hulscher and Dan Bishop are thanked for

help and companionship in the field. Les Lezards du

Tana provided logistic support, and Gabi and Tsu are

especially thanked for their patient shepherding us

around. Brian Windley is thanked for getting ASC

involved in Madagascar in the first place. Keith

Sircombe patiently and efficiently edited the manu-

script, and careful reviews by Maarten de Wit,

Ronadh Cox and Bob Stern greatly improved the

manuscript. Many of the zircon analyses were carried

out on the Sensitive High-mass Resolution Ion

Microprobe (SHRIMP II) operated by a consortium

consisting of Curtin University of Technology, the

Geological Survey of Western Australia and the

University of Western Australia with the support of

the Australian Research Council. We appreciate the

assistance of Peter Kinny, Sasha Nemchin and Allen

Kennedy during SHRIMP analysis and data reduction.

This paper is the Tectonic Special Research Centre

publication no. 209.

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