J. Earth Planet. Sci. Nagoya Univ., Vol. 44, 89 to 102, 1997

Cambrian granulite to upper amphibolite facies metamorphismof post-795 Ma sediments in Madagascar
Masahiro I TO* , Kazuhiro S UZUKI** and Setsuo Y OGO***School of Informatics and Sciences, Nagoya University, Nagoya 464-01, Japan**Department of Ert and Planetary Sciences, Graduate School of Sciences,
Nagoya University, Nagoya 464-01, Japan(Received November 6, 1997 / Accepted November 26, 1997)

ABSTRACTThe CHIME monazite and xenotime ages were determined for two paragneisses from Madagascar. The biotite-sillimanite-cordierite-quartz-microclineplagioclase schist at Ihosy in southern Madagascar contains core-mantle monazite grains as well as chronologically unzoned ones. The unzoned monazite
grains (8 of 11 analyzed grains) give a CHIME age of 527±15 Ma. Two zoned
grains show core ages of 1640±180 Ma and 795±75 Ma with mantles of 550±50
and 535±10 Ma, respectively. The kyanite- and corundum-bearing cordieritesillimanite-muscovite-biotite-plagioclase-quartz gneiss at Maevatanana innorthwestern Madagascar contains both monazite and xenotime. Monazite andxenotime grains are chronologically uniform, and give CHIME ages of 534±10
and 530±32 Ma, respectively. These chronological data suggest that the highgrade paragneisses in Madagascar, if not all, formed through a single thermal
event at ca. 530 Ma from post-795 Ma sediments. This metamorphism can belinked to the continental collision that resulted in Gondwana supercontinent.

INTRODUCTION
Gondwana supercontinent existed over 300 m.y. before its break up in the
Mesozoic. The evolution of this supercontinent, particularly of East Africa,
Madagascar, Southern India, Sri Lanka and East Antarctica, were recently discussed on the basis of lithology, metamorphism and tectonics (e.g. Yoshida et..
al., 1992; Windley et al., 1994: Shiraishi et al., 1994: Kroner et al., 1996).
The broad aspect of the picture for the Gondwana connection are generally
agreed, but considerable debates still remain in detail, especially the locationof the collision zone. Since Gondwana supercontinent formed through thelate Cambrian (-latest Proterozoic) continental collision of East and West
Gondwana, the distribution of Cambrian metamorphic rocks may play a keyrole in the analysis of the collision tectonics.
Recent geochronological studies have brought forth Cambrian metamorphic ages for high-grade paragneisses in the Kerala kondalite belt of southernmost India (Choudhary et al., 1992; Santosh et al., 1992; Bindu et al., 1996).
Similar high-grade paragneisses are known to occur in Madagascar (Windley
et al., 1994). Some of them were dated to be around 550 Ma (e.g. Knoner et al.,

89

90

M. Ito et al.

1996, and their references). But little is known about the extension of the ca.
550 Ma metamorphic rocks in Madagascar. In order to shed more light on thecollision tectonics of Gondwana, we have made the CHIME dating of monazite
and xenotime from two samples of High-grade paragneisses from Madagascar.

Fig. 1.

Simplified and schematic geologic map showing the main basements of Madagascar (from Widley et al., 1994).

Cambrian metamorphism of post-797 Ma sediments in Madagascar

91

GENERAL GEOLOGY AND SAMPLE DESCRIPTION
The basement rocks in Madagascar are divided into the southern and central-northern sectors by the northwest trending Ranotsara shear zone (Windley et al., 1994; Fig. 1). The southern sector consists predominantly of highgrade metamorphic rocks formed under the granulite and upper amphibolitefacies conditions. Particularly common are cordierite- and sillimanite-bearing
paragneisses with intercalated marble, metaquartzite and amphibolite layers.
They generally trend in NS direction. The central-northern sector is underlain
mainly by granitoids and amphibolite (greenstone belt) with a subordinate
amount of paragneiss. This sector is traversed by a north trending 100-150
km wide dextral shear zone of probable Pan-African age; the western marginof the shear zone runs through around Antananarivo. Rocks in the dextral
shear zone were metamorphosed under the granulite and upper amphibolitefacies conditions, and include reworked older basement. Two paragneiss samples were collected for the present CHIME dating; biotite-sillimanite-cordierite
gneiss from Ihosy in the southern sector and kyanite- and corundum-bearing
cordierite-sillimanite-muscovite-biotite gneiss from Maevatanana in the western part of the central-northern sector.
Biotite-sillimanite-cordierite-quartz-microcline-plagioclase gneiss from Ihosy
This rock is composed of plagioclase, microcline, quartz, biotite, cordierite,
sillimanite and opaque mineral, together with small amounts of zircon, apatiteand monazite. Plagioclase, sodic oligoclase in composition, is granoblastic andis slightly sericitized. Biotite, sillimanite and quartz together with fluid inclusions are found within plagioclase grains. Myrmekite is rarely developed.
Microcline is commonly string and film perthite with well developed microcline twinning. It includes sillimanite and plagioclase. Quartz show often undulatory extinction and includes sillimanite, biotite, plagioclase, microcline,
apatite and opaque mineral. Cordierite occurs as granoblastic grains with inclusions of sillimanite, biotite, quartz and opaque mineral. Some cordierite
grains are twinned. It is slightly altered. Sillimanite is coarse-grained, sometimes euhedral or subhedral, and include quartz, biotite, zircon and opaquemineral. It shows often undulatory extinction. The sillimanite inclusion incordierite, quartz and biotite are of small and short prismatic form, and are
frequently oriented. Biotite is oriented along the schistosity. It is pleochroic;
X=nearly colourless, Y=brown and Z=light brown. Symplectic biotite is also
found. Inclusions in biotite are sillimanite, apatite, zircon, monazite, andopaque mineral. Muscovite is very small in amount, and may be derived fromthe alteration of plagioclase. Monazite is anhedral, and ranges in size from0.02 to 0.15 mm.
Kyanite- and corundum-bearing cordierite-sillimanite-muscovite-biotite-plagioclase-quartz gneiss from Maevatanana
Biotite-rich layers are finely alternated with feldspathic layers which aresomewhere coarse-grained. This rock is composed of plagioclase, quartz,

92

M. Ito et al.

biotite, muscovite, cordierite and sillimanite together with accessory corundum, kyanite, zircon, apatite, monazite, xenotime and opaque mineral. Plagioclase (oligoclase) is granoblastic and is slightly sericitized. It includes biotite,sillimanite, muscovite and zircon. Quartz is also granoblastic with inclusionsof biotite, sillimanite and muscovite. Biotite is commonly associated withmuscovite and sillimanite. It is pleochroic; X= nearly colourless, Y= yellowish
olive green and Z= olive green. Inclusions in biotite are apatite, zircon, monazite and opaque mineral. Sillimanite is fibrous to very long prismatic, intimately intergrown with muscovite and biotite. Cordierite is granoblastic withinclusions of biotite, sillimanite and opaque mineral. It is moderately, sometimes completely, altered. Muscovite flakes include plagioclase, quartz, biotite, sillimanite, corundum and kyanite. Corundum and kyanite are always
coated with muscovite. Monazite and xenotime occur as tiny grains, but exceptionally large grains exceed 0.04 mm in size.
CHIME AGES
Monazite and xenotime grains in polished thin sections, prepared for theconventional electron microprobe analyses, were analyzed on JEOL JXA-733
electron microprobe equipped with three wavelength dispersive-type spectrometers. Instrument operating conditions were 15 kv accelerating voltage,
0.02-0.15µA current and 5 µm probe diameter. X-ray intensities were integrated 200σ for the lines and 100s for backgrounds at two optimum positions on
both sides of the lines. The detection limits of ThO 2 , UO 2 and PbO at 2σ
confidence level are 0.012, 0.007 and 0.004 wt.%, respectively, and possible errors in the determination of ThO 2 , UO 2 and PbO are about 14% for 0.1 wt.% of
the concentrations. The analytical results are listed in Table 1. CHIME ages
were calculated through the method described by Suzuki and Adachi (1991a,b,
1994), Adachi and Suzuki (1992) and Suzuki et al. (1994).
Biotite-sillimanite-cordierite-quartz-microcline-plagioclase gneiss from Ihosy
Eleven monazite grains were analysed. Nine of these grains are chronologically unzoned, and two grains show a core-mantle relation. Figure 2 shows
the PbO vs.ThO 2 * plots of chronologically unzoned monazite grains. Data
points, except those for grain margins (open circle), are arrayed linearly on
the diagram. They are regressed with an isochron of 527±15 Ma(MSWD=0.28) with an intercept value of -0.0010±0.0045.
Figures 3a shows the PbO vs.ThO 2 * plots of analytical data of a coremantle monazite grain (M02). Data points for the core (solid square) and the
mantle (solid circle) are separately arrayed, and define different isochrons of
1640±180 Ma (MSWD=0.66) and 550±50 Ma, respectively. Data points between the 1640 and 550 Ma isochrons (open square, cross and triangle) are forthe core-mantle boundary region, but do not define either a certain area in the
cross section or any isochrons. The textural feature suggests an overlap of
the 550 Ma mantle on the 1640 Ma core at analyzed points on the crossCambrian metamorphism of post-797 Ma sediments in Madagascar

Table 1.

Spot No.

93

Microprobe analyses of ThO 2 , UO 2 and PbO of monazite (M) and xenotime (X)from metamorphic rocks at Ihosy and Maevatanana, Madagscar.

ThO2 UO2 PbO Age ThO2*(wt.%)(wt.%) (wt.%) (Ma)Spot No.

(wt.%)

Biotite-sillimanite-cordierite gneiss (Ihosy)
M01-01
6.38 0.084 0.149 525 6.66
M01-02
6.32 0.049 0.147 534 6.48
M01-036.43 0.079 0.152 532 6.69
M01-045.30 0.175 0.134 534 5.88
M01-05
6.24 0.145 0.144 503 6.72
M01-065.73 0.170 0.139 517 6.29
M01-07 r
5.46 0.117 0.120 482 5.84
M01-08 r
5.47 0.174 0.119 462 6.05
M01-09 r
5.67 0.170 0.129 488 6.23
M01-10 r
5.63 0.207 0.126 469 6.31
M01-11 r
5.75 0.191 0.139 479 6.38
M02-01 4030 6.98 0.348 0.574 1592 8.25
M02-02 4033 7.26 0.245 0.572 1605 8.15
M02-03 4038 7.29 0.246 0.587 1638 8.19
M02-04 4041 7.48 0.276 0.568 1534 8.48
M02-05 2034 7.09 0.335 0.510 1415 8.28
M02-06 4628 6.06 0.143 0.194 693 6.54
M02-07 5330 6.72 0.112 0.163 538 7.09
M02-08 2510 6.80 0.114 0.157 515 7.17
M02-09 1349 6.72 0.138 0.164 536 7.18
M02-10 5343 6.82 0.154 0.169 541 7.34
M02-11 0236 6.31 0.174 0.135 460 6.88
M02-12 0826 6.21 0.143 0.130 458 6.68
M02-13 0836 6.15 0.150 0.150 530 6.65
M02-14 0641 6.00 0.123 0.133 488 6.41
M02-15 0852 5.67 0.170 0.139 524 6.23
M02-16 1417 5.57 0.141 0.131 510 6.04
M02-17 1425 5.26 0.133 0.131 540 5.70
M02-18 1433 5.53 0.097 0.127 511 5.85
M02-19 1453 5.52 0.148 0.130 509 6.01
M02-20 1459 5.25 0.127 0.119 495 5.67
M02-21 2206 5.50 0.133 0.129 509 5.94
M02-22 2212 5.56 0.124 0.137 539 5.97
M02-23 2219 5.69 0.100 0.129 503 6.02
M02-24 2224 4.10 0.040 0.119 658 4.24
M02-25 2238 5.40 0.247 0.438 1590 6.30
M02-26 2254 5.72 0.129 0.143 546 6.15
M02-27 2259 5.12 0.137 0.113 477 5.57
M02-28 3001 5.76 0.106 0.127 489 6.11
M02-29 3007 5.76 0.082 0.127 496 6.03
M02-30 3013 5.72 0.101 0.139 538 6.05
M02-31 3020 5.47 0.098 0.131 529 5.80
M02-32 3026 5.84 0.382 0.528 1667 7.24
M02-33 3031 5.42 0.591 0.546 1647 7.58
M02-34 3038 6.10 0.271 0.503 1624 7.09

M02-35 3043
M02-36 3049
M02-37 3057
M02-38 3063
M02-39 3069
M02-40 3601
M02-41 3609
M02-42 3616
M02-43 3625
M02-44 3631
M02-45 3637
M02-46 3644
M02-47 3651
M02-48 3657
M02-49 3663
M02-50 3669
M02-51 4305
M02-52 4311
M02-53 4319
M02-54 4336
M02-55 4345
M02-56 4351
M02-57 4357
M02-58 4366
M02-59 5014
M02-60 5023
M02-61 5037
M02-62 5049
M02-63 5056
M02-64 5809
M02-65 5823
M02-66 5840
M02-67 6415
M02-68 6425
M02-69 6429
M02-70 6436
M02-71 6728
M02-72 4645
M02-73 4045
M02-74 3345
M02-75 2445
M02-76 1945
M02-77 2152
M02-78 2752
M02-79 3252
M02-80 3752

ThO2 UO2 PbO Age ThO2*(wt.%)(wt.%) (wt.%) (Ma)(wt.%)

6.17
5.58
5.59
5.80
5.69
5.74
5.51
6.28
5.62
5.53
5.10
6.03
6.04
5.65
5.74
5.52
6.61
6.43
5.90
5.42
5.54
5.61
5.54
5.48
5.46
4.15
5.28
5.62
5.48
5.73
5.27
5.81
5.63
5.71
5.95
3.05
5.80
5.75
5.85
6.60
5.59
5.77
5.98
5.65
5.89
5.76

0.228 0.491 1604
0.166 0.276 1039
0.135 0.136 529
0.136 0.142 533
0.170 0.124 468
0.089 0.133 519
0.102 0.134 538
0.153 0.174 600
0.225 0.461 1634
0.302 0.471 1622
0.637 0.511 1577
0.237 0.449 1497
0.112 0.163 597
0.135 0.138 531
0.139 0.137 519
0.167 0.112 434
0.112 0.163 546
0.119 0.151 520
0.169 0.149 540
0.254 0.306 1125
0.107 0.130 519
0.105 0.137 540
0.132 0.134 527
0.177 0.116 448
0.124 0.121 485
0.093 0.103 541
0.108 0.126 527
0.137 0.137 530
0.251 0.143 532
0.150 0.118 446
0.128 0.122 505
0.153 0.128 476
0.142 0.140 538
0.136 0.126 480
0.152 0.121 442
0.765 0.102 432
0.162 0.142 526
0.113 0.137 524
0.125 0.140 524
0.253 0.545 1655
0.148 0.136 526
0.133 0.134 508
0.157 0.149 538
0.123 0.132 510
0.173 0.233 840
0.121 0.142 542

7.00
6.16
6.04
6.25
6.25
6.03
5.85
6.79
6.44
6.64
7.42
6.89
6.41
6.10
6.19
6.07
6.98
6.82
6.46
6.31
5.90
5.96
5.98
6.06
5.87
4.46
5.64
6.07
6.31
6.23
5.69
6.31
6.10
6.15
6.46
5.56
6.33
6.12
6.26
7.53
6.08
6.21
6.50
6.06
6.48
6.16

94

Table 1.

M. Ito et al.

(continued).

Spot No.
M02-81 4352
M02-82 4362
M02-83 3762
M02-84 3262
M02-85 2538
M02-86 2530
M02-87 2030
M03-01
M03-02
M03-03
M03-04
M03-05
M04-01
M04-02
M04-03
M04-04
M04-05
M04-06
M04-07
M04-08
M04-09
M04-10
M05-01
M05-02
M05-03
M05-04
M05-05
M06-01
M06-02
M06-03
M06-04
M06-05
M07-01
M07-02
M08-01 r
M08-02
M08-03
M08-04
M08-05
M08-06
M08-07
M08-08
M08-09M08-10
M08-11
M08-12

ThO2 UO2 PbO Age ThO2*(wt.%)(wt.%) (wt.%) (Ma)(wt.%)

5.93
5.80
5.78
5.77
5.95
6.15
5.52
5.48
5.72
5.56
5.35
5.66
5.68
5.59
5.58
5.60
5.65
5.55
5.77
5.69
5.73
5.65
2.90
3.08
2.97
3.26
3.31
7.52
7.56
7.45
7.53
7.45
6.85
6.25
6.66
7.03
6.96
6.67
7.00
6.84
6.63
6.74
6.70
6.45
6.63
6.74

0.104 0.136 511
0.141 0.140 524
0.133 0.144 542
0.140 0.138 520
0.194 0.473 1623
0.308 0.509 1602
0.567 0.480 1457
0.122 0.131 525
0.168 0.136 509
0.188 0.134 511
0.177 0.140 554
0.149 0.142 540
0.172 0.138 519
0.191 0.138 519
0.177 0.143 544
0.159 0.134 515
0.171 0.131 495
0.177 0.145 554
0.199 0.134 491
0.172 0.143 535
0.206 0.144 526
0.192 0.145 540
0.328 0.083 492
0.237 0.091 552
0.252 0.081 502
0.261 0.093 528
0.274 0.094 521
0.139 0.178 525
0.124 0.187 550
0.204 0.178 515
0.154 0.181 529
0.126 0.178 530
0.163 0.163 518
0.140 0.155 540
0.120 0.140 468
0.141 0.158 496
0.123 0.165 524
0.125 0.153 509
0.120 0.163 518
0.124 0.155 503
0.128 0.162 539
0.120 0.160 525
0.137 0.161 529
0.144 0.157 532
0.136 0.146 485
0.126 0.163 533

6.27
6.27
6.22
6.24
6.66
7.27
7.56
5.88
6.28
6.18
5.94
6.16
6.25
6.22
6.17
6.12
6.21
6.14
6.42
6.26
6.41
6.29
3.98
3.87
3.80
4.13
4.22
7.98
7.97
8.13
8.04
7.87
7.40
6.71
7.06
7.49
7.37
7.08
7.39
7.25
7.05
7.18
7.15
6.92
7.08
7.16

Spot No.

ThO2 UO2 PbO Age ThO2*(wt.%)

M08-13
M08-14M08-15
M08-16
M08-17
M08-18M08-19M08-20
M09-01
M09-02
M09-03
M09-04
M09-05
M09-06
M09-07
M09-08
M09-09
M09-10
M09-11
M09-12
M09-13
M09-14
M09-15
M10-01
M10-02
M10-03
M10-04
M10-05
M10-06
M10-07
M10-08
M10-09M10-10
M10-11
M10-12 r
M10-13
M10-14
M10-15
M10-16
M10-17
M10-18
M10-19M10-20
M10-21
M10-22
M10-23

6.74
6.88
6.69
6.67
6.63
6.81
6.63
6.77
6.41
6.45
6.55
6.53
6.46
6.44
6.39
6.27
5.60
6.56
6.42
6.61
6.48
5.85
5.64
7.25
10.5
13.0
9.00
9.05
9.07
6.45
6.50
6.55
6.67
6.69
6.58
5.93
5.50
6.22
5.93
6.58
11.6
5.36
5.71
5.77
4.94
6.34

(wt.%) (wt.%) (Ma)

0.131 0.161
0.141 0.156
0.138 0.155
0.136 0.159
0.144 0.152
0.126 0.164
0.123 0.157
0.128 0.157
0.167 0.154
0.161 0.150
0.181 0.161
0.158 0.163
0.163 0.155
0.142 0.151
0.173 0.159
0.141 0.153
0.185 0.140
0.175 0.160
0.165 0.161
0.146 0.156
0.176 0.161
0.191 0.148
0.200 0.142
0.136 0.170
0.195 0.253
0.233 0.313
0.157 0.206
0.150 0.216
0.166 0.216
0.176 0.160
0.170 0.152
0.120 0.152
0.119 0.155
0.165 0.162
0.125 0.137
0.140 0.143
0.150 0.135
0.137 0.147
0.153 0.147
0.123 0.160
0.181 0.273
0.140 0.127
0.142 0.140
0.147 0.139
0.148 0.124
0.169 0.157(wt.%)

526 7.17
498 7.35
510 7.15
523 7.12
502 7.11
534 7.23
525 7.04
513 7.20
521 6.96
503 6.98
528 7.15
542 7.06
521 7.00
513 6.91
537 6.96
532 6.74
529 6.22
527 7.14
543 6.97
518 7.10
535 7.06
537 6.48
529 6.31
518 7.70
531 11.18
534 13.76
509 9.52
531 9.55
526 9.62
534 7.03
505 7.06
515 6.95
517 7.07
526 7.24
460 7.00
526 6.40
531 5.99
519 6.67
536 6.43
537 6.99
527 12.2
510 5.83
533 6.18
523 6.26
537 5.43
533 6.90

Cambrian metamorphism of post-797 Ma sediments in Madagascar

Table 1.

95

(continued).

Spot No.

ThO2 UO2 PbO Age ThO2*(wt.%)

M10-24
M10-25
M10-26
M10-27 r
M10-28
M10-29M10-30
M11-01 0632
M11-02 0638
M11-03 0646
M11-04 0654
M11-05r0051
M11-06 1030
M11-07 1034
M11-08 1043
M11-09 1051
M11-10 1056
M11-11r1814
M11-12 1819
M11-13 1829
M11-14 1837
M11-15 1844
M11-16 1855
M11-17 1861
M11-18r1869
M11-19 2608
M11-20 2615
M11-21 2621
M11-22 2632
M11-23 2641
M11-24 2648
M11-25 2655
M11-26 2663
M11-27 2671
M11-28 3506
M11-29 3513
M11-30 3521
M11-31 3529
M11-32 3536
M11-33 3543
M11-34 3553
M11-35r4300
M11-36 4322
M11-37 4340
M11-38 4349
M11-39 4357

(wt.%) (wt.%) (Ma)Spot No.

(wt.%)

4.49 0.321 0.126 531 5.56
6.31 0.132 0.149 518 6.75
7.21 0.163 0.173 526 7.74
6.44 0.387 0.155 471 7.71
6.64 0.115 0.160 536 7.02
6.63 0.139 0.159 525 7.09
5.19 0.687 0.169 532 7.47
6.64 0.181 0.163 530 7.24
6.66 0.185 0.161 520 7.27
6.29 0.166 0.152 523 6.83
5.97 0.192 0.149 531 6.60
5.27 0.429 0.144 507 6.69
6.64 0.159 0.160 523 7.17
6.64 0.145 0.158 522 7.12
6.67 0.164 0.164 533 7.21
6.36 0.169 0.158 535 6.92
6.06 0.168 0.157 557 6.62
6.69 0.097 0.140 470 7.01
6.76 0.118 0.164 539 7.15
7.07 0.134 0.167 522 7.52
7.21 0.131 0.174 534 7.64
7.21 0.116 0.179 553 7.59
6.48 0.161 0.163 544 7.02
6.67 0.158 0.167 546 7.19
4.63 0.143 0.106 488 5.10
6.80 0.156 0.160 512 7.31
7.25 0.124 0.173 529 7.66
7.07 0.125 0.173 543 7.49
7.06 0.131 0.173 541 7.49
7.68 0.116 0.183 534 8.07
7.06 0.104 0.171 541 7.41
6.61 0.119 0.160 536 7.00
6.58 0.170 0.158 518 7.15
4.96 0.343 0.140 539 6.10
7.05 0.134 0.174 545 7.50
7.23 0.121 0.176 541 7.63
7.36 0.077 0.173 532 7.62
7.56 0.126 0.183 538 7.98
7.61 0.140 0.184 534 8.08
7.74 0.327 0.203 541 8.83
7.32 0.103 0.173 529 7.66
7.30 0.121 0.157 481 7.69
7.50 0.128 0.182 541 7.92
8.23 0.453 0.230 556 9.73
7.03 0.306 0.179 521 8.05
7.69 0.124 0.185 537 8.10

ThO2 UO2 PbO Age ThO2*(wt.%)

M11-40 4364
M11-41 5103
M11-42 5107
M11-43 5121
M11-44c5135
M11-45 5143
M11-46 5151
M11-47 5158
M11-48 5164
M11-49 6002
M11-50 6013
M11-51c6026
M11-52c6033
M11-53 6044
M11-54 6053
M11-55 6060
M11-56 6064
M11-57 6903
M11-58 6911
M11-59c6917
M11-60c6926
M11-61 6935
M11-62 6942
M11-63 6949
M11-64 6955
M11-65 6961
M11-66 7809
M11-67c7817
M11-68c7825
M11-39c7833
M11-70 7841
M11-71 7849
M11-72 7856
M11-73 8815
M11-74 8820
M11-75 8833
M11-76 8839
M11-77 8844
M11-78 8850
M11-79 9628
M11-80 9633
M11-81 9638
M11-82 9641

3.75
7.25
7.58
8.46
10.5
8.86
8.16
8.03
6.79
7.19
8.31
10.7
10.4
8.29
8.17
7.01
5.03
8.46
7.79
11.0
9.70
8.34
8.09
8.20
7.64
2.22
7.74
9.33
9.35
8.84
8.67
7.97
3.85
11.0
9.10
11.0
8.98
6.39
2.32
6.56
5.61
5.82
4.28

(wt.%) (wt.%) (Ma)(wt.%)

0.079 0.092 537 4.01
0.131 0.177 541 7.68
0.138 0.184 536 8.04
0.117 0.203 538 8.85
0.384 0.411 813 11.80.500 0.243 542 10.50.275 0.205 532 9.07
0.146 0.196 541 8.51
0.157 0.169 544 7.31
0.126 0.173 533 7.61
0.132 0.201 539 8.75
0.371 0.408 795 12.0
0.346 0.393 792 11.6
0.141 0.199 535 8.76
0.137 0.198 539 8.63
0.151 0.171 535 7.51
0.150 0.128 545 5.52
0.141 0.204 536 8.93
0.108 0.184 531 8.15
0.358 0.420 801 12.2
0.320 0.371 804 10.8
0.129 0.201 539 8.76
0.132 0.191 525 8.52
0.132 0.195 531 8.63
0.139 0.187 542 8.10
0.245 0.071 549 3.03
0.126 0.183 528 8.15
0.358 0.360 798 10.5
0.366 0.365 805 10.6
0.326 0.343 803 9.95
0.151 0.209 534 9.17
0.130 0.188 527 8.40
0.309 0.114 549 4.88
0.347 0.277 534 12.2
0.387 0.237 536 10.4
0.422 0.284 539 12.4
0.218 0.221 535 9.70
0.172 0.161 542 6.96
0.261 0.073 537 3.19
0.254 0.170 539 7.40
0.247 0.146 535 6.42
0.425 0.164 534 7.23
0.465 0.138 556 5.82

96

Table 1.

Spot No.
M01-01
M01-02
M01-03
M01-04
M01-05
M01-06
M01-07
M01-08
M01-09
M01-10
M01-11
M01-12
M01-13
M01-14M01-15
M01-16
M01-17
M02-01
M02-02
M02-03
M02-04
M02-05
M02-06
M02-07
M02-08
M02-09
M02-10
M02-11
M02-12
M12-13
M02-14
M02-15
M02-16
M02-17
M02-18M02-19
M02-20
M02-21
M02-22
M02-23
M02-24
M02-25

M. Ito et al.

(continued).

ThO2 UO2 PbO Age ThO2*(wt.%)(wt.%) (wt.%) (Ma)

10.7
10.8
10.4
10.6
11.1
13.9
12.9
11.7
10.2
10.8
12.9
16.0
8.88
13.6
11.4
13.0
10.8
11.6
11.8
12.0
7.09
10.9
10.3
9.58
9.81
8.97
7.31
8.05
7.85
6.85
9.36
8.11
9.02
8.90
11.9
12.2
12.0
11.6
8.84
11.4
10.6
11.3

1.34 0.350 544 15.1
1.31 0.342 531 15.1
1.19 0.323 529 14.3
0.890 0.308 534 13.5
1.13 0.343 542 14.9
0.787 0.378 537 16.5
0.787 0.361 545 15.5
0.769 0.327 540 14.2
0.974 0.306 537 13.4
1.23 0.339 536 14.9
0.941 0.364 532 16.2
0.655 0.417 538 18.2
0.599 0.251 542 10.9
1.06 0.396 543 17.1
0.781 0.318 535 14.0
1.34 0.394 533 17.4
1.37 0.352 539 15.3
0.938 0.332 532 14.7
0.941 0.339 533 14.9
0.932 0.338 529 15.0
1.28 0.259 536 11.3
1.05 0.324 532 14.3
1.12 0.320 536 14.0
1.50 0.331 534 14.5
1.59 0.343 535 15.1
1.36 0.305 530 13.5
1.52 0.281 535 12.4
1.46 0.290 528 12.9
0.776 0.236 531 10.4
0.560 0.202 544 8.71
1.07 0.295 537 12.9
1.42 0.291 532 12.8
1.45 0.313 531 13.8
1.44 0.310 532 13.7
0.904 0.333 527 14.8
0.758 0.334 532 14.8
0.869 0.343 541 14.9
1.02 0.339 531 15.0
1.33 0.300 533 13.2
1.03 0.337 535 14.8
1.24 0.341 545 14.7
1.06 0.335 530 14.9

Spot No.

(wt.%)

ThO2 UO2 PbO Age ThO2*(wt.%)

X01-01
X01-02
X01-03
X01-04
X01-05
X01-06
X01-07
X01-08
X01-09X01-10
X01-11
X01-12
X01-13
X02-01
X02-02
X02-03
X02-04
X02-05
X02-06(wt.%) (wt.%) (Ma)

0.411 1.49 0.123
0.477 1.64 0.136
0.482 2.06 0.166
0.397 1.41 0.112
0.285 1.17 0.093
0.131 1.25 0.097
0.142 1.19 0.093
0.148 1.13 0.088
0.239 1.26 0.103
0.439 1.82 0.148
0.390 1.46 0.117
0.373 1.71 0.136
0.364 1.37 0.113
0.216 1.40 0.115
0.203 1.27 0.102
0.153 1.58 0.118
0.128 1.49 0.112
0.278 1.66 0.132
0.342 1.32 0.107

545
542
537
521
532
538
541
532
548
540
533
534
545
556
546
522
525
541
535

Note
ThO 2 *: sum of the measured ThO 2 andThO 2 equivalent of the measured UO 2 .
UO 2 *: sum of the measured UO 2 and UO 2
equivalent of the measured ThO 2 .
Age: apparent age calculated throughequation 1 of Suzuki and Adachi(1991a,b, 1994), Suzuki et al. (1994)
and Adachi and Suzuki (1992).
r: rimAnalyzed position is shown for M02 andM11 monazite grains: for example,
M02-01 4030 means 40µm from rightand 30µm from top.

(wt.%)

1.612
1.789
2.209
1.531
1.255
1.294
1.233
1.178
1.335
1.955
1.575
1.825
1.475
1.470
1.332
1.622
1.528
1.745
1.427

Cambrian metamorphism of post-797 Ma sediments in Madagascar

Fig. 2.

97

Plot of PbO vs. ThO2* for unzoned monazite grains from biotite-sillimanite-cordierite gneiss from Ihosy. Open circle represents data points for grain margins.
Error bars in the figure represent 2σ analytical uncertainty, and error quoted inage is of .2σ

section. Data points (open circle) below the 550 Ma isochron may result fromPb loss.
Figure 3b shows the PbO vs. ThO 2 * plots of analytical data of a core-mantle
monazite grain (M11). Within this grain UO2 is heterogenoeously distributedwith respect to the shape of the cross section, showing a homogeneous concentration (0.320 - 0.384 wt.%) in the core and variable concentrations(0.077 - 0.500 wt.%) in the mantle. The ThO 2 content of the core (8.843 - 11.04
wt.%) is higher than that of the mantle (2.22 - 11.03 wt.% and mostly in therange between 6.5 and 8.5 wt.%). This compositional discontinuity suggests
overgrowth of the mantle under a different chemical environment. The
CHIME ages are 797±75 Ma for the core and 535±10 Ma for the mantle.
Kyanite- and corundum-bearing cordierite-sillimanite-muscovite-biotite-plagioclase-quartz gneiss from Maevatanana
Most monazite grains from this sample are very small in size, but two
grains have a size sufficient for microprobe analyses. Seventeen spots were
measured on M01 grain and 25 spots on M02 grain. The ThO 2 and UO 2 distributions are highly variable within individual grains (Table 1), but data points
are arrayed linearly on the PbO vs. ThO 2 * diagram (Fig. 4a). The best

98

Fig. 3.

M. Ito et al.

Plot of PbO vs. ThO2* for core-mantle monazite grains M02 (a) and M11(b) frombiotite-sillimanite-cordierite gneiss from Ihosy. Solid square represent data
points for the core and solid circle represent those for the mantle. Data points
for grain margin are shown with open circle, and those for the core-mantle
boundary with open square and cross. Explanation for errors is the same as
Fig. 2.

Cambrian metamorphism of post-797 Ma sediments in Madagascar

Fig. 4.

99

Plot of PbO vs. ThO2* for monazite (a) and that of PbO vs. UO2* for xenotime (b)from kyanite- and corundum-bearing cordierite-sillimanite-muscovite-biotite
gneiss from Maevatanana. Explanation for errors is the same as Fig. 2.

100

M. Ito et al.

fit regression line yields an age of 534±10 Ma (MSWD=0.09) and an intercept
value of 0.0010±0.0062. Figure 4b shows the PbO vs. UO 2 plots of 19 analytical
data for two xenotime grains. All data points are arrayed linearly, and define
an isochron of 530±32 Ma (MSWD=0.11) with an intercept value of0.0019±0.0072. Despite the detailed microprobe analysis, no age signatureolder than ca. 530 Ma can be obtained on both the monazite and xenotime
grains.
DISCUSSION AND CONCLUSIONThe CHIME dating disclosed that the Biotite-sillimanite-cordierite gneissfrom Ihosy contains chronologically zoned monazite grains as well as unzoned
ones. The core ages are 1640±180 and 795±75 Ma, although the mantles andunzoned grains give unequivocal ages of 527-550 Ma. The zoning pattern
coupled with the considerable difference in core age suggests that the older
core are of detrital origin, retaining pre-metamorphic information. Inheritanceof monazite grains in the upper most amphibolite to granulite facies paragneisses is not uncommon (Parrish, 1990; Suzuki and Adachi, 1994; Suzuki etal., 1994). Thus, we conclude that the biotite-sillimanite-cordierite gneiss at
Ihosy formed through the ca. 530 (527-550) Ma single metamorphic eventfrom a post-795±75 Ma sediment. This reinforces the previous age assignmentof late Proterozoic (< ca. 720 Ma) sedimentation and ca. 550 Ma metamorph..
ism for the gneisses exposed at Ihosy (Kroner et al., 1996).
The sample of kyanite- and corundum-bearing cordierite-sillimanite-muscovite-biotite gneiss was collected from the metamorphic sequence peripheral to
the Maevatanana greenstone belt of possible Archean age. The nature of theboundary between the peripheral rocks and those of the greenstone proper isstill unclear because of poor exposure. Thus, some may regard the metamorphic sequence as the basement unit, and others may consider that the metamorphic sequence overlain unconformably the greenstone belt. This rockgives CHIME monazite age of 534±10 Ma and xenotime age of 530±32 Ma.
The well-defined isochrons with no indication of older and younger ages (Fig4a,b) suggest a single-stage crystallization of monazite and xenotime during
the kyanite-sillimanite type metamorphism at ca. 530 Ma ago. We, therefore, consider that the paragneisses at Maevatanana represent the northern extensionof the ca. 530 Ma metamorphic rocks at Ihosy in the southern sector. Presumably the central NS trending ductile shear zone of Pan-African age is muchwider than has been thought (Windley et al., 1994).
The high-grade metamorphism at ca. 530 Ma in Madagascar appears to be
essentially synchronous with the granulite facies metamorphism in the southof the so-called Achankovil shear zone in southern India (539±20 Ma, Santosh
et al., 1992; 558±11 Ma, Choudhary et al., 1992; 527±10 Ma, Bindu et al., 1996).
Bindu et al. (1996) also reported a 640±30 Ma CHIME age for detrital zircon
from the cordierite charnockite sample that gives 527±10 Ma CHIME monazite
a ge . T hi s c l e a rl y s how s t ha t gne i s s e s i n s out he rnm os t Indi a a r e not t heCambrian metamorphism of post-797 Ma sediments in Madagascar

101

polymetamorphosed Proterozoic complex; they formed newly in the Cambrianfrom post-640 Ma protolith. Similar Cambrian high-grade paragneiss derived
from post-620-760 Ma sediments occur in the Yamato Mountains and the Sor
Rondane Mountains in East Antarctica (Shiraishi et al., 1994; Asami et al.,1997).
It has been known that metamorphic rocks in Sri Lanka bear many similarities with those in southernmost India and East Antarctica (Yoshida et al.,
1992; Shiraishi et al., 1994). We, therefore, consider that the Cambrian (ca.
530 Ma) metamorphic rocks in Madagascar, southernmost India, Sri Lanka andEast Antarctica constituted a single metamorphic terrain that can linked
directly to the collision of East and West Gondwana.
ACKNOWLEDGEMENTS
We acknowledge Prof. M. Adachi of Nagoya University for his enlightening
comments. One of us, MI, would like to express his deep gratitude to Dr. R.A.
Rambeloson of the University of Antananarivo, Dr. L.D. Ashwal of Rand
Afrikaans University, Dr. S. Muhongo of the University of Dar-es-Salaam andDr. M. Yoshida of Osaka City University for very kind coodination of UNESCOIUGS-IGCP-348/368 International Joint Field Workshop held in Madagascar in
1997. Journal reviews by Prof. Y. Nakai and an anonymous referee provided aconstructive framework which vastly improved the manuscript.
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