and LARDEAUX
1 : Laboratoire Magmas et Volcans, Université Blaise Pascal - UMR 6524. 5, rue Kessler, 63 038 Clermont-Ferrand FRANCE 2 : Laboratoire Dynamique de la Lithosphère, Université Claude Bernard - UMR 5570. 27, boulevard du 11 Novembre, 69 622 Villeurbanne FRANCE
The Andriamena complex is part of four North-South Archean mafic gneiss belt interpreted to form part of the same lithological unit. It corresponds to a synformal belt, structulary overlying the granitic and migmatitic basement. The lithologies consist of amphibolite gneiss, migmatite, metasedimentary rocks intruded by mafic-ultramafic bodies at 787 ± 16 Ma (Guerrot et al., 1993). The structural pattern (figure 2) results of the superposition of two distinctive phases of deformation. - D1 deformation can be observed outside the high strain zone D2. Structures related to this event (figure 3) are compatible with vertical shortening in a coaxial strain. - D2 event is characterized by the refolding of the S1 foliation into kilometric to centimetric folds (F2) with sub-horizontal axes. These folds with North-South axial plane are coherent with an horizontal East-West shortening. The shortening is associated with a strain partitioning between high strain zones (figure 2), characterized by uprigh F2 folds, and open folds areas. F2 folding affects also the stratoid granites dated at 630 Ma (Paquette et Nédélec, 1998) (figure 2). This tectonic evolution is the same as the one proposed by Martelat et al. (2000) for the southern Madagascar during the late Neoproterozoic.
The basement is generally divided in two parts. South of the BRSZ consists of Proterozoic rocks strongly reworked during Pan-African times (600-530 Ma). In contrast the North, consists mainly of late Archean rocks (granitoids, migmatitic gneiss...) strongly reworked during a widespread igneous and metamorphic activity of middle Neoproterozoic age (~800-770 Ma) and late Neoproterozoic (~580-520 Ma). Our study area is located in this North part and more precisely in the Andriamena mafic gneiss complex (figure1).
intrusive mafic-ultramfic rocks ~790 Ma
Bongolova-Ranotsara SZ (BRSZ)
Antongil
Late Archean mafic gneiss and metasedimentary rocks
X
Z
N
D1
Y
44
48
40
D2
37 61
folded L1 or L2 lineations
59
S1 foliation affected by a chocolate-block boudinage L1
37
Ta
21
lines (Brieville-def)
22
F1 fold axis parallel to the L1 lineation
13 12 47
6 27 35 52
39
39
69
37
71 32
33
Late Neoproterozoic high grade gneiss, granitoides
49
32
71
L1 stretching lineation
52
11 Data. Contoured at 1 3 5 x uniform
58
54 29
lines (Ambodiketsa) lines (Belavabory + ...)
47 26 46 20
44
27
50 cm
53
48 Data. Contoured at 1 3 5 x uniform
L1 lineations
35 33
39
D1
29 16 36
30 13
41 Data. Contoured at 1 3 5 7 x uniform
31 Data. Contoured at 1 3 5 7 9 x uniform
7
Nédelec et al. (1994)
35
Ampanihy SZ
WSW
ENE
F2
Angavo SZ S1
D2
L1
D2
L1
EAST
? F2
F2
dome-and-basin structures (gravity instabilities)
- monazites 1050°C , ~11 kbar), associated with a late Archean granitoid magmatism. The Southern part of India (Nilgiri, Palni Hill Ranges) is composed of granulite terranes subjected to UHT metamorphism at 2.5 Ga. It suggests that the already proposed connection between South India and North-Central Madagascar is a strong probability.
References cited
ky sil
qz sil x op rt crd g
???
9000
5000
Petrographical investigation of both samples clearly shows two different PT evolutions. The UHT and near isothermal decompression characteristic of the Al-Mg granulite are not recognized in the migmatite, whereas the isobaric cooling at about 7 kbar is recognized in the both. Nevertheless, without geochronological constraints, it is very difficult to interpret these PT paths.
A POLYMETAMORPHIC HISTORY FOR THE ANDRIAMENA COMPLEX: IMPLICATIONS FOR THE PRE-GONDWANA EVOLUTION.
Archean basement was structurally reworked during the late Neoproterozoic. The finite geometry reflects an E-W shortening related with the cratonic convergence between East and West Gondwana and contemporaneous with a granulitic metamorphism widely recognized in the South of Madagascar (Martelat et al, 1997).
1000
11000
+ sil = spr2 + crd2
spr2
0
770 Ma
In a second time, at 770 Ma, a thermal event generate partial melting, destabilisation of the crd2 into opx3-sil3-qz symplectite, and an isotopic resetting associated with a new monazite growth episode.
2) Development of decompression textures at 770 Ma.
figure 6 : Partial grid for univariant KFMASH reactions for fluid-absent metapelites, and the deduced PT path from the metapelitic migmatite.
crd2
A sequence of symplectite assemblages developed at the expense of grt, opx and sil indicate a near-ITD of the order of 3-4 kbar, at about 900-1000°C : [spl]>T>[qz]
600
bio sil
opx0
400
900
CONCLUSION
grt 4
7000
The 770 Ma metamorphic event (partial melting at 850°C, ~7 kbar) could be the consequence of a thermal perturbation caused by the emplacement of basic intrusions at this time. Handke et al. (1999) proposed a continental arc setting for the Neoprotrerozoic magmatism, in relation with the subduction of the Mozambique ocean under the North-central part of Madagascar (breakup of Rodinia).
T(°C)
T(°C)
sil1 crd2
ste k an y d
qz
1000
spr2
grt0
This very fine symplectite is visible on the photo 1-2-3.
The cooling part of this PT evolution is characterized by the late development of sil (±grt) coronas around spl and bio-sil patches interpreted as the result of back melting reactions between the spl and the silica-undersatured melt.
[bt]
[qz] il t s qz L r g d r lc sp
UHT
s
800
sil
P(bars)
gr t b t spl o sil px L
L
opx
crd
il qz bt s
sill opx spl L rd gr t c
the rm
opx1
1
770 Ma
Isothermal decompression textures define a real PT path at 2.5 Ga. After UHT-ITD stages, granulites cooled to normal thermal conditions (near the steady state geotherm) at 2.5 Ga. The conservation of the UHT assemblages is related to the refractory behaviour of Al-Mg granulites.
bt
t tb L gr l qz p s
qz
3000
2,5 Ga
???
eg eo
grt-crd
Initial prograde melting was achieved by biotite dehydrationmelting reactions at temperature below 850 °C.
l c il q rd z L
sil
- near Isothermal Decompression (ITD)
5000
L
rd
bt
6
4
py6 hcrd 5 q 87 e n77
late development of opx3-sil3-qz at the expense of crd2 suggesting a come back into the opx-sil-qz stability field probably through an IBC at ~7 kbar.
= opx3 + sil3 + qz
sil
+ qz = opx1 + sil1 [spl]
xs gr il q tc zL rd
770 Ma
sil d an
2
op
bt sil l L x p op d s cr
[opx]
backscattered electron image
crd 5
[spl]
sp
- Isobaric Cooling (IBC)
crd2
MA
bio + sil (+pl) = grt + spl
qz
opx 2
sil3 + qz
spl
gr t c
0
cooling above the P
figure 5 : petrographical PT path deduced from the Al-Mg granulites in a FMAS system (black lines = univariante reactions and dashed lines = isopleths for divariante reactions)
sil0
FA
g sp rt b lc ts rd il L
en s spr il qz
py en9 80 0 s hc il rd 95
1 spr
sil1
T
opx3 + sil3
PT path deduced from the metapelitic migmatites records a heating-cooling path at about 7 kbar without any significant change in pressure (figure 6).
7000
tat
r)
py 65 sil q hcrd87
10
8
spr0
z il q x s rd p o rt c g
Real PT path
ad ys
(sp
opx1
11000
ste
r) (sp
sil 90 95 en hcrd r sp
4
+ sil +FK +L
bt
Earliest assemblage (spr0-grt0-qz) implies peak PT conditions of ~11kbar, >1050°C
py80 n90 95 e c h r rd
UHT
What is the signification and the age of the petrographical ITD recorded by the Al-Mg granulites ?
k an y d
q sill 77 87 d hcr
2800
- monazites included in garnet yield the oldest age with systematically a maximum at 2.5 Ga (figure 7), We consider this late Archean age, as proposed by Nicollet et al. (1997), to reflect the timing of the UHT metamorphism. The conservation of this old event is related to the shielding effect of garnet for the U-Th-Pb system (Montel et al., 1996).
P(bars)
en
[qz]
5 py en8 65 1 s hc il rd 87
P
[spl-qz]
bt grt qz opx sil L
bio + sil + qz (+pl) = grt + L
- UHT metamorphism
[spl]
l 0 si py8 rd95 c h spr
3
q sil en crd h
2300
sp
2
[spl-spr]
1800
monazite reset and new growth
2,5 Ga
sil d
p sil s y80 pr e n90
1300
z il q s x op rt crd g
an
g r c qz
800
P (kbar)
grt
sil en hcrd r sp
q 65 1 py en8 sil
pr ill s l xs op rd sp c ill x s pl op grt s d cr
9000
9000
1
opx sill q grt spr pr t s px gr pl o ls sil
[qz] 7-8 kbar ~ 900°C t sill spl r d
11000
grt inclusion
large monazite (matrix)
Ma
Near, the Al-Mg granulites outcrop, occur Metapelitic Migmatites in wich quartzo-feldspathic layers alternate with restitic layers characterized by various assemblages (grt bearing and qz-absent grt-spl bearing metapeltites).
crd
[spl,py]
Y
grt inclusion
300
We suggest that the Pan-African geometry and strain pattern reflect the interference between E-W regional horizontal shortening (boundary forces) and diapiric structuctures (body forces) (figure 4).
12
opx-sil-qz
grt inclusions with cracks
APPARENT PETROGRAPHICAL PATH VS REAL PT PATH
S
pr grt s spl px crd o
Dy
figure 8 : Chondrite-normalised REE distribution of monazite at 770 Ma located in two clearly different textural position : in the matrix and associated with the late opx3-sil3-qz.
1) Decompression occured during the UHT event at 2.5 Ga.
? ? ?
Tb
CONCLUSION
qz
spr grt sill d spl cr
Gd
CONCLUSION
UHT metamorphism (>900°C, 7-13kbar) have been recognized in several terranes of the futur East Gondwana (India, Sri Lanka, Antartica). In Madagascar, it have been firstly identified by Nicollet et al. (1991). High Mg-Al granulites preserve numerous complex coronitic and symplectite textures providing plenty information to reconstruct an almost continuous petrographical PT path, near the peak temperature. PT evolution can be deduced from a FMAS petrogenetic grid (figure 5). Sapphirine-bearing granulites occur in two localities (figure 2) and compose an infinitesimal volume with respect to the Andriamena complex. Due to the tropical weathering, they form several boulders, wich certainly come from a very near locality.
l sil d x cr op spr t gr
Eu
2800
0,00
UHT METAMORPHISM
qz ill x s rd op rt c g
Sm
The Eastern part is characterized by dome-and-basin structures. It is probably the result of gravity instabilities between the dense mafic complex (forming the basin) and the overlyied low-density granitoid crust (domes) (figure 2). In the western part, the extensional mylonitic detachment between the two lithological units is not interpreted as a consequence of regional extension but as a decollement linked with the relative downward and upward moving of the two units.
?
mylonitic decollement
grt-spr-qz
Nd
- UHT metamorphism preserved in Al-Mg granulites at 2.5 Ga
high strain D2 deformation
WEST
spr q grt sill qg rt crd ) opx spr px c rd (o
Pr
- Partial melting and the IBC at ~7 kbar at 770 Ma
Stratoid granites 630 Ma
10,8 ±1 kbar [spl] 1040°C
Ce
F2 S1
F2
mylonitic layer
F2
figure 3 : Example of D1 structures observed in preserved area of the D2 deformation
10 km
Beraketa SZ
figure 4 : 3D schematic diagram showing the interference between boundary forces (horizontal regional shortening D2) and body forces (diapiric tectonic).
La
- matrix grains, 20-70 mm in size, with irregular morphologies, yield ages from about 1.8 Ga to 710 Ma with a main age population at 770 Ma (figure 7). We suggest that these monazites grew during the 2.5 Ga UHT event and they were subsquently totally (or no) reset at 770 Ma.
58
48 36 9
Late Neoproterozoic lithospheric shear zones
2300
33
27
Ifanadiana SZ
1800 Ma
50
14
22
Phanerozoic cover
1300
29
10
lines (WA-L1)
Angavo SZ
800
27
24 66
37
SQC
0,00 300
Probably old monazite (2.5 Ga ?) entirely reset at 770 Ma
100
Three populations were identified in thin-sections :
28 Data. Contoured at 1 2 3 x uniform
28
D2
Oldest age recorded in all the UHT granulites
figure 7 : Weighted-histogram representation of all the electron-microprobe monazite ages derived from one Al-Mg granulite. Inset: data from monazites include in garnets from another granulite.
51
Reworked Archean and Proterozoic gneiss, migmatites and granitoides
matrix
5000
id gra nites
Andriamena (study area)
opx III-sill III-qz
Lines (Andriamena)
30
Middle Archean granitoids and migmatites
spr-bearing granulites localities
strato
figure 1 : Simplified geological and structural map of the Precambrian of Madagascar (Martelat, 1998)
* matrix monazites * monazites associated with opx III - sil III - qz symplectites
0,04
New monazite generation at ~ 770 Ma - REE pattern quite different - characteristic textural position (figure 9) - size < 20 m m
10000
1000
2502 +/- 40 Ma
0,02
0,06
100000
0,04
0,08
0,02
figure 2 : Structural map of the Andriamena area derived from the study of satellite images (SPOT), 1/100 000 geological maps (Besairie, 1969) and our field investigations.
high strain zone D2
771 +/- 18 Ma n=16
1000000
ky sil
Madagascar forms a part of the Mozambique belt, resulting of the continental collision between East and West Gondwana. Structures related to this event, like vertical lithospheric shear zones, are in agreement with an East-West horizontal shortening (Martelat et al., 2000) (figure 1).
density of probability
REGIONAL FRAMEWORK
grt inclusions n=18
0,06
0,10
Al-Mg granulites - 770 Ma
l qz
NICOLLET
Jean-Marc2
figure 9 : backscattered electron image showing the textural relationships between this monazite population and the opx3-sil3-qz symplectite. The growth of the monazite is contemporaneous with the crystallisation of the symplectite.
bio si
GONCALVES
Christian1
U-Th-Pb electron microprobe dating have been use to constrain the metamorphic evolution from the AlMg granulites and migmatites. This in-situ technique have the advantage to combine textural observations and chemical composition to distinguish several episodes of monazite growth or reset during thermal events. This method is useful in polymetamorphic cases, like the North-Central Madagascar, where at least 3 magmatic and/or metamorphic events have been recognized (Guerrot et al., 1993; Nicollet et al., 1997; Tucker et al., 1999 and Kröner et al., 2000).
P(bars)
Philippe1,
ELECTRON MICROPROBE DATING OF MONAZITE
Monazite/Chondrite
IN-SITU ELECTRON MICROPROBE MONAZITE DATING OF THE COMPLEX RETROGRADE EVOLUTION OF UHT GRANULITES FROM ANDRIAMENA (MADAGASCAR) : APPARENT PETROGRAPHICAL PATH VS REAL PTt PATH.
600
T(°C)
800
1000
The UHT metamorphism and the cooling (near isobaric?) to the steady state geotherm were achieved in a single event at 2.5 Ga. The thermal perturbation at 770 Ma brought back the sample to high temperature (~850°C, 7kbar). The primary UHT assemblages were reequilibrated in this new conditions by a fictive PT path (isothermal decompression of about 3-4 kbar) joining the 2.5 Ga "high pressure" stability field and the lower pressure stability field associated with the 770 Ma event. In the same time, partial melting, isotopic resetting and new monazite growth occured.
Guerrot C., Cocherie A. and Ohnenstetter N., 1993, Origin and evolution of the West Andriamena Pan-African maficultramafic complex in Madagascar as shown by U-PB, Nd isotopes and trace element constraints. Terra Abstr. 5, p.387. Handke M. J., Tucker R. D. and Ashwal L. D., 1999, Neoproterozoic continental arc magmatism in west-central Madagascar: Geology, v.27, no.4, p.351-354. Kröner A., Hegner E., Collins A. S., Windley B. F., Brewer T. S., Razakamanana T., Pidgeon R. T., 2000, Age and magmatic history of the Antananarivo block, Central Madagascar, as derived from zircon geochronology and Nd isotopic systematics: American Journal of Science, v.300, p. 251-288. Martelat J. E., Evolution thermomécanique de la croûte inférieure du Sud de Madagascar: Ph.D. Thesis (unpublished), Université B.P. Clermont-Ferrand, 230 pp. Martelat J. E., Nicollet C., Lardeaux J. M., Vidal G. and Rakotondrazafy R., 1997, Lithospheric tectonic structures developed under high-grade metamorphism in the southern part of Madagascar: Geodyninamica Acta, v.10, no.3, p.94114. Martelat J. E., Lardeaux J. M., Nicollet C. and Rakotondrazafy R., 2000, Strain pattern and late Precambrian deformation history in southern Madagascar: Precambrian Research, v.102, p.1-20. Montel J. M., Foret S., Veschambre M., Nicollet C., Provost A., 1996, Electron microprobe dating of monazite: Chemical Geology, v. 131, p.37-53. Nicollet C., Rambeloson R. and Vielzeuf D., 1991, Retrograde evolution of the Very-High-Temperature granulites from andriamena, Madagascar: Terra Abstr. 3, p.441. Nicollet C., Montel JM, Foret S., Martelat JE, Lardeaux JM, Rakotondrazafy R., 1997, E-probe monazite dating : an excellent tool for dating uplift and polymetamorphic events ; examples of the granulites from Madagascar: IGCP 348/368, Intern. Symposium on Geol. and mine. Resources of Madagascar. Paquette J. L. and Nédélec A., 1998, A new insight into Pan-African tectonics in the East-West Gondwana collision zone by U-Pb zircon dating og granites from central Madagascar: Earth and Planetary Science Letters, v.155, p.45-56. Tucker R. D., Ashwal L. D., Handke M. J., Hamilton M. A., Le Grange M. and Rambeloson R., 1999, U-Pb geochronology and isotope geochemistry of the Archean and proterozoic rocks of North-Central Madagascar: Journal of Geology, v.107, p.125-153.