in-situ electron microprobe monazite dating of the ... - Christian Nicollet

3. -qz at the expense of crd. 2 suggesting a come back into the opx-sil-qz stability ..... Martelat J. E., Nicollet C., Lardeaux J. M., Vidal G. and Rakotondrazafy R., ...
720KB taille 5 téléchargements 303 vues
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.