Precambrian Research 102 2000 47 – 68
Metamorphic evolution of Pan-African granulite facies metapelites from Southern Madagascar
Gregor Markl
a,
, Ju¨rgen Ba¨uerle
b
, Djordje Grujic
b
a
Institut fu¨r Mineralogie, Petrologie und Geochemie, Uni6ersita¨t Tu¨bingen, Wilhelmstrasse
56
, D-
72074
Tu¨bingen, Germany
b
Geologisches Institut, Albert-Ludwigs-Uni6ersita¨t, Albertstrasse
23
b, D-
79104
Freiburg, Germany Received 29 March 1999; accepted 25 November 1999
Abstract
Granulite-facies, metapelitic gneisses from five localities in Southern Madagascar exhibit very similar mineral assemblages and textures and reveal a very similar P-T evolution. They show an early assemblage of Grt + Sil +
Qtz + Bio + Kfsp + Rt + Ilm 9 Pl 9 Spl which records P-T conditions of about 880 9 60°C at 8 9 1 kbar. In some samples, this assemblage is overprinted by the later assemblage Crd + Grt + Sil + Bio + Kfsp + Qtz + Mt 9 Crn
which equilibrated at about 690 9 40°C and 4 9 1 kbar. The main Crd-producing reaction Spl + Qtz = Crd was aided by the further reactions Sil + Bio + Qtz = Crd + Kfsp + H
2
O and Grt + Sil + Qtz = Crd. Crn formed from relictic Spl, probably during cordierite formation, by the oxidation reaction 3Hc + 0.5O
2
= 3Crn + Mt. H
2
O activities calculated from phase equilibria for both metamorphic events range from 0.6 to 1 which is in agreement with the
observation of partial melting during the HT stage. We interpret age data from the literature to suggest that the HTHP event occurred at about 560 – 580 Ma, while the MTMP event could be related to intrusive activity between
520 and 540 Ma, possibly during the same orogenic cycle. The similarities in textures and equilibration conditions among the Malagasy samples strongly imply — in contrast to the opinion of many previous workers — a common
tectonometamorphic history of the various parts of Southern Madagascar between 600 and 500 Ma. The P-T-condi- tions recorded by the samples and the age data correspond closely to P-T-paths deduced for other Gondwana
fragments like Southern India, Sri Lanka and parts of East Antarctica. This correspondence may help to constrain the location of the suture for the collision between East and West Gondwana in the late Proterozoic. © 2000 Elsevier
Science B.V. All rights reserved.
Keywords
:
Granulite; Thermobarometry; Pan-African; Madagascar; Gondwana www.elsevier.comlocateprecamres
1. Introduction
The Panafrican tectonometamorphic event oc- curred between 950 and 450 Ma Kro¨ner, 1984
and traces of it are recorded on almost every continent, mostly in form of large scale deforma-
Corresponding author. Tel.: + 49-7071-2972930; fax: + 49-7071-293060.
E-mail address
:
markluni-tuebingen.de G. Markl 0301-926800 - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 1 - 9 2 6 8 9 9 0 0 0 9 9 - 6
tion, intrusive activity and metamorphic over- prints. A key to the understanding of this
Panafrican epoch is the fate of supercontinents- first the break-up of Rodinia, and later the assem-
bly
of Gondwana.
The supercontinent
of Gondwana was assembled around 500 – 550 Ma
and it involved — as far as we know today — Africa, South America, East Antarctica, India,
Australia and most parts of Europe in addition to Sri Lanka and Madagascar Stern, 1994; and
references therein. The Panafrican tectonic, mag- matic andor metamorphic imprint has recently
been investigated intensively in most of these ar- eas in order to reconstruct the detailed history of
Gondwana assembly, break-up and, first of all, the internal structure. Special emphasis has been
placed on South-west Africa e.g. Windley, 1995; Jacobs et al., 1997, Southern India e.g. Santosh,
1986; Chacko et al., 1987; Braun et al., 1998, Sri Lanka e.g. Schumacher et al., 1990; Faulhaber
and Raith, 1994; Hiroi et al., 1994, East Antarc- tica Shiraishi et al., 1994; Jacobs et al., 1998;
Markl and Piazolo, 1998; Piazolo and Markl, 1999 and Madagascar Nicollet, 1985, 1990; Win-
dley et al., 1994; Kro¨ner et al., 1996; Martelat et al., 1997; Nicollet et al., 1997; Paquette and
Ne´de´lec, 1998; de Wit et al., in press. While each area investigated has some specialities, most of
them share some common features including in- trusive activity of similar tectonic affinity and
similar ages, similar metamorphic histories with similar metamorphic ages and even a similar in-
ventory of rock types. Naturally, most of these areas record most intensely or even exclusively the
late stages of Gondwana evolution, i.e. the late stages of the Panafrican event, depending on the
intensity of the overprint. This is also true for the here investigated southern part of Madagascar.
With respect to its metamorphic evolution, the southern part of Madagascar has been the subject
of some controversy within the last few years, because earlier work seemed to indicate greatly
varying P-T-conditions in Southern Madagascar e.g. Moine et al., 1985; Nicollet, 1985; Acker-
mand et al., 1989; Nicollet, 1990; Kro¨ner et al., 1996; Martelat et al., 1997. These results were
interpreted to show an W-E decrease in pressure Nicollet, 1990, to reflect ‘elevator-tectonics’
along shear zones with vertical displacements of up to 10 km Martelat et al., 1997 or to reflect
various metamorphic events. As this confusion about the tectonometamorphic history of the
southern part of Madagascar prohibits a detailed comparison within the context of Gondwana evo-
lution and break-up, the present contribution is aimed at clarifying the tectonometamorphic evo-
lution of Southern Madagascar during the late stages of the Panafrican event. We present new
petrological data derived from metapelitic rocks which were collected in an area about 80 000 km
2
large Fig. 1. These new samples, investigated with identical methods to enable a detailed com-
parison among them, place new constraints on the Panafrican effects in this area.
2. Geological setting and sample localities
