The Precambrian of southern Madagascar consists largely of medium- and high-grade paragneisses and metasediments that have a predominantly north-south ...
Geological Society, London, Special Publications The Precambrian mobile belt of southern Madagascar D. Ackermand, B. F. Windley and A. Razafiniparany Geological Society, London, Special Publications 1989; v. 43; p. 293-296 doi:10.1144/GSL.SP.1989.043.01.20
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Brian Frederick Windley on 26 January 2008
© 1989 Geological Society of London
The Precambrian mobile belt of southern Madagascar D. Ackermand, B. F. Windley & A. Razafiniparany The Precambrian of southern Madagascar consists largely of medium- and high-grade paragneisses and metasediments that have a predominantly north-south regional trend, which is parallel to that of the Mozambique belt of eastern Africa. The Precambrian geology of Madagascar is synthesized by Besairie (1971) and Hottin (1976). Much mineralogical information of relevance to metamorphic petrology is given by Lacroix (1922-23). Nicollet (1985, 1986) gives good metamorphic and P - T data from Ihosy and Vohibory (Fig. 1). Figure 1 shows the distribution of the main metamorphic belts in the south of the island. Included in the amphibolite-facies belts are low-, medium- and high-grade types, and the granulite-facies belt includes pyroxene- and hornblende-facies types as well as some highgrade amphibolite facies (Vachette 1979). Most abundant are gneisses with various combinations of cordierite, sillimanite, garnet and biotite. The widespread development of cordierite reaches a peak in cordieritites which form layers up to several tens of metres thick in the gneisses. In the gneisses, sillimanite is locally concentrated in sillimanitite layers up to several metres thick, and marble and quartzite layers reach a few metres in thickness. Diopsidites occur in the gneisses as conformable layers up to several metres thick and have pegmatitic pockets with phlogopite, diopside, scapolite and anhydrite; some phlogopite crystals are over 1 metre long and phlogopite is mined locally (Lacroix 1941). Pegmatites occur in most rocks. Foliation and layering are predominantly parallel and steep to vertical. Isoclinal folds have axes which plunge shallowly north-south, but in general there is a notable paucity of minor and macroscopic folds. There are some late minor open folds, shear zones and mylonites. On a map scale there is major intercalation of units of gneisses of different types. Geochronological data on the Precambrian of Madagascar are sparse, but the dates of Vachette (1979) and Vachette & Hottin (1988), usefully summarized and evaluated by Cahen & Snelling (1984), provide a preliminary idea of the range of ages present and of the complexity of the isotopic systematics. Broadly, there is evidence in Madagascar of metamorphic activity in the Archaean, the early, mid- and
late (Pan-African) Proterozoic. In the region we have studied south of the Ranotsara shear belt (Fig. 1), R b - S r data indicate ages over 3000 Ma, and metamorphic events at 2600 and 2150 Ma, which were responsible for partial rehomogenization of the pre-3000 Ma region. At Vohibory and Anosyan (Fig. 1) there are Pan-African ages on gneisses and granitic rocks between 811 and 643 Ma. Few P - T data are available from the Precambrian rocks of Madagascar. Because of a detailed experimental data base (Seifert 1974), natural sapphirine-bearing assemblages are especially useful for documenting P - T trajectories, e.g. south India (Lal et al. 1984, Mohan et al. 1986) and the Limpopo belt (Windley et al. 1984). In southern Madagascar, sapphirine is recorded in several localities; we report new data from one major occurrence at Vohidava (Fig. 1). The sapphirine occurs with orthopyroxene, cordierite, phlogopite, K-feldspar, rare sillimanite, and spinel in a layer about 2 m across, which is conformable with layers of diopside gneiss, cordieritite, pyroxenite, marble and anorthosite.
P--T conditions and their comparison with belts in southern Africa and India In the sapphirine-bearing rocks at Vohidava, idiomorphic blades of colourless sapphirine (Mga.6FeE+0.1A18.rSil.7020) border large grains of orthopyroxene (Mgl.s2tcorc)-l.81~rim) Fe2+0.07-0.08 A10.24_o.20Si1.87_1.9006) , and cordierite (Mgl.97FeZ+0.03A14.0Sis.oO18 with a total of 99.4 wt %) forms thin, discontinuous seams between them. All stoichometric formulae are averages of more than 20 point analyses of each phase. There are remarkably few corona structures in these rocks. Rare sillimanite and spinel (Mg0.95FeZ+0.07All.9904) granules occur within the orthopyroxene and cordierite. A notable feature of these phases is their very high XMg, and this allows us to interpret their history in terms of the MASH system. The reaction boundary between enstatite + sillimanite and sapphirine + cordierite extends from invariant point I12 (Fig. 2) at 9.5 kbar and 950°C (Schreyer
DALY,J. S., CLIFF,R. A. & YARDLEY,B. W. D. (eds) 1989, Evolution of Metamorphic Belts, Geological Society Special Publication No. 43, pp. 293-296. From
293
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& Seifert 1969). From field relations we have recorded early orthopyroxene with sillimanite, and microscopically we have found new sapphirine and cordierite from the breakdown of enstatite and sillimanite. Small grains of spinel in contact with cordierite enable us to define the change from enstatite + sapphirine to cordierite + spinel on the high temperature side of the invariant point I18 given by Seifert (1974). In the MASH system the change from invariant points 112 to 118 requires a steep P - T trajectory. These relations are supported by the following: (1) from the alumina content in orthopyroxene in contact with sapphirine we calculate a temperature of 980-950°C from core to rim, after Anastasiou & Seifert (1972). In contrast, we find no change in composition from core to rim in any other phases of the sapphirine-bearing rocks. (2) In nearby cordieritites there is a cordierite (Mgl.58Fe2+0.g9A13.95Sis.oOls with a total of 98.5 wt % ) - s p i n e l (Mgo.31Fe2+0.75Alt.93)quartz association which is a potential geobarometer (Seifert & Schumacher, 1986) and which indicates P of 6.0 kbar. Using the cordierite-garnet (Cao.09Mgl.09Fe2+ 1.81 MnoAoAll.99Si3.0012) geothermobarometer of
Thompson (1976) we calculate P of 6 kbar and T of c. 800°C. (3) In associated pyroxenites, adjacent clinopyroxene (Ca0.98Mg0.72Fe2+0.24 and orthopyroxene (Mgl.26 Fe2+o.72Alo.05Sil.9706) give T of 800°C (e.g. Powell 1978, Ellis 1980). Also in pyroxenites, P of c. 5.0 kbar was calculated by the orthopyroxene-garnet geobarometer (Harley 1984). At Ihosy (Fig. 1), which is situated along strike from Vohidava, garnet-plagioclase, garnet-cordierite and garnet-biotite geothermo- and geobarometers give T of more than 700°C and of 5.0--5.5 kbar with a PH2o of 0.3--0.4 (Nicollet 1985). Our data from the cordierite-spinel-quartz geobarometer from cordieritites at Ihosy indicate a similar pressure of 5.0-5.5 kbar. Along the central belt of southern Madagascar, due south of Ihosy (Fig. 1), there are similar mineral assemblages, such as cordierite + garnet in gneisses and cordierite + spinel + quartz in cordieritites, as well as similar calculated P - T conditions close to 5.0-6.0 kbar and 750-950°C. G a r n e t cordierite gneisses with spinel and quartz, at and near Fort Dauphin (Fig. 1), yield pressures of 4.5-5.5 kbar according to the method of
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Temp °C FIG. 2. A P - T-diagram for rocks from Vohidava showing relevant data for estimation of the P - T trajectory. Invariant points 112 (Schreyer & Seifert 1969) and 118(Seifert 1974) are in the MASH system for sapphirinebeating rocks. T is calculated from AI20 3 in orthopyroxene (Anastasiou and Seifert, 1972) associated with sapphirine (Sap on this figure). Further P - T points are calculated from rock types associated with sapphirinebearing rocks: cordieritites with the geobarometer Crd-Spl-Qtz, and the geothermometer Crd-Grt (dashed lines); and pyroxenites with the geobarometer Opx-Grt and the geothermometer Opx-Cpx (dot-dash box).
Seifert & Schumacher (1986). These pressures are significantly lower than those in the central belt referred to above, using the same geobarometric method. These results may indicate that the gneisses at Fort Dauphin were higher in the crustal thrust pile than those further west. The uplift path in southern Madagascar (Fig. 2) is not as steep as that of the Limpopo belt (Windley et al. 1984) and south India (Lal et al. 1984, Mohan et al. 1986). Whereas corona textures are abundant in the sapphirine-bearing
rocks of those regions and are consistent with a rapid uplift history, the sapphirine-bearing and associated rocks in Madagascar have very few coronas, and in constrast have abundant coarse cordierite. We suggest that these relations indicate that the gneissic crustal pile in Madagascar underwent much slower uplift which allowed continuous re-equilibration, and late stabilization at relatively low pressures which allowed the growth of abundant cordierite.
References ANASTASIOU,P. & SEIFERT,F. 1972. Solid solubility of A1203 in enstatite at high temperatures and 1-5 kb water pressure. Contributions to Mineralogy and Petrology, 34, 272-87. BESAIRIE,H. 1971. Carte geologique au 1/2 000000 et notice explicative. Documentation du Bureau Geologique, Madagascar, no. 184. CAHEN, L. • SNELLING,N. J. 1984. The Geochronology and Evolution of Africa, Clarendon Press, Oxford, 512 pp.
ELLIS, D. J. 1980. Osumilite-sapphirine-quartz granulites from Enderbyland, Antarctica: PT conditions of metamorphism, implications for garnet-cordierite equilibria and the evolution of the deep crust. Contributions to Mineralogy and Petrology, 74, 201-10. HARLEY, S. L. 1984. The solubility of alumina in orthopyroxene coexisting with garnet in FeOMgO- A1203- SiO2 and CaO- FeO- MgOA1203-SIO2. Journal of Petrology, 25, 665-96.
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Horny, G. 1976. Presentation et essai d'interpretation du Precambrien de Madagascar. Bulletin
de Bureau de Recherche Geologique et Miniere (France), Section 4, 2, 117-53. LACROIX, A. 1922-23. Mineralogie de Madagascar, - -
3 vols, E. Challamel, Paris. 1941. Les gisements de phlogopite de Madagascar et les pyroxenites qui les renferment. Annales
Geologiques du Service des Mines (Madagascar), 11, 119 pp. LAL, R. K., ACKERMAND,D., RAITH, M., RAASE,P. & SEIFERT, F. 1984. Sapphirine-bearing assemblages from Kiranur, south India: a study of chemographic relationships in the NazOF e O - M g O - A I z O 3 - S i O 2 - H 2 0 system. Neues Jahrbuch fuer Mineralogie, Abhandlungen, 150, 121-52. MOHAN, A., ACKERMAND, D. & LAL, R. K. 1986. Reaction textures and P - T - X trajectory in the sapphirine-spinel-bearing granulites from Ganguvarpatti, southern India. Neues Jahrbuch fuer Mineralogie, Abhandlungen, 154, 1-19. NICOLLE1", C. 1985. Les gneiss rubanes a cordierite et grenat d'Ihosy: un marquer thermobarometrique darts le sud de Madagascar Precambrian Research, 28, 175-85. 1986. Sapphirine et staurotide fiche en magnesium et chrome dans les amphibolites et anorthosites a corindon du Vohibory, Sud Madagascar. Bulletin de Mineralogie, 109, 599-612. POWELL, R. 1978. The thermodynamics of pyroxene
geotherms. Philosophical Transactions of the Royal Society, London. Series A, 288, 457-69. SCHREYER, W. & SEIFERa', F. 1969. Compatibility relations of the aluminium silicates in the systems M g O - AI20 3- SiO2- H20 and K 2 0 - M g O AI2OD-SiO2-H20 at high pressures. American Journal of Science, 267, 371-88. SEIFERT, F. 1974. Stability of sapphirine: a study of the aluminous part of the system MgO-AI2ODSiO2-H20. Journal of Geology, 82, 173-204. & SCHOMACHER,J. C. 1986. Cordierite-spinelquartz assemblages: a potential geobarometer.
Bulletin of the Geological Society of Finland, 58, 95-108.
THOMPSON, A. B. 1976. Mineral reactions in pelitic rocks. II. Calculation of some P - T - X ( F e - M g ) phase relations. American Journal of Science, 276, 425-54. VACHE'ITE (CAEN--VACHE'ITE), M. 1979. Radiochronologie du Precambrien de Madagascar. lOth Colloquium de Geologie, Montpellier, 25-27 Avril 1979, Resumes, pp. 20-1. & HOTTIN, G. 1988. The Precambrian of Madagascar through whole-rock Rb-Sr isochron ages. Bulletin de l'Academie Malgache, in press. WINDLEY, B. ACKERMAND,A. & HERD, R. K. 1984. Sapphirine/kornerupine-bearing rocks and crustal uplift history of the Limpopo belt, southern Africa. Contributions to Mineralogy and Petrology, 86, 342-58.
D. ACKERMAND, Mineralogisches Institut der Universit~t, D-2300 Kiel, Federal Republic of Germany. B. F. WINDLEY,Department of Geology, The University, Leicester, UK. A. RAZAFINIPARANY,Laboratoire de Geologie, Universit6 de Madagascar, Antananarivo, Madagascar.
