The Precambrian of Madagascar is divided into two sectors by the north-west trending sinistral Ranotsara shear zone, which continues in the Mozambique belt, probably as the Surma shear zone, and in Southern India as the Achankovil shear zone. South of Ranotsara six n o r t h -s o u t h trending tectonic belts are recognized that consist largely of granulite and high amphibolite facies paragneisses, phlogopite diopsidites, concordant granites and granulites. North of Ranotsara the central-northern segment is traversed by a north-trending axial 100-150 km wide dextral shear zone of probable PanAfrican age, which was metamorphosed under granulite and high amphibolite facies conditions and which has reworked older basement. This shear zone continues across southern India as the Palghat-Cauvery shear zone. Major stratiform basic-ultrabasic complexes occur in the axial zone and in the basement to the west. Well preserved low grade continental margin-type sediments (quartzites, mica schists and stromatolitic marbles) of Kibaran age are present in western Madagascar. Two partly greenschist grade sedimentary groups lie unconformably on high grade basement in north-east Madagascar. Isotopic age data suggest the presence in Madagascar of Archaean, Early and Mid-Proterozoic crustal material that was extensively reworked in Pan-African times.

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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 high-grade 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

Kornerupine-rich layers up to several centimetres thick with minor sillimanite, spinel, Fe oxide and ilmenite occur in a diopsidite in sillimanite-cordierite gneiss south of Beraketa (24~ 46~ southern Madagascar. Kornerupine, sillimanite, spinel and hematite grains up to 1 mm across have mutual polygonal boundaries indicating textural equilibrium at their crystallisation. Kornerupine has XMg 0.67--0.80 and 0.9 to 2.6 wt.% B203. Sillimanite contains up to 2.0 wt.% Fe203. Spinel is essentially (Mg,Fe 2+) A1204 with an XMg range of 0.29-0.40 and exsolution lamellae of Fe oxide. Textural relations demonstrate two limited reactions, each confined to areas less than 500 ~tm across: (1) Kornerupine and spinel reacted along grain contacts to form very fine-grained tourmaline, corundum and chlorite. The replacing phases are symmetrically zoned with a central tourmaline and hematite, bordered by an aggregate of chlorite, tourmaline and corundum, followed outwards by a rim of chlorite against the kornerupine and spinel. (2) Within kornerupine grains, zoned, round aggregates consist of very fine-grained chlorite, tourmaline and corundum of different composition than in (1). They define the terminal reaction of kornerupine breakdown.Geothermobarometry indicates that the early kornerupine-bearing assemblage was stable at 7.0 kbar and 700 ~ This P-T point lies close to the retrograde, nearly isothermal trajectory defined independently by nearby sapphirine-bearing assemblages. The fine-grained aggregates formed most likely during further cooling, or by increasing water fugacity.