PETROGENETIC IMPLICATION OF THREE CONTRASTING TERRANES IN NORTHERN MADAGASCAR

The Precambrian rocks of northern and central Madagascar may be divided into three tectonic elements: (1) a northernmost, largely juvenile Neoproterozoic (~750-725 Ma) volcanic/plutonic arc terrane, perhaps once contiguous with the Seychelles; (2) a southernmost continental shield consisting of middle-to-late Archean granitoid and mafic gneisses and lesser paragneiss; and an intervening collision zone that changes character from west to east. In the east, the zone consists principally of pelitic shist, gneiss, and migmatite intruded by rare plutons of charnockite, and in the west, it consists of migmatized and deformed calc-alkaline gneisses invaded by leucogranites. We have previously reported on the age and metamorphic conditions of collisional events in the eastern part. We report here new U-Pb geochronology and whole-rock geochemistry from samples in the western part of the area that shed light on the origin and timing of collision events in this part of Madagascar.

The western area is dominated by a series of tonalitic to granodioritic gneisses containing enclaves of amphibolite. Sheets and dikes of pink leucogranite cut this complex. The latter yield a very good U/Pb zircon age of ~560 Ma, only slightly older than the timing of charnockite plutonism and regional metamorphism in the east (~520 Ma). The ages on amphibolite and migmatite gneisses are equivocal with respect to field relationships. Field relations suggest that tonalite sheets and dikes intrude the amphibolite host, yet zircon extracted from the amphibolite yielded a concordant U-Pb age of ~575 Ma and zircon from the tonalite yielded a concordant U-Pb age of 882 Ma. Whole-rock characteristics of the tonalite/granodiorite gneisses suggest that their protoliths formed in a subduction environment.