Stenian-Tonian arc magmatism in west-central Madagascar: the genesis of the Dabolava Suite

Abstract

Madagascar is a complex, composite geological terrane that occupies an important location in Late Proterozoic plate reconstructions. The recognition in one of the Madagascan terranes of a Stenian to Tonian-aged arc magmatic suite (the Dabolava Suite) demonstrates that its host terrane resided at a plate boundary from c. 1080 to 980 Ma. Gabbroic and granitoid intrusions of the Dabolava Suite are recognized only in the Ikalamavony Domain in west–central Madagascar. The oxygen isotopic compositions of zircon indicate that the parental magmas involved crustal contributions that were fractionated by a hydrological cycle, whereas hafnium isotopic compositions reflect near depleted-mantle signatures with only minor deflection to more crustal values. Together, these trends suggest mantle derivation of parental magmas coupled with uppercrustal assimilation of Stenian-aged pre-existing plutonic and volcanic rocks. These magmatic rocks, together with the coeval sedimentary rocks of the Ikalamavony Group, are taken to represent a subduction-related magmatic arc that formed in an oceanic-arc tectonic setting in the Mozambique Ocean outboard of the Archaean to early Palaeoproterozoic shield of Madagascar. The arc accreted to the older craton before the initiation of the Imorona–Itsindro Suite magmatism at c. 850 Ma that intruded both domains.