Petrogenesis and tectonic affinities of ImoronaItsindro gabbros in the Alaotra region of central Madagascar: whole rock analyses and geodynamic implications

Abstract

During the Cryogenian period (0.84–0.76 Ga), the bimodal Imorona-Itsindro plutonic suite intruded the Precambrian basement of Madagascar and has been the subject of continuous debate regarding its origin and tectonic setting. Controversy exists between studies proposing an intraplate rift related to the mantle plume and those favoring an Andean-type suprasubduction origin. This study focuses on the whole rock analyses of the Imorona-Itsindro gabbros in the Alaotra region to explain their magmatic and tectonic affinities, and their geodynamic implications. Sampling was carried out in three different sectors around Lake Alaotra. Eight gabbro samples were collected and subjected to major and trace element analyzes using ICP-OES and ICP-MS techniques. Petrographically, gabbros are mainly composed of plagioclase, olivine, clinopyroxene and orthopyroxene, with accessory minerals such as ilmenite, magnetite, spinel, pyrite, and apatite. The main oxide compositions show high Mg# tholeiitic affinity (46.93 < Mg# < 74.24), while variations between Mg# versus CaO indicate magmatic fractionation of plagioclase and clinopyroxene. Trace element compositions reveal light rare earth and large ion lithophile elements enrichments typical of subduction-related magmas, while high field strength elements show depletions, indicative of an underplate source. The results support an Andean-type suprasubduction setting consistent with a west-east vergence subduction between the Mozambican oceanic lithosphere and the Malagasy craton and its exotic terrains in this period.