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

Resumen

During the Cryogenian period (0.84–0.76 Ga), the bimodal Imorona-Itsindro plutonic suite intruded thePrecambrian basement of Madagascar and has been the subject of continuous debate regarding its origin andtectonic setting. Controversy exists between studies proposing an intraplate rift related to the mantle plume andthose favoring an Andean-type suprasubduction origin. This study focuses on the whole rock analyses of theImorona-Itsindro gabbros in the Alaotra region to explain their magmatic and tectonic affinities, and theirgeodynamic implications. Sampling was carried out in three different sectors around Lake Alaotra. Eight gabbrosamples 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, withaccessory minerals such as ilmenite, magnetite, spinel, pyrite, and apatite. The main oxide compositions show highMg# tholeiitic affinity (46.93 < Mg# < 74.24), while variations between Mg# versus CaO indicate magmaticfractionation of plagioclase and clinopyroxene. Trace element compositions reveal light rare earth and large ionlithophile elements enrichments typical of subduction-related magmas, while high field strength elements showdepletions, indicative of an underplate source. The results support an Andean-type suprasubduction settingconsistent with a west-east vergence subduction between the Mozambican oceanic lithosphere and the Malagasycraton and its exotic terrains in this period.