The occurrence of a Tonian–Cryogenian (∼850 Ma) regional metamorphic event in Central Madagascar and the geodynamic setting of the Imorona–Itsindro (∼800 Ma) magmatic suite
Résumé
The widespread ∼800 Ma Imorona–Itsindro magmatic suite of Central Madagascar comprises calc-alkaline granitic orthogneiss (Imorona type) and meta-gabbros (Itsindro type). This suite’s geodynamic setting is controversial: it was initially related to an orogenic Andean-type continental magmatic arc however, recent studies maintain that the suite intruded undeformed supracrustal series and suggest that it is connected with continental dilation conditions. The present contribution to this debate is focused on the Itremo domain of low-grade metamorphic conditions, in which the multilayer lithologies clearly exhibit the tectonic structures. Structural and petrographic evidence is presented indicating that the Imorona–Itsindro suite postdates the main deformation phase, although in certain cases, the intrusion appears to be syn- to late-tectonic. This interpretation has been strengthened and quantified by new isotopic ages that were obtained by in situ laser ablation ICPMS on zircons and monazites. In a gneiss of rhyolitic origin that is interlayered with metasediments of the Sahatany Valley, the weighted mean 207Pb/206Pb ages of sixteen points in zircon rims is 854 ± 19 Ma; high U and a low Th/U ratio indicate that these rims are overgrowths that were crystallized during a regional metamorphic event. In schists from the contact zones of ∼790 Ma intrusions, monazites display concordant 208Pb/232Th and 206Pb/238U ages that can be interpreted as recording two distinct metamorphic events: one at approximately 820–875 Ma and one at 750–780 Ma. The older ages, often corresponding to the core of monazite grains, could be related to the regional metamorphic event detected in the Sahatany zircons. The younger ages which are mainly present in overgrowths may be related to be related to continue hydrothermal activity during the Imorona–Itsindro extensive magmatism. In the contact zone of Antoetra granite (780 Ma) and outside this zone, the mean 208Pb/232Th ages for all monazite grains are in the 525–560 Ma range, indicating full recrystallization during the Pan-African event, and indicate the difficulty in tracking older events. The explained evidence demonstrates the existence of a regional metamorphism event predating the intrusion of the Imorona–Itsindro suite, which is more appropriately called Andean-type orogenic continental arc magmatism. These results suggest the construction of a tentative geodynamic model integrating the possible relationships between the Imorona–Itsindro (800 Ma) suite and the Dabolava (1000 Ma) suite occurring along its S–W limit and exhibiting oceanic arc magmatism affinities.