Mechanisms of granulite formation in an archetypal accretion-to-collision orogen: pseudosections and monazite petrochronology from southern Madagascar

The Precambrian basement of Madagascar is comprised Archean to Neoproterozoic crust that was metamorphosed to amphibolite- to granulite-facies conditions—including ultrahigh-temperature (UHT) osumilite gneisses in southeast Madagascar—during the Ediacaran-Cambrian East-African-Malagasy collisional orogen between the Dharwar (India) and Congo (Africa) cratons. Based on the regional extent of high-grade metamorphism and structural similarities, the Malagasy basement has been described as possibly the world's best example of what the middle crust of modern Tibet might look like. These similarities with modern Tibet have led to the proposal that long-lived crustal thickening during continental collision is the primary mechanism by which regional ultrahigh-temperature metamorphism occurs. However, the timing, duration, and regional extend of metamorphism in response to different stages of the Malagasy Orogen as well as the number of accretionary events prior to the final closure of the ocean basin(s) and where the sutures lie, remain unclear. Understanding the geodynamic significance of the south-Malagasy and south-Indian granulites and whether they truly formed in an environment like the middle crust of modern Tibet, is further inhibited by uncertainty in their P-T-t paths. This contribution evaluates the P-T-t field-gradient paths of the of south-Malagasy granulites—using pseudosections, traditional thermobarometry, and LA-ICP-MS monazite petrochronology—to better understand the mechanisms of granulite formation in an archetypal accretion-to-collision orogen.