Focused radiogenic heating of middle crust caused ultrahigh temperatures in southern Madagascar

Internal heating can cause melting, metamorphism, and crustal weakening in convergent orogens. This study evaluates the role of radiogenic heat production (RHP) in a Neoproterozoic ultrahigh-temperature metamorphic (UHTM) terrane exposed in southern Madagascar. Monazite and zircon geochronology indicates that the Paleoproterozoic Androyen and Anosyen domains (i) collided with the oceanic Vohibory Arc at 630Ma, (ii) became incorporated into the Gondwanan collisional orogen by 580Ma, and (iii) were exhumed during crustal thinning at 525-510Ma. Ti-in-quartz and Zr-in-rutile thermometry reveals that UHTM occurred over >20,000km(2), mostly within the Anosyen domain. Assuming that U, Th, and K contents of samples from the field area are representative of the middle to lower crust during orogenesis, RHP was high enoughlocally >5W/m(3)to cause regional UHTM in <60 Myr. We conclude that, due in large part to the stability and insolubility of monazite at high crustal temperatures, RHP was the principal heat source responsible for UHTM, obviating the need to evoke external heat sources. Focused RHP probably thermally weakened portions of the middle crust, gravitationally destabilizing the orogen and facilitating thinning via lateral extrusion of hot crustal sections.