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Abstract Ultrahigh-temperature metamorphism (UHTM) is important for the evolution and long-term stability of continental crust. The Anosyen domain in southeastern Madagascar is a well-preserved UHTM terrane that formed during the amalgamation of Gondwana. The heat source(s) required to reach peak conditions is(are) a matter of debate. One potential cause of extreme crustal heating is the intrusion of mantle-derived melts into the crust. Foundering of the mantle lithosphere can also lead to increased heat flow. To assess the role of these heating mechanisms, we measured zircon δ18O, εHf(t) compositions, and U-Pb dates for plutonic rocks in the midcrustal UHTM domain. Our results indicate that pluton emplacement predated UHTM by as much as 40 m.y. and that all zircons have crustal O and Hf isotopic compositions. We propose that mantle lithosphere foundering caused melting in the lower crust, producing the magmas responsible for plutonism during the early stages of orogenesis. Prolonged conductive heating of the crust—combined with above-average radiogenic heating—may explain why UHTM occurred ∼40 m.y. after foundering. This suggests that foundering of the mantle lithosphere can swiftly lead to partial melting in the lower crust, as well as protracted heating of the middle crust that culminates tens of millions of years later. 

Abstract We present a comprehensive petrological and geochronological study of a single granulite sample from the lithosphere‐scale Beraketa shear zone in southern Madagascar to constrain the orogenic history of Gondwana assembly in this region. The studied sample provides a panoply of data constraining the prograde, retrograde, and late metasomatic history of the region via the application of Ti‐in‐quartz, Ti‐in‐zircon, Zr‐in‐rutile, and Al‐in‐orthopyroxene thermobarometry; phase‐equilibrium modelling; U–Pb monazite, zircon, and rutile petrochronology; and trace element diffusion chronometry in rutile. Our results reveal five stages of metamorphism along a narrow clockwiseP–Tpath that may have begun as early as 620–600 Ma and certainly by 580–560 Ma, based on the oldest concordant zircon dates. The rock was heated to >725°C at less than 7.5 kbar (Stage 1) before burial to ~8 kbar (Stage 2). Byc. 540 Ma, the rock had heated to ~970°C at ~9 kbar, and lost approximately 12% melt (Stage 3), before decompressing and cooling to the solidus at ~860°C and 6.5 kbar within 10 Ma (Stage 4). The vast majority of monazite and zircon dates record Stage 4 cooling and exhumation. Monazite and zircon rim dates as young asc. 510 Ma record subsolidus cooling (Stage 5) and associated symplectite formation around garnet. U–Pb rutile dates record partial resetting atc. 460 Ma; Zr‐ and Nb‐in‐rutile diffusion chronometry link these dates to a metasomatic event that lasted <1 Ma at ~600°C. In addition to chronicling a near‐complete cycle of metamorphism in southern Madagascar, this study constrains the rates of heating and cooling. We estimate that heating (7–14°C/Ma) outpaced reasonable radiogenic heating rates with modest mantle heat conduction. Therefore, we conclude that elevated mantle heat conduction or injection of mantle‐derived magmas likely contributed to regional ultrahigh‐temperature metamorphism (UHTM). Exhumation and cooling from peak metamorphic conditions to the solidus occurred at rates greater than 0.45 km/Ma and 14°C/Ma. 

Ancient orogens eroded to midcrustal levels provide insight into strain accommodation, metamorphism and melting in Himalaya-type continent–continent collisions. This study focuses on the Neoproterozoic–Cambrian Eastern Africa–Kuunga orogen exposed in Madagascar, where uncertainty about the terrane correlations, and therefore structural framework, of the orogen persists. We present a comprehensive dataset of monazite petrochronology and thermobarometry across the southern Madagascar basement to quantify the regional and temporal variability of metamorphism. We argue that the ultrahigh-temperature Anosyen domain and associated Androyen domain have a shared geological history, recording two successive tectonic events at 630–600 and 580–500 Ma. Other Madagascar domains record primarily the former (Vohibory domain to the west) or latter (all other domains to the NE) event. From this inference, we discuss terrane correlations with Africa and India, then present a structural framework for the orogen in which the Anosyen–Androyen domain was structurally confined in a central, lithosphere-scale transpressional shear system between divergent, diachronous thrust belts. By limiting exhumation, extrusion and collapse, the structural trapping of the Androyen–Anosyen domain facilitated longer-lasting, higher-Tmetamorphism than associated rocks in the adjacent nappe systems. Such structural trapping may be an important control on high-Tmetamorphism in the cores of Himalaya-type orogens in general.