Radiogenic heating to ultrahigh temperature: Geochronology and 4+ thermometry across southern Madagascar

Abstract:

As zones of focused metamorphism, melting, and ductile deformation, hot crustal sections are especially important for understanding the evolution of the lower crust in convergent orogens, such as Tibet. One of the best exposed crustal sections that reached ultrahigh-temperature (UHT: >900° C) is in southern Madagascar. There, the association of UHT mineral assemblages with exceptionally high concentrations of radioactive heat-producing elements begs the question of whether radiogenic heating caused UHT. Partial melting, retrogression and rapid elemental diffusion at high temperature, however, have left a complex record of radiometric dates that span 150 Myr (~650–500 Ma), making it difficult to address this question.

We apply laser-ablation split-stream ICPMS (LASS) petrochronology and 4+ cation thermometry to better constrain the lengthscales and timescales over which high temperatures were sustained in southern Madagascar. By deciphering complex intragrain (re)crystallization textures of zircon and monazite from a broad geographical area, we deduce that orogenesis lasted >60 Myr. Specifically, 600–540 Ma zircon and monazite with HREE depletion suggest (re)crystallization in the presence of garnet. All monazite show negative Eu anomalies (compatible with the presence of plagioclase), but ~570 to ~530 Ma monazite has more-pronounced anomalies. Ti-in-zircon and Zr-in-rutile thermometry confirm ultrahigh temperatures within a restricted area of ~100 x 200 km.

These thermal and chronologic constraints allow us to evaluate the potential causes of heating. Measured K, Th, and U contents of rocks within the UHT domain indicate an average heat-production rate of >4 μW/m³, sufficient to produce UHT metamorphism in 60-km thick crust within 60 Myr. We conclude that high concentrations of heat-producing elements caused focused heat production in young metasedimentary rocks that were sandwiched between older crustal domains. We speculate that ultrahigh temperatures caused by radiogenic heating led to preferential crustal melting within—and ductile extrusion of—the young metasediments.