INTRACRUSTAL MAGMATIC HEAT ADVECTION IN THE EDIACARAN UHT DOMAIN OF MADAGASCAR

We report the first observation of osumilite, along with monazite LASS U-Th-Pb chronology, pseudosections, and feldspar thermometry from the ultrahigh-temperature (UHT) rocks of southern Madagascar to evaluate their P-T-time path and test the relative importance of advective and radiogenic heating in the achievement of UHT.

Osumilite is observed in two rock types: a low-fO₂ assemblage with garnet and a high-fO₂ assemblage with magnetite + ilmenite-hematite. The stability of these assemblages has been constrained experimentally to > 900 ^oC (Fe²⁺; Carrington and Harley, 1995) and > 850 ^oC at < 0.85 GPa (hematite-magnetite buffer; Das et al., 2001).

Pseudosections yield conditions of 900-950 ^oC/6-7 kbar and 970-1000 ^oC/4.5-5.5 kbar. Feldspar thermometry indicates T > 915 ± 30 ^oC. Together with the osumilite-bearing assemblages, these results show regional equilibrium at 900-1000 ^oC and < 7 kbar. Cordierite and sillimanite inclusions in garnet—and the absence of rutile—imply a low-P prograde path.

A minimum age for UHT of 546 ± 6 Ma is constrained by 3 samples that yielded single-population monazite U-Th-Pb dates interpreted to reflect cooling below the solidus. A maximum age for UHT of 567 ± 11 Ma is constrained by three 100-500 μm monazite grains within pseudomorphs after osumilite. This date coincides with emplacement of voluminous charnockites (30-50% field area; solidi >950 ^oC, liquidi >1100 ^oC) whose Nd isotope ratios suggest formation by remelting of local Archean crust (Paquette et al., 1994).

Synthesis of all geochronology indicates prograde metamorphism began by 625 Ma and lasted 58 ± 17 Myr, UHT lasted <22 ± 13 Myr and cooling to 300 ^oC (biotite Ar/Ar dates) took another 65 ± 5 Myr. The duration of prograde metamorphism is similar to the duration of the India-Asia collision (50 Myr). However, peak T was 100-200 ^oC higher than modern Tibet at the same P. Such high T at relatively low P (6 kbar) is difficult to achieve in 60 Myr by radiogenic heating alone (Bea 2012; Clark et al., 2015; Horton et al., 2016). Based on the coincidence of UHT and the emplacement of charnockites, we propose that high-T melts generated lower in the crust and emplaced at the level of current exposure contributed to the heat needed to reach the extreme PT conditions observed in southern Madagascar.