GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 270-2
Presentation Time: 8:15 AM


HOLDER, Robert M. and HACKER, Bradley R., Department of Earth Science, University of California, Santa Barbara, Santa Barbara, CA 93106,

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.

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