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Abstract

Ultrasonic measurements of P-wave velocity (Vp) and S-wave velocity (Vs) were conducted at high pressures up to 1.0 GPa and high temperatures up to 400 °C for ultrahigh-temperature (UHT) metamorphosed rocks from Mount Riiser-Larsen, in the Archaean Napier Complex of Enderby Land. The results at 1.0 GPa and 400 °C are Vp=7.17 km s−1, Vs=4.24 km s−1, Vp/Vs=1.69, Poisson's ratio (σ)=0.23 for pyroxenite (SiO2=44.2 wt%, density (ρ)=3.41 g cm−3); Vp=6.93 km s−1, Vs=3.81 km s−1, Vp/Vs=1.82, σ=0.28 for mafic granulite (SiO2=52.2 wt%, ρ=3.02 g cm−3); Vp=6.88 km s−1, Vs=3.72 km s−1, Vp/Vs=1.85, σ=0.29 for mafic granulite (SiO2=49.5 wt%, ρ=2.88 g cm−3); and Vp=6.17 km s−1, Vs=3.59 km s−1, Vp/Vs=1.72, σ=0.24 for orthopyroxene-bearing felsic gneiss (SiO2=65.4 wt%, ρ=2.68 g cm−3). Vp and Vp/Vs of these UHT rocks are not comparable with the previously proposed seismic velocity model (Vp=6.56 km s−1, Vp/Vs=1.70) for the lower crust beneath the Mizuho Plateau of eastern Dronning Maud Land. Combining the available measured velocity and density data with the seismic velocity profile defined for the Mizuho Plateau, we suggest that relatively low Vp and Vs characteristics of the lower crust beneath the Mizuho Plateau may be attributed to higher abundance of biotite in the mafic lower crustal rocks. It is proposed that the biotite-bearing lower crustal rocks were formed by metasomatic processes associated with Pan-African orogeny.

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