Perrierite in sapphirine-quartz gneiss: geochemical and geochronological features and implications for accessory-phase paragenesis of UHT metamorphism

Abstract

The Al-rich variety of perrierite has been found in sapphirine-quartz-bearing ultrahigh-temperature (UHT) gneiss from Mt. Riiser-Larsen in the Napier Complex, East Antarctica. The major constituents of this perrierite-bearing gneiss are quartz, orthopyroxene, sapphirine, and osumilite, with minor biotite, K-feldspar, and cordierite. Perrierite, zircon, monazite, and rutile are accessory phases. The perrierite grains have diameters in the range 50-500 μm and are pale brownish-orange in color. They have relatively high Al2O3 and MgO content, and low CaO and FeO content compared to other reported perrierite compositions. [The approximate formula is (La0.7Ce1.6Pr0.2Nd0.6Sm0.1Y0.3Ca0.3)(Fe0.5Mg0.7Al1.0)(Ti2.0Al1.0)Si4.0O22]. The perrierite yields U-Th-Pb chemical ages (ca. 2460 ± 110 Ma) and chondrite-normalized rare earth element (REE) patterns (Cen/Dyn = 10-16) that are similar to those of monazite (ca. 2450 ± 30 Ma and Cen/Dyn = 12-24). Both perrierite and monazite may have been stable accessory phases during UHT metamorphism, and the occurrence of perrierite in this sample is possibly controlled by the relatively low phosphorous and high REE and titanium contents.