Linking P-T path with development of discontinuous phosphorus zoning in garnet during high-temperature metamorphism — an example from Lützow-Holm Complex, East Antarctica

Abstract

Garnet porphyroblasts contained in the garnet -sillimanite gneiss from the Lützow -Holm Complex at Skallevikshalsen, East Antarctica, have a phosphorus -poor core and phosphorus -rich rim. The core/rim boundary of the phosphorus zoning is discontinuous. The irregular shape of this core/rim boundary together with the pressure difference between the core and the rim inferred from the difference in aluminosilicate inclusions (kyanite in the core and sillimanite in the rim) suggests that this is the resorption/reprecipitation boundary. The difference between phosphorus concentrations in the garnet core and rim is accompanied by a change in phosphate inclusions in the garnet. Apatite and monazite are included in the phosphorus -poor garnet core, whereas monazite alone is included in the phosphorus -rich garnet rim. Utilizing the core/rim boundary as a contemporaneous surface when comparing different garnet grains, the timing of the discontinuous phosphorus -zoning formation (and thus, the garnet resorption) and change in the phosphate assemblage can be correlated to the pressure -temperature path of the garnet -sillimanite gneiss. The phosphorus -poor core of the garnet mainly formed during the prograde stage in the kyanite to sillimanite stability fields under which apatite probably buffered the phosphorus -content of garnet, and the phosphorus -rich garnet rim possibly crystallized from the melt at the retrograde stage near the vapor saturated solidus under which monazite alone (without apatite) probably did not buffer the phosphorus -content of garnet. Garnet resorption occurred during the decompression stage between these two garnet growth stages.

Possible polymetamorphism and brine infiltration recorded in the garnet–sillimanite gneiss, Skallevikshalsen, Lützow–Holm Complex, East Antarctica

“…Yoshimura et al (2004) considered that the garnet-sillimanite gneiss is likely a restitic product of partial melting. Partial melting of the garnet-sillimanite gneiss is also supported by the presence of 'nanogranite' (Cesare et al, 2009) or 'felsite' (Hiroi et al, 2014) inclusions in garnet porphyroblasts (Kawakami and Hokada, 2010 and this study). Peak metamorphic condition of Skallevikshalsen was previously estimated to be 770-940°C, 0.65-1.2 GPa from the garnet-biotite gneiss and 780-960°C, 0.60-1.1 GPa from the garnet-two pyroxene gneiss (Yoshimura et al, 2004).…”

“…Ultrahigh-temperature (UHT ) metamorphism of about 1000°C, 1.1 GPa and subsequent isothermal decompression are reported from Rundvågshetta (Kawasaki et al, 1993;Ishikawa et al, 1994;Yoshimura et al, 2008). The complex is deduced to have experienced a typical clockwise P-T path (Hiroi et al, 1983a(Hiroi et al, , 1983b1986;Kawakami and Motoyoshi, 2004;Kawakami and Hokada, 2010). The rocks in the Prince Olav Coast experienced the reaction staurolite = garnet + Al 2 SiO 5 + spinel + H 2 O in the sillimanite stability field whereas those in the Lützow-Holm Bay experienced the reaction in the kyanite stability field (Hiroi et al, 1983a(Hiroi et al, , 1983b(Hiroi et al, , 1991.…”

“…This sample has an advantage of including many relic inclusions indicative of P-T path in the garnet porphyroblasts (Kawakami and Motoyoshi, 2004;Kawakami and Hokada, 2010). In this study, garnet grains A, B, C and E of Kawakami and Hokada (2010) are studied in detail . Garnets A and C are from the thin section TK2002011804d, and garnets B and D are from the thin section TK2002011804c.…”

“…The sample mainly consists of garnet (~5 mm in diameter), sillimanite, K-feldspar, plagioclase and quartz with minor biotite, rutile, ilmenite, zircon, monazite and pyrite (Kawakami and Motoyoshi, 2004;Kawakami et al, 2006;Kawakami and Hokada, 2010). It is noteworthy that the rock matrix contains no hydrous mineral except for minor amount of biotite.…”

“…The garnet core includes biotite, staurolite, kyanite, kyanite + spinel + biotite (presumably after staurolite), prismatic sillimanite, spinel + quartz, plagioclase (An43-45 at the inner core, An15 and An5 at the outer core), quartz, rutile, zircon, apatite and fine-grained monazite (<52 µm; Kawakami and Hokada, 2010) . Prismatic sillimanite is only rarely included in the outermost core of the garnet.…”