Timing of attainment of the spinel+quartz coexistence in garnet-sillimanite leucogneiss from Skallevikshalsen, Luetzow-Holm Complex, East Antarctica

Abstract

Spinel in direct contact with quartz is newly found in the Grt Sil leucogneiss from Skallevikshalsen, Lützow Holm Complex, East Antarctica. It is included in the outermost core of the garnet porphyroblast and is characterized by low ZnO content (2.77 3.83 wt%) and very low Cr 2 O 3 content (0 0.09 wt%). Incorporation of 2.77 3.39 wt% ZnO stabilized the Spl + Qtz assemblage to lower temperatures for about 50 degrees. As a result, their coexistence can be attained around the peak P T conditions of Skallevikshalsen. The occurrence of relic staurolite, kyanite and breakdown texture of staurolite to spinel and kyanite included in the garnet core suggests that prograde stage is preserved in the garnet core and breakdown of staurolite itself was attained in the kyanite stability field. The Spl + Qtz assemblage is included at outermost part of the garnet core, suggesting that outermost core of the garnet was formed near the peak metamorphic conditions. Decrease in anorthite content of plagioclase included in the garnet core from that found in the rock matrix suggests that decompression took place during garnet formation. Attainment of these reaction sequences requires clockwise P T path as previously proposed for the Lützow Holm Complex.

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

“…Apatite is very rare in the rock matrix. XRF analyses suggests that the sample contains 0.07 wt% whole -rock P 2 O 5 (Kawakami and Motoyoshi, 2004). This sample is the same lithology as the garnet -sillimanite gneiss studied by Yoshimura et al (2004).…”

“…The complex is deduced to have experienced a typical "clockwise" P -T path (Fig. 2), evidenced by the presence of prograde kyanite and staurolite as relict inclusions in garnet or plagioclase (Hiroi et al, 1983a(Hiroi et al, , 1983bMotoyoshi, 1986;Kawakami and Motoyoshi, 2004;Satish -Kumar et al, 2006) and by the reaction textures in ultramafic rocks (Hiroi et al, 1986). The rocks in the Prince Olav Coast experienced the reaction staurolite = garnet + aluminosilicate + spinel + H 2 O in the sillimanite stability field whereas those in the Lützow -Holm Bay experienced the reaction in the kyanite stability field (reaction 1 in Fig.…”

“…They also considered that partial melting also occurred in the garnet -sillimanite gneiss and this rock type is likely to See the text for a detailed explanation of reactions 3a -3e. Note that the value of X Fe for the whole -rock garnet -sillimanite gneiss is 0.70, and that for the garnet is 0.67 -0.70 (Kawakami and Motoyoshi, 2004). (4) Peak metamorphic temperature of 848 ± 55 °C from Skallen (arrow; Satish - Kumar and Wada, 2000).…”

“…(6) Retrograde P -T estimate from Skallen (light grey plaid area; Ikeda, 2004). (7) Reaction line of garnet + sillimanite = spinel + quartz with the effect of ZnO in spinel being considered (grey lines; Kawakami and Motoyoshi, 2004).…”

UHT granulites of the Highland Complex, Sri Lanka II: Geochronological constraints and implications for Gondwana correlation

“…Mg-Al Spr + Qz bearing assemblages within garnet porphyroblasts are identified from the Highland Complex of Sri Lanka (Sajeev and Osanai, 2004a;Osanai et al, 2006), the Skallen Group in the Lützow-Holm Complex of East Antarctica (Yoshimura et al, 2008), the Madurai Block of southern India (Tateishi et al, 2004;Brandt et al, 2011), and Andriamena in northwestern Madagascar (Goncalves et al, 2004) and Imanombo in southern Madagascar (Jöns and Schenk, 2011). Another rare assemblage, Spl + Qz, is also reported in most of these terranes (e.g., Kawakami and Motoyoshi, 2004;Morimoto et al, 2004;Sajeev and Osanai 2004b;Tadokoro et al 2007;Kawasaki et al, 2011;Jöns and Schenk, 2011). Although the Spr + Qz assemblage is not identified from the Trivandrum Block of southern India, the Spl + Qz assemblage has been reported in this block (Morimoto et al, 2004;Tadokoro et al, 2007).…”