Kornerupine sensu stricto associated with mafic and ultramafic rocks in the Lützow-Holm Complex at Akarui Point, East Antarctica: what is the source of boron?

Abstract

Kornerupine, (A, Mg, Fe)(Al, Mg, Fe) 9 (Si, Al, B) 5 O 21 (OH, F), is known from only five mafic or ultramafic settings worldwide (of the .70 localities overall). We report a sixth occurrence from Akarui Point in the Lützow-Holm Complex, East Antarctica, where two ruby corundum (0.22-0.34 wt% Cr 2 O 3 )-plagioclase lenses are found at the same structural level as boudinaged ultrabasic rocks in hornblende gneiss and amphibolite. Ion microprobe analyses of kornerupine give 13-59 ppm Be, 181-302 ppm Li, and 5466-6812 ppm B, corresponding to 0.38-0.47 B per 21.5 O; associated sapphirine also contains B (588-889 ppm). Peak metamorphic conditions are estimated to be 770-790 8C and 7.7-9.8 kbar. Kornerupine encloses tourmaline and plagioclase, which suggests the prograde reaction tourmaline (1) þ plagioclase (.An34) þ sapphirine + spinel ! kornerupine þ corundum (ruby) þ plagioclase (,An82) + (fluid or melt). Alternatively, kornerupine and tourmaline could have formed sequentially under nearly constant P-T conditions during the infiltration of fluid that was originally B-bearing, but then progressively lost Na (or gained Ca) and B through reaction with mafic rocks. Kornerupine later reacted with H 2 O-CO 2 fluid in cracks at P-T conditions in the andalusite stability field: kornerupine þ plagioclase þ (Na, K, + Si in fluid) ! tourmaline þ biotite þ corundum (sapphire) + magnesite + andalusite þ (Ca in fluid). Secondary tourmaline differs from the included tourmaline in containing less Ti and having a higher Na/(Na þ Ca þ K) ratio. There are two possible scenarios for introducing B into the lenses: (1) infiltration of boron-bearing aqueous fluids released by prograde breakdown of muscovite in associated metasedimentary rocks; (2) hydrothermal alteration of mafic and ultramafic rocks by seawater prior to peak metamorphism. The latter scenario is consistent with an earlier suggestion that Akarui Point could be part of an ophiolite complex developed between the Yamato-Belgica and Rayner complexes.

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

“…It is important to note that this change is accompanied by the discontinuous change in the phosphorus concentration in the garnet. The discontinuous increase in the phosphorus concentration in the garnet rim is often observed in hightemperature metamorphic rocks accompanying partial melting, and the correlation with the phosphate assemblage included in the garnet, as reported in this study, has actually been observed worldwide Hiroi, 1997;Hiroi et al, 1997;Yang and Rivers, 2002;Kawakami and Motoyoshi, 2004;Kawakami et al, 2008). However, the generalization of the phosphorus zoning correlated with the change in phosphate assemblage included in garnet cannot be easily carried out at the present stage because of the lack of the detailed case studies on this problem.…”

“…However, it is well known that some minor elements like phosphorus commonly preserve chemical zoning with a non -diffusive nature even in the high -temperature metamorphic rocks Spear and Kohn, 1996;Hiroi, 1997;Hiroi et al, 1997;Chernoff and Carlson, 1999;Yang and Rivers, 2002;Yoshimura et al, 2004). Their zoning pattern is best observed under X -ray elemental mapping and it is characterized by a drastic, discontinuous change in their concentration within garnet (for examples of such zoning, see Hiroi et al, 1997;Chernoff and Carlson, 1999;Yang and Rivers, 2002;Yoshimura et al, 2004;Kawakami et al, 2008). Zoning in minor elements that is not affected by the diffusion in garnet is important because it can represent the contemporaneous surface (cf., Chernoff and Carlson, 1999) and contain considerable information about the reaction history that the rock has experienced (Spear and Pyle, 2002) even if the major element zoning in garnet is obscured by the later diffusion.…”

Metamorphic pressure–temperature conditions of the Lützow–Holm Complex of East Antarctica deduced from Zr–in–rutile geothermometer and Al₂SiO₅ minerals enclosed in garnet

“…This is the same sample studied by Kawakami et al (2008) and Nakamura et al (2014). The matrix of this sample consists mainly of garnet (~10 mm), biotite, sil-limanite, plagioclase, K-feldspar, and quartz with minor ilmenite, rutile (~100-500 µm), apatite, zircon, monazite, and spinel.…”

“…1). Peak metamorphic P-T condition was previously estimated at 770-790°C/ 7.7-9.8 kbar by applying Grt-Bt geothermometers and Grt-Al 2 SiO 5 -Qtz-Pl (GASP) geobarometers to a Sil-Bt-Grt leucogneiss (Kawakami et al, 2008) (Fig. 2a).…”

“…(a) Akarui Point. k2008, Kawakami et al (2008) Johannes and Holtz (1996). MI1997, Motoyoshi and Ishikawa (1997); KsM2006, Kawasaki and Motoyoshi (2006); Y2008, Yoshimura et al (2008); H2019, Hiroi et al (2019).…”