Title 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?
Author Kawakami, T.; Grew, E.S.; Motoyoshi, Y.; Shearer, C.K.; Ikeda, T.; Burger, P.V.; Kusachi, I.
Author Affil Kawakami, T., Kyoto University, Department of Geology and Mineralogy, Kyoto, Japan. Other: National Institute of Polar Research, Japan; Kyushu University, Japan; Chiba University, Japan; National Institute of Polar Research, Japan; University of New Mexico; Kyushu University, Japan; Okayama University, Japan
Source Geodynamic evolution of East Antarctica; a key to the east-west Gondwana connection, edited by M. Satish-Kumar, Y. Motoyoshi, Y. Osanai, Y. Hiroi and K. Shiraishi. Geological Society Special Publications, Vol.308, p.351-375, . Publisher: Geological Society of London, London, United Kingdom. ISSN: 0305-8719. ISBN: 978-1-86239-268-7
Publication Date 2008
Notes In English. 66 refs. Ant. Acc. No: 85828
Index Terms Antarctica--East Antarctica; Antarctica--Prince Olav Coast; aenigmatite group; Akarui Point; Antarctica; biotite; boron; Cambrian; chain silicates; East Antarctica; feldspar group; framework silicates; genesis; geochemistry; gneisses; igneous rocks; International Polar Year 2007- 08; ion probe data; IPY 2007-08 Research Publications; kornerupine; lenses; Lutzow- Holm Complex; mafic composition; mass spectra; metamorphic rocks; metamorphism; mica group; mineral composition; orthosilicates; oxides; P-T conditions; Paleozoic; plagioclase; plutonic rocks; Prince Olav Coast; pyroxenite; Queen Maud Land; ring silicates; sapphirine; sheet silicates; silicates; sorosilicates; spectra; spinel; tourmaline group; ultramafic composition; ultramafics; X-ray diffraction data
Abstract Kornerupine, ([], Mg, Fe)(Al, Mg, Fe)9(Si, Al, B)5O21(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 Lutzow-Holm Complex, East Antarctica, where two ruby corundum (0.22-0.34 wt% Cr2O3)- 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°C and 7.7-9.8 kbar. Kornerupine encloses tourmaline and plagioclase, which suggests the prograde reaction tourmaline (1) + plagioclase (›An34) + sapphirine ± spinel-->kornerupine + corundum (ruby) + 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 H2O-CO2 fluid in cracks at P-T conditions in the andalusite stability field: kornerupine + plagioclase + (Na, K, ± Si in fluid) --> tourmaline+biotite+corundum (sapphire) ± magnesite ± andalusite + (Ca in fluid). Secondary tourmaline differs from the included tourmaline in containing less Ti and having a higher Na/(Na+Ca+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.
URL http://hdl.handle.net/10.1144/SP308.17
Publication Type journal article
Record ID 290779