R6: Plutonic rocks, volcanic rocks and subduction factory

[R6P-11] Petrography and Rb-Sr mineral age of mafic dyke rocks on Niban-Rock, Lützow-Holm Complex (LHC), East Antarctica.

*Tomoharu MIYAMOTO¹, Yamashita Katsuyuki², Daniel J. Dunkley³, Toshiaki Tsunogae⁴, Mutsumi Kato⁵ (1. Kyushu University, 2. Okayama University, 3. Polish Academy of Sciences, 4. Univ. of Tsukuba, 5. Chiba University)

Keywords:Lützow-Holm Complex, Mafic dyke, Intrusion of after peak metamorphism

Niban Rock is a 2.5 km × 3.5 km exposure located at northeast LHC and is corresponded into the amphibolite–facies zone. Niban Rock is composed of Niban–higashi Rock and Niban–nishi Rock, and is underlain mainly by sillimanite–garnet–biotite gneiss, biotite gneiss, and biotite–hornblende gneiss along with minor metabasite, calc–silicate gneiss, granite, and aplite. Although the intrusive rocks were less voluminous than the widespread metamorphic rocks, the emplacement of the mafic dyke which cut metamorphic textures and structures was recognized in the Niban-higashi Rock. The mafic dyke was a few tens cm width intrusive to 10 m length at least, oriented mostly to the WNW-ESE direction, and cut sharply through the foliage of the surrounding host gneisses. The dyke does not exhibit textures resembling the major metamorphic structures found in the surrounding basement rocks. The boundary between the dyke and surrounding host gneisses had no reactive textures. The mafic dyke rocks are commonly holocrystalline and aphyric. They consisted mainly of alkali feldspar, plagioclase, biotite, hornblende, quartz, apatite, and titanite. The crystals often grow in parallel arrays in the intrusive direction of the dyke. The mafic dyke rocks have alkalic compositions, and enriched to incompatible elements. For chronological research, the mafic fractions and felsic fractions were separated from crushed mafic rock samples. Their ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr values are aligned on the isochron diagram, and the age showed 500.9 ± 0.2 Ma and IR = 0.704665 ± 0.000016). Occurrence of the mafic dykes at Niban-Rock suggest the presence and timing of mantle-involved igneous activity after major metamorphism at the NE LHC.