11:45 〜 12:00
[SMP25-05] Boron isotope compositions of coexisting kornerupine and tourmaline in high-grade metamafic rocks: an example from Akarui Point, Lützow-Holm Complex, East Antarctica
キーワード：ホウ素同位体、南極、沈み込み帯、大陸衝突帯、変成岩
Boron isotope compositions of kornerupine and tourmaline in rare kornerupine-plagioclase-corundum (Krn-Pl-Crn) rocks are determined. The rocks occur as lenses in a hornblende-gneiss or found at the boundary between an amphibolite lens and the hornblende-gneiss from Akarui Point (Lützow-Holm Complex, East Antarctica) where peak metamorphic conditions of ~800-900 oC and ~8-11 kbar have been reported. The δ11B values of kornerupine that is considered as a stable phase at peak metamorphism are -11.6 to -7.8 ‰ and -9.8 to -5.3 ‰ in two separate samples. Prograde tourmaline inclusions in kornerupine and ruby corundum yielded δ11B values of -2.0 to +0.6 ‰, and the secondary tourmaline replacing kornerupine yielded -4.6 to -3.7 ‰. Based on previous studies, the prograde tourmaline and kornerupine can be interpreted as isotopically in equilibrium. The prograde tourmaline shows δ11B values similar to published MORB, mantle rocks, some subduction zone metasediments and some blackwall tourmalines that is formed at the decompression stage of high-pressure metamorphism. Therefore, the syn-metamorphic B-bearing fluid to form the Krn-Pl-Crn lens is likely sourced from a mixture of sedimentary, mafic and ultramafic lithologies in a subduction setting. The metabasic and meta-ultramafic lenses found in Akarui Point may be the traces of mixing zone of a subduction zone that predates the Late Proterozoic to Cambrian high-temperature collisional metamorphism.