[S-MP25] Supercontinents and Crustal Evolution

[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

*Tetsuo Kawakami¹, Simon L. Harley² (1.Graduate School of Science, Kyoto University, 2.The University of Edinburgh)

Keywords:boron isotope, Antarctica, subduction zone, collision zone, metamorphic rocks

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.