Boron isotope compositions of coexisting kornerupine and tourmaline in high-grade metabasic rocks: an example from Akarui Point, Lützow-Holm Complex, East Antarctica

Abstract

Boron isotope compositions were measured in kornerupine and tourmaline from lenses consisting primarily of kornerupine, plagioclase and corundum. The lenses occur within hornblende-gneiss or along the boundary between this gneiss and an amphibolite lens at Akarui Point in the Lützow-Holm Complex, Prince Olav Coast, East Antarctica. The peak metamorphic conditions have been estimated to be ∼ 800-900 °C and ∼ 8-11 kbar. The δ¹¹B compositions of kornerupine, which is interpreted to have been a stable phase at the metamorphic peak, are −11.6 ± 0.4 to −7.8 ± 0.5‰ and −9.8 ± 0.3 to −6.1 ± 0.2‰ in two different samples. Grains of prograde tourmaline included in kornerupine and corundum yielded δ¹¹B = −2.1 ± 0.3 to +0.6 ± 0.3‰, and the secondary tourmaline replacing kornerupine yielded δ¹¹B = −4.6 ± 0.2 to −3.7 ± 0.2‰. Therefore, the isotopic fractionation between kornerupine and tourmaline, Δ¹¹B_Tur-Krn (= δ¹¹B_Tur − δ¹¹B_Krn), of the average prograde tourmaline and average host kornerupine is +6.7 ± 1.5‰, which is interpreted to indicate isotopic equilibrium at the metamorphic peak on the basis of previous studies of isotope fractionation between tourmaline and minerals of the kornerupine-prismatine series. The δ¹¹B values obtained on prograde tourmaline are between whole rock δ¹¹B of MORB and mantle rocks and of some sedimentary rocks, and are similar to the δ¹¹B of blackwall tourmalines that crystallized during the decompression stage following high-pressure metamorphism. We infer that the syn-metamorphic B-bearing fluid present in the kornerupine-plagioclase-corundum 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 could be interpreted as the remnant of mixing zone of Ediacaran to Cambrian subduction channel.