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

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 L & uuml;tzow-Holm Complex, Prince Olav Coast, East Antarctica. The peak metamorphic conditions have been estimated to be -800-900 degrees C and -8-11 kbar. The delta 11B 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%degrees and -9.8 +/- 0.3 to -6.1 +/- 0.2%degrees in two different samples. Grains of prograde tourmaline included in kornerupine and corundum yielded delta 11B = -2.1 +/- 0.3 to +0.6 +/- 0.3%degrees, and the secondary tourmaline replacing kornerupine yielded delta 11B = -4.6 +/- 0.2 to -3.7 +/- 0.2%degrees. Therefore, the isotopic fractionation between kornerupine and tourmaline, Delta B-11(Tur-Krn) (= delta B-11(Tur )- delta B-11(Krn)), of the average prograde tourmaline and average host kornerupine is +6.7 +/- 1.5%degrees, 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 delta B-11 values obtained on prograde tourmaline are between whole rock delta 11B of MORB and mantle rocks and of some sedimentary rocks, and are similar to the delta 11B 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.