We determined the metamorphic age and pressure-temperature conditions recorded in a sillimanite-garnet-bearing pelitic gneiss from Niban-nishi Rock, which is part of Niban Rock, on the Prince Olav Coast, eastern Dronning Maud Land, East Antarctica. Niban-nishi Rock is recognized as a component of the Lützow-Holm Complex (LHC), which is characterized by metamorphic age of 600-520 Ma and metamorphic grade of amphibolite to granulite facies. Electron microprobe U-Th-Pb monazite dating of the examined gneiss revealed that, unlike the typical exposures of the LHC, Niban-nishi Rock experienced Tonian metamorphism at 940.1 ± 9.8 Ma (2σ level), and is thus more similar to the neighboring exposures of Cape Hinode and Akebono Rock. Small numbers of younger monazite ages of 827-531 Ma were also detected, and some of which might relate to the metamorphism of the LHC. Phase equilibrium modeling and geothermobarometry indicate metamorphic conditions of 690-730 °C/0.38-0.68 GPa and 620-670 °C/0.42-0.60 GPa for peak and retrograde stages, respectively. The obtained peak temperature is lower than that of typical exposures in transitional- and granulite-facies zones of the LHC, such as Akarui Point and Skallen. Metamorphic features of Niban-nishi Rock, such as the upper amphibolite-facies condition and occurrences of garnet with retrograde zoning and sillimanite in the matrix, differ from those of Cape Hinode and Akebono Rock, with the former belonging to granulite facies and the latter showing kyanite in the matrix and garnet with growth zoning. Investigating Niban-nishi Rock is key in revealing the tectonic relation between the LHC and Cape Hinode (the Hinode Block). Further field surveys are required to reveal the metamorphic variations and the relations among exposures on the Prince Olav Coast.

This paper reports multiple fluid infiltration events during retrograde metamorphism in the Sør Rondane Mountains, East Antarctica. Pelitic gneisses from southern part of Perlebandet have cordierite-biotite intergrowth rimming garnet, implying that garnet breakdown occurred by fluid infiltration. Using the Raman peak of CO₂ in cordierite and Cl-bearing composition in biotite, this study revealed that the cordierite-biotite intergrowth was formed in equilibrium with one-phase CO₂-Cl-H₂O fluid. The intergrowth texture is cut by thin selvages composed of Cl-bearing biotite, suggesting Cl-bearing fluid infiltration. Since andalusite is exclusively observed in the selvage, near isobaric cooling path is presumed for the pressure-temperature (P-T) path of these post-peak fluid-related reactions. The inconsistence with counter-clockwise P-T path reported from northern Perlebandet is probably due to the granodiorite/leucocratic granite bodies beneath the studied metamorphic rocks. In order to understand the tectonic evolution at the final stage of Gondwana amalgamation, therefore, effect of hidden igneous rocks needs to be taken into consideration.

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.

Grandidierite, (Mg,Fe)Al₃O₂(BO₃)SiO₄, was found in a garnet-clinopyroxene-ilmenite-rich mafic granulite from Austhovde in the Late Neoproterozoic to Early Cambrian Lützow-Holm Complex (LHC), East Antarctica, the first reported occurrence of this borosilicate in a mafic granulite. It occurs in one of the many nanogranitoid inclusions (NIs) in garnet. Quartz, sodic plagioclase, myrmekite, K-feldspar, epidote and biotite are also found only as inclusions in garnet. Garnet porphyroblasts show marked compositional zoning: Ca increases and Mg decreases from the core to rim with little change in Fe and Mn contents except for the outermost rim. Anorthite content of inclusion plagioclase increases from core to rim of host garnet in parallel with increase in garnet Ca towards the rim. This together with the distinctly different mineral assemblages within and exterior to garnet porphyroblasts suggests that partial melting took place and produced melts were extracted leaving a mafic and calcic restite. Partial melting also occurred locally in garnet porphyroblasts consuming different hydrous mineral inclusions to produce various NIs ranging from K-feldspar-rich to K-feldspar-free. Subsequent decompression at high temperatures resulted in breakdown of garnet to orthopyroxene + calcic plagioclase with further consumption of quartz, such that none remained in the matrix of the granulite. Grandidierite may have formed by a reaction between a trapped boron-bearing aluminous granitic melt and host garnet upon cooling.

This paper first reports the counter-clockwise pressure-temperature (P-T) path for the Lützow-Holm Complex in East Antarctica. The metamorphic textures of kyanite-bearing pelitic gneisses from Tenmondai Rock including earlier spinel and ilmenite inclusions, geothermobarometric data, and pseudosection modeling indicate that the pressure increases prior to the peak metamorphic conditions at around 9 ± 0.5 kbar and 770-820 °C, followed by cooling to kyanite-stability field. We conclude that these gneisses underwent granulite-facies metamorphism with a counter-clockwise P-T path, but this contrasts with the widely recognized clockwise P-T path of the Lützow-Holm Complex basement rocks in general. One plausible hypothesis we proposed could be that this counter-clockwise P-T path originated from magmatism with late compression and the rocks of the different structural levels are juxtaposed, while acknowledging that this hypothesis conflicts with previous studies and that further work is needed to clarify these issues.