Piotr KRÓL, Monika A. KUSIAK, Daniel J. DUNKLEY, Martin J. WHITEHOUSE, Simon A. WILDE, Lars E. AUGLAND
The Napier Complex in Enderby Land and western Kemp Land is a unique component of the East Antarctic Shield because it records a timeline of crustal growth from the Eo- to Neoarchean. It is mainly composed of enderbitic and charnockitic gneisses and granulites that were metamorphosed at ∼ 2.5 Ga, and locally at ∼ 2.8 Ga, under high- to ultra-high-temperature conditions. Despite generating scientific interest for several decades, the geological history of the complex has not been well constrained. In this study, samples from the Napier Mountains were selected for zircon imaging and U-Pb dating by Secondary Ion Mass Spectrometry. They record metamorphic growth, recrystallization, and modification of zircon at 2800-2770, 2740-2720, and 2490-2460 Ma. For the first time, fluid-related alteration at around 2730 Ma is evident in a granitic gneiss from Grimsley Peaks. At the similar time, dioritic gneiss was formed at Mount Marr. Tonalitic and granitic gneisses from Grimsley Peaks yield protolith crystallization ages of around 3210 and 2825 Ma, respectively. The generation of granitic gneiss was coeval with ∼ 2.8 Ga metamorphism in the area. These new data from this little-known part of the complex provide a better understanding of the crustal evolution of the Napier Complex.
Tomoharu MIYAMOTO, Katsuyuki YAMASHITA, Daniel J. DUNKLEY, Kazuhiko SHIMADA, Toshiaki TSUNOGAE, Mutsumi KATO
Melanocratic dyke rocks were found at Skallevikshalsen and Rundvågshetta in the Lützow-Holm Complex. The rocks are commonly holocrystalline and aphyric, and consist mainly of alkali feldspar, biotite, augite, quartz, apatite, and titanite, with minor amounts of plagioclase. Almost all the mineral compositions tended to be homogeneous, whereas the composition of apatite sometimes showed intragranular heterogeneity in the F and Cl components. The melanocratic dyke rocks at Skallevikshalsen have an ultrapotassic mafic composition and resemble the compositional features of the lamproitic to minettic dyke rock previously found at Innhovde, located in the western region of the Lützow-Holm Complex. The dyke rocks at Rundvågshetta are considered to be a mixture of ultrapotassic mafic magma and high-Cl/F intermediate to felsic magma. Considering their occurrence and the results of Rb-Sr mineral dating, the time of intrusion was just after the metamorphism of the Lützow-Holm Complex, and the igneous activity was thought to have been caused by the collision between East and West Gondwana.
The Vengen Granite, one of early Paleozoic granitic rocks crops out of the southern end of the Vengen ridge, the Kanino-tsume Peak at the Main Shear Zone (MSZ) of the Sør Rondane Mountains, East Antarctica. This granite is composed of medium- to fine-grained mylonitic biotite granite and cuts the MSZ and Kanino-tsume Shear Zone. The fine-grained two-mica granitic dykes locally intrude the Vengen Granite. The two-mica granitic dykes have foliations parallel to mylonitic foliations of the Vengen Granite. The Vengen Granite is composed of plagioclase, quartz, K-feldspar, biotite, and muscovite with trace amounts of titanite, allanite, apatite, zircon and opaques as accessory minerals. The granite is geochemically characterized by a high-K content, which resembles adakitic affinity. These chemical data combined with rare earth elements and Sr-Nd isotope geochemistry suggest that the source magma of the Vengen Granite was derived from partial melting of the meta-tonalite with minor amounts of the pelitic gneisses in the SW-terrane subducted under the NE-terrane during collision of the West and East Gondwana continents.
This paper reports laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb ages of a suite of high-grade metamorphic rocks collected from Sinnan Rocks, Akebono Rock, Niban Rock, Gobanme Rock, Tenmondai Rock, Akarui Point, Cape Omega, and Oku-iwa Rock along the Prince Olav Coast, in the Lützow-Holm Complex (LHC), East Antarctica. The dating results indicate that a newly detected ∼ 990 Ma metamorphism of garnet-sillimanite-biotite gneiss from Niban Rock. A thermal event at 931.7 ± 9.8 Ma is recorded in zircon from staurolite-bearing garnet-gedrite-biotite-chlorite gneiss in Akebono Rock. The metamorphic zircon grains in other analyzed samples provide Ediacaran to Cambrian ages. Their multi-growth textures and age populations are possibly interpreted to exhibit three metamorphic stages during >600-580, 580-550, and 550-500 Ma. Combined with previous reports, the metamorphic rocks in Cape Hinode, Niban-nishi Rock of Niban Rock, and Akebono Rock might have experienced earlier high-temperature metamorphism at ∼ 990-930 Ma without younger overprinting. Extensive high-grade metamorphism during ∼ 650-500 Ma is recorded from not only the granulite-facies zone in the west of the LHC but also the amphibolite-facies zone in its east. The main metamorphic episode in the LHC is likely to be subdivided into a preceding thermal event (either independent single metamorphic event or prograde stage) at pre-580 Ma, near-peak condition stage during 580-560 Ma, and subsequent retrograde stage after 550 Ma. In regional context this indicates that the assembly at the central Gondwana started with the collision of early and late Neoproterozoic terranes prior to 580 Ma, as a part of the East Africa-Antarctic Orogeny. Subsequent collisions took place among late Neoproterozoic igneous terrane, above terranes collided at pre-580 Ma, and Neoarchean terrane, which were probably driven by the Kuunga Orogeny.
Toshiaki SHIMURA, Yuki HARADA, Geoffrey L. FRASER, Noriyoshi TSUCHIYA
Spinel + plagioclase symplectites in pelitic metamorphic rocks have been reported from many localities. We report in detail an occurrence of this texture from Tenmondai Rock, Lützow-Holm Complex, East Antarctica. This texture is produced by the following metamorphic reaction; garnet + sillimanite = spinel + plagioclase. This can be described as the following two net-transfer reactions; 5Grs + Alm + 12Als = 3Hc + 15An, and 5Grs + Prp + 12Als = 3Spl + 15An. We propose new geobarometers (GASpP) based on these equations. For example, one of these is P = (−155790 + 587.4T + RT lnKFe) / 29.378, for garnet + sillimanite + spinel + plagioclase assemblage in the CFASZn-system. Where P is pressure in bar, T is temperature in K, and R represents gas constant. The equilibrium constant is, KFe = (aGrs5aAlm)/(aHc3aAn15). Our proposed new geobarometers are free from quartz, corundum, orthopyroxene, and cordierite within these equations. These barometers are useful to estimate pressure conditions for spinel-bearing pelitic metamorphic rocks within a wide pressure-temperature range (andalusite, kyanite, and sillimanite fields). We estimate the metamorphic pressure-temperature (P-T) conditions at Tenmondai Rock by applying the GASP, GRIPS, GRAIL, GASpP geobarometers, and the Zr-in-Rt geothermometer. The metamorphic evolution of the rocks at Tenmondai Rock is characterized by a clockwise P-T-t path. The peak P-T condition is about 820 MPa and 850 °C. Spinel + plagioclase symplectite was produced during the decompressional stage of this metamorphic evolution, at about 450 MPa and 700 °C.
Geoffrey H. GRANTHAM, M. SATISH-KUMAR, Kenji HORIE, Henriette UECKERMANN
The ∼ 550 Ma Kuunga Orogeny extends from the Damara in Namibia, through the Zambesi and Lurio orogenic belts in Zambia and Mozambique, southern Africa, through Dronning Maud Land and Princess Elizabeth Land, Antarctica into western Australia. Sverdrupfjella is located at the western end of Dronning Maud Land where the Kuunga Orogeny is inferred to post-date and overprint the East African Orogeny.
Three complexes are recognized in Sverdrupfjella western Dronning Maud Land, Antarctica. A western basal ∼ 1140 Ma Jutulrora Complex, consisting mostly of arc-related tonalitic trondjhemitic orthogneiss with evolved Sr-Nd isotopic signatures with TDm ages >2 Ga. It is structurally overlain by the Fuglefjellet Complex, comprising supracrustal ∼ 800-900 Ma carbonates intercalated with quartzo-feldspathic gneisses with detrital zircons of ∼ 1000-1200 Ma age with ∼ 500 Ma overgrowths. The Fuglefjellet Complex is overlain in the east by the Rootshorga Complex containing paragneisses with minor orthogneisses (∼ 1100-1200 Ma), intruded by granitic orthogneiss of similar age. Strontium-Nd isotopic signatures from the Rootshorga Complex has TDm ages <1.8 Ga.
D1 and D2 planar fabrics typically dip to SE with vergence top-to-NW in all complexes. D3 deformation verges top-to-the-SE. In the Jutulrora Complex, D3 comprises ∼ 100 m scale folds with NW dipping axial planes, cut by SE dipping dilational granite sheets. In the Rootshorga Complex D3 is characterised by syntectonic granite veins with extensional and compressional displacements with top-to-the SE shear. Discordancies are consistent with low angle thrust planes at Fuglefjellet and Kvikjolen with probable repetition of carbonate layers.
Zircon ages of the granitic sheets are 490-500 Ma. Strontium and Nd isotopic signatures of the granitic sheets intruded into all complexes are consistent with melting of Jutulrora Complex crust with Archaean and Mesoproterozoic xenocrysts in some samples. Top-to-SE shear zones displace pegmatites with an inferred age of 520 Ma and are syntectonic with layer parallel ∼ 490 Ma granite sheets.
P-T-t studies from the Rootshorga Complex yield isothermal decompression paths at ∼ 800-900 °C with decompression from ∼ 1.4 GPa at ∼ 570 Ma to ∼ 700 °C and ∼ 0.7 GPa at ∼ 500 Ma whereas P-T-t estimates from the Jutulrora Complex are ∼ 600-700 °C and <∼ 0.8 GPa at ∼ 500 Ma with a path consistent with crustal loading. The Rootshorga and Fuglefjellet Complex are inferred to comprise a mega-nappe, emplaced during the Kuunga Orogeny ∼ 500 Ma ago, over the footwall Jutulrora Complex. Aerogravity, satellite gravity and seismic tomography data reflecting unusually thick crust are consistent with this interpretation.
Studies of chemical compositions and grain size of garnet in a quartzo-feldspathic gneiss from the Lützow-Holm Complex at Skallen, East Antarctica, form the basis of an interpretation of timing of garnet homogenization. The gneiss contains only garnet as mafic minerals except sporadic biotite inclusions in the garnet. The garnet represents approximately constant Mg/(Fe + Mg) within a grain. The values of small grains are higher than those of large grains, while the grossular content at their periphery is similar. The grain size shows lognormal distribution. These features indicate that the grains were mutually not in equilibrium and were poorly annealed, which implies insufficient grain-boundary diffusion. The consideration based on a qualitative phase diagram suggests that the Mg/(Fe + Mg) of the small grains and that of the large grains were homogenized before and after cessation of the grain-boundary diffusion, respectively. The small grains preserve the composition that was in equilibrium with biotite. In contrast, the large grains represent their bulk compositions of chemical zoning profiles that were formed during fractional growth. The homogeneous interior of large grains is, therefore, not always appropriate for geothermometry. The highest Mg/(Fe + Mg) among small grains would be suitable when the grain-boundary diffusion ceased before homogenization. The erroneous use of large grains causes depression of the estimated temperature by 60-80 °C, even though the compositional difference corresponds to only ∼ 10 °C in the phase diagram.
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.
Fumiko HIGASHINO, Tetsuo KAWAKAMI, Tatsuro ADACHI, Masaoki UNO
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, Δ11BTur-Krn (= δ11BTur − δ11BKrn), 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.
Yoshikuni HIROI, Tomokazu HOKADA, Tatsuro ADACHI, Kazuyuki SHIRAISHI, Yoichi MOTOYOSHI, Edward S. GREW
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.