Fluid inclusions in an osumilite-bearing granulite from Bunt Island in the Archean Napier Complex, East Antarctica: implications for a decompressional P-T path?

We report high-density CO_2-rich fluid inclusions in garnet,orthopyroxene,and quartz from an osumilite-bearing aluminous granulite from Bunt Island of the Archean Napier Complex,East Antarctica.The melting temperatures of fluids lie in the range of -56.8 to -57.8゜C being close to the triple point for pure CO_2 (-56.6゜C ). Homogenization of the CO_2,-rich fluids into the liquid phase occurs at temperatures in the range of -35.4 to 24.7゜C. This translates into CO_2 densities in the range of 0.788-1.084g/cm^3 .The estimated CO_2 ,isochore for high-density inclusions in garnet intersects the P-T trajectory of Bunt Island at around 10 kbar at 1050゜C, which corresponds to the peak metamorphic conditions of the region derived from mineral phase equilibria.We therefore infer that CO_2 ,was the dominant fluid species present during the ultrahigh-temperature metamorphism in Bunt Island.CO_2 inclusions with lower density occurring in quartz and garnet provide isochores that intersect the P-T path at <7 kbar and <950゜C indicating density reversal of originally high-density inclusions along a decompressional exhumation path of the ultrahigh-temperature rocks in the Bunt Island

Geology of ultrahigh-temperature metamorphic rocks from Tonagh Island in the Napier Complex, East Antarctica

The summer party of the 39th Japanese Antarctic Research Expedition (1997-1998) carried out geological field work on Tonagh Island, located at the southern-end of Amundsen Bay, northern Enderby Land, which belongs to the central Napier Complex and shows part of the highest-grade metamorphic region in the Napier Complex. The island is underlain by various kinds of ultrahigh-temperature (UHT) metamorphic rocks and subordinate amounts of two types of unmetamorphosed intrusive rocks (alkali-dolerite and granitic pegmatite). UHT-metamorphic rocks are subdivided into five lithologic units (Units I to V) owing to their lithologies and geological structures from north to south, bounded by thrust-shear zones, accompanied with remarkable anhydrous mylonite and later pseudotachylite-cataclasite. The distinctive metamorphosed mafic dikes with tholeiitic composition are also found along the shear zone, characteristically at the boundary between Units II and III. The unmetamorphosed alkali-dolerite dikes, which are designated Amundsen dikes, cut across not only the sequence of metamorphic rocks but also the unit boundary shear zone and metamorphosed mafic dike. A geological perspective of the metamorphic rocks from Tonagh Island is generally classified into eight types on the regional map scale such as 1) orthopyroxene-bearing quartzofeldspathic gneiss, 2) garnet-bearing quartzofeldspathic gneiss, 3) two pyroxene-bearing mafic granulite, 4) garnet-orthopyroxene gneiss and granulite, 5) magnetite-quartz gneiss, 6) metamorphosed ultramafic rocks, 7) layered gneiss 1 (composed mainly of mafic gneiss and orthopyroxene-bearing quartzofeldspathic gneiss), 8) layered gneiss 2 (composed mainly of mafic gneiss and garnet-bearing quartzofeldspathic gneiss) with subordinate meta-impure quartzite, aluminous granulite, and calc-silicate granulite. The orthopyroxene-bearing quartzofeldspathic charnockitic gneiss and garnet-bearing quartzofeldspathic gneiss are the main constituents of Tonagh Island. Unit I has a peculiarity of predominance of layered gneisses showing thin alternation (centimeters to several meters in thickness) of orthopyroxene- and garnet-bearing quartzofeldspathic gneisses, two-pyroxene mafic granulite, garnet-orthopyroxene gneiss and granulite, sapphirine-bearing aluminous gneiss, garnet-sillimanite gneiss, leucocratic quartzofeld-spathic gneiss, magnetite-quartz gneiss, meta-quartzite and metamorphosed ultramafic rocks (pyroxenite and Iherzolite). Units II and III are characterized by widespread distributions of two pyroxene-bearing mafic granulite (gneiss) and garnet-orthopyroxene gneiss at the upper structural level, although layered gneisses dominate at the lower structural level. In field appearance Unit II and Unit III have nearly the same lithology and may be considered repetition due to thrusting. The magnetite-quartz gneiss occurs only in Units I, II and III characteristically. Unit IV is underlain by garnet- and orthopyroxene-bearing quartzofeldspathic gneisses and layered gneisses, which look very close to the lower structural level of Units II and III. On the other hand, constituents of Unit V are mainly orthopyroxene- and garnet-bearing quartzofeldspathic gneisses with subordinate layered gneiss 1 and traces of aluminous, mafic and ultramafic granulites. Therefore the most effective tectonic boundary may be the shear zone among Units IV and V. In this paper the regional geology of Tonagh Island is described in detail, with a brief description of modes of occurrence and petrographic features of UHT metamorphic rocks and unmetamorphosed intrusive rocks

LREE-enriched mafic gneiss and meta-ultramafic rock from Tonagh Island in the Napier Complex, East Antarctica

Mafic gneisses and meta-ultramafic rocks are exposed on Tonagh Island These rocks are high-Mg composition and have undergone ultrahigh-temperature metamorphism Most of the mafic gneisses and the meta-ultramafic rocks occur as intercalated layers or lenses within the quartzo-feldspathic gneisses Some of the mafic gneisses and the meta-ultramafic rocks locally cut the layers or foliations of neighboring quartzo-feldspathic gneisses, suggesting that these mafic gneisses and meta-ultramafic rocks were originally intrusive rocks Major and trace element compositions of the mafic gneisses and the meta-ultramafic rocks resemble those of komatiitic basalt to komatiite from the Archaean greenstone belt Light REE-enriched and flat patterns are recognized in the mafic gneisses and meta-ultramafic rocks although the REE pattern of the deformed sample is convex upward In the Mt Riiser-Larsen region situated 40 km northeast from Tonagh Island, it has been found in a previous study that the LREE-enriched mafic gneisses and meta-ultramafic rocks have been derived from komatiitic rocks whose chemical trends show magmatic variations Therefore, the LREE-enriched patterns of mafic gneisses and ultramafic rocks on Tonagh Island and in the Mt Riiser-Larsen region could be inherited from the chemical nature of original rocks prior to undergoing ultrahigh-temperature metamorphism The LREE-enriched pattern cannot be explained by interaction between an LREE-flat parental magma and an LREE-enriched crustal material m terms of geochemical constraints Hence, the LREE-enriched and flat types of the mafic gneisses and the meta-ultramafic rocks on Tonagh Island would have been derived from different source mantles with respect to LREE signature

Petrology and metamorphism of sapphirine-bearing aluminous gneisses from Tonagh Island in the Napier Complex, East Antarctica

A variety of Mg-rich silica-undersaturated aluminous gneisses containing sapphirine, spinel and corundum occur within the ultrahigh-temperature (UHT) metamorphic sequence at Tonagh Island in the Napier Complex, East Antarctica. They occur as blocks or pods in quartzo-feldspathic gneisses or mafic granulite, or as thin layers around ultramafic rocks. The modes of occurrence, constituent minerals and mineral textures of these aluminous gneisses are different from each other, suggesting that they are derived from different protoliths or formation processes. Field occurrences suggest that some of the aluminous gneisses on Tonagh Island may not be simple pelitic precursors but were formed through processes associated with partial melting or metasomatism. Various reaction textures and compositional zoning in constituent minerals, which reflect retrograde metamorphism, are commonly observed in these rocks. Garnet-orthopyroxene geothermobarometry using the chemical compositions of the cores of garnet and orthopyroxene yields slightly lower temperatures (800-1000℃ at 0.5-1.0 Gpa) than the thermal climax (1100℃). Pressure condition of 0.8-1.1 Gpa at the thermal peak (1030-1100℃) is estimated from the garnet-orthopyroxene geobarometry

Metamorphic reactions and preliminary P-T estimates of ultrahigh-temperature mafic granulite from Tonagh Island in the Napier Complex, East Antarctica

Metamorphic reactions and P-T conditions of mafic granulites from Tonagh Island in the Archean Napier Complex were examined to evaluate the nature of mafic granulite which has suffered ultrahigh-temperature (UHT) metamorphism. Although a peak mineral assemblage is preserved in plagioclase-orthopyroxene-clinopyroxene mafic granulites, initial mineral chemistry was completely reset by retrograde metamorphism. Evidence of UHT metamorphism was therefore inferred using lamella-bearing pyroxenes as 900 to 980℃. Hornblende-quartz, biotite-quartz, and garnet-hornblende-quartz coronas around ortho- and clinopyroxenes in two-pyroxene mafic granulites are typical evidence of retrograde metamorphism in amphibolite-facies conditions. The textures suggest that the retrograde event took place in the presence of H_2O-bearing fluid. Garnet-quartz corona after pyroxenes is an another predominant texture of retrograde metamorphism throughout the studied area. As the corona is characterized by anhydrous mineral assemblages, we suggest that retrograde infiltration of H_2O-bearing fluid is a local event. The corona texture suggests that retrograde metamorphism has taken place almost isobarically, because a univariant line of the corona-forming reaction shows a gentle slope in P-T space. The isobaric cooling temperature (650-700℃ at 6.5-8.0 kbar) is, however, significantly lower than the previously reported cooling temperature of approximately 950℃ subsequent to UHT metamorphism. Mafic granulites therefore preserve the second isobaric cooling event, which is distinguished from high-temperature isobaric cooling. The mafic granulites were subsequently intruded by mafic dykes which were also metamorphosed. P-T conditions recorded in the rocks are 630-670℃ at 6.4-8.2 kbar, consistent with the retrograde event of host mafic granulites. The event marks the last stage of regional metamorphism recorded in mafic granulites

Deformation of ultrahigh-temperature metamorphic rocks from Tonagh Island in the Napier Complex, East Antarctica

The deformation history of ultrahigh-temperature metamorphic rocks from Tonagh Island, Napier Complex, East Antarctica is divided into nine stages, namely D_1 to D_9. The D_1 structure would have been formed under non- or weakly-deformational condition during thermal peak of prograde metamorphism. The D_2-D_6 structures would have been produced under retrograde granulite facies conditions. Subsequently D_7-D_9 brittle faulting modified the structures in part. The D_1 to D_8 deformation corresponds to first to third tectonometamorphic episodes, previously reported, of the whole Napier Complex before the intrusion of the Amundsen dyke. The structural geology of the Island is mainly characterized by NE-SW trending D_2 foliation (S_2) with WNW plunging mineral lineation, WNW-ESE to E-W trending D_5 folds and NE-SW to E-W trending D_6 mylonite zones. S_2 is the main foliation of this area, but was highly folded and faulted during the D_5-D_6 stages. The S_2 and S_2-parallel D_3 shear zones are the products of layer-parallel shearing due to a top-to-the-SE (dextral-reverse) displacement. The D_3 and D_6 fault rocks show that seismic faulting and plastic deformation alternated both during the D_3 stage and during the D_6 stage under retrograde granulite facies conditions. Multiphase faulting in different shear senses occurred during the D_6 stage