Yoichi Motoyoshi

F-rich phlogopite stability in ultra-high-temperature metapelites from the Napier Complex, East Antarctica

Abstract Fluorine-rich phlogopite [F content up to ~8 wt%; F/(F + OH) ~0.9] in ultra-high-temperature metapelitic granulites from the Napier Complex, East Antarctica is associated with aluminous orthopyroxene, osumilite, sapphirine, garnet, and quartz. Textural relationships imply that some of the phlogopite is of primary origin and stable under ultra-high-temperature conditions. This is in accord with recent experimental evidence on the stability of F-rich phlogopite. Because the F-rich phlogopite also occurs as rounded inclusions in aluminous orthopyroxene (Al2O3 up to 12.8 wt%), sapphirine, osumilite, and garnet, it is inferred that the ultra-high-temperature mineral assemblages, which includes these minerals formed during prograde partial melting reactions at the expense of phlogopite, at a depth of less than 30 km. Thus the coarse-grained peak metamorphic assemblages formed below 9 kbar, and there is no evidence the rocks underwent any significant degree of decompression during or soon after peak metamorphic conditions. The phlogopite breakdown reactions we suggest on the basis of textural arguments differ from those postulated from experiments on F-free systems.

Geodynamic Evolution of East Antarctica: A Key to the East–West Gondwana Connection

Geological correlations of East Antarctica with adjoining continents have been puzzling geologists ever since the concept of a Gondwana supercontinent surfaced. Despite the paucity of outcrops because of ice cover, difficulty of access and extreme weather, the past 50 years of Japanese Antarctic Research Expeditions (JARE) has successfully revealed vital elements of the geology of East Antarctica. This volume presents reviews and new research from localities across East Antarctica, especially from Dronning Maud Land to Enderby Land, where the geological record preserves a history that spans the Archaean and Proterozoic. The reviews include extensive bibliographies of results obtained by geologists who participated in the JARE. Comprehensive geological, petrological and geochemical studies, form a platform for future research on the formation and dispersion of Rodinia in the Mesoproterozoic and subsequent assembly of Gondwana in the Neoproterozoic to Early Palaeozoic.

Sapphirine+quartz association in garnet: implication for ultrahigh-temperature metamorphism at Rundvågshetta, Lützow-Holm Complex, East Antarctica

Abstract We report the occurrence of sapphirine+quartz association within garnet porphyroblast in the garnet–orthopyroxene–sillimanite granulite (Grt–Opx–Sill granulite) from Rundvågshetta in the Lützow-Holm Complex, East Antarctica. The granulites in the study area show a characteristic mineral assemblage consisting of orthopyroxene+sillimanite+quartz. The presence of sapphirine and quartz inclusions within garnet in the sapphirine-bearing Grt–Opx–Sill granulite suggests that metamorphic conditions changed from the stability field of orthopyroxene+sillimanite+quartz to that of sapphirine+quartz during the garnet growth. Peak metamorphic temperature conditions of about 1000–1100 °C are obtained by ternary feldspar thermometry for these granulites. Similar temperatures were also estimated from the Al-in-orthopyroxene geothermometer. The granulites are also characterized by coarse-grained garnet, being partly surrounded by a fine-grained symplectite composed of orthopyroxene and cordierite, whereas sapphirine+cordierite symplectitic intergrowth occurs in the matrix. These textures imply that the area underwent isothermal decompression subsequent to ultrahigh-temperature metamorphism. The sapphirine-bearing Grt–Opx–Sill granulite is likely to be the restitic product of partial melting and shows signs of segregation and movement of melt.

Geodynamic evolution of Mt. Riiser-Larsen, Napier Complex, East Antarctica, with reference to the UHT mineral associations and their reaction relations

Abstract Mt. Riiser-Larsen is the largest outcrop in the Archaean–early Proterozoic Napier Complex, East Antarctica. The area is structurally divided into the Main and the Western Blocks by the subvertical Riiser-Larsen Main Shear Zone (RLMSZ) of about 200 m width composed of mylonite and pseudotachylite. Mineral parageneses including sapphirine+quartz and osumilite, diagnostic of ultrahigh-temperature (UHT) metamorphism, are found in Mg-rich aluminous, siliceous and quartzo-feldspathic gneiss layers in both the Main and the Western Blocks of the Mt. Riiser-Larsen area. Some of the sapphirine–quartz associations are accompanied by retrograde reaction textures, which include growth of cordierite and/or garnet between sapphirine and quartz in the Main Block, and of orthopyroxene+sillimanite in the Western Block. These textures indicate the reaction 1 and 2 in the Main Block and 3 in the Western Block. Phase equilibria and P–T pseudosections for sapphirine+quartz-bearing associations suggest that these three reactions took place during a temperature drop from 1100 °C to 1000 °C at pressures of 0.6–0.8 GPa in the Main Block and 0.8–0.9 GPa in the Western Block. The geological structure and distribution of the UHT rocks provide an insight into the vertical extent of the>1000 °C UHT metamorphic zone: a minimum thickness of 4–5 km of the UHT-metamorphosed layers, which become deeper towards the west in the Main Block. The Western Block represents a c. 0.1–0.3 GPa (c. 3–10 km) deeper structural level than the Main Block. In addition to the extent of the horizontal distribution of UHT metamorphism in the Napier Complex, our results on the vertical component provide new constraints for modelling the heat source and tectonic process of the unusually high-temperature regional metamorphism in the late Archaean–early Proterozoic. Electron microprobe monazite U–Th–Pb dating for hydrated and mylonitized sapphirine–quartz gneiss gave a wide spectrum of monazite age distribution between 2300 and 800 Ma, suggesting the tectonic uplift and juxtaposition of the two blocks in the Mt. Riiser-Larsen area later than the mid–late Proterozoic.

Pre-metamorphic carbon, oxygen and strontium isotope signature of high-grade marbles from the lützow-holm complex, east antarctica: Apparent age constraints of carbonate deposition

Abstract C, O and Sr isotope geochemistry of high-grade marbles from the Lützow-Holm Complex, East Antarctica, has given clues on the depositional ages and post-depositional alterations. Dolomitic and calcitic marbles occur as thin layers with varying thickness (up to 100 m) in several outcrops in eastern Dronning Maud Land, most of which underwent post-depositional geochemical alterations. In particular, the Sr and O isotope alterations are extensive, with 87Sr/86Sr(550 Ma) ratios as high as 0.758 and δ18O values as low as −5‰. These data suggest that multiple stages of fluid–rock interaction processes during diagenesis, prograde to peak and retrograde metamorphic events have altered the depositional isotopic signatures. However, some of the marble layers, exceptionally, preserve pre-metamorphic geochemical characteristics, such as low Sr isotope ratios, high δ18O and δ13C values, and well-equilibrated unaltered trace and rare earth element patterns. Lowest 87Sr/86Sr isotopic ratios of 0.7066 and 0.7053 with high δ13C and δ18O values suggest an apparent age of deposition around 730–830 Ma, although total geochemical resetting of carbonates by seawater of this age cannot be ruled out. The apparent depositional ages are consistent with carbonate deposition in the ‘Mozambique Ocean’ that separated East and West Gondwana.

Kornerupine sensu stricto associated with mafic and ultramafic rocks in the Lützow-Holm Complex at Akarui Point, East Antarctica: What is the source of boron?

Abstract Kornerupine, (□, Mg, Fe)(Al, Mg, Fe)9(Si, Al, B)5O21(OH, F), is known from only five mafic or ultramafic settings worldwide (of the >70 localities overall). We report a sixth occurrence from Akarui Point in the Lützow-Holm Complex, East Antarctica, where two ruby corundum (0.22–0.34 wt% Cr2O3)–plagioclase lenses are found at the same structural level as boudinaged ultrabasic rocks in hornblende gneiss and amphibolite. Ion microprobe analyses of kornerupine give 13–59 ppm Be, 181–302 ppm Li, and 5466–6812 ppm B, corresponding to 0.38–0.47 B per 21.5 O; associated sapphirine also contains B (588–889 ppm). Peak metamorphic conditions are estimated to be 770–790 °C and 7.7–9.8 kbar. Kornerupine encloses tourmaline and plagioclase, which suggests the prograde reaction tourmaline (1) + plagioclase (>An34)+ sapphirine±spinel→kornerupine+corundum (ruby)+plagioclase (<An82)±(fluid or melt). Alternatively, kornerupine and tourmaline could have formed sequentially under nearly constant P–T conditions during the infiltration of fluid that was originally B-bearing, but then progressively lost Na (or gained Ca) and B through reaction with mafic rocks. Kornerupine later reacted with H2O–CO2 fluid in cracks at P–T conditions in the andalusite stability field: kornerupine+plagioclase+(Na, K, ± Si in fluid)→tourmaline+biotite+corundum (sapphire)± magnesite±andalusite+(Ca in fluid). Secondary tourmaline differs from the included tourmaline in containing less Ti and having a higher Na/(Na+Ca+K) ratio. There are two possible scenarios for introducing B into the lenses: (1) infiltration of boron-bearing aqueous fluids released by prograde breakdown of muscovite in associated metasedimentary rocks; (2) hydrothermal alteration of mafic and ultramafic rocks by seawater prior to peak metamorphism. The latter scenario is consistent with an earlier suggestion that Akarui Point could be part of an ophiolite complex developed between the Yamato–Belgica and Rayner complexes.

Experimental Constraints on the Thermal Peak of a Granulite from McIntyre Island, Enderby Land, East Antarctica

Abstract High-pressure experiments have been carried out at 11-22 kbar and 900-1200°C using a piston cylinder apparatus to constrain the thermal peak condition of a granulite characterized by the mineral assemblage of orthopyroxene+sillimanite+quartz from McIntyre Island, Enderby Land, East Antarctica. The bulk composition of the starting material is 85 wt.% McIntyre granulite+15 wt.% sillimanite. At 11 kbar, orthopyroxene, sillimanite and quartz are stable below 1000°C. At 1050°C sillimanite does not appear, and sapphirine coexists with orthopyroxene and quartz. These experimental results indicate that the McIntyre granulite has undergone the ultra high-temperature metamorphism at 1000-1050°C represented by the diagnostic mineral assemblage of orthopyroxene, sillimanite and quartz.

Electron Microprobe (EMP) Dating on Monazite from Forefinger Point Granulites, East Antarctica: Implication for Pan-African Overprint

Electron microprobe (EMP) dating on monazite in granulite-facies rocks from Forefinger Point, East Antarctica, yielded dominant ages of ∼500 Ma on matrix monazites. They are associated with secondary cordierite, biotite and sapphirine, formed during nearly isothermal decompression after the high P-T assemblages involving garnet, orthopyroxene and sillimanite. Older ages around 750–1000 Ma are detected in monazite cores and in monazite inclusions in garnet porphyroblast. Combining the available age data and the reaction textures, it becomes evident that the Forefinger Point granulites have been overprinted by a granulite-facies decompressional event of Pan-African age. Moreover, EMP monazite dating imply that the Forefinger Point granulites have experienced at least two stages of metamorphic evolution.

Origin of xenocrystic garnet and kyanite in clinopyroxene-hornblende-bearing adakitic meta-tonalites from Cape Hinode, Prince Olav Coast, East Antarctica

Abstract Xenocrystic garnet and kyanite, in addition to clinopyroxene and rare orthopyroxene, are newly found to occur in middle Proterozoic slightly metamorphosed adakitic trondhjemites and tonalites (meta-tonalites) at Cape Hinode on the eastern Prince Olav Coast in the latest Proterozoic–Early Palaeozoic Lützow-Holm Complex, East Antarctica. Textural and compositional features of garnet and kyanite suggest that these minerals formed most probably as restite phases of partial melting of mid-ocean ridge basalt (MORB) between 15 and 20 kbar pressure, and were entrained by the tonalitic magmas, which underwent fractional crystallization upon ascent to form cumulates that were also entrained and metamorphosed to basic–intermediate granulite blocks. Available geochronological data for the meta-tonalites indicate that all these events including MORB formation took place in the middle Proterozoic. The meta-tonalites and associated basic, calc-silicate, and pelitic rocks were emplaced as an allochthonous block in the Lützow-Holm Complex at the waning stage of its main regional metamorphism, most probably as a part of the final amalgamation of East and West Gondwana into the Gondwana supercontinent.

Post-peak (<530 Ma) thermal history of Lützow-Holm Complex, East Antarctica, based on Rb–Sr and Sm–Nd mineral chronology

Abstract Rb–Sr and Sm–Nd mineral dating of metamorphic rocks from Skallen, Skallevikshalsen and Rundvågshetta, in the southwestern part of the Lützow-Holm Complex, Dronning Maud Land, assists in constructing a thermal history after peak metamorphism. The results fall into two groups: (1) a record of regional cooling after peak metamorphism (524–488 Ma); (2) local resetting 50–80 Ma after peak metamorphism (474–446 Ma). This grouping is consistently observed in published ages from various localities in the Lützow-Holm Complex. A Sm–Nd age of 524 Ma is indistinguishable from published zircon and monazite ages. Ages of 511 and 488 Ma are related to cooling after peak metamorphism. The younger age group overlaps with ages of post-metamorphic magmatism and related hydrothermal activity reported from localities throughout East Antarctica. This intracontinental, post-orogenic igneous activity continued after the tectonic assembly of Gondwana.