Yasuhito Osanai

Sensitive high-resolution ion microprobe U-Pb dating of prograde and retrograde ultrahigh-temperature metamorphism as exemplified by Sri Lankan granulites

Ultrahigh-temperature (UHT) granulites of the central Highland Complex, Sri Lanka, underwent some of the highest known peak temperatures of crustal metamorphism. Zircon and monazite U-Pb systems in granulites near Kandy, the highest grade region (∼1050 °C; 0.9 GPa), preserve both a record of the timing of prograde and retrograde phases of UHT metamorphism and evidence for the ages of older protolith components. Zircon grains from a quartz-saturated granulite containing relics of the peak UHT assemblage have remnant detrital cores with dates of ca. 2.5–0.83 Ga. Date clusters of ca. 1.7 and 1.04–0.83 Ga record episodes of zircon growth in the source region of the protolith sediment. Two generations of overgrowths with contrasting Th/U record metamorphic zircon growth at 569 ± 5 and 551 ± 7 Ma, probably in the absence and presence of monazite, respectively. The age of coexisting metamorphic monazite (547 ± 7 Ma) is indistinguishable from that of the younger, low-Th/U zircon overgrowths. Zircon from a quartz-undersaturated monazite-absent UHT granulite with a mainly retrograde assemblage is mostly metamorphic (551 ± 5 Ma). The ca. 570 Ma zircon overgrowths in the quartz-saturated granulite probably record partial melting just before or at the metamorphic peak. The ca. 550 Ma zircon in both rocks, and the ca. 550 Ma monazite in the quartz-saturated sample, record post-peak isothermal decompression. A possible model for this pressure-temperature-time evolution is ultrahot collisional orogeny during the assembly of Gondwana, locally superheated by basaltic underplating, followed by fast extensional exhumation.

Evolution of the Hidaka metamorphic belt, northern Japan

The Hidaka metamorphic belt is a tilted island-arc assembly of crustal layers developed during early to middle Tertiary age. The P-T estimates of metamorphic rocks in the amphibolite and granulite facies reveal a relatively high geothermal gradient (40°C km−1), possibly caused by the large amount of gabbroic intrusions, at an early evolutional stage of the crust. The crust was subsequently displaced subhorizontally from north to south, giving rise to detachment of lower to upper crustal layers from the lowest crust, and to the formation of a duplex of crustal metamorphic layers. This was followed by dextral transpression in which the crustal layers were uplifted and tilted steeply eastward. Geological setting In Hokkaido, northern Japan, there are two Cretaceous arc-trench systems; one occupies the western part of Hokkaido and the other occupies the eastern part (Fig. 1). The western system consists of the Oshima granitic-volcanic terrane and the Sorachi-Yezo subduction-accretionary terrane on the eastern side where the Kamuikotan serpentinite melange with high-P/T schists occurs. Westward subduction occurred during early to late Cretaceous time (Okada 1982, Kiminami & Kontani 1983, Ishizuka et al. 1983). The eastern system is composed of the Tokoro subduction-accretion terrane which is situated on the western side of the Nemuro volcanic terrane. This northeast-ward subduction was active in the late Cretaceous (Sakakibara 1986, Sakakibara et al. 1986). Late Cretaceous accretionary complexes of both arc-trench systems are juxtaposed in the central part of Hokkaido where the Hidaka metamorphic belt occurs (Fig. 1). A collision between the two arc-trench

Extreme Crustal Metamorphism during a Neoproterozoic Event in Sri Lanka: A Study of Dry Mafic Granulites

Garnet‐clinopyroxene‐quartz granulites of the central Highland Complex of Sri Lanka preserve textural and compositional features indicative of high‐pressure, ultrahigh‐temperature (HP‐UHT) crustal metamorphism and multistage retrogression. Grains of the peak metamorphic assemblage, garnet‐clinopyroxene‐quartz, are commonly separated and embayed by late orthopyroxene‐plagioclase symplectites; however, in some domains, rare grain‐to‐grain associations of the peak assemblages are still preserved. Thermodynamic modeling in the CaO‐Na2O‐K2O‐FeO‐MgO‐Al2O3‐SiO2 system indicates peak metamorphic conditions of 12.5 kbar at 925°C. The temperature estimates using garnet and clinopyroxene core compositions are in the range 844°–982°C, in agreement with the thermodynamic modeling. In conclusion, the textural, geochemical, and thermodynamic modeling and thermobarometric data indicate a multistage decompression after HP‐UHT metamorphism. U‐Pb zircon (laser ablation–inductively coupled plasma mass spectrometry) ages represent the timing of the peak metamorphism at ca. 580 Ma. A Sm‐Nd internal isochron from mineral phases (garnet, clinopyroxene, orthopyroxene, and felsic fraction) and from a whole rock yields an age of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

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

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Prograde and anatectic reactions in the deep arc crust exposed in the Hidaka metamorphic belt, Hokkaido, Japan

\end{document} Ma interpreted as the time of isothermal decompression (retrogression). Our results from the central Highland Complex of Sri Lanka provide important constraints on the Neoproterozoic orogeny associated with the assembly of Gondwana.

Multiple breakdown and chemical equilibrium of silicic clinopyroxene under extreme metamorphic conditions in the Kontum Massif, central Vietnam

Abstract This paper deals with the anatectic migmatites and leucogranites developed in a high- T , low- P metamorphic terrane, Kyushu, Japan. The migmatites in this terrane are divided into metatexites and diatexites and they occur at and above the garnet-cordierite grade. Layers, lenses and pods of leucogranite (a few centimeters up to 6 m in thickness) are wide-spread in the metamorphic terrane and are most abundant, not in the highest grade zone, but in intermediate grade zones, in which migmatites do not develop. The migmatite leucosomes are typically depleted in orthoclase component, while many leucogranites contain abundant orthoclase. It is suggested that the leucogranites are crystal cumulates precipitated from peraluminous melts, that had been generated and ascended from deeper levels in the crust and that the zone of metatectic migmatites represents both the source region and passage region for the anatectic melts. It is thus clear that the anatectic melts had segregated and ascended on the order of 5 km in the crust of this region. Diatexites on the other hand represent solid-melt mixtures at more advanced stages of partial melting, dominated by biotite breakdown. Implications of the observations made in the Higo terrane are discussed in reference to an evolutionary scheme for crustal anatexis and granite magma genesis.

Empirical thermometer of TiO2 in quartz for ultrahigh-temperature granulites of East Antarctica

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.

Carbonic fluids in ultrahigh-temperature metamorphism: Evidence from Raman spectroscopic study of fluid inclusions in granulites from the Napier complex, East Antarctica

Abstract Pargasites (F/(F+Cl+OH) ratio ( X F ) of up to 0.48) from Tonagh Island in Enderby Land, East Antarctica are closely associated with typical high-grade minerals such as orthopyroxene in quartzo-feldspathic, mafic, and ultramafic granulites, and is regarded as a stable mineral at the peak metamorphic conditions (>1100 °C) calculated for the ultrahigh-temperature Archean Napier Complex. Although experimental investigations have suggested that the upper thermal stability limit of F-free pargasite is below 1050 °C, thermodynamic calculations for the present pargasite+quartz assemblage indicate that the thermal stability limit of pargasite with X F =0.5 is about 150 °C higher than that of the hydroxyl end member. Fluorine substitution in the pargasite therefore allowed the mineral to survive the ultrahigh-temperature metamorphism at Tonagh Island. A positive correlation between the F content of pargasite and coexisting biotite indicates that the minerals approach chemical equilibrium in terms of F–OH distribution. Although the fluorine composition of pargasites ( X F =0.12–0.48) and bulk rock (300–2500 ppm) varies widely, the log( f H 2 O / f HF ) values calculated for these rocks are relatively constant (3.2–3.7), which is consistent with infiltration of an F-bearing fluid during prograde metamorphism. The infiltration of such a fluid is also supported by the higher bulk F content of most of the analyzed samples compared to those of continental and oceanic basaltic rocks, that is, F had been added from an external source. A positive correlation between bulk MgO and F content suggests that F may have been selectively trapped in high- X Mg pargasite in MgO-rich rocks.