Masaaki Owada

Spatial variations of Sr and Nd isotope ratios of Cretaceous-Paleogene granitoid rocks, Southwest Japan Arc

Cretaceous-Paleogene granitoid rocks and contemporaneous volcanic rocks are widely distributed in the Inner Zone of Southwest Japan. This intense intermediate to felsic magmatism is considered to have taken place on the eastern margin of the Eurasian Continent, before the Southwest Japan Arc drifted away from the continent in the middle Miocene, resulting in the opening of the Japan Sea. The granitoid rocks show regional variations in terms of their radiometric age, petrography, Sr, Nd and O isotope ratios. Based on Sr and Nd isotope ratios, granitoid rocks can be divided into three zones (South, Transitional and North) between the Median Tectonic Line and the Japan Sea. Granitoid rocks and associated gabbros of the North Zone have low initial Sr isotope ratios (0.7048 to 0.7068) and high initial εNd values (+3 to-2.2), whereas granitoid rocks and gabbros from the South Zone have high initial Sr isotope ratios (0.7070 to 0.7088) and low initial εNd values (-3.0to-8.0). Most granitoid rocks from the Transitional Zone have Sr and Nd isotope ratios that lie between those of the North and South Zones, although there is some overlap. Contamination of magmas by upper crust cannot explain this geographical variation in Sr and Nd isotopes. Instead, the regional variation is attributed to compositionally different, magma sources (probably upper mantle and lower crust), beneath the North and South Zones. This is supported by the Sr and Nd isotopic ratios of upper mantle and lower crustal xenoliths included in Cenozoic volcanic rocks in the North and South Zones. These ratios are similar to those of the granitoid rocks in the respective zones. It is suggested that a micro-continent or island arc consisting of continental crust was underthrust beneath the South Zone before or during the Cretaceous, resulting in compositionally distinct sources for granitoid rocks of the North and South Zones. The large variation observed in Sr and Nd isotope ratios for Transitional Zone granitoid rocks is explained by variable proportions of the two different crustal and upper mantle components.

High fluorine pargasites in ultrahigh temperature granulites from Tonagh Island in the Archean 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.

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

Abstract Clinopyroxene in ultrahigh-temperature mafic granulites from the Kontum Massif in central Vietnam records multiple metamorphic stages, manifested as exsolution textures (quartz rods and orthopyroxene + hornblende + plagioclase needles), and as symplectitic intergrowths (involving clinopyroxene + plagioclase). These textures suggest a metamorphic evolution characterized by decompression and subsequent cooling from eclogite-facies to amphibolite-facies conditions through ultrahigh-temperature conditions. Quartz rods in clinopyroxene and clinopyroxene + plagioclase symplectites were formed under eclogite conditions prior to ultrahigh-temperature metamorphism. The orthopyroxene + hornblende + plagioclase needles in clinopyroxene are regarded as cooling products after ultrahightemperature metamorphism. Recalculated compositions of precursor clinopyroxene show supersilicic composition. During the metamorphic evolution, the chemical composition varies from silicic (Ca- Eskola-rich) via sodic (Jadeite-rich) to aluminous (Ca-Tschermak-rich) compositions. Presence of supersilicic clinopyroxene suggests that the granulite decompressed from possible ultrahigh-pressure conditions (ca. 800-900 °C at 2-3 GPa) preceding the ultrahigh-temperature stage (1050 °C at 1.3 GPa), which provide strong constraints on the tectonic evolution of the Indochina region, and it also provides insights on crustal exhumation at a continental collision zone. Another significant aspect of this study is that the breakdown textures of clinopyroxene and its chemical variations may provide important information in establishing pre- and post-peak evolution, especially for extremely hightemperature or high-pressure metamorphic rocks.

SHRIMP Zircon U-Pb Dating of Sapphirine-Bearing Granulite and Biotite-Hornblende Gneiss in the Schirmacher Hills, East Antarctica: Implications for Neoproterozoic Ultrahigh-Temperature Metamorphism Predating the Assembly of Gondwana

We applied SHRIMP zircon U-Pb age dating to ultrahigh-temperature (UHT) sapphirine-bearing orthopyroxene garnet (SOG) granulite and biotite-hornblende (Bt-Hbl) gneiss in the Schirmacher Hills, East Antarctica. In the Bt-Hbl gneiss, concordant ages of and Ma were obtained from zircon overgrowth rims and zircon cores, with oscillatory and irregular zoning, respectively. The zircon overgrowth rims ( Ma) with low Th/U ratios from the Bt-Hbl gneiss are interpreted as having a metamorphic origin. Oscillatory-zoned and/or irregularly zoned zircon cores may have crystallized during an igneous event at Ma; 800-Ma igneous events have not previously been recognized in central Dronning Maud Land (DML) inland nunatak. Zircons in the SOG granulite yielded a concordant age of Ma, using analyses of sector-zoned and simple-zoned grains. These zircons have relatively high Th/U ratios with a narrow range, and they occur in association with garnet breaking down to form cordierite. The -Ma age obtained from these zircons is interpreted as the timing of crystallization from a high-Th/U partial melt soon after peak metamorphism. The combination of a ca. 800-Ma igneous age and 660–640-Ma metamorphic ages obtained from Schirmacher Hills is different from that of other neighboring parts of central DML. In addition, a metamorphic PT path involving ultrahigh temperatures at early and subsequent isobaric cooling (IBC) stages at around 650 Ma has not previously been known in the central DML nunatak region. The ca. 650-Ma UHT metamorphic event probably occurred in a back-arc tectonic setting and predates the main collisional event of central DML (ca. 550–500 Ma).

Pan-African Alkali Granitoids from the Sør Rondane Mountains, East Antarctica

Abstract Alkali granitoids (500-550 Ma) representing a prominent Pan-African magmatic event are widely distributed in the Sor Rondane Mountains, Dronning Maud Land, East Antarctica. Geochemically, they are granitic to syenitic in composition and show an alkaline affinity of A-type granites. They are characterized by high K 2 O+Na 2 O (7-13 wt%) and K 2 O/Na 2 O (1-2), low to intermediate Mg#, wide ranges of SiO 2 (45-78 wt%), Sr (20-6500 ppm) and Ba (40-13000 ppm) and have Nb and Ti depletion in the primitive mantle normalized diagram. The granitoids are subdivided into Group I granites, Group II granites, Lunckeryggen Syenitic Complex and Mefjell Plutonic Complex. The Group I granites have higher Mg#, Sr/Ba, Sr/Y, (La/Yb) N and LREE/HREE, lower A/CNK, SREE and initial 87 Sr/ 87 Sr ratios and lack Eu anomalies compared to those with negative Eu anomalies in the Group II granites. The syenitic rocks from the Mefjell Plutonic Complex are higher in alkali, Ga, Zr, Ba, and have lower Mg#, Rb, Sr, Nb, Y, F and LREE/HREE with positive Eu anomaly, whereas the granites from the Mefjell Plutonic Complex have high LREE/HREE ratios with negative Eu anomaly. The Lunckeryggen syenitic rocks have intermediate Mg#, higher K 2 O, P 2 O 5 , TiO 2 , Fe 2 O 3 /FeO, Ba, Sr/Y and LREE/HREE ratios with lack of Eu anomalies and are lower in Al 2 O 3 , Ga, Y, Nb and Rb/Sr ratios. Based on chemical characteristics combined with isotopic data, we suggest that the Lunckeryggen syenitic body and Group I granitic bodies may be derived from the mantle-derived hot basic magma by fractional crystallization with minor assimilation. We also suggest that the Group II granites may be derived from assimilation with crustal rocks to varing degrees and then fractional crystallization in higher crustal levels (ACF model). The Mefjell Plutonic Complex seems to be derived from a heterogenetic magma source compared with other granitoids from the Sor Rondane Mountains. The syenitic rocks in the Mefjell Plutonic complex have a unique source (iron-enriched) and have a chemical affinity with the charnockites in Gjelsvikjella and western Muhlig-Hofmannfjella, but not like the Yamato syenites in adjacent areas.

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

Abstract We report the first quantitative compositional data on fluid inclusions in ultrahigh-temperature (UHT) granulites from the Napier Complex of Enderby Land, East Antarctica. Fluid inclusions in various high-grade minerals such as garnet, orthopyroxene and sapphirine from three UHT localities in the Amundsen Bay area were studied in terms of petrography and microthermometry as well as laser Raman spectroscopy. Measured melting temperatures of inclusions from all the three localities indicate that the trapped fluid phase is dominantly carbonic. Raman analyses confirmed a near pure CO2 composition with only minor dilutants such as N2 (<6.0 mol%), CH4 (<0.3 mol%), and H2O (<0.1 mol%). CH4-bearing fluid associated with sapphirine granulites suggests low oxygen fugacity ( fO2) conditions for the rocks, whereas CH4 was not detected from fluid inclusions in magnetite-bearing high-f O2 garnet granulite. The range of CO2 isochores computed from density measurements in fluid inclusions from the granulites pass through the peak P–T conditions of the Napier metamorphism (T= 1050–1150 °C, P=9–11 kbar) indicating synmetamorphic nature of the fluids. Inclusions in garnet from Bunt Island coexist with carbonate minerals (magnesite) and graphite along with dense CO2-rich fluid, indicating probable derivation from deep-seated primary magmatic sources. The ubiquitous association of carbonic fluids in the UHT mineral assemblages suggests CO2 influx during extreme crustal metamorphism of the Napier Complex. The carbonic fluid probably played an important role in transporting heat from mantle or mantle-derived magmas and in stabilizing the dry mineral assemblages.

Early Proterozoic Tectonothermal Events in the Napier Complex, East Antarctica: Implications for the Formation of East Gondwana

Abstract Sm-Nd internal isochron ages involving retrograde garnet determined from three ultrahigh-temperature (UHT) gneisses taken from Tonagh Island, the western part of the Napier Complex, East Antarctica gave 1870±82 Ma, 1897±50 Ma and 1557±35 Ma. These ages are younger than the late Archaean timing of UHT metamorphism in the Napier Complex. The ca. 1900 Ma age is considered to reflect an important tectonothermal event in the Napier Complex including a tholeiite dyke intrusion. On the other hand, the ca. 1600 Ma age represents a thermal modification lacking signs of deformational events, and separates from the ca. 1900 Ma event. The related East Gondwana fragments such as the Rayner Complex in Antarctica and the Eastern Ghats Belt in India record extensive tectonothermal event of ca. 1400-1600 Ma, and rare indications of ca. 1900-2000 Ma. It is stressed that the assembly of East Gondwana including the Napier Complex, the Rayner Complex, and the Eastern Ghats Belt, if it existed, should be before ca. 1600 Ma, and may be traced back to ca. 2000 Ma of the supercontinent “Columbia” era.

Contrasting metamorphic P – T path between Schirmacher Hills and Mühlig-Hofmannfjella, central Dronning Maud Land, East Antarctica

Abstract Retrograde metamorphic P–T paths of garnet–pyroxene-bearing mafic gneisses from three regions in central Dronning Maud Land (CDML) were examined. No difference in P–T conditions estimated from rocks of the three regions was recognized, and they are within the range of c. 6–8 kbar, 750–830 °C. However, localities in the Mühlig-Hofmann Range (Filchnerfjella and Jutulsessen) preserve rocks with mineral textures that indicate near-isothermal decompressional histories. In the Schirmacher Hills, an isolated exposure on the Princess Astrid Coast, metamorphic texture observed in mafic gneiss is indicative of an isobaric cooling history. Combining their P–T paths and age determinations suggests that the Schirmacher Hills was a separate terrane, together with present-day SE Africa, whereas the Grenvillian-age east–west-trending CDML inland nunatak regions are characterized by an isothermal decompressional metamorphic history related to the final amalgamation of Gondwana.

Geochemistry of post‐kinematic mafic dykes from central to eastern Dronning Maud Land, East Antarctica: evidence for a Pan-African suture in Dronning Maud Land

Abstract The region comprising central to eastern Dronning Maud Land (2°W to 40°E), East Antarctica, is underlain by Mesoproterozoic to Cambrian metamorphic rocks and post-kinematic intrusive rocks with varied compositions. The post-kinematic mafic dykes linked to the Pan-African orogen include various types of lithologies: lamprophyre and lamproite in Mühlig-Hofmannfjella in central Dronning Maud Land and lamprophyre and high-K dolerite in the Sør Rondane Mountains in eastern Dronning Maud Land. Most of the mafic dykes have been weakly affected by low-grade metamorphism, but clearly preserve their igneous textures. The mafic dykes show a high abundance of Rb, Ba, Sr and light rare earth elements with negative anomalies of Nb, Ta and Ti in a multi-element primitive mantle-normalized diagram. The geochemical characteristics of the mafic dykes suggest that they were derived from a metasomatized mantle source leaving phlogopite, rutile and/or titanite as residual phases. Considering Sr and Nd isotopic systematics of the mafic dykes and the host metamorphic rocks and coeval felsic intrusive rocks, a large crustal boundary potentially related to a suture zone of West and East Gondwana should pass between Mühlig-Hofmannfjella and the Sør Rondane Mountains.

Paleozoic Subduction-Accretion-Closure Histories in the West Mongolian Segment of the Paleo-Asian Ocean: Evidence from Pressure-Temperature-Time-Protolith Evolution of High-Mg and -Al Gneisses in the Altai Mountains

High-Mg, high-Al metasedimentary gneisses from the Altai Mountains, Mongolia, belonging to a subduction-accretion complex within the Central Asian Orogenic Belt can be divided into five rock types on the basis of mineral assemblages. Most rock types have high MgO and Al2O3 content and low CaO, Na2O, Rb, and Sr content. All rock types experienced a similar medium-pressure metamorphism characterized by a “hairpin”-shaped counterclockwise pressure-temperature path. U-Pb zircon and U-Th-Pb monazite ages indicated metamorphism at ca. 356 Ma and 277 Ma and inherited ages of 510–379 Ma, suggesting possible provenance to granitoids comparable to those in the Altai Mountains, China. The zircons that newly nucleated at ca. 356 Ma are characterized by high concentrations of light rare earth elements without a Ce anomaly—features common in zircons from hydrothermally altered rocks and a reducing environment. Petrological and geochronological results in this study suggest the following tectonic evolution: (1) continuous subduction and accretion of paleo-Asian oceanic crust during the Early Paleozoic, resulting in periodic granitoid magmatism in the period 510–380 Ma and a continuous supply of granite-derived sediments providing detrital zircon and monazite grains to the accretionary prism; (2) ridge subduction during the Late Devonian–Early Carboniferous (ca. 356 Ma), resulting in hydrothermal metamorphism of the accretionary prism and interaction with seawater that produced rocks with unusual whole-rock chemistry; and (3) closure of the ocean leading to continental collision in the Early Permian (ca. 277 Ma), with part of the accretionary prism squeezed into lower crustal levels to form medium-pressure metamorphic rocks.