Toshiaki Tsunogae

First Report of Sapphirine+Quartz Assemblage from Southern India: Implications for Ultrahigh-temperature Metamorphism

We report here for the first time, the occurrence of sapphirine+quartz assemblage in textural equilibrium from quartzo-feldspathic and pelitic granulites from southern India. The sapphirine-bearing rocks occur as layered gneisses associated with pink granite within massive charnockite in Rajapalaiyam area in the southern part of Madurai Block. Sapphirine occurs in three associations: (i) fine-grained subhedral mineral associated with quartz enclosed in garnet, (ii) intergrowth with Al-rich orthopyroxene (up to 9.7 wt.% Al2O3), and (iii) in symplectitic intergrowth with orthopyroxene (Al2O3= 5.9–6.7 wt.%) and cordierite surrounding garnet. The sapphirine in association with quartz is slightly magnesian (XMg = 0.79–0.80) and low in Si content (1.55–1.56 pfu) as compared with those associated with orthopyroxene and cordierite (XMg= 0.77–0.79, Si = 1.59–1.63 pfu). The sapphirine+quartz assemblage suggests that the granulites underwent T>1050 °C peak metamorphism. Cores of porphyroblastic orthopyroxene in the sapphirine-bearing rocks shows high-Al2O3 content of up to 9.7 wt.%, suggesting T = 1040–1060°C and P = 8 kbar. FMAS reaction of sapphirine+quartz→garnet+sillimanite+cordierite indicates a cooling from sapphirine+quartz stability field after the peak ultrahigh-temperature metamorphism. Slightly lower temperature estimates from ternary feldspar and sapphirine-spinel geothermometers (T = 950–1000°C) also support a post-peak isobaric cooling. Corona textures of orthopyroxene+cordierite (±sapphirine), orthopyroxene+sapphirine, and cordierite+spinel around garnet suggest subsequent decompression. The sapphirine-quartz association and related textures reported in this study have important bearing on the ultrahigh-temperature metamorphism and exhumation history of the Madurai Block as well as on the tectonic evolution of the continental deep crust in southern India.

Extremely high density pure CO2 fluid inclusions in a garnet granulite from Southern India

A Late Archean (2.4–2.7 Ga) garnet granulite from the southern margin of the south Indian craton preserves fluid inclusions with the highest density CO2 yet reported from lower crustal rocks. The rock comprises garnet, clinopyroxene (salite), plagioclase, amphibole (pargasite), orthopyroxene (hypersthene), biotite, scapolite, and quartz with accessory K‐feldspar, ilmenite, apatite, and ankerite. Pressure‐temperature estimates of 9–11 kbar and 740°–800°C are computed from electron microprobe data on equilibrium assemblages. We report here detailed inclusion petrography and microthermometric data on fluids trapped in garnet, feldspar, and quartz from the granulite. The dominant category in all these minerals are composed of monophase carbonic inclusions with melting temperatures close to −56.6°C, indicating a CO2‐rich composition for the fluid. Laser Raman microscopic studies confirm that the fluid is pure CO2, with no traces of impurities such as N2 and/or CH4. The inclusions homogenize into the liquid phase at temperatures of −55.3° to +9.4°C. Primary CO2 inclusions in garnet show a sharply defined homogenization temperature within the narrow range 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

First Report of Sapphirine-bearing Rocks from the Palghat-Cauvery Shear Zone System, Southern India

-52.3^{\circ }\pm 2.4^{\circ }

Ultrahigh-temperature metamorphism and decompression history of sapphirine granulites from Rajapalaiyam, southern India: implications for the formation of hot orogens during Gondwana assembly

\end{document} C, translating into CO2 densities of 1.172–1.155 g/cm3. While trail bound inclusions in garnet show densities of 1.137–0.952 g/cm3, carbonic inclusions in plagioclase and quartz yield densities of 1.095–1.070g/cm3 and 1.112–1.086 g/cm3, respectively. These homogenization temperatures of the garnet‐hosted primary inclusions are by far the lowest yet recorded for pure CO2 fluids from granulite facies rocks. The fluid densities when computed into isochores indicate entrapment of CO2 at peak pressure‐temperature conditions estimated from mineral phase equilibria. The “ultrahigh density” CO2, preserved in a pristine state within primary inclusions in garnet and unmodified by later processes, is interpreted to have been trapped from either fluids exsolved by mantle magmas or CO2 transferred from the mantle through deep‐rooted shear systems. Ample evidence for mantle magmas is present in the area in the form of alkaline felsic to mafic and ultramafic plutons, anorthosites, and carbonatites. The region also forms part of a dense network of deep‐rooted shear systems that served as pathways for mantle‐derived CO2 that caused carbonate metasomatism. This study provides unequivocal evidence for the involvement of extremely high density CO2 in granulite formation within continental deep crust.

High fluorine pargasites in ultrahigh temperature granulites from Tonagh Island in the Archean Napier Complex, East Antarctica

We report new occurrences of sapphirine- and corundum-bearing granulites intercalated within orthogneisses at Lachmanapatti and Malappatty in the northern part of Madurai Block. Sapphirine in these localities occurs either as needle-like intergrowth with cordierite and corundum in symplectites and medium- to fine-grained euhderal to subhedral crystals associated with cordierite and corundum (Lachmanapatti) or in association with plagioclase, corundum and gedrite (Malappatty). The sapphirine from Lachmanapatti is highly magnesian (XMg = 0.87-0.94) with higher Cr content (up to 0.9 pfu) as compared with those in other localities in the Madurai Block. The sapphirine-corundum association reported in this study has important bearing on the ultrahigh-temperature metamorphism and exhumation history of the northern Madurai Block as well as on the tectonic evolution of the continental deep crust in southern India.

First Report of the Spinel + Quartz Assemblage from Kodaikanal in the Madurai Block, Southern India: Implications for Ultrahigh-Temperature Metamorphism

Abstract Silica deficient Mg-Al granulites from Paramati within the Palghat-Cauvery Shear System contain sapphirine in association with corundum, spinel and sillimanite. Gedrite, which occurs commonly in this locality, coexists with cordierite, corundum and sillimanite. Mineral assemblages and reaction textures indicate peak metamorphism at ultrahigh- temperature conditions. This new locality provides evidence for extreme crustal metamorphism along the Archean-Proterozoic collision boundary in southern India.

Petrology, phase equilibria modelling and zircon U–Pb geochronology of Paleoproterozoic mafic granulites from the Fuping Complex, North China Craton

Abstract Metamorphic P-T profiles in the eastern and western parts of the Limpopo Belt, Zimbabwe, were examined on the basis of mineralogic and petrologic data from areas which include the southernmost Zimbabwe Craton, the Northern Marginal Zone, and the Central Zone. The Zimbabwe Craton consists of a granite-greenstone terrane of 2900 Ma to 3600 Ma old. The Northern Marginal Zone represents a high-grade equivalent of the granite-greenstone lithologies. The Central Zone has suffered granulite-facies metamorphism during the late Archean. Metamorphic temperature of the Zimbabwe Craton adjacent to the northern boundary of the Limpopo Belt is calculated to be 650–680°C for amphibolite and 710–720°C for quartzofeldspathic gneiss in the eastern part. These temperatures are slightly lower than that calculated from the same assemblages in the Northern Marginal Zone and Central Zone. Peak metamorphic conditions in the eastern part of the Northern Marginal Zone are estimated at about 740–790°C and 7.5–7.9 kbar from the assemblage, garnet (core)-orthopyroxene-plagioclase-quartz in pelitic gneiss. These conditions were followed by a retrograde event of about 630–660°C and 3.8–4.5 kbar which were calculated from the assemblage, garnet (rim)-cordierite-sillimanite-quartz in the same rock. The P - T path is very similar to the retrograde path from the Southern Marginal Zone. It implies that both the Northern Marginal Zone and Southern Marginal Zone may have shared a similar metamorphic evolution. Garnet-bearing mafic gneiss in the Central Zone indicates somewhat higher P - T conditions than those in the Northern Marginal Zone. Central Zone metamorphic conditions are estimated at 790–890°C and 11.0–13.3 kbar for garnet-clinopyroxene-plagioclase-quartz assemblages and 750–870°C and 7.6–8.8 kbar for garnet-hornblende-plagioclase-quartz assemblages. The difference of P - T conditions between the Northern Marginal Zone and Central Zone may be related to movement along the pre-Triangle Shear Zone. The Northern Marginal Zone appears to have been displaced along the shear zone relative to the Central Zone during uplift, carrying the deeper crust to the surface.

Ti-free hogbomite in spinel- and sapphirine-bearing Mg-Al rock from the Palghat-Cauvery shear zone system, southern India

Sapphirine-bearing Mg–Al granulites from Rajapalaiyam in the southern part of the Madurai Block provide critical evidence for Late Neoproterozoic–Cambrian ultrahigh-temperature (UHT) metamorphism in southern India. Poikiloblastic garnet in quartzo-feldspathic and pelitic granulites contain inclusions of fine-grained subidioblastic to xenoblastic sapphirine associated with quartz, suggesting that the rocks underwent T > 1000°C peak metamorphism. Quartz inclusions in spinel within garnet are also regarded as clear evidence for a UHT condition. Inclusions of orthopyroxene within porphyroblastic garnet in the sapphirine-bearing rocks show the highest Al 2 O 3 content of up to 10.3 wt%, suggesting T = 1050–1070°C and P = 8.5–9.5 kbar. Temperatures estimated from ternary feldspar and other geothermometers ( T = 950–1000°C) further support extreme thermal metamorphism in this region. Xenoblastic spinel inclusions in sapphirine coexisting with quartz suggest that the spinel + quartz assemblage pre-dates the sapphirine + quartz assemblage, probably implying a cooling from T ~ 1050°C or an anticlockwise P–T path. The FMAS reaction sapphirine + quartz + garnet → orthopyroxene + sillimanite indicates a cooling from the sapphirine + quartz stability field after the peak metamorphism. Corona textures of orthopyroxene + cordierite (± sapphirine), orthopyroxene + sapphirine + cordierite, and cordierite + spinel around garnet suggest subsequent near-isothermal decompression followed by decompressional cooling toward T = 650–750°C and P = 4.5–5.5 kbar. The sapphirine–quartz association and related textures described in this study have an important bearing on the UHT metamorphism and exhumation history of the Madurai Block, as well as on the tectonic evolution of the continental deep crust in southern India. Our study provides a typical example for extreme metamorphism associated with collisional tectonics during the Late Neoproterozoic–Cambrian assembly of the Gondwana supercontinent.