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  • 1
    Electronic Resource
    Electronic Resource
    Tectonothermal evolution of the High Himalayan Crystalline Sequence, Langtang Valley, northern Nepal (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of metamorphic geology 10 (1992), S. 0 
    Details
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The High Himalayan Crystalline Sequence in north-central Nepal is a 15-km-thick pile of metasediments that is bound by the Main Central Thrust to the south and a normal fault to the north. The Langtang section through the metasediments shows an apparent inversion of metamorphic isograds with high-P, kyanite-grade rocks exposed beneath low-P, sillimanite-grade rocks. Textural evidence confirms that the observed inversion is a result of a polyphase metamorphic history and phase equilibria studies indicate that thermal decoupling has occurred within a mechanically coherent section of crust. Rocks now exposed at the base of the High Himalayan thrust sheet underwent Barrovian regional metamorphism (M1) prior to 34 Ma in the early stages of the Himalayan orogeny, recording metamorphic conditions of T= 710 ± 30° C, P= 9 ± 1 kbar. After the activation of the Main Central Thrust, which emplaced these metapelites southwards onto the lower grade Lesser Himalayan formations, the upper part of the thrust sheet was overprinted by a second heating event (M2), resulting in sillimanite-grade metamorphism and anatexis of metapelites at T= 760 ± 30° C, P= 5.8 ± 0.4 kbar between 17 and 20 Ma. Crustally derived, leucogranite magmas have been emplaced into low-grade Tethyan sediments on the hangingwall of the normal fault that bounds the northern limit of the metapelitic sequence.The cause of the selective heating of the upper section of the metasediments during M2 cannot be reconciled with either post-thrusting thermal relaxation or advection models. The cause of M2 remains problematical but it is suggested that heat focusing has occurred at the top of the High Himalayan Crystalline Sequence as a result of movement on the normal fault blanketing metapelites of high heat productivity with low-grade sediments of low thermal conductivity. This model implies that the normal fault was active before M2, consistent with decompression textures that formed during, or shortly after, sillimanite-grade metamorphism.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1525-1314.1992.tb00095.x
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  • 2
    Electronic Resource
    Electronic Resource
    Carbon-isotope constraints on fluid advection during contrasting examples of incipient charnockite formation (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of metamorphic geology 11 (1993), S. 0 
    Details
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Abstract Incipient charnockite formation within amphibolite facies gneisses is observed in South India and Sri Lanka both as isolated sheets, associated with brittle fracture, and as patches forming interconnected networks. For each mode of formation, closely spaced drilled samples across charnockite/gneiss boundaries have been obtained and δ13C and CO2 abundances determined from fluid inclusions by stepped-heating mass spectrometry.Isolated sheets of charnockite (c.50 mm wide) within biotite–garnet gneiss at Kalanjur (Kerala, South India) have developed on either side of a fracture zone. Phase equilibria indicate low-pressure charnockite formation at pressures of 3.4 ± 1.0 kbar and temperatures of about 700°C (for XH2O= 0.2). Fluid inclusions from the charnockite are characterized by δ13C values of −8% and from the gneiss, 2 m from the charnockite, by values of −15%. The large CO2 abundances and relatively heavy carbon-isotope signature of the charnockite can be traced into the gneiss over a distance of at least 280 mm from the centre of the charnockite, whereas the reaction front has moved only 30 mm. This suggests that fluid advection has driven the carbon-isotope front through the rock more rapidly than the reaction front. The carbon-front/reaction-front separation at Kalanjur is significantly larger than the value determined from a graphite-bearing incipient charnockite nearby, consistent with the predictions of one-dimensional advection models.Incipient charnockites from Kurunegala (Sri Lanka) have developed as a patchy network within hornblende–biotite gneiss. CO2 abundances rise to a peak near one limb of the charnockite, and isotopic values vary from δ13C of c.−5.5% in the gneiss to −9.5% in the charnockite. The shift to lighter values in the charnockite can be ascribed to the formation of a CO2-saturated partial melt in response to influx of an isotopically light carbonic fluid.Thus, incipient charnockites from the high-grade terranes of South India and Sri Lanka reflect a range of mechanisms. At shallower structural levels non-pervasive CO2 influxed along zones of brittle fracture, possibly associated with the intrusion of charnockitic dykes. At deeper levels, in situ melting occurred under conditions of ductile deformation, leading to the development of patchy charnockites.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1525-1314.1993.tb00193.x
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  • 3
    Electronic Resource
    Electronic Resource
    Contrasting retrograde oxygen isotope exchange behaviour and implications: examples from the Langtang Valley, Nepal (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of metamorphic geology 12 (1994), S. 0 
    Details
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Quantitative modelling of oxygen exchange by diffusion during slow cooling has been compared to the observed oxygen isotope distributions from high-grade metamorphic and granitic rocks of the High Himalayan Crystallines, Langtang Valley, central Nepal, in order to investigate the effect of retrograde diffusional exchange on the preservation of high-temperature, oxygen isotope systematics.Modelled fractionations, using water-present diffusion data reported in the literature, predict quartz-mica fractionations to be much larger than those at peak metamorphic and igneous conditions due to low closure temperatures for micas. Quartz-feldspar fractionations may be less than those at peak conditions, and in some samples may even be slightly negative.The observed oxygen isotope fractionations in the metamorphic rocks are small and largely appear to record equilibrations close to peak conditions determined by other methods. Hence these rocks clearly do not conform to predictions of fluid-present diffusional retrograde exchange. It is suggested that their retrograde history was therefore within an anhydrous closed system in which diffusion was slow and hence mineral closure temperatures were high. The granitic rocks record rather larger quartz-biotite fractionations, approaching those predicted by the diffusion modelling. However, quartz-feldspar fractionations are large and hence, although significant retrograde exchange has clearly occurred, simple diffusion alone is not sufficient to explain the observed data and open-system exchange may be required. The presence of fluids during the retrograde history of this part of the section is supported by petrographic evidence.The different retrograde oxygen exchange histories recorded between the regional metamorphic and magmatic regimes of the Langtang section would appear to support the importance of water on the kinetics of such exchange, and suggests that in its absence, diffusional exchange may become insignificant, allowing oxygen isotope thermometry to record meaningful high-temperature data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1525-1314.1994.tb00021.x
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  • 4
    Electronic Resource
    Electronic Resource
    Isotopic constraints on fluid infiltration from an eclogite facies shear zone, Holsenøy, Norway (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of metamorphic geology 12 (1994), S. 0 
    Details
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Granulite facies anorthosites on Holsenøy Island in the Bergen Arcs region of western Norway are transected by shear zones 0.1–100 m wide characterized by eclogite facies assemblages. Eclogite formation is related to influx of fluid along the shears at temperatures of c. 700d̀C and pressures in excess of 1.7 GPa. Combined carbon and nitrogen stable isotope, 40Ar/36Ar, trace-element and petrological data have been used to determine the nature and distribution of fluids across the anorthosite-eclogite transition.A metre-wide drilled section traverses the eclogitic centre of the shear into undeformed granulite facies garnet-clinopyroxene anorthosite. Clinozoisite occurs along grain boundaries and microcracks in undeformed anorthosite up to 1 m from the centre of the shear and clinozoisite increases in abundance as the edge of the shear zone is approached. The eclogite-granulite transition, marked by the appearance of sodic pyroxene and loss of albite, occurs within the most highly sheared section of the traverse. The jadeite-in reaction coincides with increased paragonite activity in mica. The separation between paragonite and clinozoisite reaction fronts can be semiquantitatively modelled assuming advective fluid flow perpendicular to the shear zone. The inner section of the traverse (0.25 m wide) is marked by retrogressive replacement of omphacite by plagioclase + paragonite accompanied by veins of quartz-phengite-plagioclase.C-N-Ar characteristics of fluid inclusions in garnet show that fluids associated with precursor granulite, eclogite and retrogressed eclogite are isotopically distinct. The granulite-eclogite transition coincides with a marked change in CO2 abundance and δ13C (〈36ppm, δ13C=-2% in the granulite; 〈180 ppm, δ13C=-10% in the eclogite). The distribution of Ar indicates mixing between influxed fluid (40Ar/36Ar 〉 25 times 103) and pre-existing Ar in the granulite (40Ar/36Ar 〈 8 times 103). δ15N values decrease from +6% in the anorthosite to +3% within the eclogite shear. The central zone of retrogressed eclogite post-dates shearing and is characterised by substantial enrichment of Si, K, Ba and Rb. Fluids are CO2-rich (δ13C ∼ -5%) with variable N2 and Ar abundances and isotopic compositions.Both Ar and H2O have penetrated the underformed granulite fabric more than 0.5m beyond the granulite/eclogite transition during eclogite formation. Argon isotopes show a mixing profile consistent with diffusion through an interconnecting H2O-rich fluid network. In contrast, a carbon-isotope front coincides with the deformation boundary layer, indicating that the underformed anorthosite was impervious to CO2-rich fluids. This is consistent with the high dihedral angle of carbonic fluids, and may be interpreted in terms of evolving fluid compositions within the shear zone.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1525-1314.1994.tb00025.x
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  • 5
    Electronic Resource
    Electronic Resource
    The pressure–temperature–time path of migmatites from the Sikkim Himalaya (2004)
    Oxford, UK : Blackwell Science Inc
    Journal of metamorphic geology 22 (2004), S. 0 
    Details
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A combined metamorphic and isotopic study of lit-par-lit migmatites exposed in the hanging wall of the Main Central Thrust (MCT) from Sikkim has provided a unique insight into the pressure–temperature–time path of the High Himalayan Crystalline Series of the eastern Himalaya. The petrology and geochemistry of one such migmatite indicates that the leucosome comprises a crystallized peraluminous granite coexisting with sillimanite and alkali feldspar. Large garnet crystals (2–3 mm across) are strongly zoned and grew initially within the kyanite stability field. The melanosome is a biotite–garnet pelitic gneiss, with fibrolitic sillimanite resulting from polymorphic inversion of kyanite. By combining garnet zoning profiles with the NaCaMnKFMASHTO pseudosection appropriate to the bulk composition of a migmatite retrieved from c. 1 km above the thrust zone, it has been established that early garnet formed at pressures of 10–12 kbar, and that subsequent decompression caused the rock to enter the melt field at c. 8 kbar and c. 750 °C, generating peritectic sillimanite and alkali feldspar by the incongruent melting of muscovite. Continuing exhumation resulted in resorption of garnet. Sm–Nd growth ages of garnet cores and rim, indicate pre-decompression garnet growth at 23 ± 3 Ma and near-peak temperatures during melting at 16 ± 2 Ma. This provides a decompression rate of 2 ± 1 mm yr−1 that is consistent with exhumation rates inferred from mineral cooling ages from the eastern Himalaya. Simple 1D thermal modelling confirms that exhumation at this rate would result in a near-isothermal decompression path, a result that is supported by the phase relations in both the melanosome and leucosome components of the migmatite. Results from this study suggest that anatexis of Miocene granite protoliths from the Himalaya was a consequence of rapid decompression, probably in response to movement on the MCT and on the South Tibetan detachment to the north.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1525-1314.2004.00511.x
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  • 6
    Electronic Resource
    Electronic Resource
    Crustal evolution in north-east and east Africa from model Nd ages (1984)
    [s.l.] : Nature Publishing Group
    Nature 309 (1984), S. 773-776 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The Karkur Murr charnockites from Jabel Uweinat (Fig. 1) form the oldest known basement in north-east Africa4'5 and two samples have model Nd ages, relative to depleted mantle6, of 3,000 and 3,200 Myr (Fig. 2, Table 1). This confirms their Archaean age, while their low eNd(T) and high eSr(T) values ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/309773a0
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  • 7
    Electronic Resource
    Electronic Resource
    Carbon isotope compositions of fluid inclusions in charnockites from southern India (1988)
    [s.l.] : Nature Publishing Group
    Nature 333 (1988), S. 167-170 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Incipient charnockites from southern India, which occur as veins and patches within biotite and amphibole gneiss, argue for a volatile-controlled transition from amphibolite-facies gneiss to arrested charnockite development8"11. The presence of grain-margin calcite8 and abundant CO2-rich fluid ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/333167a0
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  • 8
    Electronic Resource
    Electronic Resource
    Preliminary conclusions of the Royal Society and Academia Sinica 1985 geotraverse of Tibet (1986)
    [s.l.] : Nature Publishing Group
    Nature 323 (1986), S. 501-507 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The 1985 Chinese/British expedition to the Tibetan Plateau attempted to solve the question of the origin of the very thick crustal rocks in this region. Continuing northwards movement of the Indian plate over the past 38 Myr has given rise to severe folding and thrust faulting, causing crustal ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/323501a0
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  • 9
    Electronic Resource
    Electronic Resource
    Significance of contrasting magmatism in North East Africa and Saudi Arabia (1981)
    [s.l.] : Nature Publishing Group
    Nature 289 (1981), S. 394-396 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Nearly 600 age determinations have been published from the Arabian-Nubian shield, about two-thirds of which are from Saudi Arabia. Although most of these are isolated K/Ar or Rb/Sr ages, and cannot be considered reliable indicatorsof geological events, available U-Pb zircon and Rb/Sr isochron ages ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/289394a0
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  • 10
    Electronic Resource
    Electronic Resource
    The Red Sea line and Arabian–Nubian magmatism (1982)
    [s.l.] : Nature Publishing Group
    Nature 296 (1982), S. 178-178 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] HARRIS AND GASS REPLY-Jackson and Stoeser have not distinguished between the identification of a Phanerozoic alkaline province in northeast Africa and our speculations on possible causes of this event and its apparent absence in Arabia. Jackson and Stoeser identify an undated syenite and trachytes ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/296178a0
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