bluuebunny/crossref_metadata_2025 huggingface dataset:
Low‐grade metamorphism of the Mikabu and northern Chichibu belts in central Shikoku, SW Japan: implications for the areal extent of the Sanbagawa low‐grade metamorphism
Authors: SUZUKI S. , ISHIZUKA H.
Published: October 2004

In central Shikoku, SW Japan, the Mikabu belt is bounded to the north by the Sanbagawa belt, and to the south by the northern (N) Chichibu belt. The N‐Chichibu belt can be further subdivided into northern and southern parts. There is no apparent difference in the overall geology, structure, or fossil and radiometric ages between the Mikabu belt and the northern part of the N‐Chichibu belt. Greenstones from the Mikabu belt and the northern part of the N‐Chichibu belt show evidence for similar low‐grade metamorphism, and include the following mineral assemblages with albite+chlorite in excess: metamorphic aragonite, sodic pyroxene+quartz, epidote+actinolite+pumpellyite, glaucophane+ pumpellyite+quartz, and lawsonite (not with actinolite or glaucophane). These similarities suggest that the Mikabu belt and the northern part of the N‐Chichibu belt belong to the same geological unit (the MB‐NNC complex). The mineral assemblages also indicate that the MB‐NNC complex belongs to a different metamorphic facies from the low‐grade part of the Sanbagawa belt, that is, the former represents lower temperature/higher pressure conditions than the latter. Structural and petrological continuity between the MB‐NNC complex and Sanbagawa belt has not yet been confirmed, but both have similar radiometric ages. It is therefore most likely that the MB‐NNC complex and Sanbagawa belt belong to the same subduction complex, and were metamorphosed under similar but distinct conditions. These two units were juxtaposed during exhumation. In contrast, the southern part of the N‐Chichibu belt is distinct in lithology and structure, and includes no mineral assemblages diagnostic of the MB‐NNC complex and the Sanbagawa belt. Thus, the southern part of the N‐Chichibu belt may represent a different geological unit from the MB‐NNC complex and Sanbagawa belt.
Pumpellyite–actinolite facies of the Sanbagawa metamorphism
Authors: BANNO S.
Published: October 2004

The pumpellyite–actinolite facies proposed by Hashimoto is defined by the common occurrence of the pumpellyite–actinolite assemblage in basic schists. It can help characterize the paragenesis of basic and intermediate bulk compositions, which are common constituents of various low‐grade metamorphic areas. The dataset of mutually consistent thermodynamic properties of minerals gives a positive slope for the boundary between the pumpellyite–actinolite and prehnite–pumpellyite facies in P–T  space. In the Sanbagawa belt in Japan, the mineral parageneses of hematite‐bearing and ‐free basic schists, as well as pelitic schists have been well documented. The higher temperature limit of this facies is defined by the disappearance of the pumpellyite+epidote+actinolite+chlorite assemblage in hematite‐free basic schists with XFe3+ of epidote around 0.20–0.25 and the appearance of epidote+actinolite+chlorite assemblage with XEpFe3+≤0.20. In hematite‐bearing basic schists, there is a continuous change of paragenesis to higher grade, epidote–glaucophane or epidote–blueschist facies. In pelitic schists, the albite+lawsonite+chlorite assemblage does occur but only rarely, and its assemblage cannot be used to determine the regional thermal structure. The lower temperature equivalence of the pumpellyite–actinolite assemblage is not observed in the field. The Mikabu Greenstone complex and the northern margin of the Chichibu complex, which are located to the south of the Sanbagawa belt, are characterized by clinopyroxene+chlorite or lawsonite+actinolite assemblages, which are lower temperature assemblages than the pumpellyite+actinolite assemblage. These three metamorphic complexes belong to the same subduction‐metamorphic complex. The pumpellyite–actinolite facies or subfacies can be useful to help reveal the field thermal structure of metamorphic complexes
A corundum–quartz assemblage from the Eastern Ghats Granulite Belt, India: evidence for high P–T metamorphism?
Authors: SHAW R. K. , ARIMA M.
Published: October 2004

Corundum+quartz‐bearing assemblages occur in small lenses in granulite facies metapelites in Rayagada, north‐central part of the Eastern Ghats Granulite Belt, India. Corundum porphyroblasts and quartz coexist with porphyroblastic almandine‐rich garnet, hercynite spinel, ilmenite and magnetite. Corundum and quartz are separated by sillimanite or a composite corona consisting of sillimanite and garnet, whereas corundum shows sharp grain boundaries with spinel, ilmenite and magnetite. Porphyroblastic corundum contains prismatic sillimanite inclusions in which irregularly shaped quartz is enclosed. Two distinct reactions are inferred from the textural features: corundum+quartz=sillimanite and spinel+quartz=garnet+sillimanite. From the petrographical features, we infer that corundum–quartz–garnet–spinel was the peak metamorphic assemblage. Although large uncertainties exist regarding the positions of the respective reactions in P–T  space, from several published experimental results and theoretical calculations a peak metamorphic condition of 12 kbar and 1100 °C is estimated as the lower stability limit of the corundum–quartz assemblage. Decompression from the peak P–T  condition to c. 9 kbar, 950 °C is inferred.
Well‐preserved garnet growth zoning in granulite from the Dabie Mountains, central China
Authors: CHEN NENG‐SONG , SUN MIN , YOU ZHEN‐DONG , MALPAS J.
Published: October 2004

Ultra‐high pressure eclogites and granulites both occur in the Dabie Mountains, central China. A garnet porphyroblast from felsic granulite in the Dabie Mountains has been analysed for compositional zoning by electron microprobe. Two segments of the porphyroblast have opposite compositional variations. Segment I (from centre outward 9 mm to analytical point 18) has decreasing XSps and increasing XPyr, while Segment II (from analytical point 18, 1 mm outward to the rim) has increasing XSps and XAlm and decreasing XPyr and XGrs. The compositional zoning in segment I is considered as growth zoning and that in Segment II as diffusive retrograde zoning. Garnet growth zoning records a P–T  path prior to the peak granulite metamorphism. The minimum P–T  conditions are estimated to be 1.35 GPa and 850 °C for peak metamorphism, based on the highest Mg/(Fe+Mg) composition in the garnet (analytical point 18) and matrix hypersthene, biotite and plagioclase. A symplectitic corona surrounds the porphyroblast and appears to have formed at 0.6 GPa and 700 °C. The well‐preserved growth zoning in garnet suggests a short residence time for the granulite at peak metamorphism and thus rapid tectonic uplift history. The P–T  path is consistent with that of ultra‐high‐pressure eclogite in the area. Tectonic movements during a collisional event could have brought both the granulite and the eclogite to their present positions.
Oxygen isotope study of quartz‐Al2SiO5 pairs from the Mica Creek area, British Columbia: implications for the recovery of peak metamorphic temperatures
Authors: GHENT E. D. , VALLEY J. W.
Published: October 2004

Oxygen isotope analyses of quartz‐Al2SiO5 pairs have been made for samples from the Mica Creek area, British Columbia. We have analysed quartz–kyanite nodules and quartz–kyanite and quartz–sillimanite in multiphase pelitic rocks from the staurolite–kyanite, kyanite, and sillimanite zones. Apparent temperatures calculated from oxygen isotopic fractionation range from 555 °C (staurolite–kyanite zone) to 695 °C (sillimanite zone). Temperatures from the quartz–kyanite nodules range from 630 to 675 °C. Some of the nodules show isotopic disequilibrium. Most of the results confirm predictions that bimineralic rocks will yield an estimate of peak metamorphic temperatures, when the less abundant mineral (an aluminium silicate) is the slower oxygen diffuser. Using cooling rates of 10–100 °C Ma−1 for the multiphase rocks, measured crystal sizes and modes, the Fast Grain Boundary diffusion model with ‘wet’ diffusion data (PH2O≅1.0 kbar) yields predicted apparent temperatures which are generally lower than the measured apparent temperatures. The agreement is improved if slower diffusion coefficients are used. This suggests that f (H2O) during cooling was lower than that of the hydrothermal experiments and thus that there was little interaction with aqueous fluids of internal or external origin to modify the isotopic compositions. The measured apparent isotopic temperatures and apparent garnet–biotite Fe–Mg exchange temperatures show very poor agreement for the sillimanite zone samples, with the garnet–biotite Fe–Mg exchange temperatures generally higher than the oxygen isotope temperatures. Compared with the other calibrations that we tested the measured apparent temperatures using the Sharp calibration show the best agreeement with recently published P–T  grids, although some variability in agreement is expected due to variable f (H2O) during cooling.
Low‐P / high‐T metamorphism in the Okiep Copper District, western Namaqualand, South Africa
Authors: RAITH J. G. , HARLEY S. L.
Published: October 2004

The Okiep Copper District, part of the 1.2–1.0 Ga high‐grade terrane in western Namaqualand, is composed of a mid‐Proterozoic supracrustal sequence and several pre‐ to post‐orogenic intrusive suites affected by two high‐grade events (M2a/M2b, M3) of Kibaran and one low‐grade event (M4) of Pan‐African age. Peak assemblages in quartz‐bearing pelites are characterized either by garnet+cordierite coexisting with sillimanite/biotite, or by biotite+sillimanite±garnet; a difference controlled by bulk composition and variation in water activities (0.1–0.7) during dehydration melting. Maximum P–T conditions were reached during M2a coevally with the major deformational event (D2a) and are estimated at 750–820 °C and 5–6 kbar. A counterclockwise P–T  path is indicated by regionally occurring pseudomorphs of sillimanite after andalusite and by prograde reaction textures preserved as relics in M2a porphyroblasts. Two stages of retrograde metamorphism are distinguished: M2a garnet+cordierite‐bearing assemblages were retrogressed to biotite+sillimanite+quartz (M2b) along discontinuous foliation planes and shear zones (D2b). Retrograde M3 corona assemblages formed at similar P–T  conditions (580–660 °C and 5.8±0.5 kbar) to the M2b assemblages but M3 crystallization postdates penetrative D2 deformation, intrusion of 1.06 Ga granitoids and formation of associated W–Mo deposits. It is concluded that: (a) Kibaran high‐grade metamorphism in the Okiep Copper District is thermally punctuated and (b) reaction textures documenting apparent isobaric cooling of this low‐P high‐T  terrane must be interpreted with caution.
The Meatiq dome (Eastern Desert, Egypt) a Precambrian metamorphic core complex: petrological and geological evidence
Authors: NEUMAYR (email: ) P. , HOINKES G. , PUHL J. , MOGESSIE A. , KHUDEIR A. A.
Published: October 2004

The Meatiq basement, which is exposed beneath late Proterozoic nappes of supracrustal rocks in the Central Eastern Desert of Egypt, was affected by three metamorphic events. The ophiolite cover nappes show only the last metamorphic overprint. The M1 metamorphic event (T ≥750 °C) is restricted to migmatized amphibolite xenoliths within the Um Ba′anib orthogneiss in the structurally lowest parts of the basement. Typical upper amphibolite facies M2 mineral assemblages include Grt–Zn‐rich Spl–Qtz±Bt, Grt–Zn‐rich Spl–Ms–Kfs–Bt–Sil–Qtz and locally kyanite in metasedimentary rocks. The mineral assemblages Ms–Qtz–Kfs–Sil in the matrix and Sil–Grt in garnet cores indicate that peak M2 P–T  conditions exceeded muscovite and staurolite stabilities. Diffusional equilibration at M2 peak temperature conditions caused homogeneous chemical profiles across M2 garnets. Abundant staurolite in garnet rims and the matrix indicates a thorough equilibration during M2 at decreasing temperature conditions. M2 P–T  conditions ranged from 610 to 690 °C at 6–8 kbar for the metamorphic peak and 530–600 °C at about 5.8 kbar for the retrograde stage. However, relic kyanite indicates pressures above 8 kbar, preceeding the temperature peak. A clockwise P–T  path is indicated by abundant M2 sillimanite after relic kyanite and by andalusite after sillimanite. M2 fluid inclusions, trapped in quartz within garnet and in the quartz matrix show an array of isochores. Steepest isochores (water‐rich H2O‐CO2±CH4/N2 inclusions) pass through peak M2 P–T  conditions and flatter isochores (CO2‐rich H2O‐CO2±CH4/N2 inclusions) are interpreted to represent retrograde fluids which is consistent with a clockwise P–T  path for M2. The M3 assemblage Grt–Chl in the uppermost metasedimentary sequence of the basement limits temperature to 460 to 550 °C. M3 temperature conditions within the ophiolite cover nappes are limited by the assemblage Atg–Trem–Tlc to<540 °C and the absence of crysotile to >350 °C. The polymetamorphic evolution in the basement contrasts with the monometamorphic ophiolite nappes. The M1 metamorphic event in the basement occurred prior to the intrusion of the Um Ba′anib granitoid at about 780 Ma. The prograde phase of the M2 metamorphic event took place during the collision of an island arc with a continent. The break‐off of the subducting slab increased the temperature and resulted in the peak M2 mineral assemblages. During the rise of the basement domain retrograde M2 mineral assemblages were formed. The final M3 metamorphic event is associated with the updoming of the basement domain at about 580 Ma along low‐angle normal faults.
Diffusion‐controlled corona reaction and overstepping of equilibrium in a garnet granulite, Yenisey Ridge, Siberia
Authors: ASHWORTH J. R. , SHEPLEV V. S. , BRYXINA N. A. , KOLOBOV V. Yu. , REVERDATTO V. V.
Published: October 2004

Diffusion modelling is applied to layered garnet–pyroxene–quartz coronas, formed by a pressure‐induced reaction between plagioclase and primary pyroxene in a metabasic granulite. The reconstructed reaction involves some change in composition of reactant minerals. The distribution of minerals between layers is satisfactorily explained by diffusion‐controlled reaction with local equilibrium, in which the diffusion coefficient for Al was smaller than those for Fe, Mg and Ca by a factor of approximately four. Diffusion of Mg towards plagioclase implies a chemical‐potential gradient for MgO component in a direction opposite to the changing Mg content of garnet; this is explained by the influence of Al2O3 on the chemical potential of the pyrope end‐member. Grain‐boundary diffusion is suggested to have operated, possibly with composition gradients different from those in the bulk minerals. Chemical‐potential differences across the corona are estimated from the variation in garnet composition, enabling affinity (the free energy change driving the reaction) to be estimated as 6.9±1.8 kJ per 24‐oxygen mole of garnet produced. This implies that the pressure for equilibrium among the minerals was overstepped by 1.4±0.4 kbar. The probable P–T conditions of reaction were in the range 650–790 °C, 8–10 kbar. Assuming a timescale of reaction between 106 and 108 years, estimated diffusion coefficients for Fe, Mg and Ca are in the range 9×10−23 to 5×10−20 m2 s−1. These are consistent with experimental values in the literature for solid‐state diffusion, including grain‐boundary diffusion.
Models for orthopyroxene–plagioclase and other corona reactions in metanorites, Dahomeyide orogen, West Africa
Authors: ATTOH KODJOPA
Published: October 2004

Coronal reaction textures occur in metanorite and related intrusions in the Pan‐African Dahomeyide orogen of West Africa; they apparently formed by retrograde subsolidus reactions during cooling of the intrusions from 900 to 700 °C at c. 9±1 kbar. The coronas that formed around orthopyroxene (Opx) consist of sequential layers of diopside (Dio), hornblende (Hbl), garnet (Grt) and plagioclase (Pl) with inclusions of kyanite (Ky) or sillimanite (Sil). Three well‐organized mineral assemblage sequences have been identified and modelled using steady‐state diffusion theory for closed and open systems. The mineral sequence Opx|Hbl+Qtz|Hbl|Pl+Ky, formed by diffusion‐controlled reactions in a closed system as layer thicknesses are very sensitive to relative component mobilities defined by Onsager diffusion coefficient (Lii). Models of this corona (type i) that satisfy modes require LCaCa≥LMgMg≥LFeFe>LAlAl∼LSiSi ; however, small open‐system fluxes involving loss of Al and gain of Ca are required to obtain the best fit between model and observed mineral proportions. Under steady‐state diffusion, the monomineralic hornblende layer grew by replacing plagioclase whereas the |Hbl+Qtz| grew by Opx replacement. The type ii corona, which consists of the sequence Opx|Dio|Grt+Dio|Pl+Sil, is also stable under steady‐state diffusion in a closed system. Modelling results show that the diopside grew by replacing Opx whereas most of Grt+Dio grew by replacing plagioclase. Stable solutions to the closed‐system diffusion model that approximate the mode are restricted to the L‐ratio regions where Fe, Mg and Ca are more mobile than Si and Al but are unstable when LAlAl>100 LSiSi . However, type ii corona mineral proportions were only closely matched when open‐system loss of Al and gain of small amounts of Fewere considered in the diffusion models and relative mobilities were LFeFe≥LCaCa≥LMgMg>LAlAl∼LSiSi . The modelling results indicate that Ca and Mg were the most mobile elements in the formation of type i corona whereas Fe and Ca were the most mobile components in the growth of type ii coronas. A third corona type, consisting of the mineral sequence Act|Hbl|Grt|Pl+Sil, is only stable in open systems and requires large external fluxes involving gain of Fe, Al, Mg and Na and loss of Ca to obtain a solution of the diffusion model that approximates the estimated mineral proportions. Extensive recrystallization of plagioclase to produce sillimanite or kyanite inclusions accompanying corona formation may explain the open‐system behaviour indicated by the diffusion models.
Fluid inclusions in high pressure/low temperature rocks from the Calabrian Arc (Southern Italy): the burial and exhumation history of the subduction‐related Diamante‐Terranova unit
Authors: INVERNIZZI C. , VITYK M. , CELLO G. , BODNAR R.
Published: October 2004

The Diamante‐Terranova Unit (DIATU), in the Calabrian Arc of southern Italy, is part of an ophiolitic sequence involved in a high pressure/low temperature event (P=8 kbar; T =400 °C) followed by re‐equilibration at greenschist facies conditions (P=3 kbar; T =300 °C). The rocks contain two types of quartz–calcite veins – an earlier generation of deformed, folded and faulted veins formed during or before subduction, and a later set of planar, undeformed veins formed during exhumation of the DIATU. The earlier folded quartz–calcite veins contain regularly shaped aqueous inclusions as well as inclusions with a highly irregular dendritic texture. The later planar veins contain only regularly shaped aqueous inclusions similar to those in the earlier veins. In both vein types, all inclusions are demonstrably secondary in origin. Regularly shaped inclusions from both vein types are low salinity (0–5 wt% NaCl). Most contain liquid and vapour and homogenize to the liquid (Th 135–180 °C), whereas others contain only liquid at room temperature. Both the two‐phase and monophase inclusions occur in the same fractures and are thought to record the same trapping event, with the monophase inclusions remaining metastable liquid at room temperature. No microthermometric data could be obtained from the dendritic inclusions in the earlier folded veins. Inclusions with the highly irregular dendritic texture found in the earlier veins are similar to those produced experimentally during laboratory‐induced deformation of synthetic inclusions in quartz under conditions of internal underpressure, simulating either isobaric cooling or isothermal compression. The occurrence of inclusions with the dendritic texture in the earlier folded veins, and their absence from the later planar veins, suggests that the earlier veins formed before or during subduction and were folded and faulted in a compressional environment and their contained fluid inclusions were modified to produce the dendritic texture. During later uplift of the DIATU, planar veins containing regularly shaped aqueous inclusions formed and some of the fluids forming these veins were also trapped as secondary inclusions in the earlier folded veins. The results of this study provide convincing evidence that inclusions with a highly irregular dendritic morphology represent early inclusions that have survived prograde conditions in a high pressure/low temperature metamorphic environment (but have been texturally modified). The high pressure/low temperature ‘implosion’ texture is preserved over geological time, even after being overprinted by internal overpressure conditions generated during retrograde decompression. We suggest that inclusions that have survived prograde metamorphism are common in high pressure/low temperature rocks, but are often not identified as such due to their morphology which makes their recognition difficult.
An internally consistent thermodynamic data set for phases of petrological interest
Authors: HOLLAND T. J. B. , POWELL R.
Published: October 2004

The thermodynamic properties of 154 mineral end‐members, 13 silicate liquid end‐members and 22 aqueous fluid species are presented in a revised and updated data set. The use of a temperature‐dependent thermal expansion and bulk modulus, and the use of high‐pressure equations of state for solids and fluids, allows calculation of mineral–fluid equilibria to 100 kbar pressure or higher. A pressure‐dependent Landau model for order–disorder permits extension of disordering transitions to high pressures, and, in particular, allows the alpha–beta quartz transition to be handled more satisfactorily. Several melt end‐members have been included to enable calculation of simple phase equilibria and as a first stage in developing melt mixing models in NCKFMASH. The simple aqueous species density model has been extended to enable speciation calculations and mineral solubility determination involving minerals and aqueous species at high temperatures and pressures. The data set has also been improved by incorporation of many new phase equilibrium constraints, calorimetric studies and new measurements of molar volume, thermal expansion and compressibility. This has led to a significant improvement in the level of agreement with the available experimental phase equilibria, and to greater flexibility in calculation of complex mineral equilibria. It is also shown that there is very good agreement between the data set and the most recent available calorimetric data.
A sequence of partial melting reactions at Mt Stafford, central Australia
Authors: GREENFIELD J. E. , CLARKE G. L. , WHITE R. W.
Published: October 2004

Metasedimentary gneisses show a rapid change in grade in a 10 km wide low‐P/high‐T  regional aureole at Mt Stafford in the Arunta Block, central Australia. Migmatite occurs in all but the lowermost of five metamorphic zones, which grade from greenschist (Zone 1) through amphibolite (Zones 2–3) to granulite facies (Zones 4–5). The sequence of partial melting reactions inferred for metapelitic rocks is dependant upon protolith, temperature and fluid conditions. The metapelite solidus in Zone 2 reflects vapour‐present melting at P≈3 kbar and T ≈640 °C, melting having initially been controlled by the congruent breakdown of the assemblage Crd–Kfs–Bt–Qtz. At slightly higher temperature, andalusite in leucosome formed via the reaction Kfs+Qtz+Bt+H2O→And+melt; And+melt having been stabilized by the presence of boron. Sillimanite coaxially replaces andalusite in the high‐grade portion of Zone 2. In Zone 3, large aluminosilicate aggregates in leucosome are armoured by Spl–Crd±Grt symplectites. Garnet partially pseudomorphs biotite, cordierite or spinel in high‐grade portions of Zone 3. Zone 4 Grt–Crd–Opx‐bearing metapsammite assemblages and garnet‐bearing leucosome reflect T ≈800 °C and P=2.2±0.9 kbar. In the model KFMASH system the principal vapour‐absent melting step reflected significant modal changes related to the breakdown of the As–Bt tie‐line and the establishment of the Spl–Crd tie‐line; the bulk rock geochemistry of migmatite samples straddle the Spl–Crd tie‐line. The aluminous bulk‐rock composition of the common bedded migmatite restricted its potential to witness garnet‐forming and orthopyroxene‐forming reactions, minor textural and modal changes in and above Zone 3 reflecting biotite destablization in biotite‐poor assemblages.
Evolution of a kyanite‐bearing belt within a HT‐LP orogen: the case of NW Variscan Iberia
Authors: RECHE J. , MARTINEZ F. J. , ARBOLEYA M. L. , DIETSCH C. , BRIGGS W. D.
Published: October 2004

Kyanite replaces andalusite in a belt of Ordovician and Silurian pelitic rocks that form a narrow synform pinched between high‐grade antiforms in NW Variscan Iberia. Kyanite occurs across the belt in Al‐rich, black pelites in assemblages I: kyanite–chloritoid–chlorite–muscovite and II: kyanite–staurolite– chlorite–muscovite. In I, kyanite occurs in the matrix and in kyanite–muscovite aggregates that pseudomorph earlier andalusite porphyroblasts. The aggregates are found across the belt and can still be recognized in assemblage II and even in III: andalusite–staurolite–biotite–muscovite, this latter being a hornfelsic Silurian schist where kyanite is relic and staurolite occurs in the matrix, and is resorbed inside new massive pleochroic andalusite. KFMASH and MnKFMASH pseudosections have been constructed using Thermocalc for Al‐rich and Al‐poorer compositions from the belt. Chloritoid zoning in Al‐rich rocks containing assemblage I, plus chloritoid–chlorite thermometry complemented with garnet–chlorite thermometry in Al‐poorer lithologies, mean that the path is one of increasing pressure and temperature. Conditions prior to assemblage I, with earlier andalusite stable, are those of the andalusite–chloritoid– chlorite field as testified by chloritoid enclosed in andalusite porphyroblast rims. The passage from assemblage I to II implies a prograde path within the kyanite field. Assemblage III represents peak conditions, indicating a prograde staurolite‐consuming reaction across a KFMASH field, leading eventually to a locally found andalusite–biotite–muscovite hornfels. The lowest pressure stages are recorded by cordierite–biotite in Al‐poor pelites. Garnet‐bearing MnKFMASH assemblages in Al‐poorer pelites record conditions similar to assemblages II and III. The replacement of andalusite by kyanite in assemblage I is attributed to downdragging of andalusite‐bearing rocks into a synform as testified by the strained andalusite porphyroblasts affected by a subvertical crenulation cleavage. Prograde metamorphism in the eastern contact of the belt is due to heat transferred to the belt from the ascending high grade antiform across the Vivero fault.
Transformation of magnetite to hematite and its influence on the dissolution of iron oxide minerals
Authors: LAGOEIRO L. E.
Published: October 2004

Deformed rocks of the Itabira Iron Formation (itabirites) in Brazil show microstructural evidence of pressure solution of quartz and iron oxides; it appears that magnetite was dissolved and hematite precipitated. The dissolution of magnetite seems to be related to its transformation to hematite by oxidation of Fe2+ to Fe3+. The transformation of magnetite to hematite occurs along {111} planes, and results in the development of hematite domains along {111} that are parallel to the foliation. The difference in volume created by the transformation of magnetite to hematite and the shear stress acting on the interphase boundaries allow fluids to migrate along these planes. The dissolution of magnetite involves the hydrolyzation of the Fe2+—O bonds at interphase boundaries of high normal stress. The high fugacity of oxygen in the fluid phase promotes the reaction of Fe2+ (in solution) with oxygen. Fe2+ ions oxidize to Fe3+ and precipitate as hematite platelets with their longest axes oriented parallel to the direction of maximum stretching. The transformation of magnetite to hematite during deformation plays an important role in the fabric evolution of the iron formation rocks. The transformation along {111} creates planes of weakness that facilitate fracturing. The fracturing plus the dissolution result in a reduction of magnetite grain size, and the oriented precipitation results in layers of hematite platelets. These processes produce a new fabric characterized by a penetrative foliation and lineation.
Petrology of an intrusion‐related high‐grade migmatite: implications for partial melting of metasedimentary rocks and leucosome‐forming processes
Authors: JUNG S. , MEZGER* K. , MASBERG P. , HOFFER E. , HOERNES S.
Published: October 2004

Intrusion‐related migmatites comprise a substantial part of the high‐grade part of the southern Damara orogen, Namibia which is dominated by Al‐rich metasedimentary rocks and various granites. Migmatites consist of melanosomes with biotite+sillimanite+garnet+cordierite+hercynite and leucosomes are garnet‐ and cordierite‐bearing. Metamorphic grade throughout the area is in the upper amphibolite to lower granulite facies (5–6 kbar at 730–750 °C). Field evidence, petrographic observations, chemical data and mass balance calculations suggest that intrusion of granitic magmas and concomitant partial melting of metasedimentary units were the main processes for the generation of the migmatites. The intruding melts were significantly modified by magma mixing with in situ partial melts, accumulation of mainly feldspar and contamination with garnet from the wall rocks. However, it is suggested that these melts originally represented disequilibrium melts from a metasedimentary protolith. The occurrence of LILE‐, HFSE‐ and LREE‐enriched and ‐depleted residues within the leucosomes implies that both quartzo‐feldspathic and pelitic rocks were subjected to partial melting. Isotope ratios of the leucosomes are rather constant (143Nd/144Nd (500 Ma): 0.511718–0.511754, ε Nd (500 Ma): −3.54 to −5.11) and Sr (87Sr/86Sr (500 Ma): 0.714119–0.714686), the metasedimentary units have rather constant Nd isotope ratios (143Nd/144Nd (500 Ma): 0.511622–0.511789, ε Nd (500 Ma): −3.70 to −6.93) but variable Sr isotope ratios Sr (87Sr/86Sr (500 Ma): 0.713527–0.722268). The most restitic melanosome MEL 4 has a Sr isotopic composition of 87Sr/86Sr (500 Ma): 0.729380. Oxygen isotopes do not mirror the proposed contamination process, due to the equally high δ18O contents of metasediments and crustal melts. However, the most LILE‐depleted residue MEL 4 shows the lowest δ18O value (<10). Mass balance calculations suggest high degrees of partial melting (20–40%). It is concluded that partial melting was promoted by heat transfer and release of a fluid phase from the intruding granites. High degrees of partial melting can be reached as long as the available H2O, derived from the crystallization of the intruding granites, is efficiently recycled within the rock volume. Due to the limited amounts of in situ melting, it seems likely that such regional migmatite terranes are not the sources for large intrusive granite bodies. The high geothermal gradient inferred from the metamorphic conditions was probably caused by exhumation of deep crustal rocks and contemporaneous intrusion of huge masses of granitoid magmas. The Davetsaub area represents an example of migmatites formed at moderate pressures and high temperatures, and illustrates some of the reactions that may modify leucosome compositions. The area provides constraints on melting processes operating in high‐grade metasedimentary rocks.
Dehydration‐related deformation during regional metamorphism, NW Sardinia, Italy
Authors: SIMPSON G. D. H.
Published: October 2004

The relationship between deformation and dehydration has been investigated in Hercynian regionally metamorphosed rocks exposed on NW Sardinia. Two episodes of prograde mineral growth (M1 & M2) involving dehydration are recognized: growth of chlorite/phengite porphyroblasts at anchizone metamorphic conditions, contemporaneous with the first phase of deformation, D1, and growth of biotite from chlorite and phengite coincident with the second phase of deformation, D2. Deformation during both episodes of dehydration is characterized by penetrative axial planar foliations defined by well‐developed phyllosilicate preferred orientations quantified by XRD textural goniometry, tight to isoclinal similar folds (interlimb angles <40°), and mineral‐filled veins (hydrofractures) orientated parallel to axial planar foliations, that formed contemporaneously with the development of the penetrative foliations. No prograde mineral growth occurred during D2 at chlorite‐zone conditions. D2 deformation in the absence of dehydration is characterized by non‐penetrative crenulation cleavages, poorly developed phyllosilicate preferred orientations, relatively open (interlimb angles >40°), low‐strain similar folds and minor brittle deformation. Systematic variations in macrofold interlimb angles, with respect to the timing of mineral growth, indicate that enhanced shortening (c. 80%) occurred during dehydration. Microfabrics show that the onset of dehydration is associated with the transition from a crenulation cleavage to a penetrative foliation. The presence of axial planar hydrofractures that formed coevally with dehydration and fabric development requires that supralithostatic fluid pressures and low differential stresses (<c. 20 MPa) accompanied dehydration. These features demonstrate a connection between the timing of dehydration and the style of deformation.
High‐pressure granulite facies metamorphism in the Pan‐African belt of eastern Tanzania: P–T–t evidence against granulite formation by continent collision
Authors: APPEL P. , MÖLLER* A. , SCHENK V.
Published: October 2004

To constrain the tectonic history of the Pan‐African belt in Tanzania, we have studied the P–T  evolution of granulites from northern and eastern Tanzania representative for a large part of the southern Pan‐African belt of East Africa (e.g. Pare, Usambara, Ukaguru and Uluguru Mountains). Thermobarometry (conventional and multireaction equilibria) on enderbites and metapelites gives 9.5–11 kbar and 810±40 °C during peak metamorphism at 650–620 Ma. This is consistent with the occurrence of both sillimanite and kyanite in metapelites and of the high‐P granulite facies assemblage garnet–clinopyroxene–quartz in mafic rocks. Peak metamorphic conditions are surprisingly similar over a very large area with N‐S and E‐W extents of about 700 and 200 km respectively. The prograde metamorphic evolution in the entire area started in the kyanite field but evolved mainly within the sillimanite stability field. The retrograde P–T  evolution is characterized by late‐stage kyanite in metapelites and garnet–clinopyroxene coronas around orthopyroxene in meta‐igneous rocks. This is in agreement with thermobarometric results and isotopic dating, indicating a period of nearly isobaric and slow cooling prior to tectonic uplift. The anticlockwise P–T  path could have resulted from magmatic underplating and loading of the lower continental crust which caused heating and thickening of the crust. Substantial postmetamorphic crustal thickening of yet unknown age (presumably after 550 Ma) led subsequently to the exhumation of high‐P granulites over a large area. The results are consistent with formation of the Pan‐African granulites at an active continental margin where tonalitic intrusions caused crustal growth and heating 70–100 Ma prior to continental collision. The P–T–t path contradicts recent geodynamic models which proposed tectonic crustal thickening due to continental collision between East and West Gondwana as the cause of granulite formation in the southern part of the Pan‐African belt.
Metamorphic evolution of a subduction complex, South Shetland Islands, Antarctica
Authors: TROUW R. A. J. , SIMÕES L. S. A. , VALLADARES C. S.
Published: October 2004

A subduction complex composed of ocean floor material mixed with arc‐derived metasediments crops out in the Elephant Island group and at Smith Island, South Shetland Islands, Antarctica, with metamorphic ages of 120–80 Ma and 58–47 Ma, respectively. Seven metamorphic zones (I–VII) mapped on Elephant Island delineate a gradual increase in metamorphic grade from the pumpellyite–actinolite facies, through the crossite–epidote blueschist facies, to the lower amphibolite facies. Geothermometry in garnet–amphibole and garnet–biotite pairs yields temperatures of about 350 °C in zone III to about 525 °C in zone VII. Pressures were estimated on the basis of Si content in white mica, Al2O3content in alkali amphibole, NaM4/AlIVin sodic‐calcic and calcic amphibole, AlVI/Si in calcic amphibole, and jadeite content in clinopyroxene. Mean values vary from about 6–7.5 kbar in zone II to about 5 kbar in zone VII. Results from the other islands of the Elephant Island group are comparable to those from the main island; Smith Island yielded slightly higher pressures, up to 8 kbar, with temperatures estimated between 300 and 350 °C. Zoned minerals and other textural indications locally enable inference ofP–T –ttrajectories, all with a clockwise evolution. A reconstruction in space and time of theseP–T –tpaths allows an estimate of the thermal structure in the upper crust during the two ductile deformation phases (D1& D2) that affected the area. This thermal structure is in good agreement with the one expected for a subduction zone. The arrival and collision of thickened oceanic crust may have caused the accretion and preservation of the subduction complex. In this model, D1represents the subduction movements expressed by the first vector of the clockwiseP–T–tpath, D2reflects the collision corresponding to the second vector with increasing temperature and decreasing pressure, and D3corresponds to isostatic uplift accompanied by erosion, under circumstances of decreasing temperature and pressure.
The retrograde P–T–t path for low‐pressure granulites from the Reynolds Range, central Australia: petrological constraints and implications for low‐P/high‐T metamorphism
Authors: BUICK I. S. , CARTWRIGHT I. , HARLEY S. L.
Published: October 2004

Several aspects of the petrogenesis of low‐pressure granulite facies rocks from the Reynolds Range (central Australia) are contentious, including: (a) the shape of the retrograde P–T –time path, and whether it is an artefact of repeated thermal events at different P–T  conditions; (b) the type of regional metamorphism; and (c) the causes of metamorphism. Granulite facies rocks from the Reynolds Range Group experienced three major periods of mineralogical equilibration. Metapelitic rocks underwent dehydration‐melting reactions to form migmatites under peak M2 P–T  conditions of c. 5.0–5.3 kbar and c. 750–800 °C. Metapsammitic rocks that did not melt during M2 show spectacular garnet–orthopyroxene intergrowths that developed at c. 3.5–3.7 kbar and c. 700–750 °C after penetrative regional deformation, but prior to amphibolite facies rehydration in discrete strike‐parallel zones. Rehydration occurred within the sillimanite stability field at P–T conditions close to the granite solidus (c. 3.2–3.4 kbar and 650–700 °C). Subsequently the terrane cooled into the andalusite stability field. Geochronological constraints suggest that: (a) peak‐M2 conditions were reached at c. 1594 Ma; (b) the garnet–orthopyroxene intergrowths in unmelted metapsammites probably developed between c. 1594 Ma and c. 1586 Ma; and (c) upper amphibolite facies rehydration occurred between c. 1586 Ma and 1568 Ma. The lack of petrological evidence for multiple dehydration and rehydration of the rocks suggests that the three episodes of mineralogical recrystallization can be linked to yield a single continuous retrograde P–T–t path of minor initial decompression (c. 1.5 kbar) from the M2 peak, followed by cooling (c. 100 °C) to the granite solidus over a period of c. 26 Ma. Late kyanite‐bearing shear zones that dissect the terrane are unrelated to this event and formed during the c. 300–400 Ma Alice Springs Orogeny. The shape of the P–T–t path and the duration of M2 metamorphism suggests that advective heating was not the major cause of high‐grade metamorphism, and that some other, longer lived heat source, such as the burial of anomalously radiogenic, pre‐tectonic granites, is required.
Tectonothermal history of the western Lachlan Fold Belt, Australia: insights from white mica studies
Authors: OFFLER R. , MCKNIGHT S. , MORAND V.
Published: October 2004

Detailed b lattice parameter and illite crystallinity (IC) studies of K‐white micas in slates from the Stawell and Ballarat‐Bendigo Zones (SZ, BBZ) in the western Lachlan Fold Belt of Victoria, Australia, reveal a metamorphic pattern characterized by regional metamorphism associated with crustal thickening and younger contact metamorphism accompanied by deformation. The IC data indicate that rocks regionally metamorphosed prior to the intrusion of the Early and Late Devonian granitoids, vary in grade from epizonal (greenschist facies) to diagenetic (zeolite facies) and that most are of epizonal to anchizonal (prehnite–pumpellyite facies) grade. In the BBZ, a decrease in grade from west to east occurs. Across fault zones, IC values show little change, indicating that limited vertical displacement has occurred. This is in accord with the thin skinned deformation model proposed for the western Lachlan Fold Belt. The b lattice parameters (x=9.022 Å; n=137; σn=0.009) indicate baric conditions intermediate between those of New Hampshire (P=Al2SiO5 triple point) and Otago (intermediate P ). Thus, a moderately low geothermal gradient existed 450–430 Ma ago, when these rocks were deformed. KD Fe/Mg (actinolite)/Fe/Mg (chlorite) values (0.52–0.70) obtained from coexisting actinolite and chlorite in metabasites from fault zones support the moderately high‐P (c. 4 kbar) metamorphism suggested by the b cell parameter values. The metamorphic conditions indicated by these data are contrary to the low‐P/high‐T  conditions proposed by previous authors, who inferred an intimate association between deformation, granitoid intrusion and gold mineralization. The b lattice parameter of white micas in slates adjacent to Early Devonian (c. 400 Ma) granitoids with schist bearing aureoles in the north‐eastern part of the BBZ (x=9.002 Å; n=27; σn=0.007), indicate pressures in the order of c. 2.5 kbar which are in accord with those obtained from andalusite–cordierite and zoisite–garnet bearing assemblages observed in the higher grade metapelitic and calcareous rocks. This contrasts with the higher pressure (c. 4 kbar) existing during regional metamorphism and implies that c. 6.5–8 km of metasedimentary rocks in the BBZ were removed before the emplacement of the Early Devonian granitoids. Metamorphic assemblages in hornfelses associated with Late Devonian granitoids indicate a further 5–6 km of metasediment were removed in the next 40 Ma prior to their emplacement. This study shows the value of white mica studies in elucidating the tectonothermal history of a low‐grade metamorphic terrane dominated by metapelitic rocks.
Ultrahigh temperature granulite metamorphism (1050 °C, 12 kbar) and decompression in garnet (Mg70)–orthopyroxene–sillimanite gneisses from the Rauer Group, East Antarctica
Authors: HARLEY S. L.
Published: October 2004

Highly magnesian and aluminous migmatitic gneisses from Mather Peninsula in the Rauer Group, Eastern Antarctica, preserve ultrahigh temperature (UHT) metamorphic assemblages that include orthopyroxene+sillimanite±quartz, garnet+sillimanite±quartz and garnet+orthopyroxene±sillimanite. Garnet that ranges up to XMg of 71.5 coexists with aluminous orthopyroxene that shows zoning from cores with 7.5–8.5 wt% Al2O3 to rims with up to 10.6 wt% Al2O3 adjacent to garnet. Peak P–T  conditions of 1050 °C and 12 kbar are retrieved from Fe–Mg–Al thermobarometry involving garnet and orthopyroxene, in very good agreement with independent constraints from petrogenetic grids in FeO–MgO–Al2O3–SiO2 and related chemical systems. Sapphirine, orthopyroxene and cordierite form extensive symplectites and coronas on the early phases. The specific reaction textures and assemblages involving these secondary phases correlate with initial garnet XMg , with apparent higher‐pressure reaction products occurring on the more magnesian garnet, and are interpreted to result from an initial phase of ultrahigh temperature near‐isothermal decompression (UHT‐ITD) from 12 to 8 kbar at temperatures in excess of 950 °C. Later textures that involved biotite formation and then partial breakdown, along with garnet relics, to symplectites of orthopyroxene+cordierite or cordierite+spinel may reflect hydration through back‐reaction with crystallizing melts on cooling below 900–850 °C, followed by ITD from 7 to 8 kbar to c. 5 kbar at temperatures of 750–850 °C. The tectonic significance of this P–T  history is ambiguous as the Rauer Group records the effects of Archean tectonothermal events as well as high‐grade events at 1000 and 530 Ma. Late‐stage biotite formation and subsequent ITD can be correlated with the P–T  history preserved in the Proterozoic components of the Rauer Group and hence with either 1000 or 530 Ma collisional orogenesis. However, whether the preceding UHT‐ITD history reflects a temporally unrelated event (e.g. Archean) or is simply an early stage of either the late‐Proterozoic or Pan‐African tectonism, as recently deduced for similar UHT rocks from other areas of the East Antarctica, remains uncertain.
Nimesulide and indomethacin induce apoptosis in head and neck cancer cells
Authors: Pelzmann Martina , Thurnher Dietmar , Gedlicka Claudia , Martinek Helga , Knerer Birgit
Published: October 2004

Background:  Non‐steroidal anti‐inflammatory drugs (NSAIDs) are known to inhibit the enzyme cyclooxygenase (COX). There are two isoforms of the enzyme. Recent investigations indicate that both isoforms, COX‐1 and COX‐2, are involved in carcinogenesis.

Methods:  We investigated the effects of nimesulide, a COX‐2 selective and indomethacin, a non‐selective NSAID on the head and neck squamous cell carcinoma (HNSCC) cell lines SCC‐9 and SCC‐25. Effects on cell numbers and apoptosis were assayed by cell counting, immunofluorescence and fluorescence activated cell sorting (FACS). COX expression was examined by Western blotting.

Results:  The investigated cell lines express COX‐1 and COX‐2. Nimesulide and indomethacin induce apoptosis and cause a reduction of cell number. Incubation with NSAIDs upregulated COX‐2 expression.

Conclusion:  The results of our study on HNSCC cells together with data from different studies showing anti‐cancer activity of NSAIDs suggest that COX inhibitors could play a role in HNSCC treatment and prevention.
Association of Cdc42/N‐WASP/Arp2/3 Signaling Pathway with Golgi Membranes
Authors: Matas Olga B. , Martínez‐Menárguez Josè Ángel , Egea Gustavo
Published: October 2004

Recent findings indicate that Cdc42 regulates Golgi‐to‐ER (endoplasmic reticulum) protein transport through N‐WASP and Arp2/3 (Luna et al. 2002, Mol. Biol. Cell, 13:866–879). To analyse the components of the Cdc42‐governed signaling pathway in the secretory pathway, we localized Cdc42, N‐WASP and Arp2/3 in the Golgi complex by cryoimmunoelectron microscopy. Cdc42 is found throughout the Golgi stack, particularly in cis/middle cisternae, whereas N‐WASP and Arp3 (a component of the Arp2/3 complex) are restricted to cis cisternae. Arp3 also colocalized in peri‐Golgi tubulovesicular structures with either KDEL receptor or GM130. Even though Arp3 is not found in TGN46‐positive cisternal elements, a small fraction of Arp3‐labeled tubulo‐vesicular elements showed TGN46 labeling. Active Cdc42 (GTP‐bound form) induced relocation of N‐WASP and Arp3 to the lateral rims of Golgi cisternae. These results show that the actin nucleation and polymerization signaling pathway governed by Cdc42/N‐WASP/Arp operates in the Golgi complex of mammalian cells, further implicating actin dynamics in Golgi‐associated membrane trafficking.