bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrodynamic modelling of some metamorphic processes
Authors: PERCHUK L. L. , PODLADCHIKOV Y. YU. , POLYAKOV A. N.
Published: May 2007

The P–T paths for metamorphic complexes from the Precambrian shields and fold belts of different ages may result from advection, i.e. one‐cycle convective processes in the lithosphere. This conclusion has been exemplified by the metamorphic evolution of several well‐known complexes, for which an advective model can be successfully applied. Numerical simulations of the above processes in terms of Newtonian rheology by using a two‐dimensional finite element program have been conducted.

Two representative models for intracontinental gravitational ordering initiated presumably by mantle activity are considered: (i) a thermally activated multi‐layered rhythmic sequence and (ii) huge rising diapiars causing circulation, in which crustal lithologies underwent high‐P metamorphism (above 10–15 kbar) and subsequent ascent toward the Earth's surface.
Dehydration reaction and isotope front transport induced by CO2infiltration at Nuliyam, South India
Authors: JACKSON D. H. , SANTOSH M.
Published: May 2007

The field relations from a quarry at Nuliyam, South India, illustrate dehydration of an amphibolite facies gneiss to granulite facies charnockite by CO2influx, over a scale of 30 m. Both the calc‐silicate source of the fluids and the full extent of their penetration into the gneiss are preserved in a continuous section. Fluid flow is by a hydraulic fracture mechanism, but is thought to be pervasive. The sharp reaction front predicted by the continuum mechanical theory for advective fluid transport is not observed. The front spreading is on too large a scale for either diffusive or dispersive control and is due to local kinetic disequilibrium between the fluid and rock, although the divariant nature of the reaction may also have a limited effect. The time‐integrated fluid flux varies from the instantaneous porosity at the fluid front to 20 vol. % adjacent to the calc‐silicate. Carbon isotope budgets suggest that decarbonation of the calc‐silicate by a Rayleigh fractionation process provides a sufficient source for the CO2influxing into the gneiss. Graphite abundances vary from 0.01 to 0.1% (by weight), it is principally derived by precipitation from the fluid and may be modelled from phase equilibria. Carbon isotope fronts coincide with the reaction front on the scale of sampling, although isotopic disequilibrium between graphite and inclusion‐CO2also implies local fluid‐rock disequilibrium.
The (Na–Ca)amphibole–albite–chlorite–epidote–quartz geothermobarometer in the system S–A–F–M–C–N–H2O. 1. An empirical calibration
Authors: TRIBOULET C.
Published: May 2007

The proposed geothermobarometer is based on an empirical calibration which takes account of two equilibria involving the tremolite, edenite, pargasite and hastingsite components in amphiboles. It has applications to assemblages found in metabasic rocks of widely different chemical compositions (magnesian to Fe‐rich metabasalts), and for metamorphism ranging from lower greenschist to highest amphibolite facies. Knowing the Si(T1), Aliv, Alvi, Fe3+, Fe2+, Mg, Ca, NaM4, NaAand A vacancy in an amphibole, and the Al3+andXMgin coexisting epidote and chlorite, it is possible to calculate two values of InKdfor this assemblage. These equilibria involve edenite‐tremolite and (pargasite/hastingsite)‐tremolite end‐members in amphibole (the calculation program is given). For these equilibria, the isopleths (iso‐values ofKd) have been calculated for 0.27 <XMg< 0.75 and 0 <XFe3+= Fe3+/(Fe3++ Alvi) < 0.8. It is then possible to determine pressure and temperature directly whenXMg,XFe3+, InKdfor tremoliteedenite and InKdfor (pargasite/hastingsite)‐tremolite are known. Application of this geothermobarometer is limited to Ca‐free plagioclase assemblages, and completeP–Tpaths can be drawn only if all the minerals are considered together. Phase relations at successive stages of crystallization can be constrained by studying the relationships between the coexisting minerals, their zoning and the metamorphic fabrics.
Sequential, syndeformational porphyroblast growth during Hercynian low‐pressure/high‐temperature metamorphism in the Canigou massif, Pyrenees
Authors: GIBSON R. L.
Published: May 2007

The sequence of growth of garnet, staurolite and aluminosilicate in Fe‐rich metapelitic rocks from the Canigou massif, Pyrenees, is established using evidence of inclusion, reaction and pseudomorphing textures between the different minerals, compositional zoning patterns in garnet and staurolite (that can be related to the KFMASH reaction grid), and the geometric relations between inclusion trails in the porphyroblasts and the matrix microstructures. The evidence indicates that garnet and staurolite commenced growth before aluminosilicate in all cases, even where all three are in textural equilibrium. Interpretation of the reaction textures between the porphyroblasts and of the compositional zoning in garnet and staurolite in terms of the KFMASH reaction grid indicates the importance of continuous reactions in the development of these phases. Some garnet and staurolite porphyroblasts underwent renewed growth during breakdown, producing rims enriched in Mn and Zn respectively. The presence of aluminosilicate in these assemblages (i.e. the absence of a clear andalusite‐absent zone in the field) is attributed to a strong pressure‐dependence for the aluminosilicate‐producing reactions. Porphyroblast‐matrix microstructural relations indicate that Hercynian metamorphism in the massif was synchronous with the development of the regional subhorizontal foliation (S3).
Orthopyroxene–sillimanite–sapphirine granulites from the Bamble granulite terrane, southern Norway
Authors: KIHLE J. , BUCHER‐NURMINEN K.
Published: May 2007

Silica‐deficient sapphirine‐bearing rocks occur as an enclave within granulite facies Proterozoic gneisses and migmatites near Grimstad in the Bamble sector of south‐east Norway (Hasleholmen locality). The rocks contain peraluminous sapphirine, orthopyroxene, gedrite, anthophyllite, sillimanite, sapphirine, corundum, cordierite, spinel, quartz and biotite in a variety of assemblages. Feldspar is absent.

Fe2+/(Fe2++ Mg) in the analysed minerals varies in the order: spinel > gedrite ≥ anthophyllite ≥ biotite > sapphirine>orthopyroxene > cordierite.

Characteristic pseudomorph textures indicate coexistence of orthopyroxene and sillimanite during early stages of the reaction history. Assemblages containing orthopyroxene‐sillimanite‐sapphirine‐cordierite‐corundum developed during a high‐pressure phase of metamorphism and are consistent with equilibration pressures of about 9 kbar at temperatures of 750–800°C. Decompression towards medium‐pressure granulite facies generated various sapphirine‐bearing assemblages. The diagnostic assemblage of this stage is sapphirine‐cordierite. Sapphirine occurs in characteristic symplectite textures. The major mineralogical changes can be described by the discontinuous FMAS reaction: orthopyroxene + sillimanite → sapphirine + cordierite + corundum.

The disequilibrium textures found in the Hasleholmen rocks are characteristic for reactions which have been in progress but then ceased before they run to completion. Textures such as reaction rims, symplectites, partial replacement, corrosion and dissolution of earlier minerals are characteristic of granulite facies rocks. They indicate that, despite relatively high temperatures (700–800° C), equilibrium domains were small and chemical communication and transport was hampered as a result of dry or H2O‐poor conditions.
Ionic association in H2O–CO2 fluids at mid‐crustal conditions
Authors: WALTHER J. V.
Published: May 2007

The equilibrium constant, Ka, of the association reaction to form ion pairs from charged solute species in supercritical solutions can be calculated from a model based on published equations. Log Ka at constant pressure is a linear function of the inverse in the dielectric constant of the fluid times temperature. The dielectric properties of H2O and CO2 at supercritical pressures and temperatures can also be evaluated using the Kirkwood equation. Using Looyenga mixing rules, the dielectric constant of H2O–CO2 mixtures can be obtained and the change in log Ka with addition of CO2 in aqueous solutions evaluated. These changes in log Ka with addition of CO2 are consistent with measured changes of log Ka with addition of Ar in supercritical H2O–Ar solutions.

Log Ka of KCl and NaCl increase to an increasing extent as the mole fraction of CO2 increases in H2O–CO2 solutions. For instance, at 2 kbar and constant temperature between 400 and 600° C, log Ka of KCl increases by about two orders of magnitude whilst that of NaCl increases by over four orders of magnitude as the CO2 mole fraction increases from 0.0 to 0.35. Such changes in log Ka will have dramatic effects on the solubility of minerals in CO2‐rich environments.
P‐T‐X conditions of calc‐silicate formation: evidence from fluid inclusions and phase equilibria; Llano Uplift, central Texas, USA
Authors: LETARGO C. M. R. , LAMB W. M.
Published: May 2007

Abstract
The Llano Uplift in central Texas is a Grenville aged (c. 1.1 Ga) metamorphic terrane consisting predominantly of amphibolite facies mineral assemblages. The formation of these assemblages has been attributed to the emplacement of relatively late granite plutons throughout the area. Two types of granitic intrusion have previously been recognized: (1) Town Mountain Granites, which occur as relatively large, circular‐shaped bodies of coarse‐grained granite, and (2) Younger Granites which are present as smaller and more irregular bodies of finer‐grained granite. In the central part of the uplift, wollastonite‐bearing calc‐silicate rocks occur within the Valley Spring Gneiss. The development of these calc‐silicate rocks has been linked to infiltrating fluids presumably derived from spatially associated Younger Granites. The stability of coexisting quartz, calcite, wollastonite, grossular and anorthite and coexisting quartz, calcite, wollastonite, andradite and hedenbergite shows that the calc‐silicate rocks equilibrated under H2O‐rich conditions with χCO2 <0.10. Fluid inclusions present within the calc‐silicate minerals are H2O‐rich with salinities of <17 wt% equivalent NaCl. The absence of any detectable CO2 in the fluid inclusions may indicate entrapment of the inclusions at lower pressures and more H2O‐rich conditions compared to the stability of the peak metamorphic mineral assemblage. Homogenization temperatures, measured for texturally primary inclusions, range from 360 to 368° C corresponding to a density range from 0.53 to 0.82 g/cm3. Isochores for these fluid inclusions, when combined with the stability of the solid‐solid equilibria Grs + Qtz = Wo + An, yield formation conditions of 500–550° C at 1–2 kbar. This indicates that the granitic intrusions involved in the formation of the Blount Mountain calc‐silicates were emplaced at a pressure of at least 1–2 kbar.
A garnet population in Yellowknife schist, Canada
Authors: KRETZ R.
Published: May 2007

Abstract
Data are presented on a garnet population in a specimen of garnet‐biotite‐plagioclase‐quartz schist from the cordierite zone of an Archaean thermal dome in the Southern Slave Province of the Canadian Shield. Garnet crystals are bounded by planar dodecahedral faces and by trapezohedral faces which on the 10‐μm scale are corrugated. Crystal distribution, as revealed by dissection of a small cubic volume of rock, is random. The size distribution is normal, with a mean diameter of 0.81 mm and a standard deviation of 0.32 mm. In the largest crystal of the population (mean radius 0.83 mm), [Mn] = 100 Mn/(Fe + Mg + Mn + Ca) decreases from 14.5 at the centre to 7.5 and then increases in the outer margin to 8.5; [Fe] increases continuously from 67 at the centre to 77 at the surface; [Mg] increases from 12.5 to 13.5 and then falls sharply to 11; [Ca] remains unchanged at 4.0 and then drops to 3.3. Progressively smaller crystals have progressively lower [Mn] and higher [Fe] concentrations at their centres, while all crystals have the same margin composition. Growth vectors extending from given concentration contours to crystal surfaces are of equal length regardless of the size of the crystal in which the vector is located.

A garnet‐forming model is presented in which reaction was initiated by a rise in temperature. Nucleation sites were randomly selected. The nucleation rate increased with time and then declined. Crystal faces advanced at a constant linear rate, which implies an increase in volume proportional to surface area. Initially, the composition of garnet deposited on crystal surfaces was determined by van Laar equations of equilibrium, which demanded the withdrawal of Mn and Fe from within chlorite crystals. This transfer reaction was then accompanied by an ion exchange reaction which moved Mn and Fe to garnet surfaces from biotite, in exchange for Mg. The exchange reaction provides an explanation for the high overall concentration of Mn and Fe in garnet and for the observed Mn and Mg reversals in the margins of crystals. The increase of garnet volume in the garnet population is found to be parabolic, i.e. Vαα5.
Metamorphic evolution of the late Archaean Cadillac tectonic zone, McWatters, Abitibi belt, Quebec
Authors: MORIN D. , JÉBRAK M. , BEAUFORT D. , MEUNIER A.
Published: May 2007

Abstract
Archaean greenstone belts are often cut by major shear zones, for example the Cadillac tectonic zone (CTZ) of the southern Abitibi region in north‐western Quebec. At McWatters, the CTZ contains slices of metavolcanic units bounded by corridors of highly strained and altered rocks. Mineral assemblages of the metabasites record the metamorphic evolution of the CTZ.

The McWatters metabasalts and metagabbros have similar chemistry but different mineral assemblages consisting of variable amounts of actinolite, hornblende, chlorite, albite, epidote, quartz, carbonates, titanite, biotite, rutile, magnetite, ilmenite and sulphides. The different mineral assemblages, which coexist in a single tectonic slice, can be divided into three types, characterized by (A) presence of hornblende and actinolite, (B) presence of actinolite and epidote, and (C) absence of amphibole and epidote. Partial replacements indicate that these mineral assemblages are not in equilibrium. The hornblende of the least altered and deformed samples of the type A assemblage is a relict of a prograde metamorphic event, contemporaneous with the development of the main schistosity. The prograde conditions are estimated at P= 5 kbar, T= 475° C with low Pf. The more altered and deformed samples of the type C assemblage record a later retrograde metamorphic event. Conditions of the later event are estimated at P= 4 kbar, T= 400° C with higher Pf. Widespread calcite precipitation occurred during a later episode. The diversity of the mineral assemblages results from permeability variations along the high‐strain zones of the CTZ.
Thermobarometry in a subgreenschist to greenschist transition in metabasites of the Abitibi greenstone belt, Superior Province, Canada
Authors: POWELL W. G. , CARMICHAEL D. M. , HODGSON C. J.
Published: May 2007

Abstract
Mineral equilibria in the system CaO–MgO–Al2O3–SiO2–H2O provide a basis for mapping of four reaction isograds and one bathograd in the low‐pressure transition from subgreenschist to greenschist facies. Most of the Matachewan area of the Abitibi greenstone belt is in the lower‐pressure bathozone, as indicated by the widespread occurrences of the subassemblage Prh–Chl. The higher‐pressure bathozone is indicated by two occurrences of Pmp–Act–Ep–Qtz, but in these samples the bathograd is displaced to anomalously low pressure by the high Fe content of the coexisting minerals. This illustrates the need to analyse coexisting minerals, calculate activities of end‐member species, and compute P–T curves for individual samples before interpreting the isograd/bathograd pattern.

Petrographic and microprobe analysis indicates that great care must be taken in the selection of ‘equilibrium’ assemblages. Pyroxene phenocrysts in one sample are replaced by the assemblage Pmp–Act–Ep–Chl–Qtz, whereas Prh–Act–Ep–Chl–Qtz occurs in the groundmass. Compositional variation may be more cryptic, as in a sample of metabasaltic hyaloclastite that contains two spatially distinct ‘univariant’ assemblages, Prh–Pmp–Ep–Chl–Qtz and Prh–Act–Ep–Chl–Qtz, within the devitrified matrix. Whereas chlorite compositions are similar in both assemblages, prehnite and epidote in the latter assemblage are significantly richer in Fe and poorer in Al. Accordingly, the rock is interpreted to contain two distinct ‘univariant’ assemblages, rather than one ‘invariant’ assemblage (Prh–Pmp–Act–Ep–Chl–Qtz). The displaced ‘univariant’ curves for this sample intersect at 2.2 kbar and 250°C.

Taking account of all thermobarometric implications, the low‐grade limit of the greenschist facies is at 250–270°C and 2–2.5 kbar, corresponding to depths of 7–8 km. Comparison of apparent P–T conditions on both sides of the Larder Lake – Cadillac break, a regional CO2‐metasomatized fault zone that is spatially associated with many Archaean gold deposits, provides an upper limit of not more than c. 1 km for post‐metamorphic south‐side‐up, dip‐slip displacement.
40Ar/39Ar and oxygen isotope studies of polymetamorphism from Tinos Island, Cycladic blueschist belt, Greece
Authors: BRÖCKER M. , KREUZER H. , MATTHEWS A. , OKRUSCH M.
Published: May 2007

Abstract
Petrological, oxygen isotope and 40Ar/39Ar studies were used to constrain the Tertiary metamorphic evolution of the lower tectonic unit of the Cyclades on Tinos. Polyphase high‐pressure metamorphism reached pressures in excess of 15 kbar, based on measurements of the Si content in potassic white mica. Temperatures of 450–500° C at the thermal peak of high‐pressure metamorphism were estimated from critical metamorphic assemblages, the validity of which is confirmed by a quartz–magnetite oxygen isotope temperature of 470° C. Some 40Ar/39Ar spectra of white mica give plateau ages of 44–40 Ma that are considered to represent dynamic recrystallization under peak or slightly post‐peak high‐pressure metamorphic conditions. Early stages in the prograde high‐pressure evolution may be documented by older apparent ages in the high‐temperature steps of some spectra.

Eclogite to epidote blueschist facies mineralogies were partially or totally replaced by retrograde greenschist facies assemblages during exhumation. Oxygen isotope thermometry of four quartz–magnetite pairs from greenschist samples gives temperatures of 440–470° C which cannot be distinguished from those deduced for the high‐pressure event. The exhumation and overprint is documented by decreasing ages of 32–28 Ma in some greenschists and late‐stage blueschist rocks, and ages of 30–20 Ma in the lower temperature steps of the Ar release patterns of blueschist micas. Almost flat parts of Ar–Ar release spectra of some greenschist micas gave ages of 23–21 Ma which are assumed to represent incomplete resetting caused by a renewed prograde phase of greenschist metamorphism.

Oxygen isotope compositions of blueschist and greenschist facies minerals show no evidence for the infiltration of a δ18O‐enriched fluid. Rather, the compositions indicate that fluid to rock ratios were very low, the isotopic compositions being primarily controlled by those of the protolith rocks. We assume that the fundamental control catalysing the transformation of blueschists into greenschists and the associated resetting of their isotopic systems was the selective infiltration of metamorphic fluid. A quartz–magnetite sample from a contact metamorphic skarn, taken near the Miocene monzogranite of Tinos, gave an oxygen isotope temperature of 555° C and calculated water composition of 9.1%. The value of δ18O obtained from this water is consistent with a primary magmatic fluid, but is lower than that of fluids associated with the greenschist overprint, which indicates that the latter event cannot be directly related to the monozogranite intrusion.
Orthopyroxene‐bearing, mafic migmatites at Cone Peak, California: evidence for the formation of migmatitic granulites by anatexis in an open system
Authors: HANSEN E. , STUK M.
Published: May 2007

Abstract
Orthopyroxene‐bearing migmatites, exposed at the summit of Cone Peak in the Santa Lucia Range, California, offer an opportunity to explore potential links between granulite facies metamorphism and migmatite formation. Geothermobarometry indicates that the metamorphic temperatures and pressures were in the approximate ranges of 700–750° C and 7.0–7.5 kbar. The rocks at the summit comprise three domains: relatively coarse‐grained, leucocratic veins; relatively fine‐grained, biotite‐enriched zones at the margins of the veins; and a biotite–hornblende‐bearing host rock. Orthopyroxene is concentrated in the veins, which have also the highest ratio of anhydrous to hydrous minerals of the three rock types. The composition of the veins, together with their textures and modes, suggest that they formed through anatexis involving a dehydration‐melting reaction which consumed hornblende and produced orthopyroxene. Variability in mineralogy and composition indicates that there was some local migration of magma along the veins before their final solidification. The biotite‐enriched zones formed either by the concentration of residual biotite at the margins of the vein, or through the metasomatic conversion of hornblende (and/or pyroxene) to biotite, or by a combination of the two processes. Significant differences in the chemistry of the neosome (vein + biotite‐enriched zone) and the host rock rule out simple dehydration melting in a local closed system. The model that explains best the mineralogical and chemical patterns involves triggering of melting by an influx of a low‐aH2O mixed fluid which added K and Si to and removed Ca from the neosome.
Metamorphic and tectonic domains of China
Authors: SHENBAO DONG
Published: May 2007

Abstract
Ten metamorphic domains can be distinguished in China, comprising four cratonic, three intracratonic and three intercratonic domains. Each domain contains one or more metamorphic belts, each of which, in turn, contains a characteristic metamorphic facies or facies series that was formed during a distinct metamorphic epoch.

The metamorphic domains reflect the tectonic domains and tectonic evolution of China. Ancient continental nucleii in the North China and Tarim–Alxa cratons were probably unified with the Yangtze craton during the Early Proterozoic to form the China Platform. Widespread greenschist facies metamorphism, during the Middle and Late Proterozoic, accompanied by glaucophane–greenschist facies metamorphism, represents a rifting and closure event in the China Platform; a second rifting and closure event in the China Platform occurred during the Caledonian. The China and Siberian platforms were closed during the Hercynian to form the Eurasian Continent. Closure of the ancient Tethys Ocean occurred in the Indosinian epoch, and subduction and collision within Xizang (Tibet) and Taiwan occurred during Mesozoic–Cenozoic time.

The distribution in time of types of metamorphism in China suggests cyclical changes of metamorphism known as the Archaean, Proterozoic and Phanerozoic megacycles. Each megacycle since the Archaean consists of a change from progressive, low‐ to intermediate‐grade metamorphism to lower grade, greenschist metamorphism that was superimposed on a general trend in which high‐grade metamorphism became progressively less important with time. The change in metamorphic megacycles shows a general secular decrease in regional heat supply during metamorphism punctuated by episodic high‐grade, progressive metamorphism within orogenic belts.
Regional metamorphism and tectonic evolution of the Inner Mongolian suture zone
Authors: KEDONG TANG , ZHUYUN YAN
Published: May 2007

Abstract
Regional metamorphism in central Inner Mongolia has occurred during four different periods: the middle Proterozoic, the early Palaeozoic, the middle Palaeozoic and the late Palaeozoic tectonic cycles. The middle Proterozoic and late Palaeozoic metamorphic events are associated with rifting and are characterized by low‐pressure facies series. The early Palaeozoic metamorphism occurred in two stages: (1) subduction zone metamorphism resulted in paired metamorphic belts in the Ondor Sum ophiolite and Bainaimiao island arc complex; and (2) orogenic metamorphism occurred during the collision of an island arc with the continent. Two types of middle Palaeozoic metamorphism are represented: (1) subduction zone metamorphism, which affected the melange; and (2) orogenic metamorphism that resulted from continent–continent collision.
Metamorphic history and tectonic evolution of the Qinling Complex, eastern Qinling Mountains, China
Authors: ZHENDONG YOU , YUJING HAN , SHUTIAN SUO , NENSONG CHEN , ZENQIU ZHONG
Published: May 2007

Abstract
The metamorphic history and tectonic evolution of the Qinling Complex is divided into formation and modification stages. During the Proterozoic formation stage, three deformational sequences are recognized. Andalusite–muscovite, sillimanite–muscovite and sillimanite–K‐feldspar zones of amphibolite facies regional metamorphism are earlier than, or synchronous with the first or second phase of folding. Ductile shear zones were formed and Caledonian granites were emplaced during the modification stage. The granites superimposed contact aureoles (garnet–K‐feldspar zone) on the regional metamorphic fabric.

Metamorphic reactions, P–T conditions of metamorphism and P–T–t paths were estimated by analysis of mineral textures and standard thermobarometric techniques. The P–T–t path of the Proterozoic tectonometamorphic cycle shows prominent clockwise decompression. The P–T–t path of the Caledonian tectonometamorphic cycle is characterized by an early rise of pressure and temperature, followed by isothermal decompression (rapid uplift) and finally with isobaric cooling.

The P–T–t paths of the two tectonometamorphic cycles reflect two major stages of collision and uplift in the evolution of the Qinling orogenic belt during the Proterozoic and Caledonian–Hercynian periods, respectively.
High‐pressure/low‐temperature metamorphism in northern Hubei Province, central China
Authors: ZHOU G. , LIU Y. J. , EIDE E. A. , LIOU J. G. , ERNST W. G.
Published: May 2007

Abstract
The Qinling–Dabie accretionary fold belt in east‐central China represents the E–W trending suture zone between the Sino‐Korean and Yangtze cratons. A portion of the accretionary complex exposed in northern Hubei Province contains a high‐pressure/low‐temperature metamorphic sequence progressively metamorphosed from the blueschist through greenschist to epidote–amphibolite/eclogite facies. The ‘Hongan metamorphic belt’can be divided into three metamorphic zones, based on progressive changes in mineral assemblages: Zone I, in the south, is characterized by transitional blueschist–greenschist facies; Zone II is characterized by greenschist facies; Zone III, in the northernmost portion of the belt, is characterized by eclogite and epidote–amphibolite facies sequences. Changes in amphibole compositions from south to north as well as the appearance of increasingly higher pressure mineral assemblages toward the north document differences in metamorphic P–T conditions during formation of this belt. Preliminary P–T estimates for Zone I metamorphism are 5–7 kbar, 350–450°C; estimates for Zone III eclogites are 10–22 kbar, 500 ± 50°C.

The petrographic, chemical and structural characteristics of this metamorphic belt indicate its evolution in a northward‐dipping subduction zone and subsequent uplift prior to and during the final collision between the Sino‐Korean and Yangtze cratons.
Empirical tests of carbon isotope thermometry in granulites from southern California
Authors: MORRISON J. , BARTH A. P.
Published: May 2007

Abstract
‘Peak’metamorphic carbon isotope fractionations between calcite and graphite (ΔCal–Gr) in marbles and calc‐silicates from the Cucamonga granulite terrane (San Gabriel Mountains, California) range from 3.48 to 2.90%. The data are used to test three previously published calibrations of the calcite–graphite carbon isotope thermometer. An empirical calibration of the calcite–graphite carbon isotope thermometer gives temperatures of 700–750°C; a theoretical–experimental calibration of the system gives temperatures of 760°–870°C; an experimental calibration gives temperatures of 870–1300°C. Temperatures calculated using the empirical calibration are in agreement with those calculated from garnet‐based cation exchange thermometry when uncertainty is considered. Temperatures calculated using the theoretical–experimental calibration overlap the upper range of cation exchange thermometry temperatures and range to 50°C higher. The experimental calibration yields temperatures from 50 to 480°C higher than those from cation exchange thermometry. Moreover, temperatures from the experimental calibration are also inconsistent with mineral and melt equilibria in the granulite phase assemblage.

Despite the better agreement between cation exchange thermometry and the empirical calibration of the calcite–graphite system, temperatures calculated using the theoretical–experimental calibration may be real peak metamorphic temperatures. If retrograde diffusion partially reset garnet‐based cation exchange thermometers by c. 50°C, then the cation exchange temperatures are consistent with those from the theoretical–empirical calibration. Thermometric evidence from biotite dehydration melting equilibria is consistent with either the empirical calibration if melting was fluid‐present, or the theoretical–experimental calibration if melting was fluid‐absent.
Tectonic overpressure in competent mafic layers and the development of isolated eclogites
Authors: MANCKTELOW N. S.
Published: May 2007

Abstract
Variation in the state of stress during heterogeneous deformation should be reflected in variation in the effective pressure of metamorphic reactions, whether this is mean stress or the normal stress acting across the reacting interface. The magnitude of this pressure variation will determine whether it is discernible in the preserved metamorphic mineral assemblages of heterogeneously deformed rocks. The magnitude of the mean stress difference across a non‐slipping interface between two materials with viscosity ratio >c. 20:1 is effectively equal to the maximum shear stress for flow in the more viscous material. Progressive shortening of the interface results in a higher mean stress in the more competent material, whereas extension results in a lower mean stress. For high‐P/low‐T eclogite facies conditions, current experimental data indicate that clinopyroxene‐ and garnet‐rich layers of eclogite should be very strong and that pressure differences of up to 800 MPa (8 kbar) between competent layer and weaker matrix may be possible. Such high values can be obtained in widely separated competent layers for values of bulk stress in the overall multilayer that are much lower (by a factor approaching the viscosity ratio). Extrusion of material between more rigid plates, which has been proposed as a regional mechanism of lateral ‘continental escape’for both the Alps and the Himalayas, should also be accompanied by a lateral gradient in effective pressure; otherwise extrusion could not occur. Maximum mean stresses with magnitudes that are many times the maximum shear stress required for plastic flow should develop for deformation zones that are long relative to their width (e.g. around 20 times for a width‐to‐thickness ratio of 10). Tectonic overpressure in progressively shortened competent layers, particularly in regions of extrusion between more rigid plates, might help explain the occurrence of isolated layers and pods of low‐T eclogite (<550°C) with estimated peak pressures markedly in excess of those in the surrounding matrix. It cannot explain the occurrence of isolated high‐T eclogites, because at temperatures >c. 550°C, the dramatic weakening of clinopyroxene in the power‐law creep field precludes the development of significant overpressures in eclogite layers.
Palaeopressures of South Indian two‐pyroxene garnet granulites from thermochemically calibrated CMAS barometers
Authors: ECKERT J. O. , NEWTON R. C.
Published: May 2007

AbstractThe enthalpy of reaction of plagioclase and pyroxene to produce garnet and quartz has been a major source of error in granulite geobarometry because of relatively uncertain enthalpy values available from high‐temperature solution calorimetry and compiled indirectly from experimental phase equilibria. Recent, improved calorimetric measurements of ΔHRare shown to yield palaeopressures which are internally consistent between orthopyroxene and clinopyroxene calibrations for many South Indian granulites from the Archaean high‐grade terranes of southern Karnataka and northern Tamil Nadu. This represents a considerable improvement over previous calibrations, which gave disparate results for the two independent barometers involving orthopyroxene and clinopyroxene, requiring a 2‐kbar ‘empirical adjustment’to force agreement.

Palaeopressures thus calculated for 30 well‐documented two‐pyroxene garnet granulites from South India give internally consistent pressures with a mean of 8.1°1.1 kbar at 750°C, consistent with the presence of both kyanite and sillimanite in many areas. Those samples for which garnet–pyroxene exchange thermometers give plausible granulite‐range temperatures and whose minerals are minimally zoned give the best agreement of the two barometers. Samples which yield low palaeotemperatures and different rim and core compositions of minerals yield pressures for the orthopyroxene assemblage as much as 2 kbar lower than for the assemblage with clinopyroxene. This disparity probably represents post‐metamorphic‐peak re‐equilibration. We conclude that considerable confidence may be placed in geobarometry of two‐pyroxene granulites where apparent palaeotemperatures are in the granulite facies range (>700°C) and where mineral zonation is minimal. Of the several possible sets of activity–composition relations in use, those constructed from analysis of phase equilibria give slightly higher palaeopressures and appear more consistent with analytical data from the Nilgiri Hills uplift, where kyanite is the only aluminium silicate reported to be stable in peak‐metamorphic assemblages. The present results support a palaeopressure gradient, increasing generally from south to north, across the Nilgiri Hills as inferred by previous geobarometry.
Internally consistent recalibrations of mineral equilibria for geothermobarometry involving garnet–orthopyroxene–plagioclase–quartz assemblages and their application to the South Indian granulites
Authors: LAL R. K.
Published: May 2007

AbstractThree reactions are calibrated as geothermobarometers for garnet–orthopyroxene–plagioclase–quartz assemblages, namely: 1/2 ferrosilite + 1/3 pyrope ± 1/2 enstatite + 1/3 almandine (A): ferrosilite + anorthite ± 2/3 almandine + 1/3 grossularite + quartz (B); and enstatite + anorthite ± 2/3 pyrope + 1/3 grossularite + quartz (C). The internally consistent geothermobarometers based on reactions (A), (B) and (C) are calibrated from experimental data only. The thermodynamic parameters of reaction (A) are derived from published experimental data in the FMAS system (n= 104) in the range 700–1400°C and 5–50 kbar, while those for reaction (B) are derived by summation of the existing reversed experimental data of the mineral equilibria: ferrosilite ± fayalite + quartz (D) and anorthite + fayalite ± 2/3 almandine + 1/3 grossularite (E). The retrieved thermodynamic parameters for reactions (A), (B) and (C) are, respectively: (ΔH0, cal) ‐3367 ± 209, ‐2749 ± 350 and +3985 ± 545; (ΔS0, cal K−1) ‐1.634 ± 0.163, ‐8.644 ± 0.298 and ‐5.376 ± 0.391; and (ΔV01,298, cal bar−1) ‐0.024, ‐0.60946 and ‐0.5614.

On a one‐cation basis, the derived Margules parameters of the ternary Ca–Fe–Mg in garnet are:WFe–Mg= ‐1256 + 1.0 (∼0.23)T(K),WMg–Fe= 2880 ‐1.7 (∼0.13)T(K),WCa–Mg= 4047 (∼77) ‐1.5T(K),WMg–Ca= 1000 (∼77) ‐1.5T(K),WCa–Fe= ‐723 + 0.332 (∼0.02)T(K),WFe–Ca= 1090, (cal) and the ternary constantC123= ‐4498 + 1.516 (∼0.265)T(K) cal (subregular solution model of non‐ideal mixing); and Fe–Mg–Al in orthopyroxene:WFe–Mg= 948 (∼200) ‐0.34 (∼0.10)T(K),WFe–Al= ‐1950 (∼500) andWMg–Al= 0 (cal) (regular solution model of non‐ideal mixing). The anorthite activity in plagioclase is calculated by the ‘Al‐avoidance’model of subregular Ca–Na mixing commonly used for geobarometry based on reactions (B) and (C).

When the geothermobarometers are applied to garnet–orthopyroxene–plagioclase–quartz assemblages (n= 45) of wide compositional range from the Precambrian South Indian granulites, temperature ranges of 690–860°C (X= 760 ± 45°C) and pressure ranges of 5–10 kbar were obtained. TheP–Tvalues were estimated simultaneously and there is no difference in the pressure calculated fromPMg(reaction C) andPFe(reaction B). In the existing calibrations this difference is 1 kbar or more. Furthermore, there is no compositional dependence of the lnKof the experimental data in the FMAS (n= 104) and the CFMAS (n= 78) systems at different temperatures and the estimated temperatures of the South Indian granulites.
Sigmoidal inclusion trails, punctuated fabric development, and interactions between metamorphism and deformation
Authors: WILLIAMS M. L.
Published: May 2007

Porphyroblast inclusion fabrics are consistent in style and geometry across three Proterozoic metamorphic field gradients, comprising two pluton‐related gradients in central Arizona and one regional gradient in northern New Mexico. Garnet crystals contain curved ‘sigmoidal’ inclusion trails. In low‐grade chlorite schists, these trails can be correlated directly with matrix crenulations of an older schistosity (S1). The garnet crystals preferentially grew in crenulation hinges, but some late crenulations nucleated on existing garnet porphyroblasts. At higher grade, biotite, staurolite and andalusite porphyroblasts occur in a homogeneous S2 foliation primarily defined by matrix biotite and ilmenite. Biotite porphyroblasts have straight to sigmoidal inclusion trails that also represent the weakly folded S1 schistosity. Staurolite and andalusite contain distinctive inclusion‐rich and inclusion‐poor domains that represent a relict S2 differentiated crenulation cleavage. Together, the inclusion relationships document the progressive development of the S2 fabric through six stages. Garnet and biotite porphyroblasts contain stage 2 or 3 crenulations; staurolite and andalusite generally contain stage 4 crenulations, and the matrix typically contains a homogeneous stage 6 cleavage.

The similarity of inclusion relationships across spatially and temporally distinct metamorphic field gradients of widely differing scales suggests a fundamental link between metamorphism and deformation. Three end‐member relationships may be involved: (1) tectonic linkages, where similar P‐T‐time histories and similar bulk compositions combine to produce similar metamorphic and structural signatures; (2) deformation‐controlled linkages, where certain microstructures, particularly crenulation hinges, are favourable environments for the nucleation and/or growth of porphyroblasts; and (3) reaction‐controlled linkages, where metamorphic reactions, particularly dehydration reactions, are associated with an increase in the rate of fabric development. A general model is proposed in which (1) garnet and biotite porphyroblasts preferentially grow in stage 2 or 3 crenulation hinges, and (2) chlorite‐consuming metamorphic reactions lead to pulses in the rate of fabric evolution. The data suggest that fabric development and porphyroblast growth may have been quite rapid, of the order of several hundreds of thousands of years, in these rocks. These microstructures and processes may be characteristic of low‐pressure, first‐cycle metamorphic belts.
Jadeitites, albitites and related rocks from the Motagua Fault Zone, Guatemala
Authors: HARLOW G. E.
Published: May 2007

Jadeitites from Guatemala are found as weathered blocks in tectonized serpentinite in a 15‐km zone north of the Motagua Fault Zone. Rock types found with jadeitite include albitites, albite‐mica rocks, omphacite/taramitic amphibole‐bearing metabasites, chlorite‐actinolite schists, talc‐carbonate rocks and antigorite schists. In addition to the predominant jadeitic (Jd93_100) pyroxene, common phases in jadeitite include micas (paragonite and/or phengite ± rarer phlogopite), omphacite, albite, titanite /Pm zircon, apatite and graphite. Conditions of jadeitite formation are 100‐400d̀ C, 5‐11 kbar with 0.0 > log10asio2≥= 0.7. Fluid inclusions, coarse textures, vein structures, and rhythmic zoning of pyroxene indicate an aqueuos fluid was involved. Jadeitites are either (1) metasomatic modifications of former felsic‐to‐pelitic inclusions that have undergone silica depletion plus efficient soda exchange and enrichment, or (2) solution precipitations derived from such a source. The close spatial relationship of faults and shear zones, serpentinites, and jadeitites suggests jadeitites form in a relatively high‐P/Tsetting with substantial flow of sodic fluid in a tectonized zone.

Most Guatemalan jadeitites are extensively altered to analcime, albite, taramitic amphibole, (clino)zoisite ± nepheline and preiswerkite. This alteration reflects depressurization /Pm heating to below the jadeite + fluid = analcime reaction at highaNa. With progressive alteration, analcime and nepheline are replaced by albite; the increase in silica content may result from fluid flowing up a tectonized zone reaching saturation with an albite assemblage. Albitite phases, albite, actinolite, zoisite, /Pm chlorite, phengite, K‐feldspar and quartz, record conditions ofc. 3‐8 kbar atT< 400d̀ C, indicating a clockwiseP‐Ttrajectory of the blocks.

Barium aluminosilicates—banalsite, celsian, cymrite and hyalophane—are common minor late‐stage phases in jadeitites and albite‐rich rocks. Barian phengite is common in albite‐mica rocks.
Interlaboratory standardization and calibration of day mineral crystallinity and crystallite size data
Authors: WARR L. N. , RICE A. H. N.
Published: May 2007

Clay mineral crystallinity and crystallite (domain) size data determined by X‐ray diffraction (XRD) are methods extensively used in the characterization of very low‐grade metamorphic conditions. However, the lack of sufficient interlaboratory standardization has made comparisons between different research groups unreliable due to significant variations in numerical results obtained, a consequence of the different machine conditions, measurement methods and sample preparations used during analysis. A calibration approach to the standardization of data using rock chip standards is presented, which allows data sets produced by different research groups to be directly and quantitatively compared. A standardized scale, the crystallinity index standard (CIS), is proposed, with illite crystallinity anchizonal boundary limits of 0.25d̀Δ2θ and 0.42d̀Δ2θ, and equivalent illite crystallite sizes of 52 and 23 nm, respectively, determined by the Warren‐Averbach method. Calibrating both old and new data will enable more reliable comparisons between similar and contrasting geological environments, and should improve the accuracy and reliability of correlations made between XRD data and other indicators of very low‐grade metamorphism, hence increasing the value of such clay mineral studies.
Oxygen isotope exchange and closure temperatures in cooling rocks
Authors: JENKIN G. R. T. , FARROW C. M. , FALLICK A. E. , HIGGINS D.
Published: May 2007

Retrograde exchange of oxygen isotopes between minerals in igneous and metamorphic rocks by means of diffusion is explored using a finite difference computer model, which predicts both the zonation profile of δ18O within grains, and the bulk δ18O value of each mineral in the rock. Apparent oxygen isotope equilibrium temperatures that would be observed in these rocks are calculated from the δ18O values of each mineral pair within the rock. In systems which cool linearly from a sufficiently high temperature or at a low enough cooling rate, such that the final oxygen isotope values are not dependent upon the initial oxygen isotope values (‘slow cooling’), the apparent oxygen isotope temperature derived for a rock composed of a single mineral pair can be shown to be simply related to the Dodson closure temperatures (Tc) for the two phases and the mode of the rock. Adding a third phase into a system which undergoes ‘slow’ cooling will cause the apparent temperature derived for the two minerals already present to differ from the simple relationship for a two‐phase system. In some systems oxygen isotope reversals can be developed. If cooling is not ‘slow’, then the mineral δ18O values resulting from cooling will be partly dependent upon the initial temperature of the system concerned. The model successfully simulates the mineral δ18O values that are often observed in granitic rocks. Application of the model will help in assessing the validity of oxygen isotope thermometry in different geological settings, and allows quantitative prediction of the oxygen isotope fractionations that are developed in cooling closed systems.
Interdependence of deformation, fluid infiltration and reaction progress recorded in eclogitic metagranitoids (Sesia Zone, Western Alps)
Authors: FRÜH‐GREEN G. L.
Published: May 2007

The effects of high‐strain deformation and fluid infiltration during Alpine eclogite facies metamorphism have been studied across ductile shear zones in relatively undeformed metagranitoids at Monte Mucrone (Sesia Zone, Western Alps, Italy). Microfabrics together with bulk rock and stable isotope data indicate that the mineralogical and chemical variations are related to the degree of deformation, rather than to changes in P‐T conditions or tectonic position. Transformation of meta‐quartz diorite to recrystallized eclogitic mylonites involved the breakdown of biotite and plagioclase and required the influx of H2O. Bulk‐rock geochemical data show that ductile deformation to form eclogitic mylonites involved an increase in volume with a weight percent gain in H2O and Si and variable loss of K, Na, Ca and Al. δ18O changes systematically across ductile shear zones into the undeformed country rocks. Constant values in shear zone centres indicate advection parallel to the shear zone and within 10 cm of the mylonites. A dominant component of diffusive oxygen exchange perpendicular to the shear zones produced isotopic fronts, evident from a gradual increase in δ18O values to the reference values of the country rocks. The degree of isotopic shift within the shear zones reflects increasing deformation and degree of reaction progress.

Multiple phases of Alpine deformation and mineral growth are recognized in the Monte Mucrone metagranitoids, and in some cases, eclogite facies shear zones were reactivated under greenschist facies conditions. The results of this study suggest that high‐strain deformation provided pathways for both synkinematic and post‐kinematic metamorphic fluids which were necessary for complete reactions. Relict igneous fabrics, as well as the presence of corona textures around biotite and pseudomorphs after primary igneous plagioclase in the least deformed rocks, indicate a paucity of hydrous fluids and support the conclusion that fluid movement was channelled rather than pervasive.