bluuebunny/crossref_metadata_2025 huggingface dataset:
On the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling
Authors: SPEAR F. S.
Published: May 2007

Abstract
Finite difference models of Fe‐Mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet‐biotite Fe‐Mg exchange thermometry. For rocks cooled from high grades where the garnet was initially homogeneous, the calculated temperature (Tcalc) using garnet core and matrix biotite depends on the size of the garnet, the ratio of garnet to biotite in the rock (Vgarnet/Vbiotite) and the cooling rate. For garnets with radii of 1 mm and Vgarnet/Vbiotite<1, Tcalc is 633, 700 and 777°C for cooling rates of 1, 10 and 100°C/Ma. For Vgarnet/Vbiotite= 1 and 4 and a cooling rate of 10° C/Ma, Tcalc is approximately 660 and 610° C, respectively. Smaller and larger garnets have lower and higher Tcalc, respectively. These results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below Tcalc of the garnet core, provided that Vgarnet/Vbiotite is sufficiently small (<0.1) and that the composition of the biotite at the metamorphic peak has not been altered during cooling.

Numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550–600° C generate a ‘well’in Fe/(Fe + Mg) near the rim during cooling. Maximum calculated temperatures for the assemblage garnet + chlorite + biotite + muscovite + plagioclase + quartz using the Fe/(Fe + Mg) at the bottom of the ‘well’with matrix biotite range from 23–43° C to 5–12° C below the peak metamorphic temperature for cooling rates of 1 and 100° C/Ma, respectively. Maximum calculated temperatures for the assemblage garnet + staurolite + biotite + muscovite + plagioclase + quartz are approximately 70° C below the peak metamorphic temperature and are not strongly dependent on cooling rate. The results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet‐biotite Fe‐Mg exchange thermometry on amphibolite facies rocks (Tmax > 550° C) because the rim composition of the garnet, which is required to calculate the peak temperature, is that most easily destroyed by diffusion.
Allochemical retrograde metamorphism in shear zones: an example in metapelites, Virginia, USA
Authors: GATES A. E. , SPEER J. A.
Published: May 2007

Abstract
Metapelites in the Altavista area, southwest Virginia Piedmont, USA, underwent allochemical hydrothermal retrograde metamorphism in synmetamorphic shear zones. The metapelites of the Evington Group were metamorphosed in a prograde sequence of chlorite, staurolite, and sillimanite zones. Garnet–biotite geothermometry and phase relations support eastward increasing metamorphic grade, ranging from 570° C in the staurolite zone to 650° C in the sillimanite zone at c. 5.8 kbar. Sillimanite‐zone rocks later underwent progressive retrogression around shear zones which acted as fluid conduits. Retrograde assemblages are successively zoned around the shear zones with staurolite‐, chloritoid‐ and kyanite‐bearing assemblages. The shear zones commonly contain kyanite or tourmaline veins. Applicable phase equilibria indicate that retrogression occurred during isobaric cooling through c. 200–270° C. Rock compositional changes with retrogression occurred in steps: SiO2 was gained in the early stages of the retrogression but lost in the late stages; Al2O3, K2O, and H2O were increasingly gained through the sequence; CaO was increasingly lost. Addition of H2O and decreasing temperatures resulted in new ferromagnesian minerals (staurolite, chloritoid, chlorite) and changes in H2O, SiO2, Al2O3, K2O, and CaO contents produced muscovite and sodic plagioclase.

Subsequent to prograde metamorphism, deeply derived fluids migrated upwards along shear zones, providing fluid and energy for the retrograde reactions. The sheared rocks underwent fluid infiltration with fluid fluxes of 1.8 × 107–4.3 × 107 cm3/cm2 corresponding to minimum estimated fluid‐to‐rock ratios of 7.5–21 as a function of position within the shear zone. Fluid flow was from high to low temperature early and low to high temperature later in the retrogression.
Compositional variations in mafic phyllosilicates from regional low‐grade metabasites and application of the chlorite geothermometer
Authors: BEVINS R. E. , ROBINSON D. , ROWBOTHAM G.
Published: May 2007

Abstract
Mafic phyllosilicates in metabasites affected by low‐grade regional metamorphism from Wales and eastern North Greenland show variations in their structure and chemistry. These variations are related to four mineral zones in these metabasites, which are recognized on the presence/absence of various key calc‐silicate minerals and also actinolite. Zones 1 and 2 equate with the zeolite facies, zone 3 with the prehnite–pumpellyite facies (or prehnite–actinolite facies in rocks with appropriate bulk rock composition) and zone 4 with the greenschist facies. Whilst variations in Fe/(Fe + Mg) in chlorite correlate closely with Fe/(Fe + Mg) ratios in the whole‐rock, other chemical variations are clearly unrelated to whole‐rock compositions. Contents of Aliv are seen to increase systematically in samples from zone 1 through to zone 4, which relate to an increase in temperature. Calibration of alteration temperatures, calculated using the chlorite geothermometer (based on Aliv contents) developed for meta‐andesites in the Los Azufres geothermal system (Mexico), against x values (an estimate of the proportion of chlorite to swelling component in the mafic phyllosilicates) shows a decrease in the swelling component in passing from zone 1 to zone 4, i.e. with an increase in temperature. Calculated temperatures compare favourably with published stability estimates for the various key calc‐silicates and actinolite. These data indicate that the chlorite geothermometer, although developed for meta‐andesites from a hydrothermal system, does show a correlation with temperatures estimated from calc‐silicate assemblages in metabasites affected by low‐grade metamorphism developed on a regional scale.
Chlorite crystallinity: an empirical approach and correlation with illite crystallinity, coal rank and mineral facies as exemplified by Palaeozoic and Mesozoic rocks of northeast Hungary
Authors: ÁRKAI P.
Published: May 2007

Abstract
Fairly strong (r= 0.75–0.85) positive linear correlations were found between crystallinity indices (peak widths) measured on the first two basal reflections of chlorite and those of illite–muscovite in <2‐μm fractions of a representative shale–slate–phyllite series from Palaeozoic and Mesozoic formations of northeast Hungary. The metamorphic grade ranges from late or deep diagenesis through anchizone to epizone conditions. Chlorite crystallinity values measured on air‐dried and ethylene‐glycol‐solvated samples suggest that the effects of expandable interlayers are negligable, especially in the higher grade (∼temperature) part of the series. However, the greater scattering of crystallinity values for the chlorite 001 reflection compared to those of the 002 reflection may be related to the effects of minor amounts of interlayered and/or discrete smectite and/or vermiculite. With increasing metamorphic grade and advancing equilibrium recrystallization, the chlorite compositions in different samples become more homogenous. No correlation exists between crystallinity and changes in chlorite composition as estimated from the intensity ratios of basal reflections. Hence an increase of domain size and a decrease of lattice distortion with increasing grade (∼temperature) may be decisive factors affecting chlorite crystallinity.

Chlorite crystallinity can be applied as a reliable regional, statistical technique complementary with, or instead of, the illite crystallinity method. The illite and chlorite crystallinity scales used here are related to Kübler's epi‐, anchi‐ and diagenetic zones and correlated with coal rank, conodont colour alteration and mineral facies data. As the effects of the detrital white mica can be observed even in the <2‐μm fractions of anchizonal metapelites, the anchizone boundaries determined solely on the base of ‘fixed’illite crystallinity values may vary with amounts of detrital and newly formed muscovite–illite. Hence a complex approach utilizing more than one method for determination of grade is preferred for petrogenetic purposes, even if relationships between crystallinity scales, coal rank and mineral facies also vary strongly in different tectonic settings and lithologies.
Development of slaty cleavage and degree of very‐low‐grade metamorphism: a review
Authors: KISCH H. J.
Published: May 2007

Abstract
Illite crystallinity (IC) and other indicators of the grade of very‐low‐grade metamorphism associated with the appearance of various stages of slaty cleavage in phyllosilicate‐rich rocks have been compiled from a wide variety of terranes. IC values have been converted to a Kübler‐equivalent standard scale, but the diverse characterizations of the cleavage fabrics in published descriptions do not always allow an unequivocal identification of equivalent stages of cleavage development.

Nevertheless, there exists a distinct relationship between grade and the appearance of various stages of cleavage development.

(1) Indications of incipient slaty cleavage, such as S0–S1 pencil structure, appearance of primary (S1) crenulation cleavage and of closely‐spaced cleavage without parallel fabric in the microlithons, is associated with a wide range of mostly medium‐ and high‐grade diagenetic IC values.

(2) The appearance of smooth cleavage with a strong parallel fabric in the microlithons and/or quartz–mica ‘beards’and the chlorite–mica stacks shortened at a high angle to (001), and of irregular cleavage in sandy beds is associated with a much narrower range of predominantly low‐ and medium‐grade anchimetamorphic grades (rarely high‐grade diagenetic). The first appearance of these stages of cleavage development with higher grades can often be related to post‐kinematic magmatic heating, polymetamorphism (pre‐cleavage metamorphism), or ‘static’recrystallization without cleavage formation, for example in low‐strain zones.

There exists a relationship between finite strain, fabric and metamorphic grade in mudstones and slates; in coarser clastic rocks the same finite strain–fabric relationship occurs at appreciably higher grades. A relationship between finite strain in carbonate rocks and IC in the nearby rocks has been reported from the Helvetic zone of the Swiss Alps.

The earlier stages of cleavage formation are associated with little improvement in IC; the narrow range of IC associated with smooth cleavage is concluded to represent recrystallization and grain growth concurrent with cleavage formation.
Variscan very low‐grade metamorphism in southwest England: a diastathermal and thrust‐related origin
Authors: WARR L. N. , PRIMMER T. J. , ROBINSON D.
Published: May 2007

AbstractRegional metamorphism in the external Variscides of southwest England varied from diagenetic level to greenschist facies. There is a fundamental difference in the metamorphic character between the northern and southern regions of the area. In the north, M1 metamorphism is of a sedimentary burial character associated with high heat flow, whilst to the south it is related to tectonic burial during thrust thickening processes, with lower geothermal gradients. This pattern appears to be related to the character of basin development and its subsequent tectonic evolution. The northern region has features that accord with a diastathermal (extensional) origin for the very low‐grade metamorphism whilst in the southern region the very low‐grade metamorphism is linked to thrusting as a consequence of Variscan compression. The Tintagel High‐Strain Zone presents an anomaly in this regional pattern where an M2 metamorphic phase is attributed to localized D2 thrust stacking along the southern margin of the Culm Basin.

There is no extensive overprint of the regional metamorphic pattern by the contact aureoles surrounding the granite plutons of the region. However, there is a noticeable coincidence between the areas of regional epizone grade and the extent of the geophysically defined subsurface limit of the granite batholith (excluding the North Devon area). This link is attributed largely to the late‐stage structural up‐doming of the higher grade areas over the roof of the batholith.
Diagenetic and low‐grade metamorphic terranes of Gaspé Peninsula related to the geological structure of the Taconian and Acadian orogenic belts, Quebec Appalachians
Authors: HESSE R. , DALTON E.
Published: May 2007

Abstract
Illite crystallinity (IC) measurements, determination of the proportion of 2M mica‐polytypes and organic‐matter reflectance measurements establish regional diagenetic/low‐grade metamorphic trends for the Taconian and Acadian belts of Gaspé Peninsula.

IC varies as a function of many factors besides maximum burial temperature and heating time. Correlation between IC and %2M illite polytypes for the Fortin Group and Temiscouata Formation suggests (i) that the amount of high‐grade detrital mica in the samples is low, and (ii) that IC can be used with some confidence as an estimator of regional thermal maturation levels. Correlation of these parameters with available organic reflectance values further supports this assumption. The illites of the Temiscouata and western Fortin groups are mostly phengitic in composition, whereas in the eastern outcrop belt they are more Mg‐ and Fe‐rich (celadonitic), but generally also of lower grade and lower 2M content. The d(060) values for illites measured on the unorientated <2‐μm fraction of samples fall between 1.502 and 1.503 Å (range: 1.500–1.504 Å), indicating relatively low octahedral occupancy by Mg and Fe (between one‐fifth and one‐third of the available spaces). Pyrophyllite and paragonite were not detected. Chlorites are Fe‐rich and ripidolitic.

The IC map for the Acadian belt of the peninsula displays general congruence between IC contours (2200 sample points) and structural trends for the 27,000‐km2 area. The highest grades (anchimetamorphic) are associated with the oldest rocks (Honorat and Matapedia groups) exposed in the cores of major anticlines. Anchimetamorphic grades associated with the western outcrop belt of the Lower Devonian Fortin Group require 7–8 km of subsidence to accommodate sufficient thickness of overlying younger rocks (on top of 4–5 km of Fortin Group deep‐water clastics) to explain the grades in terms of burial metamorphism assuming a geothermal gradient of 30° C km−1. The lowest‐grade diagenetic rocks occupy a large area in the northeastern part of the peninsula, smaller areas in the northwestern part of the Acadian belt, in the centre of Chaleurs Bay synclinorium, and in the Ordovician Mictaw Group. The contact between the Taconian and Acadian belt is marked by a distinct maturation discontinuity. The Grand Pabos fault juxtaposes rocks of contrasting maturation levels (Matapedia Group against Fortin Group) in the west, but shows no maturation offset further east in the Honorat Group. The fault zone limiting the Fortin Group in the north is also associated with a major IC jump.
Effects of fluid production on fluid flow during regional and contact metamorphism
Authors: HANSON R. B.
Published: May 2007

Numerical and analytical models of fluid flow that account for fluid production during prograde regional and contact metamorphism show that expulsion of metamorphic fluids dominates the convective flux when crustal permeabilities are less than 0.1–100 μD, depending primarily on the rate of fluid production. When this is the case, fluid circulation is limited or prevented, fluid pressures are elevated above hydrostatic values, and flow throughout most of the model is up and away from the region of maximum fluid production. Fluid circulation is predicted to occur where permeability is high, in dry rocks, or after rates of fluid production decrease as peak temperatures are reached. Large changes in the pattern of flow and influx of externally derived fluids may thus occur in metamorphic terranes when dehydration wanes or ceases and cooling begins. Inclusion of an impermeable horizon in the models further inhibits fluid circulation. Earlier, shallow hydrothermal models and interpretations based on the Rayleigh number may be inappropriate for characterizing fluid flow during prograde metamorphism at depth because they do not account for fluid production.
Prehnite–pumpellyite to greenschist facies transition, Smartville Complex, near Auburn, California
Authors: SPRINGER R. K. , DAY H. W. , BEIERSDORFER R. E.
Published: May 2007

The Smartville Complex is a late Jurassic, rifted volcanic arc in the northern Sierra Nevada, California. Near Auburn, California, it consists of a lower volcanic unit, dominated by basaltic flows, and an upper volcanic unit of andesitic volcaniclastic rocks, both of which have been intruded by dykes and irregular bodies of diabase. These rocks contain relict igneous minerals, and the metamorphic minerals albite, chlorite, quartz, pumpellyite, prehnite, epidote, amphibole, titanite, garnet, biotite, K‐feldspar, white mica, calcite, and sulphide and oxide minerals.

Prehnite–pumpellyite (PrP), prehnite–actinolite (PrA), and greenschist (GS) zones have been identified. The pumpellyite‐out isograd separates the PrP and PrA zones, and the prehnite‐out isograd separates the PrA and GS zones. The minerals Ab + Qtz + Mt + Tn are common to most assemblages in all three zones. The MgO/(MgO + FeO) ratio of the effective bulk composition has an important and systematic effect on the observed mineral assemblages in the PrP zone. Prehnite‐bearing assemblages contain the additional minerals, Pmp + Amp + Ep + Chl in MgO‐rich rocks, and either Pmp + Ep + Chl or Amp + Ep + Chl in less magnesian rocks. Subcalcic to calcic amphibole is common in the PrP zone. The mineral assemblage Prh + Act + Ep + Chl, without Pmp, characterizes the PrA zone, and the mineral assemblage Act + Ep + Chl, without Prh or Pmp, characterizes the GS zone. The disappearance of pumpellyite and prehnite occurred by continuous reactions.

The sequence of mineral assemblages was produced by burial metamorphism at P–T conditions of 300° 50°C at approximately 2.5 ± 0.5 kbar. During metamorphism, the composition of the fluid phase was nearly 100% H2O and the oxygen fugacity was between the hematite–magnetite and quartz–fayalite–magnetite buffers.
Metamorphism and uplift of Alpine schist in the Franz Josef–Fox Glacier area of the Southern Alps, New Zealand
Authors: GRAPES R. , WATANABE T.
Published: May 2007

Geothermometry and geobarometry of 10 garnet–oligoclase zone schists in the Franz Josef–Fox Glacier area, Southern Alps, New Zealand, give temperatures ranging from 415 to 625°C and pressures from 5.2 to 9.2 kbar, indicating a T–P array of about 50°C/kbar and inferred peak temperature conditions over a c. 15‐km‐thick section at depths between c. 20 and 34 km. The present‐day distribution of the schist samples implies that only about one‐third of the original crustal section is now exposed.

The garnet–oligoclase zone schists represent the deeper part of a metamorphosed and deformed accretionary complex that was associated with late Palaeozoic–early Mesozoic subduction along the Gondwana continental margin. Partial uplift (c. 0.2 m/Ma) and erosion of the complex during Jurassic–Cretaceous times (Rangitata uplift) was synchronous with D2 deformation and recrystallization, as recorded by the P–T array. Cenozoic (Kaikoura) uplift and exhumation of the schist since c. 30 Ma to form the Southern Alps was associated with oblique‐slip movement on the Alpine Fault. The present‐day position and steep eastward dip of isograds and D2 structures suggest considerable clockwise rotation during uplift associated with ductile attenuation and tectonic thinning by over two‐thirds of the original schist sequence, largely due to simple shear along schistosity planes. As the schist generally shows only incipient greenschist facies retrograde recrystallization, an apparently complete (although contracted) prograde mineral sequence has been preserved by rapid uplift (>5 km/Ma) of hot rock and the effects of limited shear heating near the Alpine Fault.
Silica‐undersaturated sapphirine, spinel and kornerupine granulite facies rocks, NE Strangways Range, Central Australia
Authors: GOSCOMBE B.
Published: May 2007

Small pods of silica‐undersaturated Al‐rich and Mg‐rich granulite facies rocks containing sapphirine, pleonastic spinel, kornerupine, cordierite, orthopyroxene, corundum, sillimanite and gedrite are scattered throughout the NE Strangways Range, Central Australia. These are divided into four distinct rock types, namely orthopyroxene‐rich aluminous granofels and metapelitic gneisses containing sapphirine, spinel or kornerupine. Two granulite facies metamorphic events are recognized, of which only the first (M1) is considered in this paper.

Peak metamorphic mineral parageneses indicate that the M1 thermal maximum occurred at approximately 900–950 °C and 8–9 kbar. All samples are characterized by profuse and diverse coronitic and symplectic reaction textures. These are interpreted as evidence for the sequential crossing of the following reactions in the system FMAS:

cordierite + spinel + corundum = sapphirine + sillimanite, cordierite + spinel = orthopyroxene + sapphirine + sillimanite, sapphirine + spinel + sillimanite = orthopyroxene + corundum, sapphirine + sillimanite = cordierite + orthopyroxene + corundum.

Phase stability relationships in FMAS and MASH indicate an anticlockwiseP–Tpath terminated by isobaric cooling. Such a path is exemplified by early low‐Pmineral parageneses containing spinel, corundum and gedrite and the occurrence of both prograde and retrograde corundum. Reaction textures preserve evidence for an increase inaH2O andaB2O3with progressive isobaric cooling. This hydrous retrogression resulted from crystallization of intimately associated M1 partial melt segregations. There is no evidence for voluminous magmatic accretion giving rise to the high M1 thermal gradient. The M1P–Tpath may be the result of either lithospheric thinning after both crustal thickening and burial of the supracrustal terrane, or concomitant crustal thickening and mantle lithosphere thinning.
Contrasting thermomechanical evolutions in the Southalpine metamorphic basement of the Orobic Alps (Central Alps, Italy)
Authors: DIELLA V. , SPALLA M. I. , TUNESI A.
Published: May 2007

In the Southern Alps a progressive metamorphic zonation, with an increase in the geothermal gradient from NE to SW, has been widely proposed. However, recent investigations have shown that the greenschist metamorphic imprint of the low‐grade zone corresponds to a metamorphic retrogression following amphibolite facies conditions. On the other hand, in the medium‐grade zone, a later low‐pressure, high‐temperature (LPHT) metamorphic event has also been proposed. In an attempt to resolve these different interpretations, new petrological and partly new structural data have been obtained for two sectors of the Orobic Alps, traditionally attributed to different metamorphic zones. Thermobarometric determinations, supported by microstructural analysis, indicate the following different pressure‐retrograde paths in each sector: (1) in the Val Vedello basement (VVB) rocks, a first metamorphic imprint characterized by P= 7–9 kbar and T= 570–610°C was followed by a greenschist retrogression (P≤ 4 kbar and T≤ 500° C); (2) in the Lario basement (LB) rocks, the first detectable metamorphic stage, characterized by mineral assemblages indicating P= 7–9 kbar and T= 550–630° C, was followed by a LPHT event, synkinematic with F2 extensional deformation. A greenschist retrogression marks the final uplift of these rocks.

Reinterpretation of the available geochronological data indicates a diachronism for the two thermomechanical evolutions. In the light of these data, we interpret the retrograde P–T–t path of the VVB rocks as a pre‐Permian post‐thickening uplift and the retrograde P–T–t evolution of the LB rocks as a Permo‐Mesozoic uplift related to the extensional tectonic regime of the Tethyan rifting.
P–T paths in the Handöl area, central Scandinavia: record of Caledonian accretion of outboard rocks to the Baltoscandian margin
Authors: BERGMAN S.
Published: May 2007

The Seve–Köli Nappe Complex is widespread in the Scandinavian Caledonides and is composed of units representing parts of the Baltoscandian margin (Seve Nappes) now overlain by magmatic–sedimentary rocks (Köli Nappes) derived from west of this margin. The metamorphic evolution of Köli and Seve units has been studied in the Handöl area, central Scandinavian Caledonides, where a fragmented ophiolite with cover sequence in the lower Köli units is thrust over the higher grade Seve units. Thermobarometry constrains metamorphic conditions to 490–570° C/950–600 MPa, with a slight downwards increase in grade, for the lower Köli (Bunnerviken lens), 520–620° C/1000–600 MPa for the upper Seve (Täljstensvalen Complex), 630–740° C/750–650 MPa for the middle Seve (Snasahögarna Nappe) and 480–600° C/1150–1000 MPa for the lower Seve (Blåhammarfjället Nappe).

P–T paths during garnet growth have been constructed for all units, except the highest grade middle Seve. These paths record heating at the base of the Köli and cooling in the underlying Seve units. Pressure increase during garnet growth is indicated for all units leading to anticlockwise P–T paths in the Seve. The results imply thermal convergence with time for all units and spatial convergence in metamorphic grade in the Köli. It is suggested that the contrasting metamorphic histories on either side of the Seve–Köli boundary resulted from the emplacement of relatively colder Köli rocks on top of relatively hotter Seve rocks and that emplacement of structurally higher units contributed to the increase in pressure.
Porphyroblast inclusion‐trail orientation data: eppure non son girate*!
Authors: BELL T. H. , JOHNSON S. E. , DAVIS B. , FORDE A. , HAYWARD N. , WILKINS C.
Published: May 2007

Extensive examination of large numbers of spatially orientated thin sections of orientated samples from orogens of all ages around the world has demonstrated that porphyroblasts do not rotate relative to geographical coordinates during highly non‐coaxial ductile deformation of the matrix subsequent to their growth. This has been demonstrated for all tectonic environments so far investigated. The work also has provided new insights and data on metamorphic, structural and tectonic processes including: (1) the intimate control of deformation partitioning on metamorphic reactions; (2) solutions to the lack of correlation between lineations that indicate the direction of movement within thrusts and shear zones, and relative plate motion; and (3) a possible technique for determining the direction of relative plate motion that caused orogenesis in ancient orogens.
Timing and tectonic setting of Grenvillian metamorphism—constraints from a transect along Georgian Bay, Ontario*
Authors: JAMIESON R. A. , CULSHAW N. G. , WODICKA N. , CORRIGAN D. , KETCHUM J. W. F.
Published: May 2007

Systematic mapping of a transect along the well‐exposed shores of Georgian Bay, Ontario, combined with the preliminary results of structural analysis, geochronology and metamorphic petrology, places some constraints on the geological setting of high‐grade metamorphism in this part of the Central Gneiss Belt. Correlations within and between map units (gneiss associations) have allowed us to recognize five tectonic units that differ in various aspects of their lithology, metamorphic and plutonic history, and structural style. The lowest unit, which forms the footwall to a regional decollement, locally preserves relic pre‐Grenvillian granulite facies assemblages reworked under amphibolite facies conditions during the Grenvillian orogeny. Tectonic units above the decollement apparently lack the early granulite facies metamorphism; out‐of‐sequence thrusting in the south produced a duplex‐like structure. Two distinct stages of Grenvillian metamorphism are apparent. The earlier stage (c. 1160–1120 Ma) produced granulite facies assemblages in the Parry Sound domain and upper amphibolite facies assemblages in the Parry Island thrust sheet. The later stage (c. 1040–1020 Ma) involved widespread, dominantly upper amphibolite facies metamorphism within and beneath the duplex. Deformation and metamorphism recently reported from south and east of the Parry Sound domain at c. 1100–1040 Ma have not yet been documented along the Georgian Bay transect. The data suggest that early convergence was followed by a period of crustal thickening in the orogenic core south‐east of the transect area, with further advance to the north‐west during and after the waning stages of this deformation.
Metamorphic evolution of exhumed middle to lower crustal rocks in the Mojave Extensional Belt, southern California, USA
Authors: HENRY D. J. , DOKKA R. K.
Published: May 2007

The Waterman Metamorphic Complex of the central Mojave Desert was exposed as a consequence of early Miocene detachment‐dominated extension. However, it has evidence consistent with a more extensive geological history that involves collision of a crustal fragment(s), tectonic thickening by overthrusting and two periods of extension. The metamorphic complex contains granitoid intrusives and felsic mylonitic gneisses as well as polymetamorphic rocks that include marble, calc‐silicate, quartzite. mafic granulite, pyribolite, amphibolite, migmatite and biotite schist. The latter group of rocks was affected by an initial series of high‐grade metamorphic events (M1 and M2) and a localized lower grade overprint (M3). The initial metamorphism (M1) can be separated into two stages along its high‐grade P–T path: M1a, a granulite facies metamorphism at 800–850° C and 7.5–9 kbar and Mlb, an upper amphibolite facies overprint at 750–800° C and 10–12 kbar. M1a developed mineral assemblages and textures consistent with granulite facies conditions at a reduced activity of H2O and is associated with intense ductile deformation (D1) and minor local partial melting. M1b overprinted the granulite assemblages with a series of hydrous phases under conditions of increasing pressure and H2O activity and is accompanied by little or no deformation. M2 developed at lower pressures and temperatures (650–750° C, 4.5–5.5 kbar) and is distinguished by a second local overprint of hydrous phases that reflects an input of aqueous fluids probably associated with the intrusion of a series of granitic dykes and veins. Effects of M3 are confined to the Mitchel detachment zone, an anastomosing early Miocene detachment fault, and are characterized by local ductile/brittle deformation (D2) of the pre‐existing high‐grade rocks and granitoid intrusives and by the production of mylonites and mylonitic gneisses under greenschist facies conditions (300–350° C, 3–5 kbar). The initial overprint (M1a) represents metamorphism, devolatilization and minor partial melting of supracrustal rocks under granulite facies conditions as a consequence of tectonic and, possibly, magmatic thickening. The increasing pressure transition of M1a to M1b reflects a period of continued compressional tectonism, thrusting and influx of H2O, in part, locally related to crystallization of partial melts. The near isothermal decompression between M1b and M2 probably represents a pre‐112‐Ma extensional episode that may have been the result of a decompressional readjustment of a thickened crust. Following the initial extensional event, the metamorphic complex remained at depths of 10–17 km for at least 90 Ma until it was uplifted following Miocene extension. M3 develops locally in response to this second extensional period resulting from the early Miocene detachment faulting.
Ductile shearing and migmatization at mid‐crustal levels in an Archaean high‐grade gneiss belt, northern Gallatin Range, Montana, USA
Authors: MOGK D. W.
Published: May 2007

A high‐grade Archaean gneiss terrane in the northern Gallatin Range, south‐western Montana, USA, contains a trondhjemite–tonalite gneiss (TTG) sequence that was migmatized during pervasive ductile shearing. Metamorphism of these rocks is in the upper amphibolite to granulite facies at temperatures of 680–735°C, pressures in excess of 8 kbar, and a ‘clockwise’P–T–t path is inferred. Ductile shearing occurred in metre‐scale anastomosing bands of high strain throughout the area. The TTGs have been extensively migmatized via vapour‐present melt reactions involving the incongruent melting of biotite‐bearing TTG to produce hornblende and granitic melt. The granitic melt is produced in narrow envelopes adjacent to ductile shear zones in response to infiltration of water‐rich solutions. Melt migration occurred on a local scale, and extraction of melt from the system left behind a plagioclase–hornblende residuum with minor interstitial microcline. Ductile shearing and migmatization in the TTG operated in a positive feedback mechanism; the entire volume of gneiss was chemically and mechanically reworked through the cyclical infiltration of aqueous solutions, vapour‐present melting and melt‐enhanced deformation. The proposed melt reaction may be an important crustal differentiation process considering that (1) many collisional orogens do not attain temperatures high enough to permit vapour‐absent melting, (2) pervasive networks of ductile shear zones at mid‐crustal levels may serve as channels for fluid ingress and melt extraction, and (3) the large volumes of TTGs in Archaean and Phanerozoic orogens may constitute a significant source reservoir for certain types of high‐level granites.
Silica mobility and fluid movement during metamorphism of the Connemara schists, Ireland
Authors: YARDLEY B. W. D. , BOTTRELL S. H.
Published: May 2007

Quartz veins are developed in a wide range of metasediment types in the upper amphibolite facies rocks of Connemara, and attest to considerable migration of silica. Contrary to common assumptions, there is clear evidence that these veins do not primarily result from movement of fluid to regions of lower P–T down the regional geothermal gradient. Under amphibolite facies conditions, a dilute chloride fluid moving down temperature has the potential to alter 60g of plagioclase to muscovite for each gram of vein quartz precipitated, while cooling over the temperature interval from 650 to 500° C. The absence of significant metasomatic effects in the vein walls effectively precludes a simple origin from such through‐flowing, externally derived fluids. The oxygen isotopic composition of matrix quartz shows considerable differences between different rock types (quartzite, pelite and marble), with a range of δ18OSMOW from c.+ 11.5% (quartzite) to + 18.5% (marble). In each rock type, vein quartz compositions closely match those of the matrix quartz. These results demonstrate the importance of local segregation processes in the formation of veins, and suggest that fluid convection cells were not developed during metamorphism on a scale larger than the individual sedimentary formations, if at all.

Both oxygen isotope data and the absence of metasomatism indicate that veins form primarily by segregation of quartz from the host lithologies, with only a relatively minor component of through flow of externally derived fluid. Veins are clearly not the major pathways of metamorphic dewatering.

It is proposed that abundant veins in the predominantly pelitic Ballynakill Formation formed during peak metamorphic D3 folding because the formation was embrittled by high fluid pressures but was capped by impermeable marble. Hence the pelitic formation fractured repeatedly and the pore fluid drained through the fractures to form veins, while irreversible loss through the rest of the succession was a much less important process.

In the central mountains of Connemara, rather pure, unreactive quartzites are cut by widely spaced, laterally extensive quartz veins that are axial planar to D3 folds. These veins may mark pathways whereby metamorphic fluid made its way through the massive impermeable quartzite from lower parts of the nappe pile, but here too, oxygen isotope data indicate considerable segregation of locally derived quartz, reflecting the importance of pumping of fluid between wail rocks and fractures relative to the component of through flow.
Melting and diffusion processes in closed‐system migmatization
Authors: MAALØE S.
Published: May 2007

Estimated variations in mineral concentrations across leucosomes suggest that leucosomes are generated during anatexis by a diffusive exchange between the leucosome and the mesosome, and not by the migration of melt from the mesosome. However, the presence of melt is a precondition for the diffusive exchange to take place. Initially a crack is formed due to shear stress. The formation of a crack allows a diffusive exchange to take place through the melt, which causes melting of minerals situated near the crack. The diffusive exchange of material is less efficient in the mesosome where the melt is isolated at grain corners and edges. The microcline enrichment of some granitic leucosomes is thought to be due to the diffusive depletion of the mesosome caused by growth of alkali feldspar during the consolidation of the migmatite. In general, it seems unnecessary to invoke concentrations of water in the leucosome or the intrusion of external fluids or magmas for migmatite formation.
Microstructural analysis of the classical spiral garnet porphyroblasts of south‐east Vermont: evidence for non‐rotation
Authors: HAYWARD N.
Published: May 2007

New data strongly suggest that the classical spiral garnet porphyroblasts of south‐east Vermont, USA, generally did not rotate, relative to geographical coordinates, throughout several stages of non‐coaxial ductile deformation. The continuity of inclusion trails (Si) in these porphyroblasts is commonly disrupted by planar to weakly arcuate discontinuities, consisting of truncations and differentiation zones where quartz–graphite Si bend sharply into more graphitic Si. Discontinuous, tight microfold hinges with relatively straight axial planes are also present. These microstructures form part of a complete morphological gradation between near‐orthogonally arranged, discontinuous inclusion segments and smoothly curving, continuous Si spirals. Some 2700 pitch measurements of well‐developed inclusion discontinuities and discontinuous microfold axial planes were taken from several hundred vertically orientated thin sections of various strike, from specimens collected at 28 different locations around the Chester and Athens domes. The results indicate that the discontinuities have predominantly subvertical and subhorizontal orientations, irrespective of variations in the external foliation attitude, macrostructural geometry and apparent porphyroblast‐matrix rotation angles. Combined with evidence for textural zoning, this supports the recent hypothesis that porphyroblasts grow incrementally during successive cycles of subvertical and subhorizontal crenulation cleavage development. Less common inclined discontinuities are interpreted as resulting from deflection of anastomosing matrix foliations around obliquely orientated crystal faces prior to inclusion. Most of the idioblastic garnet porphyroblasts have a preferred crystallographic orientation. Dimensionally elongate idioblasts also have a preferred shape orientation, with long axes orientated normal to the mica folia, within which epitaxial nucleation occurred.

Truncations and differentiation zones result from the formation of differentiated crenulation cleavage seams against porphyroblast margins, in association with progressive and selective strain‐induced dissolution of matrix minerals and locally also the porphyroblast margin. Non‐rotation of porphyroblasts, relative to geographical coordinates, suggests that deformation at the microscale is heterogeneous and discontinuous in the presence of undeformed, relatively large and rigid heterogeneities, which cause the progressive shearing (rotational) component of deformation to partition around them. The spiral garnet porphyroblasts therefore preserve the most complete record of the complex, polyphase tectonic and metamorphic history experienced in this area, most of which was destroyed in the matrix by progressive foliation rotation and reactivation, together with recrystallization.
Metamorphic peak geothermobarometry in the Furulund Group, Sulitjelma, Scandinavian Caledonides: implications for uplift
Authors: BOYLE A. P. , WESTHEAD R. K.
Published: May 2007

Regional metamorphism in the Sulitjelma area of the arctic Scandinavian Caledonides has produced a series of Barrovian zones, from chlorite through to kyanite in more aluminous pelites, which transect the major lithological boundaries in a large nappe unit of the Köli Nappe Complex. The metamorphic zones are inverted, and metamorphic grade increases westwards from the foreland to the hinterland. The Furulund Group comprises a mixed sequence of originally flysch‐like sediments which crop out over the whole range of the observed Barrovian zones, but are usually too calcareous to develop the characteristic Barrovian aluminous phases staurolite and kyanite. Instead, above the garnet isograd, the Furulund Group pelites and semi‐pelites have widely developed hornblende porphyroblasts in the common assemblage Grt + Pl + Bt + Ms + Qtz ± Hbl ± Ep ± Czo ± Chl ± Cal ± Dol. Thermobarometric estimates of metamorphic peak P–T conditions (i.e. at maximum recorded temperatures) from this assemblage, using three different methods, indicate a westward increase of both pressure and temperature over a distance of 14 km away from the garnet isograd towards the hinterland of the orogen, independent of topographic level and without change in the common mineral assemblage. The increased peak pressure in the west indicates greater initial burial and subsequent exhumation in the hinterland than towards the foreland. Restoration indicates that the Furulund Group has been subjected to substantial eastward bulk tilting after peak metamorphic conditions. Whilst this enhances the overturning of the metamorphic zones, the amount of tilting was not sufficient to cause the overturning.
Blue amphiboles and their significance for the metamorphic history of the Pan‐African Gariep belt, Namibia
Authors: FRIMMEL H. E. , HARTNADY C. J. H.
Published: May 2007

The metamorphic history of mafic exotic blocks from a tectonic melange zone within an allochthonous ophiolitic terrane (Marmora Terrane) of the Pan‐African Gariep orogenic belt in south‐western Namibia was studied, based on mineral parageneses and amphibole composition. Glaucophane described previously from these rocks could not be verified. Instead, two types of blue amphiboles were distinguished: (i) rims of (ferro‐) edenitic to pargasitic to barroisitic hornblende composition around brownish amphibole phenocrysts replacing magmatic clinopyroxene, and (ii) deep blue porphyroblasts of magnesio‐riebeckite with little ferro‐glaucophane component in a highly metasomatized albite‐rich rock. Textural and mineralogical evidence, particularly the existence of up to three different amphibole generations in metagabbro samples, supports a multiphase metamorphic history experienced by these exotic blocks. The first metamorphic event, M1, is interpreted as very low‐P hydrothermal oceanic metamorphism that affected the igneous protoliths at up to amphibolite facies temperatures. Subsequent M2 metamorphism was syntectonic and is characterized by temperatures similar to those attained during M1 but higher pressures indicating burial to 15–20 km. This event is related to a subduction process. The third metamorphic event, M3, was low grade and of regional nature. It is the only one recorded in the sedimentary envelope of the exotic blocks. The formation of magnesio‐riebeckite is considered a retrograde reaction at greenschist facies during M2. The results indicate that in the Gariep belt subduction and subsequent obduction have occurred, although blueschist facies metamorphism has not been reached.
Dehydration melting, fluid buffering and decompressional P–T path in a granulite complex from the Eastern Ghats, India
Authors: DASGUPTA S. , SENGUPTA P. , FUKUOKA M. , CHAKRABORTI S.
Published: May 2007

A suite of metapelites, charnockites, calc‐silicate rocks, quartzo‐feldspathic gneisses and mafic granulites is exposed at Garbham, a part of the Eastern Ghats granulite belt of India. Reaction textures and mineral compositional data have been used to determine the P–T–X evolutionary history of the granulites. In metapelites and charnockites, dehydration melting reactions involving biotite produced quartzofeldspathic segregations during peak metamorphism. However, migration of melt from the site of generation was limited. Subsequent to peak metamorphism at c. 860° C and 8 kbar, the complex evolved through nearly isothermal decompression to 530–650° C and 4–5 kbar. During this phase, coronal garnet grew in the calc‐silicates, while garnet in the presence of quartz broke down in charnockite and mafic granulite. Fluid activities during metamorphism were internally buffered in different lithologies in the presence of a melt phase. The P–T path of the granulites at Garbham contrasts sharply with the other parts of the Eastern Ghats granulite belt where the rocks show dominantly near‐isobaric cooling subsequent to peak metamorphism.
Fluid infiltration and regional metamorphism of the Waits River Formation, north‐east Vermont, USA
Authors: LÉGER A. , FERRY J. M.
Published: May 2007

Abstract
The Siluro‐Devonian Waits River Formation of north‐east Vermont was deformed, intruded by plutons and regionally metamorphosed during the Devonian Acadian Orogeny. Five metamorphic zones were mapped based on the mineralogy of carbonate rocks. From low to high grade, these are: (1) ankerite‐albite, (2) ankerite‐oligoclase, (3) biotite, (4) amphibole and (5) diopside zones. Pressure was near 4.5kbar and temperature varied from c. 450° C in the ankerite‐albite zone to c. 525° C in the diopside zone. Fluid composition for all metamorphic zones was estimated from mineral equilibria. Average calculated χco2[= CO2/(CO2+ H2O)] of fluid in equilibrium with the marls increases with increasing grade from 0.05 in the ankerite‐oligoclase zone, to 0.25 in the biotite zone and to 0.44 in the amphibole zone. In the diopside zone, χCO2 decreases to 0.06.

Model prograde metamorphic reactions were derived from measured modes, mineral chemistry, and whole‐rock chemistry. Prograde reactions involved decarbonation with an evolved volatile mixture of χCO2 > 0.50. The χCO2 of fluid in equilibrium with rocks from all zones, however, was generally <0.40. This difference attests to the infiltration of a reactive H2O‐rich fluid during metamorphism. Metamorphosed carbonate rocks from the formation suggests that both heat flow and pervasive infiltration of a reactive H2O‐rich fluid drove mineral reactions during metamorphism. Average time‐integrated volume fluxes (cm3 fluid/cm2 rock), calculated from the standard equation for coupled fluid flow and reaction in porous media, are (1) ankerite‐oligoclase zone: c. 1 × 104; (2) biotite zone: c. 3 × 104; (3) amphibole zone: c. 10 × 104; and diopside zone: c. 60 × 104. The increase in calculated flux with increasing grade is at least in part the result of internal production of volatiles from prograde reactions in pelitic schists and metacarbonate rocks within the Waits River Formation.

The mapped pattern of time‐integrated fluxes indicates that the Strafford‐Willoughby Arch and the numerous igneous intrusions in the field area focused fluid flow during metamorphism. Many rock specimens in the diopside zone experienced extreme alkali depletion and also record low χCO2. Metamorphic fluids in equilibrium with diopside zone rocks may therefore represent a mixture of acid, H2O‐rich fluids given off by the crystallizing magmas, and CO2‐H2O fluids produced by devolatilization reactions in the host marls. Higher fluxes and different fluid compositions recorded near the plutons suggest that pluton‐driven hydrothermal cells were local highs in the larger regional metamorphic hydrothermal system.