bluuebunny/crossref_metadata_2025 huggingface dataset:
Sample preparation effects on illite crystallinity measurement: grain‐size gradation and particle orientation
Authors: KRUMM S. , BUGGISCH W.
Published: May 2007

Abstract
Specimen thickness is the main experimental factor controlling the results of illite crystallinity (IC) or crystallite size measurements on sedimentation slides. Different values obtained from thick and thin preparations are due to grain‐size gradation effects, which may exclude larger and higher ordered grains from contributing to the diffraction. Orientation effects control the measured peak intensity. The change from poor particle orientation in thick slides to high orientation in very thin slides is marked by an increase in specimen density, diminishing non‐basal reflections, and by a strong increase in peak intensity. A plateau with constant peak breadths is observed if thin slides of well ordered, platy illites are used. A similar plateau can be recognized for thick preparations of specimens from less ordered materials, but not from well ordered ones. Therefore, it is suggested that IC is determined on very thin sedimentation slides with a thickness of 0.25 mg/cm2 or less. Ultrasonic and H2O2 treatments enhance the degree of particle orientation by destruction of grain aggregates and organic compounds, leading to smaller peak breadths and higher intensities.
The smectite–chlorite transition in drillhole NJ‐15, Nesjavellir geothermal field, Iceland: XRD, BSE and electron microprobe investigations
Authors: SCHIFFMAN P. , FRIDLEIFSSON G. O.
Published: May 2007

Abstract
In well NJ‐15 of the Nesjavellir geothermal field, Iceland, the transition of discrete smectite into discrete chlorite has been studied from drill cuttings recovered at depths of less than 1714 m and over a continuous range of temperatures between 60 and 300° C. At temperatures below 180° C, the clay fractions contain mixtures of di‐ and trioctahedral smectites, whose layer charge increases with depth. Between 200 and 240° C, discrete smectites have transformed into smectite‐rich, randomly interstratified chlorite and smecite (R0 C/S). Because the abundance of chlorite interlayers in this C/S is generally <20%, its presence can be detected only by electron microprobe techniques and not by X‐ray diffraction. Between 245 and 265° C, both regularly (R1) and randomly interstratified C/S are the predominant layer silicates. Discrete chlorite first appears at approximately 270° C and coexists with minor amounts of R0 C/S at higher temperatures.

R0 and R1 C/S form a nearly complete compositional series between trioctahedral saponite and discrete chlorite end‐members. The interlayer cation and Si content of smectites and C/S decrease with increasing temperature. The Mg/(Mg + Fe) content of smectite, C/S, and chlorite is unrelated to temperature. The percentage of chlorite in C/S, as determined by electron microprobe analyses, increases continuously with increasing temperature, except for occurrences of smectite‐rich C/S in fresh basaltic dykes which have not thermally equilibrated with the higher grade country rocks.
A white mica crystallinity study of the Berwyn Hills, North Wales
Authors: AWAN M. A. , WOODCOCK N. H.
Published: May 2007

Abstract
White mica crystallinity studies have been carried out on 90 samples of mudrocks, six of spotted slate, and five of accretionary lapilli tuff from the area around the Berwyn Hills, North Wales. Strain was measured for some of the spotted slate and tuff samples. The metamorphic grade increases from southeast to northwest, with values of the Kübler index varying from 0.64 to 0.20Δ2θ. Metamorphic zonal boundaries follow the strike of bedding and cleavage, but crystallinity values increase into stratigraphically younger rocks on the northwest side of the Berwyn Dome. This effect is attributed mainly to a rapid increase in the thickness of synmetamorphic overburden to the northwest, comprising exposed Silurian turbidites and inferred Lower Devonian non‐marine sediments. Strain variations have a more local influence on crystallinity, and lateral variations in the contemporary geothermal gradient cannot be ruled out. However, only with unrealistically high gradients would the need for a thick Lower Devonian component to the overburden be removed. This reasoning implies that the metamorphic peak was coeval with the Acadian (late Caledonian) event, rather than with an early diastathermal event.
The effect of whole‐rock MnO content on the stability of garnet in pelitic schists during metamorphism
Authors: SYMMES G. H. , FERRY J. M.
Published: May 2007

Garnet‐bearing mineral assemblages are commonly observed in pelitic schists regionally metamorphosed to upper greenschist and amphibolite facies conditions. Modelling of thermodynamic data for minerals in the system Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O, however, predicts that garnet should be observed only in rocks of a narrow range of very high Fe/Mg bulk compositions. Traditionally, the nearly ubiquitous presence of garnet in medium‐ to high‐grade pelitic schists is attributed qualitatively to the stabilizing effect of MnO, based on the observed strong partitioning of MnO into garnet relative to other minerals. In order to quantify the dependence of garnet stability on whole‐rock MnO content, we have calculated mineral stabilities for pelitic rocks in the system MnO–Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O for a moderate range of MnO contents from a set of non‐linear equations that specify mass balance and chemical equilibrium among minerals and fluid. The model pelitic system includes quartz, muscovite. albite, pyrophyllite, chlorite, chloritoid, biotite, garnet, staurolite, cordierite, andalusite, kyanite. sillimanite, K‐feldspar and H2O fluid. In the MnO‐free system, garnet is restricted to high Fe/Mg bulk compositions, and commonly observed mineral assemblages such as garnet–chlorite and garnet–kyanite are not predicted at any pressure and temperature. In bulk compositions with XMn= Mn/(Fe + Mg + Mn) > 0.01, however, the predicted garnet‐bearing mineral assemblages are the same as the sequence of prograde mineral assemblages typically observed in regional metamorphic terranes. Temperatures predicted for the first appearance of garnet in model pelitic schist are also strongly dependent on whole‐rock MnO content. The small MnO contents of normal pelitic schists (XMn= 0.01–0.04) are both sufficient and necessary to account for the observed stability of garnet.
Porphyroblast rotation: eppur si muove*?
Authors: PASSCHIER C. W. , TROUW R. A. J. , ZWART H. J. , VISSERS R. L. M.
Published: May 2007

In a number of recent papers, the theory has been postulated that porphyroblasts as a rule do not rotate with respect to geographical coordinates, and can be used to determine the original orientation of older foliations. Complex inclusion patterns in spiral garnets have even been used to advocate a new model of orogenesis, involving several alternating phases of horizontal shortening and extension. Critical assessment of the assumptions and data used to support the theory of irrotational porphyroblasts reveals numerous flaws. Millipede structures, used as proof for flow partitioning, can also form by other flow geometries. Evidence quoted to support irrotational behaviour of porphyroblasts is unsound. Porphyroblasts do occur in sets with a preferred orientation of the internal foliation trace, but these cannot be shown to represent original orientations. Microstructures which resemble truncation planes in spiral garnets are used as evidence that these structures developed by several phases of deformation and as proof for periodic extension and horizontal shortening in orogenesis. They can, however, also be explained by intermittent growth of a rotating porphyroblast during a single phase of deformation. Finally, porphyroblast sets in which orientation is a function of aspect ratio indicate that porphyroblast rotation with respect to kinematic axes does occur in at least some situations.
Proterozoic low‐pressure/high‐temperature metamorphism and an anticlockwise P–T–t path for the Hazeldene area, Mount Isa Inlier, Queensland, Australia
Authors: RUBENACH M. J.
Published: May 2007

In the Hazeldene area, situated in the Mount Isa Inlier, Queensland, the metamorphic grade changes from chlorite zone, through biotite and cordierite zones, to sillimanite/K‐feldspar zone.

Microstructural studies of rocks near the sillimanite isograd demonstrate that cordierite grew early during the development of a steep foliation (S2), was replaced by biotite, andalusite and sillimanite at the metamorphic peak late in S2, and in turn by kyanite + chlorite adjacent to localized small post‐D2 shear zones. Although the anticlockwise P–T–t path is well defined, the precise P–T conditions are uncertain because of problems with experimental and thermodynamic data. The best estimate for the metamorphic peak for rocks close to the sillimanite isograd is around 600° C at 4 kbar.

The metamorphism has been dated at 1544 Ma, and was synchronous with a major crustal shortening event. Because proposed extensional events occurred more than 60 Ma earlier, their contribution to the peak metamorphic thermal perturbation would have been insignificant. The syn‐metamorphic Mica Creek Pegmatites, the abundance of high heat‐producing elements in the nearby pre‐D2 Sybella Granite, and advective heat by fluids which caused considerable metasomatism in the Hazeldene area, may have each contributed to the thermal budget. However, the metamorphic thermal gradient may be 80°C km‐1 or higher, strongly suggesting a local magmatic control. As none are known in the area, such syn‐metamorphic plutons would have to lie beneath the exposed high‐grade rocks.
Tectonothermal evolution of the High Himalayan Crystalline Sequence, Langtang Valley, northern Nepal
Authors: INGER S. , HARRIS N. B. W.
Published: May 2007

The High Himalayan Crystalline Sequence in north‐central Nepal is a 15‐km‐thick pile of metasediments that is bound by the Main Central Thrust to the south and a normal fault to the north. The Langtang section through the metasediments shows an apparent inversion of metamorphic isograds with high‐P, kyanite‐grade rocks exposed beneath low‐P, sillimanite‐grade rocks. Textural evidence confirms that the observed inversion is a result of a polyphase metamorphic history and phase equilibria studies indicate that thermal decoupling has occurred within a mechanically coherent section of crust. Rocks now exposed at the base of the High Himalayan thrust sheet underwent Barrovian regional metamorphism (M1) prior to 34 Ma in the early stages of the Himalayan orogeny, recording metamorphic conditions of T= 710 ± 30° C, P= 9 ± 1 kbar. After the activation of the Main Central Thrust, which emplaced these metapelites southwards onto the lower grade Lesser Himalayan formations, the upper part of the thrust sheet was overprinted by a second heating event (M2), resulting in sillimanite‐grade metamorphism and anatexis of metapelites at T= 760 ± 30° C, P= 5.8 ± 0.4 kbar between 17 and 20 Ma. Crustally derived, leucogranite magmas have been emplaced into low‐grade Tethyan sediments on the hangingwall of the normal fault that bounds the northern limit of the metapelitic sequence.

The cause of the selective heating of the upper section of the metasediments during M2 cannot be reconciled with either post‐thrusting thermal relaxation or advection models. The cause of M2 remains problematical but it is suggested that heat focusing has occurred at the top of the High Himalayan Crystalline Sequence as a result of movement on the normal fault blanketing metapelites of high heat productivity with low‐grade sediments of low thermal conductivity. This model implies that the normal fault was active before M2, consistent with decompression textures that formed during, or shortly after, sillimanite‐grade metamorphism.
The (Na–Ca)amphibole–albite–chlorite–epidote–quartz geothermobarometer in the system S–A–F–M–C–N–H2O. 2. Applications to metabasic rocks in different metamorphic settings
Authors: TRIBOULET C. , THIÉBLEMONT D. , AUDREN C.
Published: May 2007

In order to illustrate different applications of the amphibole‐albite‐chlorite‐epidote‐quartz geothermobarometer, pressure‐temperature‐time (P–T–t) ± space (P–T–t–s) ± deformation (P–T–t–d) paths have been established from literature data. They are discussed as a function of the chemical, equilibrium and microstructural data available in each case, and compared with the conclusions already established by other methods. It is clear that it is necessary to know the relative chronology of the events (directions of zoning of minerals in successive microstructural positions) to establish precise P–T paths; this enables reconstruction of complex geodynamic histories. From this point of view, it is necessary to analyse the maximum possible number of minerals in a few well‐chosen metabasic rocks showing different generations of blastesis. The rocks should belong to different tectonic units to obtain the best overall picture of a metamorphic complex.
Archaean granulite facies metamorphism of the Lewisian of Tiree, Inner Hebrides, north‐west Scotland
Authors: CARTWRIGHT I.
Published: May 2007

The Lewisian of Tiree, north‐west Scotland, underwent granulite facies metamorphism prior to 2.4 Ga. The temperatures and pressures estimated from garnet–clinopyroxene, garnet–orthopyroxene, hornblende–plagioclase and garnet–biotite geothermometers and clinopyroxene–plagioclase–garnet–quartz and orthopyroxene–plagioclase–garnet–quartz geobarometers are 810 ± 50° C and 10.5 ± 1.5 kbar. The imprecision of pressure estimates stems largely from uncertainties in garnet activity models. Calculations of blocking temperatures for Fe–Mg interdiffusion in clinopyroxene and garnet suggest that these temperatures and pressures represent only slightly reset peak‐metamorphic conditions.

Down‐temperature re‐equilibration resulted in chemical zoning over the outer 50–100 μm of the mafic minerals. P–T paths calculated from this mineralogical zoning suggest nearly isobaric cooling. However, the growth of late sillimanite in metapelites requires that the retrograde P–T path had a significant decompression component, suggesting that the mineralogical zonation does not define the retrograde P–T path. The discrepancy between the P–T path calculated from mineralogical zonation and that implied by mineral reactions probably results from the net‐transfer geobarometry reactions closing at higher temperatures than the exchange geothermometers.

The Tiree rocks have a similar history to the mainland Scourian complex. Granulite facies metamorphism accompanied by partial melting occurred prior to the intrusion of the Scourie dykes at c. 2.4 Ga, and the rocks underwent retrogression both prior to and after dyke emplacement. However, peak metamorphic temperatures and pressures on Tiree were lower than those recorded in the Scourian complex, and the Tiree rocks may have been at a different crustal level at that time.
Variable Variscan thermal rejuvenation in the St Malo region, Cadomian Orogen, France: evidence from 40Ar/39Ar mineral ages
Authors: DALLMEYER R. DAVID , BROWN MICHAEL , D'LEMOS RICHARD S. , STRACHAN ROBIN A.
Published: May 2007

Abstract
The St Malo region in north‐west France contains migmatites and anatectic granites derived by partial melting of metasedimentary protoliths during Cadomian orogenesis at c. 540 Ma. Previously reported Rb–Sr model ages for muscovite and biotite range from c. 550 to c. 300 Ma, and suggest variable resetting of mineral isotopic systems. These rocks display microscopic evidence for variably intense Cadomian intracrystalline plastic strain but record no obvious evidence of penetrative Palaeozoic regional deformation. 40Ar/39Ar mineral ages have been determined to evaluate better the extent, timing and significance of Palaeozoic overprinting.

Eleven muscovite concentrates and one whole‐rock phyllite have been prepared from various units exposed in the St Malo and adjacent Mancellian regions. In the Mancellian region, muscovite from two facies of the Bonnemain Granite Complex record 40Ar/39Ar plateau ages of c. 527 and 521 Ma. An internally discordant 40Ar/39Ar release spectrum characterizes muscovite from protomylonitic granite within the Cadomian Alexain‐Deux Evailles‐Izé Granite Complex, and probably records the effects of Variscan displacement along the North Armorican Shear Zone. Muscovite concentrates from anatectic granite and from Cadomian mylonites along ductile shear zones within the north‐western sector of the St Malo region exhibit internally discordant 40Ar/39Ar release spectra which suggest variable and partial late Palaeozoic rejuvenation. By contrast, muscovite concentrates from samples of variably mylonitic Brioverian metasedimentary rocks exposed within the south‐eastern sector of the St Malo region display internally concordant apparent age spectra which define plateaux of 326–320 Ma. A whole‐rock phyllite sample from Brioverian metasedimentary rocks exposed along the eastern boundary of the St Malo region displays an internally discordant argon release pattern which is interpreted to reflect the effects of a partial late Palaeozoic thermal overprint. Muscovite from the Plélan granite, part of the Variscan Plélan‐Bobital Granite Complex, yields a 40Ar/39Ar plateau age of c. 307 Ma.

The 40Ar/39Ar results indicate that Cadomian rocks of the St Malo region have undergone a widespread and variable Palaeozoic (Carboniferous) rejuvenation of intracrystalline argon systems which apparently did not affect the Mancellian region. This rejuvenation was not accompanied by penetrative regional deformation, and was probably of a static thermal–hydrothermal origin. The heat source for rejuvenation was probably either the result of heating during Variscan extension or advection from Variscan granites which are argued to underlie the St Malo region.
Accretion and post‐accretion metamorphism in subduction complex terranes of the New England Fold Belt, eastern Australia
Authors: LEITCH E. C. , MORAND V. J. , FERGUSSON C. L. , HENDERSON R. A. , CARR P. F.
Published: May 2007

Abstract
Regional metamorphic rocks that form Late Palaeozoic subduction complexes in central Queensland, Australia, are products of two metamorphic episodes. Synaccretion metamorphism (M1) gave rise to prehnite‐pumpellyite and greenschist facies rocks, whereas a subsequent episode (M2) at about 250 Ma formed upper greenschist to upper amphibolite facies rocks of both intermediate‐ and low‐pressure type, probably in a compressive arc or back‐arc setting. A similar pattern can be recognized for 1000 km along the New England Fold Belt, although at several localities, where higher grade rocks are exposed, metamorphism was essentially continuous over the M1‐M2 interval, with a rapid rise in geothermal gradient at the end of accretion. Where out‐stepping of tectonic elements has occurred at long‐lived convergent margins elsewhere, similar overprinting of high‐ by lower‐pressure facies series is anticipated, complicating the tectonic interpretation of metamorphism. The discrete character of metamorphic events may be blurred where conditions giving rise to a major episode of accretion and out‐stepping are followed by the subduction of a major heat source.
Grain boundary migration of water and carbon dioxide during uplift of garnet‐zone Alpine Schist, New Zealand
Authors: CRAW D. , NORRIS R. J.
Published: May 2007

Abstract
Deformed quartz veins in garnet‐zone schist adjacent to the active Alpine Fault, New Zealand, have fluid inclusions trapped along quartz grain boundaries. Textures suggest that the inclusions formed in their present shapes during annealing of the deformed veins. Many of the inclusions are empty, but some contain carbon dioxide with densities that range from 0.16 to 0.80 g cm−3. No water, nitrogen or methane was detected. The inclusions are considerably more CO2‐rich than either the primary metamorphic fluid (<5% CO2) or fluids trapped in fracture‐related situations in the same, or related, rocks (<50% CO2). Enrichment of CO2 is inferred to have resulted from selective migration (wicking) of saline water from the inclusions along water‐wet grain boundaries after cooling‐induced immiscibility of a water‐CO2 mixture. Inclusion volumes changed after loss of water. Non‐wetting CO2 remained trapped in the inclusions until further percolation progressively removed CO2 in solution. This mechanism of fluid migration dominated in ductile quartz‐rich rocks near, but below, the brittle‐ductile transition. At deeper levels, hydraulic fracturing is also an important mechanism for fluid migration, whereas at shallower levels advection through open fractures dominates the fluid flow regime.
High‐pressure eclogites in northern Jiangsu – southern Shandong province, eastern China
Authors: ENAMI MASAKI , ZANG QUIJA , YIN YUJUN
Published: May 2007

Abstract
Eclogites are distributed for more than 500 km along a major tectonic boundary between the Sino‐Korean and Yangtze cratons in central and eastern China. These eclogites usually have high‐P assemblages including omphacite + kyanite and/or coesite (or its pseudomorph), and form a high‐P eclogite terrane. They occur as isolated lenses or blocks 10 cm to 300 m long in gneisses (Type I), serpentinized garnet peridotites (Type II) and marbles (Type III). Type I eclogites were formed by prograde metamorphism, and their primary metamorphic mineral assemblage consists mainly of garnet [pyrope (Prp) = 15–40 mol%], omphacite [jadeite (Jd) = 34–64 mol%], pargasitic amphibole, kyanite, phengitic muscovite, zoisite, an SiO2 phase, apatite, rutile and zircon. Type II eclogites characteristically contain no SiO2 phase, and are divided into prograde eclogites and mantle‐derived eclogites. The prograde eclogites of Type II are petrographically similar to Type I eclogites. The mantle‐derived eclogites have high MgO/(FeO + Fe2O3) and Cr2O3 compositions in bulk rock and minerals, and consist mainly of pyrope‐rich garnet (Prp = 48–60 mol%), sodic augite (Jd = 10–27 mol%) and rutile. Type III eclogites have an unusual mineral assemblage of grossular‐rich (Grs = 57 mol%) garnet + omphacite (Jd = 30–34 mol%) + pargasite + rutile.

Pargasitic and taramitic amphiboles, calcic plagioclase (An68), epidote, zoisite, K‐feldspar and paragonite occur as inclusions in garnet and omphacite in the prograde eclogites. This suggests that the prograde eclogites were formed by recrystallization of epidote amphibolite and/or amphibolite facies rocks with near‐isothermal compression reflecting crustal thickening during continent–continent collision of late Proterozoic age. Equilibrium conditions of the prograde eclogites range from P > 26 kbar and T= 500–750°C in the western part to P > 28 kbar and T= 810–880°C in the eastern part of the high‐P eclogite terrane. The prograde eclogites in the eastern part are considered to have been derived from a deeper position than those in the western part. Subsequent reactions, manifested by (1) narrow rims of sodic plagioclase or paragonite on kyanite and (2) symplectites between omphacite and quartz are interpreted as an effect of near‐isothermal decompression during the retrograde stage. The conditions at which symplectites re‐equilibrated tend to increase from west (P < 10 kbar and T < 580°C) to east (P > 9 kbar and T > 680°C). Equilibrium temperatures of Type II mantle‐derived eclogites and Type III eclogite are 730–750°C and 680°C, respectively.
An empirical estimate of the diffusion rate of oxygen in diopside
Authors: SHARP Z. D. , JENKIN G. R. T.
Published: May 2007

The intracrystalline diffusion rate of oxygen in diopside was constrained based on natural isotopic variations from a granulite facies marble from Cascade Slide, Adirondacks (New York, USA). The oxygen isotope compositions of the diopsides, measured as a function of grain size, are nearly constant (20.9 ± 0.3‰ vs. SMOW) over the entire measured size range (0.3–3.2 mm diameter). The δ18O values of the cores of calcite grains are 23.0‰. Temperature estimates based on the Δ18O(calcite‐diopside) are 800d̀C, in agreement with the highest previous thermometric estimates for these rocks.

The lack of isotopic variation in the diopsides as a function of grain size requires that the oxygen intracrystalline diffusion rate in diopside from the Adirondack samples was very slow. The maximum diffusion rates (D800d̀C parallel to the c‐axis) were calculated with an infinite reservoir model (IRM) and a finite reservoir model (FRM) that incorporates mineral modal abundances and initial isotopic variations. For an assumed activation energy (Q) = 100 kJ/mol, the IRM diffusion rate estimate of 1.6 times 10‐20cm2/s is two orders of magnitude faster than from the FRM; at Q=500kJ/mol, the D800d̀C estimate for both methods is c. 5.6 times 10‐20 cm2/s. The present results require that a hydrothermal fluid significantly enhances the diffusion rate of oxygen in diopside if previous data are correct.

The δ18O(SMOW) and δ13C(PDB) values of the calcite, measured in situ with a CO2 laser, are 22.9 ± 0.3, 0.1±0.3‰ in the grain cores, 22.1 ±0.3, 0.2 ±0.1‰ at the grain boundaries and 21.7 ±0.4, ‐0.6±0.1‰ abutting diopside grains. The δ18O and δ13δC values measured conventionally are: crystal cores, 22.96, ‐0.95‰; abutting diopside grains, 22.38, ‐0.93‰; bulk, 22.79, ‐0.95%. Use of the bulk δ18O(calcite) values for thermometry yields unreasonably high temperatures. The lower δ18O values at the calcite grain boundaries are not due to retrograde diffusional exchange with the diopside, they are thought to be a result of a late retrograde fluid infiltration.
A calculated petrogenetic grid for the system K2O‐FeO‐MgO‐Al2O3‐SiO2‐H2O, with particular reference to contact‐metamorphosed pelites
Authors: XU GUOWEI , WILL T. M. , POWELL R.
Published: May 2007

A petrogenetic grid is presented for the system KFMASH (K2O‐FeO‐MgO‐Al2O3‐SiO2‐H2O), including biotite, muscovite, K‐feldspar, chlorite, chloritoid, staurolite, cordierite, garnet, orthoamphibole, orthopyroxene, spinel, andalusite, sillimanite, kyanite, quartz and corundum with H2O in excess, which was calculated using the computer program THERMOCALC and the Powell and Holland internally consistent thermodynamic dataset. By removing the normal constraint of having quartz in excess, both quartz‐bearing and quartz‐absent equilibria are shown. Quartz‐absent equilibria are particularly relevant at high‐T and low‐P conditions, because of their common occurrence at these conditions. The calculated mineral assemblage and mineral compositional variations in terms of FeMg‐1 and (Fe, Mg)SiAl‐2 exchange vectors are broadly compatible with observations on natural rocks, particularly when non‐KFMASH components are taken into account.
Phase relations in andalusite‐sillimanite type Fe‐rich metapelites: Tono contact metamorphic aureole, northeast Japan
Authors: OKUYAMA‐KUSUNOSE Y.
Published: May 2007

Mineral assemblages in metapelites of the contact aureole of the Tono granodiorite mass, northeast Japan, change systematically during progressive metamorphism along an isobaric path at 2‐3 kbar. The bulk rock compositions of metapelites are aluminous with A′ values on an AFM projection larger than that of the chlorite join. The metapelites commonly contain paragonite in the low‐grade zone. With increasing temperatures, andalusite is formed by the breakdown of paragonite. The importance of pyrophyllite as a source of Al2SiO5 polymorphs is limited in typical pelitic rocks.

The most common type of metapelite in the study area has FeO/(FeO + MgO) = 0.5–0.6, and develops assemblages involving chlorite, andalusite, biotite, cordierite, K‐feldspar, sillimanite and almandine, with paragenetic changes similar to other andalusite‐sillimanite type aureoles. Rocks with FeO/(FeO + MgO) > 0.8 progressively develop chloritoid‐bearing assemblages from Bt‐Chl‐Cld, And‐Bt‐Cld, to And‐Bt at temperatures between the breakdown of paragonite and the appearance of cordierite in the more common pelitic rocks in the aureole. The paragenetic relations are explained by a KFMASH univariant reaction of Chl + Cld = And + Bt located to the low‐temperature side of the formation of cordierite by the terminal equilibrium of chlorite. A P‐T model depicting the relative stability of chloritoid and staurolite at low‐ and medium‐pressure conditions, respectively, is proposed, based on the derived location of the Chl + Cld = And + Bt reaction combined with the theoretical phase relations among biotite, chlorite, chloritoid, garnet and staurolite.
Sm‐Nd isotopic evidence on the age of eclogitization in the Zermatt‐Saas ophiolite
Authors: BOWTELL S. A. , CLIFF R. A. , BARNICOAT A. C.
Published: May 2007

Rocks within the Zermatt‐Saas ophiolite of the western Alps have undergone eclogite facies metamorphism during subduction prior to the Alpine collision. The metamorphic history of these rocks is well defined, with eclogitic assemblages being followed by the limited growth of blueschist assemblages of glaucophane and paragonite. Subsequent greenschist alteration occurs adjacent to faults, veins and metasediments. Away from such sources of water, retrogression is very limited. Sm‐Nd isotopic analyses of an essentially unretrogressed eclogitic metabasalt suggest that eclogite facies metamorphism occurred at 52 ± 18 Ma. The large uncertainty is due to the presence of very small amounts of Nd‐rich epidote present as inclusions within garnet. As the closure temperature of garnet to Sm & Nd is thought to be >600d̀C, resetting due to post‐high‐pressure diffusion is thought to be insignificant. Given the fine‐grained protolith to the sample analysed, and its extensive deformation under eclogite facies conditions, incomplete homogenization of pre‐metamorphic isotopic variations is also considered unlikely to be responsible for the young age. A Tertiary age of eclogitization means that models of early Alpine evolution based on the cessation of high‐pressure metamorphism in the Cretaceous need to be revised.
Incipient metamorphism and deformation in the Variscides of SW Dyfed, Wales: first steps towards isotopic equilibrium
Authors: SLATER D. J. , YARDLEY B. W. D. , SPIRO B. , KNIPE R. J.
Published: May 2007

Incipient metamorphism accompanying thrusting, folding and cleavage development has been investigated in a varied sequence of Palaeozoic sediments near the Variscan front in SW Dyfed, Wales. The aim was to evaluate a critical stage in the progression from heterogeneous sediment, whose detrital phases are neither in equilibrium with one another, nor with pore fluids, through indurated sedimentary rock to metamorphic rock comprising newly formed crystals that equilibrated with one another as they grew.

Quartz veins are widely developed in the area, especially in the more psammitic lithologies, while finer grained rocks became cleaved during tectonic deformation. Mineralogical constraints and fluid inclusion measurements suggest maximum temperatures around 200‐310d̀ C (slightly higher in the Marloes‐Musselwick Thrust Sheet than in other parts of the structural succession) at depths of the order of 6‐13 km.

Quartz veins yield distinctly heavier oxygen isotopic compositions than detrital quartz grains in the adjacent wall rocks, although care must be taken in interpreting the data because slivers of detrital grains may become incorporated into veins, while matrix detrital grains may incorporate veinlets or rims of newly formed quartz. It is concluded that vein quartz grew in isotopic equilibrium with a fluid phase whose isotopic composition was primarily controlled by exchange with phyllosilicates, not detrital quartz grains. Vein and matrix quartzes from the Marloes‐Musselwick Thrust Sheet are distinctly lighter (δ18Oveins=+14 to +18% and δ18Omatrix=+11 to +14%) than those from other thrust sheets (δ18O =+17 to +20% and +14 to +17%, respectively).

We conclude that vein quartz and phyllosilicate grains in cleavage domains probably attained equilibrium with a locally buffered pore fluid at the peak of metamorphism, but many relict grains of different chemical and isotopic composition remained elsewhere in the rock. Local fluid migration along veins and through cleavage lamellae facilitated the attainment of equilibrium, but there is little evidence for large‐scale infiltration of externally derived fluids. With further metamorphism the quartz in these rocks would attain an isotopic composition intermediate between that of the heavy vein material and light detritus which coexist here.
Fluid inclusion evidence for an anticlockwise metamorphic P‐T path in central Massachusetts
Authors: WINSLOW D. M. , BODNAR R. J. , TRACY R. J.
Published: May 2007

Acadian (Late Silurian to Early Devonian) metamorphism in the Central Maine Terrane (CMT) in central Massachusetts is characterized by an early low‐P, high‐T (Buchan‐type) metamorphism followed by thickening at high temperature (>650d̀ C) and then by cooling to 100‐200d̀ C below peak recorded temperatures before eventual unroofing. Mineralogical and textural evidence for this path includes sillimanite pseudomorphs after early andalusite, abundant cordierite in pelitic lithologies, replacement of low‐P cordierite‐bearing assemblages by high‐P garnet‐bearing assemblages, and recrystallization of mylonites associated with late shear zones to form lower‐T and higher‐P assemblages. Peak conditions in the highest grade rocks were 685‐780d̀ C and 5‐6 kbar; the cooling path passed through 550d̀ C at about 6.5 kbar.

The well‐constrained P‐T path documented from geological and mineralogical evidence for the CMT offers an unusual opportunity to examine characteristics of fluid inclusions that have experienced a long‐lived metamorphic event spanning a broad range of P‐T conditions. Fluid inclusion data from the CMT document a range of fluid compositions (CO2‐rich, mixed CO2‐N2‐rich, N2‐rich and H2O‐rich) and densities during metamorphism. Densities of CO2 fluid inclusions range from 0.20 to 1.03 g cm‐3. Medium‐density CO2 fluid inclusions are contained in quartz inclusions within garnets in partial melt leucosomes, and in quartz grains within migmatites. Fluid inclusions within the quartz inclusions indicate trapping conditions of 650‐700d̀ C at pressures below 5 kbar. Other CO2 fluid inclusions from matrix quartz yield isochores which pass through 700d̀ C and 5.2 kbar. The highest density inclusions associated with rocks containing the late high‐P assemblages have isochores which pass below the estimated P‐T conditions for recrystallization of the mylonite. Fluid inclusion evidence suggests an early low‐P heating event, followed by thickening at high temperature, and then by nearly isobaric cooling to about 500d̀ C with later decompression. This interpretation is also consistent with previously published petrological models and supports an anticlockwise P‐T path for the CMT of south‐central Massachusetts.
Metapelitic granulites from Jetty Peninsula, east Antarctica: formation during a single event or by polymetamorphism?
Authors: HAND M. , SCRIMGEOUR I. , POWELL R. , STÜWE K. , WILSON C. J. L.
Published: May 2007

Granulite facies metasedimentary gneiss exposed on Jetty Peninsula, east Antarctica, contains assemblages involving garnet‐sillimanite‐biotite‐cordierite‐spinel‐ilmenite‐rutile and garnet‐orthopyroxene‐cordierite‐biotite, as well as quartz and K‐feldspar. Peak assemblages involve garnet + sillimanite + ilmenite (±rutile) and garnet + orthopyroxene. P‐T calculations suggest formation conditions of approximately 800d̀ C at 7‐7.5 kbar. Cooling from peak conditions is suggested by biotite + garnet (±sillimanite) overprinting some peak assemblages. A subsequent increase in temperature is inferred from the formation of cordierite + garnet + biotite + ilmenite, garnet + sillimanite + cordierite + ilmenite and cordierite + orthopyroxene assemblages during D2. In slightly zincian bulk compositions, hercynitic spinel + cordierite + sillimanite constitutes the peak D2 assemblage. Average pressure calculations indicate peak pressures of 5.9 ±0.4 kbar at 700d̀ C for the cordierite‐bearing D2 assemblages. Available radiometric data suggest that peak metamorphism occurred at c. 1000 Ma and D2 occurred after 940 ± 20 Ma. The following two possibilities exist for the metamorphic evolution. (1) The formation of the lower pressure cordierite‐bearing assemblages is associated with a separate metamorphic event (M2), unrelated to the peak assemblage (M1), and the lower pressure assemblages have no relevance in terms of a single tectonothermal event. (2) The cordierite‐bearing assemblages formed during a progression from peak conditions. In this case, the lower pressure assemblages reflect a broadly decompressional metamorphic evolution, during which temperatures fluctuated. Comparison with P‐T paths from granulites of similar age in adjacent areas suggests that the second possibility should be preferred. The cooling interval between peak conditions and the development of cordierite‐bearing coronas and symplectites suggests affinities with isobarically cooled granulites of similar age immediately to the west, and the low‐P/high‐T post‐peak conditions are similar to the later stages of decompressional paths recognized in much of east Antarctica.
Rb‐Sr and U‐Pb isotope studies on migmatites from the Schwarzwald (Germany): constraints on isotopic resetting during Variscan high‐temperature metamorphism
Authors: KALT A. , GRAUERT B. , BAUMANN A.
Published: May 2007

Mineral and isotope studies were undertaken on migmatites from the Schwarzwald, Moldanubian zone of the Variscan belt. The aims of the study were to date the migmatite formation and to determine the processes involved in migmatization in order to evaluate their influence on isotopic resetting. Textural evidence and the comparison of mineral compositions from leucosomes and mesosomes of two centimetre‐scale migmatite profiles, respectively, suggest that migmatitic textures and mineral assemblages were formed by metamorphic segregation (deformation‐enhanced mass transport) rather than by partial melting (anatexis). The results of Rb‐Sr thin‐slab dating on these profiles indicate that Sr isotopes were not completely reset during migmatization. No true isochron ages, but ages of approximate isotopic homogenization were obtained on the thin slabs by calculating 87Sr/86Sr ratios back to various stages in their evolution. The coincidence of these Rb‐Sr data with U‐Pb ages of monazites from migmatites and non‐migmatitic gneisses shows that gneisses and migmatites were formed during the same high‐temperature event in the Carboniferous (330‐335 Ma). The observation that high‐temperature metamorphism failed to equilibrate Sr isotopes on the centimetre‐scale imposes limitations on the use of conventional whole‐rock isochron techniques in dating migmatites.
Meteoric water‐rock interaction in the lower plate of the Whipple Mountain metamorphic core complex, California
Authors: MORRISON J.
Published: May 2007

Oxygen isotope ratios, whole rock major and trace element compositions, and petrological characteristics of 52 samples from nine distinct igneous lithologies in the lower plate of the Whipple Mountain metamorphic core complex of south‐eastern California indicate that both mylonitic and non‐mylonitic lithologies underwent exchange with surface‐derived meteoric waters. Broadly granodioritic lithologies are characterized by whole rock δ18O values that range from 10.6 to 2.6‰. Isotopic compositions of quartz and feldspar mineral separates indicate that quartz has largely retained original igneous compositions but that feldspar has undergone variable and often large 18O‐depletions (up to 6.5‰).

Over 4 km of structural relief is exposed in lower plate gneisses below the Whipple detachment fault including non‐mylonitic lithologies at shallow structural levels above the mylonite front, and mylonitic gneisses at intermediate to deep levels below the mylonite front. Coupled δ18Oqtz ‐ δ18OFsp systematics of non‐mylonitic and mylonitic andesite to rhyolite dykes from shallow and intermediate structural levels of the lower plate document two episodes of hydrothermal alteration: a high‐temperature (>c.600d̀C) episode involving a metamorphic or magmatic fluid with δ18O values ∼ 7‰ and a low‐temperature (c.350d̀C) episode involving low‐δ18O meteoric fluids. All the dykes that document exchange with meteoric fluids are non‐mylonitic. Coupled δ18OFsp systematics of non‐mylonitic and mylonitic granodioritic gneisses from above and below the mylonite front also document low‐temperature (c. 350d̀ C) exchange with meteoric fluids. The data indicate that infiltration of meteoric fluids occurred as lower plate lithologies were juxtaposed against the base of the faulted upper plate. High‐angle normal faults in the upper plate served as the conduits for the downward circulation of surface‐derived fluids. Meteoric fluids were able to penetrate across the detachment fault into the lower plate.

Uplift rates coupled with independent cooling rates indicate that surface‐derived fluids penetrated to a depth of c.4km and possibly as deep as c.8km. Penetration of surface‐derived fluid into the ductile deformation regime is not required to explain the low δ18O values observed in lower plate lithologies of the Whipple Mountain metamorphic core complex.
Garnet‐forming reactions and recrystallization in high‐grade mylonite zones, MacRobertson Land, east Antarctica
Authors: WHITE R. W. , CLARKE G. L.
Published: May 2007

Proterozoic granulite facies gneisses in MacRobertson Land, east Antarctica, are cut by numerous D5 mylonite‐ultramylonite zones of probable Cambrian age. In garnet‐absent mafic two‐pyroxene gneisses and garnet‐bearing charnockitic orthogneisses, the mylonite‐ultramylonite zones are characterized by the growth of garnet at the expense of ilmenite, pyroxene and plagioclase. Textures within each mylonite zone can vary from protomylonitic to ultramylonitic. A range of mineral textures involving M5 garnet is developed corresponding to variations in deformation intensity. In protomylonites, garnet occurs as coronas on orthopyroxene‐plagioclase and ilmenite‐plagioclase boundaries, and as overgrowths on earlier garnet. In ultramylonites, fine‐grained orthopyroxene‐plagioclase‐garnet ± quartz ± clinopyroxene intergrowths and poikilitic garnet are common. Garnet growth in all shear zones is accompanied by shifts in the compositions of neoblastic minerals occurring with garnet, consistent with local chemical equilibrium having been attained during recrystallization. Mylonitization is inferred to have occurred at P∼ 6.5 kbar. Temperature estimates for M5 vary between 550 and 797d̀ C, which may reflect variations and uncertainties associated with the calibrations used and/or partial re‐equilibration during cooling. The presence of post‐tectonic, coronate garnet in some mylonite zones indicates that garnet continued to form exclusively in the mylonite zones after movement had ceased and is interpreted to reflect the effects of localized strain heating.
40Ar/39Ar evidence that formation of blueschists in continental crust was synchronous with foreland fold and thrust belt deformation, western Brooks Range, Alaska
Authors: TILL A. B. , SNEE L. W.
Published: May 2007

Abstract
40Ar/39Ar ages from white mica in rocks of the internal zone of the Brooks Range contractional orogen indicate that the Nanielik antiformal duplex developed at about 120 Ma and was remobilized on its southern boundary at c. 108 Ma. Blueschist facies metamorphism accompanied development of the antiform. The timing of the blueschist facies event and creation of the antiform overlap the period of shallow‐seated deformation in the foreland fold and thrust belt and sedimentation in the foreland basin of the Brooks Range. Blueschist facies P‐T conditions may therefore characterize the thicker parts of orogenic wedges in some orogenic systems; ancient blueschists need not necessarily be interpreted as indicators of active subduction or continent‐continent collision.

Microprobe analysis using quantitative wavelength‐dispersive and electron backscattered electron imaging methods was used to characterize the composition of white micas in the dated samples. None of the samples was compositionally homogeneous; many contained 2‐3 populations of white mica, including both potassic and sodic varieties. Samples which had undergone (in sequence) amphibolite, albite‐epidote amphibolite and blueschist facies metamorphic events retained muscovites relict of the amphibolite facies event. Samples that had undergone only the blueschist facies event also contained multiple populations of mica, some probably from detrital sources.
Reconsideration of the age of blueschist facies metamorphism on the Seward Peninsula, Alaska, based on phengite 40Ar/39Ar results
Authors: HANNULA K. A. , McWILLIAMS M. O.
Published: May 2007

Abstract
40Ar/39Ar step‐heating and single‐grain laser fusion ages from phengites from the polydeformed and polymetamorphosed blueschist‐greenschist facies Nome Group fall into two groups. Samples from the upper part of the structural section that have experienced a relatively weak metamorphic and deformational post‐blueschist facies overprint and one sample from the Cape Nome orthogneiss yield plateau ages of 116‐125 Ma. More intensely overprinted samples yield hump‐shaped spectra with minimum ages of 123 Ma and maximum ages of 334 Ma. Samples with hump‐shaped spectra are derived from a greater structural depth than most samples with plateau ages. Unreasonably old maximum ages from some of the disturbed spectra suggest that the hump‐shaped spectra result from the incorporation of excess 40Ar. This interpretation conflicts with previous interpretations of similarly disturbed spectra from the Brooks Range, which have been argued to provide minimum ages for blueschist facies metamorphism. Since the maximum temperatures achieved by all samples were probably above the blocking temperature of Ar in phengite, the 116‐125 Ma plateau ages are a minimum age for blueschist facies metamorphism on the Seward Peninsula, Alaska.
Coronas and high‐P veins in metagabbros of the Kohistan island arc, northern Pakistan: evidence for crustal thickening during cooling
Authors: JAN M. Q. , KARIM A.
Published: May 2007

Abstract
The E‐W‐trending Kohistan terrane in the NW Himalaya is a sandwich of a magmatic arc between the collided Karakoram (Asian) and Indian plates. The southern part of the Kohistan arc is principally made up of amphibolites derived from volcanic and plutonic rocks of Early Cretaceous age. Gabbroic relics in the amphibolites display calc‐alkaline character, and their mineralogy is similar to low‐P plutonic rocks reported from modern and ancient island arcs. The largest of these relics, occurring along the southern margin of the amphibolite belt near Khwaza Khela, is subcircular in outline and is about 1 km across. It consists of cumulate gabbros and related rocks displaying a record of cooling and crustal thickening. Primary olivine and anorthite reacted to produce coronas consisting of two pyroxenes +Mg‐Fe2+‐Al spinel ± tschermakitic hornblende at about 800° C, 5.5–7.5 kbar. This thermotectonic event is of regional extent and may be related to the overthrusting of the Karakoram plate onto the Kohistan arc some 85 Ma ago, or even earlier. Later the gabbros were locally traversed by veins containing high‐P assemblages: garnet, kyanite, zoisite, paragonite, oligoclase, calcite, scapolite and quartz ° Chlorite ° Corundum ± diopside. Formed in the range 510–600° C, and 10–12 kbar, these suggest further thickening and cooling of the crust before its uplift during the Tertiary. This paper presents microprobe data on the minerals, and discusses the tectonic implications of the coronitic and vein assemblages in the gabbros.