bluuebunny/crossref_metadata_2025 huggingface dataset:
Paragenetic and mineral‐chemical relationships in orthoamphibole‐bearing gneisses from Enderby Land, east Antarctica: a record of Proterozoic uplift
Authors: HARLEY SIMON L.
Published: May 2007

Abstract
Polymetamorphic orthoamphibole‐bearing gneisses from the vicinity of shear zones in Casey Bay, Enderby Land, Antarctica, record both the overprinting of Archaean granulite lithologies by Proterozoic metamorphism and the subsequent evolution of the latter episode during localized deformation.

Mineral chemistry and zoning relationships in orthoamphibole‐garnet‐kyanite‐quartz and later orthoamphibole‐garnet‐cordierite‐quartz assemblages are used together with interpretation of reaction and corona textures to constrain the Proterozoic pressure‐temperature path experienced by the rocks. Consideration of reaction topologies, P‐T‐X(Fe‐Mg‐A1) relationships in orthoamphibole‐bearing assemblages, and standard geothermobarometry indicate that the gneisses underwent a near‐isothermal decompression P‐T history (steep positive dP/dT) from ± 8 kbar and 700°C to <5.5kbar and 650°C. This uplift path is correlated with the general effects of Rayner Complex metamorphism and deformation which occurred after 1100 Ma in a major erogenic belt south of Casey Bay.
Plagioclase relations in pelites, central Menderes Massif, Turkey. II. Perturbation of garnet‐plagioclase geobarometers
Authors: ASHWORTH J. R. , EVIRGEN M. M.
Published: May 2007

Abstract
Activity‐composition relations in oligoclase near the peristerite gap are investigated in pelites from the Central Menderes Massif. The pressure of metamorphism is estimated independently, from garnet‐rutile‐ilmenite‐kyanite‐quartz, as being in the range 4–7 kbar. In the temperature range, 450–600°C approximately, both the Newton‐Haselton calibration of the garnet‐plagioclase‐kyanite‐quartz geobarometer and a related simple treatment of garnet‐plagioclase‐muscovite‐biotite give a wide range of apparent pressures, correlated with plagioclase composition and ranging up to 11–12 kbar where the plagioclase is most sodic. This effect is attributed to failure of the activity model for plagioclase used in the Newton‐Haselton treatment. It is inferred that, in the present area, γplagAn decreases with increasing XplagAn in the range An15‐An25. The data can be interpreted in terms of high γplagAn in the high‐albite structure at these temperatures, modified to lower values by ‘e’ordering in the more calcic oligoclases. The ordering appears to be independent of the peristerite gap, and the data do not support the interpretation of the gap as a solvus. Garnet‐plagioclase assemblages are unreliable as geobarometers where the plagioclase is more sodic than approximately An20 and T < 700°C, and should instead be used to investigate the γ‐X behaviour of the plagioclase where independent geobarometry can be used as a constraint.
Incomplete high P–T metamorphic transitions within the Kvamsøy pyroxenite complex, west Norway: a case study of disequilibrium
Authors: MØRK MAI BRITT E.
Published: May 2007

Abstract
The Kvamsøy pyroxenite complex consists of olivine websterite, olivine gabbro and leucogabbro‐norite which have been subjected to regional high P‐T (HPT) metamorphism. The metamorphism has resulted in a range of disequilibrium textures with the development of coronas and pseudomorphism of the igneous phases. Reactions between felsic and mafic mineral domains have been controlled by variable and selective diffusion of elements, resulting in a variety of local plagioclase‐breakdown reactions and in significant compositional variations for the product garnet. Restricted diffusion favours transient reaction stages with garnet ± spinel ± corundum ± zoisite after calcic plagioclase in olivine gabbro and olivine websterite and garnet ± spinel ± kyanite ± quartz + sodic plagioclase in leucogabbro‐norite. Complete HPT reaction has produced garnet pyroxenite which consists of garnet + diopside + hornblende + zoisite in gabbroic rocks, while amphibolitization continued during the cooling and uplift history. Grt + Ky + Pl + Qtz geobarometry suggests pressures in the range 13‐16 kbar for T= 750°C, comparable with the regional eclogite‐forming metamorphism.
Amphibole composition as an indicator of subtle grade variation in epidote‐glaucophane schists
Authors: HOSOTANI H. , BANNO S.
Published: May 2007

Abstract
The paragenetic relations of epidote‐glaucophane schists are described in terms of the system Al2O3‐Fe2O3‐Fe2O3‐MgO‐CaO with excess of quartz, albite and epidote. If alkali‐amphibole is free from Ca and AlIV, its composition when associated with epidote is invariant, univariant or divariant at a given pressure and temperature on Miyashiro's (1957) diagram of alkali‐amphibole solid solution if it is also associated, respectively, with three, two or one additional minerals in the system.

Using a group of epidote‐glaucophane schists from the Kotu area of the Sanbagawa metamor‐phic belt in Shlkoku, Japan (isophysical compositional),univariant boundary lines were determined for the assemblages that, in addition to the ubiquitous quartz + albite + phengitic mica, contain hematite + chlorite, garnet + chlorite and actinolite + chlorite, respectively. The slopes of the univariant boundary lines obtained from petrographical data are in good agreement with those calculated in a model system.

The positions of isophysical univariant boundary lines on the amphibole compositional diagram serve to distinguish the grade of metamorphism among the rocks of the same mineral facies. The hematite‐chlorite univariant boundary line can be used to divide the zone of epidote‐glaucophane schists of the Sanbagawa metamorphic belt into three, and the garnet‐chlorite‐paragonite invariant equilibrium can be used to divide the epidote zone of New Caledonia into three.
Review
Authors: THOMPSON PETER H.
Published: May 2007

MIGMATITES
Regional distribution of Al‐silicates and metamorphic zonation in the low‐grade Verrucano metasediments from the Northern Apennines, Italy
Authors: FRANCESCHELLI M. , LEONI L. , MEMMI I. , PUXEDDU M.
Published: May 2007

Abstract
Mineralogical and petrological studies of Triassic Verrucano metasediments of the Northern Apennines are reported. The widespread occurrence of Al‐silicates allows the delineation of four metamorphic zones with increasing metamorphic grade: (1) kaolinite zone (well Perugia 2, Umbria); (2) kaolinite‐pyro‐phyllite zone (Monte Argentario and part of the Verrucano of the Monticiano‐Roccastrada area and Monti Leoni); (3) pyrophyllite zone (Monti Pisani, Iano, Monti Leoni, the Monticiano‐Roccastrada area and some wells in the Larderello region); (4) kyanite zone (Massa area and some wells in the Larderello area).

The four metamorphic zones correspond to temperatures ranging from 300°C to about 450°C. On the basis of the Si content of muscovite and geological arguments, pressures of between 3 and 5 kbar are estimated. The metamorphic zones are located more or less parallel to the bent north‐west‐south‐east trending structural zonation of the Northern Apennines, with the concave side towards the Tyhrrenian Sea.

During the Alpine orogeny, the Verrucano metasediments underwent three folding phases each of which has produced an axial plane schistosity (S1, S2, S3). During the first folding phase the Verrucano sediments were buried increasingly deeply within the crust from east to west. The climax of Alpine metamorphism was attained prior to the second folding phase with crystallization of porphyroblasts of kyanite and chloritoid in a central area located between Massa and Larderello. The inferred paleo‐temperature distribution pattern resembles an asymmetric thermal high defined by the kyanite zone, and surrounded by the pyrophyllite zone. A similar pattern is still present in the Tuscan crust, as indicated by a series of geothermal anomalies passing through the Northern Apennines.
Alpine metamorphism of pelitic schists in the Nufenen Pass area, Lepontine Alps
Authors: KLAPER E. M. , BUCHER‐NURMINEN K.
Published: May 2007

Abstract
The effects of Tertiary Alpine metamorphism on pelitic Mesozoic cover rocks have been studied along a cross‐section in the central Lepontine Alps in the Nufenen Pass area, Switzerland.

Greenschist facies to amphibolite facies conditions are indicated by the formation of the index minerals chloritoid, garnet, staurolite and kyanite in pelitic rocks. Regional metamorphism reached maximum conditions during the interkinematic period between a main Alpine penetrative (D2) and a late Alpine (D3) crenulation type deformation phase or synchronous with the late Alpine deformation. Based on AFM phase relationships four different metamorphic zones can be distinguished: (1) chloritoid zone; (2) staurolite + chlorite zone; (3) staurolite + biotite zone; and, (4) kyanite zone.

The isograds that separate these zones can be modelled by univariant reactions in the KFMASH system. The conditions of metamorphism calculated from geological ther‐mobarometers for the maximum post‐D2 por‐phyroblast stage are from North to South: 500° C at 5‐6 kbar and 600° C at 7‐8 kbar.

Detailed thermobarometry of garnet por‐phyroblasts with complex textures suggests that maximum temperature was reached later than maximum pressure. Early garnet growth occurred along a prograde P‐T‐path, post‐D2 rims grew with increasing temperature but decreasing pressure, and finally post‐D3 garnet formed along a retrograde P‐T‐path.

It may be concluded from the calculated pressure and temperature difference over a short distance (3 km) across the mapped area that the isogradic surfaces of the post‐D2 metamorphism are steeply oriented. The data also suggest that isobaric and isothermal surfaces are parallel.

Much of the observed metamorphic pattern can be explained as the result of a significant post‐D2 differential uplift of the hot Pennine area relative to the Helvetic area along a tectonic contact zone. The closely spaced isograds (isotherms) in the North may then be interpreted as a thermal effect owing to the emplacement of the hot Pennine rocks against the Got‐thard massif with its cover. Whereas, in the Pennine metasediments, post‐D2 porphyroblast formation can be related to the decompression path which was steep enough for dehydration reactions to proceed. It is also remarkable that late kyanite porphyroblasts probably formed with decreasing pressure.

The interpretation given here for the Nufenen Pass area may also apply to the Luk‐manier Pass area where similar metamorphic patterns have been reported by Fox (1975). The formation of the ‘Northern Steep Belt’;, as denned by Milnes (1974b), and the associated late Alpine fold zones may, therefore, have significantly modified the metamorphic pattern of the Helvetic‐Penninic contact zone.
Sillimanite and andalusite produced by base‐cation leaching and contact metamorphism of felsic igneous rocks
Authors: VERNON R.H. , FLOOD R.H. , D'ARCY W.F.
Published: May 2007

Abstract
Contact metamorphism adjacent to a porphyritic quartz‐monzodiorite at Kentucky, New South Wales, Australia has produced hornfelses in porphyritic leucogranite at a peak temperature of about 650–700° C and a maximum confining pressure of about 2 kbar (200 MPa). A gradation appears to exist from normal slightly peraluminous to modified strongly peraluminous metagranite hornfelses, which have also been enriched in sulphur. The strongly peraluminous hornfelses, containing cordierite, andalusite, sillimanite, biotite, pyrite and pyrrhotite, retain residual porphyritic igneous microstructures. These rocks appear to have been formed by leaching of base cations, during and possibly just before the contact metamorphism. Folia of fibrous sillimanite anastomose between lenticular grains of quartz and feldspar and truncate igneous zoning in plagioclase grains, suggesting that cation leaching and solution transfer occurred during growth of the sillimanite. Fibrous sillimanite also grew in grain boundaries of polygonal aggregates formed by the contact metamorphism. Therefore, at least some of the cation leaching appears to have occurred at the highest metamorphic grade. Metasandstones that are locally strongly peraluminous adjacent to the monzodiorite stock also, have probably undergone similar leaching.
Low‐grade metamorphism and geotectonic setting of the Macuchi Formation, Western Cordillera of Ecuador
Authors: AGUIRRE L. , ATHERTON M.P.
Published: May 2007

Abstract
The Cretaceous‐Eocene basic to intermediate marine volcanic rocks of the Mucuchi Formation constitute the Western Cordillera in northern Ecuador. Their chemical features mostly correspond to those of tholeiitic basalts with some calc‐alkaline affinities and suggest an oceanic island arc setting. The Macuchi rocks are affected by low‐grade, non‐deformative metamorphism, characterized by zeolite, prehnite‐pumpellyite and lower greenschist facies assemblages. Depth‐zonation is suggested by the downward mineral sequence: (i) laumontite+ (pistacitic epidote, pumpellyite + prehnite); (ii) pumpellyite+ prehnite + pistacitic epidote; (iii) actinolite+biotite+ pistacitic epidote + chlorite. This broad zonation and the chemistry of individual minerals point to an interaction between the volcanic rocks and sea‐water under a moderate to high thermal gradient (= 75° C/km?). Alteration appears to have been dependent primarily on fluid control (volume, pressure, composition), temperature and reaction kinetics which together partly overshadow the role of load‐pressure. Compositional variations of a mineral species at the scale of a contiguous flow or even at the scale of a thin section show that intensity of alteration was spatially uneven depending on rock permeability and consequently, metastable equilibrium commonly exists. However, a progressive approximation to equilibrium as a result of P–T control is shown by the mineralogy. A high fo2 of the fluid phase is evident from the mineral chemistry. The metamorphism of the Macuchi volcanics is similar to the hydrothermal‐burial type produced during the development of a volcanic arc where lavas and volcanoclastics accumulated in a shallow marine environment. However, some of its characteristics point to a transition toward systems defined by a higher T/P ratio such as those found in ocean‐floor metamorphism.

A model is proposed in which the Macuchi volcanics are assigned to an oceanic island arc generated contemporaneously with a marginal basin which has opened as the outcome of progressive north‐south attenuation of the continental crust due to mantle diapirism.
Chemographic relationships of biotite and cordierite in the McGerrigle thermal aureole, Gaspé, Quebec
Authors: BOSSE J.Y. VAN , WILLIAMS‐JONES A.E.
Published: May 2007

AbstractBiotite and cordierite occur in a 1‐km wide zone of pelitic hornfelses around the McGerrigle pluton. These phases display systematic changes inXFethat can be attributed to continuous reactions involving chlorite or andalusite in the system KFMASH. Through much of the zone biotite and cordierite were products of the ‘breakdown’of chlorite. Close to the pluton this continuous reaction was terminated by a discontinuous reaction that introduced andalusite. Pelites which interdigitate with apophyses of the intrusive at the pluton margin contain assemblages that record a continuous reaction between biotite, cordierite, andalusite, muscovite, and quartz or, alternatively, the discontinuous breakdown of muscovite and quartz to K‐feldspar and andalusite.

The mole fraction of Fe in biotite and cordierite increased significantly with the progress of the first continuous reaction and apparently decreased during the second continuous reaction. TheKDof Fe‐Mg between the minerals decreased and apparently increased, respectively, during the two reactions.

Biotite‐cordierite‐chlorite assemblages are interpreted to have been stable at temperatures between 525° C and 615° C and biotite‐cordierite‐andalusite assemblages stable at temperatures between 615° C and 635° C. The confining pressure was estimated to have been < 2 kbar.

The results of this study suggest that theKDof Fe‐Mg between biotite and cordierite is a function of temperature, the Fe‐Mg exchange characteristics of the controlling continuous reaction and non‐ideal mixing of Fe and Mg.
Garnet–chloritoid equilibria in eclogitic pelitic rocks from the Sesia zone (Western Alps): their bearing on phase relations in high pressure metapelites
Authors: VUICHARD J.P. , BALLÈVRE M.
Published: May 2007

Abstract
A detailed study of garnet–chloritoid micaschists fom the Sesia zone (Western Alps) is used to constrain phase relations in high pressure (HP) metapelitic rocks. In addition to quartz, phengite, paragonite and rutile, the micaschists display two distinct parageneses, namely garnet + chloritoid + chlorite and garnet + chloritoid + kyanite. Talc has never been observed. Garnet and chloritoid are more magnesian when chlorite is present instead of kyanite. The distinction of the two equilibria results from different bulk rock chemistries, not from P–T conditions or redox state. Estimated P–T conditions for the eclogitic metamorphism are 550–600°C, 15–18 kbar.

The presence of primary chlorite in association with garnet and chloritoid leads us to construct two possible AFM topologies for the Sesia metapelites. The paper describes a KFMASH multisystem for HP pelitic rocks, which extends the grid of Harte & Hudson (1979) towards higher pressures and adds the phase talc. Observed parageneses in HP metapelites are consistent with predicted phase relations. Critical associations are Gt–Ctd–Chl and Gt–Ctd–Ky at relatively low temperatures and Gl–Chl–Ky and Gt–Tc–Ky at relatively high temperatures.
Widespread fluid infiltration during metamorphism of the Witwatersrand goldfields: generation of chloritoid and pyrophyllite
Authors: PHILLIPS G.N.
Published: May 2007

Abstract
Chloritoid and pyrophyllite occur together in all major goldfields of the Witwatersrand Basin and are widespread in virtually all rock types of the upper Witwatersrand Supergroup, including metaconglomeratic reefs and altered mafic rocks. Both minerals are particularly characteristic of the pelitic horizons intimately associated with reef packages, but they are also developed locally in the regionally persistent metapelites that have basin‐wide extent. Pyrophyllite is particularly common in foliated zones, adjacent to quartz veins, and near unconformably overlying auriferous conglomerates.

The wide distribution of chloritoid and pyrophyllite in metapelites of the Witwatersrand Basin is attributed to alteration of chlorite‐rich shales, rather than to unusual premetamorphic starting materials. This alteration event involved the redistribution of many elements, with up to 40% volume loss, mainly due to removal of silica. Removal of most of the Mg and some Fe accounts for the stabilization of chloritoid and pyrophyllite. Relatively immobile elements included Al, Ti, Nb, Cr, V, P, La and Ce, whereas Si, Fe, Mn, Zn, Co, Ni, Cu, Mg and Ca were lost, and K, Rb and Ba were introduced by an infiltrating fluid.

The alteration event is inferred to have been within the chloritoid and pyrophyllite stability field (and thus syn‐metamorphic) as bulk chemical changes in metapelites are from chlorite directly towards chloritoid and then pyrophyllite, rather than to lower grade minerals such as kaolinite. Muscovite–chlorite–chloritoid and muscovite–chloritoid–pyrophyllite assemblages are attributed to fluid buffering along appropriate curves, as their production by metamorphism of lower grade mineral mixes is considered unlikely, based on the present bulk rock compositional data. A metamorphic timing for the alteration accounts for the correlation of strongly foliated areas with greater degrees of inferred alteration. The transitions from chlorite to chloritoid to pyrophyllite define zones of increasing alteration.

Widespread infiltration as part of peak metamorphism is suggested by the distribution of chloritoid and pyrophyllite, quartz veining and textures. Fluid:rock ratios calculated from a silica budget in one metapelitic horizon exceed 100:1 over many square kilometres. These values need not imply multi‐pass fluid flow, as much of the silica migration may be redistribution on a scale of a few metres, from source rocks into veins. Although infiltration during metamorphism may have affected much of the upper Witwatersrand succession, channelized fluid flow within reef packages, along faults and unconformities and in certain metaconglomerates and metapelites is inferred.
Petrology of amphibolite‐facies mafic and ultramafic rocks from the Catalina Schist, southern California: metasomatism and migmatization in a subduction zone metamorphic setting
Authors: SORENSEN S.S.
Published: May 2007

Abstract
The Catalina Schist of southern California is a subduction zone metamorphic terrane. It consists of three tectonic units of amphibolite‐, high‐P greenschist‐ and blueschist‐facies rocks that are structurally juxtaposed across faults, forming an apparent inverted metamorphic gradient. Migmatitic and non‐migmatitic metabasite blocks surrounded by a meta‐ultramafic matrix comprise the upper part of the Catalina amphibolite unit. Fluid‐rock interaction at high‐P, high‐T conditions caused partial melting of migmatitic blocks, metasomatic exchange between metabasite blocks and ultramafic rocks, infiltration of silica into ultramafic rocks, and loss of an albitic component from nonmigmatitic, clinopyroxene‐bearing metabasite blocks.

Partial melting took place at an estimated P=˜8–11 kbar and T=˜640–750°C at high H2O activity. The melting reaction probably involved plagioclase + quartz. Trondhjemitic melts were produced and are preserved as leucocratic regions in migmatitic blocks and as pegmatitic dikes that cut ultramafic rocks.

The metasomatic and melting processes reflected in these rocks could be analogous to those proposed for fluid and melt transfer of components from a subducting slab to the mantle wedge. Aqueous fluids rather than melts seem to have accomplished the bulk of mass transfer within the mafic and ultramafic complex.
Thermal structure of active thrust belts
Authors: KARABINOS P. , KETCHAM R.
Published: May 2007

Abstract
In order to study the thermal structure of active thrust belts, we have developed a numerical model of conductive heat transfer between thrust sheets during deformation. Our finite difference approach alternates small, instantaneous increments of displacement and isotherm translation with conductive relaxation of perturbed isotherms. In each step, conduction occurs for a length of time equal to the displacement increment divided by the thrust velocity. Computer simulations demonstrate that conductive heat transfer is significant during deformation and that temperatures in hanging‐wall rocks decrease while temperatures in foot‐wall rocks increase over distances of up to 10 km from the thrust surface. When the effects of internal heat production are also calculated, heating of foot‐wall rocks exceeds cooling of hanging‐wall rocks. Rocks located between two thrusts may experience a complicated temperature–time path of early heating followed by cooling. These models help to explain the rapid metamorphism of rocks in the Taconian thrust belt in the northern Appalachians of New England soon after deposition of the youngest sediments.
Metamorphic evolution of the Attic Cycladic Metamorphic Belt on Naxos (Cyclades, Greece) utilizing 40Ar/39Ar age spectrum measurements
Authors: WIJBRANS J.R. , McDOUGALL I.
Published: May 2007

Abstract
40Ar/39Ar age spectrum analysis of phengite separates from Naxos, part of the Attic Cycladic Metamorphic Belt in Greece, indicates that cooling following high‐pressure, low‐ to medium‐temperature metamorphism, M1, occurred about 50 Ma ago. Phengite has 40Ar* gradients that suggest that part of the scatter observed in conventional K–Ar ages was caused by diffusion of radiogenic argon from the minerals during a younger metamorphism, M2. In central Naxos, this metamorphism (M2) has overprinted the original mineral assemblages completely, and is associated with development of a thermal dome. Excellent 40Ar/39Ar plateaus at 15.0 ± 0.1 Ma, 11.8 ± 0.1 Ma, and 11.4 ± 0.1 Ma, obtained on hornblende, muscovite and biotite, respectively, from the migmatite zone, indicate that relatively rapid cooling followed the M2 event, and that no significant thermal overprinting occurred subsequent to M2. Toward lower M2 metamorphic grade, 40Ar/39Ar plateau ages of hornblendes increase to 19.8 ± 0.1 Ma; concomitantly the proportion of excess 40Ar in the spectra increases as well. We propose that the peak of M2 metamorphism occurred beween 15.0 and 19.8 Ma ago. K–Ar ages of biotites from a granodiorite on the west coast are indistinguishable from those found in the metamorphic complex, and hornblende K–Ar ages from the same samples are in the range 12.1–13.6 Ma. As the latter ages are somewhat younger than most ages obtained from the metamorphic complex, intrusion of the granodiorite most likely followed the peak of the M2 metamorphism.

The metamorphic evolution of Naxos is consistent with rapid crustal thickening during the Cretaceous or early Tertiary, causing conditions at which supracrustal rocks experienced pressures in the range 900–1500 MPa. Transition to normal crustal thicknesses ended the M1 metamorphism about 50 Ma ago. The M2 metamorphism and granodiorite intrusion occurred during a period of heat input into the crust, possibly related to the migration of the Hellenic volcanic ar°C in a southerly direction through the area.
Compositional variation of phlogopite in a marble sample: implications for geological thermobarometry
Authors: BUCHER‐NURMINEN K.
Published: May 2007

Abstract
The textural and compositional features of phlogopites in a contact‐metamorphic dolomite marble inclusion in the Bergell intrusion (central Alps) and in a metasomatic reaction vein cutting through this marble suggest different origins for vein phlogopites:

(a) High‐Al vein phlogopite represents former marble phlogopite which has been compositionally modified by reaction with the vein forming fluid.

(b) Low‐Al vein phlogopite represents phlogopite precipitated from the vein forming fluid.

As both types of vein phlogopite were in contact with the same vein forming fluid at the same time, low‐Al phlogopite most likely represents an equilibrium phlogopite composition, whereas high‐Al phlogopite does not. High‐Al vein phlogopite retained its Al‐content from the contact‐metamorphic marble parent phlogopite and only underwent Fe‐Mg exchange with the metasomatic fluid.

All the vein phlogopites studied are strongly enriched in Fe relative to marble phlogopite. The data may suggest in general that phlogopite Al/Si ratios may be retained from the conditions under which the phlogopites first formed, whereas the Mg/Fe‐ratios may be substantially modified by exchange with other ferromagnesian solid phases and/or a metamorphic fluid at later stages in their metamorphic history. This may have significant effects on calculated pressures and temperatures from thermobarometers involving biotite.
Mineralogy, petrology and P–T–t path of Ca–Na amphibole assemblages, Saint‐Martin des Noyers formation, Vendée, France
Authors: THIEBLEMONT D. , TRIBOULET C. , GODARD G.
Published: May 2007

Abstract
The Saint‐Martin des Noyers Formation is interpreted as a slice of an island‐arc system of Lower to Middle Palaeozoic age, located in the internal part of the Variscan orogen in Vendée (Armorican Massif, France). Metamorphosed igneous rocks range in composition from ultramafic to rhyolitic. The regular increase in the FeO/(FeO+MgO) ratio, from mafic to silicic samples, results in a systematic variability in the nature and composition of the metamorphic phases. In basaltic samples, the occurrence of relict garnet‐barroisite assemblages suggests relatively high‐pressure conditions for the peak of metamorphism. During a subsequent retrograde evolution, the primary barroisitic hornblendes recrystallized to texturally complex mixtures of actinolite and hornblende. Despite this complication, it is possible to decipher a P–T‐t path based on amphibole chemistry. The P–T trajectory deduced is dominated by the effect of pressure and consistent with early underthrusting and subsequent tectonic uplift of the ancient arc of Saint‐Martin des Noyers.
Metamorphic evolution of the High Himalayan Crystallines in SE Zanskar, India
Authors: POGNANTE U. , LOMBARDO B.
Published: May 2007

ABSTRACTThe High Himalayan Crystallines (HHC) of SE Zanskar consist of biotite paragneisses, of orthogneisses that derive from early‐Palaeozoic granitoids, of minor metabasics and of post‐metamorphic leucogranites of Miocene age.

Two main metamorphic events have been documented in the HHC. The first event occurred atP= 12.0 ± 0.5 kbar andT= 750 ± 50° C in rare metabasics intruded by early‐Palaeozoic granitoids. In the biotite paragneisses, thermobarometric estimates of the first event point to comparableTatP4–5 kbar lower. The first event is followed by a pervasive syn‐tectonic crystallization characterized by lowerPandT. On the basis of the cooling ages of the metamorphic minerals and on the geological evidence, the second event is referred to the Tertiary Himalayan crystallization. Further petrological and geochronological studies are necessary to prove whether a few mineral relics ascribed to the first event define a polyphase Himalayan evolution or if they record the incomplete obliteration of an older history during the Himalayan event.

The HHC of SE Zanskar show a decrease in metamorphic grade from the middle structural levels upward, close to the Kade unit, and downward, close to the Lesser Himalaya (from sillimanite‐K‐feldspar‐biotite‐bearing assemblages to kyanite‐staurolite‐muscovite‐bearing assemblages). This metamorphic zonation is probably a consequence of the polyphase history of intracontinental thrusts and of the tectonic emplacement of hot crustal slabs within shallower and colder thrust sheets at relatively late stages of the continental collision between India and Eurasia.
The metamorphism in the Central Himalaya
Authors: PECHER A.
Published: May 2007

ABSTRACTAll along the Himalayan chain an axis of crystalline rocks has been preserved, made of the Higher Himalaya crystalline and the crystalline nappes of the Lesser Himalaya. The salient points of the metamorphism, as deduced from data collected in central Himalaya (central Nepal and Kumaun), are:

The Higher Himalaya crystalline, also called the Tibetan Slab, displays a polymetamorphic history with a first stage of Barrovian type overprinted by a lower pressure and/or higher temperature type metamorphism. The metamorphism is due to quick and quasi‐adiabatic uplift of the Tibetan Slab by transport along an MCT ramp, accompanied by thermal refraction effects in the contact zone between the gneisses and their sedimentary cover. The resulting metamorphic pattern is anapparent(diachronic) inverse zonation, with the sillimanite zone above the kyanite zone.

Conversely, the famous inverted zonation of the Lesser Himalaya is basically aprimarypattern, acquired during a one‐stage prograde metamorphism. Its origin must be related to the thrusting along the MCT, with heat supplied from the overlying hot Tibetan Slab, as shown by synmetamorphic microstructures and the close geometrical relationships between the metamorphic isograds and the thrust.

Thermal equilibrium is reached between units above and below the MCT. Far behind the thrust tip there is good agreement between the maximum temperature attained in the hanging wall and the temperature of the Tibetan Slab during the second metamorphic stage; but closer to the MCT front, the thermal accordance between both sides of the thrust is due to a retrogressive metamorphic episode in the basal part of the Tibetan Slab.
The Acadian thermal history of western Maine
Authors: DE YOREO J. J. , LUX D. R. , GUIDOTTI C. V. , DECKER E. R. , OSBERG P. H.
Published: May 2007

ABSTRACT Following the Middle Devonian Acadian deformation an extensive belt of high grade metamorphism was formed in New England. In south‐western Maine, at the northern end of this belt, there occurs a transition along the strike from regional low‐pressure/high‐temperature metamorphism to contact metamorphism in low‐grade rocks. Petrological studies indicate that this transition occurs along a surface plunging to the north‐east at about 3.5°, with respect to the Middle‐to‐Late Devonian erosion surface. In addition, detailed petrological mapping has defined a history of temporally separate, localized metamorphic events associated with plutonism and occurring at increasingly deeper levels to the south‐west. Geochronological studies constrain ambient temperatures in the transition zone at the time of metamorphism to be less than 300° C in the north‐east and between 350° C and 500° C in the south‐west. They also establish a pattern of diachronous cooling due to differential uplift and erosion, with cooling occurring later and most rapidly to the south‐west. Geophysical evidence suggests that along with this spatial variation in metamorphic style the shapes of the plutons in Maine undergo a transition from laterally extensive sheet‐like bodies in the high grade terrane to more equant‐shaped bodies in the low‐grade terrane. Using the results of these petrological, geochronological and geophysical studies, as well as those of stratigraphical and structural studies we construct a thermal model for the transition zone. The model suggests that the Acadian metamorphism in south‐western Maine is a result of deep‐level contact metamorphism near laterally extensive granitic sills dipping to the north‐east with respect to the present erosion surface. The plutons themselves are interpreted to be a result of lower crustal melting in response to crustal thickening in the presence of normal or slightly augmented mantle heat flux.
New evidence for polyphase metamorphism of glaucophane schist and eclogite exotic blocks in the Franciscan Complex, California and Oregon
Authors: MOORE D. E. , BLAKE M. C.
Published: May 2007

ABSTRACT The early metamorphic history of high‐grade exotic blocks in the Franciscan Complex may be more complicated than previously supposed. The different assemblages of high‐grade glaucophane schist, eclogite, amphibolite and hornblende schist are commonly considered to have formed at the same time from essentially unmetamorphosed oceanic crust. However, new textural and mineralogical data presented here suggest that high‐grade glaucophane schist and eclogite have replaced an earlier epidote‐amphibolite facies assemblage that is identical to the primary assemblages in many of the hornblende‐rich blocks. At least some of the hornblende‐rich blocks may therefore be well‐preserved remnants of the earlier metamorphism. Comparison of the mineral assemblages and element partitioning in the mixed‐assemblage blocks suggests that the glaucophane schist and eclogite metamorphism took place at slightly lower temperatures but at the same or higher pressures than the earlier, hornblende‐forming stage.
The effect of dislocations on bulk diffusion in feldspars during metamorphism
Authors: YUND R. A. , QUIGLEY J. , TULLIS J.
Published: May 2007

ABSTRACT There is no significant difference in the diffusion profiles across albite‐adularia bicrystals that were simultaneously deformed at a strain rate of 10‐6S‐1 and those from hydrostatic experiments at the same conditions (1500 MPa and 1000°C for 156 h). This indicates that the bulk alkali diffusion rate, which is the sum of lattice diffusion (D, 1) and dislocation pipe diffusion (Dp), is not significantly enhanced by dislocations at these conditions, and that the maximum value for the ratio of Dp/D1 is about 105. This is equal to the value previously reported for‘oxygen’diffusion in albite. If this ratio is independent of temperature, the contribution of either static (pre‐deformed) or moving (syn‐deformed) dislocations to the bulk diffusion rate of alkalis is probably minor at all metamorphic conditions. For Al and Si diffusion the ratio of Dp/D1 may be larger if D1 is lower. Thus a significant contribution from dislocations to bulk diffusion cannot be ruled out, especially during simultaneous deformation.
Low‐grade regional metamorphism in the Mesozoic‐Cenozoic volcanic sequences of the Central Andes
Authors: LEVI B. , AGUIRRE L. , NYSTRÖM J. O. , PADILLA H. , VERGARA M.
Published: May 2007

ABSTRACT All the Mesozoic and Cenozoic volcanic rocks of the Central Andes (from southern Ecuador to central Chile), except Recent ones, have been affected by episodes of regional metamorphism, without change in texture and structure. The metamorphism, which ranges from low zeolite to greenschist facies, can be classified as burial metamorphism because there is an overall increase in metamorphic grade with stratigraphic depth in the individual volcanic sequences separated by regional unconformities. Some sequences display metamorphic patterns transitional to ocean‐floor and to geothermal field types, reflecting variations along and across the Andes in tectonic setting and thermal gradients.

Volcanism was closely followed by metamorphism during each cycle characterizing the geological history of the Central Andes. The episodic nature of the metamorphism has led to breaks in metamorphic grade at regional unconformities and repetition of facies series, where strata of higher grade may even overlie those of lower grade. The existence of permeability‐controlled distribution patterns of secondary minerals within individual flows shows that gradients of chemical activity, rate of reaction and Pfluid were acting, in addition to temperature and P,tot overall gradients, during the regional metamorphism. The alteration is accompanied by chemical changes and disturbances of the K‐Ar and Rb‐Sr isotope systems. Similarities between Mesozoic facies series in the western and eastern flanks of the Andes are consistent with a mechanism of ensialic spreading‐subsidence.
A method for modelling mass balance in partial melting and anatectic leucosome segregation
Authors: BEA F.
Published: May 2007

A method is proposed for adjusting the mass balance to characterize quantitatively the behaviour of minerals in anatexis. The method is based on an unconstrained simple mixing model that can be expressed as:

image

whereB,A0, andA1‐n, are compositional vectors of segregate, source rock and source minerals, respectively. The most important concepts are: (1) degree of partial fusion:FMM= 1/a0; (2) mineral fractionation index:

image

and (3) plagioclase differentiation index:

image.

For a given mineral, theMFIvalues have the following meaning: (a)MFI<0: residual phase originated, at least partly, as a product of incongruent melting; (b) 0 >MFI<1: preferential retention in the residue; (c)MFI= 1: identical modal fraction in source and melt; (d)a0>MFI> 1: preferential incorporation into the segregate, and (e)MFI>a0: external contribution to the anatectic system defined bya0A0. To test the method and illustrate its use, it was applied to two real problems of partial melting in the Peña Negra Anatectic Complex (Central Spain). The first is a very simple case of segregation of a diktyonitic neosome from an orthogneiss through partial melting located in vertical shear zones. This process is characterized by: (1)FMM= 0.51; (2) active incorporation of K‐feldspar, plagioclase and biotite into the segregate; (3) disequilibrium melting of plagioclase; (4) residual behaviour of quartz and ilmenite. The second case concerns the formation of a cordierite‐bearing granite from granodioritoid diatexites through an anatectic process, whose most salient characteristics are: (1)FMM= 0.45; (2) incongruent melting of biotite; (3) residual behaviour of plagioclase, which melted with aPDIof 1.22; (4) preferential incorporation of quartz into the segregate; (5) total extraction of K‐feldspar from the residue.
Low‐pressure regional metamorphism in the Omeo Metamorphic Complex, Victoria, Australia
Authors: MORAND V. J.
Published: May 2007

The Omeo Metamorphic Complex forms the southern end of the Wagga Metamorphic Belt, which is the main locus of Palaeozoic low‐pressure metamorphism in the Lachlan Fold Belt, south‐eastern Australia. It comprises metamorphosed Ordovician quartz‐rich turbidites originally derived from Precambrian cratonic rocks. Prograde regional metamorphism occurred in the early Silurian, very soon after sedimentation had ceased. The sequence of metamorphic zones, with increasing grade, is: chlorite, biotite, cordierite, andalusite–K‐feldspar and sillimanite–K‐feldspar. Migmatites occur in the sillimanite–K‐feldspar zone, but large bodies of S‐type granite were derived from rocks underlying the exposed Ordovician sequence. P and T estimates for the highest grade rocks are T= 700°C and P= 3.5 kbar, indicating a very high P–T gradient of 65°C/km.

The high heat flow during prograde metamorphism probably resulted from a combination of a thermal anomaly persisting from a pre‐metamorphic back‐arc basin environment, and intrusion of hot, mantle‐derived magmas into the lower and middle crust.

Regional retrograde metamorphism coincided with a general reheating of the crust in the Siluro‐Devonian, accompanied by intrusion of many I‐type plutons and resetting of the K–Ar dates of some earlier plutons. The Omeo Metamorphic Complex was exposed to erosion at this time.
Successive overprinting granulite facies metamorphic events in the Anmatjira Range, central Australia
Authors: CLARKE G. L. , COLLINS W. J. , VERNON D. R. H.
Published: May 2007

The Anmatjira Range and adjacent Reynolds Range, central Australia, comprise early Proterozoic metasediments and othogneisses that were affected by three, and possibly four, temporally distinct metamorphic events, M1–4, and deformation events, D1–4, in the period 1820–1590 Ma. The north‐western portion of the range, around Mt Stafford, preserves the effects of ±1820 Ma M1‐D1, and shows a spectacular lateral transition from muscovite + quartz‐bearing schists to interlayered andalusite‐bearing migmatites and two‐pyroxene granofelses that reflect extremely low‐pressure granulite facies conditions, over a distance of less than 10 km. Orthopyroxene + cordierite + garnet + K‐feldspar + quartz‐bearing gneisses occur at the highest grade, implying peak conditions of ±750°C and 2.5 ± 0.6 kbar. An anticlockwise P–T path for M1 is inferred from syn‐ to late‐D1 sillimanite overprinting andalusite, petrogenetic grid considerations and quantitative estimates of metamorphic conditions for inferred overprinting assemblages. The effects of M1 have been variably overprinted to the south‐east by a c. 1760 Ma M2–D2 event. Much of the central Anmatjira Range, around Ingellina Gap, comprises orthogneiss, deformed during D2, and metapelites that have M1 andalusite and K‐feldspar overprinted by M2 sillimanite and muscovite. The south‐eastern portion of the range, around Mt Weldon, comprises metasediments and orthogneisses that were completely recrystallized during M2–D2, with metapelitic gneisses characterized by spinel + sillimanite + K‐feldspar + quartz‐bearing assemblages that suggest peak M2 conditions of >750°C and 5.5 ± 1 kbar. Overprinting parageneses in metapelitic gneisses imply that D2 occurred during essentially isobaric cooling. A third granulite facies event, M3, affected rocks in the Reynolds Range, immediately to the south of the Anmatjira Range, at c. 1730 Ma. A possible fourth event, M4, with a minimum age of c. 1590 My affected both Ranges, but resulted in only minor overprinting of M1–3 assemblages. The superimposed effects of M1–4, mapped for the entire Anmatjira–Reynolds Range area, indicate that only minor or no dislocation of the regional geology occurred during any of the metamorphic and accompanying folding, events. Although the immediate cause of each of the metamorphic events involved advection, the ultimate causes were external to the metasediments and most probably external to the crust.
An enlarged and updated internally consistent thermodynamic dataset with uncertainties and correlations: the system K2O–Na2O–CaO–MgO–MnO–FeO–Fe2O3–Al2O3–TiO2–SiO2–C–H2–O2
Authors: HOLLAND T. J. B. , POWELL R.
Published: May 2007

We present, as a progress report, a revised and much enlarged version of the thermodynamic dataset given earlier (Holland & Powell, 1985). This new set includes data for 123 mineral and fluid end‐members made consistent with over 200 P–T–XCO2–fO2 phase equilibrium experiments. Several improvements and advances have been made, in addition to the increased coverage of mineral phases: the data are now presented in three groups ranked according to reliability; a large number of iron‐bearing phases has been included through experimental and, in some cases, natural Fe:Mg partitioning data; H2O and CO2 contents of cordierites are accounted for with the solution model of Kurepin (1985); simple Landau theory is used to model lambda anomalies in heat capacity and the Al/Si order–disorder behaviour in some silicates, and Tschermak‐substituted end‐members have been derived for iron and magnesium end‐members of chlorite, talc, muscovite, biotite, pyroxene and amphibole.

For the subset of data which overlap those of Berman (1988), it is encouraging to find both (1) very substantial agreement between the two sets of thermodynamic data and (2) that the two sets reproduce the phase equilibrium experimental brackets to a very similar degree of accuracy. The main differences in the two datasets involve size (123 as compared to 67 end‐members), the methods used in data reduction (least squares as compared to linear programming), and the provision for estimation of uncertainties with this dataset. For calculations on mineral assemblages in rocks, we aim to maximize the information available from the dataset, by combining the equilibria from all the reactions which can be written between the end‐members in the minerals. For phase diagram calculations, we calculate the compositions of complex solid solutions (together with P and T) involved in invariant, univariant and divariant assemblages. Moreover we strongly believe in attempting to assess the probable uncertainties in calculated equilibria and hence provide a framework for performing simple error propagation in all calculations in thermocalc, the computer program we offer for an effective use of the dataset and the calculation methods we advocate.
Metamorphic and post‐metamorphic fluid flow in the low‐grade rocks of the Harlech Dome, north Wales
Authors: BOTTRELL S. H. , GREENWOOD P. B. , YARDLEY B. W. D. , SHEPHERD T. J. , SPIRO B.
Published: May 2007

A combination of fluid inclusion, stable isotope and geochemical techniques has been used to study the nature of fluids present and their behaviour during Caledonian low‐grade metamorphism of the Harlech Dome, north Wales. Fluid inclusion studies show that in most of the metasedimentary sequence the peak metamorphic fluid was an aqueous Na–K–Cl brine but in the graphitic Clogau Formation and in parts of the overlying Maentwrog Formation immiscible H2O‐rich and CH4‐rich fluids coexisted.

Late‐stage inclusions are of calcium‐rich brine and a dilute aqueous fluid. The chemical composition of chlorite in metamorphic veins and rocks varies between different formations and quartz‐oxygen isotopic compositions show considerable variation between different units. Both of these features are taken to indicate that there was little or no pervasive movement of fluid between different units at the peak of metamorphism. After the metamorphic peak there was focused flow of fluid upward through the sequence along fractures, in response to end‐Caledonian uplift and unloading. Where the migrating fluid crossed the graphitic shales, interaction between the fluid and the shales gave rise to the formation of the auriferous veins of the Dolgellau Gold Belt. Subsequent to this mineralizing event there was widespread development of 18O‐enriched calcites and micas.

In the case of vein minerals it is possible that these crystallized directly from late‐stage fluids at lower temperature than the quartz in the same veins. Alternatively, the original vein minerals may have re‐equilibrated with later 18O‐enriched or cooler fluid. In the case of muscovites in the rock matrix it is proposed that the isotopically heavy compositions are the result of re‐equilibration of initially light grains with an introduced fluid, requiring considerable influx of fluid. This event may relate to either of two late‐stage fluids observed as inclusions.