bluuebunny/crossref_metadata_2025 huggingface dataset:
Sequential growth of cordierite and andalusite porphyroblasts, Cooma Complex, Australia: microstructural evidence of a prograde reaction
Authors: VERNON R.H.
Published: May 2007

Abstract
Low‐pressure prograde metamorphism of pelitic rocks in the Cooma Complex, south‐east Australia, has produced cordierite‐andalusite schists at intermediate grades. The first foliation (S1) is preserved largely as inclusion trails in cordierite porphyroblasts. Microstructural evidence indicates that the cordierite porphyroblasts grew during the early stages of development of a crenulation‐foliation (S2) and that andalusite porphyroblasts grew during the development of a later crenulation‐foliation (S3). Microstructural evidence also indicates that the andalusite was a product of the prograde reaction: cordierite + muscovite ± andalusite + biotite + quartz. The occurrence of the products of this reaction in ‘beard’structures between cordierite microboudins formed by extension in S3 confirms that the andalusite grew during the development of S3. The investigation shows that porphyroblast‐matrix relationships can preserve the orientation of an early S‐surface that has been largely obliterated from the matrix, as well as providing relatively direct evidence of sequential mineral growth and metamorphic reactions.
Empirical garnet–muscovite geothermometry in low‐grade metapelites, Selwyn Range (Canadian Rockies)
Authors: HYNES A. , FOREST R.C.
Published: May 2007

Abstract
Partitioning of Fe and Mg between garnet and phengitic muscovite was calibrated as a geothermometer by Green & Hellman (1982) using experimental data at 25–30 kbar. When the thermometer is applied to pelites regionally metamorphosed at pressures of between 3 and 7 kbar it yields temperatures much higher than those from the garnet–biotite thermometer. A new empirical calibration is proposed for use with such rocks, with particular application where garnet occurs at lower grades than biotite. The new calibration is where K is given by: In K= In Kd and Xii are mole fractions in the garnets.

The calibration was derived from comparison with the garnet–biotite thermometer of Ferry & Spear (1978), assuming no pressure‐dependence for the partitioning between garnet and muscovite, no ferric iron partitioning, ideal mixing in muscovite, and the garnet mixing model of Ganguly & Saxena (1984) modified for a non‐linear Ca effect. This latter garnet mixing model was selected because it gave the geologically most reasonable results. It has not proved possible to distinguish a pressure effect from a ferric‐iron effect.

Despite the simplifying assumptions used to derive the calibration, it yields temperatures generally within 15°C of those given by the garnet–biotite thermometer, and has been used to supply thermometric data in a low‐grade region of the Canadian Rockies.
Coexisting amphiboles in an eclogite from the Western Alps: new constraints on the miscibility gap between sodic and calcic amphiboles
Authors: REYNARD B. , BALLÈVRE M.
Published: May 2007

Abstract
Crystal‐chemical relationships between coexisting sodic and calcic amphiboles have been studied in eclogitic metagabbros from the Aosta Valley, Western Alps. Textural analysis gives evidence of three successive high‐pressure parageneses:

1. Pre‐kinematic high‐grade blueschist assemblages, preserved as polymineralic inclusions in garnet cores and made of glaucophane and actinolite (stage A).

2. Synkinematic eclogite assemblages, composed of garnet + omphacite + glaucophane ± actinolite ± white mica ° Clinozoisite + quartz + rutile (stage B).

3. Post‐kinematic epitactic overgrowths of barroisitic amphibole on glaucophane and actinolite (stage C).

P–T conditions of the eclogitic metamorphism have been estimated at around 500–550°C, 16 kbar.

Glaucophane and actinolite coexist as discrete grains in stage A and B assemblages. This texture and the chemistry of the amphiboles unambiguously denotes the existence of a miscibility gap between sodic and calcic amphiboles (from NaM4= 0.80 in actinolite to NaM4= 1.70 in glaucophane at T= 500–550°C). A comparison with published analyses allows a new solvus along the glaucophane–actinolite join to be drawn.

The later barroisitic amphibole (stage C) exhibits strong chemical zonation indicating disequilibrium growth. This amphibole cannot either be used to define a miscibility gap with glaucophane or actinolite or be considered as an intermediate stage between these two end‐members.
Thermal and baric evolution of garnet granulites from Sri Lanka
Authors: SANDIFORD M. , POWELL R. , MARTIN S.F. , PERERA L.R.K.
Published: May 2007

AbstractGarnet granulites from Sri Lanka preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700–750°C and pressures in the vicinity of 6–8 kbar. Associated strain patterns suggest prograde metamorphism occurred during and immediately following an episode of crustal thickening, with the progradeP–Tconditions probably reflecting a combination of the conductive and advective transport of heat at the mid‐levels of tectonically thickened crust. The occurrence of prograde wollastonite provides evidence for internally buffered fluid compositions, or fluid absent conditions, during peak metamorphism and precludes pervasive advection of a CO2‐rich fluid. The advective heat component is therefore likely to have been provided by the transport of silicate melt. Intricate symplectitic textures record partial re‐equilibration of the garnet granulites to lower pressures (˜ 4–6 kbar) at high temperatures (600–750°C), and testify either to the erosional denudation of the overthick crust prior to significant cooling (i.e. quasi‐isothermal decompression) or to a subsequent static heating possibly of early Palaeozoic age (Pan‐African). The metamorphic history of the Sri Lankan granulites is compared with high grade terrains in the neighbouring fragments of Gondwana, with the emphasis on similarities with Proterozoic granulites of the East Antarctic craton.
On the origin of CO2‐rich fluid inclusions in migmatites
Authors: HOLLISTER L.S.
Published: May 2007

Abstract
Nearly pure CO2 fluid inclusions are abundant in migmatites although H2O‐rich fluids are predicted from the phase equilibria. Processes which may play a role in this observation include (1) the effects of decompression on melt, (2) generation of a CO2‐bearing volatile phase by the reaction graphite + quartz + biotite + plagioclase = melt + orthopyroxene + CO2‐rich vapour, (3) selective leakage of H2O from CO2+ H2O inclusions when the pressure in the inclusion exceeds the confining pressure during decompression, and (4) enrichment of grain‐boundary vapour in CO2 by subsolidus retrograde hydration reactions.
Migmatite structures in the Central Gneiss Complex, Boca de Quadra, Alaska
Authors: McLELLAN E.L.
Published: May 2007

AbstractMigmatite structures in the Coast Plutonic‐Metamorphic Complex are well exposed in the inlet of Boca de Quadra, southeast Alaska. Two types of anatectic migmatites are present.Patchmigmatites formed byin situmelting and subsequent crystallization of melt.Diktyoniticmigmatites comprise a discontinuous veined network of leucocratic material, in which leucosomes enclose boudins of host rock. The margins of these boudins show the development of both melanosomes and shear band fabrics.

Strain analysis of diktyonitic melanosomes indicates that these regions have undergone volume decreases of 20‐27%. This volume decrease is attributed to melt extraction into the adjacent fracture‐filling leucosomes. Thus, diktyonitic migmatites formed by shear‐induced segregation of partial melt, whereas in patch migmatites the lack of shear stresses inhibited melt segregation. The variable structural style of anatectic migmatites in Boca de Quadra is not related to host‐rock composition, but may be due to differences in the amount of differential stress during migmatization. These in turn may be controlled by host‐rock strength and/or diachroneity of migmatization and deformation.

Determination of volume changes during migmatization using strain analysis is potentially capable of discriminating intrusive and anatectic migmatites and consequently of documenting melt segregation and subsequent migration across crustal levels.
The mechanism of veining and retrograde alteration of Alpine eclogites
Authors: BARNICOAT A.C.
Published: May 2007

Abstract
The introduction of externally derived fluids into rocks of the Zermatt–Saas zone of the Swiss Alps gave rise to the simultaneous formation of shear and hydraulic fractures. These fractures are now filled with albite‐rich assemblages and surrounded by alteration halos up to c. 2 m wide. The alteration assemblages are zoned and an examination of reactions in P–T–aH2O space implies that the parageneses developed by the hydration of fluid‐absent eclogites. A mechanical analysis of the veins (after Sibson, 1981) shows that Pfluid/Pload must have been at least 0.96. Fluid migration into the country rocks must have been driven by excess hydraulic head either derived from the vertical extent of the veins or due to their connection to a deeper, external reservoir, possibly tapped along thrust surface(s). Diffusive and capillary transport were insignificant. The fluids may have been derived from underlying metasediments that were dehydrating during the quasi‐isothermal uplift of this part of the Alps, or they may have originated during the prograde mesoalpine metamorphism documented in the area.
Contrasting sapphirine parageneses from Wilson Lake, Labrador and their tectonic implications
Authors: CURRIE K.L. , GITTINS J.
Published: May 2007

AbstractRegionally distributed pelitic granulites in the Wilson Lake region contain the assemblage sapphirine + hypersthene + sillimanite + quartz. Geochronology and geobarometry suggest it developed in early Proterozoic rocks at temperatures approaching 900°C and pressures above 10 kbar. Vein‐like metasomatized rocks around a suite of mafic to ultramafic intrusions, emplaced near the peak of metamorphism about 1700 Ma ago, contain sapphirine, but these assemblages developed at temperatures near 750°C and pressures of 4.5 kbar. Both types of assemblage occur as relics in amphibolite‐grade (biotite–sillimanite) migmatites.

P–Tdeterminations indicate rapid isothermal uplift of 20 km accompanied by mafic intrusion and hydration. The metamorphic history and tectonic setting suggest exposure of deep continental crust by thrusting during continental collision, followed by essentially isothermal decompression.
Thermobarometric constraints on the thermal history of the Main Central Thrust Zone and Tibetan Slab, eastern Nepal Himalaya
Authors: HUBBARD M. S.
Published: May 2007

ABSTRACT The Main Central Thrust (MCT) south of Mt Everest in eastern Nepal is a 3 to 5km thick shear zone separating chlorite‐bearing schist in the lower plate from sillimanite‐bearing migmatitic gneiss in the overlying Tibetan Slab. The metamorphic grade increases through the MCT zone toward structurally higher levels. Previous workers have suggested that either post‐ or synmetamorphic thrust movement has caused this inversion of metamorphic isograds. In an effort to quantify the increase in grade and to constrain proposed structural relations between metamorphism and slip on the fault, four well‐calibrated thermobarometers were applied to pelitic samples collected along two cross‐strike transects through the MCT zone and Tibetan Slab. Results show an increase in apparent temperature up‐section in the MCT zone from 778 K to 990 K and a decrease in temperature to ∼850 K in the lower Tibetan Slab, which is consistent with synmetamorphic thrust movement. A trend in calculated pressures across this section is less well‐defined but, on average, decreases up‐section with a gradient of ∼28MPa/km, resembling a lithostatic gradient. Pressure‐temperature paths for zoned garnets from samples within the MCT zone, modelled using the Gibbs' Method, show a significant decrease in temperature and a slight decrease in pressure from core to rim, which might be expected for upper plate rocks during synmetamorphic thrust movement. Samples from the uppermost Tibetan Slab yield higher temperatures and pressures than those from the lower Tibetan Slab, which may be evidence for later‘resetting’ of thermobarometers by intrusion of the large amounts of leucogranite at that structural level.
The blueschists along the Indus Suture Zone in Ladakh, NW Himalaya
Authors: HONEGGER K. , Le FORT P. , MASCLE G. , ZIMMERMANN J.‐L.
Published: May 2007

ABSTRACT Blueschists occur along the Indus Suture Zone in Ladakh as tectonic thrust slices, as isolated blocks within mélange units and as pebbles within continental detrital series. In the Shergol‐Baltikar section high‐pressure rocks within the Mélange unit lie between the Dras‐Naktul‐Nindam nappes in the north and the Lamayuru units in the south. The blueschists are imbricated with mélange formation of probably upper Cretaceous age. They are overlain discordantly by the Shergol conglomerate of post Eocene (Oligo‐Miocene ?) age. Blueschist lithologies are dominated by volcanoclastic rock sequences of basic material with subordinate interbedding of cherts and minor carbonates. Mineral assemblages in metabasic rocks are characterized by lawsonite‐glaucophane/crossite‐Na‐pyroxene‐chlorite‐phengite‐titanite ± albite ± stilpnomelane. In the quartz bearing assemblages garnet is present but omphacite absent. P‐T estimates indicate temperatures of 350 to 420°c and pressures around 9–11 kbar. Geochemical investigations show the primary alkaline character of the blueschist, which suggests an oceanic island or a transitional MORB type primary geotectonic setting. K/Ar isotopic investigations yield middle Cretaceous ages for both whole rocks and minerals. Subduction related HP‐metamorphism affecting the Mesozoic Tethyan oceanic crust developed contemporaneously with magmatism in the Dras volcanic are and the Ladakh batholith. Subsequent collision of India with Asia obducted relics of subduction zone material which later became involved in nappe emplacement during the Himalayan mountain building.
Polyphase metamorphism and the development of the Main Central Thrust
Authors: STÄUBLI A.
Published: May 2007

ABSTRACT Along a cross‐section through the Lesser and Higher Himalayan units at the Kishtwar window area (north‐west India), a polyphase, Barrovian‐type metamorphism has been delineated in relation to the development of the Main Central Thrust (MCT). In the metapelitic mineral assemblages, three metamorphic phases have been distinguished:

conditions up to amphibolite grade at moderate to high pressures (alm + rut + ilm + kya + qtz) characterize the M1 phase;

pressure release and/or temperature increase as a result of movement along the MCT and the formation of gneiss domes in the Higher Himalaya, as expressed by oriented (N70°‐100° E) fibrolite, defines the M2 phase; and,

finally during uplift of the Kishtwar window area, a retrogressive M3 phase is characterized by the assemblage quartz‐muscovite‐chlorite.

Both optically zoned and single‐stage garnets have been examined with the electron microprobe to determine their element partitioning. Normal zoning has been found in samples below the MCT in the Lesser Himalaya, indicating prograde growth during the M2 phase, whereas tectonically above, in the Higher Himalaya unit, the garnets reveal double‐stage growth with a complex zoning pattern due to reaction‐partitioning during M1 and M2 and reverse‐zoning at their rims during the retrogressive M3 phase. Geothermometry on metapelites along a cross‐section through the MCT zone and the Higher Himalaya imply distinct readjustments of garnet‐biotite exchange equilibria and indicate isothermal conditions (500‐600° C) throughout the section during the M3 retrogression. Pressure calculations (gro‐an‐kya‐qtz and alm‐rut‐ilm‐kya‐qtz) suggest a decrease in pressure towards the top of the section (6‐7.5 to 4.5‐5 kbar), as corroborated by fibrolite replacing kyanite. The spatially inverse metamorphism exposed within the Lesser Himalaya of the Kishtwar window is regarded as a product of polyphase metamorphism combined with ongoing thrusting and shearing and is reflected by condensed M2 isograds around the Kishtwar window.
Geothermobarometry and development of inverted metamorphism in the Darjeeling‐Sikkim region of the eastern Himalayan
Authors: MOHAN A. , WINDLEY B. F. , SEARLE M. P.
Published: May 2007

ABSTRACT The Darjeeling‐Sikkim region provides a classic example of inverted Himalayan metamorphism. The different parageneses of pelitic rocks containing chlorite, biotite, garnet, staurolite, kyanite, sillimanite, plagioclase and K‐feldspar are documented by a variety of textures resulting from continuous and discontinuous reactions in the different zones. Microprobe data of coexisting minerals show that XMg varies in the order: garnet < staurolite < biotite < chlorite. White mica is a solid solution between muscovite and phengite. Garnet is mostly almandine‐rich and shows normal growth zoning in the lower part of the Main Central Thrust (MCT) zone, and reverse zoning in the upper part of the zone. Chemographical relations and inferred reactions for different zones are portrayed in AFM space. In the low‐grade zones oriented chlorites and micas and rolled garnets grew syntectonically, and were succeeded by cross‐cutting chlorites and micas and garnet rims. In the upper zones sillimanite, kyanite and staurolite crystallized during a static inter‐kinematic phase. P‐T contitions of metamorphism, estimated through different models of geothermobarometry, are estimated to have been 580°c for the garnet zone to a maximum of 770°c for the sillimanite zone. The preferred values of pressure range from 5.0 kbar to 7.7 kbar. Models to explain the inverted metamorphism include overthrusting of a hot high Himalayan slab along a c. 5 km wide ductile MCT zone and the syn‐ or post‐metamorphic folding of isograds.
Thermal model for the Zanskar Himalaya
Authors: SEARLE M. P. , REX A. J.
Published: May 2007

ABSTRACT Crustal thickening along the northern margin of the Indian plate, following the 50 Ma collision along the Indus Suture Zone in Ladakh, caused widespread high‐temperature, medium‐pressure Barrovian facies series metamorphism and anatexis. In the Zanskar Himalaya metamorphic isograds are inverted and structurally telescoped along the Main Central Thrust (MCT) Zone at the base of the High Himalayan slab. Along the Zanskar valley at the top of the slab, isograds are the right way‐up and are also telescoped along northeast‐dipping normal faults of the Zanskar Shear Zone (ZSZ), which are related to culmination collapse behind the Miocene Himalayan thrust front. Between the MCT and the ZSZ a metamorphic‐anatectic core within sillimanite grade rocks contains abundant leucogranite‐granite crustal melts of probable Himalayan age. A thermal model based on a crustal‐scale cross‐section across the Zanskar Himalaya suggests that M1 isograds, developed during early Himalayan Barrovian metamorphism, were overprinted during high‐grade MCT‐related anatexis and folded around a large‐scale recumbent fold developed in the hanging wall of the MCT.
The dynamics of the suture between the Kohistan island arc and the Indian plate in the Himalaya of Pakistan
Authors: CHAMBERLAIN C. P. , ZEITLER P. K. , JAN M. Q.
Published: May 2007

ABSTRACT The pressure‐temperature and temperature‐time paths derived for rocks in the Kohistan arc and adjacent Nanga Parbat‐Haramosh massif record the dynamics of the collision between the island arc and the Indian plate. Studies of P‐T‐t paths show that the Kohistan arc was thrust over the Nanga Parbat‐Haramosh massif at least 25 Ma ago, but not more than 30–35 Ma ago. Rocks in the Kohistan arc followed decreasing pressure paths, with the early metamorphism beginning at high pressures (9.5 kbar) and later metamorphism occurring at 8.0 kbar. In contrast, rocks in the Nanga Parbat‐Haramosh massif (Indian plate) experienced increasing pressure and temperature paths. Prior to thrusting, the massif was at low pressures (4.0 kbar) and low temperatures (450°c). Later, the pressure and temperature increased to 8 kbar and 580°c. The authors interpret the convergence (to approximately the same pressure and temperature) of the P‐T paths in the two terranes as being the result of thrusting and thermal equilibration between the thrust sheets. 40Ar/39Ar cooling ages of hornblendes and other geochronological data suggest that the time of peak metamorphism and hence the completion of thickening was approximately 30–35 Ma ago.

Temperature‐time paths show that after thrusting, during the period 25–10 Ma, the Kohistan arc and Nanga Parbat‐Haramosh massif were uplifted at similar rates (0.5 km Ma). However, in the past 10 Ma the Nanga Parbat‐Haramosh massif has been uplifted more rapidly than the adjacent Kohistan arc. Rapid uplift has been accommodated by late faults along the edge of the massif.
The role of deformation partitioning in the deformation and recrystallization of plagioclase and K‐feldspar in the Woodroffe Thrust mylonite zone, central Australia
Authors: BELL T. H. , JOHNSON S. E.
Published: May 2007

ABSTRACT In the Woodroffe Thrust mylonite zone, central Australia, recrystallization in plagioclase and K‐feldspar involved subgrain rotation, assisted by grain‐boundary or kink band boundary bulging, without contribution from a change in the chemical composition from host grains to new grains. The size of subgrains and new grains changes across the mylonite zone, apparently as a function of the strain rate and the H2O content of the rock.

The partitioning of deformation into zones of progressive shearing and progressive shortening controls the sites of recovery and recrystallization in feldspar during mylonitization. The size of feldspar porphyroclasts in well developed mylonites is governed by the scale of deformation partitioning reached in the earlier stages of mylonitization, before the formation of a large proportion of fine‐grained matrix that can accommodate the progressive shearing component of the deformation.

Recrystallization occurs in microcline, apparently without involving a translation to a monoclinic structure, as microcline‐twinned new grains are common adjacent to microcline‐twinned host grains. K‐feldspar triclinicity values calculated from XRD traces increase from the margins to the interior of the mylonite zone, in conjunction with deformation intensity. K‐feldspar host grains locally have cores of orthoclase or untwinned microcline, surrounded by mantles of twinned microcline, suggesting a relationship between the presence of microcline twinning and the degree of K‐feldspar triclinicity.
The geodynamic evolution of the Internal Zone of the Betic Cordilleras (south‐east Spain): a model based on structural analysis and geothermobarometry
Authors: BAKKER H. E. , JONG K. DE , HELMERS H. , BIERMANN C.
Published: May 2007

ABSTRACT The Internal Zone of the Betic Cordilleras consists of several superimposed major thrust sheets with different P‐T‐t evolutions. On the basis of an integrated field, microscopic and laboratory study, the tectono‐metamorphic history of the Mulhacen Complex and Almanzora Unit has been reconstructed in detail. The Mulhacen Complex has been affected by at least five phases of penetrative deformation, which have been labelled Dx‐1, Dx, Dx+1, Dx+2 and Dx+3. Dx‐1, and Dx are related to continent‐continent collision, which is indicated by high pressure‐low temperature (HP/LT) and subsequent intermediate P/T metamorphic conditions. Dx+1 is related to crustal thinning and heterogeneous extension. During this event the Almanzora Unit was juxtaposed against the Mulhacen Complex. This phase was succeeded by the establishment of low pressure‐high temperature (LP/HT) conditions and at least two phases of folding and overthrusting. The Almanzora Unit shows a comparable tectono‐metamorphic evolution post Dx+1. However, the P/T conditions prior to Dx+1 indicate a higher crustal position with respect to the Mulhacen Complex during the collisional event.
Microstructure, metamorphism and tectonics of the western Cape Breton Highlands, Nova Scotia
Authors: PLINT H. E. , JAMIESON R. A.
Published: May 2007

ABSTRACT Microstructural and petrological data from the Jumping Brook metamorphic suite, western Cape Breton Highlands, suggest that a single episode of syntectonic prograde metamorphism, followed by uplift, cooling and associated retrogression, affected these rocks during mid‐Palaeozoic times. Microstructures indicative of progressive crenulation foliation development can be traced from low‐grade (chlorite zone) through high‐grade (kyanite zone) rocks, allowing a clear sequence of porphyroblast growth to be established. Metamorphic reactions and P‐T calculations suggest metamorphic conditions of 700‐750°C at 8‐10 kbar were achieved in kyanite zone rocks. Although a complete P‐T‐t path was not defined, combined petrological and geochronological data can be used to constrain computed P‐T‐t models. These models suggest that a component of post‐metamorphic tectonic exhumation is required to explain the observed times of cooling and uplift. The microstructural and petrological data to not support the interpretation that the high‐grade rocks represent pre‐existing crystalline basement. Indeed, the metamorphic history, geochronology and computed tectonic models all point to a single, short‐lived episode of Silurian‐Devonian volcanism, intrusion, convergence, regional metamorphism and uplift, probably resulting from collision tectonics at an irregular continental margin.
Pressure, temperature and fluid composition during amphibolite facies metamorphism of graphitic metapelites, Howard Ridge, British Columbia
Authors: SEVIGNY J. H. , GHENT E. D.
Published: May 2007

ABSTRACT Graphitic metapelites from the Howard Ridge area, British Columbia, have been studied to estimate the pressure, temperature and fluid composition attending amphibolite facies metamorphism. Results from thermobarometric calculations indicate that P‐T conditions of 610–625°C and 6.7kbar were reached during metamorphism. The equilibrium paragonite‐quartz‐albite‐kyanite‐H2O gives significantly different estimates of XH2O in the metamorphic fluid using different paragonite solution models. Estimates of XH2O range from a maximum of 0.93 (Eugster et al., 1972) to a minimum of 0.29 (Chatterjee & Flux, 1986). H2O estimates obtained using the Eugster et al. (1972) and Chatterjee & Froese (1975) solution models give similar results (i.e. 0.8 ± 0.1 versus 0.7 ± 0.1, respectively). Non‐ideal mixing in the C‐O‐H system provides an XH2O estimate of 0.74 at H2O maximum conditions, 0.5 log units below the QFM buffer. The Chatterjee & Flux (1986) paragonite solution model provides unrealistically low estimates of XH2O relative to other paragonite solution models, C‐O‐H equilibria, and published fluid inclusion and mineral equilibria data. Consistent estimates of fluid composition between C‐O‐H and mineral equilibria suggest that a H2O‐rich fluid attended metamorphism of graphitic metapelites at Howard Ridge.
Estimation of Fe3+ from electron microprobe analyses: observations on calcic amphibole and chlorite
Authors: JACOBSON C. E.
Published: May 2007

ABSTRACT Metabasites and metagreywackes from the Pelona and Rand Schists of southern California were analysed using three different electron microprobes. For all three instruments, the estimated Fe3+ contents of calcic amphibole, chlorite and epidote are positively correlated. For some samples, there is an additional correlation between high estimated Fe3+ and the presence of magnetite. These results imply that microprobe analyses can be used to discern relative differences in Fe3+. However, microprobe data and calculations on the sensitivity of the correction procedures to systematic analytical errors indicate that estimated values of Fe3+ are not significant in an absolute sense. Thus, estimates of Fe3+ are meaningful when comparing samples analysed with a single microprobe, but must be used with caution when comparing analyses obtained on more than one probe.
Aluminum silicates in the Mount Raleigh pendant, British Columbia
Authors: KERRICK D. M. , WOODSWORTH G. J.
Published: May 2007

In regionally metamorphosed pelites of the Mount Raleigh pendant, the fibrolite isograd occurs 5km downgrade from the sillimanite isograd. Fibrolite formed from the decomposition of biotite, a reaction that probably resulted from the late‐stage influx of acidic volatiles. In contrast, sillimanite formed by the direct,‘volume‐for‐volume’replacement of andalusite. Andalusite and sillimanite coexist in a 3 km‐wide zone above the sillimanite isograd. Electron probe analyses of these phases reveal low minor element contents and yield KD[=X] values close to unity; the low Fe2O3 contents are compatible with reducing conditions implied by the ubiquity of graphite. Because KD→ 1.0, the zone of coexisting andalusite + sillimanite cannot be attributed to multivariancy resulting from partitioning of minor elements between these phases. Rather, the metastable persistence of andalusite into the sillimanite P‐T stability field is suggested. The modal proportions of sillimanite versus andalusite imply that minimal (<5%) and alusitesillimanite reaction occurred in a zone 1.5km above the sillimanite isograd; in contrast, there was a marked increase in reaction progress immediately above this zone. With an estimated thermal gradient (in the plane of exposure) of approximately 20°C/km, the 1.5 km‐wide zone of nil reaction suggests that the andalusite‐sillimanite equilibrium boundary was overstepped by about 30 °C before significant reaction occurred. Inclusion‐rich areas in andalusite provided favourable sites for sillimanite nucleation; however, the growth of sillimanite may have been impeded by‘pinning’of sillimanite grain boundaries by inclusions.
The upper thermal stability of chlorite + quartz: an experimental study in the system MgO‐Al2O3‐SiO2‐H2O
Authors: MASSONNE H.‐J.
Published: May 2007

Experiments up to water pressures of 21 kbar have been undertaken to bracket the reactions chlorite + quartz = talc + kyanite + H2O, chlorite + quartz = talc + cordierite + H2O, and talc + kyanite + quartz = cordierite ± H2O by reversed runs in the system MgO‐Al2O3‐SiO2‐H2O (MASH). These reaction curves intersect at an invariant point (IP1) at PH2O = 6.4 ± 0.2 kbar and a temperature of 624 ± 4°C. The curve of the chlorite + quartz breakdown to talc + kyanite + H2O at water pressures above 6.4 kbar shows a negative dP/dT, with the slope decreasing with rising pressure, whereas the slope of the breakdown curve to talc + cordierite + H2O at water pressures is clearly positive.

The composition of the chlorite solid solution reacting with quartz has been estimated to be approximately Mg4.85Al1.15[Al1.15Si2.85O10](OH)8 over the entire pressure range investigated. The composition of the talc solid solution forming by the breakdown of chlorite + quartz appears to be Mg2.94Al0.06[Al0.06Si3.94O10](OH)2 at PH2O = 2kbar. With increasing pressure, the Al content of talc decreases, reaching a value of about 0.06 atoms per formula unit at P,H2O = 21 kbar.

As a consequence of the new experimental data, the existing phase topologies of the MASH‐system and K2O‐MASH‐system have been revised. For example, the invariant point IP1 and the univariant reaction curve kyanite + talc + H2O = chlorite + cordierite are stable. For this reason, the development of medium‐ to high‐temperature metamorphic rocks compositionally approximating the MASH‐system must be reconsidered. The whiteschists from Sar e Sang, Afghanistan, are treated as an example. The application of the present experimental data to metamorphic rocks of more normal composition requires the examination of the influence of further components. This leads to the conclusion that the introduction of Fe2+ into magnesian chlorite extends its stability field in the presence of quartz by 10°‐15°C in comparison with pure Mg‐chlorite.
Tectonic implications from sapphirine‐bearing lithologies, south‐west Grenville Province, Canada
Authors: GRANT S. M.
Published: May 2007

Sapphirine has been found in two types of magnesian, metabasic lenses from tectonite zones within the Central Gneiss Belt of the south‐west Grenville Province, Canada. The first type (association I) comes from a lenticular mafic lens within highly tectonized anorthosite, the second type (association II) comes from meta‐eclogitic pods with foliated amphibolite rims. In each case the sapphirine‐bearing assemblages record a wealth of reaction textures. The primary mineralogy in association II is represented by high alumina clinopyroxene, garnet and kyanite ± plagioclase and records pressures of around 14‐16 kbar; in association I the primary mineralogy is represented by plagioclase, two pyroxenes and possibly olivine but here the equilibrium pressure is unknown.

The host gneisses equilibrated at approximately 8 to 10 kbar and 700‐750°C by continuous cation exchange reactions during and after the culmination of the Grenvillian orogeny at 1.16‐1.0 Ga. It is unlikely that the higher pressures recorded in the meta‐eclogitic pods represent an earlier high‐pressure metamorphism as the pods are restricted to shear zones. A tectonic mode of emplacement into a crust undergoing granulite facies metamorphism is more likely. Sapphirine formed by discontinuous decompression reactions; in association II this involved a reaction between garnet and kyanite and resulted in the formation of magnesian granulite facies assemblages. At the same time primary clinopyroxene became much less aluminous by evolving plagioclase. Pressures and temperatures from coexisting phases, that are believed to have equilibrated at the same time as sapphirine formation, are estimated as 11 to 12 kbar and 750°C. These probably represent the peak conditions for granulite facies metamorphism in the south‐west Grenville Province.
Petrology and retrograde P‐T path in granulites of the Kanskaya formation, Yenisey range, Eastern Siberia
Authors: PERCHUK LEONID , GERYA TARAS , NOZHKIN ALEX
Published: May 2007

The Kanskaya formation in the Yenisey range, Eastern Siberia is a newly studied example of retrogression of granulite facies rocks. The formation consists of two stratigraphical units: the lower Kuzeevskaya group and the upper Atamanovskaya group. Rocks from both of these units show rare reaction textures such as replacement of cordierite by garnet, sillimanite and quartz, silimanite coronas around spinel and corundum, and garnet rims around plagioclase in metabasites, while plagioclase rims around garnet can be seen in associated metapelites. The paragenesis quartz + orthopyroxene + sillimanite is a feature of the Kuzeevskaya group. In many samples, chemical zoning of garnet and cordierite shows an increase in Mg from core to rim as well as the reverse.

Biotite‐garnet‐cordierite‐sillimanite‐quartz as well as spinel±biotite‐garnet°Cordierite±sillimanite‐quartz assemblages were studied using geothermometers and geobarometers based on both exchange and net‐transfer reactions (Perchuk & Lavrent'eva, 1983; Aranovich & Podlesskii, 1983; Gerya & Perchuk, 1989). Detailed investigation of 10 samples including 1000 microprobe analyses revealed decompression (first stage) followed by the near isobaric cooling of the granulites. From geological studies, the 7 km total thickness of the sequence closely corresponds to the pressure difference (∼ 2.2kbar) measured by geobarometers in the samples taken from different levels in the sequence. Individual samples yield P‐T paths ranging from 100°C/kbar to 140°C/kbar depending on their locations with respect to the large Tarakskiy granite pluton. In places the 100°C/kbar path changed to the 140°C/kbar due to the influence of the intrusion. In a P‐T diagram these trajectories are subparallel lines, whose P‐T maxima define the Archaean geotherm between 3.1 and 2.7 Ga, determined isotopically. A petrological model for P‐T evolution of the Kanskaya formation is proposed.
Lack of porphyroblast rotation in the Otago schists, New Zealand: implications for crenulation cleavage development, folding and deformation partitioning
Authors: JOHNSON S. E.
Published: May 2007

Porphyroblasts of garnet and plagioclase in the Otago schists have not rotated relative to geographic coordinates during non‐coaxial deformation that post‐dates their growth. Inclusion trails in most of the porphyroblasts are oriented near‐vertical and near‐horizontal, and the strike of near‐vertical inclusion trails is consistent over 3000 km2. Microstructural relationships indicate that the porphyroblasts grew in zones of progressive shortening strain, and that the sense of shear affecting the geometry of porphyroblast inclusion trails on the long limbs of folds is the same as the bulk sense of displacement of fold closures. This is contrary to the sense of shear inferred when porphyroblasts are interpreted as having rotated during folding.

Several crenulation cleavage/fold models have previously been developed to accommodate the apparent sense of rotation of porphyroblasts that grew during folding. In the light of accumulating evidence that porphyroblasts do not generally rotate, the applicability of these models to deformed rocks is questionable.

Whether or not porphyroblasts rotate depends on how deformation is partitioned. Lack of rotation requires that progressive shearing strain (rotational deformation) be partitioned around rigid heterogeneities, such as porphyroblasts, which occupy zones of progressive shortening or no strain (non‐rotational deformation). Therefore, processes operating at the porphyroblast/matrix boundary are important considerations. Five qualitative models are presented that accommodate stress and strain energy at the boundary without rotating the porphyroblast: (a) a thin layer of fluid at the porphyroblast boundary; (2) grain‐boundary sliding; (3) a locked porphyroblast/matrix boundary; (4) dissolution at the porphyroblast/matrix boundary, and (5) an ellipsoidal porphyroblast/shadow unit.
The Narryer Gneiss Complex of the Yilgarn Block, Western Australia: a segment of Archaean lower crust uplifted during Proterozoic orogeny
Authors: MUHLING J. R.
Published: May 2007

The Narryer Gneiss Complex of the Yilgarn Block is a key segment of the Western Australian Precambrian Shield. It is a regional granulite facies terrain comprised of predominantly quartzo‐feldspathic gneisses derived from granitic intrusions c. 3.6–3.4 Ga old. Granulite facies metamorphism occurred c. 3.3 Ga ago, and conditions of 750–850°C and 7–10 kbar are estimated for the Mukalo Creek Area (MCA) near Errabiddy in the north. The P–T path of the MCA has been derived from metamorphic assemblages in younger rocks that intruded the gneisses during at least three subsequent events, and this path is supported by reaction coronas in the older gneisses. There is no evidence for uplift immediately following peak metamorphism of the MCA, and a period of isobaric cooling is inferred from the pressures recorded in younger rocks. Pressures and temperatures estimated from metadolerites, which intruded the older gneisses during ‘granite–greenstone’tectonism at about 2.6 Ga and during early Proterozoic thrusting show that the Errabiddy area remained in the lower crust, although it was probably reheated during the younger events. Isothermal uplift to upper crustal levels occurred at c. 1.6 Ga ago, and was followed by further deformation and patchy retrogression of high‐grade assemblages. The effects of younger deformation, cooling and reheating can be discerned in the older gneisses, but as there has been no pervasive deformation or rehydration, the minerals and microstructures formed during early Archaean granulite facies metamorphism for the most part are retained.

The MCA remained in the lower crust for about 1700 Ma following peak metamorphism and some event unrelated to the original metamorphism was required to exhume it. Uplift occurred during development of the Capricorn Orogen, when some 30–35 km were added to the crust beneath the Errabiddy area. The recognition of early Proterozoic thrusting, plus crustal thickening, suggests that the Capricorn Orogen is a belt of regional compression which resulted from convergence of the Yilgarn and Pilbara Cratons.
A model for low‐pressure facies metamorphism during crustal thickening
Authors: LOOSVELD R. J. H. , ETHERIDGE M. A.
Published: May 2007

Low‐pressure metamorphic facies (i.e. high T/P ratios) are widespread in a wide range of tectonic settings. Explanations offered for the occurrence of these facies include extensional and/or magmatic models. However, these fail to explain that the low‐P facies metamorphism is commonly coeval with a phase of pervasive crustal thickening, with T/P ratios increasing during, or slightly lagging behind, the thickening. We propose an alternative explanation based on the approximate synchroneity of crustal thickening and erosion (thinning) of the mantle lithosphere.
Metamorphism of an Early Palaeozoic continental margin, western Baie Verte Peninsula, Newfoundland
Authors: JAMIESON R. A.
Published: May 2007

Metamorphic mineral assemblages and textures from Early Palaeozoic continental margin rocks in north‐western Newfoundland indicate that different structural levels have contrasting metamorphic histories. Rocks of the East Pond Metamorphic Suite, which represent the older, structurally lower level of the margin, experienced an early high‐pressure–low‐temperature stage of metamorphism (10–12 kbar minimum, 450–500°C) which produced eclogite in mafic dykes and phengite–garnet assemblages in pelites. This was overprinted by higher temperature–lower pressure amphibolite facies metamorphism (700–750°C, 7–9 kbar minimum) which produced complex symplectic textures in rocks of all compositions. Rocks of the Fleur de Lys Supergroup, which were deposited in the stratigraphically higher levels of the rifted margin, reached pressures of 7–8.5 kbar at about 450°C during the early stages of metamorphism, overprinted by assemblages which indicate maximum temperatures of 550–600°C at about 6.5 kbar. The metamorphic history of both units is interpreted to be the result of thermal relaxation following initial burial of a continental margin by overriding thrust sheets. Since there is no evidence that maximum pressures or temperatures within the Fleur de Lys Supergroup were ever as high as those reached in the East Pond Metamorphic Suite, these rocks may have followed parallel, ‘nested’P–T–t paths, with the more deeply buried East Pond Metamorphic Suite subjected to greater thermal relaxation effects. Quantitative modelling of P–T–t paths is not possible with the present data, owing to both large uncertainties in P–T estimates, and in the time of metamorphism.
Eclogites and polyphase P–T cycling in the Caledonian Uppermost Allochthon in Troms, northern Norway
Authors: KROGH E. J. , ANDRESEN A. , BRYHNI I. , BROKS T. M. , KRISTENSEN S. E.
Published: May 2007

Eclogites in the Tromsø area, northern Norway, are intimately associated with meta‐supracrustals within the Uppermost Allochthon of the Scandinavian Caledonides (the Tromsø Nappe Complex). The whole sequence, which includes pelitic to semipelitic schists and gneisses, marbles and calc‐silicate rocks, quartzofeldspathic gneisses, metabasites and ultramafites, has undergone three main deformational/metamorphic events (D1/M1, D2/M2 and D3/M3). Detailed structural, microtextural and mineral chemical studies have made it possible to construct separate P–T paths for these three events.

Chemically zoned late syn‐ to post‐D1 garnets with inclusions of Bt, Pl and Qtz in Ky‐bearing metapelites indicate a prograde evolution from 636°C, 12.48 kbar to c. 720°C, 14–15 kbar. This latter result is in agreement with Grt–Cpx geothermometry and Grt–Cpx–Pl–Qtz geobarometry on eclogites and trondhjemitic to dioritic gneisses. Maximum pressures at c. 675°C probably reached 17–18 kbar based on Cpx–Pl–Qtz inclusions in eclogitic garnets, and Grt–Ky–Pl–Qtz and Jd–Ab–Qtz in trondhjemitic gneisses. Post‐D1/pre‐D2 decompressional breakdown of the high‐P assemblages indicates a substantial drop in pressure at this stage. Inclusions and chemical zoning in syn‐ to post‐D2 garnets from metapelites record a second episode of prograde metamorphism, from 552°C, 7.95 kbar, passing through a maximum pressure of 10.64 kbar at 644°C, with final equilibration at c. 665°C, 9–10 kbar. The corresponding apparently co‐facial paragenesis Grt + Cpx + Pl + Qtz in metabasites yields c. 635°C, 8–10 kbar. In the metapelites post‐D3, Grt in apparent equilibrium with Bt, Phe and Pl yield c. 630°C, 9 kbar. The D1/M1 and D2/M2 episodes are exclusively recorded in the Tromsø Nappe Complex and must thus pre‐date the emplacement of this allochthonous unit on top of the underlying Lyngen Nappe, while the D3/M3 episode is common for the two units.

A previously published Sm–Nd mineral isochron (Grt–Cpx–Am) on a partly retrograded and recrystallized ecologite of 598 ± 107 Ma represents either the timing of formation of the eclogites or the post‐eclogite/pre‐D2 decompression stage, while a Rb–Sr whole rock isochron of an apparently post‐D1/pre‐D2 granite of 433 ± 11 Ma is consistent with a K–Ar age of post‐D1/pre‐D2 amphiboles from a retrograded eclogite of 437 ± 16 Ma which most likely record cooling below the 475–500°C isotherm after the M3 metamorphism.
The illite ‘crystallinity’technique: a critical appraisal of its precision
Authors: ROBINSON D. , WARR L. N. , BEVINS R. E.
Published: May 2007

Analysis of the precision of the illite ‘crystallinity’technique shows that machine errors are <5%, while intra‐ and inter‐sample errors are variable but are up to 12% and 14%, respectively (1σ). Consideration of this error analysis shows that the isocryst approach, which involves close contouring (e.g. 0.03 Δ2°) of illite ‘crystallinity’data, has a very low degree of confidence (<0.5) and thus is not regarded as statistically valid. If contouring is to be undertaken with a high degree of confidence (>0.8) it is necessary that contours should be at intervals of 0.1 ΔΘ2°, which is equivalent to subdivision of the anchizone into upper and lower units. Where previous interpretations have relied upon an isocryst method of contouring at less than 0.1 ΔΘ2° the conclusions must be regarded as unsubstantiated.

Centrifuge separation of clay fractions (based on a Stokes’law application) gives separations in which a significant, but variable, percentage of grains have long axes greater than the size calculated. For the typical <2‐μm fraction utilized, some 20% of grains lie in the 2–4‐μm range, although the proportion is not believed to have a significant effect upon ‘crystallinity’values. The formula is applicable for grain‐sizes down to 0.5 μm. Illite ‘crystallinity’values on samples prepared by an ultrasonic disaggregation method show a small increase on those prepared by ball mill crushing. The differences are minimal at the epi/anchizone level but increase to some 10% at the anchizone/diagenetic level. The effect on grade determinations is again thought to be minimal and indicates that concern over unsuitability of the ultrasonic disaggregation method is unfounded.