Spinel-anthophyllite rocks of the Kokchetav massif (northern Kazakhstan)

Spinel-anthophyllite rocks that may be classified as ultrabasic low-Ca spinel amphibolites have been first discovered in the Kokchetav collision zone (northern Kazakhstan). They outcrop 2 km west of Enbek-Berlyk Village among schists and quartzites and are closely associated with spinel harzburgites and garnet pyroxenites. The main hosted minerals are spinel (hercynite) and anthophyllite. The rocks bear magnetite-hornblende-spinel-anthophyllite pseudomorphs with rounded and polygonal sections, which might have been resulted from the replacement of garnet grains. The prismatic anthophyllite crystals and scarce olivine relics contain elongate parallel spinel inclusions resembling spinel-olivine syntactic intergrowths in the Enbek-Berlyk spinel harzburgites. The spinel-anthophyllite rocks are similar to the associated spinel harzburgites in CaO, MnO, SiO₂, and Al₂O₃ contents but are richer in FeO and poorer in MgO (F = FeO/(FeO + MgO) = 57% against 35% in the harzburgites). Geological, mineralogical, and geochemical data suggest that the spinel-anthophyllite rocks formed during the isochemical contact metamorphism of garnet-bearing spinel harzburgites, which contained more FeO and less MgO than garnet-free harzburgites of the same area. Variations in FeO and MgO contents in both types of harzburgites seem to be due to different chemical compositions of the chlorite protoliths of these rocks.     

Structural events and metamorphic consequences in Almora Nappe,during Himalayan collision tectonics

Almora Nappe in Uttarakhand, India, is a Lesser Himalayan representative of the Himalayan Metamorphic Belt that was tectonically transported over the Main Central Thrust (MCT) from Higher Himalaya. The Basal Shear zone of Almora Nappe shows complicated structural pattern of polyphase deformation and metamorphism. The rocks exposed along the northern and southern margins of this nappe are highly mylonitized while the degree of mylonitization decreases towards the central part where the rocks eventually grade into unmylonitized metamorphics.Mylonitized rocks near the roof of the Basal Shear zone show dynamic metamorphism (M₂) reaching upto greenschist facies (～450 °C/4 kbar). In the central part of nappe the unmylonitized schists and gneisses are affected by regional metamorphism (M₁) reaching upper amphibolite facies (～4.0–7.9 kbar and ～500–709 °C). Four zones of regional metamorphism progressing from chlorite–biotite to sillimanite–K-feldspar zone demarcated by specific reaction isograds have been identified. These metamorphic zones show a repetition suggesting that the zones are involved in tight F₂ – folding which has affected the metamorphics. South of the Almora town, the regionally metamorphosed rocks have been intruded by Almora Granite (560 ± 20 Ma) resulting in contact metamorphism. The contact metamorphic signatures overprint the regional S₂ foliation. It is inferred that the dominant regional metamorphism in Almora Nappe is highly likely to be of pre-Himalayan (Precambrian!) age.     

A Comparative geochemical study of pelitic schists and metamorphosed carbonate rocks from south-central Maine,USA

Whole-rock major element chemical analyses of progressively metamorphosed impure carbonate rocks and pelitic schists, collected from the same metamorphic terrain, reveal similarities and differences in the chemical response of these rock types to the metamorphic event. Relative to a constant aluminum reference frame, both schist and carbonate exhibit no detectable change in their contents of Fe, Mg, Ti, Si, and Ca with change in metamorphic grade. Carbonate rocks become progressively depleted in K and Na with increasing grade of metamorphism, while schists exhibit no statistically significant change in their contents of K and Na. Both rock types become depleted in volatiles (principally CO₂ and H₂O) with increasing grade.Whole-rock chemical data permit two mechanisms for migration of K and Na from the carbonate rocks during metamorphism: (a) diffusion of alkalis from carbonate to adjacent schist; (b) transport of alkalis by through-flowing metamorphic fluid (infiltration). Mineral equilibria in schist and metacarbonate rock from the same outcrops allow calculation of the affinity for cation exchange between the two rock types during metamorphism. Measured affinities indicate that if mass transport of K and Na occurred by diffusion, chemical potential gradients would have driven the alkalis from schist into carbonate rock. Because diffusion cannot produce the observed chemical trends in the metacarbonates, K and Na are believed to have been removed during metamorphism by infiltration.The disparity in chemical behavior between the pelitic schists and metacarbonate rocks may be a result of enhanced fluid flow through the carbonates. The carbonate rocks may have acted as metamorphic aquifers; the greater flow of fluid through them would then have had a correspondingly greater effect on their whole-rock chemistry.     

Petrology of thermally metamorphosed pelitic rocks in the Champawat Area,Kumaun Himalaya

In the Champawat area, Kumaun Himalaya, greenschist facies regionally metamorphosed rocksviz chlorite-phyllite and schist have been subjected to thermal metamorphism due to emplacement of batholithic granite/granodiorite body. As a consequence, biotite, garnet, andalusite, fibrolite, sillimanite and perthite minerals have formed in the contact rocks. The conspicuous absence of cordierite and staurolite reported from such aureole rocks is due to higher FeO/MgO ratio of the bulk rock composition in the former while the absence of staurolite is due to low Al₂O₃/FeO+MgO ratio in the schists. AFM diagram demonstrates that in muscovite-bearing schist, the bulk composition of chlorite- and cordierite-bearing rocks are restricted to low FeO/MgO side and thus the restricted occurrence of former and the absence of latter in the contact rocks of the area. This is further evident from the common occurrence of almandine-rich garnet in the rocks.     

A model for phlogopite development in low magnesium rocks

Phlogopites containing anomalously high values of Zn, Mn and Li have been identified in low-magnesium feldspathic schists and quartzites from the Precambrian of northern New Mexico. The phlogopite is volumetrically minor (<2%) and coexists with muscovite. The presence of hematite and piemontite in the phlogopite-bearing samples indicates high oxidizing conditions, but their presence in phlogopite-free mica schists nearby demonstrates that high fo₂ is insufficient to account for the phlogopite occurrences. A model for phlogopite development is presented which is based upon (1) high fo₂ conditions, (2) limited H⁺ during metamorphism, and (3) early mobilization of Mg (and Zn, Mn, Li) during incipient metamorphism. The absence of phlogopite in associated rocks which refect high fo₂ conditions is due to the larger amount of H⁺ available during metamorphism, resulting in the development of abundant muscovite rather than rare phlogopite.     

Probable anticlockwise P,–T evolution in extending crust: Hlinsko region, Bohemian Massif

ABSTRACT In the Hlinsko region (Variscan Bohemian Massif, Czech Republic) a major extensional shear zone separates low-grade metasedimentary series (Hlinsko schists) and high-grade rocks of the Moldanubian terrane (Svratka Crystalline Unit). During late-Variscan extension, a tonalite intruded syntectonically into the normal ductile shear zone, and caused contact metamorphism of the overlying schists. Concurrent syntectonic sedimentation of a flysch series took place at the top of the hangingwall schists. In order to decipher the detailed petrological evolution of the Hlinsko unit situated in the hangingwall of this tectonic contact, a phase diagram approach and petrogenetic grids, calculated with the thermocalc computer program, were used. The crystallization/deformation relationships and the paragenetic analysis of the Hlinsko schists define a P–T path with an initial minor increase in pressure followed by cooling. Calculated pseudosections constrain this anticlockwise P-T evolution to the upper part of the andalusite field between 0.36 and 0.40 GPa for temperatures ranging from 570 to 530°C. A low a_H2O is required to explain the presence of andalusite-biotite-bearing assemblages, and could be related to the presence of abundant graphite. In contrast, the footwall rocks of the Svratka Crystalline Unit record decompression from around 0.8 GPa at a relatively constant temperature, followed by cooling. Thus, the footwall and the hangingwall units display opposite, but convergent P–T histories. Decompression in the footwall rocks is related to a rapid exhumation. We propose that the inverse, anticlockwise P–T path recorded in the hangingwall pelites is related to the rapid, extension-controlled sedimentation of the overlying flysch series.     

HP–LT Variscan metamorphism in the Cubito-Moura schists (Ossa-Morena Zone, southern Iberia)

Multi-equilibrium thermobarometry shows that low-grade metapelites (Cubito-Moura schists) from the Ossa–Morena Zone underwent HP–LT metamorphism from 340–370 °C at 1.0–0.9 GPa to 400–450 °C at 0.8–0.7 GPa. These HP–LT equilibriums were reached by parageneses including white K mica, chlorite and chloritoid, which define the earliest schistosity (S₁) in these rocks. The main foliation in the schists is a crenulation cleavage (S₂), which developed during decompression from 0.8–0.7 to 0.4–0.3 GPa at increasing temperatures from 400–450 °C to 440–465 °C. Fe³⁺ in chlorite decreased greatly during prograde metamorphism from molar fractions of 0.4 determined in syn-S₁ chlorites down to 0.1 in syn-S₂ chlorites. These new data add to previous findings of eclogites in the Moura schists indicating that a pile of allochtonous rocks situated next to the Beja-Acebuches oceanic amphibolites underwent HP–LT metamorphism during the Variscan orogeny. To cite this article: G. Booth-Rea et al., C. R. Geoscience 338 (2006).     

Field and textural relationship in pelitic schists and gneisses from the area around Mangpu, Darjeeling district, West Bengal

The metasedimentary rocks of the area around Mangpu constitute a portion of the hinge zone of the northern limb of the major synform of Lower Darjeeling Himalaya. The rocks display evidences of multiple deformation and at least three major phases of deformation have been recognized. The time relations between the phases of deformation (D₁, D₂, D₃) and metamorphic crystallization reveal a single major prograde metamorphic event that initiated with the D₁ deformation and finally outlasted it. The earlier phase of this metamorphism is essentially regional syn-tectonic lowgrade (greenschist facies) which may be designated (M₁, early). This was followed by regional static metamorphism (M₁, late) in the post-tectonic phase between D₁ and D₂ deformations (upper green schist and amphibolite facies). This M1 metamorphism is superposed by later retrogressive metamorphism (M₂) during the D₂ and D₃ deformations (lower greenschist facies). Within the study area four isograds have been delineated by the first appearance of index minerals in the pelitic schists and gneiss which display Barrovian type of metamorphism.     

Oxygen isotope study of metamorphic rocks from the Ouégoa district,New Caledonia

48 minerals from 18 in situ metamorphic rocks (mostly metasediments) from the Ouégoa district have been studied. Particular emphasis was placed on obtaining isotopic data for quartz, calcite and muscovite but some pyroxenes and amphiboles were also examined. Data for Ouégoa rocks show they have tended to be isotopically homogenized by metamorphism and that the effect of increasing metamorphism is to progressively deplete the rocks of heavy C and O isotopes. These results indicate that during metamorphism the rocks isotopically exchanged through the medium of a widespread oxygen-carrying fluid phase. Tentatively assigned temperatures obtained from isotopic data for quartz-calcite and quartz-muscovite pairs, using the calibration curves of Epstein and Taylor (1967), indicate lawsonite in the Ouégoa schists to be stable over a temperature range of 250 to 400° C and epidote from ca. 380° to at least 550° C. Temperatures for metamorphic zones in Ouégoa blueschists closely parallel those obtained for Type III and IV glaucophane-bearing rocks from Ward Creek, California (Taylor and Coleman, 1968). The measured tectonic thickness of lawsonite-bearing schists has been used to calculate a lithostatic pressure increment of 2 Kb and geothermal gradient of 20° C per km for the lawsonite zone. Comparison of lithostatic pressure increment with total pressure increment estimated from the stability relations of lawsonite over the temperature range 250–400° C (3.5 Kb Nitsch, 1972) suggest P _total ≠ P _lithostatic and that that the pressure of the fluid phase may have exceeded lithostatic pressure.     

Lithological subdivision and petrology of the Great Himalayan Vaikrita Group in Kumaun,India

The Vaikrita Group made up of coarse mica-garnet-kyanite and sillimanite-bearing psammitic metamorphics constituting the bulk of the Great Himalaya in Kumaun is divisible into four formations, namely theJoshimath comprising streaky, banded psammitic gneisses and schists, the Pandukeshwar consisting predominantly of quartzite with intercalations of schists, thePindari made up of gneisses and schists with lenses of calc-silicate rocks and overwhelmingly injected by Tertiary pegmatites and granites (Badrinath Granite) leading to development of migmatites, and theBudhi Schist comprising biotite-rich calc-schists. The Vaikrita has been thrust along the Main Central Thrust over the Lesser Himalayan Munsiari Formation made up of highly mylonitized low-to meso-grade metamorphics, augen gneisses and phyllonites. Petrological studies demonstrate contrasting nature of metamorphism experienced by the Vaikrita and the Munsiari rocks. Sillimanite-kyanite-garnet-biotite-muscovite (±K-feldspar and ± plagioclase).—quartz metapelites and interbanded calc-schists and calc-gneisses with mineral assemblages of calcite-hornblende-grossular garnet, labradorite (An₅₀?An₆₅), (± K-feldspar)-quartz (± biotite), and hornblende-diopside ± labradorite ± quartz, suggest medium to high grade of metamorphism or indicate upper amphibolite facies experienced by the rocks of the Vaikrita Group. The associated migmatites, granite-gneisses and granites of the Pindari Formation were formed largely as a result of anatexis of metapelites and metapsammites. While, the sericite-chlorite-quartz and muscovite-chlorite-chloritoid-garnet-quartz, assemblages in metapelites and epidote-actinolite-oligoclase (An₂₀)-quartz and epidote-hornblende-andesine (An₂₉) ± quartz in the metabasites suggest a low-grade metamorphism (greenschist facies) for the Munsiari Formation, locally attaining the lower limit of medium-grade (epidote-amphibolite) facies. The inferred P-T conditions obtained from textural relations of various mineral phases and the stability relationship of different coexisting phases in equilibrium, suggest that the temperature ranged between 600° and 650° C and pressure was over 5 kb for the Vaikrita rocks. The mineral assemblages of the Munsiari Formation indicate comparatively lower P-T conditions, where the temperature reached approximately 450° C and pressure was near 4 kb. The rocks of the two groups were later subjected to intense shearing, cataclasis and attendant retrograde metamorphism within the zone of the Main Central (=Vaikrita) Thrust.     

Metamorphism and Metasomatism at King Island Scheelite Mine

Abstract

The King Island Scheelite Mine lies in the contact aureole of a granodiorite stock. Its open cut and numerous drill cores expose a contact metamorphosed and metasomatized series of finely interbedded argillaceous and calcareous sediments, with interleaved flows of picrite-basalt and basic pyroclastics, the scheelite ore being limited to two limestone horizons. The range and gradation in composition of the original rocks has resulted in an unusual variety of meta-morphic rocks, including forsterite-phlogopite-spinel-tremolite hornfels, anthophyllite-cordierite hornfels, biotite hornfels, actinolite hornfels, a variety of calc-flinta, and marbles. The original sedimentation gave rise to a rapid alternation of limestone and shale, many times repeated, and during metamorphism these rocks reacted with each other to produce narrow bands of calc-flinta.

Subsequent pyrometasomatism selectively converted the greater part of the marble beds to scheelite-bearing andradite skarn, leaving the various hornfels and calc-flinta very little affected. The replacement of the marble was a volume for volume process, and the conversion of 1,000,000 tons of marble to average grade ore involved the introduction of about 350,000 tons of SiO₂, 250,000 tons of Fe₂O₃, 55,000 tons AI₂O₃, 30,000 tons of H₂O and 82,500 tons of CaO.

The temperature of the contact metamorphism attained over 500° C, and the rocks cooled to about 400° C. before the pyrometasomatism occurred. The rocks giving rise to the various hornfels underwent varying degrees of contraction during metamorphism, whereas the limestones probably expanded during metamorphism, and became more permeable to solutions.     

Evidence for structural discordance in the inverted metamorphic sequence of Sikkim Himalaya: Towards resolving the Main Central Thrust controversy

Inverted metamorphism in the Himalayas is closely associated with the Main Central Thrust (MCT). In the western Himalayas, the Main Central Thrust conventionally separates high grade metamorphic rocks of the Higher Himalayan Crystalline Sequence (HHCS) from unmetamorphosed rocks of the Inner sedimentary Belt. In the eastern Himalayas, the Inner sedimentary Belt is absent, and the HHCS and meta-sedimentary Lesser Himalayan Sequence (LHS) apparently form a continuous Barrovian metamorphic sequence, leading to confusion about the precise location of the MCT. In this study, it is demonstrated that migmatitic gneisses of the sillimanite zone in the higher structural levels of the HHCS are multiply deformed, with two phases of penetrative fabric formation (S_1HHCS and S_2HHCS) followed by third folding event associated with a spaced, NW-SE trending, north-east dipping foliation (S_3HHCS). The underlying LHS schists (kyanite zone and lower) are also multiply deformed, with the bedding S₀ being isoclinally folded (F_1LHS), and subsequently refolded (F_2LHS and F_3LHS). The contact zone between the HHCS and LHS is characterized by ductile, top-to-the southwest shearing and stabilization of a pervasive foliation that is consistently oriented NW-SE and dips northeast. This foliation is parallel to the S_3HHCS foliation in the HHCS, and the S_2LHS in the LHS. Early lineations in the HHCS and LHS also show different dispersions across the contact shear zone, implying that pre-thrusting orientations of the two units were distinct. The contact shear zone is therefore interpreted to be a plane of structural discordance, shows a shear sense consistent with thrust movement and is associated with mineral growth during Barrovian metamorphism. It may well be considered to represent the MCT in this region.     

The composition of Alpine marine sediments (Bündnerschiefer Formation,W Alps) and the mobility of their chemical components during orogenic metamorphism

The Bündnerschiefer of the Swiss-Italian Alps is a large sedimentary complex deposited on the Piemonte–Liguria and Valais oceans and associated continental margins from the upper Jurassic to Eocene. It is made of a large variety of sequences associated or not with an ophiolitic basement. The Bündnerschiefer makes an accretionary prism that developed syn-tectonically from the onset of alpine subduction, and it records orogenic metamorphism following episodes of HP metamorphism. The Bündnerschiefer shares important similarities with the Otago schists of New Zealand and with the Wepawaug schists of Connecticut, both of which form accretionary prisms and have an orogenic metamorphic imprint.With the aim of testing the hypothesis of mobility of chemical components as a function of metamorphic grade, in this work I present fifty-five bulk chemical analyses of various lithological facies of the Bündnerschiefer collected along the well-studied field gradient of the Lepontine dome of Central Switzerland, in the Prättigau half window of East Switzerland, and in the Tsaté Nappe of Valle d'Aosta (Italy). The dataset includes the concentration of major components, large ion lithophile elements (Rb, Sr, Ba, Cs), high field strength elements (Zr, Ti, Nb, Th, U, Ta, Hf), fluid-mobile light elements (B, Li), volatiles (CO₂, S), REEs, and Y, V, Cr, Co, Sn, Pb, Cu, Zn, Tl, Sb, Be, and Au. These data are compared against the compositions of the global marine sediment reservoir, typical crustal reservoirs, and against the previously measured compositions of Otago and Wepawaug schists. Results reveal that, irrespective of their metamorphic evolution, the bulk chemical compositions of orogenic metasediments are characterized by mostly constant compositional ratios (e.g., K₂O/Al₂O₃, Ba/Al₂O₃, Sr/CaO, etc.), whose values in most cases are undistinguishable from those of actual marine sediments and other crustal reservoirs. For these rocks, only volatile concentrations decrease dramatically as a function of metamorphic temperature, and significant deviations from the reservoir signatures are evident for SiO₂, B, and Li. These results are interpreted as an indication of residual enrichment in the sediments, a process taking place during syn-metamorphic dehydration from the onset of metamorphism in a regime of chemical immobility. Residual enrichment increased the absolute concentrations of the chemical components of these rocks, but did not modify significantly their fundamental ratios. This poor compositional modification of the sediments indicates that orogenic metamorphism in general does not promote significant mass transfer from accretionary prisms. In contrast, mass transfer calculations carried out in a shear zone crosscutting the Bündnerschiefer shows that significant mass transfer occurs within these narrow zones, resulting in gains of H₂O, SiO₂, Al₂O₃, K₂O, Ba, Y, Rb, Cu, V, Tl, Mo, and Ce during deformation and loss of Na₂O, CO₂, S, Ni, B, U, and Pb from the rock. These components were presumably transported by an aquo-carbonic fluid along the shear zone. These distinct attitudes to mobilize chemical elements from orogenic sediments may have implications for a potentially large number of geochemical processes in active continental margins, from the recycling of chemical components at plate margins to the genesis of hydrothermal ore deposits.     

Metamorphic zonal sequences of pelitic schists and gneisses from the area around Kandra (Jharkhand): Constraints from field and textural relationship

The pelitic schists of the area around Kandra, Singhbhum district, Jharkhand belong to the Chaibasa Formation of the Singhbhum Group, which constitute a part of the youngest Precambrian orogenic cycle of the Singhbhum region. Structurally, the area represents the Singhbhum anticlinorium and is overlain by Dalma traps which form the synclinorium towards the north of the area around Kandra. This area mainly consists of medium to high grade rocks belonging to greenschist and amphibolite facies. These rocks are folded in the E-W trending doubly plunging folds (F₁) overturned towards the south with low plunges and superposed by cross-folds (F₂). The spatial distribution of the index minerals in the pelitic schists of the area shows Barrovian type of metamorphism. Four isograds, viz. biotite, garnet, staurolite and sillimanite have been delineated by the first appearance of the index minerals and also by isograd reactions. The textural relation suggests that sillimanite is formed from staurolite consumption reaction instead of kyanite consumption.     

Age and origin of sillimanite schist from the Chinese Altai metamorphic belt: implications for late Palaeozoic tectonic evolution of the Central Asian Orogenic Belt

Reconstructing late Palaeozoic metamorphism of the Central Asian Orogenic Belt (CAOB) can provide a better understanding of how the CAOB formed. The petrology of sillimanite-bearing metapelitic schists from high-grade portions of the Permian Chinese Altai metamorphic belt (andalusite-type) reflects the effects of poorly understood high-T, low-P metamorphism. Phase equilibria modelling in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–TiO₂–O (NCKFMASHTO) system restricts P–T conditions of the sillimanite schists to approximately 635–670°C at approximately 5.8–6.8 kbar. SHRIMP U–Pb analyses of zircon from the rocks yield a concordant age of 299.2 ± 3.4 Ma. Combined with the slightly younger (292.8 ± 2.3 Ma) areally restricted pelitic granulite with peak P?T conditions of approximately 780–800°C at approximately 5–6 kbar and high-T granulite with P?T conditions of approximately 860°C at approximately 6 kbar, these metamorphic rocks reflect prograde heating at relatively low pressure in early Permian time. Together with contemporary and widespread magmatic activities, they are best explained in the context of a post-orogenic extensional environment related to a mantle plume.     

Silica diffusion at Ascutney Mountain,Vermont

A suite of pelitic schists from the contact metamorphic aureole around the syenite stock at Ascutney Mountain, Vermont, shows a decrease in the amounts of cordierite, quartz, and andalusite and an increase in pleonaste and corundum as the contact is approached. Whole rock chemical analyses show a distinct SiO₂ concentration gradient from the wall rocks toward the intrusion; however, the syenite has a higher weight percent SiO₂ than the adjacent schists. It is suggested that silica diffused down an SiO₂ activity gradient into the intrusive; other major oxide components were apparently immobile. The diffusion of silica was probably through intergranular fluids, and the amount of silica available for diffusion was governed by the solubilities of cordierite, andalusite, and quartz in the interstitial fluid.     

Gradients in fluid composition across metacarbonate layers of the Wepawaug Schist, Connecticut, USA

Metamorphic index mineral zones, pressure-temperature (P-T) conditions, and CO₂-H₂O fluid compositions were determined for metacarbonate layers within the Wepawaug Schist, Connecticut, USA. Peak metamorphic conditions were attained in the Acadian orogeny and increase from ~420 °C and ~6.5 kb in the low-grade greenschist facies to ~610 °C and ~9.5 kb in the amphibolite facies. The index minerals oligoclase, biotite, calcic amphibole, and diopside formed with progressive increases in metamorphic intensity. In the upper greenschist facies and in the amphibolite facies, prograde reaction progress is greatest along the margins of metacarbonate layers in contact with surrounding schists, or in reaction selvages bordering syn-metamorphic quartz veins. New index minerals typically appear first in these more highly reacted contact and selvage zones. It has been postulated that this spatial zonation of mineral assemblages resulted from infiltration, largely by diffusion, of water-rich fluids across lithologic contacts or away from fluid conduits like fractures. In this model, the infiltrating fluids drove prograde CO₂ loss and were derived from surrounding dehydrating schists or sources external to the metasedimentary sequence. The model predicts that significant gradients in the mole fraction of CO₂ (X_CO2 X_{CO_2 } ) should have been present during metamorphism, but new estimates of fluid composition indicate that differences in X_CO2 X_{CO_2 } preserved across layers or vein selvages were very small, ~0.02 or less. However, analytical solutions to the two-dimensional advection-dispersion-reaction equation show that only small fluid composition gradients across layers or selvages are needed to drive prograde CO₂ loss by diffusion and mechanical dispersion. These gradients, although typically too small to be measured by field-based techniques, would still be large enough to dominate the effects of fluid flow and reaction along regional T and P gradients. Larger gradients in fluid composition may have existed across some layers during metamorphism, but large gradients favor rapid reaction and would, therefore, seldom be preserved in the rock record. Most of the H₂O needed to drive prograde CO₂ loss probably came from regional dehydration of surrounding metapelitic schists, although H₂O-rich diopside zone conditions may have also required an external fluid component derived from syn-metamorphic intrusions or the metavolcanic rocks that structurally underlie the Wepawaug Schist.     

The origin of scapolite in the regionally metamorphosed rocks of Mary Kathleen,Queensland, Australia

Scapolite at Mary Kathleen (North-Western Queensland) occurs in calcareous and non-calcareous metapelites, acid and basic metavolcanics and metadolerites. Graphical treatment of the relationship between scapolite composition (Me%) and the host rock oxide ratios CaO/Na₂O and Al₂O₃/(CaO + Na₂O) reveals the following points:

1. The calcareous metapelites are also very sodic.
2. Scapolite in calcareous metapelites is more marialitic than that in low-calcium equivalents.
3. In graphs of Me% against CaO/Na₂O and Al₂O₃/(CaO + Na₂O) the metasediments and the metaigneous rocks show markedly different trends.

It is concluded that scapolite in the metasediments originated by isochemical metamorphism of shales and marls containing evaporitic halite. The local abundance of halite was the main control on the composition and distribution of the scapolite, but the relative abundance of CaO and Na₂O was a modifying factor. In the metaigneous rocks scapolite formed metasomatically during regional metamorphism by the introduction of volatile-rich fluids derived from the adjacent evaporitic sediments. The relative availability of CO₂ and Cl₂ again appears to have been the primary control on scapolite composition and may in turn have been controlled by bulk rock composition.     

The mineral–geochemical evidence of contact transformation of ores of the Dzhusinskoe pyrite–polymetallic deposit (Southern Urals)

The Dzhusinskoe pyrite–polymetallic deposit is characterized by an abundant concentration of dykes of basic and intermediate rocks. Thermal metamorphism of ore-host rocks and the recrystallization of ore minerals are associated with the intrusion of post-ore dykes. A regular increase in the homogenization temperature from 156° at a distance from a dyke to 287–305°C in the contact zone was established. Highly saline (6.4–15.7 wt % NaCl eq.) CO₂–H₂O–NaCl fluids under high pressure (up to 1500 bar) can be associated with the processes of contact and regional metamorphism.     

Tectonic interpretation of the thermochronological data and PT-conditions of rock metamorphism in the Bodonchin zonal complex (Mongolian Altay)

Based on the new petrological and thermochronological data, analysis of the metamorphism conditions and tectonic evolution of the Bodonchin zonal complex in the Mongolian Altay was performed. Using mineral geothermometers and geobarometers, the parameters of the thermal state of the Mongolian Altay crust site during the collision of terranes were estimated, and the paleogeotherm at the peak of syncollisional metamorphism was reconstructed. The thermal state of the crust was determined by either a high concentration of radioactive heat sources or a high mantle heat flow. The estimated metamorphism temperatures and pressures of rocks in two zones (staurolite-kyanite schists and migmatites) of the Bodonchin complex correspond to the paleogeotherms with average temperature gradients ∂T/∂z = 25.5 and 27.2 ºC/km. The results of isotope dating of zircons and metamorphic minerals were used to construct a thermochronological model for the regressive stage of evolution of the polymetamorphic complex. The rates of the ascent of metamorphic rocks to the surface as a result of thrusts in the Bulgan Fault zone were estimated at 0.3–1 mm/year.