Metamorphic evolution of the assemblage tremolite + talc + calcite + dolomite + quartz within a sample of siliceous dolomite from the southern Damara Orogen (Namibia)

Magnesian calcite temperatures of calcites formed during metamorphic reactions and fluorine distribution between OH-bearing minerals have been measured for the assemblage {ie180-1} within a sample of siliceous dolomite from the southern Damara Orogen. Two groups of temperatures were found: A group with higher temperatures in the range 583 to 545°C for reaction sites where tremolite was formed, and a second group with lower temperatures from 522 to 476°C for reaction sites where talc has grown. From special reaction textures it is evident that the reaction sites of the higher temperature group related with temolite is older, whereas talc has grown after the formation of tremolite with decreasing temperature. The transition from the tremolite stability field to the talc stability field occurred at a temperature between the two temperature groups. Each measured magnesian calcite temperature can be assigned to a microscopic fluid vein which connects single reaction sites either with tremolite or with talc. Single microveins developed at different times, because the prevailing temperature during the formation of individual fluid veins and the connected reaction sites are different. The mineral associations and the reaction textures of the sample were formed within a period of time necessary for cooling the rock at least from about 580 to 480°C. The textural relations after the successive reaction processes appear to be the result of a single univariant reaction. Distribution of fluorine between tremolite-phlogopite pairs and tremolite-talc pairs show that the fluorine content of the tremolite may be inherited in the latter case. A plausible P-T path is outlined explaining the evolution of assemblages in dolomites and metapelites of the same area which contain staurolite and kyanite. The first appearance of staurolite and kyanite in the field is situated on the lower grade side of the zone containing assemblage (I). The conversation of the present field occurrence of assemblage (I) is due to the fading of fluid infiltration into the dolomites.     

Phase relations of talc and tremolite in metamorphic calcite-dolomite sediments in the southern portion of the Damara Belt (South West Africa)

The occurrence of talc and tremolite in a temperature gradient was investigated in siliceous calcite-dolomite sediments exposed along a strip in the southeastern part of the Damara Orogen. Five bivariant reactions may lead to the formation of talc and tremolite:

1. 3 dolomite+4 quartz+1 H₂O ? 1 talc+3 calcite+3 CO₂
2. 5 talc+6 calcite+4 quartz ? 1 tremolite+6 CO₂+2 H₂O
3. 2 talc+3 calcite ? 1 tremolite+1 dolomite+1 CO₂+1 H₂O
4. 5 dolomite+8 quartz+1 H₂O ? 1 tremolite+3 calcite+7 CO₂
5. 2 dolomite+1 talc+4 quartz ? 1 tremolite+4 CO₂.

The common paragenesis of four mineral assemblages tc+cc+dol+qtz¹ and tre+tc+ cc+qtz with increasing temperature over an extended area show that the reactions must have taken place along the equilibrium curve or when fluid pressure is not constant along the equilibrium plane of reactions (1) or (2). The described occurrence of the five mineral assemblage tre+tc+cc+dol+qtz can be stable only on the isobaric intersection point, or when P _f is variable on the univariant intersection curve of the equilibrium planes of all five reactions. The genetic relations of the described parageneses are illustrated with the help of a phase diagram. Minimum P-T conditions which prevailed during metamorphism in this part of the Damara Orogen have been estimated to be about 590° C and 5 kb.     

韩国春川软玉的矿物学研究

采用常规宝石学测试、偏光显微镜观察与电子探针(EPMA)分析对具有矿化分带的韩国春川软玉进行研究。结果表明,春川软玉主要矿物组成为透闪石,其次有透辉石、方解石、白云石、滑石等。矿化分带分为大理岩带、蚀变带、软玉+蚀变带、软玉带,是白云质大理岩经过多次交代与变质作用形成的。     

Sub—microscopic Textures and Retrogressive Metamorphic Origin of Longxi Nephrite

Nephrite specimens from Longxi,sichuan,prepared by ion-thinning and dispersion techniques have been studied using TEM and SAED.A series of sub-microscopic textures such as (010)multiple-chain faults and related fault terminations.(001)mechanical twinning,sub-grain boundaries or fault walls,tremolite fibrous pseudoform of talc and intergrowth of tremolite and talc with (010)as interface are revealed .By analogy of metallographic textures,characteristics and mechanisms of the process of “deformation-recovery-recrystallization“ are discussed in detail,Topotactic reaction mechanism by which tremolite retrogresses to talc was studied and a retrogressive metamorphic origin of Longxi nephrite is proposed.     

Geothermobarometry in metasediments of the southern Grossvenediger area (Tauern Window, Austria)

Metasediments in the southern Grossvenediger area (Tauern Window, Austria) were studied along a cross-section through rocks of increasing metamorphic grade from the margin of the Tauern Window in the south to the base of the Upper Schieferhülle, including the Eclogite Zone, in the north. In the southern part of the cross-section there is no evidence for a pre-late Alpine metamorphic history in the form of high-pressure relics or pseudomorphs. Mineral assemblages are characterized by the stability of tremolite + calcite, biotite + calcite and biotite + chlorite + calcite. In the northern part a more complete Alpine metamorphic evolution is preserved. Primary high-pressure assemblages are dolomite + quartz, tremolite + zoisite, zoisite + dolomite + quartz + phengite I and probably tremolite + dolomite + phengite I. Secondary, post-kinematic assemblages [tremolite + calcite, talc + calcite, phengite II + chlorite + calcite (+ quartz), biotite + chlorite + calcite, biotite + zoisite + calcite] formed as a result of the dominant late Alpine metamorphic overprint. The occurrence of biotite + zoisite + calcite is confined to the northernmost area and defines a biotite–zoisite–calcite isograd. P–T estimates based on standard thermobarometric techniques and on stability relationships of tremolite + calcite + dolomite + quartz and zoisite give consistent results. P–T conditions of the main Tertiary metamorphic overprint were 525° C, P= 7.5 ± 1 kbar in the northern part of the cross-section. The southern part was metamorphosed at lower temperatures of 430–470° C. The Si-content of phengites from this area is almost as high as that of phengites from the Eclogite Zone (Si^max= 3.4 pfu). Pressures > 10 kbar at 420° C are suggested by phengite barometry according to Massone & Schreyer (1987). In the absence of high-pressure relics or pseudomorphs, these phengites, which lack late Alpine re-equilibration, are the only record that rocks of the southern part probably also experienced an early non-eclogitic high-pressure metamorphism.     

Prograde and retrograde fluid flow during contact metamorphism of siliceous carbonate rocks from the Ballachulish aureole,Scotland

 Siliceous dolomites and limestones contain abundant retrograde minerals produced by hydration-carbonation reactions as the aureole cooled. Marbles that contained periclase at the peak of metamorphism bear secondary brucite, dolomite, and serpentine; forsterite-dolomite marbles have retrograde tremolite and serpentine; wollastonite limestones contain secondary calcite and quartz; and wollastonite-free limestones have retrograde tremolite. Secondary tremolite never appears in marbles where brucite has replaced periclase or in wollastonite-bearing limestones. A model for infiltration of siliceous carbonates by CO₂-H₂O fluid that assumes (a) vertical upwardly-directed flow, (b) fluid flux proportional to cooling rate, and (c) flow and reaction under conditions of local equilibrium between peak temperatures and ≈400 °C, reproduces the modes of altered carbonate rocks, observed reaction textures, and the incompatibility between tremolite and brucite and between tremolite and wollastonite. Except for samples from a dolomite xenolith, retrograde time-integrated flux recorded by reaction progress is on the order of 1000 mol fluid/cm² rock. Local focusing of flow near the contact is indicated by samples from the xenolith that record values an order of magnitude greater. Formation of periclase, forsterite, and wollastonite at the peak of metamorphism also required infiltration with prograde time-integrated flux approximately 100–1000 mol/cm². The comparatively small values of prograde and retrograde time-integrated flux are consistent with lack of stable isotope alteration of the carbonates and with the success of conductive thermal models in reproducing peak metamorphic temperatures recorded by mineral equilibria. Although isobaric univariant assemblages are ubiquitous in the carbonates, most formed during retrograde metamorphism. Isobaric univariant assemblages observed in metacarbonates from contact aureoles may not record physical conditions at the peak of metamorphism as is commonly assumed. Received: 19 September 1995 / Accepted: 14 March 1996     

The role of fluids in the metamorphism of komatiites,Agnew nickel deposit,western Australia

The Agnew nickel sulfide deposit is spatially associated with a lenticular body of ultramafic rocks which shows a concentric zonation in metamorphic mineralogy. Olivine + tremolite + chlorite + cummingtonite ±enstatite assemblages occur at the margin of the ultramafic lens, giving way to olivine + anthophyllite, olivine + talc and olivine + antigorite assemblages successively inwards. These rocks are interpreted as having crystallized from komatiitic lavas, and exhibit a spectrum of compositions from those of original flow tops to pure olivine adcumulates. The relative modal abundances of metamorphic olivine, tremolite and chlorite reflect original proportions of cumulus olivine and komatiite liquid in the protolith. Peak metamorphic conditions are estimated at 550° C, based on garnet-biotite thermometry, at a maximum pressure of 3 kb. This temperature falls within the narrow range over which metamorphic olivine may co-exist with enstatite, anthophyllite, talc or antigorite depending upon the fugacity of water in the metamorphic fluid. The observed mineralogical zonation is therefore attributed to infiltration by CO₂-rich fluids, generated by decarbonation of talc-carbonate rocks formed during pre-metamorphic marginal alteration of the ultramafic lens. Metamorphic fluids were essentially binary mixtures of water and CO₂, with minor H₂S having a maximum partial pressure less than 1 percent of total pressure. Enstatite-bearing assemblages formed in the presence of CO₂-rich fluids at fluid: rock volume ratios close to one, while anthophyllite, talc and antigorite bearing assemblages formed in the presence of progressively more water-rich fluids at progressively lower fluid-rock ratios.     

Oxide and sulphide isograds along a Late Archean, deep-crustal profile in Tamil Nadu, south India

Oxide–sulphide–Fe–Mg–silicate and titanite–ilmenite textures as well as their mineral compositions have been studied in felsic and intermediate orthogneisses across an amphibolite (north) to granulite facies (south) traverse of lower Archean crust, Tamil Nadu, south India. Titanite is limited to the amphibolite facies terrane where it rims ilmenite or occurs as independent grains. Pyrite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade. Pyrrhotite is confined to the high‐grade granulites. Ilmenite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade and occurring primarily as hemo‐ilmenite in the high‐grade granulite facies rocks. Magnetite is widespread throughout the traverse and is commonly associated with ilmenite. It decreases in abundance with increasing metamorphic grade. In the granulite facies zone, reaction rims of magnetite + quartz occur along Fe–Mg silicate grain boundaries. Magnetite also commonly rims or is associated with pyrite. Both types of reaction rims represent an oxidation effect resulting from the partial subsolidus reduction of the hematite component in ilmenite to magnetite. This is confirmed by the presence of composite three oxide grains consisting of hematite, magnetite and ilmenite. Magnetite and magnetite–pyrite micro‐veins along silicate grain boundaries formed over a wide range of post‐peak metamorphic temperatures and pressures ranging from high‐grade SO₂ to low‐grade H₂S‐dominated conditions. Oxygen fugacities estimated from the orthopyroxene–magnetite–quartz, orthopyroxene–hematite–quartz, and magnetite–hematite buffers average 2.5 log units above QFM. It is proposed that the trends in mineral assemblages, textures and composition are the result of an external, infiltrating concentrated brine containing an oxidizing component such as CaSO₄ during high‐grade metamorphism later acted upon by prograde and retrograde mineral reactions that do not involve an externally derived fluid phase.     

The Metamorphism of the Blue River Ultramafic Body, Cassiar, British Columbia, Canada

The Blue River ultramafic body is an ‘Alpine’-typeperidotite tectonically emplaced within spilitic volcanic rocksin northern British Columbia. The intrusive margins were shearedand serpentinized to a lizardite-chrysotile plus brucite assemblageduring emplacement, prior to thermal metamorphism in the aureoleof a younger batholith. Relatively anhydrous peridotite andhydrous serpentinite were both affected by thermal metamorphism.The body has been subdivided into units defined by the mineralassemblages observed in meta-peridotite and meta-serpentiniteabove and below the isograd for the advent of the mineral talc.Isograds were also established for prograde metamorphic olivine,tremolite, and enstatite. The intrusive was subjected to two metamorphic processes, oxidationand dehydration. The nucleation of metamorphic olivine in weaklymetamorphosed serpentinite was erratic, and turbid porphyroblastcores are enriched in Fe and Mn. The dehydration reaction isthought to have been metastable. Above the talc isograd, serpentine, in both peridotite and serpentinite,reacted with original spinel to form ferritchromit and chlorite.The chlorite becomes progressively more aluminous with increasein grade. The oxidation process inhibited dehydration in meta-peridotiteas a stable chlorite was formed. The process also served toreduce the Fe content of the silicate system, as shown by thecomposition of the olivine generated from excess serpentinein high grade meta-serpentinite.     

The origin of major talc deposits in the Eastern Desert of Egypt: relict fragments of a metamorphosed carbonate horizon?

Detailed geochemical and mineralogical investigation of four talc deposits in the Eastern Desert of Egypt (Atshan, Abu Gurdi, Darhib and Kashira) suggests that the deposits form a distinct lithological unit within the Shadli metavolcanic rocks. The talc crystallized from the replacement of siliceous carbonate beds locally intercalated with clastic sediments. Th/Yb vs. Ta/Yb ratios of the rocks suggest that the sediments and the host volcanic rocks formed in an active continental margin (ACM) environment. Thus, the talc deposits may represent relict fragments of an ancient, regionally extensive carbonate horizon within the arc-related metavolcanics. The talc-rich rocks, which contain relict carbonate, serpentinized olivine and tremolite, have low (<3 wt%) Al₂O₃, Cr, Ni (<20 ppm), Co and Sc (<15 ppm) concentrations, precluding mafic or felsic igneous protoliths. The deposits were locally affected by contact metamorphism, giving rise to pyroxene-hornfels and granulite facies assemblages, and by regional metamorphism which produced greenschist-amphibolite grade assemblages. Disseminated sulfides commonly occur in the talc-tremolite-rich rocks (having low Al₂O₃ concentrations), suggesting that the metals were probably present in the original carbonate beds, but were remobilized and reconcentrated during the various metamorphic events.     

F-OH substitution in natural tremolite,talc, and phlogopite

The distribution of F between tremolite and talc has been determined in metamorphosed siliceous carbonates from the Grenville Province, Ontario. Wavelength dispersive electron microprobe analyses of contiguous, texturally compatible tremolite-talc pairs indicate that the substitution of F for OH is the most significant deviation from end-member stoichiometry in the samples studied. Mixing of F and OH components has been represented by an ideal solution model for F in tremolite and an asymmetric model for F in talc. Both linear and nonlinear regression techniques have been used to derive activity coefficients for the exchange of one equivalent of OH and F components in talc. The following expressions are the result of nonlinear regression of 32 analyses from coexisting mineral pairs: $$\begin{gathered} \ln \gamma _{TC(OH)} = X_{TC(F)}^2 [2.447 - 2.845X_{TC(OH)} ] \hfill \\ \ln \gamma _{TC(F)} = X_{TC(OH)}^2 [1.024 + 2.845X_{TC(F} ] \hfill \\ \end{gathered} $$ Isobaric \(T - X_{CO_2 } \) sections constructed using these equations show an enhanced stability for the assemblages talc+calcite and phlogopite+quartz+calcite with F substituting for OH. Projection of isothermal invariant points into P-T space predicts a shift in the stability of the assemblage talc-calcite from lower grade into the sillimanite field with increasing substitution of F for OH in talc.     

Mineralogical Evidence for Fluid-Rock Interaction Accompanying Prograde Contact Metamorphism of Siliceous Dolomites: Alta Stock Aureole, Utah, USA

Contact metamorphism of siliceous dolomite in the southern partof the metamorphic aureole of the Alta stock (Utah, USA) producedthe prograde isograd sequence: talc (Tc), tremolite (Tr), forsterite(Fo), and periclase (Per). Calcite (Cc)–dolomite (Do)geothermometry and phase equilibria define a general progradeT–X(CO₂) path of decreasing X(CO₂) with rising temperaturefor the dolomite. High-variance assemblages typify the aureole.Per + Cc and Fo + Cc + Do characterize the inner aureole (Perand Fo zones), and Tr + Do + Cc and Tc + Do + Cc are widespreadin the outer aureole (Tr and Tc zones). Low-variance assemblagesare rare and the thickness of reaction zones (coexisting reactantand product minerals) at the isogradic reaction fronts are narrow(tens of metres or less). The mineral assemblages, calculatedprogress of isograd reactions, and the prograde T–X(CO₂)path all indicate that massive dolomite was infiltrated by significantfluxes of water-rich fluids during prograde metamorphism, andthat the fluid flow was down-temperature and laterally awayfrom the igneous contact. Fluid infiltration continued throughat least the initial retrograde cooling of the periclase zone.Down-T fluid flow is also consistent with the results of Cc–Dogeothermometry and patterns of ¹⁸O depletion in this area. Theclose spatial association of reacted and unreacted chert nodulesin both the tremolite and talc zones plus the formation of tremoliteby two reactions indicate that the outer aureole varied in X(CO₂),and imply that fluid flow in the outer aureole was heterogeneous.The occurrence of dolomite-rich and periclase (brucite)-absent,high-     

Contact Metamorphosed Ultramafic Rocks in the Western Sierra Nevada Foothills, California

Within the western Sierra Nevada metamorphic belt, linear bodiesof alpine-type ultramafic rock, now composed largely of serpentineminerals, parallel the regional strike and commonly coincidewith major fault zones. Within this metamorphic belt, east ofSacramento, California, ultramafic rocks near a large maficintrusion, the Pine Hill Intrusive Complex, have been emplacedduring at least two separate episodes. Those ultramafic rocks,evidently unaffected by the Pine Hill Intrusive Complex andcomposed largely of serpentine minerals, were emplaced alonga major fault zone after emplacement of the Pine Hill IntrusiveComplex. Those ultramafic rocks, contact metamorphosed by thePine Hill Intrusive Complex, show a zonation of mineral assemblagesas the igneous contact is approached: olivine+antigorite+chlorite+tremolite+Fe-Cr spinel olivine+talc+chlorite+tremolite+Fe-Crspinel olivine+anthophyllite+chlorite+tremolite+Fe-Cr spinel olivine+orthopyroxene+aluminous spinel+hornblende+Fe-Cr spinel.Superimposed on these mineral assemblages are abundant secondaryminerals (serpentine minerals, talc, chlorite, magnetite) whichformed after contact metamorphism. Correlation of observed mineralassemblages with the experimental systems, MgO-SiO₂-H₂O andMgO-Al₂O₃-SiO₂-H₂O suggests an initial contact temperature of775±25 °C for the Pine Hill Intrusive Complex assumingPtotal P_fluid PH₂O. The pressure acting on the metamorphic rockduring emplacement of the intrusion is estimated to be a minimumof 1.5 kb.     

A new occurrence of garnetiferous skarn rocks in Saudi Arabia: a case study from Bahrah area, Jeddah–Makkah Al Mukaramah highway

Metacarbonate rocks (including marble and skarn deposits) at Bahrah area are confined to a Precambrian island-arc suite made up mostly of massive basalts and volcaniclastics aligned in a NE-trending belt. The marbles are either pure (almost made up of calcite) or contain considerable amounts of tremolite, actinolite, epidote, and diopside. Garnet-bearing rocks at Bahrah area are classified into garnetiferous marble and skarn calc-silicate assemblages that are described here for the first time. The calc-silicates become more abundant when the marble becomes interbedded with foliated metabasalt. Such contact is delineated by an epidote zone of variable thickness. Microscopically, the skarns are enriched in Ca-bearing minerals such as grossular garnet, epidote, titanite, diopside, and augitic salite. There are evidence that calc-silicate skarns were formed due to a thermal effect of a concealed underground shallow granitic intrusion. The basaltic rocks furnished Mg²⁺, Fe²⁺, Ti⁴⁺, and Al³⁺ that were first concentrated in the epidote zone. This was followed by pervasive replacement of epidote by large idiomorphic garnet (grossularite) that attains up to ～1.5 cm wide. It is evident that diopside is earlier than garnet with no replacement fabrics between the two minerals. Two types of titanite (sphene) can be distinguished: The first is secondary in the metabasalt host where titanite develops after titanomagnetite during regional metamorphism (i.e., metamorphic). On the other hand, the second type of titanite is found in the garnet-bearing calc-silicate skarn where it is typically euhedral with no link to any opaque phase and it is believed to be formed due to the event of superimposed thermal metamorphism (i.e., metasomatic). There are several evidence of the thermal metamorphic effect such as distinct granoblastic and annealing textures and K-metasomatism and formation of phlogopite at the expense of tremolite in the marble, in addition to poikiloblastic hornblende in the metabasalt host with distinct recrystallization. Also, there are some evidence of shearing such as brecciation along microshear planes, microfolding, introduction of fine euhedral pyrite, and presence of injected silica postdating crystallization of garnet in the calc-silicates.     

Using the geologic setting of talc deposits as an indicator of amphibole asbestos content

This study examined commercial talc deposits in the U.S. and their amphibole-asbestos content. The study found that the talc-forming environment directly influenced the amphibole and amphibole-asbestos content of the talc deposit. Large talc districts in the U.S. have mined hydrothermal talcs that replaced dolostone. Hydrothermal talcs, created by siliceous fluids heated by magmas at depth, consistently lack amphiboles as accessory minerals. In contrast, mineable talc deposits that formed by contact or regional metamorphism consistently contain amphiboles, locally as asbestiform varieties. Examples of contact metamorphic deposits occur in Death Valley, California; these talc-tremolite deposits contain accessory amphibole-asbestos. Talc bodies formed by regional metamorphism always contain amphiboles, which display a variety of compositions and habits, including asbestiform. Some industrial mineral deposits are under scrutiny as potential sources of accessory asbestos minerals. Recognizing consistent relations between the talc-forming environment and amphibole-asbestos content may be used in prioritizing remediation or monitoring of abandoned and active talc mines.     

Metamorphism and Fluid Infiltration of the Calc-silicate Aureole of the Beinn an Dubhaich Granite, Skye

The Tertiary Beinn an Dubhaich granite intruded at 75OC and05 kb into siliceous dolostones and limestones of the UpperDurness Group in Strath, Skye, with the consequent developmentof talc, tremolite, diopside, olivine, and periclase in thebulk of the aureole, and abundant fluoro-borosilicate skarnsimmediately adjacent to the pluton. With increasing grade thelimestones develop the mineral sequence talc, tremolite, diopside,and olivine, whereas the dolostones develop the sequence talc,tremolite, olivine, and periclase. The abundant chert nodulesin the dolostones take either of the two reaction paths, dependenton their size. Those below 2–7 mm in dimension followthe dolostone reaction path, whereas larger nodules follow thelimestone reaction path. The presence of monticellite in thehighest-grade rocks points to the flushing of the contact byvolumes of water-rich fluid, derived presumably from the granite.Consideration of low-grade olivine-bearing veins and fracturesin the dolostones also points to the presence of extremely water-richfluid in the more distal parts of the aureole. Using simplebox models with constant porosity, it is shown that the observedreaction progress in oli vine-grade and talc-grade rocks canonly be accounted for if the rocks were infiltrated by substantialvolumes of water-bearing fluid. Minimum estimates for volumesof infiltrated fluid show that rocks nearest the pluton wereprobably infiltrated by greater amounts of fluid than thosefurther away. Low-grade rocks which suffered greatest amountsof infiltration are the brecciated dolostones nearest the Thrust.     

Assembly of central Gondwana along the Zambezi Belt: Metamorphic response and basement reactivation during the Kuunga Orogeny

Central Gondwana was assembled by three continental collisions in relatively quick succession: late Cryogenian East Africa Orogen, early Ediacaran West Antarctica Orogen and late Ediacaran Kuunga Orogen. The Kuunga Orogen involved diachronous closure of the South Adamastor–Khomas–Mozambique Oceans and accretion of Kalahari Craton and cratonic elements in Antarctica, with a previously assembled North Gondwana. The two older orogens were still hot and deforming at the time of final assembly by the Kuunga Orogen, and were therefore reworked and re-metamorphosed. The Central Kuunga Orogen is comprised of the Lufilian Arc, Zambezi Belt, Malawi–Unango Complex and the Lurio Belt. This region was the site of earliest collision in the Kuunga Orogen at ~575 Ma, and involved collision of two buoyant, previously metamorphosed rigid basement promontories. Pivoting on the Zambezi Belt indenters led to clockwise rotation of the Kalahari Craton and oblique collision within the Damara Belt ~20–30 m.y. later. The Central Kuunga Orogen is a relatively cold collisional belt dominated by eclogite, whiteschist and Barrovian series metamorphic parageneses, and contrasts with the paired metamorphic response in the Damara Belt to the west, and low-P/high-T metamorphism in the East Kuunga Orogen. Metamorphic parageneses are preserved from each stage of the full Wilson Cycle: from initiation of continental lithosphere thinning at ~940 Ma, widespread rifting between 725 and 805 Ma, and passive margin sedimentation until ~580 Ma. Eclogite-facies subduction parageneses indicate consumption of ocean lithosphere was underway by ~630–660 Ma. Collision at ~575 Ma involved deep burial of continental crust and formation of very high-P, low T/depth metamorphic parageneses, followed by Barrovian series thermal peaks at ~545 and ~525 Ma. Isostatic compensation and stress switches associated with plate reconfigurations once Gondwana was assembled, resulted in exhumation and local extension in an intra-continental setting from ~518 Ma.     

The muscovite-melt bathograd and low-P isograd suites in north-central Wopmay Orogen, Northwest Territories, Canada

In north-central Wopmay Orogen, syntectonic low-P(Buchan-type) suites of mineral isograds outline regional metamorphic temperature culminations that are associated, at the higher structural levels, with emplacement of early Proterozoic plutons in the west part of a deformed and eastward transported continental margin prism. The mapped isograds mark the first occurrence of biotite, staurolite, andalusite, sillimanite, sillimanite-K feldspar and K feldspar-plagioclase-quartz ± muscovite (granitic) pods in metapelites, with increasing proximity to the plutons.
Microprobe analyses and field observations have resulted in the formulation of reactions for the 'ideal'pelitic system K₂O-Na₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-Al₂O₃-SiO₂-H₂O, to account for the various mineral assemblages of each metamorphic zone. A P-T petrogenetic grid showing erosion surface P-T curves for the northern Wopmay Orogen pelites, compiled on the basis of the mapped isograds and the inferred reaction(s) for each metamorphic zone, documents a variation in exposed metamorphic pressure ranging between 2 and 4 kbar.
The configuration of a new bathograd, based on the invariant model reaction sillimanite + K feldspar + plagioclase + biotite + quartz + vapor ± muscovite + liquid and interpolated across three metamorphic suites, is consistent with a major regional structure culmination and with independently determined pressures obtained from anorthite-grossular-quartz-Al₂SiO₅ geobarometry. The positive correlation between the configuration of the bathograd and the structural and pressure culmination points to the pressure-dependence of anatectic-granitic-pod mineral associations.     

The Fluid Evolution Related to Talc Mineralization in the Hwanggangri Area, South Korea

Abstract: Scores of talc deposits distributed in the Hwanggangri Mineralized Zone, South Korea are interpreted to have resulted from the contact metamorphic process related to the igneous intrusion of the Muamsa Granite. Talc usually occurs as an alteration product from tremolite, which is crystallized during the prograde stage. It commonly forms fine–grained, fibrous aggregates, or pseudomorphs after tremolite, and abundant tremolite is included as impurities in the talc ore. Quantitative X-ray diffraction analysis has shown that the ores can be divided into three types depending on the concentrations of talc; talc-tremolite ore (talc > 50 %), tremolite-talc ore (10 % < talc < 50 %), and tremolite (talc < 10 %) ores. Oxygen isotope compositions of those talc ores become higher from 10.9–12.1 through 11.6–12.3 to 11.9–13.2% with increasing tremolite concentrations, while hydrogen isotope compositions are not so variable, –73 to –69%. The high value of the calculated δ¹⁸O_H2O of the fluids equilibrated with the tremolite may have resulted from the mixing of magmatic water with ¹⁸O–enriched CO₂ evolved from the decarbonation reaction during the prograde stage. However, as the decarbonation reaction ceased during the retrograde stage, the inferred oxygen isotope compositions of talc-forming fluids show similar values to those of the igneous fluids.     

Antigorite-ophicarbonates: Contact metamorphism in Valmalenco,Italy

Outside the Bergell tonalite contact aureole, ophicarbonate rocks consist of blocks of antigorite schist embedded in veins of calcite ± tremolite. An antigorite schistosity predates some of these calcite veins. Mono- and bimineralic assemblages occur in reaction zones associated with the veins. Within the aureole, the ophicarbonate veining becomes less distinct and polymineralic assemblages become more frequent. A regular sequence of isobaric univariant assemblages is found, separated by isograds corresponding to isobaric invariant assemblages. In order of increasing grade the invariant assemblages are: antigorite+diopside+olivine+tremolite+calcite antigorite+dolomite+olivine+tremolite+calcite antigorite+olivine+talc+magnesite antigorite+dolomite+olivine+tremolite+talc These assemblages match a previously derived topology in P-T-X_CO2 space for the system CaO-MgO-SiO₂-H₂O-CO₂; the field sequence can be used to adjust the relative locations of calculated invariant points with respect to temperature. Isobaric univariant and invariant assemblages are plotted along a profile map to permit direct comparison with the phase diagram.It is inferred that, during the formation of the ophicarbonate veins, calcite precipitated from fluid introduced into the serpentinite. During contact metamorphism, however, the compositions of pore fluids evolved by reaction in the ophicarbonate rocks were largely buffered by the solid phases. This control occurred on a small scale, because there are local variations in the buffering solid assemblages within a centimeter range.