Two-feldspar geothermometry in granulite facies metamorphic rocks

A geothermometric technique based on equilibria between coexisting plagioclase and alkali feldspar was applied to quartzo-feldspathic granulites from Salvador, BA, Brazil. The conditions of metamorphism were determined to be in the range 750 ° C–800 ° C, 4–8 Kb, by comparison with experimental data on the stabilities of sapphirine, phlogopite and other minerals occurring in the associated rocks. Selected feldspar data gives temperatures near, but slightly below, this range. Several variants of the Wood and Banno model, as well as an empirical two-pyroxene geothermometer, were also tested and found to give temperatures which were apparently 50 °–100 ° high. The solubility of Al₂O₃ in orthopyroxene indicates temperatures which are about 200 ° to high, suggesting that Fe in the natural assemblages significantly changes relationships observed experimentally in MgO-Al₂O₃-SiO₂ systems.     

Stratabound tungsten mineralization in regional metamorphic calc-silicate rocks from the Austroalpine Crystalline Complex,Austria

Stratabound tungsten mineralization in regional metamorphic calc-silicate rocks of probably Lower Paleozoic age is described from the polymetamorphic Austroalpine Crystalline Complex (ACC) of the Eastern Alps. Scheelite-bearing calc-silicate rocks which are often associated with marbles and tourmalinites are intercalated conformably with metaclastic rocks. Alkalipoor calc-silicate rocks with high amounts of clinozoisite/ zoisite, grossular, quartz, plagioclase, etc. are the most important host rocks for tungsten mineralization. These unusual calc-silicate rocks are products of regional metaorphism and are interpreted as reaction skarns. They have formed in the presence of a water-dominated fluid phase with very low X_CO2.In the Koralpe estimated P-T conditions are 650–700 °C at 5–7 kb. The mineralogical composition and the mineral zoning of the calc-silicate rocks is controlled by the degree of the Hercynian and Eoalpine metamorphism. There are no signs of graniteelated skarn formation. Tungsten preconcentration is thought to be syngenetic/syndiagenetic. It is genetically linked to exhalative hydrothermal processes in other Lower Paleozoic terrains of the Eastern Alps.     

Plagioclase and other mineral equilibria in a contact metamorphic aureole

Plagioclase, microcline, amphibole, clinozoisite, clinopyroxene and biotite from alternating pelitic and calcareous hornfelses of the Wyman Formation, Blanco Mountain Quadrangle, California, were analyzed using an electron microprobe. The metamorphic aureole formed at temperatures of 300–600° C, total pressure 2–3 Kb, and low but variable partial pressure of CO₂. The minerals show some compositional changes with metamorphic grade as well as differences from one assemblage to another. The plagioclases developed in the aureole do not form a continuous series. Rather, coexisting grains of plagioclase in individual rock layers form at certain distinct compositions: An 1–3, 15–17, 28–32, 38–45, 51–55, 59–65, 75 and 80. There is no evidence of disequilibrium in the rocks, although diffusion was limited; the volume for chemical equilibrium for most samples was less than 1 mm. Inspection of the changes in mineral assemblages with increasing degree of metamorphism and with changes in fluid composition suggests a number of reactions between the phases. Neither these reactions nor the compositions of coexisting minerals provide an obvious explanation for the observed gaps in the plagioclase series. Therefore it is postulated that the compositional clustering is structurally controlled.     

Ordering and composition of scapolite: Field observations and structural interpretations

Field observations, experimental and crystallographic data and thermodynamic considerations all suggest that Al-Si order-disorder is a crucial factor in explaining composition and stability of the mineral scapolite. Over the whole compositional range, scapolites have Al^IV-O-Al^IV bonds with the exception of one intermediate member with an Al/Si ratio of 1/2. Scapolites of this composition are the lowest temperature form and appear in areas with argillaceous carbonates and evaporites which have been subjected to progressive metamorphism. In similar areas without evaporites, the onset of the CO₃-scapolite stability field is approx. 150° C higher with an Al/Si ratio in the scapolite of about 5/7. This particular CO₃-scapolite is a compromise between the number of Al-O-Al bonds and the volume of the anion site occupied by CO₃. Based on field- and experimental data, temperature-composition diagrams for scapolite, plagioclase and calcite have been constructed. These diagrams may be explained in the light of contrasting Al-Si order-disorder in plagioclase and scapolite, i.e. at low temperature, plagioclase endmembers and intermediate scapolite members are stable, towards higher temperatures the ∩-shaped temperature-composition field of plagioclase and the V-shaped one of scapolite interfere in a complicated way. Electron microscopy of Al-rich scapolite, 632/n extinction rules. But these scapolites (with or without Cl-anion) show domain boundaries. We interpret them as APB's in the Al/Si ordering pattern on T₂-T₃ sites which reverses when a displacement R=1/2 [111] is applied.     

Halite and sylvite as solid inclusions in high-grade metamorphic rocks

Solid inclusions of halite and sylvite, formed during regional and contact metamorphism have been identified by microscopy and by electron microprobe analysis in rocks from Campolungo, Switzerland and Cornone di Blumone, Italy. The solid inclusions occur in several of the major minerals crystallized during metamorphism and have been observed as idiomorphic crystals and dendrites. The compositions measured in 100 analyses from Campolungo, Switzerland and 40 analyses from Cornone di Blumone, Italy extend across the two-phase region in the system, KCl-NaCl, indicating that the salt inclusions are high temperature precipitates. In both localities compositionally zoned and unzoned crystals have been found. Measured compositions on the temperature maximum of the two-phase region indicate at least 500° C which can be compared with 500°±20° C determined by Mercolli (1982) and Walther (1983) from the Mg content of calcites from Campolungo. The solid inclusions have been trapped apart from CO₂-rich and saline, H₂O-rich fluid inclusions which have been described by Mercolli (1982) as the earliest preserved fluid inclusions in the rocks. The early precipitation of salt minerals at Campolungo indicates that fluids were saturated with NaCl and KCl at 500° C and pressures of 2,000 bars or higher. Similar relationships exist between solid and fluid inclusions in the rocks of Cornone di Blumone which formed at temperatures as high as 800° C and pressures between 0.5 and 1 kilobar (Ulmer 1983). The entrapment of halite and sylvite as solid inclusions preserves the composition of the minerals which may therefore be useful as geothermometers.     

Middle Miocene thermal metamorphism due to the infiltration of high-temperature fluid in the Sanbagawa metamorphic belt,southwest Japan

 The Middle Miocene Tobe hornfels in the Sanbagawa metamorphic belt, western Shikoku, southwest Japan, is characterized by an abnormally steep metamorphic gradient compared with other hornfelses associated with intrusive bodies. The basic hornfels, originally Sanbagawa greenschist rocks, is divided into the following three metamorphic zones: plagioclase, hornblende, and orthopyroxene. The plagioclase zone is defined by the appearance of calcic plagioclase, the hornblende zone by the assemblage of hornblende+calcic plagioclase+quartz, and the orthopyroxene zone is characterized by the assemblage of orthopyroxene + clinopyroxene + plagioclase + quartz. Calcic amphibole compositions change from actinolite to hornblende as a result of the continuous reactions during prograde metamorphism. Petrographical and thermometric studies indicate a metamorphic temperature range of 300–475°C for the plagioclase zone, 475–680°C for the hornblende zone, and 680–730°C for the orthopyroxene zone. The temperature gradient based on petrological studies is approximately 5°C/m, which is unusually high. Geological and petrological studies demonstrate that the hornfelses were formed by the focusing of high-temperature fluids through zones of relatively high fracture permeability. The steep thermal gradient in the Tobe hornfels body is consistent with a large fluid flux, greater than 8.3 × 10^–7 m³ m^–2S^–1, over the relatively short duration of metamorphism, approximately 100 years. Received: 10 October 1995 / Accepted: 28 May 1996     

Evidence for pre-regional metamorphic fluid infiltration of the Lower Calcsilicate Unit, Reynolds Range Group (central Australia)

Grandite garnet-rich calcsilicate rocks from the Lower Calcsilicate Unit of the regionally metamorphosed Reynolds Range Group (central Australia) crop out along a strike-parallel section in which a transition zone from M2₂ amphibolite to granulite facies rocks is exposed. Across this transition the grandite-rich layers do not show systematic changes in mineral assemblages, compositions and modes, or stable isotope compositions. These layers are deformed by F2₂ folds that are associated with the peak of regional low-pressure/high-temperature metamorphism. Therefore, the grandite-rich layers appear to pre-date regional metamorphism and to have acted as closed chemical systems during prograde M2₂ metamorphism. Mineral assemblages in the grandite-rich layers are consistent with their formation through the infiltration of oxidized, water-rich fluids (X_co2 < 0.1–0.3; log f_o2 -16 to -14). The stable isotope values of calcite (Δ¹³C=-4.2 to -0.8%₀ PDB; Δ¹⁸O = 10.5–14.0%⁰ V-SMOW) and bulk-silicate fractions (Δ¹⁸O = 6.1 to 10.8%) of the grandite-rich layers are most consistent with the infiltrating fluid being from a magmatic source. It is most likely that fluid infiltration occurred during the pre-M2₂ contact metamorphism (M2₁) that affected much of the Reynolds Range Group. The preservation of these assemblages is probably due to their high variance and little pervasive fluid-rock interaction having occurred during M2₂. The clinopyroxene- and feldspar-rich calcsilicate rocks that host the grandite-rich layers contain poikiloblastic grandite garnet that formed during prograde M2₂ metamorphism. Thin marbles that locally occur with the grandite-rich layers contain a third garnet generation that is post- or late M2₂. This grossular-rich garnet occurs in coronas around calcite, plagioclase, clinopyroxene, wollastonite and scapolite. These coronas are consistent with cooling and/or compression. However, because the marble assemblages are themselves overprinted by M2₁ grandite-rich layers the development of coronal garnet does not reflect a continuous P-T-t path. Rather, it more probably reflects the partial re-equilibration of M2₁ contact metamorphic assemblages to post-M2₂ conditions.     

Calcareous inclusions in lavas and agglomerates of Santorini volcano

Thermally metamorphosed and metasomatised fragments of basement actinolite-chlorite-calcite-quartz schists and quartz-bearing marbles are found as inclusions in Quaternary agglomerates and historic (197 B. C.—1950) dacitic lavas of Santorini volcano, Greece.Inclusions in agglomerates preserve the structure of parent schists in the alternation of bands rich in diopside or salite with bands rich in plagioclase. By contrast, inclusions in historic dacites are not banded. Most develop a thin zone of hybrid material at the contact with enclosing lava. The assemblage calcic clinopyroxene-wollastonite-plagioclase is commonly developed. The clinopyroxene is a Fe³⁺-rich salite or ferrosalite. Andradite-rich garnet and sphene are accessory minerals. Most examples carry interstitial siliceous glass of distinctive chemical composition, and several show minor olivine, augite, hypersthene and calcic plagioclase of magmatic origin.Other inclusions exhibit the assemblage anhydrite-calcic clinopyroxene, the latter mineral ranging widely in Al content. A single example has been observed to develop two distinct assemblages, the first coarsely crystalline melilite-wollastonite-magnetite, the second finely intergrown melilite-wollastonite-andraditic garnet (-xonotlite).Stability data for hedenbergite and andradite as constituents of skarn assemblages suggest that the clinopyroxene-rich assemblages of inclusions in historic dacites formed at temperatures near to or above 800° C and oxygen fugacity (fO₂) considerably greater than that which could be imposed upon the inclusions by dacite magma (T 900° C, fO₂10^–13 atm.). Thermal breakdown of original carbonates of the inclusions probably supplied the necessary oxygen. T-fO₂ data for the reaction 4 Magnetite+18 Wollastonite 6 Andradite indicate that the assemblage melilite-wollastonite-magnetite of the last inclusion described formed at higher T and/or lower fO₂ than the assemblage melilite-wollastonite-garnet. The latter assemblage undoubtedly formed during inclusion of the fragment by dacite magma, while metamorphism by a more basic, high temperature magma may have produced the former. Temperature data for reactions limiting the stability of melilite in the system CaO-Al₂O₃-SiO₂-H₂O indicate a minimum temperature of around 800° C for formation of both assemblages.     

Occurrence and origin of marialitic scapolite in the Humboldt lopolith,N.W. Nevada

The occurrence and origin of marialitic scapolite in the Humboldt lopolith was investigated in the field and in the laboratory using petrographic and experimental techniques. Scapolite occurs in three modes: as a pervasive replacement of plagioclase and other minerals in gabbro, diorite and extrusive rocks; as a poikiloblastic mineral in scapolitite dikes; and as a fracture-filling mineral with analcime, albite and sphene in scapolite veins. Additional secondary minerals associated with scapolite include epidote, prehnite, hornblende and diopside-salite clinopyroxene. Relations with these minerals suggest that most marialitic scapolite grew at temperatures around 400° C. Scapolite composition varies from EqAn₁₂ to EqAn₃₇, containing from 72 to 96 atomic% Cl in the R position. Experiments on systems of similar compositions indicate that NaCl-H₂O fluid having more than 40 mol% NaCl is needed to stabilize the scapolite.Variation in scapolite compositions is due to thermal and fluid compositional gradients normal to conduits of hydrothermal fluids, and occurs on a scale up to 100 m. The likely source of Na and Cl is pre-existing evaporites or evaporitic brine derived from the wallrocks. Salinity could have been increased to a level sufficient to stabilize scapolite by hydration of an originally dry magma, possibly aided by hydrothermal boiling. Results may be applied to hydrothermal alteration in areas of rifting or back-arc spreading, and in mid-ocean ridge hydrothermal systems.     

Retrograde formation of NaCl-scapolite in high pressure metaevaporites from the Cordilleras Béticas (Spain)

A Permo-Triassic pelite-carbonate rock series (with interacalated metabasitic rocks) in the Cordilleras Béticas, Spain, was metamorphosed during the Alpine metamorphism at high pressures (P _min near 18 kbar). The rocks show well preserved sedimentary features of evaporites such as pseudomorphs of talc, of kyanite-phengitetalc-biotite, and of quartz after sulfate minerals, and relicts of baryte, anhydrite, NaCl, and KCl, indicating a salt-clay mixture of illite, chlorite, talc, and halite as the original rock. The evaporitic metapelites have a whole rock composition characterized by high Mg/(Mg+Ca) ratios>0.7, variable alkaline and Sr, Ba, contents, but are mostly K₂O rich (<8.8 wt%). The F (<2600 ppm), Cl (<3600 ppm), and P₂O₅ (<0.24 wt%) contents are also high. The pelitic member of this series is a fine grained biotite rock. Kyanite-phengite-talc-biotite aggregates in pseudomorphs developed in the high pressure stage. Albite-rich plagioclase was formed when the rocks crossed the albite stability curve in the early stages of the uplift. Scapolite, rich in NaCl (Ca/(Ca+Na) mol% 24–40) and poor in SO₄, with Cl/(Cl+CO₃) ratios between 0.6 and 0.8, formed as porphyroblasts, sometimes replacing up to 60% of the rock in a late stage of metamorphism (between 10 and 5 kbar, near 600°C). No reaction with albite is observed, and the scapolite formed from biotite by: $$\begin{gathered} Al - biotite + CaCO_3 + NaCl + SiO_2 \hfill \\ = Al - poor biotite + scapolite + MgCO_3 + KCl \hfill \\ + MgCl_2 + H_2 O \hfill \\ \end{gathered}$$ Calculated fluid composition in equilibrium with scapolite indicates varying salt concentrations in the fluid. Distribution of Cl and F in biotite and apatite also indicates varying fluid compositions.     

Margarite in kyanite- and corundum-bearing anorthosite,amphibolite, and hornblendite from central Fiordland,New Zealand

Margarite is both abundant and widespread throughout a sequence of interstratified amphibolite, hornblendite, and metamorphosed anorthosite from the upper Lyvia River, central Fiordland. These rock types comprise part of a metamorphosed layered intrusion. Assemblages recorded from these rocks are the product of two distinct phases of metamorphism. First generation assemblages typically comprise plagioclase (An_84–96), hornblende, kyanite, and minor corundum. Clinozoisite and chlorite occur as late stage breakdown products of plagioclase and hornblende. Margarite developed during the second phase of metamorphism.Within the corundum-bearing rocks replacement of corundum or plagioclase by margarite can be observed directly. On the basis of these observations the following reaction is evident: 1 corundum+1 anorthite+1H₂O=1 margarite.In other assemblages the formation of margarite can be attributed to the breakdown of kyanite and clinozoisite according to the reaction: 2 kyanite+2 clinozoisite=1 margarite+3 anorthite.Margarite is found, however, to contain appreciable amounts of paragonite solid-solution (up to 28 mol%) and plagioclase produced (second generation) is not pure anorthite but of intermediate compositions (An_46–62). The reaction therefore involves the introduction of both soda and silica. Margarite also crystallized independently of clinozoisite according to a reaction of the general form: 5 pargasite+17 kyanite+19 H₂O =8 margarite+4 chlorite+7 plagioclase.Application of available experimental data suggests that the margarite formed between 550 and 720 ° C up to a maximum pressure of 9.5 kb. Whereas the involvement of albite component (second generation plagioclase) will tend to lower the temperatures and pressures necessary for the occurrence of margarite, this effect is partially offset by the significant amounts of paragonite end-member held within the margarite. An independent estimate of the metamorphic conditions in metapelites suggests that the introduction of albite lowers equilibration temperatures by about 2 ° C for every 1% albite.     

Metasomatism of gabbro – mineral replacement and element mobilization during the Sveconorwegian metamorphic event

Widespread metasomatism affected the 100 km long and 25 km wide Proterozoic Bamble and Modum‐Kongsberg sectors, South Norway, resulting in the chemical and mineralogical transformation of wide segments of continental crust. Scapolitization was associated with veining, and was followed by albitization, transforming metagabbros pervasively over large areas. Fluids played an active role in these reactions, forming H₂O‐, CO₂‐ and Cl‐bearing phases at the expense of the primary volatile‐free minerals, causing depletion in Fe and infiltration of K, Mg, Na, B and P. The transformation of gabbro to scapolite metagabbro is observed as a fluid front replacing the primary magmatic mineral assemblage in three stages: during an incipient amphibolitization stage, the primary mafic minerals were replaced by anthophyllite or hastingsite, followed by pargasitic and edenitic Ca‐amphibole. Magnetite was dissolved, while rutile formed by the breakdown of ilmenite. Plagioclase was replaced by Cl‐rich scapolite (Me_19‐42) reflecting Cl‐saturation, while K‐ and Mg‐saturation produced phlogopite, enstatite, sapphirine and rare corundum. The high modal contents of chlorapatite and tourmaline in the scapolite metagabbro imply infiltration of B and P. The albitites consist dominantly of albite (Ab_95‐98) with varying, generally small, amounts of chlorite, calcite, rutile, epidote and pumpellyite. They formed from a H₂O–CO₂‐fluid rich in Na. The gabbro yields a zircon U–Pb age of 1149 ± 7 Ma and tonalite 1294 ± 38 Ma, whereas rutile from scapolite metagabbro and albitite has U–Pb ages of 1090–1084 Ma, and phlogopite produced during scapolitization Rb–Sr ages of 1070–1040 Ma. Temperature conditions for the scapolitization are inferred to have been 600–700 °C. The reported ages, combined with mineralogical and petrographic observations and inferred P–T conditions, indicate that the metasomatism was a part of the regional Sveconorwegian amphibolite facies metamorphic phase. Initial ⁸⁷Sr/⁸⁶Sr of the scapolite ranges from 0.704 to 0.709. The Sr‐signature, the Cl‐ and B‐rich environment and regional distribution of lithologies suggest that the fluid may have originated from evaporites that were mobilized during the regional metamorphism.     

T-XCO2 stability relations and phase equilibria of a calcic carbonate scapolite

At a total pressure of 5 kb, calcic, Cl-free scapolite (Me₈₃) is stable relative to plagioclase-bearing assemblages at $\text{T ≧ 625°C}$, . With decreasing temperature, scapolite breaks down to plagioclase + calcite. Scapolite is replaced by plagioclase + grossular + cancrinite + CO₂ in the presence of H₂O-rich fluids. The stable coexistence of scapolite and calcite, an assemblage typical of most natural occurrences of calcic scapolite, is limited by the reaction: scapolite + calcite → grossular + cancrinite + CO₂, which occurs at 750°C, X_CO2 = 0.46; 700°C, X_CO2 = 0.33; 650°C, X_CO2 = 0.18, for the chosen bulk composition.Generalization of the experimental results to encompass the complete range of fully carbonated scapolite compositions indicates that mizzonite (Me₇₅) has the largest T-X_CO2 stability field. For scapolite more calcic than mizzonite, stable growth is restricted to conditions of increasingly higher temperature and X_CO2.The experimental results are consistent with various petrologic features of scapolite-bearing rocks, particularly scapolite-clinopyroxene granulites, and indicate that such rocks were formed in the presence of CO₂-rich fluids.     

Formation of monazite via prograde metamorphic reactions among common silicates: implications for age determinations

Three lines of evidence from schists of the Great Smoky Mountains, NC, indicate that isogradic monazite growth occurred at the staurolite-in isograd at ∼600°C: (1) Monazite is virtually absent below the staurolite-in isograd, but is ubiquitous (several hundred grains per thin section) in staurolite- and kyanite-grade rocks. (2) Many monazite grains are spatially associated with biotite coronas around garnets, formed via the reaction Garnet + Chlorite + Muscovite = Biotite + Plagioclase + Staurolite + H₂O. (3) Garnets contain high-Y annuli that result from prograde dissolution of garnet via the staurolite-in reaction, followed by regrowth, and rare monazite inclusions occur immediately outside the annulus and in the matrix, but not in the garnet core. Larger monazite grains also exhibit quasi-continuous Th zoning with high Th cores and low Th rims, consistent with monazite growth via a single reaction and fractional crystallization during prograde growth. Common silicates may host sufficient P and LREEs that reactions among them can produce observable LREE phosphate. Specifically phosphorus contents of garnet and plagioclase are hundreds of parts per million, and dissolution of garnet and recrystallization of plagioclase could form thousands of phosphate grains several micrometers in diameter per thin section. LREEs may be more limiting, but sheet silicates and plagioclase can contain tens to ∼100 (?) ppm LREE, so recrystallization of these silicates to lower LREE contents could produce hundreds of grains of monazite per thin section. Monazite ages, determined via electron and ion microprobes, are ∼400 Ma, directly linking prograde Barrovian metamorphism of the Western Blue Ridge with the “Acadian” orogeny, in contrast to previous interpretations that metamorphism was “Taconian” (∼450 Ma). Interpretation of ages from metamorphic monazite grains will require prior chemical characterization and identification of relevant monazite-forming reactions, including reactions previously viewed as involving solely common silicates.     

High pressure basic inclusions from the Kayrunnera kimberlitic diatreme in New South Wales,Australia

Basic inclusions of two types occur in a kimberlitic diatreme at Kayrunnera in northwestern New South Wales. Type I inclusions comprise assemblages of clinopyroxene+garnet+rutile±plagioclase ±quartz±K-feldspar±scapolite±sphene±apaite. Type II inclusions have assemblages of clinopyroxene +garnet+kyanite+quartz±plagioclase and are lower in Ti, total Fe, and higher in Al and have a higher Mg/Mg+gSFe ratio than the Type I inclusions. Experimental and theoretical data indicate that both inclusion types equilibrated at between 850–900 ° C and 18–23 kb. Due to their low concentrations of incompatible elements, the Type I inclusions are considered to represent a basaltic melt derived from an Fe-rich mantle source rock, and not to be the product of fractionation. The Type II inclusions are believed to represent cumulates which formed from a basaltic magma. The presence of sulphur rich scapolite in the Type I inclusions extends the range of P-T conditions from which this mineral has been reported thus adding further credence to the hypothesis that it may act as a stable repository for S and CO₂ in the crust and upper mantle.     

Lapis lazuli from Baffin island — a precambrian meta-evaporite

On Baffin Island, the major minerals in lapis lazuli are calcite, lazurite, diopside, nepheline and phlogopite in the Main Occurrence and calcite, lazurite, diopside, plagioclase and scapolite in the North Occurrence. The abundances of Na, K, S, Cl, Br, F and Fe, the well-developed layering parallel to the regional foliation, and the scarcity of intrusive rocks, support the hypothesis that the deposit evolved from an evaporite parent during regional metamorphism. After sedimentation, considerable reduction of sulfur was effected by CO, perhaps aided by H₂, Cl^? and anaerobic bacteria. Chemographic analysis suggests that the observed phase assemblages represent local equilibrium within small volumes, where SiO₂-Al₂O₃-MgO-Na₂O-K₂O behaved as inert components at the peak of metamorphism. The lack of systematic zoning of phase assemblages, and the uniformity of mineral compositions within the deposit, also argue against a metasomatic origin. Most phases equilibrated at or near the peak of metamorphism (granulite facies).     

Gypsum as a precursor to pyrrhotite in metamorphic rocks

During low-grade regional metamorphism, pyrrhotite can form from gypsum by the reaction: CaSO₄·2H₂O+Fe _sol. ²⁺ +2C_org.FeS+Ca _sol. ²⁺ +2H₂O+2CO₂. This reaction takes place in the anchizone, below 350°C and might be initiated by the thermal dehydration of gypsum (200°C) and aided by the generation of gaseous hydrocarbons. Evidence for the reaction is the occurrence in dolomitic layers in the Ballachulish Slate, East Larroch quarry, Argyll of lath and swallow-tail shaped quartz+dolomite+pyrrhotite pseudomorphs after gypsum. Quartz+pyrrhotite bodies in slate represent replaced and deformed (mainly flattened) crystals, concretions and possible veinlets of gypsum. Pyrite porphyroblast growth, after the peak of metamorphism and under relatively high fS₂ conditions, failed to destroy some early pyrrhotite because it is encapsulated in quartz. Pyrite-silicate reactions and hydrothermal exhalations have been suggested previously to account for pyrrhotite-enriched horizons in regionally metamorphosed rocks. Replacement of gypsum by pyrrhotite is an additional explanation for pyrrhotite-enriched horizons, especially in dolomitic and graphitic lithologies.     

The peristerite gap in low-grade metamorphic rocks

Coexisting Na-plagioclases from greenschists both in the thermal aureole of the Kasugamura Granite, Japan, and in the low-P metamorphic zone of Yap Island, western Pacific were analyzed in great detail; the peristerite solvus was determined for each suite. The asymmetric solvus has steep albite-rich and gentle oligoclase-rich limbs that are similar to those for higher pressure series. The present results together with those from Vermont, New Zealand, and the Sanbagawa belt indicate that the peristerite solvus shifts toward the albite component and higher temperature with increasing pressure. With increasing pressure, albite co-existing with oligoclase (An=100 Ca/Ca+ Na=20) varies in composition from An 8–9 (in Kasugamura), through An 3 (in Yap Island and Vermont), to An 1 (in New Zealand) and An less than 0.5 (in the Sanbagawa belt). The consolute temperatures for the peristerite solvus estimated from available geothermometry are 420° C in Kasugamura, 450–550° C in Vermont and 550°–600° C in the Sanbagawa belt. The variation of plagioclase composition in progressive metamorphic zones is explained by intersection of a plagioclase-forming reaction and the peristerite immiscibility gap in an isobaric T-X _An diagram. The greenschist zone is characterized by albite, the transition zone by occurrence of peristerite pairs and the amphibolite zone by plagioclase of An 20–50.     

Geologic setting,genesis and transformation of sulfide deposits in the northern part of Khetri copper belt,Rajasthan, India — an outline

The present study is confined to the northern part of the Khetri copper belt that extends for about 100 km in northern Rajasthan. Mineralization is more or less strata-bound and is confined to the garnetiferous chlorite schist and banded amphibolite quartzite, occurring towards the middle of the Proterozoic Delhi Supergroup. Preserved sedimentary features and re-estimation of the composition of the pre-metamorphic rocks suggest that the latter were deposited in shallow marine environment characterized by tidal activity. Cordierite-orthoamphibole-cummingtonite rock occurring in the neighbourhood of the ores is discussed, and is suggested to be isochemically metamorphosed sediment. The rocks together with the ores were deformed in two phases and metamorphosed in two progressive and one retrogressive events of metamorphism. Study of the host rocks suggests that the maximum temperature and pressure attained during metamorphism are respectively 550–600°C and < 5.5 kb. Principal ore minerals in Madan Kudan are chalcopyrite, pyrrhotite, pyrite and locally magnetite. In Kolihan these are chalcophyrite, pyrrhotite and cubanite. Subordinate phases are sphalerite, ilmenite, arsenopyrite, mackinawite, molybdenite, cobaltite and pentlandite. The last two are very rare. Gangue minerals comprise quartz, chlorite, garnet, amphiboles, biotite, scapolite, plagioclase and graphite. The ores are metamorphosed at temperatures > 491°C. Sulfide assemblages are explained in terms of f_{S 2} during metamorphism. Co-folding of the ore zone with the host rocks, confinement of the ores to the carbonaceous pelites or semi-pelitic rocks, strata-bound and locally even stratiform nature of the orebodies, lack of finite wall rock alteration, metamorphism of the ores in the thermal range similar to that for the host rocks, absence of spatial and temporal relationship with the granitic rocks of the region led the authors to conclude that the entire mineralization was originally sedimentary-diagenetic. Any loss of primitive features and development of incongruency are due to subsequent deformation and metamorphism to which the ores and their hosts were together subjected.     

The influence of crystallography and kinetics on phengite breakdown reactions in a low-pressure metamorphic aureole

A natural example of phengite that had undergone partial thermal decomposition at a pressure of about 0.5 kbar and a temperature of about 680° C in a contact aureole was exmined in the transmission electron microscope (TEM). Partially pseudomorphed phengites were found to consist of combinations of phengite, biotite, K-feldspar, mullite, sillimanite, spinel and cordierite. Different areas within individual, partially pseudomorphed, phengite grains show various degrees of reaction and different reaction products; the cores are the least reacted and the margins have reacted most. In the cores the assemblage Al-, Mg-enriched phengite+biotite +K-feldspar+mullite±spinel has formed, whereas the assemblage K-feldspar+mullite+sillimanite+spinel +biotite+cordierite has formed at the edges. According to our thermodynamic calculations, the breakdown of phengite should have produced cordierite+spinel +corundum+K-feldspar in regions isolated from the influx of SiO₂ and cordierite+andalusite+quartz+K-feldspar in regions near the edge of the grains that were essentially saturated with SiO₂. Chemical equilibrium was not achieved in any part of the partially pseudomorphed phengites on a micron scale or larger. Breakdown theoretically should have been complete by about 550° C; the reaction temperature was overstepped by at least 130° C for 20–25 years. The variations in the degree and type of reaction are probably due partly to the availability of suitable nucleation sites in different regions, partly to the need to remove H₂O from reaction sites and partly to the influence of SiO₂, which diffused into the grains during metamorphism. The presence of SiO₂ lowers the equilibrium temperatures. Thus there is a higher driving force for breakdown near the grain boundaries than in the cores. Most of the products show an orientation relationship with the parent phengite and have consistent habit planes; they have their closest-packed planes and closest-packed directions parallel to one another and to those of phengite. Such relationships minimize the strain and surface energies at nucleation and favour most rapid nucleation and growth of the reaction products. The great structural similarity of biotite to phengite resulted in its having the highest rate of nucleation and growth of any product and it occurred in all areas of the phengite pseudomorphs studied. Mullite and sillimanite were produced metastably. Mullite has more rapid nucleation kinetics than other aluminosilicates because it is structurally disordered. Sillimanite formed rather than andalusite in regions of the partially pseudomorphed phengites where the reaction reached an advanced stage, because the reaction from phengite to andalusite requires an energetically unfavourable change in aluminium co-ordination state.