Humite- and scapolite-bearing assemblages in marbles and calcsilicates of Dronning Maud Land, Antarctica: new data for Gondwana reconstructions

This study investigates marbles and calcsilicates in Central Dronning Maud Land (CDML), East Antarctica. The paleogeographic positioning of CDML as part of Gondwana is still unclear; however, rock types, mineral assemblages, textures and P–T conditions observed in this study are remarkably similar to the Kerala Khondalite Belt in India. The CDML marbles and calcsilicates experienced a Pan-African granulite facies metamorphism at c. 570 Ma and an amphibolite facies retrogression at c. 520 Ma. The highest grade assemblage in marbles is forsterite+spinel+calcite+dolomite, in calcsilicates the assemblages are diopside+spinel, diopside+garnet, scapolite+wollastonite+clinopyroxene±quartz, scapolite±anorthite±calcite+clinopyroxene+wollastonite. These assemblages constrain the peak metamorphic conditions to 830±20 °C, 6.8±0.5 kbar and X _CO2>0.46. During retrogression, highly fluoric humite-group minerals (humite, clinohumite, chondrodite) replaced forsterite, and garnet rims formed at the expense of scapolite during reactions with wollastonite, calcite or clinopyroxene but without involvement of anorthite. Metamorphic conditions were about 650 °C, 4.5±0.7 kbar, 0.2< X _CO2^fluid<0.36, and the co-existence of garnet, clinopyroxene, wollastonite and quartz constrains f_O2 to FMQ-1.5 log units. Mineral textures indicate a very limited influx of H₂O-rich fluid during amphibolite facies retrogression and point to significant variations of fluid composition in mm-sized areas of the rock. Gypsum was observed in two samples; it probably replaced metamorphic anhydrite which appears to have formed under amphibolite facies conditions. The observed extensive anorogenic magmatism (anorthosites, A-type granitoids) and the character of metamorphism between 610 and 510 Ma suggest that the crustal thermal structure was characterized by a long-lived (50–100 Ma) rise of the crustal geotherm probably caused by magmatic underplating.     

Fluid infiltration during contact metamorphism of interbedded marble and calc-silicate hornfels, Twin Lakes area, central Sierra Nevada, California

Abstract In the Twin Lakes area, central Sierra Nevada, California, most contact metamorphosed marbles contain calcite + dolomite + forsterite ± diopside ± phlogopite ± tremolite, and most calc-silicate hornfelses contain calcite + diopside + wollastonite + quartz ± anorthite ± K-feldspar ± grossular ± titanite. Mineral-fluid equilibria involving calcite + dolomite + tremolite + diopside + forsterite in two marble samples and wollastonite + anorthite + quartz + grossular in three hornfels samples record P± 3 kbar and T± 630° C. Various isobaric univariant assemblages record CO₂-H₂O fluid compositions of χCO₂= 0.61–0.74 in the marbles and χCO₂= 0.11 in the hornfelses. Assuming a siliceous dolomitic limestone protolith consisting of dolomite + quartz ° Calcite ± K-feldspar ± muscovite ± rutile, all plausible prograde reaction pathways were deduced for marble and hornfels on isobaric T-XCO₂ diagrams in the model system K₂O-CaO-MgO-Al₂O₃-SiO₂-H₂O-CO₂. Progress of the prograde reactions was estimated from measured modes and mass-balance calculations. Time-integrated fluxes of reactive fluid which infiltrated samples were computed for a temperature gradient of 150 °C/km along the fluid flow path, calculated fluid compositions, and estimated reaction progress using the mass-continuity equation. Marbles and hornfelses record values in the range 0.1–3.6 × 10⁴ cm³/cm² and 4.8–12.9 × 10⁴ cm³/cm², respectively. For an estimated duration of metamorphism of 10⁵ years, average in situ metamorphic rock permeabilities, calculated from Darcy's Law, are 0.1–8 × 10^?6 D in the marbles and 10–27 × 10^?6 D in the hornfelses. Reactive metamorphic fluids flowed up-temperature, and were preferentially channellized in hornfelses relative to the marbles. These results appear to give a general characterization of hydrothermal activity during contact metamorphism of small pendants and screens (dimensions ± 1 km or less) associated with emplacement of the Sierra Nevada batholith.     

Evolution of metamorphic fluid recorded in granulite facies metacarbonate rocks from the middle segment of the Mogok metamorphic belt in central Myanmar

The Mogok metamorphic belt of Palaeogene age, which records subduction‐ and collision‐related events between the Indian and Eurasian plates, lies along the western margin of the Shan plateau in central Myanmar and continues northwards to the eastern Himalayan syntaxis. Reaction textures of clinohumite‐ and scapolite‐bearing assemblages in Mogok granulite facies metacarbonate rocks provide insights into the drastic change in fluid composition during exhumation of the collision zone. Characteristic high‐grade assemblages of marble and calcsilicate rock are clinohumite+forsterite+spinel+phlogopite+pargasite/edenite+calcite+dolomite, and scapolite+diopside+anorthite+quartz+calcite respectively. Calculated petrogenetic grids in CaO–MgO–Al₂O₃–SiO₂–H₂O–CO₂ and subsets of this system were employed to deduce the pressure–temperature–fluid evolution of the clinohumite‐ and scapolite‐bearing assemblages. These assemblages suggest higher temperature (>780–810°C) and [=CO₂/(CO₂+H₂O) >0.17–0.60] values in the metamorphic fluid for the peak granulite facies stage, assuming a pressure of 0.8 GPa. Calcite grains commonly show exsolution textures with dolomite particles, and their reintegrated compositions yield temperatures of 720–880°C. Retrograde reactions are mainly characterized by a reaction zone consisting of a dolomite layer and a symplectitic aggregate of tremolite and dolomite grown between clinohumite and calcite in marble, and a replacement texture of scapolite by clinozoisite in calcsilicate rock. These textures indicate that the retrograde reactions developed under lower temperature (<620°C) and (<0.08–0.16) conditions, assuming a pressure of 0.5 GPa. The metacarbonate rocks share metamorphic temperatures similar to the Mogok paragneiss at the peak granulite facies stage. The values of the metacarbonate rock at peak metamorphic stage are, however, distinctly higher than those previously deduced from carbonate mineral‐free paragneiss. Primary clinohumite, phlogopite and pargasite/edenite in marble have F‐rich compositions, and scapolite in calcsilicate rock contains Cl, suggesting a contrast in the halogen compositions of the metamorphic fluids between these two lithologies. The metamorphic fluid compositions were probably buffered within each lithology, and the effective migration of metamorphic fluid, which would have extensively changed the fluid compositions, did not occur during the prograde granulite facies stage throughout the Mogok metamorphic belt. The lower conditions of the Mogok metacarbonate rocks during the retrograde stage distinctly contrast with higher conditions recorded in metacarbonate rocks from other metamorphic belts of granulite facies. The characteristic low conditions were probably due to far‐ranging infiltration of H₂O‐dominant fluid throughout the middle segment of the Mogok metamorphic belt under low‐amphibolite facies conditions during the exhumation and hydration stage.     

Contrasting fluid evolution of granulite-facies marbles: implications for a high-T intermediate-P terrain in the Famatinian Range, San-Juan, Argentina

High-temperature, intermediate-pressure calc-silicate marbles occur in the granulite-facies terrain of the La Huerta Range in the Province of San Juan, NW-Argentina, in three bulk-compositional varieties: Type (1) dolomite-absent scapolite-wollastonite-grandite-clinopyroxene-quartz—calcite marbles; Type (2) diopside-forsterite-spinel-corundum—calcite marbles with dolomite exolution lamellae in calcite; Type (3) serpentinized forsterite-spinel-dolomite marbles. An isobaric cooling path from peak-metamorphic conditions of 860°C to 750°C at 6.5 kbar is inferred from scapolite-wollastonite-grandite reaction textures in Type (1) and is consistent with cooling after an advective heat input from related gabbroic and tonalitic intrusive bodies. Stable carbon and oxygen isotope geochemistry was used to decipher the fluid/rock evolution of the three marble types. An interpreted four-stage temperature-time-fluid flow path comprises: (1) infiltration of pre-peak-metamorphic fluids, depleted in δ¹⁸O, that caused a shift of primary sedimentary δ¹⁸O ratios to lower values (19.6–20.0); (2) syn-metamorphic fluid liberation from Type (1) marbles with evidence for processes close to batch devolatilization that caused a weak coupled ¹³C and ¹⁸O depletion during prograde metamorphism. A different devolatilization behaviour, close to Rayleigh fractionation, texturally associated with fold settings indicates that granulite-facies fluid flow was focused rather than pervasive; (3) H₂O-absent conditions were dominant when coronal grandite formed during incipient high-temperature isobaric cooling at the expense of scapolite and wollastonite in the Type (1) marbles; (4) intense post-peak- hydration of Type (2) and Type (3) marbles is the last recognizable metasomatic event. In combination, the three marble types record fluid infiltration both before and after the metamorphic peak.     

Contrasting Carbon and Oxygen Isotopic Evolution in Metacarbonates from the Kerala Khondalite Belt,Southern India

The Kerala Khondalite belt is a Proterozoic metasupracrustal granulite facies terrain in southern India comprising garnet-biotite gneiss, garnet-sillimanite gneiss and orthopyroxene granulites as major rock types. Calc-silicate rocks and marbles, occurring as minor lithologies in the Kerala Khondalite Belt, show different mineral assemblages and reaction histories of which indicate a metamorphic P-T-fluid history dominated by internal fluid buffering during the peak metamorphism, followed by external fluid influx during decompression. The carbon and oxygen isotopic compositions of calcite from three representative metacarbonate localities show contrasting evolutionary trends. The Ambasamudram marbles exhibit carbon and oxygen isotope ratios (δ¹³C ∼ 0‰ and δ¹⁸O ∼ 20‰) typical of middle to late Proterozoic marine carbonate sediments with minor variation ascribed to the isotopic exchange due to the devolatilization reactions. The δ¹³C and δ¹⁸O values of ∼ −9‰ and 11‰, respectively, for calcite from calc-silicate rocks at Nuliyam are considerably low and heterogeneous. The wollastonite formation here, possibly corresponds to an earlier event of fluid infiltration during prograde to peak metamorphism, which resulted in decarbonation and isotope resetting. Further, petrologic evidence supports a model of late carbonic fluid infiltration that has partially affected the calc-silicate rocks, with subsequent isotope resetting, more towards the contact between calc-silicate rock and charnockite. At Korani, only oxygen isotopes have been significantly lowered (δ¹⁸O ∼ 13‰) and the process involved might be a combination of metamorphic devolatilization accompanied by an aqueous fluid influx, supported by petrologic evidence. The stable isotope signatures obtained from the individual localities, thus indicate heterogeneous patterns of fluid evolution history within the same crustal segment.     

Occurrence, Mineral Chemistry, and Metamorphism of Precambrian Carbonate Rocks in a Portion of the Grenville Province

Marble occurs abundantly in a 31,000 km² segment of the southernGrenville Province of the Canadian Precambrian Shield, whereit is associated with quartzite, biotite-garnet gneiss, andamphibolite to form the Grenville Group. An 1800 km² area onthe western margin of this segment, north of the Ottawa river,displays a great variety of carbonate rocks, which may be dividedinto two groups: (I) major marble, with calcite, dolomite, graphite, phlogopite,Ca amphibole, Ca pyroxene, forsterite, humite group minerals, (II) minor marble, with pink calcite, phlogopite, Ca amphibole,Ca pyroxene, K feldspar, scapolite, sphene. Rocks of the first group are associated with plagioclase gneissand amphibolite, and are metamorphosed limestone, little affectedby metasomatism; rocks of the second group, which are less common,are associated with potassium feldspar gneiss and heterogeneousgranitic and syenitic rocks, and are inferred to be metasomaticrocks. Numerous mineral reactions have taken place in the carbonaterocks during metamorphism. The calcite-dolomite reaction, whichgoverns the Mg content of calcite, indicates a metamorphic temperatureof about 650 °C. Forsterite was possibly produced from low-Alamphibole, and forsterite + spinel from high-Al amphibole. Thecrystallization of some silicate minerals in the minor marbleunits, and the enrichment in the contained calcite in Fe andSr are attributed to metasomatic reactions. Metamorphic ion-exchangereactions involving carbonates produced the following distributioncoefficients: Sr in calcite/Sr in dolomite = 2.5 Mn in calcite/Mn in dolomite = 0.89 Fe in calcite/Fe in dolomite = 0.29 from which inferences may be drawn concerning the distributionof these elements between the Ca and Mg sites within dolomiteduring metamorphic crystallization. Ion-exchange reactions involvingsilicates produced the following distribution of Mn: humite group Ca pyroxene.Ca amphibole phlogopite where the numbers are distribution coefficients. An equilibriumdistribution of Fe between silicates and calcite in the minormarble was evidently not attained during metasomatic crystallization.Numerous retrograde reactions have taken place, including thealteration of pyroxene to amphibole, forsterite to serpentine,and the exsolution of dolomite from calcite. Forsterite in marble, and orthopyroxene in the associated gneissesand amphibolites crystallized sporadically in the Laurentianhighlands, but not in the lowlands of the Ottawa rift valley,where peak metamorphic temperatures may have been slightly lower.In the highlands, reactions to produce forsterite and orthopyroxenewere initiated in response to a local increase in temperature,local peculiarities in the chemical composition of amphibole,which produced these minerals, or a local decrease in the activityof CO₂ and H₂O in the grain-boundary phase.     

Stable isotopic and petrological constraints on scapolitization of the Whitestone meta-anorthosite,Grenville Province,Ontario*

The Whitestone Anorthosite (WSA), located in the Central Gneiss Belt of the south-western Grenville Province, Ontario, exhibits a nearly concentric metamorphic envelope characterized by an increase in modal scapolite, hornblende, epidote and garnet, developed around a core of granulite facies clinopyroxene ± orthopyroxene ± garnet meta-anorthosite. Scapolite- and hornblende-bearing assemblages develop mainly at the expense of plagioclase and pyroxene within the envelope. Stable isotopic and petrological data for scapolite-bearing mineral assemblages within meta-anorthosite constrain the source of carbon responsible for CO₃-scapolite formation and the extent of fluid/rock interaction between the anorthosite and adjacent lithologies. Stable isotopic data indicate increasing δ¹⁸O and δ¹³C from core to margin of the meta-anorthosite and for samples from the southern extension of the WSA, where it is ductilely deformed within the Parry Sound Shear Zone (PSSZ). The average δ¹⁸O_SMOW value (whole rock) for the WSA core is 6.9‰, increasing to 11.5‰ where the WSA is in tectonic contact with marble breccia. The average δ¹³C_PBD value of scapolite in meta-anorthosite from the centre of the WSA is -3.4‰, increasing to -0.5‰ at the eastern (marble) contact. Average values of δ¹³C for scapolite and whole-rock δ¹⁸O for samples from the shear zone are -1.0 and 8.0‰, respectively. Marbles have average δ¹⁸O and δ¹³C values of 19.2 and -0.4‰, respectively. The sulphate content of texturally primary scapolite decreases from the core of the WSA (X_SO4= 0.48) to the eastern contact (≤0.05). Texturally late scapolite after plagioclase and garnet tends to be CO₃-rich relative to texturally primary scapolite, and some scapolite grains show zoning in the anion site with CO₃-enriched rims. Scapolite composition may vary at any scale from a single grain to outcrop. The pattern of isotopic enrichment in ¹³C and ¹⁸O preserved in the eastern margin of the WSA is consistent with marble as the major source of fluid contributing to the formation of the metamorphic envelope. The decrease in X_SO4 and increase in X_CO3 in scapolite toward the margin of the WSA indicate that the volatile content was reset by, or developed from, a CO₂-bearing fluid. Assuming derivation of fluid from marble, minimum fluid/rock values at the margin of the WSA range from 0.03 for the least enriched, to 0.30 for the most isotopically enriched samples. Although marble is not found in immediate contact with samples of sheared meta-anorthosite from the PSSZ, a marble source is also consistent with the C and O isotope composition and anion chemistry of scapolite within these samples.     

Constraints on the application of carbon isotope thermometry in high- to ultrahigh-temperature metamorphic terranes

Nine marble horizons from the granulite facies terrane of southern India were examined in detail for stable carbon and oxygen isotopes in calcite and carbon isotopes in graphite. The marbles in Trivandrum Block show coupled lowering of δ¹³C and δ¹⁸O values in calcite and heterogeneous single crystal δ¹³C values (? 1 to ? 10‰) for graphite indicating varying carbon isotope fractionation between calcite and graphite, despite the granulite facies regional metamorphic conditions. The stable isotope patterns suggest alteration of δ¹³C and δ¹⁸O values in marbles by infiltration of low δ¹³C–δ¹⁸O‐bearing fluids, the extent of alteration being a direct function of the fluid‐rock ratio. The carbon isotope zonation preserved in graphite suggests that the graphite crystals precipitated/recrystallized in the presence of an externally derived CO₂‐rich fluid, and that the infiltration had occurred under high temperature and low f_O2 conditions during metamorphism. The onset of graphite precipitation resulted in a depletion of the carbon isotope values of the remaining fluid+calcite carbon reservoir, following a Rayleigh‐type distillation process within fluid‐rich pockets/pathways in marbles resulting in the observed zonation. The results suggest that calcite–graphite thermometry cannot be applied in marbles that are affected by external carbonic fluid infiltration. However, marble horizons in the Madurai Block, where the effect of fluid infiltration is not detected, record clear imprints of ultrahigh temperature metamorphism (800–1000 °C), with fractionations reaching <2‰. Zonation studies on graphite show a nominal rimward lowering δ¹³C on the order of 1 to 2‰. The zonation carries the imprint of fluid deficient/absent UHT metamorphism. Commonly, calculated core temperatures are > 1000 °C and would be consistent with UHT metamorphism.     

Fluid flow and mass transport at the Valentine wollastonite deposit, Adirondack Mountains, New York State

The Valentine wollastonite skarn in the north-west Adirondack Mountains, New York, is a seven million ton deposit which resulted from channellized infiltration of H₂O-rich, silica-bearing fluids. The wollastonite formed by reaction of these fluids with non-siliceous calcite marble. The skarn formed at the contact of the syenitic Diana Complex and was subsequently overprinted by Grenville-age granulite facies metamorphism and retrograde hydrothermal alteration during uplift. Calcite marbles adjacent to the deposit have generally high δ¹⁸O values (c. 21‰), typical of Grenville marbles which have not exchanged extensively with externally derived fluids. Carbon isotopic fractiona-tions between coexisting calcite and graphite in the marbles indicate equilibration at 675d? C, consistent with the conditions of regional metamorphism. Oxygen isotopic ratios from wollastonite skarn are lower than in the marbles and show a 14‰ variation (-1‰ to 13‰). Some isotopic heterogeneity is preserved from skarn formation, and some represents localized exchange with low-δ¹⁸O retrograde fluids. Detailed millimetre- to centimetre-scale isotopic profiles taken across skarn/marble contacts reveal steep δ¹⁸O gradients in the skarn, with values increasing towards the marble. The gradients reflect isotopic evolution of the fluid as it reacted with high δ¹⁸O calcite to form wollastonite. Calcite in the marble preserves high δ¹⁸O values to within <5 mm of the skarn contact. The preservation of high δ¹⁸O values in marbles at skarn contacts and the disequilibrium fractionation between wollastonite skarn and calcite marble across these contacts indicate that the marbles were not infiltrated with significant quantities of the fluid. Thus, the marbles were relatively impermeable during both the skarn formation and retrograde alteration. Skarn formation may have been episodic and fluid flow was either chaotic or dominantly parallel to lithological contacts. Although these steep isotope gradients resemble fluid infiltration fronts, they actually represent the sides of the major flow system. Because chromatographic infiltration models of mass transport require the assumption of pervasive fluid flow through a permeable rock, such models are not applicable to this hydrothermal system and, by extension, to many other metamorphic systems where low-permeability rocks restrict fluid migration pathways. Minimum time-integrated fluid fluxes have been calculated at the Valentine deposit using oxygen isotopic mass balance, reaction progress of fluid buffering reactions, and silica mass balance. All three approaches show that large volumes of fluid were necessary to produce the skarn, but silica mass balance calculations yield the largest minimum flux and are hence the most realistic.     

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.     

Genesis of wollastonite- and grandite-rich skarns in a suite of marble-calc-silicate rocks from Sittampundi, Tamil Nadu: constraints on the P–T–fluid regime in parts of the Pan-African mobile belt of South India

The Pan-African tectonothermal activities in areas near Sittampundi, south India, are characterized by metamorphic changes in an interlayered sequence of migmatitic metapelites, marble and calc-silicate rocks. This rock sequence underwent multiple episodes of folding, and was intruded by granite batholiths during and subsequent to these folding events. The marble and the calc-silicate rocks develop a variety of skarns, which on the basis of mineralogy; can be divided into the following types: Type I: wollastonite?+?clinopyroxene (mg#?=?71–73)?+?grandite (16–21 mol% Adr)?+?quartz?±?calcite, Type II: grandite (25–29 mol% Adr )?+?clinopyroxene (mg#?=?70)?+?calcite?+?quartz, and Type III: grandite (36–38 mol% Adr)?+?clinopyroxene (mg#?=?55–65)?+?epidote?+?scapolite?+?calcite?+?quartz. Type I skarn is 2–10 cm thick, and is dominated by wollastonite (>70 vol%) and commonly occurs as boudinaged layers parallel to the regional foliation Sn₁ related to the Fn₁ folds. Locally, thin discontinuous lenses and stringers of this skarn develop along the axial planes of Fn₂ folds. The Type II skarn, on the other hand, is devoid of wollastonite, rich in grandite garnet (40–70 vol%) and developed preferentially at the interface of clinopyroxene-rich calc-silicates layers and host marble during the later folding event. Reaction textures and the phase compositional data suggest the following reactions in the skarns: 1. calcite?+?SiO₂?→?wollastonite?+?V, 2. calcite?+?clinopyroxene?+?O₂?→?grandite?+?SiO₂?+?V, 3. scapolite?+?calcite?+?quartz?+?clinopyroxene?+?O₂?→?grandite?+?V and 4. epidote?+?calcite?+?quartz?+?clinopyroxene?+?O₂?→?grandite?+?V Textural relations and composition of phases demonstrate that (a) silica metasomatism of the host marble by infiltration of aqueous fluids (X_CO2?<?0.15) led to production of large volumes of wollastonite in the wollastonite-rich skarn whereas mobility of FeO, SiO₂ and CaO across the interface of marble and calc-silicate and infiltration of aqueous fluids (X_CO2?<?0.35) were instrumental for the formation of grandite skarns. Composition of minerals in type II skarn indicates that Al₂O₃ was introduced in the host marble by the infiltrating fluid. Interpretation of mineral assemblages observed in the interlayered metapelites and the calcareous rocks in pseudosections, isothermal P-X_CO2 and isobaric T-X_CO2 diagrams tightly bracket the “peak” metamorphic conditions at c.9?±?1 kbar and 750°?±?30°C. Subsequent to ‘peak’ metamorphic conditions, the rocks were exhumed on a steeply decompressive P–T path. The estimated ‘peak’ P–T estimates are inconsistent with the “extreme” metamorphic conditions (>11 kbar and >950°C) inferred for the Pan-African tectonothermal events from the neighboring areas. Field and petrological attributes of these skarn rocks are consistent with the infiltration of aqueous fluid predominantly during the Fn₁ folding event at or close to the ‘peak’ metamorphic conditions. Petrological features indicate that the buffering capacity of the rocks was lost during the formation of type I and II skarns. However, the host rock could buffer the composition of the permeated fluids during the formation of type III skarn. Aqueous fluids derived from prograde metamorphism of the metapelites seem to be the likely source for the metasomatic fluids that led to the formation of the skarn rocks.     

Wollastonite at Nuliyam,Kerala, southern India: a reassessment of CO2-infiltration and charnockite formation at a classic locality

The occurrence of a charnockitised felsic gneiss adjacent to a marble/calc-silicate horizon at Nuliyam, southern India, has been cited in recent literature as a classic example of the dehydration of crustal rocks resulting from the advective infiltration of CO₂-rich fluids generated from a local carbonate source. Petrographic study of the Nuliyam calc-silicate, however, reveals it to consist of abundant wollastonite and scapolite and contain locally discordant veins rich in wollastonite. At the pressure—temperature conditions proposed for charnockite formation in recent studies, 5 kbar and 725°C, this wollastonite-bearing mineral assemblage was stable in the presence of a fluid phase only if X _CO2 was near 0.25 and could not have coexisted with the fluid causing biotite breakdown and charnockite development in adjacent rocks (X _CO2>0.85). The stable coexistence of wollastonite and scapolite prohibits the calc-silicate from being a source for fluid driving charnockitisation at the required P-T conditions. Textural observations such as the limited replacement of wollastonite by calcite+quartz symplectites and mosaics, are consistent with late fluid infiltration into the calc-silicate. The extensive isotopic, chemical and mineral abundance data of Jackson and Santosh (1992) are re-interpreted and integrated with these observations to develop a model involving the infiltration of an externally derived CO₂-rich fluid during high-temperature decompression. Increased charnockite development next to the calc-silicate has arisen because the calc-silicate acted as a relatively unreactive and impermeable barrier to fluid transport and caused fluid ponding beneath antiformal closures. The Nuliyam charnockite/calc-silicate locality is an example of a structural trap in a metamorphic setting rather than a site where charnockite formation can be attributed to local fluid sources.     

Calc-silicate reactions and bedding-controlled isotopic exchange in the Notch Peak aureole, Utah: implications for differential fluid fluxes with metamorphic grade

Fluid compositions and bedding‐scale patterns of fluid flow during contact metamorphism of the Weeks Formation in the Notch Peak aureole, Utah, were determined from mineralogy and stable isotope compositions. The Weeks Formation contains calc‐silicate and nearly pure carbonate layers that are interbedded on centimetre to decimetre scales. The prograde metamorphic sequence is characterized by the appearance of phlogopite, diopside, and wollastonite. By accounting for the solution properties of Fe, it is shown that the tremolite stability field was very narrow and perhaps absent in the prograde sequence. Unshifted oxygen and carbon isotopic ratios in calcite and silicate minerals at all grades, except above the wollastonite isograd, show that there was little to no infiltration of disequilibrium fluids. The fluid composition is poorly constrained, but X(CO₂)_fluid must have been >0.1, as indicated by the absence of talc, and has probably increased with progress of decarbonation reactions. The occurrence of scapolite and oxidation of graphite in calc‐silicate beds of the upper diopside zone provide the first evidence for limited infiltration of external aqueous fluids. Significantly larger amounts of aqueous fluid infiltrated the wollastonite zone. The aqueous fluids are recorded by the presence of vesuvianite, large decreases in δ¹⁸O values of silicate minerals from c. 16‰ in the diopside zone to c. 10‰ in the wollastonite zone, and extensive oxidation of graphite. The carbonate beds interacted with the fluids only along margins where graphite was destroyed, calcite coarsened, and isotopic ratios shifted. The wollastonite isograd represents a boundary between a high aqueous fluid‐flux region on its higher‐grade side and a low fluid‐flux region on its lower‐grade side. Preferential flow of aqueous fluids within the wollastonite zone was promoted by permeability created by the wollastonite‐forming reaction and the natural tendency of fluids to flow upward and down‐temperature near the intrusion‐wall rock contact.     

Fluid-rock interaction and thermal evolution during thrusting of an Alpine metamorphic complex (Tinos island, Greece)

This study examines the fluid-rock interaction and thermal evolution along a thrust that juxtaposes calcite-rich marbles of high P-T metamorphic unit of the Attic-Cycladic Massif (Greece) on top of a lower-grade dolomite marble unit. The Tertiary thrust represents a major phase of tectonic movement related to the decompression of the Alpine orogen in the Hellenides. The stable isotope signatures of the thrust plane and adjacent sections of the footwall and hanging wall rocks are characterized by significant carbon and oxygen isotope depletions. The depletion is most pronounced in calcite, but is almost entirely missing in coexisting dolomite. The isotopic patterns in the thrust zone can be explained by the infiltration of an externally derived water-rich H₂O-CO₂-CH₄ fluid [X _C (=X _{CO 2}+X _{CH 4})<0.05] at water-rock ratios on the order of 0.1 to 0.5 by weight. The fluid-induced calcite recrystallization is viewed as an important rheological control during thrusting. The temperature evolution of the footwall, hanging wall and mylonitic tectonic contact was determined by calcite-dolomite solvus thermometry. Histograms of calcite-dolomite temperatures are interpreted as indicating a heating of the footwall dolomite marble during the thrusting of the hotter upper plate. Conversely, the hanging wall marble unit was cooled during the thrusting. The calcite-dolomite thermometry of the thrust plane gives temperatures intermediate between the initial temperatures of the lower and upper marble units, and this leads to the conclusion that conductive heat transfer rather than fluid infiltration controlled the thermal evolution during thrusting. Received: 14 April 1998 / Accepted: 9 December 1998     

Carbonate-hosted talc deposits in the contact aureole of an igneous intrusion (Hwanggangri mineralized zone,South Korea): geochemistry,phase relationships,and stable isotope studies

Many talc deposits occur in the Hwanggangri Mineralized Zone (HMZ) in dolomitic marbles of the Cambro-Ordovician Samtaesan Formation within 1 km of the contact with the Cretaceous Muamsa Granite. Talc commonly forms fine-grained, fibrous aggregates, or pseudomorphs after tremolite; abundant tremolite is included as impurities in the talc ore. Talc generally was derived from tremolite in calc-silicate rock within the dolomitic marble. Calc-silicate rock, consisting mainly of tremolite and diopside, was generated from silicic metasomatism during the prograde stage, which promoted decarbonation reactions until dolomite was exhausted locally. Hydrothermal alteration of calc-silicate rock to talc is marked by the addition of Mg and Si, and the leaching of Ca; Cr, Co, and Ni were relatively immobile during the retrograde stage. Contact metamorphism related to the granite intrusion generated the successive appearance of tremolite, diopside, and forsterite, or wollastonite-bearing assemblages in the marble, depending on the bulk rock composition. The X_CO2 content of the metamorphic fluids rose initially above X_CO2=0.6, and decreased steadily toward a water-rich composition with increasing temperature above 600 °C in the calcitic marble, while buffered reaction of the dolomitic marble occurred at higher X_CO2 conditions above 600 °C. Talc mineralization developed under metastable conditions with infiltration of large amounts of igneous fluids along a fault-shattered zone during the retrograde stage and is characterized by the loss of Ca²⁺ with the addition of Mg²⁺. Oxygen and carbon isotopic variations of carbonate and calc-silicate minerals are in agreement with theoretical relationships determined for decarbonation products of contact metamorphism. Talc formation temperatures obtained from oxygen isotope fractionation, T–X_CO2 relationships, and activity diagrams range from 380 to 400 °C.     

Interaction of aqueous fluids with calcareous metasediments during high-T, low-P regional metamorphism in the Qadda area, southern Arabian Shield

The S.W. Nabitah Mobile Belt, Saudi Arabia, contains a Proterozoic island-arc complex. In the Qadda area, the metavolcanic-dominated supracrustal sequence records amphibolite facies regional metamorphism of high-T , low-P type. Calcsilicate rocks and aluminous dolomitic marbles within the supracrustal sequence have been studied in detail to refine estimates of peak metamorphic P–T conditions and assess the role of fluids during prograde and retrograde metamorphism. Fluid-independent thermobarometers (including the calcite–dolomite thermometer and P-sensitive equilibria involving grossular, wollastonite, anorthite, meionite, quartz and calcite) yield peak P–T conditions of c. 650–660 °C, 4 kbar, both higher than previous estimates, giving a revised average thermal gradient of c. 45 °C km^–1. The close match between the peak temperatures implied by calcite–dolomite thermometry and those recorded by univariant devolatilization equilibria suggests that the calcareous rocks were fluid-bearing during late-prograde and peak metamorphic stages. These fluids were essentially binary H₂O–CO₂ mixtures with low NaCl and HF concentrations. Most were H₂O-rich, with X_CO2 between 0.02 and 0.2, but values of c. 0.6 are recorded by two samples. High modal abundances of the solid products of decarbonation reactions (e.g. c. 10–50% wollastonite) in many of the rocks that record low-X_CO2 equilibrium fluids implies infiltration of significant quantities of externally derived aqueous fluid during late-prograde metamorphism, but not enough to exhaust the buffering capacity of the rocks. Calculated minimum time-integrated fluid-to-rock ratios of five wollastonite-bearing calcsilicate rocks range from 0.7±0.22 to 1.39±0.46 (1σ); those of six marbles range from c. 0 to 4±1.4. The latter variation occurs on a metre-scale, implying focusing of fluid flow. Diopside-rich rocks record fluid-to-rock ratios of up to 88±48. Penetrative wollastonite lineations indicate a temporal link between infiltration and distributed ductile deformation. Infiltrating fluids were probably derived both from the prograde dehydration of adjacent metabasalts and metatuffs and from crystallization of voluminous pretectonic granitoid intrusions. In general, fluid-to-rock ratios deduced for the metavolcanic-dominated Qadda area are similar to those recorded by rocks in the metasediment-dominated terrane of N. New England. The occurrence of post-tectonic retrograde hydration textures in both carbonate-bearing and carbonate-free rocks otherwise lacking hydrous minerals testifies to infiltration of aqueous fluids during retrograde metamorphism in the absence of penetrative deformation. Minimum fluid-to-rock ratios calculated for secondary grossular reaction rims in some calcsilicates are c. 0.04. Later patchy hydration of scapolite probably utilized static, pore-filling fluids remaining after the early retrograde infiltration.     

Uplift-related retrogression history of aragonite marbles in Western Crete (Greece)

In the low-grade, high-pressure (400°C, 10 kbar) metamorphic Phyllite-Quartzite Unit of Western Crete, widespread occurrences of aragonite marbles have been discovered recently. A sedimentary precursor is proved by relic structures (bedding, fossils). Partial or complete transformation of aragonite into calcite is ubiquitous. Compositional and microstructural features reflect the metamorphic history: (1) The high-pressure stage is documented by aragonite that is chemically characterized by incorporation of variable amounts of Sr and the lack of Mg. The most Sr-rich aragonite has about 9 wt% SrO (X _Sr ^arag =0.09). A compositional zoning observed in some aragonite crystals may be due to the prograde divariant calcitearagonite transformation in the system CaCO₃-SrCO₃. Because the parent rocks probably were Sr-poor calcite limestones, one can speculate that strontium has been supplied from an external source under high-pressure conditions. (2) During uplift, calcite replacing aragonite did not equilibrate with unreplaced aragonite. Disequilibrium is indicated by highly variable compositions of calcite crystals that show topotactic relations to the host aragonite. The calcite compositions range from that of the host aragonite (Sr-rich and Mg-free) to Mg-bearing and Sr-poor. (3) Calcite that recrystallized during retrogression is generally Sr-poor (mean value ofX _Sr<0.005), Mg-bearing (X _Mg0.010), and chemically homogeneous. Because practically no Sr remains in the calcite, an interaction with a fluid phase is indicated. In fine-grained calcite marbles rich in solid organic matter, microstructural features indicative of former aragonite may be present. (4) The last stage of retrogression is documented by the appearance of radiating aragonite in veins and nodules. This aragonite, which shows neither deformation nor retrogression, was probably formed metastably in a near-surface environment.     

Textural and isotopic development of marble assemblages during the Barrovian-style M2 metamorphic event,Naxos, Greece

A detailed petrological analysis of the marble assemblages observed within the M2 metamorphic complex on Naxos is presented. Two distinct periods of mineral growth are documented; the first is associated with prograde M2 metamorphism and the second with retrograde M2 metamorphism occurring during ductile extensional thinning of the complex. The textural and miner-alogical characteristics and the carbon and oxygen isotope compositions of each generation are described, and the P-T-X _{CO 2} conditions at which these two mineral generations were stable, and the compositions of the fluids present during metamorphism are characterised. Whereas the low variance and stable isotope compositions of prograde siliceous dolomite assemblages are consistent with internally buffered fluid evolution, the retrograde mineral generation is shown to have grown as a result of the infiltration of a water-rich fluid phase that transported silica, Al₂O₃, Na₂O and FeO into the host rocks. This observation, together with the stable isotope compositions of the retrograde calcite, and the fact that occurrences of veins of this type are limited to marbles in the highest grade areas (T>600° C) of the metamorphic complex, suggests that the fluids responsible for vein formation were generated during the crystallisation of melts as the metamorphic complex cooled from peak temperatures. The existence of this second generation of minerals has significant implications for previous studies of heat transport by fluid flow on Naxos, because many of the unusually low ¹⁸O compositions of pelites at high grades may be ascribable to the effects of interaction with retrograde M2 fluids, rather than with prograde fluids.     

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     

Phase equilibria involving humite minerals in impure dolomitic limestones

The natural occurrence of critical assemblages among the phases clinohumite, chondrodite, norbergite, tremolite, forsterite, brucite, periclase, diopside, calcite and dolomite, together with experimental and thermochemical data, permits the calculation of phase equilibria governing the stability of the humite group minerals in impure dolomitic limestones. The phase relations are described by 29 divariant (OH-F) continuous reactions, and 11 univariant discontinuous reactions. The equilibrium conditions for these reactions have been calculated and plotted in isobaricT-X(OH-F) andT- phase diagrams. Continuous reactions govern the compositions of (OH-F) solid solutions and the consequent movement of three-phase triangles on the chemographic diagram. Discontinuous reactions result in the appearance or disappearance of a distinct phase assemblage. The pure OH-humite minerals are metastable relative to forsterite+brucite. With increasing fluorine content, clinohumite, followed in turn by chondrodite and norbergite, becomes stable. The stability fields for the individual humite minerals expand to more CO₂-rich fluid compositions with increasing fluorine content and decreasing total pressure. At 1,000 bars, clinohumite can contain a maximum of 58 mole percent fluorine before reacting discontinuously to form chondrodite (X _F=0.61) and forsterite. The stability field for clinohumite+calcite is restricted to fluids with <0.40. At temperatures less than 700°C, the minimum fluorine mole fractions required to stabilize chondrodite and norbergite are 0.31 and 0.62 respectively. At the same conditions, chondrodite can contain a maximum of approximately 85 mole % F. The calculated phase equilibrium boundaries, the range of stable humite compositions and the compositions of coexisting (OH-F) phases are in good overall agreement with natural assemblages. Owing to steepdT/d slopes for several of the humite continuous reactions, the F/(F+OH) ratio of a given humite mineral is a useful indicator of the CO₂/H₂O ratio of the fluid phase.