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1.
ABSTRACT The metasedimentary sequence of the Deep Freeze Range (northern Victoria Land, Antarctica) experienced high-T/low-F metamorphism during the Cambro-Ordovician Ross orogeny. The reaction Bt + Sil + Qtz = Grt + Crd + Kfs + melt was responsible for the formation of migmatites. Peak conditions were c. 700–750° C, c. 3.5–5 kbar and xH2Oc. 0.5). Distribution of fluid inclusions is controlled by host rock type: (1) CO2-H2O fluid inclusions occur only in graphite-free leucosomes; (2) CO2–CH4± H2O fluid inclusions are the most common type in leucosomes, and in graphite-bearing mesosomes and gneiss; and (3) CO2–N2–CH4 fluid inclusions are observed only in the gneiss, and subordinately in mesosomes. CO2–H2O mixtures (41% CO2, 58% H2O, 1% Nad mol.%) are interpreted as remnants of a synmig-matization fluid; their composition and density are compatible P–T–aH2O conditions of migmatization (c. 750° C, c. 4 kbar, xH2Oc. 0.5). CO2-H2O fluid in graphite-free leucosomes cannot originate via partial melting of graphite-bearing mesosomes in a closed system; this would have produced a mixed CO2–CH4 fluid in the leucosomes by a reaction such as Bt + Sil + Qtz + C ± H2O = Grt + Crd + Kfs + L + CO2+ CH4. We conclude that an externally derived oxidizing CO2-H2O fluid was present in the middle crust and initiated anatexis. High-density CO2-rich fluid with traces of CH4 characterizes the retrograde evolution of these rocks at high temperatures and support isobaric cooling (P–T anticlockwise path). In unmigmatized gneiss, mixed CO2–N2–CH4 fluid yields isochores compatible with peak metamorphic conditions (c. 700–750° C, c. 4–4.5 kbar); they may represent a peak metamorphic fluid that pre-dated the migmatization.  相似文献   

2.
The Southern Marginal Zone of the Limpopo Belt in South Africa is characterised by a granulite and retrograde hydrated granulite terrane. The Southern Marginal Zone is, therefore, perfectly suitable to study fluids during and after granulite facies metamorphism by means of fluid inclusions and equilibrium calculations. Isolated and clustered high-salinity aqueous and CO2(-CH4) fluid inclusions within quartz inclusions in garnet in metapelites demonstrate that these immiscible low H2O activity fluids were present under peak metamorphic conditions (800-850 °C, 7.5-8.5 kbar). The absence of widespread high-temperature metasomatic alteration indicates that the brine fluid was probably only locally present in small quantities. Thermocalc calculations demonstrate that the peak metamorphic mineral assemblage in mafic granulites was in equilibrium with a fluid with a low H2O activity (0.2-0.3). The absence of water in CO2-rich fluid inclusions is due to either observation difficulties or selective water leakage. The density of CO2 inclusions in trails suggests a retrograde P-T path dominated by decompression at T<600 °C. Re-evaluation of previously published data demonstrates that retrograde hydration of the granulites at 600 °C occurred in the presence of H2O and CO2-rich fluids under P-T conditions of 5-6 kbar and ~600 °C. The different compositions of the hydrating fluid suggest more than one fluid source.  相似文献   

3.
Well-formed, texturally-early fluid inclusions in garnets from the Archean Pikwitonei granulite domain, Manitoba, Canada, have been analyzed using microthermometric methods. The mean CO2 homogenization temperature (to liquid) for inclusions in 12 of 13 samples from the Cauchon Lake-Nelson River area is +15.2° C (n=125, 2σ=8.2° C), corresponding to a CO2 density of 0.82 g/cm3. Inclusions in the remaining sample have somewhat lower CO2 homogenization temperatures (mean=+5.4° C, n=24). The studied inclusions contain an estimated 10 to 20 vol. percent H2O, with minor amounts of other fluid species such as CH4, N2, and/or H2S. The fluid inclusions were probably trapped during early garnet growth at relatively low pressures (≤5 kbar if at 750° C), and appear to have undergone only limited or possibly no subsequent re-equilibration. This interpretation is consistent with the “anti-clock-wise” P-T-t path (heating before loading) determined for the Pikwitonei region by other workers. For such a prograde path, inclusions entrapped early, at high temperatures but at relatively low pressures, would experience internal underpressures during most of the subsequent prograde and retrograde phases of metamorphism. The texturally-early fluid inclusions in garnets from the Pikwitonei region therefore cannot be used to provide direct information about the highest metamorphic temperature and pressure conditions (750° C and 7 kbar). However, the results obtained in this study suggest that texturally-early fluid inclusions in garnets may, in some cases, retain evidence of the prograde metamorphic path.  相似文献   

4.
Fluid inclusions have been studied in three pegmatite fields in Galicia, NW Iberian Peninsula. Based on microthermometry and Raman spectroscopy, eight fluid systems have been recognized. The first fluid may be considered to be a pegmatitic fluid which is represented by daughter mineral (silicates)-rich aqueous inclusions. These inclusions are primary and formed above 500 °C (dissolution of daughter minerals). During pegmatite crystallization, this fluid evolved to a low-density, volatile-rich aqueous fluid with low salinity (93% H2O; 5% CO2; 0.5% CH4; 0.2% N2; 1.3% NaCl) at minimum P–T conditions around 3 ± 0.5 kbar and 420 °C. This fluid is related to rare-metal mineralization. The volatile enrichment may be due to mixing of magmatic fluids and fluids equilibrated with the host rock. A drop in pressure from 3 ± 0.5 to 1 kbar at a temperature above 420 °C, which may be due to the transition from predominantly lithostatic to hydrostatic pressure, is recorded by two-phase, water-rich inclusions with a low-density vapour phase (CO2, CH4 and N2). Another inclusion type is represented by two-phase, vapour-rich inclusions with a low-density vapour phase (CO2, CH4 and N2), indicating a last stage of decreasing temperature (360 °C) and pressure (around 0.5 kbar), probably due to progressive exhumation. Finally, volatile (CO2)-rich aqueous inclusions, aqueous inclusions (H2O-NaCl) and mixed-salt aqueous inclusions with low Th, are secondary in charac- ter and represent independent episodes of hydrothermal fluid circulation below 310 °C and 0.5 kbar. Received: 14 October 1999 / Accepted: 5 October 1999  相似文献   

5.
The Degdekan and Gol’tsovsky gold-quartz deposits are located in the southeastern Yana-Kolyma gold belt. The orebodies occur as quartz veins hosted in metaterrigenous rocks and cut by postmineral basic-intermediate dikes. It was established that metamorphism of sulfides and gangue quartz was restricted to a few centimeters off the dike contact. According to sulfide geothermometers, the metamorphic temperatures close to the contact of dikes attained 700°C at the Degdekan deposit and were no higher than 491°C at the Gol’tsovsky deposit. The formation of the forbidden assemblage of quartz and loellingite and its fine-grained texture indicate that the thermal effect on the Degdekan ore was short-term. The prolonged heating of the ore at the Gol’tsovsky deposit gave rise to the aggradation recrystallization of quartz and the formation of equilibrium sulfide aggregates that show only insignificant differences in composition from the primary phases. The average homogenization temperature of primary and pseudosecondary fluid inclusions is 206 ± 40°C in the unmetamorphosed veins and 257 ± 33°C in the metamorphosed veins. The salinity of fluids in the primary and pseudosecondary inclusions in quartz veins of both types varies from 0.5 to 14.0 wt % NaCl equiv. The melting temperature of liquid CO2 in the carbon dioxide inclusions, ranging from ?57.0 to ?60.8°C, suggests an admixture of CH4 and/or N2. The unmetamorphosed quartz veins were formed at a fluid pressure varying from 0.7 to 1.3 kbar, while quartz veins at the contact with dikes crystallized at a pressure of 0.8–1.5 kbar. The results of gas chromatography showed the presence of CO2 and H2O, as well as N2 and CH4. The average bulk of volatiles contained in the fluid inclusions in quartz from the metamorphosed veins is 1.5–2 times lower than in the unmetamorphosed veins; this proportion is consistent with the occurrence of decrepitated gas inclusions in the heated quartz.  相似文献   

6.
Aqueous-carbonaceous and later pure aqueous fluid inclusions in quartz from a ferberite (Fe.95Mn.05 WO4) vein within the low-grade metamorphic aureole of the Borne granite (French Massif Central) have been studied by microthermometry and Raman spectrometry. The bulk V?-X properties of the aqueous-carbonaceous inclusions have been derived using the equation of state of Heyenet al. (1982) for the low-temperature CO2-CH4 system. A P-T path has been proposed for their trapping using the equations of state of Jacobs and Kerrick (1981a) for the H2O-CO2-CH4 system. Two main episodes were reconstructed for the history of the aqueous-carbonaceous fluid. (1) Primary H2O-CO2-CH4 vapourrich inclusions in quartz indicated the early circulation of a low-density fluid (65 mole% H2O-34 mole% CO2-1 mole% CH4 and traces of N2: d = 0.35 gcm?3) at around 550° ± 50°C and 700 ± 100 bar. Fluid cooled approximately isobarically to 450°-400°C and was progressively diluted by H2O with a concomitant increase in density. The fO2 of the H2OCO2-CH4 fluid, estimated from the equilibrium CO2 + 2H2O CH4 + 2O2, first ranged from 10?22 to 10?27 bar, close to the Q-F-M buffer. Within analytical errors, these values were consistent with the presence of graphite in equilibrium with the fluid. (2) A drop in PCO2, and therefore a drop in fO2, was recorded by the secondary liquid-rich inclusions in quartz. The inclusions, formed at and below 400°C, were composed of H2O and CH4 only, and fO2 at that stage was below that fixed by the graphite-fluid equilibrium. This second episode in the fluid-rock system could be explained by the drop of temperature below the blocking temperature of the graphite-fluid equilibrium. According to this interpretation, the blocking of the graphite-fluid equilibrium occurred at T ≥ 370°C and probably at 400°C on account of the pressure correction. Mass spectrometric data show that ferberite contains H2O, CO2 and CH4 in fluid inclusions, which lie in the gap of the V?-X properties of the aqueouscarbonaceous fluid in quartz. Deposition of ferberite probably occurred at around 400°C, the previously inferred blocking temperature, resulting from either the drop in PCO2, the drop fO2 and/or the related pH-increase.It is concluded that the existence of a blocking-temperature for the graphite-fluid chemical equilibrium may be a critical factor for maintaining a stable fluid pressure gradient in geothermal systems occurring under greenschist facies conditions in graphite-bearing rocks.  相似文献   

7.
The transformations (density, composition, or shape alterations) of fluid inclusions under isobaric cooling and isothermal compression have been experimentally modeled. The H2O-CO2-CH4 inclusions used in the model experiments have been synthesized in quartz at 1 or 3 kbar and 700°C. The parameters of the model experiments were as follows: for isobaric cooling, 700–400°C at 1 or 3 kbar; for isothermal compression, 1–3 and 3–5 kbar at 400 and 700°C, respectively. The shape of the inclusion walls changed as a result of the experiments because of the dissolution and reprecipitation of the host phase. The intensity of these changes was directly affected by the pressure and temperature differences and the amount of the aqueous phase in the inclusions. Fluid densities increased in many inclusions in the run products: the increase ranged from insignificant (0.06 g/cm3) to fairly noticeable (0.15 g/cm3). The composition of the fluid inclusions also changed. The results of the experiments, in particular of those carried out with a labeled fluid, show that the penetration of a denser external fluid into inclusions with a lower fluid density is the basic reason for the increase in the density and change in the composition of fluid inclusions. In several experiments, the methane concentration changed with an attendant change in the melting temperature of carbon dioxide but without noticeable changes in the carbon dioxide homogenization temperature. Original Russian Text ? O.V. Vasyukova, V.I. Fonarev, 2006, published in Geokhimiya, 2006, No. 12, pp. 1266–1278.  相似文献   

8.
Abstract The Berzosa fault is a major ductile shear zone, the Berzosa Shear Zone (BSZ), which separates the ‘Ollo de Sapo’anticline from the inner higher-grade crystalline axis of the Iberian Hercynian Belt. This shear zone is the site of abundant early kinematic quartz (± Al-silicates) segregations, rich in fluid inclusions. Host rocks are medium-grade staurolite schists and sillimanite gneisses. Fluid inclusions in selected quartz segregations across the Berzosa shear zone have been studied by microthermometric methods as well as, in some instances, by Raman analysis. The recorded fluid inclusion history begins at the end of an intense secondary recrystallization period during late-peak metamorphic conditions and lasts until late in the uplift history of the zone. Three types of inclusions have been found, which in a time sequence are: CO2± H2O; H2O+salt (B-type); and, N2+CH4. Three types of B inclusion may be distinguished in turn, depending on whether they were trapped during an earlier dynamic-recovery phase (B1-type), formed later as intergranular trails (B2-type), or were trapped apparently along with N2+CH4 in clusions from a heterogeneous fluid (B3-type). Considerations from isochores confirm that CO2± H2O inclusions were trapped during late-peak and high-T retrograde metamorphic conditions (in the range 650–500°C and 5–2 kbar), whilst N2+CH4 inclusions, along with the B3-type of inclusions, formed at low-pressures (<1 kbar) and temperatures (± 300°C). B2-type inclusions were trapped chronologically between these two in a period in which strong inverse lateral thermal gradients developed in the zone. Inferred P-T paths for the area are convex to the T-axis.  相似文献   

9.
《Ore Geology Reviews》1999,14(3-4):203-225
The auriferous veins at Yirisen, Masumbiri, Sierra Leone, occurring mainly in the form of sericitic quartz-sulphide lodes and stringers, are hosted in metamorphosed volcano-sedimentary assemblages invaded by at least two generations of granitic intrusions. Detailed microthermometric studies of fluid inclusions from the veins coupled with laser Raman spectroscopic analysis show that the inclusions contain aqueous fluids of variable salinity (5 to 60 wt.% NaCl equivalent) and dense carbonic fluids (pure CO2: 1.08>d>0.88 g/cm3). Optical observations and analysis on opened inclusions by scanning electron microscopy (SEM) reveal that some of the aqueous inclusions contain a number of daughter minerals: halite, sylvite, Ca-, Fe-, Mg- and possibly Li-bearing chlorides, and anhydrite; nahcolite occurs also in some of the CO2 inclusions. The SEM runs also detected a small amount of electrum, suggesting that silver might be a bi-product of the mineralisation. The aqueous and carbonic fluids remained immiscible throughout the formation and evolution of the hydrothermal veins. A few mixed (H2O+CO2) inclusions apparently resulted from accidental trapping of both fluids in the same cavity. The wide range of salinities observed in the aqueous inclusions is attributed to the mixing of relatively hot, low-salinity aqueous fluids and colder, high-salinity brines. The CO2-rich and low-salinity H2O inclusions are considered to be derived from the metamorphic decarbonation/dehydration of the greenstone pile whilst the high-salinity brines are believed to be basinal in origin. Pressure–temperature (P–T) conditions of entrapment, inferred from the intersection of representative isochores of the immiscible fluids, indicate that the formation of the veins started at T=400°C and P about 4 kbar, in the presence of the high-density CO2 and low-salinity H2O fluids. At about 200°C, pressure fluctuations (incremental opening of the vein) correspond to the trapping of the lower-density CO2 inclusions and high-salinity brines. It is proposed that the decarbonation/dehydration processes (possibly aided by later magmatic processes) expelled and mobilised the gold from the greenstone pile and concentrated it in the CO2-bearing hydrothermal fluid in the form of Au–chloride complexes. High thermal gradients are believed to have caused the upward migration of this fluid from the bottom of the greenstone pile through structurally controlled conduits. We contend that phase separation of the H2O–CO2 metamorphic fluid, aided possibly by some wall–rock alteration, most probably triggered a decrease in ligand activity and thus, precipitation of the gold into lodes. Percolation of the basinal brines is thought to have remobilised some of the gold together with some silver.  相似文献   

10.
H2O, CO2, and H2OCO2 inclusions were observed in quatz from deep-seated granitic intrusions belonging to the Precambrian Farsund plutonic complex, south Norway. These inclusions represent solidus and/or sub-solidus fluids that were present in these rocks at some period between the initial melt and the present. Early CO2 and H2OCO2 inclusions with about 20 mole% CO2 contain up to 10 mole% CH4 in the CO2 phase and have densities from 0.96 to 0.85 g/cc. These inclusions are considered to most nearly approximate solidus vapour phases and suggest conditions of final solidification of the magma at 5 to 6 Kb and 700°C to 800°C. The H2O inclusions have salinities between 2 and 60 wt%; the majority contain 5 to 20 equivalent wt.% NaCl and have densities from 1.05 to 0.85 g/cc. Microthermometry indicates that other cations such as K+, Ca2+ and / or Mg2+ are present in these aqueous fluids. The H2O inclusions primarily represent fluids present at a post-magmatic stage of fracturing and healing of these rocks during uplift.  相似文献   

11.
The Talate Pb-Zn deposit,located in the east of the NW-SE extending Devonian Kelan volcanic-sedimentary basin of the southern Altaides,occurs in the metamorphic rock series of the upper second lithological section of the lower Devonian lower Kangbutiebao Formation(D_1k_1~2).The Pb-Zn orebodies are stratiform and overprinted by late sulfide—quartz veins.Two distinct mineralization periods were identified:a submarine volcanic sedimentary exhalation period and a metamorphic hydrothermal mineralization period.The metamorphic overprinting period can be further divided into two stages:an early stage characterized by bedding-parallel lentoid quartz veins developed in the chlorite schist and leptite of the ore-bearing horizon,and a late stage represented by pyritechalcopyrite-quartz veins crosscutting chlorite schist and leptite or the massive Pb-Zn ores.Fluid inclusions in the early metamorphic quartz veins are mainly CO_2-H_2O-NaCI and carbonic(CO_2±CH_4±N_2) inclusions with minor aqueous inclusions.The CO_2-H_2O-NaCl inclusions have homogenization temperatures of 294-368℃,T_(m,CO2) of-62.6 to-60.5℃,T_(h,CO2) of 7.7 to 29.6℃(homogenized into liquid),and salinities of 5.5-7.4 wt%NaCl eqv.The carbonic inclusions have T_(m,CO2)of-60.1 to-58.5℃,and T_(h,Co2) of-4.2 to 20.6℃.Fluid inclusions in late sulfide quartz veins are also dominated by CO_2-H_2O-NaCl and CO_2±CH_4 inclusions.The CO_2-H_2O-NaCl inclusions have T_(b,tot) of142 to 360℃,T_(m,CO2)of-66.0 to-56.6℃,T_(h,CO2) of-6.0 to 29.4℃(homogenized into liquid) and salinities of 2.4-16.5 wt%NaCl eqv.The carbonic inclusions have T_(m,Co2)of-61.5 to-57.3℃,and T_(h,CO2) of-27.0to 28.7℃.The aqueous inclusions(L-V) have T_(m,ice) of-9.8 to-1.3℃ and T_(h,tot) of 205 to 412℃.The P-T trapping conditions of CO_2-rich fluid inclusions(100-370 MPa,250-368℃) are comparable with the late- to post-regional metamorphism conditions.The CO_2-rich fluids,possibly derived from regional metamorphism,were involved in the reworking and metal enrichment of the primary ores.Based on these results,the Talate Pb-Zn deposit is classified as a VMS deposit modified by metamorphic fluids.The massive Pb-Zn ores with banded and breccia structures were developed in the early period of submarine volcanic sedimentary exhalation associated with an extensional subduction-related back-arc basin,and the quartz veins bearing polymetallic sulfides were formed in the late period of metamorphic hydrothermal superimposition related to the Permian-Triassic continental collision.  相似文献   

12.
Abstract The Llano Uplift in central Texas is a Grenville aged (c. 1.1 Ga) metamorphic terrane consisting predominantly of amphibolite facies mineral assemblages. The formation of these assemblages has been attributed to the emplacement of relatively late granite plutons throughout the area. Two types of granitic intrusion have previously been recognized: (1) Town Mountain Granites, which occur as relatively large, circular-shaped bodies of coarse-grained granite, and (2) Younger Granites which are present as smaller and more irregular bodies of finer-grained granite. In the central part of the uplift, wollastonite-bearing calc-silicate rocks occur within the Valley Spring Gneiss. The development of these calc-silicate rocks has been linked to infiltrating fluids presumably derived from spatially associated Younger Granites. The stability of coexisting quartz, calcite, wollastonite, grossular and anorthite and coexisting quartz, calcite, wollastonite, andradite and hedenbergite shows that the calc-silicate rocks equilibrated under H2O-rich conditions with χCO2 <0.10. Fluid inclusions present within the calc-silicate minerals are H2O-rich with salinities of <17 wt% equivalent NaCl. The absence of any detectable CO2 in the fluid inclusions may indicate entrapment of the inclusions at lower pressures and more H2O-rich conditions compared to the stability of the peak metamorphic mineral assemblage. Homogenization temperatures, measured for texturally primary inclusions, range from 360 to 368° C corresponding to a density range from 0.53 to 0.82 g/cm3. Isochores for these fluid inclusions, when combined with the stability of the solid-solid equilibria Grs + Qtz = Wo + An, yield formation conditions of 500–550° C at 1–2 kbar. This indicates that the granitic intrusions involved in the formation of the Blount Mountain calc-silicates were emplaced at a pressure of at least 1–2 kbar.  相似文献   

13.
Fluid infiltration into retrograde granulites of the Southern Marginal Zone(Limpopo high grade terrain)is exemplified by hydration reactions.shear zone hosted metasomatism,and lode gold mineralisation.Hydration reactions include the breakdown of cordierite and orthopyroxene to gedrite kyanite,and anthophyllite,respectively.Metamorphic petrology,fluid inclusions,and field data indicate that a low H_2O-activity carbon-saturated CO_2-rich and a saline aqueous fluid infiltrated the Southern Marginal Zone during exhumation.The formation of anthophyllite after orthopyroxene eslablished a regional retrograde anthophyllite-in isograd and occurred at P-T conditions of- 6 kbar and 610 C,which fixes the minimum mole fraction of II.0 in the CO_2-rich fluid phase at- 0.1.The maximum H_2O mole fraction is hxed by the lower temperature limit(~800℃) for partial melting at ~0.3.C-O-H fluid calculations show that the CO_2-rich fluid had an oxygen fugacity that was 0.6 log10 units higher than that of the fayalite-magnetitequartz buffer and that the CO_2/(CO_2+CH_4) mole ratio of this fluid was 1.The presence of dominantly relatively low density CO_2-rich fluid inclusions in the hydrated granulites indicates that the fluid pressure was less than the lithostatic pressure.This can be explained by strike slip faulting and/or an increase of the rock permeability caused by hydration reactions.  相似文献   

14.
The Meatiq basement, which is exposed beneath late Proterozoic nappes of supracrustal rocks in the Central Eastern Desert of Egypt, was affected by three metamorphic events. The ophiolite cover nappes show only the last metamorphic overprint. The M1 metamorphic event (T ≥750 °C) is restricted to migmatized amphibolite xenoliths within the Um Ba′anib orthogneiss in the structurally lowest parts of the basement. Typical upper amphibolite facies M2 mineral assemblages include Grt–Zn-rich Spl–Qtz±Bt, Grt–Zn-rich Spl–Ms–Kfs–Bt–Sil–Qtz and locally kyanite in metasedimentary rocks. The mineral assemblages Ms–Qtz–Kfs–Sil in the matrix and Sil–Grt in garnet cores indicate that peak M2 P–T conditions exceeded muscovite and staurolite stabilities. Diffusional equilibration at M2 peak temperature conditions caused homogeneous chemical profiles across M2 garnets. Abundant staurolite in garnet rims and the matrix indicates a thorough equilibration during M2 at decreasing temperature conditions. M2 P–T conditions ranged from 610 to 690 °C at 6–8 kbar for the metamorphic peak and 530–600 °C at about 5.8 kbar for the retrograde stage. However, relic kyanite indicates pressures above 8 kbar, preceeding the temperature peak. A clockwise P–T path is indicated by abundant M2 sillimanite after relic kyanite and by andalusite after sillimanite. M2 fluid inclusions, trapped in quartz within garnet and in the quartz matrix show an array of isochores. Steepest isochores (water-rich H2O-CO2±CH4/N2 inclusions) pass through peak M2 P–T conditions and flatter isochores (CO2-rich H2O-CO2±CH4/N2 inclusions) are interpreted to represent retrograde fluids which is consistent with a clockwise P–T path for M2. The M3 assemblage Grt–Chl in the uppermost metasedimentary sequence of the basement limits temperature to 460 to 550 °C. M3 temperature conditions within the ophiolite cover nappes are limited by the assemblage Atg–Trem–Tlc to<540 °C and the absence of crysotile to >350 °C. The polymetamorphic evolution in the basement contrasts with the monometamorphic ophiolite nappes. The M1 metamorphic event in the basement occurred prior to the intrusion of the Um Ba′anib granitoid at about 780 Ma. The prograde phase of the M2 metamorphic event took place during the collision of an island arc with a continent. The break-off of the subducting slab increased the temperature and resulted in the peak M2 mineral assemblages. During the rise of the basement domain retrograde M2 mineral assemblages were formed. The final M3 metamorphic event is associated with the updoming of the basement domain at about 580 Ma along low-angle normal faults.  相似文献   

15.
The ultrahigh-temperature (UHT) metamorphism of the Napier Complex is characterized by the presence of dry mineral assemblages, the stability of which requires anhydrous conditions. Typically, the presence of the index mineral orthopyroxene in more than one lithology indicates that H2O activities were substantially low. In this study, we investigate a suite of UHT rocks comprising quartzo-feldspathic garnet gneiss, sapphirine granulite, garnet-orthopyroxene gneiss, and magnetite-quartz gneiss from Tonagh Island. High Al contents in orthopyroxene from sapphirine granulite, the presence of an equilibrium sapphirine-quartz assemblage, mesoperthite in quartzo-feldspathic garnet gneiss, and an inverted pigeonite-augite assemblage in magnetite-quartz gneiss indicate that the peak temperature conditions were higher than 1,000 °C. Petrology, mineral phase equilibria, and pressure-temperature computations presented in this study indicate that the Tonagh Island granulites experienced maximum P-T conditions of up to 9 kbar and 1,100 °C, which are comparable with previous P-T estimates for Tonagh and East Tonagh Islands. The textures and mineral reactions preserved by these UHT rocks are consistent with an isobaric cooling (IBC) history probably following an counterclockwise P-T path. We document the occurrence of very high-density CO2-rich fluid inclusions in the UHT rocks from Tonagh Island and characterize their nature, composition, and density from systematic petrographic and microthermometric studies. Our study shows the common presence of carbonic fluid inclusions entrapped within sapphirine, quartz, garnet and orthopyroxene. Analysed fluid inclusions in sapphirine, and some in garnet and quartz, were trapped during mineral growth at UHT conditions as 'primary' inclusions. The melting temperatures of fluids in most cases lie in the range of -56.3 to -57.2 °C, close to the triple point for pure CO2 (-56.6 °C). The only exceptions are fluid inclusions in magnetite-quartz gneiss, which show slight depression in their melting temperatures (-56.7 to -57.8 °C) suggesting traces of additional fluid species such as N2 in the dominantly CO2-rich fluid. Homogenization of pure CO2 inclusions in the quartzo-feldspathic garnet gneiss, sapphirine granulite, and garnet-orthopyroxene gneiss occurs into the liquid phase at temperatures in the range of -34.9 to +4.2 °C. This translates into very high CO2 densities in the range of 0.95-1.07 g/cm3. In the garnet-orthopyroxene gneiss, the composition and density of inclusions in the different minerals show systematic variation, with highest homogenization temperatures (lowest density) yielded by inclusions in garnet, as against inclusions with lowest homogenization (high density) in quartz. This could be a reflection of continued recrystallization of quartz with entrapment of late fluids along the IBC path. Very high-density CO2 inclusions in sapphirine associated with quartz in the Tonagh Island rocks provide potential evidence for the involvement of CO2-rich fluids during extreme crustal temperatures associated with UHT metamorphism. The estimated CO2 isochores for sapphirine granulite intersect the counterclockwise P-T trajectory of Tonagh Island rocks at around 6-9 kbar at 1,100 °C, which corresponds to the peak metamorphic conditions of this terrane derived from mineral phase equilibria, and the stability field of sapphirine + quartz. Therefore, we infer that CO2 was the dominant fluid species present during the peak metamorphism in Tonagh Island, and interpret that the fluid inclusions preserve traces of the synmetamorphic fluid from the UHT event. The stability of anhydrous minerals, such as orthopyroxene, in the study area might have been achieved by the lowering of H2O activity through the influx of CO2 at peak metamorphic conditions (>1,100 °C). Our microthermometric data support a counterclockwise P-T path for the Napier Complex.  相似文献   

16.
Fine-grained peraluminous synkinematic leuco-monzogranites (SKG), of Cambro-Ordovician age, occur as veins and sills (up to 20–30 m thick) in the Deep Freeze Range, within the medium to high-grade metamorphics of the Wilson Terrane. Secondary fibrolite + graphite intergrowths occur in feldspars and subordinately in quartz. Four main solid and fluid inclusion populations are observed: primary mixed CO2+H2O inclusions + Al2SiO5 ± brines in garnet (type 1); early CO2-rich inclusions (± brines) in quartz (type 2); early CO2+CH4 (up to 4 mol%)±H2O inclusions + graphite + fibrolite in quartz (type 3); late CH4+CO2+N2 inclusions and H2O inclusions in quartz (type 4). Densities of type 1 inclusions are consistent with the crystallization conditions of SKG (750°C and 3 kbar). The other types are post-magmatic: densities of type 2 and 3 inclusions suggest isobaric cooling at high temperature (700–550°C). Type 4 inclusions were trapped below 500°C. The SKG crystallized from a magma that was at some stage vapour-saturated; fluids were CO2-rich, possibly with immiscible brines. CO2-rich fluids (±brines) characterize the transition from magmatic to post-magmatic stages; progressive isobaric cooling (T<670°C) led to a continuous decrease off O 2 can entering in the graphite stability field; at the same time, the feldspars reacted with CO2-rich fluids to give secondary fibrolite + graphite. Decrease ofT andf O 2 can explain the progressive variation in the fluid composition from CO2-rich to CH4 and water dominated in a closed system (in situ evolution). The presence of N2 the late stages indicates interaction with external metamorphic fluids.Contribution within the network Hydrothermal/metamorphic water-rock interactions in crystalline rocks: a multidisciplinary approach on paleofluid analysis. CEC program: Human Capital and Mobility  相似文献   

17.
We present a study of carbonate-bearing polyphase inclusions in garnets from leucocratic granitoids intruding metapelitic granulites of the Southern Marginal Zone (SMZ) of the Neoarchean Limpopo high-grade complex, South Africa, during the post-peak stage (2710–2650 Ma; U-Pb ages for zircons and monazites). Ternary feldspar thermometry suggests that the granitoid magma cooled from temperatures 800–900 °C at a pressure of ca. 6.5 kbar. Abundant CO2 fluid inclusions in quartz and T-XCO2 phase equilibria modeling via PERPLE_X imply action of an essentially carbonic fluid in the granitoids. Cores of almandine-rich garnet grains from the granitoids contain polyphase carbonate-bearing inclusions with a distinct negative crystal shape. The major carbonate in the inclusions is a strongly zoned magnesite-siderite variety, whereas pyrophyllite is the predominant silicate phase. Raman spectra of unexposed inclusions revealed a presence of CO2, as well as CH4 and H2O. The carbonate-bearing inclusions coexist with larger polyphase inclusions composed of biotite, quartz, K-feldspar, plagioclase, sillimanite, which are interpreted as relics of granitic melts. Modeling the mineral assemblage preserved within the carbonate-bearing inclusions shows that their present mineral and chemical compositions are a product of interaction of the trapped aqueous‑carbonic fluid with host garnet during cooling below 400 °C. Despite strong modifications, the inclusions bear evidence for initial saturation of the fluid with Mg‑carbonate. This is taken as an evidence for an origin of the fluids by devolatilization of the Mg-rich carbonate-bearing ultrabasic greenstone rocks of the Kaapvaal Craton that were buried under the SMZ. Being generated at temperatures between 650 and 700 °C, the fluid subsequently participated in anatexis and coexisted with the granite magma during exhumation and interaction of the SMZ granulites with cratonic rocks.  相似文献   

18.
Three successive metamorphic stages M1, M2 and M3 have been distinguished in polymetamorphic granulite facies quartz-feldspathic gneisses from the Seiland Igneous Province, Caledonides of northern Norway. An early period of contact metamorphism (M1; 750–950°C, ca. 5 kbar) was followed by cooling, accompanied by strong shearing and recrystallization at intermediate-P granulite facies conditions (M2; 700–750°C, 5–6kbar). High-P granulite facies (M3; ca. 700°C, 7–8 kbar) is related to recrystallization in narrow ductile shear zones and secondary growth on M2 minerals. On the basis of composition, fluid inclusions in cordierite, quartz and garnet can be divided into three major types: (1) CO2 inclusions; (2) mixed CO2–N2 inclusions; (3) N2 inclusions. Fluid chronology and mineral assemblages suggest that the earliest inclusions consist of pure CO2 and were trapped at the M1 contact metamorphic episode. A carbonic fluid was also present during the intermediate-P granulite facies M2 metamorphism. The CO2-rich inclusions in M2 garnet can be divided into two generations, an early lower-density and a late higher-density, with isochores crosscutting the P-T box of M2 and M3, respectively. The nitrogen-rich fluids were introduced at a late stage in the fluid evolution during the high-P M3 event. The mixed CO2–N2 inclusions, with density characteristics compatible with M3 conditions, are probably produced from intersection between pre-existing pure CO2 inclusions and N2 fluids introduced during M3. The fluid inclusion data agree with the P-T evolution established from mineral assemblages and mineral chemistry.  相似文献   

19.
Fluid inclusions in mineralized graphite-sillimanite-mica schist from the Rampura-Agucha Pb-Zn-(Ag) deposit, Rajasthan, northwest India, have been investigated by microthermometry and Raman microspectrometry. Three different main types of fluid inclusions in quartz can be distinguished: (1) gaseous (CO2, partially mixed with CH4-N2), (2) low salinity aqueous inclusions (0–8 eq. wt% NaCl) and (3) high salinity aqueous inclusions (NaCl ± MgCl2-CaCl2). Low density CO2-rich and low salinity H2O inclusions are contemporaneous and occur, together with CH4-N2 inclusions, in close association with sulfide mineral inclusions. This indicates immiscibility between the gaseous and aqueous phase and participation of these fluids during the deposition or remobilization of the ore, which occurred over a wide P (1220 to 200 bar) and T (450 to 250 °C). Raman spectra of graphite indicate upper greenschist-facies metamorphic conditions, although host rocks have been metamorphosed at upper amphibolite-facies metamorphic conditions. This indicates that graphite re-equilibrated with the CO2-rich phase during retrograde metamorphism.  相似文献   

20.
 Petrologic and oxygen isotope data indicate that water-rich fluids infiltrated metasedimentary rocks of the Waterville and Sangerville (formally Vassalboro) Formations, south-central Maine, during peak metamorphism, and depleted Sangerville rocks in alkalis but not equivalent Waterville rocks. Fluid inclusion data from two outcrops, ∼1 km apart, one of the Waterville and the other of the Sangerville Formations, suggest a cause for the geochemical difference between the two units. Postulated peak metamorphic inclusions, the texturally earliest of aqueous inclusions in the metasediments, approximate the water-rich compositions of peak fluids predicted by mineral-fluid equilibria, and have average salinity in the Sangerville Formation ∼ three times that of equivalent Waterville inclusions. The higher salinity in the Sangerville fluids could explain the greater alkali depletion in these rocks. Probable pre-peak or prograde inclusions are preserved in metasediments as the texturally earliest carbonic inclusions which contain CO2, CH4, N2±H2O, as determined by microthermometry and Raman spectrometry. They may have formed by breakdown of organic matter. Probable retrograde inclusions occur as texturally late aqueous inclusions in healed fractures with salinity ranges indistinguishable between the two formations. Synmetamorphic granitic dikes present in the two outcrops were ruled out as a source for fluids in metasediments because composition and density ranges of inclusions in dikes and metasediments are fundamentally different, and because there is no correlation between the abundance or composition of inclusions in a sample and proximity to dikes. Isochores for many of the inclusions in both metasediments and dikes are not consistent with the inferred P−T conditions of their trapping, but intersect at ∼300° to 400° C and 1 to 2 kbar. The intersections probably resulted because inclusion densities continued to equilibrate during uplift and cooling until quartz became rigid. The present densities are those at the last equilibration, not the time of trapping. In contrast, the clear distinctions in inclusion compositions between dikes and between dike and country rock show that the original compositional differences generally have been preserved. Received: 4 February 1994 / Accepted: 22 June 1994  相似文献   

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