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1.
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 δ18OSMOW 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 δ13CPBD 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 (XSO4= 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 XSO4 and increase in XCO3 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.  相似文献   

2.
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   

3.
A combined study of major and trace elements, fluid inclusions and oxygen isotopes has been carried out on garnet pyroxenite from the Raobazhai complex in the North Dabie Terrane (NDT). Well‐preserved compositional zoning with Na decreasing and Ca and Mg increasing from the core to rim of pyroxene in the garnet pyroxenite indicates eclogite facies metamorphism at the peak metamorphic stage and subsequent granulite facies metamorphism during uplift. A P–T path with substantial heating (from c. 750 to 900 °C) after the maximum pressure reveals a different uplift history compared with most other eclogites in the South Dabie Terrane (SDT). Fluid inclusion data can be correlated with the metamorphic grade: the fluid regime during the peak metamorphism (eclogite facies) was dominated by N2‐bearing NaCl‐rich solutions, whereas it changed into CO2‐dominated fluids during the granulite facies retrograde metamorphism. At a late retrograde metamorphic stage, probably after amphibolite facies metamorphism, some external low‐salinity fluids were involved. In situ UV‐laser oxygen isotope analysis was undertaken on a 7 mm garnet, and impure pyroxene, amphibole and plagioclase. The nearly homogeneous oxygen isotopic composition (δ18OVSMOW = c. 6.7‰) in the garnet porphyroblast indicates closed fluid system conditions during garnet growth. However, isotopic fractionations between retrograde phases (amphibole and plagioclase) and garnet show an oxygen isotopic disequilibrium, indicating retrograde fluid–rock interactions. Unusual MORB‐like rare earth element (REE) patterns for whole rock of the garnet pyroxenite contrast with most ultra‐high‐pressure (UHP) eclogites in the Dabie‐Sulu area. However, the age‐corrected initial εNd(t) is ? 2.9, which indicates that the protolith of the garnet pyroxenite was derived from an enriched mantle rather than from a MORB source. Combined with the present data of oxygen isotopic compositions and the characteristic N2 content in the fluid inclusions, we suggest that the protolith of the garnet pyroxenite from Raobazhai formed in an enriched mantle fragment, which has been exposed to the surface prior to the Triassic metamorphism.  相似文献   

4.
Granulite xenoliths within alkali olivine basalts of the Pali-Aike volcanic field, southern Chile, contain the mineral assemblage orthopyroxene + clinopyroxene + plagioclase + olivine + green spinel. These granulites are thought to be accidental inclusions of the lower crust incorporated in the mantle-derived basalt during its rise to the surface. Symplectic intergrowths of pyroxene and spinel developed between olivine and plagioclase imply that the reaction olivine+plagioclase = Al-orthopyroxene + Al-clinopyroxene + spinel (1) occurred during subsolidus cooling and recrystallization of a gabbroic protolith of the granulites.Examination of fluid inclusions in the granulites indicates the ubiquitous presence of an essentially pure CO2 fluid phase. Inclusions of three different parageneses have been recognized: Type I inclusions occur along exsolution lamellae in clinopyroxene and are thought to represent precipitation of structurally-bound C or CO2 during cooling of the gabbro. These are considered the most primary inclusions present. Type II inclusions occur as evenly distributed clusters not associated with any fractures. These inclusions probably represent entrapment of a free fluid phase during recrystallization of the host grains. IIa inclusions are found in granoblastic grains and have densities of 0.68–0.88 g/cm3. Higher density (=0.90–1.02 g/cm3) IIb inclusions occur only in symplectite phases. Secondary Type III CO2+glass inclusions with =0.47–0.78 g/cm3 occur along healed fractures where basalt has penetrated the xenoliths. Type III inclusions appear related to exsolution of CO2 from the host basalt during its ascent to the surface. These data suggest that CO2 is an important constituent of the lower crust under conditions of granulite facies metamorphism, indicated by Type I and II fluid inclusions, and of the mantle, as indicated by Type III inclusions.Correlation of fluid inclusion densities with P-T conditions calculated from both two-pyroxene geothermometry and reation (1) indicate emplacement of a gabbroic pluton at 1,200–1,300° C, 4–6 kb; cooling was accompanied by a slight increase in pressure due to crustal thickening, and symplectite formation occurred at 850±35° C, 5–7 kb. Capture of the xenoliths by the basalt resulted in heating of the granulites, and CO2 from the basalt was continuously entrapped by the xenoliths over the range 1,000–1,200° C, 4–6 kb. Examination of fluid inclusions of different generations can thus be used in conjunction with other petrologic data to place tight constraints on the specific P-T path followed by the granulite suite, in addition to indicating the nature of the fluid phase present at depth.  相似文献   

5.
CO2–CH4 fluid inclusions are present in anatectic layer-parallel leucosomes from graphite-bearing metasedimentary rocks in the Skagit migmatite complex, North Cascades, Washington. Petrological evidence and additional fluid inclusion observations indicate, however, that the Skagit Gneiss was infiltrated by a water-rich fluid during high-temperature metamorphism and migmatization. CO2-rich fluid inclusions have not been observed in Skagit metasedimentary mesosomes or melanosomes, meta-igneous migmatites, or unmigmatized rocks, and are absent from subsolidus leucosomes in metasedimentary migmatites. The observation that CO2-rich inclusions are present only in leucosomes interpreted to be anatectic based on independent mineralogical and chemical criteria suggests that their formation is related to migmatization by partial melting. Although some post-entrapment modification of fluid inclusion composition may have occurred during decompression and deformation, the generation of the CO2-rich fluid is attributed to water-saturated partial melting of graphitic metasedimentary rocks by a reaction such as biotite + plagioclase + quartz + graphite ± Al2SiO5+ water-rich fluid = garnet + melt + CO2–CH4. The presence of CO2-rich fluid inclusions in leucosomes may therefore be an indication that these leucosomes formed by anatexis. Based on the inferences that (1) an influx of fluid triggered partial melting, and (2) some episodes of fluid inclusion trapping are related to migmatization by anatexis, it is concluded that a free fluid was present at some time during high-temperature metamorphism. The infiltrating fluid was a water-rich fluid that may have been derived from nearby crystallizing plutons. Because partial melting took place at pressures of at least 5 kbar, abundant free fluid may have been present in the crust during orogenesis at depths of at least 15 km.  相似文献   

6.
Following ultrahigh temperature granulite metamorphism at ∼1 Ga, the Eastern Ghats Province of India was intruded by the Koraput Alkaline Complex, and was subsequently re-metamorphosed in the granulite facies in the mid-Neoproterozoic time. Fluid inclusion studies were conducted on silica undersaturated alkali gabbro and syenites in the complex, and a pre-metamorphic pegmatitic granite dyke that intrudes it. High density (1.02–1.05 g/cc), pseudo-secondary pure CO2 inclusions are restricted to metamorphic garnets within the gabbro and quartz within the granite, whereas moderate (∼0.92–0.95 g/cc) and low density (∼0.75 g/cc) secondary inclusions occur in garnet, magmatic clinopyroxene, plagioclase, hornblende and quartz. The isochores calculated for high density pseudo-secondary inclusions pass very close to the peak metamorphic window (∼8 kbar, 750 °C), and are interpreted to represent the fluid present during peak metamorphism that was entrapped by the growing garnet. Microscopic round inclusions of undigested, relict calcite in garnet suggest that the CO2 present during metamorphism of the complex was internally derived through carbonate breakdown. Pure to low salinity (0.00–10.1 wt% NaCl equivalent) aqueous intra-/intergranular inclusions showing unimodal normal distribution of final ice-melting temperature (Tm) and temperature of homogenization (Th) are present only in quartz within the granite. These represent re-equilibrated inclusions within the quartz host that were entrapped at the metamorphic peak. Rare, chemically precipitated graphite along the walls of carbonic inclusions is interpreted as a post-entrapment reaction product formed during decompression. The fluid inclusion evidence is consistent with rapid exhumation of a thickened lower crust following the mid-Neoproterozoic granulite facies metamorphic event. The study suggests that mantle CO2, transported by alkaline magma into the crust, was locked up within carbonates and released during granulite metamorphism.  相似文献   

7.
The Archean Shawmere anorthosite lies within the granulite facies portion of the Kapuskasing Structural Zone (KSZ), Ontario, and is crosscut by numerous linear alteration veins containing calcite + quartz ± dolomite ± zoisite ± clinozoisite ± margarite ±paragonite ± chlorite. These veins roughly parallel the trend of the Ivanhoe Lake Cataclastic Zone. Equilibria involving clinozoisite + margarite + quartz ± calcite ± plagioclase show that the vein minerals were stable at T < 600 °C, XCO2 < 0.4 at P ≈ 6 kbar. The stabilities of margarite and paragonite in equilibrium with quartz are also consistent with T < 600 °C and XCO2 < 0.4 at 6 kbar. Additional assemblages consisting of calcite + clinochlore + quartz + talc + margarite indicate T < 500 °C with XCO2 > 0.9. Thus, vein formation, while clearly retrograde, spanned a range of temperatures, and fluid compositions evolved from H2O-rich to CO2-rich. The calcite in the retrograde veins has δ18O values that range from 8.4 to 11.2‰ (average = +9.7 ± 0.9‰) and δ13C values that range from −3.9 to −1.6‰ (average = −3.1 ± 0.6‰). These values indicate that the fluids from which calcite precipitated underwent extensive exchange with the anorthosite and other crustal lithologies. The fluids may have been initially derived either from devolatilization of metamorphic rocks or crystallization of igneous rocks in the adjacent Abitibi subprovince. Vein quartz contains CO2-rich fluid inclusions (final melting T = −57.0 to −58.7 °C) that range in size from 5 to 17 μm. Measured homogenization temperatures (T h) range from −44.0 to 14.5 °C, however for most inclusions (46 of S1), T h = −44.0 to −21.1 °C (ρCO2 ≈ 1.13 to 1.05 g/cm3). At 400 to 600 °C, these densities correspond to pressures of 3.5 to 7 kbar, which is the best estimate of pressures of vein formation. It has been argued that some high density CO2-rich fluid inclusions found in the KSZ were formed during peak metamorphism and thus document the presence of a CO2-rich fluid during peak granulite facies metamorphism (Rudnick et al. 1984). The association of high density CO2-rich fluid inclusions with clearly retrograde veins documents the formation of similar composition and density inclusions after the peak of metamorphism. Thus, the coincidence of entrapment pressures calculated from fluid inclusion density measurements with peak metamorphic pressures alone should not be considered strong evidence for peak metamorphic inclusion entrapment. All fluid inclusion results are consistent with an initially semi-isobaric retrograde P–T path. Received: 2 April 1996 / Accepted: 15 November 1996  相似文献   

8.
Abstract The prograde metamorphism of eclogites is typically obscured by chemical equilibration at peak conditions and by partial requilibration during retrograde metamorphism. Eclogites from the Eastern Blue Ridge of North Carolina retain evidence of their prograde path in the form of inclusions preserved in garnet. These eclogites, from the vicinity of Bakersville, North Carolina, USA are primarily comprised of garnet–clinopyroxene–rutile–hornblende–plagioclase–quartz. Quartz, clinopyroxene, hornblende, rutile, epidote, titanite and biotite are found as inclusions in garnet cores. Included hornblende and clinopyroxene are chemically distinct from their matrix counterparts. Thermobarometry of inclusion sets from different garnets record different conditions. Inclusions of clinozoisite, titanite, rutile and quartz (clinozoisite + titanite = grossular + rutile + quartz + H2O) yield pressures (6–10 kbar, 400–600 °C and 8–12 kbar 450–680 °C) at or below the minimum peak conditions from matrix phases (10–13 kbar at 600–800 °C). Inclusions of hornblende, biotite and quartz give higher pressures (13–16 kbar and 630–660 °C). Early matrix pyroxene is partially or fully broken down to a diopside–plagioclase symplectite, and both garnet and pyroxene are rimmed with plagioclase and hornblende. Hypersthene is found as a minor phase in some diopside + plagioclase symplectites, which suggests retrogression through the granulite facies. Two‐pyroxene thermometry of this assemblage gives a temperature of c. 750 °C. Pairing the most Mg‐rich garnet composition with the assemblage plagioclase–diopside–hypersthene–quartz gives pressures of 14–16 kbar at this temperature. The hornblende–plagioclase–garnet rim–quartz assemblage yields 9–12 kbar and 500–550 °C. The combined P–T data show a clockwise loop from the amphibolite to eclogite to granulite facies, all of which are overprinted by a texturally late amphibolite facies assemblage. This loop provides an unusually complete P–T history of an eclogite, recording events during and following subduction and continental collision in the early Palaeozoic.  相似文献   

9.
Granulite facies anorthosites on Holsenøy Island in the Bergen Arcs region of western Norway are transected by shear zones 0.1–100 m wide characterized by eclogite facies assemblages. Eclogite formation is related to influx of fluid along the shears at temperatures of c. 700d?C and pressures in excess of 1.7 GPa. Combined carbon and nitrogen stable isotope, 40Ar/36Ar, trace-element and petrological data have been used to determine the nature and distribution of fluids across the anorthosite-eclogite transition. A metre-wide drilled section traverses the eclogitic centre of the shear into undeformed granulite facies garnet-clinopyroxene anorthosite. Clinozoisite occurs along grain boundaries and microcracks in undeformed anorthosite up to 1 m from the centre of the shear and clinozoisite increases in abundance as the edge of the shear zone is approached. The eclogite-granulite transition, marked by the appearance of sodic pyroxene and loss of albite, occurs within the most highly sheared section of the traverse. The jadeite-in reaction coincides with increased paragonite activity in mica. The separation between paragonite and clinozoisite reaction fronts can be semiquantitatively modelled assuming advective fluid flow perpendicular to the shear zone. The inner section of the traverse (0.25 m wide) is marked by retrogressive replacement of omphacite by plagioclase + paragonite accompanied by veins of quartz-phengite-plagioclase. C-N-Ar characteristics of fluid inclusions in garnet show that fluids associated with precursor granulite, eclogite and retrogressed eclogite are isotopically distinct. The granulite-eclogite transition coincides with a marked change in CO2 abundance and δ13C (<36ppm, δ13C=-2% in the granulite; <180 ppm, δ13C=-10% in the eclogite). The distribution of Ar indicates mixing between influxed fluid (40Ar/36Ar > 25 times 103) and pre-existing Ar in the granulite (40Ar/36Ar < 8 times 103). δ15N values decrease from +6% in the anorthosite to +3% within the eclogite shear. The central zone of retrogressed eclogite post-dates shearing and is characterised by substantial enrichment of Si, K, Ba and Rb. Fluids are CO2-rich (δ13C ～ -5%) with variable N2 and Ar abundances and isotopic compositions. Both Ar and H2O have penetrated the underformed granulite fabric more than 0.5m beyond the granulite/eclogite transition during eclogite formation. Argon isotopes show a mixing profile consistent with diffusion through an interconnecting H2O-rich fluid network. In contrast, a carbon-isotope front coincides with the deformation boundary layer, indicating that the underformed anorthosite was impervious to CO2-rich fluids. This is consistent with the high dihedral angle of carbonic fluids, and may be interpreted in terms of evolving fluid compositions within the shear zone.  相似文献   

10.
Sapphirine has been found in two types of magnesian, metabasic lenses from tectonite zones within the Central Gneiss Belt of the south-west Grenville Province, Canada. The first type (association I) comes from a lenticular mafic lens within highly tectonized anorthosite, the second type (association II) comes from meta-eclogitic pods with foliated amphibolite rims. In each case the sapphirine-bearing assemblages record a wealth of reaction textures. The primary mineralogy in association II is represented by high alumina clinopyroxene, garnet and kyanite ± plagioclase and records pressures of around 14-16 kbar; in association I the primary mineralogy is represented by plagioclase, two pyroxenes and possibly olivine but here the equilibrium pressure is unknown.
The host gneisses equilibrated at approximately 8 to 10 kbar and 700-750°C by continuous cation exchange reactions during and after the culmination of the Grenvillian orogeny at 1.16-1.0 Ga. It is unlikely that the higher pressures recorded in the meta-eclogitic pods represent an earlier high-pressure metamorphism as the pods are restricted to shear zones. A tectonic mode of emplacement into a crust undergoing granulite facies metamorphism is more likely. Sapphirine formed by discontinuous decompression reactions; in association II this involved a reaction between garnet and kyanite and resulted in the formation of magnesian granulite facies assemblages. At the same time primary clinopyroxene became much less aluminous by evolving plagioclase. Pressures and temperatures from coexisting phases, that are believed to have equilibrated at the same time as sapphirine formation, are estimated as 11 to 12 kbar and 750°C. These probably represent the peak conditions for granulite facies metamorphism in the south-west Grenville Province.  相似文献   

11.
The enderbites from Tromøy in the central, granulite facies part of the Proterozoic Bamble sector of southern Norway contain dominantly CO2 and N2 fluid inclusions. CO2 from fluid inclusions in quartz segregations in enderbites was extracted by mechanical (crushing) and thermal decrepitation and the δ13C measured. Measurement was also made on samples washed in 10% HCl, oxidized with CuO at high temperatures, and step-wise extracted with progressive heating. Results between the different techniques are systematic. The main results show δ13C of -4.5±1.5% for crushing and -7±2% for thermal decrepitation. δ13C is about constant for CO2 extracted at different temperatures and points to a homogeneous isotopic composition. Due to the presence of carbonate particles and/or induced contaminations for the extraction by thermal decrepitation, the results for the crushing experiments are assumed the most reliable for fluid-inclusion CO2. Very low values of δ13C have not been found in enderbite samples and δ13C combined with δ18O of the host quartzes (8-11%) indicates juvenile values. In addition, the fluid inclusions were examined by microthermometry and Raman analysis and host quartz by acoustic emission and cathodoluminescence. CO2 fluid inclusions have varying densities with a frequency maximum of 0.92 g cm-3 and generally do not concur with trapping densities at granulite conditions. Textures show that CO2 must have been trapped in fluid inclusions in one early event, but transformed to different extents during late isothermal uplift without important fractionation of isotope compositions. The present data support a model of intrusion and crystallization of a CO2-rich enderbitic magma at granuiite conditions.  相似文献   

12.
The Eastern Ghats Belt (EGB), characterised by pervasive Grenvillian granulite facies metamorphism, is the host to several 950–1000 Ma old massif-type anorthosite complexes. The present work describes one such complex near Udayagiri from the northern margin of the EGB, reported for the first time as “Udayagiri anorthosite complex” (UAC). The ‘massif type’ UAC comprises mainly of anorthosite, leuconorite-olivine leuconorite and norite in the decreasing order of areal extent. Mineralogically, these rocks dominantly consist of cumulates of moderately calcic plagioclase (～An50–60), moderately magnesian intercumulus olivine (XMg: ～0.6) and orthopyroxene (XMg: 0.47 to 0.70). Metamorphic garnet (Alm: ～50 mol%) is also common in these rocks. Anorthosite and leuconorite of the UAC exhibit a moderate ‘+ve’ Eu anomaly. Norite occurs locally as schlierens and is relatively rich in Fe, P, Rb, Sr, Th, Nb, Ta, Y and REE which could be a residual melt product. These rocks exhibit both relict magmatic mineralogy and textures with a metamorphic impress manifested by the development of multilayered corona involving olivine, orthopyroxene, garnet, phlogopite, ilmenite and plagioclase during cooling of the pluton. The corona development is a result of combination of significant magmatic and metamorphic reactions which have the potential to provide important clues for deciphering the magmatic and metamorphic evolution of such plutons in ambient granulite facies conditions.  相似文献   

13.
Oscillatory zoning in low δ18O skarn garnet from the Willsboro wollastonite deposit, NE Adirondack Mts, NY, USA, preserves a record of the temporal evolution of mixing hydrothermal fluids from different sources. Garnet with oscillatory zoning are large (1–3 cm diameter) euhedral crystals that grew in formerly fluid filled cavities. They contain millimetre‐scale oscillatory zoning of varying grossular–andradite composition (XAdr = 0.13–0.36). The δ18O values of the garnet zones vary from 0.80 to 6.26‰ VSMOW and correlate with XAdr. The shape, pattern and number of garnet zones varies from crystal to crystal, as does the magnitude of the correlated chemistry changes, suggesting fluid system variability, temporal and/or spatial, over the time of garnet growth. The zones of correlated Fe content and δ18O indicate that a high Fe3+/Al, high δ18O fluid mixed with a lower Fe3+/Al and δ18O fluid. The high δ18O, Fe enriched fluids were likely magmatic fluids expelled from crystallizing anorthosite. The low δ18O fluids were meteoric in origin. These are the first skarn garnet with oscillatory zoning reported from granulite facies rocks. Geochronologic, stable isotope, petrologic and field evidence indicates that the Adirondacks are a polymetamorphic terrane, where localized contact metamorphism around shallowly intruded anorthosite was followed by a regional granulite facies overprint. The growth of these garnet in equilibrium with meteoric and magmatic fluids indicates an origin in the shallow contact aureole of the anorthosite prior to regional metamorphism. The zoning was preserved due to the slow diffusion of oxygen and cations in the large garnet and protection from deformation and recrystallization in zones of low strain in thick, rigid, garnetite layers. The garnet provide new information about the hydrothermal system adjacent to the shallowly intruded massif anorthosite that predates regional metamorphism in this geologically complex, polymetamorphic terrane.  相似文献   

14.
Fluid inclusions in quartz grains from five samples of high-grade rocks (two paragneisses, an amphibolite, a mafic gneiss and a tonalite dike) from the 2.7 Ga Kapuskasing structural zone (KSZ), Ontario, were examined with petrographic, microthermometric and laser Raman techniques. Three types of fluid inclusions were observed: CO2-rich, H2O-rich and mixed CO2-H2O. CO2-rich fluid inclusions are pseudosecondary or secondary in nature and are generally pure CO2; a few contain varying amounts of CH4·H2O-rich fluid inclusions are secondary in nature, contain variable amounts of dissolved salts, and generally contain daughter crystals. Mixed CO2-H2O fluid inclusions occur where trails of H2O-rich inclusions intersect trails of CO2-rich inclusions. Isochores for high density (p=1.03 g/cm3) pseudosecondary, pure CO2 inclusions intersect the lower pressure portion of the estimated P-T field for high-grade metamorphism, implying that pure CO2 was the peak metamorphic fluid. The variable CH4 content of CO2 inclusions within graphite-bearing samples suggests that CH4 was introduced locally after the formation of the CO2 inclusions; however the origin of the CH4 remains problematic. An aqueous fluid clearly penetrated the gneisses after the peak metamorphism (during uplift/erosion), forming secondary inclusions and contributing to the minor retrogressive hydration observed in these rocks. The presence of the pseudosecondary, high-density CO2 inclusions in quartz crystals in the KSZ rocks constrains the uplift/ erosion path for the KSZ to one of simultaneous decrease in pressure and temperature.  相似文献   

15.
http://www.sciencedirect.com/science/article/pii/S1674987112000564   总被引：10，自引：0，他引：10  
High-pressure（HP） granulites widely occur as enclaves within tonalite-trondhjemitegranodiorite （TTG） gneisses of the Early Precambrian metamorphic basement in the Shandong Peninsula, southeast part of the North China Craton（NCC）.Based on cathodoluminescence（CL）,laser Raman spectroscopy and in-situ U-Pb dating,we characterize the zircons from the HP granulites and group them into three main types:inherited（magmatic） zircon,HP metamorphic zircon and retrograde zircon.The inherited zircons with clear or weakly defined magmatic zoning contain inclusions of apatites,and 207Pb/206Pb ages of 2915—2890 Ma and 2763—2510 Ma,correlating with two magmatic events in the Archaean basement. The homogeneous HP metamorphic zircons contain index minerals of high-pressure metamorphism including garnet,clinopyroxene.plagioclase,quartz,rutile and apatite,and yield 207Pb/206Pb ages between 1900 and 1850 Ma,marking the timing of peak HP granulite fades metamorphism.The retrograde zircons contain inclusions of orthopyroxene.plagioclase.quartz,apatite and amphibole.and yield the youngest 207Pb/206Pb ages of 1840—1820 Ma among the three groups,which we correlate to the medium to low-pressure granulite fades retrograde metamorphism.The data presented in this study suggest subduction of Meso- and Neoarchean magmatic protoliths to lower crust depths where they were subjected to HP granulite facies metamorphism during Palaeoproterozoic（1900—1850 Ma）.Subsequently, the HP granulites were exhumated to upper crust levels,and were overprinted by medium to low-pressure granulite and amphibolite facies retrograde event at ca.1840—820 Ma.  相似文献   

16.
The Lac St-Jean anorthosite massif underlies an area of over 20,000 km2 and has been emplaced into migmatitic gneisses of the central granulite terrain of the Grenville Province of the Canadian shield. Field data and petrography in an area straddling the anorthosite-gneiss contact, close to Chicoutimi (Quebec) permits an outline of its tecto-magmatic evolution. Depositional magmatic textures in the massif reveals that it crystallized from a magma in a relatively calm tectonic environment. The absence of fusion in pelitic gneisses at the contact proves that the crystallization did not take place at the level presently exposed. The parallelism of subvertical foliation in the enveloping gneisses and the anorthosite indicates that both were deformed together. It is suggested that the deformation results from a diapiric ascent of the anorthosite massif after its consolidation at depth. The depth of consolidation of the anorthosite is estimated at 25–30 km from subsolidus reaction between plagioclase and olivine. The diapiric ascent is further substantiated by the fact that three sets of mafic dykes of different ages, intrusive into the anorthosite, have a mineralogy which indicates successively decreasing P, T conditions of emplacement from granulite fades to amphibolite facies. An evolution of the basement gneisses and the anorthosite is proposed as a working hypothesis; it relies on the fact that metabasite dyke swarms in the basement gneisses represent a period of major crustal extension and could be used as a stratigraphic subdivision of the Grenville Province.  相似文献   

17.
Abstract Fluid evolution paths in the COHN system can be calculated for metamorphic rocks if there are relevant data regarding the mineral assemblages present, and regarding the oxidation and nitrodation states throughout the entire P-T loop. The compositions of fluid inclusions observed in granulitic rocks from Rogaland (south-west Norway) are compared with theoretical fluid compositions and molar volumes. The fluid parameters are calculated using a P-T path based on mineral assemblages, which are represented by rocks within the pigeonite-in isograd and by rocks near the orthopyroxene-in isograd surrounding an intrusive anorthosite massif. The oxygen and nitrogen fugacities are assumed to be buffered by the coexisting Fe-Ti oxides and Cr-carlsbergite, respectively. Many features of the natural fluid inclusions, including (1) the occurrence of CO2-N2-rich graphite-absent fluid inclusions near peak M2 metamorphic conditions (927° C and 400 MPa), (2) the non-existence of intermediate ternary CO2-CH4-N2 compositions and (3) the low-molar-volume CO2-rich fluid inclusions (36–42 cm3 mol?1), are reproduced in the calculated fluid system. The observed CO2-CH4-rich inclusions with minor N2 (5 mol%) should also include a large proportion of H2O according to the calculations. The absence of H2O from these natural high-molar-volume CO2-CH4-rich inclusions and the occurrence of natural CH4-N2-rich inclusions are both assumed to result from preferential leakage of H2O. This has been previously experimentally demonstrated for H2O-CO2-rich fluid inclusions, and has also been theoretically predicted. Fluid-deficient conditions may explain the relatively high molar volumes, but cannot be used to explain the occurrence of CH4-N2-rich inclusions and the absence of H2O.  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.
Calc-silicate boudins from the Rauer Group, East Antarctica, were metamorphosed under granulite facies conditions during late Proterozoic (ca. 1,000 Ma) M3 metamorphism. Boudin cores contain low to moderate aCO 2 assemblages including wollastonite, grossularandradite (grandite) garnet, clinopyroxene, scapolite, plagioclase, quartz±calcite. Petrological and stable isotopic evidence suggests that these core assemblages resulted from pre-peak M3 infiltration of water-rich fluids; there is no evidence for a pervasive fluid phase under peak M3 conditions. The boudins are separated from the surrounding Fe-rich pelites and semi-pelites by a series of concentric, high-variance reaction zones developed under peak M3 conditions. Variations in mineral assemblage, mineral composition and whole rock composition across these zones suggest that they formed by diffusional masstransfer, controlled principally by a chemical potential gradient in Ca across the original calc-silicate-paragneiss lithological boundary. As a consequence of the nearcomplete decarbonation of the calc-silicatesbefore the M3 peak, development of the diffusion-controlled reaction zones did not liberate significant CO2 during granulite facies metamorphism. Similar calcite-poor, low aCO 2 calc-silicate horizons in other granulite facies terrains are unlikely to have been important local fluid sources during deep crustal metamorphism.  相似文献   

20.
Summary ?Fluid inclusions from two Mesoproterozoic, metamorphosed layered intrusive complexes, Niquelandia and Barro Alto, Goiás State, Brazil record multiple fluid influx events from the magmatic to granulitic and retrograde metamorphic stages. 1. The oldest inclusions contain high density CO2 ± N2 ± CH4 and are found as primaries in plagioclase and orthopyroxene in mafic granulite with homogenization temperatures between − 48 and − 28 °C. These inclusions may correspond to the early, magmatic stage. This type was found in samples from both the Niquelandia and the Barro Alto complexes. 2. Intragranular, relatively high density CO2 + N2 inclusions (Th between − 33 and − 26 °C) together with decrepitated and reequilibrated N2 inclusions (Th between − 160 and − 151 °C) in the rock-forming minerals can be associated with the granulite facies metamorphism. Such inclusions were found only in the Barro Alto complex. 3. Transgranular, high density, CO2–N2 inclusions (93% CO2 and 7% N2, according to Raman analysis, with Th between − 66.6 and − 50.4 °C) as well as the low density, secondary CO2 ± N2 ± CH4 inclusions (Th between − 13.0 and + 18.7 °C) and the H2O–NaCl–CaCl2 hypersaline inclusions (with halite dissolution temperature between 132 and 354 °C, and Th between 212 and 490 °C) are attributed to different fluid influx events during the retrograde metamorphism. This inclusion type can be found both in the Niquelandia and in the Barro Alto complexes. The fluid inclusion textures and compositions show several stages of fluid evolution. The fluid inclusion measurements and the geothermobarometric data indicate an anticlockwise P-T path for both the Barro Alto and the Niquelandia complexes. Received October 16, 2000; revised version accepted November 20, 2001  相似文献   

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