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1.
Structurally controlled fluid flow during contact metamorphism in the Ritter Range pendant, California, USA 总被引:1,自引:0,他引:1
John M. Ferry Sorena S. Sorensen Douglas Rumble III. 《Contributions to Mineralogy and Petrology》1998,130(3-4):358-378
The mineralogy and O-isotope geochemistry of siliceous limestones from the Ritter Range pendant constrain the geometry and
amount of fluid flow during contact metamorphism associated with emplacement of a pluton of the Sierra Nevada Batholith. Wollastonite
(Wo) replaces calcite (Cal) + quartz (Qtz) on a layer-by-layer basis in homoclinal beds that strike NW and dip almost vertically.
At the peak of metamorphism (P≈ 1500 bars, T≈ 600 °C) fluid in equilibrium with Cal, Qtz, and Wo has composition XCO2=0.28, requiring that the Wo-forming reaction was driven by infiltration of reactive H2O-rich fluid. The spatial distribution of Wo and Cal + Qtz records that peak metamorphic fluid flow was layer-parallel, upward.
Bounds on the prograde time-integrated fluid flux associated with formation of Wo are set by: (1) the overlap in O-isotope
composition between Wo-bearing and Wo-free rocks (>245 mol fluid/cm2 rock); (2) the amount of fluid that would drive the Wo-reaction front upward to the present level of exposure from a point
at depth where Cal, Qtz, and Wo would be in equilibrium with pure CO2 (<1615 mol/cm2). Back-reaction of Wo to Cal + Qtz records an additional time-integrated retrograde fluid flux of ≈ 200–1000 mol/cm2. The direction and amount of flow inferred from mineralogical and isotopic data agree with the results of the hydrologic
model for metamorphic fluid flow in the area of Hanson et al. (1993). Fingers of Wo-bearing rock that extend farthest from
the fluid source along contacts between limestone and more siliceous rocks point to strong control of flow geometry at the
0.1–100 m scale exerted by premetamorphic structures. Studies that neglect structural control at this scale may fail to predict
correctly fundamental aspects of contact metamorphic fluid flow.
Received: 27 January 1997 / Accepted: 2 October 1997 相似文献
2.
Fluid infiltration and regional metamorphism of the Waits River Formation, north-east Vermont, USA 总被引:2,自引:0,他引:2
Abstract The Siluro-Devonian Waits River Formation of north-east Vermont was deformed, intruded by plutons and regionally metamorphosed during the Devonian Acadian Orogeny. Five metamorphic zones were mapped based on the mineralogy of carbonate rocks. From low to high grade, these are: (1) ankerite-albite, (2) ankerite-oligoclase, (3) biotite, (4) amphibole and (5) diopside zones. Pressure was near 4.5kbar and temperature varied from c. 450° C in the ankerite-albite zone to c. 525° C in the diopside zone. Fluid composition for all metamorphic zones was estimated from mineral equilibria. Average calculated χco2[= CO2/(CO2+ H2O)] of fluid in equilibrium with the marls increases with increasing grade from 0.05 in the ankerite-oligoclase zone, to 0.25 in the biotite zone and to 0.44 in the amphibole zone. In the diopside zone, χCO2 decreases to 0.06. Model prograde metamorphic reactions were derived from measured modes, mineral chemistry, and whole-rock chemistry. Prograde reactions involved decarbonation with an evolved volatile mixture of χCO2 > 0.50. The χCO2 of fluid in equilibrium with rocks from all zones, however, was generally <0.40. This difference attests to the infiltration of a reactive H2O-rich fluid during metamorphism. Metamorphosed carbonate rocks from the formation suggests that both heat flow and pervasive infiltration of a reactive H2O-rich fluid drove mineral reactions during metamorphism. Average time-integrated volume fluxes (cm3 fluid/cm2 rock), calculated from the standard equation for coupled fluid flow and reaction in porous media, are (1) ankerite-oligoclase zone: c. 1 × 104; (2) biotite zone: c. 3 × 104; (3) amphibole zone: c. 10 × 104; and diopside zone: c. 60 × 104. The increase in calculated flux with increasing grade is at least in part the result of internal production of volatiles from prograde reactions in pelitic schists and metacarbonate rocks within the Waits River Formation. The mapped pattern of time-integrated fluxes indicates that the Strafford-Willoughby Arch and the numerous igneous intrusions in the field area focused fluid flow during metamorphism. Many rock specimens in the diopside zone experienced extreme alkali depletion and also record low χCO2. Metamorphic fluids in equilibrium with diopside zone rocks may therefore represent a mixture of acid, H2O-rich fluids given off by the crystallizing magmas, and CO2-H2O fluids produced by devolatilization reactions in the host marls. Higher fluxes and different fluid compositions recorded near the plutons suggest that pluton-driven hydrothermal cells were local highs in the larger regional metamorphic hydrothermal system. 相似文献
3.
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
4.
John M. Ferry 《Contributions to Mineralogy and Petrology》1996,124(3-4):235-254
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 相似文献
5.
Jay J. Ague 《Contributions to Mineralogy and Petrology》1998,132(2):180-197
Simple one-dimensional numerical models are presented for coupled advection-hydrodynamic dispersion and kinetically controlled
oxidatioin-reduction reactions in graphite-free porous media containing magnetite coexisting with silicate assemblages. Fluid-solid
interactions involving either OH (O2-H2O-H2) or COH (O2-H2O-H2-CO2-CO-CH4) fluids are considered at ∼500 ∘C and 5 kbar. The major implications of the modeling are as follows: (1) Regional (km scale) reduction of typical magnetite-bearing
rocks originally at f
O2 near NNO may be possible during long-term metamorphic fluid flow if the infiltrating fluids have sufficiently low f
O2 and sufficiently large concentrations of CH4 and/or H2. Regional oxidation of such rocks by highly oxidized OH or COH fluids appears to be difficult to achieve. (2) Nearly identical
mineral assemblages and modes may be produced by very different kinetic reaction pathways. The model implies that “equilibrium”
assemblages preserved in rocks may not always reflect the true kinetic reaction path that evolved during fluid flow, and highlights
the need for quantitative measurements of metamorphic reaction rates. (3) Preservation of sharp lithologic contacts between
rocks of very different redox states containing accessory amounts of oxides may be unlikely if fluid-rock interaction times
exceed 103104 years. Substantial contact disruption over these times scales is predicted even for oxide-rich rocks if redox contrasts between
layers are large. Flow across lithologic contacts may produce asymmetric patterns of metasomatic mineral zonation that may
prove useful for mapping flow directions in metamorphic sequences. (4) For fluid flow in typical T gradients through originally homogeneous rock, significant major element metasomatism (e.g., K, Na, Ca) may be possible without
producing large changes in oxide abundances.
Received:12 November 1997 / Accepted: 9 March 1998 相似文献
6.
Colin M. Graham John W. Valley John M. Eiler Hideki Wada 《Contributions to Mineralogy and Petrology》1998,132(4):371-389
Using a recently developed ion microprobe technique, a detailed oxygen isotope map of calcite grains in a coarse-grained
marble has been constructed, supported by trace element (Mn, Sr, Fe) analysis and cathodoluminescence (CL) imaging, in order
to constrain scales of oxygen isotope equilibrium, timescales and mechanisms of metamorphic fluid infiltration, and fluid
sources and pathways. Results are compared with a previous study of this sample (Wada 1988) carried out using a cryo-microtome
technique and conventional oxygen isotope analysis. The marble, from the high temperature/low pressure Hida metamorphic belt
in north-central Japan, underwent granulite facies followed by amphibolite facies metamorphic events, the latter associated
with regional granite intrusion. The CL imaging indicates two types of calcite, a yellow luminescing (YLC) and a purple luminescing
(PLC) variety. The YLC, which occupies grain boundaries, fractures, replacement patches, and most of the abundant deformation
twin lamellae, post-dates the dominant PLC calcite and maps out fluid pathways. Systematic relationships were established
between oxygen isotope and trace element composition, calcite type and texture, based on 74 18O/16O and 17 trace element analyses with 20–30 μ m spatial resolution. The YLC is enriched in Mn and Fe, and depleted in 18O and Sr compared to PLC, and is much more 18O depleted than is indicated from conventional analyses. Results are interpreted to indicate infiltration of 18O-depleted (metamorphic or magmatic) fluid (initial δ18O = 9‰–10.5‰) along grain boundaries, fractures and deformation twin lamellae, depleting calcite grains in Sr and enriching
them in Mn and Fe. The sample is characterised by gross isotopic and elemental disequilibrium, with important implications
for the application of chromatographic theory to constrain fluid fluxes in metacarbonate rocks.
Areas of PLC unaffected by “short-circuiting” fluid pathways contain oxygen diffusion profiles of ∼10‰/∼200 μm in grain boundary
regions or adjacent to fractures/patches. When correction is made for estimated grain boundary/fracture and profile orientation
in 3D, profiles are indistinguishable within error. Modelling of these profiles gives consistent estimates of Dt (where D is the diffusion coefficient and t is time) of ∼0.8 × 10−8 m2, from which, using experimental data for oxygen diffusion in calcite, timescales of fluid transport along grain boundaries
at amphibolite facies temperatures of ∼103 to ∼104 years are obtained. These short timescales, which are much shorter than plausible durations of metamorphism, imply that
rock permeabilities may be transiently much higher during fluid flow than those calculated from time integrated fluid fluxes
or predicted from laboratory measurements. The preservation of 18O/16O profiles requires either rapid cooling rates (∼100–600 °C/million years), or, more plausibly, loss of grain boundary fluid
such that a dry cooling history followed the transient passage of fluid. The δ18O/trace element correlations are also consistent with volume diffusion-controlled transport in the PLC. Fluid transport and
element exchange occurred by two inter-related mechanisms on short timescales and on different lengthscales – long-distance
flow along cracks, grain boundaries and twin lamellae coupled to ∼200 μm-scale volume diffusion of oxygen.
Received: 8 December 1997 / Accepted: 18 May 1998 相似文献
7.
Ian Cartwright Ian S. Buick Roland Maas 《Contributions to Mineralogy and Petrology》1997,128(4):335-351
The Jervois region of the Arunta Inlier, central Australia, contains para- and orthogneisses that underwent low-pressure
amphibolite facies metamorphism (P = 200–300 MPa, T = 520–600 °C). Marble layers cut by metre-wide quartz + garnet ± epidote veins comprise calcite, quartz, epidote, clinopyroxene,
grandite garnet, and locally wollastonite. The marbles also contain locally discordant decimetre-thick garnet and epidote
skarn layers. The mineral assemblages imply that the rocks were infiltrated by water-rich fluids (XCO2 = 0.1–0.3) at ∼600 °C. The fluids were probably derived from the quartz-garnet vein systems that represent conduits for fluids
exsolved from crystallizing pegmatites emplaced close to the metamorphic peak. At one locality, the marble has calcite (Cc)
δ18O values of 9–18‰ and garnet (Gnt) δ18O values of 10–14‰. The δ18O(Gnt) values are only poorly correlated with δ18O(Cc), and the δ18O values of some garnet cores are higher than the rims. The isotopic disequilibrium indicates that garnet grew before the
δ18O values of the rock were reset. The marbles contain ≤15% garnet and, for water-rich fluids, garnet-forming reactions are
predicted to propagate faster than O-isotopes are reset. The Sm-Nd and Pb-Pb ages of garnets imply that fluid flow occurred
at 1750–1720 Ma. There are no significant age differences between garnet cores and rims, suggesting that fluid flow was relatively
rapid. Texturally late epidote has δ18O values of 1.5–6.2‰ implying δ18O(H2O) values of 2–7‰. Waters with such low-δ18O values are probably at least partly meteoric in origin, and the epidote may be recording the late influx of meteoric water
into a cooling hydrothermal system.
Received: 29 April 1996 / Accepted: 12 March 1997 相似文献
8.
Detlef W. Fasshauer Niranjan D. Chatterjee Ladislav Cemic 《Contributions to Mineralogy and Petrology》1998,133(1-2):186-198
Heat capacity, thermal expansion, and compressibility data have been obtained for a number of selected phases of the system
NaAlSiO4-LiAlSiO4-Al2O3-SiO2-H2O. All C
p
measurements have been executed by DSC in the temperature range 133–823 K. The data for T ≥ 223 K have been fitted to the function C
p
(T) = a + cT
−2 + dT
−0.5 + fT
−3, the fit parameters being The thermal expansion data (up to 525 °C) have been fitted to the function V
0(T) = V
0(T) [1 + v
1 (T−T
0) + v
2 (T−T
0)2], with T
0 = 298.15 K. The room-temperature compressibility data (up to 6 GPa) have been smoothed by the Murnaghan equation of state.
The resulting parameters are These data, along with other phase property and reaction reversal data from the literature, have
been simultaneously processed by the Bayes method to derive an internally consistent thermodynamic dataset (see Tables 6 and
7) for the NaAlSiO4-LiAlSiO4-Al2O3-SiO2-H2O quinary. Phase diagrams generated from this dataset are compatible with cookeite-, ephesite-, and paragonite-bearing assemblages
observed in metabauxites and common metasediments. Phase diagrams obtained from the same database are also in agreement with
the cookeite-free, petalite-, spodumene-, eucryptite-, and bikitaite-bearing assemblages known to develop in the subsolidus
phase of recrystallization of␣lithium-bearing pegmatites. It is gratifying to note that the cookeite phase relations predicted
earlier by Vidal and Goffé (1991) in the context of the system Li2O-Al2O3-SiO2-H2O agree with our results in a general way.
Received: 19 May 1998 / Accepted: 25 June 1998 相似文献
9.
Small tectonic slices of undeformed eclogites and ultrahigh-pressure granulites occur in three tectonic units of the Śnieżnik
Mts. (SW Poland). Ultrahigh-pressure granulite/eclogite transitions with peak metamorphic conditions between 21 and 28 kbar
at 800 to 1000 °C occur only in the Złote unit. Conventional U-Pb multigrain analyses of zircons from a mafic granulite provided
207Pb/206Pb ages between 360 to 369 Ma which are interpreted to approximate timing of original crystallisation from a melt. Diffusion
kinetics and the restricted availability of a fluid phase mainly controlled the conversion from granulite to eclogite, although
some bulk-chemical differences were also recognised. The ultrahigh-pressure granulites from the Złote unit exclusively contain
H2O-rich inclusions with variable salinities which distinguishes them from high-temperature (HT)-granulites world-wide. This is also in contrast to the fluid regime (H2O-N2-CO2) recognised in the lower-temperature eclogites (600–800 °C) from the closely associated Międzygórze and Śnieżnik units. The
variation in fluid composition between the lower-temperature eclogites and ultrahigh-pressure granulites on the one hand and
ultrahigh-pressure granulites and HT-granulites on the other hand probably indicates contrasting P-T-t paths as a result of different tectonic environments.
Received: 15 June 1998 / Accepted: 2 March 1999 相似文献
10.
Sarah Sherlock Simon Kelley Simon Inger Nigel Harris Aral Okay 《Contributions to Mineralogy and Petrology》1999,137(1-2):46-58
Geochronological investigations in high- and ultra-high-pressure metamorphic rocks are problematic since firstly the low
temperatures lead to fine grain size and disequilibrium assemblages, and secondly the problem of “excess argon” affects 40Ar-39Ar systematics, the most commonly used isotopic system. The Tavsanli Zone is a belt of high-pressure low-temperature (HP-LT) rocks spanning NW Turkey and is one such region where previous geochronological studies have produced a range of estimates
for the age of HP-LT metamorphism, raising the question of whether they are geologically significant. This study presents new data from the Tavsanli
Zone; 40Ar-39Ar ages are in the range 60 Ma to 175 Ma, whilst Rb-Sr ages are restricted to 79.7 Ma to 82.8 Ma, confirming the presence
of excess argon. Detailed ultra-violet laser ablation microprobe (UVLAMP) studies have revealed younger 40Ar-39Ar ages in the cores of coarser white micas, which in conjunction with 40Ar-39Ar ages from the finest grained lithologies and the Rb-Sr white mica crystallisation ages, constrain the post-HP-LT metamorphism exhumation rates of these rocks. Petrological and regional constraints suggest that syn-subduction exhumation
and cooling took place initially by synchronous subduction and exhumation by underplating. This is followed by a phase of
syn-continent-continent collision at a rate of approximately 1.5 mma−1 and exhumation to the surface via thrusting. The 40Ar-39Ar hornblende data from a granodiorite intruding the HP-LT rocks constrain the later parts of exhumation path. This study highlights the importance of a multi-system geochronological
approach when attempting to determine the history of HP-LT rocks.
Received: 12 May 1998 / Accepted: 21 April 1999 相似文献
11.
Reductive deposition of graphite at lithological margins in East Central Vermont: a Sr, C and O isotope study 总被引:3,自引:0,他引:3
K. A. Evans M. J. Bickle A. D. L. Skelton M. Hall H. Chapman 《Journal of Metamorphic Geology》2002,20(8):781-798
O, Sr and C isotopes from east‐central Vermont are used to provide information on the timing and volume of metamorphic fluid flow. The results are then used to assess the evidence for redox transformations between C species. Oxygen profiles are homogenised on a metre scale; comparison with Sr isotopes suggest that O alteration may have occurred over a significantly larger timescale than that of Sr, possibly because O was modified during dewatering and diagenesis in addition to the high temperature alteration recorded by strontium. Sr isotope distributions are consistent with cross‐layer fluid fluxes of 104?106 moles m?2; absolute values depend on the Sr fluid‐rock distribution coefficient which is poorly known; however, reaction progress constraints suggest that fluxes were towards the lower end of this range. High δ13C values observed at lithological boundaries cannot be explained by volume loss or closed system processes and are taken to indicate reductive precipitation of graphite as a result of mixing between CO2 and CH4‐bearing fluids. Mass balance calculations indicate that redox reactions occurring under metamorphic conditions convert a minimum of 10% of the CO2 released from limestones into graphite, thus providing a potentially important control on the average residence time of C within the crust with implications for C cycling models. 相似文献
12.
The complication introduced by solid solutions in the analysis of infiltration-driven mineral reactions is that the mole fraction of tracer component i in fluid (X i) changes with reaction progress (ξ). The effect was incorporated into transport models for coupled fluid flow and mineral reaction by parameterizing the relation between X i and ξ. With specific reference to carbonation and hydration during regional metamorphism of the peridotite body in Val d’Efra, whose constituent minerals are all solid solutions, infiltration of a disequilibrium fluid produces a single sharp reaction front if rock is assumed uniform in composition. The reaction front separates completely unreacted rock downstream from rock upstream with ξ at a steady-state limit (ξss ≤ ξmax) that depends on input fluid composition (ξmax is the maximum possible value). Novel phenomena develop, however, if the flow medium, like the metaperidotite body, is composed of many small domains that differ in initial mineral modes and compositions but with X i homogenized at a spatial scale larger than the size of the domains (e.g., by diffusion). In this case, infiltration of a disequilibrium fluid produces up to as many different reaction fronts along the flow path as there are domains with 0 ≤ ξ < ξss in all domains except upstream from the slowest moving front where ξ = ξss in all domains. Measured values of ξ in the metaperidotite, (all 0 < ξ < ξmax) are best reproduced by down-temperature infiltration of a disequilibrium fluid with X\textCO2 = 0. 1 9 6 X_{{{\text{CO}}_{2} }} = \, 0. 1 9 6 into a multi-domain medium with uniform X\textCO2 X_{{{\text{CO}}_{2} }} at each spatial point along the flow path (homogenized across the domains at the m-scale by diffusion), and time-integrated fluid flux ≥1,836 mol fluid/cm2 rock. Results resolve the paradox of the widespread spatial distribution of reactants and products of infiltration-driven decarbonation/dehydration reactions in regional metamorphic terrains (which in the absence of solid solution and compositional domains indicate up-temperature flow) and the prediction of hydrodynamic models that regional metamorphic fluid flow normally is directed vertically upward and down temperature. 相似文献
13.
Andreas Tennie Radegund Hoffbauer Stephan Hoernes 《Contributions to Mineralogy and Petrology》1998,133(4):346-355
The oxygen isotope fractionation between kyanite and calcium carbonate has been investigated experimentally at four temperatures
in the range between 625 and 775 °C at 13 kbar. Because of low exchange rates, the isotopic reaction was enhanced by polymorphic
transformation of andalusite to kyanite. With this experimental modification a close approach to equilibrium was reached in
all runs. The temperature dependence of the equilibrium fractionation is described by the equation 1000 ln ky-cc=−2.62×106/T
2. Application of the experimental results to natural quartz-kyanite-garnet assemblages indicates the preservation of the oxygen
isotope composition of kyanite acquired during its formation, reflecting its extremely low oxygen diffusivity. This refractory
behaviour restricts the use of kyanite for thermometry but opens the possibility to use its O-isotope composition as an indicator
for recognition of polymetamorphic rock histories and reconstruction of the prograde evolution of a metamorphic sequence.
Received: 8 June 1998 / Accepted: 24 August 1998 相似文献
14.
Regional Metamorphism of the Waits River Formation, Eastern Vermont: Delineation of a New Type of Giant Metamorphic Hydrothermal System 总被引:4,自引:0,他引:4
Metamorphic isograds and time-integrated fluid fluxes were mappedover the 1500 km2 exposure of the Waits River Formation, easternVermont, south of latitude 4430'N. Isograds based on the appearanceof oligoclase, biotite, and amphibole in metacarbonate rocksdefine elongated metamorphic highs centered on the axes of twolarge antiforms. The highest-grade isograd based on the appearanceof diopside is closely associated spatially with synmetamorphicgranitic plutons. Pressure, calculated from mineral equilibria,was fairly uniform in the area, 7 1.5 kb; calculated temperatureincreases from {small tilde} 480C at the lowest grades in thearea to {small tilde} 575C in the diopside zone. CalculatedXco2f equilibrium metamorphic fluid increases from <0-03at the lowest grades to 0.2 in the amphibole zone and decreasesto 0.07 in the diopside zone. Time-integrated fluid fluxesincrease with increasing metamorphic grade, with the followingmean values for each metamorphic zone (in cm3/cm2): ankerite-oligoclasezone, 1 x 104; biotite zone, 7 x 104; amphibole zone, 2 x 105;diopside zone, 7 x 105. The mapped pattern of time-integrated fluxes delineates twolarge deep-seated ({small tilde} 25-km depth) regional metamorphichydrothermal systems, each centered on one of the major antiforms.Fluid flowed subhorizontally perpendicular to the axis of theantiforms from their low-temperature flanks to their hot axialregions and drove prograde decarbonation reactions as they went.Along the axes of the antiforms fluid flow was further focusedaround synmetamorphic granitic intrusions. In the hot axialregion fluid changed direction and flowed subvertically outof the metamorphic terrane, precipitating quartz veins. Estimatesof the total recharge, based on progress of prograde decarbonationreactions, nearly match estimates of the total discharge, basedon measured quartz vein abundance, (2-10) x 1012 cm3 fluid percm system measured parallel to the axes of the antiforms. Withinthe axial regions fluids had lower XCO2 and rocks record greatertime-integrated fluxes close to the intrusions than at positionsmore than {small tilde} 5 km from them. The differences in bothfluid composition and time-integrated flux can be explainedby mixing close to the intrusions of regional metamorphic fluidsof XCO2/ with fluids from another source with XCO2{small tilde}0 in the approximate volume ratio of 1:2. 相似文献
15.
The northeastern portion of the Mont Blanc massif in western Switzerland is predominantly comprised of the granitic rocks
of the Mont Blanc intrusive suite and the Mont Blanc basement gneisses. Within these metamorphic rocks are a variety of sub-economic
Fe skarns. The mineral assemblages and fluid inclusions from these rocks have been used to derive age, pressure, temperature
and fluid composition constraints for two Variscan events. Metamorphic hornblendes within the assemblages from the basement
amphibolites and iron skarns have been dated using 40Ar/39Ar, and indicate that these metamorphic events have a minimum age of approximately 334 Ma. Garnet-hornblende-plagioclase thermobarometry
and stable isotope data obtained from the basement amphibolites are consistent with metamorphic temperatures in the range
515 to 580 °C, and pressures ranging from 5 to 8 kbar. Garnet-hornblende-magnetite thermobarometry and fluid inclusion studies
indicate that the iron skarns formed at slightly lower temperatures, ranging from 400 to 500 °C in the presence of saline
fluids at formational pressures similar to those experienced by the basement amphibolites.
Late Paleozoic minimum uplift rates and geothermal gradients calculated using these data and the presence of Ladinien ichnofossils
are on the order of 0.32 mm/year and 20 °C/km respectively. These uplift rates and geothermal gradients differ from those
obtained from the neighbouring Aiguilles Rouges massif and indicate that these two massifs experienced different metamorphic
conditions during the Carboniferous and Permian periods. During the early to late Carboniferous period the relative depths
of the two massifs were reversed with the Aiguilles Rouges being initially unroofed at a much greater rate than the Mont Blanc,
but experiencing relatively slower uplift rates near the termination of the Variscan orogeny.
Received: 23 April 1996 / Accepted: 27 August 1996 相似文献
16.
The Ballachulish Igneous Complex consists of an outer quartz diorite and an inner granite, emplaced at about 300 MPa, initially
at 1000 to 1050 °C. The contact aureole (0.5–2 km wide) occurs in metapelites and metapsammites plus minor graphitic slates,
carbonate rocks and metaquartzites. A textural examination of the arkosic Appin Quartzite, which was previously believed to
have melted only within a few metres of the intrusion, demonstrates that partial melting occurred up to 500 m away from the
vertical eastern contact. Coupling petrographic observations with Qtz-Ab-Or-H2O phase relations, we determined both the amounts of actual melt and the maximum possible amounts of melt in the samples.
Melting efficiency was everywhere less than 100% and decreased with distance from the intrusion. Though perhaps not the only
possible source of fluid throughout the aureole, simple models demonstrate that H2O evolution from the pluton would have been volumetrically sufficient and persisted long enough to account for the observed
partial melting. A time-integrated fluid flux of 7000 kg/m2 from the pluton is necessary to account for the observed amounts of partial melt in the Appin Quartzite. From its inefficiency,
we infer that infiltration of the Appin Quartzite cannot have occurred along interconnected grain-edge channels. Rather, it
was controlled by hydraulic fracturing, with fracture density determining melting efficiency. Bulk-rock permeability is calculated
to be 10−20 m2, an order of magnitude lower than that necessary to permit pervasive flow of all the fluid exsolving from the pluton. There
is little difference between the calculated time-integrated fluid flux through the Appin Quartzite on the eastern flank and
an estimate of the infiltrating flux through the pelitic Leven Schist on the western flank in the time interval during which
both rock types were above their solidus temperature, despite differences in their equilibrium quartz-H2O dihedral angles at temperatures immediately below the solidus, and differences in the attitude of the contact. The rates
of H2O expulsion from the cooling pluton are consistent with highly efficient fracture-dominated flow, allowing insufficient time
for textural equilibration.
Received: 26 March 1998 / Accepted: 8 March 1999 相似文献
17.
John D. Clemens Giles T. R. Droop Gary Stevens 《Contributions to Mineralogy and Petrology》1997,129(4):308-325
The system KAlO2–MgO–SiO2–H2O–CO2 has long been used as a model for the processes of granulite-facies metamorphism and the development of orthopyroxene-bearing
mineral assemblages through the breakdown of biotite-bearing assemblages. There has been considerable controversy regarding
the role of carbon dioxide in metamorphism and partial melting. We performed new experiments in this system (at pressures
of 342 to 1500 MPa with T between 710 and 1045 °C and X
Fl
H2O between 0.05 and 1.00), accurately locating most of the dehydration and melting equilibria in P-T-X
Fl
H2O space. The most important primary result is that the univariant reaction Phl + Qtz + Fl = En + Sa + melt must be almost coincident
with the fluid-absent reaction (Phl + Qtz = En + Sa + melt) in the CO2-free subsystem. In conjunction with the results of previous measurements of CO2 solubility in silicate melts and phase equilibrium experiments, our theoretical analysis and experiments suggest that CO2 cannot act as a flux for partial melting. Crustal melting in the presence of H2O–CO2 mixed fluids will always occur at temperatures higher than with pure H2O fluid present. Magmas produced by such melting will be granitic (s.l.) in composition, with relatively high SiO2 and low MgO contents, irrespective of the H2O–CO2 ratio in any coexisting fluid phase. We find no evidence that lamprophyric magmas could be generated by partial fusion of
quartz-saturated crustal rocks. The granitic melts formed will not contain appreciable dissolved CO2. The channelled passage of hot CO2-rich fluids can cause local dehydration of the rocks through which they pass. In rock-dominated (as opposed to fluid-dominated)
systems, minor partial melting can also occur in veins initially filled with CO2-rich fluid, as dehydration and local disequilibrium drive the fluid towards H2O-rich compositions. However, CO2 is unlikely to be a significant agent in promoting regional granulite-grade metamorphism, melting, magma generation, metasomatism
or long-range silicate mass transfer in Earth's crust. The most viable model for the development of granulite-facies rocks
involves the processes of fluid-absent partial melting and withdrawal of the melt phase to higher crustal levels.
Received: 28 November 1996 / Accepted: 25 June 1997 相似文献
18.
Rubbo Marco Borghi Alessandro Compagnoni Roberto 《Contributions to Mineralogy and Petrology》1999,137(4):289-303
Samples of the metagranodiorite from M. Mucrone (Sesia zone, Western Alps) show pseudomorphic and coronitic textures where
the igneous minerals were partially replaced by high-pressure metamorphic assemblages. The original magmatic paragenesis consisted
of quartz, plagioclase, K-feldspar, biotite and minor phases. During the eclogitic event the original plagioclase was fully
replaced by zoisite, jadeite and quartz ± K-feldspar pseudomorphic symplectites and the biotite was in part replaced by phengitic
mica. Moreover, a composite corona often developed around the biotite. This corona consists of a layer of phengite I and garnet
and, where the igneous biotite and feldspars were in contact, of an outer layer of phengite II intergrown with quartz. Biotite,
phengite and K-feldspar are homogeneous while garnet shows a strong composition zoning recording the kinetics of the metamorphic
reactions. A numerical simulation of the observed garnet zoning is performed assuming that intercrystalline diffusion and
plagioclase resorption were the slowest rate-determining processes during the prograde P-T path. The metamorphic paragenesis constrains the P-T path chosen in the simulation. The comparison between measured and calculated garnet zoning permits evaluation of the relative
weights of interface kinetics, grain-boundary and lattice diffusion.
Received: 26 November 1997 / Accepted: 6 August 1999 相似文献
19.
Experimental reconstruction of sodic dolomitic carbonatite melts from metasomatised lithosphere 总被引:3,自引:0,他引:3
Investigations of peridotite xenolith suites have identified a compositional trend from lherzolite to magnesian wehrlite
in which clinopyroxene increases at the expense of orthopyroxene and aluminous spinel, and in which apatite may be a minor
phase. Previous studies have shown that this trend in mineralogy and chemical composition may result from reaction between
sodic dolomitic carbonatite melt and lherzolite at pressures around 1.7 to 2 GPa. This reaction results in decarbonation of
the carbonatite melt, releasing CO2-rich fluid. In this study, we have experimentally reversed the decarbonation reaction by taking two natural wehrlite compositions
and reacting them with CO2 at a pressure of 2.2 GPa and temperatures from 900 to 1150° C. Starting materials were pargasite-bearing wehrlites, one with
minor apatite (composition 71001*) and one without apatite (composition 70965*). At lower temperatures (900° C) the products were apatite+pargasite+magnesite harzburgite for runs using composition 71001*, and pargasite+dolomite lherzolite for runs using composition 70965*. At and above 1000° C, carbonatite melt with harzburgite residue (olivine+orthopyroxene+spinel) and with lherzolite residue
(olivine+orthopyroxene+clinopyroxene+ spinel) were produced respectively. Phase compositions in reactants and products are
consistent with the documented carbonatite/lherzolite reactions, and also permit estimation of the carbonatite melt compositions.
In both cases the melts are sodic dolomitic carbonatites. The study supports the hypothesis of a significant role for ephemeral,
sodic dolomitic melts in causing metasomatic changes in the lithosphere at P≤2 GPa. The compositions of wehrlites imply fluxes of CO2, released by metasomatic reactions, which are locally very large at around 5 wt% CO2.
Received: 15 December 1995/Accepted: 14 February 1996 相似文献
20.
The effect of fluid flow on mixed-volatile reactions in metamorphic rocks is described by an expression derived from the standard equation for coupled chemical-reaction and fluid-flow in porous media. If local mineral-fluid equilibrium is assumed, the expression quantitatively relates the time-integrated flux at any point in a flow-system to the progress of devolatilization reactions and the temperature- and pressure-gradients along the direction of flow. Model calculations indicate that rocks are generally devolatilized by fluids flowing uptemperature and/or down-pressure. Flow down-temperature typically results in hydration and carbonation of rocks. Time-integrated fluid fluxes implied by visible amounts of mineral products of devolatilization reactions are on the order of 5·102–5·104 mol/cm2. The model was applied to regionally metamorphosed impure carbonate rocks from south-central Maine, USA, to obtain estimates of fluid flux, flow-direction, and in-situ metamorphic-rock permeability from petrologic data. Calculated time-integrated fluxes are 104–106 cm3/cm2 at 400°–450° C, 3,500 bars. Fluid flowed from regions of low temperature to regions of high temperature at the peak of the metamorphic event. Using Darcy's Law and estimates for the duration of metamorphism and hydrologic head, calculated fluxes are 0.1–20·10-4 m/year and minimum permeabilities are 10-10–10-6 Darcy. The range of inferred permeability is in good agreement with published laboratory measurements of the permeability of metamorphic rocks. 相似文献