首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
A detailed study of retrograde alteration of a staurolite porphyroblast and its surrounding matrix of mica schist has made use of petrographic, modal, and microprobe analysis. Retrogression was to the garnet zone of metamorphism and apparently occurred largely after a temperature decline of 70–100° C. The event caused metasomatic removal of Zn but may have been isochemical relative to other analyzed elements. The best estimate of the overall reaction is: 1 staurolite+3.018 biotite+3.550 quartz+0.629 albite +0.014 anorthite+0.678 NaCl+14.004 H2O =3.274 Na-rich muscovite+3.561 chlorite +0.273 ilmenite+0.110 chloritoid+0.039 garnet +0.339 ZnCl2.Non-systematic variation in composition of analyzed minerals is revealed by statistical treatment of replicate analyses. Such variation involves monovalent and divalent cations within many minerals, but is most pronounced within retrograde muscovite. Muscovite variation involves Si and Al as well as FM and alkalis and does not follow a phengite law of charge-coupled substitution.Relative to the core of the retrograded staurolite crystal, zoning is seen in averaged muscovite compositions and in development of incompatible mineral assemblages, which include chloritoid well within retrograded staurolite but biotite within the matrix. A local gradient in the chemical potential of an Al-bearing component was likely present during retrogression.Alteration of staurolite was probably accomplished by reaction and diffusion through the medium of an intergranular fluid phase. Relative to staurolite, migration of elements involved immigration of considerable amounts of Mg, Na, K, and H and expulsion of Al, Fe, Zn, and O. It is inferred that concentration of Al within the fluid phase was considerably lower than those of monovalent and divalent cations.Preservation of considerable staurolite and evidence for a local concentration gradient of Al in the fluid phase suggest that limited amounts of H2O were available. Expulsion of Zn suggests that much water was not consumed locally but exited the terrane. An attempt at resolution of this dilemna involves fracture-channelized infiltration of H2O into the rock. A more regional petrographic study of retrogression suggests that H2O which entered the rock may have been liberated initially by prograde dehydration at a moderately greater depth of 2–3 km.Results of this study, especially the non-phengitic nature of crystal-chemical substitution within muscovite, indicate chemical reaction under conditions of disequilibrium. Apparently, extent of retrogression was controlled by availability of H2O rather than by thermochemical equilibria.  相似文献   

2.
A model for trondhjemite genesis is proposed where granite is transformed to trondhjemite via infiltration by a Na-rich metamorphic fluid. The Rockford Granite of the Northern Alabama Piedmont serves as the case example for this process and is characterized as a synmetamorphic, peraluminous trondhjemite-granite suite. The major process operative in the conversion of granite to trondhjemite involves cation exchange of Na for K in the feldspar and mica phases through a volatile fluid medium. Whole-rock 18O values for the trondhjemites are negatively correlated with atomic proportion K/Na ratio indicating a partial reequilibration of the altered granitoids with a Na- and18O-rich metamorphically derived fluid. Biotite decomposition to an Al-epidote-paragonitic muscovite-secondary quartz assemblage is also associated with the sodium metasomatism, as are apatite replacement by Al-epidote and secondary zircon crystallization. Rare albitization of primary magmatic plagioclase and discontinuous grossularite reaction rim growth on magmatic garnet are present in the trondhjemites indicating the mobility of Ca during alkali metasomatism. The replacement of magmatic phases by me tasomatic phases exemplifies the chemical changes produced during infiltration metasomatism where the trondhjemites are depleted in P2O5, Th, Rb, U, K2O, V, Sn, F, MgO, Pb, TiO2, FeO* and Li and enriched in CaO, Na2O, Zr and Sr relative to the granites. Other elements, such as Cr, MnO, Cu, Zn, Co, Ba, SiO2, Ni, Al2O3, are shown to be relatively immobile during the metasomatism. The infiltration metasomatism probably occurred during prograde regional metamorphism, when a discrete fluid phase was produced in the surrounding amphibolite-grade metasediments. Foliation planes in the granite apparently served as conduits for fluid flow with reaction-enhanced permeability accompanying the 8% molar volume reduction during Na-for-K exchange in the feldspars. A source for the Na and Sr in the metamorphic fluid may have been paragonitic muscovite in the metasedimentary country rocks. Rubidium and K were probably retained in metasedimentary biotite. The Silent Lake pluton in southeastern Ontario is a possible analogue to the alkali metasomatic processes affecting the Rockford Granite.  相似文献   

3.
At Bangriposi, variable stages in replacement of staurolite by chloritoid – Na–K–Ca mica shimmer aggregates in muscovite schists provides insight into the complex interplay between fluid flow, mass transfer, and dissolution–precipitation during pseudomorph growth. Idioblastic chloritoid growing into mica caps without causing visible deformation, and monomineralic chloritoid veins (up to 300 μm wide) within shimmer aggregates replacing staurolite attest to chloritoid nucleation in fluid‐filled conduits along staurolite grain boundaries and crystallographic planes. The growth of shimmer aggregates initiated along staurolite margins, and advanced inwards into decomposing staurolite along networks of crystallographically controlled fluid‐filled conduits. Coalescence among alteration zones adjacent to channel fills led to dismemberment and the eventual demise of staurolite. Mass balance calculation within a volume‐fixed, silica‐conserved reference frame indicate the shimmer aggregates grew via precipitation from fluids in response to mass transport that led to the addition of H2O, K2O, Na2O and CaO in the reaction zone, and Al2O3 was transported outward from the inward‐retreating margin of decomposing staurolite. This aided precipitation of chloritoid in veins and in the outer collars, and as disseminated grains in the shimmer aggregates at mid‐crustal condition (~520 ± 20 °C, 5.5 ± 2.0 kbar). Computation using one‐dimensional transport equation suggests that staurolite decomposition involved advection dominating over diffusive transport; the permeation of externally derived H2O caused flattening of chemical potential gradients in H2O and aqueous species, for example, and , computed using the Gibbs method. This suggests that staurolite decomposition was promoted by the infiltration of a large volume of H2O that flattened existing chemical potential gradients. In the initial stages of replacement, chloritoid super‐saturation in fluid caused preferential nucleation and growth of chloritoid at staurolite grain boundaries and in crystallographic planes. As reaction progressed, further chloritoid nucleation was halted, but chloritoid continued to grow as the 3‐mica aggregates continued to replace the remaining staurolite in situ, while the chloritoid‐compatible elements were transported in the water‐rich phase facilitating continued growth of the existing chloritoid grains.  相似文献   

4.
Greenstone, blueschist and eclogite metabasaltic blocks from the Franciscan complex of California preserve extensive petrographic and chemical evidence for interaction with hydrous fluids at high-P, low-T metamorphic conditions. The Nd and Sr isotope variations within and among the blocks constrain the origin of the basaltic protoliths, the nature of the fluid metasomatism that occurred within the upper levels (15–45 km) of the paleosubduction zonc, and the character and provenance of the rock that generated the hydrous fluids within the paleosubduction zone. Samples with little or no petrographic evidence of retrograde alteration and unaltered garnet separates have Nd. With increasing degrees of retrograde alteration, Nd isotope compositions are consistently lower, ranging down to Nd(160)=5. Actinolitic alteration rinds which are present on some blocks have the least radiogenic compositions with Nd=1.6 to 6.1. While Nd isotope compositions of unaltered blockes are in the range expected for basalt derived from normal depleted mantle, the Sr isotope compositions are more radiogenic ranging from Sr(160)=–5 to +11. Compositions of unaltered eclogite and blue-schist blocks are consistent with a protolith origin in normal oceanic crust derived from depleted mantle. The Sr isotopy systematics indicate that the protoliths were modified by seawater alteration in an ocean-floor hydrothermal system. Isotopic compositions of samples from parts of blocks that have a retrograde metamorphic overprint show a strong correlation between less radiogenic Nd compositions and the extent of retrograde metamorphism. Maximum Nd isotope ratios of the metasomatizing fluid are provided by analyses of actinolitic rinds, and range from Nd(160)=1.6 to 6.1. A possible source for fluids of this composition is subducted sediment that was derived from a continental craton. Because rind formation occurred while the basaltic blocks were within an ultramafic matrix, the fluids must have migrated from sediments in the accretionary wedge into an overlying wedge of mantle material imbricated with blocks of oceanic crust. This suggests possibly km-scale movement of fluids that carry an amount of the rare-earth elements sufficient to significantly modify the trace-element budget of subducted basalt.  相似文献   

5.
The Shuanghe ultrahigh-pressure (UHP) slab in the Dabie Mountains consists of layered coesite-bearing eclogite, jadeite quartzite, marble and biotite gneiss, and is fault bounded against hosting orthogneiss. Representative assemblages of eclogite are Grt+Omp+Coe+Rt±Ky±Phn±Mgs; it formed at P>27 kbar and 680–720±50 °C. During exhumation, these UHP rocks experienced multistage retrograde metamorphism. Coesite was overprinted by quartz aggregates, phengite by biotite±muscovite and rutile by titanite. Garnet was successively replaced by a thin rim of Amp, Amp+Pl, and Amp+Ep±Bt+Pl (minor). Omphacite and kyanite were replaced by Amp+Pl±Cpx (or ±Bt) and by Zo+Pl+Ms±Mrg±Bt, respectively. Secondary calcite occurs as irregular pockets in some layers. An outcrop near the UHP slab border is composed of 20 thin, concordant layers of foliated eclogites, amphibolite and gneissic rocks of variable bulk composition. These layers exhibit mineral assemblages and textures transitional from less altered through extensively retrograded eclogite to gneissic rock of low-amphibolite facies through hydration, metasomatism and recrystallization. Retrograde metamorphism has caused oxygen and hydrogen isotope disequilibria between some of the minerals, but the fluid for retrograde reactions was internally buffered in the stable isotope compositions. Retrograde metamorphism of variable extent may be attributed to selective infiltration of retrograde fluids of CO2-rich and low-salinity aqueous, intensity of deformation and mineral resistance to alteration. The fluid phase for retrogression may have occurred either as discontinuous flow along grain boundaries in completely retrograded eclogites, and/or as isolated pockets in extensive or less altered eclogite layers.  相似文献   

6.
Metasomatism and fluid flow in ductile fault zones   总被引：8，自引：0，他引：8  
Observed major element metasomatism in 5 amphibolite facies ductile fault zones can be explained as the inevitable consequence of aqueous fluid flow along normal temperature gradients under conditions of local chemical equilibrium. The metasomatism does not require the infiltration of chemically exotic fluids. Calculations suggest that metasomatized ductile fault zones are typically infiltrated by 105 moles H2O/cm2, fluid flow is in the direction of decreasing temperature, and fluids contain about 1.0 molal total chloride. Where available, stable isotopic alteration data confirm both flow direction and fluid fluxes calculated from major element metasomatism. The fluid fluxes inferred from metasomatism do not require large-scale fluid recirculation or mantle sources if significant lateral fluid flow occurs in the deep crust. Time-integrated fluid fluxes are combined with estimates of flow duration to constrain average flow rates and average permeabilities. Rocks in ductile fault zones are probably much more permeable during metasomatism (average permeabilities of 10-17 to 10-15 m2) than rocks normally are during regional metamorphism (10-21 to 10-18 m2). Estimated average fluid flow rates (3.5×10-3 to 0.35 m/yr) are insufficient, however, to significantly elevate ambient temperatures within ductile faults. Fluid flow in the direction of decreasing temperature may increase the ductility of silicate rocks by adding K to the rocks and thereby driving mica-forming reactions.  相似文献   

7.
The most recent of two metamorphic events (M2) in the Snow Peakarea caused progressive changes in mineral parageneses in peliticrocks ranging from chlorite-biotite to kyanite grade. Systematicpartitioning of elements between coexisting phases indicatesa close approach to equilibrium during M2. Temperature estimatesfor M2 range from 440 ?C in the chlorite-biotite zone to 565?C in the kyanite zone. Coexistence of kyanite, garnet, ilmenite,and quartz places an upper pressure limit of approximately 60kb, and an upper temperature limit at the kyanite-sillimaniteboundary. Equilibrium of garnet, kyanite, plagioclase, and quartzindicates that total pressure of equilibration of kyanite-bearingassemblages was approximately 6 kb. Pressure estimates basedon equilibrium of garnet, muscovite, biotite, and plagioclaseindicate a pressure gradient between garnet and lower staurolitezone samples, which equilibrated at approximately 3? 5 kb, andupper staurolite to kyanite zone samples, which equilibratedat 5? 5 kb. Equilibrium of paragonite component of muscovitewith plagioclase, kyanite and quartz, distribution of speciesin C-O-H fluids in equilibrium with graphite, and the presenceof zoisite in adjacent calc-silicate rocks indicate that themetamorphic fluid in kyanite-bearing assemblages contained 65-90mole per cent H2O. However, the experimentally calibrated equilibriumof staurolite, quartz, garnet, and kyanite can be reconciledwith estimated temperature only if XH2O in the fluid was verylow ( 33 mole per cent). T-X(Fe-Mg) relations among chlorite, biotite, garnet, staurolite,kyanite, muscovite and quartz are calculated at 6 kb on thebasis of 3 independent Fe-Mg exchange equilibria: garnet-biotite,chlorite-biotite (empirical, this study), garnet-staurolite(empirical, this study), and three independent net transferequilibria. Alternative sets of data for Mg-chlorite and Fe-stauroliteare evaluated by comparing observed and calculated changes inmineral paragenesis and mineral composition with grade. Chloritedata from Helgeson et al., 1978 give T-X(Fe-Mg) relations consistentwith trends observed in these rocks, whereas data derived frombreakdown of clinochlore and clinochlore + dolomite do not.Calculation of T-X(Fe-Mg) relations consistent with observationsrequires lower values of and than those consistent with experiments on the breakdown of staurolite+quartz.  相似文献   

8.
The contact aureole developed in siliceous carbonates surrounding the Beinn an Dubhaich granite, Skye, shows textural and stable isotope evidence for infiltration of aqueous fluids during both prograde and retrograde metamorphism. Strongly depleted isotope compositions of reaction-product calcite correlate with high silica and fluorine contents, demonstrating a strong link between isotopic alteration and metasomatism by fluids with a significant magmatic component, even at the margins of the aureole. The oxygen and carbon isotope compositions of the carbonates form a linear cluster with a positive slope of about five, consistent with the depletion of isotope compositions by the infiltration of magmatic and/or meteoric fluids. Rayleigh fractionation during devolatilization played a minor role in determining the final isotope composition. Stable isotope compositions of coexisting calcite–dolomite pairs show varying amounts of isotopic disequilibrium, which correlate with the inferred fluid infiltration mechanism. Much of the calcite in dolostones is the product of infiltration-driven reactions along fractures, and is greatly depleted isotopically relative to the host dolomite, especially at talc grade. At higher grades the calcite–dolomite fractionation is smaller, probably due to both increased fluid–rock interaction and a greater tendency for fluid infiltration to be pervasive on the grain-scale. Limestones generally show near-equilibrium fractionation of oxygen and carbon owing to the overwhelming compositional influence of the host calcite. Veins formed during late-stage hydrothermal circulation have strongly 18O-depleted compositions relative to the host rock. No small-scale spatial patterns to the isotopic depletion were observed, but the extent of fluid infiltration was greatest in the west of the aureole. Fluid infiltration was clearly highly heterogeneous, with no evidence of a consistent flow direction. It is not possible to determine fluid fluxes or flow directions from one-dimensional flow models based on continuum flow in the Beinn an Dubhaich aureole.  相似文献   

9.
The metamorphic evolution of rocks cropping out near Stoer, within the Assynt terrane of the central region of the mainland Lewisian complex of NW Scotland, is investigated using phase equilibria modelling in the NCKFMASHTO and MnNCKFMASHTO model systems. The focus is on the Cnoc an t’Sidhean suite, garnet‐bearing biotite‐rich rocks (brown gneiss) with rare layers of white mica gneiss, which have been interpreted as sedimentary in origin. The results show that these rocks are polymetamorphic and experienced granulite facies peak metamorphism (Badcallian) followed by retrograde fluid‐driven metamorphism (Inverian) under amphibolite facies conditions. The brown gneisses are inferred to have contained an essentially anhydrous granulite facies peak metamorphic assemblage of garnet, quartz, plagioclase and ilmenite (±rutile, K‐feldspar and pyroxene) with biotite, hornblende, muscovite, chlorite and/or epidote as hydrous retrograde minerals. P–T constraints imposed by phase equilibria modelling imply conditions of 13–16 kbar at >900 °C for the Badcallian granulite facies metamorphic peak, consistent with the field evidence for partial melting in most lithologies. The white mica gneiss comprises a muscovite‐dominated matrix containing porphyroblasts of staurolite, corundum, kyanite and rare garnet. Previous studies have suggested that staurolite, corundum, kyanite and muscovite all grew at the granulite facies peak, with partial melting and melt loss producing a highly aluminous residue. However, at the inferred peak P–T conditions, staurolite and muscovite are not predicted to be stable, suggesting they are retrograde phases that grew during amphibolite facies retrograde metamorphism. The large proportion of mica suggests extensive H2O‐rich fluid‐influx, consistent with the retrograde growth of hornblende, biotite, epidote and chlorite in the brown gneisses. P–T conditions of 5.0–6.5 kbar at 520–550 °C are derived for the Inverian event. In situ dating of zircon from samples of the white mica gneiss yield apparent ages that are difficult to interpret. However, the data are permissive of granulite facies (Badcallian) metamorphism having occurred at c. 2.7–2.8 Ga with subsequent fluid driven (Inverian) retrogression at c. 2.5–2.6 Ga, consistent with previous interpretations.  相似文献   

10.
Mineralisation of uraninite and brannerite occurs in the albitised metasedimentary and intrusive igneous rocks of Kerpura-Tiwari-ka-bas area in Sikar district, Rajasthan. Samples collected from well dumps contain 0.016 to 1.52% U3O8 with very low ThO2. The host rocks show wide variation in chemical composition due to varying degree of alkali metasomatism, associated alteration and composition of protoliths. The spatial distribution of uranium in groundwater as well as rock samples from well dumps indicates localisation of the uranium mineralisation along NNE-SSW and NWSE directions in Kerpura block. Petrographic and lithogeochemical studies point towards close genetic relationship between alkali metasomatism and uranium mineralisation. The mineralisation seems to be due to mobilisation of uranium and other LILEs by metasomatising fluids and their deposition along shear zones at a later stage, which may not be spatially related to zones of intense albitisation.  相似文献   

11.
Peridotite xenoliths entrained in Plio-Pleistocene alkali basalts from Sardinia represent fragments of the uppermost lithospheric mantle, and are characterised by an anhydrous four-phase mineral assemblage. They range in bulk rock composition from fertile spinel-lherzolites to residual spinel-harzburgites. The Sr-Nd isotope and trace element composition of clinopyroxene mineral separates varies between LREE-depleted samples with 87Sr/86Sr as low as 0.70262 and 143Nd/144Nd up to 0.51323 and LREE-enriched samples with 87Sr/86Sr up to 0.70461 and 143Nd/144Nd down to 0.51252. The available data suggest that all the studied peridotite samples suffered variable degrees of partial melting during Pre-Mesozoic times (based on Nd model ages relative to CHUR and DMM). The overprinted enrichment is related to a subsequent metasomatism, induced by fluids rising through the lithosphere that preferentially percolated the originally most depleted domains. Despite the occurrence of orogenic volcanism in the area, preferential enrichment in elements typically associated with slab derived fluids/melts (K, Rb, Sr, Th) relative to LREE has not been detected, and metasomatism seems to be more likely related to the infiltration of highly alkaline basic melts characterised by an EM-like Sr-Nd isotopic composition. Similar 87Sr/86Sr-143Nd/144Nd compositions, characterised by an EM signature, are observed in anorogenic mafic lavas and peridotite xenoliths from widespread localities within the "European" plate, whereas they have not previously been recorded in peridotite xenoliths and associated alkaline mafic lavas from the stable "African" lithospheric domain.  相似文献   

12.
魏正宇  张玮 《江苏地质》2014,38(1):150-153
以芨岭地区铀矿床碱交代蚀变岩石为研究对象,对该区碱交代作用蚀变的特征进行详细研究。结果表明:芨岭钠交代岩型铀矿床属断裂碱交代型铀矿床,矿床蚀变种类繁多,主要有钠长石化、绿泥石化、碳酸盐化、赤铁矿化、绢云母化和硅化等。蚀变组合及分布特征较明显,并与铀矿化关系密切。  相似文献   

13.
地幔流体作用——地幔捕虏体中流体包裹体的研究   总被引：8，自引：0，他引：8  
被碱性玄武岩和金伯利岩带到地表的地幔捕虏体是认识地球深部信息的窗口 ,是人们能够直接观察到的一种上地幔样品 ,其矿物中流体包裹体的存在提供了上地幔流体活动的直接证据。流体 /地幔矿物之间元素的分配对约束地幔交代过程中流体相的作用和上地幔流体的组成 ,揭示俯冲带壳幔物质的再循环过程 ,解释岛弧玄武岩高场强元素亏损的原因有重要意义。文章对近年来有关地幔捕虏体中流体包裹体的研究进行了评述 ,并结合近年来流体 /地幔矿物之间元素分配的高温高压实验研究讨论了流体在地幔中的重要作用。  相似文献   

14.
Quartz‐kyanite veins, adjacent alteration selvages and surrounding ‘precursor’ wall rocks in the Dalradian Saxa Vord Pelite of Unst in the Shetland Islands (Scotland) were investigated to constrain the geochemical alteration and mobility of Al associated with channelized metamorphic fluid infiltration during the Caledonian Orogeny. Thirty‐eight samples of veins, selvages and precursors were collected, examined using the petrographic microscope and electron microprobe, and geochemically analysed. With increasing grade, typical precursor mineral assemblages include, but are not limited to, chlorite+chloritoid, chlorite+chloritoid+kyanite, chlorite+chloritoid+staurolite and garnet+staurolite+kyanite+chloritoid. These assemblages coexist with quartz, white mica (muscovite, paragonite, margarite), and Fe‐Ti oxides. The mineral assemblage of the selvages does not change noticeably with metamorphic grade, and consists of chloritoid, kyanite, chlorite, quartz, white mica and Fe‐Ti oxides. Pseudosections for selvage and precursor bulk compositions indicate that the observed mineral assemblages were stable at regional metamorphic conditions of 550–600 °C and 0.8–1.1 GPa. A mass balance analysis was performed to assess the nature and magnitude of geochemical alteration that produced the selvages adjacent to the veins. On average, selvages lost about −26% mass relative to precursors. Mass losses of Na, K, Ca, Rb, Sr, Cs, Ba and volatiles were −30 to −60% and resulted from the destruction of white mica. Si was depleted from most selvages and transported locally to adjacent veins; average selvage Si losses were about −50%. Y and rare earth elements were added due to the growth of monazite in cracks cutting apatite. The mass balance analysis also suggests some addition of Ti occurred, consistent with the presence of rutile and hematite‐ilmenite solid solutions in veins. No major losses of Al from selvages were observed, but Al was added in some cases. Consequently, the Al needed to precipitate vein kyanite was not derived locally from the selvages. Veins more than an order of magnitude thicker than those typically observed in the field would be necessary to accommodate the Na and K lost from the selvages during alteration. Therefore, regional transport of Na and K out of the local rock system is inferred. In addition, to account for the observed abundances of kyanite in the veins, large fluid‐rock ratios (102–103 m3fluid m−3rock) and time‐integrated fluid fluxes in excess of ∼104 m3fluid m−2rock are required owing to the small concentrations of Al in aqueous fluids. It is concluded that the quartz‐kyanite veins and their selvages were produced by regional‐scale advective mass transfer by means of focused fluid flow along a thrust fault zone. The results of this study provide field evidence for considerable Al mass transport at greenschist to amphibolite facies metamorphic conditions, possibly as a result of elevated concentrations of Al in metamorphic fluids due to alkali‐Al silicate complexing at high pressures.  相似文献   

15.
Concordant igneous-looking bands of ferruginous bulk composition occur in a highly aluminous Precambrian metasedimentary series composed predominantly of kyanite quartzite. The bands consist of quartz, staurolite, and magnetite (partially martitized) with accessory amounts of muscovite, chlorite (pseudomorphous after biotite), chloritoid, apatite, and monazite. Quartz is found in three types (I–III) differing in appearance as well as in origin. Staurolite, in combination with quartz-II, shows peculiar radial sieve textures caused by mimetic crystallization after preexisting chloritoid rosettes. The chloritoid has been largely consumed, either by a reaction with hypothetical former kyanite to produce staurolite+ quartz with rock composition unchanged, or, possibly, by metasomatic introduction of oxygen (oxidation) to yield staurolite+quartz+magnetite; the remaining chloritoid, however, persisted in stable equilibrium with the other minerals of the rock. The staurolite quartzites are thus considered to represent original sedimentary bands which have undergone several stages of recrystallisation and (possibly) metasomatic modification during their metamorphic history. Their igneous aspect results from annealing crystallisation during a late static, i.e. postdeformational, thermal event of regional metamorphism.Chemical analysis of the staurolite shows no unusual features. For all staurolites plotted there is a positive relationship of the excess H+ over 2.0 and the Si+4-deficiency in the unit cell. This suggests partial substitution of 4 H+ for Si+4.The formation of staurolite in regional metamorphic rocks with excess silica, low alkali contents, and (FeO+MgO)/Al2O3 ratios < 1 showing chloritoid at lower grades appears to be governed, in many cases, by the reaction chloritoid+Al-silicate=staurolite+quartz+H2O.The assemblage chloritoid-staurolite may be stable in regional metamorphism over a limited pressure-temperature range.  相似文献   

16.
Calcsilicate xenoliths occur in large numbers in some lavas and pyroclastic flows of Lascar Volcano. Their whole-rock major element and REE compositions indicate that the protolith was the Upper Cretaceous Yacoraite Formation, which crops out extensively in NW Argentina. The whole-rock major element compositions of the xenoliths fall into specific groups suggesting a strong geochemical zonation in the skarn zone. Three geochemical zones have been identified; (1) an outer metamorphic zone rich in wollastonite; (2) a middle zone rich in pyroxene and garnet; (3) an inner zone rich in pyroxene and magnetite. The two innermost zones have developed from the wollastonite zone by infiltration of metasomatic fluids rich in Fe, Mn, Mg, Ti and Al. Whole-rock REE patterns have not changed significantly during prograde metamorphism and metasomatism, indicating REE immobility in the altering fluids. Retrograde alteration by acid-sulphate fluids produced anhydrite skarns and secondary calcite and wilkeite veins in the wollastonite zone. The carbon and oxygen isotopic compositions of this calcite indicate that it formed by Rayleigh crystallization from a low-temperature (<200 °C) fluid containing dissolved H2CO3. The calculated δ18O of the water in this fluid suggests a magmatic origin whereas the calculated δ13C of the dissolved carbonate is consistent with derivation from rocks of the Yacoraite Formation at 350 °C. It is suggested that the magmatic acid-sulphate fluid was responsible for leaching carbonate from the surrounding carbonate rocks and redepositing it in the skarn zone. REEs were mobilized during the retrograde acid-sulphate and acid-carbonate alteration. A negative Ce anomaly associated with this carbonate and sulphate indicates high oxygen fugacities in the mineralizing fluids.  相似文献   

17.
Chloritoid, and the Isochemical Character of Barrow's Zones   总被引：1，自引：0，他引：1  
It is argued that despite poverty of outcrop the apparent restrictionof chloritoid to a wedge-shaped area at the north-eastern extremityof Barrow's zones is real. Two possible interpretations of thisrestriction are considered: (a) That the chloritoid producingreaction (as yet unidentified) was characterized by a lowerP/T than that of the reaction muscovite+ chlorite+chloritoid+quartz staurolite+biotite+H2O, whereby, with increasing grade, chloritoidgives way to staurolite. A pressure gradient increasing fromnorth-east to south-west (postulated on separate grounds, Chinner,1966) would then result in the convergence of the chloritoidand staurolite isograds towards the south-west, and the eventualsuppression of the chloritoid isograd to give the wedge-shapedoutcrop actually found, (b) The lack of low-grade hydrous assemblagesaluminous enough to give chloritoid or staurolite with increasinggrade suggests that the low-grade limit of chloritoid (and,to the south-west, of staurolite) may not be an isograd, buta chemical boundary. Such a boundary could either be metasedimentary,or metasomatic, representing an alkali gradient of the typestudied by Orville, in which, essentially, potassium and waterreleased within the high-grade metamorphic zones have migratedto low-grade zones to form more micaceous assemblages. The widespreadexistence of ‘shimmer aggregate‘ muscovite alterationof aluminous minerals in thesillimanite, kyanite, and staurolitezones provides evidence of potassium transfer during the waneof metamorphic temperatures on a scale comparable to that which,during the main metamorphic imprint, would have been requiredto mask the development of peraluminous assemblages in the chlorite,biotite, and garnet zones.  相似文献   

18.
Brittle intra-crystal fracturing during a micro-seismic event under amphibolite facies P–T conditions (ca. 4.5 kbar, 555°C) formed micro-shear zones and brittle fragments in cordierite from migmatites of the Ordovician Sierras Pampeanas (NW Argentina). During post-seismic static recovery and coarsening of crystal fragments, primary cordierite (XMg?=?0.65) underwent partial breakdown within the fractured and subsequently recovered and recrystallized part of primary cordierite. Post-kinematic partial primary cordierite breakdown results in the formation of secondary sillimanite, magnetite, staurolite, quartz, and secondary cordierite (XMg?=?0.70–0.80) within a ca. 300–500 μm thick alteration zone. Secondary cordierite, volumetrically the most important secondary phase, essentially forms by iron and magnesium diffusion and substitution. All other secondary phases form by nucleation. The retrograde breakdown occurred under near-isochemical conditions with only limited influx of hydrothermal fluids containing minor zinc, potassium, silica, and excess oxygen. The observed modal composition of the alteration zone is similar to results of thermodynamic modeling of primary cordierite breakdown, using multiphase equilibria software. This indicates that the observed reaction reflects conditions of, or close to, thermodynamic equilibrium. Although fluid was present during, and may have been an important factor contributing to, partial mineral breakdown, we consider post-kinematic recovery and recrystallization, effected by a combination of cation diffusion and elimination of dislocations, as the key process facilitating retrograde metamorphism and the generation of the secondary mineral assemblage in (near-) equilibrium modes.  相似文献   

19.
Metapelites from the southern aureole of the Vedrette di Ries tonalite (eastern Alps) were variably overprinted by contact and earlier regional metamorphic events during pre-Alpine and Alpine metamorphic cycles. In these rocks, starting from a primary garnet mica-schist (garnet stage), a complex sequence of transformations, affecting the site of the garnet, has been recognized. In the outermost part of the aureole, the primary garnet sites are occupied by nodules of kyanite (kyanite stage). Closer to the tonalite, kyanite is replaced by staurolite (staurolite stage), which in turn is pseudomorphed by muscovite (muscovite stage). The aggregates of kyanite do not overgrow garnet directly; they post-date a stage (fibrolite stage) represented by the pseudomorphic alteration of garnet into fibrolitic sillimanite plus biotite. A further sericite stage is likely to have occurred between the fibrolite and kyanite stages. Preservation of the sub-spherical garnet shape during all these transformations and persistence of mineralogical and textural relicts from earlier stages were favoured by the very low strain experienced by the rocks since the garnet stage. The textural sequence is in agreement with the metamorphic history of this part of the Austroalpine basement of the Eastern Alps: the garnet and fibrolite stages, and the coeval main foliation of the samples, are referred to the high-grade Hercynian metamorphism; the kyanite stage to the Eo-Alpine metamorphism; the staurolite and muscovite stages to the Oligocene contact metamorphism. It is suggested that kyanite growth as microgranular aggregates took place in polymetamorphic rocks where static, high- P /low- T  metamorphism overprinted high- T  assemblages that contained sillimanite or andalusite.  相似文献   

20.
Prograde metamorphic reactions involving the growth of phyllosilicates and accompanying cleavage development have been investigated in Dalradian metasediments from the biotite zone of eastern Scotland. Crystallization of muscovite within the psammites of the Southern Highland Group is linked to the replacement of plagioclase porphyroclasts. This reaction is triggered by significant alkali metasomatism during active deformation and plays an important role in the formation of a prominent spaced cleavage within the psammites. The Si content of most of these early-formed muscovites is partially buffered by the quartz content of the rock, although close to the Highland Boundary Fault, evidence of greater influence from externally derived fluids on muscovite compositions is preserved. Locally higher fluid fluxes adjacent to the fault are also indicated by a relatively high δ 18O(SMOW) signature in the rocks. The biotite-producing reaction in these greenschist-facies rocks is linked to the later production of a celadonite-poor muscovite which formed as overgrowths around pre-existing white micas. This reaction is sensitive to the initial composition of muscovite and preferentially occurs in quartz-rich metasediments containing a celadonite-rich muscovite. A systematic increase in the progress of the biotite-producing reaction northwards across the biotite zone confirms the presence of high geothermal gradients along the southern margin of the Dalradian block, adjacent to the Highland Boundary Fault. Received: 26 February 1999 / Accepted: 8 November 1999  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号