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1.
周志强 《地质与勘探》2021,57(5):981-993
青海茫崖石棉矿区超基性岩体是由原岩以纯橄岩、辉橄岩和橄辉岩为主体组成的富镁质超基性岩体,经历自变质和后期多期热液的叠加变质蚀变作用,经蛇纹石化后形成蚀变完全的蛇纹岩岩体,其中部分蛇纹岩又进一步发生滑石化及碳酸盐化蚀变为滑石菱镁片岩、菱镁滑石片岩、滑石片岩和菱镁岩等。本文在野外地质调查基础上,在室内通过镜下岩矿综合鉴定、全岩化学成分分析以及电子探针成分分析等手段进行了岩石化学特征、矿物学特征及其蚀变演化过程研究。结果表明,该变质超基性岩体蛇纹岩主要特征组合矿物为蛇纹石(利蛇纹石、叶蛇纹石、纤蛇纹石)、磁铁矿、菱镁矿、滑石、水镁石、铬铁矿,变余矿物有斜方辉石、单斜辉石和铬铁矿,滑石菱镁片岩类主要组成矿物为菱镁矿、滑石、蛇纹石及磁铁矿,局部可见石英脉。该地区变质超基性岩体较完整地记录了橄榄岩水化、滑石化及碳酸盐化作用过程的各个阶段,超基性岩蚀变演化过程主要有两个作用阶段:(Ⅰ)橄榄石、辉石类矿物的蛇纹石化作用及蛇纹石绿泥石化作用;(Ⅱ)富Ca、CO2流体交代蛇纹石、滑石及水镁石的碳酸盐化作用。蛇纹石化等变质蚀变作用促进了Si、Mg及Fe元素化学活动性,使元素发生富集与迁移,对于次生矿物的形成与演化起到了一定的催化作用。多期不同组成流体热液的交代作用过程,清晰地展示了利蛇纹石、纤蛇纹石和叶蛇纹石的演化序列,以及滑石、水镁石、铬铁矿和磁铁矿的形成过程及标形特征。  相似文献   

2.
The Dabie-Sulu ultrahigh-pressure (UHP) terrane is the largest in the world. Mafic-ultramafic rocks occur as ubiquitous minor components in it, and preserve UHP index minerals such as coesite and microdiamond. Eclogites and garnet peridotites together with their country rocks including quartzofeldspathic, pelitic, psammatic, and carbonate rocks were subjected to coeval UHP meta-morphism during the Triassic collision between the Sino-Korean and Yangtze cratons. This review summarizes petrological and geochemical characteristics of eclogites, garnet peridotites, and UHP metasediments from published data and ongoing research in the Dabie-Sulu belt of east-central China.

Parageneses of minerals from coesite-bearing eclogites encased in gneiss, garnet peridotite, and marble delineate clockwise P-T paths characterized by nearly isothermal decompression. Many eclogites have been overprinted by Barrovian-zone amphibolite- to granulite-facies assemblages, whereas others preserve primary igneous minerals and textures in the cores of coesite-bearing eclogitic blocks. In addition, eclogites contain UHP hydrous and carbonate phases including talc, epidote, zoisite, magnesite, and dolomite; these together with Ti-clinohumite, phlogopite, amphibole, chlorite, and possible talc in garnet peridotites and OH-topaz in kyanite quartzites document the role of hydrous mineral transport to the deep mantle in fluid-deficient UHP metamorphic regions. Both crustal- and mantle-derived garnet peridotites from the Dabie-Sulu region were recrystallized within the diamond stability field, with the Earth's lowest recorded geothermal gradient. <5°C/km, suggesting that UHP metamorphic recrystallization took place in a previously unrecognized, forbidden P-T region.

Geochemical and isotopic data indicate that Dabie-Sulu mafic-ultramafic rocks have diverse origins; their compositions in some cases have been complicated by metamorphic recrystallization, crustal contamination, and fluid metasomatism. Nevertheless, REE geochemical and Nd isotope data clearly indicate that they have “continental” affinities and cannot represent a subducted Tethyan oceanic slab. Garnet peridotites and their enclosing eclogites display variable isotopic compositions; mantle-derived fragments preserve a mantle signature, whereas crust-hosted mafic-ultramafics display distinct crustal contamination and metasomatism. Among the many outstanding projects remaining to be investigated, geochemical and isotopic constraints of mantle-derived garnet peridotites and eclogites should provide an additional window to our understanding of mantle heterogeneity, metasomatism, slab/mantle interactions, and lithospheric evolution of the Sino-Korean craton.  相似文献   

3.
The Raobazhai ultramafic body of the North Dabie Complex is re-interpreted as a mantle-derived peridotitic slice enclosed in, and isofacially metamorphosed with, surrounding granulite-to-amphibolite facies gneisses. The ultramafic sheet consists mainly of metaharzburgite, but includes subunits of metadunite and mylonitic lherzolite. The rocks contain spinel but neither garnet nor plagioclase. However, in the mylonitic lherzolite, fine-grained intergrowths of spinel, orthopyroxene and clinopyroxene outline domains resembling the habit of garnet in two dimensions; broad-beam microprobe analyses imply pseudomorphs after a pyropic garnet precursor. The mineral assemblage of the metadunite and metaharzburgite is: olivine (Fo92)+orthopyroxene (En92)+tremolitic-to-magnesiohornblende+Mg–Al-chromite, indicating amphibolite facies recrystallization. The mineral assemblage of the mylonitic lherzolite is: olivine (Fo90)+orthopyroxene (En90)+clinopyroxene+Cr-bearing spinel+pargasitic amphibole, indicative of granulite-to-amphibolite facies metamorphism. Phase equilibria and geothermometric estimations show that the Raobazhai meta-ultramafics have undergone at least three stages of recrystallization: (I) 950–990 °C, (II) 750–860 °C, and (III) 670–720 °C, assuming equilibrium in the spinel peridotite stability field ( c. 6–15 kbar), although an early, high-pressure stage (≥18 kbar) is probable, based on the inferred garnet pseudomorphs. Petrochemical and geothermobarometric data suggest that the ultramafic slice represents a fragment of the mantle wedge, tectonically incorporated into subducted continental crust and re-equilibrated at granulite-to-amphibolite facies conditions while being exhumed to shallow levels.  相似文献   

4.
A large mass of dolomitic marble including many eclogite blocks occurs in orthogneisses of the Rongcheng area of the Su-Lu province, eastern China. The marble consists mainly of dolomite, calcite (formerly aragonite), graphite, forsterite, diopside, talc, tremolite and phlogopite. Aggregates of talc and calcite occur at the boundary between dolomite and diopside. Tremolite is a reaction product between talc and calcite. Eclogite blocks are rimmed by dark green amphibolite. The primary mineral assemblage in the core of eclogite is Na-bearing garnet (up to 0.2  wt% Na2O), omphacitic pyroxene, clintonite and rutile. Secondary minerals are pargasitic/edenitic amphibole, plagioclase, sodic diopside, chlorite, zoisite and titanite. The peak metamorphic conditions, based on stability of the dolomite+forsterite+aragonite (now calcite)+graphite assemblage, under conditions where tremolite is unstable, are estimated at T  =610–660 °C and P =2.5–3.5  GPa (for X CO=0.001). A reaction between dolomite and diopside to form talc under tremolite-unstable conditions indicates a temperature decrease under ultra-high-pressure conditions ( P >2.4  GPa, X CO<0.0013). The formation of secondary tremolite is consistent with a nearly adiabatic pressure decrease post-dating the ultra-high-pressure metamorphism. The temperature decrease under ultra-high-pressure conditions preceding decompression may reflect the underplating of a cold slab, and the rapid decompression probably corresponds to the upwelling stage promoted by the delamination of a downwelling lithospheric root. The P – T  conditions of the amphibolitization stage are estimated at <0.9  GPa and <460 °C, and are similar to conditions recorded by the surrounding orthogneisses.  相似文献   

5.
The Smartville Complex is a late Jurassic, rifted volcanic arc in the northern Sierra Nevada, California. Near Auburn, California, it consists of a lower volcanic unit, dominated by basaltic flows, and an upper volcanic unit of andesitic volcaniclastic rocks, both of which have been intruded by dykes and irregular bodies of diabase. These rocks contain relict igneous minerals, and the metamorphic minerals albite, chlorite, quartz, pumpellyite, prehnite, epidote, amphibole, titanite, garnet, biotite, K-feldspar, white mica, calcite, and sulphide and oxide minerals.
Prehnite–pumpellyite (PrP), prehnite–actinolite (PrA), and greenschist (GS) zones have been identified. The pumpellyite-out isograd separates the PrP and PrA zones, and the prehnite-out isograd separates the PrA and GS zones. The minerals Ab + Qtz + Mt + Tn are common to most assemblages in all three zones. The MgO/(MgO + FeO) ratio of the effective bulk composition has an important and systematic effect on the observed mineral assemblages in the PrP zone. Prehnite-bearing assemblages contain the additional minerals, Pmp + Amp + Ep + Chl in MgO-rich rocks, and either Pmp + Ep + Chl or Amp + Ep + Chl in less magnesian rocks. Subcalcic to calcic amphibole is common in the PrP zone. The mineral assemblage Prh + Act + Ep + Chl, without Pmp, characterizes the PrA zone, and the mineral assemblage Act + Ep + Chl, without Prh or Pmp, characterizes the GS zone. The disappearance of pumpellyite and prehnite occurred by continuous reactions.
The sequence of mineral assemblages was produced by burial metamorphism at P–T conditions of 300° 50°C at approximately 2.5 ± 0.5 kbar. During metamorphism, the composition of the fluid phase was nearly 100% H2O and the oxygen fugacity was between the hematite–magnetite and quartz–fayalite–magnetite buffers.  相似文献   

6.
G. Rebay  B. Messiga 《Lithos》2007,98(1-4):275-291
In the coronitic metagabbroic rocks of the Corio and Monastero metagabbro bodies in the continental Sesia–Lanzo zone of the western Italian Alps, a variety of mineral reactions that testify to prograde conditions from greenschist to eclogite-facies can be recognised. A microstructural and microchemical study of a series of samples characterized by coronitic textures and pseudomorphic replacement of the original igneous minerals has allowed the prograde reactions undergone by the rocks to be established.

In completely eclogitized coronitic samples, paragonite, blue amphibole, garnet, epidote, fine grained jadeite and chloritoid occur in plagioclase microdomains (former igneous plagioclase). The mafic mineral microdomains consist of glaucophane and garnet. Complexly-zoned amphiboles constrain changing metamorphic conditions: cores of pre-Alpine brown hornblende and/or tremolite are preserved inside rims of a sodic–calcic amphibole that are in turn surrounded by a sodic amphibole. The main high-pressure mineral assemblage, as seen in mylonites, involves glaucophane, chloritoid, epidote, garnet ± phengite, ± paragonite. Some layers within the gabbro contain garnet, omphacite, ± glaucophane, and acid dykes crosscutting the gabbro body contain jadeite, quartz, garnet, epidote and paragonite.

The presence of chloritoid-bearing high-pressure assemblages reflects hydration of the gabbros during their pre-Alpine exhumation prior to subduction, as well as the composition of the microdomains operating during subduction. The pressure and temperature conditions of gabbro transformation during subduction are inferred to be 450–550 °C at up to 2 GPa on the basis of the chloritoid-bearing assemblages. The factors controlling the reaction pathway to form chloritoid-bearing high-pressure assemblages in mafic rocks are inferred from these observations.  相似文献   


7.
P–T  paths based on parageneses in the immediate vicinity of former high-temperature contact zones between mantle peridotites and granulitic country rocks of the Central Vosges (NE France) were derived by applying several conventional thermometers and thermobarometric calculations with an internally consistent dataset. The results indicate that former garnet peridotites and garnet–spinel peridotites were welded together with crustal rocks at depths corresponding to 1–1.2 GPa. The temperature of the crustal rocks was about 650–700 °C at this stage, whereas values of 1100 °C (garnet peridotites) and 800–900 °C (garnet–spinel peridotites) were calculated for the ultramafic rocks. After emplacement of the mantle rocks, exhumation of the lower crust took place to a depth corresponding to 0.2–0.3 GPa. The temperatures of the incorporated peridotite slices were still high (900–1000 °C) at this stage. This is indicated by the presence of high- T  /low- P parageneses ( c . 800 °C, 0.2–0.3 GPa) in a small (1–10 m) contact aureole around a former garnet peridotite. Crustal rocks distant to the peridotites equilibrated in the same pressure range at lower temperature (650–700 °C). High cooling rates (102–103 °C Ma−1) were calculated for a garnet–biotite rock inclusion in the peridotites and for the crustal rocks at the contact by applying garnet–biotite diffusion modelling. Minimum rates of 0.75–7.5 cm a−1 are required for vertical ascent of rock units (30 km vertical distance) derived from the crust–mantle boundary, resulting in a late Variscan (340 Ma) high- T  /low- P event.  相似文献   

8.
The Luliangshan garnet peridotite massif is an ultramafic complex in the North Qaidam UHPM belt, NW China. The strongly layered complex comprising garnet-bearing dunite, garnet-harzburgite, garnet-lherzolite and garnet-pyroxenite and garnet-free dunite, occurs together with eclogite embedded in various continental gneisses. The geological setting, the internal structure, bulk-composition, rare earth elements, isotopic and mineral composition data show that the garnet peridotite derives from a middle Ordovician Alaskan-type layered sub-arc cumulate intrusion of ascending mantle wedge melts. An abyssal peridotite protolith can be excluded. During the Ordovician-Silurian continental collision, thickening and foundering, the Luliangshan peridotite complex was exposed to ultrahigh pressures (UHP) reaching 5.5 GPa possibly >6 GPa at temperatures of 900 °C (perhaps up to 1000 °C) corresponding to a depth of ∼200 km. The extreme pressure conditions have been derived from thermobarometry using mineral compositions of the garnet peridotite assemblages, but they are supported by a wealth of decompression-induced mineral exsolutions in UHP minerals and by diamond inclusion in zircon. The Luliangshan garnet peridotite has experienced four stages of retrograde overprint during exhumation that lasted into the Devonian: (i) decompression-induced unmixing of the UHP minerals; (ii) garnet kelyphitisation; (iii) amphibole overprinting and (iv) serpentinization. Hydrous minerals occurring within peak metamorphic assemblage represent pseudo-inclusions, that is reaction products of reactions related to various stages of decompression and cooling rather than prograde inclusions during porphyroblast growth.  相似文献   

9.
A second natural occurrence of yoderite   总被引：3，自引：0，他引：3  
A second example of yoderite has been discovered in whiteschists from the Southern Chewore Hills of northern Zimbabwe. The mineral is pale green in colour and occurs in an equilibrium assemblage with talc+chlorite+kyanite+dravite+hematite. There is no quartz present. Recalculated microprobe analyses give a structural formula of Mg2Al5.7Fe0.3Si4O18(OH)2, similar to that obtained for the type locality at Mautia Hill, Tanzania, i.e. Mg2Al5.6Fe0.4Si4O18(OH)2. Textural relationships and relative proportions of minerals suggest that the yoderite was formed by reaction between talc, chlorite, kyanite and hematite. Experimental evidence suggests high-water-pressure metamorphic conditions at temperatures exceeding a reaction curve that extends between 13  kbar at 590  °C and 21  kbar at 650  °C. The yoderite-bearing whiteschist is associated with a 1.4  Ga dismembered ophiolite. It is proposed that this yoderite occurrence is associated with a relict subduction/suture zone.  相似文献   

10.
The Mary granite, in the East Athabasca mylonite triangle, northern Saskatchewan, provides an example and a model for the development of non-migmatitic gneissic texture. Gneissic compositional layering developed through the simultaneous evolution of three microdomains corresponding to original plagioclase, orthopyroxene and matrix in the igneous rocks. Plagioclase phenocrysts were progressively deformed and recrystallized, first into core and mantle structures, and ultimately into plagioclase-rich layers or ribbons. Garnet preferentially developed in the outer portions of recrystallized mantles, and, with further deformation, produced garnet-rich sub-layers within the plagioclase-rich gneissic domains. Orthopyroxene was replaced by clinopyroxene and garnet (and hornblende if sufficient water was present), which were, in turn, drawn into layers with new garnet growth along the boundaries. The igneous matrix evolved through a number of transient fabric stages involving S-C fabrics, S-C-C' fabrics, and ultramylonitic domains. In addition, quartz veins were emplaced and subsequently deformed into quartz-rich gneissic layers. Moderate to highly strained samples display extreme mineralogical (compositional) segregation, yet most domains can be directly related to the original igneous precursors. The Mary granite was emplaced at approximately 900 °C and 1.0 GPa and was metamorphosed at approximately 750 °C and 1.0 GPa. The igneous rocks crystallized in the medium-pressure granulite field (Opx–Pl) but were metamorphosed on cooling into the high-pressure (Grt–Cpx–Pl) granulite field. The compositional segregation resulted from a dynamic, mutually reinforcing interaction between deformation, metamorphic and igneous processes in the deep crust. The production of gneissic texture by processes such as these may be the inevitable result of isobaric cooling of igneous rocks within a tectonically active deep crust.  相似文献   

11.
Hydration of eclogite, Pam Peninsula, New Caledonia   总被引：2，自引：0，他引：2  
Garnet glaucophanite and greenschist facies assemblages were formed by the recrystallization of barroisite-bearing eclogite facies metabasites in northern New Caledonia. The mineralogical evolution can be modelled by calculated P–T and P–X H2O diagrams for appropriate bulk rock compositions in the model system CaO–Na2O–FeO–MgO–Al2O3–SiO2–H2O. The eclogites, having developed in a clockwise P–T path that reached P ≈19 kbar and T  ≈590 °C, underwent decompression with the consumption of free H2O as the volume of hydrous minerals increased. Eclogite is preserved in domains that experienced no fluid influx following the loss of this fluid. Garnet glaucophanite formed at P ≈16 kbar during semi-pervasive fluid influx. Fluid influx, after further isothermal decompression, was focused in shear zones, and resulted in chlorite–albite-bearing greenschist facies mineral assemblages that reflect P ≈9 kbar.  相似文献   

12.
Diffusion modelling is applied to layered garnet–pyroxene–quartz coronas, formed by a pressure-induced reaction between plagioclase and primary pyroxene in a metabasic granulite. The reconstructed reaction involves some change in composition of reactant minerals. The distribution of minerals between layers is satisfactorily explained by diffusion-controlled reaction with local equilibrium, in which the diffusion coefficient for Al was smaller than those for Fe, Mg and Ca by a factor of approximately four. Diffusion of Mg towards plagioclase implies a chemical-potential gradient for MgO component in a direction opposite to the changing Mg content of garnet; this is explained by the influence of Al2O3 on the chemical potential of the pyrope end-member. Grain-boundary diffusion is suggested to have operated, possibly with composition gradients different from those in the bulk minerals. Chemical-potential differences across the corona are estimated from the variation in garnet composition, enabling affinity (the free energy change driving the reaction) to be estimated as 6.9±1.8  kJ per 24-oxygen mole of garnet produced. This implies that the pressure for equilibrium among the minerals was overstepped by 1.4±0.4  kbar. The probable P–T conditions of reaction were in the range 650–790  °C, 8–10  kbar. Assuming a timescale of reaction between 106 and 108 years, estimated diffusion coefficients for Fe, Mg and Ca are in the range 9×10−23 to 5×10−20 m2 s−1. These are consistent with experimental values in the literature for solid-state diffusion, including grain-boundary diffusion.  相似文献   

13.
In this work, the factors controlling the formation and preservation of high-pressure mineral assemblages in the metamorphosed orthopyroxene-bearing metagranitoids of the Sandmata Complex, Aravalli-Delhi Mobile Belt (ADMB), northwestern India have been modelled. The rocks range in composition from farsundite through quartz mangerite to opdalite, and with varying K2O, Ca/(Ca + Na)rock and FeOtot + MgO contents. A two stage metamorphic evolution has been recorded in these rocks.
An early hydration event stabilized biotite with or without epidote at the expense of magmatic orthopyroxene and plagioclase. Subsequent high-pressure granulite facies metamorphism (∼15 kbar, ∼800 °C) of these hydrated rocks produced two rock types with contrasting mineralogy and textures. In the non-migmatitic metagranitoids, spectacular garnet ± K-feldspar ± quartz corona was formed around reacting biotite, plagioclase, quartz and/or pyroxene. In contrast, biotite ± epidote melting produced migmatites, containing porphyroblastic garnet incongruent solids and leucosomes.
Applying NCKFMASHTO T–M (H2O) and P–T pseudosection modelling techniques, it is demonstrated that the differential response of these magmatic rocks to high-pressure metamorphism is primarily controlled by the scale of initial hydration. Rocks, which were pervasively hydrated, produced garnetiferous migmatites, while for limited hydration, the same metamorphism formed sub-solidus garnet-bearing coronae. Based on the sequence of mineral assemblage evolution and the mineral compositional zoning features in the two metagranitoids, a clockwise metamorphic P–T path is constrained for the high-pressure metamorphic event. The finding has major implications in formulating geodynamic model of crustal amalgamation in the ADMB.  相似文献   

14.
In the Austroalpine Mont Mary nappe (Italian Western Alps) discrete zones of mylonites–ultramylonites developed from coarse-grained, upper amphibolite facies metapelites of pre-Alpine age. The syn–mylonitic mineral assemblage is quartz–biotite–muscovite–plagioclase–garnet–sillimanite–ilmenite–graphite, and formed via the model hydration reaction: Grt1+Kfs+H2O=Bt2+Ilm2+Qtz+Ms± Sil .Grain-size reduction of about three orders of magnitude was accompanied by extensive recrystallization of all minerals except sillimanite, and by compositional changes of garnet and biotite. Deformation took place at temperatures of 510–580  °C under low-pressure conditions (0.25–0.45 GPa) and corresponds to the latest stages of pre-Alpine metamorphic evolution. The pre-Alpine mylonitization conditions were close to the brittle-ductile transition, as indicated by syn–mylonitic generation of pseudotachylytes and high differential stress inferred from quartz grain-size piezometry. The brittle-ductile behaviour at a relatively high temperature, and the absence of annealing textures in quartz aggregates, are suggestive of water-deficient conditions during mylonitization. These were accomplished through progressive consumption of water by syn–kinematic hydration reaction and by adsorption onto the greatly increased grain boundary area resulting from dynamic recrystallization.  相似文献   

15.
Garnet peridotites occur as lenses, blocks or layers within granulite–amphibolite facies gneiss in the Dabie-Sulu ultra-high-pressure (UHP) terrane and contain coesite-bearing eclogite. Two distinct types of garnet peridotite were identified based on mode of occurrence and petrochemical characteristics. Type A mantle-derived peridotites originated from either: (1) the mantle wedge above a subduction zone, (2) the footwall mantle of the subducted slab, or (3) were ancient mantle fragments emplaced at crustal depths prior to UHP metamorphism, whereas type B crustal peridotite and pyroxenite are a portion of mafic–ultramafic complexes that were intruded into the continental crust as magmas prior to subduction. Most type A peridotites were derived from a depleted mantle and exhibit petrochemical characteristics of mantle rocks; however, Sr and Nd isotope compositions of some peridotites have been modified by crustal contamination during subduction and/or exhumation. Type B peridotite and pyroxenite show cumulate structure, and some have experienced crustal metasomatism and contamination documented by high 87Sr/86Sr ratios (0.707–0.708), low εNd( t ) values (−6 to −9) and low δ18O values of minerals (+2.92 to +4.52). Garnet peridotites of both types experienced multi-stage recrystallization; some of them record prograde histories. High- P–T  estimates (760–970 °C and 4.0–6.5±0.2 GPa) of peak metamorphism indicate that both mantle-derived and crustal ultramafic rocks were subducted to profound depths >100 km (the deepest may be ≥180–200 km) and experienced UHP metamorphism in a subduction zone with an extremely low geothermal gradient of <5 °C km−1.  相似文献   

16.
Characterization of some Egyptian serpentinites used as ornamental stones   总被引：1，自引：0，他引：1  
Egypt is characterized by wide occurrence of serpentinites, particularly in the central and southern parts of Eastern Desert. There are several cooperate factors that affect the serpentinites used as ornamental stones. These factors involve mineralogical characteristics （including mineral composition and microstructure parameter）, physical properties and mechanical properties. Antigorite is an essential serpentinite mineral, with a minor amount of chrysotile. Talc, carbonates （magnesite and dolomite） and tremolite are the main associated minerals. Quality and quantity of associated minerals such as talc and carbonates （dolomite and magnesite） affect the properties of serpentinites used as ornamental stones. Carbonates are resistant to weathering but suffer from acidic cleaning agents in interior use, whereas serpentinites with a high content of talc used on external faces undergo an increase in volume and a consequent rapid degradation. Studied serpentinites are characterized by relatively small grain size with foliated texture, low water absorption, low porosity, and high abrasion resistance. In the samples studied the uniaxial compressive strength （UCS） varies between 89 and 189 MPa, with an average of 152 MPa. According to the classification of Bell （1992）, sample No. B8 is very high in strength while the rest high in strength.  相似文献   

17.
High-pressure metamorphic rocks exposed in the Bantimala area, c . 40  km north-east of Ujung Pandang, were formed as a Cretaceous subduction complex with fault-bounded slices of melange, chert, basalt, turbidite, shallow-marine sedimentary rocks and ultrabasic rocks. Eclogites, garnet–glaucophane rocks and schists of the Bantimala complex have estimated peak temperatures of T  =580–630 °C at 18  kbar and T  =590–640 °C at 24  kbar, using the garnet–clinopyroxene geothermometer. The garnet–omphacite–phengite equilibrium is used to estimate pressures. The distribution coefficient K D1=[( X pyr)3( X grs)6/( X di)6]/[(Al/Mg)M2,wm (Al/Si)T2,wm]3 among omphacite, garnet and phengite is a good index for metamorphic pressures. The K D1values of the Bantimala eclogites were compared with those of eclogites with reliable P–T  estimates. This comparison suggests that peak pressures of the Bantimala eclogites were P =18–24  kbar at T  =580–640 °C. These results are consistent with the P–T  range calculated using garnet–rutile–epidote–quartz and lawsonite–omphacite–glaucophane–epidote equilibria.  相似文献   

18.
Abstract Three types of mineral associations are described from calc-silicate granulites from the Eastern Ghats, India, where geothermobarometry in associated rocks suggests extremely high P–T conditions of metamorphism ( c . 9 ± 1 kbar, 950° C). These mineral associations are: (i) calcite + quartz + scapolite + plagioclase, (ii) calcite + scapolite + wollastonite + porphyroblastic garnet + coronal garnet and (iii) calcite + quartz + wollastonite + scapolite + porphyroblastic garnet + coronal garnet, all coexisting with K-feldspar, titanite and clinopyroxene. The first two associations evolved through nearly isobaric cooling retrograde paths, whereas the third evolved through a nearly isothermal decompression path followed by an isobaric cooling retrograde path. Textural and compositional characteristics suggest the following mineral reactions in the calc-silicate granulites: calcite + quartz = wollastonite + CO2, calcite + plagioclase = scapolite, calcite + scapolite + wollastonite = porphyroblastic garnet ± quartz + CO2, CaTs + wollastonite = coronal garnet (association ii) and wollastonite + scapolite = coronal garnet (association iii) + quartz + CO2. Andradite content in garnet was buffered by the redox equilibria wollastonite + hedenbergite + O2= andradite + quartz (association iii) and wollastonite + andradite + CaTs + scapolite = hedenbergite + calcite + grossular + O2 (association ii). The contrasting mineral parageneses have been ascribed to interplay of variables such as X CO2, f O2, f HCl in the fluid, bulk Na content and the nature of the retrograde P–T–X CO2 paths through which the rocks evolved.  相似文献   

19.
Abstract The central sector of Mühlig-Hofmannfjellet (3°E/71°S) in western Dronning Maud Land (East Antarctic shield) is dominated by large intrusive bodies of predominantly orthopyroxene-bearing quartz syenites (charnockites). Metasedimentary rocks are rare; however, two distinct areas with banded gneiss–marble–quartzite sequences of sedimentary origin were found during the Norwegian Antarctic Research Expedition NARE 1989/90. Cordierite-bearing metapelitic gneisses from two different localities contain the characteristic mineral assemblage: cordierite + garnet + biotite + K-feldspar + plagioclase + quartz ± sillimanite ± spinel. Thermobarometry indicates equilibration conditions of about 650°C and 4 kbar. Associated orthopyroxene–garnet granulites, on the other hand, revealed pressures of about 8 kbar and temperatures of 750°C. The earlier granulite facies metamorphism is not well preserved in the cordierite gneisses as a result of excess K-feldspar combined with interaction with an H2O-rich fluid phase, probably released by the cooling intrusives. These two features allowed the original high-grade K-feldspar + garnet assemblages to recrystallize as cordierite–biotite–sillimanite gneisses, completely re-equilibrating them. Phase relationships indicate that the younger metamorphic event occurred in the presence of a fluid phase that varied in composition between the lithologies.  相似文献   

20.
Abstract The Rockley Volcanics from near Oberon, New South Wales occur within the aureole of the Carboniferous Bathurst Batholith and have been contact metamorphosed at P ∼ 100 ± 50MPa (10.5kbar) and a maximum T ∼ 565°C in the presence of a C–O–H fluid. Prior to contact metamorphism the volcanics were regionally metamorphosed and altered with the extensive development of actinolite, chlorite, plagioclase, quartz and calcite. The contact metamorphosed equivalents of these rocks have been subdivided into: Ca-poor (cordierite + gedrite), Mg-rich (amphibole + olivine + spinel), mafic (amphibole + plagioclase) and Ca-rich (amphibole + garnet + diopside; diopside + plagioclase; garnet + diopside + wollastonite) rocks.
The chemistry of the minerals in the hornfelses was controlled by the bulk rock chemistry and fluid composition. Pargasites and hastingsites as well as an unusual phlogopite with blue green pleochroism, are found in Ca-rich hornfelses. A comparison of the assemblages with experimentally derived equilibria suggests that the fluid phase associated with the Ca-rich hornfelses was water-rich (Xco2= 0.1 to 0.3) while that associated with the Mg-rich hornfelses was enriched in CO2 (Xco2 > 0.7). The different hornfels types have reacted to contact metamorphism independently in both their solid and fluid chemistries.  相似文献   

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