摘要:

AbstractRecognition of several ultrahigh‐pressure (UHP) metamorphic terranes in continental collision belts has revolutionized the concept of geodynamic processes. In order to facilitate better communication and focus among active investigators, the Task Group III‐6 of the International Lithosphere Program‘Ultrahigh‐Pressure Metamorphism and Geodynamics in Collision‐type Orogenic Belts’held the first two day workshop at Stanford University in December, 1994. Petrotectonic settings, mineral paragenesis, geochronoldgy, and geochemical characteristics of UHP rocks from several recognized and suspected UHP terranes were addressed. This special issue presents 11 papers from the more than 50 contributions from the 88 participants representing 15 countries. Many challenging petrotectonic and petrochemical problems remain to be investigated. These include detailed P‐T time paths for both the UHP unit and adjacent units, the role of fluids at mantle depths, deep seismic profiles and mechanisms and rate of exhumation of

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00146.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractPetrogenetic grids for ultrahigh‐pressure (UHP) metamorphism were calculated at differentXco2conditions in the model system CaO‐MgO‐SiO2‐CO2‐H2O involving coesite (Co), diopside (Di), dolomite (Do), enstatite (En), forsterite (Fo), magnesite (Ms), quartz (Qz), talc (Tc), tremolite (Tr) using a published internally consistent thermodynamic data set. Two P‐T grids atXco2= 0.01 and 0.5 are described. In the calculated P‐T grid atXco2= 0.01, four out of 10 stable invariant points, Co‐En‐Ms‐Tc, Co‐Di‐En‐Tc‐Tr, Co‐Di‐Ms‐Tc‐Tr and Di‐En‐Ms‐Tc‐Tr lie within the stability field of coesite. If the fluid phase hasXco2= 0.5, no invariant point is stable under UHP conditions. Some magnesite‐bearing assemblages are stabilized by the following three reactions: Di + Ms = Do + Fo + CO2, Ms + Tr = Do + Fo + CO2+ H2O and Ms + Tc = Fo+ CO2+ H2O atXco2= 0.01 and by reaction Ms + Tc = Fo + CO2+ H2O together with these three atXco2= 0.5. Ten possible UHP assemblages for mafic and ultramafic compositions at very lowXco2conditions include the following: Co‐Do‐Ms, Co‐Di‐Ms, Co‐Di‐Tc, Di‐Ms‐Tc, Di‐En‐Tc‐, Di‐En‐Ms, Co‐Di‐En, Di‐En‐Fo, Di‐Fo‐Ms, Di‐Do‐Fo. Among them, talc‐bearing assemblages are restricted toXco2<0.02 and their high‐P limit is 31.7 kb (749°C) atXco2= 0.01. Dolomite‐magnesite‐silica assemblages have large P‐T stability fields even ifXco2is as low as 0.1, and could occur in cold subduction zones with very low geothermal gradients. Reported UHP coesite‐dolomite assemblage is restricted only to a calc‐silicate rock interlayered with marble whereXco2is relatively higher; no such assemblage appears for mafic and ultramafic rocks with lowXco2evidenced by the occurrence of diopside (or omphacite) at the expense of dolomite + coesite. The effect ofXco2on the stability of coesite‐dolomite‐magnesite, diopside‐enstatite‐magnesite, diopside‐talc assemblages is examined and the occurrence of coesite‐dolomite, magnesite

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00147.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe Maksyutov Complex, situated in the southern Ural Mountains of Russia, is the first location where quartz aggregates within garnets exhibiting radial fractures were identified as coesite pseudomorphs (Chesnokov&Popov 1965). The complex consists of two tectonic units: a structurally lower eclogite‐bearing schist unit and an overlying meta‐ophiolite unit. Both units show evidence for multiple stages of metamorphism and deformation. The high‐pressure metamorphism of the eclogite‐bearing schist unit, discussed in this report, is suspected to be related to a collision between the Russian platform and a fragment of the Siberian continent during the early Cambrian. At least three stages of metamorphism (M1‐3) and two stages of deformation (S1and S2) were observed in thin sections: M1) garnet (Alm55‐60, Prp22‐28, Grs16‐20) + omphacite (Jd46‐56) + phengite (Si ≅ 3.5) + rutile; M2) garnet + glaucophane ± lawsonite + white mica; and M3) epidote + chlorite ± albite ± actinolite + white mica. Observed mineral parageneses define a retrograde P‐T path for the eclogite. Mineral assemblages within the most representative eclogite from the lower unit of the Maksyutov Complex indicate minimum peak pressures of 15 kbar at temperatures of approximately 600°C. If the presence of coesite pseudomorph is confirmed, the peak ultrahigh‐pressure metamorphism may be

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00148.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe Kokchetav Complex is a tectonic mega‐melange consisting of seven pre‐Ordovician units (units I‐VII) of contrasting lithologies and P–T conditions of metamorphism, overlain and/or intruded by four post‐recrystallization entities. Most of the constituent rock types display affinities with continental crust; paraschists and paragneisses, which carry biogenically produced carbon, clearly were laid down near the surface of the Earth. Microdiamond (and rare coesite) inclusions are contained in strong, refractory garnet, zircon, clinopyroxene, and kyanite, some of the constituent neoblastic phases of this metasedimentary unit. Systematic mineral parageneses and textural relationships support the hypothesis that the metamorphic assemblages represent a close approach to chemical equilibrium at the time of formation. Metamorphism of diamond‐bearing paragneisses and schists transpired at 535 ± 5 Ma; physical conditions included minimum pressures of 40 kbar and temperatures exceeding 900 °C. Other associated units contain mineralogic evidence of somewhat lower to considerably lower pressures and temperatures: observed magnesite + diopside pairs, coesite, grossular‐pyropic garnet, potassic clinopyroxene, Si‐rich phengite, barroisite‐crossite(?), aluminous titanite and/or Al‐rutile, and the assemblage talc + kyanite + garnet all testify to relatively elevated pressures of formation. The metamorphosed lithotectonic units represent individual, discrete stages in what initially may have been a continuous P‐T series, but intense post‐metamorphic dislocation has resulted in the preservation of a chaotically mixed sequence rather than an unbroken gradation in preserved conditions of metamorphism. Only units I‐III, and probably VIb may represent portions of a dismembered subduction zone lithologie assemblage. The uplift to mid‐crustal levels and cooling of the mega‐melange took place by about 515–517 Ma, at which time the complex was stabilized as a part of the Kazakhstan microcontinental collage. An hypothesized Late Vendian‐Early Cambrian subduction of the Kazakhstan‐North Tianshan(?) microcontinental salient to depths exceeding 125 km, followed by decoupling from the descending oceanic crust‐capped lithospheric plate is held responsible for the ultrahigh‐pressure metamorphism of the Kokchetav Complex. Inasmuch as vestiges of a calc‐alkaline volcanic/plutonic arc of approximately Early Cambrian age are preserved as only scattered relics in the general region, the plate‐tectonic setting may have involved an intra‐oceanic, Marianas‐type, incipient arc which was subsequen

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00149.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractEclogites occur in three districts of the northern and southern parts of Tien‐Shan. Three eclogites collected from the Aktyuz, Makbal and Atbashy districts were analyzed; the P‐T paths of three eclogites were estimated by analyzing compositional growth zoning and retrograde reaction of garnet and omphacite. Aktyuz and Makbal eclogites have not preserved the prograde path. An Aktyuz eclogite that underwent a quartz eclogite facies metamorphism (about T = 600°C, P = 12 kbar) has recorded three stages of retrograde metamorphism. Four stages of retrograde metamorphism were recognized in a Makbal eclogite; the garnet‐omphacite geothermometer gave about T = 560°C at 20 kbar as the highest metamorphic condition. Garnet from a garnetchloritoid‐talc schist of the Makbal district includes quartz pseudomorphs after coesite; some units evidently underwent a low‐temperature part of coesite eclogite fades metamorphism. Prograde and retrograde paths were recognized in an Atbashy eclogite; five stages of metamorphic reaction were observed in the Atbashy sample. The prograde path from stage I to stage III has been recorded in garnet and omphacite in which quartz pseudomorphs after coesite are included. The peak metamorphism of stage III took place at about 660°C at 25 kbar. The stages IV and V are retrograde. UHP eclogite facies metamorphism took place twice in Kyrghyzstan. The Aktyuz and Atbashy eclogites gave Rb‐Sr mineral‐isochron ages of about 750 Ma and 270 Ma, respectively. The K‐Ar age of paragonite from the Makbal eclogi

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00150.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractAltered quartz‐rich and nearly quartz‐free eclogitic rocks and completely retrograde quartz‐rich garnet amphibolites occur as blocks or lenses in gneisses at Weihai, northeastern tip of the Sulu ultrahigh‐P belt. Eclogitic rocks with assemblage garnet ± clinopyroxene ± coesite + rutile have experienced three‐stage metamorphic events including ultrahigh‐pressure eclogite, granulite and amphibolite facies. Granulite metamorphic event is characterized by formation of the hypersthene + salite + plagioclase ± hornblende corona between garnet and quartz + clinopyroxene. P‐T conditions for the three‐stage recrystallization sequence are 840 ± 50°C,>28 kbar, about 760±50°C, 9 kbar, and ~650°C,<8 kbar respectively. Most country rock gneisses contain dominant amphibolite‐facies assemblages; some garnet‐bearing clinopyroxene gneisses recrystallized under granulite‐facies conditions at about 740±50°C and 8.5 kbar; similar to granulite‐facies retrograde metamorphism of the enclosed eclogitic blocks. Minor cale‐silicate lenses within gneisses containing an assemblage grossular + salite + titanite + quartz with secondary zoisite and plagioclase may have formed within a large pressure range of 14‐35 kbar. Eclogitic boudins and quartzo‐feldspathic country rocks may have experienced coevalin situUHP and subsequent retrograde metamorphism. The established nearly isothermal decompression P‐T path suggests that this area may represent the interior portion of a relatively large subducted sialic block. The recognized UHP terrane may extend eastward acro

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00151.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractGarnet clinopyroxenite containing porphyroclastic clinopyroxene with garnet lamellae from the Su‐Lu ultrahigh‐pressure (UHP) province, eastern China, records a three‐stage evolutionary history. Stage A: precursor aluminous clinopyroxene associated with Mg‐rich spinel was stable. Stage B: Mg‐rich spinel and aluminous clinopyroxene recrystallized to form porphyroblasts of garnet and diopside with garnet lamellae, respectively. Thereafter, these porphyroblasts were granulated and recrystallized to form equigranular neoblasts in the matrix, being driven by subsolidus deformation. Stage C: the assemblage developed retrograde hornblende, epidote, spinel, chlorite, calcite, and dolomite. The estimated composition of the precursor aluminous clinopyroxene indicates that equilibrium conditions of the stage A were 900‐1000°C, and 1.0‐1.5 GPa. The neoblast garnet‐clinopyroxene pairs, not conjectural, give 740 ± 50°C and higher than 2.5 GPa for the climax conditions of stage B. This implies that the Hujialin garnet clinopyroxenite was transported from a low dP/dT setting to a high dP/dT setting, probably related to subduction during stage B. The Hujialin garnet clinopyroxenite as well as adjacent UHP eclogite which records 700°C and 3.0 GPa as the maximum physical conditions, experienced amphibolite facies retrogression, suggesting that they shared a common P‐T hist

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00152.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractPetrological studies of a serpentinized garnet lherzolite body in Rongcheng of the Su‐Lu region of eastern China revealed unusually high pressure. Spinel lherzolite probably in a subducting slab was transformed to garnet lherzolite at mantle depth. During exhumation, they were subsequently subjected to the granulite and then amphibolite overprinting and a phase of serpentinization. The peak P–T conditions of the garnet lherzolite estimated after detailed analysis of the metamorphic texture are 4–5 GPa and 820°C or 5–6 GPa and 780°C, depending on the chosen geothermobarometers. The lower dP/dT of the garnet lherzolite can be interpreted as the results of subduction of an old (say 100 Ma older than the time of collision) and cold, slab underneath the margin of the Sino–Ko

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00153.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractKyanite‐anthophyllite schist preserves the first record of high pressure in the amphibolite‐facies unit of the SW Dabie Mountains, whereas ultrahigh‐ and high‐pressure (UHP and HP) metamorphism has been well documented by the occurrence of coesite, diamond and mafic eclogite in the SE Dabie Mountains. Textural evidence indicates that minerals of the kyanite‐anthophyllite schist formed mainly in two stages: (i) garnet + kyanite + antho‐phyllite + rutile formed at pressure in excess of 1.2 GPa at T<650°C; (ii) cordierite±staurolite formed by reaction of anthophyllite + kyanite at P<0.5 GPa, T∼530°C. Plagioclase and ilmenite replaced garnet and rutile respectively during decompression. In a still later stage, secondary biotite recrystallized, accompanied by sillimanite replacing kyanite, and spinel replacing staurolite. The P‐T information suggests that the amphibolite unit in the SW Dabie Mountains is part of the Triassic collision belt between the Sino‐Korean and Yangtze cratons. The P‐T paths of the UHP eclogite in the eastern Dabie Mountains and the HP kyanite‐anthophyllite schist in the SW Dabie Mountains show similar decompression and equivalent late stage Barrovian‐style metamorphism. Emplacement of voluminous granitoid at middle crustal levels between 134–118 Ma contributed to the development of the Barrovian‐type metam

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00154.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractAccording to field occurrence and P‐T condition, eclogites of southern Henan and northern Hubei Provinces can be divided into two types: medium temperature (MT) and low temperature (LT) eclogites. MT eclogite occurs as layers or lenticular bodies within migmatized gneiss of the Dabie Group. This study is the first to report an occurrence of the assemblages coesite and kyanite + talc in this area. Garnet exhibits a distinct prograde compositional zoning and has mineral inclusions with rotational textures indicating syntectonic growth. Five evolutionary stages are outlined. (1) Pre‐eclogite stage, determined by the inclusions of barroisite + zoisite + quartz in the cores of zoned garnets. (2) Eclogite stage, characterized by garnet + omphacite + kyanite ± talc + coesite + rutile, represents the peak metamorphism. The peak conditions are estimated to be T = 600‐700°C, P>27 kb. (3) Glaucophane stage, without an appearance of plagioclase, is assigned to a transitional stage. Blades of glaucophane form rims around garnet grains as a result of the reaction talc + jadeite = glaucophane. This marks the beginning of retrograde metamorphism. (4) Symplectite stage, where eclogitic minerals break down, and Amp + Pl symplectite develops around garnet or omphacite; (5) Later retrograde stage is represented by epidote‐amphibolite assemblages. Low temperature eclogite appears as blocks in the Qijiaoshan Formation (part of the Susong Group). Four stages can be identified: (1) Pre‐eclogite stage, amphibole + epidote + sphene inclusions occur in garnet core; (2) Eclogite stage, consists of garnet + omphacite + rutile + quartz + phengite + glaucophane + zoisite. The peak conditions are T = 490‐560°C, P<15 kb; (3) Symplectitic stage, is characterized by the breakdown of eclogitic minerals; (4) Greenschist facies stage, is recorded by a greenschist facies assemblage. The difference between the two types of eclogites suggests contrasting processes. A model is proposed whereby partial melting of continental crust and the emplacement of tonalite occurs during the exhumation of ultrahigh‐pressure

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00155.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY