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1.
Margarite is both abundant and widespread throughout a sequence of interstratified amphibolite, hornblendite, and metamorphosed anorthosite from the upper Lyvia River, central Fiordland. These rock types comprise part of a metamorphosed layered intrusion. Assemblages recorded from these rocks are the product of two distinct phases of metamorphism. First generation assemblages typically comprise plagioclase (An84–96), hornblende, kyanite, and minor corundum. Clinozoisite and chlorite occur as late stage breakdown products of plagioclase and hornblende. Margarite developed during the second phase of metamorphism.Within the corundum-bearing rocks replacement of corundum or plagioclase by margarite can be observed directly. On the basis of these observations the following reaction is evident: 1 corundum+1 anorthite+1H2O=1 margarite.In other assemblages the formation of margarite can be attributed to the breakdown of kyanite and clinozoisite according to the reaction: 2 kyanite+2 clinozoisite=1 margarite+3 anorthite.Margarite is found, however, to contain appreciable amounts of paragonite solid-solution (up to 28 mol%) and plagioclase produced (second generation) is not pure anorthite but of intermediate compositions (An46–62). The reaction therefore involves the introduction of both soda and silica. Margarite also crystallized independently of clinozoisite according to a reaction of the general form: 5 pargasite+17 kyanite+19 H2O =8 margarite+4 chlorite+7 plagioclase.Application of available experimental data suggests that the margarite formed between 550 and 720 ° C up to a maximum pressure of 9.5 kb. Whereas the involvement of albite component (second generation plagioclase) will tend to lower the temperatures and pressures necessary for the occurrence of margarite, this effect is partially offset by the significant amounts of paragonite end-member held within the margarite. An independent estimate of the metamorphic conditions in metapelites suggests that the introduction of albite lowers equilibration temperatures by about 2 ° C for every 1% albite.  相似文献   

2.
Thermodynamic calculations have shown that the dP/dT slope of the reaction 4 margarite+3 quartz5 kyanite +2 zoisite+3 H2O as determined by Storre and Nitsch (1974) is too steep. This reaction has been reinvestigated using synthetic margarite, zoisite, kyanite, and natural quartz in the starting mixtures and using infrared spectroscopy to examine the run products. The experimentally determined dP/dT slope ranges between –2.2 and –17 bars/ K, which is in excellent agreement with predictions based on thermodynamics. An internally consistent set of univariant curves could be fitted to the experimental reversals for the above reaction and for the reactions margarite+ quartz anorthite+kyanite+H2O and 2 zoisite+kyanite +quartz 4 anorthite+H2O investigated by Nitsch et al. (1981) and Goldsmith (1981), respectively. Addition of up to 40 mol % of the component NaAl2(Si3Al) ·O10(OH)2 (paragonite) to margarite will increase the stability of the margarite solid solution plus quartz by 2–3 kbar without significantly affecting the dP/dT slope, making the paragenesis margarite plus quartz a good geobarometer.  相似文献   

3.
An increasing number of occurrences of margarite have been reported in the last years. However, previous experimental investigations in the system CaO-Al2O3-SiO2-H2O are limited to the synthesis of margarite and to the upper stability limit according to the reaction (1) 1 margarite?1 anorthite +1 corundum +1 H2O (Chatterjee, 1971; Velde, 1971). Since margarite often occurs together with quartz, the upper stability limit of margarite in the presence of quartz is of special interest. Therefore, the reactions (5) 1 margarite +1 quartz ?1anorthite +1 kyanite/andalusite +1 H2O and (6) 4 margarite+3 quartz ? 2 zoisite+5 kyanite+3 H2O were investigated experimentally using mixtures of natural margarite (from Chester, Mass., USA), quartz, kyanite, andalusite, zoisite, and synthetic anorthite. The indicated equilibrium temperatures at water pressures equal to total pressure are: 515± 25°C at 4 kb, 545 ±15°C at 5 kb, 590±10°C at 7 kb, and 650±10°C at 9 kb for reaction (5), and 651±11°C at 10 kb, 648 ± 8°C at 12.5kb, and 643±13°C at 15kb for reaction (6), respectively. Besides this, additional brackets for equilibrium temperatures were determined for the above cited reaction (1): 520±10°C at 3 kb, 580±10°C at 5 kb, and 640± 20°C at 7 kb. On the basis of these experimentally determined reactions (1), (5), and (6) and of the reactions (3) 2 zoisite +1 kyanite? 4 anorthite +1 corundum +1 H2O (7) 2 zoisite +1 kyanite +1 quartz ? 4 anorthite +1 H2O and (10) 1 pyrophyllite ? 1 andalusite/kyanite+3 quartz+1 H2O for which experimental or, in the case of reaction (3), calculated data were already available, a pressure-temperature diagram with 3 invariant points and 11 univariant reactions was developed using the method of Schreinemakers. This diagram, summarizing both experimental and phase relation studies, allows conclusions about the conditions under which margarite has been formed in nature. Margarite is limited to low grade metamorphism at water pressures up to approximately 3.5 kb; in the presence of quartz, margarite is even limited to low grade metamorphism at water pressures up to 5.5 kb. Only at water pressures higher than the values stated before margarite, and margarite+quartz, respectively, can occur in medium grade metamorphism (as defined by Winkler, 1970 and 1973). For the combined occurrence of margarite+quartz and staurolite as reported by Harder (1956) and Frey (personal communication, 1973) it may be estimated that water pressure has been greater than approximately 5.5 kb, wheras temperature has been in the range from 550 to 650°C. Furthermore, the present study shows that the assemblage zoisite+kyanite (+ H2O) is an indicator of both pressure [P H 2 O> approximately 9kb]and temperature [T> approximately 640 to 650° Cat water Pressures up to 15 kb].  相似文献   

4.
Geothermometry and geobarometry of 10 garnet–oligoclase zone schists in the Franz Josef–Fox Glacier area, Southern Alps, New Zealand, give temperatures ranging from 415 to 625°C and pressures from 5.2 to 9.2 kbar, indicating a T–P array of about 50°C/kbar and inferred peak temperature conditions over a c. 15-km-thick section at depths between c. 20 and 34 km. The present-day distribution of the schist samples implies that only about one-third of the original crustal section is now exposed.
The garnet–oligoclase zone schists represent the deeper part of a metamorphosed and deformed accretionary complex that was associated with late Palaeozoic–early Mesozoic subduction along the Gondwana continental margin. Partial uplift ( c. 0.2 m/Ma) and erosion of the complex during Jurassic–Cretaceous times (Rangitata uplift) was synchronous with D2 deformation and recrystallization, as recorded by the P–T array. Cenozoic (Kaikoura) uplift and exhumation of the schist since c. 30 Ma to form the Southern Alps was associated with oblique-slip movement on the Alpine Fault. The present-day position and steep eastward dip of isograds and D2 structures suggest considerable clockwise rotation during uplift associated with ductile attenuation and tectonic thinning by over two-thirds of the original schist sequence, largely due to simple shear along schistosity planes. As the schist generally shows only incipient greenschist facies retrograde recrystallization, an apparently complete (although contracted) prograde mineral sequence has been preserved by rapid uplift (>5 km/Ma) of hot rock and the effects of limited shear heating near the Alpine Fault.  相似文献   

5.
F.S. Spear  G. Franz 《Lithos》1986,19(3-4):219-234
Petrologic data on the paragenesis of (I) kyanite-zoisite marbles and (II) garnet-chloritoid quartz-mica schists are presented with the goal of providing constraints on the pressure-temperature evolution of the Eclogite Zone, Tauern Window, Austria. The peak metamorphic assemblages in the two rock types are: (I) kyanite + zoisite + dolomite + quartz; zoisite + muscovite + dolomite + calcite + quartz; and (II) garnet + chloritoid + kyanite + muscovite + quartz + epidote ± dolomite ± Zn-staurolite. The estimated peak metamorphic conditions are 19 ± 2 kbar, 590 ± 20°C.

Secondary alteration of the kyanite-zoisite marbles was accomplished in two stages. The early stage resulted in the production of margarite, paragonite, secondary muscovite and chlorite and the later stage resulted in the formation of sudoite (a di/trioctahedral Mg---Al layer silicate) and kaolinite. The early alteration is bracketed at conditions between 3 and 10 kbar, 450–550°C and the later alteration between 200 and 350°C, P 3 kbar.

The P-T path is characterized by maximum burial to approximately 19 kbar (60–70 km) (at≈590°C), followed by nearly isothermal decompression to approximately 10 kbar (30 km), and then more gradual decompression with cooling to approximately 3 kbar (10 km). Alteration was apparently accomplished by the influx of H2O-rich fluids, with the composition of the fluid locally buffered by the mineral assemblage.  相似文献   


6.
The Southern Alps of New Zealand is an actively rising mountain belt which displays a thermal anomaly adjacent to the Alpine Fault, the Australian-Pacific plate boundary. Extensive fluid movement occurs in this uplift zone, resulting in metallic vein mineralization. Gold mineralization is confined to greenschist facies rocks, while younger veins in amphibolite facies rocks near the Alpine Fault are enriched in copper. Transport and deposition of metals in this complex hydrothermal system is governed by interaction between rising metamorphic fluids and downward-percolating meteoric fluid. Metamorphic fluids have equilibrated with graphitic schist country rock and are relatively reduced. Infiltration and mixing of meteoric water increases oxygen activity and decreases sulphur activity in the fluid. Oxidised meteoric water heats up and dissolves Cu during downward percolation. This Cu is deposited as the fluid becomes more reduced. Hence, there is a progressive increase in copper content in the middle portions of the hydrothermal system, especially in the more permeable highly fractured rocks near the Alpine Fault.  相似文献   

7.
Margarite and Paragonite are found coexisting in amphibolites of the Untere Schieferhülle in the area of the upper Schlegeistal (Zillerthal Alps, Northern Tyrol). These amphibolites are metamorphosed under conditions of the low grade amphibolite facies. The chemical composition of the two micas was determined by the electron microprobe. A maximum of 14 Mol-% margarite and 18 Mol-% muscovite enters into the paragonite, the margarite being entered by 20 to 50 Mol-% paragonite and a maximum of 10 Mol-% muscovite. There seems to be a solubility gap between margarite and paragonite in a range between 15 and 50 Mol-% margarite.At their margins the margarites and paragonites breakdown into a mixture of feldspar and into a fine, microscopically not identifiable phase. Plagioclases having An 28 to An 42 result from breakdown of paragonite, feldspars between An 50 and An 60 probably arose from breakdown of margarite. A definite statement on this probelem is not possible because the smallness and the inhomogeneity of the feldspar grains.Based on the experimental data concerning the stability of margarite, paragonite (±quartz, ±CO2) and kyanite, the P-T-range of the metamorphosis is discussed.  相似文献   

8.
Abstract The E-W-trending Kohistan terrane in the NW Himalaya is a sandwich of a magmatic arc between the collided Karakoram (Asian) and Indian plates. The southern part of the Kohistan arc is principally made up of amphibolites derived from volcanic and plutonic rocks of Early Cretaceous age. Gabbroic relics in the amphibolites display calc-alkaline character, and their mineralogy is similar to low-P plutonic rocks reported from modern and ancient island arcs. The largest of these relics, occurring along the southern margin of the amphibolite belt near Khwaza Khela, is subcircular in outline and is about 1 km across. It consists of cumulate gabbros and related rocks displaying a record of cooling and crustal thickening. Primary olivine and anorthite reacted to produce coronas consisting of two pyroxenes +Mg-Fe2+-Al spinel ± tschermakitic hornblende at about 800° C, 5.5–7.5 kbar. This thermotectonic event is of regional extent and may be related to the overthrusting of the Karakoram plate onto the Kohistan arc some 85 Ma ago, or even earlier. Later the gabbros were locally traversed by veins containing high-P assemblages: garnet, kyanite, zoisite, paragonite, oligoclase, calcite, scapolite and quartz ° Chlorite ° Corundum ± diopside. Formed in the range 510–600° C, and 10–12 kbar, these suggest further thickening and cooling of the crust before its uplift during the Tertiary. This paper presents microprobe data on the minerals, and discusses the tectonic implications of the coronitic and vein assemblages in the gabbros.  相似文献   

9.
The Bixiling mafic-ultramafic metamorphic complex is a 1•5km2 tectonic block within biotite gneiss in the southern Dabieultrahigh-pressure terrane, central China. The complex consistsof banded eclogites that contain thin layers of garnet-bearingcumulate ultramafic rock. Except for common eclogitic phases(garnet, omphacite, kyanite, phengite, zoisite and rutilc),banded eclogites contain additional talc and abundant coesiteinclusions in omphacite, zoisite, kyanite and garnet. Some metaultramaficrocks contain magnesite and Ti-clinohumite. Both eclogites andmeta-ultramafic rocks have undergone multi-stage metamorphism.Eclogite facies metamorphisrn occurred at 610–700C andP>27 kbar, whereas amphibolite facies retrograde metamorphismis characterized by symplectites of plagioclase and hornblendeafter omphacite and replacement of tremolite after talc at P<6–15kbar and T <600C. The meta-ultramafic assemblages such asolivine + enstatite + diopside + garnet and Ti-clinohumite +diopside + enstatite + garnet + magnesite olivine formed at700–800C and 47–67 kbar. Investigation of the phaserelations for the system CaO-MgO-SiO2-H2O-CO2 and the experimentallydetermined stabilities of talc, magnesite and Ti-clinohumiteindicate that (1) UHP talc assemblages are restricted to Mg-Algabbro composition and cannot be an important water-bearingphase in the ultramafic mantle, and (2) Ti-clinohumite and magnesiteare stable H2O-bearing and CO2-bearing phases at depths >100km. The mafic-ultramafic cumulates were initially emplaced atcrustal levels, then subducted to great depths during the Triassiccollision of the Sine-Korean and Yangtze cratons. KEY WORDS: eclogite; magnesite; meta-ultramafics; talc; ultrahigh-P metamorphism *Corresponding author  相似文献   

10.
Abstract In the Su-Lu ultrahigh- P terrane, eastern China, many coesite-bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet-quartz-jadeite rock and kyanite quartzite. In addition to garnet + omphacite + rutile + coesite, other peak-stage minerals in some eclogites include kyanite, phengite, epidote, zoisite, talc, nyböite and high-Al titanite. The garnet-quartz-jadeite rock and kyanite quartzite contain jadeite + quartz + garnet + rutile ± zoisite ± apatite and quartz + kyanite + garnet + epidote + phengite + rutile ± omphacite assemblages, respectively. Coesite and quartz pseudomorphs after coesite occur as inclusions in garnet, omphacite, jadeite, kyanite and epidote from both eclogites and metasediments. Study of major elements indicates that the protolith of the garnet-quartz jadeite rock and the kyanite quartzite was supracrustal sediments. Most eclogites have basaltic composition; some have experienced variable 'crustal'contamination or metasomatism, and others may have had a basaltic tuff or pyroclastic rock protolith.
The Su-Lu ultrahigh- P rocks have been subjected to multi-stage recrystallization and exhibit a clockwise P-T path. Inclusion assemblages within garnet record a pre-eclogite epidote amphibolite facies metamorphic event. Ultrahigh- P peak metamorphism took place at 700–890° C and P >28 kbar at c . 210–230 Ma. The symplectitic assemblage plagioclase + hornblende ± epidote ± biotite + titanite implies amphibolite facies retrogressive metamorphism during exhumation at c . 180–200 Ma. Metasedimentary and metamafic lithologies have similar P-T paths. Several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in-situ ultrahigh- P metamorphism during the Triassic collision between the Sino-Korean and Yangtze cratons.  相似文献   

11.
Beginning of melting and subsolidus relationships in the system K2O-CaO-Al2O3-SiO2-H2O have been experimentally investigated at pressures up to 20 kbars. The equilibria discussed involve the phases anorthite, sanidine, zoisite, muscovite, quartz, kyanite, gas, and melt and two invariant points: Point [Ky] with the phases An, Or, Zo, Ms, Qz, Vapor, and Melt; point [Or] with An, Zo, Ms, Ky, Qz, Vapor, and Melt.The invariant point [Ky] at 675° C and 8.7 kbars marks the lowest solidus temperature of the system investigated. At pressures above this point the hydrated phases zoisite and muscovite are liquidus phases and the solidus temperatures increase with increasing pressure. At 20 kbars beginning of melting occurs at 740 °C. The solidus temperatures of the quinary system K2O-CaO-Al2O3-SiO2-H2O are almost 60° C (at 20 kbars) and 170° C (at 2kbars) below those of the limiting quaternary system CaO-Al2O3-SiO2-H2O.The maximum water pressure at which anorthite is stable is lowered from 14 to 8.7 kbars in the presence of sanidine. The stability limits of anorthite+ vapor and anorthite+sanidine+vapor at temperatures below 700° C are almost parallel and do not intersect. In the wide temperature — pressure range at pressures above the reaction An+Or+Vapor = Zo+Ms+Qz and temperatures below the melting curve of Zo+Ms+Ky+Qz+Vapor, the feldspar assemblage anorthite+sanidine is replaced by the hydrated phases zoisite and muscovite plus quartz. CaO-Al2O3-SiO2-H2O. Knowledge of the melting relationships involving the minerals zoisite and muscovite contributes to our understanding of the melting processes occuring in the deeper parts of the crust. Beginning of melting in granites and granodiorites depends on the composition of plagioclase. The solidus temperatures of all granites and granodiorites containing plagioclases of intermediate composition are higher than those of the Ca-free alkali feldspar granite system and below those of the Na-free system discussed in this paper.The investigated system also provides information about the width of the P-T field in which zoisite can be stable together with an Al2SiO5 polymorph plus quartz and in which zoisite plus muscovite and quartz can be formed at the expense of anorthite and potassium feldspar. Addition of sodium will shift the boundaries of these fields to higher pressures (at given temperatures), because the pressure stability of albite is almost 10kbars above that of anorthite. Assemblages with zoisite+muscovite or zoisite+kyanite are often considered to be products of secondary or retrograde reactions. The P-T range in which hydration of granitic compositions may occur in nature is of special interest. The present paper documents the highest temperatures at which this hydration can occur in the earth's crust.  相似文献   

12.
A new occurrence of kyanite eclogite in the Pirin Mountains of southwestern Bulgaria within the rocks belonging to the Obidim Unit of the Rhodope Metamorphic Complex is presented. This eclogite provides important information about the peak–pressure conditions despite strong thermal overprint at low pressure. Textural relationships, phase equilibrium modelling and conventional geothermobarometry were used to constrain the metamorphic evolution. Garnet porphyroblasts with inclusions of omphacite (up to 43 mol.% Jd), phengite (up to 3.5 Si p.f.u.), kyanite, polycrystalline quartz, pargasitic amphibole, zoisite and rutile in the Mg‐rich cores (XMg = 0.44–0.46) record a prograde increase in P–T conditions from ～2.5 GPa and 650 °C to ～3 GPa and 700–750 °C. Maximum pressure values fall within the stability field of coesite. During exhumation, the peak–pressure assemblage garnet + omphacite + phengite + kyanite was variably overprinted by a lower pressure one forming symplectitic textures, such as diopside + plagioclase after omphacite and biotite + plagioclase after phengite. The development of spinel (XMg = 0.4–0.45) + corundum + anorthite assemblage in the kyanite‐bearing domains at ～1.1 GPa and 800–850 °C suggests a thermal overprint in the high‐pressure granulite facies stability field. This thermal event was followed by cooling at ～0.8 GPa under amphibolite facies conditions; retrograde kelyphite texture involving plagioclase and amphibole was developed around garnet. Our results add to the already existing evidence for ultra high pressure (UHP) metamorphism in the Upper Allochthon of the Rhodope Metamorphic Complex as in the Kimi Unit and show that it is more widespread than previously known. Published age data and field structural relations suggest that the Obidim Unit represents Variscan continental crust involved into the Alpine nappe edifice of the Rhodopes and that eclogite facies metamorphism was Palaeozoic, in contrast to the Kimi Unit where age determinations suggest a Jurassic or Cretaceous age for UHP metamorphism. This implies that UHP metamorphism in the Upper Allochthon of the Rhodopes may have occurred twice, during Alpine and pre‐Alpine orogenic events, and that two independent HP/UHP provinces of different age overlap in this area.  相似文献   

13.
The Marguerite Amphibolite and associated rocks in northern Fiordland, New Zealand, contain evidence for retention of Carboniferous metamorphic assemblages through Cretaceous collision of an arc, emplacement of large volumes of mafic magma, high‐P metamorphism and then extensional exhumation. The amphibolite occurs as five dismembered aluminous meta‐gabbroic xenoliths up to 2 km wide that are enclosed within meta‐leucotonalite of the Lake Hankinson Complex. A first metamorphic event (M1) is manifest in the amphibolite as a pervasively lineated pargasite–anorthite–kyanite or corundum ± rutile assemblage, and as diffusion‐zoned garnet in pelitic schist xenoliths within the amphibolite. Thin zones of metasomatically Al‐enriched leucotonalite directly at the margins of each amphibolite xenolith indicate element redistribution during M1 and equilibration at 6.6 ± 0.8 kbar and 618 ± 25 °C. A second phase of recrystallization (M2) formed patchy and static margarite ± kyanite–staurolite–chlorite–plagioclase–epidote assemblages in the amphibolite, pseudomorphs of coronas in gabbronorite, and thin high‐grossular garnet rims in the pelitic schists. Conditions of M2, 8.8 ± 0.6 kbar and 643 ± 27 °C, are recorded from the rims of garnet in the pelitic schists. Cathodoluminescence imaging and simultaneous acquisition of U‐Th‐Pb isotopes and trace elements by depth‐profiling zircon grains from one pelitic schist reveals four stages of growth, two of which are metamorphic. The first metamorphic stage, dated as 340.2 ± 2.2 Ma, is correlated with M1 on the basis that the unusual zircon trace element compositions indicate growth from a metasomatic fluid derived from the surrounding amphibolite during penetrative deformation. A second phase of zircon overgrowth coupled with crosscutting relationships date M2 to between 119 and 117 Ma. The Early Carboniferous event has not previously been recognized in northern Fiordland, whereas the latter event, which has been identified in Early Cretaceous batholiths, their xenoliths, and rocks directly at batholith margins, is here shown to have also affected the country rock. However, the effects of M2 are fragmentary due to limited element mobility, lack of deformation, distance from a heat source and short residence time in the lower crust during peak P and T. It is possible that many parts of the Fiordland continental arc achieved high‐P conditions in the Early Cretaceous but retain earlier metamorphic or igneous assemblages.  相似文献   

14.
Abstract

Two groups of stretching lineations can be distinguished in the Central Alpine " root zone " between Ticino and Mera :

1) Steeply plunging lineations formed during retrograde metamor-Phism under amphibolite/greenschist facies conditions indicate an uplift movement of the Central Alps. The lineations can be related to an important back-thrusting event of late Oligocene/early Miocene age.

2) Gently plunging lineations formed under lower greenschist facies conditions display a pattern typical of a dextral strike-slip system. These lineations are of early Miocene age.

This cpmbined movement, achieved by ductile deformation along the lnsubric line was followed by a stage of brittle deformation in a dextral strike-slip system (= Tonale line).

The signification of this interpretation is shown in a new crustal cross section through the Central Alpine/Southern Alpine border zone in the Iicino area.  相似文献   

15.
 The combined information about the stratigraphies from the foreland basins surrounding the Swiss Alps, exhumation mechanisms and the structural evolution of the Alpine orogenic wedge allow an evaluation of the controls of erosion rates on large-scale Alpine tectonic evolution. Volumetric data from the Molasse Basin and fining-upward trends in the Gonfolite Lombarda indicate that at ∼20 Ma, average erosion rates in the Alps decreased by >50%. It appears that at that time, erosion rates decreased more rapidly than crustal uplift rates. As a result, surface uplift occurred. Because of surface uplift, the drainage pattern of the Alpine hinterland evolved from an across-strike to the present-day along-strike orientation. Furthermore, the decrease of average erosion rates at ∼20 Ma coincides with initiation of a phase of thrusting in the Jura Mountains and the Southern Alpine nappes at ∼50 km distance from the pre-20-Ma thrust front. Coupled erosion-mechanical models of orogens suggest that although rates of crustal convergence decreased between the Oligocene and the present, the reduction of average erosion rates at ∼20 Ma was high enough to have significantly influenced initiation of the state of growth of the Swiss Alps at that time. Received: 8 June 1998 / Accepted: 30 October 1998  相似文献   

16.
Abstract The effects of Tertiary Alpine metamorphism on pelitic Mesozoic cover rocks have been studied along a cross-section in the central Lepontine Alps in the Nufenen Pass area, Switzerland. Greenschist facies to amphibolite facies conditions are indicated by the formation of the index minerals chloritoid, garnet, staurolite and kyanite in pelitic rocks. Regional metamorphism reached maximum conditions during the interkinematic period between a main Alpine penetrative (D2) and a late Alpine (D3) crenulation type deformation phase or synchronous with the late Alpine deformation. Based on AFM phase relationships four different metamorphic zones can be distinguished: (1) chloritoid zone; (2) staurolite + chlorite zone; (3) staurolite + biotite zone; and, (4) kyanite zone. The isograds that separate these zones can be modelled by univariant reactions in the KFMASH system. The conditions of metamorphism calculated from geological ther-mobarometers for the maximum post-D2 por-phyroblast stage are from North to South: 500° C at 5-6 kbar and 600° C at 7-8 kbar. Detailed thermobarometry of garnet por-phyroblasts with complex textures suggests that maximum temperature was reached later than maximum pressure. Early garnet growth occurred along a prograde P-T-path, post-D2 rims grew with increasing temperature but decreasing pressure, and finally post-D3 garnet formed along a retrograde P-T-path. It may be concluded from the calculated pressure and temperature difference over a short distance (3 km) across the mapped area that the isogradic surfaces of the post-D2 metamorphism are steeply oriented. The data also suggest that isobaric and isothermal surfaces are parallel. Much of the observed metamorphic pattern can be explained as the result of a significant post-D2 differential uplift of the hot Pennine area relative to the Helvetic area along a tectonic contact zone. The closely spaced isograds (isotherms) in the North may then be interpreted as a thermal effect owing to the emplacement of the hot Pennine rocks against the Got-thard massif with its cover. Whereas, in the Pennine metasediments, post-D2 porphyroblast formation can be related to the decompression path which was steep enough for dehydration reactions to proceed. It is also remarkable that late kyanite porphyroblasts probably formed with decreasing pressure. The interpretation given here for the Nufenen Pass area may also apply to the Luk-manier Pass area where similar metamorphic patterns have been reported by Fox (1975). The formation of the ‘Northern Steep Belt’;, as denned by Milnes (1974b), and the associated late Alpine fold zones may, therefore, have significantly modified the metamorphic pattern of the Helvetic-Penninic contact zone.  相似文献   

17.
The regional distribution of metamorphic mineral assemblages in Mesozoic carbonate rocks of the Western Hohe Tauern allows the mapping of isograds based on the appearance of biotite+calcite and biotite+zoisite+calcite. The latter isograd corresponds approximately to the thermal maximum of the alpidic metamorphism in the central part of this area. An estimate of P, T, X fluid conditions can be obtained from phase relations among muscovite, biotite, chlorite, margarite, tremolite, zoisite, anorthite, quartz, calcite, and dolomite in the system K2O-CaO-MgO-Al2O3-SiO2-H2O-CO2 which approximates the composition of marls. Calculations based on various experimental and thermodynamic data have been made with emphasis on phase relations pertinent to a group of carbonate rocks with very low Fe and Na contents in non-opaque minerals. Significant and opposite deviations from the phase relations for stochiometric end member mineral compositions are due to the substitutions F-OH and Mg+Si-2Al. Consistency of observed and calculated phase relations is favoured by high F-contents. For the majority of carbonate rocks in the high metamorphic zone, maximum temperatures around 550° C, minimum pressures of 4–6 kb, and relatively low XCO2 values within the stability field of zoisite and of biotite+calcite+quartz are indicated.  相似文献   

18.
The Archaean lode-gold deposits at Norseman, Western Australia, consist of auriferous quartz veins in dextral-reverse ductile-brittle shear zones within tholeiitic metabasalts of upper-greenschist to amphibolite facies metamorphic grade. Three types of deposits (Northern, Central, Southern) are delineated on the basis of their spatial distribution, veining style, alteration mineraloty and metamorphic grade of host rocks. Northern deposits, hosted in upper-greenschist to lower-amphibolite facies rocks, comprise massive to laminated quartz veins with selvedges of quartz-chlorite-calcite-biotite-plagioclase assemblages. Central deposits, hosted in lower-amphibolite facies rocks, consist of laminated to massive quartz veins with selvedges of quartz-actinolite-biotite-plagioclasecalcite assemblages. Southern deposits, hosted in middleamphibolite facies metabasalts, consist of banded quartz-diopside-calcite-microcline-zoisite veins. All deposits exhibit variable ductile deformation of veins and contiguous alteration haloes, consistent with a syn-deformational genesis at high temperatures. From Northern to Southern deposits, the alteration assemblages are indicative of higher temperatures of formation, and there are progressively greater degrees of dynamically recovered textures in alteration and gangue minerals. These observations imply that a thermal variation of gold-related hydrothermal alteration exists within the Norseman Terrane over a distance of 40 km, with TNorthern<TCentral<TSouthern This thermal zonation is corroborated by T−XCO 2 phase relations between vein selvedge assemblages, which signify formation temperatures of approximately 420°–475°C, 470°–495°C and >500°C for Northern, Central and Southern deposits, respectively. The sum of structural, petrographic and mineral chemistry data indicates that the alteration assemblages formed in high-temperature, open hydrothermal systems and have not been subsequently metamorphosed. The thermal differences between the deposit groups may reflect (1) a temperature gradient, at relatively constant P, corresponding to the proximity of the deposits to regional granitoid complexes, or (2) formation of the deposits at progressively deeper crustal levels from north to south. In either case the deposits represent a continuum of gold deposition from upper-greenschist to amphibolite facies, now exposed in an oblique section through the Archaean crust at Norseman.  相似文献   

19.
Petrology of high-pressure granulites from the eastern Himalayan syntaxis   总被引：36，自引：0，他引：36  
The eastern Himalayan syntaxis, situated at the eastern terminus of the Himalayas, is the least-known segment of the Himalayas. Recent research in this area has revealed that the syntaxis consists of the Gangdise, the Yarlung Zangbo, and the Himalayan units, each of which is bounded by faults. The Himalayan unit, the northernmost exposed part of the Indian plate, mainly contains amphibolite facies rocks, marked by the assemblages staurolite+kyanite+plagioclase+biotite+muscovite±sillimanite and garnet+amphibole+plagioclase, in the south; to the north, low- to medium-pressure granulite grade pelitic gneisses and marbles are present and are characterized by the assemblages garnet+sillimanite+K-feldspar+plagioclase or antiperthite+biotite+quartz±spinel±cordierite±orthopyroxene in gneisses, and anorthite+diopside±wollastonite and plagioclase+diopside+quartz+phlogopite+calcite in marbles. Within this unit, the Namula thrust system is a series of moderately north-dipping structures that displaced the granulite facies rocks southwards over the amphibolite facies rocks. High-pressure granulites occur as relics within these granulite facies rocks and contain garnet–kyanite granulite and garnet clinopyroxenite. The peak assemblage of the garnet–kyanite granulite includes garnet (core part)+kyanite+ternary feldspar+quartz+rutile. Sillimanite+garnet (rim part)+K-feldspar+ oligoclase+ilmenite+biotite and spinel+albite+biotite or spinel+cordierite±orthopyroxene, which are coronas around sillimanite and garnet, are retrograde products of this peak assemblage. Another peak assemblage includes very-high-Ca garnet (CaO 32–34 wt%, Alm10±Grs>80) and diopside (CaO 22–24 wt%), scapolite, meionite, quartz, and accessory Al-bearing titanite (Al2O3 4–4.5 wt%). The diopside has kink bands. Partial or complete breakdown of Ca-rich garnet during post-peak metamorphism produced pseudomorphs and coronas consisting of fine-grained symplectic intergrowths of hedenbergite and anorthite. Thermobarometric estimates in combination with reaction textures, mineral compositions, and recent experimental studies indicate that these peak assemblages were formed at P=c. 1.7–1.8 GPa, T =c. 890 °C, and the retrograde assemblages experienced near-isothermal decompression to P=0.5±0.1 GPa, T =850±50 °C. The whole-rock compositions indicate that marble and pelite are plausible candidates for the protoliths. These facts suggest the following (1) sedimentary rocks were transported to upper-mantle depths and equilibrated at those conditions to form these high-pressure granulites, which were then emplaced into the crust quickly. During the rapid exhumation of these rocks, the earlier high-pressure assemblages were overprinted by the later low- to medium-pressure assemblages, that is, the high-pressure granulite belt formed in the syntaxis. (2) The Namula thrust system is an important tectonic boundary in the syntaxis, or even in the Higher Himalaya more generally.  相似文献   

20.
The petrology and geochemistry of a newly discovered suite of high-pressure garnet + clinopyroxene-bearing rocks from the Monotonous Series of the Moldanubian Zone of the Bohemian Massif, southwest Czech Republic have been investigated. Three types [common eclogites, quartz ± kyanite ± (clino)zoisite eclogites and garnet-hornblende-clinopyroxenites] are distinguished by petrography and geochemistry. All underwent a significant degree of partial breakdown under granulite and amphibolite facies conditions during exhumation. Important features include the growth of orthopyroxene in breakdown domains after garnet and omphacite and anorthite + spinel ± corundum ± exceedingly peraluminous sapphirine replacing kyanite. Garnet zoning and inclusion patterns support a prograde evolution from low pressures for at least some of the samples. The post-eclogite stage granulite facies overprint indicates that high temperatures prevailed during exhumation, but preservation of zoning in some garnets and the results of diffusion modelling suggest that this overprint took place over a very short time-scale. The geochemical and petrological results allow characteristic differences to be recognized between these eclogites and metabasites found in other tectonic units of South Bohemia and consequently the assigning of all high-pressure rocks to a single, now disrupted, tectonic unit is a gross simplification that seriously misrepresents the tectono-metamorphic history of the region.  相似文献   

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