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1.
The cordierite-bearing gneisses occurring as elongate patches in an 8- to 10-km-wide zone along the Achankovil fault-lineament at the northern margin of the southern Kerala crustal segment represent an important lithological unit in the Archaean granulite terrane of south India. The textural relationships in these rocks are consistent with the following main reactions: (1) garnet+quartz=cordierite+hypersthene; (2) garnet+sillimanite+quartz=cordierite; (3) hypersthene+sillimanite+quartz=cordierite; (4) sillimanite+spinel=cordierite+corundum; and (5) biotite+quartz+sillimanite=cordierite+K-feldspar. Many of the mineral associations and reaction textures, including the remarkable preservation of symplectites, are indicative of partial replacement of high-pressure assemblages by cordierite-bearing lower-pressure ones during an event of rapid decompression. Temperature estimates from coexisting mineral phases show 710° (garnet-biotite), 791° (garnet-cordierite) and 788° C (garnet-orthopyroxene). Pressure estimates from mineral assemblages range from 5.4 to 7 kb. Detailed fluid inclusion studies in quartz associated with cordierite show high-density CO2 (0.80–0.95 g/cm3) as the dominant fluid phase, with traces of probable CH4 (?) in the sillimanite-bearing rocks. The isochore for the higher-density fluid inclusions defines a pressure of 5.5 kb. The fracture-bound CO2 and CO2-H2O (±CH4?) inclusions indicate simultaneous entrapment at 400° C and 1.7 kb in the cordierite-hypersthene assemblage and 340° C and 1.2 kb in the cordierite-sillimanite assemblage. The P-T path delineated from combined solid and fluid data corresponds to the piezothermic array of the gneisses and is characterized by T-convex nature, indicative of rapid and virtually isothermal crustal uplift, probably aided by extensional tectonics.  相似文献   

2.
The mineral assemblages in the eclogites and meta-sediments of the Münchberg gneiss complex suggest minimum pressures of about 15 kbar, and temperatures of 600±30° C for the eclogite-facies metamorphism. Both rock types underwent a subsequent amphibolite-facies metamorphism at almost the same temperature range. In the Weissenstein borehole the eclogites and meta-sediments show an intimate interlayering on a centimetre scale. Contacts between the two rock types are often sharp and untectonized suggesting in-situ metamorphism of the eclogites. The following features indicate that the gneisses were subjected to eclogite-facies metamorphism:
  1. Phengite relics and phengite inclusions in garnet contain up to 3.45 Si per formula unit while amphibolite facies phengites have considerably lower Si-contents (3.0–3.25 p.f.u.).
  2. Omphacite relics occur in the form of Na-augite (6–9 mol% jadeite)-oligoclase symplectites.
  3. Garnets with up to 40 mol% of the pyrope component occur.
The geological and mineral-chemical data indicate that large volumes of crustal material have been buried to depths possibly exceeding 70 km. The preservation of primary eclogitic textures as well as symplectitic textures in the presence of a fluid phase, are indicative of a rapid decompression. This would suggest a tectonic uplift (e.g. underthrusting) as is also indicated by the virtually constant temperature range during the uplift.  相似文献   

3.
Thermally metamorphosed and metasomatised fragments of basement actinolite-chlorite-calcite-quartz schists and quartz-bearing marbles are found as inclusions in Quaternary agglomerates and historic (197 B. C.—1950) dacitic lavas of Santorini volcano, Greece.Inclusions in agglomerates preserve the structure of parent schists in the alternation of bands rich in diopside or salite with bands rich in plagioclase. By contrast, inclusions in historic dacites are not banded. Most develop a thin zone of hybrid material at the contact with enclosing lava. The assemblage calcic clinopyroxene-wollastonite-plagioclase is commonly developed. The clinopyroxene is a Fe3+-rich salite or ferrosalite. Andradite-rich garnet and sphene are accessory minerals. Most examples carry interstitial siliceous glass of distinctive chemical composition, and several show minor olivine, augite, hypersthene and calcic plagioclase of magmatic origin.Other inclusions exhibit the assemblage anhydrite-calcic clinopyroxene, the latter mineral ranging widely in Al content. A single example has been observed to develop two distinct assemblages, the first coarsely crystalline melilite-wollastonite-magnetite, the second finely intergrown melilite-wollastonite-andraditic garnet (-xonotlite).Stability data for hedenbergite and andradite as constituents of skarn assemblages suggest that the clinopyroxene-rich assemblages of inclusions in historic dacites formed at temperatures near to or above 800° C and oxygen fugacity (fO2) considerably greater than that which could be imposed upon the inclusions by dacite magma (T 900° C, fO210–13 atm.). Thermal breakdown of original carbonates of the inclusions probably supplied the necessary oxygen. T-fO2 data for the reaction 4 Magnetite+18 Wollastonite 6 Andradite indicate that the assemblage melilite-wollastonite-magnetite of the last inclusion described formed at higher T and/or lower fO2 than the assemblage melilite-wollastonite-garnet. The latter assemblage undoubtedly formed during inclusion of the fragment by dacite magma, while metamorphism by a more basic, high temperature magma may have produced the former. Temperature data for reactions limiting the stability of melilite in the system CaO-Al2O3-SiO2-H2O indicate a minimum temperature of around 800° C for formation of both assemblages.  相似文献   

4.
This study investigates marbles and calcsilicates in Central Dronning Maud Land (CDML), East Antarctica. The paleogeographic positioning of CDML as part of Gondwana is still unclear; however, rock types, mineral assemblages, textures and P–T conditions observed in this study are remarkably similar to the Kerala Khondalite Belt in India. The CDML marbles and calcsilicates experienced a Pan-African granulite facies metamorphism at c. 570 Ma and an amphibolite facies retrogression at c. 520 Ma. The highest grade assemblage in marbles is forsterite+spinel+calcite+dolomite, in calcsilicates the assemblages are diopside+spinel, diopside+garnet, scapolite+wollastonite+clinopyroxene±quartz, scapolite±anorthite±calcite+clinopyroxene+wollastonite. These assemblages constrain the peak metamorphic conditions to 830±20 °C, 6.8±0.5 kbar and X CO2>0.46. During retrogression, highly fluoric humite-group minerals (humite, clinohumite, chondrodite) replaced forsterite, and garnet rims formed at the expense of scapolite during reactions with wollastonite, calcite or clinopyroxene but without involvement of anorthite. Metamorphic conditions were about 650 °C, 4.5±0.7 kbar, 0.2< X CO2fluid<0.36, and the co-existence of garnet, clinopyroxene, wollastonite and quartz constrains fO2 to FMQ-1.5 log units. Mineral textures indicate a very limited influx of H2O-rich fluid during amphibolite facies retrogression and point to significant variations of fluid composition in mm-sized areas of the rock. Gypsum was observed in two samples; it probably replaced metamorphic anhydrite which appears to have formed under amphibolite facies conditions. The observed extensive anorogenic magmatism (anorthosites, A-type granitoids) and the character of metamorphism between 610 and 510 Ma suggest that the crustal thermal structure was characterized by a long-lived (50–100 Ma) rise of the crustal geotherm probably caused by magmatic underplating.  相似文献   

5.
A petrographic and electron microprobe study of an interbedded calc-mica schist from Gassetts, Vermont reveals the complexities of Fe-Mg crystalline solution and gradients in X H2O/X CO2 during regional metamorphism. The common association of microcline+diopside ± zoisite formed from biotite +calcite+quartz may be produced by continuous (Fe-Mg) exchange reactions, despite the implied metastability in the CaO-K2O-MgO-Al2O3-SiO2-H2O-CO2 system. Different assemblages are produced in a reaction zone between carbonate and pelite beds of varying thickness. This illustrates the importance of exhaustion of fluid buffering assemblages due to mineral reactions proceeding on a local scale. Siliceous dolomites at the same metamorphic grade have produced significantly different assemblages to the interlayered carbonatepelite sequence.  相似文献   

6.
The pre-Cambrian granulites of Enderby Land Antarctica, contain coexisting spinel-quartz, sapphirine-quartz, hypersthene-sillimanite-quartz and osumilite on a regional extent. Osumilite is present in a variety of mineral assemblages, most of which are documented in granulites for the first time. The mineral assemblages, reactions and compositional zoning in minerals are discussed in terms of continuous and discontinuous reactions in response to changing conditions of metamorphism. The development of many of the mineral coronas can be explained by continuous rather than discontinuous reactions, due to the effects of Mg-Fe and (Mg,Fe)-2Al exchange equilibria with decreasing temperature. The highest P-T conditions of metamorphism (8–10 kb, 900 °–980 ° C, Ellis, in preparation) were beyond the stability limit of coexisting garnet-cordierite. Secondary cordierite has developed through a large number of mineral reactions in response to cooling of these granulites.A theoretical analysis of the phase relations involving osumilite in the chemical systems K2O-MgO-Al2O3-SiO2 and K2O-MgO-FeO-Al2O3-SiO2 is presented. In the pure Mg-system the lower temperature stability limit of Mg-osumilite is inferred to be defined with increasing pressure by the reactions OsCd+En+Kfeld+Qtz, OsSa+En+Kfeld+Qtz, OsSill+En+Kfeld+Qtz. In iron-bearing systems an important reaction involving osumilite is Os+GtCd+Hy+Kfeld+Qtz.At moderate temperatures and pressures, osumilite is limited to rocks which lie on the Mg-rich side of the Cd-Hy stable tie line on an AFM diagram. At higher pressures and temperatures osumilite occurs in a widerrange of rock compositions because of the stability of coexisting garnet and osumilite. Petrographic data, as well as chemographic relations indicate that for many common rock compositions, garnet, cordierite, hypersthene, sapphirine and sillimanite cannot coexist with both osumilite and K-feldspar.Published with the permission of the Director, Bureau of Mineral Resources  相似文献   

7.
Granulites are important component of high-grade metamorphic rocks reflecting intense conditions observed for crustal rocks in terms of temperature, and pressure. This review paper demonstrates how these high-grade granulites are critical to understanding the evolution of the lower continental crust with special reference to southern India. Geothermobarometric traverse across different granulite blocks in southern India shows wide ranging P-T conditions of metamorphism (700–1000 ° C, and 5–10 kbar). The sapphirine-, orthopyroxene-sillimanite, and spinel -bearing quartz-deficient granulites recognised from parts of southern granulite terrain (Ganguvarpatti, Kiranur, and Palani hill ranges etc.) show oriented sillimanite aggregates pseudomorph after course twinned kyanite, staurolite + kyanite assemblages, and corroded blebs of gedrite within orthopyroxene, suggesting a prograde stage of a clockwise P-T evolution. Evidence of ITD history comes from the textures in which an early Mg-rich garnet (XMg 52–60) with orthopyroxene (up to 10% Al2O3) involving sillimanite breakdown forming variety of symplectites having combinations of orthopyroxene, sapphirine, cordierite, and spinel. These spectacular reaction textures, and mineralogic sensors from the high-grade rocks establish a prograde clockwise P-T-t path with notable decompressive history (ITD) in the southern granulite terrain. The inferred P-T-t paths have been further integrated with the recent geochronological, and isotopic data to constrain the timing, and duration of metamorphism, emplacement of the magmatic protolith for characterising the evolution of the granulites, and their bearing on the geodynamic implications. Based on the emerging evidence for Neoproterozoic tectonothermal imprints in the southern granulite terrain, history of the assembly of dispersed fragments is also addressed within the East Gondwana framework.  相似文献   

8.
Osumilite, approximate composition K(Mg,Fe)2 Al5Si10O30, has been reported recently from two granulite localities. The mineral has been synthesised in a model pelitic composition at 1000 and 1100 ° C and 3.6–6.3 kb under conditions of low water and oxygen fugacity. Osumilite coexists, apparently stably, with hypersthene, cordierite and quartz (?) thus duplicating the mineral assemblage of one of the natural occurrences. Osumilite is in a divariant reaction relationship with cordierite and hypersthene i.e. osumilite ? cordierite + hypersthene + orthoclase+quartz. This reaction runs to the right with increasing pressure. Experimental data and field observations suggest that the joins osumilite-garnet and osumilite-sillimanite are not stable. It is suggested osumilite is involved in an invariant point in the system K2O-MgO-FeO-A12O3-SiO2 with the phases cordierite, hypersthene, sapphirine, spinel, orthoclase and quartz. The invariant point should occur at 1000 ± 100 °C and 7± 2kb.  相似文献   

9.
The theoreticalP-T grid for stability relations of the phases cordierite (Cd), sapphirine (Sa), hypersthene (Hy), garnet (Ga), spinel (Sp), sillimanite (Si), and quartz (Qz) of Hensen (1971), has proved useful in the interpretation of metamorphic mineral assemblages formed at low oxygen fugacity. Both experimental data and evidence from natural rocks indicate that at high oxygen fugacity compatability relations change as a result of the enlargement of the stability field of spinel, which causes a topological inversion and the stabilisation of the invariant points [Sa], [Ga], and [Cd]. This implies the stable existence of the univariant equilibria (for buffered conditions): Sp+Qz=Ga+Hy+Si+O2 (Sa, Cd), Cd+Sp+Qz=Hy+Si+O2 (Sa, Ga) and Sa+Sp+Qz=Hy+Si+O2 (Ga, Cd) and the divariant reaction: Sp+Qz=Hy+Si+O2 (Sa, Ga, Cd). These redox equilibria are restricted to conditions of high oxygen fugacity. The proposed theoreticalP-T grids, for both low and high oxygen fugacity, satisfactorily explain all experimental data and metamorphic mineral assemblages so far found in granulites.  相似文献   

10.
Sapphirine and spinel can accommodate significant ferric iron and therefore the mineral equilibria involving these phases must be sensitive to a(O2). In this paper we examine the theoretical phase relationships involving sapphirine and spinel in addition to sillimanite, garnet, cordierite, rutile, hematite-ilmenite solid solution (henceforth ilmenite), and magnetite-ulvospinel solid solution (henceforth magnetite), in the presence of quartz and hypersthene in the system FeO-MgO-Al2O3-SiO2-TiO2-O2 (FMASTO), with particular reference to the topological inversion in P-T postulated by Hensen (Hensen 1986). Documented natural associations suggest that the appropriate topology for assemblages involving magnetite and ilmenite is Hensen's higher a(O2) one, while, in contrast, the topology for assemblages involving ilmenite and rutile is the lower a(O2) one. The exact configuration of the inversion between these two topologies remains uncertain because of uncertainties in the ferric/ferrous iron partitioning between sapphirine and spinel-cordierite at high temperatures. By comparison with experimental data and natural occurences, the sillimanite-sapphirine-cordierite-garnet-hypersthene-quartz assemblage is in equilibrium at about 1000°–1020° C and 7–8 kbars, while sapphirine-cordierite-spinel-garnet-hypersthene-quartz occurs at temperatures in excess of those attainable during crustal metamorphism, for ilmenite-rutile buffered assemblages. This implies that sapphirine-rutil-ehypersthene-quartz assemblages, as found in the Napier Complex, Antarctica, can only occur at > 1000° C. Also, spinel-rutile-hypersthene-quartz assemblages should not be found in rocks because temperatures in excess of 1100° C are expected to be involved in their formation. The temperatures of formation of spinel-sillimanite-sapphirine-garnethypersthene-quartz, sapphirine-spinel-cordierite-sillimanite-hypersthene-quartz, and sillimanite-spinel-cordieritegarnet-hypersthene-quartz in assemblages buffered by magnetite and ilmenite are less well constrained, but are likely to be in the range 900°–1000° C. These conclusions apply to rocks with compositions close to FMASTO; the perturbing effects of substantial concentrations of additional components, in particular Ca, mainly in garnet, and Zn and Cr, mainly in spinel, may invalidate these conclusions.  相似文献   

11.
This paper reports a study of the metamorphic evolution of pelitic, semi-pelitic migmatites and mafic granulites of the Chafalote Metamorphic Suite (CMS), Uruguay, which represents the southernmost exposures of high-grade metamorphic rocks in the Dom Feliciano Belt, Uruguain—Sul-Rio-Grandense shield, South America. This belt is one of the Brasiliano orogens that crop out along the Brazilian and Uruguayan Atlantic margin, and the CMS is one of several disconnected segments of supracrustal rock in a dominantly granitic terrain. Petrological evidence from CMS mafic granulites and semi-pelitic migmatites indicates four distinct metamorphic assemblages. The early prograde assemblage (M1) is preserved only as inclusions in porphyroblasts of the peak-metamorphic (M2) assemblage. Peak-metamorphism was followed by near-isothermal decompression (M3), which resulted in symplectites and coronitic textures in the mafic granulites and compositional zoning of Ca in garnet (decreasing rimwards) and plagioclase (increasing rimwards) in the semi-pelitic migmatites. The retrograde metamorphic assemblage (M4) is represented by hydration reaction textures replacing minerals of the M2 and M3 assemblages. Average P–T calculations using the program THERMOCALC and conventional thermobarometric methods yield peak-metamorphic (M2) P–T conditions of 7–10 kbar and 830–950 °C, near-decompressional (M3) P–T conditions of 4.8–5.5 kbar and 788–830 °C and M4 retrograde P–T conditions of 3–6 kbar and 600–750 °C. The calculated P–T path for the CMS rocks is ‘clockwise’ and incorporates a near-isothermal decompression segment followed by minor cooling, consistent with a history of crustal thickening followed by extensional collapse at ca. 650–600 Ma. The metamorphism recorded by rocks of this crustal segment may be correlated with 650 Ma metamorphism in the Coastal Terrane of the Kaoko Belt in Namibia, being the first unequivocal match between South America and Africa provided by crystalline rocks south of the Congo Craton.  相似文献   

12.
A quartz-absent magnesian paragneiss layer from Mount Sones, in the Archaean Napier complex of Enderby Land, Antarctica, contains the stable divariant FMAS assemblage sapphirine (X Mg=78) — cordierite (X Mg=87) — garnet (X Mg=51) — sillimanite. Rare green spinel (X Mg=53.5, ZnO=2.65wt%) occurs as inclusions mainly within sapphirine, but also within sillimanite and garnet. Late thin coronas of cordierite (X Mg=90.5) mantle sapphirine in contact with extensively exsolved anorthoclase. The mineral textures are interpreted to indicate the former stability of a hypersthene-quartz absent assemblage followed by the development of the FMAS equilibrium assemblage sapphirine-cordierite-garnet-sillimanite (sp, hy, qz) and further divariant reaction involving the consumption of sapphirine. The (sp, hy, qz) assemblage uniquely defines the stable P-T reaction topology appropriate to granulites from the Napier Complex, as this paragenesis is allowed in the grids of Hensen (1971, 1986) but is not possible in other grids which assume the stability of a sapphirine-absent ([sa]) FMAS invariant point involving the phases spinel, garnet, hypersthene, cordierite, sillimanite and quartz. The observed mineral assemblages and textures are consistent with peak metamorphism between the [sp] and [hy] invariant points of Hensen (1971), at temperatures of 930–990° C, followed by cooling on a lower dP/dT trajectory towards the (sp, qz) univariant line. The initial spinel-bearing assemblage was stabilized by Zn and to a lesser extent by Ni and Cr, and hence does not require a marked decrease in temperature and increase in pressure to produce the (sp, hy, qz) assemblage. It is inferred that fO 2 conditions substantially lower than those used in the experiments of Annersten and Seifert (1981) prevailed in the high-grade metamorphism in the Napier Complex.  相似文献   

13.
In pelitic rocks, under conditions of low f O 2 and low f H 2 O, the stability of the mineral pair cordierite-garnet is limited by five univariant reactions. In sequence from high pressure and low temperature to high temperature and low pressure these are: cordierite+garnet hypersthene+sillimanite+quartz, cordierite+garnet hypersthene+sapphirine+quartz, cordierite+garnet hypersthene+spinel+quartz and cordierite+garnet olivine+spinel +quartz. In this sequence of reactions the Mg/Mg+Fe2+ ratio of all ferro-magnesian minerals involved decreases continuously from the first reaction to the fifth. The five univariant boundaries delimit a wide P-T range over which cordierite and garnet may coexist.Two divariant equilibria in which the Mg/Mg+ Fe2+ ratio of the coexisting phases are uniquely determined by pressure and temperature have been studied in detail. P-T-X grids for the reactions cordierite garnet+sillimanite+quartz and cordierite+hypersthene garnet+quartz are used to obtain pressure-temperature estimates for several high grade metamorphic areas. The results suggest temperatures of formation of 700–850° C and load pressures of 5–10 kb. In rare occasions temperatures of 950–1000° C appear to have been reached during granulite metamorphism.On the basis of melting experiments in pelitic compositions it is suggested that Ca-poor garnet xenocrysts found in calc-alkaline magmas derive from admixed pelitic rocks and did not equilibrate with the calc-alkaline magma.  相似文献   

14.
The volume of fluid and amount of heat involved in a portion of a metamorphic event around three synmetamorphic granitic stocks has been quantitatively estimated using mineral composition and modal data from carbonate rocks. Values of volumetric fluid-rock ratios range, with respect to a reference zoisite isograd, from 0.001 to 0.434. Amounts of heat involved range from –25 to 134 cal/cm3 rock. Contours of constant fluid-rock ratio and of constant amount of heat are generally concentric about the granitic stocks indicating that the stocks are foci of high heat and fluid fluxes during metamorphism. In addition, contours of fluid-rock ratios and amount of heat outline NE-SW-trending channelways of high fluid and heat fluxes that alternate with regions of lower fluid and heat fluxes. The NE-SW-trending vertical bedding and schistosity in the area — of premetamorphic origin — probably constrained fluid and heat transfer to occur preferentially in NE-SW directions. Large values of heat involved in metamorphism are strongly correlated with large fluid-rock ratios, suggesting that fluids are an important carrier of heat during metamorphism. Configurations of mapped isograds in the area mimic the patterns of contours of constant fluid-rock ratio and of heat content, indicating that configurations of isograds may contain useful information about regional patterns of heat and fluid transfer during metamorphism.Notation T Last temperature recorded by metacarbonate rocks (°C) - P Lithostatic pressure (bars) - Pi Partial pressure of component i (bars) - of last fluid in equilibrium with carbonate rocks during metamorphism - R 1.987 cal/bar-degree - K s Activity constant for an assemblage of solid mineral phases - In Natural logarithm - c v Volumetric heat capacity (cal/cm3-degree) - Q Heat added to or subtracted from a rock during metamorphism in the zoisite zone (kcal/100 cm3 rock; cal/cm3 rock) - Q{ibrxn} Heat added to or subtracted from a rock due to mineral reactions during metamorphism in the zoisite zone (kcal/100 cm3 rock; cal/cm3 rock) - Std. Dev. Standard Deviation - Average of fluid in equilibrium with carbonate rocks during their metamorphism in the zoisite zone - of fluid in equilibrium with carbonate rocks at the zoisite isograd - T Temperature at the zoisite isograd (°C) - X i,j Mole fraction of component i in phase j - H i Molar enthalpy of reaction i at 0 bars pressure - ¯V i Change of molar volume due to reaction ii - i Measure of progress of reaction i - V Change in rock volume due to fluid-rock reactions - iV Initial rock volume before metamorphism within the zoisite zone - ¯V s,i Change in molar volume of solid minerals due to reaction i Component notation an CaAl2Si2O8 Phase notation Pl Plagioclase - Am Amphibole - Cc Calcite - Qz Quartz - Di Diopside - Zo Zoisite - Ga Garnet - Bi Biotite - Kf Microcline - Mu Muscovite  相似文献   

15.
ABSTRACT Sequential reaction textures in Archaean garnet-corundum-sapphirine granulites from the Central Zone of the Limpopo Belt document a progression from early, coarse-grained, high-pressure (P > 9.5 kbar) granulite-facies assemblages (M1) to late, low-pressure (P <6 kbar) granulite-facies sub-assemblages (M2). The stable M1 assemblage was garnet (57% pyrope; Mg/(Mg + Fe) = 62) + sapphirine + corundum + gedrite + phlogopite + rutile. Late-M1 boron-free kornerupine grew at the expense of garnet and corundum, and coexisted with garnet, sapphirine and gedrite. Partial or complete breakdown of coarse garnet and kornerupine during M2 resulted in the development of pseudomorphs and coronas consisting of fine-grained symplectic intergrowths of cordierite, gedrite and sapphirine (later, spinel). The majority of reaction textures can be explained in terms of a stable reaction sequence, and a model time-sequence of mineral facies can be constructed. When compared with a qualitative petrogenetic grid of (Fe, Mg)-discontinuous reactions in the FMASH multisystem sapphirine-garnet-corundum-spinel-cordierite-gedrite-kornerupine, the facies-sequence indicates decompression at essentially constant T assuming constant a(H2O). Exhumation of M1 corundum inclusions during M2 breakdown of kornerupine resulted in production of metastable spinel by a disequilibrium reaction with gedrite. A second disequilibrium reaction of the spinel with cordierite produced sapphirine. The operation of such reaction while pressure was decreasing (the opposite dP from that implied by the texture if assumed to be the product of an equilibrium reaction) has serious implications for the use of reaction textures in the construction of P-T vectors. Garnet-biotite thermometry on garnet interiors and phlogopite inclusions in corundum yields temperatures of ca. 850°C for the M1 stage. A minimum late-M1 pressure of ca. 7 kbar is indicated by the former association of kornerupine and corundum. Relict M1 kyanites reported by other workers indicate a minumum early-M1 pressure of 9.5 kbar, implying metamorphism at depths of at least 33 km (probably 38km). The high-pressure granulite-facies metamorphism was followed by an almost isothermal pressure decrease of > 5 kbar, indicative of rapid uplift. The P-T path is interpreted as the product of a single metamorphic cycle which probably took place in response to tectonic thickening of the crust. Such a process contrasts with the extensional origin recently proposed for isobarically cooled granulite-facies terranes.  相似文献   

16.
 The melting reaction: albite(solid)+ H2O(fluid) =albite-H2O(melt) has been determined in the presence of H2O–NaCl fluids at 5 and 9.2 kbar, and results compared with those obtained in presence of H2O–CO2 fluids. To a good approximation, albite melts congruently at 9 kbar, indicating that the melting temperature at constant pressure is principally determined by water activity. At 5 kbar, the temperature (T)- mole fraction (X (H2O) ) melting relations in the two systems are almost coincident. By contrast, H2O–NaCl mixing at 9 kbar is quite non-ideal; albite melts ∼70 °C higher in H2O–NaCl brines than in H2O–CO2 fluids for X (H2O) =0.8 and ∼100 °C higher for X (H2O) =0.5. The melting temperature of albite in H2O–NaCl fluids of X (H2O)=0.8 is ∼100 °C higher than in pure water. The P–T curves for albite melting at constant H2O–NaCl show a temperature minimum at about 5 kbar. Water activities in H2O–NaCl fluids calculated from these results, from new experimental data on the dehydration of brucite in presence of H2O–NaCl fluid at 9 kbar, and from previously published experimental data, indicate a large decrease with increasing fluid pressure at pressures up to 10 kbar. Aqueous brines with dissolved chloride salt contents comparable to those of real crustal fluids provide a mechanism for reducing water activities, buffering and limiting crustal melting, and generating anhydrous mineral assemblages during deep crustal metamorphism in the granulite facies and in subduction-related metamorphism. Low water activity in high pressure-temperature metamorphic mineral assemblages is not necessarily a criterion of fluid absence or melting, but may be due to the presence of low a (H2O) brines. Received: 17 March 1995/Accepted: 9 April 1996  相似文献   

17.
The evolution of the mineral assemblages and P–T conditions during partial melting of upper‐amphibolite facies paragneisses in the Orue Unit, Epupa Complex, NW Namibia, is modelled with calculated P–T–X phase diagrams in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O system. The close concordance of predictions from the phase diagrams to petrographic observations and thermobarometric results documents that quantitative phase diagrams are suitable to explain the phase relationships in migmatitic upper‐amphibolite facies low‐ and medium‐pressure metapelites, which occur in many high‐grade metamorphic terranes worldwide. Different mineral assemblages in the migmatitic metapelites of the Orue Unit reflect regional discrepancies in the metamorphic grade: in a Northern Zone, early biotite–sillimanite–quartz assemblages were replaced via melt‐producing reactions by cordierite‐bearing assemblages. In a Southern Zone, they were replaced via melt‐producing reactions by garnet‐bearing assemblages while cordierite is restricted to rare metapelitic granofelses, which preserve Grt–Sil–Crd–Bt peak assemblages. Peak‐metamorphic conditions of 700–750 °C at 5.5–6.7 kbar in the Southern Zone and of ～750 °C at 4.5 kbar in the Northern Zone are estimated by integrating thermobarometric calculations with data from calculated mineral composition isopleths. Retrograde back‐reactions between restite and crystallizing melt are recorded by the replacement of garnet by biotite–sillimanite and/or biotite–muscovite intergrowths. Upper‐amphibolite facies metamorphism and partial melting (c. 1340–1320 Ma) in the rocks of the Southern Zone of the Orue Unit, which underwent probably near‐isobaric heating–cooling paths, are attributed to contact metamorphism induced by the coeval (c. 1385–1319 Ma) emplacement of the Kunene Intrusive Complex, a huge massif‐type anorthosite body. The lower‐pressure metapelites of the Northern Zone are interpreted to record contact metamorphism at an upper crustal level.  相似文献   

18.
《Gondwana Research》2013,23(3-4):799-809
Externally derived, pure CO2 that mixes with a carbon-(under)saturated C-O-H fluid in lower crustal granulites may result in graphite precipitation if the host-rock oxygen fugacity (fO2rock) is below the upper fO2 limit of graphite. The maximum relative amount of graphite that can precipitate varies between a few mol% up to more than 25 mol%, depending on pressure, temperature, and host-rock redox state. The maximum relative amount of graphite that can precipitate from an infiltrating CO2 fluid into a dry granulite (CO fluid system) varies between zero and a few mol%. Thermodynamic evaluation of the graphite precipitation process shows that CO2 infiltration into lower crustal rocks does not always result in a carbon (super)saturated fluid. In that case, graphite precipitation is only possible if carbon saturation can be reached as a result of the reaction CO2 → CO + ½ O2. Graphite that has been precipitated during granulite facies metamorphic conditions can subsequently be absorbed by a COH fluid during retrograde metamorphism. It is also possible, however, that significant amounts of graphite precipitate from a COH fluid during retrograde metamorphism. This study shows that interpreting the presence or absence of graphite in granulites with respect to CO2 infiltration requires detailed information on the P–T–fO2rock conditions, the relative amount of CO2 that infiltrates into the rock, and whether H2O is present or not.  相似文献   

19.
In the metabasites of Val Chiusella, metamorphic assemblages are present, corresponding to the glaucophane schist facies, i.e. garnet glaucophanites to omphacite-garnet glaucophanites, as well as to the eclogite facies, i.e., glaucophane eclogites, eclogites, and omphacite felses. Both groups of assemblages are divided by the critical reaction 1 zoisite +1 glaucophane 1.2 omphacite+0.8 garnet+0.7 paragonite +1.4 quartz+0.8 H2O. From textural evidence it is clear that in the investigated area this reaction proceeded to the right according to a prograde metamorphism. Correspondingly, K garn-cpx D(Fe/Mg) values of coexisting garnet-omphacite pairs in the glaucophane schist facies assemblages are higher than in the eclogite facies assemblages and reflect a temperature increase from about 450 ° C to about 550 ° C at minimum water vapour pressures of 12 to 16 kb.  相似文献   

20.
An inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions. For mafic and ultramafic sequences, the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits (XCO2= 0.2–0.3). This result follows from the calculation of mineral equilibria in the model system CaO–MgO–FeO–Al2O3–SiO2–H2O–CO2, using a new, expanded, internally consistent dataset. Greenstone metamorphism cannot have involved much crustal over-thickening, because very shallow levels of greenstone belts are preserved. Such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere. In this case, the observed metamorphism, which was contemporaneous with deformation, is of the low-P high-T type. For this type of metamorphism, the metamorphic peak should have occurred earlier at deeper levels in the crust; i.e. the piezothermal array should be of the ‘deeper-earlier’type. However, at shallow crustal levels, the piezothermal array is likely to have been of ‘deeper-later’type, as a consequence of erosion. Thus, while the lower crust reached maximum temperatures, and partially melted to produce the observed granites, mid-crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde. We propose that these fluids are responsible for the gold mineralization. Thus, the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution, and does not mean that the mineralization has to be a consequence of igneous processes. Upward migration of metamorphic fluid, via appropriate structurally controlled pathways, will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower XCO2. These assemblages are therefore grossly out of equilibrium with the fluid. In the case of infiltrated metabasic rocks, intense carbonation and sulphidation is predicted. If, as seems reasonable, gold is mobilized by the fluid generated by devolatilization, then the combination of processes proposed, most of which are an inevitable consequence of the metamorphism, leads to the formation of greenstone gold deposits predominantly from metamorphic fluids.  相似文献   

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