摘要:

AbstractAn outcrop of staurolite‐bearing pelitic schist from the Solitude Range in the south‐western Rocky Mountains, British Columbia, was examined in order to determine the nature of prograde garnet‐ and staurolite‐producing reactions using information from garnet zoning and inclusion mineralogy. Although not present as a matrix phase, chloritoid is present as inclusions in garnet and is interpreted to have participated in the simultaneous growth of garnet and staurolite by a reaction such as chloritoid + quartz = garnet + staurolite + H2O.A garnet zoning trend reversal, which is most pronounced with respect to almandine and grossular components, is present in the outer core of garnets. The location of the zoning reversal corresponds to the outer limit of chloritoid inclusions in garnet. As there is no evidence for polymetamorphism, the zoning reversal is interpreted to indicate continued garnet growth by prograde reaction(s) during a single metamorphic event after the exhaustion of chloritoid as a matrix phase.Metamorphic conditions recorded by mineral rim compositions are 550–600° C at 6–7 kbar. Because there is no evidence for partial resorption of garnet during production of staurolite, we interpret these results to represent pea

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00188.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Abstract‘Peak’metamorphic carbon isotope fractionations between calcite and graphite (ΔCal–Gr) in marbles and calc‐silicates from the Cucamonga granulite terrane (San Gabriel Mountains, California) range from 3.48 to 2.90%. The data are used to test three previously published calibrations of the calcite–graphite carbon isotope thermometer. An empirical calibration of the calcite–graphite carbon isotope thermometer gives temperatures of 700–750°C; a theoretical–experimental calibration of the system gives temperatures of 760°–870°C; an experimental calibration gives temperatures of 870–1300°C. Temperatures calculated using the empirical calibration are in agreement with those calculated from garnet‐based cation exchange thermometry when uncertainty is considered. Temperatures calculated using the theoretical–experimental calibration overlap the upper range of cation exchange thermometry temperatures and range to 50°C higher. The experimental calibration yields temperatures from 50 to 480°C higher than those from cation exchange thermometry. Moreover, temperatures from the experimental calibration are also inconsistent with mineral and melt equilibria in the granulite phase assemblage.Despite the better agreement between cation exchange thermometry and the empirical calibration of the calcite–graphite system, temperatures calculated using the theoretical–experimental calibration may be real peak metamorphic temperatures. If retrograde diffusion partially reset garnet‐based cation exchange thermometers byc.50°C, then the cation exchange temperatures are consistent with those from the theoretical–empirical calibration. Thermometric evidence from biotite dehydration melting equilibria is consistent with either the empirical calibration if melting was fluid‐present, or the theoretical–experimen

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00189.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractVariation in the state of stress during heterogeneous deformation should be reflected in variation in the effective pressure of metamorphic reactions, whether this is mean stress or the normal stress acting across the reacting interface. The magnitude of this pressure variation will determine whether it is discernible in the preserved metamorphic mineral assemblages of heterogeneously deformed rocks. The magnitude of the mean stress difference across a non‐slipping interface between two materials with viscosity ratio>c.20:1 is effectively equal to the maximum shear stress for flow in the more viscous material. Progressive shortening of the interface results in a higher mean stress in the more competent material, whereas extension results in a lower mean stress. For high‐P/low‐Teclogite facies conditions, current experimental data indicate that clinopyroxene‐ and garnet‐rich layers of eclogite should be very strong and that pressure differences of up to 800 MPa (8 kbar) between competent layer and weaker matrix may be possible. Such high values can be obtained in widely separated competent layers for values of bulk stress in the overall multilayer that are much lower (by a factor approaching the viscosity ratio). Extrusion of material between more rigid plates, which has been proposed as a regional mechanism of lateral ‘continental escape’for both the Alps and the Himalayas, should also be accompanied by a lateral gradient in effective pressure; otherwise extrusion could not occur. Maximum mean stresses with magnitudes that are many times the maximum shear stress required for plastic flow should develop for deformation zones that are long relative to their width (e.g. around 20 times for a width‐to‐thickness ratio of 10). Tectonic overpressure in progressively shortened competent layers, particularly in regions of extrusion between more rigid plates, might help explain the occurrence of isolated layers and pods of low‐Teclogite (c.550°C, the dramatic weakening of clinopyroxene in the power‐law creep field precludes the development of significant overpre

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00190.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00191.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractAt Kottavattam, southern Kerala (India), late Proterozoic homogeneous leptynitic garnet–biotite gneisses of granitic composition have been transformed on a decimetric scale into coarse‐grained massive charnockitesensu strictoalong a set of conjugate fractures transecting the gneissic foliation. Charnockitization post‐dates the polyphase deformation, regional high‐grade metamorphism and anatexis, and evidently occurred at a late stage of the Pan‐African tectonothermal history. Geothermobarometric and fluid inclusion data document textural and chemical equilibration of the gneiss and charnockite assemblages at similarPlith–Tconditions (650–700°C, 5–6 kbar) in the presence of carbonic fluids internally buffered by reaction with graphite and opaque mineral phases (XCO2= 0.7–0.6;XH2O= 0.2–0.3;XN2= 0.1; logfO2= ‐17.5).Mineralogical zonation indicates that charnockitization of the leptynitic gneiss involved first the breakdown of biotite and oxidation of graphite in narrow, outward‐migrating transition zones adjacent to the gneiss, followed by the breakdown of garnet and the neoblastesis of hypersthene in the central charnockite zone. Compared to the host gneiss, the charnockite shows higher concentrations of K, Na, Sr, Ba and Zn and lower concentrations of Mg, Fe, Ti, V, Y, Zr and the HREE, with a complementary pattern in the narrow transition zones of biotite breakdown. ThePlith–T–XH2Odata and chemical zonation patterns indicate charnockitization through subsolidus‐dehydration reaction in an open system. Subsequent residence of the carbonic fluids in the charnockite resulted in low‐grade alteration causing modification of the syn‐charnockitic elemental distribution patterns and the properties of entrapped fluids. We favour an internally controlled process of arrested charnockitization in which, during near‐isothermal uplift, the release of carbonic fluids from decrepitating inclusions in the host gneiss into simultaneously developing fracture zones led to a change in the fluid regime from ‘fluid‐absent’in the gneiss to ‘fluid‐present’in the fracture zones and to the development of an initial fluid‐pres

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00192.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractIncipient charnockite formation within amphibolite facies gneisses is observed in South India and Sri Lanka both as isolated sheets, associated with brittle fracture, and as patches forming interconnected networks. For each mode of formation, closely spaced drilled samples across charnockite/gneiss boundaries have been obtained and δ13C and CO2abundances determined from fluid inclusions by stepped‐heating mass spectrometry.Isolated sheets of charnockite (c.50 mm wide) within biotite–garnet gneiss at Kalanjur (Kerala, South India) have developed on either side of a fracture zone. Phase equilibria indicate low‐pressure charnockite formation at pressures of 3.4 ± 1.0 kbar and temperatures of about 700°C (forXH2O= 0.2). Fluid inclusions from the charnockite are characterized by δ13C values of −8% and from the gneiss, 2 m from the charnockite, by values of −15%. The large CO2abundances and relatively heavy carbon‐isotope signature of the charnockite can be traced into the gneiss over a distance of at least 280 mm from the centre of the charnockite, whereas the reaction front has moved only 30 mm. This suggests that fluid advection has driven the carbon‐isotope front through the rock more rapidly than the reaction front. The carbon‐front/reaction‐front separation at Kalanjur is significantly larger than the value determined from a graphite‐bearing incipient charnockite nearby, consistent with the predictions of one‐dimensional advection models.Incipient charnockites from Kurunegala (Sri Lanka) have developed as a patchy network within hornblende–biotite gneiss. CO2abundances rise to a peak near one limb of the charnockite, and isotopic values vary from δ13C ofc.−5.5% in the gneiss to −9.5% in the charnockite. The shift to lighter values in the charnockite can be ascribed to the formation of a CO2‐saturated partial melt in response to influx of an isotopically light carbonic fluid.Thus, incipient charnockites from the high‐grade terranes of South India and Sri Lanka reflect a range of mechanisms. At shallower structural levels non‐pervasive CO2influxed along zones of brittle fracture, possibly associated with the intrusion of charnockitic dykes. At deeper levels,in situmelting occurred under conditions of ductile deformation, leading to

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00193.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe enthalpy of reaction of plagioclase and pyroxene to produce garnet and quartz has been a major source of error in granulite geobarometry because of relatively uncertain enthalpy values available from high‐temperature solution calorimetry and compiled indirectly from experimental phase equilibria. Recent, improved calorimetric measurements of ΔHRare shown to yield palaeopressures which are internally consistent between orthopyroxene and clinopyroxene calibrations for many South Indian granulites from the Archaean high‐grade terranes of southern Karnataka and northern Tamil Nadu. This represents a considerable improvement over previous calibrations, which gave disparate results for the two independent barometers involving orthopyroxene and clinopyroxene, requiring a 2‐kbar ‘empirical adjustment’to force agreement.Palaeopressures thus calculated for 30 well‐documented two‐pyroxene garnet granulites from South India give internally consistent pressures with a mean of 8.1°1.1 kbar at 750°C, consistent with the presence of both kyanite and sillimanite in many areas. Those samples for which garnet–pyroxene exchange thermometers give plausible granulite‐range temperatures and whose minerals are minimally zoned give the best agreement of the two barometers. Samples which yield low palaeotemperatures and different rim and core compositions of minerals yield pressures for the orthopyroxene assemblage as much as 2 kbar lower than for the assemblage with clinopyroxene. This disparity probably represents post‐metamorphic‐peak re‐equilibration. We conclude that considerable confidence may be placed in geobarometry of two‐pyroxene granulites where apparent palaeotemperatures are in the granulite facies range (>700°C) and where mineral zonation is minimal. Of the several possible sets of activity–composition relations in use, those constructed from analysis of phase equilibria give slightly higher palaeopressures and appear more consistent with analytical data from the Nilgiri Hills uplift, where kyanite is the only aluminium silicate reported to be stable in peak‐metamorphic assemblages. The present results support a palaeopressure gradient, increasing generally from south to north, across the Nilgiri Hills as

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00194.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThree reactions are calibrated as geothermobarometers for garnet–orthopyroxene–plagioclase–quartz assemblages, namely: 1/2 ferrosilite + 1/3 pyrope ± 1/2 enstatite + 1/3 almandine (A): ferrosilite + anorthite ± 2/3 almandine + 1/3 grossularite + quartz (B); and enstatite + anorthite ± 2/3 pyrope + 1/3 grossularite + quartz (C). The internally consistent geothermobarometers based on reactions (A), (B) and (C) are calibrated from experimental data only. The thermodynamic parameters of reaction (A) are derived from published experimental data in the FMAS system (n= 104) in the range 700–1400°C and 5–50 kbar, while those for reaction (B) are derived by summation of the existing reversed experimental data of the mineral equilibria: ferrosilite ± fayalite + quartz (D) and anorthite + fayalite ± 2/3 almandine + 1/3 grossularite (E). The retrieved thermodynamic parameters for reactions (A), (B) and (C) are, respectively: (ΔH0, cal) ‐3367 ± 209, ‐2749 ± 350 and +3985 ± 545; (ΔS0, cal K−1) ‐1.634 ± 0.163, ‐8.644 ± 0.298 and ‐5.376 ± 0.391; and (ΔV01,298, cal bar−1) ‐0.024, ‐0.60946 and ‐0.5614.On a one‐cation basis, the derived Margules parameters of the ternary Ca–Fe–Mg in garnet are:WFe–Mg= ‐1256 + 1.0 (∼0.23)T(K),WMg–Fe= 2880 ‐1.7 (∼0.13)T(K),WCa–Mg= 4047 (∼77) ‐1.5T(K),WMg–Ca= 1000 (∼77) ‐1.5T(K),WCa–Fe= ‐723 + 0.332 (∼0.02)T(K),WFe–Ca= 1090, (cal) and the ternary constantC123= ‐4498 + 1.516 (∼0.265)T(K) cal (subregular solution model of non‐ideal mixing); and Fe–Mg–Al in orthopyroxene:WFe–Mg= 948 (∼200) ‐0.34 (∼0.10)T(K),WFe–Al= ‐1950 (∼500) andWMg–Al= 0 (cal) (regular solution model of non‐ideal mixing). The anorthite activity in plagioclase is calculated by the ‘Al‐avoidance’model of subregular Ca–Na mixing commonly used for geobarometry based on reactions (B) and (C).When the geothermobarometers are applied to garnet–orthopyroxene–plagioclase–quartz assemblages (n= 45) of wide compositional range from the Precambrian South Indian granulites, temperature ranges of 690–860°C (X= 760 ± 45°C) and pressure ranges of 5–10 kbar were obtained. TheP–Tvalues were estimated simultaneously and there is no difference in the pressure calculated fromPMg(reaction C) andPFe(reaction B). In the existing calibrations this difference is 1 kbar or more. Furthermore, there is no compositional dependence of the lnKof the experimental data in th

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00195.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractGanguvarpatti is part of a Precambrian terrane characterized by granulite facies rocks, including charnockites, mafic granulites, sapphirine‐bearing granulites, leptynites and gneisses. A sequence of reactions deduced from the multiphase reaction textures provide information on the metamorphic history of this area, as they formed in response to decompression during uplift. Geothermobarometry and constraints from reaction textures define a segment of aP–Tpath traversed by the granulites of Ganguvarpatti. Near‐peak metamorphic conditions ofc.800°C and 8 kbar were succeeded by a symplectitic stage at a significantly lower pressure (c.700°C and 4.5 kbar), documenting a nearly isothermal decompressionP–Tpath and rapid uplift (c.12 km) followed by cooling. The presence of many fluid inclusions of extremely low density in the charnockites is consistent with a nearly isothermal uplift path. Attainment of a maximum pressure ofc.8 kbar indicatesc.27 km depth of burial during metamorphism. This would imply a total crustal thickness ofc.65–70 km at 2.6–2.5 Ga. Such a profound crustal thickness and a clockwise decompressiveP–Tpath is interpreted as a consequence of tectonic thickening of crust, accomplished by collision tectonics of the southern granulite terrane against the Dharwar craton along the Palghat–Cauvery shear zone via nor

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00196.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractA major episode of continental crust formation, associated with granulite facies metamorphism, occurred at 2.55–2.51 Ga and was related to accretional processes of juvenile crust. Dating of tonalitic–trondhjemitic, granitic gneisses and charnockites from the Krishnagiri area of South India indicates that magmatic protoliths are 2550–2530 ± 5 Ma, as shown by both U–Pb and207Pb/206Pb single zircon methods. Monazite ages indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510 ± 10 Ma. These data provide precise time constraints and Sr–Nd isotopes confirm the existence of late tonalitic–granodioritic juvenile gneisses at 2550 Ma. Pb single zircon ages from the older Peninsular gneisses (Gorur–Hassan area) are in agreement with some previous Sr ages and range between 3200 ± 20 and 3328 ± 10 Ma. These gneisses were derived from a 3.3–3.5‐Ga mantle source as indicated from Nd isotopes. They did not participate significantly in the genesis of the 2.55‐Ga juvenile magmas. All these data, together with previous work, suggest that the 2.51‐Ga granulite facies metamorphism occurred near the contact of the ancient Peninsular gneisses and the 2.55–2.52‐Ga ‘juvenile’tonalitic–trondhjemitic terranes during synaccretional processes (subduction, mantle plume?). Rb–Sr biotite ages between 2060 and 2340 Ma indicate late cooling probably related to the dextral major east–west shearing which displaced the 2

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00197.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY