摘要:

ABSTRACTIn the inner aureole of the Bufa del Diente alkali syenite (north‐east Mexico), thin calcareous argillite bands horizontally embedded in impure marbles acted as contact‐metamorphic aquifers for hypersaline brines of magmatic origin. Thick‐bedded marbles were largely impervious. From 180 m up to the intrusion contact, argillites were completely decarbonated, resulting in melilite + wollastonite + phlogopite + perovskite‐bearing parageneses. In marbles, this assemblage is confined to a narrow 7‐12‐m‐wide infiltration zone adjacent to the contact. Up to this distance, calcite + wollastonite + diopside + alkali feldspar + titanite was stable, indicating that the fluid evolution in these marbles was internally buffered.Brine infiltration from the metaargillite aquifer into the marbles occurred perpendicular to the marble‐metaargillite boundaries and was confined to a zone 4‐6 cm wide above the boundaries. This is documented by the three reactionsCc + Di = Mel + CO2, (1)Cc + Kfs + Di + H2O = Phl + Wo + CO2, (2)Cc + Ttn = Prv + Wo + CO2, (3)Melilites (Ak32‐45Gh13‐32Sm32‐40to Ak52‐72Gh0‐1Sm28‐48) occur as rims around diopsides and become continuously thicker towards the metaargillite beds. Fluid inclusion observations suggest that the infiltrating brine was hypersaline (NaCl + KClcq∼ 65 wt%) and that the reactions took place at the water‐rich side of the H2O‐CO2‐salts immiscibility field at about 600d̀ C (2, 3) and 660 to 680d̀ C (1) atP∼ 1200 bar andXco2∼ 0.02.Mass balance calculations show that the amount of brine infiltrated from the aquifer into the marble was very low and decreased continuously with increasing distance from the boundary. The maximum width of brine infiltration was about 6 cm. This confirms that brine flow was largely parallel to the aquifer, not perpendicular to it. The CO2produced by the decarbonation reactions probably escaped as an immiscible low‐density H2O‐CO2fluid ofXco2≤ 0.5 into overlying marble via grain‐edge flow. The metaargillite‐marble boundary acted as a semipermeable membrane 6 cm in thickness keeping back the brine in the aquifer and

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00028.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTAcadian (Late Silurian to Early Devonian) metamorphism in the Central Maine Terrane (CMT) in central Massachusetts is characterized by an early low‐P, high‐T(Buchan‐type) metamorphism followed by thickening at high temperature (>650d̀ C) and then by cooling to 100‐200d̀ C below peak recorded temperatures before eventual unroofing. Mineralogical and textural evidence for this path includes sillimanite pseudomorphs after early andalusite, abundant cordierite in pelitic lithologies, replacement of low‐Pcordierite‐bearing assemblages by high‐Pgarnet‐bearing assemblages, and recrystallization of mylonites associated with late shear zones to form lower‐Tand higher‐Passemblages. Peak conditions in the highest grade rocks were 685‐780d̀ C and 5‐6 kbar; the cooling path passed through 550d̀ C at about 6.5 kbar.The well‐constrainedP‐Tpath documented from geological and mineralogical evidence for the CMT offers an unusual opportunity to examine characteristics of fluid inclusions that have experienced a long‐lived metamorphic event spanning a broad range ofP‐Tconditions. Fluid inclusion data from the CMT document a range of fluid compositions (CO2‐rich, mixed CO2‐N2‐rich, N2‐rich and H2O‐rich) and densities during metamorphism. Densities of CO2fluid inclusions range from 0.20 to 1.03 g cm‐3. Medium‐density CO2fluid inclusions are contained in quartz inclusions within garnets in partial melt leucosomes, and in quartz grains within migmatites. Fluid inclusions within the quartz inclusions indicate trapping conditions of 650‐700d̀ C at pressures below 5 kbar. Other CO2fluid inclusions from matrix quartz yield isochores which pass through 700d̀ C and 5.2 kbar. The highest density inclusions associated with rocks containing the late high‐Passemblages have isochores which pass below the estimatedP‐Tconditions for recrystallization of the mylonite. Fluid inclusion evidence suggests an early low‐Pheating event, followed by thickening at high temperature, and then by nearly isobaric cooling to about 500d̀ C with later decompression. This interpretation is also consistent with previously published petrological models and supports an ant

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00029.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTDuring the Alice Springs Orogeny, deformation at Ormiston Gorge, central Australia, occurred under lower‐ to middle‐greenschist facies conditions. Dolomites of the Bitter Springs Formation and quartzites. metagreywackes, and metapelites of the Heavitree Quartzite contain abundant early‐, syn‐, and post‐tectonic veins. However, though vein densities locally approach 15%, the distribution of veins and the oxygen isotope geochemistry of wallrocks and veins suggest that fluid movement was on a local scale. The Heavitree Quartzite contains quartz veins that, even along the main thrust plane, have similar δ18O values (13.5–16.9%o) to those of their wallrocks (13.6–16.9%o), with Δ18O(vein‐wallrock) values of ‐0.6 to 0.4%o. In contrast, the Bitter Springs Formation contains predominantly dolomite veins that have δ18O values of 23.4 to 27.7%o. These differences are observed even at the boundary between the Heavitree and Bitter Springs rocks, implying that significant fluid exchange between these rocks has not occurred, or that fluid flow was channelled through areas outside those sampled for this study. By contrast with the Heavitree Quartzite, δ18O values of wallrocks in individual samples of the Bitter Springs Formation are significantly higher (23.3–29.1%o) than those of the veins, with δ18O(vein‐wallrock) values up to ‐4%o (average of ‐2.1%o). These systematic differences in δ18O values most likely result from oxygen isotope fractionation caused by fluid immiscibility or disequilibrium dissolution. Smaller differences in δ13C values between some dolomite veins and wallrocks [δ13C(vein‐wallrock) up to ‐1.9%o, average of ‐0.5%o] are also explained by these processes. This study indicates that large volumes of veins may be produced by repeated fracturing and fluid migration within particular rock units, without involving lar

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00030.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTCompositional maps of orthopyroxene and garnet of contrasting grain size and in contact with different minerals were made from two paragneiss granulites from the Minto terrane of northern Quebec. The compositional maps provide clear evidence of late exchange of Fe/(Fe + Mg) after Ca in garnet and Al in orthopyroxene had been quenched‐in. The extent of late Fe‐Mg exchange was controlled by neighbouring minerals, with negligible Fe‐Mg gradients against plagioclase and quartz, and substantial gradients against exchangeable Fe‐Mg minerals. Cores of grains in contact with exchangeable Fe‐Mg neighbours are progressively more reset in Fe/(Fe + Mg) as grain size decreases, whereas cores of even small grains surrounded by only plagioclase and quartz are not significantly different in Fe/(Fe + Mg) than cores of the largest grains. Gradients of Ca in garnet and of Al in orthopyroxene in grains of uniform Fe/(Fe + Mg) preserve a high‐temperature retrograde history during which intergranular exchange effected compositional uniformity of mineral rims and intragranular Fe‐Mg diffusion in garnet and orthopyroxene was rapid enough to homogenize Fe/(Fe + Mg). The transition from efficient intergranular exchange at relatively high temperatures to local Fe‐Mg exchange at lower temperatures may have been controlled by loss of an intergranular exchange medium in the rock, possibly an internally generated dehydration melt phase. Implications for geothermometry of granulites include the following (numerical values are particular to this study): (1) core compositions of garnet and orthopyroxene grains in contact with exchangeable neighbours may be reset in Fe/(Fe + Mg) relative to the most refractory compositions by an amount equivalent to 120d̀ C; (2) Fe‐Mg exchange thermometry using even the most refractory Fe/(Fe + Mg) compositions may not record peak granulite conditions, possibly recording instead the temperature at which an intergranular exchange medium was lost from the rock; and (3) temperature‐sensitive net transfer equilibria involving Al solubility in orthopyroxene yield temperatures up to 150d̀ C higher than maximum Fe‐Mg exchange temperatures, even in grains with flat Fe/(Fe ‐ Mg) compositional profiles, making them a better means of estimating peak granulite temperatures than F

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00031.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTIn the central Minto Block of northern Québec, the Lake Minto and Goudalie domains are dominated, respectively, by orthopyroxene‐bearing plutonic suites (granite‐granodiorite and diatexite) and a tonalitic gneiss complex, both of which contain scattered remnant paragneisses. Two main granulite‐grade mineral assemblages are observed in the paragneiss: garnet (Grt)‐orthopyroxene (Opx)‐plagioclase‐quartz (GOPQ) and garnet (Grt)‐cordierite (Crd)‐sillimanite‐plagioclase‐quartz (GCSPQ). These show distinct lithological associations, with the GCSPQ assemblages occurring exclusively within the diatexite in the Lake Minto domain. Petrogenetic grid considerations demonstrate that the GOPQ rocks are higher grade than the GCSPQ rocks. Maximum temperatures for GOPQ rocks, obtained from equilibria based on Al solubility in orthopyroxene in equilibrium with garnet, range from 950 to 1000d̀ C, significantly higher than garnet‐orthopyroxene Fe‐Mg exchange temperatures of 700 ± 50d̀ C, the latter probably representing a closure temperature below peak conditions. The Al temperatures were corrected for late cation exchange by adjusting the Fe/(Fe + Mg) ratios in garnet and orthopyroxene, to achieve internal consistency between the GOPQ thermometers and barometers. Grt‐Crd thermometry records temperatures of 750±50d̀ C. PeakP‐Tconditions range from 5‐6 kbar and 750‐800d̀ C in the Goudalie and eastern Lake Minto domains, to 7‐10 kbar and 950‐1000d̀ C in the western and central Lake Minto domain. This variability contrasts with the uniform crustal pressures of 5 ± 1 kbar recorded by the GCSPQ assemblages in the diatexites and the hornblende granodiorites (c. 4‐5 kbar) across the same area. The GOPQ rocks are inferred to record earlierP‐Tconditions that prevailed before the formation of GCSPQ assemblages and the intrusion of the granodiorites. PartialP‐Tpaths in GOPQ rocks from both domains, based on net transfer equilibria corrected for Fe‐Mg resetting, document cooling of 100‐250d̀ C from thermal‐peak conditions, concomitant with a modest pressure decrease of 2‐3 kbar. Although textures diagnostic of isobaric cooling are not developed, the paths are consistent with a tectonic model in which granulite metamorphism and crustal thickening in the Minto Block were consequences of magmatic underplating. The progression from higherP‐Tconditions recorded by GOPQ assemblages to lowerP‐Tconditions recorded by GCSPQ assemblages is attributed to variable amounts of synmagmatic uplift and cooling in a single, continuous thermal event in the Minto crust, associated with protracted crustal magmatism. In the Goudalie and eastern Lake Minto domains, where GOPQ and GCSPQ rocks and Hbl granodiorites have similarP‐Tconditions of equilibration, the crust may not have been thickened as much as further west, where GOPQP‐Tconditions are significantly hi

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00032.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTCalcite and quartz veins have formed, and are forming, in steeply dipping fissures in the actively rising Alpine Schist metamorphic belt of New Zealand. The fluids that deposited these minerals were mostly under hydrostatic pressure almost down to the brittle‐ductile transition, which has been raised to 5‐6 km depth by rapid uplift. Some fluids were trapped under lithostatic pressures. Fluids in the fissure veins were immiscible H2O + NaCl‐CO2mixtures at 200‐350d̀ C. Bulk fluid composition is 15‐20 mol% CO2and<4.3 total mol CH4+ N2+ Ar/100mol H2O. Water hydrogen isotopic ratio δDH2Oin the fissure veins spans ‐29 to ‐68‰, δ18OH2O‐0.7 to 8.5‰, and bulk carbon isotopic ratio δ13C ranges from ‐3.7 to ‐11.7‰. The oxygen and hydrogen isotopic data suggest that the water has a predominantly meteoric source, and has undergone an oxygen isotope shift as a result of interaction with the host metamorphic rock. Similar fluids were present during cooling and uplift. Dissolved carbon is not wholly derived from residual metamorphic fluids; part may be gener

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00033.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTP‐Tpaths for a simple situation appropriate to many low‐P, high‐T(LPHT) terranes in which metamorphism and deformation are localized by advection of heat in magmas, has been modelled assuming a medium with a power‐law rheology with an inverse exponential dependence of stress and temperature and capable of sustaining deviatoric stress, τ, in the order of 100 MPa at 400d̀ C and strain rates of up to 10‐13s‐1. Numerical simulations and analytical approximations forP‐Thistories appropriate for simple convergent deformation histories show that the destruction of the deviatoric stress field around large intrusions may result in significant decompression near the metamorphic temperature peak. Moreover, for a specified strain rate and temperature evolution,P‐Tpaths may vary from clockwise to anticlockwise merely as a function of vertical distance from the heat source. Inasmuch as mounting independent evidence suggests that the crust can support deviatoric stresses of up to about 100 MPa at temperatures of 400‐500d̀ C, and that the shear strength of the crust is strongly temperature‐dependent in the range 400‐800d̀ C, these results suggest that caution should be taken in the tectonic interpretation ofP‐Tpaths involving decompression of the order of 100 MPa or less in LPHT terranes. The results illustrate a plausible mechanism for the close spatial association of both clockwise and anticlockwiseP‐Tpaths doc

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00034.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe north‐west Turkish blueschists represent a subducted passive continental margin sequence dominated by metaclastic rocks and marble. The depositional age of the blueschist protoliths are probably Palaeozoic to Mesozoic, while the age of the high‐pressure/low‐temperature metamorphism is Late Cretaceous. Blueschists are tectonically overlain by a volcanosedimentary sequence made up of accreted oceanic crustal material that locally shows incipient blueschist metamorphism and by spinel peridotite slices.The metaclastic rocks with regional jadeite and glaucophane, which comprise the lower part of the blueschist unit, make up an over 1000‐m‐thick coherent sequence in the Kocasu region of north‐west Turkey. Rare metabasic horizons in the upper parts of the metaclastic sequence with sodic amphibole + Iawsonite but no garnet indicate lawsonite blueschist facies metamorphism. The blueschist metaclastics in the Kocasu region are practically free of calcium and ferric iron and closely approximate the NFMASH system in bulk composition. Two low‐variance mineral assemblages (with quartz and phengite) are jadeite + glaucophane + chlorite + paragonite and chloritoid + glaucophane + paragonite. The metaclastics comprise up to several‐metres‐thick layers of jadeite schist with quartz, phengite and nearly 100 mol% jadeite. Phase relations in the metaclastics show that the chloritoid + glaucophane assemblage, even in Fe2+‐rich compositions, is stable in the jadeite stability field. In the NFASH system the above assemblage without the accompanying garnet has a narrow thermal stability field.Mineral equilibria in the metaclastics involving chloritoid, glaucophane, jadeite, paragonite and chlorite indicate metamorphicP‐Tconditions of 20 ± 2 kbar and 430 ± 30 d̀ C, yielding geothermal gradients close to 5d̀ C km‐1, one of the lowest geotherms recorded. Blueschists in the Kocasu region, which have been buried to 70 km depth, are tectonically overlain by the volcanosedimentary sequence and by peridotite buried not deeper than 30 km.Phengites from two jadeite schists were dated by Ar/Ar laser probe; they give an age of 88.5 ± 0.5 Ma, interpreted as the age of metamorphism. Blueschists and the overlying peridotite bodies are intruded by 48‐53‐Ma‐old granodiorite bodies that were emplaced at 10 km depth. This suggests that the exhumation of blueschists by underplating of cold continental crust, and normal faulting at the blueschist‐peridotite, interface occurred during the Late

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00035.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTA sequence of regional metamorphic isograds indicating a range from prehnite‐pumpellyite to lower amphibolite facies was mapped in metabasites near Flin Flon, Manitoba. The lowest grade rocks contain prehnite + pumpellyite and are cut by younger brittle faults containing epidote + chlorite + calcite. Isobaric temperature‐XCO2and pressure‐temperature (constantXCO2) diagrams were calculated to quantify the effects of CO2in the metamorphic fluid on the stability of prehnite‐pumpellyite facies minerals in metabasites containing excess quartz and chlorite. Prehnite and, to a lesser extent, pumpellyite are stable only in fluids withXco20.002, epidote + chlorite + calcite assemblages are stable. Our calculated phase relations are consistent with regional metamorphism in the Flin Flon area in the presence of an H2O‐rich fluid and a more CO2‐rich fluid in the later fault zones. We believe that the potential effects of small amounts of CO2in the metamorphic fluid should be assessed when considering the pressure‐temperature implications of mineral assemblages in low‐gr

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00036.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe Palghat Gap region is located near the centre of the large southern Indian granulite terrane. at the northern edge of the Kodaikanal charnockite massif. The dominant rock types in the region are hornblende‐biotite ± orthopyroxene gneisses and charnockites along with minor amounts of intercalated mafic granulite, metapelite and calc‐silicate. TheP‐Testimates from garnetiferous mafic granulites and metapelite samples are generally in the range 9‐10 kbar and 800‐900d̀ C using both conventional thermobarometric methods and the TWEEQU thermobarometry program. TheseP‐Testimates, which should be taken as minimum values, are among the highest yet reported for South Indian and Sri Lankan granulites. The occurrence of orthopyroxene + plagioclase symplectites around embayed garnet grains in the mafic granulites and cordierite rims around garnet grains in metapelite suggest an isothermal decompression‐type path. Similarly, a core‐rimP‐Ttrajectory indicatesc.3 and 7 kbar decompression at high temperature in the mafic granulites and metapelite, respectively. In both rock types, the key to the determination of the retrogradeP‐Tpath was the recognition of small amounts of second generation plagioclase with a more anorthitic composition than the matrix plagioclase. The preservation of high garnet‐pyroxene temperatures in the mafic granulites (despite small garnet grain size) suggests rapid cooling of the terrane. Calculated minimum cooling rates range from 8 to 80d̀ C Ma‐1. Such cooling rates are more rapid than those associated with normal isostatic processes and suggest that the terrane was tectonically e

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00037.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY