摘要:

ABSTRACTStructures in rocks altered by the infiltration of magmatic fluids provide key constraints on the relative timing of plutonism and regional deformation in the central Old Woman Mountains, south‐eastern California. In this well‐exposed area the Scanlon shear zone, a belt of attenuated, shallowly dipping, amphibolite facies Palaeozoic strata, is in contact with two tabular, Late Cretaceous (∼74 Ma) granitoid plutons. The shear zone contains massive wollastonite‐bearing skarns ∼40 m above the contact with the Old Woman granodiorite. Field evidence, petrological data and stable isotope compositions indicate that the mineral assemblages in these skarns formed when the granodiorite crystallized and exsolved water‐rich, magmatic fluids. Structural features in the skarns, which include transposed wollastonite foliations, syntectonic vesuvianite and garnet porphyroblasts, and quartz ± wollastonite veins, afford an opportunity to monitor the regional strain at the time of pluton crystallization. These structures yield a broad range of timing relationships that indicate synkinematic mineral growth with deformation ending first, atc. 74 Ma. The metasomatic structures are kinematically compatible with the regional deformation observed in other lithologies and no post‐tectonic features overprint the fabrics in the skarns. Observations of isoclinally folded and boudinaged dykes and pegmatites, granodiorite saddle reefs in isoclinal fold hinges, and undeformed, locally pegmatitic dykes also yield a broad range of timing relationships indicative of synkinematic plutonism, with deformation ending first. Our results show that coupling petrological and stable isotope data with structural analysis is effective for unravelling the relative timing of heat and mass transfer processes in pluton‐

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00054.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe Old Woman Mountains in south‐eastern California are a Late Cretaceous low‐pressure metamorphic terrane where multiple magmatic intrusions generated broad regions of elevated metamorphic temperatures. In the Scanlon Gulch area, two sheet‐like, Late Cretaceous granitoid plutons are in contact with the Scanlon shear zone, a 1‐km‐thick sheet of isoclinally folded and transposed metamorphic rocks. The metaluminous Old Woman granodiorite underlies the shear zone and the peraluminous Sweetwater Wash granite overlies it. Both plutons record emplacement ages of ∼74 Ma. Thermobarometry and phase relations in the shear zone suggest that peak metamorphism was at 650 ± 50† C and 4.3 ± 0.5 kbar. Late Cretaceous metamorphic temperatures were less elsewhere in the Old Woman Mountains, away from the intrusions.One‐dimensional thermal models are used to investigate how differences in the time between the emplacement of plutons would affect the thermal evolution of the central Old Woman Mountains. The prediction of a thermal history inferred from petrological and thermochronological data requires the rapid emplacement of the two plutons around the shear zone; simulations with delays of more than 1 Myr in the emplacement of the second pluton failed to predict peak metamorphic temperatures. Calculations which consider only the emplacement of a single pluton yield metamorphic temperatures that are too low. The time separating the intrusions is by far the most sensitive parameter in the calculations; assumptions concerning the treatment of the initial geothermal gradient and the latent heat of crystallization have relatively small effects on the predicted thermal histories. Our results suggest that for certain geometries, relatively short‐lived magmatic events involving rapid emplacement of multiple intrusions can produce low‐

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00055.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTCalc‐silicate rocks occur as elliptical bands and boudins intimately interlayered with eclogites and high‐pressure gneisses in the Münchberg gneiss complex of NE Bavaria. Core assemblages of the boudins consist of grossular‐rich garnet, diopside, quartz, zoisite, clinozoisite, calcite, rutile and titanite. The polygonal granoblastic texture commonly displays mineral relics and reaction textures such as post kinematic grossular‐rich garnet coronas. Reactions between these mineral phases have been modelled in the CaO‐Al2O3‐SiO2‐CO2‐H2O system with an internally consistent thermodynamic data base. High‐pressure metamorphism in the calc‐silicate rocks has been estimated at a minimum pressure of 31 kbar at a temperature of 630d̀ C withXH2, O ≥ 0.03. Small volumes of a CO2‐N2‐rich fluid whose composition was buffered on a local scale were present at peak‐metamorphic conditions. TheP‐Tconditions for the onset of the amphibolite facies overprint are about 10 kbar at the same temperature.XCo2of the H2O‐rich fluid phase is regarded to have been<0.03 during amphibolite facies conditions. TheseP‐Testimates are interpreted as representing different stages of recrystallization during isothermal decompression. The presence of multiple generations of mineral phases and the preservation of very high‐pressure relics in single thin sections preclude pervasive post‐peak metamorphic fluid flow as a cause of a re‐equilibration within the calc‐silicates. The preservation of eclogite facies, very high‐pressure relics as well as amphibolite facies reactions textures in the presence of a fluid phase is in agreement with fast,

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00056.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTMineral stable isotopic and trace element studies in 2 GPa banded eclogites of the Tauern Window, eastern Alps, record mm‐ to cm‐scale heterogeneities that reflect compositional variations in the accompanying metamorphic fluids. A close correlation between dolomite mode and dolomite δ18O is consistent with equilibrium partitioning among coexisting minerals and fluids. Small variations in dolomite δ13C values correspond with δ18O variations, but an overall decrease in dolomite δ13C byc. 1%o across a 12‐cm sample is a relict feature that pre‐dates eclogite equilibration. Garnet, omphacite, and clinozoisite rims show little systematic mineral‐mineral partitioning behaviour for Ti, V, Cr, Y, Sr, or Zr; major elements, however, are well equilibrated among these same minerals. Despite the apparent lack of mineral‐mineral trace element equilibration, most of the trace elements vary systematically with water activity calculated in each layer. Trace element behaviour during the eclogite metamorphism thus appears to have been controlled largely by mineral‐fluid interactions along grain boundaries.Shallow structural levels in other subduction complexes (c. 10‐45 km) typically exhibit fracture‐controlled permeability and extensive metasomatism, but there is no field or geochemical evidence for extensive fluid advection during high‐pressure metamorphism in the Tauern eclogites. Because most dewatering and devolatilization during tectonic burial occurs prior to eclogite conditions, the volumetric fluid/rock ratio in eclogites should generally be low. Low fluid/rock ratios, coupled with the possible non‐wetting nature of the fluids, permits the production and preservation of fine‐scale chemical heterogeneities in deeply subducted eclogites and associated fluids. However, the eventual breakdown at greater depth of volatile‐bearing dolomite, phengite, clinozoisite, zoisite, or amphibole could lead to renewed fracture‐controlled fluid release from the subducted rocks to regions appro

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00057.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTCalc‐silicate boudins within Proterozoic granulite facies gneisses of the northern Prince Charles Mountains, East Antarctica, preserve a number of reaction textures including garnet coronas between calcite and scapolite; garnet‐quartz coronas between scapolite and wollastonite and between plagioclase and wollastonite; calcite‐quartz intergrowths in wollastonite; and calcite‐plagioclase symplectites in scapolite. These textures have been modelled using petrogenetic grids for reactions in the CaO‐Al2,O3‐SiO2‐CO2system, but with reduced mineral activities to account for additional components in real mineral compositions. Such fixed‐composition reduced‐activity grids are strictly valid only at the point inP‐T‐aCO2space where an assemblage last equilibrated, and do not show the true positions of reactions away from this point because mineral compositions change with reaction progress. In this case, however, mineral compositions close to end‐member values and low extents of reaction progress mean that compositional change was limited and the grids are good approximations to true pseudosections over the entireP‐T‐aco2range of interest.The grids show that the textures are consistent with near‐isobaric cooling from about 850 to 700d̀ C at 7 kbar, aP‐Tpath compatible with thermobarometric studies of other lithologies from the area. Phase relationships indicate that CO2activities were buffered by the local mineral assemblage during peak and retrograde metamorphism, either under fluid‐absent conditions or within a non‐pervasive fluid phase. Previous studies of garnet coronas in scapolite‐wollastonite calc‐silicates have used qualitative grids based on limited experimental data to invoke garnet growth during water infiltration at high temperature, but the grids used here show that garnet coronas can form on co

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00058.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTChemical data for 139 natural paragonite‐muscovite (Pg‐Ms) pairs illustrate the effects of ferromagnesian components on theP‐T‐Xtopology of the Pg‐Ms solvus. The pairs were selected on the basis of: reasonably accurate knowledge of theP‐Tconditions of formation; evidence for close approach to equilibrium at peak metamorphic conditions; exclusion of pairs in which paragonite contains more than 5 mol% margarite; and exclusion of pairs from polymetamorphic rocks that contain more than one set of cogenetic Pg‐Ms pairs. Graphical analysis reveals considerable scatter in the data; nevertheless, it is evident that the muscovite limb of the solvus shifts markedly toward end‐member muscovite with increasing pressure from approximately 7 kbar to 21 kbar. This shift is attributed to a pressure‐induced increase of the ferromagnesian content of muscovite, which increases the size of the XII alkali site ‐ to the effect that K is more readily accommodated than Na. The data also suggest that the paragonite limb of the solvus migrates slightly toward end‐member paragonite with increasing pressure. Broadening of the Pg‐Ms solvus with increasing pressure reflects increasingly nonideal Na‐K mixing as the phengite content of muscovite increases. Due to the wide scatter of data for Pg‐phengitic‐Ms pairs, it is concluded that, at the present time, Pg‐Ms solvus thermometry is only via

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00059.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTGrandite garnet‐rich calcsilicate rocks from the Lower Calcsilicate Unit of the regionally metamorphosed Reynolds Range Group (central Australia) crop out along a strike‐parallel section in which a transition zone from M22amphibolite to granulite facies rocks is exposed. Across this transition the grandite‐rich layers do not show systematic changes in mineral assemblages, compositions and modes, or stable isotope compositions. These layers are deformed by F22folds that are associated with the peak of regional low‐pressure/high‐temperature metamorphism. Therefore, the grandite‐rich layers appear to pre‐date regional metamorphism and to have acted as closed chemical systems during prograde M22metamorphism.Mineral assemblages in the grandite‐rich layers are consistent with their formation through the infiltration of oxidized, water‐rich fluids (Xco2<0.1–0.3; logfo2‐16 to ‐14). The stable isotope values of calcite (Δ13C=‐4.2 to ‐0.8%0PDB; Δ18O = 10.5–14.0%0V‐SMOW) and bulk‐silicate fractions (Δ18O = 6.1 to 10.8%) of the grandite‐rich layers are most consistent with the infiltrating fluid being from a magmatic source. It is most likely that fluid infiltration occurred during the pre‐M22contact metamorphism (M21) that affected much of the Reynolds Range Group. The preservation of these assemblages is probably due to their high variance and little pervasive fluid‐rock interaction having occurred during M22.The clinopyroxene‐ and feldspar‐rich calcsilicate rocks that host the grandite‐rich layers contain poikiloblastic grandite garnet that formed during prograde M22metamorphism. Thin marbles that locally occur with the grandite‐rich layers contain a third garnet generation that is post‐ or late M22. This grossular‐rich garnet occurs in coronas around calcite, plagioclase, clinopyroxene, wollastonite and scapolite. These coronas are consistent with cooling and/or compression. However, because the marble assemblages are themselves overprinted by M21grandite‐rich layers the development of coronal garnet does not reflect a continuousP‐T‐tpath. Rather, it more probably reflects the partial re‐equilibrati

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00060.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTMetamorphism of the Gile Mountain Formation and Waits River Formation in the Strafford Dome and Townshend‐Brownington Syncline in east‐central Vermont records two nappe‐style events, D1 and D2, followed by doming. D1 formed a muscovite + biotite ± ilmenite schistosity subparallel to compositional layering, SO, and was followed by heating to garnet grade. The temperature and pressure at the end of D1 are estimated to bec. 450d̀ C and 6‐8 kbar. D2 variably crenulated and folded S1 during a nearly isothermal pressure increase of 1‐2 kbar, calculated from compositions of garnet, which have inclusions trails with progressive crenulation and rotation of the S1 fabric. SimilarP‐Tpaths are computed for most of the area, suggesting that the later schistosity developed during emplacement of a regional nappe 3‐6 km thick. There is a general lack of D3 (dome‐stage) microstructures.Near the Strafford‐Willoughby Arch, staurolite and kyanite overgrew S2 in pelites, and plagioclase with increasingXAnovergrew S2 in calcic pelites, reflecting post‐D2 heating to a maximum of 550‐600d̀ C. Metamorphic pressures at the end of D2 are fairly constant on the west side of the dome, indicating minor dome‐stage uplift. In contrast, pressures at the thermal peak of metamorphism decrease by more than 4 kbar east of the dome. The observed pattern of isotherms and isobars is mainly the result of post‐metamorphic, differential uplift and unroofing.Finally, a minor, retrograde metamorphism produced the assemblage albite + epidote + K‐feldspar + muscovite + chlorite, with grade increasing eas

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00061.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTOxygen isotope ratios, whole rock major and trace element compositions, and petrological characteristics of 52 samples from nine distinct igneous lithologies in the lower plate of the Whipple Mountain metamorphic core complex of south‐eastern California indicate that both mylonitic and non‐mylonitic lithologies underwent exchange with surface‐derived meteoric waters. Broadly granodioritic lithologies are characterized by whole rock δ18O values that range from 10.6 to 2.6‰. Isotopic compositions of quartz and feldspar mineral separates indicate that quartz has largely retained original igneous compositions but that feldspar has undergone variable and often large18O‐depletions (up to 6.5‰).Over 4 km of structural relief is exposed in lower plate gneisses below the Whipple detachment fault including non‐mylonitic lithologies at shallow structural levels above the mylonite front, and mylonitic gneisses at intermediate to deep levels below the mylonite front. Coupled δ18Oqtz‐ δ18OFspsystematics of non‐mylonitic and mylonitic andesite to rhyolite dykes from shallow and intermediate structural levels of the lower plate document two episodes of hydrothermal alteration: a high‐temperature (>c.600d̀C) episode involving a metamorphic or magmatic fluid with δ18O values ∼ 7‰ and a low‐temperature (c.350d̀C) episode involving low‐δ18O meteoric fluids. All the dykes that document exchange with meteoric fluids are non‐mylonitic. Coupled δ18OFspsystematics of non‐mylonitic and mylonitic granodioritic gneisses from above and below the mylonite front also document low‐temperature (c. 350d̀ C) exchange with meteoric fluids. The data indicate that infiltration of meteoric fluids occurred as lower plate lithologies were juxtaposed against the base of the faulted upper plate. High‐angle normal faults in the upper plate served as the conduits for the downward circulation of surface‐derived fluids. Meteoric fluids were able to penetrate across the detachment fault into the lower plate.Uplift rates coupled with independent cooling rates indicate that surface‐derived fluids penetrated to a depth ofc.4km and possibly as deep asc.8km. Penetration of surface‐derived fluid into the ductile deformation regime is not required to explain the low δ18O values observed in lower plate lithologi

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00062.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe prograde amphibole that coexists with chlorite, epidote, muscovite, albite, quartz and hematite in Sanbagawa schists was examined to investigate the relationship between the progradeP‐Tpaths of individual rocks and the metamorphic field gradient in the Sanbagawa metamorphic belt, central Shikoku. The amphibole changes from actinolite, through ferri‐winchite and crossite, to barroisite and hornblende with increasing grade along the metamorphic field gradient. However, the sequence of prograde amphibole compositions in each sample varies in different mineral zones. The general scheme can be summarized as: magnesioriebeckite‐riebeckite crossite in the upper chlorite zone of lower‐grade rocks; crossite or glaucophane barroisite in the garnet zone of medium‐grade rocks; and actinolite or winchite barroisite hornblende in the albite‐biotite zone of higher‐grade rocks. Changes of amphibole composition indicate that the progradeP‐Tpath recorded in the higher‐grade rocks was situated on the higher‐temperature side of that of the lower‐grade rocks and on the lower‐pressure side of the metamorphic field gradient. The systematic change ofP‐Tpaths implies an increasing dP/dTduring continuous subduction. These features can be interpreted as documenting prograde metamorphism within a young subduction zone that has a

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00063.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY