摘要:

AbstractA metasomatic diopside rock occurs at the top of the dolomitic Connemara Marble Formation of western Ireland and contains titanite and K‐feldspar in addition to around 90% diopside (XMg= 0.90–0.97). U–Pb isotopic measurements on this mineral assemblage show that the titanite is both unusually uranium‐rich and isotopically concordant, with the result that a precise U–Pb age of 478 ± 2.5 Ma can be determined. The age is identical within error to a less precise Rb–Sr age of diopside–K‐feldspar of 483 ± 6 Ma. Petrological evidence indicates that the assemblage crystallized atc. 620° C close to or below the closure temperature of titanite. The age thus provides a precise estimate of the time of metamorphism; this age is 11 ± 3 Ma younger than the 490 Ma age for nearby gabbroic plutons which has previously been used to constrain the peak metamorphic age. This difference accords well with geological evidence that the gabbros were emplaced prior to the metamorphic peak. Analysis of minerals with high closure temperature from assemblages whose crystallization is unambiguously associated with a specific episode of fluid infiltration at the peak of metamorphism provides the basis for a new approach to dating metamorphism. The success of this approach is demonstrated by the res

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00141.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThree types of mineral associations are described from calc‐silicate granulites from the Eastern Ghats, India, where geothermobarometry in associated rocks suggests extremely highP–Tconditions of metamorphism (c. 9 ± 1 kbar, 950° C). These mineral associations are: (i) calcite + quartz + scapolite + plagioclase, (ii) calcite + scapolite + wollastonite + porphyroblastic garnet + coronal garnet and (iii) calcite + quartz + wollastonite + scapolite + porphyroblastic garnet + coronal garnet, all coexisting with K‐feldspar, titanite and clinopyroxene. The first two associations evolved through nearly isobaric cooling retrograde paths, whereas the third evolved through a nearly isothermal decompression path followed by an isobaric cooling retrograde path. Textural and compositional characteristics suggest the following mineral reactions in the calc‐silicate granulites: calcite + quartz = wollastonite + CO2, calcite + plagioclase = scapolite, calcite + scapolite + wollastonite = porphyroblastic garnet ± quartz + CO2, CaTs + wollastonite = coronal garnet (association ii) and wollastonite + scapolite = coronal garnet (association iii) + quartz + CO2. Andradite content in garnet was buffered by the redox equilibria wollastonite + hedenbergite + O2= andradite + quartz (association iii) and wollastonite + andradite + CaTs + scapolite = hedenbergite + calcite + grossular + O2(association ii). The contrasting mineral parageneses have been ascribed to interplay of variables such asXCO2,fO2,fHClin the fluid, bulk Na content and the nature of the retrogradeP–T–XCO2paths through which the

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00142.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe main porphyroblastic minerals in schists and phyllites of the Foothills terrane, Western Metamorphic Belt, central Sierra Nevada, California, are cordierite and andalusite (mostly chiastolite). Less commonly, biotite, muscovite, chlorite, garnet or staurolite are also present as porphyroblasts. The variety of porphyroblast and matrix microstructures in these rocks makes them suitable for testing three modern hypotheses on growth and deformation of porphyroblasts: (1) porphyroblast growth is always syndeformational; (2) porphyroblasts nucleate only in low‐strain, largely coaxially deformed, quartz‐rich (Q) domains of a crenulation foliation and are dissolved in active high‐strain, non‐coaxially deformed, mica‐rich (M) domains, the spacing between which limits the size of the porphyroblasts; and (3) porphyroblasts generally do not rotate, with respect to geographical coordinates, during deformation, provided they do not deform internally, so that they may be used as reliable indicators of the orientation of former regional structural surfaces, even on the scale of orogenic belts.Some porphyroblast–matrix relationships in the Foothills terrane are inconsistent with hypotheses 1 and 2, and others are equivocal. For example, in many rocks it cannot be determined whether the porphyroblasts grew where the strain had already been partitioned into M and Q domains, whether the porphyroblasts caused this partitioning, or both. Although most porphyroblasts appear to be syndeformational, as predicted by hypothesis 1, observations that do not support the general application of hypotheses 1 and 2 to rocks of the Foothills terrane include: (a) lack of residual crenulations in many strain‐shadows and alternative explanations where they are present; (b) absence of porphyroblasts smaller than the distance between nearest mica‐rich domains; (c) nucleation of crenulations on existing porphyroblasts, rather than nucleation of porphyroblasts between existing crenulations; (d) presence of micaceous ‘arcs’in an undifferentiated matrix against some porphyroblasts, suggesting static growth; (e) absence of crenulations in porphyroblastic rocks showing sedimentary bedding; and (f) porphyroblasts with very small, random inclusions, which are probably pre‐deformational. Similarly, porphyroblasts that have overgrown sets of crenulations and porphyroblasts with micaceous ‘arcs’are probably post‐deformational, at least on the scale of a large thin section and probably over much larger areas, judging from mesoscopic structural evidence.Some porphyroblasts in rocks of the Foothills terrane do not appear to have rotated, with respect to geographical coordinates, during matrix deformation, in accordance with hypothesis 3, at least on the scale of a large thin section. However, other porphyroblasts evidently have rotated. In some instances, this appears to be due to mutual interference, but many apparently rotational porphyroblasts are too far apart to have interfered with each other, which indicates that the rotation was associated with deformation of the matrix. The occurrence of planar bedding surfaces adjacent to porphyroblasts about which bedding and/or foliation surfaces are folded suggests rotation of the porphyroblasts during non‐coaxial flow parallel to bedding, rather than crenulation of the matrix foliation around static porphyroblasts. It appears that porphyroblasts may rotate during deformation if the matrix is relatively homogeneous, so that the strain is effectively non‐coaxial. This may occur after homogenization of a matrix in response to the strongest degree of crenulation folding, whereas the same porphyroblasts may have been inhibited from rotating previously, when strain accumulatio

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00143.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractPetrological, oxygen isotope and40Ar/39Ar studies were used to constrain the Tertiary metamorphic evolution of the lower tectonic unit of the Cyclades on Tinos. Polyphase high‐pressure metamorphism reached pressures in excess of 15 kbar, based on measurements of the Si content in potassic white mica. Temperatures of 450–500° C at the thermal peak of high‐pressure metamorphism were estimated from critical metamorphic assemblages, the validity of which is confirmed by a quartz–magnetite oxygen isotope temperature of 470° C. Some40Ar/39Ar spectra of white mica give plateau ages of 44–40 Ma that are considered to represent dynamic recrystallization under peak or slightly post‐peak high‐pressure metamorphic conditions. Early stages in the prograde high‐pressure evolution may be documented by older apparent ages in the high‐temperature steps of some spectra.Eclogite to epidote blueschist facies mineralogies were partially or totally replaced by retrograde greenschist facies assemblages during exhumation. Oxygen isotope thermometry of four quartz–magnetite pairs from greenschist samples gives temperatures of 440–470° C which cannot be distinguished from those deduced for the high‐pressure event. The exhumation and overprint is documented by decreasing ages of 32–28 Ma in some greenschists and late‐stage blueschist rocks, and ages of 30–20 Ma in the lower temperature steps of the Ar release patterns of blueschist micas. Almost flat parts of Ar–Ar release spectra of some greenschist micas gave ages of 23–21 Ma which are assumed to represent incomplete resetting caused by a renewed prograde phase of greenschist metamorphism.Oxygen isotope compositions of blueschist and greenschist facies minerals show no evidence for the infiltration of a δ18O‐enriched fluid. Rather, the compositions indicate that fluid to rock ratios were very low, the isotopic compositions being primarily controlled by those of the protolith rocks. We assume that the fundamental control catalysing the transformation of blueschists into greenschists and the associated resetting of their isotopic systems was the selective infiltration of metamorphic fluid. A quartz–magnetite sample from a contact metamorphic skarn, taken near the Miocene monzogranite of Tinos, gave an oxygen isotope temperature of 555° C and calculated water composition of 9.1%. The value of δ18O obtained from this water is consistent with a primary magmatic fluid, but is lower than that of fluids associated with the greenschist overprint, which indicates that the latter event cannot be dire

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00144.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractEclogites with a wide range in bulk composition are present in the Münchberg Massif, part of the Variscan basement of the Bohemian Massif in north‐east Bavaria. New analyses of the primary phases garnet, omphacite, phengite and amphibole, as well as the secondary phases clinopyroxene II, various amphiboles, biotite/phlogopite, plagioclase, margarite, paragonite, prehnite and pumpellyite, reveal a complex uplift history. New discoveries were made of samples with very jadeite‐rich primary omphacite as well as a secondary omphacite in a symplectite with albite. Various geothermobarometric techniques, together with thermodynamic databases (incorporating separately determined activity–composition values) and experimental data have clustered the minimum conditions for the primary assemblages to theP–Trange 650 ± 60° C, 14.3 ± 1 kbar. However, jadeite (in omphacite)–kyanite–paragonite (in phengite) and zoisite–grossular (in garnet)–kyanite–quartz relationships suggests pressures of 25–28 kbar at the same temperatures. The fact that the secondary omphacite–plagioclase assemblage yields pressures within a few hundred bars of the minimum pressures for the plagioclase‐free assemblages strongly suggests that the minimum values are serious underestimates.Zoning, inclusion suites and breakdown reactions of primary phases, in addition to new minerals formed during uplift, define a polyphase metamorphic evolution which, from geochronological evidence, occurred solely within the Variscan cycle. The complex breakdown in other Bohemian Massif eclogites and the distinct variation in their temperatures during uplift suggest a multi‐stage thrusting model for the regional evolution of the eclogites. Such an evolution has significance with respect to incorporation of mantle slices into crustal sequences and fluid derivation from successively subducted units, possibly dr

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00145.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe D'Entrecasteaux Islands of eastern Papua New Guinea consist of a number of active metamorphic core complexes formed under an extensional tectonic setting related to sea‐floor spreading in the west Woodlark Basin. The complexes are defined by mountainous domes (>2500 m high) of fault‐bounded, high‐grade metamorphic rocks (including eclogite facies) intruded by 2–4‐Ma granodiorite plutons. Garnet–clinopyroxene exchange thermometers indicate that the temperature of equilibration of the eclogites was 730–900° C. The jadeite component of omphacite indicates minimum pressure of 21 kbar, suggesting depths of>70 km.The metamorphic rocks have undergone widespread retrogression to amphibolite facies. Retrogression of the metamorphic basement is associated with shearing and formation of the metamorphic core complexes.P–Tconditions in the early stages of shear zone activity, determined using the garnet–biotite exchange thermometer and the GASP and GRIPS barometers, were 570–730° C and 7–11 kbar. A second phase of re‐equilibration at much lower pressures appears to be related to the widespread intrusion of granodiorite plutons. One re‐equilibrated gneiss indicated maximum temperature of 730° C at estimated pressures of approximately 4 kbar. This late, high‐temperature metamorphism is also indicated by reactions involving the production of hercynite and corundum in aluminous gneisses and formation of sillimanite at the expense of kyanite.Two major episodes of granodiorite intrusion occurred during uplift and exhumation of the core complexes. Both closely coincide spatially with high‐temperature metamorphic rocks, the onset of deformation in extensional shear zones and subsequent uplift of the metamorphic basement. These observations indicate a fundamental link between uplift and granodiorite intrusion during continental extension and the formation of the D'Entrecasteaux Is

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00146.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractBlueschists and retrogressed eclogites are located along the Pinchi Fault Zone (near 54°30′N and 124°W) in central British Columbia. The Pinchi Fault separates rocks of contrasting geological histories, and the blueschists and eclogites occur with ultramafic rocks as fault‐bounded blocks. The retrogressed eclogites occur as tectonic blocks, now in glacial debris, a few metres across; blueschists occur in coherent kilometre‐sized tracts. Eclogites contain garnet–omphacite–rutile–quartz, with glaucophane, lawsonite and titanite. Some of the lawsonite appears to be stable with omphacite and garnet. Tectonic blocks of eclogite from two different localities have recorded differentP–Thistories. In one tectonic block,P–Testimates for garnet inclusions in clinopyroxene and lawsonite (c. 565° C,>13.1 kbar) suggest that garnet and omphacite initially equilibrated outside the stability field of lawsonite. A decrease in temperature, as recorded in garnet rims and matrix clinopyroxene, resulted in crystallization of lawsonite and other retrogressive minerals. Later crystallization of stilpnomelane (locally pseudomorphing garnet), howieite, winchite and actinolite was triggered by an influx of fluid underP–Tconditions outside the stability field of garnet. Lawsonite appears to have been stable, suggesting a minimum pressure of about 3 kbar. In the second tectonic block, clinopyroxene inclusions in garnet suggest temperatures near 350° C (P>10 kbar) and garnet rims equilibrated with matrix clinopyroxene suggest temperatures near 450° C at pre

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00147.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractOrthopyroxene‐bearing migmatites, exposed at the summit of Cone Peak in the Santa Lucia Range, California, offer an opportunity to explore potential links between granulite facies metamorphism and migmatite formation. Geothermobarometry indicates that the metamorphic temperatures and pressures were in the approximate ranges of 700–750° C and 7.0–7.5 kbar. The rocks at the summit comprise three domains: relatively coarse‐grained, leucocratic veins; relatively fine‐grained, biotite‐enriched zones at the margins of the veins; and a biotite–hornblende‐bearing host rock. Orthopyroxene is concentrated in the veins, which have also the highest ratio of anhydrous to hydrous minerals of the three rock types. The composition of the veins, together with their textures and modes, suggest that they formed through anatexis involving a dehydration‐melting reaction which consumed hornblende and produced orthopyroxene. Variability in mineralogy and composition indicates that there was some local migration of magma along the veins before their final solidification. The biotite‐enriched zones formed either by the concentration of residual biotite at the margins of the vein, or through the metasomatic conversion of hornblende (and/or pyroxene) to biotite, or by a combination of the two processes. Significant differences in the chemistry of the neosome (vein + biotite‐enriched zone) and the host rock rule out simple dehydration melting in a local closed system. The model that explains best the mineralogical and chemical patterns involves triggering of melting by an influx of a low‐aH2Omixed fluid which added K and Si to and rem

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00148.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.tb00149.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY