Pressure Shadows Research Articles

Formation of the Yakuno ophiolite; accretionary subduction under medium-pressure-type metamorphic conditions

The notion that the Yakuno ophiolite and overlying Maizuru Group represents an accretionary prism formed during the Permian evolution of Japan on the Yakuno eruptive sequence, association of hemipelagic mudstone with silicic tuff, exotic fossiliferous limestones derived from previously accreted sea-mounts, upward coarsening of sequences terrigenous sandstone and conglomerate, and mildly deformed Permian and Triassic forearc basin formations. The most important indicator, however, is the seaward imbrication and repetition observed in both the Maizuru Group and the ophiolite itself. D1 deformation structures include axial–planar foliations (pressure-solution cleavage for the Maizuru Group and granulite–amphibolite metamorphic layering in the ophiolite), flattening type strain, symmetric pressure shadows and fringes, and isoclinal folds showing axial–planar foliations and thrust faulting at their overturned limb. The exceptional asymmetry observed indicates seaward-directed shearing near the thrust, while D1 structures in the Maizuru zone are explained by off-scraping, above the basal decollement. The later Jurassic D2 kink fold structure includes a first-order asymmetric kink with a brittle thrust at its overturned limb, more-or-less coeval with M2 retrograde metamorphism. Medium-pressure M1 prograde metamorphism in the Yakuno ophiolite produced layering of granulite and amphibolite, and in the Maizuru Group, formation of illite along pressure-solution cleavage of mudstones. The metamorphic grade is controlled by the stratigraphic relationships and appears typical of that in ocean floor regions. However, there was only one episode of M1 prograde metamorphism which occurred contemporaneously with D1 off-scraping. Given that subduction zones are normally characterized by high P/ T metamorphic regimes, the observed P/ T history appears to reflect relatively unusual conditions. Such high thermal gradients may plausibly reflect the approach of a young, hot oceanic plate which continued subducting beneath the Japanese arc. Accordingly, the Yakuno ophiolite was probably formed at the trench–trench–ridge triple junction.

Geochemical and textural evidence for early (shallow) diagenetic growth of stratigraphically confined carbonate concretions, Upper Devonian Rhinestreet black shale, western New York

Laterally persistent carbonate concretionary horizons are conspicuous to the Upper Devonian (Frasnian) Rhinestreet black shale of the western New York State Appalachian Plateau. Field observations, including randomly tilted concretions and differential compaction of host sediment laminae around concretions, are consistent with early diagenetic growth in unconsolidated sediment. Further, estimates of pre-cementation host sediment porosity based on the volume percentage of CaCO 3 cement (74–93%) and, perhaps most importantly, the preservation of a cardhouse clay fabric in mudstones collected from concretion pressure shadow regions suggest that concretionary growth occurred rapidly within perhaps a meter of the seafloor. Petrographic examination (SEM) of matrix carbonate samples collected from six concretions reveals a relatively porous (∼4–7%) framework of calcite micrite and microsparite, euhedral pyrite (most abundant near concretion rims), disseminated pyrite framboids and randomly oriented detrital clay platelets and domains. C isotopic compositions of Rhinestreet concretion matrix carbonate range from −13.9‰ to +1.7‰ PDB suggesting derivation of C from a mixed source that included depleted and isotopically heavy C. Slightly to moderately depleted O isotopic compositions (−3.8‰ to−6.8‰ PDB) do not corroborate the shallow depth of concretion growth suggested by field and textural observations. The vertically confined nature of most Rhinestreet concretions, their large size, oblate ellipsoidal shape, variable C isotope compositions and absence of any evidence that concretions were sited at local concentrations of organic matter suggest an origin by anaerobic CH 4 oxidation. Early pre-concretion sulfate reduction and associated pyrite framboid formation that occurred throughout the host sediment was renewed in thin, shallow zones of uncompacted sediment—the concretionary horizons—by CH 4 and isotopically heavy dissolved carbonate diffusing upward from the zone of biogenic methanogenesis to the base of the sulfate reduction zone, probably little more than a meter below the seabed. Local variations in the mix of isotopically light CH 4 and heavier dissolved carbonate may have been responsible for the wide range of δ 13C isotopic compositions of Rhinestreet concretions. Crucial to this mechanism is a pause in deposition that would have held the zone of carbonate precipitation at a fixed distance below the ocean floor long enough for the large concretions to grow. Evidence that sedimentation slowed or even stopped during concretion growth includes (1) laminae that can be traced through concretions with no observed systematic changes in thickness and (2) minimal concretion center-to-edge variations in estimated pre-cementation porosity. A resumption of sedimentation and related burial brought concretion growth in each horizon to an end. The moderately depleted δ 18O values of Rhinestreet concretions probably reflect the “resetting” of O isotopic compositions by warm fluids migrating through the permeable concretions during burial.

Origin of Shale Fabric by Mechanical Compaction of Flocculated Clay: Evidence from the Upper Devonian Rhinestreet Shale, Western New York, U.S.A.

ABSTRACT Most workers attribute the preferred grain orientation that defines the shale fabric to the mechanical collapse of a high-porosity clay microfabric under increasing overburden pressure. Scanning electron microscope analysis of samples of the Upper Devonian (Frasnian) Rhinestreet shale of western New York State collected from near early diagenetic carbonate concretions provides further evidence of this mechanism. Specifically, mudstones recovered from pressure shadows immediately adjacent to lateral edges of concretions are characterized by an open fabric of randomly arranged clay domains--the cardhouse fabric. Laterally equivalent shale samples collected only 0.2 to 0.3 m distant the pressure shadows, however, reveal a low-porosity, strongly oriented shale fabric in which clay particles mold around incompressible quartz silt and euhedral pyrite grains and pyrite framboids. These observations suggest that the organic Rhinestreet shale accumulated as water-rich flocculated clay. The open clay fabric observed in pressure-shadow mudstones was preserved by the incompressible concretions during burial. Flocculated clay beyond the shielding effect of the concretions, however, collapsed during progressive burial to form the strongly oriented shale fabric.

Zn-Pb mineralisation in the Silvermines district, Ireland: a product of burial diagenesis

Carbonate-sulphide cement stratigraphic relationships in the host rock and ore have been used to constrain the age of mineralisation at the Silvermines zinc-lead-barium deposit. The base-metal sulphides post-date planar dolomite and replace stylolites. Furthermore, the pre-mineralisation planar dolomites also replace stylolites. These and other diagenetic observations indicate that the base-metal sulphides formed at burial depths greater than 800 m, but probably predate the Variscan deformation (since pressure shadows overgrow base metal sulphides). This indicates that the sulphides are of epigenetic origin, constraining the age of mineralisation to between the late Chadian (≈347 Ma) and the late Westphalian (≈307 Ma). However, the most likely age for mineralisation, (based on widespread macro-stylolite development) is Asbian (≈339 Ma) or younger. No evidence of synsedimentary sulphides (in the form of hydrothermal chimneys, vent faunas, or sulphides intergrown with marine cements) was observed at Silvermines. Mineralised breccias (black matrix breccias), late-stage internal sediments, and dissolution zones within the carbonate cements all appear to be produced by hydrothermal karsting that occurred during the mineralisation process. Fluid inclusion homogenisation temperatures for ore-stage calcites (up to 300 °C) approach the peak temperature estimates derived from regional maturation parameters (270 to 310 °C from conodont alteration indices and vitrinite reflectance). This suggests that homogenisation temperatures represent maximum heating temperatures (probably during Variscan time) rather than mineralisation temperatures.

Partial eclogitisation of gabbroic rocks in a late Precambrian subduction zone (Zambia): prograde metamorphism triggered by fluid infiltration

Gabbros and eclogites occur closely associated in a 200-km-long and up to 40-km-wide area of the Zambezi Belt in central Zambia. This area is interpreted to represent part of a late Precambrian suture zone, with the mafic rocks being relics of subducted oceanic crust. Gradual stages of prograde transformation from gabbro to eclogite are preserved by disequilibrium textures of incomplete reactions. This resulted in kyanite-omphacite-bearing assemblages for eclogites that have Al-poor bulk compositions. Undeformed eclogites typically preserve features of a former gabbroic texture, reflected by replacements of plagioclase and magmatic pyroxene by eclogite facies minerals. Textures of deformed eclogites range from sheared porphyroclastic to porphyroblastic. Relics of magmatic pyroxene are common and complete eclogitisation occurred only in millimetre to centimetre-scale domains in most of the rocks. No evidence for prograde blueschist or amphibolite facies mineral assemblages was found in eclogites. In contrast, the fine grained intergrowth of omphacite, garnet, kyanite and quartz, which replace former plagioclase or was formed in the pressure shadow of magmatic pyroxene relics, indicates that eclogitisation might have affected the gabbroic protoliths directly without any significant intervening metamorphic reactions. Eclogitisation took place under P-T conditions of 630-690 degreesC and 26-28 kbar, suggesting a large overstepping (>10 kbar) of reaction boundaries. Eclogitisation was initialised and accompanied by a channelised fluid flow resulting in veins with large, subhedral grains of omphacite, kyanite and garnet. The gabbro-to-eclogite transformation was enhanced by a fluid which allowed the necessary material transport for the dissolution-precipitation mechanism that characterises the metamorphic mineral replacements. The process of eclogitisation was limited by reaction kinetics and dissolution-precipitation rates rather than by the metamorphic P-T conditions. Even though ductile deformation occurred and equilibrium phase boundaries were overstepped, the infiltration of fluids was necessary for triggering the gabbro-to-eclogite transformation.

Evidence for symmetric extension and exhumation of the Yaounde nappe (Pan-African fold belt, Cameroon)

It is commonly assumed that the Yaounde nappe was deformed and emplaced onto the Congo craton during two compressional Pan-African deformation phases ( D 1 and D 2). A detailed field and microstructural study of the mica schists, gneisses and pyriclasites of the Yaounde nappe in the Yaounde area was carried out and it appeared, however, that the D 2 deformation phase more likely corresponds to a major N–S and moderate E–W ductile pure shear extension, rather than compression. This was inferred from the study of various strain indicators, including the geometry of shear zones, the shape of boudins, the shape of pressure shadows, the orientation of deformed veins, of fold axes, the mineral lineations, as well as the preferred orientation pattern of quartz c-axes. The P– T conditions pertaining during this extensional phase were variably recorded by the mica schists (675 °C and 9 kbar), gneisses (800 °C and 12 kbar) and pyriclasites (750 °C and 10 kbar). Pure shear extension turned into minor top-to-the-south simple shear at lower temperature during retrogression into the greenschist facies. We therefore suggest that the syn- D 2 extension associated with subsequent uplift and erosion led to (Pan-African) exhumation of the Yaounde nappe. The D 1 phase of deformation is poorly known due to strong D 2 phase overprinting. However, the high P– T conditions indicated by the metasedimentary rocks in the area suggest that D 1 was compressional. The peak conditions of the related prograde metamorphism correspond to D 1– D 2 transition at the amphibolite–granulite interface. We propose that D 1 and D 2 respectively represent the compressional and extensional phases of a Pan-African tectono-metamorphic cycle within the Yaounde nappe.

CARACTERIZAÇÃO ESTRUTURAL DOS VEIOS DE QUARTZO AURÍFEROS DA REGIÃO DE CUIABÁ (MT)

The gold mineralization in the Cuiaba region occurs in quartz veins found in metasedimentary rocks (phyllites, metasandstones, metadiamictites, marbles and metasiltstones) of the Cuiaba Group, internal zone of the Paraguai Fold Belt. Structural analyses carried out in areas located at the overturned limb (Jardim Italia and Casa de Pedra Mining), normal limb (CPA gold field) and hinge zone (Mineiro and Abdala gold fields) of the Bento Gomes Antiform, confirm the poliphase evolution (four deformation phases) and shows that this fold is a first phase (Dn) related structure with an axial plane slaty cleavage and a stretching/mineral lineation, interpreted as the X direction of the strain ellipsoid. Symmetrical linear features (pressure shadows and elongated clasts) is considered evidence that Dn flow was essencially coaxial. Three different quartz vein types were identified: parallel to S0 (type 1); parallel to Sn (type 2); perpendicular to Sn (type 3). The type 3 veins are also perpendicular to the stretching lineation, and appear to be late Dn tension fractures. They are both the most abundant and the richest, with 2-5 g/ton ore grade. So, kinematics and geometric constraints show that the type 3 veins formed related to Dn tension fractures, associated with metamorphic gold remobilisation during late Dn.

Mineral growth in rocks: kinetic-rheological models of replacement, vein formation, and syntectonic crystallization

We model three cases of coupling between mineral growth kinetics and mechanical response of the rock: (i) dispersed spherical crystals growing by replacement in a hydrostatically stressed elastic rock; (ii) growth of veins or vein networks accommodated by viscous relaxation of surrounding rock; and (iii) syntectonic crystallization in a rock undergoing bulk pure shear. Such models for the microscopic environment of mineral growth, together with additional assumptions or knowledge about rheological behavior and aggregate geometry, provide refined estimates of the behavior of a macroscopic volume element, which could be combined with geochemical reaction-transport models. Crucial in the models are the various consequences—pressure solution, creep, fracturing—of the local stress that is necessarily generated by mineral growth in rocks (other than in pores).In the first model, the dispersed spherical crystals of mineral A are assumed to grow within a spherical volume of rock consisting of mineral B, the “mineralized zone” (MZ), itself embedded in elastic rock. The macroscopic stress in the MZ and the far-field stress in the surrounding rock are uniform and hydrostatic. Mineral growth of the A crystals is driven by supersaturation with respect to mineral A, is accommodated by replacement of B grains, and leads to an expansion of the MZ described by an infinitesimal strain. The radial growth rate of a spherical crystal of mineral A, with replacement of mineral B, is da/dt=kARTln(ΩA)[kBV0B/(kBV0B+kAV0A)], where kA and kB are kinetic constants, R is the gas constant, T the temperature in kelvin, and V0A and V0B are specific volumes. Reference saturation states of mineral A, ΩA > 1, and host mineral B, ΩB = 1, are specified at the far-field hydrostatic stress, σ0. The microscopic environment of each crystal of A is modeled by a representative volume element (RVE) consisting of a sphere of mineral A embedded in a spherical shell of mineral B. In each RVE, stress is neither uniform nor hydrostatic. The model links the local microscopic stress with the macroscopic stress in the MZ and surrounding rock.The second model refers to veins that make room for themselves by growing, not to veins that form by cementation of previously opened, or opening, cracks. If the growth of mineral A as propagating veins or as an interconnected vein network takes place by deformation and expansion of the MZ—with the deformation itself driven by the stress arising from the supersaturation-driven growth—the surrounding rock undergoes deformation and is treated as viscous. The rate of growth in vein width, w, is dw/dt=2kA[RTln(ΩA)−SV0A]/(1+M), where M = 16ηkAV0Ac2/(3b3), η is the medium viscosity, c is the vein radius, and each vein is treated as centered in a spherical RVE of radius b. S is an effective tensile stress required for vein propagation. For a vein network, in which veins surround equant polyhedra of rock of radius b, we set c = b and drop the term in S for simplicity. Veins may also widen by replacement. The ratio of vein widening by expansion to that by replacement is (dw/dt)expansion/(dw/dt)replacement={(3b/16η)/[(3b/16η)+kAV0A]}/[kBV0B/(KBV0B+kAV0A)]. Both mechanisms of accommodation contribute equally when η = 3b/(16kBV0B); at higher viscosity, replacement is dominant.The incipient growth of a cylindrical crystal with circular cross section in a medium deforming in pure shear simulates syntectonic crystallization. Both dissolution and deformation of the host accommodate growth. In the model, the crystal tends to grow faster in the direction of maximum rate of extension. In this direction, the host mineral may either dissolve to accommodate growth, or precipitate to form a pressure shadow. Accommodation by host dissolution is greatest in the direction of maximum rate of shortening.

Microstructures of sediment flow deposits and subglacial sediments: a comparison

Microstructural analysis of glacial deposits has recently been used as a research tool to determine sediment genesis. However, the occurrence of microstructures in deposits of known origin has not been sufficiently documented, hindering our ability to confidently interpret microstructures in sediments of unknown origin. Our objective is to present a calibration study of microstructures of recent sediment flow deposits and associated sediments from the Matanuska Glacier, Alaska, and to evaluate the degree of commonality with micro‐structures found in subglacially deformed sediments. Microstructures in sediment flow deposits can be formed as a result of sediment transport, deposition, and/or post‐depositional processes, and are related to the viscosity regime of the source flow. Characteristic microstructures formed during brittle deformation include shears, faults, and brecciation; microstructures formed during ductile deformation include folds, pressure shadows, reorientation of clasts around a ‘core’ stone, fine laminations, basal shear zones, imbrication, and flow fabrics. Other microstructures include fluid escape and injection structures, clast haloes, and fissility. The results of our comparison suggest that sediment flow deposits share many microstructures in common with subglacially deformed sediments.

Pressure anomalies around cylindrical objects in simple shear

We model the pressure distribution around cylindrical objects in simple shear deformation using the finite element method in two dimensions and present an analytical solution for a special case. Parameter space is explored numerically for viscosity contrast between object and matrix, η, and for the non-linearity of flow, as expressed by n. We show that the geometry and size of the pressure shadow is independent of η, but strongly dependent on n. For example, at n=1, pressure shadows are roughly circular in shape, while for n>1, pressure shadows disintegrate into two branches. We also show that pressure may only exceed 100 MPa in pressure shadows at resolvable distances from the object if η>4 for strain rates of 10 −14 s −1 and matrix viscosities around 10 22 Pa s. As these values for strain rate, viscosity and viscosity contrast are geologically reasonable, we emphasise that it is conceivable that geobarometric results obtained from syndeformational mineral parageneses near rigid porphyroblasts may be influenced by non-lithostatic components of pressure.

Local deformation in compacting flows: development of pressure shadows

Small scale structures produced by deformation and compaction are common in igneous, metamorphic, and sedimentary rocks, and are often used to define foliation and lineation directions. Analytic solutions for models of flow round a rigid stationary sphere are obtained using the equations that describe viscous compaction. The finite deformation associated with these flows shows many of the features observed in deformed rocks, some of which resemble those generated when the ratio of the bulk viscosity ζ to the shear viscosity η is small. Under these conditions the particle paths of the matrix are almost unaffected by the presence of the sphere. It is encouraging that the continuum compaction equations can describe the observed behaviour, even though the length scale on which the deformation occurs is only mm to cm. It is unlikely that the observations can be used to distinguish between D’Arcy flow of the low viscosity phase and mass transport by diffusion.

Sequential kinetics of a muscovite-out reaction; a natural example

A natural example of sequential kinetics for the muscovite dehydration reaction in highly deformed mylonitic gneisses is analyzed. Studied textures consist of deformed pegmatitic muscovite crystals surrounded by fibrolitic sillimanite tightly intergrown with biotite and with potassium feldspar in the pressure shadows. Potassium feldspar, andalusite, biotite, and quartz appearing in the crystal core constitute the products of muscovite breakdown produced by topotactic replacement of muscovite within a single crystal. Non-isochemical decomposition of muscovite is proposed in which diffusion of K+ outward from the crystal took place along the muscovite interlayer and shear planes parallel to (001). The chemical potential gradient calculated for K capable of producing the texture observed within the pegmatitic crystals, results not only from the overstepping of the muscovite-out reaction but also from the difference in free energy between andalusite and sillimanite under P-T conditions where andalusite is stable. The overall reaction within the andalusite stability field consists of the transformation 1 Ms+1 Qtz-->1 And+1 Kfs+1 H 2 O and of the indirect replacement 0.25 Sil-->0.25 And. Andalusite nucleation and growth took place completely inside muscovite, whereas most of the potassium feldspar was produced outside, preferentially at pressure shadows, after out-ward intracrystalline diffusion of K (super +) and fibrolitic sillimanite dissolution in the matrix. The rate-limiting step of this reaction process was initially the migration rate of the muscovite-andalusite and muscovite-potassium feldspar interfaces. However, it was followed by a diffusion-controlled step. The change in the relative rates of the reaction mechanisms was due to the coarsening of the reaction products, the low amount of energy and time required for non-reconstructive transformations (periodic bond chains of the tetrahedral sheet of muscovite are inherited by potassium feldspar and those of the octahedral sheet by andalusite), the magnitude of the heat flow associated with a possible contact metamorphic episode, and the slow diffusion of K+ within the crystal. Intracrystalline diffusion took place through a double monolayer of absorbed molecules of water ( approximately 10 Aa) localized at microcleavages produced by basal slip in the interlayer level, instead of a proper lattice diffusion process. The derived rate law at constant P and T (considering the cases of 2 kbar and 605 degrees C and 3 kbar and 639 degrees C) for the diffusion-controlled process gives a time span of about 10 000 years for the growth of these metamorphic textures, which seems to be a reasonable estimate for a contact metamorphic event.

Syn‐tectonic pluton intrusion during contractional deformation: microstructural and metamorphic evidence from the aureole of the Acadian Victory Pluton, north‐eastern Vermont, USA

Structures within the aureole of the Acadian (Devonian) Victory Pluton suggest that movement along the Monroe Fault occurred during pluton intrusion. Pre‐ or syn‐tectonic garnet textures, including discordant internal and external foliations, sigmoidal inclusion trails and deflection of the matrix foliation around garnet, are found along the Monroe Fault. In sillimanite+K‐feldspar grade rocks, the matrix foliation, defined by biotite and fibrolitic sillimanite, wraps around garnet and sillimanite porphyroblasts. Granite dykes and sills near the pluton contact are folded and boundinaged, and leucosome fills garnet pressure shadows and shear bands. Away from the fault and pluton contact, microstructures indicate much less deformation during metamorphism, suggesting that deformation was partitioned into the fault or the pluton. Deformation ceased before crystallization of the main body of the Victory Pluton was complete. It is possible that magmatism facilitated deformation along the Monroe Fault, and/or that the magma was transported to the mid‐ to upper crust along the Monroe Fault. This study suggests that Acadian deformation in north‐eastern Vermont occurred as late as 390–370 Ma.

The geology of the Taparko gold deposit, Birimian greenstone belt, Burkina Faso, West Africa

The Taparko gold deposit, located in the eastern branch of the Proterozoic Birimian Bouroum-Yalogo greenstone belt (Burkina Faso) consists of a network of quartz veins developed in a N 170° trending shear zone (250 m wide, 4 km long) superimposed on the regional Birimian structural pattern. The quartz vein network is composed of: (a) a dominant array of quartz veins (type 1), parallel to the shear zone and comprising strongly deformed dark quartz exhibiting foliation, layering, ribbon, tension gashes, etc.; (b) oblique and subparallel related veins (type 2) of gray to white weakly deformed quartz crosscutting the dominant quartz veins resulting in breccia structures; and (c) shallow dipping veins (type 3), cross-cutting veins types 1 and 2 and filled by undeformed white buck structure quartz. Cross-cutting relationships and different quartz types in different veins and within individual veins imply a concomitant filling of the veins during the progressive deformation. Initial sinistral transcurrent shearing evolved with time to sinistral reverse shearing. Metallic minerals occur only in type 1 and 2 veins and were deposited in two stages, with native gold being related to second stage sulfides. Gold (and chalcopyrite) precipitated preferentially upon the surfaces of fractured pyrite grains in low-pressure sites (pressure shadow zones) around and/or within the sulfide grains (along subsequently annealed fractures). The formation of the South Taparko deposit can be divided into a succession of events: (a) during the first event, N 170°-directed sinistral transcurrent shearing resulted in a N 20° mylonitic foliation and fractured rock which allowed H2O-, CO2- and SiO2-rich fluids to circulate and deposit quartz with buck texture; (b) during the second event, type 1 quartz was strongly deformed and type 2 veins formed with sigmoidal shapes as viewed on a horizontal plane; and (c) during the third event, the sinistral transcurrent shearing evolved to sinistral reverse shearing and the deformation style evolved correspondingly from ductile to brittle-ductile. During the last phase of deformation gold nucleated and deposited in low-pressure zones.

Graphite pseudomorphs after diamond? A carbon isotope and spectroscopic study of graphite cuboids from the Maksyutov Complex, south Ural Mountains, Russia

Unusual cuboid graphite aggregates (up to 13 mm edge length) from the eclogitic gneiss unit of the Maksyutov Complex deflect a foliation defined by groundmass graphite and phengite, and pressure shadows have developed around these blocky aggregates. Carbon isotope ratios, δ 13C/ 12C, for the cuboid graphite range from about −24 to −42‰, demonstrating that these rocks have retained an original biogenic carbon signature. X-ray diffraction, laser Raman spectroscopy, infrared spectroscopy, and transmission electron microscopy indicate that graphite is well-crystallized with minor defects; no relict organic compounds were detected. Comparisons of these cuboid aggregates with thin sections and scanning electron microscope images of proven graphitized diamonds from the Beni Bousera peridotite massif show that Maksyutov graphite is similar. Laboratory experiments by other workers on graphite demonstrate that this intriguing morphology could not be the result of deformation, because graphite returns to its original shape and size on stress release. Existing experiments on diamond graphitization do not adequately replicate the conditions of natural rocks being exhumed from subduction zones characterized by ultrahigh pressures, and thus cannot be applied with confidence to the Maksyutov Complex. Our spectroscopic and microscopic studies suggest that these cuboid aggregates probably are diamond pseudomorphs.

Ductile fault rocks and metamorphic zonation in the Dalradian of the Highland Border SW of Stonehaven, Kincardineshire

Synopsis High strain zones, characterized by shear fabrics and mylonites, have been identified within lower amphibolite-grade, coarse granular metasandstones of the Glen Lethnot Grit Formation of the Southern Highland Group (Dalradian) along the Highland Border, south-west of Stonehaven. The rocks comprise part of a polydeformed, regionally metamorphosed, Neoproterozoic sequence, bounded to the north by the post-orogenic Mount Battock Granite (Late Caledonian) and to the south-east by the Highland Boundary Fault Zone. Early regional D1 and D2 structures have been rotated by a major monoform, the Highland Border Downbend (D4), to produce a steeply dipping, downward facing succession. The mylonitic metasand-stones contain a variety of kinematic indicators, including asymmetrical pressure shadows, S-C fabrics, shear bands and an extensional crenulation cleavage, which yield a sense of shear of top-towards-the SE (present structural position). Regional syn- to post-D2 metamorphism resulted in the development of prograde pelitic mineral assemblages which overgrow earlier Dl structures. Mylonitic fabrics within the high strain zones, developed during D2, both deform and are also overgrown by syn- to post-kinematic biotite porphyroblasts. The high strain zones may represent part of a major ductile structure active during the Dalradian D2–D3 tectonothermal event, which contributed to the development of strong metamorphic gradients and narrowing of mineral assemblage zones within the Highland Border.

40Ar/39Ar age of shear zones in the southwest Meguma Zone between Yarmouth and Meteghan, Nova Scotia

The Cape St. Marys, Cranberry Point, and Chebogue Point shear zones lie within a zone of D2 deformation that overprinted D1 fabrics and reactivated F1 folds between the shear zones. D2 was accompanied by greenschist-facies metamorphism that variably overprinted earlier metamorphism of varying grade. A collection of single muscovite grains from pressure shadows defining L2 in a sample from the Cape St. Marys shear zone yields an average apparent age of 316 ± 5 Ma. The age obtained for these grains from an isotope correlation diagram is 320 ± 5 Ma. Ages for muscovite separates for two samples from the Cranberry Point shear zone are 325 ± 4 and 319 ± 4 Ma. An amphibolite from this zone yields a discordant spectrum interpreted to reflect the combined effects of Acadian metamorphism and excess argon. The muscovite ages approximate the time of syntectonic deformation because muscovite closure temperatures approximate the temperature of metamorphism during D2. Thus, together with published data from the Chebogue Point shear zone, our data indicate a Mid-Carboniferous age, ca. 320 Ma, for D2 within this system of three shear zones and reactivated folds.

Structural signature of sediment accretion in a Palaeozoic accretionary complex, southeastern Australia

Chaotic block-in-matrix melange, broken formation along high strain zones associated with large scale imbrication, early bedding-parallel cleavage, recumbent folds in turbidites, and structural complexity in cherts are key elements of a mid-Palaeozoic subduction complex in the eastern part of the Lachlan Fold Belt, southeastern Australia. The complex consists of an imbricated turbidite, chert and basalt sequence of mid-Cambrian to Late Ordovician age. Structural and biostratigraphic controls require a complexly imbricated sequence with reversals in younging seawards towards the inferred former trench/subduction zone, as in young subduction accretionary complexes such as the Kodiak accretionary complex in the Aleutians and the Shimanto complex of Japan. Subduction accretion or underplating in the Narooma accretionary complex is not typified by duplexing, but shows a strain-dependent transition from an “inland” belt of chevron-folding cut by reverse faults, to a coastal belt with an early bedding-parallel fabric, isoclinal-recumbent folding, poly-deformation and differentiated layering in quartzrich turbiditic greywackes. Greywacke units show multiple cleavage fabrics formed in one folding event. Cherts below the turbidites show multiple folding events and zones of broken formation with intense veining and stylolitisation. The structurally lowest units, including deformed pillow basalts and block-in-matrix melange, show strong planar-linear fabrics. In the block-in-matrix melange, prolate-shaped pods of greywacke and chert are aligned sub-parallel to the bulk extension direction defined by mica pull-aparts and pressure shadows on pyrite within the mudstone matrix. Mica neocrystallisation and pressure solution are the dominant deformation mechanisms at the base of the complex.

Extensional tectonics within a subduction-type orogen. The case study of the Wugongshan dome (Jiangxi Province, southeastern China)

In southeastern China, the Wugongshan massif consists of metamorphic rocks and granites generally considered as “basement” rocks formed during an early Paleozoic orogeny. Our structural and radiometric reappraisal of this massif shows that it results from a Mesozoic doming. In the metamorphic rocks, the foliation defines an E-W elliptical shape and bears a N-S stretching lineation. The axial zone of the dome is occupied by oriented plutons. The outer part of the dome consists of Devonian quartzite, Carboniferous-Permian limestone and Triassic-Jurassic sandstone. Ductile deformation observed in the Permian carbonates and older units develops contrasting kinematics. The northern and southern flanks present top-to-the-north and top-to-the-south motions, respectively. The same divergent pattern of motion is recorded from brittle structures up to the Jurassic sandstone. The late Cretaceous red beds are undeformed. In the axial zone, coaxial flow indicated by symmetric pressure shadows and quartz fabrics predominates. Along the N and S flanks of the dome, post-folial recumbent folds are symmetrically overturned to the N and S respectively, but at the two extremities of the dome, the fold axes are parallel to the stretching lineation. 40 Ar 39 Ar ages of biotite and muscovite from mylonite and granite suggest a late Triassic age for the ductile deformation and an early Cretaceous age for the final doming. The Wugongshan dome is similar to a metamorphic core complex formed in an extensional tectonic regime. A model which emphasizes plutonism is discussed. From Permian to early Cretaceous, southeastern China was a subduction-type orogen, the crust of which was thickened by accretion of mantle derived magmas. The emplacement of a large amount of Mesozoic granitoids in southeastern China promoted thermal softening and gravitational instability that allowed the crust to extend. The formation of the Wugongshan dome and other metamorphic core complexes appears to be a the direct consequence of plutonic activity and crust rheology.

Hydrothermal alteration at the Mineral Hill Mine, Jardine, Montana; a lower amphibolite facies Archean lode gold deposit of probable synmetamorphic origin

The Mineral Hill Mine is an Archean lode gold deposit in lower amphibolite facies grunerite-bearing banded iron-formation and metaturbidites. Three types of gold ore are mined: type I ore is dominant, developed in wall-rock alteration envelopes marginal to 0.01- to 1-m quartz veins; type II ore is in 1 formed large isoclinal folds and pervasive axial-planar S 1 schistosity. Concurrent M 1 metamorphism crystallized early grunerite, followed by zoned synkinematic garnet porphyroblasts in iron-formation, and garnet, biotite, staurolite, sillimanite,and andalusite in metaclastic rocks. Garnet growth was followed by formation of quartz-filled pressure shadows during M 1 . M 2 -D 2 created upright open F 2 folds, S 2 crenulation cleavage, folding of pressure shadows around garnet, and replacement of biotite and garnet by chlorite. D 2 was followed by D 3 folding and poorly developed S 3 cleavage with no metamorphic mineral growth. M 1 was the single prograde metamorphic event in the region; M 2 and D 3 were retrograde events. Petrogenetic grids and mineral thermometry place peak M 1 metamorphic conditions at 535 degrees to 560 degrees C and 3 kbars. Type I hydrothermal alteration has a characteristic zoning pattern: from quartz veins outward, it consists of the arsenopyrite, pyrrhotite, hornblende, and grunerite zones. The arsenopyrite zone ( 4 ), and visible native gold, with rare galena, sphalerite, and siderite.