Mineral Fabrics Research Articles

Improved isolation of archeomagnetic signals by combined low temperature and alternating field demagnetization

Conventional alternating field (AF) demagnetization of the magnetite-bearing claystone foundations of a Saxon or late medieval lime kiln in Lincolnshire, England fail to isolate stable characteristic remanences, or remanences compatible with possible contemporary geomagnetic field orientations. Consolidation of the material prevented thermal demagnetization. When low temperature demagnetization (LTD) precedes AF demagnetization, however, the vector plots show a stable characteristic (primary) component. Magnetic anisotropy measurements show that the LTD did not significantly disturb the mineral fabric of the claystone, that the mineral fabric did not deflect the palaeofield, and that AF demagnetization did not induce a field-impressed anisotropy during the experiments. Anisotropy of low-field magnetic susceptibility (AMS) is affected by all minerals, and therefore the anisotropy of the magnetite was isolated by measuring anisotropy of anhysteretic remanence (AARM); this is of more relevance in evaluating the potential for palaeofield deflection. Thus, we conclude that LTD preceding AF demagnetization is responsible for improving the isolation of a characteristic remanence, which then favours a late medieval age for the kiln foundation.

Localization of dislocation creep in the lower mantle: implications for the origin of seismic anisotropy

Recent seismological observations reveal the presence of seismic anisotropy in localized regions at the base of the mantle within an otherwise isotropic lower mantle. These regions can be placed in a tectonic context, corresponding to locations of paleosubduction and plume upwelling. This project works toward determining whether the observed seismic anisotropy may be explained by the development of a mineral fabric by lattice-preferred orientation (LPO). Numerical modeling is used to explore whether the conditions at the base of upwelling and downwelling regions are consistent with those required for fabric development. Specifically, we examine whether dislocation creep dominates these regions within a background mantle that flows primarily by diffusion creep. The key to our study is the use of a composite rheology that includes both mechanisms of diffusion and dislocation creep and is based on mineral physics experiments. Results show that it is possible to produce a localization of dislocation creep near slabs within a background mantle dominated by diffusion creep. In contrast, upwelling regions are characterized by a domination of diffusion creep. These results indicate that LPO may be the cause of lowermost mantle seismic anisotropy near paleoslabs, but other mechanisms such as shape-preferred orientation may be required to produce the anisotropy observed near upwellings.

Magnetic fabrics reveal Upper Mantle Flow fabrics in the Troodos Ophiolite Complex, Cyprus

The foliations and lineations recognized in outcrops of the Troodos mantle-sequence provide relatively few and imprecise observations of the mineral fabric. Thus, their kinematic interpretation, and any inferences about the metamorphic flow patterns are tenuous. However, every outcrop yields the anisotropy of magnetic susceptibility (AMS) from which we may infer the combined orientation–distribution response of silicates (pyroxene, serpentine) and magnetite. Furthermore, in many outcrops, the anisotropy of anhysteretic remanence (AARM) isolates the contribution of magnetite to the orientation–distribution. AMS and AARM each provide a magnitude ellipsoid whose principal directions define the orientation–distribution of several hundred minerals in each sample, and thus their flow fabric.Within the Troodos mantle-sequence, high-temperature, semi-cataclastic silicate flow directions were uniform through volumes of ∼1km3 but differ between adjacent subareas. This reflects the scale of the heterogeneity of solid-state flow. However, averaged over the entire area of ∼100km2, the global orientation–distribution of silicate alignments were consistent, indicating early, silicate flow up to the west, away from a spreading axis that lay east of the Troodos range. On the same regional scale, later magnetite alignments indicate flow up to the north-west. The non-horizontal and non-coaxial flow stages suggest that the mantle sequence was affected by diapiric rise in the solid-state, radiating from centres which may originally have been magma chambers. The one responsible for the recorded flow patterns may have been approximately 30km in diameter and located SE of Mount Troodos.

Magmatic fabrics in batholiths as markers of regional strains and plate kinematics: example of the Cretaceous Mt. Stuart batholith

Abstract The Cretaceous Mt. Stuart batholith was syntectonically emplaced within amphibolite grade metasedimentary rocks of the Cascades Crystalline Core, Washington State. The northern part of the batholith defines a NW—SE trending antiformal fold. We present fabric data from that part of the batholith, collected during field mapping and anisotropy of magnetic susceptibility (AMS) measurements. The significance of the data is discussed in terms of regional tectonic deformation and plate kinematics. The data were collected from rocks with well preserved igneous textures and the fabrics therefore formed during magmatic deformation. The AMS provides measurements of the preferred orientations of Fe-rich minerals (biotite ± hornblende ± traces of pyrrhotite and magnetite) which are consistent with field measurements of the mesoscopic fabrics defined by plagioclase, biotite and hornblende crystals. The magnetic fabrics are also consistent with the orientations of folds, mineral fabrics and boudinage structures that record high-temperature subsolidus deformation in the margin of the pluton and in its host rocks. The lineations are parallel to the stretching direction associated with small increments of strain that occurred during deformation of the magmatic arc, as the batholith was crystallizing and deforming in the tectonic stress field, ca. 93 Ma. The fabrics in the Mt. Stuart batholith are used to infer emplacement in a magmatic arc during either 1) plate displacement perpendicular to a NW-SE trending plate margin, or 2) wrench dominated transpression. In the second case the analysis suggests a nearly N-S plate vector along the western North American margin during plutonism. The results demonstrate the potential usefulness of magmatic fabrics in syntectonic plutons for plate tectonic analyses of orogenic belts.

Age and tectonic evolution of Neoproterozoic ductile shear zones in southwestern Madagascar, with implications for Gondwana studies

Southern Madagascar comprises a complex Precambrian terrain of high‐grade metamorphic rocks with a history of polyphase deformation and metamorphism. Two prominent N‐S trending late Neoproterozoic ductile shear zones, the Ampanihy and Vorokafotra shears, each with projected strike length of > 450 km and between 10 and 20 km in width, crosscut the region. A third set of en echelon shears forms part of the early Paleozoic Ranotsara Shear Zone that cuts the basement in a NW‐SE direction over a combined strike length of > 400 km. The host rocks of these shears comprise paragneisses (metasediments) with detrital zircons ranging in age between 720 and 1900 Ma. A felsic layer, interpreted as a metavolcanic rock, gives a date of 722±1 Ma. Remnants of late Archean orthogneisses in the central part of the study area may represent basement to the paragneisses. Four episodes of deformation and metamorphism have been recognized on the combined basis of field observations, petrogenesis, and U/Pb analyzes of zircons, monazites, sphenes, and rutiles. Two episodes of early simple shear deformation (D1 and D2) at midcrustal levels occurred between 627 and 647 Ma, during which northeast verging recumbent sheath folds and ductile thrusts were formed and peak prograde metamorphism reached 7–12 kbar at 750°–900°C. Early prolate mineral fabrics (L1/L2) are preserved in massif‐type anorthosite bodies and their marginal country rocks. D1 occurred between 630 and 647 Ma, while D2 occurred at 627‐628 Ma. This was followed by a 10–15 Myr period of static, annealing metamorphism until 609–614 Ma when bulk shortening (D3) took place. D2 and D3 are coaxial but are separated in time by leucocratic dykes that intruded between 610 and 620 Ma. D3 was focused zonally, forming the prominent N‐S shear zones between 607 and 609 Ma; its oblate strain resulted in a strong composite D2/D3 fabric defined by subvertical S‐tectonites and subhorizontal intersection lineations. A variety of post‐D3 pegmatites accompanied ∼85 Myr of relatively static annealing and metasomatic/metamorphic mineral growth, during which numerous occurrences of phlogopite, uranium, and rare earth elements formed. A continuum of concordant monazite dates suggests that this thermal event is part of an extended period of low‐pressure (3–5 kbar) charnockite‐producing processes between 520 and 605 Ma. The continuum, however, appears to be punctuated at ∼580, 550, and 520 Ma. Deformation (D4) recorded within the Ranotsara Shear Zone overlaps with the youngest parts of the regional metamorphic conditions between 520 and 550 Ma. Prevailing low‐pressure, high‐temperature amphibolite‐granulite facies rapidly gave way to greenschist facies conditions between 490 and 530 Ma, as is evident from overlapping ages of zircon, monazite, sphene, and rutile. We conclude that D1 to D3 represents a period of 40 Myr of compressional deformation that we interpret to be related to collisional events during the amalgamation of Gondwana. The first part of the thermal continuum between 550 and 605 Ma reflects ∼55 Myr of slow cooling and annealing at midcrustal levels, while the onset of the last episode, between 520 and 530 Ma, heralds accelerated exhumation accompanied by extensional tectonics between 490 and 520 Ma. We believe that this postcollisional time span represents a prolonged period of evolution of a Tibetan‐style plateau into an Aegean‐style extensional terrain. This ∼100 Myr event in southern Madagascar is similar to that recorded throughout large sectors of the East African Orogen between ca. 500 and 600 Ma. We believe that this type of postconvergent thermotectonism best represents the original definition of “Pan‐African” [Kennedy, 1964], which in today's terminology equates with “postorogenic extensional collapse” [Dewey, 1988], or “destabilization of an orogen” [Lipps, 1998]. Kennedy's Pan‐African was widespread throughout the interior a supercontinent, when Gondwana's periferal margins were subjected to far‐field tensional forces. This suggests that neither gravitational collapse of the Pan‐African‐Braziliano Orogens nor delamination were the sole or even the dominant driving forces for the postconvergent extension.

On the dynamics of magma mixing by reintrusion: implications for pluton assembly processes

Many plutons have formed by repeated intrusion, with complex internal contacts characterized by sharp and diffuse kinematic and compositional domains. We performed numerical experiments of mixing following magma chamber recharge that explicitly considers the fluid dynamics of multiphase mixtures. In the limit of zero diffusivity of intensive scalar quantities and low Stokes number, mingling by fluid instability is equivalent to deformation, and persistent fluid structures are kinematic ‘attractors’. Three distinct regimes are exemplified, and can be described by their multiphase Reynolds (or Grashof) number. For a Reynolds number greater than about 100, an internal intrusive contact will collapse by internal wave-breaking, and chaotic magma mingling and mixing yield a nearly chamber-wide stratification. This flow may scour mushy regions at the walls and widely distribute previously crystallized material. For Reynolds numbers from 10 to 100, the internal wave does not break, and the stratification occupies less of the chamber, leaving islands of unmixed material. For Reynolds numbers around 1, internal slumping and folding can occur, and is the most likely to be preserved by a mineral fabric. Internal, sub-vertical contacts require a high absolute viscosity in the resident magma, representing a time-break between episodes of reintrusion. The shape of perched mafic domains captures the progress of sinking from higher levels into regions of increasing strength and crystallinity.

Microscopic Physical Biomarkers in Carbonate Hot Springs: Implications in the Search for Life on Mars

Physical evidence of life (physical biomarkers) from the deposits of carbonate hot springs were documented at the scale of microorganisms—submillimeter to submicrometer. The four moderate-temperature (57 to 72°C), neutral pH springs reported on in this study, support diverse communities of bacteria adapted to specific physical and chemical conditions. Some of the microbes coexist with travertine deposits in endolithic communities. In other cases, the microbes are rapidly coated and destroyed by precipitates but leave distinctive mineral fabrics. Some microbes adapted to carbonate hot springs produce an extracellular polymeric substance which forms a three-dimensional matrix with living cells and cell remains, known as a biofilm. Silicon and iron oxides often coat the biofilm, leading to long-term preservation. Submicrometer mineralized spheres composed of calcium fluoride or silica are common in carbonate hot spring deposits. Sphere formation is biologically mediated, but the spheres themselves are apparently not fossils or microbes. Additionally, some microbes selectively weather mineral surfaces in distinctive patterns. Hot spring deposits have been cited as prime locations for exobiological exploration of Mars. The presence of preserved microscopic physical biomarkers at all four sites supports a strategy of searching for evidence of life in hot spring deposits on Mars.

Characterization of crack distribution: fabric analysis versus ultrasonic inversion

SUMMARY We analyse the relation between rock fabric, expressed by the preferred orientation of rock-forming minerals and microcracks, and elastic anisotropy of crystalline rock from the KTB pilot well. Detailed analyses of mineralogical composition, textures and microcrack fabrics were performed. In addition, ultrasonic velocity measurements of spherical samples in several directions were carried out at various confining pressures, and inverted in terms of the complete set of 21 elastic constants. By comparing the elastic tensors of the rocks at the final confining pressure (at which most of the microcracks are closed) with those at a lower pressure level, it is possible to separate the anisotropy induced by microcracks from that caused by mineral alignment. In contrast to previous work, no a priori knowledge of the type of anisotropy (triclinic, monoclinic, orthotropic etc.), or of the spatial orientation of the symmetry elements (planes, axes) of the cracked rock or of the intact rock is assumed. Furthermore, no restrictive assumptions on the orientation distribution function and the shape of the cracks are needed. The results show that the elastic anisotropy characteristics, whether they are related to the microcracks or to the rock-forming minerals, are clearly correlated with the directly observed rock fabrics. We show that the symmetry directions of the mineral fabric and of microcrack fabric agree. A further result is that the microcrack-induced anisotropy dominates the other causes of anisotropy at confining pressures smaller than a few tens of megapascals, the situation being reversed at higher pressures. The laboratory data are quantitatively compared with sonic log data from the KTB well, showing the influence of pore fluids, eVective pressure and crack density reduction on the anisotropy in situ.

P-wave anisotropy in eclogites and relationship to the omphacite crystallographic fabric

Measurements of P-wave velocity at room temperature and confining pressures up to 500 MPa were carried out on three eclogite mylonites collected from a shear zone in the Monviso area (Western Alps). P-wave velocities at a pressure of 400 MPa range from 7.7 km/s to 7.9 km/s, yielding to a maximum anisotropy of 6%. From the CPO of omphacite we estimated a maximum contribution of omphacite to the P-wave anisotropy of only 1.3%. These results suggest that primarily the compositional layering and secondary the fabric of minor constituent minerals significantly contribute to the seismic anisotropy. Because of the anisotropy, the seismic reflectivity of subduction zones may vary with the direction of observation.

Structures on interfaces of mingled magmas, Stewart Island, New Zealand

Strain analysis of magmatic rocks has been attempted using the shapes of enclaves of contrasting magmas and analysis of mineral fabrics. In composite plutonic rocks of the Anglem Complex, Stewart Island, New Zealand, interfaces between different magma types are deformed into fold-like patterns which have the potential to provide new data on strain in magmatic processes. The fold-forms have shorter wavelength and higher amplitude on interfaces at a high angle to layering whereas fold-forms are either absent or of low amplitude on interfaces parallel to layering. The fold-forms may be amplified/de-amplified irregularities or buckle folds, and in either case they indicate shortening perpendicular to rock layering. Whole-rock fabrics of the plagioclase–hornblende–biotite rocks, analysed by shape preferred orientation and centre-to-centre methods, record less intense alignment and strain than predicted by free rotation of particles in viscous media. The inhibition of crystal rotation at high crystal concentrations and the growth of late poorly aligned crystals present difficulties for calibrating whole-rock fabrics with the strain incurred by magmatic flow.

In situ imaging of microorganisms in geologic material

In order to fully delineate the interactions of microorganisms with geological substrates, unequivocal identification of intact microbial cells within geologic samples is required without the disruption of either the rock texture or the relationship of the microorganisms to the mineral fabric. To achieve this objective we developed a protocol that enables the visualization of intact microbial cells in petrographic thin sections, avoids detaching the cells from their host mineral surfaces and avoids microbial contamination during the lapidary process. Propidium iodide and POPO-3, nucleic acid stains that specifically target double-stranded DNA and RNA were utilized for in situ visualization of cells in surface and subsurface basalts from northeastern Idaho. Additionally, examination of samples incubated with acetic acid–UL– 14C via phosphor imagining facilitated the in situ visualization of 14C labeled biomass. Biomass observed was low (<10 7 cells/g). These observations indicate that the microbial distribution in these rocks exhibits a high degree of spatial heterogeneity at the sub-centimeter scale.

Tectonic pattern of South America inferred from tidal gravity anomalies

The knowledge of several physical parameters that characterize a tectonic unit is important to evaluate its dynamic evolution and, therefore, its role in the continental evolution. Several methodologies have been used to measure the elastic properties of tectonic units using: seismic waves, the level of coherence between the wavelength pattern of the topography and gravity fields, borehole deformation rate, mineral fabric, etc. In this study, the Earth's dynamic elastic response to gravity tides is applied for measuring the lithosphere effective elastic thickness. This methodology proved to be a good alternative to the conventional coherence method. It was applied over the continent, taking into account the poor gravity coverage in South America and the restricted area of some tectonic units. Results are presented as lateral distribution of the effective elastic thickness in South America.

Palaeomagnetism, rock magnetism and magnetic fabrics in Precambrian metamorphic terranes of the Huabei Shield, China

This study has investigated magnetic remanence, rock magnetism and anisotropy of magnetic susceptibility (AMS) in granulite and amphibolite grade metamorphic terranes of the Huabei Shield between Inner Mongolia in the west and the Bohai Sea in the east. Rock magnetic studies identify annealed metamorphic magnetite grains with multidomain properties as the remanence carriers; a widely recorded stable remanence was probably fixed by grain shape effects. Granulite facies terranes are typically between one and two orders more strongly magnetised than amphibolite terranes and AMS fabrics correlate mostly with metamorphic mineral fabrics observed in the country rocks. Progressive thermal demagnetisation identifies a range of two and three component structures resident in magnetite. An important component recognised as a partial or complete remagnetisation by Late Mesozoic–Tertiary tectonic/magmatic activity is present in basement at the southern margin of the outcrop (Miyun terrane) and where extensive granite plutonism has occurred (Zhunhua terrane). These components have directions corresponding to remanence in the Yunmeng Shan Granite (119–114 Ma, D/I=33/58°, 39 samples, a 95=3.5°, palaeopole at 201°E, 64°N). Most remanence elsewhere was probably acquired during post-tectonic uplift and cooling of the basement between ∼2200 and 1850 Ma because palaeomagnetic directions are removed from the Phanerozoic palaeofield path and they are distinct from the palaeomagnetic record in the overlying Jixian Supergroup deposited at ∼1840–900 Ma. These latter magnetisations are considered reliable indicators of the palaeofield during Late Palaeoproterozoic times because deformation of overlying supracrustal rocks is mostly slight and no prominent deflection of magnetic remanence by magnetic fabrics is observed. Palaeofield directions and poles attributed to the time of uplift-related cooling are: Qian’an Terrane (D/I=215/71°, a 95=9°, 17 samples, pole at 99°E, 10°N) and North Qianxi Terrane (D/I=44/−45°, a 95=4°, 41 samples, pole at 79°E, 11°S). In addition, a more widely-preserved shallow northerly component correlates with a NW→E swathe of components recorded by uplift-related cooling within the Datong–Huan’an granulite terrane in the west of the shield. A preliminary Palaeo-Mesoproterozoic apparent polar wander path for the Huabei Shield is defined from the Palaeoproterozoic record in the metamorphic basement rocks and the Meso-Neoproterozoic record in the overlying Jixian Supergroup. It incorporates a loop between ∼2200 and 1850 Ma and exhibits a general east to west trend in subsequent times.

Graphic derivation of the local sense of shear strain components in stretched walls of lithotectonic boundaries

Lithotectonic boundaries (LTBs) are deformed interfaces between rock masses such as sheared strata, midcrustal thrust slices or sutured microcontinents. Structural analysis of strained LTB walls have focussed attention on rock cuts through the foliation normal and mineral lineation direction, but this practice is commonly inappropriate for investigating the tangential shear strain ( γ). A simple graphic technique is introduced whereby one may derive the sense of large shear strain components in two sections across an LTB wall segment; one section parallel to the lineation direction, the other normal to the foliation. The orientation of the strain ellipse is used on each section to deduce the sense of its shear strain component by visual inspection. The technique is applicable to metamorphic rocks with L– S mineral fabrics, but without reliable gauges of large incremental or total longitudinal strain. If the factor of prolateness/oblateness of the strain ellipsoid is unknown, then the γ direction cannot be fixed within an angle of <90° on the LTB surface (cf. accompanying paper by R. J. Lisle, Journal of Structural Geology, 20, 969–973). However, the γ direction may be accurately determined if an LTB cross section fortuitously contains the normal to foliation as well as the mineral lineation direction. Similarly, the γ direction can be determined if the local strain ellipsoid corresponds to an oblate or prolate spheroid. Where the ellipsoid is nearly spheroidal, one can therefore discern which of the two shear strain components under consideration has the larger magnitude. This proves advantageous in assessing the sense of Alleghanian tangential shear below the Brevard fault zone (Grandfather Mountain area), southern Appalachians. Here γ has the sense of a ductile thrust, >1 km into the stretched footwall. Close to the fault zone, however, γ has a large sinistral component, unless the LTB dip is >40°.

Magmatic fabric acquisition mechanisms in a syenite: Results of a combined anisotropy of magnetic susceptibility and image analysis study

The structural significance of the anisotropy of magnetic susceptibility (AMS) and acquisition mechanisms of silicate and ferrimagnetic mineral fabrics are investigated in the Archean Lebel syenite intrusion, Ontario, Canada. Bulk mineral subfabrics are determined by intercept counting of mineral phases in classified digital images of thin sections cut parallel to the principal planes of the magnetic fabric ellipsoids of the samples. Intensities, shapes and principal direction orientations of the resulting intercept ellipsoids are generally similar to the corresponding AMS ellipsoids. Differences are due mainly to bimodal distribution of mineral fabrics observed on foliation planes of some samples showing pre‐full crystallization textures, and structural overprinting in samples deformed after crystallization. Subfabrics of clinopyroxene and magnetite in samples showing pre‐full crystallization microstructure are investigated by measuring inertia tensors of individual grain shapes. The resulting inertia tensor ellipsoids indicate that magnetic fabrics in these rocks are strongly correlated to the shape preferred orientation of magnetite, which is similar to the overall petrofabric of the samples. Crystal size distributions of clinopyroxene and magnetite indicate that they formed during in situ crystallization and that magnetite probably appeared late in the crystallization and flow history of the magma. Analysis of preferred orientations of these grains suggests that the petrofabrics of the samples were acquired during flow with strongly interacting grains, analogous to a logjam in a river. The final characteristics of the AMS and silicate petrofabric are the end points of a continuous crystallization and flow history in the magma, rather than transient late features.

The petrophysical behaviour of crustal rocks under the influence of fluids

Fluid phases of different composition have been found to be present ubiquitously in most crustal rocks down to deep levels in the lower crust. They are stored either in isolated fluid inclusions within the mineral grains or in open and frequently interconnected cracks and fissures. Although the porosity of typical basement rocks usually is very small, the presence or absence of fluids is found to be of paramount importance for rock strength and stability.Experiments of the ‘triaxial deformation cell’ type were performed with two typical basement rocks, a monzogranite and a two-mica-granite at a confining pressure of ≤55 MPa, and deformation rates ≥10−7 s−1 at temperatures of 80°, 250°, and 300°C, respectively. The rock samples were pressurized by fluid phases independent of the confining pressure; fluids applied were pure H2O, Ar, CO2.Compared to drained rock samples (i.e. samples in which internal fluids could leave the rock specimen and the sample chamber during the experiment and no fluid pressure could build up), the presence of fluids generally caused a strong reduction in rock strength. Internal fluid pressures up to 65 MPa independent of the fluid contents were controlled externally.The mode of failure also changed sigificantly from a simple shear fracture in the fluid-absent case to a subaxial multiple splitting mode when fluids were present. The deformation style prior to brittle failure in the wet experiments shows a distinct phase of ductile yielding preceding it. Intergranular creep, i.e. ductile mineral deformation sensu stricto, has not been observed at this stage, but change in the mineral fabric has been. The interpretation of the brittle-to-ductile transition of rocks in the earth's crust has to take into account these influences exerted by fluid phases.

Age constraints on the regional deformation within the eastern segment, S. Sweden: Late sveconorwegian granite dyke intrusion and metamorphic‐deformational relations

A post‐deformational granitic dyke and the metamorphic‐deformational relations of the rocks in the Gällared area place important constraints on the timing of regional deformation and metamorphism within the southern, internal part of the Eastern Segment of the SW Swedish Gneiss Province. The petrologic data suggest a Sveconorwegian clockwise P‐T evolution for the rocks in the Ullared‐Gällared area, characteristic for tectonically thickened continental crust. Metamorphic‐deformational relations show that the deformation of the rocks in the Ullared‐Gällared area took place under Sveconorwegian high‐ to medium‐grade metamorphic conditions. The intrusion of the Gällared granite dyke sets a clear lower limit for the deformation and the high‐grade metamorphism. Five Pb‐evaporation datings of single idiomorphic zircons from the dyke have provided a very well defined plateau age of 956±7 Ma. The granite dyke is unmetamorphosed except for very low‐grade local alterations, and truncates a high‐grade metamorphic, strongly planar mineral fabric in the host amphibolite. The Gällared granite dyke shows that granitic melts were present during late stages of the Sveconorwegian orogeny. The P‐T evolution of the rocks in the area implies that high geothermal gradients prevailed during exhumation; the late granitic and pegmatitic dykes can therefore have been generated by partial melting of underlying crustal levels. The P‐T‐t history and the metamorphic‐deformational relations evidence a pervasive Sveconorwegian tectonometamorphic evolution. Möller, C. & Söderlund, U., 1997: Age constraints on the regional deformation within the Eastern Segment, S. Sweden: Late Sveconorwegian granite dyke intrusion and metamorphic‐deformational relations. GFF, Vol. 119 (Pt. 1, March), pp. 1–12. Stockholm. ISSN 1103–5897.

The Renison Granite, northwestern Tasmania; a petrological, geochemical and fluid inclusion study of hydrothermal alteration

The Renison granite in northwestern Tasmania is genetically related to cassiterite sulfide mineralization (carbonate replacement) at Renison Bell, one of the largest Sn deposits in the world. The magmatic hydrothermal system responsible for the mineralization in the roof sediments is also responsible for the development of three zones of extensive hydrothermal alteration in the granite. The tourmaline zone (tourmaline + quartz + topaz + fluorite + sericite + cassiterite) is in part surrounded by a sericite zone (sericite + quartz + tourmaline + calcite + cassiterite). An albite zone is characterized by albite + quartz + chlorite + sericite along with primary plagioclase and K feldspar. The Renison granite is a reduced felsic granite with biotite as the only mafic mineral. It is characterized by high contents of SiO 2 (70.9-75.3%) and K 2 O (4.69-5.91%), but low Na 2 O (2.52-3.12%). High amounts of Rb (up to 918 ppm) and Sr (as low as 19 ppm), as well as other trace element abundances in the most felsic rocks, indicate that parts of the magma were highly fractionated, and as a result, enriched in volatile constituents, mostly aqueous. During hydrothermal alteration, B, Fe, and F were introduced into the tourmaline zone, whereas Na and K were removed. In the sericite and albite zones, chemical and petrographic effects of alteration are less conspicuous and are marked by changes in the concentrations of Na and Ca and small changes in K. Norms of unaltered granites project as a tight cluster on the haplogranite plane (quartz-albite-orthoclase) whereas altered rocks are widely scattered. Normal trace element concentrations defining-fractionation trends of igneous origin have surveyed the effects of alteration in albite and sericite zone rocks as well as in weakly to moderately altered, tourmaline zone rocks. However, depletion in Rb, St, and Ba occurs in strongly tourmalinized rocks in which the primary mineral fabric is completely replaced by secondary (hydrothermal) mineral fabric...

Décollement processes at the Nankai accretionary margin, southeast Japan: Propagation, deformation, and dewatering

The décollement zone, expressed on seismic profiles and observed in drill cores from the Nankai accretionary margin off the southeast coast of Japan, reveals several unique characteristics which appear to distinguish it from thrust faults identified in the same setting. Physical manifestations of these include evidence for the asymmetric distribution of deformation structures about the décollement, the extension of this fault zone well in front of the tectonic deformation front, and the absence of features indicative of precursory shear, for example, folded sediments, shear bands, and penetrative mineral fabrics. These characteristics suggest that the mode of formation and evolution of this décollement zone may be unique from that of most thrust faults. We propose that the décollement zone propagates not as a shear fracture controlled by tectonic stress conditions but rather as a subhorizontal tension fracture propagating under high pore pressures. To test this possibility, physical property measurements and clay mineral fabrics were obtained for several samples from the Nankai décollement zone using computed tomography and X ray texture methods. Our findings suggest that deformation within the décollement zone is partitioned into a volumetric component, preserved as reduced porosities within coherent fragments, and a localized shear component, evidenced by mineral preferred orientations along discrete slip surfaces. We suggest that the reduced porosities result from the destruction of “cementation” in the sediments during the early stages of deformation and may arise from cyclic fatiguing of the sediment induced by fluctuating pore pressures. The nonpenetrative shear fabrics probably develop as the tectonic deformation front migrates seaward, and the weakened protodécollement subsequently accommodates shear displacements along discrete fractures.

Triassic-Jurassic silica-undersaturated and silica-saturated alkalic intrusions in the Cordillera of British Columbia: Implications for arc magmatism

Alkalic igneous rocks of early Mesozoic age are found in both the Quesnel and Stikine terranes in the Canadian Cordillera and include both silica-undersaturated and silica-saturated types. The saturated complexes are most abundant in Quesnellia and are multiphase complexes dominated by monzonite to diorite intrusions. Undersaturated complexes are distributed through both terranes, are dominated by syenite with lesser monzonite and pyroxenite, and, when present as a single intrusion, are characterized by concentric zoning, igneous layering, and planar mineral fabrics. Both types of complex are associated with Cu-Au mineralization accompanied by potassic and distinctive sodic and calc-potassic alteration assemblages. Although undersaturated and saturated alkalic intrusions are petrographically distinct, a petrogenetic association is suggested by their spatial coincidence in some districts, and similarities in their tectonic environment and associated alteration. The undersaturated complexes represent a distinctive suite of alkalic intrusion with magmatic arc affinities, and their emplacement into both Stikinia and Quesnellia between 210 and 200 Ma suggests that these terranes were either linked at that time or have shared unusual but similar magma-generating tectonic events at identical times.