Deformation processes and rheology of pyroxenites under lithospheric mantle conditions

We combined microstructural observations and high-resolution crystallographic preferred orientation (CPO) mapping to unravel the active deformation mechanisms in garnet clinopyroxenites, garnet–spinel websterites, and spinel websterites from the Beni Bousera peridotite massif. All pyroxenites display microstructures recording plastic deformation by dislocation creep. Pyroxene CPOs are consistent with dominant slip on [001]{110} in clinopyroxene and on [001](100) or [001](010) in orthopyroxene. Garnet clinopyroxenites have however high recrystallized fractions and finer grain sizes than spinel websterites. Recrystallization mechanisms also differ: subgrain rotation dominates in garnet clinopyroxenites, whereas in spinel websterites nucleation and growth also contribute. Elongated shapes and strong intracrystalline misorientations suggest plastic deformation of garnet, but CPOs are weak. Clinopyroxene porphyroclasts in spinel websterites show deformation twins underlined by orthopyroxene exsolutions. Thermodynamic calculations indicate that garnet clinopyroxenites deformed at 2.0 GPa and 950–1000 °C and spinel pyroxenites at 1.8 GPa and 1100–1150 °C. The lower temperatures may explain the faster work rates implied by the finer grained microstructures in garnet clinopyroxenites. Greater stresses may have also reduced the competence contrast between garnet and pyroxene in the garnet pyroxenites and, at the outcrop scale, lowered the competence contrast between pyroxenites and peridotites, favoring mechanical dispersion of pyroxenites in the cooler lithospheric mantle.     

Weakening and strain localization produced by syn-deformational reaction of plagioclase

There are many observations in naturally deformed rocks on the effects of mineral reactions on deformation, but few experimental data. In order to study the effects of chemical disequilibrium on deformation we have investigated the hydration reaction plagioclase + H₂OM more albitic plagioclase + zoisite + kyanite + quartz. We utilized fine-grained (2-6 µm) plagioclase aggregates of two compositions (An54 and An60), both dried and with 0.1-0.4 wt% H₂O present, in shear deformation experiments at two sets of conditions: 900 °C, 1.0 GPa (in the plagioclase stability field) and 750 °C, 1.5 GPa (in the zoisite stability field). Dry samples and those deformed in the plagioclase stability field underwent homogeneous shearing by dislocation creep, but samples with 0.1 to 0.4 wt% water deformed in the zoisite stability field showed extreme strain localization into very narrow (~1-3 µm) shear bands after low shear strain. In these samples the microstructures of reaction products in the matrix differ from those in the shear bands. In the matrix, large (up to 400 µm) zoisite crystals grew in the direction of finite extension, and relict plagioclase grains are surrounded by rims of recrystallized grains that are more albitic. In the shear bands, the reaction products albitic plagioclase, zoisite, white mica, and traces of kyanite form polyphase aggregates of very fine-grained (<0.1 µm) dislocation-free grains. Most of the sample strain after % ~2 has occurred within the shear bands, within which the dominant deformation mechanism is inferred to be diffusion-accommodated grain boundary sliding (DAGBS). The switch from dislocation creep in dry samples deformed without reaction to DAGBS in reacted samples is associated with a decrease in flow stress from ~800 to <200 MPa. These experiments demonstrate that heterogeneous nucleation driven in part by chemical disequilibrium can produce an extremely fine-grained polyphase assemblage, leading to a switch in deformation mechanism and significant weakening. Thus, localization of deformation in polyphase rocks may occur on any pressure (P),temperature (T)-path where the equilibrium composition of the constituent minerals changes.     

Microfabric of folded quartz veins in metagreywackes: dislocation creep and subgrain rotation at high stress

The microfabrics of folded quartz veins in fine‐grained high pressure–low temperature metamorphic greywackes of the Franciscan Subduction Complex at Pacheco Pass, California, were investigated by optical microscopy, scanning electron microscopy including electron backscatter diffraction, and transmission electron microscopy. The foliated host metagreywacke is deformed by dissolution–precipitation creep, as indicated by the shape preferred orientation of mica and clastic quartz without any signs of crystal‐plastic deformation. The absence of crystal‐plastic deformation of clastic quartz suggests that the flow stress in the host metagreywacke remained below a few tens of MPa at temperatures of 250–300 °C. In contrast, the microfabric of the folded quartz veins indicates deformation by dislocation creep accompanied by subgrain rotation recrystallization. For the small recrystallized grain size of ～8 ± 6 μm, paleopiezometers indicate differential stresses of a few hundred MPa. The stress concentration in the single phase quartz vein is interpreted to be due to its higher effective viscosity compared to the fine‐grained host metagreywacke deforming by dissolution–precipitation creep. The fold shape suggests a viscosity contrast of one to two orders of magnitude. Deformation by dissolution–precipitation creep is expected to be a continuous process. The same must hold for folding of the vein and deformation of the vein quartz by dislocation creep. The microfabric suggests dynamic recrystallization predominantly by subgrain rotation and only minor strain‐induced grain boundary migration, which requires low contrasts in dislocation density across high‐angle grain boundaries to be maintained during climb‐controlled creep at high differential stress. The record of quartz in these continuously deformed veins is characteristic and different from the record in metamorphic rocks exhumed in seismically active regions, where high‐stress deformation at similar temperatures is episodic and related to the seismic cycle.     

On the petrology of brittle precursors of shear zones – An expression of concomitant brittle deformation and fluid–rock interactions in the ‘ductile’ continental crust?

The inherited localization model for shear zone development suggests that ductile deformation in the middle and lower continental crust is localized on mechanical anisotropies, like fractures, referred to as shear zone brittle precursors. In the Neves area (Western Tauern Window, Eastern Alps), although the structural control of these brittle precursors on ductile strain localization is well established, the relative timing of the brittle deformation and associated localized fluid flow with respect to ductile deformation remains in most cases a matter of debate. The present petrological study, carried out on a brittle precursor of a shear zone affecting the Neves metagranodiorite, aims to determine whether brittle and ductile deformations are concomitant and therefore relate to the same tectonic event. The brittle precursor consists of a 100–500 µm wide recrystallized zone with a host mineral‐controlled stable mineral assemblage composed of plagioclase–garnet–quartz–biotite–zoisite±white mica±pyrite. Plagioclase and garnet preserve an internal compositional zoning interpreted as the fingerprint of Alpine metamorphism and fluid–rock interactions concomitant with the brittle deformation. Phase equilibrium modelling of this garnet‐bearing brittle precursor shows that metamorphic garnet and plagioclase both nucleated at 0.6 ± 0.05 GPa, 500 ± 20°C and then grew along a prograde path to 0.75 ± 0.05 GPa, 530 ± 20°C. These amphibolite facies conditions are similar to those inferred from ductile shear zones from the same area, suggesting that both brittle and ductile deformation were active in the ductile realm above 500°C for a depth range between 17 and 21 km. We speculate that the Neves area fulfils most of the required conditions to have hosted slow earthquakes during Alpine continental collision, that is, coupled frictional and viscous deformation under high‐fluid pressure conditions ~450°C. Further investigation of this potential geological record is required to demonstrate that slow earthquakes may not be restricted to subduction zones but are also very likely to occur in modern continental collision settings.     

Late‐stage orogenic loading revealed by contact metamorphism in the northern Appalachians,New York

Pelitic schists from contact aureoles surrounding mafic–ultramafic plutons in Westchester County, NY record a high‐P (~0.8 GPa) high‐T (~790 °C) contact overprint on a Taconic regional metamorphic assemblage (~0.5 GPa). The contact metamorphic assemblage of a pelitic sample in the innermost aureole of the Croton Falls pluton, a small (<10 km²) gabbroic body, consists of quartz–plagioclase–biotite–garnet–sillimanite–ilmenite–graphite–Zn‐rich Al‐spinel. Both K‐feldspar and muscovite are absent, and abundant biotite, plagioclase, sillimanite, quartz and ilmenite inclusions are found within subhedral garnet crystals. Unusually low bulk‐rock Na and K contents imply depletion of alkalic components and silica through anatexis and melt extraction during contact heating relative to typical metapelites outside the aureole. Thermobarometry on nearby samples lacking a contact overprint yields 620–640 °C and 0.5–0.6 GPa. In the aureole sample, WDS X‐ray chemical maps show distinct Ca‐enriched rims on both garnet and matrix plagioclase. Furthermore, biotite inclusions within garnet have significantly higher Mg concentration than matrix biotite. Thermobarometry using GASP and garnet–biotite Mg–Fe exchange equilibria on inclusions and adjacent garnet host interior to the high‐Ca rim zone yield ~0.5 ± 0.1 GPa and ~620 ± 50 °C. Pairs in the modified garnet rim zone yield ~0.9 ± 0.1 GPa and ~790 ± 50 °C. Thermocalc average P–T calculations yield similar results for core (~0.5 ± ~0.1 GPa, ~640 ± ~80 °C) and rim (~0.9 ± ~0.1 GPa, ~800 ± ~90 °C) equilibria. The core assemblages are interpreted to record the P–T conditions of peak metamorphism during the Taconic regional event whereas the rim compositions and matrix assemblages are interpreted to record the P–T conditions during the contact event. The high pressures deduced for this later event are interpreted to reflect loading due to the emplacement of Taconic allochthons in the northern Appalachians during the waning stages of regional metamorphism (after c. 465 Ma) and before contact metamorphism (c. 435 Ma). In the absence of contact metamorphism‐induced recrystallization, it is likely that this regional‐scale loading would remain cryptic or unrecorded.     

Variscan amphibolite-facies rocks from the Kurtoğlu metamorphic complex (Gümüşhane area,Eastern Pontides,Turkey)

The Kurtoğlu metamorphic complex, that forms part of the pre-Liassic basement of the Sakarya zone in northern Turkey, consists of at least two tectonic units. Blueschist-facies rocks of unknown metamorphic age in the southern part of the complex are tectonically overlain by Variscan low-pressure high-temperature metamorphic rocks. The latter comprise mica schists and fine-grained gneisses, cut by metaleucogranitic dikes, as well as migmatitic biotite gneisses and subordinate amphibolite intercalations. Structural data indicate that metamorphism and penetrative deformation occurred after dyke intrusion. Peak metamorphic conditions of the mica schists, fine-grained gneisses and metaleucogranites are estimated to ∼650°C and ∼0.4 GPa, based on phase relationships in the system NCKFMASH, Fe–Mg partitioning between garnet and biotite as well as garnet-aluminosilicate-quartz-plagioclase (GASP) and garnet-plagioclase-biotite-quartz (GBPQ) barometry. Peak temperatures of the migmatitic biotite gneisses and amphibolite intercalations are not well constrained but might have been significantly higher (690–740°C), as suggested from hornblende-plagioclase thermometry. ⁴⁰Ar–³⁹Ar incremental dating on muscovite and biotite fractions from the mica schists and fine-grained gneisses yielded plateau ages of ∼323 Ma. Significantly older model ages of ∼329 and ∼337 Ma were obtained on muscovite fractions from two metaleucogranite samples. These fractions contain both relict igneous and newly formed metamorphic muscovite.     

Plastic Deformation and Recrystallization of Garnet: A Mechanism to Facilitate Diffusion Creep

Elongate and deformed garnets from Glenelg, NW Scotland, occurwithin a thin shear zone transecting an eclogite body that hasundergone partial retrogression to amphibolite facies at circa700°C. Optical microscopy, back-scattered electron imaging,electron probe microanalysis and electron back-scatter diffractionreveal garnet sub-structures that are developed as a functionof strain. Subgrains with low-angle misorientation boundariesoccur at low strain and garnet orientations are dispersed, aroundrational crystallographic axes, across these boundaries. Towardshigh-strain areas, boundary misorientations increase and thereis a loss of crystallographic control on misorientations, whichtend towards random. In high-strain areas, a polygonal garnetmicrostructure is developed. The garnet orientations are randomlydispersed around the original single-crystal orientation. Somegarnet grains are elongate and Ca-rich garnet occurs on thefaces of elongate grains oriented normal to the foliation. Commonly,the garnet grains are admixed with matrix minerals, and, wherein contact with other phases, garnet is well faceted. We suggestthat individual garnet porphyroclasts record an evolution fromlow-strain conditions, where dislocation creep and recoveryaccommodated deformation, through increasing strain, where dynamicrecrystallization occurred by subgrain rotation, to higheststrains, where recrystallized grains were able to deform bydiffusion creep assisted grain boundary sliding with associatedrotations. KEY WORDS: diffusion creep; EBSD; garnet; plastic deformation; recrystallization     

Microstructural evolution during strain localization in dolomite aggregates

Dolomite aggregates deformed by dislocation creep over a wide range of conditions (T = 700–1000 °C, effective pressure of 900 MPa, strain rates of 10⁻⁷ – 10⁻⁴/s) strain weaken by up to 75% of the peak differential stress. Microstructural study of samples shortened to different finite strains beyond the peak differential stress shows that strain becomes highly localized within shear zones by high-temperature creep processes, with no contribution of brittle cracking. At low strains (8%), dolomite deforms homogeneously by recrystallization-accommodated dislocation creep. At progressively higher sample strains, deformation is localized into narrow shear zones made up of very fine (∼3 μm) recrystallized grains and relict porphyroclasts (20–100 μm). Finely-recrystallized dolomite grains in the shear zones are largely dislocation free and localized shear is facilitated by diffusion creep. In contrast, original dolomite grains and porphyroclasts in shear zones have high dislocation densities and do not deform after shear zone formation. Calculated strain rates in the shear zones are two to three orders of magnitude faster than the imposed bulk strain rate of the samples and these strain rates are consistent with predictions of the diffusion creep flow law for fine-grained dolomite.     

The geodynamics of collision of a microplate (Chilenia) in Devonian times deduced by the pressure–temperature–time evolution within part of a collisional belt (Guarguaraz Complex,W-Argentina)

The Guarguaraz Complex in West Argentina formed during collision between the microplate Chilenia and South America. It is composed of neritic clastic metasediments with intercalations of metabasic and ultrabasic rocks of oceanic origin. Prograde garnet growth in metapelite and metabasite occurred between 1.2 GPa, 470°C and 1.4 GPa, 530°C, when the penetrative s₂-foliation was formed. The average age of garnet crystallization of 390 ± 2 Ma (2σ) was determined from three four-point Lu–Hf mineral isochrones from metapelite and metabasite samples and represents the time of collision. Peak pressure conditions are followed by a decompression path with slight heating at 0.5 GPa, 560°C. Fluid release during decompression caused equilibration of mineral compositions at the rims and also aided Ar diffusion. An ⁴⁰Ar/³⁹Ar plateau age of white mica at 353 ± 1 Ma (1σ) indicates the time of cooling below 350–400°C. These temperatures were attained at pressures of 0.2–0.3 GPa, indicative of an average exhumation rate of ≥1 mm/a for the period 390–353 Ma. Late hydrous influx at 0.1–0.3 GPa caused pervasive growth of sericite and chlorite and reset the Ar/Ar ages of earlier coarse-grained white mica. At 284–295 Ma, the entire basement cooled below 280°C (fission track ages of zircon) after abundant post-collisional granitoid intrusion. The deeply buried epicontinental sedimentary rocks, the high peak pressure referring to a low metamorphic geotherm of 10–12°C/km, and the decompression/heating path are characteristics of material buried and exhumed within a (micro) continent–continent collisional setting.     

Quartz microstructures developed during non-steady state plastic flow at rapidly decaying stress and strain rate

Synseismic loading to very high stresses (>0.5 GPa) and subsequent creep during stress relaxation in the uppermost plastosphere at temperatures of ca. 300–350 °C, near the lower tip of an inferred once seismically active crustal scale fault, was proposed based on peculiar microstructures identified in rocks exposed over >100 km² in the Sesia Zone, European Western Alps. Here we discuss the conspicuous and highly heterogeneous microstructural record of quartz in disseminated small-scale shear zones. Sub-basal deformation lamellae and arrays of elongate subgrains on the TEM-scale indicate an early stage of glide-controlled deformation at high stresses. Distributed brittle failure is indicated by healed microcracks. Very fine-grained recrystallised aggregates with a pronounced crystallographic preferred orientation reflect intense plastic flow by dislocation creep. Locally, a fine-grained foam microstructure indicates a final stage of static grain growth at low differential stress. For the previously inferred peak stresses of about 0.5 GPa and given temperatures, initial strain rates on the order of 10⁻¹⁰ s⁻¹ are predicted by available flow laws for dislocation creep of quartz. We emphasise the importance of short-term non-steady state deformation in the uppermost plastosphere underlying seismically active upper crust. The related heterogeneous record of quartz is governed by the local stress history at constant temperature.     

Ductile deformation of garnet in mylonitic gneisses from the Münchberg Massif (Germany)

Mylonitic gneisses from the Münchberg Massif contain single grains (type I) and polycrystalline aggregates (type II) of garnet displaying a distinct elongation parallel to a macroscopic lineation which is interpreted as the result of ductile deformation. Lattice-preferred orientations of quartz (textures) symmetrical to the macroscopic foliation and lineation and the lack of rotational microfabrics indicate that the bulk deformation was pure shear at least during the latest strain increments. Garnet textures measured by EBSD together with microprobe analyses demonstrate that these two structural types of garnet can be related to two different processes of ductile deformation: (1) For the single grains stretching can be attributed to diffusion creep along grain boundary zones (Coble creep). The related mass transfer is indicated by the fact that primary growth zones are cut off at the long faces of the grains while the related strain shadow domains do not show comparable chemical zoning. Pressure solution and precipitation suitable to produce similar structures can be largely ruled out because retrogressive reactions pointing to the presence of free hydrous fluids are missing. (2) For the polycrystalline garnet aggregates consisting of cores grading into fine-grained mantles, dislocation creep and associated rotation recrystallization can be assumed. Continuous lattice rotation from the core to the outer polycrystalline rim allow a determination of the related dominant slip systems which are {100}<010> and equivalent systems according to the cubic lattice symmetry. The same holds for garnets which appear to be completely recrystallized. For this type of fine-grained aggregates an alternative nucleation model is discussed. Due to penetrative dislocation glide in connection with short range diffusion and the resulting lattice rotation, primary growth zones are strongly disturbed.Since for the considered rock unit of the Münchberg Massif peak metamorphic temperatures between 630 and 670 °C can be assumed, this study clearly demonstrates that the inferred processes of ductile garnet deformation can occur not only in HT regimes as often suggested in the literature even if embedded within a matrix of “low-strength” minerals like quartz, feldspars and micas.     

Quartz deformation mechanisms during Barrovian metamorphism: Implications from crystallographic orientation of different generations of quartz in pelites

The behaviour of quartz during metamorphism is studied based on two case studies from the Barrovian terrains of Sulitjelma in arctic Scandinavia and Loch Tay in the Central Highlands Dalradian of Scotland. Both terrains preserve evidence for metamorphism in pelites involving nucleation and growth of garnet at different times in the deformation history. Data are presented on the size, shape and crystallographic orientation of quartz preserved as inclusions in garnet and as grains in the surrounding matrix. While quartz-grains remain small and dispersed between mica grains, deformation appears to be dominated by grain-boundary sliding accommodated by dissolution–precipitation. At amphibolite facies, textural coarsening occurs by dissolution of small quartz grains and growth of larger quartz grains, coupled with segregation of quartz from mica. As a result, quartz deforms by dislocation creep, developing crystallographic preferred orientations (CPO) consistent with both coaxial and non-coaxial strain. Quartz CPOs with <0001> axes lying parallel to foliation and stretching direction are commonly developed, and best explained by mechanical rotation of inequant (detrital?) quartz grains. There is no evidence for selective entrapment of quartz inclusions in garnet on the basis of quartz crystallographic orientation.     

Crystallographic preferred orientation in albite samples deformed experimentally by dislocation and solution precipitation creep

The crystallographic preferred orientations of a series of experimentally deformed fine-grained albite aggregates were measured by synchrotron source X-ray diffraction. Most samples were deformed and extensively recrystallized by low-temperature recrystallization-accommodated dislocation creep. In axial compression as well as simple shear these samples developed weak but distinct crystallographic preferred orientations consistent with intracrystalline slip on {001}<100>; the sheared samples have a marked asymmetry of the <100> maxima with respect to the shear zone boundaries. One sample was axially compressed by solution precipitation creep; it developed a somewhat different but equally strong preferred orientation, perhaps reflecting crystallographic anisotropy in rates of dissolution and growth.     

Deformation of amphibolites via dissolution–precipitation creep in the middle and lower crust

Continuous compositional zoning in amphibole grains in strongly deformed and lineated amphibolites from the Eastern Blue Ridge, North Carolina indicates that most of the deformation was accommodated by dissolution–precipitation creep. Amphibole in most samples shows moderate prograde and/or retrograde zoning parallel to the long‐axis with compositions ranging between magnesiohornblende and tschermakite. In one sample, grains are zoned from actinolitic (Si = 7.9 p.f.u.) cores to tschermakitic (Si = 6.2 p.f.u) rims. Amphibole‐plagioclase thermometry suggests prograde growth temperatures as low as 400 °C, but typically range from 650 to 730 °C and retrograde growth temperatures <700 °C. These estimates are corroborated quantitatively with amphibole‐garnet‐plagioclase thermobarometry and qualitatively with a positive correlation between TiO₂ concentration in amphibole and calculated temperature. This growth zoning provides persuasive evidence that amphibole precipitation produced the fabric, but evidence for dissolution is less common. It is present, however in the form of truncations of complicated zoning patterns produced by healed fractures and overgrowths in low‐temperature cores by high‐temperature tschermakitic grains lacking similar internal structures. The preservation of this network of straight cracks filled with optically continuous amphibole also provides evidence against the operation of dislocation creep even to temperatures >700 °C because dislocation‐creep would have deformed the fracture network. Thus, these amphibolites deformed by dissolution–precipitation creep that produced a strong linear fabric under upper amphibolite facies, middle‐to‐lower crustal conditions. The significance of this discovery is that dissolution–precipitation creep is activated at lower stresses than dislocation creep and that the strength of the lower crust, where amphibole is the dominant mineral is probably lower than that derived from experimental studies.     

The field and microstructural signatures of deformation‐assisted melt transfer: Insights from magmatic arc lower crust,New Zealand

Melt must transfer through the lower crust, yet the field signatures and mechanisms involved in such transfer zones (excluding dykes) are still poorly understood. We report field and microstructural evidence of a deformation‐assisted melt transfer zone that developed in the lower crustal magmatic arc environment of Fiordland, New Zealand. A 30–40 m wide hornblende‐rich body comprising hornblende ± clinozoisite and/or garnet exhibits 'igneous‐like' features and is hosted within a metamorphic, two‐pyroxene–pargasite gabbroic gneiss (GG). Previous studies have interpreted the hornblende‐rich body as an igneous cumulate or a mass transfer zone. We present field and microstructural characteristics supporting the later and indicating the body has formed by deformation‐assisted, channelized, reactive porous melt flow. The host granulite facies GG contains distinctive rectilinear dykes and garnet reaction zones (GRZ) from earlier in the geological history; these form important reaction and strain markers. Field observations show that the mineral assemblages and microstructures of the GG and GRZ are progressively modified with proximity to the hornblende‐rich body. At the same time, GRZ bend systematically into the hornblende‐rich body on each side of the unit, showing apparent sinistral shearing. Within the hornblende‐rich body itself, microstructures and electron back‐scatter diffraction mapping show evidence of the former presence of melt including observations consistent with melt crystallization within pore spaces, elongate pseudomorphs of melt films along grain boundaries, minerals with low dihedral angles as small as <10° and up to <60°, and interconnected 3D melt pseudomorph networks. Reaction microstructures with highly irregular contact boundaries are observed at the field and thin‐section scale in remnant islands of original rock and replaced grains, respectively. We infer that the hornblende‐rich body was formed by modification of the host GG in situ due to reaction between an externally derived, reactive, hydrous gabbroic to intermediate melt percolating via porous melt flow through an actively deforming zone. Extensive melt–rock interaction and metasomatism occurred via coupled dissolution–precipitation, triggered by chemical disequilibrium between the host rock and the fluxing melt. As a result, the host plagioclase and pyroxene became unstable and were reacted and dissolved into the melt, while hornblende and to a lesser extent clinozoisite and garnet grew replacing the unstable phases. Our study shows that hornblendite rocks commonly observed within deep crustal sections, and attributed to cumulate fractionation processes, may instead delineate areas of deformation‐assisted, channelized reactive porous melt flow formed by melt‐mediated coupled dissolution–precipitation replacement reactions.     

信阳-舒城断裂带西段石榴云母片岩中石榴石的矿物学特征及其构造意义

桐柏山地区信阳-舒城断裂带西段石榴云母片岩中石榴石变斑晶保留了较多岩石形成过程中的变形-变质信息,它真实地反映了中国南北两大板块缝合带的形成条件和演化历史.石榴石探针成分分析结果表明其属于铁铝榴石,反映出经受中级区域变质作用的特征.在Nadi石榴石成分与变质带的关系图上主要投影在略偏石榴石带的蓝晶石带内,显示岩石遭受了...     

The effects of quartz recrystallization and reaction on weak phase interconnection,strain localization and evolution of microstructure

We conducted axial compression and general shear experiments, at T = 900 °C and P = 1.5 GPa, on samples of banded iron formation (BIF) and synthetic aggregates of quartz, hematite and magnetite to investigate how dynamic recrystallization of quartz promotes strain localization, and the role of weak second phases (oxides) on the rheology and microstructural evolution of the aggregates. Experiments showed strain localization into oxide rich layers, and that the oxide content and oxide distribution are key factors for the strength of the aggregate. Only 2–10 wt.% hematite leads to pronounced weakening and increasing hematite content above ∼10% has only a minor additional effect. Where oxide grains are dispersed, the initial strength contrast with quartz induces stress concentrations at their tips, promoting high stress recrystallization-accommodated dislocation creep of quartz. Fine recrystallized quartz reacts with oxide, forming trails of fine reaction product (ferrosilite/fayalite) leading to the interconnection/percolation of a weaker matrix. The strength contrast between the quartz framework and these fine-grained trails promotes strain localization into micro-shear zones, inducing drastic strain weakening. Thus dynamic recrystallization of quartz promotes syn-deformational reactions leading to a microstructurally-controlled evolution of phase strength contrast. It results in a rheologic transition from load-bearing framework to a matrix-controlled rheology, with transition from S–C′ to S–C fabric with increasing strain.     

Petrogenetic relations among titanium‐rich minerals in an anatectic high‐P mafic granulite

The petrogenetic relations among Ti‐rich minerals in high‐grade metabasites is illuminated here through a detailed petrological investigation of an anatectic garnet–clinopyroxene granulite from the Grenville Province, Ontario, Canada containing rutile, titanite and ilmenite in distinct microtextural settings. Garnet porphyroblasts exhibit zoned Ti concentrations (up to 0.15 wt% TiO₂ in their cores), as well as a variety of rutile inclusion types, including clusters of small, variably elongate grains and thin (≤1 μm) oriented needles. Calcite inclusions in garnet, commonly observed surrounding garnet cores containing quartz and clinozoisite, indicate the presence of evolving C–O–H fluids during garnet growth and suggest that the rutile clusters may have formed from subsequent Ti diffusion and rutile precipitation within existing fluid inclusions. Titanite forms large subhedral crystals and typically occurs where the primary garnet–clinopyroxene assemblage is in contact with leucosome containing megacrystic hornblende, silvialitic scapolite and calcic plagioclase. Many titanite crystals exhibit marginal subgrains that correspond with sharp changes in their major and trace element composition, likely related to a dissolution–precipitation or recrystallization process following primary crystallization. Clinopyroxene–ilmenite symplectite coronas surround titanite in most locations, likely forming from reaction with the hornblende‐plagioclase matrix (±fluids/melt). Integration of multi‐equilibria thermobarometry and Zr thermometry in rutile and titanite with phase equilibrium modelling allows definition of a clockwise P–T path evolving to peak pressures of ~1.5 GPa at ~750°C during garnet and rutile growth, followed by peak temperature conditions of ~1.2 GPa and ~820–880°C associated with melt‐present titanite growth, and finally cooling and decompression to regional amphibolite facies conditions (~1.0 GPa and ~750°C) associated with the formation of clinopyroxene–ilmenite symplectites surrounding titanite. P–T pseudosections calculated for the pristine (leucosome‐ and titanite ‐free) metabasite bulk composition reproduce much of the prograde phase relations, but predict rutile as the stable Ti‐rich mineral at the peak thermal conditions associated with melt‐present titanite growth. The P–M(CaO) and T–M(CaO) models show that bulk CaO concentrations have a significant effect on the stability ranges of titanite and rutile. Increased bulk CaO tends to stabilize titanite to higher pressure and temperature at the expense of rutile, with a ≥15% increase in CaO producing the observed titanite‐bearing assemblage at high‐P granulite facies conditions. Thus, the model results are consistent with the textural observations, which suggest that titanite stability is associated with a chemical exchange between the host metabasite and a Ca‐rich melt.     

How does shear zone nucleate? An example from the Suretta nappe (Swiss Eastern Alps)

In order to address the question of the processes involved during shear zone nucleation, we present a petro-structural analysis of millimetre-scale shear zones within the Roffna rhyolite (Suretta nappe, Eastern central Alps). Field and microscopic evidences show that ductile deformation is localized along discrete fractures that represent the initial stage of shear zone nucleation. During incipient brittle deformation, a syn-kinematic metamorphic assemblage of white mica + biotite + epidote + quartz precipitated at ca. 8.5 ± 1 kbar and 480 ± 50 °C that represent the metamorphic peak conditions of the nappe stacking in the continental accretionary wedge during Tertiary Alpine subduction. The brittle to ductile transition is characterized by the formation of two types of small quartz grains. The Qtz-IIa type is produced by sub-grain rotation. The Qtz-IIb type has a distinct CPO such that the orientation of c-axis is perpendicular to the shear fracture and basal and rhombhoedric slip systems are activated. These Qtz-IIb grains can either be formed by recrystallization of Qtz-IIa or by precipitation from a fluid phase. The shear zone widening stage is characterized by a switch to diffusion creep and grain boundary sliding deformation mechanisms. During the progressive evolution from brittle nucleation to ductile widening of the shear zone, fluid–rock interactions play a critical role, through chemical mass-transfer, metasomatic reactions and switch in deformation mechanisms.     

Influence of time, temperature, confining pressure and fluid content on the experimental compaction of spherical grains

Theoretical models of compaction processes, such as for example intergranular pressure-solution (IPS), focus on deformation occurring at the contacts between spherical grains that constitute an aggregate. In order to investigate the applicability of such models, and to quantify the deformation of particles within an aggregate, isostatic experiments were performed in cold-sealed vessels on glass sphere aggregates at 200 MPa confining pressure and 350 °C with varying amounts of fluid. Several runs were performed in order to investigate the effects of time, fluid content, pressure and temperature, by varying one of these parameters and holding the others fixed. In order to compare the aggregates with natural materials, similar experiments were also performed using quartz sand instead of glass spheres. Experiments with quartz show evidence of IPS, but the strain could not be quantified. Experiments with glass spheres show evidence of several types of deformation processes: both brittle (fracturing) and ductile (plastic flow and fluid-enhanced deformation, such as IPS). In experiments with a large amount of water (≥ 5 vol.%), dissolution and recrystallization of the glass spheres also occurred, coupled with crystallization of new material filling the initial porosity. Experiments performed with a fluid content of less than 1 vol.% indicate creep behavior that is typical of glass deformation, following an exponential law. These experiments can also be made to fit a power law for creep, with a stress exponent of n = 10.5 ± 2.2 in both dry and wet experiments. However, the pre-factor of the power law creep increases 5 times with the addition of water, showing the strong effect of water on the deformation rate. These simple and low-cost experiments provide new insights on the rheology of soda-lime glass, which is used in analogue experiments, and of glass-bearing rocks under mid-crustal P–T conditions. They also highlight the strong enhancement of plasticity of natural rocks in presence of fluid or of a glassy phase.