Quartz Microfabrics Research Articles

Deformational Temperatures Across the Lesser Himalayan Sequence in Eastern Bhutan and Their Implications for the Deformation History of the Main Central Thrust

AbstractWe postulate that the inverted metamorphic sequence in the Lesser Himalayan Sequence of the Himalayan orogen is a finite product of its deformation and temperature history. To explain the formation of this inverted metamorphic sequence across the Lesser Himalayan Sequence with a focus on the Main Central Thrust (MCT) in eastern Bhutan, we determined the metamorphic peak temperatures by Raman spectroscopy of carbonaceous material and established the deformation temperatures by Ti‐in‐quartz thermobarometry and quartz c axis textures. These data were combined with thermochronology, including new and published 40Ar/39Ar ages of muscovite and published apatite fission track, and apatite and zircon (U‐Th)/He ages. To obtain accurate metamorphic, deformation, and closure temperatures of thermochronological systems, pressures and cooling rates for the period of interest were derived by inverse modeling of multiple thermochronological data sets, and temperatures were determined by iterative calculations. The Raman spectroscopy of carbonaceous material results indicate two temperature sequences separated by a thrust. In the external sequence, peak temperatures are constant across the structural strike, consistent with the observed hinterland‐dipping duplex system. In the internal temperature sequence associated with the MCT shear zone, each geothermometer yields an apparent inverted temperature gradient although with different temperature ranges, and all temperatures appear to be retrograde. These observations are consistent with the quartz microfabrics. Further, all thermochronometers indicate upward younging across the MCT. We interpret our data as a composite peak and deformation temperature sequence that formed successively and reflects the broadening and narrowing of the MCT shear zone in which the ductile deformation lasted until ~11 Ma.

Heterogeneous brittle-ductile deformation at shallow crustal levels under high thermal conditions: The case of a synkinematic contact aureole in the inner northern Apennines, southeastern Elba Island, Italy

We present an example of interaction between magmatism and tectonics at shallow crustal levels. In the Late Miocene the metamorphic units of the eastern Elba Island (northern Apennines) were intruded at very shallow crustal levels by a large pluton (>60km2) with the development of an hectometre-sized contact aureole defined by growth of low-pressure/high-temperature mineral assemblages (Pmax<0.2GPa, Tmax~650°C). Structural data show that the contact aureole is associated with a km-sized antiform of the foliation and by several metre- to decametre-thick high-strain domains consisting of strongly foliated rocks containing synkinematic HT/LP mineral assemblages and ductile shear zones of variable thickness. These shear zones are characterized by a mylonitic foliation variably overprinted by cataclasis. Quartz microfabrics indicate that the dynamic crystallization processes progressively changed from grain boundary migration, associated with the thermal peak of contact metamorphism, to subgrain rotation and bulging recrystallization, the latter mostly associated with the cataclastic overprint. These transitions of recrystallization mechanisms in quartz are related to a progressive decrease of temperature during deformation. Deformation accompanied the development and cooling of the contact aureole, which recorded the switch from high temperature ductile to low temperature brittle conditions. The geometry of the studied deformation structures is consistent with the constraints of the regional tectonic evolution and its local interaction with the localized and transient thermal anomaly related to the coeval emplacement of igneous rocks.

Recrystallization of quartz after low-temperature plasticity – The record of stress relaxation below the seismogenic zone

Quartz microfabrics in rocks from the Silvretta basal thrust and the Defereggen-Antholz-Vals (DAV) shear zone in the Eastern Alps, are analysed by polarized light and electron microscopy. The microfabrics from both shear zones record a switch from low-temperature plasticity at transient high stress to recrystallization at relaxing stresses at greenschist facies conditions. The development of new grains is dominantly by subgrain rotation and subsequent strain-induced grain-boundary migration in areas of localized high strain developed during initial low-temperature plasticity. The findings suggest that new grains develop at almost random crystallographic orientations at fast rates of stress relaxation (i.e. at low stress), as indicated by recrystallized quartz zones in the Silvretta fault rocks. In contrast, at slow rates of stress relaxation, new grains develop at moderately high stresses with crystallographic preferred orientation characterized by high Schmid factor for basal <a> glide, as indicated by vein quartz samples from the DAV shear zone. Both recorded histories with transient peak stresses and different rates of stress relaxation are interpreted to be related to seismic activity of the fault systems. This study demonstrates that characteristic microfabrics provide important information about the deformation history of natural shear zones developed in different tectonic regimes.

Transpression and tectonic exhumation in the Heimefrontfjella, western orogenic front of the East African/Antarctic Orogen, revealed by quartz textures of high strain domains

The metamorphic basement of the Heimefrontfjella in western Dronning Maud Land (Antarctica) forms the western margin of the major ca. 500 million year old East African/East Antarctic Orogen that resulted from the collision of East Antarctica and greater India with the African cratons. The boundary between the tectonothermally overprinted part of the orogen and its north-western foreland is marked by the subvertical Heimefront Shear Zone. North-west of the Heimefront Shear Zone, numerous low-angle dipping ductile thrust zones cut through the Mesoproterozoic basement. Petrographic studies, optical quartz c-axis analyses and x-ray texture goniometry of quartz-rich mylonites were used to reveal the conditions that prevailed during the deformation. Mineral assemblages in thrust mylonites show that they were formed under greenschist-facies conditions. Quartz microstructures are characteristic of the subgrain rotation regime and oblique quartz lattice preferred orientations are typical of simple shear-dominated deformation. In contrast, in the Heimefront Shear Zone, quartz textures indicate mainly flattening strain with a minor dextral rotational component. These quartz microstructures and lattice preferred orientations show signs of post-tectonic annealing following the tectonic exhumation. The spatial relation between the sub-vertical Heimefront Shear Zone and the low-angle thrusts can be explained as being the result of strain partitioning during transpressive deformation. The pure-shear component with a weak dextral strike-slip was accommodated by the Heimefront Shear Zone, whereas the north–north-west directed thrusts accommodate the simple shear component with a tectonic transport towards the foreland of the orogen. Keywords: Dronning Maud Land; quartz microfabrics; X-ray texture goniometry; shear zones; mylonites. (Published: 15 June 2016) To access the supplementary material for this article, please see the supplementary file under Article Tools, online. Citation: Polar Research 2016, 35 , 25420, http://dx.doi.org/10.3402/polar.v35.25420

Quartz microfabrics of shear zones of the western orogenic front of the East African/Antarctic Orogen: result of transpressive deformation?

Quartz microfabrics of shear zones of the western orogenic front of the East African/Antarctic Orogen: result of transpressive deformation?

Microfabric memory of vein quartz for strain localization in detachment faults: A case study on the Simplon fault zone

This manuscript deals with the adaptation of quartz-microfabrics to changing physical deformation conditions, and discusses their preservation potential during subsequent retrograde deformation. Using microstructural analysis, a sequence of recrystallization processes in quartz, ranging from Grain-Boundary Migration Recrystallization (GBM) over Subgrain-Rotation Recrystallization (SGR) to Bulging Nucleation (BLG) is detected for the Simplon fault zone (SFZ) from the low strain rim towards the internal high strain part of the large-scale shear zone. Based on: (i) the retrograde cooling path; (ii) estimates of deformation temperatures; and (iii) spatial variation of dynamic recrystallization processes and different microstructural characteristics, continuous strain localization with decreasing temperature is inferred. In contrast to the recrystallization microstructures, crystallographic preferred orientations (CPO) have a longer memory. CPO patterns indicative of prism <a> and rhomb <a> glide systems in mylonitic quartz veins, overprinted at low temperatures (≤400 °C), suggest inheritance of a high-temperature deformation. In this way, microstructural, textural and geochemical analyses provide information for several million years of the deformation history. The reasons for such incomplete resetting of the rock texture is that strain localization is caused by change in effective viscosity contrasts related to temporal large- and small-scale temperature changes during the evolution of such a long-lived shear zone. The spatially resolved, quantitative investigation of quartz microfabrics and associated recrystallization processes therefore provide great potential for an improved understanding of the geodynamics of large-scale shear zones.

Quartz c-axis fabrics in constrictionally strained orthogneisses: implications for the evolution of the Orlica-Śnieżnik Dome, the Sudetes, Poland

ABSTRACT Żelaźniewicz, A., Kromuszczyńska, O. and Biegała, N. 2013. Quartz c-axis fabrics in constrictionally strained orthogneisses: implications for the evolution of the Orlica-Śnieżnik Dome, the Sudetes, Poland. Acta Geologica Polonica, 63(4), 697-722, Warszawa. The Orlica-Śnieżnik Dome (OSD), NE Bohemian Massif, contains in its core several gneiss variants with protoliths dated at ~500 Ma. In the western limb of the OSD, rodding augen gneisses (Spalona gneiss unit) are mainly L&gt;S tectonites with a prominent stretching lineation. The few quartz LPO studies have produced somewhat discrepant results. Reexamination of these rocks revealed that texture formation was a protracted, multistage process that involved strain partitioning with changing strain rate and kinematics in a general shear regime at temperatures of the amphibolite facies (450-600°C). Quartz c-axis microfabrics show complex yet reproducible patterns that developed under the joint control of strain geometry and temperature; thus the LPOs are mixed features represented by pseudogirdle patterns. Domainal differences in quartz microfabrics (ribbons, tails, quartzo-feldspathic aggregate) are common in the Spalona orthogneisses but uncommon in the sheared migmatitic gneisses. In the latter rocks, the constrictional strain was imposed on the originally planar fabric defined by high-temperature migmatitic layering. The constrictional fabric of the Spalona gneisses may have developed in the hinge zones of kilometer-scale folds, where the elongation occurred parallel to the fold axes. Other occurrences of rodding gneisses throughout the Orlica-Śnieżnik Dome are thought to occupy similar structural positions, which would point to the significance of large-scale folds in the tectonic structure of the dome.

Metamorphism and kinematics of the early deformation in the Variscan suture of SW Iberia

AbstractThe early (Devonian) collisional stage in SW Iberia has been investigated through the analysis of deformation in the Cubito‐Moura schists, the main lithology of an Allochthonous Complex putatively rooted in the suture between the Ossa‐Morena and South Portuguese zones. The first deformation in these schists (D1) is recorded as a S1‐L1 mylonitic fabric well preserved in early quartz veins. Subsequent D2 deformation caused the main folds and the main (S2) foliation. After restoration, the stretching lineation (L1) trends at a small angle with the Ossa‐Morena/South Portuguese suture. This trend, together with the top‐to‐the‐east kinematics determined from quartz microfabric is indicative of an oblique left‐lateral collisional scenario in SW Iberia. Chlorite–white K‐mica–quartz ± chloritoid multi‐equilibrium calculations yield P–T conditions in the range 0.9–1.2 GPa and 300–400 °C, during the first collisional stage. P–T conditions during D2 were 0.3–0.8 GPa and 400–450 °C, thus indicating an important stage of exhumation of the Allochthonous Complex during these two collisional events, after subduction of the Ossa‐Morena Zone margin under the South Portuguese Zone continental crust. In the general context of the Variscan orogen, dominated by dextral collision, the left‐lateral convergence in SW Iberia can be explained in terms of the Avalonian salient represented by the South Portuguese Zone, which would impinge between Iberia and Morocco.

Matrix deformation mechanisms in HP-LT tectonic mélanges — Microstructural record of jadeite blueschist from the Franciscan Complex, California

Exhumed high pressure–low temperature metamorphic mélanges of tectonic origin are believed to reflect high strain accumulated in large scale interplate shear zones during subduction. Rigid blocks of widely varying size are embedded in a weak matrix, which takes up the deformation and controls the rheology of the composite. The microfabrics of a highly deformed jadeite-blueschist from the Franciscan Complex, California, are investigated to help understand deformation mechanisms at depth. The specimen shows a transposed foliation with dismembered fold hinges and boudinage structures. Several generations of open fractures have been sealed to become veins at high-pressure metamorphic conditions. The shape of these veins, frequently restricted to specific layers, indicates distributed host rock deformation during and after sealing. Small cracks in jadeite and lawsonite are healed, with tiny quartz inclusions aligned along the former fracture surface. Large jadeite porphyroblasts show strain caps and strain shadows. Open fractures are sealed by quartz and new jadeite epitactically grown on the broken host. Monophase glaucophane aggregates consist of undeformed needles with a diameter between 0.1 and 2μm, grown after formation of isoclinal folds. Only quartz microfabrics indicate some stage of crystal-plastic deformation, followed by annealing and grain growth. Aragonite in the latest vein generation shows retrogression to calcite along its rims. The entire deformation happened under HP-LT metamorphic conditions in the stability field of jadeite and quartz, at temperatures between 300 and 450°C and pressures exceeding 1–1.4GPa. The microfabrics indicate that dissolution precipitation creep was the predominant deformation mechanism, accompanied by brittle failure and vein formation at quasi-lithostatic pore fluid pressure. This indicates low flow strength and, combined with high strain rates expected for localized deformation between the plates, a very low viscosity of material in the interplate shear zone at a depth>30–45km.

Quartz as indicator mineral in the Central Swiss Alps: the quartz recrystallization isograd in the rock series of the northern Aar massif

Earlier observations on the deformation of rock-forming quartz during Alpine metamorphism that were made by Voll (1976) along a traverse following the Reuss valley (Aar massif, Central Swiss Alps) are still relevant but can be explained in more modern term. Voll found that a few kilometres south of the contact between the Aar massif and its sedimentary cover quartz has partly undergone a combination of dynamic recrystallization and nucleation along grain boundaries and fractures at the expense of the deformed parent grains. Further to the south, increasing grain growth of quartz under nearly static conditions is observed, with polygonal grain shapes typical of annealed microfabrics. Comparable stages of recrystallization could be found in several N–S traverses through the Aar massif so that “points of first occurrence” of newly formed quartz could be connected to define a quartz recrystallization isograd. This isograd extents over 90 km through the northern Aar massif, following a course parallel to the sanidine/microcline isograd but at a distance of 10–15 km further to the north. The evolution of the quartz microfabric is discussed with respect to recent estimates of the T-t path of the Aar massif.

Quartz veins record earthquake-related brittle failure and short term ductile flow in the deep crust

Discordant quartz veins in a retrograded eclogite–amphibolite (Western Gneiss Region, Norway) formed at temperatures of about 500 to 550 °C. The parental fractures crosscut the foliation and show branching, indicating rapid crack propagation at high differential stress. The shape of the quartz veins indicates intense inhomogeneous ductile deformation of the host rock during opening and sealing, controlled by the presence of the cohesionless fluid-filled fractures. The vein quartz microfabrics indicate crystallization in an open cavity, implying a stage of very high permeability and fluid flow. The event of brittle failure at 500 to 550 °C, followed by an episode of inhomogeneous ductile deformation, is attributed to stress changes related to the earthquake cycle, i.e. coseismic loading and postseismic creep. The record suggests seismic activity at the nearby low angle normal fault at 15 to 25 km depth and provides insight into transient crustal properties and processes near active faults at great depth.

Origin of migmatites by deformation‐enhanced melt infiltration of orthogneiss: a new model based on quantitative microstructural analysis

AbstractA detailed field study reveals a gradual transition from high‐grade solid‐state banded orthogneiss via stromatic migmatite and schlieren migmatite to irregular, foliation‐parallel bodies of nebulitic migmatite within the eastern part of the Gföhl Unit (Moldanubian domain, Bohemian Massif). The orthogneiss to nebulitic migmatite sequence is characterized by progressive destruction of well‐equilibrated banded microstructure by crystallization of new interstitial phases (Kfs, Pl and Qtz) along feldspar boundaries and by resorption of relict feldspar and biotite. The grain size of all felsic phases decreases continuously, whereas the population density of new phases increases. The new phases preferentially nucleate along high‐energy like–like boundaries causing the development of a regular distribution of individual phases. This evolutionary trend is accompanied by a decrease in grain shape preferred orientation of all felsic phases. To explain these data, a new petrogenetic model is proposed for the origin of felsic migmatites by melt infiltration from an external source into banded orthogneiss during deformation. In this model, infiltrating melt passes pervasively along grain boundaries through the whole‐rock volume and changes completely its macro‐ and microscopic appearance. It is suggested that the individual migmatite types represent different degrees of equilibration between the host rock and migrating melt during exhumation. The melt topology mimicked by feldspar in banded orthogneiss forms elongate pockets oriented at a high angle to the compositional banding, indicating that the melt distribution was controlled by the deformation of the solid framework. The microstructure exhibits features compatible with a combination of dislocation creep and grain boundary sliding deformation mechanisms. The migmatite microstructures developed by granular flow accompanied by melt‐enhanced diffusion and/or melt flow. However, an AMS study and quartz microfabrics suggest that the amount of melt present did not exceed a critical threshold during the deformation to allow free movements of grains.

Vein quartz microfabrics indicating progressive evolution of fractures into cavities during postseismic creep in the middle crust

Discordant quartz veins in metamorphic rocks formed at temperatures similar to 300 °C in an extensional tectonic regime. The vein shape is characterized by an aperture to length ratio exceeding the maximum value feasible for purely elastic deformation by orders of magnitude. Non-elastic ductile deformation of the host rock must have controlled widening of the arrested cracks. Fracture parallel shortening is low. Due to the low accumulated strain, the microstructural record of this stage of deformation cannot be isolated in the polyphase host rock. Information is exclusively obtained from the microfabrics of the vein sealing quartz. There, the density of geometrically necessary dislocations, derived from orientation maps created by electron backscatter diffractometry (EBSD), decreases markedly from the vein wall to the centre. The gradient is attributed to progressive cavity formation and buckling of the vein walls during sealing. Early grown crystals at the vein margins record a prolonged deformation history compared to lately grown grains in the centre of the vein. The gradient and the lack of significant crystal plastic deformation of the crystals in the vein centre show that deformation of the completed vein is insignificant compared to deformation during progressive opening and sealing. In view of the precursory brittle deformation and the intense crystal plastic deformation at the vein margins, the monogenetic veins are interpreted to record a single mid-crustal stress transient, related to the earthquake cycle. Fracturing is proposed to be a consequence of coseismic loading, while progressive opening and sealing record the deformation during postseismic stress relaxation by creep.

Microfabrics of UHP metamorphic granites in the Dora Maira Massif, western Alps – no evidence of deformation at great depth

AbstractIn the Dora Maira Massif, western Alps, essentially undeformed ultrahigh‐pressure (UHP) metamorphic granites (Brossasco granite) are embedded in, and locally grade into, granite gneisses or augengneisses and mylonites. In this study, the quartz microfabrics of the undeformed granites are compared against the augengneisses and mylonites in a representative number of samples from several locations. In the undeformed granites, the fine‐grained quartz aggregates that formed from coesite upon decompression are characterized by a foam structure and random crystallographic orientation. In the deformed granites, the quartz microstructures and the crystallographic preferred orientation (CPO) indicate deformation by dislocation creep. Most of the deformation of the granites (if not all) must have happened at a late stage during exhumation, after transformation of coesite to quartz, at greenschist facies conditions in the middle crust. The deformed granites provide no evidence of deformation during subduction, at (U)HP metamorphic conditions, and in the earlier stages of exhumation. The diameter of internally undeformed slices of continental crust subducted to and exhumed from about 100 km can exceed that of the presently exposed Brossasco granite, i.e. it can be on the kilometre scale.

Fabric development during shear deformation in the Main Central Thrust Zone, NW-Himalaya, India

Quartz microfabrics and associated microstructures have been studied on a crustal shear zone—the Main Central Thrust (MCT) of the Himalaya. Sampling has been done along six traverses across the MCT zone in the Kumaun and Garhwal sectors of the Indian Himalaya. The MCT is a moderately north-dipping shear zone formed as a result of the southward emplacement of a part of the deeply rooted crust (that now constitutes the Central Crystalline Zone of the Higher Himalaya) over the less metamorphosed sedimentary belt of the Lesser Himalaya. On the basis of quartz c- and a-axis fabric patterns, supported by the relevant microstructures within the MCT zone, two major kinematic domains have been distinguished. A noncoaxial deformation domain is indicated by the intensely deformed rocks in the vicinity of the MCT plane. This domain includes ductilely deformed and fine-grained mylonitic rocks which contain a strong stretching lineation and are composed of low-grade mineral assemblages (muscovite, chlorite and quartz). These rocks are characterized by highly asymmetric structures/microstructures and quartz c- and a-axis fabrics that indicate a top-to-the-south sense that is compatible with south-directed thrusting for the MCT zone. An apparently coaxial deformation domain, on the other hand, is indicated by the rocks occurring in a rather narrow belt fringing, and structurally above, the noncoaxial deformation domain. The rocks are highly feldspathic and coarse-grained gneisses and do not possess any common lineation trend and the effects of simple shear deformation are weak. The quartz c-axis fabrics are symmetrical with respect to foliation and lineation. Moreover, these rocks contain conjugate and mutually interfering shear bands, feldspar/quartz porphyroclasts with long axes parallel to the macrosopic foliation and the related structures/microstructures, suggesting deformation under an approximate coaxial strain path. On moving towards the MCT, the quartz c- and a-axis fabrics become progressively stronger. The c-axis fabric gradually changes from random to orthorhombic and then to monoclinic. In addition, the coaxial strain path gradually changes to the noncoaxial strain path. All this progressive evolution of quartz fabrics suggests more activation of the basal, rhomb and 〈 a〉 slip systems at all structural levels across the MCT.

Cretaceous structural evolution of the Pelagonian crystalline in western Voras Mt (Macedonia, Northern Greece)

The kinematic of the Cretaceous deformation and the relationship between deformation and metamorphism of the Pelagonian crystalline was studied in Voras Mt (northern Greece). The Pelagonian crystalline in this area has been subdivided into a lower, core part, consisting mainly of gneissic rocks and schists and an upper, cover part, consistine of schists and quartzites with marble intercalations. Intensely deformed granitic rocks of Upper Paleozoic age intrude the Pelagonian crystalline basement. An S j foliation is the oldest fabric recognized in the Pelagonian crystalline. Sj is mainly defined by syn-Sjgarnet(Gr1), biotite(Btj), white mica (Wnij), chloritoid, kyanite and plagioclase in the metapelitic rocks and green amphibole, epidote, plagioclase, and biotiteiBtj) in the amphibolite. Garnet grows also in some cases post-kinematically. Ilmenite and tourmaline are often found in the pelitic rocks as well. S is overprinted by an S2 foliation that developed as a crenulation cleavage. In most places, however, S2 has destroyed all earlier fabrics and a single S2 fabric is present related to, isoclinal or sheath folds intrafolial in places. S2 in the metapelitic rocks is characterized by the syn-S2 development of chlorite, white mica(Wm2) and plagioclase. In the amphibolite S2 is mainly defined by the syn-kinematic development of actinolite, plagioclase, biotite(Bt2), white mica(Wm2) and chlorite. During D2 garnet(Gr1) and biotiteiBtj) are partially replaced by chlorite, while green amphibole is replaced by actinolite and chlorite. Chloritoid remains generally stable along the S2-planes but in some places transforms to chlorite and sericite. Furthermore, D2 was locally followed by a static post-kinematic annealing indicated by polygonal quartz microfabrics with equilibrated grain boundaries and triple points. The overall orientation of S2 is dome shaped with a gentle SW-ward and NE-ward dip in the southwestern and northeastern flanks of the dome respectively. Syn-S2 minerals defined a very well exposed NW-SE trending stretching lineation. Kinematic indicators show a main top to the SE sense of movement. An S3 crenulation cleavage associated with asymmetric NW-SE trending folds is also present in most parts of the core and cover rocks, possibly, related to a constrictional type of deformation. A well developed, S4 shear band cleavage is mainly present in the upper parts of the metamorphic dome and formed under cooler conditions. S4 shear bands are associated with a NW-SE developed stretching lineation defined by elongated and dynamically recrystallized quartz grains and a preferred orientation of white mica and chlorite. Along the S4 shear bands a transformation of garnet, biotite, chloritoid and amphibole into chlorite is always observed. S4 shear bands indicate a main top to the SE sense of movement. The P-T metamorphic conditions were derived from textural equilibria and mineral assemblages, as well as from the spatial distribution of the metamorphic minerals. Syn-Dj metamorphism reached the conditions of the boundaries between greenschist and amphibolite facies. Syn-D2 retrogression took place under greenschist facie conditions. K/Ar radiometric datings on coarse-grained syn-St and younger fine-grained syn-S2 micas define an Early Cretaceous cooling age ('135Ma) for the older event and a Mid- to Late Cretaceous age ('90-80Ma) for the second event. A white mica age of ca. 65Ma correlates with S4 shear band clevage. Furthermore, the intrusion age of a granitic body into the Pelagonian crystalline is dated using the Pb/Pb single zircon evaporation method. The estimated intrusion age of 300±3Ma suggests that the Pelagonian crystalline was affected by a pre-kinematic magmatic activity relative to its Cretaceous deformation.

40Ar/ 39Ar thermochronology from the northwestern Dabie Shan: constraints on the evolution of Qinling–Dabie orogenic belt, east-central China

Seven structural units with distinct kinematics have been recognized in the northwestern Dabie Shan (114°E–115°E and 31°N–32°N), east-central China. They are, from north to south: (1) the north-verging Mafan fold belt (MFB) involving early Paleozoic metamorphic rocks; (2) the Liangting melange belt (LMB), which is separated from the MFB by a south-dipping thrust and consists of gneissic and marble blocks surrounded by highly sheared greenschist; (3) the north-verging Laoshan ductile fold belt (LFB), which was thrust northward over the Liangting melange belt and involves Proterozoic schists; (4) the Suhe thrust sheet (STS), which consists of phyllite and slate above a folded thrust fault dipping both to the north and south; (5) the 5–8-km-wide Huwan shear belt (HSB) in the footwall of the folded thrust, consisting of mylonitic gneisses and sheared ecologite; (6) the Kafang eclogite-bearing gneiss dome (KGD), which is defined by broadly warped foliations; and (7) the 10-km-wide Pendian ductile shear belt (PSB) characterized by dextral strike-slip deformation. Nine phengite and muscovite samples in eclogites, mylonitic shear zones, and metamorphic gneisses from four of the seven structural units have been dated by the 40Ar/ 39Ar step-heating method. Phengites from deformed eclogites in the HSB yield complex age spectra that we interpret as most likely indicating closure with respect to Ar loss between 350 and 430 Ma. The rock samples collected from the eclogite-bearing gneiss display a significant static recrystallization of quartz, which suggests temperatures in excess of ca 350–400°C. The ages of 350–430 Ma from samples 9669B and 9669C are, therefore, interpreted as retrograde metamorphic ages for the eclogites. Six additional phengites sampled from mylonitic rocks yield two groups of ages, 263–249 and 236–214 Ma. The older group samples (96-5, 9667 and 9692) were collected from mylonitic rocks with top-to-north shear sense in the LMB, HSB and KGD, respectively. 9667 and 9692 exhibit a strong static recrystallization, which means that ductile deformation took place at a temperature in excess of 350–400°C. We therefore adopt the age of 263–249 Ma as the cooling ages of white micas. Considering sample 96-5 shows microfabrics formed at lower temperature around or slight below the closure temperature of white mica, there is a strong possibility that the age of 254±1 Ma represents a crystallization age rather than a cooling age. Because these samples are taken from the top-to-north shear zones, we also interpret the 263–249 Ma event as a result of thrusting-induced denudation. The younger group samples (96106, 96124 and 96113) collected from dextral shear belts in the HSB and PSB exhibit a strong static recrystallization at a temperature in excess of 350–400°C. Their ages (236–214 Ma) are therefore interpreted as the cooling ages related to later dextral strike-slip faulting. Sample 96101 collected from weakly foliated rock without penetrative deformation on the north limb of the KGD shows poorly recovered quartz microfabrics at the temperature around, or slightly below, the closure temperature of white mica. Its age of 187±1 Ma is, therefore, interpreted as crystallization age during the downdip detachment event related to doming. The 40Ar/ 39Ar data from the northwestern Dabie Shan suggest four distinctive cooling events. On the basis of the local structural framework and regional tectonic relationships, we suggest that the 410–350 Ma cooling event was related to either subduction of an oceanic plate beneath the Sino-Korean plate or collision between the Sino-Korean plate and a microcontinental block in the middle Paleozoic. The three younger episodes of cooling between the late Permian and the late Triassic may all have been related to the collision between the Sino-Korean and Yangtze plates. Specifically, the 263–249 Ma event could have been a result of thrusting-induced denudation, whereas the 236–214 Ma cooling event is likely related to strike-slip faulting. The younger ages of 195–187 Ma obtained from the gneiss dome are interpreted to be a result of regional denudation related to doming during the late stage of collision between the Sino-Korean and Yangtze plates.

Thermochronometry and microstructures of quartz—a comparison with experimental flow laws and predictions on the temperature of the brittle–plastic transition

A gradient in quartz microfabrics across a major strike-slip shear zone (with a minor vertical component), active during the Oligocene in the Eastern Alps (Alto Adige, Italy), is correlated with new zircon fission track thermochronometric data and available Rb–Sr biotite ages to constrain the depth/temperature range of the recorded rheologic regimes. Distributed deformation in the semibrittle regime (i.e. beneath the brittle–ductile transition) is effective near the closure temperature for fission tracks in zircon (which we estimate as 280±30°C), with high-stress dislocation creep of quartz, microcracking, and pressure solution being active simultaneously. Steady state dislocation creep of quartz at moderate stress in the fully plastic regime is effective at temperatures above the closure temperature for the Rb–Sr and K–Ar systems of biotite (ca. 310±30°C) and below that for the K–Ar system of white mica (ca. 350±50°C). For the inferred temperatures and correlated flow stresses derived by paleopiezometry, the majority of available experimental flow laws for wet quartzite predict strain rates on the order of 10 −13–10 −14 s −1, consistent with the geological constraints. This finding supports the validity of the extrapolation of experimental flow laws to natural strain rates.

General features of the gold deposits in the Rio Itapicuru greenstone belt (RIGB, NE Brazil): discussion of the origin, timing and tectonic model

Anomalous concentration of gold is associated with relatively narrow shear zones in the supracrustal sequence of the Rio Itapicuru Greenstone Belt (RIGB). Both geological mapping and geochemical / geophysical surveys led to the discovery of several gold mines. Among them, the Fazenda Brasileiro mine (1401 Au) is the largest one. This paper draws the general features of the RIGB gold deposits and puts constraints on the relationship between mineralization and the structural evolution of the belt. Mineralization is essentially associated (direct or indirectly) with carbonaceous lithologies and is represented by quartz veins/veinlets thot define the main orebodies. In the main mines, Au is almost always related to the occurrence of arsenopyrite, appearing as inclusions, within microfractures or attached to the crystal faces. Hydrothermal effects have been recognized within the host-rocks. Recent structural and geochronological data show that gold precipitated late (2,083-2,031 Ga) with respect to the regional tectonic events (Ca - 2,100-2,080 Ga). A coupling between thrusting and late stage of granitic magmatism to account for the generation of the mineralizing fluids is suggested. By leaching Au from the volcanic pile, the metamorphic and pluton-related fluids were driven along early planar and linear anisotropies. Hence this pathway within the shear zones led to a geometric disposition of the oreshoots in which their long axes are parallel to the stretching lineation. Quartz microfabrics and <oaxis preferred orientation indicate diffusion-recrystallization as the main deformation mechanism acting in the Au-bearing quartz veins. This scenario explains the metamorphic and magmatic evidences for the origin of the mineralization as well as the ambiguous control of the orebodies (sulphides overprint the mylonitic foliation) by the stretching lineation.

Alpine deformation and 40Ar/39Ar geochronology of synkinematic white mica in the Siviez‐Mischabel Nappe, western Pennine Alps, Switzerland

We explore the timing of deformation and exhumation of the Siviez‐Mischabel Nappe (western Swiss Alps), which has been considered a classic example of a midcrustal crystalline nappe since the studies of Argand [1916]. This study presents 40Ar/39Ar ages obtained on both synkinematic white mica from Permo‐Triassic cover sediments and more complex white mica populations from basement gneisses of the Siviez‐Mischabel and middle Pennine Nappes. Primary foliation developed in cover units by nucleation, growth, and rigid rotation of mica grains during noncoaxial Alpine deformation. Although some samples show a crenulation of this primary foliation, mica growth appears to have occurred only during the development of primary foliation, the main phase of greenschist facies deformation related to imbrication of the Siviez‐Mischabel Nappe and other middle Pennine Nappes. Good agreement exists between independent estimates of the timing of deformation and reported 40Ar/39Ar white mica ages from cover units of the central and southern Siviez‐Mischabel Nappe. In cover units from the central and southern Siviez‐Mischabel regions of the study area, 40Ar/39Ar ages appear to date synkinematic white mica growth. Results suggest that the Siviez‐Mischabel Nappe was emplaced and developed foliation during a 5 m.y. period from 41 to 36 Ma. In cover units from the eastern Siviez‐Mischabel, however, 40Ar/39Ar white mica ages appear to date postkinematic thermal events. These thermal events may be related to Oligocene magmatic activity in the lower Pennine Nappes or to Miocene development of the Simplon fault zone. Variations in the relation between Alpine age and grain size for cover samples from the central, eastern, and southern Siviez‐Mischabel correlate well with the regional variations in temperature inferred from quartz microfabrics and the pattern of regional metamorphism. When considered in concert with other recent isotopic studies on the timing of major tectonic and thermal events in the western Swiss Alps, these data support arguments that the relative timing of events such as thrusting and back thrusting of crystalline nappes in hinterland units and exhumation of high‐pressure units in the suture zone of the western Alps are intimately related and synchronous on the scale of a few million years.