Quartz C-axis Orientations Research Articles

Reassessing the timing of high-strain deformation in the Strangways Metamorphic Complex, Central Australia, by in situ mica Rb–Sr and titanite U–Pb geochronology

Dating the timing of deformation within shear zones is critical to quantifying orogenic processes and developing time-resolved regional tectonic frameworks. Such work requires the integration of detailed microstructural analysis with in situ geochronology to quantify when deformation mechanisms were active at the micro-scale. The Proterozoic, granulite-facies Strangways Metamorphic Complex was exhumed during the Devonian to Carboniferous intracontinental Alice Springs Orogeny. New microstructural observations, quartz c -axis orientation analyses, mica chemistry, Ti-in-biotite thermometry, in situ mica Rb–Sr and titanite U–Pb geochronology outline a detailed history of ductile shearing across the complex. Movement along the north boundary shear zone of Strangways Metamorphic Complex appears to have initiated c. 382 Ma, preceding peak metamorphism in the area during the Alice Springs Orogeny. Widespread reverse-sense ductile shearing occurred within the Strangways Metamorphic Complex between c. 365 and 355 Ma, and correlates with rapid cooling of the region. Late-stage ductile deformation is recorded at c. 335 Ma, likely reflecting the terminal exhumation of the Strangways Metamorphic Complex. Finally, the new in situ muscovite and biotite Rb–Sr data collected herein permit comparison with previous two-point mica Rb–Sr isochrons and 40 Ar– 39 Ar dates from the same specimens. In the rocks analysed, the biotite Rb–Sr system returned dates similar to the previous 40 Ar– 39 Ar white mica dates, perhaps indicating a similar effective closure temperature. Supplementary material: Test results and previous geochronology data from the Alice Springs Orogeny are available in tabular form at https://osf.io/yjp24/?view_only=0bf5bec9f5c94a1aa4fb413320ec24e8

Contrasting kinematics of brittle-shears within the Salem–Attur and Bhavani shear zone, south India: Tectonic implications

We document kinematics and rheological behaviour of brittle shears (~50 cm wide) postdating solid-state tectonic fabric in the Salem–Attur (SASZ) and Bhavani (BSZ) shear zone that constitute a Paleoproterozoic (~2500 Ma) suture juxtaposing disparate granulite blocks in south India. We constrain brittle deformation mechanisms from established relationship between changing orientation of deflected strain marker (quartz vein) and foliation within the shear band with respect to their orientation outside the shear band. Quartz c-axis orientation in charnockite (host lithology) and phyllonite (reworked charnockite) from the SASZ show presence of mixed basal 〈a〉 (low-T) and prism 〈a〉 (high-T) slip, and single basal 〈a〉 slip mechanism, respectively. This suggests considerable cooling of the granulite block prior to the onset of brittle shearing. Distribution of strain parameters – effective shear strain (Γ), shear strain (γ), stretch K2 along intermediate strain axis Y – from margin to the centre of the shear band, show peaked distribution with a single maximum at the shear zone centre. This implies rheological-weakening/strain-softening induced localizing shear zone character. Kinematically heterogeneous strain distribution during brittle shearing varies from transpression dominated for the BSZ to transpression-to-transtension switchover for the SASZ. Demonstrably, contrasting cooling-exhumation, hitherto unexplored, characterizes post-accretionary tectonics along the paleo-suture zone.

Quartz preferred orientation in naturally deformed mylonitic rocks (Montalto shear zone–Italy): a comparison of results by different techniques, their advantages and limitations

In the geologic record, the quartz c-axis patterns are widely adopted in the investigation of crystallographic preferred orientations (CPO) of naturally deformed rocks. To this aim, in the present work, four different methods for measuring quartz c-axis orientations in naturally sheared rocks were applied and compared: the classical universal stage technique, the computer-integrated polarization microscopy method (CIP), the time-of-flight (TOF) neutron diffraction analysis , and the electron backscatter diffraction (EBSD). Microstructural analysis and CPO patterns of quartz, together with the ones obtained for feldspars and micas in mylonitic granitoid rocks, have been then considered to solve structural and geological questions related to the Montalto crustal scale shear zone (Calabria, southern Italy). Results obtained by applying the different techniques are discussed, and the advantages as well as limitations of each method are highlighted. Importantly, our findings suggest that patterns obtained by means of different techniques are quite similar. In particular, for such mylonites, a subsimple shear (40% simple shear vs 60% pure shear) by shape analysis of porphyroclasts was inferred. A general tendency of an asymmetric c-maximum near to the Z direction (normal to foliation) suggesting dominant basal slip, consistent with fabric patterns related to dynamically recrystallization under greenschist facies, is recognized. Rhombohedral slip was likely active as documented by pole figures of positive and negative rhombs (TOF), which reveal also potential mechanical Dauphine twinning. Results showed that the most complete CPO characterization on deformed rocks is given by the TOF (from which also other quartz crystallographic axes can be obtained as well as various mineral phases may be investigated). However, this use is restricted by the fact that (a) there are very few TOF facilities around the world and (b) there is loss of any domainal reference, since TOF is a bulk type analysis. EBSD is a widely used technique, which allows an excellent microstructural control of the user covering a good amount of investigated grains. CIP and US are not expensive techniques with respect the other kind of investigations and even if they might be considered obsolete and/or time-consuming, they have the advantage to provide a fine and grain by grain “first round” inspection on the investigated rock fabric.

P-T path development derived from shearband boudin microstructure

This work focuses on the development of a regional P-T-path from the Malpica-Lamego Ductile Shear Zone, NW Portugal, based on the microstructures of shearband boudins evolved during progressive simple shear. The combination of microstructural analysis, fluid inclusion studies, crystallographic preffered orientation and fractal geometry analyses, allows to link several stages in the internal evolution of the boudin to regional P-T conditions. The boudinage process is initiated under differential stress after the original layer achieved sufficient viscosity contrast relative to the surrounding matrix. Two main transformations occur simultaneously: i) change in the external shape with continuous evolution from tabular rigid body to sigmoidal asymmetric morphology (shearband boudin) and ii) localized dynamic recrystallization in the sharp-tips of the structure (acute edge of shearband boudin), and along the boudin's margin and grain boundaries. Smaller recrystallized grains, particularly in the sharp-tip domains, accommodate most of the external strain, and larger relict grains are preserved in the centre. Dynamic recrystallization under constant strain rates and strain partitioning inside the boudins is indicated by fractal geometry based on grain boundary and grain area analysis. Progressive deformation leads to the generation of structural and textural heterogeneous domains inside the boudins, and is recorded by quartz c-axis orientation analysis and fluid inclusion studies. The last deformation episode shows the final formation of the blunt-tip domain and internal secondary shear planes. The regional P-T path begins with the crystallization of andalusite after an internal shearband boudin dilation event and ends with quartz dynamic recrystallization on boudin tips. The main deformation stage (310/315 Ma) led to reactivation of internal secondary shear zones with sillimanite crystallization.

Quartz c-axis orientation patterns in fracture cement as a measure of fracture opening rate and a validation tool for fracture pattern models

We evaluate a published model for crystal growth patterns in quartz cement in sandstone fractures by comparing crystal fracture-spanning predictions to quartz c-axis orientation distributions measured by electron backscatter diffraction (EBSD) of spanning quartz deposits. Samples from eight subvertical opening-mode fractures in four sandstone formations, the Jurassic–Cretaceous Nikanassin Formation, northwestern Alberta Foothills (Canada), Cretaceous Mesaverde Group (USA; Cozzette Sandstone Member of the Iles Formation), Piceance Basin, Colorado (USA), and upper Jurassic–lower Cretaceous Cotton Valley Group (Taylor sandstone) and overlying Travis Peak Formation, east Texas, have similar quartzose composition and grain size but contain fractures with different temperature histories and opening rates based on fluid inclusion assemblages and burial history. Spherical statistical analysis shows that, in agreement with model predictions, bridging crystals have a preferred orientation with c-axis orientations at a high angle to fracture walls. The second form of validation is for spanning potential that depends on the size of cut substrate grains. Using measured cut substrate grain sizes and c-axis orientations of spanning bridges, we calculated the required orientation for the smallest cut grain to span the maximum gap size and the required orientation of the crystal with the least spanning potential to form overgrowths that span across maximum measured gap sizes. We find that within a 10° error all spanning crystals conform to model predictions. Using crystals with the lowest spanning potential based on crystallographic orientation (c-axis parallel to fracture wall) and a temperature range for fracture opening measured from fluid inclusion assemblages, we calculate maximum fracture opening rates that allow crystals to span. These rates are comparable to those derived independently from fracture temperature histories based on burial history and multiple sequential fluid inclusion assemblages. Results support the R. Lander and S. Laubach model, which predicts that for quartz deposited synchronously with fracture opening, spanning potential, or likelihood of quartz deposits that are thick enough to span between fracture walls, depends on temperature history, fracture opening rate, size of opening increments, and size, mineralogy, and crystallographic orientation of substrates in the fracture wall (transected grains). Results suggest that EBSD maps, which can be more rapidly acquired than measurement of tens to hundreds of fluid inclusion assemblages, can provide a useful measure of relative opening rates within populations of quartz-filled fractures formed under sedimentary basin conditions. Such data are useful for evaluating fracture pattern development models.

Ductility contrast induced by silicification in pelitic schist of the Ryoke metamorphic belt, Japan

Contrasting ductility is recognized in the rocks of Cretaceous Ryoke metamorphic belt in Iwakuni area, southwest Japan. Pelitic schist is ubiquitous in the region and differences in mineral assemblages mark increase in metamorphic grade. The area has been graded as chlorite-biotite zone in the north progressing into biotite- and muscovite-cordierite zones in the south. Pelitic schist near the boundary between the biotite- and muscovite-cordierite zones has undergone partial silicification to form whitish silicified schist layers which contain two types of quartz veins: those parallel to foliation in the host rock are called schistosity-concordant veins, and those inclined to host rock foliation, schistosity-discordant veins. In this study we examined the quartz structure in the silicified schist and in both types of veins to understand the ductility contrast induced by the silicification process. Crystallographic orientations of quartz in the veins and silicified schist rocks were studied using the Scanning Electron Microscopy (SEM) based Electron Back Scatter Diffraction (EBSD) technique. Quartz c-axis orientations in the silicified schist are nearly random, demonstrating an absence of post-silicification ductile deformation. Quartz grains in the schistosity-concordant veins have preferred c-axis orientations perpendicular to the schistosity indicating ductile shortening. In contrast, schistosity-discordant veins display distinct quartz c-axis fabric than that found in the schistosity-concordant veins. This is because the two types of host rocks exhibit a difference in ductility during deformation. The presence of deformed quartz veins in the undeformed silicified schist indicates transformation of the ductile pelitic schist into the brittle silicified schist at mid-crustal levels where these rocks originate, hence forming contrasting rock layers. Schistosity-concordant veins in the biotite-rich pelitic schist deformed with its host rock in a ductile manner while the schistosity-discordant veins in the neighboring silicified schist were left intact. Silicification of the pelitic schist may have been caused by the silica-rich geofluids produced by subsurface processes. Geofluids responsible for the occurrence of such mechanically contrasting layers mark an increase in seismic reflectivity at mid-crustal depths and may be potential reflectors of seismic waves giving rise to the so-called “bright spots”.

Quartz c-axis orientations of the silicified rock from the Kasuga gold deposit in the southern Satsuma Peninsula, Southwest Japan

Quartz c-axis orientations of the silicified rock from the Kasuga gold deposit in the southern Satsuma Peninsula, Southwest Japan

The Hengshan low-angle normal fault zone: Structural and geochronological constraints on the Late Mesozoic crustal extension in South China

Abstract We document in this paper a significant low-angle normal fault, namely the Hengshan detachment fault, which extends more than 150 km long in NNE orientation and bounds an elongated Early Cretaceous basin in Hunan Province, central South China. Detailed structural and geochronological analyses have been conducted along its southern segment, where a well exposed, ~ 3 km thick, flat ductile shear zone develops along the western margin of the Hengshan granitic massif. This zone is featured by shallow-dipping foliations of varied trend from NE to NW, which bear penetrative stretching lineations varying from NW to SW trend. Shear sense criteria indicate top-to-the-NW and top-to-the-SW motions along its northern and southern parts, respectively. Quartz c-axis orientations of mylonitic rocks from the shear zone exhibit asymmetric single or crossed girdle patterns, and the distributions of fabric point maxima suggest a simultaneous operation of basal and prism slips, indicating a deformation temperature ranging from 400 °C to 550 °C. One zircon U–Pb age of the sheared albitite and three muscovite 40 Ar/ 39 Ar ages of the mylonitic rocks indicate that the ductile shearing initiated at 136 Ma and lasted till 97 Ma. The zircon U–Pb dating results of five granitic samples from the Hengshan granitic pluton yield two phases of crystallization ages of 232–228 Ma and 150–151 Ma respectively, indicating two preceding magmatic events prior to the ductile shearing in the lower plate. Our structural and geochronological data allow defining the Hengshan massif as an extensional dome, rather than a metamorphic core complex as previously predicted, due to the lack of syn-tectonic plutonism and the origin of detachment fault associated with the reactivation of the inherited thrust. This study provides crucial evidence for understanding how the mid–upper crust deformed during the process of extension, which testifies to a coeval occurrence of non-coaxial deformation of the ductile flow in the middle crust and brittle faulting in the upper crust. It also sheds new insights into the timing of the regional crustal extension in South China, and permits to constrain its onset time to be ca. 136 Ma.

The effects of Dissolution–Precipitation Creep on quartz fabrics within the Purgatory Conglomerate, Rhode Island

Quartz Crystallographic Preferred Orientation (CPO) patterns are most commonly a result of deformation by dislocation creep. We investigated whether Dissolution–Precipitation Creep (DPC), a process that occurs at lower differential stresses and temperatures, may result in CPO in quartz. The Purgatory Conglomerate is part of the SE Narragansett basin where strain intensity increases from west to east and is associated with top-to-the-west transport and folding during the Alleghanian orogeny. Within the Purgatory Conglomerate, DPC led to quartz dissolution along cobble surfaces perpendicular to the shortening direction, and quartz precipitation in overgrowths at the ends of the cobbles (strain shadows), parallel to the maximum extension direction. Quartz c-axis orientations as revealed by Electron Backscatter Diffraction (EBSD) methods were random in all analyzed domains within the cobbles and strain shadows irrespective of the intensity of strain or metamorphic grade of the sample. Quartz dissolution probably occurred exclusively along the cobbles' margins, leaving the remaining grains unaffected by DPC. The fact that quartz precipitated in random orientations may indicate that the strain shadows were regions of little or no differential stress.

EBSD study of Quartz c-axis orientations in the silicified host rocks of the Kasuga gold deposit, Southwest Japan

The Kasuga High Sulfidation gold deposit is hosted in the silicified Late Miocene andesite of Nansatsu Volcanic Rocks in the southern part of the Satsuma Peninsula, Southwest Japan. Optical microscopic observations and X-ray fluorescence spectrometry (Horiba XGT-5000) analysis of two silicified andesite samples from the Kasuga ore body revealed that the rocks underwent different degrees of hydrothermal alteration and no tectonic deformation afterward. Crystallographic orientations of quartz composing the silicified andesite samples studied using Electron Backscatter Diffraction, EBSD (EDAX GE-1, TSL-OIM) displayed different trends in the c-axis orientations between the samples. The c-axes of quartz grains are mainly oriented sub-horizontal or plunging at low angles in the less-silicified andesite. The highly silicified sample has larger quartz grains with c-axes plunging at high angles. The difference in c-axis orientations is provably related to the silicification processes in the earlier and later stages of hydrothermal alteration. The results of this study suggest that quartz c-axis orientations of rocks deformed under low strain regimes are potentially inherited from initial undeformed state, for example crystallization from hydrothermal solutions.

Complex fringes around magnetite porphyroclasts: Growth and deformation history

Deformed strain fringes in iron formation rocks show complex quartz fiber patterns that grew alongside magnetite porphyroclasts embedded in a matrix of quartz and iron oxides during coaxial to non-coaxial deformation. These rocks have been deformed by a combination of processes involving microfracturing, pressure solution and dislocation glide at temperatures of approximately 300 °C. Detailed microstructural observation and crystallographic analysis show that quartz fiber growth is not controlled by the crystal faces. Quartz c-axis orientations of fibrous quartz indicate that these grains initially grew with their c-axes parallel to the fiber length. Late solid-state deformation of crystallized fibers comprised dislocation glide along basal plane and subsequent recrystallization. This produces aggregates of recrystallized grains consisting of equant to elongated quartz grains with straight and orthogonal grain boundaries.

Triassic high-strain shear zones in Hainan Island (South China) and their implications on the amalgamation of the Indochina and South China Blocks: Kinematic and 40Ar/ 39Ar geochronological constraints

A kinematic and geochronological study has been carried out on the Triassic high-strain shear zones in Hainan Island, the southern South China Block. There are WNW- and NE-trending high-strain shear zones with greenschist- to amphibolite-facies metamorphism in this island. Kinematic indicators suggest a dextral top-to-the-NNE thrust shearing for the WNW-trending high-strain shear zones and a sinistral top-to-the-SE thrust shearing for the NE-trending shear zones. The quartz c-axis orientations of mylonitic rocks exhibit the domination of basal slip and some activation of a rhombohedra gliding system. The timing of shearing for these shear zones has been constrained by the 40Ar/ 39Ar dating analyses of synkinematic minerals. Middle Triassic (242–250 Ma) and late Triassic–early Jurassic (190–230 Ma) have been identified for the WNW- and NE-trending shear zones, respectively. A synthesis of these kinematic and thermogeochronological data points to a two-stage tectonic model for Hainan Island, that is, top-to-the-NNE oblique thrusting at 240–250 Ma followed by top-to-the-SE oblique thrusting at 190–230 Ma. In combination with the available data from the southern South China and Indochina Blocks, it is inferred that South Hainan and North Hainan have affinity to the Indochina and South China Blocks, respectively. The tectonic boundary between South Hainan and North Hainan lies roughly along the WNW-trending Changjiang–Qionghai tectonic zone probably linking to the Song Ma and Ailaoshan zones. The middle Triassic structural pattern of Hainan Island is spatially and temporally compatible with those of the South China and Indochina Blocks, and thus might be a derivation from the amalgamation of the Indochina with South China Blocks in response to the closure of the Paleotethys Ocean and subsequent subduction/collision.

Metamorphic evolution of preserved Hercynian crustal section in the Serre Massif (Calabria–Peloritani Orogen, southern Italy)

This paper presents and discusses the results of an integrated structural and petrological study, in order to delineate the entire tectono metamorphic history of a still little-known crystalline fragment of the southern Hercynian European Belt, currently framed within the central Mediterranean region after the superposition of the Alpine tectonics. These results were obtained by correlating P– T constraints yielded step by step with the sequence of the identified blasto-deformational relationships in an intermediate continental crustal level outcropping in the southern Serre Massif (Calabria). This allowed a detailed P– T evolution characterised by a multi-stage metamorphic history to be reconstructed. Structural investigations showed the presence of a pervasive mylonitic foliation, that obliterated most of the previous metamorphic textures. This fabric contains kinematic indicators consistent with an average top-to-ENE–NE sense of shear in the present-day geographic coordinates. In addition, the occurrence of late-tectonic leucogranite rocks partly affected by sub-solidus deformation, cut in turn by later undeformed ones, allowed the final stages of the shearing event to be bracketed at the same time as the Late Hercynian magmatic activity in the area. Microstructural investigation by quartz c-axis orientation pattern analysis allowed the temperature of shearing to be constrained as occurring under greenschist to amphibolite facies conditions. The latter are set in relation with the influence of the heat deriving from the intrusion of the Late Hercynian granitoids. Lastly, pressure temperature ( P– T) pseudosection computations in the MnNaCaKFMASH system allowed a detailed P– T path to be reconstructed, consisting of an initial orogenic cycle characterised by a prograde lower amphibolite facies evolution, developing from P of 590 MPa at T of 500 °C to peak P– T conditions of 900 MPa at 530 °C. This stage was followed by retrograde quasi-adiabatic decompressional ( P = 400 MPa; T = 500 °C), evolving towards an extensional deep-seated shearing, with P of 300 MPa at T of 470 °C. This last orogenic stage played a role in favouring the intrusion of granitoid bodies, which were indeed found to be partly affected by sub-solidus non-coaxial deformation. Progressive emplacement of large volumes of granitoid bodies gave rise to a gradually distributed thermal metamorphic overprint with thermal peak conditions at P of 300 MPa and T of 685 °C. This episode was finally followed by a low-pressure cooling path ( P = 150 MPa; T = 500 °C), consistent with the final unroofing stage of the former crystalline basement complex. A detailed reconstruction of the tectono-metamorphic evolution of this Hercynian continental crustal portion allowed a Late Palaeozoic geodynamic scenario to be envisaged, in which tectonic and magmatic processes mutually interacted to define, in a feedback-type evolution, the tectono-thermal regime operating during the gravitational collapse of a previously thickened Hercynian crust.

Microstructural Investigation of Naturally Deformed Leucogneiss from an Alpine Shear Zone (Southern Calabria – Italy)

In order to reveal any correlation between the preferred orientation of quartz and progressive intensity of strain, a suite of deformed leucogneiss collected within a ductile shear zone outcropping in Calabria (southern Italy) was investigated. Based on the microstructural approach and matrix-clast relationships, the method applied here may be useful for appraising the connection between deformation mechanisms of constituent minerals and bulk textural properties of naturally sheared rocks. Accordingly, quartz c-axis orientation patterns were determined by image-assisted analysis. Results revealed a correlation between finite strain and textural features: As strain increases, the matrix develops at the expense of the clast counterpart, which is instead progressively reduced in both size and amount.

Structural signatures and 40Ar/ 39Ar geochronology of the Indosinian Xuefengshan tectonic belt, South China Block

To advance our understanding of the kinematics of the Xuefengshan tectonic belt (XFSTB) and the Mesozoic tectonics of the South China Block (SCB), this paper presents new data and interpretations from our structural studies and thermochronological dating. The XFSTB is characterized by greenschist-facies metamorphism and development of S and S–L tectonic fabrics. An asymmetric positive flower structural pattern is composed of NW–WNW- and SE–ESE-dipping cleavages, faults and shear zones. Kinematic indicators indicate a dominant top-to-WNW-thrusting with a sinistral strike-slip component on the ESE-dipping shear zones and top-to-ESE-back-thrusting on the WNW-dipping shear zones. The quartz c-axis orientations of mylonitic rocks exhibit monoclinic point-maximum asymmetry, indicative of a sinistral shear sense under the 〈a〉 basal gliding condition. The timing of the major deformation event has been constrained to the middle Triassic to early Jurassic (244–195 Ma) on the basis of 40Ar/ 39Ar geochronology and other geological observations. The structural characteristics of the XFSTB are probably related to an Indosinian oblique convergent zone, in combination with tectonic wedging and associated back thrusting, all above a low-angle, SE-dipping basal detachment. Oblique northwestward and upward movement along the basal detachment was partitioned into the NW–WNW-directed thrusting, SE–ESE-directed back-thrusting and subsidiary strike-slip movement along the NE- and NNE-trending faults and shear zones. The XFSTB may represent part of a huge structural fan between the Yangtze and Cathaysian blocks, and can be interpreted as a product of the Indosinian intracontinental collision involving a weak zone.

Flow patterns during exhumation of the Sambagawa metamorphic rocks, SW Japan, caused by brittle-ductile, arc-parallel extension

Abstract Mesoscopic and microscopic structural analyses of the high-pressure/temperature Sambagawa metamorphic rocks (accretion complexes), SW Japan, have been carried out. Deformation characterized by extreme layer-normal thinning and nearly arc-parallel stretching occurred during exhumation in the Late Cretaceous. Asymmetric quartz c -axis fabrics and orientation of shear band cleavages reveal a pervasive top-to-the-west sense of shear in the Sambagawa metamorphic rocks during the exhumation stage. The 3D strain geometries, inferred from quartz c -axis fabric patterns, vary from plane strain to flattening across the metamorphic belt. We hypothesize that the data are most reasonably explained by a model of counter-flow in the subduction channel. The counter-flow was induced by a left-lateral oblique subduction of the oceanic (Izanagi) plate, which was strongly coupled with the subducting sediments. The 3D strain geometries suggest that the counter-flow (i.e. simple shear in the model) must have been accompanied by some arc-normal ‘press’ component. The mode of deformation changed from ductile to brittle arc-parallel extension, when the rocks were elevated and cooled below the temperature condition for the brittle-ductile transition of quartz ( c. /it 300 °C). The normal faulting (i.e. brittle extension) at subgreenschist conditions was often accompanied by the precipitation of actinolite. This change in deformation mechanism with decreasing temperature is recorded by a conjugate set of normal faults found in the oligoclase-biotite zone in the study area, for which the palaeostress directions conform to the ductile strain geometries.

Extensional collapse of the late Neoproterozoic-early Palaeozoic East African-Antarctic Orogen in central Dronning Maud Land, East Antarctica

Abstract The East African-Antarctica Orogen resulted from the continent-continent collision of East and West Gondwana, or parts thereof, during the Pan-African event at c . 650–510 Ma. The collision overprinted large areas of older, mainly Mesoproterozoic, crust up to granulite facies grade in East Antarctica. The collision history is well documented by folding and thrusting, isothermal decompression and metamorphic zircon growth at c. 580–560 Ma (Pan-African I). The convergence was succeeded by an extensional phase, probably representing orogenic collapse. This Pan-African II event at c . 530–510 Ma is characterized by large-scale extensional structures, finally resulting in the post-tectonic intrusion of voluminous A2-type granitoids. In central Dronning Maud Land the Pan-African II event started with the intrusion of syntectonic igneous rocks within an overall extensional setting. Two new SHRIMP data from gabbro zircons of the Zwiesel Gabbro give ages of 521±5.6 and 527±5.1 Ma. These ages are interpreted as crystallization ages and confirm the interpretation that the gabbro was emplaced early during the Pan-African II event. The gabbro was intruded by a network of leucogranite dykes and veins. Whereas the gabbro appears entirely undeformed, the leucogranite dykes are strongly mylonitized along extensional shear zones, indicating pronounced strain partitioning of the gabbro complex. Within the leucogranite mylonites, large tension gashes developed during mylonitization, indicating very high strain rates. Quartz c-axis orientations from quartz of the tension gashes show a distinct cross-girdle that formed during pure shear deformation. Fluid inclusion data from the leucogranite mylonites and the associated tension gashes mainly reveal recrystallization-related intracrystalline CO 2 -dominant inclusions with relatively low densities of < 1 g cm −3 . The fluid inclusion data are interpreted to represent the last stages of a retrograde P-T path that is characterized by simultaneous cooling and decompression during extensional exhumation, probably succeeding the collapse of overthickened crust. A comparable orogenic collapse of the East African-Antarctic Orogen is reported from other parts of the orogen, such as from western Madagascar and the northern Arabian-Nubian Shield.

Quartz c-axes orientation determination using the rotating polarizer microscope

A fast and accurate optical method to calculate quartz c-axis orientations using the rotating polarizer stage and standard thin sections is presented. c-Axis orientations are calculated for each pixel and Achsenverteilungsanalyse (AVA) can easily be constructed to study problems that would normally require a prohibitive amount of tedious work. The computer controlled rotating polarizer stage replaces the polarizer and analyzer of the standard petrographic microscope and allows a thin section to remain fixed while the polarizing filters are rotated by stepper motors. Data are collected by stepping the polarizers through a 180° rotation, capturing a frame at each step and extracting information on the intensity of the pixel and position of the polarizers. The position data are used to calculate the trend or trend+180° of the quartz c-axes, while a simple mathematical relationship between intensity and plunge is derived which allows the plunges of c-axes to be calculated.

Quartz microstructures and c-axis preferred orientations in high-grade gneisses and mylonites around the Morin anorthosite (Grenville Province)

Quartz in deformed rocks from two large, high-grade shear zones around the Morin anorthosite (Morin terrane, Grenville Province) displays distinctive microstructures, as well as c-axis preferred orientations. In the west-dipping Morin shear zone, east of the Morin anorthosite, four distinct quartz microstructures (types 1–4) are identified, based on deformation features and grain size. The c-axis orientations are characterized by a single maximum near the stretching lineation and two maxima in type 1 microstructure, and by an asymmetrical, single girdle in type 2 microstructure. Quartz c axes show crossed-girdle pattern in type 3 microstructure. Both quartz microstructures and c-axis preferred orientations suggest that crystal–plastic slip and dynamic recrystallization are the dominant deformation mechanisms. The asymmetry of c-axis orientations with respect to the mylonitic foliation, as well as the substructures developed in quartz, indicates a dextral sense of shear in the Morin shear zone. Type 4 microstructure, which developed in some gneisses and granulites, is interpreted to record influence of postdeformation annealing by which quartz c-axis orientations were partially modified. In the north-northeast-trending, subvertical Labelle shear zone that separates the Morin terrane from the Mont-Laurier terrane, metamorphic assemblages and structural elements suggest that an early, sinistral strike-slip deformation occurred under granulite-facies conditions. This was overprinted by a late downdip movement of the Mont-Laurier terrane under retrogressive conditions. Quartz in felsic gneisses from this zone shows two types of microstructures: one is similar to type 4 from the Morin shear zone, the other is named type 5. Quartz c-axis orientations are complex and less systematic, due to overprinting by two episodes of deformation and possible annealing. These complexities limit the utility of quartz microstructures and c-axis data in the structural analysis of the Labelle shear zone.

The analysis of three-dimensional orientation data

For many observed orientation patterns of three-dimensional geological data no statistical tests are available that define the probability with which a measured sample represents the parent population. Consequently, there is no accepted definition of an appropriate sample size and patterns can only be compared qualitatively by visual inspection. Empirical solutions to these problems are proposed and applied to quartz c-axis orientation patterns. An estimate of the distribution of orientations in a parent population is provided by an orientation diagram prepared by contouring a measured sample represented on the surface of a sphere using a counting circle 100/n% of the area of the projected hemisphere, where n is the sample size. Contouring the sample as it is accumulated allows identification of the sample size beyond which there is no significant change in the measured pattern. Data from similar-sized samples of computer-simulated random patterns provide outside estimates of the likely differences between the measured sample and the parent population. Pairs of patterns are compared using the mean absolute difference between the matrices used to prepare the contoured diagrams. A measure of their similarity is provided by the magnitude of this difference as a function of the departure of the two patterns from randomness, as indicated by the amount of empty space present. Characterization of the traditionally recognized types of quartz c-axis orientation patterns, using the parameters of the orientation patterns or their normalized eigenvectors, is successful only with point maximum, partial girdle, girdle, and random orientation patterns.