Preferred Orientation Of Quartz Research Articles

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.

Geology, geochemistry, and U–Pb geochronology of the Archean (2.74 Ga) Serra do Rabo granite stocks, Carajás Metallogenetic Province, northern Brazil

The Carajás region, located in the southeastern part of the Amazon Craton, has been considered one of the most important mineral provinces in the world. The Serra do Rabo Granite (SRG) crops out near the eastern termination of the Carajás fault as two granite stocks, elongated approximately in an E–W direction, concordant with the regional structures. Leucomicrocline granite, hornblende–microcline granite, biotite–hornblende–microcline granite, hornblende syenogranite, and subordinate aplite are identified. The granites are grayish pink and coarse to medium grained and have mainly hypidiomorphic granular texture. Granophyric textures are common. The accessory minerals are ilmenite, apatite, zircon, allanite, and rare pyroxene. The SRG rocks are either massive or foliated, with a slightly anastomosed continuous S 1 foliation (E–W/subvertical) outlined by the preferred orientation of quartz, feldspars, and mafic minerals. Locally, decimeter- to meter-wide mylonite/ultramylonite bands ( S 1m) occur along the E–W foliation. The S 1 foliation was developed under higher temperatures than those of the S 1m mylonite foliation. The SRG structural evolution was controlled by progressive deformation under decreasing temperature, indicative of syntectonic emplacement. The SRG also has relatively high SiO 2, K 2O, and Na 2O contents; high FeO*/(FeO*+MgO) ratios; high Zr, Ba, Nb, and Ga; and very high rare-earth element contents. The chemical signature is moderately alkaline and metaluminous, comparable to that of the A-type, A2, and ALK-3 granites. The origin of the SRG magmas may be related to the partial melting of crustal sources, such as previously metamorphosed calc-alkaline granites. The SRG crosscuts supracrustal rocks, promoting low-pressure/high-temperature metamorphism. The interaction between regional compressive stresses and the ballooning effect of the granite stocks promoted slight aureole flattening and rheological changes in the supracrustal rocks. The U–Pb zircon age of 2743±1.6 Ma is interpreted as the age of zircon crystallization, granite stock emplacement, and regional horizontal shortening. Other 2.7 Ga syntectonic alkaline granites (e.g. Estrela, Plaquê, Planalto) have been reported in the region.

Textures and microstructures of naturally deformed amphibolites from the northern Cascades, NW USA: methodology and regional aspects

Abstract Neutron texture analyses of quartz-bearing and quartz-free amphibolite mylonites from the Windy Pass thrust, Cascades Crystalline Core (Washington/USA) reveal pronounced textures of plagioclase and clino-amphiboles (hornblende, cummingtonite) but no preferred orientation of quartz. A reliable strategy for amphibolite fabric analysis is presented by a systematic analytical approach to the experimental diffraction data processing. Clino-amphiboles show transitional textures between ideal single crystal orientations and axial symmetric great circle distributions. Plagioclase reveals a -axes distributions scattering along a great circle approximating the foliation plane as well as a -axes maxima close to the macroscopic lineation. Correlation of the textures with grain shape anisotropies of horn-blende and plagioclase and comparison with data from the literature suggest that the texture variations are due to different strain regimes rather than due to different crystallographic reorientation mechanisms. The kinematic directions deduced from the microfabric correlate well with the regional tectonic interpretations. In contrast, individual deformation paths are not yet established for the different tectonic units, as the significance of the separating Windy Pass thrust requires further structural analysis and fabric studies.

The deformation and recrystallization of quartz in a mylonite zone, Central Australia

Hydrolytic weakening in quartz has been extensively demonstrated by experimental deformation of single crystals and aggregates. This paper describes the deformation and recrystallization microstructures and preferred orientations of quartz in a mylonite zone separating granulite facies (0.2% H 2O) from amphibolite facies (1.0% H 2O) acid gneisses. The transition from slightly deformed country rock on both sides to the ultimate product of mylonitization (a phyllite) is described and the following major differences are noted: 1. (1) The strain prior to recrystallization is higher on the granulite side. 2. (2) Misorientations across deformation-band boundaries are much higher on the granulite side. 3. (3) Subgrains and new grains are considerably smaller at the same stage of recrystallization on the granulite side. 4. (4) Preferred orientation of [0001] developes more rapidly with respect to strain on the amphibolite side. 5. (5) There is a closer orientation relationship between host and recrystallized (new) grains on the granulite side. The microstructures and preferred-orientation development on both sides are related to concurrent ductile deformation, dynamic recovery and recrystallization processes. The differences between the two sides is attributed to the difference in bulk H 2O content of the rocks and a resultant difference in strain rate. The suggested effect of water on recovery processes is favoured over its possible role in slip processes.

Discussion

D r A. L. H arris remarked that the recognition by Drs Powell and Johnson of two orogenic periods in the western Highlands emphasized the need for strict criteria in the recognition of pre- and post tectonic gneissification. Each orogenic episode is apparently associated with the development of pelitic gneisses, referred to on the Geological Survey maps as 'migmatites'; the tracing of the Caledonian overprint on rocks metamorphosed in the Precambrian will depend partly on the recognition of deformed 'migmatites.' The quartzofeldspathic fraction of the pelitic gneisses occurring as pegmatitic veinlets or lits commonly conforms to minor fold structures, but this in itself is an insufficient criterion of deformation. Such 'folded' veinlets may be the result of post deformation segregation along already folded surfaces. The nature of the fabric within the quartzofeldspathic lits is critical. In the 'old' pegmatites, demonstrated by Dr van Breemen and his colleagues to have suffered D 2 deformation, there is in addition to a mica foliation, a strong planar fabric defined by the preferred dimensional orientation of quartz and feldspar. The fact that this has survived prolonged Caledonian metamorphism in at least mid-amphibolite facies, is a strong argument in favour of the use of such fabrics as the only safe criterion for pre-tectonic gneisses. Clearly if the preferred orientation of quartz and feldspar could be broken down by prolonged static crystallization, the deformed gneisses would come closely to resemble the product of post tectonic gneissification. Their recognition as pre-tectonic gneisses would then be a matter of subjective judgement.

An X-ray Technique for Distinguishing Between Detrital and Secondary Quartz in the Fine-grained Fraction of Sedimentary Rocks

ABSTRACT Fine-grained disseminated quartz in sedimentary rocks may often be characterized as being secondary in origin on the basis of its X-ray diffractogram. The existence of a diffractogram intensity ratio I100/I101 substantially greater than that obtained from a random sample of crushed quartz appears to be a sufficient (but not necessary) criterion for recognizing quartz of secondary origin in the absence of a volcanic component. The intensity ratio anomaly reflects preferred orientation of quartz in the sample mount and results from the presence of faces on the (100) form (prism face) on some of the grains. These results are consistent with both geologic and oxygen isotopic evidence.

Syntectonic Recrystallization and Preferred Orientation of Quartz

Predictions of preferred orientations of quartz to be expected during syntectonic recrystallization are made in terms of a recently proposed hypothesis. The new predictions are made considering combinations of slip mechanisms in various stress fields. Quartz maximum II is shown to represent stability in terms of combined basal and r or z slip. If r or z slip alone is important, maximum IV should be formed.

X-Ray Analysis of Preferred Orientation in Fine-Grained Quartz Aggregates

The preferred orientations of quartz in a number of small fine-grained specimens of experimentally deformed and recrystallized flint were determined with a Norelco pole figure goniometer. The device was modified by introducing small (0.5-1.0 mm.) collimators to examine restricted areas of flat specimens in both the reflection and transmission modes. Scans in the two modes were combined to yield complete pole figures. The preferred orientations in the specimens are shown to be axially symmetric and are given by intensity profiles extending 0°-90° from the symmetry axis. Profiles for as many as fifteen diffraction peaks from individual specimens are subjected to spherical harmonic analysis to obtain a complete determination of the preferred orientation (the crystallite distribution function), which is represented concisely in the form of an inverse pole figure. From the inverse pole figure the preferred orientation of any crystallographic direction (such as the [0001] direction in quartz, which is difficult to measure directly) may be generated. Similarly, the preferred orientations of positive and negative trapezohedral and rhombohedral forms (such as the pairs , respectively), which cannot be resolved by standard X-ray measurements, are separately determined. Earlier work with polymers and metals using these methods is reviewed. The methods are illustrated by means of two flint cylinders recrystallized during compression at high temperature and pressure; one (GB-3) is relatively homogeneous and the other (GB-11) shows marked variation in preferred orientation. There are differences in the orientations of the positive and negative forms in both specimens. The preferred orientations may be characterized as a mixture of two components: (a) parallelismof c = 0001 with the compression axis and (b) parallelism of the poles of the positive unit rhomb with the compression axis. The latter component dominates in the more coarsely recrystallized (hotter) regions of specimen GB-11.

Ripple marks and pseudo-ripple marks in deformed quartzite

Study of regionally metamorphosed Precambrian quartzites from Tasmania indicates that ripple marks may be preserved in rocks that have been sufficiently deformed to have a preferred orientation of quartz and mica. Ripple marks may be distinguished from tectonic structures of similar appearance (pseudo-ripple marks) only if sufficient diagnostic sedimentary features are retained. Petrofabric analysis is inconclusive in distinguishing ripples from pseudo-ripples because tectonic and sedimentary structures may be similar in orientation.

The Boyndie syncline of the Dalradian of the Banffshire Coast

Summary The paper deals with the great fold affecting the Dalradian rocks exposed on the Banffshire coast, for which the name Boyndie syncline is now proposed. It is a large asymmetrical syncline whose axis runs north-north-east and emerges on the coast west of Banff. Its western limb is steeply inclined and, as Read showed, youngs eastward; over short stretches the rocks of this limb are intensely corrugated by subsidiary folds. The eastern limb is, as a whole, very gently inclined and is flexed to form the shallow Turriff syncline. Much of this limb is violently corrugated on a small scale. Variations in the attitude of the axial planes of small folds within the Boyndie syncline are described and discussed. It is suggested that the corrugation of the eastern limb is due to the squeezing out of incompetent beds from the core of the major fold. The central part of the steep western limb of the syncline has been intensely deformed and boudinage, mullioning and allied processes have led to the formation in this zone of a tectonic melange in which elonaated or lenticular masses of rock are enclosed in a less competent matrix. The axiil plunge of small-scale structures in this zone is very variable and is often steep. Preliminary petrofabric studies show that the preferred orientation of quartz is much stronger here than elsewhere in the fold. An oblique cleavage is imposed on rocks to the east and west of the intensely deformed sector. The possible significance of the structures seen on the Banffshire coast is discussed and their bearing on current views on the tectonics of North-East Scotland is debated.