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1.
Anisotropy of magnetic susceptibility (AMS) in micaceous quartzites with mean susceptibility (K
m) >50 × 10−6 SI units is known to be on account of the orientation distribution of the para/ferromagnetic minerals (e.g. micas, magnetite),
which comprise the minor phase in the rocks. However, the strain in such deformed micaceous quartzites is dominantly accommodated
by the quartz grains, which are the major phase in them. The objective of this paper is to explore the extent to which AMS
data from micaceous quartzites provide information about the shape of the strain ellipsoid. AMS analysis of 3 quartzite blocks
is performed, and the shape of the AMS ellipsoid is recorded to be oblate. From AMS data, the three principal planes of the
AMS ellipsoid are identified in each block and thin sections are prepared along them. Quartz grain shape (aspect ratio, R
q), intensity of quartz and mica shape preferred orientation (κq and κmi, respectively) and 2D strain (E) recorded by quartz are measured in each section. R
q, κq, κmi and E are all noted to be minimum in the section parallel to the magnetic foliation plane as compared to the other two sections.
This indicates that the quartz grains have oblate shapes in 3D and accommodated flattening strain, which is similar to the
shape of the AMS ellipsoid. The role of mica in causing Zener drag and pinning of quartz grain boundaries is discussed. It
is concluded that during progressive deformation, migration of pinned grain boundaries is inhibited. This causes enhanced
recrystallization at the grain boundaries adjacent to the pinned ones, thus guiding the shape modification of quartz grains.
A strong correlation is demonstrated between κq and κmi as well as κmi and E. It is inferred that fabric evolution of quartz was controlled by mica. Hence, the shape of the AMS ellipsoid, which is on
account of mica, provides information about shape of the strain ellipsoid. 相似文献
2.
糜棱岩韧性变形发生的应变局部化过程,尤其是多相糜棱岩第二相对基质相变形的影响一直是显微构造研究难点.研究表明糜棱岩借助颗粒边界滑移实现多相混合,形成多矿物相集合体.在多相糜棱岩内,第二相在基质相颗粒边界施加齐纳阻力,牵制基质相颗粒边界的迁移速率,破坏基质相颗粒的动态平衡过程,使基质相颗粒位于古应力计对应的颗粒粒度以下,导致基质相整体的表面积增大,促进扩散交换过程,提高了扩散蠕变,降低了基质相位错蠕变和结晶学优选方位(CPO)形成的效率,使变形机制从颗粒粒径不敏感蠕变机制(GSI)过渡为颗粒粒径敏感蠕变机制(GSS).另外,多相糜棱岩内的第二相具有诱导应变局部化的效应,使塑性应变局部化更为强烈,引起物质强度的变化,进而引起岩石变形过程和岩石圈流变行为的改变.选取秦岭群花岗质糜棱岩进行多相矿物糜棱岩定量化研究,结果显示花岗质糜棱岩伴随着云母含量的增多以及各相混合程度的增大,石英的颗粒粒度明显减小,CPO强度显著降低,基质相显微变形受第二相控制逐渐增强. 相似文献
3.
Kyuichi Kanagawa Hirobumi Shimano Yoshikuni Hiroi 《Journal of Structural Geology》2008,30(9):1150-1166
The mylonitization of the Pankenushi gabbro in the Hidaka metamorphic belt of central Hokkaido, Japan, occurred along its western margin at ≈600 MPa and 660–700 °C through dynamic recrystallization of plagioclase and a retrograde reaction from granulite facies to amphibolite facies (orthopyroxene + clinopyroxene + plagioclase + H2O = hornblende + quartz). The reaction produced a fine-grained (≤100 μm) polymineralic aggregate composed of orthopyroxene, clinopyroxene, quartz, hornblende, biotite and ilmenite, into which strain is localized. The dynamic recrystallization of plagioclase occurred by grain boundary migration, and produced a monomineralic aggregate of grains whose crystallographic orientations are mostly unrelated to those of porphyroclasts. The monomineralic plagioclase aggregates and the fine-grained polymineralic aggregates are interlayered and define the mylonitic foliation, while the latter is also mixed into the former by grain boundary sliding to form a rather homogeneous polymineralic matrix in ultramylonites. However in both mylonite and ultramylonite, plagioclase aggregates form a stress-supporting framework, and therefore controlled the rock rheology. Crystal plastic deformation of pyroxenes and plagioclase with dominant (100)[001] and (001)1/2 slip systems, respectively, produced distinct shape- and crystallographic-preferred orientations of pyroxene porphyroclasts and dynamically recrystallized plagioclase grains in both mylonite and ultramylonite. Euhedral to subhedral growth of hornblende in pyroxene porphyroclast tails during the reaction and its subsequent rigid rotation in the fine-grained polymineralic aggregate or matrix produced clear shape- and crystallographic-preferred orientations of hornblende grains in both mylonite and ultramylonite. In contrast, the dominant grain boundary sliding of pyroxene and quartz grains in the fine-grained polymineralic aggregate of the mylonite resulted in their very weak shape- and crystallographic-preferred orientations. In the fine-grained polymineralic matrix of the ultramylonite, however, pyroxene and quartz grains became scattered and isolated in the plagioclase aggregate so that they were crystal-plastically deformed leading to stronger shape- and crystallographic-preferred orientations than those seen in the mylonite. 相似文献
4.
MCT Zone of Alakhnanda valley is a major ductile shear zone in Garhwal Himalaya, which is characterised by different types
of mylonite rocks. On the basis of grain size and the percentage of matrix in the rock, zones comprising protomylonite, augen
mylonite, mylonite and ultramylonite have been identified. The study of microstructures, grain size and crystallographic preferred
orientation of quartz c-axis fabric reveals that the rocks of the MCT zone were deformed by a combination of intracrystalline creep (power law creep)
and grain boundary migration (sliding super plasticity). 相似文献
5.
The Eclogite Zone, of the Tauern Window is an exhumed subduction channel comprising eclogites with different grades of retrogression in a matrix of high-pressure metasediments. The rocks were exposed to 600 °C and 20–25 kbars, and then retrogressed during their exhumation, first under blueschist facies and later under amphibolite facies metamorphism. To gain insights into the deformation within the subduction channel during subduction and exhumation, both fresh and retrogressed eclogites, as well as the surrounding metasediments were investigated with respect to their deformation microstructures and crystallographic preferred orientations (CPOs). Pristine and retrogressed eclogites show grain boundary migration and subgrain rotation recrystallization microstructures in omphacite. A misorientation axes analysis reveals the activity of complementary deformation mechanisms including grain boundary sliding and dislocation creep. The omphacite CPOs of the eclogites correspond to dominant SL-fabrics characteristic of plane strain deformation, though there are local variations towards flattening or constriction within the paleosubduction channel. The glaucophane CPOs in retrogressed eclogites match those of omphacite, suggesting that a constant strain geometry persisted during exhumation at blueschist facies conditions. Plastic deformation of the host high-pressure metasediments outlasted that of the eclogites, as indicated by white mica fabrics and quartz CPO. The latter is consistently asymmetric, pointing to the operation of non-coaxial deformation. The microstructures and CPO data indicate a continuous plastic deformation cycle with eclogite and blueschist facies metamorphism related to subduction and exhumation of the different rock units. 相似文献
6.
The Moresby Seamount detachment (MSD) in the Woodlark Basin (offshore Papua New Guinea) is a large active low-angle detachment excellently exposed at the seafloor, and cutting through mafic metamorphic rocks. Hydrothermal infiltration of quartz followed by that of calcite occurred during cataclastic deformation. Subsequent deformation of these a priori softer minerals leads to mylonite formation in the MSD. This study aims at a better understanding of the deformation mechanism switch from cataclastic to plastic flow. Deformation fabrics of the fault rocks were analyzed by light-optical microscopy. Rheologically critical phases were mapped to determine distributions and area proportions, and EBSD was used to measure crystallographic preferred orientation (CPO). Strong calcite CPOs indicate dominant dislocation creep. Quartz CPOs, however, are weak and more difficult to interpret, suggesting at least some strain accommodation by diffusion creep mechanisms. When quartz aggregates are intermixed with the polymineralic mylonite matrix diffusion creep grain boundary sliding may be dominant. The syntectonic conversion from mafic cataclasites to more siliceous and carbonaceous mylonites induced by hydrothermal processes is a critical weakening mechanism enabling the MSD to at least intermittently plastic flow at low shear stresses. This is probably a crucial process for the operation of low-angle detachments in hydrated and dominantly mafic crust. 相似文献
7.
Timon F. Fliervoet Stanley H. White Martyn R. Drury 《Journal of Structural Geology》1997,19(12):1495-1520
Microstructural and textural investigations by scanning (SEM) and transmission electron microscopy (TEM) techniques have been performed on samples taken across two quartzo-feldspathic mylonite zones from the Redbank Deformed Zone, Central Australia. One has been deformed at greenschist-facies (GS), the second at amphibolite-facies (Am), conditions. With increasing strain the rock type changes from protomylonite to mylonite to ultramylonite. The protomylonites and mylonites consist of alternating quartz and polymineralic quartz-feldspar bands. At the highest strains a homogeneous, fine-grained polymineralic ultramylonite occurs. Shear-zone geometry and microscale structures indicate that these ultramylonites experienced higher strains and were weaker than the encapsulating protomylonites and mylonites. TEM and SEM studies of the ultramylonites reveal a rectangular to square grain shape, a continuous alignment of grain and interphase boundaries across several grain diameters, a grain size (GS 0.5 μm; Am 5–11 μm) less than the equilibrium subgrain size, and open and void-containing grain and interphase boundaries. Analysis of local textures by electron back-scatter diffraction (EBSD) in the SEM showed a very weak crystallographic preferred orientation (CPO) for the quartz. The grain misorientation relationships are not consistent, with dislocation creep being the dominant deformation mechanism. All structures are of the type expected if grain-boundary sliding processes had contributed significantly to the deformation. Consequently, the deformation of such quartzo-feldspathic rocks, and by implication the rheology of the Redbank Deformed Zone, must have been controlled by the mechanical properties of these fine-grained polymineralic ultramylonites, deforming by grain-boundary sliding processes. This is in contrast to the pure quartz bands which deformed by dislocation-creep mechanisms and were less important in the rheology of the Redbank Deformed Zone. 相似文献
8.
Quartzofeldspathic ultramylonites from the Alpine Fault Zone, one of the world's major, active plate boundary-scale fault zones have quartz crystallographic preferred orientations (CPO) and abundant low-angle (<10° misorientation) boundaries, typical microstructures for dislocation creep-dominated deformation. Geometrically necessary dislocation density estimates indicate mean dislocation densities of ∼109 cm−2. A significant proportion (∼30%) of grain boundaries (>10° misorientation) are decorated by faceted pores, commonly with uniformly-oriented pyramidal shapes. Only grain boundaries with >10° misorientation angles in polymineralic aggregates are decorated by pores. Mean grain boundary pore densities are ∼5 × 108 cm−2. Grain boundary pores are dissolution pits generated during syn-deformational transient grain boundary permeability, nucleating on dislocation traces at dilatant grain boundary interfaces. They have not been removed by subsequent grain boundary closure or annealing. Pore decoration could have led to grain boundary pinning, triggering a switch in the dominant deformation mechanism to grain boundary sliding, which is supported by evidence of CPO destruction in matrix quartz. Pore-decorated grain boundaries have significantly reduced surface area available for adhesion and cohesion, which would reduce the tensile and shear strength of grain boundaries, and hence, the bulk rock. Grain boundary decoration also significantly decreased the mean distance between pores, potentially facilitating dynamic permeability. Consequently, these microstructures provide a new explanation for strain weakening and evidence of fluid flow along grain boundaries in mylonites at mid-crustal conditions. 相似文献
9.
A detailed structural and microstructural analysis of the Miocene Raft River detachment shear zone (NW Utah) provides insight into the thermomechanical evolution of the continental crust during extension associated with the exhumation of metamorphic core complexes. Combined microstructural, electron backscattered diffraction, strain, and vorticity analysis of the very well exposed quartzite mylonite show an increase in intensity of the rock fabrics from west to east, along the transport direction, compatible with observed finite strain markers and a model of ``necking'' of the shear zone. Microstructural evidence (quartz microstructures and deformation lamellae) suggests that the detachment shear zone evolved at its peak strength, close to the dislocation creep/exponential creep transition, where meteoric fluids played an important role on strain hardening, embrittlement, and eventually seismic failure.Empirically calibrated paleopiezometers based on quartz recrystallized grain size and deformation lamellae spacing show very similar results, indicate that the shear zone developed under stress ranging from 40 MPa to 60 MPa. Using a quartzite dislocation creep flow law we further estimate that the detachment shear zone quartzite mylonite developed at a strain rates between 10−12 and 10−14 s−1. We suggest that a compressed geothermal gradient across this detachment, which was produced by a combination of ductile shearing, heat advection, and cooling by meteoric fluids, may have triggered mechanical instabilities and strongly influenced the rheology of the detachment shear zone. 相似文献
10.
Petrology of a non-classical Barrovian inverted metamorphic sequence from the western Arunachal Himalaya, India 总被引：1，自引：0，他引：1
In this study, we reconstruct the inverted metamorphic sequence in the western Arunachal Himalaya using combined structural and metamorphic analyses of rocks of the Lesser and Greater Himalayan Sequences. Four thrust-bounded stratigraphic units, which from the lower to higher structural heights are (a) the Gondwana rocks and relatively weakly deformed metasediments of the Bomdila Group, (b) the tectonically interleaved sequence of Bomdila gneiss and Bomdila Group, (c) the Dirang Formation and (d) the Se La Group are exposed along the transect, Jira–Rupa–Bomdila–Dirang–Se La Pass. The Main Central thrust, which coincides with intense strain localization and the first appearance of kyanite-grade partial melt is placed at the base of the Se La Group.Five metamorphic zones from garnet through kyanite, kyanite migmatite, kyanite-sillimanite migmatite to K-feldspar-kyanite-sillimanite migmatites are sequentially developed in the metamorphosed low-alumina pelites of Dirang and Se La Group, with increasing structural heights. Three phases of deformation, D1–D2–D3 and two groups of planar structures, S1 and S2 are recognized, and S2 is the most pervasive one. Mineral growths in all these zones are dominantly late-to post-D2, excepting in some garnet-zone rocks, where syn-D1 garnet growths are documented. Metamorphic isograds, which are aligned parallel to S2 were subsequently folded during D3. The deformation produced plane-non-cylindrical fold along NW–SE axis.In the garnet-zone, peak metamorphism is marked by garnet growth through the reaction biotite + plagioclase → garnet + muscovite. An even earlier phase of syn-D1 garnet growth occurred in the chlorite stability field with or without epidote. In the kyanite-zone metapelites, kyanite appeared via the pressure-sensitive reaction, garnet + muscovite → kyanite + biotite + quartz. Staurolite was produced in the same rock by retrograde replacement of kyanite following the reaction, garnet + kyanite + H2O → staurolite + quartz. These reactions depart from the classical kyanite- and staurolite-isograd reactions in low-alumina pelites, encountered in other segments of eastern Himalaya. In the metapelites, just above the kyanite-zone, melting begins in the kyanite field, through water-saturated and water-undersaturated melting of paragonite component in white mica. Leucosomes formed through these reactions are characteristically free of K-feldspar, with sodic plagioclase and quartz as the dominant constituents. With increasing structural height, the melting shifts to water-undersaturated melting of muscovite component of white mica, producing an early K-feldspar + kyanite and later K-feldspar + sillimanite assemblages and granitic leucosomes.Applications of conventional geothermobarometry and average P–T method reveal near isobaric (at P 8 kbar) increase in peak metamorphic temperatures from 550 °C in the garnet-zone to >700 °C for K-feldspar-kyanite-sillimanite-zone rocks. The findings of near isobaric metamorphic field gradient and by the reconstruction of the reaction history, reveal that the described inverted metamorphic sequence in the western Arunachal Himalaya, deviates from the classical Barrovian-type metamorphism. The tectonic implication of such a metamorphic evolution is discussed. 相似文献
11.
Shear and extensional veins formed during the reactivation of the Magdala shear system at Stawell in western Victoria, Australia,
contribute to the formation of the auriferous Central and Basalt Contact lodes. Within this shear system is a range of fault
rocks accompanied by steep-dipping (>65°) quartz-rich laminated shear veins and relatively flat-lying extensional veins. Both
vein sets appear to have been a primary source for the host rock permeability during fluid flow in a regime of significant
deviatoric stresses. The macro- and microstructures suggest that the dilatancy, that produced mineralized veins, formed under
conditions of overpressure generated by fluid infiltration late in a tectonic regime. A new microfabric analysis technique
is used to investigate the quartz-rich veins, which allows rapid integration of the microstructure with the crystallographic
preferred orientations (CPOs). Both the shear and extensional quartz veins have a random CPO with ∼120° dihedral angles between
the quartz–quartz grains, which is typical of a metamorphic equilibrium microfabric. The microstructures indicate that the
quartz has undergone extensive grain adjustment in the solid-state, with grain shape and size affected by interfacial solution
(pressure solution) effects. These features are consistent with inferences from experimental rock deformation studies, where
grain boundary migration is enhanced in a water-rich environment. The onset of solution-transfer processes (pressure solution)
developed as the quartz microfabric stabilized and continued to modify the CPO and microstructure significantly. It is concluded
that grain growth and pressure solution are coupled diffusive mass transfer processes, related to fluctuations in pore fluid
pressures in a region undergoing deformation at near lithostatic pressures. 相似文献
12.
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. 相似文献
13.
Lattice Preferred Orientation (LPO) of quartz and muscovite are measured in a micaceous quartzite using SEM based Electron Backscatter Diffraction (EBSD) analysis. The measurements were done in a thin section prepared parallel to the K1K3 plane of the anisotropy of magnetic susceptibility (AMS) ellipsoid, which is equivalent to the XZ plane of the strain ellipsoid. Bulk data from the sample reveal that quartz c-axis develop an oblique single girdle LPO pattern indicating a dextral sense of shear. A similar pattern is produced by poles to the basal planes (001) of muscovite, which implies that muscovite controls the LPO of quartz in the rock. Petrofabric analysis of quartz is performed in domains viz. (I) not containing muscovite, (II) between two muscovite grains, and (III) rim of quartz grains around a muscovite grain. Quartz LPO pattern in Domain-III is noted to be similar to the bulk LPO recorded from the whole sample, thus confirming that muscovite was critical in textural evolution of the rock. The study thus establishes that a minor mineral phase in a rock can control the LPO of the major mineral phase, thus playing a significant role in development of bulk texture of the rock. The advantages of domainal petrofabric analysis in evaluation of slip systems in minerals in different parts of a rock, and vorticity quantification are discussed. 相似文献
14.
Klaus WEBER 《《地质学报》英文版》2002,76(3):292-299
Optical, cathodoluminescence and transmission electron microscope (TEM) analyses were conducted on four groups of calcite fault rocks, a cataclastic limestone, cataclastic coarse-grained marbles from two fault zones, and a fractured mylonite. These fault rocks show similar microstructural characteristics and give clues to similar processes of rock deformation. They are characterized by the structural contrast between macroscopic cataclastic (brittle) and microscopic mylonitic (ductile) microstructures. Intragranular deformation microstructures (i.e. deformation twins, kink bands and microfractures) are well preserved in the deformed grains in clasts or in primary rocks. The matrix materials are of extremely fine grains with diffusive features. Dislocation microstructures for co-existing brittle deformation and crystalline plasticity were revealed using TEM. Tangled dislocations are often preserved at the cores of highly deformed clasts, while dislocation walls form in the transitions to the fine-grained 相似文献
15.
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. 相似文献
16.
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. 相似文献
17.
Tectonic pseudotachylytes, i.e. quenched friction-induced silicate melts, record coseismic slip along faults and are mainly reported from the brittle crust in association with cataclasites. In this study, we document the occurrence of recrystallization of quartz to ultrafine-grained (grain size 1–2 μm) aggregates along microshear zones (50–150 μm thick) in the host rock adjacent to pseudotachylytes from two different faults within quartzite (Schneeberg Normal Fault Zone, Eastern Alps), and tonalite (Adamello fault, Southern Alps) in the brittle crust. The transition from the host quartz to microshear zone interior includes: (i) formation of high dislocation densities; (ii) fine (0.3–0.5 μm) polygonization to subgrains defined by disordered to well-ordered dislocation walls; (iii) development of a mosaic aggregate of dislocation-free new grains. The crystallographic preferred orientation (CPO) of quartz towards the microshear zone shows a progressive misorientation from the host grain, by subgrain rotation recrystallization, to a nearly random CPO possibly related to grain boundary sliding. These ultrafine aggregates appear to be typically associated with pseudotachylytes in nature. We refer the crystal plastic deformation of quartz accompanied by dramatic grain size refinement to the coseismic stages of fault slip due to high differential stress and temperature transients induced by frictional heating. Microshear zones localized on precursory fractures developed during the stages of earthquake rupture propagation and the very initial stages of fault slip. Thermal models indicate that the process of recrystallization, including recovery processes, occurred in a time lapse of a few tens of seconds. 相似文献
18.
武功山东区存在一条大型韧性剪切带。鞘褶皱倒向以及旋转变形构造(如S-C面理组构、旋转碎斑系、雪球构造和粒内显微破裂构造等)显示此剪切带为由南向北逆冲推覆性质。砾石、黄铁矿还原斑和石英斑晶的有限应变分析表明剪切带西段和东段岩石分别以收缩型椭球和压扁型椭球变形为特征。剪切带的主要变形时代是早古生代,可能与早古生代华夏陆块和扬子陆块之间的碰撞造山作用有关。 相似文献
19.
Discontinuous grain growth of quartz in metacherts: the influence of mica on a microstructural transition 总被引：2，自引：0，他引：2
Abstract The microstructure of quartz in metacherts of the Ryoke metamorphic belt in central Japan develops from polygonal, through duplex to irregular with increasing metamorphic grade. The polygonal microstructure is composed of small (mostly 90–160 μm), equant, equigranular, polygonal quartz grains, whereas the irregular microstructure is characterized by large (>300 μm) grains with irregular grain boundaries. The duplex microstructure is a mixture of small polygonal and large irregular grains. The development of these microstructures is interpreted as being due to secondary recrystallization. The size of polygonal grains is greatly influenced by the presence of second-phase minerals, such as mica, whereas that of large irregular grains is unaffected by second-phase minerals. There seems to be a critical grain size for quartz to occur as polygonal aggregates: no polygonal aggregates occur in rocks with larger than the critical grain size. The size (about 140 μm) decreases slightly with increasing volume fraction of mica. The mean grain sizes of polygonal quartz ( D ) and coexisting mica ( d ) in the duplex microstructure are systematically related to the volume fraction of mica ( f ) by D = 0.728 d (1/ f )0.629 . 相似文献
20.