首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 703 毫秒
1.
Three periods of mineral growth and three generations of spiral‐shaped inclusion trails have been distinguished within folded rocks of the Qinling‐Dabie Orogen, China, using the development of three successive and differently trending sets of foliation intersection axes preserved in porphyroblasts (FIAs). This progression is revealed by the consistent relative sequence of changes in FIA trends from the core to rim of garnet porphyroblasts in samples with multiple FIAs. The first and second formed sets of FIAs trend oblique to the axial planes of macroscopic folds that dominate the outcrop pattern in this region. The porphyroblasts containing these FIAs grew prior to the development of the macroscopic folds, yet the FIAs do not change orientation across the fold hinges. The youngest formed FIAs (set 3) lie subparallel to the axial planes of these folds and the porphyroblasts containing these FIAs formed in part as the folds developed. The deformation associated with all three generations of spiral‐shaped inclusion trails in garnet porphyroblasts involved the formation of subhorizontal and subvertical foliations against porphyroblast rims accompanied by periods of garnet growth; pervasive structures have not necessarily formed in the matrix away from the porphyroblasts. The macroscopic folds are heterogeneously strained from limb to limb, doubly plunging and have moderately dipping axial planes. The consistent orientation of Set 1 FIAs indicates that the development of spiral‐shaped inclusion trails in porphyroblasts with FIAs belonging to Set 2 did not involve rotation of the previously formed porphyroblasts. The consistent orientation of Sets 1 and 2 FIAs indicate that the development of spiral‐shaped inclusion trails in porphyroblasts with FIAs belonging to Set 3 did not involve rotation of the previously formed porphyroblasts during folding. This requires a fold mechanism of progressive bulk inhomogeneous shortening and demonstrates that spiral‐shaped inclusion trails can form outside of shear zones.  相似文献   

2.
The behaviour of spherical versus highly ellipsoidal rigid objects in folded rocks relative to one another or the Earth’s surface is of particular significance for metamorphic and structural geologists. Two common porphyroblastic minerals, garnet and staurolite, approximate spherical and highly ellipsoidal shapes respectively. The motion of both phases is analysed using the axes of inflexion or intersection of one or more foliations preserved as inclusion trails within them (we call these axes FIAs, for foliation inflexion/intersection axes). For staurolite, this motion can also be compared with the distribution of the long axes of the crystals. Schists from the regionally shallowly plunging Bolton syncline commonly contain garnet and staurolite porphyroblasts, whose FIAs have been measured in the same sample. Garnet porphyroblasts pre-date this fold as they have inclusion trails truncated by all matrix foliations that trend parallel to the strike of the axial plane. However, they have remarkably consistent FIA trends from limb to limb. The FIAs trend 175° and lie 25°NNW from the 020° strike of the axial trace of the Bolton syncline. The plunge of these FIAs was determined for six samples and all lie within 30° of the horizontal. Eleven of these samples also contain staurolite porphyroblasts, which grew before, during and after formation of the Bolton syncline as they contain inclusion trails continuous with matrix foliations that strike parallel to the axial trace of this fold. The staurolite FIAs have an average trend of 035°, 15°NE from the 020° strike of the axial plane of this fold. The total amount of inclusion trail curvature in staurolite porphyroblasts, about the axis of relative rotation between staurolite and the matrix (i.e. the FIA), is greater than the angular spread of garnet FIAs. Although staurolite porphyroblasts have ellipsoidal shapes, their long axes exhibit no tendency to be preferentially aligned with respect to the main matrix foliation or to the trend of their FIA. This indicates that the axis of relative rotation, between porphyroblast and matrix (the FIA), was not parallel to the long axis of the crystals. It also suggests that the porphyroblasts were not preferentially rotated towards a single stretch direction during progressive deformation. Five overprinting crenulation cleavages are preserved in the matrix of rocks from the Bolton syncline and many of these result from deformation events that post-date development of this fold. Staurolite porphyroblast growth occurred during the development of all of these deformations, most of which produced foliations. Staurolite has overgrown, and preserved as helicitic inclusions, crenulated and crenulation cleavages; i.e. some inclusion trail curvature pre-dates porphyroblast growth. The deformations accompanying staurolite growth involved reversals in shear sense and changing kinematic reference frames. These relationships cannot all be explained by current models of rotation of either, or both, the garnet and staurolite porphyroblasts. In contrast, we suggest that the relationships are consistent with models of deformation paths that involve non-rotation of porphyroblasts relative to some external reference frame. Further, we suggest there is no difference in the behaviour of spherical or ellipsoidal rigid objects during ductile deformation, and that neither garnet nor staurolite have rotated in schists from the Bolton syncline during the multiple deformation events that include and post-date the development of this fold.  相似文献   

3.
过去还无人指出过板块相对运动的方向与缓倾斜叶理、逆断层和断层上的线状指示物有直接关系,这是因为缓倾斜构造上的运动方向只和变厚了的造山地层的重力塌陷有关,它们和俯冲板块传递给仰冲板块的推力没有关系。缓倾斜叶理上的运动方向的线状指示物和斑状变晶中的叶理弯曲或叶理交切轴（FIA）并无直接关系,这是因为FIA的指向受缓倾斜叶理和斑状变晶边缘上产生的、近乎垂直的叶理之间的交切面控制。在班状变晶边缘上形成的、近乎垂直的叶理在基质中的方位可能在较大范围内变动,因为它们会在稍早期间形成的叶理再活化作用影响下发生转动或遭到破坏。斑状变晶边缘上近乎垂直的叶理,与形成于早期或晚期的缓倾斜叶理的交线,在后期的生长中被圈闭在班状变晶里,此交线规定出了FIA的方位,而与叶理上的运动方向无关。从美国佛蒙特州阿巴拉契亚山脉采集的FIA资料指出,在125km×35km的一片地区内,在该地岩层所发生的多次变形中,从未曾使早期形成的FIA组的方位发生变动。这种情况要求：后来的每一代褶皱都是由于渐进的。总体不均匀缩短作用造成的。这种情况表明：FIA保存着原始的运动方向,此方向未因以后的变形而转动。非洲板块与欧洲板块的相对运动方向和由阿尔卑斯期变质岩中叶理交切轴（FIAs）所指示  相似文献   

4.
Inclusion trails in garnet and albite porphyroblasts in the Fleur de Lys Supergroup preserve successive generations of microstructures, some of which correlate with equivalent microstructures in the matrix. Microstructure–porphyroblast relationships provide timing constraints on a succession of seven crenulation cleavages (S1–S7) and five stages of porphyroblast growth. Significant destruction and alteration of early fabrics has occurred during the microstructural development of the rock mass. Garnet porphyroblasts grew episodically through four growth stages (G1–G4) and preserve a succession of five fabrics (S1–S5) as inclusion trails. Garnet growth during each of the four growth phases did not occur on all pre-existing porphyroblasts, resulting in contrasting growth histories between individual garnet porphyroblasts from the same outcrop. Albite porphyroblasts grew during a single stage of growth and have overgrown microstructures continuous with the matrix. The garnet and albite porphyroblast inclusion trails record a succession of crenulation cleavages without any rotation of the porphyroblasts relative to other porphyroblasts in the population.
Complex microstructural histories are best resolved by preparing multiple oriented thin sections from a large number of samples of different rock types within the area of study. The succession of matrix foliations must be understood, as it provides the most useful time-frame against which to measure the relative timing of phases of porphyroblast growth. Comparable microstructures must be identified in different porphyroblasts and in the rock matrix.  相似文献   

5.
Detailed 3‐D analysis of inclusion trails in garnet porphyroblasts and matrix foliations preserved around a hand‐sample scale, tight, upright fold has revealed a complex deformation history. The fold, dominated by interlayered quartz–mica schist and quartz‐rich veins, preserves a crenulation cleavage that has a synthetic bulk shear sense to that of the macroscopic fold and transects the axis in mica‐rich layers. Garnet porphyroblasts with asymmetric inclusion trails occur on both limbs of the fold and display two stages of growth shown by textural discontinuities. Garnet porphyroblast cores and rims pre‐date the macroscopic fold and preserve successive foliation inflection/intersection axes (FIAs), which have the same trend but opposing plunges on each limb of the fold, and trend NNE–SSW and NE–SW, respectively. The FIAs are oblique to the main fold, which plunges gently to the WSW. Inclusion trail surfaces in the cores of idioblastic porphyroblasts within mica‐rich layers define an apparent fold with an axis oblique to the macroscopic fold axis by 32°, whereas equivalent surfaces in tabular garnet adjacent to quartz‐rich layers define a tighter apparent fold with an axis oblique to the main fold axis by 17°. This potentially could be explained by garnet porphyroblasts that grew over a pre‐existing gentle fold and did not rotate during fold formation, but is more easily explained by rotation of the porphyroblasts during folding. Tabular porphyroblasts adjacent to quartz‐rich layers rotated more relative to the fold axis than those within mica‐rich layers due to less effective deformation partitioning around the porphyroblasts and through quartz‐rich layers. This work highlights the importance of 3‐D geometry and relative timing relationships in studies of inclusion trails in porphyroblasts and microstructures in the matrix.  相似文献   

6.
Spiral garnet porphyroblasts are known to record lengthy periods of deformation and metamorphism by preserving single or multiple FIAs (Foliation Intersection Axis) formed normal to tectonic shortening directions. Thanks to technological advances in X-ray computed micro-tomography (XCMT), FIAs can now be readily determined in relatively large samples in contrast to previous methods that require the preparation of a set of radial vertical and horizontal thin sections of samples. XCMT scanning not only alleviates tedious thin section based procedures but also illuminates the complete internal architecture of a rock sample allowing three-dimensional (3D) quantitative shape analysis of an individual porphyroblast as well as precise measurement of FIAs. We applied the technique to a sample from the Hunza Valley in the Karakoram metamorphic complex (KMC), NW Himalayas, containing numerous garnet porphyroblasts with spiral-shaped inclusion trails. The XCMT imaging reveals an E–W trending FIA within the sample, which is consistent with orthogonal N–S collision of the India-Kohistan Island Arc with Asia. Garnet long axes (XGT) have variable plunges that define a broad sub-vertical maximum and a small sub-horizontal maximum. The XGT principle maxima lie at N-090 and N-120. Smaller maxima lie at N-020 and N-340. Geometric relationships between XGT axes and FIA orientation in the sample suggest that porphyroblast shapes are controlled by the geometry of the lens-shaped microlithons in which they tend to nucleate and grow. The orientation of inclusion trails and matrix foliations in the sample are correlated with three discrete tectono-metamorphic events that respectively produced andalusite, sillimanite and kyanite in the KMC. Late staurolite growth in the sample reveals how the rocks extruded to the surface via a significant role of roll-on tectonics, which can be correlated with the Central Himalayas.  相似文献   

7.
In the Littleton Formation, garnet porphyroblasts preserve three generations of growth that occurred before formation of the Bolton Syncline. Inclusion trails of foliations overgrown by these porphyroblasts are always truncated by the matrix foliation suggesting that garnet growth predated the matrix foliation. In contrast, many staurolite porphyroblasts grew synchronously with formation of the Bolton Syncline. However, local rim overgrowths of the matrix foliation suggest that some staurolite porphyroblasts continued to grow after development of the fold during younger crenulation producing deformations. The axes of curvature or intersection of foliations defined by inclusion trails inside the garnet porphyroblasts lie oblique to the axial plane of the Bolton Syncline but do not change orientation across it. This suggests the garnets were not rotated during the subsequent deformation associated with fold development or during even younger crenulation events. Three samples also contain a different set of axes defined by curvature of inclusion trails in the cores of garnet porphyroblasts suggesting a protracted history of garnet growth. Foliation intersection axes in staurolite porphyroblasts are consistently orientated close to the trend of the axial plane of the Bolton Syncline on both limbs of the fold. In contrast, axes defined by curvature or intersection of foliations in the rims of staurolite porphyroblasts in two samples exhibit a different trend. This phase of staurolite growth is associated with a crenulation producing deformation that postdated formation of the Bolton Syncline. Measurement of foliation intersection axes defined by inclusion trails in both garnet and staurolite porphyroblasts has enabled the timing of growth relative to one another and to the development of the Bolton Syncline to be distinguished in rocks where other approaches have not been successful. Consistent orientation of foliation intersection axes across a range of younger structures suggests that the porphyroblasts did not rotate relative to geographical coordinates during subsequent ductile deformation. Foliation intersection axes in porphyroblasts are thus useful for correlating phases of porphyroblastic growth in this region.  相似文献   

8.
The three-dimensional geometry of spiral inclusion trails from the Canton Schist were measured to determine whether the spirals were a product of porphyroblast rotation within a shear zone, or porphyroblast growth during a series of overprinting fold events. The spiral inclusion trails are composed of three separate, sub-planar inclusion trail surfaces occupying texturally distinct parts of the porphyroblasts. These surfaces are correlated across a >10 km2 area using textural criteria and relative timing. Measured patterns of inclusion trail orientation and asymmetry suggest they did not form by porphyroblast rotation within a non-coaxial shear zone. Rather, the porphyroblasts grew during three successive overprinting fold events (F2–F4), and the spiral inclusion trails represent the accumulated curvature associated with folding of successive axial plane foliations. The data show that spiral garnets are not peculiar to shear zones, and can form by overprinting crenulations and folds. This is consistent with the common occurrence of spiral garnets in multiply-deformed, regionally metamorphosed fold belts.  相似文献   

9.
Recent studies have used the relative rotation axis of sigmoidal and spiral‐shaped inclusion trails, known as Foliation Inflexion/Intersection Axis (FIA), to investigate geological processes such as fold mechanisms and porphyroblast growth. The geological usefulness of this method depends upon the accurate measurement of FIA orientations and correct correlation of temporally related FIAs. This paper uses new data from the Canton Schist to assess the variation in FIA orientations within and between samples, and evaluates criteria for correlating FIAs. For the first time, an entire data set of FIA measurements is published, and data are presented in a way that reflects the variation in FIA orientations within individual samples and provides an indication of the reliability of the data. Analysis of 61 FIA trends determined from the Canton Schist indicate a minimum intrasample range in FIA orientations of 30°. Three competing models are presented for correlation of these FIAs, and each of the models employ different correlation criteria. Correlation of FIAs in Model 1 is based on relative timing and textural criteria, while Model 2 uses relative timing, orientation and patterns of changing FIA orientations, and Model 3 uses relative timing and FIA orientation as correlation criteria. Importantly, the three models differ in the spread of FIA orientations within individual sets, and the number of sets distinguished in the data. Relative timing is the most reliable criterion for correlation, followed by textural criteria and patterns of changing FIA orientations from core to rim of porphyroblasts. It is proposed that within a set of temporally related FIAs, the typical spread of orientations involves clustering of data in a 60° range, but outliers occur at other orientations including near‐normal to the peak distribution. Consequently, in populations of FIA data that contain a wide range of orientations, correlation on the basis of orientation is unreliable in the absence of additional criteria. The results of this study suggest that FIAs are best used as semiquantitative indicators of bulk trends rather than an exact measurement for the purpose of quantitative analyses.  相似文献   

10.
In the metamorphic cores of many orogenic belts, large macroscopic folds in compositional layering also appear to fold one or more pervasive matrix foliations. The latter geometry suggests the folds formed relatively late in the tectonic history, after foliation development. However, microstructural analysis of four examples of such folds suggests this is not the case. The folds formed relatively early in the orogenic history and are the end product of multiple, near orthogonal, overprinting bulk shortening events. Once large macroscopic folds initiate, they may tighten further during successive periods of sub-parallel shortening, folding or reactivation of foliations that develop during intervening periods of near orthogonal shortening. Reactivation of the compositional layering defining the fold limbs causes foliation to be rotated into parallelism with the limbs.Multiple periods of porphyroblast growth accompanied the multiple phases of deformation that postdated the initial development of these folds. Some of these phases of deformation were attended by the development of large numbers of same asymmetry spiral-shaped inclusion trails in porphyroblasts on one limb of the fold and not the other, or larger numbers of opposite asymmetry spirals on the other limb, or similar numbers of the same asymmetry spirals on both limbs. Significantly, the largest disparity in numbers from limb to limb occurred for the first of these cases. For all four regional folds examined, the structural relationships that accompanied these large disparities were identical. In each case the shear sense operating on steeply dipping foliations was opposite to that required to originally develop the fold. Reactivation of the folded compositional layering was not possible for this shear sense. This favoured the development of sites of approximately coaxial shortening early during the deformation history, enhancing microfracture and promoting the growth of porphyroblasts on this limb in comparision to the other. These distributions of inclusion trail geometries from limb to limb cannot be explained by porphyroblast rotation, or folding of pre-existing rotated porphyroblasts within a shear zone, but can be explained by development of the inclusion trails synchronous with successive sub-vertical and sub-horizontal foliations.  相似文献   

11.
Understanding the relationships of inclusion trail geometries in porphyroblasts relative to matrix foliations is vital for unravelling complex deformation and metamorphic histories in highly tectonized terranes and the approach used to thin sectioning rocks is critically important for this. Two approaches have been used by structural and metamorphic geologists. One is based on fabric orientations with sections cut perpendicular to the foliation both parallel (P) and normal (N) to the lineation, whereas the other uses geographic orientations and a series of vertical thin sections. Studies using P and N sections reveal a simple history in comparison with studies using multiple-vertical thin sections. The reason for this is that inclusion trails exiting the porphyroblasts into the strain shadows in P and N sections commonly appear continuous with the matrix foliation whereas multiple vertical thin sections with different strikes reveal that they are actually truncated. Such truncations or textural unconformities are apparent from microstructures, textural relationships, compositional variations and FIA (foliation intersection axis) trends. A succession of four FIA trends from ENE–WSW, E–W, N–S to NE–SW in the Robertson River Metamorphics, northern Queensland, Australia, suggests that these truncations were formed because of the overprint of successive generations of orthogonal foliations preserved within porphyroblasts by growth during multiple deformation events. At least four periods of orogenesis involving multiple phases of porphyroblast growth can be delineated instead of just the one previously suggested from an N and P section approach.  相似文献   

12.
Schists from the foothills of the Central Sierra Nevada contain one dominant matrix foliation and yet four phases of growth of both cordierite and andalusite porphyroblasts can be distinguished. These occurred early during four separate deformation events that formed successive steep and shallow foliations. A fifth deformation event pre-dates the growth of all porphyroblasts studied. The multiple phases of porphyroblast growth allow correlation of structures across and along the region. A repeated pattern of deformation, in terms of the curvature of earlier foliations against the overprinting one, allows samples containing porphyroblasts with simpler inclusion trail geometries to be interpreted with confidence. The large-scale fold structures in this region formed before or during the second of the five deformation events recorded by the porphyroblasts. However, the matrix foliation is predominantly a product of the fourth deformation, which has commonly reactivated or re-used older foliations, and is dominated by east-side-up shear. The intervening third deformation produced locally intense foliations and was accompanied by top-to-the-east shear. The very weak fifth deformation produced weak crenulations with subhorizontal axial planes and was coaxial. Multiple phases of episodic but synchronous growth of cordierite and andalusite were produced by the KFMASH univariant equilibrium Ms+Chl+Qtz=And+Crd+Bt+H2O. The rocks crossed this reaction at a pressure just below the intersection with the KFMASH divariant equilibrium Ms+Chl+Qtz=Crd+Bt+H2O; the latter being overstepped in favour of the former as there is no evidence for cordierite growth prior to andalusite in these rocks. Subsequent multiple episodes of synchronous growth of cordierite and andalusite indicate that the possible variation in P–T during subsequent deformations was not large. This requires the high-amplitude macroscopic fold to form prior to porphyroblast growth and then be simply tightened and modified by the younger deformations.  相似文献   

13.
An ～W–E belt of maximum bulk horizontal shortening (the orogen core) moved North relative to the overlying crust to form the Himalayan Syntaxes due to roll‐on of this portion of the Indian plate. This displacement occurred below a lengthy succession of gently dipping decollements that formed episodically at a depth of ～30 km along the orogen core due to numerous periods of gravitational collapse and spreading of the overlying ductile crust. Successively developed basal decollements were deformed when continued bulk horizontal shortening of the orogen core below reasserted dominance over the effects of gravitational collapse above causing refolding about steeply dipping axial planes. This resulted in northwards migration of the orogen core above depths of ～30 km causing rocks metamorphosing at depths of ～22 km on the north side of the orogen core to be moved to its south side with no change in depth as roll‐on progressed. Garnet porphyroblasts record this lengthy history of lateral migration across the orogen within their inclusion trails. The ～6.4 kbar average pressures accompanying it were obtained from the Mn, Fe and Ca contents of successive garnet cores. Garnet grew at depths of ～22 km until movement towards the surface initiated on successively developed decollements that accommodated the volume constraints of gravitational collapse and spreading on both sides of the orogen. The speed of extrusional displacement increased the further the rocks migrated from the orogen core developing mylonitic schists around the porphyroblasts. This truncated inclusion trails against all matrix foliations as the porphyroblasts were carried towards the surface. Indeed, these rocks were multiply deformed during at least four distinct periods of deformation after mylonitization began and prior to exposure above the Main Central Thrust (MCT). Three or more sub‐vertical and sub‐horizontal foliations were formed during each of the five changes in FIA trend (foliation inflection/intersection axes in porphyroblasts) preserved in these rocks. The inclusion trail asymmetries and P‐T of garnet core growth accompanying each FIA reveal that the first four changes in FIA trend, which define periods of tectonism about one direction of horizontal bulk shortening (relative plate motion), occurred on the north side of the orogen core. The fifth occurred on the south side of the orogen core and the switch in shear sense on gently dipping foliation planes that resulted from this shift to the south eventually led to the development of the MCT. When magnetic anomaly 22 that formed in the Southern Indian Ocean Ridge is taken into account, these five changes in FIA trend correlate markedly with changes in the motion of India relative to a constant Eurasia from 50 to c. 25 Ma. They reveal that Eurasia moved NNW during FIAs 1, 3 and 4 and SSE during FIA 5 when the shear sense on gently dipping foliations switched to top to the S. They suggest collision of India with Eurasia took place at 50 Ma, immediately prior to the development of FIA 1.  相似文献   

14.
Abstract Reactivation of early foliations accounts for much of the progressive strain at more advanced stages of deformation. Its role has generally been insufficiently emphasized because evidence is best preserved where porphyroblasts which contain inclusion trails are present. Reactivation occurs when progressive shearing, operating in a synthetic anastomosing fashion parallel to the axial planes of folds, changes to a combination of coarse- and finescale zones of progressive shearing, some of which operate antithetically relative to the bulk shear on a fold limb. Reactivation of earlier foliations occurs in these latter zones. Reactivation decrenulates pre-existing or just-formed crenulations, generating shearing along the decrenulated or rotated pre-existing foliation planes. Partitioning of deformation within these foliation planes, such that phyllosilicates and/or graphite take up progressive shearing strain and other minerals accommodate progressive shortening strain, causes dissolution of these other minerals. This results in concentration of the phyllosilicates in a similar, but more penetrative manner to the formation of a differentiated crenulation cleavage, except that the foliation can form or intensify on a fold limb at a considerable angle to the axial plane of synchronous macroscopic folds. Reactivation can generate bedding-parallel schistosity in multideformed and metamorphosed terrains without associated folds. Heterogeneous reactivation of bedding generates rootless intrafolial folds with sigmoidal axial planes from formerly through-going structures. Reactivation causes rotation or ‘refraction’of axial-plane foliations (forming in the same deformation event causing reactivation) in those beds or zones in which an earlier foliation has been reactivated, and results in destruction of the originally axial-plane foliation at high strains. Reactivation also provides a simple explanation for the apparently ‘wrong sense’, but normally observed ‘rotation’of garnet porphyroblasts, whereby the external foliation has undergone rotation due to antithetic shear on the reactivated foliation. Alternatively, the rotation of the external foliation can be due to its reactivation in a subsequent deformation event. Porphyroblasts with inclusion trails commonly preserve evidence of reactivation of earlier foliations and therefore can be used to identify the presence of a deformation that has not been recognized by normal geometric methods, because of penetrative reactivation. Reactivation often reverses the asymmetry between pre-existing foliations and bedding on one limb of a later fold, leading to problems in the geometric analysis of an area when the location of early fold hinges is essential. The stretching lineation in a reactivated foliation can be radically reoriented, potentially causing major errors in determining movement directions in mylonitic schistosities in folded thrusts. Geometric relationships which result from reactivation of foliations around porphyroblasts can be used to aid determination of the timing of the growth of porphyroblasts relative to deformation events. Other aspects of reactivation, however, can lead to complications in timing of porphyroblast growth if the presence of this phenomenon is not recognized; for example, D2-grown porphyroblasts may be dissolved against reactivated S1 and hence appear to have grown syn-D1.  相似文献   

15.
Porphyroblast inclusion trails: the key to orogenesis   总被引：8，自引：0，他引：8  
Detailed microstructural analysis of inclusion trails in hundreds of garnet porphyroblasts from rocks where spiral-shaped inclusion trails are common indicates that spiral-shaped trails did not form by rotation of the growing porphyroblasts relative to geographic coordinates. They formed instead by progressive growth by porphyroblasts over several sets of near-orthogonal foliations that successively overprint one another. The orientations of these near-orthogonal foliations are alternately near-vertical and near-horizontal in all porphyroblasts examined. This provides very strong evidence for lack of porphyroblast rotation.
The deformation path recorded by these porphyroblasts indicates that the process of orogenesis involves a multiply repeated two-stage cycle of: (1) crustal shortening and thickening, with the development of a near-vertical foliation with a steep stretching lineation; followed by (2) gravitational instability and collapse of this uplifted pile with the development of a near-horizontal foliation, gravitational spreading, near-coaxial vertical shortening and consequent thrusting on the orogen margins. Correlation of inclusion trail overprinting relationships and asymmetry in porphyroblasts with foliation overprinting relationships observed in the field allows determination of where the rocks studied lie and have moved within an orogen. This information, combined with information about chemical zoning in porphyroblasts, provides details about the structural/metamorphic ( P-T-t ) paths the rocks have followed.
The ductile deformation environment in which a porphyroblast can rotate relative to geographic coordinates during orogenesis is spatially restricted in continental crust to vertical, ductile tear/transcurrent faults across which there is no component of bulk shortening or transpression.  相似文献   

16.
Abstract Low-pressure/high-temperature (low-P/high-T) metamorphic rocks of the Cooma Complex, southeastern Australia, show evidence of an anticlockwise pressure-temperature-time-deformation (P-T-t-D) path, similar to those of some other low-P/high-T metamorphic areas of Australia. Prograde paths are reasonably well constrained in cordierite-andalusite schists, cordierite-K-feldspar gneisses and andalusite-K-feldspar gneisses. These paths are inferred to be convex to the temperature axis, involving increase in pressure with increase in temperature. Evidence of the retrograde path is inconclusive, but is consistent with approximately isobaric cooling, as are available isotopic data on the Cooma Granodiorite, which indicate initially rapid cooling following attainment of peak temperatures. The retrograde path is inconsistent with either a clockwise P-T-t-D path involving rapid or even moderate decompression immediately post-dating the peak of metamorphism, or a path in which the retrograde component simply reverses the prograde component, because both these paths should cross reactions forming cordierite from aluminosilicate, for which no evidence has been observed. Determination of the deformational-metamorphic history of the complex is not straightfoward and depends on careful examination of critical samples. Evidence necessary for successful elucidation of the prograde, and part of the retrograde, deformational-metamorphic history in the Cooma Complex includes: (1) sequentially grown porphyroblasts that can be timed relative to surrounding foliations; (2) partial replacement microstructures providing relative timing of metamorphic reactions that cannot be timed relative to foliation development; (3) a tectonic marker foliation (S4 at Cooma) that allows correlation of foliations from one location to another; and (4) single samples containing all of the foliations and all generations of porphyroblast growth within a single metamorphic zone. The latest two or three foliations involve low strain accumulation, allowing relative timing relationships between foliations and porphyroblasts to be more clearly determined. Sequential porphyroblast growth and foliation development in the cordierite-andalusite schists is examined for situations involving rotation and non-rotation of porphyroblasts relative to geographically fixed coordinates. Although the number of foliations developed varies in the rotational situation, depending on the deformation history proposed, the sequential order of porphyroblast growths does not differ from the non-rotational situation. Thus, whether or not porphyroblasts rotated in the Cooma rocks, the sequence of reactions, and therefore P-T-t paths inferred from the relative timing of porphyroblast growths, remain the same, for the deformational histories evaluated.  相似文献   

17.
Successions of FIAs(foliation inflection/intersection axes preserved within porphyroblasts) provide a relative time scale for deformation and metamorphism.In-situ dating of monazite grains preserved as inclusions within garnet and staurolite porphyroblasts within the foliations defining each FIA from such successions provides a rigorous approach to grouping ages that formed over extended periods of deformation and metamorphism.Matching age and FIA progressions confirms the suitability of this approach pl...  相似文献   

18.
Porphyroblasts of garnet and plagioclase in the Otago schists have not rotated relative to geographic coordinates during non-coaxial deformation that post-dates their growth. Inclusion trails in most of the porphyroblasts are oriented near-vertical and near-horizontal, and the strike of near-vertical inclusion trails is consistent over 3000 km2. Microstructural relationships indicate that the porphyroblasts grew in zones of progressive shortening strain, and that the sense of shear affecting the geometry of porphyroblast inclusion trails on the long limbs of folds is the same as the bulk sense of displacement of fold closures. This is contrary to the sense of shear inferred when porphyroblasts are interpreted as having rotated during folding.
Several crenulation cleavage/fold models have previously been developed to accommodate the apparent sense of rotation of porphyroblasts that grew during folding. In the light of accumulating evidence that porphyroblasts do not generally rotate, the applicability of these models to deformed rocks is questionable.
Whether or not porphyroblasts rotate depends on how deformation is partitioned. Lack of rotation requires that progressive shearing strain (rotational deformation) be partitioned around rigid heterogeneities, such as porphyroblasts, which occupy zones of progressive shortening or no strain (non-rotational deformation). Therefore, processes operating at the porphyroblast/matrix boundary are important considerations. Five qualitative models are presented that accommodate stress and strain energy at the boundary without rotating the porphyroblast: (a) a thin layer of fluid at the porphyroblast boundary; (2) grain-boundary sliding; (3) a locked porphyroblast/matrix boundary; (4) dissolution at the porphyroblast/matrix boundary, and (5) an ellipsoidal porphyroblast/shadow unit.  相似文献   

19.
Microstructural measurements of FIAs in staurolite reveal at least 3 periods of growth in the Proterozoic Colorado Front Range and 5 in the Paleozoic Western Maine. Dated monazite inclusions in staurolite have an absolute age of 1760±12 Ma (FIA 1), 1720±7 Ma (FIA 2), 1682±18 Ma (FIA 3) in Colorado, and 408±10 Ma (FIA 2), 388±8 Ma (FIA 3), 372±6 Ma (FIA 4), 352±4 Ma (FIA 5) in Maine, supporting the multiple periods of deformation and metamorphism indicated by the FIA succession in each region. Multiple phases of growth by similar reactions in the same as well as in diverse adjacent rocks in both regions suggest that PT and X are not the only factors controlling the commencement and cessation of metamorphic reactions. The FIAs preserved by the staurolite porphyroblasts indicate that the local partitioning of deformation at the scale of a porphyroblast was the eventual controlling factor on whether or not the staurolite forming reactions took place.  相似文献   

20.
Microstructural measurements of FIAs in staurolite reveal at least 3 periods of growth in the Proterozoic Colorado Front Range and 5 in the Paleozoic Western Maine. Dated monazite inclusions in staurolite have an absolute age of 1760±12 Ma (FIA 1), 1720±7 Ma (FIA 2), 1682±18 Ma (FIA 3) in Colorado, and 408±10 Ma (FIA 2), 388±8 Ma (FIA 3), 372±6 Ma (FIA 4), 352±4 Ma (FIA 5) in Maine, supporting the multiple periods of deformation and metamorphism indicated by the FIA succession in each region. Multiple phases of growth by similar reactions in the same as well as in diverse adjacent rocks in both regions suggest that PT and X are not the only factors controlling the commencement and cessation of metamorphic reactions. The FIAs preserved by the staurolite porphyroblasts indicate that the local partitioning of deformation at the scale of a porphyroblast was the eventual controlling factor on whether or not the staurolite forming reactions took place.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号