Foliation development and refraction in metamorphic rocks: reactivation of earlier foliations and decrenulation due to shifting patterns of deformation partitioning

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.     

Behaviour of rigid objects during deformation and metamorphism: a test using schists from the Bolton syncline, Connecticut, USA

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.     

Shear sense

Investigation of microstructural relationships in major movement zones in metamorphic rocks, where the sense of displacement is known from regional geological relationships, indicates numerous problems with current concepts of shear-sense criteria and their application. The direction of apparent shearing commonly conflicts from one criterion to another (e.g. from the symmetry of quartz c -axis orientation diagrams to the asymmetry of extensional crenulation cleavages). This implies that interpretations of shear sense along foliations from some mesoscale and microscale criteria have been erroneous.
A new approach to interpreting shear sense, involving the use of strain fields, resolves conflicts in mesoscopic and microscopic criteria and provides a method for determining coherent shear-sense histories extending back before the last shearing event for 'any foliated metamorphic rock'. It also provides a powerful tool for determining the structural/metamorphic path that a rock has followed within an orogen. For determination of the shear sense on the last foliation developed in a rock, this approach uses geometries developed around competent heterogeneities such as quartz pebbles, pegmatite pods, veins, porphyroclasts, porphyroblasts and breccia clasts. A shear-sense history is derived by applying this approach to earlier foliations preserved within the heterogeneities and their strain shadows.     

Spiral and staircase inclusion trail axes within garnet and staurolite porphyroblasts from schists of the Bolton Syncline, Connecticut: timing of porphyroblast growth and the effects of fold development

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.     

Testing models for the development of spiral-shaped inclusion trails in garnet porphyroblasts: to rotate or not to rotate, that is the question

Abstract The formation of spiral-shaped inclusion trails (SSITs) is problematical, and the two viable models for their formation involve opposite shear senses along the foliation in which the porphyroblasts are growing. One model argues for porphyroblast rotation, with respect to a geographically fixed reference frame, whereas the other argues for no such porphyroblast rotation, but instead rotation of the matrix foliation around the porphyroblast. Thus, porphyroblasts with SSITs cannot be used as shear-sense indicators until it is conclusively determined which model best explains them.
Any successful model must explain features associated with SSITs, including: (1) foliation truncation zones, (2) smoothly curving SSITs, (3) millipede microstructure, (4) total inclusion-trail curvature in median sections, (5) porphyroblasts with SSITs that have grown together, (6) evidence for relative porphyroblast displacements, (7) shear-sense indicators inside and outside porphyroblasts; (8) crenulations associated with porphyroblasts and (9) geometries in sections subparallel to spiral axes (axes of rotation). A detailed study of these features suggests that most, if not all, can be explained by both the rotational and non-rotational models, in spite of these models involving diametrically opposed movement senses. Therefore, geometrical analysis of individual porphyroblast microstructures may not determine which model best explains SSITs until the kinematics required to form these microstructures are better understood, in particular the sense of shear along a developing crenulation cleavage. Specific tests for determining the shear sense along crenulation cleavages are proposed, and results of such tests may conclusively resolve the debate over how SSITs form.     

Early formed regional antiforms and synforms that fold younger matrix schistosities: their effect on sites of mineral growth

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.     

Episodic porphyroblast growth in the Fleur de Lys Supergroup, Newfoundland: timing relative to the sequential development of multiple crenulation cleavages

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.     

Distinguishing and Correlating Multiple Phases of Metamorphism across Multiply Deformed Regions Using the Axes of Inclusion Trails in Porphyroblasts

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...     

Episodic gravitational collapse and migration of the mountain chain during orogenic roll‐on in the Himalayas

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.     

Secondary cleavages in ductile shear zones

Secondary cleavages developed at late stages in ductile shear zones show several features that are inconsistent with progressive simple shear in the zone. These are: the orientation of a single secondary cleavage oblique to the shear zone boundaries; conjugate sets with opposite senses of shear, and multiple sets with the same sense of shear. These features can be explained if the bulk flow is partitioned into slip along discrete failure planes parallel to the primary foliation (S), coaxial stretching along the foliation, and spin.     

The drawbacks of sectioning rocks relative to fabric orientations in the matrix: A case study from the Robertson River Metamorphics (Northern Queensland, Australia)

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.     

Garnet porphyroblast timing and behaviour during fold evolution: implications from a 3-D geometric analysis of a hand-sample scale fold in a schist

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.     

How useful are ‘millipede’ and other similar porphyroblast microstructures for determining synmetamorphic deformation histories?

ABSTRACT Oppositely concave microfolds (OCMs) in and adjacent to porphyroblasts can be classified into five nongenetic types. Type 1 OCMs are found in sections through porphyroblasts with spiral-shaped inclusion trails cut parallel to the spiral axes, and commonly show closed foliation loops. Type 2 OCMs, commonly referred to as ‘millipede’ microstructure, are highly symmetrical, the foliation folded into OCMs being approximately perpendicular to the overprinting foliation. Type 3 OCMs are similar to Type 2, but are asymmetrical, the foliation folded into OCMs being variably oblique to the overprinting foliation. Type 4 OCMs are highly asymmetrical, only one foliation is present, and this foliation is parallel to the local shear plane. Type 5 OCMs result from porphyroblast growth over a microfold interference pattern. Types 1 and 2 are commonly interpreted as indicating highly noncoaxial and highly coaxial bulk deformation paths, respectively, during porphyroblast growth. However, theoretically they can form by any deformation path intermediate between bulk coaxial shortening and bulk simple shearing. Given particular initial foliation orientation and timing of porphyroblast growth, Type 3 OCMs can also form during these intermediate deformation paths, and are commonly found in the same rocks as Type 2 OCMs. Type 4 OCMs may indicate highly noncoaxial deformation during porphyroblast growth, but may be difficult to distinguish from Type 3 OCMs. Thus, Types 1–3 (and possibly 4) reflect the finite strain state, giving no information about the rotational component of the deformation(s) responsible for their formation. Furthermore, there is a lack of unequivocal independent evidence for the degree of noncoaxiality of deformation (s) during the growth of porphyroblasts containing OCMs. Type 2 OCMs that occur independently of porphyroblasts or other rigid objects might indicate highly coaxial bulk shortening, but there is a lack of supporting physical or computer modelling. It is possible that microstructures in the matrix around OCMs formed during highly noncoaxial and highly coaxial deformation histories might have specific characteristics that allow them to be distinguished from one another. However, determining degrees of noncoaxiality from rock fabrics is a major, longstanding problem in structural geology.     

Formation of Foliations and their Related Minerals from Diagenetic to Medium-grade Metamorphic Rocks: A Case Study of the Hongyanjing and Liao-Ji Backarc Basins,China

Deciphering the relationship between polyphase tectonic foliations and their associated mineral assemblages is significant in understanding the process from diagenesis to low-/medium-/high-grade metamorphism. It can provide information related to strain, metamorphic conditions and overprinting relationships and so help reveal the tectonic evolution of orogenesis. In this study, we predominately focus on the formation of foliations and their related minerals, as developed in two separate basins. First of all, two stages of axial plane cleavages (S1 and S2) were recognized in the Hongyanjing inter-arc basin, the formation of the S1 axial plane cleavage is associated with mica rotation and elongation in mudstones in the local area. The pencil structure of S2 formed during the refolding phase, the minerals in the sedimentary rocks not changing their shape and orientation. Secondly, in the Liao-Ji backarc basin, foliations include diagenetic foliation (bedding parallel foliation), tectonic S1 foliation (secondary foliation or axial plane cleavage of S0 folding) and crenulation cleavage (S2). The formation mechanism of foliation changes from mineral rotation or elongation and mineral solution transfer in S1 to crystal-plastic deformation, dynamic recrystallization and micro-folding in S2. Many index metamorphic minerals formed from low-grade to medium-grade consist of biotites, garnets, staurolite and kyanite, constituting a typical Barrovian metamorphic belt. Accordingly, a new classification of foliation is presented in this study. The foliations can be divided into continuous and disjunctive foliations, based on the existence of microlithons, detectable with the aid of a microscope. Disjunctive foliation can be further sub-divided into spaced foliation and crenulation cleavage, according to whether (or not) crenulation (micro-folding) is present. The size of the mineral grains is also significant for classification of the foliations.     

Porphyroblast inclusion trails: the key to orogenesis

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.     

雪球状石榴子石变斑晶的形成机制:以南迦巴瓦地区雅鲁藏布江缝合带西侧石英片岩为例

雅鲁藏布江缝合带米林地区的石英片岩糜棱岩化强烈,线理及面理构造发育。S-C组构、"σ"残斑以及不对称褶皱等指示了上盘相对下盘向NW下滑的剪切运动趋势。电子背散射衍射(EBSD)测试结果表明:雪球状石榴子石变斑晶边部面理(S2)中石英包裹体晶格优选方位模式图指示的运动指向与石英岩基质面理(或外部面理;S3)中石英包裹体晶格优选方位模式图指示的运动指向一致,都是上盘向NW正滑。然而,雪球状石榴子石的核部(S1)石英包裹体优选方位(LPO)模式图指示相反运动指向。能量色散显微分析(EDS)测试结果表明石榴子石的成分环带显示连续生长环带特征。连接石榴子石核部面理(S1)可以恢复得到石英岩早期不对称褶皱形状的面理轨迹。这些说明文章样品中雪球状石榴子石变斑晶是生长在不对称褶皱之上的。此过程主要是剪切方向发生了旋转,而不是石榴子石自身旋转。这种雪球状石榴子石变斑晶的存在说明南迦巴瓦地区雅鲁藏布江缝合带西侧岩石最初经历向SE的逆冲作用,后期经历由SE向NW的拆离滑脱事件。     

Complex microstructures preserved in rocks with a simple matrix: significance for deformation and metamorphic processes

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.     

Deformation partitioning and porphyroblast rotation in meta-morphic rocks: a radical reinterpretation

Abstract Most porphyroblasts never rotate during ductile deformation, provided they do not internally deform during subsequent events, with the exception of relatively uncommon but spectacular examples of spiralling garnets. Instead, the surrounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast. Consequently, porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails, even where these structures have been rotated or obliterated in the matrix due to subsequent deformation. These relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subsequent deformation. They can also be used to determine the deformation history, as porphyroblasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component.     

The development of spiral‐shaped inclusion trails during multiple metamorphism and folding

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.     

S-C Mylonites

Two types of foliations are commonly developed in mylonites and mylonitic rocks: (a) S-surfaces related to the accumulation of finite strain and (b) C-surfaces related to displacement discontinuities or zones of relatively high shear strain. There are two types of S-C mylonites. Type I S-C mylonites, described by Berthé et al., typically occur in deformed granitoids. They involve narrow zones of intense shear strain which cut across (mylonitic) foliation.Type II S-C mylonites (described here) have widespread occurrence in quartz-mica rocks involved in zones of intense non-coaxial laminar flow. The C-surfaces are defined by trails of mica ‘fish’ formed as the result of microscopic displacement discontinuities or zones of very high shear strain. The S-surfaces are defined by oblique foliations in the adjacent quartz aggregates, formed as the result of dynamic recrystallization which periodically resets the ‘finite-strain clock’. These oblique foliations are characterized by grain elongations, alignments of segments of the grain boundary enveloping surfaces, and by trails of grains with similar c-axis orientations.Examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of S-C tectonites, and a deviation from the general tradition of purely geometric analysis of foliation and time relationships. Kinematic indicators such as those discussed here allow the recognition of kilometre-scale zones of intense non-coaxial laminar flow in crustal rocks, and unambiguous determination of the sense of shear.