Differentiated Crenulation Cleavage Research Articles

Distinct metamorphic evolution of alternating silica-saturated and silica-deficient microdomains within garnet in ultrahigh-temperature granulites: An example from Sri Lanka

Here we report the occurrence of garnet porphyroblasts that have overgrown alternating silica-saturated and silica deficient microdomains via different mineral reactions. The samples were collected from ultrahigh-temperature (UHT) metapelites in the Highland Complex, Sri Lanka. In some of the metapelites, garnet crystals have cores formed via a dehydration reaction, which had taken place at silica-saturated microdomains and mantle to rim areas formed via a dehydration reaction at silica-deficient microdomains. In contrast, some other garnets in the same rock cores had formed via a dehydration reaction which occurred at silica-deficient microdomains while mantle to rim areas formed via a dehydration reaction at silica-saturated microdomains. Based on the textural observations, we conclude that the studied garnets have grown across different effective bulk compositional microdomains during the prograde evolution. These microdomains could represent heterogeneous compositional layers (paleobedding/laminations) in the precursor sediments or differentiated crenulation cleavages that existed during prograde metamorphism. UHT metamorphism associated with strong ductile deformation, metamorphic differentiation and crystallization of locally produced melt may have obliterated the evidence for such microdomains in the matrix. The lack of significant compositional zoning in garnet probably due to self-diffusion during UHT metamorphism had left mineral inclusions as the sole evidence for earlier microdomains with contrasting chemistry.

The structural interpretation of domainal trace lineation: an example from the Mona Complex, Anglesey

Domainal trace lineation is defined by the intersection of two foliations, where at least one of these foliations is a composition banding formed by deformational processes (e.g. differentiated crenulation cleavage). The diagnostic feature of this lineation is a pronounced striping observed on the foliation surface that intersects the banding. An example of domainal trace lineation is displayed on the dominant foliation surfaces in New Harbour Group metasediments, near Rhoscolyn, Anglesey. The lineation is produced by the intersection of two foliations of domainal type, which produces prismatic domains resembling stretched objects. This probably explains the previous identifications of the lineation as a stretching lineation. The new interpretation of the lineation has potentially important implications for the deformation history of the New Harbour Group.

Radical changes in bulk shortening directions during orogenesis: Significance for progressive development of regional folds and thrusts

Polyphase folding and metamorphism of rocks in the eastern Mount Isa Inlier resulted from four distinct orogenic cycles. These consist of NE–SW shortening (O 00) from 1672 to ∼1648 Ma, involving a succession of five phases of deformation, which was responsible for the development of the first generation NW–SE striking macroscopic folds, plus cleavages that are mainly only preserved in porphyroblasts. N–S shortening (O 1) followed to 1600 Ma, involving at least three phases of deformation. This period of N–S directed orogenesis rotated large portions of the NW–SE trending regional folds into W–E trends and the dominance of locally well-preserved W–E trending sub-vertical matrix foliations as well as the intensification of foliation parallel to bedding. Subsequent W–E shortening (O 2) from 1600 to ∼1515 Ma was accompanied by the development of N–S trending folds. These dominate the Mount Isa Inlier and where best developed involved the formation of at least three near orthogonal foliations. Where these large-scale folds regionally are weakly developed or not visible, only one sub-vertical foliation has developed. Finally NW–SE shortening (O 3) produced locally developed SW–NE trending folds and weak crenulations until ∼1500 Ma. The asymmetry of steep to gentle pitch changes in inclusion trails preserved in garnet porphyroblasts that developed during O 00 indicates thrusting to the SW. Similar inclusion trail asymmetries in garnet, staurolite and andalusite porphyroblasts that developed in O 1, indicate thrusting to the N confirming a change in thrust direction across the Wonga Duchess belt to the W. Formation of the Meerenurker roof thrust near Mount Isa by thrusting to the SW during O 00 with subsequent development of the Kokkalukkanurker duplex below by thrusting to the S during O 1 reduces the displacement required for the roof thrust from 234 to ∼65 kms. Foliations preserved in O 00 garnets are truncated by foliations associated with the development of O 1 in the matrix. O 1 garnet, staurolite and andalusite porphyroblasts preserve inclusion trails that are commonly truncated against a differentiated crenulation cleavage in the matrix that formed during O 2. The inclusion trails entrapped within O 00 and O 1 generated porphyroblasts reveal that the foliations that progressively developed in the matrix during these orogenies were composite fabrics resulting from the effects of multiple discrete orthogonal increments in each case. Numerous phases of sub-vertical foliation development due to horizontal bulk shortening were followed by phases of gently dipping foliation formation due to gravitational collapse that accompanied thrusting during O 00 and O 1.

Foliations and shear sense: A modern approach to an old problem

Argument about shear on foliations began in the mid 19th century and continues to the present day. It results from varying interpretations of what takes place during the development of different types of foliations ranging from slaty cleavages through differentiated crenulation cleavages, schistosity and gneissosity to mylonites. Computer modelling, quantitative microstructural work and monazite dating have provided a unique solution through access to the history of foliation development preserved by porphyroblasts. All foliations involve shear in their development and most can be used to derive a shear sense. The shear sense obtained is consistent between foliation types and accords with recent computer modelling of these structures preserved within porphyroblasts relative to those in the matrix. The asymmetry of curving foliation into a locally developing new one allows determination of the shear sense along the latter foliation in most rocks. The problem of shear on fold limbs and parallelism of foliation and the flattening plane of the strain ellipse is resolved through the partitioning of shearing and shortening components of deformation into zones that anastomose around ellipsoidal domains lying parallel to the XY plane. Conflicts in shear sense occur if multiple reuse or reactivation of foliations is not recognized and allowed for but are readily resolved if taken into account.

Contact metamorphism and partial melting of Dalradian pelites and semipelites in the southern sector of the Etive aureole

Synopsis Contact-metamorphic mineral assemblages are described from Dalradian non-graphitic pelites and semipelites in the southern sector of the Etive thermal aureole, adjacent to the Quarry Diorite ring intrusion. Six metamorphic zones were mapped on the basis of incoming index minerals: I, Biotite; II, Cordierite; III, Andalusite+K-feldspar; IV, Corundum; V, Spinel; VI, Sillimanite. Garnet and orthopyroxene occur locally in high-grade quartz-bearing assemblages. Rocks of the Biotite and Cordierite zones retain their regional-metamorphic textures to a large extent, particularly S 2 crenulation cleavage. Above the Andalusite+K-feldspar isograd the rocks are hornfelses, but relict features of S 2 persist. Rocks of the upper Spinel and Sillimanite zones are agmatic/stromatic migmatites which preserve evidence of local partial melting in the form of S 2 -parallel leucosomes and thin fracture-filling vein leucosomes. All leucosomes are K-feldspar-rich. Vein leucosomes consist mainly of leucogranite with a metaluminous composition and, in the Sillimanite Zone, locally contain crystals of peritectic orthopyroxene and/or cordierite. The compositions of coexisting minerals, determined by EPMA, were used to constrain metamorphic reactions and, with equilibrium thermodynamic calculations, to refine estimates of the P-T conditions of metamorphism. Isograd reactions can generally be modelled satisfactorily in the CNKFMASH system as an isobaric sequence at P=2.2 kbar. An exception is the spinel isograd which occurs at anomalously low T due to the partitioning of Zn into spinel. Estimated isograd temperatures for the And+Kfs, Crn, Spl, melt and Sil isograds are 610°C, 630°C, 660°C, 680°C and 705°C, respectively. Calculated dehydration equilibria yield low values of water activity in the Sillimanite Zone, confirming that partial melting in the innermost aureole occurred under fluid-absent conditions. Leucosome textures and structures suggest that the siliceous microlithons of the former S 2 differentiated crenulation cleavage were the likely sites of melting and provided channels for the movement of melt into brittle fractures to form the vein leucosomes.

Progressive deformation partitioning and deformation history: Evidence from millipede structures

The progressive development and migration of patterns of deformation partitioning at all scales through the rock matrix commonly destroys any record of the ductile history associated with previous events making the problem of similar structures developing through multiple pathways generally intractable. However, records of the small-scale geometries that form as deformation commences and begins to partition through a rock are routinely trapped and protected by porphyroblasts because these large crystals nucleate and/or grow at this time. This allows examination of the geometry of microstructures formed at the start of deformation partitioning that were destroyed by the same event in the matrix, or which formed during an event prior to any preserved in the matrix. Porphyroblasts locally preserve oppositely concave microfolds (“millipedes”), which, in all examples that we have found, exclusively indicate a deformation history of bulk inhomogeneous shortening. Very similar structures have been formed experimentally during inhomogeneous simple shear but can readily be distinguished from those trapped in porphyroblasts that form during progressive bulk inhomogeneous shortening. Oppositely concave microfolds in some porphyroblasts reveal that deformation near orthogonal to a previously developed foliation occurred by axial plane shear driven rotation that led to rapid reactivational “card-deck-like” collapse of the pre-existing foliation. Differentiated crenulation cleavages may result from the same process providing yet another reason for the cessation of porphyroblast growth at the start of differentiation.

Microstructural evidence for N–S shortening in the Mount Isa Inlier (NW Queensland, Australia): the preservation of early W–E-trending foliations in porphyroblasts revealed by independent 3D measurement techniques

3D microstructural analyses of porphyroblast inclusion trails using both the ‘asymmetry switch’ method for determining foliation intersection axes preserved in porphyroblasts (FIAs) and the recently developed ‘FitPitch’ method, reveal W–E- and N–S-trending FIA sets in the White Blow Formation of the Mount Isa Inlier. Each method reveals two subsets of FIAs centered on each of these major trends. These were distinguished based on the relative timing, trend, and orientation of inclusion trail patterns. Thirty-six samples were analyzed using both techniques and produced very similar results. Pitches of the inclusion trails preserved within the porphyroblasts in vertically oriented thin-sections and trends in horizontal sections yield distinct near-orthogonal modal orientations from all the analyzed samples. This indicates that the porphyroblasts host successive fabrics as crenulation foliations and did not rotate with respect to geographical axes. W–E- and N–S-trending FIAs have been obtained from both garnet and staurolite porphyroblasts hosting differentiated crenulation cleavages. Garnet and staurolite growth during bulk north–south shortening recorded the development of multiple foliations and an associated succession of metamorphic events at middle-amphibolite facies conditions that predates the metamorphic history generally recognized in this terrain. This period of bulk shortening and associated metamorphism formed during a period of orogenesis called O 1. W–E shortening formed N–S striking foliations that preserve a period of orogenesis (O 2), and another succession of metamorphism involving more phases of porphyroblast growth preserving N–S-trending FIAs. Overprinting of successive FIA trends (WSW–ENE, WNW–ESE, NNW–SSE, and SSW–NNE) suggests a relative clockwise rotation of the bulk shortening direction through time as it switches from N–S to W–E overall, with a major ‘tectonic break’ or decompression between O 1 and O 2. The porphyroblast inclusion trail patterns preserving W–E-trending FIAs provide a window into the lengthy period of earlier deformation and metamorphism that is no longer preserved within the matrix foliations.

Partitioning of deformation along an orogen and its effects on porphyroblast growth during orogenesis

An identical succession of foliation inflection/intersection axis trends in porphyroblasts (FIAs) is present in three separate areas affected by Acadian deformation and metamorphism along 130 km of the Appalachians from East Central Vermont to North Central Massachusetts. The FIAs trend successively NW–SE, SW–NE, W–E, NNW–SSE and SSW–NNE and appear to have formed during the same succession of deformations, which have been dated in SE Vermont as follows; not yet dated, pre-425, 425–404, 404–385 and 385–360 Ma, respectively. The two areas that are furthest apart, in east central Vermont and north central Massachusetts, have strikingly similar total distributions of FIAs in garnet porphyroblasts that show a somewhat inverse relationship to those observed in the Chester–Athens dome area of SE Vermont that separates them. That is, the number of samples containing the first formed FIA set is greatest for the two regions lying furthest apart and decreases in younger sets, whereas the number of samples containing the first FIA set is the least in the central region and increases for more recently developed sets. Partitioning of deformation into zones of progressive shearing and shortening has been demonstrated to control the sites of development of differentiated crenulation cleavage versus porphyroblast growth and sites of porphyroblast growth on one limb of a regional fold versus the other. We argue that partitioning of the deformation around competent feldspathic gneiss bodies at an orogen scale produced the variation in the number of samples containing the various FIA sets between the central and outer areas described above. Porphyroblasts most commonly grew during SW–NE directed shortening to the north and south of the Chester–Athens dome region of the Appalachians, but only rarely grew in the latter region. This suggests that deformation at this time partitioned around an outlier of competent basement feldspathic gneiss that underlies the Chester–Athens dome region, and this protected the rocks above and in the strain shadow to the NW and SE from the effects of the great bulk of the deformation that occurred over the period of time during which NW–SE-trending FIAs formed. We attribute the increase in the number of samples containing progressively younger FIAs in the Chester–Athens dome area during subsequent periods of NW–SE-, N–S-, WSW–ENE-, and WNW–ESE-directed shortening to the rise in temperature and pressure recorded in the rocks above the Chester–Athens dome with peak conditions occurring during the development of the NNW–SSE oriented FIA set between 404 and 385 Ma. As the temperature and pressure increased, the gneiss below would have become less competent, allowing the deformation associated with the succession of FIA sets to partition more pervasively through the dome core. This provided more sites for porphyroblast nucleation and growth and hence the number of samples recording these FIA sets increased. The concomitant decrease in the number of samples in which porphyroblasts grew in the rock within the areas to the north and south with successive FIAs possibly resulted from a progressive increase in competency of these rocks as the number of porphyroblasts increased.

A new approach to distinguishing multiple phases of metamorphism and deformation: application to the Northeastern Appalachian

Garnet, staurolite and aluminosilicates in St, Ky, and Sil zones on the northeastern flank of the Pelham Dome, Massachusetts, USA show microstructural and petrographic features suggesting multiple episodes of mineral growth. Sigmoidal, staircase, spiral and differentiated crenulation cleavage inclusion trail geometries as well as other textural truncations are preserved in both garnet and staurolite indicating these porphyroblasts developed by at least three periods of growth. These periods were distinguished using the trends and relative timing of the foliation inflexion/intersection axes (called FIA) preserved as inclusion trails within porphyroblasts. A total of 122 FIA trends were determined from garnet, staurolite, biotite and plagioclase porphyroblasts. FIA trends of garnet and staurolite are divided into four FIA sets as follows; NW-SE (set A), NE-SW (set B), E-W (set C) and NNW-SSE to NNE-SSW (set D). The relative timing of each FIA set was determined from porphyroblasts containing two or three different FIA trends consistently developed from core to rim. The succession from the oldest to youngest FIA sets are designated A, B, C and D. The variation of FIA trend populations around the Northfield syncline may be also attributed to difference in metamorphic grade and fold geometry, and may have been controlled by modal changes in metamorphic minerals involved in garnet/staurolite growth and consumption reactions accompanying the multiple deformation and metamorphic events. Based on Temperature—time (T—t) geochronological data and late-Paleozoic tectonic model in the study area, garnet and staurolite porphyroblasts containing four FIA sets may have developed during Devonian-Mississipian, and the matrix may have remetamorphosed during Permian.

Are crenulation cleavage zones mylonites on the microscale?

Mylonites commonly show characteristic structures such as S– C fabric and C′ type shear bands. In the present paper, the presence of similar structures on the microscale is reported from the cleavage zones of differentiated crenulation cleavage in garnet biotite schists belonging to the Lunavada Group of Proterozoic metasedimentary rocks, India. These rocks have experienced three episodes of deformation. A differentiated crenulation cleavage ( S 2), characterized by alternating cleavage zones and microlithons developed during D 2 by microfolding of the S 1 foliation. Although the schists under investigation do not show any macroscopic- or mesoscopic-scale evidence of mylonitization, they show the presence of shear structures within the cleavage zones. The fabric resembling S– C and C′ shear bands within these zones indicates shearing within them during D 2 deformation. A model incorporating shearing along the cleavage zones is proposed to explain the genesis of shear structures within them. Accordingly, it is invoked that solution transfer and grain rotation are important deformation mechanisms during the early stages of crenulation and this results in the migration of quartz from the limbs to the hinges of the microfolds. At the later stages of crenulation the phyllosilicates (micas) forming the limbs of the microfolds are at an oblique angle to the direction of shortening and most of the mobile material like quartz has already been removed from the limbs by solution transfer. Therefore, the stress conditions are ideal for shearing and intracrystalline crystal–plastic deformation to occur along the limbs during the later stages of crenulation. It is proposed that the fabric resembling S– C, embryonic C′ type shear bands and well developed C′ (in that order) develop with increasing strain and shearing within the cleavage zones. At still higher strains, the shear bands may rotate into parallelism with the domain boundary between the cleavage zones and the microlithons. Composition of muscovite constituting cleavage zones and microlithons is discussed and it is concluded that the deformation mechanisms that operate during the later stages of crenulation, especially under upper greenschist to lower amphibolite conditions, are similar to those during mylonitization.

The progressive development of a macroscopic upright fold pair during five near-orthogonal foliation-producing events: complex microstructures versus a simple macrostructure

A macroscopic anticline in the Adelaide Fold Belt, with a profile similar to first deformation folds with an axial-plane slaty cleavage elsewhere in the orogen, folds bedding, schistosity, and a differentiated crenulation cleavage. All these structures are readily visible in outcrop and a young crenulation cleavage has locally developed parallel to the axial plane of the fold. Yet foliations preserved as inclusion trails, in andalusite, garnet and staurolite porphyroblasts, reveal a history of five foliations that formed successively in steep, shallow, steep, shallow and finally steep orientations without producing a refold geometry in bedding at the macroscopic scale. The predominantly dextral (looking north) asymmetry of curvature of overprinting foliations on both limbs of this fold tightly constrains any model for its development. The anticline formed as one hinge of an open fold couplet with a sub-vertical axial plane slaty cleavage during the first deformation. It was progressively tightened during each subsequent event, without producing a refold geometry in bedding. Steep axial-plane foliations were overprinted by shallow differentiated crenulated cleavages that transected the fold couplet. These shallow foliations were then folded during the next deformation, producing another steeply dipping foliation, as the fold couplet tightened. Thus simple macroscopic folds, in orogens with a multideformational history, may appear to have formed as late structures because they fold differentiated crenulation cleavages and a bedding-parallel schistosity, but actually formed very early during orogenesis and were then tightened by a succession of near-orthogonal foliation-producing deformations. Such deformation histories provide a simple explanation for the lack of large-scale refold geometries in many fold belts with a complex history of deformation. Small-scale structures are not always a direct reflection of larger-scale geometries in fold belts.

On the deformation sequence in the New Harbour Group of Holy Island, Anglesey, North Wales

Four phases of deformation are recorded by minor structures in the New Harbour Group (NHG) of southern Holy Island. The regional schistosity in these rocks is a differentiated crenulation cleavage of D2 age. An earlier preferred orientation (S1) is commonly preserved as crenulations within the Q-domain microlithons of the S2 schistosity and is demonstrably non-parallel to bedding. F3 folds are widely developed in S2 and, to a lesser extent, in bedding. S3 crenulation cleavage is sporadically developed but can be intense locally. A major antiformal fold exists in the NHG near Rhoscolyn. This fold is of D3 age since it clearly deforms S2 schistosity and is consistent with the vergence of F3 minor structures. All planar structures are deformed by folds of D4 age. © 1997 John Wiley & Sons, Ltd.

Microstructure, metamorphism, thermochronology and P‐T‐t‐deformation history of the Port aux Basques gneisses, south‐west Newfoundland, Canada*

Abstract The Port aux Basques gneisses comprise three lithostratigraphic units separated by major fault zones: the Grand Bay Complex; the Port aux Basques Complex; and the Harbor le Cou Group. A similar regionally developed polyphase history of penetrative deformation characterizes each of these units. Thickening during D1 produced rare recumbent folds (F1) and an axial planar schistosity (S1), overprinted by D2 recumbent folds (F2), and transposed during development of a locally penetrative, differentiated crenulation cleavage (S2). In western sectors of the area, D2 was associated with NW‐directed reverse shearing. The NE‐trending structural grain reflects D3 transpression, partitioned into dextral transcurrent movement along major shear zones and development of upright‐to‐steeply inclined, periclinal folds (F3) and a variably penetrative schistosity (S3).Amphibolite facies metamorphism increases in grade from west to east across the area. Microstructures, including porphyroblast‐matrix foliation relations and internal textural unconformities in garnet, indicate episodic porphyroblast nucleation and growth, which reflect a prograde traverse sequentially across univariant reactions during syntectonic metamorphism. Garnet, kyanite and staurolite porphyroblasts are wrapped by the S2 foliation, but each may contain trails of inclusions that define S1; commonly these trails preserve early stages of S2 crenulation cleavage development. Progressive and sequential reaction out of kyanite, staurolite and muscovite in favour of sillimanite, garnet, biotite and K‐feldspar, and the development of an increasing volume of anatectic migmatite in south‐eastern sectors of the area record syn‐ to late‐D2 peak metamorphic conditions. Microstructural relationships and petrogenetic grid considerations indicate clockwise trajectories in P‐T space for units of the Port aux Basques gneisses. Peak metamorphic conditions are estimated to have been 620–650° C at ≤8kbar in the west and 700–750° C at ≤8 kbar in the east.Titanite from an upper amphibolite facies calc‐silicate gneiss yields U‐Pb ages of c. 420 Ma, interpreted to date cooling shortly after the thermal peak in these gneisses. Variable D3 strain was associated with some recrystallization of hornblende and micas. 40Ar/39Ar hornblende plateau isotope correlation ages range from c. 419 to c. 393 Ma, from east to west across the area, and are interpreted to record cooling through c. 500° C coeval with or soon after D3 deformation. The range in ages may record the effects of heterogeneous D3 deformation and differential uplift from south‐east to north‐west associated with displacement on major shear zones. 40Ar/39Ar muscovite plateau ages cluster at c. 390 Ma, and date cooling through c. 375° C during regional exhumation. Cooling rates are moderate to fast and may indicate a component of tectonic exhumation.The Port aux Basques gneisses are a product of Silurian collisional tectonics. The higher grade of metamorphism in comparison with adjacent areas of the Canadian Appalachians is interpreted to reflect greater thickening due to juxtaposition of the St Lawrence promontory (Laurentian margin) with the Cabot promontory (Avalonian margin) during closure of the Iapetus Ocean.

Regional-scale foliation reactivation and re-use during formation of a macroscopic fold in the Robertson River Metamorphics, north Queensland, Australia

The rocks defining a macroscale antiform (25 km 2) in the Proterozoic Robertson River Metamorphics have been affected by four deformations of distinctive style, with the fourth deformation, D 4, being responsible for formation of the macroscale geometry of the fold. Redistribution of progressive shearing strain, due to strain partitioning during progressive D 4 deformation, resulted in the accommodation of D 4 shearing strain along S 2 differentiated crenulation cleavages that had been synchronously rotated into favourable orientations. On the antiformal limbs this commonly resulted in reactivation of S 2 because shearing during D 4 was in a sense that was antithetic relative to that on the bulk scale of the fold. Continued D 4 deformation caused unfolding of D 2 crenulations, resulting in straightening of sigmoidally folded S 1 and its rotation toward the axial plane of the synchronously forming macroscopic D 4 fold. In zones where the sense of shear during D 4 was the same as that operating on the bulk scale of the fold (i.e. synthetic), D 4 shearing strain was accommodated dominantly by the favourably oriented, approximately axial-planar S 1 fissility. In zones where the progressive shearing component of D 4 deformation was relatively more intense, the S 2 cleavage was also rotated into parallelism with S 4 and was also re-used, as opposed to reactivated. Detailed microstructural analysis, particularly of porphyroblast-matrix relationships, combined with field observations have resolved the processes operating during folding and reveal that, despite the intensity of D 4 deformation, a separate cross-cutting S 4 cleavage has rarely been produced at either the meso- or microscale. Similar processes have probably operated at all scales in other orogenic belts.

Sigmoidal inclusion trails, punctuated fabric development, and interactions between metamorphism and deformation

ABSTRACTPorphyroblast inclusion fabrics are consistent in style and geometry across three Proterozoic metamorphic field gradients, comprising two pluton‐related gradients in central Arizona and one regional gradient in northern New Mexico. Garnet crystals contain curved ‘sigmoidal’ inclusion trails. In low‐grade chlorite schists, these trails can be correlated directly with matrix crenulations of an older schistosity (S1). The garnet crystals preferentially grew in crenulation hinges, but some late crenulations nucleated on existing garnet porphyroblasts. At higher grade, biotite, staurolite and andalusite porphyroblasts occur in a homogeneous S2 foliation primarily defined by matrix biotite and ilmenite. Biotite porphyroblasts have straight to sigmoidal inclusion trails that also represent the weakly folded S1 schistosity. Staurolite and andalusite contain distinctive inclusion‐rich and inclusion‐poor domains that represent a relict S2 differentiated crenulation cleavage. Together, the inclusion relationships document the progressive development of the S2 fabric through six stages. Garnet and biotite porphyroblasts contain stage 2 or 3 crenulations; staurolite and andalusite generally contain stage 4 crenulations, and the matrix typically contains a homogeneous stage 6 cleavage.The similarity of inclusion relationships across spatially and temporally distinct metamorphic field gradients of widely differing scales suggests a fundamental link between metamorphism and deformation. Three end‐member relationships may be involved: (1) tectonic linkages, where similar P‐T‐time histories and similar bulk compositions combine to produce similar metamorphic and structural signatures; (2) deformation‐controlled linkages, where certain microstructures, particularly crenulation hinges, are favourable environments for the nucleation and/or growth of porphyroblasts; and (3) reaction‐controlled linkages, where metamorphic reactions, particularly dehydration reactions, are associated with an increase in the rate of fabric development. A general model is proposed in which (1) garnet and biotite porphyroblasts preferentially grow in stage 2 or 3 crenulation hinges, and (2) chlorite‐consuming metamorphic reactions lead to pulses in the rate of fabric evolution. The data suggest that fabric development and porphyroblast growth may have been quite rapid, of the order of several hundreds of thousands of years, in these rocks. These microstructures and processes may be characteristic of low‐pressure, first‐cycle metamorphic belts.

Microstructural analysis of the classical spiral garnet porphyroblasts of south‐east Vermont: evidence for non‐rotation

ABSTRACTNew data strongly suggest that the classical spiral garnet porphyroblasts of south‐east Vermont, USA, generally did not rotate, relative to geographical coordinates, throughout several stages of non‐coaxial ductile deformation. The continuity of inclusion trails (Si) in these porphyroblasts is commonly disrupted by planar to weakly arcuate discontinuities, consisting of truncations and differentiation zones where quartz–graphite Si bend sharply into more graphitic Si. Discontinuous, tight microfold hinges with relatively straight axial planes are also present. These microstructures form part of a complete morphological gradation between near‐orthogonally arranged, discontinuous inclusion segments and smoothly curving, continuous Si spirals. Some 2700 pitch measurements of well‐developed inclusion discontinuities and discontinuous microfold axial planes were taken from several hundred vertically orientated thin sections of various strike, from specimens collected at 28 different locations around the Chester and Athens domes. The results indicate that the discontinuities have predominantly subvertical and subhorizontal orientations, irrespective of variations in the external foliation attitude, macrostructural geometry and apparent porphyroblast‐matrix rotation angles. Combined with evidence for textural zoning, this supports the recent hypothesis that porphyroblasts grow incrementally during successive cycles of subvertical and subhorizontal crenulation cleavage development. Less common inclined discontinuities are interpreted as resulting from deflection of anastomosing matrix foliations around obliquely orientated crystal faces prior to inclusion. Most of the idioblastic garnet porphyroblasts have a preferred crystallographic orientation. Dimensionally elongate idioblasts also have a preferred shape orientation, with long axes orientated normal to the mica folia, within which epitaxial nucleation occurred.Truncations and differentiation zones result from the formation of differentiated crenulation cleavage seams against porphyroblast margins, in association with progressive and selective strain‐induced dissolution of matrix minerals and locally also the porphyroblast margin. Non‐rotation of porphyroblasts, relative to geographical coordinates, suggests that deformation at the microscale is heterogeneous and discontinuous in the presence of undeformed, relatively large and rigid heterogeneities, which cause the progressive shearing (rotational) component of deformation to partition around them. The spiral garnet porphyroblasts therefore preserve the most complete record of the complex, polyphase tectonic and metamorphic history experienced in this area, most of which was destroyed in the matrix by progressive foliation rotation and reactivation, together with recrystallization.

Episodic metamorphic reactions during orogenesis: the control of deformation partitioning on reaction sites and reaction duration

Abstract Textural ‘unconformities’or truncations are common in porphyroblasts with complex inclusion trails. They reflect cycles of successive foliations that develop against competent porphyroblasts during orogenesis and are preserved by successive growth increments. Their truncational character results from shear and dissolution along a particular foliation generating a differentiated crenulation cleavage. The increment of porphyroblast growth that follows a textural ‘unconformity’may or may not mark a significant compositional change, depending on the amount of movement of the rock through P–T space between cleavage‐forming events.Although historically interpreted to result from a significant metamorphic hiatus, most textural unconformities indicate that the reactions involved in the formation of these minerals are episodic during continuous prograde metamorphism, starting and stopping as a function of the stage of crenulation of the matrix foliation and the pattern of deformation partitioning. Such episodic reaction behaviour can only occur for multivariant reactions, or successive but different univariant reactions. The reason why garnet is the most common porphyroblast to exhibit evidence for episodic reactions is probably the fact that it grows by multivariant reactions over a much wider P–T range than most other common porphyroblast phases.Porphyroblast growth is micrometasomatic. It is episodic because a significant reduction of strain occurs within domains of progressive shortening each time continuous progressive shearing domains form on their margins. This stops microfracture development across the progressive shortening domains, thereby preventing rapid access and interaction of fluid, ions and complexes with porphyroblast boundaries. Shifting patterns of deformation partitioning and resulting small‐scale juxtaposition of different compositional layers spreads the duration and location of multivariant reactions and causes differential timing of porphyroblast growth along a particular stratigraphic horizon. It may also locally preserve metastable metamorphic assemblages.In regionally metamorphosing/deforming pelites, near‐simultaneous cessation of porphyroblast growth on all rims, once continuous differentiated progressive shearing domains have formed nearby, precludes fluid recirculation as a significant process for removal of material during cleavage development. Alternatively, diffusion of simple molecules and dissociated ions along actively shearing and micro‐gaped phyllosilicates, with recomplexing in fluid‐filled microfractures, readily explains the control of deformation partitioning on reaction site and reaction duration.

Grenville-age orogeny in the Llano Uplift of central Texas: Deformation and metamorphism of the Rough Ridge Formation

Field and petrographic study of the Precambrian Rough Ridge Formation of the Packsaddle Group documents a more complex tectonic history for the Llano Uplift than has previously been reported. The Rough Ridge Formation was affected by at least two metamorphic events and five phases of deformation that may be part of a single progressive event. The predominant mineral assemblages in pelitic and mafic rocks are characteristic of the amphibolite facies at relatively low pressure. Ubiquitous granoblastic polygonal textures overprinting multiple foliations indicate that post-tectonic recrystallization followed dynamothermal metamorphism, a probable consequence of regional reheating during later granite intrusion. The most intense deformation produced isoclinal folds and pervasive axial planar foliation (S2), resulting in complete transposition of original bedding. This foliation cuts an earlier, local foliation (S1) and is folded by small F3 folds and a crenulation cleavage, S3. Both S2 and S3 are cut by a differentiated crenulation cleavage, S4. The most prominent folds in the area and the associated pervasive axial planar cleavage, S5, overprint all earlier structures. In one pelitic unit, oriented inclusions show that garnet formed after S3 and cordierite after S5. Growth and recrystallization of micas, quartz, and feldspars span all phases of deformation. The timing of tectonic and metamorphic events is bracketed by the pretectonic intrusion of the 1,167 ± 15-m.y.- old Red Mountain Gneiss and the post-tectonic intrusion of a 1,080 ± 30-m.y. old melarhyolite dike. Structural and petrologic characteristics of the Rough Ridge Formation indicate that the Llano Uplift underwent a complex tectonic and metamorphic development similar to that described for the Grenville orogeny elsewhere in North America.

Lack of porphyroblast rotation in noncoaxially deformed schists from Petrel Cove, South Australia, and its implications

Inclusion trails representing an S 1 cleavage demonstrate the lack of porphyroblast rotation during subsequent highly non-coaxial deformations. The pelitic schists of the Kanmantoo Group at Petrel Cove, South Australia contain two generations of porphyroblasts. The first one consists of cordierite porphyroblasts that formed early in D 2 and contain straight to slightly sigmoidal inclusion trails of S 1 . The second generation consists of andalusite porphyroblasts that overgrew crenulated S 2 late during D 3. Several hundred inclusion trail traces from cordierites measured from oriented specimens taken throughout a strongly folded area show a horizontal great circle distribution when plotted and contoured on a stereographic projection. Hence, S 1 was planar and horizontal prior to D 2. S 1 measurements on limbs and hinges of a mesoscale D 2 foldpair show that folding had little effect on porphyroblast orientation as the S 1 orientation remained constant and subhorizontal around the fold. Hence, porphyroblasts have not rotated during any of the non-coaxial deformations accompanying and following their growth. This is interpreted as a result of the partitioning of the deformation around them. S 1 in the matrix has been totally destroyed by the formation of S 2 as a fully differentiated crenulation cleavage. What has previously been regarded as inconsistent senses of shear recorded by porphyroblasts around folds is resolved by the fact that the matrix foliation rotated rather than the porphyroblasts due to the effects of deformation partitioning. The presence of a subhorizontal S 1 foliation suggests horizontal movements (e.g. thrusting or detachment faulting) during the earliest phase of the Adelaidian orogeny.

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.