Recrystallization of quartzofeldspathic rocks in a paleozoic thrust belt, Grand Lake, Newfoundland: Implications for garnet-biotite thermometry

Migmatitic granitic gneiss and associated garnetiferous granite at Grand Lake are tectonically interleaved with high-pressure ( 9 kbar) metapelites that contain Barrovian (Ky-St-Grt-Bt-Ms-Rt) mineral assemblages. The migmatites contain metabasites that are compositionally similar to the latest Proterozoic Long Range dykes, suggesting that the gneisses correlate to Grenvillian inliers in western Newfoundland.

In the metapelites, the Barrovian porphyroblastic assemblage overprints microfolds and is itself overprinted by greenschist-facies assemblages in thrust-related high-strain zones. Evidence that the quartzofeldspathic rocks also experienced Barrovian metamorphism is provided by low Ti/>Al amphiboles in metabasites in the gneiss, and, despite the migmatitic character of these rocks, the presence of growth-zoned garnets with bell-shaped compositional profiles (e.g., rimward increase in X_Prp and decrease in X_Sps) similar to those exhibited by garnet in the metapelite. The range of garnet-biotite paleotemperatures for the quartzofeldspathic rocks is only marginally higher (T_max=530–660°C) than that determined for the metapelites (T_max=500–615°C). There are no systematic differences in Grt-Bt temperatures between (proto) mylonitic rocks and nearby precursors.

These results indicate that the quartzofeldspathic rocks and metapelites share a common, post-migmatization (of the gneisses) metamorphic history; they may well have originally been linked by a basement/cover relationship. Mineralogical evidence for the earlier, higher grade metamorphic history of the migmatites has virtually been eradicated even outside the high-strain zones. In this area, there is no clear relation between qualitatively-estimated superimposed strain and the degree of resetting of the Grt-Bt geothermometer.     

Spatially-focussed melt formation in aluminous metapelites from Broken Hill, Australia

Large garnet poikiloblasts hosted by leucosome in metapelitic gneiss from Broken Hill reflect complex mineral–melt relationships. The spatial relationship between the leucosomes and the garnet poikiloblasts implies that the growth of garnet was strongly linked to the production of melt. The apparent difficulty of garnet to nucleate a large number of grains during the prograde breakdown of coexisting biotite and sillimanite led to the spatial focussing of melting reactions around the few garnet nuclei that formed. Continued reaction of biotite and sillimanite required diffusion of elements from where minerals were reacting to sites of garnet growth. This diffusion was driven by chemical potential gradients between garnet‐bearing and garnet‐absent parts of the rock. As a consequence, melt and peritectic K‐feldspar also preferentially formed around the garnet. The diffusion of elements led to the chemical partitioning of the rock within an overall context in which equilibrium may have been approached. Thus, the garnet‐bearing leucosomes record in situ melt formation around garnet porphyroblasts rather than centimetre‐scale physical melt migration and segregation. The near complete preservation of the high‐grade assemblages in the mesosome and leucosome is consistent with substantial melt loss. Interconnected networks between garnet‐rich leucosomes provide the most likely pathway for melt migration. Decimetre‐scale, coarse‐grained, garnet‐poor leucosomes may represent areas of melt flux through a large‐scale melt transfer network.     

Multi-textured garnets from a single growth event: an example from northern Norway

Abstract Textural relationships between porphyroblasts of biotite and garnet in metasediments in the Nordkinn Peninsula area of the Finnmarkian Caledonides of North Norway are apparently complex. There is evidence for two textural zones in both mineral phases and superficially the development of these appears to have overlapped, at least in part, in time and space. This apparently complex porphyroblast growth history can be considerably simplified if only one period of garnet growth occurred and if different inclusion fabrics developed where garnet replaced biotite porphyroblasts and where it overgrew the matrix foliation. The possibility that porphyroblasts with textural evidence for multiphase growth histories actually grew during a single crystallization event is of importance in the interpretation and elucidation of tectonometamorphic relationships.     

Intergranular diffusion rates from the analysis of garnet surfaces: Implications for metamorphic equilibration

Novel approaches to garnet analysis have been used to assess rates of intergranular diffusion between different matrix phases and garnet porphyroblasts in a regionally metamorphosed staurolite‐mica‐schist from the Barrovian‐type area in Scotland. X‐ray maps and chemical traverses of planar porphyroblast surfaces reveal chemical heterogeneity of the garnet grain boundary linked to the nature of the adjacent matrix phase. The garnet preserves evidence of low temperature retrograde exchange with matrix minerals and diffusion profiles documenting cation movement along the garnet boundaries. Garnet–quartz and garnet–plagioclase boundaries preserve evidence of sluggish Mg, Mn and Fe diffusion at comparable rates to volume diffusion in garnet, whereas diffusion along garnet–biotite interfaces is much more effective. Evidence of particularly slow Al transport, probably coupled to Fe³⁺ exchange, is locally preserved on garnet surfaces adjacent to Fe‐oxide phases. The Ca distribution on the garnet surface shows the most complex behaviour, with long‐wavelength heterogeneities apparently unrelated to the matrix grain boundaries. This implies that the Ca content of garnet is controlled by local availability and is thought likely to reflect disequilibrium established during garnet growth. Geochemical anomalies on the garnet surfaces are also linked to the location of triple junctions between the porphyroblasts and the matrix phases, and imply enhanced transport along these channels. The slow rates of intergranular diffusion and the characteristics of different boundary types may explain many features associated with the prograde growth of garnet porphyroblasts. Thus, minerals such as quartz, Fe‐oxides and plagioclase whose boundaries with garnet are characterized by slow intergranular diffusion rates appear to be preferentially trapped as inclusions within porphyroblasts. As such grain boundary diffusion rates may be a significant kinetic impediment to metamorphic equilibrium and garnet may struggle to maintain chemical and textural equilibrium during growth in pelites.     

Controls on solid-phase inclusion during porphyroblast growth: insights from the Barrovian sequence (Scottish Dalradian)

A series of Barrovian sequence samples ranging from garnet to sillimanite zones were investigated to infer their porphyroblast-forming reactions and mineral inclusion histories. Quartz is overgrown and partly consumed during garnet formation and remains as inclusion-rich layers in porphyroblasts of the garnet zone. Staurolite crystals in the staurolite zone display sharp transitions between inclusion-rich and inclusion-free areas, suggesting two stages of growth with a different role of quartz in each. The inclusion-rich domains formed similarly to those in garnet by simple overgrowth and resorption of matrix minerals, with thermodynamic constraints suggesting that this staurolite-forming reaction required the presence of chloritoid that is now absent from the examined samples. The participation of garnet was limited in staurolite formation, with chloritoid breakdown supplying sufficient material to form the large amounts (c. 25 vol%) of staurolite found in the rock. This reaction produces an excess of SiO₂, which leaves the crystal domain as SiO_2aq and thus caused the formation of the inclusion-free areas in the staurolite and precipitation of quartz in the matrix. In the sillimanite zone, staurolite is consumed forming new garnet. The newly formed garnet has less quartz inclusions than its core due to a proportionally greater consumption of quartz by the second garnet-forming reaction than by the initial, garnet-grade reactions. Textural and thermodynamic data both suggest that inclusions in these porphyroblasts represent leftovers of a preferentially overgrown matrix than co-products of the porphyroblast-forming reaction.     

Juxtaposition of Barrovian and migmatite domains in the Chinese Altai: a result of crustal thickening followed by doming of partially molten lower crust

Ordovician metasedimentary rocks are the oldest and most extensive sedimentary sequence in the Chinese Altai. They experienced two major episodes of deformation (D1 and D2) resulting in the formation of juxtaposed Barrovian‐type and migmatite domains. D1 is characterized by a penetrative sub‐horizontal fabric (S1), and D2 is marked by upright folds (F2) with NW–SE‐trending axial planes in shallow crustal levels and by sub‐vertical transposition foliations (S2) in the high‐grade cores of large‐scale F2 antiforms. In the Barrovian‐type domain, successive growth of biotite, garnet and staurolite is observed in the S1 fabric. Kyanite included in garnet and plagioclase in the migmatite domain is interpreted to have formed also in S1. In the biotite and garnet zones, the spaced S2 cleavage is marked by biotite and muscovite, and in the staurolite and kyanite zones, the penetrative S2 fabric is characterized by sillimanite, locally with late cordierite. Phase equilibria modelling indicates that the S1 fabric was associated with an increase in pressure and temperature under Barrovian‐type conditions in both domains. The S2 fabric was related to decompression, in which rocks in the biotite and garnet zones well preserve the peak assemblage, and the higher grade rocks in the staurolite and kyanite zones re‐equilibrated to different degrees under high‐temperature/low‐pressure (HT/LP) conditions. The D1 metamorphic history is attributed to the progressive burial related to Early–Middle Palaeozoic crustal thickening and the metamorphism associated with D2 is interpreted to result from exhumation by vertical extrusion. The extrusion of hot rocks was contemporaneous with the formation of gneiss domes accompanied by the intrusion of juvenile magmas at middle crustal levels during the Middle Palaeozoic. Consequently, there is a genetic link between the Barrovian‐type and migmatite domains related to continuous transition of the Barrovian‐type fabric into the HT/LP one during development of domal structures in the southern Altai orogenic belt. This study has a broad impact on the understanding of the thermo‐mechanical behaviour of accretionary orogenic systems worldwide. The lower crustal flow and doming of hot crust, so far reported only in continental collisional settings, seems to be also an integral mechanism responsible for both horizontal and vertical redistribution of accreted material prior to continental collision.     

Pseudomorphs and associated microstructures of western Maine,USA

In metapelitic rocks of western Maine, a pluton-related M3 metamorphic gradient ranging in grade from garnet to upper sillimanite zone was superposed on a fairly uniform M2 regional metamorphic terrain characterized by the assemblage andalusite+staurolite+biotite+/−garnet. As a result, M2 assemblages re-equilibrated to the P, T, and aH₂O conditions of M3, and both prograde and retrograde pseudomorphism of M2 porphyroblasts occurred. The type of pseudomorph and degree of development is directly related to the rock's position within the M3 metamorphic gradient, a function of its proximity to the Mooselookmeguntic pluton. Several ‘hinge’ zones occur in which the M3 minerals that pseudomorphed a particular M2 phase change. For example, M2 garnet was replaced by M3 chlorite or biotite, depending on its position within the M3 gradient. Similarly, in a transition zone between M3 upper staurolite and lower sillimanite zones, M2 staurolite was stable and shows M3 growth rims. Downgrade from this transition zone, staurolite was pseudomorphed by chlorite and muscovite, whereas upgrade, the pseudomorphs contain muscovite and some biotite. M3 pseudomorphs commonly retain crystal shapes of the original M2 porphyroblasts, reflecting relatively low regional deviatoric stress during and after M3. Although evidence for textural disequilibrium is common, chemical equilibrium was closely approached during M3. This study demonstrates for M3 that: (1) the pseudomorphic replacement was a constant volume process, and (2) fabrics produced by tectonic events can be erased by subsequent deformation and/or sufficiently intense subsequent recrystallization.     

Microtextures of opaque inclusions: their use as indicators for hiatuses during garnet porphyroblast growth

Within an analysed garnet porphyroblast, opaque inclusions imaged with the backscatter facility of a scanning electron microscope show different microtextures depending on their position within the porphyroblast. Three different zones can been distinguished: Zone 1 contains a Ti‐rich magnetite that has decomposed to a symplectite of fine and narrowly spaced exsolution lamellae of ilmenite and magnetite. Zone 2 shows a Ti‐rich magnetite symplectite with thicker and more widely spaced exsolution lamellae of ilmenite and magnetite. Within zone 3, Ti‐rich magnetite symplectite has totally been replaced by recrystallized magnetite crystals bordered by a thin ilmenite rim. Similar microtextures within ulvöspinel‐rich magnetite have elsewhere shown to be the result of an increase in oxidation and rate of diffusion. During metamorphism of metapelites, such an increase can be reasonably envisaged because of dehydration reactions progressing during rising temperatures, and this has occurred during the overgrowth of the three different microtextures by the garnet porphyroblast. Because the microtextures are homogeneous within the three different zones, it is deduced that the oxidation reaction rate of the opaque inclusions was substantially lower than the garnet growth rate. As a consequence, hiatuses in the garnet growth history must have occurred between the evolution from one microtexture to the next. A comparison between the inclusion trail geometry and the microtextural zone boundaries shows a perfect coincidence between these and the sites where inclusion trails become strongly deflected and truncated. This correlation confirms that, in the studied case, sharp microstructural boundaries (as truncation zones or deflection zones) coincide with growth hiatuses. The study therefore highlights the potential use of opaque inclusions to confirm or reject the occurrence of growth hiatuses within garnet porphyroblasts, especially in cases where discontinuities in the inclusion trail patterns are otherwise arbitrarily associated with growth hiatuses.     

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.     

Microstructural features of albite porphyroblasts as indicators of sequential Barrovian metamorphic mineral growth in the Caledonides of the SW Scottish Highlands

Inclusion – porphyroblast and porphyroblast – porphyroblast relationships show that abundant albite in mica schists in the Caledonides of the SW Scottish Highlands are part of the Barrovian metamorphic assemblage. Growth early in the D2 deformational phase of porphyroblast cores followed the growth of Mn‐rich garnet but preceded the growth of porphyroblasts of the index mineral almandine. Two sets of inclusion trails in the albite correspond to the regionally expressed S1 and S2. Straight trails of muscovite, chlorite, quartz, epidote and the earliest growth of biotite make up S1. Crenulated trails express deformation of S1 early in D2 with muscovite, chlorite, biotite, quartz, epidote and the Mn‐rich garnet associated with the development of S2 crenulation cleavage. The geometries of these trails uniquely record early stages of D2 deformational history. An _0?3 growth is related to the temporal coincidence of the formation of S1–S2 crenulation cleavage hinges as favourable sites for nucleation and the release of large amounts of water from prograde reactions during tectonothermal reconstitution of first cycle immature sediments with a volcanic component. The main characteristics of the regionally expressed D2 schistosity were developed during the major grain coarsening that followed both albite and almandine porphyroblast growth. Essentially inclusion‐free An _4?19 rims grew on the inclusion‐containing cores in the almandine zone in the later stages of schistosity growth and unoriented porphyroblasts of muscovite, biotite and chlorite indicate that mineral growth extended from the later stages of D2 to post‐D2. Previous interpretations of the albite porphyroblast growth having been during D4 to post‐D4 contemporaneous with retrogression are inconsistent with the microstructural evidence.     

Reaction relationships during retrograde metamorphism at Olary, South Australia

Abstract In metapelitic schists of the north-eastern Weekeroo Inliers, Olary Block, Willyama Supergroup, South Australia, syn-S1 and syn-S2 assemblages involving staurolite, garnet, biotite and another mineral, most probably cordierite, were overgrown by large syn-S3 andalusite porphyroblasts, owing to isobaric heating from metamorphic conditions that existed during the development of S2. Conditions during the development of S3 probably just reached the andalusite—sillimanite transition. During the development of S4, at somewhat lower temperatures than those that accompanied the development of S3, the following reaction occurred:
staurolite + chlorite + muscovite ± biotite + andalusite + quartz + H₂O.
The amount of retrogression is controlled primarily by the amount of H₂O added by infiltration. As the syn-S3 matrix assemblage was stable during the development of S4, but the andalusite porphyroblasts were no longer stable with the matrix when H₂O was added, the retrogression is focused in and around the porphyroblasts. With enough H₂O available, and if quartz was consumed before biotite in a porphyroblast, then the following reaction occurred:
staurolite + chlorite + muscovite + corundum ± biotite + andalusite + H₂O.
This reaction allowed corundum inclusions in the andalusite to grow, regardless of the presence of quartz in the matrix assemblage.     

The effect of metamorphic fluid flow on the nucleation and growth of garnets from Troms, North Norway

A spatial association is observed between the size distribution of garnet porphyroblasts and the size distribution of quartz veins in greenschist facies metapelites from Troms, North Norway. The size distribution of quartz veins reflects the flow regime of metamorphic fluids. The hypothesis that the flow regime of metamorphic fluids is also responsible for the size distribution of garnet crystals was tested by ascribing empirical acceleration parameters to the nucleation and growth rates of garnet crystals.
In regions where fluid flow was interpreted as pervasive', acceleration parameters for nucleation were high, whereas in regions where fluid flow was interpreted as channelled', acceleration parameters for growth were high. Accelerated crystal growth is further implied from the chemical zoning and crystal morphologies of garnets collected near discrete veins.
This spatial association may imply that fluid flow can be instrumental in controlling garnet crystallization. Fluid flow could affect garnet crystallization kinetics by facilitating thermal advection and/or mass transfer. In the study area, rhodochrosite (MnCO₃) veins provide evidence for mass transfer of Mn by fluid flow. An influx of Mn would expand the stability field of garnet to lower temperatures. The resulting thermal overstep could accelerate nucleation and/or growth of garnets.
The corollary of this study is that size distributions and chemical zoning of garnets, or other porphyroblast phases, can be used to study metamorphic fluid flow.     

Garnet evolution in pre-Variscan pelitic rocks from the Lake Emosson area, Aiguilles Rouges Massif, Western Alps

Abstract The chemical evolution of garnets from pelitic rocks of probable Palaeozoic age corresponds to a complex metamorphic evolution of the host rocks.
Among the almandine-rich garnets (Alm_60–80), two main types of evolution can be distinguished. Early Mn-rich garnets coexisting with kyanite may be replaced by plagioclase and then, during a late stage, by biotite and/or sillimanite. The second type of evolution corresponds to an overgrowth of Mn-poor late-stage garnet on older Mn-rich garnets which corresponds to a thermal peak with sillimanite-type of metamorphism. This new garnet may appear either as an overgrowth with a strong discontinuity, or as small, new euhedral garnet or as skeletal garnet.
This chemical evolution of garnet corresponds to an early collisional stage of metamorphism (of high pressure type with high Mn values) of probable Ordovician age followed by uplift and a thermal peak (low Mn values) in Devonian times.     

Deformation-induced garnet zoning

Compositional zoning patterns in garnet porphyroblasts from kyanite-bearing samples of the Devonian Littleton Formation, north-central Massachusetts, reveal complex patterns of growth that are related to multiple deformation and metamorphic events. Garnet porphyroblasts exhibit asymmetrical and irregular zoning patterns in X_Mn, X_Ca and Fe/(Fe + Mg). Zoning reversals in Mn and Fe/(Fe + Mg) and patch distribution in Ca appear to occur around the boundaries of the textural zones. Also, the compositions of the garnet at the textural boundaries are variable for all traverses. These observations suggest that the garnet zoning was not only modified from diffusion processes, but was also influenced by pre-existing microfabrics through the effects of preferential dissolution and resorption in partial disequilibrium. Relationships between chemical and textural truncations indicate that the zoning patterns of garnet were strongly modified from preferential dissolution and precipitation during the development of successive foliations that occurred in zones of high strain/stress (cleavage seams) and zones of low strain/stress, respectively.     

Sequential growth of cordierite and andalusite porphyroblasts, Cooma Complex, Australia: microstructural evidence of a prograde reaction

Abstract Low-pressure prograde metamorphism of pelitic rocks in the Cooma Complex, south-east Australia, has produced cordierite-andalusite schists at intermediate grades. The first foliation (S₁) is preserved largely as inclusion trails in cordierite porphyroblasts. Microstructural evidence indicates that the cordierite porphyroblasts grew during the early stages of development of a crenulation-foliation (S₂) and that andalusite porphyroblasts grew during the development of a later crenulation-foliation (S₃). Microstructural evidence also indicates that the andalusite was a product of the prograde reaction: cordierite + muscovite ± andalusite + biotite + quartz. The occurrence of the products of this reaction in 'beard'structures between cordierite microboudins formed by extension in S₃ confirms that the andalusite grew during the development of S₃. The investigation shows that porphyroblast-matrix relationships can preserve the orientation of an early S-surface that has been largely obliterated from the matrix, as well as providing relatively direct evidence of sequential mineral growth and metamorphic reactions.     

An Exotic Eclogite/Blueschist Slice in a Barrovian Style Metamorphic Terrain, Alanya Nappes, Southern Turkey

A 600 m thick, more than 40 km long slice of eclogite faciesrocks, called the Sug?z? Nappe, occurs in the Alanya area, southernTurkey, sandwiched between two other crystalline nappes whichdo not show HP/LT metamorphism. All three nappcs were affectedby a later Barrovian-type metamorphism and penetrative deformationwhich welded the nappes into a single tectonic unit. The SugOzuNappe consists predominantly of garnet-mica schists with lensesof eclogite and blueschist metabasites. The mineral assemblagein the eclogites, garnet+omphacite+glaucophane+paragonite+quartz+phengite+rutile, is estimated to have formed at 13?5?1?5 kb and510?25?C. The eclogite assemblage is variably overprinted bya later Barrovian metamorphism with the development of barroisite,chlorite, and albite. The extent of the Barrovian overprintwas controlled by the supply of fluid to the rocks. The Barrovian metamorphism increases in grade downwards in thestructural sequence; biotite and garnet isograds are mappedin the lowermost Mahmutlar Nappe, made up mostly of metapelitesand metapsammites. The metapelites in the garnet zone consistof garnet+biotite+chlorite+muscovite+albite+oligoclase+quartz+ilmenite;metamorphic conditions are estimated as 6?5?1?0 kb and 469?13?C. The HP/LT rocks of the Sug?z? Nappe underwent a cooling of about100?C during a dry uplift from a depth of about 48 km to 21km where they were intercalated with the other nappes, and affectedby a Barrovian metamorphism caused by the incoming fluids. Thecase of the Alanya Nappcs illustrates that the Barrovian overprintobserved in many eclogites and blueschists may not be due toincreasing temperature during uplift, but simply due to theintroduction of a fluid phase during part of the uplift P-Tpath.     

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

Porphyroblast 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.     

Constraining the conditions of Barrovian metamorphism in Sikkim,India: P–T–t paths of garnet crystallization in the Lesser Himalayan Belt

Garnet crystallization in metapelites from the Barrovian garnet and staurolite zones of the Lesser Himalayan Belt in Sikkim is modelled utilizing Gibbs free energy minimization, multi‐component diffusion theory and a simple nucleation and growth algorithm. The predicted mineral assemblages and garnet‐growth zoning match observations remarkably well for relatively tight, clockwise metamorphic P–T paths that are characterized by prograde gradients of ～30 °C kbar^?1 for garnet‐zone rocks and ～20 °C kbar^?1 for rocks from the staurolite zone. Estimates for peak metamorphic temperature increase up‐structure toward the Main Central Thrust. According to our calculations, garnet stopped growing at peak pressures, and protracted heating after peak pressure was absent or insignificant. Almost identical P–T paths for the samples studied and the metamorphic continuity of the Lesser Himalayan Belt support thermo‐mechanical models that favour tectonic inversion of a coherent package of Barrovian metamorphic rocks. Time‐scales associated with the metamorphism were too short for chemical diffusion to substantially modify garnet‐growth zoning in rocks from the garnet and staurolite zones. In general, the pressure of initial garnet growth decreases, and the temperature required for initial garnet growth was reached earlier, for rocks buried closer toward the MCT. Deviations from this overall trend can be explained by variations in bulk‐rock chemistry.     

Two-stage decompression in orthopyroxene–sillimanite granulites from Forefinger Point, Enderby Land, Antarctica: implications for the evolution of the Archaean Napier Complex

Magnesian metapelites of probable Archaean age from Forefinger Point, SW Enderby Land, East Antarctica, contain very-high-temperature granulite facies mineral assemblages, which include orthopyroxene (8–9.5 wt% Al₂O₃)–sillimanite ± garnet ± quartz ± K-feldspar, that formed at 10 ± 1.5 kbar and 950 ± 50°C. These assemblages are overprinted by symplectite and corona reaction textures involving sapphirine, orthopyroxene (6–7 wt% Al₂O₃), cordierite and sometimes spinel at the expense of porphyroblastic garnet or earlier orthopyroxene–sillimanite. These textures mainly pre-date the development of coarse biotite at the expense of initial mesoperthite, and the subsequent formation of orthopyroxene (4–6 wt% Al₂O₃)–cordierite–plagioclase rinds on late biotite.
The early reaction textures indicate a period of near-isothermal decompression at temperatures above 900°C. Decompression from 10 ± 1.5 kbar to 7–8 kbar was succeeded by biotite formation at significantly lower temperatures (800–850°C) and further decompression to 4.5 ± 1 kbar at 700–800°C.
The later parts of this P–T evolution can be ascribed to the overprinting and reworking of the Forefinger Point granulites by the Late-Proterozoic ( c . 1000 Ma) Rayner Complex metamorphism, but the age and timing of the early high-temperature decompression is not known. It is speculated that this initial decompression is of Archaean age and therefore records thinning of the crust of the Napier Complex following crustal thickening by tectonic or magmatic mechanisms and preceding the generally wellpreserved post-deformational near-isobaric cooling history of this terrain.     

Ductile-brittle transition in pre-Alpine amphibolite facies mylonites during evolution from water-present to water-deficient conditions (Mont Mary nappe, Italian Western Alps)

In the Austroalpine Mont Mary nappe (Italian Western Alps) discrete zones of mylonites–ultramylonites developed from coarse-grained, upper amphibolite facies metapelites of pre-Alpine age. The syn–mylonitic mineral assemblage is quartz–biotite–muscovite–plagioclase–garnet–sillimanite–ilmenite–graphite, and formed via the model hydration reaction: Grt₁+Kfs+H₂O=Bt₂+Ilm₂+Qtz+Ms± Sil .Grain-size reduction of about three orders of magnitude was accompanied by extensive recrystallization of all minerals except sillimanite, and by compositional changes of garnet and biotite. Deformation took place at temperatures of 510–580  °C under low-pressure conditions (0.25–0.45 GPa) and corresponds to the latest stages of pre-Alpine metamorphic evolution. The pre-Alpine mylonitization conditions were close to the brittle-ductile transition, as indicated by syn–mylonitic generation of pseudotachylytes and high differential stress inferred from quartz grain-size piezometry. The brittle-ductile behaviour at a relatively high temperature, and the absence of annealing textures in quartz aggregates, are suggestive of water-deficient conditions during mylonitization. These were accomplished through progressive consumption of water by syn–kinematic hydration reaction and by adsorption onto the greatly increased grain boundary area resulting from dynamic recrystallization.