Porphyroblast Growth Research Articles

Igneous origin of K-feldspar megacrysts in deformed granite of the Papoose Flat Pluton, California, USA

Many points of evidence, especially igneous microstructures and structures resulting from solid-state deformation, indicate that K-feldspar megacrysts in deformed granites of the Papoose Flat pluton are residual phenocrysts, not porphyroblasts. Evidence of an igneous origin includes features such as crystal shapes, simple twinning, zonally arranged euhedral plagioclase inclusions, oscillatory compositional zoning, and local occurrence in microgranitoid enclaves. Evidence of solid-state deformation of the megacrysts (which is consistent with their existence prior to the mylonitic deformation) includes marginal recrystallization and neocrystallization, microcline twinning, marginal replacement by myrmekite, and recrystallized/neocrystallized “tails”. Evidence of porphyroblastic growth, such as overgrown inclusion trails, is absent. This appears to be the situation in most felsic augen gneisses and mylonites.

Low Pressure Metamorphism, Deformation and Syntectonic Granite Emplacement in the Palaeoproterozoic Mahakoshal Supracrustal Belt, Central India

Porphyroblast assemblage of andalusite, garnet, staurolite and cordierite in the metasedimentary rocks of the Palaeoproterozoic Mahakoshal supracrustal belt suggests low pressure/medium temperature metamorphism. These porphyroblasts exhibit both dynamic (syn-kinematic) and static (post-kinematic) growth. Field and petrographic studies indicate that the growth of andalusite and garnet porphyroblasts initiated during the early stages of D 2 (S 2 cleavage) deformation and outlasted it. The staurolite porphyroblasts appeared during the late stages of D 2 deformation and continued after it. Restricted occurrence of these syn-kinematic porphyroblasts to the vicinity of the shear zone suggest that intense deformation and repetitive cleavage development, apart from the elevated temperature conditions, enhanced the growth of the porphyroblasts. The cordierite porphyroblasts, whose growth is controlled essentially by the lithological composition, exhibit static growth, post-dating D 1 (S 1 cleavage) deformation. The mineralogical assemblages in the metasedimentary rocks suggest a peak P and T around 3.5 kb and 550–600°C respectively, which was attained during the D 2 deformation. The D 2 deformation was coeval with the development of reverse-oblique slip ductile shear zone and emplacement of linear, syn-kinematic granites along the southern margin of the belt. This indicates a contractional tectonic regime, wherein rising granitic plutons caused advective heat transfer to the middle and upper crust and created relatively higher temperature conditions at lower pressure.

Experimental investigation of the kinetics of the reaction wollastonite + calcite + anorthite = grossular + CO2

Mechanisms and kinetics of the reaction wollastonite + calcite + anorthite ⇒ grossular + carbon dioxide were studied in powder experiments in the presence of a H₂O-CO₂ fluid with 8 to 13 mole percent CO₂ at 665° to 750°C and 400 MPa. The reaction was studied in terms of overall reaction rates, nucleation rates, and crystal growth rates. Grossular was the only solid reaction product. The crystals were grown around minute andradite-rich cores, which enriched the grossular with traces of iron. At a given temperature, the conversion to grossular + CO₂ increased linearly with time at least up to 25 percent. Reaction rates increased in the temperature range from 710° to 750°C by a factor of 22. At the constant temperature of 730°C the reaction rate increased by a factor of 2.4 when overstepping of the equilibrium temperature was increased from 55° to 95°C due to a decrease in the CO₂-concentration in the fluid. Nucleation and growth rates were measured at 730°C as a function of time. There was a sharp nucleation rate maximum during the second day of the experiments followed by a steady state nucleation rate at a much lower level during the rest of the experiment. Crystal growth rates permanently decreased either with respect to diameter, surface area, or volume. In experiments with varied surface area for either the reactants or for the grossular, no consistent relationship between surface area and reaction rate was found and thus no single rate-limiting step could be identified. For the rate-controlling process of the overall reaction a model similar to competitive, diffusion-controlled growth of garnet porphyroblasts is suggested. In this model each grossular crystal is surrounded by a diffusion halo where nutrients deplete with time. Once the diffusion halos impinge throughout the reactant mixture, the diffusion gradients are leveled. Consequently, the nucleation rate drops down to a steady state level and crystal growth rates decrease. Nucleation never totally ceases because the nucleation barrier is overcome by enrichment of andradite component in the garnet cores. Analog enrichment of trace elements during nucleation might be common in metamorphic crystallization. The results achieved will serve to better understand garnet formation in porphyroblastic rocks.

Crack-fill porphyroblastesis

Garnet-mica schists from the Scottish Highlands provide new insight into an important mechanism of phyllosilicate growth, termed ‘crack-fill porphyroblastesis’. It is shown that grain boundary dilatancy, microcracking and porphyroblast-matrix decoupling all play a significant role in facilitating growth in regimes of noncoaxial shear. With respect to chlorite porphyroblasts, there are three growth stages. Following nucleation, the initial phase of growth is by progressive matrix replacement, to preserve inclusion trails of fine carbonaceous material. The second growth stage produced new optically continuous inclusion-free chlorite on the {001} margins of those crystals at a high angle to the schistosity. This growth results from decoupling at the porphyroblast–matrix contact on those margins at a high angle to the principal axis of extension. The development of dilatant cracks at porphyroblast margins provides a sink for material migrating down Pf and chemical potential gradients. This causes precipitation of new optically continuous ‘clear’ chlorite on the pre-existing, heavily included core. The porphyroblast–matrix boundary continues to dilate after porphyroblast growth had terminated, producing plano-convex quartz-rich strain shadows. Similar growth behaviour is recognised in biotite porphyroblasts, indicating that ‘crack-fill porphyroblastesis’ is an important growth mechanism for phyllosilicates in actively deforming metamorphic rocks. It also indicates that decoupling and crack-fill development at porphyroblast margins could be important in controlling the pattern of material transfer, and may have significant implications for matrix permeability and fluid-flow characteristics.

Fractionation of bulk rock composition due to porphyroblast growth: effects on eclogite facies mineral equilibria, Pam Peninsula, New Caledonia

AbstractChemically zoned porphyroblasts in metamorphic rocks indicate that diffusional processes could not maintain equilibrium conditions on a grain scale during porphyroblast growth or establish it afterwards. An effect of this inability to maintain equilibrium is the progressive removal of elements forming garnet cores from any metamorphic reaction that occurs at the porphyroblast boundaries or in the matrix of the rock. To examine this effect on mineral assemblages, the Bence–Albee matrix correction was applied to X‐ray intensity maps collected using eclogite samples from northern New Caledonia in order to determine the chemical composition of all parts of the sample. The manipulation of these element maps allows a quantitative analysis of the fractionation of the bulk rock composition between garnet cores and the matrix. A series of calculated equilibrium‐volume compositions represents the change in matrix chemistry with progressive elemental fractionation as a consequence of prograde garnet growth under high‐P conditions. Pressure–temperature pseudosections are calculated for these compositions, in the CaO–Na2O–FeO–MgO–Al2O3–SiO2–H2O system. Assemblages, modal proportions and mineral textures observed in the New Caledonian eclogites can be closely modelled by progressively ‘removing’ elements forming garnet cores from the bulk rock composition. The pseudosections demonstrate how chemical fractionation effects the peak metamorphic assemblage, prograde textures and the development of retrograde assemblages.

Assessing the extent of disequilibrium and overstepping of prograde metamorphic reactions in metapelites from the Bushveld Complex aureole, South Africa

AbstractAndalusite–staurolite–biotite hornfels metamorphosed beneath the mafic layered rocks of the Bushveld Complex, South Africa, preserves a detailed record of the relative timing of porphyroblast growth and metamorphic reactions. The sequence inferred from microstructures shows considerable overlap of the period of growth of porphyroblasts of staurolite, cordierite, biotite and andalusite, and the persistence over a similar interval of the reactant porphyroblastic phase chloritoid. This is inconsistent with calculations of equilibrium phase relations, and implies that disequilibrium processes controlled the prograde reaction sequence, despite the slow heating rates involved (1 °C per 10 000 yr). The early appearance of cordierite by a metastable reaction and its subsequent disappearance indicates that delayed nucleation of porphyroblastic phases, rather than simply sluggish reaction, is required to account for the sequence of growth. The predicted reactions for the first appearance of andalusite and staurolite have low entropy of reaction, and do not occur until they have been overtaken in terms of reaction affinity by high‐entropy devolatilisation reactions involving the breakdown of chlorite. Once the porphyroblasts have nucleated, metastable chloritoid‐breakdown reactions also contribute to their growth. The implied magnitude of the critical overstepping for andalusite nucleation is around 5 kJ mole−1 (equivalent to 40 °C for the chlorite‐breakdown reaction), and that for other phases is expected to decrease in the order andalusite>staurolite>cordierite. Coupling between nucleation rate, crystal growth rates and the resulting grain size distribution suggests that the rate constants of natural reactions are at least an order of magnitude lower than those measured in the laboratory. Pseudomorphs after chloritoid and cordierite conserve volume but not Al or other species of low mobility, suggesting a breakdown mechanism controlled by an interface process such as the slow dissolution of the refractory porphyroblast phase, rather than by a transport step.

Shear zone vs folding origin for spiral inclusion trails in the Canton Schist

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

Development of garnet porphyroblasts by multiple nucleation, coalescence and boundary misorientation-driven rotations

Two types of garnet porphyroblast occur in the Schneeberg Complex of the Italian Alps. Type 1 porphyroblasts form ellipsoidal pods with a centre consisting of unstrained quartz, decussate mica and small garnet grains, and a margin containing large garnet grains. Orientation contrast imaging using the scanning electron microscope shows that the larger marginal garnet grains comprise a number of orientation subdomains. Individual garnet grains without subdomains are small (< 50 µm), faceted and idioblastic, and have simple zoning profiles with Ca-rich cores and Ca-poor rims. Subdomains of larger garnet grains are similar in size to the individual, small garnet grains. Type 2 porphyroblasts comprise only ellipsoidal garnet, with small subdomains in the centre and larger subdomains at the margin. Each subdomain has its own Ca high, Ca dropping towards subdomain boundaries. Garnet grains, with or without subdomains, all have the same Ca-poor composition at rims in contact with other minerals. The compositional zonation patterns are best explained by simultaneous, multiple nucleation, followed by growth and amalgamation of individual garnet grains. The range of individual garnet and garnet subdomain sizes can be explained by a faster growth rate at the porphyroblast margin than in the centre. The difference between Type 1 and Type 2 porphyroblasts is probably related to the growth rate differential across the porphyroblast. Electron backscatter diffraction shows that small, individual garnet grains are randomly oriented. Large marginal garnet grains and subdomain-bearing garnet grains have a strong preferred orientation, clustering around a single garnet orientation. Misorientations across subdomain boundaries are small and misorientation axes are randomly oriented with respect to crystallographic orientations. The only explanation that fits the observational data is that individual garnet grains rotated towards coincident orientations once they came into contact with each other. This process was driven by the reduction of subdomain boundary energy associated with misorientation loss. Rotation of garnet grains was accommodated by diffusion in the subdomain boundary and diffusional creep and rigid body rotation of other minerals (quartz and mica) around the garnet. An analytical model, in which the kinetics of garnet rotation are controlled by the rheology of surrounding quartz, suggests that, at the conditions of metamorphism, the rotation required to give a strong preferred orientation can occur on a similar time-scale to that of porphyroblast growth.

Development of garnet porphyroblasts by multiple nucleation, coalescence and boundary misorientation‐driven rotations

AbstractTwo types of garnet porphyroblast occur in the Schneeberg Complex of the Italian Alps. Type 1 porphyroblasts form ellipsoidal pods with a centre consisting of unstrained quartz, decussate mica and small garnet grains, and a margin containing large garnet grains. Orientation contrast imaging using the scanning electron microscope shows that the larger marginal garnet grains comprise a number of orientation subdomains. Individual garnet grains without subdomains are small (&lt; 50 µm), faceted and idioblastic, and have simple zoning profiles with Ca‐rich cores and Ca‐poor rims. Subdomains of larger garnet grains are similar in size to the individual, small garnet grains. Type 2 porphyroblasts comprise only ellipsoidal garnet, with small subdomains in the centre and larger subdomains at the margin. Each subdomain has its own Ca high, Ca dropping towards subdomain boundaries. Garnet grains, with or without subdomains, all have the same Ca‐poor composition at rims in contact with other minerals. The compositional zonation patterns are best explained by simultaneous, multiple nucleation, followed by growth and amalgamation of individual garnet grains. The range of individual garnet and garnet subdomain sizes can be explained by a faster growth rate at the porphyroblast margin than in the centre. The difference between Type 1 and Type 2 porphyroblasts is probably related to the growth rate differential across the porphyroblast.Electron backscatter diffraction shows that small, individual garnet grains are randomly oriented. Large marginal garnet grains and subdomain‐bearing garnet grains have a strong preferred orientation, clustering around a single garnet orientation. Misorientations across subdomain boundaries are small and misorientation axes are randomly oriented with respect to crystallographic orientations. The only explanation that fits the observational data is that individual garnet grains rotated towards coincident orientations once they came into contact with each other. This process was driven by the reduction of subdomain boundary energy associated with misorientation loss. Rotation of garnet grains was accommodated by diffusion in the subdomain boundary and diffusional creep and rigid body rotation of other minerals (quartz and mica) around the garnet. An analytical model, in which the kinetics of garnet rotation are controlled by the rheology of surrounding quartz, suggests that, at the conditions of metamorphism, the rotation required to give a strong preferred orientation can occur on a similar time‐scale to that of porphyroblast growth.

The role of conjugate crenulation cleavage in the development of ‘millipede’ microstructures

Millipede microstructures are described from Archean metaturbidites surrounding the Back River volcanic complex, eastern Slave Structural Province. They are locally developed within andalusite porphyroblasts that grew in response to the intrusion of late syn-kinematic granitoids. The porphyroblasts acted as a mechanical heterogeneity and resulted in the nucleation of conjugate crenulations, which were then helicitically overgrown by the porphyroblasts. This process produced the millipede morphology through a simple, consistent mechanism. The model presented here for the development of millipede microstructures represents an alternative to other models requiring complex deformation partitioning and restricted porphyroblast growth.

Crystallization processes in migmatites

An important prerequisite for interpreting microstructures in plutonic and metamorphic rocks is understanding crystallization processes. This study uses crystal size distribution (CSD) and grain shape parameters to investigate mineral-reaction-controlled crystal growth and melt solidification in migmatites of the Bayerische Wald (Variscan Orogeny, Germany). Partially molten rocks were studied because they allow the simultaneous investigation of crystallization of a melt and the growth of solid products of a metamorphic reaction. CSD and shape factors were obtained for three different minerals: (1) cordierite in a diatexite, where the melt remained in the system; (2) K-feldspar in a segregated K-feldspar + quartz leucosome; and (3) plagioclase from a segregated plagioclase + quartz leucosome. Cordierite crystals exhibit a nearly linear CSD pattern and subeuhedral crystal shapes. K-feldspar grains display an approximately linear CSD except for a marked increase at large crystal size classes. K-feldspar grains are characterized by strongly lobate grain boundaries. The CSD of plagioclase crystals is nearly linear, with a slight decrease at small crystal size classes. Plagioclase shape factors also indicate lobate grain boundaries, however, with lower values than K-feldspar. Cordierite CSD and shape data are consistent with interface-controlled crystallization. The CSD indicates that nucleation and growth is the rate limiting step during the cordierite-producing reaction, in contrast to porphyroblast growth in solid rocks where mass transfer is typically rate limiting. CSD combined with information on the duration of crystallization yields growth rates of 10 −17 to 10 −18 m/s. The plagioclase data are consistent with magmatic crystallization and are well-described by the communicating neighbor theory. The K-feldspar data also indicate magmatic crystallization. The “overproduction” of large grains of K-feldspar reflects the interplay between nucleation and growth rates at the initial stage of crystallization. Because of different environments between solid and anatectic rocks, the CSD may help to decipher metamorphic from anatectic migmatites.

Elle: the numerical simulation of metamorphic and deformation microstructures

We present a generalised framework for the numerical simulation of the evolution of rock microstructures during deformation and metamorphism. This approach is based upon a data structure that describes a polycrystalline material using a two-dimensional network of nodes and connecting boundaries that allows micro-processes to be analysed at a range of scales. The nodes may possess attributes of position, topology and chemistry; and polygonal domains defined by these nodes may possess attributes of mineralogy, rheology and lattice orientation. We represent the complex behaviour of deforming and metamorphosing rocks at the grain scale as the interaction of a set of locally-defined driving forces and micro-processes, calculated for small time steps. A central program controls the evolution of extrinsic variables such as temperature and defines the history of deformation or metamorphic processes. The central program then passes the data structure to distinct process algorithms, which interact with this data structure, both by using it to determine the local values of driving forces, and by altering the attributes to simulate the progress of the process. We outline the function of the different aspects of the modelling task, provide simple examples showing porphyroblast growth during deformation and static grain growth, and describe the data structure that we have developed which enables us to handle multi-process simulations.

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 ulvospinel-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.

The significance of monazite U–Th–Pb age data in metamorphic assemblages; a combined study of monazite and garnet chronometry

In this study, a coupled U–Th–Pb isotopic and EMP chemical study was carried out in situ on monazite (micro-inclusions within garnet as well as matrix grains) from rocks recovered from two chronologically well-constrained areas of the Himalayan orogen. Monazite inclusions within garnets from three samples yield ages of ∼44–36 Ma, whereas matrix grains within one sample are typically younger (30–26 Ma). Y depletion of these younger matrix grains indicates that they grew after garnet had crystallised. The recognition of episodic monazite growth during regional metamorphism, first at greenschist facies (before garnet growth) and secondly at upper amphibolite facies (post-dating garnet growth), allows relative growth ages of the occluding garnet to be calculated. These are in excellent agreement with Sm–Nd garnet ages from surrounding units. This approach not only provides a ready means of obtaining porphyroblast growth ages but also allows the combination of precise U–Th–Pb data from metamorphic monazite with thermobarometric information obtained from rock-forming minerals.

Polyphase evolution and reaction sequence of compositional domains in metabasalt: a model based on local chemical equilibrium and metamorphic differentiation

AbstractEclogitic garnet amphibolite samples from the Southern Steep Belt of the Central Alps show evidence of several stages of metamorphic evolution and exhumation. A method for unravelling this evolution is presented and applied to these samples. It involves a combination of detailed petrographic analysis and microchemical characterization with quantitative models of the thermodynamically stable phase relations for specific compositional domains of each sample. Preserved mineral relics and textural evidence are compared to model predictions to identify the important irreversible reactions. The interpretation of the exhumation history is thus based on the consistency of a wide spectrum of observations with predicted phase diagrams, leading to robust reconstruction of a pressure–temperature (P–T) path even where the mineralogical relics in samples are insufficient, due to retrogression, to warrant application of multi‐equilibrium thermobarometric techniques.The formation of compositionally different domains in the metabasalt samples studied is attributed to prograde growth of porphyroblasts (e.g. garnet, plagioclase, zoisite) in the matrix, implying substantial metamorphic differentiation at the scale of a few millimetres. Chemical interaction among different domains during the subsequent P–T evolution is shown to have been very limited. This led to different reaction sequences during exhumation, in which relics preserved in different domains reflect a range of continually changing P–T conditions. For samples from a single outcrop, we deduce a Barrovian prograde path to eclogite facies (23 ± 3 kbar, 750 ± 50°C), followed by (rapid) decompression to 8 ± 1 kbar and 675 ± 25°C, and a final heating phase at similar pressures reaching 750 ± 40°C. This evolution is attributed to the Alpine cycle involving subduction→collision and slab breakoff→extrusion of tectonic fragments that make up the Southern Steep Belt of the Central Alps. Copyright © 2000 John Wiley &amp; Sons, Ltd.

Dome emplacement and formation of kilometre-scale synclines in a granite–greenstone terrain (Quadrilátero Ferrı́fero, southeastern Brazil)

The Quadrilátero Ferrı́fero is a portion of the Brazilian Precambrian shield containing a granite–greenstone terrain characterized by granite–gneiss domes encircled by kilometre-scale synclines. Integration of porphyroblast-matrix microtextures, c-axis fabrics analysis and strain analysis with outcrop-scale structural relationships indicates that the main phase of domes and synclines evolution occurred during the westerly directed 0.8–0.6 Ga Pan-African-Brasiliano regional shortening. This tectonic event has produced the single pervasive foliation observed in the folded supracrustal sequences of the Quadrilátero Ferrı́fero. Dip-slip ductile shear zones developed at the dome–supracrustal interface are synchronous with dome evolution during the Pan-African-Brasiliano deformation, as indicated by contemporaneous growth of aureole porphyroblasts in the shear planes. These shear zones facilitated steeply directed downward flow of country rock material around the granite–gneiss domes. The marked density contrast between granite–gneiss material and the iron-rich supracrustals provided a gravitationally favourable environment for downward wallrock flow. Variations in shear zone kinematics away from the domes, combined with kilometer-scale viscous drag, contributed to produce kilometre-scale synclines in the supracrustal rocks constrained between upward moving domes.

Porphyroblast inclusion trail geometries in the Grand Canyon: evidence for non-rotation and rotation?

Porphyroblasts in Paleoproterozoic supracrustal rocks of the Grand Canyon grew during a progressive deformation that involved shortening and heterogeneous development of NE-striking subvertical foliation ( S 2) that crenulates and transposes an earlier foliation ( S 1). Inclusion trails in syn- S 2 porphyroblast cores show a remarkably consistent NW strike across the regional transect, suggesting a lack of appreciable porphyroblast rotation relative to the trace of the NE-striking S 2 foliation. These apparently non-rotated porphyroblast cores thus preserve a regional NW-striking S 1 orientation. Porphyroblast rims and younger porphyroblasts locally overgrow NE-striking S 2 indicating that porphyroblast growth (and continued non-rotation) spanned S 2. Apparent non-rotation of most porphyroblasts across the transect is interpreted to reflect a dominance of coaxial shortening. In contrast, single thin sections show domains of these ‘non-rotated’ garnets with sigmoidal inclusion trails adjacent to domains where the characteristic sigmoids are rotated by about 45°. This documents local rotation of garnets relative to S 2 during a heterogeneous late or post- S 2 ‘reactivation’ of the foliation. Thus these types of studies should continue to look for both rotational and non-rotational behaviors at all scales and continue to specify the frame of reference.

Porphyroblast microstructures; a review of current and future trends

Many recent papers show how porphyroblast microstructures play an important role in a wide range of structural and metamorphic studies. This paper reviews ten current applications of these microstructures: (1) porphyroblast growth-timing criteria; (2) tracking progressive foliation development relative to changing metamorphic conditions; (3) timing of pluton emplacement relative to deformation and metamorphism; (4) finite longitudinal strain determinations; (5) kinematics and porphyroblast rotation; (6) use of linear fabrics preserved in porphyroblasts; (7) porphyroblasts and folding mechanisms; (8) inclusion-trail orientations and orogenic processes; (9) inferring shear-strain rates from porphyroblast growth rates; and (10) in-situ age determinations. Although there is still no concensus on the interpretation of some porphyroblast microstructures, a bright future lies ahead as traditional and newly developed techniques of microstructural analysis are combined with modern chemical and microprobe techniques to provide an increased understanding of the relationships between deformation and metamorphism in a wide range of metamorphic settings.

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.

Pan-African pressure-temperature evolution of the Merelani area in the Mozambique Belt in northeast Tanzania

The bedrock of the Merelani area in the Mozambique Belt in northeast Tanzania was multiply deformed and metamorphosed in Pan-African (Neoproterozoic) times. The first metamorphic event took place at the granulite grade and later the rocks suffered retrogression during isobaric cooling (IBC) at the amphibolite grade. Thermobarometric data from the Merelani gneisses indicate temperatures in the range 670–610°C and pressures of 6.5–6.0 kbar. Isobaric cooling is exemplified by late, post-tectonic growth of kyanite porphyroblasts at the expense of sillimanite in graphitic paragneisses as well as by garnet zoning in the same rock. Further retrogression was due to hydration along shear zones and faults. This led to the alteration of forsterite, enstatite and pargasite into serpentine minerals. Crystallisation of vanadiferous zoisite (tanzanite) in hydrothermally altered rocks took place at this late stage when X H 2O = 0.5 in calcsilicate rocks. These findinds, combined with the textural and thermobarometric data from the Uluguru Mountains and the Furua area, suggest that the IBC P- T path was prominent in the metamorphic evolution(s) of the Pan-African Mozambique Belt rocks in Tanzania. This was most probably caused by addition of igneous rocks (underplating) at the initial stage of the formation of the granulite-facies rocks in the belt: followed by their residence and coolingin the middle and upper crustal levels. The slight decompression during this stage could be explained by either extensional or transtensional deformation. Finally, the Merelani rocks, like those in the Uluguru Mountains, were rapidly exhumed.