Zoning and recrystallization of phengitic micas: implications for metamorphic equilibration

White micas (phengites) in the metasediments of the Scottish Dalradian display a large range of compositions within single samples. The variations in the composition of these phengites are strongly controlled by their structural age, with early fabrics containing a paragonite-poor, celadonite-rich phengite whereas in later fabrics the micas are generally paragonite-rich and celadonite-poor. Retrograde phengite growth, identified using back scattered electron imaging, occurs as celadonite-rich rims on micas within all existing fabrics and appears to be preferentially developed along existing white mica-plagioclase grain boundaries. The presence of these chemically distinct phengite populations within single samples implies that chemical exchange between the individual micas was inefficient. It is proposed that diffusion-controlled exchange reactions in phengites have relatively high closure temperatures below which major element exchange is effectively impossible. This closed system behaviour of micas questions the ease with which phengites may equilibrate with other phases during prograde greenschist and lower amphibolite facies metamorphism. Many of the chemical variations preserved in phengites from such metamorphic rocks may reflect deformation/recrystallization controlled equilibria.     

Modelling white mica pressure‐temperature‐time (P‐T‐t) paths using thermobarometric and 40Ar/39Ar thermochronologic data

White micas are major rock forming minerals in igneous and metamorphic rocks, and their chemical and isotopic variations can be used to determine pressure, temperature and time (P‐T‐t) histories. We apply ⁴⁰Ar/³⁹Ar multi‐diffusion domain modelling to white micas from blueschists blocks in serpentinite matrix mélange from the exhumed Baja California subduction complex. Thermal history models yielded T‐t paths suggesting that ⁴⁰Ar* resides within multiple diffusion domains with varying ⁴⁰Ar* retentivity. Modelled white mica thermal histories and thermobarometric data were used to forward model continuous P‐T‐t paths. P‐T‐t paths are consistent with previous studies and are interpreted to constrain blueschist block exhumation paths within the Baja accretionary wedge. Our P‐T‐t models use temperature controlled ⁴⁰Ar/³⁹Ar step heat data in which argon loss by volume diffusion can be demonstrated, and for which the white mica petrogenesis is known.     

Coupling of deformation and reactions during mid-crustal shear zone development: an in situ frictional–viscous transition

A well preserved strain and reaction gradient records the progressive transformation of a megacrystic Kfs+Cpx+Opx+Bt₁±Qtz syenitic pluton to a strongly sheared Kfs+Act+Bt₂+Ab+Qtz tectonite within the exhumed Norumbega Fault System, Maine, USA. Detailed microstructural analysis indicates that fracturing and localized fluid infiltration initiated the deconstruction of the existing K-feldspar and two-pyroxene load-bearing framework, and that feedback among metamorphic reactions, fabric development and enhanced permeability during progressive shearing led to the development of an interconnected, biotite- and actinolite-rich foliation. The activation of dislocation creep in biotite and quartz, and dissolution–precipitation creep in actinolite and feldspar, with increasing strain ultimately resulted in a transition from dominantly frictional to dominantly viscous deformation processes. Petrological data show that various scales of geochemical disequilibrium exist across the strain and reaction gradient, and that reaction progress was limited by slow chemical diffusion during the early stages of deformation. Petrological modelling results indicate that the existing plutonic assemblage was metastable at mid-crustal conditions, and that fluid infiltration and deformation allowed the product assemblage to advance towards chemical equilibration. Comparison of the observed microstructures and deformation mechanisms with experimental and numerical modelling results suggest that the development of an interconnected biotite-dominated fabric probably caused a major (up to three fold) reduction in bulk rock strength and localization of strain into the foliated margin.     

The Occurrrence of Garnet in the Granulite-Facies Terrane of the Addirondack Highlands and Elsewhere, an Amplification and a Reply

Elaborating on the theme of a previous paper (1965) and in responseto Buddington‘s criticism (1966) the present discussionis primarily concerned with the high-grade meta-morphic reactionsand regional-metamorphic zoning in the Adirondacks. The hypersthene isograd, delineated by the first appearanceof orthopyroxene as a product of P_w-T-controlted reactions involvinghornblende and biotite in quartz-buartz-bearing rocks. separatesthe almandite-amphibolite-facies terrane of the lowlands fromthe granulite-facies region of the highlands. In the almandite-amphibolite facies of the lowlands at leastone isograd, the orthoclase-garnet isograd, (Ruddington's lstgarnet isograd), can be recognized which is delineated by theP_w-T-controlled appearance of these minerals at the expenseof muscovite and part of the biotite and quartz. The granulite-facies terrane of the highlands is characterizedby the predominance of assemblages of the hornblende-granulitefacies in which orthopyroxene cocxists with hydrous mineralssuch as biotite and hornblende in apparent equilibrium. In theAdlrondacks there is no sizeable, regional development of thepyroxene-granulite facies where hydrous minerals are virtuallyabsent. Within the hornblende-granulite-facies terrane two isogradscan be recognized. The garent-clinopyroxene isograd (Buddington's3rd garnet isograd) delineates the regional development of garnet,clinopyroxene, and quartz by P₁-T-controlled reaction betweenorthopyroxene and plagioclase. The isograd defines in the easternAdirondack a region of the hornblende-clinopyroxene-almanditesubfacies of the honblende-granulite facies. In the westernAdirondacks, near the hypersthene isograd, a yet uncharted cordieriteisograd may define the areal extent of the biotite-cordierite-almanditesubfacies of the hornblende-granulite facies. The central portionof the Adirondacks, where cordierite is absent and garnet andclinopyroxene do not coexist except in silica-deficient rocks,is the region of the hornblende-orthopyroxene-plagioclase subfaciesof the hornblende-granulite facies.     

Scales of equilibrium and disequilibrium during cleavage formation in chlorite and biotite-grade phyllites, SE Vermont

Detailed electron microprobe analyses of phyllosilicates in crenulated phyllites from south‐eastern Vermont show that grain‐scale zoning is common, and sympathetic zoning in adjacent minerals is nearly universal. We interpret this to reflect a pressure‐solution mechanism for cleavage development, where precipitation from a very small fluid reservoir fractionated that fluid. Multiple analyses along single muscovite, biotite and chlorite grains (30–200 μm in length) show zoning patterns indicating Tschermakitic substitutions in muscovite and both Tschermakitic and di/trioctahedral substitutions in biotite and chlorite. Using cross‐cutting relationships and mineral chemistry it is shown that these patterns persist in cleavages produced at metamorphic conditions of chlorite‐grade, chlorite‐grade overprinted by biotite‐grade and biotite‐grade. Zoning patterns are comparable in all three settings, requiring a similar cleavage‐forming mechanism independent of metamorphic grade. Moreover, the use of ⁴⁰Ar/³⁹Ar geochronology demonstrates this is true regardless of age. Furthermore, samples with chlorite‐grade cleavages overprinted by biotite porphyroblasts suggest the closure temperatures for the diffusion of Al, Si, Mg and Fe ions are greater than the temperature of the biotite isograd (>～400 °C). Parallel and smoothly fanning tie lines produced by coexisting muscovite–chlorite, and muscovite–biotite pairs on compositional diagrams demonstrate effectively instantaneous chemical equilibrium and probably indicate simultaneous crystallization. These results do not support theories suggesting cleavages form in fluid‐dominated systems. If crenulation cleavages formed in systems in which the chemical potentials of all major components are fixed by an external reservoir, then the compositions of individual grains defining these cleavages would be uniform. On the contrary, the fine‐scale chemical zoning observed probably reflects a grain‐scale process consistent with a pressure‐solution mechanism in which the aqueous activities of major components are defined by local dissolution and precipitation. Thus the role of fluids was probably limited to one of catalysing pressure‐solution and fluids apparently did not drive cleavage development.     

Progressive sequence of reactions of the Ryoke metamorphism in the Yanai district, southwest Japan: the formation of cordierite

The compositions of biotite and muscovite were examined in terms of the paragenesis and the metamorphic grade in low- to medium-grade pelitic rocks of the Ryoke metamorphism in the Yanai district, southwest Japan. The biotite and muscovite that coexist with K-feldspar have a higher K component in an A'KF diagram than those in rocks lacking K-feldspar. This fact reflects an increase in the K₂O content in muscovite, but in biotite it reflects an increase of not only the K₂O content but also of the octahedral vacancy.
At higher metamorphic grade beyond the cordierite isograd, where cordierite coexists with neither chlorite nor K-feldspar, the biotite shows an increase in illite, K Aliv □xii−1 Si−1, and Tschermak components, Alvi Aliv R+−1 Si−1, where □xii and R+ denote the interlayer vacancy and (Fe+Mg+Mn), respectively. A reaction to define the cordierite isograd is proposed by treating this chemical change as being responsible for the first appearance of cordierite, i.e. K,Al-poor biotite+phengitic muscovite=K,Al-rich biotite+cordierite+quartz+water .By treating this as a key reaction in medium-grade metamorphism, a set of reaction in a progressive metamorphism is established for the Ryoke metamorphism, a typical low-pressure type metamorphism. Some textures in one of the high-grade areas, the K-feldspar-cordierite zone, suggest that a further two prograde reactions have taken place, i.e. andalusite+biotite+quartz=cordierite+K-feldspar+water
and   andalusite=sillimanite.quartz=cordierite+K-feldspar+water
This implies that this zone probably has a P–T  path involving isobaric heating.     

The sillimanite-potash feldspar isograd in Western Maine,U.S.A.

The techniques of electron probe microanalysis and x-ray diffractometry have been utilized in a study of the sillimanite-potassium feldspar isograd in western Maine. The isograd reaction is theoretically a discontinuous one, calling for the nearly instantaneous loss of muscovite and crystallization of sillimanite and orthoclase, with a small contribution of albite from the pre-existing plagioclase. In fact, muscovite coexists with orthoclase, sillimanite, and plagioclase for a distance of at least seven miles from the isograd (marked by the initial coexistence of orthoclase and sillimanite). In this assemblage, muscovite has an extremely narrow range of composition, about an average of Ms_93.5Pg_6.5. A possible explanation for the divariant character of the isograd reaction is that, during dehydration, PH₂O slowly increased from initial values less than P_total + rock strength, under conditions of low permeability, the actual value of PH₂O being controlled by a buffer assemblage and local conditions of P and T. An alternative explanation postulates the flattening of thermal gradients following the onset of fractional melting. The isograd reaction is dependent in only a minor way upon the anorthite content of the plagioclase. Below the isograd, a continuous reaction takes place leading to a diminution in paragonite content of muscovite stable in the presence of quartz. It is possible that this reaction leads to the nearly ubiquitous normal zoning of the plagioclase. Changes in the composition of biotite at the isograd are not conspicuous, and can be satisfactorily explained by the release of Mg, Fe, and Ti impurity from the muscovite, and a continuous reaction between ilmenite, quartz, and muscovite. Garnets are not abundant and are high in Mn, both facts probably due to the low pressure of metamorphism, The presence of garnet probably relates to the Mn content of the rock, and seems to be independent of the Mg/Fe ratio of the biotite. The garnets are zoned with respect to Mn and Mg, but often Mn is enriched and Mg depleted in the marginal zone. The Mg/Fe ratio of the biotite varies twofold depending on the presence or absence of pyrrhotite. The transition: microcline → orthoclase depends upon the amount of dissolved albite; the polymorph is orthoclase in the pelitic schists but microcline in the calc-silicate rocks which are much lower in sodium. The plagioclases are of “low” structural type, although is slightly greater than many other “low” plagioclases. A correlation of d(002) of muscovite and paragonite solid solution for the range 0 to 20 % paragonite is given. An appreciable positive volume of mixing for the binary system muscovite-paragonite is indicated.     

Deformation of biotite and muscovite: Optical microstructure

An optical comparison illustrates the difference in behaviour of the two mica minerals biotite and muscovite; their response to deformation, and to chemical processes such as grain dissolution. Non-passive mechanical rotation, segmentation of deformed grains by a recovery-recrystallization type process and syntectonic growth of the phyllosilicates all contribute to the development of a strong tectonic foliation within a deformed pegmatite from the Italian Alps. There are significant mechanical differences between the two micas. Biotite readily deforms by kinking whereas muscovite forms sinusoidal folds and seldom kinks. If kink-like structures (deformation zones) are present in muscovite they are generally accompanied by fracturing. Fracturing and displacements are obvious in most deformed muscovites both parallel to axial surfaces and between (001) cleavages. Fracturing is occasionally recognised in biotite. However, it is often obscured by extensive dissolution and new grain nucleation. Evidence for dissolution processes in biotite is more abundant than in muscovite.     

On micas from magmatic and metamorphic rocks

Micas from magmatic and metamorphic rocks differ from one another in chemical composition and in trace element content. The chemical composition of micas is discussed in relation to their occurrence, paragenesis and sequence of crystallization. On the basis of previous studies of the relationship between the physical properties and the chemical composition of 34 chemically analysed micas, reliable physical methods have been established which permit identification of different mica varieties in the same rock. Structural formulae and trace element content of micas from basic and granitic rocks, as well as from skarns, schists, ortho- and paragneisses are discussed. The relationship between the components of the tetrahedral and octahedral layers and of the interlayer are illustrated as ratios. Poorly differentiated, hybrid and metasomatic rocks often contain more than one variety of mica. Some prophyritic basalts and lamprophyres contain an early phlogopite which is paragenetically related to pyroxene phenocrysts and late biotite which occurs in the groundmass and in the fractures as a result of the crystallization of residual magma. The biotitemuscovite assemblage was observed in granodiorites, quartz-monzonites, schists and gneisses. In the albite-K-feldspar granites, muscovite predominates and the biotite is usually altered. The chemical composition of micas from metamorphic rocks depends on the grade of metamorphism and on the nature of associated minerals. The biotite from paragneisses contains considerable quantities of octahedral alumina. Pre-metamorphic micas show variable deficiencies of the (OH, F) group. The micas are useful minerals in determining the degree of differentiation and subsequent alteration of igneous rocks. The present study was carried out on the basis of 34 recent complete chemical analyses andca 100 X-ray fluorescence analyses. Dedicated to Professor Dr.Carl W. Correns on the occasion of his 70th birthday.     

A Petrogenetic Grid for Reactions Producing Biotite and other Al-Fe-Mg Silicates in the Greenschist Facies

This report presents an analysis of phase relations among biotite,muscovite, chlorite, stilpnomelane, actinolite and K-feldsparin a ten component system within the greenschist facies. Itis based on study of the chemical composition of these minerals,on calculations to balance chemical equations-among them, andon their field distribution. A petrogenetic grid resulting fromthis treatment consists of a single invariant point and multipleunivariant lines, the number depending on what part of the assemblageset is held constant. The reactions which involve biotite aresimilar to previously proposed reactions for the biotite isograd.At high pressure, biotite is produced from muscovite+stilpnomelane+actinolite?K-feldspar.At low pressure, chlorite+K-feldspar?stilpnomelane?actinolitereact to form biotite. A biotite-free reaction, not previouslyidentified, divides the chlorite zone into high pressure andlow pressure fields, characterized by the assemblages muscovite+stilpnomelane+actinoliteand chlorite+K-feldspar, respectively. In the blueschist facies,muscovite plus stilpnomelane and/or actinolite are stable insteadof biotite.     

Constraining absolute deformation ages: the relationship between deformation mechanisms and isotope systematics

Isotope diffusion in a mineral is strongly temperature dependent but is also a function of grain size. Deformation must, therefore, be an important consideration in the interpretation of isotopic data because it provides a means of modifying grain size and shape. We illustrate the range of different deformation mechanisms common in micas and use simple models to investigate the relationship between these and isotope diffusion. We consider three different thermal scenarios with deformation taking place during: (a) the prograde heating path, (b) at the closure temperature of the deforming mineral, and (c) at temperatures significantly below the closure temperature. We have modelled these simple systems using a finite difference algorithm that simulates argon diffusion profiles and bulk ages. This modelling illustrates that obtaining deformation ages is critically dependent on an understanding and recognition of the different deformation mechanisms that have affected the sample. In the cases where deformation causes a change in grain size, it is important to characterise both the temperature at which deformation takes place and the closure temperature of grains formed during the deformation. The development of grains with T_c greater than the deformation temperature may record a deformation age. Examples of this condition include: (i) neocrystallisation; (ii) grain size reduction occurring at temperatures below T_c (of the reduced grain size) where the deformation mechanism has reset the grains; and (iii) deformation-induced grain coarsening.     

An electron microscopy study of deformation microstructures in granitic mylonites from southwestern Sweden, with special emphasis on the micas

Summary ¶The lithology, age, geological setting, structural and metamorphic history of the granitic mylonites from the Mylonite Zone (MZ) in southwestern Sweden have been studied extensively. The deformation history, growth of microstructures, intensity of deformation, changes in mineral compositions, and pressure-temperature conditions of deformation have, however, not been addressed. In this study, powder X-ray diffraction, optical microscopy, electron microprobe analysis and transmission electron microscopy of micas, chlorite, and plagioclase are combined to understand the physical and textural changes experienced by the rocks during mylonitization. It is shown that the occurrence of foliated micas in shear bands, recrystallization of quartz and biotite, and undulatory extinction in quartz grains were not uniform throughout the samples studied. Occurrence of dislocations and low-angle grain boundaries confirm that deformation occurred largely by glide dislocations. The low-angle grain boundaries observed are formed by the re-arrangement of these dislocations during grain size reduction processes. The micas show a high degree of spatial stacking order, but spatial stacking disorder in micas and chlorites has also been found.Ordered stacking faults are formed during low strain while disordered stacking faults are formed under high strain conditions. Occurrence of both ordered and disordered stacking faults indicates that the intensity of deformation was not uniform through the entire MZ. Moreover, the chemical composition of plagioclase shows that the exsolution lamellae observed with optical and electron microscopy are due to Ca-subsolidus reactions during low temperature deformation. Several substitution reactions occurring in the micas indicate that deformation took place between 0.3 and 0.4GPa, at a temperature higher than 500°C.Received October 15, 2001; revised version accepted December 25, 2002 Published online June 2, 2003     

A Petrogenetic Grid for Reactions Producing Biotite and other Al--Fe--Mg Silicates in the Greenschist Facies

This report presents an analysis of phase relations among biotite,muscovite, chlorite, stilpnomelane, actinolite and K-feldsparin a ten component system within the greenschist facies. Itis based on study of the chemical composition of these minerals,on calculations to balance chemical equationsamong them, andon their field distribution. A petrogenetic grid resulting fromthis treatment consists of a single invariant point and multipleunivariant lines, the number depending on what part of the assemblageset is held constant. The reactions which involve biotite aresimilar to previously proposed reactions for the biotite isograd.At high pressure, biotite is produced from muscovite+stilpnomelane+actinolite±K-feldspar.At low pressure, chlorite+K-feldspar±stilpnomelane±actinolitereact to form biotite. A biotite-free reaction, not previouslyidentified, divides the chlorite zone into high pressure andlow pressure fields, characterized by the assemblages muscovite+stilpnomelane+actinoliteand chlorite+K-feldspar, respectively. In the blueschist facies,muscovite plus stilpnomelane and/or actinolite are stable insteadof biotite.     

Metamorphic reactions in the biotite zone, eastern Scotland: high thermal gradients, metasomatism and cleavage formation

Prograde metamorphic reactions involving the growth of phyllosilicates and accompanying cleavage development have been investigated in Dalradian metasediments from the biotite zone of eastern Scotland. Crystallization of muscovite within the psammites of the Southern Highland Group is linked to the replacement of plagioclase porphyroclasts. This reaction is triggered by significant alkali metasomatism during active deformation and plays an important role in the formation of a prominent spaced cleavage within the psammites. The Si content of most of these early-formed muscovites is partially buffered by the quartz content of the rock, although close to the Highland Boundary Fault, evidence of greater influence from externally derived fluids on muscovite compositions is preserved. Locally higher fluid fluxes adjacent to the fault are also indicated by a relatively high δ ¹⁸O_(SMOW) signature in the rocks. The biotite-producing reaction in these greenschist-facies rocks is linked to the later production of a celadonite-poor muscovite which formed as overgrowths around pre-existing white micas. This reaction is sensitive to the initial composition of muscovite and preferentially occurs in quartz-rich metasediments containing a celadonite-rich muscovite. A systematic increase in the progress of the biotite-producing reaction northwards across the biotite zone confirms the presence of high geothermal gradients along the southern margin of the Dalradian block, adjacent to the Highland Boundary Fault. Received: 26 February 1999 / Accepted: 8 November 1999     

Carbonaceous material in pelitic schists of the Sanbagawa metamorphic belt in central Shikoku,Japan

Carbonaceous material in pelitic schists of the Sanbagawa metamorphic belt in central Shikoku, Japan, was separated from the host rocks and its X-ray diffraction and chemical composition were studied. Its crystal structure and chemistry change continously with increasing metamorphic grade and approach those of well-ordered graphite near the biotite isograd. As graphitization is a rate process, the temperature of complete graphitization differs from one metamorphic terrain to another as a function of the duration of metamorphism. In an individual metamorphic terraan, however, the degree of graphitization is a useful indicator of relative metamorphic temperature in lower-grade rocks.     

Experimental investigation of the formation of cordierite-orthopyroxene parageneses in pelitic rocks

Cordierite and orthopyroxene (or orthoamphibole) are widespread in migmatitic terranes, and partial melting of pelitic rocks may be important in their production. In particular, the reaction quartz +albite+biotite+garnet+water vapor = cordierite +orthopyroxene or orthoamphibole+melt was among reactions discussed by Grant (1973) but poorly constrained in pressure-temperature space.This reaction involves too many phases to be readily studied experimentally. Therefore simpler melting and dehydration reactions involving quartzalbite-biotite-cordierite-orthopyroxene were investigated.In conjunction with the work of Hoffer (1976, 1978) these experiments place useful constraints on the above reaction and on the reaction quartz+albite+aluminosilicate+biotite+vapor = cordierite+garnet+melt. In pelitic rocks near the second illimanite isograd, cordierite and garnet may coexist with melt as low as 660° C and cordierite and orthopyroxene may coexist with melt at temperatures less than 675° C. In the absence of significant Mn or Ca, in pelitic rocks within the realm of melting, biotite+garnet assemblages are probably limited to pressures greater than 2kb and aluminosilicate+biotite assemblages to pressures greater than 3kb.     

Argon loss during deformation of micas: constraints from laboratory deformation experiments

The role of internal deformation in resetting argon ages of micas has been investigated by measuring ⁴⁰Ar/³⁹Ar ratios of biotite and muscovite, before and after experimentally deforming them. Neither mica crushed cataclastically at room temperature on-line with a mass spectrometer showed any measurable change in ⁴⁰Ar/³⁹Ar age. Muscovite crystals either sheared along the K-interlayer and/or kinked at 400 °C and 100–200 MPa confining pressure, exhibit small (0.7–1.0%) reductions in age and marked increases in bulk diffusion coefficients, as determined from argon release during the initial stages of step-heating between 550 and 810 °C. We conclude that the relatively young ages of fine-grained mica populations in naturally deformed mylonites result primarily from grain size refinement and reductions in length scale for volume diffusion and/or by syntectonic neocrystallization. Internal deformation involving dislocation slip and kinking may contribute to some argon loss by pipe diffusion, but reductions in closure temperature associated with multipath diffusion are small.     

Deformation, mass transfer and mineral reactions in an eclogite facies shear zone in a polymetamorphic metapelite (Monte Rosa nappe, western Alps)

This study analyses the mineralogical and chemical transformations associated with an Alpine shear zone in polymetamorphic metapelites from the Monte Rosa nappe in the upper Val Loranco (N‐Italy). In the shear zone, the pre‐Alpine assemblage plagioclase + biotite + kyanite is replaced by the assemblage garnet + phengite + paragonite at eclogite facies conditions of about 650 °C at 12.5 kbar. Outside the shear zone, only minute progress of the same metamorphic reaction was attained during the Alpine metamorphic overprint and the pre‐Alpine mineral assemblage is largely preserved. Textures of incomplete reaction, such as garnet rims at former grain contacts between pre‐existing plagioclase and biotite, are preserved in the country rocks of the shear zone. Reaction textures and phase relations indicate that the Alpine metamorphic overprint occurred under largely anhydrous conditions in low strain domains. In contrast, the mineralogical changes and phase equilibrium diagrams indicate water saturation within the Alpine shear zones. Shear zone formation occurred at approximately constant volume but was associated with substantial gains in silica and losses in aluminium and potassium. Changes in mineral modes associated with chemical alteration and progressive deformation indicate that plagioclase, biotite and kyanite were not only consumed in the course of the garnet‐and phengite‐producing reactions, but were also dissolved ‘congruently’ during shear zone formation. A large fraction of the silica liberated by plagioclase, biotite and kyanite dissolution was immediately re‐precipitated to form quartz, but the dissolved aluminium‐ and potassium‐bearing species appear to have been stable in solution and were removed via the pore fluid. The reaction causes the localization of deformation by producing fine‐grained white mica, which forms a mechanically weak aggregate.     

Prograde destruction and formation of monazite and allanite during contact and regional metamorphism of pelites: petrology and geochronology

The conditions at which monazite and allanite were produced and destroyed during prograde metamorphism of pelitic rocks were determined in a Buchan and a Barrovian regional terrain and in a contact aureole, all from northern New England, USA. Pelites from the chlorite zone of each area contain monazite that has an inclusion-free core surrounded by a highly irregular, inclusion-rich rim. Textures and ²⁰⁸Pb/²³²Th dates of these monazites in the Buchan terrain, obtained by ion microprobe, suggest that they are composite grains with detrital cores and very low-grade metamorphic overgrowths. At exactly the biotite isograd in the regional terrains, composite monazite disappears from most rocks and is replaced by euhedral metamorphic allanite. At precisely the andalusite or kyanite isograd in all three areas, allanite, in turn, disappears from most rocks and is replaced by subhedral, chemically unzoned monazite neoblasts. Allanite failed to develop at the biotite isograd in pelites with lower than normal Ca and/or Al contents, and composite monazite survived at higher grades in these rocks with modified texture, chemical composition, and Th-Pb age. Pelites with elevated Ca and/or Al contents retained allanite in the andalusite or kyanite zone. The best estimate of the time of peak metamorphism at the andalusite or kyanite isograd is the mean Th-Pb age of metamorphic monazite neoblasts that have not been affected by retrograde metamorphism: 364.3Dž.5 Ma in the Buchan terrain, 352.9NJ.9 Ma in the Barrovian terrain, and 403.4LJ.9 Ma in the contact aureole. Some metamorphic monazites from the Buchan terrain have ages partially to completely reset during an episode of retrograde metamorphism at 343.1Nj.1 Ma. Interpretation of Th-Pb ages of individual composite monazite grains is complicated by the occurrence of subgrain domains of detrital material intergrown with domains of material formed or recrystallized during prograde and retrograde metamorphism.     

Chemical and textural equilibration of garnet during amphibolite facies metamorphism: The influence of coupled dissolution–reprecipitation

Metamorphic equilibration requires chemical communication between minerals and may be inhibited through sluggish volume diffusion and or slow rates of dissolution in a fluid phase. Relatively slow diffusion and the perceived robust nature of chemical growth zoning may preclude garnet porphyroblasts from readily participating in low‐temperature amphibolite facies metamorphic reactions. Garnet is widely assumed to be a reactant in staurolite‐isograd reactions, and the evidence for this has been assessed in the Late Proterozoic Dalradian pelitic schists of the Scottish Highlands. The 3D imaging of garnet porphyroblasts in staurolite‐bearing schists reveals a good crystal shape and little evidence of marginal dissolution; however, there is also lack of evidence for the involvement of either chlorite or chloritoid in the reaction. Staurolite forms directly adjacent to the garnet, and its nucleation is strongly associated with deformation of the muscovite‐rich fabrics around the porphyroblasts. “Cloudy” fluid inclusion‐rich garnet forms in both marginal and internal parts of the garnet porphyroblast and is linked both to the production of staurolite and to the introduction of abundant quartz inclusions within the garnet. Such cloudy garnet typically has a Mg‐rich, Mn‐poor composition and is interpreted to have formed during a coupled dissolution–reprecipitation process, triggered by a local influx of fluid. All garnet in the muscovite‐bearing schists present in this area is potentially reactive, irrespective of the garnet composition, but very few of the schists contain staurolite. The staurolite‐producing reaction appears to be substantially overstepped during the relatively high‐pressure Barrovian regional metamorphism reflecting the limited permeability of the schists in peak metamorphic conditions. Fluid influx and hence reaction progress appear to be strongly controlled by subtle differences in deformation history. The remaining garnet fails to achieve chemical equilibrium during the reaction creating distinctive patchy compositional zoning. Such zoning in metamorphic garnet created during coupled dissolution–reprecipitation reactions may be difficult to recognize in higher grade pelites due to subsequent diffusive re‐equilibration. Fundamental assumptions about metamorphic processes are questioned by the lack of chemical equilibrium during this reaction and the restricted permeability of the regional metamorphic pelitic schists. In addition, the partial loss of prograde chemical and textural information from the garnet porphyroblasts cautions against their routine use as a reliable monitor of metamorphic history. However, the partial re‐equilibration of the porphyroblasts during coupled dissolution–reprecipitation opens possibilities of mapping reaction progress in garnet as a means of assessing fluid access during peak metamorphic conditions.