Muscovite dehydration melting: Reaction mechanisms,microstructures, and implications for anatexis

Dehydration melting of muscovite in metasedimentary sequences is the initially dominant mechanism of granitic melt generation in orogenic hinterlands. In dry (vapour-absent) crust, muscovite reacts with quartz to produce K-feldspar, sillimanite, and monzogranitic melt. When water vapour is present in excess, sillimanite and melt are the primary products of muscovite breakdown, and any K-feldspar produced is due to melt crystallization. Here we document the reaction mechanisms that control nucleation and growth of K-feldspar, sillimanite, and silicate melt in the metamorphic core of the Himalaya, and outline the microstructural criteria used to distinguish peritectic K-feldspar from K-feldspar grains formed during melt crystallization. We have characterized four stages of microstructural evolution in selected psammitic and pelitic samples from the Langtang and Everest regions: (a) K-feldspar nucleates epitaxially on plagioclase while intergrowths of fibrolitic sillimanite and the remaining hydrous melt components replace muscovite. (b) In quartzofeldspathic domains, K-feldspar replaces plagioclase by K⁺–Na⁺ cation exchange, while melt and intergrowths of sillimanite+quartz form in the aluminous domains. (c) At 7–8 vol.% melt generation, the system evolves from a closed to open system and all phases coarsen by up to two orders of magnitude, resulting in large K-feldspar porphyroblasts. (d) Preferential crystallization of residual melt on K-feldspar porphyroblasts and coarsened quartz forms an augen gneiss texture with a monzogranitic-tonalitic matrix that contains intergrowths of sillimanite+tourmaline+muscovite+apatite. Initial poikiloblasts of peritectic K-feldspar trap fine-grained inclusions of quartz and biotite by replacement growth of matrix plagioclase. During subsequent coarsening, peritectic K-feldspar grains overgrow and trap fabric-aligned biotite, resulting in a core to rim coarsening of inclusion size. These microstructural criteria enable a mass balance of peritectic K-feldspar and sillimanite to constrain the amount of free H₂O present during muscovite dehydration. The resulting modal proportion of K-feldspar in the Himalayan metamorphic core requires vapour-absent conditions during muscovite dehydration melting and leucogranite formation, indicating that the generation of large volumes of granitic melts in orogenic belts is not necessarily contingent on an external source of fluids.     

Andalusite–sillimanite–quartz veins as indicators of low-pressure–high-temperature deformation during late-stage unroofing of a metamorphic core complex, Turkey

Ductilely deformed veins consisting of quartz+andalusite, in which the andalusite is partially replaced by fibrous sillimanite, locally occur in garnet–sillimanite schist near a margin of the Niğde metamorphic core complex in south-central Turkey. Mineral assemblages, reaction textures and structural features of the veins record low-pressure–high-temperature deformation during exhumation of mid-crustal rocks. The partial replacement of andalusite by sillimanite may indicate a late-stage increase in temperature and/or fluid pressure, possibly related to Miocene magmatism, during extensional unroofing of the core complex. Aluminosilicate-bearing veins are observed at the eastern margin of the massif where metapelitic rocks were deformed during unroofing of the core complex. Veins formed in aluminous rocks where deformation-enhanced permeability allowed fluid flow during extensional shear. The cm-scale veins are typically boudinaged and form asymmetric lenses concordant with the host rock foliation and are parallel to the down-dip lineation defined by sillimanite and stretched biotite. Aluminosilicate-bearing boudins record top-to-the-east shear sense, which is compatible with the extensional shear sense displayed by structures in the host rock.     

Metasomatic albitites and related biotite-rich schists from a low-pressure polymetamorphic terrane, Snake Creek Anticline, Mount Isa Inlier, north-eastern Australia: microstructures and P–T–d paths

Rocks of the Snake Creek Anticline are mainly pelitic schists, psammitic schists and quartzites that were metamorphosed during multiple high‐T/low‐P events extending from D1 to D5, with the metamorphic peak occurring late to post‐D3. Albitites are widespread, but are concentrated in five areas. They are typically fine‐ to medium‐grained, and consist of albite, with or without combinations of quartz, biotite, staurolite, cordierite, garnet, andalusite, sillimanite, kyanite, gedrite and tourmaline. From the presence or absence of albite inclusions in porphyroblasts, the albitites are interpreted as forming early in the D3 event as a result of infiltration of external fluids. Psammitic schists and quartzites were preferentially altered, but pelitic schists were also albitized in localities where the alteration was more extreme, with the replacement of muscovite total and the replacement of quartz and biotite variable. Structural controls on albitization include fracturing and syn‐D3 shear zones in fold hinges. Biotite schists with abundant porphyroblasts (combinations of staurolite, garnet, andalusite and cordierite) occur adjacent to albitites, and it is argued that they formed by the addition of Fe and Mg sourced from the albitites. In several albitite‐rich areas, cordierite grew early in D3 and was partly or entirely replaced during or after D3 by combinations of biotite, andalusite, tourmaline, staurolite and sillimanite. A postulated P–T–d path involved an increase in pressure (with or without a decrease in temperature) subsequent to early D3 albitization, followed by an increase in temperature up to the metamorphic peak (late D3 to early D4. The metamorphism was contemporary in part with the emplacement of the Williams Batholith (c. 1550–1500 Ma), which probably supplied the Na‐rich fluids.     

Transition of metamorphic series from the Kyanite- to andalusite-types in the Altai orogen, Xinjiang, China: Evidence from petrography and calculated KMnFMASH and KFMASH phase relations

Metamorphic zones in the Chinese Altai orogen have previously been separated into the kyanite- and andalusite-types, the andalusite-type being spatially more extensive. The kyanite-type involves a zonal sequence of biotite, garnet, staurolite, kyanite, sillimanite and, locally, garnet–cordierite zones. The andalusite-type zonal sequence is similar: it includes biotite, garnet and staurolite zones at lower-T conditions and sillimanite and garnet–cordierite zones at higher-T conditions, but additionally contains staurolite–andalusite and andalusite–sillimanite zones at intermediate-T conditions. As relic kyanite-bearing assemblages commonly persist in the staurolite–andalusite, andalusite–sillimanite and sillimanite zones, it is not clear that the distinction is valid. On the basis of a reevaluation of phase relations modelled in KMnFMASH and KFMASH pseudosections, kyanite and andalusite-bearing rocks of the Chinese Altai orogen record, respectively, the typical burial and exhumation history of the terrane. Mineral assemblages distributed through the various zones reflect a mix of portions of the ambient P–T array and the effects of evolving P–T conditions. The comparatively low-T biotite, garnet and staurolite zones mostly preserve kyanite-type peak assemblages that only experienced minor changes during exhumation. Rocks in the comparatively high-T sillimanite and garnet–cordierite zones are dominated by mineral assemblages of a transitional sillimanite type, having formed by the extensive modification of earlier higher pressure assemblages during exhumation. Only rocks in the intermediate-T kyanite and probably some lower sillimanite zones were clearly recrystallized by late stage andalusite metamorphism, producing the staurolite–andalusite and andalusite–sillimanite zones. This andalusite metamorphism could not reach an equilibrium state because of limited fluid availability.     

Aluminum silicates in the Mount Raleigh pendant, British Columbia

In regionally metamorphosed pelites of the Mount Raleigh pendant, the fibrolite isograd occurs 5km downgrade from the sillimanite isograd. Fibrolite formed from the decomposition of biotite, a reaction that probably resulted from the late-stage influx of acidic volatiles. In contrast, sillimanite formed by the direct,'volume-for-volume'replacement of andalusite. Andalusite and sillimanite coexist in a 3 km-wide zone above the sillimanite isograd. Electron probe analyses of these phases reveal low minor element contents and yield K _D [= X ] values close to unity; the low Fe₂O₃ contents are compatible with reducing conditions implied by the ubiquity of graphite. Because K _D → 1.0, the zone of coexisting andalusite + sillimanite cannot be attributed to multivariancy resulting from partitioning of minor elements between these phases. Rather, the metastable persistence of andalusite into the sillimanite P-T stability field is suggested. The modal proportions of sillimanite versus andalusite imply that minimal (<5%) and alusitesillimanite reaction occurred in a zone 1.5km above the sillimanite isograd; in contrast, there was a marked increase in reaction progress immediately above this zone. With an estimated thermal gradient (in the plane of exposure) of approximately 20°C/km, the 1.5 km-wide zone of nil reaction suggests that the andalusite-sillimanite equilibrium boundary was overstepped by about 30 °C before significant reaction occurred. Inclusion-rich areas in andalusite provided favourable sites for sillimanite nucleation ; however, the growth of sillimanite may have been impeded by'pinning'of sillimanite grain boundaries by inclusions.     

Oxygen isotope ratios in magnetite core-and-mantle structures

In a polymetamorphic, felsic, biotite-bearing gneiss, biotite has reacted to form magnetite and microcline. The resulting structure is a magnetite core surrounded by a mantle of feldspar and quartz normally not exceeding 20mm in diameter. Measurements of oxygen isotope ratios disclose disequilibrium between mantle microcline and mantle quartz and also between mantle and matrix minerals of the same species. A clustering of temperature estimates from the oxygen isotope distribution between magnetite and quartz and between magnetite and microcline in the interval 550 to 600°C suggests an approach to oxygen isotope equilibrium. No signs of a re-equilibriation of the reacting biotite can be found.     

LPHT metamorphism in a late orogenic transpressional setting, Albera Massif, NE Iberia: implications for the geodynamic evolution of the Variscan Pyrenees

During the Late Palaeozoic Variscan Orogeny, Cambro‐Ordovician and/or Neoproterozoic metasedimentary rocks of the Albera Massif (Eastern Pyrenees) were subject to low‐pressure/high‐temperature (LPHT) regional metamorphism, with the development of a sequence of prograde metamorphic zones (chlorite‐muscovite, biotite, andalusite‐cordierite, sillimanite and migmatite). LPHT metamorphism and magmatism occurred in a broadly compressional tectonic regime, which started with a phase of southward thrusting (D1) and ended with a wrench‐dominated dextral transpressional event (D2). D1 occurred under prograde metamorphic conditions. D2 started before the P–T metamorphic climax and continued during and after the metamorphic peak, and was associated with igneous activity. P–T estimates show that rocks from the biotite‐in isograd reached peak‐metamorphic conditions of 2.5 kbar, 400 °C; rocks in the low‐grade part of the andalusite‐cordierite zone reached peak metamorphic conditions of 2.8 kbar, 535 °C; rocks located at the transition between andalusite‐cordierite zone and the sillimanite zone reached peak metamorphic conditions of 3.3 kbar, 625 °C; rocks located at the beginning of the anatectic domain reached peak metamorphic conditions of 3.5 kbar, 655 °C; and rocks located at the bottom of the metamorphic series of the massif reached peak metamorphic conditions of 4.5 kbar, 730 °C. A clockwise P–T trajectory is inferred using a combination of reaction microstructures with appropriate P–T pseudosections. It is proposed that heat from asthenospheric material that rose to shallow mantle levels provided the ultimate heat source for the LPHT metamorphism and extensive lower crustal melting, generating various types of granitoid magmas. This thermal pulse occurred during an episode of transpression, and is interpreted to reflect breakoff of the underlying, downwarped mantle lithosphere during the final stages of oblique continental collision.     

Petrology of metapelites in the Bugaboo aureole,British Columbia,Canada

Contact metamorphism of greenschist facies Neoproterozoic turbidites by the Cretaceous Bugaboo Batholith in southeastern British Columbia has resulted in a well‐developed contact aureole. The aureole is about 1 km wide and can be divided into three main zones: (i) spotted phyllite zone, extending from the first appearance of spots of cordierite or andalusite to the last occurrence of primary chlorite; (ii) cordierite + andalusite + biotite zone, comprising hornfelses or schists with abundant porphyroblasts of cordierite and andalusite and, at higher grades, fibrolitic sillimanite; and (iii) K‐feldspar zone, characterized by hornfelses and schists that, in the inner part of this zone, are variably migmatitic. Four parts of the aureole were examined, three of which are characterized by schists, and one of which (Cobalt Lake area) is characterized by hornfelses and has exceptional exposure and comparatively unaltered rocks. Petrographic, modal, mineral‐compositional and whole rock‐compositional data were collected from the Cobalt Lake transect, allowing the prograde reaction sequence to be inferred. Notable features of the aureole at Cobalt Lake include: initial development of andalusite and plagioclase at the expense of paragonite‐rich white mica; a narrow interval across which cordierite, andalusite and biotite increase markedly at the expense of chlorite; gradual development of andalusite and biotite at the expense of cordierite and muscovite upgrade of chlorite consumption; and near‐simultaneous development of andalusite + K‐feldspar and sillimanite, the latter indicating a pressure of contact metamorphism of ~3 kbar. In other parts of the aureole, the development of sillimanite downgrade of the initial development of K‐feldspar suggests slightly higher pressures of contact metamorphism. Lack of correspondence between the observed sequence of reactions in the aureole and those predicted thermodynamically suggests that modifications to some of the thermodynamic data or activity–composition models may be required. Textural features in the aureole suggest the influence of kinetic factors on metamorphic recrystallization, including: (i) deformation‐catalysed reaction in the schists compared to the hornfelses, as indicated by different mineral‐growth sequences inferred from microstructures, and (ii) heating rate‐controlled recrystallization, as indicated by the decrease in grain size of hornfelses with increasing metamorphic grade.     

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.     

Petrology and Evolution of an Archaean Metamorphic Aureole in the Slave Craton, Canada

In the Slave Craton of northern Canada, extensive areas weremetamorphosed in broad aureoles (typically ca. 10–15 kmwide) around granitie batholiths emplaced about 2575 m.y. ago.Meta-greywackes and meta-pelites from two areas traversing oneof these aureoles near Yellowknife have been studied. New petrographicdata are given and integrated with previously published mineralogicaldata to elucidate the metamorphic history of the area. Metasedimentsin the aureole contain the concentrically zoned succession ofindex minerals chlorite, biotite, cordierite, gedrite, andalusite,sillimanite. In addition, garnet, staurolite, and parageneticallylate andalusite occur more irregularly, and cummingtonite characterizessubordinate calcic rock-types. The chemistry of all these mineralsis given and their origins discussed. The aureole evolved by the development and decay of a thermaldome. This was a continuous process, but three recognizablemetamorphic phases can be correlated as follows with establisheddeformational phases. The cycle began with a deformation phase(D₁) unaccompanied by metamorphism. This evolved into D₂ whichwas accompanied by broad regional metamorphism M₂ (characterizedby the index succession chlorite, biotite, garnet, staurolite)as thermal doming began. With continued updoming of the isotherms,the third phase (D₃) produced only minor folding but causedmajor metamorphic recrystallization (M₃), culminating in theemplacement of granite at the core of the thermal dome. A concentriczonation of the metamorphic index minerals biotite, cordierite,gedrite, andalusite+sillimanite was superimposed on earlierassemblages. This M₃ phase occurred at lower pressure (2.5–3.5kb) than M₂ because of erosional unloading, but the temperatureswere more extreme, ranging up to about 700 °C. With deformationthen complete, the thermal dome decayed, and minor mineralogicalchanges occurred in this (M₄) decay phase. The region has sincebeen effectively stable.     

Biotite and garnet compositional variation and mineral equilibria in Grenville gneisses of the Otter Lake area, Quebec

Garnet-biotite gneisses, some of which contain sillimanite or hornblende, are widespread within the Otter Lake terrain, a portion of the Grenville Province of the Canadian Shield. The metamorphic grade is upper amphibolite to, locally, lower granulite facies. The atomic ratio Fe²⁺/(Fe²⁺+ Fe³⁺) in biotite ranges from 0.79 to 0.89 (ferrous iron determinations in 10 highly pure separates), with a mean of 0.86. Mg and Fe²⁺ atoms occupy 67–78% of the octahedral sites, the remainder are occupied by Fe³⁺, Ti, and Al, and some are vacant. Mg/(Mg + Fe²⁺), denoted X, in the analysed samples ranges from 0.32 to 0.65. Garnet contains 1–24% grossular, 1–12% spessartine and X ranges from 0.07 to 0.34. Compositional variation in biotite and garnet is examined in relation to three mineral equilibria: (I) biotite + sillimanite + quartz = garnet + K-feldspar + H₂O; (II) pyrope + annite = almandine + phlogopite; (III) anorthite = grossular + sillimanite + quartz. Measurements of X (biotite) and X (garnet) are used to construct an illustrative model for equilibrium (I) which relates the observed variation in X to a temperature range of 70°C or a range in H₂O activity of 0.6; the latter interpretation is preferred. In sillimanite-free gneisses, the distribution of Mg and Fe²⁺ between garnet (low in Ca and Mn) and biotite is adequately described by a distribution coefficient (KD) of 4.1 (equilibrium II). The observed increase in the distribution coefficient with increasing Ca in garnet is ln K_D= 1.3 + 2.5 × 10^?2 [Ca] where [Ca] = 100 Ca/(Mg + Fe²⁺+ Mn + Ca). The distribution coefficient is apparently unaffected by the presence of up to 12% spessartine in garnet. In several specimens of garnet-sillimanite-plagioclase gneiss, the Ca contents of garnet and of plagioclase increase in unison, as required by equilibrium (III). The mean pressure calculated from these data (n= 17) is 5.9 kbar, and the 95% confidence limits are ±0.5 kbar.     

Potential protonation sites in the Al<Subscript>2</Subscript>SiO<Subscript>5</Subscript> polymorphs based on polarized FTIR spectroscopy and properties of the electron density distribution

Potential protonation sites for, kyanite, sillimanite, and andalusite, located in a mapping of the (3, −3) critical points displayed by their L(r) = −∇²ρ(r) distributions, are compared with polarized single-crystal FTIR spectra of kyanite and sillimanite determined earlier and with andalusite measured in this study. For andalusite, seven peaks were observed when the electric vector, E, is parallel to [100]: four intense ones at 3,440, 3,460, 3,526, and 3,597 cm⁻¹ and three weaker ones at 3,480, 3,520, and 3,653 cm⁻¹. Six peaks, three intense ones at 3,440, 3,460, and 3,526 cm⁻¹ and three weaker ones at 3,480, 3,520, and 3,653 cm⁻¹ when E parallels [010]. No peaks were observed when E is parallel to [001]. The concentration of water in andalusite varies between 110 and 168 ppm by weight % H₂O. Polarized FTIR spectra indicate that the OH vector is parallel to (001) in andalusite and sillimanite and in kyanite. Examination of the L(r) (3, −3) critical points in comparison with the polarized FTIR indicates that H prefers to bond to the oxygen atoms O1 and O2 in andalusite and O2 and O4 in sillimanite which correspond to the underbonded oxygen atoms and those with the largest L(r) maxima. In kyanite, comparison of the FTIR spectrum and the critical points indicates that H will preferentially bond to the two 4-coordinated O2 and O6 atoms.     

A petrogenetic grid for pelitic schists in the system SiO2-Al2O3-FeO-MgO-K2O-H2O

A quantitative petrogenetic grid for pelitic schists in the system KFMASH that includes the phases garnet, chlorite, biotite, chloritoid, cordierite, staurolite, talc, kyanite, andalusite, sillimanite, and pyrophyllite (with quartz, H₂O and muscovite or K-feldspar in excess) is presented. The grid is based on thermodynamic data of Berman et al. (1985) and Berman (1988) for endmember KFASH and KMASH equilibria and natural Fe-Mg partitioning for the KFMASH system. Calculation of P-T slopes and the change in Fe/(Fe+Mg) along reactions in the KFMASH system were made using the Gibbs method. In addition, the effect on the grid of MnO and CaO is evaluated quantitatively. The resulting grid is consistent with typical Buchan and Barrovian parageneses at medium to high grades. At low grades, the grid predicts an extensive stability field for the paragenesis chloritoid+biotite which arises because of the unusual facing of the reaction chloritoid+biotite + quartz+H₂O = garnet+chlorite+muscovite, which proceeds to the right with increasing T in the KFMASH system. However, the reaction proceeds to the left with increasing T in the MnKFASH system so the assemblage chloritoid + biotite is restricted to bulk compositions with high Fe/(Fe+Mg+Mn). Typical metapelites will therefore contain garnet+chlorite at low grades rather than chloritoid + biotite.     

Formation and stability of viridine in microcline leptite from Ultevis District,Northern Sweden

In the Ultevis District, Northern Sweden, viridine occurs in a potassium rich, microcline-bearing leptite together with quartz, plagioclase, phlogopite, opaques, and a retrogradely formed muscovite. The formation of viridine in this rock may be described by a dealkalization process of microcline, in which opaque minerals are involved. A diffusion of K⁺ and H₂O out of the centres of viridine formation and of H⁺ into these localities, which is a prerequisite of such type of reaction, may have been favoured by a contemporaneous migmatization of the rock, since the granite-like melts can be regarded as sinks for potassium and water. The following decomposition of viridine is initiated by the reverse reaction. Texturally this is indicated by small microcline seams along the contacts between viridine and quartz grains. The final alkalization of the viridine-quartz assemblage to form muscovite reflects increasing K⁺/H⁺-ratios and H₂O-contents of the vapor phase and/or decreasing temperatures. The potassium and water amounts necessary for the viridine decomposition reactions may have been generated by the cooling of the granitic liquids. An intergrowth of sillimanite and the muscovite can be described by a degenerated reaction. It can be interpreted as giving way to an equilibration of the alumosilicates und the actual metamorphic conditions, that is here P, T, , a _k ⁺, a _h ⁺ and a _Mn ³⁺.The viridine investigated has the lowest Mn-contents recorded so far for andalusite-type minerals with viridine optics (1.3–2.2 mol-% Mn₂SiO₅-component). Since no andalusite with lower Mn-contents occurs in the rock and since no other Al₂SiO₅-phase formed at this metamorphic stage it may be assumed that the incorporation of manganese stabilizes the andalusite structure under the P-, T-, ^–, a _k ^+-, a _h ^+- and a _Mn ³⁺-conditions verified in this particular leptite.The temperature of viridine formation is estimated to be about 600 ° C, the decomposition of viridine +quartz into the muscovite-sillimanite assemblage may have happened between 650–500 ° C at 6–2.5 kb.     

Disequilibrium in the Ross of Mull Contact Metamorphic Aureole, Scotland: a Consequence of Polymetamorphism

The Ross of Mull pluton consists of granites and granodioritesand intrudes sediments previously metamorphosed at amphibolitefacies. The high grade and coarse grain size of the protolithis responsible for a high degree of disequilibrium in many partsof the aureole and for some unusual textures. A band of metapelitecontained coarse garnet, biotite and kyanite prior to intrusion,and developed a sequence of textures towards the pluton. InZone I, garnet is rimmed by cordierite and new biotite. In ZoneII, coarse kyanite grains are partly replaced by andalusite,indicating incomplete reaction. Coronas of cordierite + muscovitearound kyanite are due to reaction with biotite. In the higher-gradeparts of this zone there is complete replacement of kyaniteand/or andalusite by muscovite and cordierite. Cordierite chemistryindicates that in Zone II the stable AFM assemblage (not attained)would have been cordierite + biotite + muscovite, without andalusite.The observed andalusite is therefore metastable. Garnet is unstablein Zone II, with regional garnets breaking down to cordierite,new biotite and plagioclase. In Zone III this breakdown is welladvanced, and this zone marks the appearance of fibrolite andK-feldspar in the groundmass as a result of muscovite breakdown.Zone IV shows garnet with cordierite, biotite, sillimanite,K-feldspar and quartz. Some garnets are armoured by cordieriteand are inferred to be relics. Others are euhedral with Mn-richcores. For these, the reaction biotite + sillimanite + quartz garnet + cordierite + K-feldspar + melt is inferred. Usinga petrogenetic grid based on the work of Pattison and Harte,pressure is estimated at 3·2 kbar, and temperature atthe Zone II–III boundary at 650°C and in Zone IV asat least 750°C. KEY WORDS: contact metamorphism; disequilibrium     

Petrogenesis of andalusite–kyanite–sillimanite veins and host rocks, Sanandaj-Sirjan metamorphic belt, Hamadan, Iran

Quartz‐rich veins in metapelitic schists of the Sanandaj‐Sirjan belt, Hamadan region, Iran, commonly contain two Al₂SiO₅ polymorphs, and, more rarely, three coexisting Al₂SiO₅ polymorphs. In most andalusite and sillimanite schists, the types of polymorphs in veins correlate with Al₂SiO₅ polymorph(s) in the host rocks, although vein polymorphs are texturally and compositionally distinct from those in adjacent host rocks; e.g. vein andalusite is enriched in Fe₂O₃ relative to host rock andalusite. Low‐grade rocks contain andalusite + quartz veins, medium‐grade rocks contain andalusite + sillimanite + quartz ± plagioclase veins, and high‐grade rocks contain sillimanite + quartz + plagioclase veins/leucosomes. Although most andalusite and sillimanite‐bearing veins occur in host rocks that also contain Al₂SiO₅, kyanite‐quartz veins crosscut rocks that lack Al₂SiO₅ (e.g. staurolite schist, granite). A quartz vein containing andalusite + kyanite + sillimanite + staurolite + muscovite occurs in andalusite–sillimanite host rocks. Textural relationships in this vein indicate the crystallization sequence andalusite to kyanite to sillimanite. This crystallization sequence conflicts with the observation that kyanite‐quartz veins post‐date andalusite–sillimanite veins and at least one intrusive phase of a granite that produced a low‐pressure–high‐temperature contact aureole; these relationships imply a sequence of andalusite to sillimanite to kyanite. Varying crystallization sequences for rocks in a largely coherent metamorphic belt can be explained by P–T paths of different rocks passing near (slightly above, slightly below) the Al₂SiO₅ triple point, and by overprinting of multiple metamorphic events in a terrane that evolved from a continental arc to a collisional orogen.     

The selective replacement of the aluminum silicates by white mica

From considerations of relativeG-T surfaces inferred from publishedP-T data and the occurrence of replacement textures of Al₂SiO₅ polymorphs in rocks, the relative positions of curves representing the following equation in _K+ —T — pH ispace on substituting Al₂SiO₅ different polymorphs are derived.3 Al₂SiO₅ + 3 SiO₂ (quartz) + 2 K⁺ + 3 H₂O 2 KAl₂[AlSi₃O₁₀](OH)₂ (muscovite)+ 2 H⁺. The curves are different because of the differentG-T values for the polymorphs which, in the field, is borne out by the observation that in a rock containing two or three Al₂SiO₅ polymorphs, in nearly all instances only one polymorph is replaced by white mica. Instances of textural relations showing the interpreted selective replacement of one Al₂SiO₅ polymorph by a white mica in the presence of one (or two) other Al₂SiO₅ polymorph(s) are cited both from the literature and various field examples. The selective replacement of kyanite if sillimanite and/or andalusite is/are present, and of andalusite if only sillimanite is present are interpreted to show that generally during the muscovitization reaction, the field of sillimanite in the above reaction (left hand side) at a particular pH (H⁺ concentration) and is larger in _K+ —T space than that of andalusite which in turn is larger than that of kyanite. Theoretically it is shown that variations to this can exist but the field evidence suggests these only occur under rare geological conditions. Although this is not totally conclusive, the selectiveness of the replacement is interpreted to show that the fluid was buffered with respect to K⁺ and H⁺ on or near the curve of the polymorph showing the lowest stability field until that polymorph is totally consumed, after which the fluid composition moves to the next lowest curve for the remaining polymorph(s) present in the rock. The alteration of more than one polymorph by an apparently simultaneous process of alteration is rare and usually occurs at a low grade of metamorphism. This is interpreted to show that the buffering reaction could not keep pace with the influx of fluid and change the composition of this fluid (in most cases).     

The influence of crystallography and kinetics on phengite breakdown reactions in a low-pressure metamorphic aureole

A natural example of phengite that had undergone partial thermal decomposition at a pressure of about 0.5 kbar and a temperature of about 680° C in a contact aureole was exmined in the transmission electron microscope (TEM). Partially pseudomorphed phengites were found to consist of combinations of phengite, biotite, K-feldspar, mullite, sillimanite, spinel and cordierite. Different areas within individual, partially pseudomorphed, phengite grains show various degrees of reaction and different reaction products; the cores are the least reacted and the margins have reacted most. In the cores the assemblage Al-, Mg-enriched phengite+biotite +K-feldspar+mullite±spinel has formed, whereas the assemblage K-feldspar+mullite+sillimanite+spinel +biotite+cordierite has formed at the edges. According to our thermodynamic calculations, the breakdown of phengite should have produced cordierite+spinel +corundum+K-feldspar in regions isolated from the influx of SiO₂ and cordierite+andalusite+quartz+K-feldspar in regions near the edge of the grains that were essentially saturated with SiO₂. Chemical equilibrium was not achieved in any part of the partially pseudomorphed phengites on a micron scale or larger. Breakdown theoretically should have been complete by about 550° C; the reaction temperature was overstepped by at least 130° C for 20–25 years. The variations in the degree and type of reaction are probably due partly to the availability of suitable nucleation sites in different regions, partly to the need to remove H₂O from reaction sites and partly to the influence of SiO₂, which diffused into the grains during metamorphism. The presence of SiO₂ lowers the equilibrium temperatures. Thus there is a higher driving force for breakdown near the grain boundaries than in the cores. Most of the products show an orientation relationship with the parent phengite and have consistent habit planes; they have their closest-packed planes and closest-packed directions parallel to one another and to those of phengite. Such relationships minimize the strain and surface energies at nucleation and favour most rapid nucleation and growth of the reaction products. The great structural similarity of biotite to phengite resulted in its having the highest rate of nucleation and growth of any product and it occurred in all areas of the phengite pseudomorphs studied. Mullite and sillimanite were produced metastably. Mullite has more rapid nucleation kinetics than other aluminosilicates because it is structurally disordered. Sillimanite formed rather than andalusite in regions of the partially pseudomorphed phengites where the reaction reached an advanced stage, because the reaction from phengite to andalusite requires an energetically unfavourable change in aluminium co-ordination state.     

Deformation-induced polymorphic transformation: experimental deformation of kyanite, andalusite, and sillimanite

Torsion experiments were performed on the Al₂SiO₅ polymorphs in the sillimanite stability field to determine basic rheological characteristics and the effect of deformation on polymorphic transformation. The experiments resulted in extensive transformation of andalusite and kyanite to sillimanite. No transformation occurred during the hot-press (no deformation) stage of sample preparation, which was carried out at similar P–T conditions and duration as the torsion experiments. Experiments were conducted on fine-grained (< 15 µm) aggregates of natural andalusite, kyanite and sillimanite at 1250 °C, 300 MPa, and a constant shear strain rate of 2 × 10^− 4/s to a maximum shear strain of 400%. Electron back-scattered diffraction (EBSD) analysis of the experiments revealed development of lattice-preferred orientations, with alignment of sillimanite and andalusite [001] slightly oblique to the shear plane. The kyanite experiment could not be analyzed using EBSD because of near complete transformation to sillimanite. Very little strain ( 30%) is required to produce widespread transformation in kyanite and andalusite. Polymorphic transformation in andalusite and kyanite experiments occurred primarily along 500 µm wide shear bands oriented slightly oblique and antithetic to the shear plane and dominated by sub-µm (100–150 nm) fibrolitic sillimanite. Shear bands are observed across the entire strain field preserved in the torsion samples. Scanning transmission electron microscope imaging shows evidence for transformation away from shear bands; e.g. fibrolitic rims on relict andalusite or kyanite. Relict grains typically have an asymmetry that is consistent with shear direction. These experimental results show that sillimanite is by far the weakest of the polymorphs, but no distinction can yet be made on the relative strengths of kyanite and andalusite. These observations also suggest that attaining high bulk strain energy in strong materials such as the Al₂SiO₅ polymorphs is not necessary for triggering transformation. Strain energy is concentrated along grain boundaries, and transformation occurs by a dynamic recrystallization type process. These experiments also illustrate the importance of grain-size sensitive creep at high strains in a system with simultaneous reaction and deformation.     

A comment on diffusion and the displacement of atoms in metamorphic transformations

Experimental data on diffusion in olivine , are used to define certain terms – diffusion coefficient, jump frequency, characteristic distance, random walk – that are useful in a discussion of atom displacements under natural conditions. Examples of atom displacements in two metamorphic terranes of the Canadian Precambrian Shield are then examined, as follows. (i) In a high‐grade metamorphic terrane in the Mid‐Proterozoic Grenville Province (Otter Lake Area), Mg concentration gradients about dolomite microcrystals in calcite and Na gradients about albite microcrystals in K‐feldspar are viewed as stranded Mg–Ca and Na–K interdiffusion gradients, formed by exsolution during slow cooling from ~700 to ~400 °C. (ii) In the Archean Slave Province (Yellowknife area), the crystallization of sillimanite, near andalusite but within crystals of quartz, possibly occurred by coupled Al–Si and oxygen–vacancy interdiffusion in quartz at ~550 °C. And the crystallization of garnet from chlorite occurred by the two‐way crystal‐boundary diffusion of several kinds of atoms across distances ranging to 3 mm. (iii) In the Otter Lake area, the crystallization of orthopyroxene–hornblende–spinel reaction zones at boundaries between crystals of olivine and plagioclase in metagabbro, evidently occurred by the mechanism of interstitial diffusion, that transported Mg, Fe, Mn and O atoms across the reaction zone from olivine to the plagioclase–(hornblende+spinel) boundary, and Si, Al, Ca and Na atoms from plagioclase to the olivine–orthopyroxene boundary, accompanied by NaSi–CaAl interdiffusion in plagioclase, and the addition of hydrogen and minor Ti, Zn, F, Cl and K from beyond the reaction zone. Also, centimetric reaction zones, with abundant biotite and plagioclase, at boundaries between K‐feldspar gneiss and deformed amphibolite dykes, evidently formed by the reaction, strained hornblende (in amphibolite) + K‐feldspar (in gneiss)→biotite (in amphibolite) + plagioclase (in gneiss), with crystal‐boundary diffusion of (Na + Ca) atoms and of K atoms across the reaction zone.