Conditions of Dalradian Metamorphism

J. geol. Soc. London, Vol. 142, 1985, pp. 1-5. Printed in Northern Ireland

Conditions of Dalradian metamorphism

Editorial note: A meeting on ‘Conditions of Dalradian chemical and thermochemical data. Powell emphasizes Metamorphism’, convened by Drs C. M. Graham and the importance of assessing the uncertainties inherent B. Harte (Edinburgh University) and DrR. Powell in these P-T calculations with specific reference to (Leeds University) on behalf of the Metamorphic both widely used and new (phengite barometer and Studies Group was held at the University of Edinburgh garnet-hornblende thermometer) methods. on 20 and 21 September 1983, and was followed by a Much discussion in the papers is focussed upon the field trip to the Dalradian of the Scottish Highlands. problems of actually estimating P, T and other vari- ables. The garnet-biotite Fe-Mg exchange thermo- Drs Colin M. Graham and Ben Harte have provided meter receives widespread usage,but problems are the followingintroduction to the proceedings of the identified over its application. The tendency for low conference and the published papers: temperature estimates (by comparison with other geothermometersand FASH experimental mineral The metamorphosed rocks of the DalradianSuper- equilibria) when CaO and MnO in garnet are ignored group, with their clearly developedand contrasting widelyis noted(e.g. Hudson, Moles,Watkins) (Barrovian and Buchan) metamorphic zonal sequences together with theuncertainties in applying the CaO and varied (pelitic-psammitic, basic, calcareous) litho- and MnO correction procedures of Hodges & Spear logies, have played an important part in the develop- (1982) and Pigage & Greenwood (1982); thelatter ment of metamorphic petrology through the studies of method tends to result in very high temperatures (e.g. such early workersas Barrow, Horne, Tilley, Read, Moles). The well-recognized (Harte & Hudson 1979; Wisemen andHarker. More recently, models of Pigage & Greenwood 1982) dichotomy between thermo-tectonicdevelopment have shown abroad relatively low-temperatureestimates given by uncor- array of ideas: regional depth control in a ‘tectogen’ rected garnet-biotite and oxygen isotope data (which (Kennedy 1948); self-heating of crustal a pile tend to favour the A12Si05 triple point of Holdaway (Richardson & Powell 1976); importance of high-level 1971) and other FASH experimental data and now the magma (Atherton 1977; Yardley 1980); the develop- new garnet-hornblende thermometer (which favour ment of inverted zonal sequences (Chinner 1978); and the A12Si05 triple point of Richardson et al. 1969) is suggestions of tectonically controlledlateral heat still apparent. transfer (Harte & Hudson 1979). The diversity of A significant feature of quite recentDalradian these suggestions no doubt partly reflects the complex- studies has been the large differences in estimates of ity of the Scottish-Irish Dalradian terrain as a whole, a pressureconditions of metamorphism with their feature whichwas further brought out inseveral implications of differences in tectonics or erosion papers at the meeting. In order to provide a setting for rates. In particular the estimates of Wells & Richard- viewing thebroader aspects of metamorphicstudies son (1979) of9-13 kbarfor Barrovian facies series and their relation to structural-tectonic and temporal rocks of the Lower Dalradian of the Central Highlands evolution, the meeting commenced with the first three are very much higher than those of 5-7 kbar derived papers in this issue, namely: Hall on geophysical by Richardson & Powell (1976) for Lower Dalradian aspects of the crustal structure; Bradbury on compari- rocks of Spean Bridge (western Highlands) and Harte sons of structures in the Dalradian Supergroup with & Hudson (1979) for the eastern Highlands. those other orogenic cores and especially the Alps; In the present papers a wide degree of consistency is and Cliff onthe dating of events in metamorphic seen in the pressures estimated by many workers. The terrains. calibration of the garnet-plagio~lase-A1~SiO~quartz The development of quantitativetectono-thermal geobarometer by Newton & Haselton (1981) is found models obviously requires accurate estimates ofP-T often to fit well with estimates based on the Ghent & conditions. The assignment of P-T conditions and Stout (1981) muscovite-biotite-garnet-plagioclase- histories to Dalradian rocks using petrogenetic grids of quartz barometer, even though they involve different mineralreactions has progressed with the develop- calibrations of the sameequilibrium. Both these ment of experimental data andSchreinemakers’ methods give similar results to the new phengite- methodsfrom studies centredaround the AI2SiO5 biotite-chlorite barometer and the KFMASH grids polymorphs (Chinner 1966) to integrated KFMASH (e.g. Baker,Dempster, Moles,Watkins). Whether grids (Harte & Hudson 1979). The latter approach is these agreements are largely a function of large error represented in this set of papers by Pattison & Harte’s ranges remains to be seen. discussion of thermal aureole assemblages. In recent The earlierpressure estimates for the Central years this approach has been augmented by the new Highlands of Wells & Richardson (1979) are found methods of geothermobarometry based on mineral- to be generally 24kbar too high. None the less,

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/1/1/4888395/gsjgs.142.1.0001.pdf by guest on 28 September 2021 2 C. M. Graham & B. Harte extensive evidence of relatively high pressures (7-10 metamorphic zones. Amongst his results the evidence kbar) from the SW through tothe Central Scottish of relatively rapid and slow periods of uplift through Highlands is found by many of the workers cited time andthe occurrence of spatially localised uplift above (see also Graham (abstr.)). Pressure estimates within the Dalradian are particularly important, since of 5-6 kbar for the eastern Highlands (Baker, Demps- they indicatediscontinuities in P-T-time paths,and ter)are found to be in line with previousestimates the possibilities of partial discontinuities in meta- (Harte & Hudson 1979). A paper with a comparison morphism between rocks at different depths and of of a wide range of barometricand thermometric lateral heat transfer as a consequence of tectonics. In methods using both silicate and sulphide assemblages anotherpaper, Dempster (abstr.) shows how rocks is presented by Moles. partially uplifted atan early date andthen held at Severalworkers examine the conditions of meta- moderate temperatures provide evidence of extensive morphism in theConnemara and NE Scotland re-equilibration amongst the ferromagnesian phases. (Buchan)regions where regional metamorphic rocks The importance of local tectonics is further indi- are closely associated with synmetamorphic basic- cated by Harte, Dempster & Booth (abstr.) who note ultrabasicmagmatism. In NE Scotland Droop & how Central Highland chloritoid-biotite assemblages Charnley and Leslie (abstr.) provide consistent press- (whose moderately low-pressure nature is supported ure estimates of 4-5 kbar for crystallization of high- by geobarometric data from other localities) are temperature aureole rocks of several Newer Gabbro apparentlysuperseded by higherpressure assemb- intrusions, and these pressure estimates are compati- lages, asituation whichis the reverse of normally ble with estimates for ‘regional’ metamorphic assemb- expected P-T-time paths but which is also indicated lages for the NE coast of the Buchan area studied by locally by the replacement of andalusite by kyanite. Hudson. The possibly important role of the development of the Comparable pressureestimates for aureoles in Tummell Steep Belt in this metamorphic evolution is Connemara to those provided by Droop & Charnley noted. In his comparison of Dalradianand Alpine are given by Treloar, while Barber & Yardley suggest structuralhistories Bradbury emphasises the import- evidence for falling pressure during melting and ance of both latesteep-belt generating dome and uplift crystallization of migmatites in E Connemara. A structures as well as early nappe structures in affecting lithosphericstretching model for low-pressure meta- the metamorphic pattern. Baker draws attention to a morphism is presented by Kneller (abstr.), who also line of metamorphic discontinuities from Portsoy via gives evidence of reactionsgenerating corundum in Glen Muick to Duchray Hill, though the nature and regionally metamorphosed pelites in the Buchan area. possibly multiple(early and late) origin of these The relationships of andalusite, sillimanite and discontinuities is far from certain. kyanitein the Buchan-Barrovian transition area of In his review of recent geophysical data on crustal NE Scotland inevitably attracted some attention, and structure in theDalradian region of Scotland, Hall are discussed by Baker and Chinner (abstr). Booth arguesfrom gravity datafor the presence of an (abstr.)and McLellan (abstr.)debate the nature of important crustallineament (the CruachanLine) sillimanite-forming reactions. trending NW-SE N of the Loch Awe area. Relatively The acquisition ofP-T data on regional metamor- low-density crust to the NE of the lineament contains phic rocks has of course other objectives besides those abundant late Caledonian granite; to the SW a higher of understanding details of petrogenesis, and provides density crust may be the result of the lack of major a basis in conjunction with detailed structural studies graniticintrusions, crustal thinning and voluminous and thermal modelling for interpreting the larger-scale basaltic igneous activity during early Dalradian evolu- processes operating in orogenic belts. In assessing such tion, andthe suggested absence of Moine rocks at evolution the time element is of considerable import- depth. Aseparation possibly along this line in the ance, and data concerning the temporal constraints are nature of the metamorphic evolution is also advocated largely gathered by both structural-petrographicstu- by Graham (abstr.) who notesthe relatively high- dies andradiometric dating. The differentclosure pressurebut low-temperature conditions in the SW temperatures of variousmineral and radiometric Highlands, and their apparent relatively early time of isotope systems (discussed by Cliff) are here particu- metamorphism. Low crustal heat production associ- larly useful in potentially providing several data points ated with the absence of Moine rocks at depth and the forany one rock which can be combined with abundance of mafic rocks in the Dalradian pile are metamorphic P-T data and petrographic histories to suggested by Graham to be important features in this construct P-T-time paths. These paths may in turn be evolution, whichmay have occurred beneath a thick combined with constraints on crustal thermal prop- nappe pile now removed by erosion. erties in thermal modelling techniques (Richardson & The evidence for inverted metamorphic zonation in Powell 1976; England & Richardson 1977). Dempster the Balquidder and Duchray Hill areas, and the presents his results from such combineda study difficulties of accountingfor them tectonically, are involving two detailed traverses across the Barrovian stressed by Chinner (abstr.) and Watkins. The prob-

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lems of documenting inversion in some cases, and the Processes of metamorphic fluid circulation and generalproblems of how such thermal inversions infiltration receive attention in the studies of Hall & could bepreserved during thermal relaxation of a Banks (abstr.)onlate pyriteformation in the thickened crustal pile, possibly modified by convective Ballachulish Slates,and of Greig (abstr.) on the transfer of heat by metamorphic fluid circulation, genesis of calc-mica-schist and calc-silicate assemb- received much debate. lages in the Loch Tay Limestone.

Abstracts of papers presented at the meeting but which are not included in the full length articles

The origins and mechanismsof formation of silliianite in the by the expansion of sillimanite into regions heated by Newer Stonehavenmetamorphic sequence John E. Booth (Uni- Gabbro passage and by the much more subsequent Newer versity of Edinburgh) Granites. It is asserted that evidence of andalusite overprinting sillimanite,on which the ‘andalusite-kyanite’ isograd Unlike the Barrovian and Buchan sequences the Stonehaven would be abolished,refers toNewer Granite ‘contact’ metamorphic sequence does not include zones of kyanite or andalusite superimposed on ‘regional’ sillimanite: the isograd andalusite-bearing rocks at lower grade than the sillimanite lives. zone. Staurolite and garnet give way to sillimanite via a series of Low-temperature re-equilihriationin the kyanite and sillima- continuous and discontinuous reactions, of which St = Sill nite zone pelites of Glen Esk, Angus Timothy J. Dempster + Biot andGnt = Sill + Biot arethe most important.In (University of Edinburgh) detailthese reactions may be demonstrated by Carmichael type reaction loops, in which textures showing the replace- Geothermometry estimates, using garnet-biotite Fe-Mg ment of staurolite by large muscovite plates may be linked exchange relations, on adjacent mineral grains in the kyanite with textures showing sillimanite-biotite intergrowths. The and sillimanite-zone pelites from Glen Esk, show anomalous- staurolitepseudomorphs appearto be post D3 which, ly low temperatures (<500”C).Microprobe investigations therefore, indicates a post D3 age for sillimanite growth. show that small zones of diffusion controlled equilibria exist Below the sillimanite-in isograd,garnets have normal between all the Fe-Mg phases in these rocks and that within bell-shaped chemical profiles or, more rarely, show no these diffusion domains a low temperature re-equilibration is zonation. With theabsorption of garnet in sillimanite- pervasive. producingreactions the profiles become modified by a Geothermometry estimates of peakmetamorphic condi- diffusion surface moving into the garnet about 10@200 tionscompatible with the observedmineral assemblages microns ahead of the absorptionsurface. This prograde (-650°C) can beobtained by using garnetcompositions process is responsiblefor producing Mn-rich garnet rims unaffected by re-equilibration in conjunction with matrix which are not a product of retrograde re-equilibration. biotite compositions away from other Fe-Mg phases. These unusuallow-temperature exchange phenomena occur in an area with a distinctive early cooling history, in Ancientisograds in amodern world GrahamA. Chinner which peak metamorphicconditions rapidly gave way to a (University of Cambridge) long period of moderately elevated temperatures. During this The distribution of classical isograds actsas a datum for periodsome re-equilibration occurred despite the probable moderating the excesses of thermochemical enthusiasm and lack of an intergranular fluid. gives informationnot otherwise attainable. Topographic High-pressuregreenschist to epidote-amphibolitefacies evidence at Balquhidder of garnet isograd inversion, original- metamorphismof Dalradian rocks of the S-W Scottish ly recognized by Tilley, has recently been confirmed by Highlands:isograds and P-T conditions Colin Graham Watkins. Inversion of a kyanite isograd has been proposed (University of Edinburgh) on the basis of fault displacement near Glen Shee. Having strained at these gnats we may swallow the camel that all Greenschist facies metamorphism of Dalradian rocks of the Barrow’s zones areinverted. Isogradscutting F, and Fz SW Highlands is of biotite grade. The scarcity of biotite in structuresindicate thatthe inversion is not primarily the variably calcareousmetasediments is consequencea of result of post metamorphic folding but represents a synmeta- variable X,--, (fluid), commonly stabilizing white mica + morphic thermal gradient. The boundary between ‘inverted’ dolomite-ankerite at theexpense of biotite + chlorite + and ‘right-way-up’ isograds is marked by the high- calcite. temperature axis that runs along thegarnet zone spine in In Fe-rich metabasic rocks, the assemblages (1) ferrostilp- Cowal and the Duchray Hill Gneiss into Deeside. It is at the nomelane + ferroactinolite + chlorite, (2) edenitichorn- NEend of this axis that much Dalradian sillimanite is blende + chlorite + ferrostilpnomelane, and (3) pargasitic concentrated and recentstudies do suggest that a small hornblende + garnet i chlorite (all + quartz + albite f bio- sillimanite zone coeval with the regional andalusite-kyanite tite) are developed with increasing grade. An isograd development did exist in Deeside. If so this sillimanite spread separating (1) and (2) (ferrostilpnomelane + ferroactinolite = with time to overprint at least the kyanite zone now to the edenitic hornblende + chlorite) in the lower greenschist SW: to deny this is to propound the paradox that the highest facies (near Tayvallich) appears to befolded by the ‘primary’ temperature rocks contain the greatest evidence of disequilib- Loch Awe Syncline. Almandine-grossular garnet appears at rium, Delineation of ‘primary’ sillimanite zone is hampered about the same grade in both metabasites and pelites.

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Temperatures of metamorphism ranged from about 410°C The form of metamorphic isograds and zones in the central upto about 530°C acrossKnapdale to thehighest-grade and SE Scottish Highlands is compared with P-T estimates garnet-zone rocks near Tarbert. Pressures deduced using the based on garnet-biotite, chlorite-phengite-biotite-quartz, phengite-chlorite-biotite barometer were S10kbar, consis- and garnet-plagioclase-quartz either with AIzSiO, or musco- tent with high celadonite contents of white micas and vite-biotite. Temperature estimates may be similar but are consistently albitic plagioclase. Metasediments along the axis up to 50°C lower than those given by comparison with the of the secondary Tarbert Monoform are variably retrogres- pelitepetrogenetic grid calibrated by H,O-saturated end- sed, with local development of lower greenschist facies member reactions (Harte & Hudson 1979). K-feldspar + chloriteassemblages at pressures c about Pressure estimates for the structurally defined ‘flat belt’ of 6 kbar. the Southern Highlands show relativevariations broadly Highpressures of metamorphism imply thickening of consistent with the pelite grid; and within this area textures Dalradian crust by nappeemplacement, probablyduring indicate a relatively well-defined peak of metamorphism close continental collision. in age to the D, structural event. N of the ‘flat-belt’ pressure indicators yield some conflicting evidence with possible overprinting of pre-D,lower-pressure (chloritoid-biotite) Metamorphosed carbonate rocks from Knapdale, SW Scot- assemblages near Blair Atholl by post-D,higher-pressure tishHighlands KennethM. Greig(University of Edin- (9-10 kbar) assemblages. An increase of pressure is also seen burgh) in the conversion of andalusite to kyanite assemblages in Carbonate horizons in theDalradian rocks of Knapdale, GlenClunie by Braemar. Such pressureincreases imply a SW Highlands have had a complex metamorphic history. The thickening of the metasedimentary pile whose origin is mineralassemblages found in one such horizon,the thin, uncertain but which may be related to: (a) development of impure ‘Loch Tay Limestone’, have been studied in detail, major steep belts (e.g. Tummel Steep Belt; (b)further major where these rocks outcrop on the shores of Loch Fyne near nappe emplacement (e.g. of the Highland Border Series) or Tarbert. movement of existing nappes; (c) thickening by widespread The mineralassemblages are the end result of a compli- minor fold (D,) accommodation of lateral shortening; (d) cated interplay of deformation, fluid generation and move- some combination of these. ment,and pressure, temperatureand bulk composition during metamorphism. A variety of standard techniques have Dalradian basin evolution and metamorphism B. C. Kneller been applied to aspatially small area, in anattempt to (University of Aberdeen) develop a holistic view of these metamorphic processes. Alithospheric stretching model for the evolution of the Latepyrite in metamorphicrocks-evidence of post- Dalradian basin predictsa history of enhanced heat flow metamorphicfluid migration A. J. Hall & D. Banks throughthe base of the pile throughoutits deposition, (University of Strathclyde) rejuvenated with each successive stretching increment. Such amodel is capable of generating low-P metamorphics and The following reaction can account for late pyrite growth in anatexites at appropriate stretching values. Some, at least, of metamorphic rocks: the present-dayvariation in Dalradian facies series can be interpreted in terms of large scale deformation of a pile with 2FeZ++ 2s’- + FeSz + FeZ++ 2e~. initially high temperature gradient, followed by some thermal This reaction requires that an acid, possibly saline, solution relaxation. dissolves ironand sulphur, transports them along micro- fractures to a cooler, lower pressure, reducing environment Regional corundum in the NE Dalradian B. C. Kneller and precipitates them as pyrite. (University of Aberdeen) Pyrite grows by replacement of quartz and other silicates. Early pyrrhotite or pyritegrains act as nuclei for pyrite Corundum occurs over a wide area in NE Scotland, as part of growth.Trapped HzO and CO2 released on uplift of the the regional low-pressure assemblage in silica-undersaturated metamorphosed terrain provide a source of the acid solution. pelites. These characteristically high-K high-alumina pelites Pyrrhotite of primary ormetamorphic originprovides the are invariably associated with calc-silicate rocks. The origin source of soluble FeZ+ and S’-. Surplus FeZ+ andsilicates in of this association is discussed. Muscovite breakdown by the solution must be removed from the region of growing pyrite reaction crystals and presumably precipitate in late veins. The loss of Mu = 2Cor + 2Kfsp + 2HZ0 electrons may be accomplished by the following reaction: is believed to have occurred at lower temperature than the silica-saturated equivalent C + 4H+ + 4e- + CH,. Mu 2Qz = 2Sil+ 2Kfsp 2Hz0. Texturaland sulphur-isotopestudies of Lower Dalradian + + Ballachulish Slate support the processoutlined above. One Conditions of thermal metamorphism in the aureole of the significance of the above is thatlate pyritegrowth could ‘YoungerBasic’ mafic intrusions of Aberdeenshire A. conceal early relationships. pyrite-pyrrhotite-silicate Graham Leslie (University of Aberdeen) P-T estimates and mineral facies variation in space and time Zonedgarnet porphyroblastsoccur in xenolithic rocks in the Centraland SE ScottishHighlands Ben Harte, formed by partial melting of metasedimentary country rocks, TimothyJ. Dempster & John E. Booth (University of and in thoroughly indurated, metasedimentary hornfelses, at Edinburgh) or nearthe margin of the Insch mafic igneous mass,

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Aberdeenshire.Both rock types display pyroxenehornfels thereactions responsible for the formation of theindex facies parageneses,but thegarnets display partitioning of minerals was strongly controlled by bulk M/FM. iron and magnesium which reflect slightly different crystal- The staurolite breakdown reactions (A-D) are given below: lization histories for each lithology. Compositions of co-existing garnet, cordierite and biotite st + musc + qz = bi + ky + HZO(A) from both lithologies imply that temperatures of 720-75OoC, and pressures in the range 3-4 kbar (PHz0/P = 0.2-0.3), would have been experienced by rocks at the margins of the st + musc + qz = bi + sill + H,O (B) intrusion. Data from other ‘Younger Basic’ masses imply broadly st bi qz = gt musc H20 (C) equivalent conditions and suggest that the suite of ‘Younger + + + + Basic’ intrusions has been intruded at or around the same level in the Buchan terrain. st + musc + qz = gt + sill + bi + H,O (D)

Staurolitebreakdown and the formation of kyaniteand In addition to dehydration reactions B to D above sillimanite sillimanite in the Barroviantype area Eileen McLellan formation occurs by the even higher temperature reaction (Yale University) gt + musc = bi + sill qz (E) Peak metamorphic conditions in the Barrovian kyanite and + sillimanite zones of GlensShee, Isla, Clova and Esk (SE There is also evidence from white mica compositions of the Grampians)were attained synchronous with or justpost- reaction datingthe last major episode of Caledonidedeformation (D3).The metamorphic peak was achieved laterand at pg + qz = ab + sill + HzO (F) higher temperatures (700°C cf. 580°C) progressively further to the E in the structurally lower levels of Glens Clova and in which pg represents paragonite component of white mica. Esk than in the structurally higher levels of Glens Shee and In kyanite-bearing rocks indirect inversion of kyanite to Isla. Metamorphism was progressive but the exact nature of sillimanite may also be recognized.

References ATHERTON,M. P. 1977. The metamorphism of the Dalradian KENNEDY,W. G. 1948. On the significance of thermal rocks of Scotland. Scott. J. Geol. W, 331-70. structure in the Scottish Highlands. Geol. Mug. 85, CHINNER,G. A. 1966. The distribution of pressure and 229-34. temperature during Dalradian metamorphism. Q. J. NEWTON,R. C. & HASELTON,H. T. 1981. Thermodynamics geol. Soc. London, 122, 159-86. of the garnet-plagi~clase-Al~SiO~quartzgeobarometer. -1978. Metamorphic zones and fault displacement in the In: NEWTON,R. C., NAVROTSKY,A. & WOOD, B. J. Scottish Highlands. Geol. Mug. 115, 37-45. (eds) Thermodynamics of Minerals and Melts. Springer ENGLAND,P. C. & RICHARDSON,S. W. 1977. The influence Verlag, New York, 131-48. of erosionupon the mineral facies of rocksfrom PIGAGE, L. C. & GREENWOOD,H. J. 1982. Internally differentmetamorphic environments. J. geol.Soc. consistentestimates of pressure andtemperature: the London, W4,201-14. staurolite problem. Am. J. Sci. 282, 943-69. GENT, E. D. & STOUT,M. Z. 1981. Geobarometryand RICHARDSON, S. W. & POWELL, R. 1976. Thermal causes of geothermometry of plagioclase-biotite-garnet-muscovite Dalradian metamorphism in the Central Highlands of assemblages. Contrib. Mineral Petrol. 76, 92-7. Scotland. Scott. J. Geol. 12, 237-68. HARTE, B.& HUDSON,N. F. C. 1979. Pelite facies series and -, GILBERT,M. C. & BELL, P. M. 1969. Experimental thetemperatures and pressures of Dalradian meta- determinations of kyanite-andalusite and andalusite- morphism in E. Scotland. In: HARRIS, A.L., HOLLAND, sillimanite equilibria; the aluminum silicate triple point. C.H. & LEAKE,B. E. (eds) The Caledonides of the Am. J. Sci. 267, 259-72. British Isles-Reviewed. Spec. Publ. geol. Soc. London, WELLS, P. R. A. & RICHARDSON,S. W. 1979. Thermal 8, 323-37. evolution of metamorphic rocks in the Central Highlands HODGES,K. V. & SPEAR, F. S. 1982. Geothermometry, of Scotland. In: HARRIS,A. L., HOLLAND, C. H. & geobarometry and the AI2SiO5 triplepoint at Mt. LEAKE,B. E. (eds) The Caledonides of the British Moosilauke, New Hampshire. Am. Mineral. 67, 1118 Isles-Reviewed. Spec.Publ. geol. Soc. London, 8, -34. 339-45. HOLDAWAY,M. J. 1971. Stability of andalusiteand the YARDLEY,B. W. D. 1980. Metamorphism and orogeny in aluminosilicate phasediagram. Am. J. Sci. 271, the Irish Dalradian. J. geol. Soc. London, 137, 303-9. 97-132.

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