Chloritoid, and the Isochemical Character of Barrow's Zones

It is argued that despite poverty of outcrop the apparent restrictionof chloritoid to a wedge-shaped area at the north-eastern extremityof Barrow's zones is real. Two possible interpretations of thisrestriction are considered: (a) That the chloritoid producingreaction (as yet unidentified) was characterized by a lowerP/T than that of the reaction muscovite+ chlorite+chloritoid+quartz staurolite+biotite+H²O, whereby, with increasing grade, chloritoidgives way to staurolite. A pressure gradient increasing fromnorth-east to south-west (postulated on separate grounds, Chinner,1966) would then result in the convergence of the chloritoidand staurolite isograds towards the south-west, and the eventualsuppression of the chloritoid isograd to give the wedge-shapedoutcrop actually found, (b) The lack of low-grade hydrous assemblagesaluminous enough to give chloritoid or staurolite with increasinggrade suggests that the low-grade limit of chloritoid (and,to the south-west, of staurolite) may not be an isograd, buta chemical boundary. Such a boundary could either be metasedimentary,or metasomatic, representing an alkali gradient of the typestudied by Orville, in which, essentially, potassium and waterreleased within the high-grade metamorphic zones have migratedto low-grade zones to form more micaceous assemblages. The widespreadexistence of ‘shimmer aggregate‘ muscovite alterationof aluminous minerals in thesillimanite, kyanite, and staurolitezones provides evidence of potassium transfer during the waneof metamorphic temperatures on a scale comparable to that which,during the main metamorphic imprint, would have been requiredto mask the development of peraluminous assemblages in the chlorite,biotite, and garnet zones.     

The Origin of Sillimanite in Glen Clova, Angus

At the sillimanite isograd in Glen Clova, sillimanite appearsto have formed within biotite, rather than in kyanite. Biotiteis thought to have been a nucleating agent, the trigonally arrangedoxygen octahedra and tetrahedra in the alternate mica layersacting as nuclei for the growth of the octahedral Al–Oand the tetrahedral (Al, Si)–O chains that constitutethe sillimanite structure. Nucleation seems to have been dominantlyepitaxial; no permanent breakdown of biotite was involved, andit is suggested that Al and Si for sillimanite growth was mainlyderived from the solution of unstable kyanite.     

Some High-pressure Parageneses of the Allalin Gabbro, Valais, Switzerland

Hydrous products from the alteration of olivine crystals inthe Gabbro of the Allalinhorn show a variety of kyanite-bearingparageneses, maximum-phase assemblages being (1) kyanite-talc-garnet-glaucophane-chloritoid;(2) kyanite-talc-garnet-glaucophane-chloritoid; (3) kyanite-talc-garnet-glaucophane.The assemblages examined are consistent with the attainmentof local equilibrium at the time of their formation, and suggestgrowth under silica-deficient conditions, but a sufficiencyof water. The coexistence of the essentially iron-free phases kyanite-chlorite-talc,coupled with the occurrence of zoisite-kyanite-quartz as alterationproducts of plagioclase, suggests the possible range of crystallizationconditions of P = 10–15 kb; T = 500–700 °C.Possible reactions giving the upper pressure limit of allalinitealteration are: 3 kyanite+talc+2 water = 3 Mg-chloritoid+4 quartz; 8 kyanite+chlorite = 10 Mg-chloritoid+4 quartz+ 7 water.     

Almandine in Thermal Aureoles

The concept that almandine-rich garnet is an anomalous phasein thermal metamorphic assemblages is based partly upon therarity of almandine in hornfelses, and partly upon evidenceof breakdown of ‘regionally’ formed garnets whentheir host-rocks have been thermally metamorphosed. Where amphibolitefacies regional gneisses have been re-metamorphosed within thepyroxene-hornfels facies conditions of the late Caledonian Lochnagaraureole, garnet of composition Alm₈₀Py₁₁p⁶ (Gr+And)₄ has reactedwith biotite, sillimanite, and muscovite to give cordierite-richpseudomorphs; where armoured from reaction by immersion in quartzor feldspar, no signs of dissolution are seen. In some totallyreconstituted hornfelses, almandine garnet of composition AIm₈₀P₁₃Sp₄(Gr+And)3 appears to have coexisted stably with cordierite andorthoclase. It is concluded that the ‘unstable’garnet was breaking down not because its P/T stability fieldhad been exceeded, but because the alman-dine-bearing rock-compositionfield in the thermal (pyroxene-hornfels) facies was more restrictedthan that in the regional (amphibolite) facies. Chemical datafrom the hornfelses suggest that this restriction was mainlydue to the stability of cordierite—the plane spinel-cordierite-quartzrestricts garnet to those rocks with effective mol. (FeO+MgO-t-MnO)A12O3>1, while within that range cordierite-biotite tie-linesrestrict garnet to rocks of high (FeO+MnO)/MgO ratio (Fig. 4b). Recorded instances of garnet ‘instability’ in thermalaureoles show similar features to the Lochnagar aureole—garnetbreaks down by reaction; unequivocal instances of isochemicalbreakdown are rare. This, combined with the widespread occurrenceof almandine in volcanic and plutonic igneous rocks, suggeststhat almandine is a physically stable phase not only in thehornblende-hornfels facies, but also in the pyroxene-hornfelsfacies and possibly in portion of the sanidinite facies as well.The rarity of almandine in thermal aureoles is the result ofits very narrow rock composition field under the P/T conditionsof such environments. Comparison of thermal and granulite facies garnet-cordieriteassemblages suggests that P and T modify the almandine-bearingrock composition field mainly by modifying the limiting Mg/Feratio of garnet and the limiting Fe/Mg ratio of cordierite (Fig.10). The wide rock-composition field of almandine in the amphibolitefacies may be contingent upon the inhibition of cordierite byhydrous minerals under relatively high partial pressures ofwater.