A diagcnctic through anchizone to epizone transition is demonstrated in pelitic rocks of thc Lower Palaeozoic marginal basin of Walcs by cxamination of variations in phyllosilicate mineralogy. illite crystallinity and h,, parameter of white micas. This transition represents a temperature range from -150°C to -400°C and the metamorphism is of a lowpressure facies series type. with a geothermal gradient of -4O"Ckm-'. Variations in grade can be correlated largely with the original basin and shelf form, suggesting a depth-related metamorphism. However, in areas closer to the site of Caledonian plate collision an increasingly syn-tectonic metamorphic event is apparent.Correlation of pelite data with metabasite assemblages is variable. the most consistent relationship being between epizone crystallinity values and epidote-actinolite (greenschist facies) assemblages. Diagenetic clay mineral assemblages are found associated with prehnitepumpellyite assemblages in metabasites and it is suggested that the latter represent nonbuffered, and therefore non-diagnostic, assemblages.

Mafic phyllosilicates in metabasites affected by low-grade regional metamorphism from Wales and eastern North Greenland show variations in their structure and chemistry. These variations are related to four mineral zones in these metabasites, which are recognized on the presence/absence of various key calc-silicate minerals and also actinolite. Zones 1 and 2 equate with the zeolite facies, zone 3 with the prehnite-pumpellyite facies (or prehnite-actinolite facies in rocks with appropriate bulk rock composition) and zone 4 with the greenschist facies. Whilst variations in Fe/(Fe + Mg) in chlorite correlate closely with Fe/(Fe + Mg) ratios in the whole-rock, other chemical variations are clearly unrelated to whole-rock compositions. Contents of Al" are seen to increase systematically in samples from zone 1 through to zone 4, which relate to an increase in temperature. Calibration of alteration temperatures, calculated using the chlorite geothermometer (based on Al" contents) developed for meta-andesites in the Los Azufres geothermal system (Mexico), against x values (an estimate of the proportion of chlorite to swelling component in the mafic phyllosilicates) shows a decrease in the swelling component in passing from zone 1 to zone 4, i.e. with an increase in temperature. Calculated temperatures compare favourably with published stability estimates for the various key calc-silicates and actinolite. These data indicate that the chlorite geothermometer, although developed for meta-andesites from a hydrothermal system, does show a correlation with temperatures estimated from calc-silicate assemblages in metabasites affected by low-grade metamorphism developed on a regional scale.

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Analysis of the precision of the illite 'crystallinity' technique shows that machine errors are <5%, while intra-and inter-sample errors are variable but are up to 12% and 14%, respectively (la). Consideration of this error analysis shows that the isocryst approach, which involves close contouring (e.g. 0.03 A 2 8 ) of illite 'crystallinity' data, has a very low degree of confidence (CO.5) and thus is not regarded as statistically valid. If contouring is to be undertaken with a high degree of confidence (>0.8) it is necessary that contours should be at intervals of 0.1 A 2 8 , which is equivalent to subdivision of the anchizone into upper and lower units. Where previous interpretations have relied upon an isocryst method of contouring at less than 0.1 A 2 8 the conclusions must be regarded as unsubstantiated.Centrifuge separation of clay fractions (based on a Stokes' law application) gives separations in which a significant, but variable, percentage of grains have long axes greater than the size calculated. For the typical <2-pm fraction u t h d , some 20% of grains lie in the 2-4-pm range, although the proportion is not believed to have a significant effect upon 'crystallinity' values. The formula is applicable for grain-sizes down to 0.5 pm. Mite 'crystallinity' values on samples prepared by an ultrasonic disaggregation method show a small increase on those prepared by ball mill crushing. The differences are minimal at the epi/anchizone level but increase to some 10% at the anchizone/diagenetic level. The effect on grade determinations is again thought to be minimal and indicates that concern over unsuitability of the ultrasonic disaggregation method is unfounded. Key w &anchizone; error and precision; illite 'crystallinity'; isocryst method. K i d , H. 1.. 1990. Calibration of the anchizone: a critical comparison of illite 'crystallinity' scales used for definition. l o d of Mctunorphic Geology, 8, 31-46. Kisch, H. J. & Frcy, M., 1987. Appendix: Effect of sample preparation on the measured 10A peak width of illite (illite 'crystallinity'). la-Low Tcmpcranue Mctomorphism (ed. Frey, M.), pp. 301-304, Blackie & Son Ltd, Glasgow. Kublcr, B., 1967. Lo cristallinit6 de I'illite ct les z o n a tout A fait supCricurcs du mttamorphisme. In: Etaga ~c c l~n i q u s , Colloquc dc Ncrrddrcl 1W. pp. 105-121, A la Bamnnitre, NeuchBtel, Suisse. Kubler, B., 1984. Les indicateurs des transformations physiques et chimiques dans la dagenbc. tempCrature er calorim6trie. In: ThmnomtrriC ct Boromfnic gCologqucs (ed. Lagache, M.) pp. 6,623-636. 157-180.