LETTERS TO THE EDITORS
321
itself be regarded as an adequate criterion for diagnosing the presence of Arenig strata.
Cherts, ashy mudstones, and black mudstones or shales in fact form a recurrent association near the base of the Moffat Shales. Neither may an inlier relationship to the
associated (?Caradoc) greywackes be assumed. Arenig sediments proven by fossils are
probably confined to the outcrops in Raven Gill, near Abington, Lanarkshire (Peach
and H o m e 1899, p. 288), where olive-grey mudstones overlie basaltic lavas and contain
a diagnostic Upper Arenig fauna, correlating the succession with that of Bennane Head,
near Ballantrae.
REFERENCES
BULMAN, O. M. B. 1962. On the genus Amplexograptus Lapworth, Elles and Wood. Geol. Mag.
99, 459-467.
ELLES, G. L. and WOOD, E. M. R. 1901-18. A monograph of British graptolites. Palaeontogr.
Soc. [Monogr.].
LAMONT, A. and LINDSTROM, M. 1957. Arenigian and Llandeilian cherts indentified in the
Southern Uplands by means of conodonts, etc. Trans. Edinb. geol. Soc. 17, 60-70.
PEACH, B. N. and HORNE, J. 1899. The Silurian rocks of Britain: Vol. 1, Scotland. Mem.
geol. Surv. U.K.
STRACHAN, I. 1971. A synoptic supplement to "A monograph of British graptolites by Miss
G. L. Elles and Miss E. M. R. Wood". Palaeontogr. Soc. [Monogr.]
WALTON, E. K. i960. Middleton and Heriot. In Mitchell, G. H., Walton, E. K., and Grant, D. eds.
Edinburgh geology—an excursion guide, 115-125. Edinburgh.
WILLIAMS, A., STRACHAN, I., BASSETT, D. A., DEAN, W. T., INGHAM, J. K., WRIGHT,
A. D. and WHITTINGTON, H. B. 1972. A correlation of Ordovician rocks in the British
Isles. Geol. Soc. Lond. Spec. Rep. No. 3.
E. K. W A L T O N
J. A. WEIR
Department of Geology,
University of St. Andrews.
MS received yth February 1973
Revised MS received 6th April 1973
METAMORPHIC INDEX MINERALS IN THE EASTERN DALRADIAN
SIRS
This paper (Porteous 1973) is interesting particularly with regard to n e w data
on the spatial distribution of the aluminosilicates in the eastern Dalradian. The author
relates major index mineral distribution, especially the western termination of the
chloritoid zone, solely to variations in temperature and pressure, and discounts the effect
of rock composition.
During a larger study of Dalradian rocks w e found that there are distinct differences
in composition between areas and even between zones, and maintain that Porteous's
f
m
o
-—/en
o
f
FIG. I . Mol. afm, aknf, acf diagrams as in Hoschek (1967). Open circles: chloritoid-free rocks
(Atherton 1968; Mather 1970); Closed circles: chloritoid-bearing rocks (unpublished work;
Snelling 1957; Williamson 1953); Stippled circle: rock containing kaolinite (unpublished work).
Size of circles represents approximate analytical error.
a
^-c
f
LETTERS TO THE EDITORS
323
(1973) evidence for his dismissal of the effect of rock composition is erroneous on two
counts. Firstly, Hoschek's (1967) diagrams were plotted using molar values, not weight
percent, (p. 126 "Folgende Molverhaltnisse sind in den Diagrammen dargestellt"), and
secondly, Porteous has apparently misunderstood Hoschek's condition that a composition is suitable for the formation of chloritoid-namely that the rock must plot
inside his field(s) on all three diagrams simultaneously. When the data of Atherton
(1968) and Mather (1970) are plotted correctly (Fig. 1, open circles) it is found that only
one rock, out of 32, definitely fulfils this condition and a further two could be judged to
do so within the limits of experimental error. Since the single rock contains small
amounts of kyanite and staurolite, we would not expect it to contain chloritoid because
of its high grade. It is not quite clear what Porteous (1973) means by the sentence "the
points plot across all three fields, and the majority, in fact, plot within the field of
chloritoid-bearing compositions." However, it can be seen that even on the incorrect
aknf diagram of Porteous (1973 fig. 3) that over half the rocks are eliminated by this
diagram alone as being suitable for the formation of chloritoid, and the other two
diagrams may eliminate further rocks.
In order to emphasize the difference between the rocks from the south-west and
those from the north-east, we have included eleven chloritoid rocks from the Stonehaven
area (seven unpublished and four from Snelling (1957) and Williamson (1953); Fig. 1,
closed circles). All these plot as required, within the chloritoid field, except for two on
the acf diagram. Both these contain calcite, which is uncommon in chloritoid rocks,
and such rocks would form a small part of Hoschek's (1967) compilation. Only 90
percent, approximately, of his data lie inside the designated fields (pp. 126-7). Furthermore, these diagrams are compositional and no corrections have been made for accessory minerals.
An even more revealing plot is provided by the more usual AKF phase diagram
(Fig. 2), which is, of course, closely related to the aknf of Hoschek (1967). Here it is
plain that the chloritoid rocks plot as they should, above the muscovite-chlorite join,
(if we allow for the fact that the join indicated is between pure muscovite and chlorite,
whereas analysed muscovites plot somewhat lower in the diagram), while most of the
rocks from the central and south-western parts of the Dalradian plot in the muscovitechlorite-biotite field. It is also interesting to note that a rock containing kaolinite, but
not chloritoid, from the chloritoid zone near Stonehaven (unpublished work) plots as
expected above the muscovite-chlorite join.
It is evident that we are dealing with rocks of different composition so that the
published analysed rocks of Atherton (1968) and Mather (1970) would not, in general,
contain chloritoid even if the PT conditions were suitable. Therefore, we cannot accept
that pressure (or temperature) is the sole reason for the lack of chloritoid, and consequently it is not possible to use the termination of the chloritoid zone to indicate a
continuous increase in pressure towards the south-west in the Barrovian rocks. This
does not deny that some increase in pressure may occur, and our argument is not in
conflict with the aluminosilicate inversion isograd in the north-east.
324
LETTERS TO THE EDITORS
FIG. 2. Mol. AKF diagram. Symbols as in Figure i.
A = A1 2 0 3 —CaO—Na 2 0—K 2 0;
K = K 2 0 ; F = FeO + MnO + MgO;
CaO is corrected for C 0 2 and P 2 O s . In the case of three rocks of Mather (1970) paragonite is
present (unpublished X-ray diffraction data) and one-half of the Na 2 0 is subtracted as 3NazO
and the rest as Na 2 0.
REFERENCES
ATHERTON, M. P. 1968. The variation in garnet, biotite and chlorite composition in medium
grade pelitic rocks from the Dalradian, Scotland, with particular reference to zonation in garnet.
Contr. Mineral. Petrol. 18, 347-371.
HOSCHEK, G. 1967. Untersuchungen zum Stabilitatsbereich von Chloritoid und Staurolith.
Contr. Mineral. Petrol. 14, 123-162.
MATHER, J. D. 1970. The biotite isograd and the lower greenschist facies in the Dalradian rocks
of Scotland. J. Petrology n» 253-275.
PORTEOUS, W. G. 1973. Metamorphic index minerals in the eastern Dalradian. Scott. J. Geol.
9, 29-43SNELLING, N. J. 1957. Notes on the petrology and mineralogy of the Barrovian metamorphic
zones. Geol. Mag. 94, 297-304.
WILLIAMSON, D. H. 1953. Petrology of chloritoid and staurolite rocks north of Stonehaven,
Kincardineshire. Geol. Mag. 90, 353-361.
M. P. ATHERTON
M. S. BROTHERTON
Department of Geology
University of Liverpool
MS received 12th June 1973