THE AMERICAN MINERALOGIST, VOL. 56, MAY-JUNE, 1971

IRON-RICH PIGEONITES FROM ACID ROCKS IN THE

TERTIARY IGNEOUS PROVINCE OF SCOTLAND

C. H. EuBr.avs, Department of Geology, South Road,Durham, England

A. C. DuNuAM AND R. N. TnolrPsoN,l Department oJ Geology,The (lniaersity, Manchester M13 9PL, Englantl.

Assrnacr

Exceptionally iron-rich pigeonites, ranging up to CaeMg::Fe6s, have been identified in

high-level acid intrusive rocks from the Isle of Rhum, Scotiand. Electron microprobe

analyses of the pigeonites are presented, together with comparative data on orthopyroxenes

and Ca-rich clinopyroxenes co-existing with them, and analyses of the three pyroxene as-

semblage in a vitrophyric dacitic lava from Ben Hiant, Ardnamurchan, Scotland. Silica

activity and oxygen fugacity are thought to predominate amongst the many factors con-

trolling igneous iron-rich pyroxene parageneses.

IxrnotucrroN

The hypabyssal acid and subacid intrusions of the British Tertiary

Igneous Province often contain pyroxene and olivine as their principal

mafic phenocrysts. Recent studies of these rocks have tended to concen-

trate on their more abundant feldspar phenocrysts' but Carmichael(1960, 1967) has given a detailed account of the olivine and pyroxene in a

suite of acid vitrophyres. A number of other analyses of pyroxenes, am-

phiboles, and biotites from British Tertiary igneous rocks have beenpublished (Hall imond, 1914; Anwar, 1955; Brown, 1956; Bell, 1966).

This account describes iron-rich pigeonites and associated pyroxenes

from four Tertiary igneous rocks localities in Scotland. Since the pyrox-

enes are generally present in small amounts, all the analyses have been

made with the electron microprobe; details of the analytical techniques

are given in Appendix 2.

OccunruNcns ol Inom-Rrcn Prcnor'rrrrs

Hallimond (1914) first noted iron-rich pigeonite in the Scottish Tertiary tocks, present-

ing an analysis of "uniaxial augite" from an inninmorite pitchstone from Mull (Table 1,

no. 1). This rock can be classified in modern terms as a pigeonite ferrodacite, by use of the

hitherto unpublished "Guide to the naming of volcanic rocks", prepared by the Associate

committee on Geology and Geophysics of canada (Baragar and Irvine, personal communi-

cation, 1969). Rhornbic pyroxene and "uniaxial enstatite-augite" were described by Richey

and Thomas (1930) from pitchstones on Ben Hiant, Ardnamurchan, but these minerals

were not analyzed. It is this occurrence, together with three examples from the IsIe of

Rhum, which are now considered.

I Present address: Department of Geochemistry & Mineralogy, The Pennsylvania

State University, University Park, Pa. 16802. U.S.A.

PIGEONITES FROM SCOTLAND

Taur,r 1. Alrllvsrs aNl CIPW Nonus or Acrn ano Sul-Acrn Rocrs

FRoM TrrE Scorrrsrr Trnr:rlnv IcNrous Pnovrucn

941

SiOr 64 13Al rOa 13.15FerOs 1 08F e O 6 . 3 1MnO O 2 iMgO 1 .08CaO 3 62Na:O 3 -64KrO 2 32HzO+ 2 .71HzO- 0 36TiOz 1 19PrOo 0 31COrOthers 0 09

66.06 ',rO 2013 14 12 612 2 7 2 . 8 92 8 4 2 . i 20 3 1 0 1 1o 7 7 1 7 42 7 5 1 8 84 2 8 4 2 81 . 5 4 3 1 63 3 8 0 5 10 r'4 0. 101 0 8 0 . 6 80 . 0 9 0 . 1 1o 3 70 1 8 0 . 1 3

68 76 69 3914 65 13 821 9 0 2 . 7 03 1 5 2 4 30 0 4 0 1 40 7 7 0 . 8 12 5 5 2 . 2 43 76 4 .062 . 9 9 2 . 6 8o 7 0 0 2 4

o . 7 7 0 . 8 10 2 0 0 2 0

0 8 8o . 1 2

Q 22 69 29.86or 14-11 9 53ab 31 .70 37 92an 13 06 11 22c - 0 5 3d i 3 0 2hy 10 .72 4 .24m t l 6 1 3 4 5i l 2 3 3 2 1 5a p 0 7 6 0 2 2c c 0 8 8

28 15 31 841 7 7 5 1 5 8 13 1 . 9 6 3 4 3 01 1 4 0 4 4 41 . 0 8 2 . 6 1

4.99 3 092 , 7 7 3 9 1t 4 7 1 . 5 40 4 8 0 . 4 7

o . 2 4

28 0618 7836 425 . 9 0

2 . 4 02 6 64 2 11 3 0o 2 6

Total 1O0 26 99 84 1OO.12 IOO 24 99.82

1. Vitrophyric inninmorite Intrusive sheet Beinn an Lochain, Mull (Richey and Thomas, 1930, table III,no. A).

2 Vitrophyric inninmorite. Lava. Ben Hiant, Ardnamurchan (Richey and Thomas, 1930, table III, no II).3. Felsite. Cnapan Breaca, Rhum (Dunham, 1968, table 3, no. A).4. Felsite (I{ 3318). Beinn nan Stac, Rhum (see Hughes, 1960, p 117); New analysis: Geochemical Labora-

tories, Manchester University.5. Granophyre Long Loch, Rhum (Dunham, 1968, table 3, no. C).

Pilchsl'one, Ben Hiant, Ardnanturchan. This is thought to be a lava or series of lava flows on

the floor of a volcanic vent (Richey and Thomas, 1930). It consists of phenocrysts of

plagioclase, ortho- and clinopyroxene, opaques, and apatite in a brown glass containingplagioclase and pyroxene microlites. The Geological Survey of Great Britain anaiysis of

this lava is quoted in Table l,no.2. Petrographic data are given in Appendlt 1. The three

different pyroxenes identified in the specimen examined (MS 66) show the following inter-

relationships. Firstly, each can be found as discrete phenocrysts; secondly, pigeonite some-

times forms cores to augite phenocrysts and; thirdly, pigeonite itself sometimes contains

hypersthene cores.Both augite and pigeonite vary in composition (Table 2, group 1). In part this results

from compositional zoning, with marginal enrichment in Fe relative to Mg. However, one

pigeonite crystal (Table 2, no. 3a) is considerably more magnesian than the others analysed

in this rock It for,ms the core of a composite grain, with a mantle oI augite (Table 2, no.

3b). The analysed hypersthene (Table 2, no. 1a) is enclosed in pigeonite (Table 2, no. lb),'lvith a siighly higher Fe/Mg ratio than the hlpersthene.

These complex grains, in which one pyroxene species jackets another and seals it from

the residual liquid, are thought to be sequential rather than equilibrium features; giving

indications of the course of pyroxene crystaliization in the rock. The iron-enrichment of the

outer pyroxene relative to the inner one supports this view. However, insufficient is known

about the pre-extrusion P.T X. history of this specimen to make any attempt to specily

in detail what processes caused individual relationships, such as pigeonite surrounding

hlpersthene.

Porphyriti,c Felsite,West Minishal,, Rhtm. The felsite is from one of many simiiar boulders

3 3 5 1 R 3 \ 3 R 8 3 o . -. o o c + o o € o o cg - ; o o c o o d o . i - : K g E

i l 3 g i 3 s g l R 3 5 ^ . o3 - i d o d o c ; o o . . i . i B g g

r ? 3 9 3 3 ; 9 3 8 i - - =' o o o ] @ o i o o o

8 J d o o o o d d " i . i i l g -

o r . o D n 9 t s o 9 0 €6 6 n N o N r O N O € r - b €

. a o o + N o N o o o . r / € t so H o o o o o o o N i N o o

: ; E : R R S € 3 8 e * * -. r o o N O O - O O O

3 J . j d o J c ; o d d . ; 9 8 -

o h N 6 0 + o i- o + 4 F - o d + F 1 4

, c o O o T O N O O O . N r o- . * O O o O O O O N N T D S :

: o + o 4 € t s 4 0 0 NN O N : O O 4 $ - O O N 4 +

. 6 O O O F O * O O 3

3 J d o o o o d o . i J i l ? -

6 0 @ d € : 4 N * o cE = D - i O + 6 < O N b 4 N

' o 4 +- N o o o o o o o N - 4 +

- =, J -

; 4 , t r E g S , i f " . r x d J d

EMELEUS. DUNHAM, AND TITOMPSON

+ 3 $ NN O t s 6 O r O 6

H N O + N t s O C4 i * a

$ €€ + F O 6 + O O

o i o t s * 9 o o

3 r o o $ @ O : a

+ o N ++ t s € o Q @ o €

o i o + N 6 0 0

6 H S N :t - t s + € + N : 9

o o o o N + o i

r oh h o 6 9 6 a €

6 0 6 € Nb r < r € @ i 9

o o o r + H o *

Egf,3g3EE

I

d.

! : €

o a

c

E

o

z

a

z

O

F

F

ts3(.)az

zt']xc

t

z

o

O

zotsO|]

N

F

943

=O

: ?

9pas6+! X t la r : a

9 x c

9 ' i >

r r . 9 =

o ( J d

< o o c. . : . i d <

E

I N S F * $ € t s €o : N i t s i D 4 i € @ €

+ i s6 N O O i O O O O € N H

N * O O S t s O N @€ H o o b 6 o $ N

. o o a N 0 o € o

N € 6 € t s 4 S hr , 9 N i € + $ N

o o o t s $ o N5 . . . .

N i + o o € @ HN € * O N i N t s

. 6 0 0 N h o Ho .

b € s o o c r o t si € o N N r b @

. o o o N $ o F6 . . € 3 6a H o o H o o o € N

b N r - O : N O O: € N N 9 0 6

o o o o N o -€ N @

4 0 N b * o @ so N q * t s 6 + F + O r. o o o o t s o -o . i d @

E-t

d > d

PIGEONITES FROM SCOTLAN D

N 6 € O E N H T €@ 6 $ S O O O 6 N

@

o € + $ o € o q €o € 6 r H o o + € N

o .

O - b @ € € @ H €€ o N * € $ 4 € i

o o o N + o H o6

* 9 * b N N O O €N O N i N S b t s N

6 0 0 N $ o i o

€ @ h b 9 € N € 6s 6 N d 4 b 6 € i N € N

. 6 O O N * O - O@ . . . . N S 6o H o o H o o o o € N

o * * $ o o + Hs € N s € < 4 $

' 6 C O O N O Ho .

a s 4 N $ o N i€ a N : $ b h 6

. o o o o N o i€ .

6 9 N O h N O €6 € N i o b b 9

a o o o N o *q . . . .

b < i i d S 5 , S *

. 5 o - B - o 3o c i * + € o i oY N

o o b b + o< 4 3 H i r q +q i o o ' s o o c *

D O O € q @o n € o o € N €

o d o - t s o o H

- A O N : Ov v D 9 0 0 9 *

r \ o o 9 € o o i

€ n N o t oN $ h 6 r 6 N O

@ Q O O € h O d

@ NH € O + € O O O

o c j - . i o < j + * c ;

q * : F € aN A S D + O

o o o r N €S N

6 0 i € b o€ 9 0 H + { N

s d o - o o . ;

N € o o o o @€ N - u / 5 0

t s o o € N + N

N O $ @ 1 \ € Oi h v : o N

o o o i N +

o @ : o : @$ € b $ + o +

3 0 0 0 0 +

6 0 N A 6 0 0o s v N o N I

€ o o N N $

: o O h N :9 4 0 0 b N

@ o o H N +

o 6 $ o 9 € €4 b v N N O

9 o o H N o

a Q d a " * Q E ?6 < r i d > d Z E

a b $ N $ N N b

@ o o € N o o i

d '

- g

a

9M EMELEUS. DUNHAM, AND THOMPSON

in the conglomerate underlying the lavas of West Minishal (Black, 1952). The rocks ap-

pear to have been derived during rapid weathering of the Long Loch-Cnapan Breaca fel-

sites, which they resemble petrographically (Dunham, 1968; Dunham and Emeleus, 1967).

The specimen (DU 9870c) contains phenocrysts of bipyramidal qtattz, plagioclase,

pyroxene, and opaques in a microcrystalline matrix. Petrographic details are given in Ap-

pendix 1. An analysis of a modally similar felsite from Rhum is given in Table 1, no. 3'

The rock contains pigeonite (Table 2, group 2) and very lare hlpersthene phenocrysts.

Extensive alteration of both phases precludes any conclusions about their textural inter-

relations. As in the Ben Hiant specimen, the pigeonite phenocrysts have a slightly higher

Fe/Mg ratio than the hypersthene. Unfortunately the scarcity of hlpersthene in both

rocks prevents making enough analyses to be sure whether this Fe/Mg relationship re-

mains when the compositional range of the mineral is known.porphyritic Felsi.te, Bei.nn nan slac, Rhum. This forms a small sheet-Iike intrusion on the

north side of Dibidil. Its general geology and petrography have been described by Hughes

(1960). The specimen (H. 3313) examined contained phenocrysts of plagioclase (two gen-

erations; Anra and Anss cores), pigeonite, quartz, and opaques in a dull black microcrystal-

line matrix. Hypersthene phenocrysts have been recorded by Hughes (1960) but none'!vas

found in our specimen. A new analysis of this rock is presented in Table 1, no. 4; petro-

graphic details in Appendix 1.

The pigeonite phenocrysts form stout 1 mm prisms, rimmed by occasional granules of

ferroaugite, which also occurs separately. The compositions of seven pigeonite grains

(Table 2, group 3) fall into two groups. It is possible that these represent the extremes of

compositional Fe/Mg zoning; strongly zoned pigeonites have been observed in another

sample from this intrusion (DU 13s76). However, as there are two compositions of plagio-

clase phenocrysts in this rock, the possibility arises of two distinct pigeonite generations.

This interpretation has been used in the construction of Figure 2.

Granophyre, Loch a'Gbi.trtie Reamhra, Rhum, The granophyre specimen (DU 9904) comes

from the northern part of the large granitic intrusion in SW Rhum (Black, 1954; Dunham

and Emeleus, 1967). The rock contains phenocrysts of plagioclase and pyroxene in a mat-

rlr of quartz, alkali feldspar, biotite, amphiboie, and opaques. Petrographic details are

given in Appendlt 1. An analysis of a modally similar granophyre is given in Table 1, no. 5.

The pyroxene phenocrysts are ferroaugite and ferriferous pigeonite; both often mantled

and partly replaced by amphibole and opaques. The analyses (Table 2, group 4) show a

spread in compositions, which is attributable to compositional zoning, since marginal en-

richment in Fe was found in the less altered crystals. The average compositions of the

pyfoxenes (Figure 2) are the most iron-rich of the samples examined.. In addition, they

appear to show a smaller compositional gap between the ca-rich and ca-poor crystals than

in the other specimens, although this is largely the result of low Ca in one ferroaugite

(Figure 1, 5a) and high Ca in one pigeonite (Figure 1, 5c).

CoupanarrvE CHEMTsTRY oF THE PvnoxnNns

The microprobe analytical data on a total of 30 pyroxene crystals from

the four rocks are presented in Table 2. Variations in the atomic propor-

tions of Fe (calculated as Fe2+), Mg, and Ca have been noted in the pre-

vious section (see Figures 1 and 2). There appears to be no positive or

negative correlation of AI, Mn, Ti, and Na with increasing Fe content

among these crystals. This is surprising in the case of Al and Mn, which

PIGEONITES FROM SCOTLAN D

6 ' a ' Fa 6 d

O o

= . - -i ' l ? Ni i o

c A +

! o - ^

4 . i g

h N O

d p .3" i l xi ; <O M H

; 5 F 1

X 3 ' ^

oIL

A

Io

IL

.R. 9Eo

+oII

uf=

946 EMDLEUS. DANHAM. AND THOMPSON

Frc. 2. Average compositions of py'roxenes from Hebridean Tertiary acid rocks in rela-tion to pyroxene compositions in the Skaergaard Intrusion (Wager and Brown, 1967) andN Atlantic Tertiary acid glasses (Carmichael, 1960). The Ben Hiant magnesian pigeonite(Table 2, group 1, no.3a) has been omitted from assemblage 1 on this diagram, as it isthought to pre-date the main pyroxene crystallization period in the rock. The point Iabelied3a on this figure is the average composition of the more magnesian pigeonite group in theBeinn nan Stac felsite (Table 2, group 3, nos 1-5). Further discussion of these two complexspecimens may be found in the text.

Key:7,2,3, 4 as in Figure 1; lM, Analysed pigeonite, Mull (Hallimond, 1914); B,Pyroxenes from Beaver Bay, Minnesota, U.S.A. (Muir, 1954); SK, Trend of analysedpyroxenes, Skaergaard intrusionl TG, Trend of analysed pFoxenes from Tertiary acidglasses; K, Subcalcic ferroaugite, eulite and fayalite from dacite, Asio, Japan, (Kuno,1969); pyroxene s],'rnbols as on Fig. 1; l, fayalite.

generally show good negative and positive correlations respectively withincreasing Fe in pyroxenes.

Carmichael (1960, 1967) has analysed many Ca-rich clinopyroxenesand some orthopyroxenes from rocks in similar environments, hypabyssaland volcanic, to those of the specimens described here. The Rhum ferro-augites have similar Mn and Al, and higher Ti and Na contents than thosestudied by Carmichael, at the same range of Fe/VIg ratios. There isappreciably less Ca in the Rhum ferroaugites than is usual in igneousferroaugites (Figure 2). However, the Rhum and Ardnamurchan augiteshave similar Ca contents to those in augites from the Skaergaard intru-sion (Brown and Vincent, 1963).

The Rhum pigeonites are among the most iron-rich so far discovered.Iron-rich, Ca-poor igneous pyroxenes have been reported by Muir (1954)from the Beaver Bay diabase, Minnesota; by Edwards (1942) from theMount Wellington sill; Tasmania; by Kuno (1969) from dacite in theAsio district, Japan; and by Philpotts (1966) from a qlrartz mangerite in

r00Fe

0M9

PIGEONITES FROM SCOTLAND

Southern Quebec, Canada. Some of these data have been plotted onFigure 2.

DrscussroN

The average composition of the predominating iron-rich Ben Hiantpitchstone pigeonite (CasMgreFer2) is virtually identical to that in theMull inninmorite (CasMgeoFep). The Iatter is unusual in containingpigeonite as its only pyroxene. These pigeonitic rocks of Mull clearlyneed further investigation. The jacketing relations among the threepyroxenes in MS 66 suggest that the orthopyroxene may have been thefirst of them to crystallize.

It should be clear from the descriptions and analyses given above thatthe pyroxenes in the rocks used in this study are rather complex, and donot approach equilibrium assemblages. Zoning of the crystals is as might

be expected in a volcanic and subvolcanic environment. The tie-Iines

drawn on Figure 2 are between average compositions of groups of py-

roxene analyses; giving approximately the same orientations as wouldhave been obtained by separation and wet chemical analysis of theminerals.

Further evidence of disequilibrium is provided by the occurrence oftwo distinct compositional groups of pigeonite in the Ben Hiant and

Beinn nan Stac specimens. Whether this feature can be ascribed to the in-

termingling of two chemically distinct magmas, a common occurrence in

Tertiary Hebridean igneous rocks, or to intermittent pigeonite crystal-Iization during the complex subvolcanic cooling history of a singlemagma, it underlines the point that the tie lines in Figure 2 are not based

on equilibrium assemblages. When constructing this diagram only themore iron-rich pigeonite groups have been joined to the average augitecompositions, because, unlike the more magnesium groups, these give tie-lines with similar orientations to those from previouslv-described augite-

ttffi :Tl::*,'',iltJl?l'o..n.,, in th e Rhum . *rc r' u,,. . o*po,i,io.,, .',, g-

ing up to the margin of the "no-pyroxene" field (Deer, Howie, and Zuss-man, 1963). Furthermore, one crystal in the granophyre shows a trend to-wards calcium-rich ferriferous pigeonite (Ca1aMg22Feo+). The zoning in

the pigeonites follows the normal patternl from magnesian cores to iron-

rich margins, as determined by the electron probe' Therefore, we

assume that the precipitation of magnetite in these rocks was not suffi-

cient to reverse the trend to iron-enrichment of the residual liquids during

the crystallization process. Thus, the Rhum pyroxenes should givesome

clues as to the nature and compositional range of the closure of the two

pyroxene fields.

948 EMELEUS, DUNHAM, AND THOMPSON

The two main interrelated problems are: firstly, why should such iron-rich pyroxenes precipitate and, secondly, at what stage does pigeonitedisappear and ferrohedenbergiteffayalitef quartz become the stableassemblage? Lindsley and Munoz (1969) have recently emphasized theimportance of silica activity in controlling the compositions of pyroxenes.They predict that where a(Sioz) is high, reaching almost unity when asil ica phase is precipitating (see Carmichael, Nicholls, and Smith, 1970).two pyroxenes should persist to very iron-rich compositions. Sparse high-qrartz phenocrysts in the Rhum acid rocks indicate that o(SiO) ap-proached unity in them. Dunham (1968) has deduced that this quattzwas co-precipitated with the iron-rich portions of the pyroxenes.

Carmichael et al. (1970) have taken up Lindsley and Munoz's pointthat o(SiO) is but one of many parameters controll ing pyroxene precipi-tation. They stress that, although a high a(SiOz) favours pyroxene at theexpense of olivine, the oxygen fugacity must also plav a vital part, wher-ever reactions involving iron take place. Thus, if o(SiO) is held constant,it follows that o(FeeOa) must be high, if /(Ot is high. The presence ofiron-rich pyroxenes in these rocks is a function of both high o(SiO) andlow/(Or).

The augites and ferroaugites in Scottish Tertiary acid glassy rocksstudied by Carmichael (1960) are all much richer in calcium, for a givenFe/Mg ratio in the pyroxene, than those from the Rhum rocks. The moreiron-rich calcium-rich pyroxenes described by him coexist with olivine.However, thetie-l ine joining the mostmagnesian of these pyroxene-olivineassemblages is still more iron-rich than any determined in the presentstudy. It is notable that quartz is reported in only two of the phenocrystassemblages analysed by Carmichael (1960), and that in both cases theferroaugite is less calcic than the main trend. I{owever, the amount ofCaO in Carmichael's rocks is less, for a given SiOz content, than in theRhum acid rocks.

The phenocryst phases in both the specimens considered here andCarmichael's suite probably formed prior to emplacement in their presentposition. Hence another possible factor in the development of thesepyroxenes may have been the length of time that the magma remainedat depth before extrusion or emplacement. Lindsley and Munoz (1969)suggest that a metastable solvus in the pyroxene system, with a less calciccalcium-rich limb than in the stable system, may exist; outlining a twopyroxene field, rather than a pyroxeneffayalitefquartz assemblage.This might indicate that the pyroxenes in the pitchstone have morenearly equilibrated than those in the felsites and granophyres. ft is stillnot clear which, if any, of these controls is the dominant one. Ifowever,many factors clearly influence the nature of the pyroxenes.

PIGEONITES FROM SCOTLAND

AcrNowmocBuruts

We are most grateful lo Professor E. A. Vincent for the gift of specimen H 3318 of theBeinn nan Stac felsite from the Oxford University Collection, and to Dr. J. C. Rucklidge,

University of Toronto, for making the microprobe analyses of its pyroxenes. Professor

J V. Smith ofiered C.H.E. microprobe facilities at the University of Chicago u-hich werefinanced by N.S F. Grant GA-572. The Nature Conservancy kindly gave permission forfieldwork on Rhum and provided every assistance there Professor G. M. Brown offereddetailed constructive criticism of an early version of the manuscript.

APPENDIX 1

Petrographic notes

Ben Hi,ant pilchslone, Ardnamu,rchan Generally referred to as "inninmorite", this rock

closely resembles the icelandites (tholeiitic andesites) defined by Carmichael (1964), but is

a little more siliceous than them. Our specimen (MS 66) contains 74 3a/6 plagioclase,4.T0/6

p).'aoxene, and, | 3/6 Fe-Ti oxide phenocrysts, all of which occur singly or in clusters. Theplagioclase phenocrysts (<3mm long) have uniform An{2 cores and sharply demarcated

Aq6 rims, determined optically using the twin orientation curves of Siemmons (1962).

The orientations of the albite, albite-Carlsbad, Carlsbad, pericline, AIa, B and albite-Ala

B twin laws present indicate high temperature optics. The augite crystals ((3 mm long;

2 Vr:4go', are generally larger than the pigeonites ({1 mm long;2 Vr:2-8").2Va of a

single hypersthene grain is 53o.

Porphyri,li,c Fel'sile, conglomerate,West Minishal', Rhum. This rock is a typical Rhum felsite,

resembling those described by Dunham (1968). The mode of specimen DU 9870 C isplagioclase phenocrysts (A.*-ro) l7.8/6, quartz with resorbed margins 0.4/6, pyroxenes

4.40/6, and Fe-Ti oxides 2.2fl1, in a microgranular groundmass. Much of the pyroxene is

altered to aggregates of amphibole and opaques, a process which has gone to completion in

many Rhum acid rocks.

Porphyr'i.tic Fel'sil,e, Bei.nn nan Stac, Rhum. This intrusion, described by Hughes (1960), is

anomalous among Rhum acid rocks. Despite its normal-looking mode, with plagioclase

phenocrysts !8.6o/p, qrartz 1.7/6,pyroxenes 5.0/6, and Fe-Ti oxides l.60/6 $n H 3318), the

compositions of plagioclase cores fall into two distinct groups: An6 and Anzs (volcanic

optics). The margins of both types are zoned to Anre. Similar compositional grouping in thepyroxenes suggests a mixed (phenocryst-bearing) magma rock, a common phenomenon in

the Tertiary Hebridean igneous province. The intermediate and ferriferous pigeonites,

often twinned on (110), have a range of 2Vrftom 3o to 20o. 27" of the ferroaugite is 50o

G'anophyre,Loch a'Ghi.Ili.e Reamhra,Rhum.This rock tlpeistlpicalof theRhumacid

intrusives with micrographic groundmasses (Dunham, 1968). The mode of specimen DU

9904 is: plagioclase phenocrysts 25.0/6, quartz 0.4/6, pyroxenes 9.3/s, and Fe-Ti oxides

3.1/6.The ferroaugite sometimes contains minute, parallel lamellae (? exsoiution; not re-

solved on the microprobe).

APPENDIX 2

Analytical Techniques

The pyroxenes were analyzed in polished thin sections using three microprobes, as

foilows: 1. ARL-EMX at the University of Toronto (for H 3318), operated by Dr. J. C.

9+9

9s0 EMELEUS, DUNHAM. AND THOMPSON

Rucklidge, 2. ARL-EMX at the University of Chicago, and 3 Geoscan I at the Universityof Durham (for the other three specimens), operated by Dr. C H. Emeleus.

Rucklidge used synthetic diopside, ferrosilite, and anorthite, and Mn rnetal as stan-dards. He determined Mn at 25 kV, other elements at 15 kV, and corrected run countdata for instrument drift, background, dead-time absorbtion, and fluorescence.

Emeleus used analysed Fe- and Ca-rich clinopyroxenes and the Mull "inninmorite"pigeonite (see text) as standards. He determined all elements at 15 kV and corrected forinstrument drift, background, and dead-time.

Considering the variety of machines and techniques used, the analyses are encouraginglSrconcordant.

RnrrntNcns

Axwet, Y. M. (1955) A clinopyroxene from the granophyre of Meall Dearg, Skye. Geol.Mog. 92, 367-373.

Bnr,l, J. D. (1966) Granites and associated rocks from the eastern part of the WesternRedhills Complex, Isle of Skye. Trans. Roy. Soc. Edinb.66,307-343.

Br.ecr, G. P. (1952) The Tertiary volcanic succession of the Isle of Rhum, Inverness shire.Trans. Ed.inb. GeoL Soc.15, 39-51.

- (1954) The acid rocks of Western Rhum. Geol,. Mag.91,257-272.Bnowrv, G. M., aNn E. A. VrNcrNr (1963) Pyroxenes from the late stages of fractionation

of the Skaergaard intrusion, East Greenland. L Petrology 4, 175-197 .Bnowx, P. E. (1956) The Mourne Mountains granites-a further study. Geol. Mag 93,

72-84.C,lnurcnall, I. S. E. (1960) The pyroxenes and olivines from some Tertiary acid glasses.

J. Pelrology l, 309 336.- (1964) The petroiogy of Thingmuli, a Tertiary volcano in Eastern Iceland. "I.

Petrology 5, 435-460.- (1967) The iron-titanium oxides of salic volcanic rocks and their associated ferro-

magnesian silicates. Contrib. Mineral. Petrology 14, 36-64.--t J. Nrcnor.rs, aNn A. L. Surru (1970) Sil-ica activity in igneous rocks. Amer.

M i.ner al,. 55, 246-263.Donn, W. A , R. A. Howrn, eNo J. Zussuax (1963). Roch-forming minerals: Vol 2, Chai.n

silicates. Longmans, London.DuNrrau, A. C. (1968) The felsites, granophyre, explosion breccias and tuffisites of the

north-eastern margin of the Tertiary igneous complex of Rhum, Inverness-shire.

QuarL f. Geol,. Soc. London L23,327-352.- AND C. H Emrnus (1967) The Tertiary geology of Rhum, Inner Hebrides. Prac.

Geol. A ssoc. 78, 391-418.Eowan-ns, A. B (1942) Difierentiation of the dolerites of 'fasmania.

II. I. Geol.s0, 579-610.

HerrruoNn, A. F. (1914) Optically uniaxial augite from M,t11. Mi,neral. Mag. 17,97-99.Hucrrs, C. J (1960) The Southern Mountains igneous complex, IsIe of Rhulr'. Quart. J.

Geol,. Soc. Lond.on 116, 111-138.Kuno, H. (1969) Pigeonite-bearing andesite and associated dacite from Asio, Japan. Amu.

J. Sci.. 276A. 257-268.LrNnsr,nv, D. H , ,l,Nr J. L. Muroz (1969) Subsolidus relations along the join hedenbergite-

ferrosilite. Amu. J. Sci., 276A, 295-324.Murn, L D. (1954) Crystallization of pyroxenes in an iron-rich diabase from Minnesota.

Minual. M ag. 3O, 37G388.

PIGEON ITES FROM SCOTLAND

Pntlrotts, A. R. (1966) Origin of the anorthosite-mangerite rocks in southern Quebec. "I.Petrology 7, 7-64.

Rrcrnv, J. E., ann II. H. Tnolrrs (1930) The geology of Ardnamurcha4, northwest Mulland Coll. Mem. Geol. Surl. Scotlard., H.M.S.O., Edinburgh.

Sr.nuuotts, D. B. (1962) Determination of volcanic and plutonic plagioclases using a three-or four-axis universal stage. Geol,. Soc. Amer. Spec. Pap. No.69

Wacrn, L. R., nNn G. M. BnowN 11967) Loyered. Igenous .Rocfts. Oliver and Boyd, Edin-burgh.

Ifarruscri,pt rueiaeil, Jody 28, 1970; acceptd. Jor publication, Octoba 27, 1970.