a note on dalradian pillow lavas, strabane, co. tyrone

A Note on Dalradian Pillow Lavas, Strabane, Co. TyroneAuthor(s): William J. McCallienSource: Proceedings of the Royal Irish Academy. Section B: Biological, Geological, andChemical Science, Vol. 43 (1936/1937), pp. 13-22Published by: Royal Irish AcademyStable URL: http://www.jstor.org/stable/20490421 .

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1I 13 ]

II.

A NOTE ON DALRADIAN PILLOW LAVAS, STRABANE;, CO. TYRONE.

By WILLIAM J. McCALLIEN, D.Sc., F.R.S.E.,

Glasgow University.

(PLATE I.)

[Read 27 JANUARY. Published 19 MARCH, 1936.]

Introduction.

THE present note is intended to record the discovery for the first time

of pillow lavas in the Dalradian rocks of Ireland. For convenience it is

suggested at the outset that these rocks should be called the Strabane

Pillow Lavas, because it seems that Strabane, 14 miles south of

Londonderry, is likely to remain the type locality for the rocks (Fig. 1).

Since their discovery in 1932 there has been reason to believe that the

Strabane Pillow Lavas have a wide distribution, for similar rocks have

been found in recent years in the Sperrin Mountains by J. J. Hartley of

Belfast University. Professor E. B. Bailey and the writer have seen the

continuation of the lavas on the western side of the River Foyle (River

Mourne) in the Lifford district.

There are also outcrops of igneous schists at localities to the north of

Strabane, between -it and Burn Dennet, notably at Artigarvan (Fig. 1),

but it is not yet known whether these represent different stratigraphical horizons or repetitions by folding of the Strabane lavas. Unfortunately the district is thickly covered with drift and exposures are poor.

The Strabane district is included in sheet 17 of the Geological Survey map of Ireland, and the rocks are described in the accompanying memoir

(1889). Portlock in his great work "Report on the Geology of Londonderry">

(1843) recorded the discovery of "greenstone slate" containing epidote and albitw in the neighbourhood of Strabane. Unfortunately this was not followed up by the Survey officers. The present writer suspected that Portlock's greenstone slate might be the continuation of the Green Beds of Antrim, and so visited Strabane at Easter in 1932. Hp was not long

there when he wrote to his chief, Professor E. B. Bailey, that the Antrim

Green Beds had been found. In June of the same year Professor Bailey

PROC. R.I.A., VOL. XLIn, SECT. B. [C]



14 Proceedings of the Royal Irish Academy.

acoompanied the writer to Strabane, and on this visit they discovered pillows in the green schists of the quarry on the north-eastern outskirts of the town. The rocks of Patten's Glen (Fig. 3) which the writer had taken to be the Green Beds were then seen to be merely sedimentary layers of the same nature interbedded with the lavas.

L04ONDOt4> p , st

u~~~~~~~fue wono-srt Utea

FIG. 1.

Sketch-map of the district north-east of Strabane.

Stratigraphy.

It is not yet possible to indicate the position of the Strabane Pillow

Lavas in the Dalradilan sequence. Tdhis will probably follow from

Hartley's work in the, Sperrns to the east and north-east of the Straeba;ne

district. Onue or two general remaks may, however, be made at this istage.

The Strabane lavas lie a eonsiderable distance below the grits and

slates whichl, running south-west through southern Inishowen, bend east

ward between Ljondonderry and ILetterkenny, and cross the River Fsoyle in

an easterly and north-easterly direction some mles south of Ljondonderry.

Further, the most northerly lavas recorded here underlie an important

limestone which runs approximately east and west a short diistance north

of Artigarvan, and which in all probability connects with the great

Dungiven Limestone to the eat.



MCOALLIE -A Note on lrad4an Pillow Lavas, Strabane. 15

In the Strabane district the best outcrop of this limestone occurs on the northern bank of a glacial spillway falling eastward from Ballylaw School, a little over a mile from Artigarvan. A hornblende-schist also outcrops in this spillway south of the limestone. South of the epidiorite, sehistose grits with biotite are exposed in many places. They are often associated with mica-schists and black schists, and in Glenmornan River these pass into calcareous schists and limestone below the Smithy at

Artigarvan. From the Smithy southward to beyond Green Lodge-400 yards-the river is occupied by pillow lavas, with limestone between the pillows.

CA

Arlt'iarvamL.9~ iC

"D PILLoW LAvAs.

| 3S Scnsrzrs. Catherine.

FIG. 2.

Sketch-map of the Artigarvan district.

Under the Artigarvan pillow lavas, mica-schists and pebbly grits are exposed for 200 yards in Glenmornan River, and the same belt of grits and schists (with biotite) outcrops in Owenreagh Burn (2 miles north-east of Knockavoe), in Strabane Glen (a glacial spillway runnimg north and south, one mile east of Strabane, Plate I, fig. 2), and in Knoekavoe and

Craigore.



18 Proceedings of the Royal Irish Acadenvy.

The Strabane Pillow Lavas structurally underlie this spread of grits and, as has already been said, they are best exposed in the neighbourhood of Strabane itself. Quarries have been opened in them in several places, and a good natural section occurs in Patten's Glen.

Underlying the lavas in the Curly Hill district of the town a good coarse crystalline limestone, associated with biotite-schist, has been exten sively quarried for burning purposes.

l_A house r0 0 0 0

-- ~0 000 --

~0 0 0

0 0 0

000

2~~~~~~~~~~~

JclwXst ___________, _

FmG. 3.

Sketch-map showing the distribution of the rocks in the immediate neighbourhood of Strabane.



MCCALIJEN-A Note on Dilraddan Pillow Lavas, Strabane. 17

In addition to thie isolated outcrops of the different rocks which have

been mentioned above, reference may be made to good quartzite exposures which occur a mile north of Silverbrook Bridge, 4 miles east of Artigarvan,

bnd which overlie an important limestone exposed in the Burn Dennet

half a mile east of Silverbrook. This limestone in its turn rests on siliceous

and micaceous schists containing poor exposures of tremolite-schist. Another outcrop of green sehist, probably of extrusive igneous origin, occurs at Dunnamanagh.

Strabane Pillow Lavcas.

In hand-specimen the typical rocks of this group consist of massive pale ,green schists in which fibrous amphibole and felspar can be seen with

a lens. The felspar often forms knotty projections from broken surfaces. Many specimens effervesce with dilute acid. Quartz veins, aggregates of large epidote crystals, and veins of asbestos also occur, and although the rocks are free from any appearance of bedding -such as occurs in green

schists of sedimentary origin, there are often streaks and irregular patches of dark slaty material in them. Sediments do oceur between the lavas and

are indistinguishable from Green Beds but they are subordinate to the other varieties. Where pillow structure is best developed the interspaces between the pillows are filled with dark coarsely crystalline limestone and dark slate (Plate I, fig. 1). Some of the lavas are very porphyritic, with undeformed phenocrysts over half an inch in length. The coarsest rock which was seen in the Strabane district occurs in a prominent knoll on

the western side of the high road a short distance north of Strabane

Water Works. In some outcrops the phenocrysts are irregularly distributed, sometimes sparse, sometimes crowded together in one specimen, and often drawn out into augen.

There are in nearly all exposures amphibole-schists in which all macro scopic igneous structures are entirely destroyed.

Petrograpky.-The rocks which may be considered the most typical igneous members of the 'group consist of large clear crystals of albite with abundant inclusions of zoisite and epidote, and sometimes of hornblende, muscovite, and biotite, in a groundmass consisting essentially of thin horublende needles and epidote, the hornblende defining a good foliation (Fig. 4, A and B). Calcite and chlorite are usually present in the ground

mas and sometimes there are large patches of multiply-twinned chlorite and bright brown biotite. The latter also occurs sporadically in the groundmass of most slides.



?8 Proceedings of the toyad Irish Academy.

H

0 0- at a003 i nAch

A B C)

FIG. 4.

Miero-sections of Strabane Pillow Lavas. (A), large felspar phenocrysts consisting of albite (elear) and epidote (Imed), in a groundmass of epidote (dotted) and biotite. (B), albite phenocryst with hornblende (H) and epidote inclusions, m a groundmass of chlonte, epidote, and albite. (0) Turbid albite, with hornblende needles, in a groundmass of epidote, chlorite, hornblende, and albite.

Although the albite of the phenocrysts is for the most part free from twinning, multiple twinning sometimes occurs. Sometimes the albite is cloudy, the cloudy feispar running as veins through the clear. In some cases there axe veins of acicular hornblende running through the felspar phenocrysts (Fig. 4, B and C) and sometimes chlorite occurs in the same way.

As a rule the above rocks have much more of groundmass than of phenocryst.

In addition to these porphyritic varieties there are others which resemble them in hand-specimen but which show varietal differences under the

mieroseope. In one slide albite is very abundant and the horublende occurs in large bright green prisms associated with ehlorite and biotite. Epidote and zoisite are sparse and there are abundant areas of granular sphene.

Another slide consists of a dense fine-grained aggregate of epidote, with some chlorite and biotite, in a base of albite and calcite (Fig. 5, B). The absence of hornblende is a noticeable feature of this slide-some minute needles only oceurring in a thick calcite vein. Good prismatic hornblende,



MCCALLEN-A Note on Dalradian Pillow Lavas, Strabane. 19

wi6th sphene, calcite, biotite, and chlorite, occurs in slides closely resembling the last one but differing also in having abundant epidote and zoisite in large plates as well as in granules (Fig. 5, C).

A B C

FIG. 5.

Micro-sections of Strabane Pillow Lavas. (A), biotite (lined) in a groundmass of

chlorite, albite, and sphene. (B), granular epidote in albite, with caleite and

chlorite in the north-west half of the field. (C), hornblende and epidote in a

groundmass of albite and chlorite.

Another non-hornblendic variety differs from the others in the large quantity of biotite which it contains. This rock consists essentially of large plates of albite (with inclusions of muscovite, biotite, and chlorite), ealcite, sphene, zoisite, and epidote. The biotite, chlorite, and stringers of sphene define the foliation. The biotite is also grouped in large clots (Fig. 5, A).

In the rocks which have been described the phenocrysts are not essentially drawn out along the foliation (cf. Fig. 4, A). A porphyritic

variety from the western side of the road north of Strabane Water Works is in the condition of a chlorite-schist. The phenocrysts can still be easily recognised, but they are very elongated, and the constituent granules of albite and epidote show a tendency to parallel alignment. Only a few of the larger plates of epidote lie athwart the foliation. The felspar occurs

in two varieties, one perfectly fresh and water clear, the other extremely turbid. The matrix is essentially a chlorite-schist consisting of chlorite, albite, and epidote, with sphene and relatively scarce biotite. Muscovite



20 Proceedings of the Royal iris/ Academy.

occurs as a secondary product in the alteration of the felspar. Homblende is extremely rare, being confined to one or two small prisms in a large albite.

0~~~~~~~~~~~~~~~~~~~~~~~1

FIG. 6.

Epidote crystal in Strabane Pillow Lavas.

The remainder of the slides to be noticed are all more or less fine grained hornblende-schists consisting of horublende, albite, epidote, and sphene, with biotite in most examples. Calcite and chlorite are sometimes abundant, and in the calcareous varieties the albite has a definite porphyro blastic habit. The hornblende is bright green in colour (Fig. 7).

A B

FIG. 7.

Micro-sections of igneous sehists associated with the Strabane Pillow Lavas. (A), albite porphyroblasts (clear) in calcite (dotted), with horublende

and chlorite defining the foliation in western half of field. (B), albite porphyroblasts in calcite. Heavy dots and granules represent epidote, lines hornblende and chlorite.



MCCALLiEN-A Note on Dalradian Pillow Lavas, Strabane. 21

The green schists of sedimentary origin which are associated with the

Strabane Pillow Lavas are albite-epidote-biotite-chlorite-schlists similar to Green Beds. iThe albite has typical porphyroblastic habit and is usually full of inclusions. Green prismatic horublende is abundant in one slide

and calcite in another.

The limestones which occur between the pillows consist essentially of calcite and zoisite (with epidote), some albite and numerous prisms of

hornblende. Pyrites also occurs. The very dark varieties are rich in graphite, which occurs with quartz between the calcareous bands as well

as being widely disseminated through the calcite. These graphite-schists often carry nodular masses of zoisite. One of the graphite-rich rocks

consists for the most part of zoisite and epidote with albite and horublende. A section has also been cut of one of the fine-grained slaty inclusions

in the lavas in Strabane Quarry. Under the, microscope it is seen to be

composed of minute hornblende needles embedded in a grounndmass

consisting of felspar and granular zoisite. Towards the junction with the

lava there is a marked concentration of dark material, mostly graphite.

The igneous rock which is associated with this inclusion consists of calcite,

large plates of albite, epidote, and biotite.

Igneous schists from other horizons.

The igneous rocks which have been described are all from the horizon

of the Strabane Pillow Lavas in the immediate neighbourhood of Strabane.

It was mentioned under Stratigraphy, however, that similar rocks occur

in other places at structurally higher levels. Some of these will now be

described briefly. In hand-specimen they are similar to the Strabane rocks except that

rio coarse porphyritic varieties have been seen.

Two slides from the Artigarvan outcrop are very different from one another as seen under the microscope. One is a typical hornblende-schist

consisting of hornblende, epidote, and albite, with a fair amount of chlorite.

The other is very rich m chlorite to the exclusion of hornblende, has

abundant albite, with calcite and dark clots of sphene and epidote.

A green schist from Dunnamanagh resembles a chlorite-schist of

sedimentary origin except that there are nodular masses of epidote-zoisite,

suggestive of pseudomorphs after porphyritic felspar, and also abundant

aggregates of sphene. In addition to the larger eyes of epidote this

mineral is evenly distributed through the slide in small granules. The

chlorite shows blue polarization colours and both quartz and felspar occur

with it. PROC. R.I.A., VOL. XLIII, SECT. B. [D]



22 Proceedings of the Royal Irish Academy.

The hornblende-schist near Ballylaw School is probably of intrusive origin. As seen under the mlroscope it is very much richer in horublende

than any of the rocks hitherto described. The hornblende occurs as large plates and crystals and also as thin needles and fibrous bunches. Albite occurs as large plates with multiple twinning and as granular aggregates Sphene and epidote are abundant and there is some chlorite.

EXPLANATION OF PLATE.

Fig. 1.-Pillow Lavas dn Strabane Quarry, showing pillow structure and sediments between the pillows.

F'ig. 2.-Strabane Glen, a glacial spillway cut through grits structurally overlying the Strabane Pillow Lavas.

Fig. 3.-Entrance to Strabane Quarry showing weathering of the Pillow Lavas into lenticular masses resembling pillows.



PROC. R. I. AcwD., VOL. XLIII, SECT. B. PLATz L

74;:

f ~~~~~~~~~~~~l. 1.

'Pm. 21. Pro,. 3.

MCCALLIEN-DALRADIAN PILLOW LAVAS, STRABANE.



a note on dalradian pillow lavas, strabane, co. tyrone

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