Some chemical data on the Mourne Mountain granite G2

Geochimica etCosmochimlca Acts, 1960, Vol.18.pp.193to199.Pergamon Prams Ltd. Printed inNorthern Ireland

Some chemical data on the Mourne Mountain granite 62 P. E. BROWN and B. J. RUSHTON Departmentof Geology, SheffieldUniversity, England, and Geological Survey, Tanganyika (Received 16 July 1959)

Abstract-New spectrographic analyses of Mourne granite G2 are presented. These indicate a high degree of uniformity of the mass. The implications of this are discussed.

THE Mourne

Mountains in County Down, Northern Ireland, are the site of a Tertiary granite complex emplaced in Silurian shales and grits. There are several closely synchronous granites intruded by ring dyke formation and cauldron subsidence (RICHEY, 1928). In the eastern Mournes there are three granites distinguished as Gl, G2 and G3 in order of emplacement and situated one within the other with marked eccentricity towards the south east. EMELEUS (1955) has shown that the western granites may be subdivided into G4 and G5, also emplaced The western granites are in contact with the eastern by cauldron subsidence. granites along a length of about 2 miles, and the area1 extent of the whole complex is of the order of 53 miles2 of which G2 occupies some 16 miles2. The granites have sharply defined contacts with the country rock and transgress the sedimentary features. In places the contact is irregular in detail, there being some veining of the shales by granite, but the junction granite to shale is always knife-sharp and the metamorphic effect of the granite confined to hornfelsing. Chemical data previously published on the granites include analyses of Gl and G4 in RICHEY (1928), G2 in NOCKOLDS and RICHEY (1939) and in the Geologicab Szcrvey Szlmmary of Progress for 1953, G3 in PATTERSON (1953), G4 and G5 in EMELEUS (1955), and further rock and mineral analyses in BROWN (1956). Spectrographic determinations have been carried out on seventeen specimens of Mourne granite GS, demonstrating a use of the method of analysis described by RUSHTON and NICHOLLS (1957). The results confirm the chemical uniformity predicted by BROWN (1956). G2 is a medium to coarse grained rock composed of orthoclase microperthite, abundant quartz, plagioclase (albite-oligoclase) and minor amounts of biotite. The whole mass has a conspicuous drusy texture and its margin is fine grained against Gl and the country rock. THE CHEMICAL ANALYSES (TABLE 1)

The localities of the analysed specimens (Fig. 1) though biased towards the north of the mass, have a fair scatter and cover a vertical interval of the order of 1500 ft. The specimen numbers (Fig. 1) are those of Table 1 and include the older analyses made by the classical method. Of the new spectrographic analyses PB7, 193

8, 30 and 102 are of specimens taken between some :‘,O and 90 ft of the contact, nl~ct the mass. The analyses were carried out on material crushed from a large sized hand specimen, but it is not known to what, extent! the small variations fout~d have bet:~~ influenced by the volume of granite crushed. An omission in hhe elements determined for the new analj,sex is silica, but in view of t’he const,ancy of SiO, in the older analyses, this is not considered serious.

-.-.

-.

wa,,

Pig. 1. Sketch nq~ of the Mourn% granites (after IkXxEY, 1328) indicating the &xx&ties of the analysed speeimerts.

PRECISION OF THE SPECTROGRAPHIC ANALYSES (TABLE 2)

It is usual to test the accuracy of an analytical technique by repeated analyses At the time of this investigation the only of a sample of proved composition. available reliable rock standards were the Westerly Rhode Island granite (GI) and the Centerville diabase (Wl) (GOLDICH and OSLUXD, 1956). As these were used for the basis of working curves for calcium, magnesium, manganese and total iron, they could only be used for aluminium and titanium measurements (see Table 2). The reproducibility of the calcium, magnesium, manganese and total iron determinations was estimated from repeated analyses of one specimen of G2 granite. It is concluded that the precision of the spectrographic method is between -&2 per cent and 14 per cent for all the elements under consideration. Alkalis were determined by a flame photometric method. Specimens PB 88 analysed by the spectrographic method and specimen A analysed by the classical method, are from the same exposure and comparison of the analyses reveaIs close similarity except for the total iron figure. The most serious difference between the analyses carried out by the olassical method and those by the spectrographic method is magnesia. It is evident that 194

t; or

0.078

@063

0.480

MgC

cao

@67

Na,O/K,O

River

5+0 0.65

5.05

0.61

W410

0.285 325

0.100

0050

3.10

0.024

0.013

11.5

128

30 yd from

067

435

290

0.325

0.085

0.027

1.50

12.2

0.110

P13102

country

20 yd

rock.

0.63

455

2%

0438

0.083

0,022

1.30

11.9

0.120

PBXll

Mountain

0.61

485

2.95

0.420

0,072

0022

1.30

12.0

0105

PB121

62

0.67

480

320

0.420

0.072

0.022

1.30

122

O-100

PB17.8

granite

D.

C.

B.

0.65

4.85

3.15

0.455

0147

0.023

1.30

12.0

0.110

PR156

445 0.67

4-95 0.59

0.435

0.325

3.00

O-094

0.102

2.99

0.024

0.025

12.5

11.7

1.15

0150

0.110

1.55

PB158

PB157

K and

0.66

471.

3.11

0.66

Tr.

002

1.54

12.49

0.13

77e,7

Na by

0.86

4.52

3.87

0.61

Tr.

Tr.

1.38

12.57

0.12

76%

0.70

4.94

3.44

0.71

0.16

0.02

1.57

U-50

0.13

77%

Fe

NasO/K,O

KsO

Was0

CaO

MgO

MnO

Fe@,

Total

AlsOs

TiO,

SiOs

I

flamephotometer.

0.69

4.84

3.32

071

Tr.

0.03

1.37

12.03

0.09

77?51

Classical

Slievenaglough. Trassey River. Between Slieve Bearnagh and Slievemeel &for. _ Slieve Bearnagh. Head of Bloody Biver associated with PB.88. (analvat P. E. BROWN (19561). iFine grained contact ori Eaife Rock, Slieve Donard (analyst P. E. Brown (1956)). Granite Quarries, Newcastle, Co. Down. Analyst H. F. H&RWOOD (NOCROLDS and RICHEY, 1939). Annalong water tunnel. Analysts A. D. WILSON and P. COOMBS (Geol. Sum Summ. Pmq. 1953).

--._-_. Cnvn __._Tarn - -.-.

PR.11.1. PB.128. PB.160. PB.167. PH.158. A.

Mn and Fe by spectrograph.

0.62

485

3-00

0.400

0.075

0.023

1.30

11.9

0.110

PB109

of Mourne

Al, Ti, Ca, Mg,

@69

475

3.30

0.525

0.074

0024

I*20

12.8

0.105

PBS8

Analyses

approximately

with

066

4.65

3-05

0380

0.085

0.024

1.50

11.7

0.105

PB87

1.

Annalong ;vater tunnel. Head of Bloody River. ditto. Eagle Rock Slieve Donard about 10 yd from contact with Gl. Slieve Corragh, north side. Glen River between Slieve Commedagh and Slieve Donard.

Beg

contact

0.64

4.90

3.15

0.530

0.076

0.018

1.10

0.130

0.105

1.40

PB46

PB42

0.95

12.0

0.105

PB30

by B. J. RUSATON.

0.78

4.85

3.80

@320

0130

0.022

I.30

12.7

0115

PB16

approximately

analyses

@71

4.75

3.35

0.320

0.065

0,027

1.10

11.8

0090

PB12

Glen ‘River north west of Slieve Donard. ditto. Between Slievemeel Mar and Slievemeel ~n&.act with Gl.

2;;;

Spectrographic

495

6-10

KsG

0.66

3‘25

3.40

i-so

0.440

0.028

0.022

MU0

1.95

12.5

0088

PB8

1.15

Fe aa

11.4

0.089

PB7

Fe&s

Total

AWs

TiO,

SiOs

Table

aa

Oxitlc

Oxide specimen no.

Gl

14.0 14.6

14.6 14.0

AlA Wl

15.3 15.7

15.x 15.3

*120,

I-%% G2. PB 128 XII0 G2. PB

128

MgO G2. PB

15.5 I

I

I

0.252 0.260

1.10 1.08

1.10 1.09

1.28 1.25 1.32

1.30 1.25

0.0222 0.0230 0.0222

~ 0.0745 0.0’716

~

Fo” as

Recommended values (1954) 13.99; C:OLI)ICH and OSLCNJ:, (1956) 14.2%

AHRENS

I 0.247 0.249

I),‘O

oxide

I TiO, Gl TiO W12

man

(30,

cao G2. PB 128

(1956) 15.0(15

0.252 1.09

j j I

1.28

0.0241 0.0220

I

0.0227

0.0735 0.0705

;

0.0710 0.410 0.420

0.430 0.405

values

(1954) 14.7%

Cior,rlIcrf and OsLuNn

0.0722 128

hxommended AHRENS

Kccommonded value AHBEYY (1954) 0.250/; Rwommended value AHRENS (1954) 1.10%

No other analyses available

! 0.415

0.410

when magnesia is present in such small amounts in analysis by the classica, method.

there is a danger of inaccuracy

REVIEW OF THE AXALYTICAL R’ESULTS The new spectrographic analyses indicate that granite GZ is essentially a uniform mass. This uniformity is most marked with respect to the figures for The relative variations of total iron, magnesium and manganese and titanium. calcium are moderately high, but the variation in the absolute amount present is small. The variations in aluminium content are not of a geologically significant order. The most striking and probably most important variations are found in the figures for alkalis. A similar variation was found in granite Gl where, by a study of optical axial angles of the potash feldspar in conjunction with X-ray 196

Some chemical data on the Mourne Mountain granite 62

spectrometer data of individual crystals, it was found that there was a considerable variation in the composition of the potash feldspar (BROWN, 1956). Frequently, where potash feldspar and plagioclase adjoin, there is present between the grains a narrow rim of albite, a feature attributed to exsolution by TUTTLE (1952). Variation in the degree of exsolution of the potash and soda components of the alkali feldspar, variations in the composition of individual crystals and variations in the total amounts of alkalis present in a given specimen may probably be attributed to volatile action since the drusy texture of the granite indicates an

Average of G2 by classical method (4 analyses) -

A&+‘, Total Fe as Fe,O, MnO MgO CaO Na,O K,O

-

(3) Average of G2 by spectrographic method contact “zone” specimens. (4 analyses)

(2)

(1)

SiO, TiO,

Table 3

-

-

Average of G2 by spectrographic method Iminus specimens from contact “zone” (13 analyses)

0.11 12.12

1.29 0.23 0.090 0.412 3.15 4.78

1.47 0.02 0.04* 0.67 3.44 4.75

L

-I

Average of all ~~~t~~~ specimens by for col. spectrographic (4) method. (17 analyses) I

-

-

77.20 0.12 12.25

(4)

0.098 12.03

0.106 12.09

0.015 0.42

1.39 0.022 0.069 0.383 3.16 4.86

1.31 0.023 0.085 0.405 3.15 4.80

0.22 0.004 0.023 0.071 0.23 0.20

-

* Three of the analyses show only trace of MgO and one 0.16.

abundant volatile content in the original magma. A similar apparently random variation in the composition of individual crystals of potash feldspar was found by TUTTLE and KEITH (1954) in the Beinn a Dubhaich granite of Skye. Untwinned plagioclases are fairly common in the Mourne granites and this makes determination of composition difficult. However, from measurements of the optic angle, the plagioclase, like the potash feldspar, appears to vary in composition. Thus two plagioclases from granite in the Bloody River section on the eastern side of the Mournes had two Vs of 73” and 83”. An X-ray spectrometer pattern of plagioclase from the chilled margin of granite Gl showed the mineral to contain a high proportion of potash feldspar impurity, thus preventing accurate identification. This information, scanty as it is, seems to indicate that the soda feldspar will be as complex in behaviour as the potash. Comparison of the average of the spectrographic analyses of specimens from the marginal area of G2 with that of analyses of the rest of the mass (columns 2 and 3, Table 3) does not reveal any significant differences. There is no systematic 197

I'.E. BROWN

and B. J. RUSBTOX

variation in the amount of any element with respect to posit,ion in the mass a~rcl it is evident that G2 may be considered an essentially uniform body. The majority of granites may be classified into synorogenic serorogenic and post-erogenic (ESKOLA, 1932; WAHL, 1936), the synorogenic group generally briirg soda dominant granodiorites whereas the granites proper occur in t’he later members of the series at high st,ructural levels. The Mourne granites are somewhat unusual in their high silica, low calcium and very low magnesium content; furthermore their regional tectonic setting, high structural level and the fact that, t,hey are the site of an abnormal gravit)y high ((YOOK and &~VI~PII.Y. i!).‘,?) indicate t)hat they may be classed as anorogenic (ESKOLA, 1950). Uniformity and lack of serial variation have a considerable bearing on the origin of the gral utes. ( ii? corresponds very closely wit’h t’he minimum point for low pressures of Tvater vapour iu i-he system orthoclase-albite--quartz and it follows that a liquid of this composit,ion would rapidly solidify to a uniform mass. The emplacement of the ma,ss by cauldron subsidence (RICHICY, 1!128), sharp contacts, chilling, and veining of t,he country rocks by fine grained granit’ic material of t’he same composition as the main body (BROWK> 1956), all indicate considerable mobility 011 the part of the intruding material. It is concluded that the granite comes illto place as a vola,tile rich magma, rapid cooling leading to t,he development of the overall drusy texture. With respect to the temperat~ures involved it may be noted that the potash feldspar in t,he chilled margin of gra1nt.e (:l is a member of the low albite-orthoclase series (BRONX, 1966) in which there has been little unmixing of t,he potash aud soda component,s. In the average granite unmixing of t,he feldspars is complete. COOK and MURPHY ( 1952) found a large positive anomaly in the Slieve GullionCarlingford-Mourne MounOain area, which they attributed to two large bodies of basic igneous rock, one beneath t,he Sieve Gullion and C’arlingford area and the second beneath the southern border of the Mourne Mountains. They suggest that though the surface outcrop of acid rock predominates, the total mass of basic rock underlying the region mustj be at least one hundred times greater. The present authors preferred hypothesis for the origin of the granibe magma is one of refusiou due to high level intrusion of t’he basic mass indicated by the gravity data (Bnowx, 1956) and perhaps some of the chemical variation found in the analysed specimens, for example in t,he Pe/Mg ratio, favours the hypothesis of refusion which would not be expected to produce complete homogeneity in relatively minor constitnent’s. Authors note. One of us (B. J. IL.) made the spectrographic analyses, the other (P. E. B.) carried out the field and petrological studies. Acknoulledgementa-The writers are greatly indebted to Professor W. A. DEER and Dr. (:. D. NICHOLLS for much advice and encouragement.

REFERENCES AHRENS L. H. (1954) Quantitative Spectrochemical Analysis offh&cates. Pergamon Press, London. BROWN P. E. (1956) The Mourne Granites-a further study. Geol. 1Mag. 93, 72-84. COOK A. H. and MURPHY T. (1952) Measurements of gravity in Ireland. Gravity survey of Ireland north of a line Sligo-Dundalk. Dublin Inst. Adv. Studies, Mem. 2, Pt. 4. EMELEUS C. H. (1955) The granites of the western Mourne Mountains. Sci. Proc. R. Dublin sot. 27, 35-50.

198

Some chemical data on the Mourne Mount&n gmniti 62 ESKOLA P. (1932) On the origin of granite megmas. &finer. petrogr. Mitt. 52, 455481. ESKOLA P. (1950) Nature of metesomatism in the process of grsnitisation. Rep. 18th Sees. Int. Ueol. Congr. Land. 1948, Part 3, 5-13. GOLDICHS. S. end O~LUXIXE. H. (1956) Composition of Westerly granite (Gf) snd Centervi~e diabsse (WI). Bull. Ueol. Sot. Amer. 67, Ml-811i. NOCKOLDSS. R. end RICHEY J. E. (1939) Replacement veins in the Mourne granites. Amer. J. Sci. 237, 27-47. PATTERSONE. M. (1953) Petrochemical data for some acid int’rusive rocks from the Mourne Mountains and Slieve Gullion. Proc. Roy. Irish Acad. B 4, 171-188. RICFIEYJ. E. (1928) The structural relations of the Mourne granites, Northern Ireland. Quart. J. CeoE.Sot. 83, 653-658. RUSHTONB. J. and NICHOLLSG. D. (1957) A spectrographic scheme for the determination of aluminium, titanium, iron, calcium, magnesium and manganese in silicates,. Spectrochim. Acta 9, 287-296. TUTTLE 0. F. (1952) Origin of the contrasting mineralogy of extrusive and plutonic salic rockr. J. C&ok 60, 107-124. TUTTLE 0. F. and KEITH M. L. (1954) The granite problem-evidence from the quartz and feldspar of a Tertiary granite. @eel. i%g. 91,67-72. WAHL W. (1936) The granites of the Finnish part of the Sveeofemmm archean mountain chain. Bull. Comm. Geol. Finlande 115.

199