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Some chemical features of basalts from the British Tertiary Province
EARTH AND PLANETARY SCIENCE LETTERS 11 (1971) 435-439. NORTH-HOLLAND PUBLISHING COMPANY
SOME CHEMICAL FEATURES THE BRITISH TERTIARY
OF BASALTS FROM PROVINCE
W.I. R1DLEY Geochemistry Branch, Manned Spacecraft Center, Houston, Texas 77058, USA Received 27 October 1970 Revised version received 17 May 1971
Mildly alkaline and transitional basalts are abundant on Rhum, Eigg, Canna and Muck. These islands, together with Skye, make up the Inner Hebrides, an important part of the British Tertiary Volcanic Province. They are chemically distinct from Tertiary basalts described from the Faroes and Iceland, but have the low concentrations of Rb, Sr found in some mildly alkaline basalts dredged front the Atlantic. The low concentration of Rb, Sr, and the almost complete absence of accumulate rocks in the Plateau regions suggests that the transitional basalts have ascended rapidly to the surface without extensive pre-eruptive fractionation. In contrast, the tholeiitic rocks of the British Tertiary are closely associated with shallow crustal magma reservoirs, and are probably more evolved than the transitional basalts. Experimental evidence indicates that the transitional basalts could be parental to both the tholeiites and high-alumina basalts that are the two other major rock types in the British Tertiary Province.
1. Introduction Alkali olivine basalts and tholeiites were first recognised in the volcanic rocks o f Western Scotland in 1924 [ 1], and their recognition led to the discovery of volcanics with similar compositions on a world-wide basis [2, 3]. The subsequent elevation o f alkali olivine basalt and tholeiite to the status of magma types laid the foundation of modern basalt petrogenesis. Nonetheless, the chemical data available for volcanics from the British Tertiary Province is still very limited, and o f variable quality, as reflected in the inability o f any two authors to agree on an average composition for alkali olivine basalt in this region. This enigma has resuited partly from a bias towards the compilation o f data for the spectacular central intrusive complexes (Skye, Rhum, Ardnamurchan, Mull, etc.) which have tholeiitic affinities, and partly from a real sampling problem in a region strongly affected by secondary pneumatolytic alteration. Consequently, an attempt has been made to chemically characterise the basahs from four islands, Rhum, Eigg, Canna, Muck, that together with Skye, comprise the Inner Hebrides. The freslaness of the samples aria-
lysed was determined on strict petrographic criteria, similar to those used for Icelandic basalts [4,5]. Fresh basalts were determined to be those that carried titanomagnetite without ilmenite oxidation lamellae, and were without maghemite or chlorophaiete. The detailed petrology and mineralogy o f these rocks are to be reported elsewhere, but the main chemical characteristics are discussed here.
2. Results Analyses for rocks from Eigg, Canna and Muck are shown in table 1. The spectrum of basaltic compositions are shown in fig. 1. Essentially the analysed basalts are a transitional series in the sense o f Coombs [6], containing either a few percent normative nepheline or normative hypersthene. They are not all alkali olivine basalts, since many that are mineralogically fresh fall in the Di-Fo-Hy field (fig. 1), but overall the group straddles the low pressure thermal divide Di-FoAn in the basalt tetrahedron. In all the aphyric rocks examined, olivine is the most common groundmass phase, but even relatively
436
t¢.L Ridley, Chemical features o f basalts
Table 1 Major element analyses of Small Isles basalts *. Number
1
2
3
SiO2 AI20a TiO2 t:%O 3 FeO MnO - MgO CaO N%O KzO P2Os ttaO
46.04 15.09 0.90 1.52 8.79 0.14 11.71 11.51 2.20 0.23 0.14 1.37
45.68 16.15 1.12 4.24 6.81 0.18 9.02 12.70 2.29 0.11 1.48
45.29 15.19 1.24 3.09 8.65 0.18 10.02 10.25 2.83 0.24 0.19 2.32
Total
99.63
99.78
99.49
C.I.P.W.norms Or Plag Ne Di Ol Mt 11 Ap
1.36 43.22 1.18 20.70 23.75 2.20 1.71 0.31
0.65 51.72 0.65 23.67 13.38 6.15 2.13 -
1.42 49.05 1.59 17.53 20.33 4.48 2.36 0.41
An %
63.7
63.3
55.7
Diff. index
18.5
20.4
25.0
* All analysed basalts are fresh, judging from an extensive ore mineralogy examination [5]. A few samples contained interstitial chlorophaiete in minor quantities. 1: Alkali basalt, Eigg; 2: Alkali basalt, Canna; 3: Alkali basalt, Muck.
coarsely crystallised lavas show no olivine-liquid reaction. In the rare porphyritic rocks, olivine and plagioclase are the most common phenocrystal minerals, pyroxene and spinel are almost completely absent. This is in marked contrast to many alkali olivine basalt series, and testifies to the transitory nature of the group. These basalts are chemically distinct from British Tertiary tholeiites (fig. 1, ref. [7, 8] ) with which they are geographically associated, but have major element similarities to some Icelandic basalts. However, most Tertiary basalts from Iceland are tholeiitic [ 10]. Analyses of basalts from the Faroes (table 2) show a preponderance of tholeiites, although transitional basalts
characterise the upper parts of the lava pile [ 11, 12]. These transitional basalts, however, are olivine tholeiites, in that they lie close to the the Hy-Di join in the system Di-Fo-Hy and appear to have affinities with the tholeiites. In this respect they are very similar to the Tertiary basalts from Iceland. Tertiary lavas from both the Faroes and Iceland are distinctly more enriched in FeO, TiO2, A1203, and P2Os compared to recent basalts erupted at the Mid-Atlantic Ridge, and this is also true of the British Tertiary tholeiites. This is not true of the basalts analysed here which have distinct alkaline affinities, and are similar to some post-Tertiary basalts erupted at the Mid-Atlantic Ridge (fig. 1, table 3). Rb, Sr, concentration (table 2) indicate that the basalts are depleted in these elements relative to other continental transitional basalts [15, 16] and to mildly alkaline volcanics on Hawaii (table 3). Of the MidAtlantic Ridge basalts, the transitional basalts with tholeiitic affinities are very strongly depleted in Rb, Sr, and other incompatible elements [17, 18], but the few with definite alkaline affinities [ 19] have Rb, Sr concentrations similar to those described here.
3. Implications Noe-Nygaard [ 11, 12] has suggested that the Wyville-Thompson Ridge, that cuts obliquely across the present day axis o f the Mid-Atlantic Ridge in a N W - S E direction represented the site of initial spreading in the North Atlantic, and that there should be a close genetic connection between the basaltic volcanism in Western Scotland (dominated by NW trending dike swarms), the Faroes, and Tertiary volcanics in Iceland. It seems more likely, however, that the Tertiary volcanism in Britain heralded the initial continental break up [20, 21], prior to the formation of an oceanic region, and that the Faroes and Iceland arose later in an oceanic region devoid of continental crust. Major element studies support this hypothesis, in that there is a close chemical correlation between Tertiary lavas on Iceland and the Faroes, where the volcanism was dominantly tholeiitic, but little correlation with the voluminous transitional basalts of Western Scotland, which are dominantly mildly alkaline. The similarity between the latter group and mildly alkaline basalts extruded at the Mid-Atlantic Ridge,
437
I¢.L Ridley, Chemical features of basalts NE
.
DI
OZ
Mid-Atlant~c rLdge /.5°N
o
J~an°¢e Fuca o °°o o ~ a n d Gorda ridges o 0 0
•
0 o
0
0
0 o
Hawaiian
v
•
v
v
v
~
,Ntd-
v
~'
OZ
ridge
V
v
OL
V
HY
Fig. 1. Normative basaltic components o f British Tertiary volcaoics plotted in an expanded basalt tetrahedron. Solid circles axe t/ansitional and mildly alkaline basalts [261, and open circles are tholeiites [7, 81. Fields of other basalts axe from Kay et al. [25].
in terms of major element and Rb, Sr abundances, clearly suggests that basalts extruded during the initial separation of Britain and Greenland could have been derived from a mantle region geochemically similar (depleted in Rb, Sr?) to that underlying the present day Mid-Atlantic Ridge. A further implication is that the thermal regime of the mantle beneath the ridge has remained stable throughout the development of the north Atlantic, from the earliest period of incipient continental drift. Experimental evidence [22, 23] indicates that the zone of generation of the mildly alkaline transitional basalts cannot be the same as the tholeiites, the latter being produced at less than 15 km depth. The close relationship between the tholeiitic volcanism in the British Tertiary and the development of shallow level crustal magma reservoirs is consistent with the experimental evidence, and suggests that the tholeiites have evolved by fractional crystallisation within the crust and are not primitive magma types. The higher SiO2, "IiO2, FeO, A1203, and K20 of the tholeiites
could result by fractionation of olivine from a parent of similar composition to the transitional basalts described here.
4. Conclusions
A spectrum of compositions is observed in the basalts of Eigg, Canna, Rhum and Muck, that straddle the low pressure thermal divide Di-Hy-Ol in the basalt tetrahedron. Thus their chemical characteristics were determined at relatively high pressures, probably within the Upper Mantle. The transitional nature of the basalts, their low concentrations ofRb, Sr, and the abundance of aphyric rocks, suggests they have not undergone extensive pre-eruptive fractionation. This contrasts with the closely associated tholeiites of the British Tertiary Province which appear to have evolved in shallow crustal magma chambers. The transitional basalts may be parental to the tholeiites. The close chemical correspondence between these
I¢.L Ridley, Chemical features of basalts
438
Table 2 Concentrations of Rb, Sr, and K20 in Small Isles basalts. Rb
Sr
K20
Canna 1 2 3 4 5 6
4 4 5 8 7 1
342 326 544 368 340 116
0.67 0.47 0.50 0.53 0.49 0.11
Muck 1 2 3 4 5
7 3 3 4 4
404 422 242 215 306
0.51 0.47 0.28 0.24 0.45
Eigg
1 2
3 1
207 365
0.23 0.30
Rhum 1 2
3 6
300 334
0.17 0.62
K20 determined by flame photometry. Precision -+2%. Rb, Sr determined by XRF analysis using a Phillips 1212 spectrometer. Counts corrected for deadtime, drift, tail effects, and matrix effects using program XTRACE (Dr. B.Gunn, University of Montreal). Precision _*1.5 ppm. Values obtained for Rb on USGS standards BCR and G-2 were 46 ppm (45), and 170 ppm (164), and for Sr were 350 ppm (312) and 460 ppm (463). Figures in parentheses are those obtained by Gunn (1969). t r a n s i t i o n a l basalts a n d mildly alkaline t r a n s i t i o n a l basalts f r o m the M i d - A t l a n t i c Ridge i n d i c a t e s t h a t the z o n e o f g e n e r a t i o n o f basaltic liquids h a s c h a n g e d little in this region since the p e r i o d o f i n c i p i e n t o c e a n formation.
Acknowledgements This w o r k was carried o u t w h e n the a u t h o r h e l d a P o s t - D o c t o r a l F e l l o w s h i p at Imperial College, L o n d o n University. A g r a n t f r o m the I n s t i t u t e o f Geologic',d Sciences is g r a t e f u l l y a c k n o w l e d g e d .
References [ 1 ] E.B.Bailey, C.T.Clough, W.B.Wright, J.E.Richey and G.V.Wilson, Tertiary and Post-Tertiary Geology of Mull, Loch Aline, and Oban, Mem. Geol. Surv. Scotland (1924) 1.
Table 3 Rb, Sr, K~O concentrations in basalts from continental volcanics, oceanic islands, and the Mid-Atlantic Ridge. Locality
Type
Aden
Sr
K20
Reference
Transitional 21 Transitional 70 Transitional 22
437 360 431
0.95 0.85 0.99
115]
Nandawar Mts. Transitional 19 N.S.W. Transitional 30
813 897
1.08 1.43
[161
Mt. Taylor New Mex.
Mildly alk.
753
1.29
[9]
Iceland
Transitional 4 Transitional 32 Tholeiite 8 Tholeiite 13 Tholeiite 4.6 Mildly alk. 16
161 539 225 182 318 413
0.22 1.55 0.42 0.54 n.d. n.d.
[13]
Mildly alk. Transitional Transitional Transitional Transitional Transitional Transitional Mildly alk. Transitional Transitional Transitional
230 106 69 115 95 105 170 105 195 195 200
n.d. n.d. n.d. n.d. n.d. 0.13 0.15 0.49 0.49 0.42 0.50
Hawaii Mid-Atlantic Ridge
Rb
25
10.6 0.98 0.75 9.5 5.6 1.1 2.5 12.9 16 13 :8
[141 [ 24 ] [ 17 I
1251 1191
[2] W.Q.Kennedy, Trends of differentiation in basaltic magmas, Am. J. Sci. 25 (1933) 239. [ 3 ] C.E.Tilley, Some aspects of magmatic evolution, Quart. J. Geol. Soc. Lond. 106 (1950) 37. 14l R.L.Wilson and S.E.Haggerty, Reversals of the Earth's magnetic field, Endeavour 25 (1966) 104. [51 N.D.Watkins and S.E.Haggerty, Primary oxidation variation and petrogenesis in a single lava, Contrib. Mineral. Petrol. 15 (1967) 251. [ 6 ] D.S.Coombs, Trends and affinities of basaltic magmas and pyroxenes as illustrated on the diopside-olivinesilica diagram, Min. Soc. Amer. Spec. Paper 1 (1963) 227. [7] A.Holmes and H.F.Harwood, The age and composition of the Whin Sill and related dikes of the North of England, Min. Mag. 21 (1928) 493. [8] A.Holmes and H.F.Harwood, The tholeiitic dikes of the North of England, Min. Mag. 22 (1929) 1. [9] l.Baker and W.l.Ridley, Field evidence and K, Rb, Sr, data bearing on the origin of the MT. Taylor volcanic field, New Mexico, USA, Earth Planet. Sci. Letters 10 (1970) 106. [ 10l G.P.L.Walker, Quart. J. Geol. Soc. Lond. 119 (1963) 29.
141.1.Ridley, Chemical features o f basalts [ 111 A.Noe-Nygaard, Chemical composition of basalts from the Wyville-Thompson Ridge, Belt. Nature 212 (1966) 272. [12] A.Noe-Nygaard, Variation in titania and alumina content through a three kilometre thick lava pile in the Faroes, Medd. Dansk. Geol. Foren. 17 (1967) 125. [ 13] K.S.Heier, B.W.Chappell, P.A.Arriens and J.W.Morgan, The geochemistry of four Icelandic basalts, Norsk. Geol. Tiddsk. 46 (1966) 427. [14] E.J.D.Kable, A.J.Erlank and R.D.Cherry, Geochemical features of lavas from Marion and Prince Edward Islands, in: Scientific results of the South African Marion and Prince Edward Islands Expedition, 1965/1966 (in press). [15] K.G.Cox, l.G.Gass and D.I.J.Mallick, The peralkaline volcanic suite of Aden and Little Aden, South Arabia J. Petrol. 11 (1970)433. [ 16] M.J.Abbott, Petrology of the Nandawar Volcano, N.S.W., Australia, Contrib. Mineral. Petrol. 20 (1969) 115. [ 17] P.W.Gast, Terrestial ratios of potassium to rubidium and the composition of the Earth's mantle, Science 147 (1965) 858.
439
[ 181 M.Tatsumoto, C.E.Hedge and A.E.J.Engel, Potassium, rubidium, strontium, thorium, uranium, and the ratio of strontium-87 to strontium-86 in oceanic tholeiitic basalts, Science 150 (1965) 886. [ 19] J.R.Cann, Rb, St, Y, Zr, Nb in ocean floor basaltic rocks Earth Planet. Sci. Letters 10 (1970) 7. [20] X.Le Pichon, Sea floor spreading and continental drift, J. Geophys. Res. 73 (1968) 3661. [21] F.J.Vine, Spreading of the ocean floor: New evidence, Science 154 (1966) 1405. [22] D.H.Green and A.E.Ringwood, The genesis of basaltic magmas, Contrib. Mineral. Petrol. 15 (1967) 103. [23] M.J.O'Hara, Primary magmas and the origin of basalts, Scot. J. Geol. 1 (1965) 19. [241 E.I.Hamilton, Distribution of some trace elements and the isotopic composition of strontium in Hawaiian lavas, Nature 206 (1965) 251. [25] R.Kay, N.J.Hubbard and P.W.Gast, Chemical characteristics and origin of oceanic ridge volcanic rocks, J. Geophys. Res. 75 (1970) 1585. [26] W.l.Ridley, The petrology of some volcanic rocks from the Small Isles of Invernessshire, Inst. Geol. Sci. Spec. Paper ( 1971 ) in press.
SOME CHEMICAL FEATURES THE BRITISH TERTIARY
OF BASALTS FROM PROVINCE
W.I. R1DLEY Geochemistry Branch, Manned Spacecraft Center, Houston, Texas 77058, USA Received 27 October 1970 Revised version received 17 May 1971
Mildly alkaline and transitional basalts are abundant on Rhum, Eigg, Canna and Muck. These islands, together with Skye, make up the Inner Hebrides, an important part of the British Tertiary Volcanic Province. They are chemically distinct from Tertiary basalts described from the Faroes and Iceland, but have the low concentrations of Rb, Sr found in some mildly alkaline basalts dredged front the Atlantic. The low concentration of Rb, Sr, and the almost complete absence of accumulate rocks in the Plateau regions suggests that the transitional basalts have ascended rapidly to the surface without extensive pre-eruptive fractionation. In contrast, the tholeiitic rocks of the British Tertiary are closely associated with shallow crustal magma reservoirs, and are probably more evolved than the transitional basalts. Experimental evidence indicates that the transitional basalts could be parental to both the tholeiites and high-alumina basalts that are the two other major rock types in the British Tertiary Province.
1. Introduction Alkali olivine basalts and tholeiites were first recognised in the volcanic rocks o f Western Scotland in 1924 [ 1], and their recognition led to the discovery of volcanics with similar compositions on a world-wide basis [2, 3]. The subsequent elevation o f alkali olivine basalt and tholeiite to the status of magma types laid the foundation of modern basalt petrogenesis. Nonetheless, the chemical data available for volcanics from the British Tertiary Province is still very limited, and o f variable quality, as reflected in the inability o f any two authors to agree on an average composition for alkali olivine basalt in this region. This enigma has resuited partly from a bias towards the compilation o f data for the spectacular central intrusive complexes (Skye, Rhum, Ardnamurchan, Mull, etc.) which have tholeiitic affinities, and partly from a real sampling problem in a region strongly affected by secondary pneumatolytic alteration. Consequently, an attempt has been made to chemically characterise the basahs from four islands, Rhum, Eigg, Canna, Muck, that together with Skye, comprise the Inner Hebrides. The freslaness of the samples aria-
lysed was determined on strict petrographic criteria, similar to those used for Icelandic basalts [4,5]. Fresh basalts were determined to be those that carried titanomagnetite without ilmenite oxidation lamellae, and were without maghemite or chlorophaiete. The detailed petrology and mineralogy o f these rocks are to be reported elsewhere, but the main chemical characteristics are discussed here.
2. Results Analyses for rocks from Eigg, Canna and Muck are shown in table 1. The spectrum of basaltic compositions are shown in fig. 1. Essentially the analysed basalts are a transitional series in the sense o f Coombs [6], containing either a few percent normative nepheline or normative hypersthene. They are not all alkali olivine basalts, since many that are mineralogically fresh fall in the Di-Fo-Hy field (fig. 1), but overall the group straddles the low pressure thermal divide Di-FoAn in the basalt tetrahedron. In all the aphyric rocks examined, olivine is the most common groundmass phase, but even relatively
436
t¢.L Ridley, Chemical features o f basalts
Table 1 Major element analyses of Small Isles basalts *. Number
1
2
3
SiO2 AI20a TiO2 t:%O 3 FeO MnO - MgO CaO N%O KzO P2Os ttaO
46.04 15.09 0.90 1.52 8.79 0.14 11.71 11.51 2.20 0.23 0.14 1.37
45.68 16.15 1.12 4.24 6.81 0.18 9.02 12.70 2.29 0.11 1.48
45.29 15.19 1.24 3.09 8.65 0.18 10.02 10.25 2.83 0.24 0.19 2.32
Total
99.63
99.78
99.49
C.I.P.W.norms Or Plag Ne Di Ol Mt 11 Ap
1.36 43.22 1.18 20.70 23.75 2.20 1.71 0.31
0.65 51.72 0.65 23.67 13.38 6.15 2.13 -
1.42 49.05 1.59 17.53 20.33 4.48 2.36 0.41
An %
63.7
63.3
55.7
Diff. index
18.5
20.4
25.0
* All analysed basalts are fresh, judging from an extensive ore mineralogy examination [5]. A few samples contained interstitial chlorophaiete in minor quantities. 1: Alkali basalt, Eigg; 2: Alkali basalt, Canna; 3: Alkali basalt, Muck.
coarsely crystallised lavas show no olivine-liquid reaction. In the rare porphyritic rocks, olivine and plagioclase are the most common phenocrystal minerals, pyroxene and spinel are almost completely absent. This is in marked contrast to many alkali olivine basalt series, and testifies to the transitory nature of the group. These basalts are chemically distinct from British Tertiary tholeiites (fig. 1, ref. [7, 8] ) with which they are geographically associated, but have major element similarities to some Icelandic basalts. However, most Tertiary basalts from Iceland are tholeiitic [ 10]. Analyses of basalts from the Faroes (table 2) show a preponderance of tholeiites, although transitional basalts
characterise the upper parts of the lava pile [ 11, 12]. These transitional basalts, however, are olivine tholeiites, in that they lie close to the the Hy-Di join in the system Di-Fo-Hy and appear to have affinities with the tholeiites. In this respect they are very similar to the Tertiary basalts from Iceland. Tertiary lavas from both the Faroes and Iceland are distinctly more enriched in FeO, TiO2, A1203, and P2Os compared to recent basalts erupted at the Mid-Atlantic Ridge, and this is also true of the British Tertiary tholeiites. This is not true of the basalts analysed here which have distinct alkaline affinities, and are similar to some post-Tertiary basalts erupted at the Mid-Atlantic Ridge (fig. 1, table 3). Rb, Sr, concentration (table 2) indicate that the basalts are depleted in these elements relative to other continental transitional basalts [15, 16] and to mildly alkaline volcanics on Hawaii (table 3). Of the MidAtlantic Ridge basalts, the transitional basalts with tholeiitic affinities are very strongly depleted in Rb, Sr, and other incompatible elements [17, 18], but the few with definite alkaline affinities [ 19] have Rb, Sr concentrations similar to those described here.
3. Implications Noe-Nygaard [ 11, 12] has suggested that the Wyville-Thompson Ridge, that cuts obliquely across the present day axis o f the Mid-Atlantic Ridge in a N W - S E direction represented the site of initial spreading in the North Atlantic, and that there should be a close genetic connection between the basaltic volcanism in Western Scotland (dominated by NW trending dike swarms), the Faroes, and Tertiary volcanics in Iceland. It seems more likely, however, that the Tertiary volcanism in Britain heralded the initial continental break up [20, 21], prior to the formation of an oceanic region, and that the Faroes and Iceland arose later in an oceanic region devoid of continental crust. Major element studies support this hypothesis, in that there is a close chemical correlation between Tertiary lavas on Iceland and the Faroes, where the volcanism was dominantly tholeiitic, but little correlation with the voluminous transitional basalts of Western Scotland, which are dominantly mildly alkaline. The similarity between the latter group and mildly alkaline basalts extruded at the Mid-Atlantic Ridge,
437
I¢.L Ridley, Chemical features of basalts NE
.
DI
OZ
Mid-Atlant~c rLdge /.5°N
o
J~an°¢e Fuca o °°o o ~ a n d Gorda ridges o 0 0
•
0 o
0
0
0 o
Hawaiian
v
•
v
v
v
~
,Ntd-
v
~'
OZ
ridge
V
v
OL
V
HY
Fig. 1. Normative basaltic components o f British Tertiary volcaoics plotted in an expanded basalt tetrahedron. Solid circles axe t/ansitional and mildly alkaline basalts [261, and open circles are tholeiites [7, 81. Fields of other basalts axe from Kay et al. [25].
in terms of major element and Rb, Sr abundances, clearly suggests that basalts extruded during the initial separation of Britain and Greenland could have been derived from a mantle region geochemically similar (depleted in Rb, Sr?) to that underlying the present day Mid-Atlantic Ridge. A further implication is that the thermal regime of the mantle beneath the ridge has remained stable throughout the development of the north Atlantic, from the earliest period of incipient continental drift. Experimental evidence [22, 23] indicates that the zone of generation of the mildly alkaline transitional basalts cannot be the same as the tholeiites, the latter being produced at less than 15 km depth. The close relationship between the tholeiitic volcanism in the British Tertiary and the development of shallow level crustal magma reservoirs is consistent with the experimental evidence, and suggests that the tholeiites have evolved by fractional crystallisation within the crust and are not primitive magma types. The higher SiO2, "IiO2, FeO, A1203, and K20 of the tholeiites
could result by fractionation of olivine from a parent of similar composition to the transitional basalts described here.
4. Conclusions
A spectrum of compositions is observed in the basalts of Eigg, Canna, Rhum and Muck, that straddle the low pressure thermal divide Di-Hy-Ol in the basalt tetrahedron. Thus their chemical characteristics were determined at relatively high pressures, probably within the Upper Mantle. The transitional nature of the basalts, their low concentrations ofRb, Sr, and the abundance of aphyric rocks, suggests they have not undergone extensive pre-eruptive fractionation. This contrasts with the closely associated tholeiites of the British Tertiary Province which appear to have evolved in shallow crustal magma chambers. The transitional basalts may be parental to the tholeiites. The close chemical correspondence between these
I¢.L Ridley, Chemical features of basalts
438
Table 2 Concentrations of Rb, Sr, and K20 in Small Isles basalts. Rb
Sr
K20
Canna 1 2 3 4 5 6
4 4 5 8 7 1
342 326 544 368 340 116
0.67 0.47 0.50 0.53 0.49 0.11
Muck 1 2 3 4 5
7 3 3 4 4
404 422 242 215 306
0.51 0.47 0.28 0.24 0.45
Eigg
1 2
3 1
207 365
0.23 0.30
Rhum 1 2
3 6
300 334
0.17 0.62
K20 determined by flame photometry. Precision -+2%. Rb, Sr determined by XRF analysis using a Phillips 1212 spectrometer. Counts corrected for deadtime, drift, tail effects, and matrix effects using program XTRACE (Dr. B.Gunn, University of Montreal). Precision _*1.5 ppm. Values obtained for Rb on USGS standards BCR and G-2 were 46 ppm (45), and 170 ppm (164), and for Sr were 350 ppm (312) and 460 ppm (463). Figures in parentheses are those obtained by Gunn (1969). t r a n s i t i o n a l basalts a n d mildly alkaline t r a n s i t i o n a l basalts f r o m the M i d - A t l a n t i c Ridge i n d i c a t e s t h a t the z o n e o f g e n e r a t i o n o f basaltic liquids h a s c h a n g e d little in this region since the p e r i o d o f i n c i p i e n t o c e a n formation.
Acknowledgements This w o r k was carried o u t w h e n the a u t h o r h e l d a P o s t - D o c t o r a l F e l l o w s h i p at Imperial College, L o n d o n University. A g r a n t f r o m the I n s t i t u t e o f Geologic',d Sciences is g r a t e f u l l y a c k n o w l e d g e d .
References [ 1 ] E.B.Bailey, C.T.Clough, W.B.Wright, J.E.Richey and G.V.Wilson, Tertiary and Post-Tertiary Geology of Mull, Loch Aline, and Oban, Mem. Geol. Surv. Scotland (1924) 1.
Table 3 Rb, Sr, K~O concentrations in basalts from continental volcanics, oceanic islands, and the Mid-Atlantic Ridge. Locality
Type
Aden
Sr
K20
Reference
Transitional 21 Transitional 70 Transitional 22
437 360 431
0.95 0.85 0.99
115]
Nandawar Mts. Transitional 19 N.S.W. Transitional 30
813 897
1.08 1.43
[161
Mt. Taylor New Mex.
Mildly alk.
753
1.29
[9]
Iceland
Transitional 4 Transitional 32 Tholeiite 8 Tholeiite 13 Tholeiite 4.6 Mildly alk. 16
161 539 225 182 318 413
0.22 1.55 0.42 0.54 n.d. n.d.
[13]
Mildly alk. Transitional Transitional Transitional Transitional Transitional Transitional Mildly alk. Transitional Transitional Transitional
230 106 69 115 95 105 170 105 195 195 200
n.d. n.d. n.d. n.d. n.d. 0.13 0.15 0.49 0.49 0.42 0.50
Hawaii Mid-Atlantic Ridge
Rb
25
10.6 0.98 0.75 9.5 5.6 1.1 2.5 12.9 16 13 :8
[141 [ 24 ] [ 17 I
1251 1191
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439
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