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K and Rb in the losberg intrusion, transvaal, South Africa
EARTH AND PLANETARY SCIENCE LETTERS 5 (1968) 41-44. NORTH-HOLLANDPUBI.ISEIINGCOMP., AMSTERDAM
K A N D R b IN T H E L O S B E R G I N T R U S I O N , T R A N S V A A L , S O U T H A F R I C A R. V. DANCHIN Del~artJnentof Geochemistry, Uuiverslt7of Cape ?lbwn,South Africa
Received 14 August 1968
K and Rb have been determined in rocks from the LosbergItntrusion,Transvaal,South Africa.It is one of a ~oup of related marie:bodies with tholeiitic affinities including the BushveldIgneousComplex, and the Great Dyke of Rhodesia. Compared with equivalent rocks from the Bushveld Complexthere is an absolute enrichment of both K and L,'Lb.This eca'iehmenthas =asultcdin the crystallitzationof primary potash minerals, and a correspondingd.ectease in the K/Rb ratios of the Losbergrocks. It is, therefore, postulated that prior to its emplacement tile Losberg magma experienced s o m e degree of f.~actionalcrystallizationat doplh.
1. INTRODUCTION The importance of the K/Rh ratio with regard to understanding the nature of the source materials of igneous reeks, and ~heir differentiation trends has been repeatedly emphasized [1-4]. Recent studies on e variety of reek types have shown that the ratio varies more widely than was previously believed. Notable in this respect are the dredged oceanid tholeiites, where ratios as high as 2 CI00have been reported [S, 6], amphibole peridofites from SL Paul's rocks having ratios up to 1000 [3], and the rocks of the Bushveld Igneous Complex, where ratios of up to 1000 have been confirmed.t7]. In this study K and Rb have been determined in rocks from the Losberg Intrusion, which is of particular interest since it is one of a group of marie bodies with tholeiitic affilaities extending along a north-south line from Rhodesia into the ,Orange Free State, South Africa, and includdng the Great Dyke and the Bushveld Igneous Complex. The lineaT' distribution of these intrusions, and the fact that they are possibly related to the same period of igneous activity [8, 9], have prompted the suggestion that their origins may be coeval and comagmatic [19]. The intrusion is of further interest since its members represent a comprehensive ~dfferantiation sequence ranging from an olivine..orthopyroxene cumulate
(harzburgite) at the exposed base, grading through a plagioclase - orthopyroxene - clinopyroxene cumulate (quartz norite), which is in turn succeeded by tl~e largest unit, a plagioclase clinopyroxene cumulate (quartz gabbro). This latter unit, with a progressive increase in quartz and orthodase, grades into a roof development of aug'ite granophyre [10]. The intrusion is approximately 400 ft thick, and is underlain by shales and capped by quartzites. A comprehensive account of the geochemistry will be presented elsewhere [11].
2. ANALYTICAL PROCEDURE The X-ray spectrographic methods for the deterruination of K and Rb have been described in detail elsewhere [12], except that a LiF (220) crystal was used in the Rb analyses. A.n indication of the accuracy and precision of these methods has been discussed in some detail [7] and the K/Rb ratios obtained are considered to be accurate and precise to within 10% of the values quoted. The results |br K and Rb given in table 1 are averages of duplicate determinations.
R. V. D A N C H I N
42
"fable i Ar~alytiealdata Specimen numbeI'
Height :,feet) above the base of the intrusion
Potassium (%)
Rubidium (ppm)
KIP.b
Harzbu~rgitel~ LB. 53 LB. 54 LB. 55 LB,. 57 LB. 2
0 10 20 40 50
0,35 0.26 0.39 0.43 0.38
14 10 16 20 15
250 260 244 215 253
Quart,: Norires LB. 4 LB. 6 Li~,.7
60 70 80
0.24 0.61 0.55
9 29 25
26 ? 210 220
Quartz Gabbros LIL 10 LIL l I LB. 68 LB. 12 LB. 35 LB. t3 LB. 27 LB. 31 LB, 24 LB. 42
90 ] 20 140 160 210 240 270 290 3,~.0 3!)0
0.68 0.60 0.65 1.02 0.86 l,O7 ,0.94 0,78 0.94 0.84
31 28 32 57 40 53 54 38 55 42
3. RESULTS Eighteen samples taken at close intervals from various heights in tile intrusion have been analysed and the results are given in table I. Fig, 1 shows a plot of K versus Rb and it is seen that these elements show the customary close coherence and that Rb enriches relative to K with increasing differentiation: the K/Rb ratio decreases accordingly from an average value of 245 in the harzburgites to about 200 in the quartz gabbros, and in doing so transgresses the K/Rb - 230 litheof Ahrens et al. [ 13], as modified by Taylor et al. [14].
intrusions. These data are given in table 2. Rocks from the Bushveld Comp)ex show a similar but far more pronounced decrease in file K/Rb ratio with increasing K, and the ratio valies from more than 1000 to less titan 100 [7]. Tiffs wi4e variation in the basic and ultrabasic rocks was ascr:ibed to mineralogical control of the K/Rb ratio. It was proposed that
, ~'~t
/ / ¸:~el /
4. DISCUSSION in view of ~he postulated genetic relationship between the l~ushve]dand Losberg intrusion,,;, it is pertinent to compare the K and Rb contents and the K/Rb ratios of equivalent rock types from the two
219 214 203 179 215 202 174 205 171 200
i !
, "' O'2 1 / ~ I0
s ~wtau~GLr~s R~ 230 J
, 3()
.
,
,
, ..,_,..J I00
p,pm Rb
Fig. 1. K versus Rb in the l.osber~ Intrusion.
K AND Rb IN THE LOSBERGINTRUSI,ON.TRANSVAAL,SOUTIJ AFRICA
43
Tabl,; 2 Compztdson of K and Rh contents of rocks Ifromthe Losbergand Bus]weldIntrusions Potassium (%)
Rock type Harzburgites Noritcs Gabbros
Rubidium (ppm)
K/Rb
l,osbetg
Bustwcld *
Losbetg
I~uslweld *
Losbcrg
Bushve.ld "
0.36 0.47 0.84
0,11 0,12 OA5
15 21 43
3.3 1.5 2.4
245
324 800 625
224 195
* Bushvcld K and Rb data from Erlank. Danchin and FuBarcl [7]. where no primary potash phase was present, plagk~clase (and to a lesser extent pyroxene), having no natural K site in its Lattice, discriminates against the entry of Rb thereby producing high K/Rb ratios. ~n the Losberg rocks, K and Rb are notably enriched compared with equivalent Boshveld members, as may be seen from table 2. The additional amounl:s of K and Rb are located in intercumulus biotite, and (in the quartz gabbros) orthoclase [ 10] : hence the K/Rb ratio is stabilized to a far greater extent than in the Bushveld rocks. This enrichment of K and Rb in the Losberg rocks suggests a certain degree of fractional crys~.allizatio:a at depth prior to intrusion and ultimate solidification. t m absolute enrichment of Ba, U, Th, Zr and Cs [ 111, compared with equivalent Bushveld rocks supports this suggestion. It is also o f interest that while Cs is enriched, th.e K/Us ratio in both Losberg [ 11] and Boshveld [ 15] sequences increases with progressive fractional crystallization. A similar reversal in the usual K/Us treltd was also observed in the Skanrgaard Intrusion, and was ascribed by Heier and Adams [16] to leaching of Cs+. It is felt, however, that the repeated observation of this feature warrants further systematic study. It is conclude,:l that while the rocke of the Losberg Intrusion provide another example where the K-Rb t'elationship in marie rock types cannot be directly invoked to provide information on the K and Rb abundances of the source areas, the K/Rb ratio can fre. quently provide usefut information as to the differentiation history of the rocks involved. ACKNOWLEDGEMENTS J. Ferguson kiJ,dly supplied the samples used :in
this study, and ~ am grateful to Professor L. H. Ahreas and A. J. Erlank for critically reading the manuscript.
REFERENCES [ l I P. W. Gast, Limitations tm the composition of tile Upper' Mantl,,,, J. Gcophys, P,cs. 65 (~960) 1297. [2 [ B. M. Gunn, K/Rb a;nd K/Ba ratios in Antarctic and New
Zealand tholeiitesand alkali basalts, J. Geophys. R,~'s.70 ([965) 6241. [ 3] S, R. Ilart and L. T. Aldrich, Fractionation of potassmm/ rubidium by amphiboles:implicationsregardingmantle composition. Science 155 (1967) 325. ~41 A. M. Stueber ~'ndV, R. Murthy, Potassium-.-uhidium ratios in ultramafic rocks and tile differentiation history of the Upper Mantle, Science 153 t 1966) 740. [S I P- W, Gast. Terrestrial ratio of potassium and rubidium and the compositionof the Earth's mantle, Science 147 0965) 858. [6] M. Tatsumoto, C. E. Hedge and A. E. J. Engel,Potassium, rubidium, strontium, thorium, uranium and the ratio of strontiura- 87 to strorttluln-1~6in oceanic the. leiitic basalt, Science 150 (1965) 886. [ 7 ] A.J. Erlank. R. V. Dancbin and C. C. Fullard, Iligh K/Rb ratios in rocks I~omthe BushveldIgneousCornpie):. South Africa, Earth Planet.Set.Letters 4 (1968) 22. [81 G. l=aure. P, M, lturlcy, H. W. Fairbairnand W. H, Pinsore Age of the Great Dyke of Southern Rhodesia, N~lture 200 (1963) 769. [9] L. O. Nicolaysen, J. W, t. De Villiers,A. J. Burgerand F. W. E. Strelow, New measurements relatingto the absolute age of the Transvaalsy:~temand the Bu~hveld~gneous Complex. Geol. Soc. S. Africa Trans. 61 (19'78) 137, I 1Ol D. Abbott and J. Ferguson. The LosbergIntrusion. Fochville. Transvaal. Geol, Soc. S. AfricaTrans. 68 (1965) 31. [ 11] R.V. Danchin and J. Fcrguson. Differentiation trcn,Js in the Bushveldand Losbergintrasions, accepted t'o~ presentation at the Symposiumon tile Bushveldand other layered intrusions. Pretoria, South Africa. [969.
44
R, V. DANCHIN
(1~] A. J, EIlank and P. K. Hofmeyr, K/R:b and K/Cs rallies in Karroo dolcrltes from South Africa;, L Gcophys. Res. 71 (t966) $439. [ 13.~ L. H. Ahrans, W, H. Pinson and M. M. Ke.ams, Assocgation of rubidium and potassium and (:heir ebut'Manco in common igneous ;rocks and meteorites, Gcochim. C0smochim. Ac(:a 2 (1952) 229. l 14] S.R. Taylor, C. H. Emeleus and C. S. Exlcy, Some anomalous K/Rb ratios in igneous rocks and t2aeir pc2~o,~oglcalsignilicanee, Geochim. Cosmochim. Acta 10 t195fi) 224.
[ 15] C. J. Liebenhcrg, The trace elements of the rocks of the Bushveld Igneeus Complex, PuhL Univ. Pretoria. New Series No. I2 (1960) TL [16] g. S. Hei~ and L A. 5. Adams, The geochemistry of the alkali metals, Phys. Chem. Earth 5 (1964) 2$E.
K A N D R b IN T H E L O S B E R G I N T R U S I O N , T R A N S V A A L , S O U T H A F R I C A R. V. DANCHIN Del~artJnentof Geochemistry, Uuiverslt7of Cape ?lbwn,South Africa
Received 14 August 1968
K and Rb have been determined in rocks from the LosbergItntrusion,Transvaal,South Africa.It is one of a ~oup of related marie:bodies with tholeiitic affinities including the BushveldIgneousComplex, and the Great Dyke of Rhodesia. Compared with equivalent rocks from the Bushveld Complexthere is an absolute enrichment of both K and L,'Lb.This eca'iehmenthas =asultcdin the crystallitzationof primary potash minerals, and a correspondingd.ectease in the K/Rb ratios of the Losbergrocks. It is, therefore, postulated that prior to its emplacement tile Losberg magma experienced s o m e degree of f.~actionalcrystallizationat doplh.
1. INTRODUCTION The importance of the K/Rh ratio with regard to understanding the nature of the source materials of igneous reeks, and ~heir differentiation trends has been repeatedly emphasized [1-4]. Recent studies on e variety of reek types have shown that the ratio varies more widely than was previously believed. Notable in this respect are the dredged oceanid tholeiites, where ratios as high as 2 CI00have been reported [S, 6], amphibole peridofites from SL Paul's rocks having ratios up to 1000 [3], and the rocks of the Bushveld Igneous Complex, where ratios of up to 1000 have been confirmed.t7]. In this study K and Rb have been determined in rocks from the Losberg Intrusion, which is of particular interest since it is one of a group of marie bodies with tholeiitic affilaities extending along a north-south line from Rhodesia into the ,Orange Free State, South Africa, and includdng the Great Dyke and the Bushveld Igneous Complex. The lineaT' distribution of these intrusions, and the fact that they are possibly related to the same period of igneous activity [8, 9], have prompted the suggestion that their origins may be coeval and comagmatic [19]. The intrusion is of further interest since its members represent a comprehensive ~dfferantiation sequence ranging from an olivine..orthopyroxene cumulate
(harzburgite) at the exposed base, grading through a plagioclase - orthopyroxene - clinopyroxene cumulate (quartz norite), which is in turn succeeded by tl~e largest unit, a plagioclase clinopyroxene cumulate (quartz gabbro). This latter unit, with a progressive increase in quartz and orthodase, grades into a roof development of aug'ite granophyre [10]. The intrusion is approximately 400 ft thick, and is underlain by shales and capped by quartzites. A comprehensive account of the geochemistry will be presented elsewhere [11].
2. ANALYTICAL PROCEDURE The X-ray spectrographic methods for the deterruination of K and Rb have been described in detail elsewhere [12], except that a LiF (220) crystal was used in the Rb analyses. A.n indication of the accuracy and precision of these methods has been discussed in some detail [7] and the K/Rb ratios obtained are considered to be accurate and precise to within 10% of the values quoted. The results |br K and Rb given in table 1 are averages of duplicate determinations.
R. V. D A N C H I N
42
"fable i Ar~alytiealdata Specimen numbeI'
Height :,feet) above the base of the intrusion
Potassium (%)
Rubidium (ppm)
KIP.b
Harzbu~rgitel~ LB. 53 LB. 54 LB. 55 LB,. 57 LB. 2
0 10 20 40 50
0,35 0.26 0.39 0.43 0.38
14 10 16 20 15
250 260 244 215 253
Quart,: Norires LB. 4 LB. 6 Li~,.7
60 70 80
0.24 0.61 0.55
9 29 25
26 ? 210 220
Quartz Gabbros LIL 10 LIL l I LB. 68 LB. 12 LB. 35 LB. t3 LB. 27 LB. 31 LB, 24 LB. 42
90 ] 20 140 160 210 240 270 290 3,~.0 3!)0
0.68 0.60 0.65 1.02 0.86 l,O7 ,0.94 0,78 0.94 0.84
31 28 32 57 40 53 54 38 55 42
3. RESULTS Eighteen samples taken at close intervals from various heights in tile intrusion have been analysed and the results are given in table I. Fig, 1 shows a plot of K versus Rb and it is seen that these elements show the customary close coherence and that Rb enriches relative to K with increasing differentiation: the K/Rb ratio decreases accordingly from an average value of 245 in the harzburgites to about 200 in the quartz gabbros, and in doing so transgresses the K/Rb - 230 litheof Ahrens et al. [ 13], as modified by Taylor et al. [14].
intrusions. These data are given in table 2. Rocks from the Bushveld Comp)ex show a similar but far more pronounced decrease in file K/Rb ratio with increasing K, and the ratio valies from more than 1000 to less titan 100 [7]. Tiffs wi4e variation in the basic and ultrabasic rocks was ascr:ibed to mineralogical control of the K/Rb ratio. It was proposed that
, ~'~t
/ / ¸:~el /
4. DISCUSSION in view of ~he postulated genetic relationship between the l~ushve]dand Losberg intrusion,,;, it is pertinent to compare the K and Rb contents and the K/Rb ratios of equivalent rock types from the two
219 214 203 179 215 202 174 205 171 200
i !
, "' O'2 1 / ~ I0
s ~wtau~GLr~s R~ 230 J
, 3()
.
,
,
, ..,_,..J I00
p,pm Rb
Fig. 1. K versus Rb in the l.osber~ Intrusion.
K AND Rb IN THE LOSBERGINTRUSI,ON.TRANSVAAL,SOUTIJ AFRICA
43
Tabl,; 2 Compztdson of K and Rh contents of rocks Ifromthe Losbergand Bus]weldIntrusions Potassium (%)
Rock type Harzburgites Noritcs Gabbros
Rubidium (ppm)
K/Rb
l,osbetg
Bustwcld *
Losbetg
I~uslweld *
Losbcrg
Bushve.ld "
0.36 0.47 0.84
0,11 0,12 OA5
15 21 43
3.3 1.5 2.4
245
324 800 625
224 195
* Bushvcld K and Rb data from Erlank. Danchin and FuBarcl [7]. where no primary potash phase was present, plagk~clase (and to a lesser extent pyroxene), having no natural K site in its Lattice, discriminates against the entry of Rb thereby producing high K/Rb ratios. ~n the Losberg rocks, K and Rb are notably enriched compared with equivalent Boshveld members, as may be seen from table 2. The additional amounl:s of K and Rb are located in intercumulus biotite, and (in the quartz gabbros) orthoclase [ 10] : hence the K/Rb ratio is stabilized to a far greater extent than in the Bushveld rocks. This enrichment of K and Rb in the Losberg rocks suggests a certain degree of fractional crys~.allizatio:a at depth prior to intrusion and ultimate solidification. t m absolute enrichment of Ba, U, Th, Zr and Cs [ 111, compared with equivalent Bushveld rocks supports this suggestion. It is also o f interest that while Cs is enriched, th.e K/Us ratio in both Losberg [ 11] and Boshveld [ 15] sequences increases with progressive fractional crystallization. A similar reversal in the usual K/Us treltd was also observed in the Skanrgaard Intrusion, and was ascribed by Heier and Adams [16] to leaching of Cs+. It is felt, however, that the repeated observation of this feature warrants further systematic study. It is conclude,:l that while the rocke of the Losberg Intrusion provide another example where the K-Rb t'elationship in marie rock types cannot be directly invoked to provide information on the K and Rb abundances of the source areas, the K/Rb ratio can fre. quently provide usefut information as to the differentiation history of the rocks involved. ACKNOWLEDGEMENTS J. Ferguson kiJ,dly supplied the samples used :in
this study, and ~ am grateful to Professor L. H. Ahreas and A. J. Erlank for critically reading the manuscript.
REFERENCES [ l I P. W. Gast, Limitations tm the composition of tile Upper' Mantl,,,, J. Gcophys, P,cs. 65 (~960) 1297. [2 [ B. M. Gunn, K/Rb a;nd K/Ba ratios in Antarctic and New
Zealand tholeiitesand alkali basalts, J. Geophys. R,~'s.70 ([965) 6241. [ 3] S, R. Ilart and L. T. Aldrich, Fractionation of potassmm/ rubidium by amphiboles:implicationsregardingmantle composition. Science 155 (1967) 325. ~41 A. M. Stueber ~'ndV, R. Murthy, Potassium-.-uhidium ratios in ultramafic rocks and tile differentiation history of the Upper Mantle, Science 153 t 1966) 740. [S I P- W, Gast. Terrestrial ratio of potassium and rubidium and the compositionof the Earth's mantle, Science 147 0965) 858. [6] M. Tatsumoto, C. E. Hedge and A. E. J. Engel,Potassium, rubidium, strontium, thorium, uranium and the ratio of strontiura- 87 to strorttluln-1~6in oceanic the. leiitic basalt, Science 150 (1965) 886. [ 7 ] A.J. Erlank. R. V. Dancbin and C. C. Fullard, Iligh K/Rb ratios in rocks I~omthe BushveldIgneousCornpie):. South Africa, Earth Planet.Set.Letters 4 (1968) 22. [81 G. l=aure. P, M, lturlcy, H. W. Fairbairnand W. H, Pinsore Age of the Great Dyke of Southern Rhodesia, N~lture 200 (1963) 769. [9] L. O. Nicolaysen, J. W, t. De Villiers,A. J. Burgerand F. W. E. Strelow, New measurements relatingto the absolute age of the Transvaalsy:~temand the Bu~hveld~gneous Complex. Geol. Soc. S. Africa Trans. 61 (19'78) 137, I 1Ol D. Abbott and J. Ferguson. The LosbergIntrusion. Fochville. Transvaal. Geol, Soc. S. AfricaTrans. 68 (1965) 31. [ 11] R.V. Danchin and J. Fcrguson. Differentiation trcn,Js in the Bushveldand Losbergintrasions, accepted t'o~ presentation at the Symposiumon tile Bushveldand other layered intrusions. Pretoria, South Africa. [969.
44
R, V. DANCHIN
(1~] A. J, EIlank and P. K. Hofmeyr, K/R:b and K/Cs rallies in Karroo dolcrltes from South Africa;, L Gcophys. Res. 71 (t966) $439. [ 13.~ L. H. Ahrans, W, H. Pinson and M. M. Ke.ams, Assocgation of rubidium and potassium and (:heir ebut'Manco in common igneous ;rocks and meteorites, Gcochim. C0smochim. Ac(:a 2 (1952) 229. l 14] S.R. Taylor, C. H. Emeleus and C. S. Exlcy, Some anomalous K/Rb ratios in igneous rocks and t2aeir pc2~o,~oglcalsignilicanee, Geochim. Cosmochim. Acta 10 t195fi) 224.
[ 15] C. J. Liebenhcrg, The trace elements of the rocks of the Bushveld Igneeus Complex, PuhL Univ. Pretoria. New Series No. I2 (1960) TL [16] g. S. Hei~ and L A. 5. Adams, The geochemistry of the alkali metals, Phys. Chem. Earth 5 (1964) 2$E.