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REE in charnockite and associated rocks, southwest Sweden
FRED H. HUBBARD & JOHN E. WHITLEY
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Hubbard, F. H. & Whitley, J. E. 1979: REE in charnockite and associated r~ks, southwest Sweden. Lithos 12, 1--ii. Oslo. ISSN 0024,4937. The rare earth element compositions of a suite e:' high-grade metamorphic and plutonic locks which outcrop in the Varberg region of southwest Sweden are presented. A plutonic complex containing both charnockitic and non-charnockit~c elements intrudes country rocks of upper amphibolite and granulite facies grade. The REE <:~atafor the country rocks sustain the view that they are of supracrustal origin and support the impcr:ance placed on these elements in investigations of metamo,phic rock progenitors. The progressive change in REE characteristics found for the various units of the plutoniic association confirms that they represent rational stages in &ifferentiation from a common parent. The end-product of this d~fferentiation is a granite with the REE concentration and distribution found, elsewhere in the Baltic Shield, to characterize Rapakivi granite. The progressive change in REE with time suggested for the Baltic Shield by other authors finds support in the results of this study. F. H. Hubbard, Department of Geology, The University, Dundee, Scotland. J. E. Whitley~ Scottish Universities Research and Re,~ctor Centre, East Kilbride, Scotland.
Charnockitic rocks of three distinctive modes outcrop within the high-grade metamorphic rock complex of the Varberg region of southwest Sweden (Quensel 1951; Hubbard 1975). The term charnockite is here used to distinguish massive, granular rocks in the compositional range tonalite to granite which bear two pyroxenes and have distinctively dark-coloured quartz and feldspar. Major quartzofeldspathic horizons within belts of layered'granufites have developed charnockitic mineralogies and fal:~rics during a granulite facies metamorphism. SucI~ layer-paralllel,/n situ, metamorphic charnocl~ite~s are thought to be derived from original greywo:ke sediments. The granulites occur as belts within widespread granite gneisses interpreted as meta-arkoses (Hubbard 1978). Charnockitic characters are also developed in these rocks but patchily, with no apparent control of distribution by the compositional layering which persists in the meta-arkoses. Irregularities of intergranular water distribution in the poorly permeable, earlier recrystallized, meta-arkoses may have localized this in situ cbarnockitizat~on. The most striking charnockite developments of the region, however, are those found in an intrusive complex with both charnockit:,c and non-charn~ckitic members which dominates the ~::oastai outcrop in the r~orth of the area. Dcamafic juxtaposition of coarse 'dry" charnockitic and 'wet' non-charnockitic ~ranite elements is a feagure of this discrete, piuto~fic complex 1 -- Lithos 1/79
which has been called the Varberg CharnockiteGranite Association (CGA) (Hubbard 1975). This complex was initiated, mobilized and emplaced during the metamorphism which produced the in situ ~:harnockitization. Its components and their associations are interpreted in terms of a discrete, internally differentiated, plutonic association in which the non-charnockific (and more Si-K rich) phases fractionated during emplacement; a process determiined and controlled by volatile aggregation. The less dense, less viscous, hydrous fractions w:re more mobile and advanced higher in the sequence than their drier, d~irnockitic, residues. The larger of these aggregates (the Torpa granite) rose to penetrate younger cover sequences which were being metamorphosed with their basement rocks. Crystallization arrested the ascent of the granite, however, before complete separation from its source was achieved. The association displayed at Varberg illustrates a mechanism of granite magma generation and expulsioh from the catazone which in this case failed to reach completion. An outline petrological map of the region is given as Fig. 1. A study of the rare earth element (REE) distribution in the rocks of the Varberg region was indicated for a variety of reasons. The intense recrystallization of all the rocks of the region has made the establishment of a stratigraphy difficult. In view of the indicated stability of REE in
LITHOS ~2 (1979)
2 F.H. Hubbard & J. E. Whitley
quenc¢ of the Baltic Shield of the Swedish charnoekitie and associated 'nozmal' granites, as found at Varberg, is not yet established with certainty. Provisional results suggest an age in the region of 1450 m.y. (Welin, pers. comm.) and more precise radiometric age determinatiop.s are currently being undertaken by Professor Welin. Establishment of the REE patterns of the Varberg granitoids would allow their comparison with the Finnish and Russian data and test their confortuity with the development pattern suggested by Koljonen & Rosenberg (1974).
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Fig. 1. Petrological map of the Varberg region of SW Sweden. !. Granite gneiss. 2. Bar~ded granulite. 3. Garnet pyribolite/Amphibolite. 4. Charnockite-Granite Association (VC = Varberg Charnockite, AGC = Apeiviken-Getter6n Charnockite, TC =Tr6nningen[is Charnockite, TG --=Tor'~a Granite). 5. Metasupracrustal gneisses, in part mi[,m'aatitic (not included in the REE investigation).
metamorphism REE characteristics might provide insight into the premetamorphic natures of the rocks. The occurrence of charnockitic rocks,, whi~.h are thought to have attained their common characteristics by divergent routes, suggests it to be a suitable region to test for a characteristic charnockite REE dist~'ibution pattern. The admixture of charnockitic and non-charnockitic granitic rocks in the Charnockite-Granite Association has b ~ n interpreted on the basis of differen~:iation from a common parent. An alternative interpretation of the granite enclaves in the charnockites as xenoliths was favoured by Quensel (1951). If the model developed by Hubbard (1975), based on field iv.vestigations, is correct the: rocks of the assc¢i,ation ~ould be expected to have a common basiic REE distribution pattern with a differentiation trend following the major element fractionation. Koljohen & Rosenberg (1974) discussed the REE distributions found in Finnish and Russian granites from the Baltic Shield, and identified a pattern of change with age. The position within the time se-
Analytical procedure The determination of REE was carried out by instrumenlal neutron activation analysis at the Scottish Universities Research and Reactor Centre. Samples were processed in two batches of ! 2, in~,'luding in each case a standard USGS rock sample (G2 o1~ BCR !) for assessment o,f accuracy, and elementa~ standards for the elements to be determined (as described by Gordon et al. 1968). :Significant interferences of 232Pa to the peaks of iS3Gd, J:6°Tb and l*SYb were removed by spectrum stripping. Wher~ possible results were calculated with reference to more th~n one photopeak for each element as a check on the absence of interferences. For La, Ce, Sin, Eu and Lu the pre,eision associated with counting statistics was between _+ I c'',Dand _+5%; for Nd, Gd and Yb the precision was in the region of + 10%. For the quartzofeldspathic granulites, with lower rare earth contents, these figures were + 10% and .+_20~i respectively. The results obtained for the standard rocks agreed with the accepted values within the quoted limits of precision. Due to peak interference it was not possible to obtain accurate Gd values for all lthe sampler!; analysed. Gd values are only shown in Table:~ 4 and 5 where the accuracy achieved was within the quoted limits, and absence of a value does not necessarily imply p~k-ticulady low levels of Gd. Because of the irregularity of availabililty of Gd data this element is omitted from distribution diagrams and calculations. Y ~as det,ermined by standard XRF procedures by refe}'ence to a calibration derived from spiked standards. The major element analyses, with the exception of those of specimens HV219 and HV75/52,, were carried out at the Department of Geology, University of St. Andrews, (Analyst: R. A. Batchelor) by wet chemical methods. The rem~finder were analysed on fused and pressed powder samgle pellets by XRF methods at the Department of Geology, University of Dundee (Analyst: R. McGill). More detaih of the analytical procedures may be obtained from the t~uthors.
The samples "Fr~e samples used were selected from collections made in the course of the gfneral geological inve::tigation of the area by one of the authors (F H.H.). The samples varied ~n size from two to
REz~ in eharnock ite
LITHOS 12 (1979)
3
Table 1. Mineral assemblages of analysed specimens.
Rock type
Mineral assemblage
1 HV 185 2 HV 219
Quartzofeldsp~:thic granulite Quartzofeldspathic granulite
pl, mp, qz; bi, hb; ore, ap, zi. pl,* qz; di, hb, hy, bi; ore, ap.
3 4 5 6
Charnockitic granite gneiss Charnockitic granite gneiss Granite gneiss Granite gneiss
pl.* qz, mp; hb, by, di, bi; ap, ore, zi. pl,* mp, qz; hb, hy, di, bi; ore, ap, zi. rap, qz, pl; bi, hb; ore, ap, zi. 'rap, qz, pl; bi; ore, ap, cc.
7 HV 299
Garnet pyribolite
pl, qz; gt, hb, di, hy; bi, ore, ap.
8 HV 251
Intermediate charnockite (VC) "
pl, mp, qz; hy, di, hb; ore, ap, bi, gt.
9 HV 165 10 HV 167 11 HV 33G
Fine-grained eharnockite (AGC) Fine-grained charnockite (AGC) Fine-grained charnockite (AGC)
pl,* mp, qz; by, di, hb; gt, ore, av, bi. pl,* mp0 qz; hy, di, gt, hb; ore, ap. rap, pl, qz; hb, hy, di; ore, ap, zL
12 HV 126 13 HV 117
Coarse-grained charnockite (TC) Coarse-grained charnockite (TC)
mp, pl, qz; hb, di, hy, b~; ore, a-~ ~t. m~, pl, qz; hb, di, hy; ore, ap, gt, bi, zi.
14 H¥ 166a 15 HV 166b 16 HV 331
Coarse charnockitic aggregate (AGC) Coarse charnockitic aggregate (AGC) Coarse charnockit~c aggregate (AGC)
mp, pl, qz; hb, hy, bi; ore, ap, zi rap, pl, qz; hb, hy, di; ore, ap, zi. rap, pl, qz; hb, h~,. di, bi; ore, ap, zi.
17 HV 176 18 HV 177 19 HV 175
Coarse chamockite phase (AGC) Transitional phase (AGC) Microcline granite (AGC)
nap, pl, qz; hb, px (alt.); ore, ap, zi, bi. rap, pl, qz; hb, hy, di; ore, ap, zi. qz, mi, pl; hb, bi; ore, ap, c¢, zi.
20 HV 93 21 HV 74/2 22 HV 74/10
Microcline granite (TG) Microcline granite fiG) Microciine granite (TG)
mi, qz, pl; hb. hi; ore, zi, st,. mi, qz, p!; bi, tab; ore, sp, zi, ap. mi, qz, pl; bi, hb; ore, sp, ap, zi.
Specimen
HV 75/52 HV 3OO HV 327 HV 328
The mineralogies are grouped into felsic, mafic and accessory minerals. Within each of these groups the minerals are arranged in rough descending order of abundance, pl-plagioclase; pl*= antiperthite; mp= microperthite; mi = mieroeline; qz = quartz; hy = hypersthene; di = diopside: hb = hornblende; bi = biotite; gt = garnet; ap=apatite; z~=zircon; co=calcite; sp=sphene. Recks 1 to 7 are examples of the country rocks, while the CGA is represented by specimens 8 to 22.
five kilograms The mineral assemblages of the selected suite are given in Table 1 and the bulk chemical compositions in Tables 2 and 3.
F'etrography The granulite samples are from massive quartzofeldspathic horizons which may most reasonably be compared and contra'~ted with the granite f,~leisses and, like the granite gueisses, may equate with the source rocks for the anr~tectic plutonic C G A . One specimen (HV 219) is charnockitic and of intermediate bulk composition whilst the other (HV 185) is more acidic and non-eharnockitic (Tables 1 andL 2). The granite gneiss samples fall into two catei~,ories based on their bulk compositions - mineralogies and petrographies. HV 75/52 and HV 1300 are charnockitic and intermediate in composition (thou~:h compositiona~ly comparable with
the more acidic granulite). HV 327 and HV 328 are truly acidic, are non-pyroxenic (though HV 327 has the dark feldspar characteristic of the charnockites) and show evidence of lit-par-lit anatectic migmatization on a very fine scale (Tables 1 and 2). Many basic igneous events are represented in the Varberg region; their products are difficult to distinguish. The selected garnet-pyribolite is believed derived from a volcanic parent associated with the depositional stage of the enclosing granulites. Several sub-units are distinguished in the Charnockite-Granite Association which represent stages in the emplacement differentiation. The principle sub-divisions are (a) the Varberg Charnockite (VC), (b) the Apelviiken-Getter6n compound unit which is dominantly charnockitic (AGC), (c) the TrSnningen~is C h a ~ o c k i t e (TC), and (d) the Torpa Granite f i G ) . The VC is a massive characckite varying from fine to medium grain and lc~:ally sparsely porphyritic. It shows intrusive relation-
LITHOS 12 (1979)
4 F.H. Hubbard & J. E. Whitley T,.rble2. Major oxide analysesof the Varbcrg region country rt~eks (wt.%). 1
2
3
4
5
6
7
SiO2 68.83 56.08 66.83 TiO2 0.39 0.59 0.53 AI:,O3 15.17 16.01 14.90 Fe~Os 1.08 9.24* 4.04* FeO !.73 MnO 0.10 0. ! 7 0.07 MgO 1.70 3.07 1.64 CaO 3.26 7.99 3.21 Na20 4.16 5.14 3.64 K20 3.17 0.77 4.00 P2Os 0.15 0.22 0.115 H20 0.39 0.63 0.65
64.9 7 4 . 1 71.9 42.8 0.56 0.25 0.32 2.58 15.97 12.32 13.'78 !4.79 2.24 n.d. !.10 4.60 1.81 !.89 1.10 11.82 0.1 i 0.05 0.06 0.23 | .40 0.27 0. 51 7.51 3.48 0.77 1.20 11.19 4.52 2.43 3.63 2.52 4.31 6.93 5.87 1.18 0.15 0.02 0.196 0.25 0.08 0.29 0.20 0.37
Total
99.53
100.13 99.91
99.66
99.32
99.73
99.84
Specimen numbering follows Table I. * Total Fe as FezOs
ships with the coun.try rocks (both granite gneiss and granulite) with xenoliths and apophys~al sheeting. The AGC comprises a dominant 'matrix' phase which is closely similar, eompasitionally and petrographieaily, to the VC though generally slightly more acid and alkaline and more universally porphyritic. Within this component there occur coarser aggregates of two contrasting types: (a) Irregular masses, usually and characteristically with diffuse margins, marked by intense feldspar megacryst growth and marie mineral aggregation. The felsic minerals usually wholly maintain the dark charnockitic colouration but pyroxene may be largely replaced by amphibole and, to a lesser extent, biotite. The feldspar megacrysts are late porphyritic or porphyroblastie. These coarsely crystalline zones are compo,:itionally indistinguish-
able from their hosts. (see Table 3). (b) Less common, coa:sely crystalline, bodies distinguished by their repe~ted pear.shat~e, sharp margins and internal compositional differentiation. "l'he narrower extremity of ~these elongate aggregates is always to the south (interpreted as 'downward' during emplaeementJ; it :is of intermediate composition, and charno~¢kitic. This rock rapidly, but progressively, changes through a sub-eharnockitie transitioeal phase, to a sili~:eous, leucocratic, microdine-granite at the broad northern limit. Distinct, sharp interfaces are main~lained throughout to the enclosing AGC resulting iin striking, abrupt juxtaposition of 'wet' and 'dr.y' granitoids. Some form of aggregation, resulting i[a marked viscosity and density contrasts must be inw~Ived. The Tr6nningen/is Charr~ockite and Torpa Granite together comprise, on a major scale, a similar association to that found it1 the discrete aggregates of the AGC. ~'he TC cons tit~ltes the charnockitic base and the TG the corre..q~onding granitic head. The passage from TC to TG :is transitional with a sub-charnockit,ic stage, of variable exte~L bearing dark-cored megacrysfic feidb;pars identical with those developed in the AGC ',~ggregate tlansitional zones. Samples have been ~elec~,ed from each of the sub-units of the CGA for comparison of their REE characteristics.
The REE results The REE compositions de~:ermined fi~r the 22 rocks analysed are presented in Tables 4 and 5. Chondr~te-normalized values are used in all distri-
Table 3. Major oxide analyses of the rocks of the Charnockite-Granite Association (~t.?/o).
8
9
10
11
12
13
14
15
16
17
13
S~Oz TiO2 A!203 Fe203 FeO MnO MgO CaO Na20 KzO P:O5 H20
55.9 65.26 1.91 !.39 13.28 12.88 2.60 2.04 8.39 5.51 0.25 0.28 2.55 1.10 4.95 2.79 4.16 3.50 4.04 4.48 I]!.81 0.70 0.16 0.42
62.0 64.7 64.72 60,.3 0.79 i.! 7 0.97 !.28 16.61 13.33 15.45 14.69 2.02 2.77 1.48 2.87 3.87 4.59 2.99 4.41 0.15 0.18 0.11 0.14 0.75 1.31 !.75 1.85 3.27 2.fl4 2.76 4.31 4.86 3.73 3.77 3.96 4.86 5.04 4.57 4.56 0.24 0.41 0.37 0.51 0.20 0.32 0.42 0.41
63.99 64.6 63.1 1.35 1.15 1.03 13.31 14.11 14.46 1.33 2.01 1.85 5.35 4.35 4.64 0.25 0.14 0.17 !.48 1.5l !.33 3.25 2.90 3. i 3 3.56 3.74 4.20 4.54 5.06 4.37 0.67 0.36 0.34 0.51 0.40 0.48
Total
99.00 100.35
99.62 100.39
99.59 100.33 99.10 100.38 101.3
Specim~'n numbering follows Table I.
99.36
99.29
19
20
62.85 6~.41 71.03 67.70 1~34 0.83 0.49 0.70 15.00 14.75 13.24 15.03 2.41 1.66 1.02 0.84 3.93 3.19 I.t18 2.65 0.11 I).ll 0 . , 0 6 0.10 1.98 !.08 0.61 0.63 2.77 3.39 1.22 1.82 3.76 3.76 3.36 3.52 5.15 5.54 5.54 6.06 0.44 0.33 0.19 0.24 0.64 !.03 0.7.t 0.40
21
22
70.0 74.4 0.56 0.33 14.52 13.61 0.82 0.54 255 1.50 008 0.05 0,62 0.32 1 93 ! .06 334 3.03 531 5.38 019 0.08 0 24 0.18
99100 99.69 100.16 100.48
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R E E in charnockite
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5
Table 4. Rare earth element analyses of the Varberg region country rocks (p.p.m.).
La Ce Nd Sm
Eu Gd Tb Yb Lu Y Y~Y,La-La* La]Yb
1
2
3
24.1 48 16 3.6 1.0 . 0.4 3.0 0.37 11 107 8
21.3 47.2 51 97.0 14 28 6.6 4.8 1.6 1.1 . . . 0.6 0.5 3.5 1.9 0.44 0.31 7 15 106 196 6 25
4
5
52.2 87.8 ~ 9.9 1.7 . 0.7 5.2 0.8 24 216 10
493 79.8 29 5.2 1.1 . 1.I 2.3 0.4 22 !90 21
6
.
7
50.3 77.9 54 6.8 1.2
32.7 60.4 34 9.8 3.0
1.1 3.9 0.6 22 218 13,
1.7 5.0 0.8 11 158 6
* For the range of lanthanides analysed (includes Gd where relevant).
bution diagrams. Where distribution diagrams and total R E E values derived from published sources are presented only those elements analysed for the rocks of this study are used in the calculations and constructions. The general REE distribution characteristics of the rock units of the Varberg region are portrayed in the distribution diagrams of Fig. 2. Fig. 2a shows that the lithological groupings erected on field, fabric, and lithological criteria remain distinct in terms of their R E E compositions. All groups show some differentiation with enrichment in the light R E E relative to the: heavy elements. ~Y, La-Lu shows a progressive increase from the granulites, through the granite gne~:sses to the C G A . The total REE for the: latter is well above average for granitic rocks with particularly high values for La and Ce. The garnet-pyribolite is the least differentiated, with L a / Y b = 6 . 5 compared with 17
for the granite gneisses and 14.5 for t h e C G A , but compares closely with the 7.0 value obtained for the granulites. Both reflect the lower light F~EE contents found in the granulites. The strongly differing major oxide compositions of the granulite specimens have no apparent strong control on either the R E E contents or their distribution in these rocks. The migmatitic granite gneisses (Tables 1 and 2) both show a slight negative Eu anomaly in their REE distributions; a feature absent from those of the other analysed gneiss category (Fig. 2c). The average curve, used in comparisons, includes both groups. It adequately portrays the general REE characteristics common to both groups, i.e. strong differentiation with a mal ked inflexion, whilst repressing the internal distinctions. Fig. 3 shows the general similarity of REE content and distribution in the various sub-units of the C G A . Numerically the Torpa Granite is
Table 5. Rare earth element analyses of th ro:ks of the Charnockite-Granite Association
La Ce Nd Sm Eu Gd Tb Yb Lu Y ~Y, La-Lu* La/Yb
8
9
10
11
12
13
14
15
88.4 166 84 20.3 5.8 2.5 8.2 1.2 24 400 !|
65.5 156 62 15.4 ! .0 6.9 4.8 0.87 24 336 14
50.3 90.3 54 10.3 5.0 1.2 4.9 0.8 17 234 10
82.!) 144.5 71 16.3 4.1) .!.5 6.0 1.l 28 355 14
56.9 143 57 14.7 2.5 10.5 1.8 6.2 0.97 34 327 9
87.3 71.4 74.6 142 193 158 69 86 69 16.4 19.9 16.8 4.1 3.3 3.9 8.9 2.4 2.1 2.4 5.3 7.3 6.0 1.0 1.2 1.1 35 36 35 351 380 429 12 I0 7
* For the range of lanthanides analysed (includes Gd where relevant). Specimen numbering follows Table I.
16
17
18
(p.p.m.)
19
20
21
22
5 5 . 5 6 2 . 3 42.0 140 104 212 150 151 94 327 227 449 124 62 65 40 116 78 1 7 . 6 15.2 9.9 20 16.3 18.9 i.8 3.0 2.4 1.5 2.5 2.3 7.2 tl.4 g.9 6.8 8.1 1.6 2.1 2.3 1.6 1.3 2.0 2.0 4.0 7.6 6.1 5.8 3.2 7.1 6.0 1.1 0.98 0 . 9 4 0 . 5 9 1.15 1.03 0.68 33 31 34 32 24 35 36 852 471 343 345 223 651 481 53 15 9 11 13 23 1~
117.3 204 81 22.7 3.8
/ii! :i ~' ii i~ill ~ ::LITHOS 12 (1979)
6 F.H. Hubbard & J. E. Whitley !
I.... ~L:aCe
Nd
I 2.~
,~n E'u
Yb
Tb
a.Major
ieoe
.5oo
R.
RockAve~ge~
"~
Cha,-r.~ite- Granite Asmciation
,134~.
, Ih'lllNK~ lOO
o o
\
°
\
~! i~. 'I..,. '~- ~,~,
\
,oo "%
~
" \ .-,,'
..:.~:.....f
b,Means& Rall,4~,~
....
liG
- - ' , 1o.
tl
//I
/// ,~j
10 La C e
.io . I.~ C~.,.__ N#__..
~
"~ Sm Eu
""
~...i-
"~-
~.~ '~o
~...,..~
Tb
Y.b I.u
Fig. 2. a. Chrondrite-normalized ?,EE distribution patterns for the major rock units of the ~/arberg region. I. Mean of variation within the CGA (407; 14.5). 2. Average granite gneiss (265: 17). ~. Average granulite 006,5; 7). 4. Garnetpyriboli~e {158; 6.5). The numerical vaJucs in paienthe~es are the IEY, La-Lu and La/Yb values; for the respective dala set:, and this format is repeated i~a subsequent figure caplion,.~, b. Chondrite-normalized REE distribution patt,:rn,; for the analysed granulites. 1. HV 219 (106; 6). 2. ItV I g5 (107; 8). The broken line is the average, trace used in comFari'.;ens, c. Chondrite-normalized REE distribution pattern:~ for the analysed granite gneisses. !. HV 300 (216; 10). 2. HV 328 (218; 13). 3. HV 327 (190; 21). 4. HV 75/52 (1~96; 25). The average trend (broken line) i.~, left incomplete in the light REE area to avoid line confusion.
distinctive, however, as a result of' its relatively extreme differentiation with abnonnally high concentrations of La, Ce a:ld Nd. Development of a slight negative Eu anomaly is also evident in this group.
Nd ~_ ~m E,u
Tb
Y~ I~u_
Fig. 3. Chondrite-rkormalized REE distribution patterns for the rocks of Ithe Charnockite-Granite Association. a. Averaged values; for the individual sub-units of the CGA. 1. Main phase charnockite (VG and AGC) (330; 12). 2. Coarse sub-,~harnockitic aggregates (AGC) (414; 12). 3. Coars,e charnockite (TC) (348; 8). 4. Differentiated aggregate (AGC) (294; I1). 5. Microcline granite (TG) (656; 31). b. 1. Average of all charrt~ockitic members of CGA (357; 11). 2. Average of all non-charnockiitic members of CGA (546; 27). The broken lines delimit/,.he total variation for the suite of CGA rocks analysed.
Petrological implications of the gEE data All the rocks of the Varberg regitm were extensively recrystallized during a major Proterozoic orogenic event characterized by long-held high temperatures. The near-equilibrium conditions attained, both mineralogical and textural, re~;r:lted in the virtual elimination of earlier fabrics and p~rageneses. It is r~ecessmT, therefor,.=, to seek aad ~nvestigate any more subtle parameters which may have survived the intense reconstitutive aetiVriitLyif rock progenitors are to be tirmly determine~J!. Though the data
~
R E E in charnockite
LITHOS 120979)
on the behaviour of REE in metamorphism is limited,, there is general consensus that the REE distribution remains essentially unchanged~ even in the passage to the very highest grades (e.g. Green et at. 1969, 1972; Hermann 1970; Wildeman & Condie 1973; Wildeman & Hat;kin 1973). Thus REE are potentially valuable indicators of premetamorphic rock chara~:ters if primary patterns can be established for reference. I n practice, the application of REE methods to t]he Varberg rock complex has given some support for earlier interpretations of the derivation of the metamorphic rocks but has given no clear-cut, substantive, proof or"the nature of the rock progenitors. The granite gneisses of the region are tho~aght to be dominantly meta-arkoses. Little REE dam on arkosic sediments are available from the literature. Nance & Taylor (1976) have ro:ently published REE values for post-Archaean seciimentary rocks from Australia which include sub-greywackes and a Triassic arkose. Values for a R,ussian Devonian continental arid sandstone are also available (Balashov et at. 1964). Although the degrees and styles of differentiation are similar for these, and for the Varberg gneisses (Fig. 4a), there is a rather ,,vide divergence in total REE. Fig. 4b, on the other hand, highlights the very close similarity of the distrit~ution patterns and total REE for the Varberg gneiss~s, the Finnish Svecokarelian granites (Koljonen & Rosen~berg 1974) and tl~e 60-70% ]iOz average granite (Haskin et al. 1968). The REE data, 1herefore, give no clear indication of whether the Varberg gre~nite gneisses are ortho- or paragneiss~rS.Their distinction from the plutonic granites of the region (the Torpa granite) and the Finnish Archaean granite (Sahama 1945), however, is clearly displayed. Balashov et al. (1964) and Ronov et al. (1967) emphas~e the important control of REE content of the p~mary r~:k on that of the derived sediments and emphasize that in an arid environment of erosion and sedimentation there is no significant difference in the REE content and fractionation between the source rocks and the derived clastic sediments. The most likely source, and basement, for the Varberg sequences, if they do represent metasediments, is rocks related to the Svecofenrfian granitoid complex which dominates the geology of southern Sweden, east of the eaajor dislocation zone• No REE data are known from these rocks but the Svecokarelian rocks of Finland analysed by Koljonen & Rosenberg (1974) are of similar type and age and may be representative of the same event in the development o1~the Baltic Plat-
7
emparil~nswith~
t
•1 0
~
LO C.e
I~d
~m Ep
~ib
"~".210
Yb Lu
Fig. 4. Comparisons of the average Varberg gran~.te gneiss REE distribution with published valaes (see text for sources and discussion), a. 1. Average Varberg granite gneiss (205; 17). 2. Russian platform sandstone (93;* I 1). 3. Australian Trias~sic sandstone (263;'* 9). 4. Average Australian Upper Proterozoic sub-greywacke (127;** 13), 5. Average Austr~dian Palaeozoic and Mesozoic subgreywackes (175;** 13). *No values for Eu, Tb and Lu:**no values for Lu. b. !. Average Varberg gratdte gneis~ (205; i 7). 2. Finnish Svecok~Lrelian granite (172;* 16). 3. 60-70% $iO~ glani~e (218;**13). 4. Torpa granite, of CGA (656; 31). 5. Fmr~ish Lappl~.nd granite gneiss (102;*** 2). *No value for Y; **no values for Tb; ***no values for Lu.
forum. Although the new REE data from the Varberg region provide no absolute conclusion regarding the nature an~J origin of the granite gneisses, they are consiste:~* with the gneisses having been ~Lerived from arkoses developed in the erosion of a granite source area of Svecofennian type in a continental environment. The gneiss categories distinguished by the presence or absence of a Eu anomaly (Fig. 2e) also differ in bulk composition and hydration (Tables 1 and 2). The micromigmatization fabric suf~gests confined anat~tic redistribution, while the char-
L]iTHOS 12 (1979)
8 F.H. Hubbard & J. E. Whitley
~r~)~
al. Granulite V Greywacke
\;,,~ ~.~ •
•
\.
\.,,,::%
,\
J ae~
,10
~. Pyribolite V Basalt
~
;1 .4
5' La Ce
_~d .PmS.m E.u ~
,Tb
_
Yb L.u
Fig. 5. a. Co~aparisons of the average Varberg granulite chondrite-normalized REE distribu~tion with published values (see te~t for sources and discussion). 1. Average Varberg granulite (! 06; 7). 2. Average Australian Cambrian greywacke (127;* 13). 3. Karelian greywacke (47.5; 15). The shaded field encompasses the variation in REE distribution of the average Wyoming greywacke (140; 19) the average Fig Tree greywacke (99;** 14) and the average Svecofennian greywacke (129; 16). Tht,' distribution patterns of these plot so closely that separate presentation would cause line confusion. *No value for Lu; **no value for Y. b. Comparison of the Varherg garnet-pyribolite chondritenormalized RFE distribution with published values. 1. Varberg garnet-pyribolite (158; 6.5). 2. Columbia~ plateau basalt (147; 6.5). 3. Average basalt (79; 1.9). 4. Alkaline olivine basatlt,, Japan (-; 22). There is insufficient correspondence of elements analysed I : : 4 to allow cah:ulafion of a comparable Y-Y, La-Lu.
nockitic gneisses may be residual after expulsion of the SiO2- K 2 0 - H 2 0 anatectic fluid. The differing Eu distributions accord with the K-feldspar/plagiocHase (K/Ca) ratios of the gneiss types. The quartzofeldspathic granulites analysed have REE contents and distributions which conform well with published data for slightly metamorphosed ~:o unmetamorphosed Precambrian and Cambrian greywackes (Fig. 5a) (Wildeman & Condie 1973; Wildeman & Haskin !973; N~ace & Taylor 1976). The heavy REE contents of the
Varberg rocks mar the cot~aparisor~; particularly the high values of Yb which cause a>a exaggerat~l reduction of the La/Yb diff(!xentiation ratios. Tile; high values of Yb, and Lu, are fouxid in both the granulite and the amphibolite faci,t:s representatives and are probably relict from the parent sediments rather tha~n relating to the higher grade of metamorphism of the Varbetg rocks. The Finnish Karelian (Archaean) greywacke has a markedly lower total REE content but sh¢,ws a similar differentiation pattern to the other Prccambrian greywackes.. The granulites have much lower REE contents than the granite gneisses and show less differentiation (LafYb for the granite gneiss= 17; for the granulite=7). These differences in REE presumably persist from the sedimentary stage and reflect the ~iffering compositions of the sediment sources which, in the case of the granulites/ greywackes, probably included besic volcanics. REE data are available from only one example of the basic intercalations which characterize the passage from granite gneiss Io the mixed granulite sequences. The major element chemistry data (Table 2) sugge,:;t that the parent rock of this garnet-pyribolite was alkaline olivine basalt. The chondrite-norma,lized REE data plot most closely with the Columbian Plateau bas+alts (Schmitt et al. 1964) (Fig. 5b) and conform wi~:h these rocks in both to~ial REE and differentiation. The Japanese island a~rc alkaline olivine basalts (Masuda 1966, 1968) show lower total REE and more pronounced differentiation. More extensive data would be required to draw any finer conclusion concerning these basic rocks. Either continental or island arc volcanic environments would be consistent 'with the sedimentary lithofacies change fi'om arkose to greywacke indicated for the Varberg rocks; either by inception of volcanism along associated rifting or by the development of an offshore island arc system. Comparison of the REE characteristics of the three groups of charnockitic rocks outcropping in the Varberg area, both with each other and with associated rocks of similar type but lacking charnockitic characteristics (Fig. 6a) give no evidence of theiir being a type pattern of REE distribution for charnc~ckite. The metamorphic charnockites maintain closely the predicted REE characters of the pa~rent rocks. Only when a melt phase becomes impo~ant, as in the CGA, does significant redistrib~;tion of REE ensue. If the CGA represents an anatectic plutonite derived from rocks of como parable type to i~tscountry rocks then a significant total REE acc:etion is involved, accompanied by
LITHOS 12 (1979) further accentuation of light/heavy REE differentiation. The consistency of the REE distribution curves derived for the members of the CGA leaves little doubt that they are kindred rock~; (Fig. 3a). This contention is reinforced by the rational nature of the REE change with bulk compositional differentiation from intermediate charnockite to acid granite with increase in total REE and accentuation of the light/heavy REE differentiation (LEt/Yb©hamockite-" 11, La/Ybgrantt® = 2 7 ) (Fig. 3b). The change in REE distribution includes progressive relative depletion in Eu to the extent that a slight negative Eu anomaly is developed in the granites (Figs. 3a and b). This parallels the increasing dominance of alkali feldspar over plagioclase. ~tese distribution trends suggest a preferential aggregation of' the light REE (La to Nd) in the mobile phase of the differentiating CGA. The Torpa Granite may be considered tile 'product' of the charnockitic plutonism shown in the CGA. It represents the aggregatio~ of the most mobile components and is the phase most likely to leave the environment of its formation to intrude higher levels. This was only partially achieved in the case of the CGA and the connection with its source is still maintained (Hubbard 1975; Hubbard & Whitley 1978). Some intrusive granites of the higher crustal levels may have had their origin in such plutonic charnockite activity as is observed, arrested before completion, in the; Varberg CGA. The most closely comparable granite REE data is found in the Rapakivi granites of Finland (Koljonen & Rosenberg 1974; Hubbard & Whitley 1978) (Fig. 5c). A similar trend is showrl also by a Rapakivi granite from the Ukraine (Balashov 1963), but a different range of REE was determined and it is not possible to show whether a Eu anomaly exists. The comparability of the Torpa Granite and the Ra?akivi granites of the Baltic Shidd is of some interesl. A ?ossibte link between the high-level Rapakivi granites ~nd the deep-level anorthositemangerite complexes h,~s been discussed (e.g. Eckerrearm 1936, 1939; Kranci~ 1969). In tSis region of SW Sweden, which has clo~e parallels with the anorthosite province of S Norway:, a link has been established be.tween granites of Rapakivi type and their charnockitic source. The poss,ibili~y ,~merges that the apparent anorthosite-Rapaldvi association may be an indirect one resulting from the derivation of a Rapakivi type mag~aa fraction from crustal charno~:kitic plutonism associated with anorthosite emplacement (Hubbard & Whitley 1978). Granites of the Torpv type are clearly distin-
REEincharnockite
9
100,
"~.~
"~ -
\\
.10
L.a Ce.
N,d
,Sin eu
~
Yb L.u ]
Fig. 6. a. Comparison of chondrite-normalized REE distribution patterns of the various charnockitic rocks of the Varberg region, l. Charnockitic granite gneiss (201; 16). 2. Charnockitic granulite (106; 7). 3. Average plutonic charnockite from CGA (357; i 1.5) (Specim~:ns 8-18 inclusive), b. Comparison of the chondrite-normalized REE distributions of the Torpa Granite of the CGA with other granites, l. Torpa Granite (656; 31). 2. Finnish Rapakivi granite (447;* 17). 3. >70% SiOz granite (254, 10). 4. Average Svecokarelian granite 072;* 16). Sources and discussion in text. *No Y value.
guishable from the Finrish Svecokarelian granites =b" 5b). The on the basis of their REE contents ~,= 'v:~, REE distribution of the latter is closer to that of the > 70% SiOz average granite of Haskin et al. (1968). A variety of granite bodies intrude the highc~ level metasedimentary sequences to the north of the Varberg region (e.g. Gorbatschev 1976; Samuellsson & Ahlin 1976). It is hoped that it will prove possible to compare their REE .=on* tents and determine whether any are of the Torpa type. An as yet incomplete U/Th investigal:ien of the granites of the region suggests that thi~; will I~: the case. Koljonen & Rosenberg (1974) r,~ogn~zed a
LITHOS 12 (1979)
10 F. H. Hubbard & J. E. Whitley
crude variation in REE with age in the rocks of the Finmsh region of the Baltic Shield. "Fo~tl REE increased and enrichment in light REE relative to heavy REE was enhanced in progressively younger sequences. This concept of secular variation finds sorae support in the ,data from Var.berg. The granite gneisses and granulites have greater REE contents a~.~[ more advanced differentiation than the Finnish Archaean equivalents and the granite gneisses co~ ~,t well be derived from granites with Sv,eeokarelil, I:~cREE characteristics. The Torpa granite equate~ with the yotmger Rapakivi granites. 'l~tis would ~:.ag.gestthat the Varberg sedimentation post-dates ~3~.• 1800-1950 m.y. Sveeokarelidic granite event ai~d that the plutonism associated with the metamor;~:;hism of the product sediments would dat~.• with ti'~e 1750 m.y. Rapakivi event. The provisional date for the CGA, however, is 14.50m.y.
in a granite s ~ m to be of greater influence in determining its REE characteristics than its gross composition. Acknowledgements. - The fiaan,:ial help of the Carn~:g~e Trust for the Universities of Scotland and the Univerfity of Dundee Travel Fund in defraying fiela expenses is gratefully acknowledged. Professor H. W. Wihon (S.U.R.R.C.) is also thanked for his interest and encouragement.
References
Balashov, Yu. A., Ronov, A. B., Migidsov, A. A. & Turanskaya, N. V. 1964: The ,.;fleet of climate and facies environment on the fractionafion of the rare earths during sedimentation. Geochemistry (USSR), English Translation, 951"-969. Eckermann, H. yon 1936: The Loos-Hamra region. GeoL F#ren. Stockh. Fi~rh. 58, i 29-343. Eckermann, H. yon 19:39: The anorthosite and kennigite of the Nordingr~i-R6d6 region. Geol. Fiir~m. Stockh. Fiirh. 60, 243-284. Gorbatschev, R. 1975: Fundamental subdivisions of Precambrian granitoids in the Areal mega-unit and the The REE study of the Varberg complex rocks evolution of the south-western Baltic Shield. Geol. affirms the significance of REE stability during Fi~ren. Stockh. Fiirh. 97, 107-114. Gordon, G. E., Randle, K., Goles, G. G., Corliss, J. B., solid-state transformations to studies of the deveBeeson, M. H. & Ox|ey, S. S. 1968: Instrumental activalopment of high-grade metamorphic regions. Orition analysis of standard rocks with high resolution ginal REE contents and distributions persist from gamma-ray detectors. Geochim. Cosmochim. Acta 32, sedilnentation to the highest grade of metamorph369-396. Green, T. H., Brunfelt, A. I. & Heier, K. S. 1969: Rare ism and allow the identification of rock progeniearth element distribution in anorthosites and as,,;ociated tors. In arkosic sedimentation the REE characters high-grade metamorphic rocks, Lofoten-Vesteraalen, Norof the source granitoids may be largely pt'eserved way. Earth Planet. Sci. l~tt. 7, 93-98. in their denudation products. Mcrdification of Green, T. H., Brunfelt, A. I. & Heier, K. S. 1972: Rare earth element distribution and K/Rb ratios in granulit~-s, REE disltribufion in orogenesis ensues onlfy when mangerites and ano~thosites, Lofoten-Vesteraalen, Normelting begins; but then abruptly. The melt phase way. Geochim. Cosmochim. Acta 36, 241-257. accumulates REE irregularly showing a preference Haskin, L. A., Ha~kin. M. A., Frey, F. A. & Wildeman, for the light elements. T. R. 1968: Relative and absolute terrestrial abundances of the rare earths. ~n Ahrens, L. H. (ed.): OrJigin and There is no l~arti¢;ular REE distribution cl~aracteDistribution of the Elements, pp. 889-912. Pergamon ~igtic of charnockite. The in situ metamorphic charPress, London. nockites have the REE of their parent. Plutonic Hermann, A. G. 1970: Yttrium and lanthanides, fi~ Wedecharnackites developing in a granulite facies enpohl, K. H. (ed.): Handb~.~ok of Geochemistry 39, 57-71-Ei-19, Springer Verlag, Berlin-Heidelberg-New vironment accumulate REE, particularly light REE, York. in the magrnatic component and differewtiate to Hubbard, F. H. 1975: The Preeambrian crystalline ,:omplex p oduce a non-charnockitic granite with the REE of south-western Sweden. The geology and petrogenetic characteristics indicated for granites of t!he Rapadevelopment of the Varberg region. Geol. Fiiren. Stockh. kivi tyl~.', which include a negative Eu anomaly. F6rh. 97, 223-236. Hubbard, F. H. 1978: A geochemical investigation of the Rapakivi magma generation may therefore be a premetamo~'phie nature of the granite gneisse~; of the direct consequence of charnockitic plutonism and Varberg Series. Geol. F6ren. Stockh. Fi#h. 100. the postulated link with anorthosite may be an Hubbard, F. H. & Whitley, J. E. 1978: Rapakivi granite, anorthosite and charnockitic plutonism. Nature 2_71, indirect ,one. 439--440. A wide-rang~ing study of the REE characteristics Koljonen, J. & Rosenberg, R. J. 1974: Rare earth elements t~f granitoids of known derivation miight provide a in granitic rocks. Lithos 7~ 249-261 useful key for determining the origin of granites of Kranek, E. H. 1970: Anorthosites and rapakivi, magmas from the lower crust. New York State Mus. Sci. Sere. more enigmatic history. Development mechanisms 18, 93-97. and the number of crustal reworkings represented
Summary of conclusions
L]ITHOS 12 (1979) Masuda, A. 1966: Lanthanides in basalt:s of Japan with three distinct types. Geochem. J. 1~ 11-26. Masuda, A. 1968: Geochemistry of lan~hanides in basalts of Central Japan. Earth Planet. Sci. Lett. 4, 284-292. Nakamura, N. & Masud~., A. 1973: Chondrites with peculiar rare earth patterl~s~ Earth Pl,~met. Sci. Lett. 1~, 429-437. Nance, W. B. & Taylor, S. R. 1976: Rare earth element patterns and crustal evolution- ,. Aus~tralianpost-Archaean sedimentary rocks. Geochim. Cosmochim. Acta 40, 1539-155 I. Quensel, P. 1951: The charnockite series of the Varberg district on the south-western coast of Sweden..,Irk. Mineral. Geol. I, Stockholm. Ronov, A. B., Balashov, Yu. A. & Migdisov, A. A. 1967: Geochemistry of the rare earths in the sedimentary cycle. Geochemistry Int. 4, 1-17. Sahama, Th. G. 1945: Spurelemente der Gesteine im sfidlichen Finnisch-Lappland. Bull. Comm. gdol. Finl. !35, 86 pp.
R E E in charnockite
11
Samuelsson,.L. & A hlir., S. 1976: The Precambriat~ crystal° line complex of south-western Sweden: The geology and petrogenetic development of the Varberg region. A comment. Geol. F~ren. Stockh. FOr/~. 98, 168-170. Schmitt, R. A., Smith, R. H. & Olehy, D. A. 19i~4: Rare earth, yttrium and scandium abundances in meteoric and terrestrial matter-I!. Geochim. Cosmochim. Aeta 28, 67-86. Wildeman, T. R. & Condie, K. C. ~973: Rare earths in Ar~haean greywackes from Wyoming and from the F~g Tree Group, South Africa. Geochim. Cosmochim. dcta 37~ 439-453° Wildeman, T. R. & Haskin, L A. 1973: Rare earths in Precambrian sediments. Geor.;)im. Cosmochim. Acta 37, 419-438. A~epted for publication March 1978 Printed January 1979
"l~'tL~f~Q Jl..zlL l! J["JL~..J~ I
~
Hubbard, F. H. & Whitley, J. E. 1979: REE in charnockite and associated r~ks, southwest Sweden. Lithos 12, 1--ii. Oslo. ISSN 0024,4937. The rare earth element compositions of a suite e:' high-grade metamorphic and plutonic locks which outcrop in the Varberg region of southwest Sweden are presented. A plutonic complex containing both charnockitic and non-charnockit~c elements intrudes country rocks of upper amphibolite and granulite facies grade. The REE <:~atafor the country rocks sustain the view that they are of supracrustal origin and support the impcr:ance placed on these elements in investigations of metamo,phic rock progenitors. The progressive change in REE characteristics found for the various units of the plutoniic association confirms that they represent rational stages in &ifferentiation from a common parent. The end-product of this d~fferentiation is a granite with the REE concentration and distribution found, elsewhere in the Baltic Shield, to characterize Rapakivi granite. The progressive change in REE with time suggested for the Baltic Shield by other authors finds support in the results of this study. F. H. Hubbard, Department of Geology, The University, Dundee, Scotland. J. E. Whitley~ Scottish Universities Research and Re,~ctor Centre, East Kilbride, Scotland.
Charnockitic rocks of three distinctive modes outcrop within the high-grade metamorphic rock complex of the Varberg region of southwest Sweden (Quensel 1951; Hubbard 1975). The term charnockite is here used to distinguish massive, granular rocks in the compositional range tonalite to granite which bear two pyroxenes and have distinctively dark-coloured quartz and feldspar. Major quartzofeldspathic horizons within belts of layered'granufites have developed charnockitic mineralogies and fal:~rics during a granulite facies metamorphism. SucI~ layer-paralllel,/n situ, metamorphic charnocl~ite~s are thought to be derived from original greywo:ke sediments. The granulites occur as belts within widespread granite gneisses interpreted as meta-arkoses (Hubbard 1978). Charnockitic characters are also developed in these rocks but patchily, with no apparent control of distribution by the compositional layering which persists in the meta-arkoses. Irregularities of intergranular water distribution in the poorly permeable, earlier recrystallized, meta-arkoses may have localized this in situ cbarnockitizat~on. The most striking charnockite developments of the region, however, are those found in an intrusive complex with both charnockit:,c and non-charn~ckitic members which dominates the ~::oastai outcrop in the r~orth of the area. Dcamafic juxtaposition of coarse 'dry" charnockitic and 'wet' non-charnockitic ~ranite elements is a feagure of this discrete, piuto~fic complex 1 -- Lithos 1/79
which has been called the Varberg CharnockiteGranite Association (CGA) (Hubbard 1975). This complex was initiated, mobilized and emplaced during the metamorphism which produced the in situ ~:harnockitization. Its components and their associations are interpreted in terms of a discrete, internally differentiated, plutonic association in which the non-charnockific (and more Si-K rich) phases fractionated during emplacement; a process determiined and controlled by volatile aggregation. The less dense, less viscous, hydrous fractions w:re more mobile and advanced higher in the sequence than their drier, d~irnockitic, residues. The larger of these aggregates (the Torpa granite) rose to penetrate younger cover sequences which were being metamorphosed with their basement rocks. Crystallization arrested the ascent of the granite, however, before complete separation from its source was achieved. The association displayed at Varberg illustrates a mechanism of granite magma generation and expulsioh from the catazone which in this case failed to reach completion. An outline petrological map of the region is given as Fig. 1. A study of the rare earth element (REE) distribution in the rocks of the Varberg region was indicated for a variety of reasons. The intense recrystallization of all the rocks of the region has made the establishment of a stratigraphy difficult. In view of the indicated stability of REE in
LITHOS ~2 (1979)
2 F.H. Hubbard & J. E. Whitley
quenc¢ of the Baltic Shield of the Swedish charnoekitie and associated 'nozmal' granites, as found at Varberg, is not yet established with certainty. Provisional results suggest an age in the region of 1450 m.y. (Welin, pers. comm.) and more precise radiometric age determinatiop.s are currently being undertaken by Professor Welin. Establishment of the REE patterns of the Varberg granitoids would allow their comparison with the Finnish and Russian data and test their confortuity with the development pattern suggested by Koljonen & Rosenberg (1974).
:::::::::::::::::::::::::::::::::::::::: ....... =========================== ::::::::::::::::::::::::::::::::::::::: ~iiii!~i!ii!!i!!ii!!~!:::::::.~.--:~
i:
E3'
|11'
o
Fig. 1. Petrological map of the Varberg region of SW Sweden. !. Granite gneiss. 2. Bar~ded granulite. 3. Garnet pyribolite/Amphibolite. 4. Charnockite-Granite Association (VC = Varberg Charnockite, AGC = Apeiviken-Getter6n Charnockite, TC =Tr6nningen[is Charnockite, TG --=Tor'~a Granite). 5. Metasupracrustal gneisses, in part mi[,m'aatitic (not included in the REE investigation).
metamorphism REE characteristics might provide insight into the premetamorphic natures of the rocks. The occurrence of charnockitic rocks,, whi~.h are thought to have attained their common characteristics by divergent routes, suggests it to be a suitable region to test for a characteristic charnockite REE dist~'ibution pattern. The admixture of charnockitic and non-charnockitic granitic rocks in the Charnockite-Granite Association has b ~ n interpreted on the basis of differen~:iation from a common parent. An alternative interpretation of the granite enclaves in the charnockites as xenoliths was favoured by Quensel (1951). If the model developed by Hubbard (1975), based on field iv.vestigations, is correct the: rocks of the assc¢i,ation ~ould be expected to have a common basiic REE distribution pattern with a differentiation trend following the major element fractionation. Koljohen & Rosenberg (1974) discussed the REE distributions found in Finnish and Russian granites from the Baltic Shield, and identified a pattern of change with age. The position within the time se-
Analytical procedure The determination of REE was carried out by instrumenlal neutron activation analysis at the Scottish Universities Research and Reactor Centre. Samples were processed in two batches of ! 2, in~,'luding in each case a standard USGS rock sample (G2 o1~ BCR !) for assessment o,f accuracy, and elementa~ standards for the elements to be determined (as described by Gordon et al. 1968). :Significant interferences of 232Pa to the peaks of iS3Gd, J:6°Tb and l*SYb were removed by spectrum stripping. Wher~ possible results were calculated with reference to more th~n one photopeak for each element as a check on the absence of interferences. For La, Ce, Sin, Eu and Lu the pre,eision associated with counting statistics was between _+ I c'',Dand _+5%; for Nd, Gd and Yb the precision was in the region of + 10%. For the quartzofeldspathic granulites, with lower rare earth contents, these figures were + 10% and .+_20~i respectively. The results obtained for the standard rocks agreed with the accepted values within the quoted limits of precision. Due to peak interference it was not possible to obtain accurate Gd values for all lthe sampler!; analysed. Gd values are only shown in Table:~ 4 and 5 where the accuracy achieved was within the quoted limits, and absence of a value does not necessarily imply p~k-ticulady low levels of Gd. Because of the irregularity of availabililty of Gd data this element is omitted from distribution diagrams and calculations. Y ~as det,ermined by standard XRF procedures by refe}'ence to a calibration derived from spiked standards. The major element analyses, with the exception of those of specimens HV219 and HV75/52,, were carried out at the Department of Geology, University of St. Andrews, (Analyst: R. A. Batchelor) by wet chemical methods. The rem~finder were analysed on fused and pressed powder samgle pellets by XRF methods at the Department of Geology, University of Dundee (Analyst: R. McGill). More detaih of the analytical procedures may be obtained from the t~uthors.
The samples "Fr~e samples used were selected from collections made in the course of the gfneral geological inve::tigation of the area by one of the authors (F H.H.). The samples varied ~n size from two to
REz~ in eharnock ite
LITHOS 12 (1979)
3
Table 1. Mineral assemblages of analysed specimens.
Rock type
Mineral assemblage
1 HV 185 2 HV 219
Quartzofeldsp~:thic granulite Quartzofeldspathic granulite
pl, mp, qz; bi, hb; ore, ap, zi. pl,* qz; di, hb, hy, bi; ore, ap.
3 4 5 6
Charnockitic granite gneiss Charnockitic granite gneiss Granite gneiss Granite gneiss
pl.* qz, mp; hb, by, di, bi; ap, ore, zi. pl,* mp, qz; hb, hy, di, bi; ore, ap, zi. rap, qz, pl; bi, hb; ore, ap, zi. 'rap, qz, pl; bi; ore, ap, cc.
7 HV 299
Garnet pyribolite
pl, qz; gt, hb, di, hy; bi, ore, ap.
8 HV 251
Intermediate charnockite (VC) "
pl, mp, qz; hy, di, hb; ore, ap, bi, gt.
9 HV 165 10 HV 167 11 HV 33G
Fine-grained eharnockite (AGC) Fine-grained charnockite (AGC) Fine-grained charnockite (AGC)
pl,* mp, qz; by, di, hb; gt, ore, av, bi. pl,* mp0 qz; hy, di, gt, hb; ore, ap. rap, pl, qz; hb, hy, di; ore, ap, zL
12 HV 126 13 HV 117
Coarse-grained charnockite (TC) Coarse-grained charnockite (TC)
mp, pl, qz; hb, di, hy, b~; ore, a-~ ~t. m~, pl, qz; hb, di, hy; ore, ap, gt, bi, zi.
14 H¥ 166a 15 HV 166b 16 HV 331
Coarse charnockitic aggregate (AGC) Coarse charnockitic aggregate (AGC) Coarse charnockit~c aggregate (AGC)
mp, pl, qz; hb, hy, bi; ore, ap, zi rap, pl, qz; hb, hy, di; ore, ap, zi. rap, pl, qz; hb, h~,. di, bi; ore, ap, zi.
17 HV 176 18 HV 177 19 HV 175
Coarse chamockite phase (AGC) Transitional phase (AGC) Microcline granite (AGC)
nap, pl, qz; hb, px (alt.); ore, ap, zi, bi. rap, pl, qz; hb, hy, di; ore, ap, zi. qz, mi, pl; hb, bi; ore, ap, c¢, zi.
20 HV 93 21 HV 74/2 22 HV 74/10
Microcline granite (TG) Microcline granite fiG) Microciine granite (TG)
mi, qz, pl; hb. hi; ore, zi, st,. mi, qz, p!; bi, tab; ore, sp, zi, ap. mi, qz, pl; bi, hb; ore, sp, ap, zi.
Specimen
HV 75/52 HV 3OO HV 327 HV 328
The mineralogies are grouped into felsic, mafic and accessory minerals. Within each of these groups the minerals are arranged in rough descending order of abundance, pl-plagioclase; pl*= antiperthite; mp= microperthite; mi = mieroeline; qz = quartz; hy = hypersthene; di = diopside: hb = hornblende; bi = biotite; gt = garnet; ap=apatite; z~=zircon; co=calcite; sp=sphene. Recks 1 to 7 are examples of the country rocks, while the CGA is represented by specimens 8 to 22.
five kilograms The mineral assemblages of the selected suite are given in Table 1 and the bulk chemical compositions in Tables 2 and 3.
F'etrography The granulite samples are from massive quartzofeldspathic horizons which may most reasonably be compared and contra'~ted with the granite f,~leisses and, like the granite gueisses, may equate with the source rocks for the anr~tectic plutonic C G A . One specimen (HV 219) is charnockitic and of intermediate bulk composition whilst the other (HV 185) is more acidic and non-eharnockitic (Tables 1 andL 2). The granite gneiss samples fall into two catei~,ories based on their bulk compositions - mineralogies and petrographies. HV 75/52 and HV 1300 are charnockitic and intermediate in composition (thou~:h compositiona~ly comparable with
the more acidic granulite). HV 327 and HV 328 are truly acidic, are non-pyroxenic (though HV 327 has the dark feldspar characteristic of the charnockites) and show evidence of lit-par-lit anatectic migmatization on a very fine scale (Tables 1 and 2). Many basic igneous events are represented in the Varberg region; their products are difficult to distinguish. The selected garnet-pyribolite is believed derived from a volcanic parent associated with the depositional stage of the enclosing granulites. Several sub-units are distinguished in the Charnockite-Granite Association which represent stages in the emplacement differentiation. The principle sub-divisions are (a) the Varberg Charnockite (VC), (b) the Apelviiken-Getter6n compound unit which is dominantly charnockitic (AGC), (c) the TrSnningen~is C h a ~ o c k i t e (TC), and (d) the Torpa Granite f i G ) . The VC is a massive characckite varying from fine to medium grain and lc~:ally sparsely porphyritic. It shows intrusive relation-
LITHOS 12 (1979)
4 F.H. Hubbard & J. E. Whitley T,.rble2. Major oxide analysesof the Varbcrg region country rt~eks (wt.%). 1
2
3
4
5
6
7
SiO2 68.83 56.08 66.83 TiO2 0.39 0.59 0.53 AI:,O3 15.17 16.01 14.90 Fe~Os 1.08 9.24* 4.04* FeO !.73 MnO 0.10 0. ! 7 0.07 MgO 1.70 3.07 1.64 CaO 3.26 7.99 3.21 Na20 4.16 5.14 3.64 K20 3.17 0.77 4.00 P2Os 0.15 0.22 0.115 H20 0.39 0.63 0.65
64.9 7 4 . 1 71.9 42.8 0.56 0.25 0.32 2.58 15.97 12.32 13.'78 !4.79 2.24 n.d. !.10 4.60 1.81 !.89 1.10 11.82 0.1 i 0.05 0.06 0.23 | .40 0.27 0. 51 7.51 3.48 0.77 1.20 11.19 4.52 2.43 3.63 2.52 4.31 6.93 5.87 1.18 0.15 0.02 0.196 0.25 0.08 0.29 0.20 0.37
Total
99.53
100.13 99.91
99.66
99.32
99.73
99.84
Specimen numbering follows Table I. * Total Fe as FezOs
ships with the coun.try rocks (both granite gneiss and granulite) with xenoliths and apophys~al sheeting. The AGC comprises a dominant 'matrix' phase which is closely similar, eompasitionally and petrographieaily, to the VC though generally slightly more acid and alkaline and more universally porphyritic. Within this component there occur coarser aggregates of two contrasting types: (a) Irregular masses, usually and characteristically with diffuse margins, marked by intense feldspar megacryst growth and marie mineral aggregation. The felsic minerals usually wholly maintain the dark charnockitic colouration but pyroxene may be largely replaced by amphibole and, to a lesser extent, biotite. The feldspar megacrysts are late porphyritic or porphyroblastie. These coarsely crystalline zones are compo,:itionally indistinguish-
able from their hosts. (see Table 3). (b) Less common, coa:sely crystalline, bodies distinguished by their repe~ted pear.shat~e, sharp margins and internal compositional differentiation. "l'he narrower extremity of ~these elongate aggregates is always to the south (interpreted as 'downward' during emplaeementJ; it :is of intermediate composition, and charno~¢kitic. This rock rapidly, but progressively, changes through a sub-eharnockitie transitioeal phase, to a sili~:eous, leucocratic, microdine-granite at the broad northern limit. Distinct, sharp interfaces are main~lained throughout to the enclosing AGC resulting iin striking, abrupt juxtaposition of 'wet' and 'dr.y' granitoids. Some form of aggregation, resulting i[a marked viscosity and density contrasts must be inw~Ived. The Tr6nningen/is Charr~ockite and Torpa Granite together comprise, on a major scale, a similar association to that found it1 the discrete aggregates of the AGC. ~'he TC cons tit~ltes the charnockitic base and the TG the corre..q~onding granitic head. The passage from TC to TG :is transitional with a sub-charnockit,ic stage, of variable exte~L bearing dark-cored megacrysfic feidb;pars identical with those developed in the AGC ',~ggregate tlansitional zones. Samples have been ~elec~,ed from each of the sub-units of the CGA for comparison of their REE characteristics.
The REE results The REE compositions de~:ermined fi~r the 22 rocks analysed are presented in Tables 4 and 5. Chondr~te-normalized values are used in all distri-
Table 3. Major oxide analyses of the rocks of the Charnockite-Granite Association (~t.?/o).
8
9
10
11
12
13
14
15
16
17
13
S~Oz TiO2 A!203 Fe203 FeO MnO MgO CaO Na20 KzO P:O5 H20
55.9 65.26 1.91 !.39 13.28 12.88 2.60 2.04 8.39 5.51 0.25 0.28 2.55 1.10 4.95 2.79 4.16 3.50 4.04 4.48 I]!.81 0.70 0.16 0.42
62.0 64.7 64.72 60,.3 0.79 i.! 7 0.97 !.28 16.61 13.33 15.45 14.69 2.02 2.77 1.48 2.87 3.87 4.59 2.99 4.41 0.15 0.18 0.11 0.14 0.75 1.31 !.75 1.85 3.27 2.fl4 2.76 4.31 4.86 3.73 3.77 3.96 4.86 5.04 4.57 4.56 0.24 0.41 0.37 0.51 0.20 0.32 0.42 0.41
63.99 64.6 63.1 1.35 1.15 1.03 13.31 14.11 14.46 1.33 2.01 1.85 5.35 4.35 4.64 0.25 0.14 0.17 !.48 1.5l !.33 3.25 2.90 3. i 3 3.56 3.74 4.20 4.54 5.06 4.37 0.67 0.36 0.34 0.51 0.40 0.48
Total
99.00 100.35
99.62 100.39
99.59 100.33 99.10 100.38 101.3
Specim~'n numbering follows Table I.
99.36
99.29
19
20
62.85 6~.41 71.03 67.70 1~34 0.83 0.49 0.70 15.00 14.75 13.24 15.03 2.41 1.66 1.02 0.84 3.93 3.19 I.t18 2.65 0.11 I).ll 0 . , 0 6 0.10 1.98 !.08 0.61 0.63 2.77 3.39 1.22 1.82 3.76 3.76 3.36 3.52 5.15 5.54 5.54 6.06 0.44 0.33 0.19 0.24 0.64 !.03 0.7.t 0.40
21
22
70.0 74.4 0.56 0.33 14.52 13.61 0.82 0.54 255 1.50 008 0.05 0,62 0.32 1 93 ! .06 334 3.03 531 5.38 019 0.08 0 24 0.18
99100 99.69 100.16 100.48
"
~
R E E in charnockite
~ ~
5
Table 4. Rare earth element analyses of the Varberg region country rocks (p.p.m.).
La Ce Nd Sm
Eu Gd Tb Yb Lu Y Y~Y,La-La* La]Yb
1
2
3
24.1 48 16 3.6 1.0 . 0.4 3.0 0.37 11 107 8
21.3 47.2 51 97.0 14 28 6.6 4.8 1.6 1.1 . . . 0.6 0.5 3.5 1.9 0.44 0.31 7 15 106 196 6 25
4
5
52.2 87.8 ~ 9.9 1.7 . 0.7 5.2 0.8 24 216 10
493 79.8 29 5.2 1.1 . 1.I 2.3 0.4 22 !90 21
6
.
7
50.3 77.9 54 6.8 1.2
32.7 60.4 34 9.8 3.0
1.1 3.9 0.6 22 218 13,
1.7 5.0 0.8 11 158 6
* For the range of lanthanides analysed (includes Gd where relevant).
bution diagrams. Where distribution diagrams and total R E E values derived from published sources are presented only those elements analysed for the rocks of this study are used in the calculations and constructions. The general REE distribution characteristics of the rock units of the Varberg region are portrayed in the distribution diagrams of Fig. 2. Fig. 2a shows that the lithological groupings erected on field, fabric, and lithological criteria remain distinct in terms of their R E E compositions. All groups show some differentiation with enrichment in the light R E E relative to the: heavy elements. ~Y, La-Lu shows a progressive increase from the granulites, through the granite gne~:sses to the C G A . The total REE for the: latter is well above average for granitic rocks with particularly high values for La and Ce. The garnet-pyribolite is the least differentiated, with L a / Y b = 6 . 5 compared with 17
for the granite gneisses and 14.5 for t h e C G A , but compares closely with the 7.0 value obtained for the granulites. Both reflect the lower light F~EE contents found in the granulites. The strongly differing major oxide compositions of the granulite specimens have no apparent strong control on either the R E E contents or their distribution in these rocks. The migmatitic granite gneisses (Tables 1 and 2) both show a slight negative Eu anomaly in their REE distributions; a feature absent from those of the other analysed gneiss category (Fig. 2c). The average curve, used in comparisons, includes both groups. It adequately portrays the general REE characteristics common to both groups, i.e. strong differentiation with a mal ked inflexion, whilst repressing the internal distinctions. Fig. 3 shows the general similarity of REE content and distribution in the various sub-units of the C G A . Numerically the Torpa Granite is
Table 5. Rare earth element analyses of th ro:ks of the Charnockite-Granite Association
La Ce Nd Sm Eu Gd Tb Yb Lu Y ~Y, La-Lu* La/Yb
8
9
10
11
12
13
14
15
88.4 166 84 20.3 5.8 2.5 8.2 1.2 24 400 !|
65.5 156 62 15.4 ! .0 6.9 4.8 0.87 24 336 14
50.3 90.3 54 10.3 5.0 1.2 4.9 0.8 17 234 10
82.!) 144.5 71 16.3 4.1) .!.5 6.0 1.l 28 355 14
56.9 143 57 14.7 2.5 10.5 1.8 6.2 0.97 34 327 9
87.3 71.4 74.6 142 193 158 69 86 69 16.4 19.9 16.8 4.1 3.3 3.9 8.9 2.4 2.1 2.4 5.3 7.3 6.0 1.0 1.2 1.1 35 36 35 351 380 429 12 I0 7
* For the range of lanthanides analysed (includes Gd where relevant). Specimen numbering follows Table I.
16
17
18
(p.p.m.)
19
20
21
22
5 5 . 5 6 2 . 3 42.0 140 104 212 150 151 94 327 227 449 124 62 65 40 116 78 1 7 . 6 15.2 9.9 20 16.3 18.9 i.8 3.0 2.4 1.5 2.5 2.3 7.2 tl.4 g.9 6.8 8.1 1.6 2.1 2.3 1.6 1.3 2.0 2.0 4.0 7.6 6.1 5.8 3.2 7.1 6.0 1.1 0.98 0 . 9 4 0 . 5 9 1.15 1.03 0.68 33 31 34 32 24 35 36 852 471 343 345 223 651 481 53 15 9 11 13 23 1~
117.3 204 81 22.7 3.8
/ii! :i ~' ii i~ill ~ ::LITHOS 12 (1979)
6 F.H. Hubbard & J. E. Whitley !
I.... ~L:aCe
Nd
I 2.~
,~n E'u
Yb
Tb
a.Major
ieoe
.5oo
R.
RockAve~ge~
"~
Cha,-r.~ite- Granite Asmciation
,134~.
, Ih'lllNK~ lOO
o o
\
°
\
~! i~. 'I..,. '~- ~,~,
\
,oo "%
~
" \ .-,,'
..:.~:.....f
b,Means& Rall,4~,~
....
liG
- - ' , 1o.
tl
//I
/// ,~j
10 La C e
.io . I.~ C~.,.__ N#__..
~
"~ Sm Eu
""
~...i-
"~-
~.~ '~o
~...,..~
Tb
Y.b I.u
Fig. 2. a. Chrondrite-normalized ?,EE distribution patterns for the major rock units of the ~/arberg region. I. Mean of variation within the CGA (407; 14.5). 2. Average granite gneiss (265: 17). ~. Average granulite 006,5; 7). 4. Garnetpyriboli~e {158; 6.5). The numerical vaJucs in paienthe~es are the IEY, La-Lu and La/Yb values; for the respective dala set:, and this format is repeated i~a subsequent figure caplion,.~, b. Chondrite-normalized REE distribution patt,:rn,; for the analysed granulites. 1. HV 219 (106; 6). 2. ItV I g5 (107; 8). The broken line is the average, trace used in comFari'.;ens, c. Chondrite-normalized REE distribution pattern:~ for the analysed granite gneisses. !. HV 300 (216; 10). 2. HV 328 (218; 13). 3. HV 327 (190; 21). 4. HV 75/52 (1~96; 25). The average trend (broken line) i.~, left incomplete in the light REE area to avoid line confusion.
distinctive, however, as a result of' its relatively extreme differentiation with abnonnally high concentrations of La, Ce a:ld Nd. Development of a slight negative Eu anomaly is also evident in this group.
Nd ~_ ~m E,u
Tb
Y~ I~u_
Fig. 3. Chondrite-rkormalized REE distribution patterns for the rocks of Ithe Charnockite-Granite Association. a. Averaged values; for the individual sub-units of the CGA. 1. Main phase charnockite (VG and AGC) (330; 12). 2. Coarse sub-,~harnockitic aggregates (AGC) (414; 12). 3. Coars,e charnockite (TC) (348; 8). 4. Differentiated aggregate (AGC) (294; I1). 5. Microcline granite (TG) (656; 31). b. 1. Average of all charrt~ockitic members of CGA (357; 11). 2. Average of all non-charnockiitic members of CGA (546; 27). The broken lines delimit/,.he total variation for the suite of CGA rocks analysed.
Petrological implications of the gEE data All the rocks of the Varberg regitm were extensively recrystallized during a major Proterozoic orogenic event characterized by long-held high temperatures. The near-equilibrium conditions attained, both mineralogical and textural, re~;r:lted in the virtual elimination of earlier fabrics and p~rageneses. It is r~ecessmT, therefor,.=, to seek aad ~nvestigate any more subtle parameters which may have survived the intense reconstitutive aetiVriitLyif rock progenitors are to be tirmly determine~J!. Though the data
~
R E E in charnockite
LITHOS 120979)
on the behaviour of REE in metamorphism is limited,, there is general consensus that the REE distribution remains essentially unchanged~ even in the passage to the very highest grades (e.g. Green et at. 1969, 1972; Hermann 1970; Wildeman & Condie 1973; Wildeman & Hat;kin 1973). Thus REE are potentially valuable indicators of premetamorphic rock chara~:ters if primary patterns can be established for reference. I n practice, the application of REE methods to t]he Varberg rock complex has given some support for earlier interpretations of the derivation of the metamorphic rocks but has given no clear-cut, substantive, proof or"the nature of the rock progenitors. The granite gneisses of the region are tho~aght to be dominantly meta-arkoses. Little REE dam on arkosic sediments are available from the literature. Nance & Taylor (1976) have ro:ently published REE values for post-Archaean seciimentary rocks from Australia which include sub-greywackes and a Triassic arkose. Values for a R,ussian Devonian continental arid sandstone are also available (Balashov et at. 1964). Although the degrees and styles of differentiation are similar for these, and for the Varberg gneisses (Fig. 4a), there is a rather ,,vide divergence in total REE. Fig. 4b, on the other hand, highlights the very close similarity of the distrit~ution patterns and total REE for the Varberg gneiss~s, the Finnish Svecokarelian granites (Koljonen & Rosen~berg 1974) and tl~e 60-70% ]iOz average granite (Haskin et al. 1968). The REE data, 1herefore, give no clear indication of whether the Varberg gre~nite gneisses are ortho- or paragneiss~rS.Their distinction from the plutonic granites of the region (the Torpa granite) and the Finnish Archaean granite (Sahama 1945), however, is clearly displayed. Balashov et al. (1964) and Ronov et al. (1967) emphas~e the important control of REE content of the p~mary r~:k on that of the derived sediments and emphasize that in an arid environment of erosion and sedimentation there is no significant difference in the REE content and fractionation between the source rocks and the derived clastic sediments. The most likely source, and basement, for the Varberg sequences, if they do represent metasediments, is rocks related to the Svecofenrfian granitoid complex which dominates the geology of southern Sweden, east of the eaajor dislocation zone• No REE data are known from these rocks but the Svecokarelian rocks of Finland analysed by Koljonen & Rosenberg (1974) are of similar type and age and may be representative of the same event in the development o1~the Baltic Plat-
7
emparil~nswith~
t
•1 0
~
LO C.e
I~d
~m Ep
~ib
"~".210
Yb Lu
Fig. 4. Comparisons of the average Varberg gran~.te gneiss REE distribution with published valaes (see text for sources and discussion), a. 1. Average Varberg granite gneiss (205; 17). 2. Russian platform sandstone (93;* I 1). 3. Australian Trias~sic sandstone (263;'* 9). 4. Average Australian Upper Proterozoic sub-greywacke (127;** 13), 5. Average Austr~dian Palaeozoic and Mesozoic subgreywackes (175;** 13). *No values for Eu, Tb and Lu:**no values for Lu. b. !. Average Varberg gratdte gneis~ (205; i 7). 2. Finnish Svecok~Lrelian granite (172;* 16). 3. 60-70% $iO~ glani~e (218;**13). 4. Torpa granite, of CGA (656; 31). 5. Fmr~ish Lappl~.nd granite gneiss (102;*** 2). *No value for Y; **no values for Tb; ***no values for Lu.
forum. Although the new REE data from the Varberg region provide no absolute conclusion regarding the nature an~J origin of the granite gneisses, they are consiste:~* with the gneisses having been ~Lerived from arkoses developed in the erosion of a granite source area of Svecofennian type in a continental environment. The gneiss categories distinguished by the presence or absence of a Eu anomaly (Fig. 2e) also differ in bulk composition and hydration (Tables 1 and 2). The micromigmatization fabric suf~gests confined anat~tic redistribution, while the char-
L]iTHOS 12 (1979)
8 F.H. Hubbard & J. E. Whitley
~r~)~
al. Granulite V Greywacke
\;,,~ ~.~ •
•
\.
\.,,,::%
,\
J ae~
,10
~. Pyribolite V Basalt
~
;1 .4
5' La Ce
_~d .PmS.m E.u ~
,Tb
_
Yb L.u
Fig. 5. a. Co~aparisons of the average Varberg granulite chondrite-normalized REE distribu~tion with published values (see te~t for sources and discussion). 1. Average Varberg granulite (! 06; 7). 2. Average Australian Cambrian greywacke (127;* 13). 3. Karelian greywacke (47.5; 15). The shaded field encompasses the variation in REE distribution of the average Wyoming greywacke (140; 19) the average Fig Tree greywacke (99;** 14) and the average Svecofennian greywacke (129; 16). Tht,' distribution patterns of these plot so closely that separate presentation would cause line confusion. *No value for Lu; **no value for Y. b. Comparison of the Varherg garnet-pyribolite chondritenormalized RFE distribution with published values. 1. Varberg garnet-pyribolite (158; 6.5). 2. Columbia~ plateau basalt (147; 6.5). 3. Average basalt (79; 1.9). 4. Alkaline olivine basatlt,, Japan (-; 22). There is insufficient correspondence of elements analysed I : : 4 to allow cah:ulafion of a comparable Y-Y, La-Lu.
nockitic gneisses may be residual after expulsion of the SiO2- K 2 0 - H 2 0 anatectic fluid. The differing Eu distributions accord with the K-feldspar/plagiocHase (K/Ca) ratios of the gneiss types. The quartzofeldspathic granulites analysed have REE contents and distributions which conform well with published data for slightly metamorphosed ~:o unmetamorphosed Precambrian and Cambrian greywackes (Fig. 5a) (Wildeman & Condie 1973; Wildeman & Haskin !973; N~ace & Taylor 1976). The heavy REE contents of the
Varberg rocks mar the cot~aparisor~; particularly the high values of Yb which cause a>a exaggerat~l reduction of the La/Yb diff(!xentiation ratios. Tile; high values of Yb, and Lu, are fouxid in both the granulite and the amphibolite faci,t:s representatives and are probably relict from the parent sediments rather tha~n relating to the higher grade of metamorphism of the Varbetg rocks. The Finnish Karelian (Archaean) greywacke has a markedly lower total REE content but sh¢,ws a similar differentiation pattern to the other Prccambrian greywackes.. The granulites have much lower REE contents than the granite gneisses and show less differentiation (LafYb for the granite gneiss= 17; for the granulite=7). These differences in REE presumably persist from the sedimentary stage and reflect the ~iffering compositions of the sediment sources which, in the case of the granulites/ greywackes, probably included besic volcanics. REE data are available from only one example of the basic intercalations which characterize the passage from granite gneiss Io the mixed granulite sequences. The major element chemistry data (Table 2) sugge,:;t that the parent rock of this garnet-pyribolite was alkaline olivine basalt. The chondrite-norma,lized REE data plot most closely with the Columbian Plateau bas+alts (Schmitt et al. 1964) (Fig. 5b) and conform wi~:h these rocks in both to~ial REE and differentiation. The Japanese island a~rc alkaline olivine basalts (Masuda 1966, 1968) show lower total REE and more pronounced differentiation. More extensive data would be required to draw any finer conclusion concerning these basic rocks. Either continental or island arc volcanic environments would be consistent 'with the sedimentary lithofacies change fi'om arkose to greywacke indicated for the Varberg rocks; either by inception of volcanism along associated rifting or by the development of an offshore island arc system. Comparison of the REE characteristics of the three groups of charnockitic rocks outcropping in the Varberg area, both with each other and with associated rocks of similar type but lacking charnockitic characteristics (Fig. 6a) give no evidence of theiir being a type pattern of REE distribution for charnc~ckite. The metamorphic charnockites maintain closely the predicted REE characters of the pa~rent rocks. Only when a melt phase becomes impo~ant, as in the CGA, does significant redistrib~;tion of REE ensue. If the CGA represents an anatectic plutonite derived from rocks of como parable type to i~tscountry rocks then a significant total REE acc:etion is involved, accompanied by
LITHOS 12 (1979) further accentuation of light/heavy REE differentiation. The consistency of the REE distribution curves derived for the members of the CGA leaves little doubt that they are kindred rock~; (Fig. 3a). This contention is reinforced by the rational nature of the REE change with bulk compositional differentiation from intermediate charnockite to acid granite with increase in total REE and accentuation of the light/heavy REE differentiation (LEt/Yb©hamockite-" 11, La/Ybgrantt® = 2 7 ) (Fig. 3b). The change in REE distribution includes progressive relative depletion in Eu to the extent that a slight negative Eu anomaly is developed in the granites (Figs. 3a and b). This parallels the increasing dominance of alkali feldspar over plagioclase. ~tese distribution trends suggest a preferential aggregation of' the light REE (La to Nd) in the mobile phase of the differentiating CGA. The Torpa Granite may be considered tile 'product' of the charnockitic plutonism shown in the CGA. It represents the aggregatio~ of the most mobile components and is the phase most likely to leave the environment of its formation to intrude higher levels. This was only partially achieved in the case of the CGA and the connection with its source is still maintained (Hubbard 1975; Hubbard & Whitley 1978). Some intrusive granites of the higher crustal levels may have had their origin in such plutonic charnockite activity as is observed, arrested before completion, in the; Varberg CGA. The most closely comparable granite REE data is found in the Rapakivi granites of Finland (Koljonen & Rosenberg 1974; Hubbard & Whitley 1978) (Fig. 5c). A similar trend is showrl also by a Rapakivi granite from the Ukraine (Balashov 1963), but a different range of REE was determined and it is not possible to show whether a Eu anomaly exists. The comparability of the Torpa Granite and the Ra?akivi granites of the Baltic Shidd is of some interesl. A ?ossibte link between the high-level Rapakivi granites ~nd the deep-level anorthositemangerite complexes h,~s been discussed (e.g. Eckerrearm 1936, 1939; Kranci~ 1969). In tSis region of SW Sweden, which has clo~e parallels with the anorthosite province of S Norway:, a link has been established be.tween granites of Rapakivi type and their charnockitic source. The poss,ibili~y ,~merges that the apparent anorthosite-Rapaldvi association may be an indirect one resulting from the derivation of a Rapakivi type mag~aa fraction from crustal charno~:kitic plutonism associated with anorthosite emplacement (Hubbard & Whitley 1978). Granites of the Torpv type are clearly distin-
REEincharnockite
9
100,
"~.~
"~ -
\\
.10
L.a Ce.
N,d
,Sin eu
~
Yb L.u ]
Fig. 6. a. Comparison of chondrite-normalized REE distribution patterns of the various charnockitic rocks of the Varberg region, l. Charnockitic granite gneiss (201; 16). 2. Charnockitic granulite (106; 7). 3. Average plutonic charnockite from CGA (357; i 1.5) (Specim~:ns 8-18 inclusive), b. Comparison of the chondrite-normalized REE distributions of the Torpa Granite of the CGA with other granites, l. Torpa Granite (656; 31). 2. Finnish Rapakivi granite (447;* 17). 3. >70% SiOz granite (254, 10). 4. Average Svecokarelian granite 072;* 16). Sources and discussion in text. *No Y value.
guishable from the Finrish Svecokarelian granites =b" 5b). The on the basis of their REE contents ~,= 'v:~, REE distribution of the latter is closer to that of the > 70% SiOz average granite of Haskin et al. (1968). A variety of granite bodies intrude the highc~ level metasedimentary sequences to the north of the Varberg region (e.g. Gorbatschev 1976; Samuellsson & Ahlin 1976). It is hoped that it will prove possible to compare their REE .=on* tents and determine whether any are of the Torpa type. An as yet incomplete U/Th investigal:ien of the granites of the region suggests that thi~; will I~: the case. Koljonen & Rosenberg (1974) r,~ogn~zed a
LITHOS 12 (1979)
10 F. H. Hubbard & J. E. Whitley
crude variation in REE with age in the rocks of the Finmsh region of the Baltic Shield. "Fo~tl REE increased and enrichment in light REE relative to heavy REE was enhanced in progressively younger sequences. This concept of secular variation finds sorae support in the ,data from Var.berg. The granite gneisses and granulites have greater REE contents a~.~[ more advanced differentiation than the Finnish Archaean equivalents and the granite gneisses co~ ~,t well be derived from granites with Sv,eeokarelil, I:~cREE characteristics. The Torpa granite equate~ with the yotmger Rapakivi granites. 'l~tis would ~:.ag.gestthat the Varberg sedimentation post-dates ~3~.• 1800-1950 m.y. Sveeokarelidic granite event ai~d that the plutonism associated with the metamor;~:;hism of the product sediments would dat~.• with ti'~e 1750 m.y. Rapakivi event. The provisional date for the CGA, however, is 14.50m.y.
in a granite s ~ m to be of greater influence in determining its REE characteristics than its gross composition. Acknowledgements. - The fiaan,:ial help of the Carn~:g~e Trust for the Universities of Scotland and the Univerfity of Dundee Travel Fund in defraying fiela expenses is gratefully acknowledged. Professor H. W. Wihon (S.U.R.R.C.) is also thanked for his interest and encouragement.
References
Balashov, Yu. A., Ronov, A. B., Migidsov, A. A. & Turanskaya, N. V. 1964: The ,.;fleet of climate and facies environment on the fractionafion of the rare earths during sedimentation. Geochemistry (USSR), English Translation, 951"-969. Eckermann, H. yon 1936: The Loos-Hamra region. GeoL F#ren. Stockh. Fi~rh. 58, i 29-343. Eckermann, H. yon 19:39: The anorthosite and kennigite of the Nordingr~i-R6d6 region. Geol. Fiir~m. Stockh. Fiirh. 60, 243-284. Gorbatschev, R. 1975: Fundamental subdivisions of Precambrian granitoids in the Areal mega-unit and the The REE study of the Varberg complex rocks evolution of the south-western Baltic Shield. Geol. affirms the significance of REE stability during Fi~ren. Stockh. Fiirh. 97, 107-114. Gordon, G. E., Randle, K., Goles, G. G., Corliss, J. B., solid-state transformations to studies of the deveBeeson, M. H. & Ox|ey, S. S. 1968: Instrumental activalopment of high-grade metamorphic regions. Orition analysis of standard rocks with high resolution ginal REE contents and distributions persist from gamma-ray detectors. Geochim. Cosmochim. Acta 32, sedilnentation to the highest grade of metamorph369-396. Green, T. H., Brunfelt, A. I. & Heier, K. S. 1969: Rare ism and allow the identification of rock progeniearth element distribution in anorthosites and as,,;ociated tors. In arkosic sedimentation the REE characters high-grade metamorphic rocks, Lofoten-Vesteraalen, Norof the source granitoids may be largely pt'eserved way. Earth Planet. Sci. l~tt. 7, 93-98. in their denudation products. Mcrdification of Green, T. H., Brunfelt, A. I. & Heier, K. S. 1972: Rare earth element distribution and K/Rb ratios in granulit~-s, REE disltribufion in orogenesis ensues onlfy when mangerites and ano~thosites, Lofoten-Vesteraalen, Normelting begins; but then abruptly. The melt phase way. Geochim. Cosmochim. Acta 36, 241-257. accumulates REE irregularly showing a preference Haskin, L. A., Ha~kin. M. A., Frey, F. A. & Wildeman, for the light elements. T. R. 1968: Relative and absolute terrestrial abundances of the rare earths. ~n Ahrens, L. H. (ed.): OrJigin and There is no l~arti¢;ular REE distribution cl~aracteDistribution of the Elements, pp. 889-912. Pergamon ~igtic of charnockite. The in situ metamorphic charPress, London. nockites have the REE of their parent. Plutonic Hermann, A. G. 1970: Yttrium and lanthanides, fi~ Wedecharnackites developing in a granulite facies enpohl, K. H. (ed.): Handb~.~ok of Geochemistry 39, 57-71-Ei-19, Springer Verlag, Berlin-Heidelberg-New vironment accumulate REE, particularly light REE, York. in the magrnatic component and differewtiate to Hubbard, F. H. 1975: The Preeambrian crystalline ,:omplex p oduce a non-charnockitic granite with the REE of south-western Sweden. The geology and petrogenetic characteristics indicated for granites of t!he Rapadevelopment of the Varberg region. Geol. Fiiren. Stockh. kivi tyl~.', which include a negative Eu anomaly. F6rh. 97, 223-236. Hubbard, F. H. 1978: A geochemical investigation of the Rapakivi magma generation may therefore be a premetamo~'phie nature of the granite gneisse~; of the direct consequence of charnockitic plutonism and Varberg Series. Geol. F6ren. Stockh. Fi#h. 100. the postulated link with anorthosite may be an Hubbard, F. H. & Whitley, J. E. 1978: Rapakivi granite, anorthosite and charnockitic plutonism. Nature 2_71, indirect ,one. 439--440. A wide-rang~ing study of the REE characteristics Koljonen, J. & Rosenberg, R. J. 1974: Rare earth elements t~f granitoids of known derivation miight provide a in granitic rocks. Lithos 7~ 249-261 useful key for determining the origin of granites of Kranek, E. H. 1970: Anorthosites and rapakivi, magmas from the lower crust. New York State Mus. Sci. Sere. more enigmatic history. Development mechanisms 18, 93-97. and the number of crustal reworkings represented
Summary of conclusions
L]ITHOS 12 (1979) Masuda, A. 1966: Lanthanides in basalt:s of Japan with three distinct types. Geochem. J. 1~ 11-26. Masuda, A. 1968: Geochemistry of lan~hanides in basalts of Central Japan. Earth Planet. Sci. Lett. 4, 284-292. Nakamura, N. & Masud~., A. 1973: Chondrites with peculiar rare earth patterl~s~ Earth Pl,~met. Sci. Lett. 1~, 429-437. Nance, W. B. & Taylor, S. R. 1976: Rare earth element patterns and crustal evolution- ,. Aus~tralianpost-Archaean sedimentary rocks. Geochim. Cosmochim. Acta 40, 1539-155 I. Quensel, P. 1951: The charnockite series of the Varberg district on the south-western coast of Sweden..,Irk. Mineral. Geol. I, Stockholm. Ronov, A. B., Balashov, Yu. A. & Migdisov, A. A. 1967: Geochemistry of the rare earths in the sedimentary cycle. Geochemistry Int. 4, 1-17. Sahama, Th. G. 1945: Spurelemente der Gesteine im sfidlichen Finnisch-Lappland. Bull. Comm. gdol. Finl. !35, 86 pp.
R E E in charnockite
11
Samuelsson,.L. & A hlir., S. 1976: The Precambriat~ crystal° line complex of south-western Sweden: The geology and petrogenetic development of the Varberg region. A comment. Geol. F~ren. Stockh. FOr/~. 98, 168-170. Schmitt, R. A., Smith, R. H. & Olehy, D. A. 19i~4: Rare earth, yttrium and scandium abundances in meteoric and terrestrial matter-I!. Geochim. Cosmochim. Aeta 28, 67-86. Wildeman, T. R. & Condie, K. C. ~973: Rare earths in Ar~haean greywackes from Wyoming and from the F~g Tree Group, South Africa. Geochim. Cosmochim. dcta 37~ 439-453° Wildeman, T. R. & Haskin, L A. 1973: Rare earths in Precambrian sediments. Geor.;)im. Cosmochim. Acta 37, 419-438. A~epted for publication March 1978 Printed January 1979