The Geochemistry of Meta-Igneous Rocks from the Amphibolite Facies Terrain of South Norway

The geochemistry of meta-igneous rocks from the amphibolite facies terrain of south Norway

ROBERT BEESON

Beeson, R.: The geochemistry of meta-igneous rocks from the amphibolite facies terrain of south Norway. Norsk Geologisk Tidsskrift, Vol. 58, pp. 1-16. Oslo 1978. ISSN 0029-I%X.

The field relations, petrography, and geochemistry of a suite of amphibolite facies metamorphic rocks from the Bamble Sector of the Fennoscandian Shield in south Norway are described and discussed.

Field and geochemical parameters suggest · an igneous origin for the suite, in particular the mineralogical and chemical continuum from åcid to basic members, element associations and trends, and the abundance of major constituents. The Na and K contents are consistent with calc alkaline igneous suites. Element diffusion trends typical of high-grade metamorphism, particularly those involving K and related elements, contrast with those observed in the meta-igneous rocks, and the metamorphism is considered to be largely isochemical. Therefore the pre-metamorphic chemical characteristics of the area, e.g. high Th concentrations, can be used to assess the metamorphic processes which have affected adj;1cent granulite and granitic terrains.

R. Beeson, Geo/ogical Survey, Private Bag Xll2, Pretoria 0001, South A/rica. Present address: Uriran Private Company, P.O. Box Il/ 1664, Tehran, Iran.

The Songe-Ubergsmoen area of south Norway is rocks of three recognisable ages and varying in the Bamble Sector of the Fennoscandian metamorphism (Starmer 19691?, Elliott 1973). Shield. Songe is 250 km south of Oslo, and 40 km The spatia! distribution of these lithologies has · north of Arendal (Fig. l). been previously described (Beeson 1975). The Bamble Sector in the Arendal region The meta-igneous rocks and to a lesser extent consists of high-grade metamorphic rocks of the metasedimentary gneisses of the Songe upper amphibolite and granulite facies (Bugge Ubergsmoen area offer an excellent opportunity 1943, Touret 1968). The Songe-Ubergsmoen to study lithologies which have not been exten area Iies in the upper amphibolite facies terrain. sively affected by the movement of elements The grade of metamorphism increases southward during granitisation or K�feldspathisation proc towards Arendal, where the granulite facies esses prevalent in adjacent areas. These proc rocks are located (Cooper 1971, Andreae 1974). esses have occured in the area, but detailed The geological history proposed by Starmer mapping has shown that the meta-igneous rocks (1972) can be applied in general to the Songe are only partially affected. This has enab1ed the Ubergsmoen area. The main metamorphic sampling of a suite of rocks which offers an episode seen today is the Svenconorwegian indication of the primary nature of the pre orogeny (1)3 of Starmer 1972) at an approximate metamorphic lithologies, and the processes ac age of 1160-1200 m.y. (O'Nions & Baadsgaard tive in high-grade metamorphism. 1971). This was followed by a major period of The field relations, petrography and geo granitisation, which had a profound effect on the chemistry of the meta-igneous rocks are dis gneisses around Tvedestrand, immediately to cussed, and compared with both metamorphic the south of the area under discussion (Field and igneous rocks suites from elsewhere in the 1969). world with which they show affinities. The Songe-Ubergsmoen has been mapped by the author at a scale of 1:15,000. Four major Iithological units occur in the area: metasedi Field relations mentary gneisses, typified by the presence of sillimanite and graphite (Beeson 1975); the meta The meta-igneous rocks consist typically of igneous rocks described here; granitic and quartz, plagioclase, biotite, and hornblende. The granodioritic gneisses (Starmer 1969a); and basi hornblende is normally restricted to the in- . �

l -N. GeologiskTidsskr. 2 R. Beeson NORSK GEOLOGISKTIDSSKRIFT l (1978) '',,,, o 5 lO 20km]) ��4-��------�,, ":;'!;ONDELED l: 'l N ,, o.. Jl /l . . . • . • . . • • . ' · . <-.:: : :-.:.: :_:: :: :::->:.":·.:-.... :-·. ..-( · _ _ _ _ . . . -.::: . · : ...... · : :�,' .. · · ·. · . . · . · · t . . . . : . . . · · · · · ·: :· ·: ·: .· · .: · _ ...... : ·:< :::: : ...... -. :. .::-:��': �;{� ONGE . . " " ." . . · . " " " : : UBERGSMOEN•-" · · . . . . . •• •• •• o·••••• · · •• o,., • • • • • • • • • • • • • • • ,'/'//1 • • • • • • • • • • • • • / • o • o•••• o•• o• o ol/ • • ••••• •• o ••••/l • • • • ...... ; / o · . . • • o • . ·. · ·o o o "/ / .. · o· · ·: o·o·o ·· : �-:.�' / . . :...... ·. ; •• o ••o. • • fl / • . • • • • • • • !l • . . . • • / . , ...l STRAND ,--�� TVEDE ' , / (/ ,, ,, ,, ,, u ,, IJ 11 1/ .,.""'/ / ,,/!� / ,"'"'/'" / ;t / ,, . o}/ t A' ·" /:: / ti LEG END 11 ,, " / 11 ,, / �� ARENDAL Towns ®

Villages e

Mapped area :::;:;;:; :=::·:-:

Approximate amphibolit e l ".... ".... granulite facies boundary

Fig. l. Location map of the Songe-Ubergsmoen area, south Norway.

termediate and basic varieties of the rock suite. iron-rich varieties. On! y rocks with a mineralogy Garnet, when present, is red in colour. K of biotite-quartz-plagioclase are common to both feldspar is rare in the meta-igneous rocks, but the meta-igneous and metasedimentary lithol when present the field and petrographic evi ogies. dence indicates that this mineral replaces The rock suite normally has a well-developed plagioclase. This mineralogy contrasts with that gneissic texture, but it is locally massive. It of the metasedim�ntary gneisses in the area, and varies from leucocratic quartz-plagioclase .allows distinction between the two rock groups gneisses with minor contents of biotite to in the field. The metasedimentary gneisses com amphibolites consisting of only plagioclase and monly have sillimanite and graphite in the more hornblende. Banding of leucocratic and mela acid varieties, and a pale pink garnet in relative! y . nocratic layers occurs, commonly with a pre- . NORSK GEOLOGISK TIDSSKRIFT I (1978) Geochemistry of meta-igneous rocks 3

lO lO

8 8

>- 6 6 u z w ::::> o w ff:4 4

2 2

o o 20 26 28 30 32 34 36 38 40 42 A NORTH l TE CONTENT

Fig. 2. Frequency distribution of the anorthite content of plagioclase in the meta-igneous rocks.

dominance of the former. The individual layers considered to have formed during two metamor vary from l centimetre to several metres in phic stages, i.e. syn- and post- the main metamor width. The amphibolite horizons within the phic episode (Beeson 1975). quartz-plagioclase-biotite gneisses (± horn Plagioclase (2-82 %) is normally present as blende) can be much wider than these banded equidimensional grains (�3 mm), although gneisses, being tens or hundreds of metres in slightly elongate, coarser varieties (�7 mm) are width. These amphibolite horizons can extend present. The grain shape can be rounded, angu for more than a kilometre along strike. lar or irregular. Plagioclase composition shows a Metasedimentary gneisses rarely occur as thin continuous increase in Ah content with silica in intercalations within the meta-igneous rocks, but this rock type (Fig. 2). The plagioclase is occa are commonly at least 50 min width. However, sionally antiperthitic. In such cases the anti ' the reverse is not true, and thin intercalations of perthitic types are patch, flame and rod forms. the meta-igneous rocks are not found in the Biotite (0-69 %) is predominantly pleochroic metasedimentary gneisses. Units of the meta pale to dark brown (X,Y) or brown (Z), although igneous rocks can be extremely persistent, one pale phlogopitic micas and iron-rich medium 500 m wide unit continuing for 15 km throughout brown to black varieties are represented. Pri the area. mary mica varies from stubby, shortened grains to thin, platelike crystals (0.5-4 mm). Mica defines the foliation plane. particularly when Petrography abundant, but the foliation can be distorted by either the growth of garnet or quartz veinlets. The petrographic description tendered here is a Biotite commonly replaces hornblende, both in generalised summary of the features of the meta the presence and absence of K-feldspar. Two igneous rocks as a whole. replacement forms are recognised: distinct stub Two main varieties of quartz (3-59%) are by or elongate laths cutting across the horn present. The first occurs as sub-rounded grains blende grains, or a felted mosaic of fine-grained (0.1-5 mm) with a non- or partially-undulose mica. The biotite also replaces garnet and rarely extinction and curved or lobate margins. The epidote. Minerals occuring as inclusions in the second variety has an irregular shape, frequently biotite are zircon, apatite, and opaque minerals. displays sutured margins, and undulose extinc The biotite alters to chlorite and prehnite. tion. The latter may occasionally contain trains The hornblende (0-33 %) is exclusively green of inclusions parallel to the foliation. These are in colour, commonly with X, Y pleochroic pale 4 R. Beeson NORSK GEOLOGISK TIDSSKRIFT l (1978)

Table l. Meta-igneous rocks, Songe-Ubergsmoen area: sum- rarely as individual grains cutting across the mary statistics. Major elements in oxide percentages, trace foliation. elements in ppm (n =50) Other minerals occurring in the meta-igneous

Element Mean Standard MaximumMinimum Crustal rocks are K-feldspar, cordierite, and epidote. deviation average Accessory minerals present include opaque minerals (magnetite and pyrite), zircon, and

Si02 66.12 7.87 78.19 47.62 60.3 apati te. Al.o. 14.63 2.15 21.32 10.51 15.6 Ti02 0.73 0.44 2.13 0.17 1.0 Fe,03 2.50 2.02 8.20 0.29 7.2 FeO 2.67 1.85 8.97 0.43 Geochernistry Mg() 2.40 2.47 13.14 0.14 3.9 Ca() 3.99 1.85 8.17 0.21 5.8 50 samples of the meta-igneous rocks have been Na.O 3.72 1.50 8.46 0.30 3. 2 analysed for Il major and 24 trace elements. K.o 1.83 1.19 5.62 0.14 2.5 Sample collection was based on 2 principles. To MnO 0.07 0.07 0.20 O.ot 0.10 P.o, 0.17 0.12 0.52 0.02 0.11 establish a representative collection of the rock group, samples were taken at 100 m intervals s 292 458 3011 7 260 Cl 358 318 1302 25 130 along road traverses perpendicular to the strike Se 14 9 45 2 22 of the foliation. In addition samples were col V 127 85 523 13 135 lected in all parts of the mapped area to establish Cr 27 35 149 o• 100 the regional variation. Co 20 12 53 o• 25 Ni 17 23 86 o• 75 Only the summary statistics of the data are C u 25 30 198 l 55 presented here (fable l). The individual ana Zn 76 47 216 6 70 lyses , Niggli values, cation percentages, modal Ga 21 4 34 12 15 analyses and katanorms have been presented 45 o• Rb 65 215 90 1972). Sr 254 138 587 13 375 elsewhere (Beeson y 35 38 188 o• 33 The majority of elements in this lithology are Zr 294 197 1754 70 165 typified by a lognormal distribution with a Sn l 4 23 o• 2 positive skew (fable 1). Only Si02, Al203, and Ba 860 335 1886 276 425 La 29 24 100 l 30 Ga have normal distributions. The distributions Ce 88 53 261 24 60 of CaO, NazO, Zr, and Th are irregular, which Pr 21 Il 45 t• 8 may indicate the presence of sub-populations. Nd 26 15 74 7 28 Only limited data are available for amphibolite Sm Il 6 28 3* 6 Pb 14 JO 52 3* 13 grade metamorphic rocks from other areas but Th 9 9 35 o• JO there are similarities in chemistry between the u l l 4 o• 3 meta-igneous rocks from the Songe-Ubergsmoen (l) From Taylor (1964) area and elsewhere. The major-element composi • indicates value below the direction limit tion of the meta-igneous rocks can be compared with the sub-acid and intermediate rocks from Brazil (Sighinolfi 1971), and with amphibolite facies rocks from northem Norway (Heier & green to medium green-brown or emerald green Thoresen 1971). However, trace-element compo and Z pale to dark green-brown. Grain size is sitions, particularly the elements associated with variable, and increases proportionally with the K, are less telated to other areas. homblende content of the rock. Commonly the Cluster analysis is used in this study to sum homblende is xenoblastic or elongate xeno marise element associatiohs. Cluster analysis is blastic, although it is also often prismatic. based on all inter-element correlations, and Quartz, plagioclase, apatite and opaque minerals hence identifies groups of related elements. The occur as inclusions in the homblende. degree of correlation between the groups can The gamet (0- 15 %) is present as large also be identified. The results of the clustering poikiloblastic grains ( .,; 15 mm) which are ir are shown as a dendrogram, which is a simplifi regular, angular, rounded or elongate in shape. cation of the multi-dimensional correlations in Included minerals are quartz, opaques, apatite, two dimensional form (Fig. 3). This gives the and altered plagioclase. Elongate grains com following associations: Mg, V, Co , Cr and Ni; monly form in hands parallel to the foliation or . Fe, Ti, Mn, Se, P and Zn; K, Rb and Ba; Zr, Y, NORSK GEOLOGISK TIDSSKRIFf l (1978) Geochemistry of meta-igneous rocks 5

V l

Co l

- p

Se l

C u

SIMILARITY 0,9 0/J 0,7 0,6 0,5 0,4 0,3 COEFFICIENT

Fig. 3. The R-mode dendrogram for the duster analysis of elements in the meta-igneous rocks. Clustering terminated at the 95% confidence leve! (0.25). 6 R. Beeson NORSK GEOLOOISK TIDSSKRIFT l (1978)

1,6

1,2 o

.!! ""' � o >- ' " U) ""'

0,8 ""' ""' o � o o

o � '-.__ o ...... co"" t + o 0,4 � � +� o + o � ""' +

Fig. 4. The Se/Y ratio versus percentage Si02 diagram for the meta�igneous rocks, divided into biotite gneisses (closed circles), biotite-hornblende gneisses (open circles), and garnet-bearing gneisses (crosses).

Ga, Th and the rare-earth elements; Ca, Na, Al, elements are only correlated with Zr. Zr normal Cl and Sr; S and Cu; and the less significantly ly occurs in zircon in acid and intermediate clustered elements Si, Sn, Pb, and U. The more rocks (Taylor 1965), and hence it is probable that important associations are discussed below. the other elements of this group are present as isomorphous inclusions in the zircon structure in Silicon has a wide range of values (47-78 %); this the meta-igneous rocks (Vlassov 1%6). gives it strong negativ� correlations with the 15 All elements of this association excepting Y elements which are most abundant in basic rocks are positively correlated with Si, although only because of the closed percentage system. La, Ce, Sm, and Th are correlated at the 99 % confidence leve!. Y decreases with Niggli Si, and Zirconium group (Zr, Y, Th, and the rare-earth hence has positive correlations with both the elements): Zr is clustered with Y, Th and the mafic elements and Zr. A strong, positive rare-earth elements in both the metasedimentary correlation between Zr and Ga exists, particu and meta-igneous rocks of the Songe-Ubergs larly at high concentrations of both elements, moen area, but a correlation with N a in the but this correlation cannot be explained. former lithology suggests that at !east a propor Taylor (1965) suggests that the Se/Y ratio is a tion of these elements is located in plagioclase guide to igneous fractionation. The Se/Y ratio (Beeson 1975). In the meta-igneous rocks the shows two individual and distinct decreasing NORSK GEOLOGISK TIDSSKRIFT l (1978) Geochemistry of meta-igneous rocks 7

trends with increasing Si02 (Fig. 4), excepting Table 2. Mean K/Rb and K/Ba ratios for the meta-igneous seven high values of the ratio. This suggests rocks and metamorphic rocks from elsewhere in the world. that this lithology has an igneous origin. 2 3 4 5 6 7 8 9 10 Fig. 4 also indicates that garnet only occurs in rocks with a Se/Y ratio of less than 0. 5. As Se is 2 K/Rb 292 378, 1330 240 217 258 688 232 297 539 strongly correlated with Fe + in these rocks, the K/Ba 19 18 20 40 28 48 13 - high Se/Y ratios would normally be expected in . garnet-bearing rocks. l. Meta-igneous rocks, Songe-Ubergsmoen area. 2. Amphibolites, Sogne-Ubergsmoen area. 3. Charnockitic gneisses (Cooper & Field i977). Potassium group (K, Rb and Ba): K-feldspar 4. Granitic gneisses, Arendal area (Cooper 1971). constitutes over l % of the mode in only 20 % of 5. Sub-acid gneisses, Musgrave Range, Australia (Lambert & the meta-igneous rocks, and hence K and Rb are Heier 1968). 6. & normally located in biotite. In K-feldspar-bearing Acid gneisses, Musgrave Range, Australia (Lambert Heier 1968). · rocks the K content is markedly increased. 7. Basic rocks, Musgrave Range, Australia (Lambert & Heier When present, K-feldspar replaces plagioclase 1968). in this lithology, and consequently both K and 8. Crustal average (Taylor 1964). & Rb show a negative correlation with Ca and Na. 9, 10. Amphibolite facies rocks, northern Norway (Heier Thoresen 1971). The negative correlation between the latter two elements and Ba is less pronounced than that of K and Rb, and hence it is considered that at !east Solomon Islands (Jakes & White 1970). The part of the Ba abundance is present in the meta-igneous trend is also not markedly di plagioclase. vergent from the Lewis & Spooner (1973) The mean K/Rb ratio (292) is higher in the granulite trend, although Field & Clough (1976) meta-igneous rocks than crustal average esti indicate that this is for a composite sample mates (e.g. Taylor 1965, table Il). However, and not necessarily typical of observed in it has a wide range of values (138-1200), and the dividual trends. Six meta-igneous rocks with relatively high mean is caused by the high ratios below 15 ppm Rb do not conform to the regular in rocks deficient in K and Rb, where the mean trend of the remainder of the rock suite. These is 739 for rocks with less than 0.5 % K. The samples are deficient in K-bearing minerals, median value for the meta-igneous rocks is con having no K-feldspar and biotite being absent or . sequently lower (238). less than l% of the mode. Comparative data in the Bamble Sector are Although K/Ba ratios in the meta-igneous currently only available for metabasites from the rocks are similar to those recorded in the area of the granulite facies transition south of the amphibolite facies terrains of Australia Songe-Ubergsmoen area (Field & Clough 1976), (Lambert & Heier 1968, table Il), and the adja and the charnockitic gneisses from the granulite cent granulite facies rocks (Cooper & Field facies (Cooper & Field 1977). Although a similar 19i7), the Ba values (mean 860 ppm) are consid mean K/Rb ratio is obtained for the metabasites erably higher than the estimates for the average from amphibolite facies samples (378), both the of the continental crust (e.g. Taylor 1965, 425 granulite facies metabasites (mean 567) and ppm). particularly the charnockitic gneisses (mean Comparison of the K, Th and U concerittd 1323) show the Rb depletion typical of many tions of the meta-igneous rocks with a range of high grade metamorphic terrains (e.g. Sighinolfi rock types from elsewhere in the world indicates 1969, 1971). Consequently the linear trend exhib that Th is relatively enriched (fable 3). This ited by the metabasites and the field of the results in high values of the Th/U and Th/K charnockitic gneisses are both on the K-rich side ratios in comparison with granitic rocks (Heier of the regression lines calculated by Shaw ( 1968) & Rojers 1963). Granitic rocks from the Sogne for igneous rock suites, and have an atypical Ubergsmoen area are similar in respect of the se orientation (Fig. 5). In contrast the field of the elements to those reported by Killeen & Heier meta-igneous rocks falls well within that of (1974) for gndsses from elsewhere in the Bamble examples given by Shaw, and have similar K/Rb Sector. ratios to several unmetamorphosed rock suites, e.g. the hornblende dacites from the western The sodium group (Na, Ca, Sr, Al and Cl): the USA and the andesites and dacites from the elements of this association are variably located 8 R. Beeson NORSK GEOLOGISKTIDSSKRIFT l (1978)

/ /• /..

5 lO 50 100 500 1000 ppm Rb

Fig. 5. K versus Rb for the meta-igneous rocks. The dashed line represents the ratio calculated by Lewis & Spooner (1973) for granulites, the continuous line the regression calculated for metabasites (Field& Clough 1976), and the hatched area is the upper part of the chamockitic gneiss field (Cooper& Field 1977).

in plagioclase and hornblende in the meta within both hornblende and plagioclase. The igneous rocks. The Na is largely restricted to the latter normal! y constitutes 30-65% of the mode former, and it displays a reasonable correlation of these rocks, and no systematic variation · with the plagioclase con tent (Fig. 6). The reia- . betweenCa and modal abundance was observed tively limited range of anorthite contents of the (Fig. 7a). However, it is evident that Ca concen plagioclase (Fig. 2) is not sufficient to disturb trations above 3 % are directly related to the this relationship markedly. Ca is contained abundance of modal hornblende (Fig. 7b). Only two samples which contain less than 3 %Ca also contain hornblende. At the higher abundances of Table 3. Mean values ofTh, U, K and ratios for metamorphic Ca (>9%) and modal hornblende (>40%), and granitic rocks. plagioclase and hornblende are the main constitu 2 3 4 5 ents and their linear relationship noted above is disturbed. Th 9 21 20 10 17 Sr correlates with both CaO and N �O at the u 3 5 2 5 99% significance level, and shows a similar re K 1.52 3.17 3.09 2.58 3.79 lationship as N a in respect of modal plagioclase. Th/U 5.86 9.05 5.2 3.5 Hence it can be assumed that Sr substitutes for Th/k (E4) 6.06 8.55 6.7 3.9 VIK (E4) 1.00 1.45 1.5 0.95 1.2 both Na and Ca in that mineral. The association between Ca and modal horn l. Meta-igneous rocks, this study blende causes strong positive correlations be 2. Granitic gneisses, this study. tween the former and the elements of the Fe and 3. Levang gneisses, Killeen & Heier (1974). 4. Canadian Shield, Shaw et al. (1967). Mg associations. Al has a similar behaviour 5. Granitic rocks, Heier & Rogers (1963). patternto both Ca and Na, giving correlations at NORSK GEOLOGISK TIDSSKRIFT l (1978) Geochemistry of meta-igneous rocks 9

o 60 o + o o o o o o o

.. o .. l+- o o o + o

o 20 ++ • •

•

0+------.------.------.------.------,--- 0 2 4 8 lO % Na

Fig. 6. Modal plagioclase versus percentage Na for the meta-igneous rocks. Key as for Fig. 4, plus amphibolites (squares).

the 99 % significance leve! with Ca and at the with an increase in the Niggli value Si. Cr and Ni 95% level with Na, indicating the major element have a sympathetic relationship with each other association in plagioclase and hornblende. Cl (Fig. 8) and with Mg except in samples with would normally be expected to associate with particularly high Mg values. Hornblende-bearing the elements in mica, and its presence in this rocks have both high and low values of Cr and grouping cannot be readily explained. Ni (Fig. 8). The Ni/Cr ratio does not vary with increasing silica and, as stated by Taylor ·(1965), Magnesium and associated elements (Mg, V, is not a good indicator of fractionation. Co, Cr, Ni): the meta-igneous rocks are divided The Cr/FeO ratio splits the meta-igneous into two associations centred on Mg and Fe. rocks into two distinct fields which are not Although correlations between the two groups related to any contrasts in modal abundances. are commonly above the 99 % significance level for individual elements, V, Cr, Co, and Ni have Iran and the associated elements (Ti, Mn, Se,

a · more close association with Mg. Cluster Zn): this forms the closest association found analysis (Fig. 3) displays these two associations, within the meta-igneous rocks, except that be although it shows that Cr and Ni have no direct tween the individual rare-earth .elements. This relationship with Co and V except through Mg. relationship of elements has been previously The Co/Mg and V/Mg ratios show no variation noted by Sighinolfi (1971) in granulite facies 10 R. Beeson NORSK GEOLOGISK TIDSSKRIFT l (1978)

• (a)

• • lO •

• • 9

• • • o 8 o

o 7 •

o o o o 6 ... " o o + o 'l 5 o ... + ex:P o -t 4 ... Q:) o o o ·o+

+ 2 o

o l o 20 40 60 80 o/o Modol Plooloclose •

• {b) lO •

• 9

• o • • 8 o

o 7 " o te o o o 6 o +

5 +o o 00 o o 4 +g o o o 3 o lO 20 30 40 50 60 o/o Modol Horn blende

Fig. 7. Percentage Ca versus modal plagioclase (7a) and modal homblende (7b) for the meta-igneous rocks. Key as for Fig. 4. NORSK GEOLOOISK TIDSSKRIFf l (1978) Geochemistry of meta-igneous rocks Il

o o

60 o o o

z E R 40

o o

+ o 20

. o. o + o

20 40 60 80 100 120 140 ppm Cr

Fig. 8. Cr versus Ni for the meta-igneous rocks. Key as for Fig. 4.

metamorphic rocks in Brazil, and is common in (faylor 1965), decreases with increasing silica both igneous (e.g. Gruza 1965) and sedimentary content (Fig. 9). rocks (e. g. Condie 1967). Both Mn and Zn are The close association of Fe and related ele closely correlated with Fe2+, and appear to ments is considered to be a primary, igneous substitute for that element. feature, oarticularly as there is a contrast with With increasing silica the ratios Mn/Fe2+ and the element ratio behaviour in the meta Zn/Fe2+ exhibit no change, but the ratios Ti/Fe2+ sedimentary gneisses of this area (Beeson 1975). and Sc/Fe2+ both decrease. The Mn/Fe2+ ratio is The clear distinction of two groups of mafic normally constant during igneous fractionation. elements in the meta-igneous rocks is difficult to (e. g. Putman & Bumham 1963) and appears to explain for certain elements, particularly V. This remain so in the meta-igneous rocks. The element occurs chiefly as va+ and should enter Zn/Fe2+ ratio, however, decreases in the later Fe3+ position in the later stages of igneous stages of igneous fractionation (e.g. Papezik fractionation in igneous rocks, but V and Fe3+ 1965). are barely correlated here. This is the converse Ti/Mg, Fe2+/Mg, and Zn/Mg ratios show no of the findings of Field & Elliott (1974), who variation with increasing silica, whereas the established a complimentary increase of these Se/Mg ratio decreases. The Co/Fe2+ ratio de two elements in the amphibolitisation of basic creases sharplywith increasing Fe2+, despite the rocks in the southern Bamble Sector. Cr and V strong correlation between the two elements. have similar characteristics except in their ionic The Ti/Zr ratio, an indicator of fractionation radii, normally resulting in Cr fractionating in 12 R. Beeson NORSK GEOLOGISK TID SSKRIFT l (1978)

120 o o

o 80 � -+ � + 2 o r!:i ..... o o o i=

40 o o . + . o . + o o • o o+ + o . o o o + . o . o• o +-+ 0+------.------.------.------.------.------.------.----- 48 52 56 60 64 68 72 76 % SiOz

Fig. 9. The Ti/Zr ratio versus percentage Si02 for the meta)igneous rocks. Key as for Fig. 4.

the more basic rocks. In view of their similar tion in modal abundance which is controlled by distributions here, this may explain the Mg-Cr-V the major element chemistry (see above). association in the meta-igneous rocks. The mean values of the major elements in the meta-igneous rocks are comparable with those of intermediate and acid rocks compiled by Nockolds (1954). As the meta-igneous rocks Discussion have a wide compositional range it is difficult to make direct comparisons, but dacites and quartz lgneous characteristics of the meta-igneous diorites have similar cornpositions (fable4). The rocks N�O + K20 contents of meta-igneous rocks are Several features combine to justify the classifi similar in relation to Si02 for the majority of acid cation of the above described lithology as being and intermediate volcanic rocks (Fig. 10). meta-igneous. It is an individual rock suite In contrast, metasedimentary gneisses from which can be recognised in the field, and the Songe-Ubergsmoen area have values dis contrasted with other lithologies of previously similar to igneous rocks, particularly for MgO, determined origin, e.g. the metasedimentary CaO and N�O (fable 4). In addition, major and gneisses which contain meta-graywackes, trace elements show fractionation trends charac -pelites, and -quartzites (Beeson 1975). The teristic of known igneous models, e.g. N�O + 2 2 2 individual members of the meta-igneous rocks K20, Se/Y, Ti/Zr,Cr/Fe +, Ti/Fe + andMn/Fe +. have sharp contacts with other lithologies which are parallel to the foliation. A.ffinities of the meta-igneous rocks The lithology constitutes a chemical con tinuum from basic 'amphibolitic' rocks to rocks The geochemistry of the meta-igneous rocks containing up to 78 % silica. The continuum is suggests that the element concentrations have reflected in the observed mineralogy, in which undergone little modification during the high the four essential minerals - quartz,plagioclase, grade metamorphism. Consequently compari biotit�<_ and homblende - all have a wide varia- sons can be made with modem equivalents with NORSK GEOLOGISK TIDSSKRIFf l (1978) Geochemistry of meta-igneous rocks 13

some confidence. The meta-igneous rocks dis Table 4. Comparison of the major-element compositions of play dose affinities with several tholeitic rock meta-igneous rocks (l) and metasedimentary gneisses (2) of the Songe-Ubergsmoen area with charnockitic gneisses (3, suites, e.g. the Scottish Tertiary Province (Kuno Cooper & Field 1977), and averages for dacites (4) and quartz- l968fF'ig. lO. Some high silica rocks do not fit diorites (5, Nockolds 1954). directly onto the tholeitic trend, and it is suggested that these are clastic rocks of which a 2 4 5 proportion of the material was derived from an igneous source. An example of such sedimentary SiO, 66.12 70.05 68.35 63.58 66.15 Iithologies has been recorded by Kuno (1968), AI,O• 14.63 13.07 13.83 16.67 15.56 Ti02 0.73 0.74 0.53 0.64 0.62 and given in Fig. 10. Fe.Oa 2.50 1.37 6.01 2.24 1.36 FeO 2.67 3.61 3.00 3.42 MgO 2.40 2.72 1.90 2.12 1.94 Significance of this study to the Bamble CaO 3.99 2.44 3.64 5.53 4.65 Sector Na.O 3.72 2.03 4.67 3.98 3.90 K,O 1.83 2.57 0.47 1.40 1.42 The southem Bamble Sector is an excellent MnO O.o7 0.06 0.09 0.11 0.08 region for the study of metamorphic processes. P .Os 0.17 0.15 0.12 0.17 0.21 Detailed geological mapping has defined the nature of the amphibolite and granulite facies metamorphism and a transitional zone with intense K-feldspathisation. The meta-igneous a reference against which the rocks of adjacent rocks of the Songe-Ubergsmoen area Iie adja areas can be compared to determine high-grade cent to the zone of K-feldspathisation ånd are metamorphic processes. apparently Jittle affected by this process. Con Of particular interest to the Bamble Sector is sequently this lithology exhibits igneous charac the marked similarity for the majority of ele teristics which have not been substantially modi ments between the meta-igneous rocks and the fied by subsequent metamorphism, and provides chamockitic gneisses of the adjacent granulite

···· ·· ···· ··· ·····

8 :\: ///�:��----,,, ALKALINE )."" ·· · FIELD BASALT FIELD · . // \ ·· \ . .. . . · // \ / / ··......

. · - 6 // ·· . � . . · ..- "'" .· ...... ·· . � ... - z .. . · .. �.",- . . ·· · .. ,.,.. . _ .... ""' __ ...... -- . - · 4 - · -- .. . - ·· ...... _ .... . _ ··· . _,., THO LEITE ·· . · . ..- "' _ ·· FIELO ...... - ·· .. ··

· .. ··

2+--.------,------�------.------.------r------50 55 60 65 70 75 % Si02

Fig. JO. Percentage (Na.O + K,O) versus SiO, for the meta-igneous rocks (after Kuno 1968). Represented are the fields ofthe tholeitic series of the Scottish TertiaryP rovince (dashed line) and geosynclinal deposits from Japan (dotted line). The mean values for the igneousrock types compiled by Nockolds (1954) are given as diamonds: l - diorite, 2 -andesite, 3- dacite, 4- rhyodacite, 5 - rhyolite, 6 - quartz diorite. 14 R. Beeson NORSK GEOLOGISK TIDSSKRIFT l (1978)

CHARNOCKITE TREND OF COOPER ( 1971)

Fig. 11. ACF diagram for the amphibolite facies showing the composition of the meta-igneous rocks (after Winkler 1967).

facies. This is well illustrated by the ACF Acknowledgements. - This work was carried out with the diagram (Fig. 11), in which the fields of both assistance of an NERC Studentship Award at the Department rocks are identical. Consequently both rock of Geology, University of Nottingham. The author thanks Professor the Lord Energlyn for allowing the use of de types may well have had the same parentage, partmental facilities and equipment, and Drs. R. B. Elliott and and subsequent metamorphic processes have P. K. Harvey for supervision. The author is also indebted to not affected the abundances of Al, Ca, Mg, Fe, the Director of the South African Geological Survey for the use of facilities in the preparation of the p aper, and to Dr. and other elements. C. Frick for criticising the manuscript. Comparisons are drawn by Cooper & Field June 1976 (1977) between the chamockitic gneisses and Fe Revised November 1977 rich rapakivi suites of the Archaean in the northem hemisphere. However, the rapakivi intrusives are Rb-rich with relatively low K/Rb Appendix ratios, whereas the chamockitic gneisses and the meta-igneous rocks are predominantly poor in Rb. Consequently this comparison may not be l. Optical determinations valid. The modal analyses were made by the point counting method covering the whole of the thin Relationship to other metamorphic terrains section on a 0,3 x 0,6 mm grid. This totalled approximately 1500 counts per section. The evidence given above substantiates a model Plagioclase compositions were determined us of isochemical metamorphism for the meta ing the Michel-Levy technique on random igneous rocks, and hence the geochemical para grains,making 6 determinations per thin section. meters are original. The majority of elements have similar concentrations to that of estimates of the earth's crust. However, Rb and U, two elements important to. the study of high grade 2. Analytical methods metamorphic terrains, are depleted in relation to Major and trace elements were determined by X associated elements. As both metasedimentary ray fluorescence spectrometry using a Philips and meta-igneous rocks have high K/Rb ratios, it PW 1212 spectrograph. Major-element analysis can only be concluded that this is an original was carried out on fused glass discs according to characteristic of the Bamble Sector in this area. the method described by Harvey et al (1973). · NORSK GEOLOGISK TIDSSKRIFT l (1978) Geochemistry of meta-igneous rocks 15

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