GEOCHRONOLOGIC INVESTIGATION OF THE METAMORPHIC TERRAIN OF SOUTHWESTERN NORWAY

PAUL PASTEELS & JEAN MICHOT

Pasteels, P. & Michot, J.: Geochronologic investigation of the metamorphic terrain of southwestern Norway. Norsk Geologisk Tidsskrift, Vol. 55, pp. 111-134. Oslo 1975.

The granulite facies grading into amphibolite facies metamorphism in Roga­ land and Vest-Agder is Dalslandian, i.e. about 1,000 m.y. old, as the anortho­ sites and anorthosite kindred rocks outcropping in the same region. This conclusion is based on the 1,000 m.y. figure recorded by several monazites, as the zircon recovered from augengneisses and granitic gneisses in general. Higher apparent ages are given, however, by the zircon extracted from typical metasedimentary rocks, as most total-rock Rb-Sr systems what­ ever. The following interpretation is given to those results: 1,000 m.y. ago, the metamorphic activity was at its climax or had reached its waning stage. The higher figures would correspond to pre-metamorphic events, or are geo­ logically meaningless 'mixed ages'. They cannot be considered as a proof of a polycyclic evolution for the considered crustal segment. The zircon from metasediments has not been 'reset to zero' by the granulite facies metamorphism. This was the case only in rocks of granitic com­ position. A distinct phase of igneous or metamorphic activity seems to have oc­ curred some 1,200 m.y. ago.

Paul Pasteels, Laboratorium voor Geochronologie, Vrije Universiteit Brussel, Belgium. Jean Michot, Laboratoire de Mineralogie-Petrologie, Universite libre de Bruxelles, Belgium.

Geological outlook The investigated region is part of the Precambrian basement of south Nor· way and includes granulite facies and amphibolite facies rocks. The former have been the subject of various publications of P. Michot, some in col­ laboration with J. Michot (P. Michot 1956a, b, 1957, 1960, 1965, 1969, J. Michot & P. Michot 1969). A salient feature of this 'deep catazonal' realm is the presence of an important igneous complex composed of anorthosites and associated rocks: norites, monzonorites (jotunites), and mangerites. The present paper is devoted to the gneisses adjacent to this South Rogaland lgneous Complex. It complements an earlier report on the South Rogaland Igneous Complex itself (Pasteels, J. Michot & Lavreau 1970). According to J. & P. Michot (1969), the succession of geological events recognizable in the field is the following: Recumbent folding about N-S trending axes. Emplacement of the Egersund-Ogna anorthositic massif. 112 P. PASTEELS & J. MICHOT

ø f!}JJ c. l,r· '•,

.,,. .

�-:.·. _ -· caledonian boundary 1[1 JIX>

VEST-AGDER

Fig. l. Sketch map with sample location. Eg-Og: Egersund-Ogna, H-H: Håland-Hel­ leren, Bj-8g: Bjerkrem-8ogndal, Å-8: Ona-Sira, G: Garsaknatt, H: Hidra massifs, F: Farsundite. The hypersthene boundary to the east: after Dahlberg 1969.

The marginal noritic intrusion of Lakssvelefjellet-Koldal was emplaced between this massif and its gneissic country rock. Migmatitization of part of this body produces locally same peculiar rock types (zone Puntavoll­ Lien). A second catazonal tectonic phase consisting of recumbent falding along E-W trending axes is recognized. At the same time the emplace- GEOCHRONOLOGIC INVESTIGATION IN SW NORWAY 113

ment of a third magmatic unit, the Bjerkrem-Sogndal lopolith started by the settling of an anorthosite layer, followed by rhythmically altemating leuconorites and banded norites. The deep catazonal evolution ended with a third tectonic phase, with E-W trending vertical axial planes ( towards the S-E, this direction changes progressively to N-S). The above mentioned magmatic units were involved, according to J. & P. Michot, in an anatectic process, the 'basic palingenesis' which gives rise to a leuconorite ichor and a plagioclasic restite (Håland-Helleren massif). The evolution of the Bjerkrem-Sogndal lopolith proceeded with the occurrence of a monzonoritic phase, ending with a mangeritic phase. Uplift and erosion occurred soon afterwards. The gneissic cover which is the subject of the present paper consists of a 'Chamockitic Series' composed of migmatitic gneisses and pyroxenic grana­ feis with concordant pegmatite bodies, and a metasedimentary sequence (the Gjesdal Series) including quartzites, diopsidic gneisses, kinzigitic gneisses with gamet, cordierite and sillimanite, silicate-bearing marbles (L. Franssen pers. comm. 1972), and granitic gneisses. Metaigneous material is probably predominant within the chamockitic series. A few samples have been collected outside this area of granulite facies metamorphism. In the Strand district N-E of Stavanger, amphibolite facies porphyroblastic and aphanitic layered gneisses occur. '(hey are more or less affected by Caledonian diaphtoresis and, under the Caledonian 'discon­ formity', strongly mylonitized. To the east, in the Flekkefjord-Feda region, more or less similar rocks are encountered, but generally exhibit a granulite facies metamorphism and are free of diaphtoresis. Only ane rock belonging to the amphibolite facies has been collected east of the hypersthene boundary mapped by Dahlberg (1969) (Fig. 1). According to Falkum (1966), a large part of the meta­ morphic sequence may be of volcanic origin in this eastem area. The same author considers the Feda augengneiss to be an old basement. However, for the more or less similar rocks outcropping in the Bamble district, Touret (1968, 1969) has proposed a mechanism of formation involving alkaline solutions percolating during the waning stage of metamorphism, at the favour of a pressure release.

Previous geochronological investigations From the data obtained at an early stage of this study, tentative conclusions were drawn which may now be considered again in the light of many more results (J. Michot & Pasteels 1968, 1969a). The climax of the Dalslandian metamorphism has been tentatively dated at 1,000 to 1,050 m.y. aga. Its conclusion, 950 ± 20 m.y. ago, is marked by the emplacement of late-tectonic intrusives (massive and gneissic mangerites). Later on, it was recognized that the farsundite ( eastem facies, ane sample being erroneously assigned to the western 'melanofeldspathic' facies) is es- 114 P. PASTEELS & J. MICHOT

Fig. 2. Pa66/F. Gamet gneiss. Subrounded faceted zircon of the type most commonly encountered. Stereoscan picture. Scale X 1900.

sentially of the same age as the mangerites or very slightly older (Pasteels, J. Michot & Lavreau 1970). A similar 950 ± 20 m.y. figure has also been obtained on the monzonorites from the Bjerkrem-Sogndal massif, and on the Skjelset adamellite (unpublished). Mention must be made, however, of the much higher Rb-Sr total-rock ages obtained by Versteeve (1971): 1,420 ± 100 m.y. for the northern part of the gneissic cover, and 1,620 ± 100 m.y. for its south-eastern part (recal­ culated with J.. S7Rb = 1.39 X 10-uy-1).

Experimental techniques

The techniques used are essentially the same as described by Deutsch et al. (1965), to which some evolutionary changes were made. For example, the dithizone purification of lead and hexone extraction of uranium were re­ }Jlaced by a simple ion-exchange procedure in the course of this investiga­ tion. Borax was used throughout for zircon dissolution but, for monazite, different techniques have been experimented with (H2S04, or borax, or a mixture HC104-HF). GEOCHRONOLOGIC INVESTIGATION IN SW NORWAY 115

Fig. 3. Pa66/J. Gamet gneiss. Corroded zircon or coalescent faceted zircons. Stereoscan picture. Scale X 950.

Except in a few instances, an electron multiplier was used for the lead isotopic composition runs. Comparison made subsequently, using a Farady cup as a collector, indicates that some of the t207 /206 values reported here may be too high by some 10 to 20 m.y. To all U-Pb ratios and t206f238 values, a 2% error can be assigned. Except for the few earlier reported mea­ surements (J. Michot & Pasteels 1968), the activation technique with silicagel or zirconium silicate has been used for the mass spectrometry of lead.

Zircon morphology

The zircon from granulite facies gneisses shows some special morphological characteristics: it is often subrounded or oval-shaped, with many facets which are vaguely recognizable under the ordinary microscope, but easy to observe using a scanning electron microscope (Figs. 2 and 3). These features among others are sometimes considered as a proof of metamorphic origin (Pidgeon & Aftalion 1972). In some of the zircon populations investigated here, a simple internal zoning combining prism faces with one dipyramid (presumably 111) is displayed by many grains, suggesting a magma tie origin. - Table l. lsotopic data and apparent U-Pb ages. - 0\

Concentrations Sample Mineral Lead isotopic composition Apparent ages :-c (ppm) "C > u til Ph rad(l) 204 206 207 208 t206/238 t207/235 t207/206 ...., ti1 Augengneisses from central Rogaland ti1 Pa69/A ZR200 313 43.2* 0.2232 100 10.190 16.149 870 891 942 ± 30 f;; Ro ZP400 588 87.0* 0.2239 100 10.318 19.09 892 905 934 ± 30 :--- 154 Sphene 28.9 4.72* 1.939 100 34.93 86.25 891 927 1,012 ± 176 -� Pa69/B Za 2,260 258* 0.338 100 11.658 16.765 735 772 881 ± 45 () :I: Zb 2,890 237* 0.979 100 20.57 44.69 514 560 753 ± 45 o Monazi te 2,840 2,940 0.0859 100 8.328 625.8 974 974 973 ± 20 ....,

South Rogaland, granitic gneisses Pa69/J ZR270a 2,546 368 0.0622 100 8.045 7.012 912 936 990 ± 10 ZP400b 2,716 352* 0.1591 100 9.320 12.864 802 847 963 ± 10 Monazite 6,625 5,793 0.0728 100 8.303 484.8 1,019 1,020 1,021 ± 15 Pa66/K Za 932 127.1 0.0697 100 7.932 12.96 826 854 924 ± 20 Zb 3,070 308* 0.2393 100 10.029 19.97 614 661 824 ± 30 Pa66/C Zircon { 394 60.3 0.1166 100 8.972 13.74 { 924 957 1,030 ± 15 (repeated) 396 61.3 934 967

South Rogaland, gamet gneiss Pa66/J Za 1,144 172.7 0.0403 100 7.826 10.091 919 950 1,018 ± 20 Zb 1,426 184.9 0.0504 100 7.748 8.677 810 849 951 ± 15 Zc 1,516 181.4 0.0743 100 7.952 9.776 750 792 910 ± 30 Pa69/C ZR200 769 135.9 0.0525 100 8.614 6.778 1,091 1,124 1,184 ± 30 ZP400 983 160.0 0.0809 100 8.828 7.167 1,018 1,056 1,134 ± 45 Pa66/F ZR150 192.1 32.9 0.1316 100 10.192 18.00 983 1,086 1,298 ± 20 ZP270 { 634 106.8 0.0536 100 8.997 11.125 { 1,004 { 1,094 1,276 ± 25 (repeated) 632 106.8 1,008 1,096 South Rogaland, banded gneiss Pa69/H Zircon 526 107.9 0.0360 100 9.111 11.478 1,192 1,254 1,362 ± 15

South Rogaland, quartzite Pa66/l Za 219 41.1 0.1229 100 9.829 19.33 1,064 1,123 1,239 ± 15 Zb 220 41.6 0.2203 100 11.119 24.18 1,048 1,112 1,215 ± 35

South Rogaland, 'anatectic synkinematic granites' Pa69/E Za 1,216 169.0 0.0277 100 7.957 5.524 977 1,017 1,104 ± 15 0.1575 100 9.524 12.591 715 796 1,030 ± 30 o Zb 1,822 208* m Mo nazite 3,696 3,210 0.0966 100 8.504 525.1 950 960 980 ± 35 o n Pa69/D Za 2,137 212* 0.1434 100 9.058 10.143 637 711 951 ± 20 ::X:: � Zb 2,858 229* 0.3123 100 11.100 18.083 515 577 831 ± 35 o Mo nazi te 6,453 3,556 0.0974 100 8.604 283.0 998 1,001 1,006 ± 25 z o Pa66/D Za 745 127.4 0.1037 100 9.061 15.85 1,001 1,036 1,109 ± 15 t"" o Zb 896 141.2 0.1237 100 9.286 15.38 936 984 1,092 ± 15 o..... Zc 1,701 221* 0.784 100 18.03 48.41 724 765 882 ± 60 n - 187 ..... Allanite 169.9 778* 2.027 100 35.97 2,216 938 956 992 + 165 z < m South Rogaland, augengneisses Cll ...., Pa66/L Za 574 100.8 0.1199 100 9.006 18.73 1,009 1,016 1,029 ± 40 - Zb 1,298 147.9* 0.4303 100 13.281 27.55 678 759 1,005 ± 45 o � Pa69/l Za 553 93.0 0.0502 100 8.050 16.83 964 988 1,041 ± 25 ..... Zb 945 148.0 0.0376 100 7.729 20.51 876 913 1,001 ± 25 o z ..... Vest-Agder, granitic gneiss z Pa66/R Za 826 187.5 0.0439 100 9.156 11.456 1,304 1,374 1,484 ± 10 Cll Zb 1,094 225 0.0273 100 9.295 12.655 1,178 1,270 1,430 ± 15 � Zc 1,395 241 0.0472 100 9.270 11.912 1,021 1,132 1,362 ± 15 z Zd 1,866 243 0.0799 100 9.648 10.823 797 954 1,343 ± 20 o � (l) For all samples following correction lead has been used: 1 18.6, 15.7, 38.9, except for the samples marked with *. For these latter, the following leads have been used: for Pa69/A ZR200, ZP400, sphene: lead from Pa69/A K-feldspar (1 17.49, 15.57, 37.02); for Pa69/B Za, Zb: lead from � Pa69/B K-feldspar (1 17.97, 15.64, 37.62); for Pa66/L Zb: lead from Pa66/L K-feldspar (1 16.93, 15.34, 36.26); for Pa66/D Zc, allanite, Pa69/D Za, Zb, Pa69/E Zb, Pa66/K Zb, Pa69/J Zb, lead from Pa66/D K-feldspar (1 17.28, 15.45, 37.69)...... -.l 118 P. PASTEELS & J. MICHOT

This zoning is often truncated by the outer faceted surface, which seems to imply metamorphic corrosion. In other instances clear overgrowth is ob­ served around the zoned core. This outer shell is much more important in volume than the zoned core itself in some cases, for example in the Feda and Liland augengneisses. In some other rocks clear, subrounded zircon is also present, which may either represent an old detritic component or else, a metamorphic genera­ tion. The following combinations of features have indeed been noted previ­ ously in the case of zircon undoubtedly generated under granulite facies conditions: coarse, uranium-poor, transparent and colourless, and, concem­ ing the extemal shape, either subrounded with facets, or else elongated but with subrounded and faceted terminations (Silver et al. 1963, Silver 1969, Pidgeon & Bowes 1972). Such zircon may not be easily distinguishable from detrital zircon of low radioactivity, remoulded by metamorphism.

Interpretation of the data The analytical data and apparent ages relative to U-Pb systems are listed in Table l, and illustrated by Figs. 4, 5, 6, 8. The Rb-Sr data are presented in Table 2 and Fig. 7. In Table l the following symbols are used: Za, Zb, Zc, . . . . represent zircon fractions recovered from a rock sample, of increasing magnetic susceptibility (if only two fractions have been analysed, Zb is always the most radioactive pure fraction which could be recovered in sufficient amount); R200: retained by the 200 mesh sieve, etc ... Calculation of the best straight line was done using the York 1966 programme, adapted for this purpose by S. Deutsch. No correlation factor between X and Y is con­ sidered. (It has been verified, however, that the introduction of an arbitrary correlation coefficient yields essentially the same straight line as without correlation. ) The constants used are the following: A:238U = 1.537 X 10-wy-1 A:285U = 9.72 X 10-10y-1 238Uf235U = 137.8 A:87Rb = 1.39 X 10-11a-1 The quoted data obtained by other workers have been recalculated, if necessary, with these constants. It should be kept in mind that the extrapolation of a 'true' age by con­ sidering a suite of more or less discordant fractions is valid only if these latter represent a cogenetic assemblage, which is by no means evident for the zircon from metamorphic rocks. Three main are as will be considered in turn: The amphibolite facies zone of central Rogaland, Strand district. The granulite facies zone of south Rogaland. The amphibolite facies zone of southem Vest-Agder. GEOCHRONOLOGIC JNVESTIGATION IN SW NORWAY 119

The amphibolite facies zone of central Rogaland, Strand district In this amphibolite facies zone, the most widespread petrographic type is the augengneiss, constituting a rather important mass to the north, but which, further south, is interlayered with amphibolites, leucogranitic and quartzo­ feldspathic gneisses. Of the two rock samples investigated, one represents a rather exceptional massive variety where the gneissic structure completely disappears (Pa69/ A). It could therefore be in fact a true intrusive granite emplaced within augen­ gneisses. In the field, it was not possible to observe the relation between this rock and the common augengneiss type. The zircon population is homo­ geneous, similar to that of intrusive granites. The second rock (Pa69/B) is

_ gneissic and more leucocratic. (Sample descriptions and locations are given in an appendix.) All zircon fractions and a monazite extracted from Pa69/B define a straight line on the Concordia diagram (Fig. 4). (The fact that the point relative to the sphene Iies somewhat out of this line is probably not signifi-

QW,------.

Ol5

0,10

1,5 1.0 207Pbrad/235u

Fig. 4. Concordia diagram for zircon, sphene and monazite from augengneisses (M refers to monazite, S to sphene). Circles: central Rogaland (amphibolite facies) samples, open: Pa69/B, black: Pa69/A. Squares: Rogaland-Vest-Agder (granulite facies) samples, open: Pa66/L, black: Pa69/l. Jmprecise data under brackets. 120 P. PASTEELS & J. MICHOT cant, considering the rather large experimental error on the ratio 207Pb rad/ 206Pb rad in this case.) The straight line computed from the five samples (sphene excluded) yields an upper intercept of 974 m.y., in perfect agreement with the concordant ages given by the monazite, and a lower intercept of 287 m.y. This latter seems to be geologically meaningless, and its interpretation in terms of con­ tinuous or episodic loss of lead will be considered in another section. The age indication fumished by the upper intercept is similar to that obtained on late tectonic intrusives in south Rogaland. One of the two rocks, Pa69f A, may in fact be a late tectonic intrusive, but this cannot be the case for Pa69fB, interlayered with other gneisses. lts zircon exhibits what seem to be metamorphic overgrowths around euhedral zoned cores. This is suggestive of a magmatic, probably volcanic or sub­ volcanic origin for the rock. The abundance of amphibolite in the sequence, and the absence of quartzite, peraluminous rocks and marble, is perspicuous. The 974 m.y. figure thus obtained seems to correspond to the age of the Dalslandian metamorphism, possibly its waning stage (Touret 1969). The zircon does not appear to be of metamorphic neoformation but nonetheless yields a metamorphic age.

The granulite facies zone of south Rogaland This region of granulite facies metamorphism has been investigated in greater detail and a larger number of samples, representing various rock types, have been collected. These samples originate from two main areas, the classical 'nappes region' north of the Egersund-Ogna massif, and that bordering the South-Rogaland lgneous Complex to the east respectively. However, because of the similarity of mineral facies on the ane hand and the similar behaviour of the isotopic systems on the other, there is no reason to consider separately the northem and eastem outcrops. On first inspection of the data, it appears that many zircons yield apparent ages higher than the 1,050 m.y. figure which would correspond to the metamorphic climax according to J. Michot & Pasteels (1968). Thus we must consider either that this conclusion is erroneous or only partially correct (in the sense, that besides the 1,050 m.y. old phase still older anes exist), or that same zircons have kept an isotopic record of their pre-metamorphic history. The data do not allow in general the definition of individual chords for every sample. Analysing a larger number of fractions would have represented considerable work with little reward, since in many instances the total spread of radioactivities and of 'amounts of disturbance' is toa narrow. Thus in order to define good straight lines we must group the samples as logically as possible.

The 1,000-1,050 m.y. old metamorphic event. - Despite the restrictions made above, the existence of a 1,000-1,050 m.y. old metamorphic event is in fact confirmed by the new data when these are carefully considered. GEOCHRONOLOGIC INVESTIGATION IN SW NORWAY 121

Q25

0,20

700

600

500

400 0,150

1,50 1,75 1,0 2.5 3,0 3,5

Fig. 5. Concordia diagram for South Rogaland and Vest-Agder metasedimentary (and metaigneous) gneisses. All points represent zircon fractions, except that marked M (monazite). Open circles: granitic gneiss, amphibolite facies. Black circles: granitic gneiss, granulite facies. Black squares: metasedimentary rocks (q: quartzite, g: garnet gneiss, b: banded gneiss).

Arguments can be found indeed to discard or consider separately the higher figures. First, the argument put forward by J. Michot & Pasteels (1968, 1969a), i.e. that if hypersthene-'bearing late-tectonic intrusives were emplaced 970 m.y. ago, the climax of granulite facies metamorphism cannot be much older, remains essentially valid. The three analysed monazite samples yield concordant or nearly con­ cordant figures very dose to 1,000 m.y. The 'best' age indication is that provided by Pa69fJ monazite: 1,020 ± 20 m.y. The two other samples Pa69/D and Pa69/E may be somewhat younger, 1,000 ± 25 m.y., though the difference Iies within error limits. The corresponding extrapolated zircon ages either confirm or are signifi­ cantly higher than the figure given by the monazite. Samples Pa69/J illustrate the first case, the two analysed zircon fractions defining a 220-1,010 m.y. chord (Fig. 5). It is difficult to understand why, on the contrary, the four zircon fractions relative to the other two above-mentioned monazite-bearing rocks define a 300-1,161 m.y. chord instead (Fig. 6). This problem will be discussed below. 122 P. PASTEELS & J. MICHOT

The zircons extracted from the two porphyroblastic (or el astie) gneisses investigated, Pa66fL and Pa69fl (this latter typically representing the so­ called 'Feda augengneiss'), yield a four-point 75-1,034 m.y. chord (Fig. 4). We thus note that, as in the amphibolite facies area east of Stavanger (Strand district), the data yielded respectively by monazite (in general) and zircon, separated from augengneisses, are in good agreement, and thus seemingly point to the real age of an important metamorphic episode. The augengneisses either have the necessary composition for allowing zircon reequilibration and isotopic resetting, or, more probably, were the locus of alkaline fluid circulation which caused the development of K-spar porphyro­ blasts, and favoured the zircon reequilibration process. The zoned euhedral cores of the zircons may be remnants of the metaigneous(?) substrate on which the migmatitization proceeded. Besides the augengneisses and the above-mentioned Pa69/J granitic gneiss adjacent to the Feda augengneiss, other rocks contain zircon which point to rather similar ages, but either slightly lower or slightly higher. Sample Pa66/K represents a special case, being collected 5 m from the contact of the Bjerkrem-Sogndal lopolith norites. Thus it is quite conceivable a priori that the lead has been expelled from the zircon by thermal effects during the emplacement of this magmatic body. A two-point chord yields 264 and 964 m.y. intercepts (Fig. 5). The higher figure compares well with the 967 ± 12 m.y. obtained on the mangerites belonging to the same lopo­ lith, in accordance with the thermal effect interpretation. No other zircon suite has provided a 'primary age' (upper intercept) lower than 1,000 m.y. in the granulite facies zone (the zircon of the late tectonic intrusives ex­ cepted.)

Possible existence of a 1,100 m.y. old metamorphic event?- More delicate is the question of the zircons which seem to point to a slightly older age than 1,000 m.y. As we shall see, it is very clear that in same rocks, which are beyond any doubt metasedimentary, detrital zircon not reset to zero is present: then the extrapolated age is much older than 1,000 m.y. But when the extrapolated age is of about, say, 1,100 m.y., should we accept the reality of this age, or simply the presence of an old detrital component in subordinate amounts, which has, however, the effect of biasing the extra­ polated age towards higher values? A best fit line with 467-1,101 m.y. intercepts has been computed combin­ ing the data from Pa66fC (granitic gneiss) and Pa66fJ (gamet gneiss) (Figs. 4 and 5). The Feda augengneiss zircons have their representative points fall­ ing very dose to this line. Considering that the choice of the data which should be discussed together in order to derive an 'age' is arbitrary to same extent, it may not be evident a priori why the 1,100 m.y. figure thus obtained is not a 'real age' like the 1,020 ± 20 m.y. figure discussed previously. In fact, in this computation, the weight of the three zircon fractions extracted from Pa66fJ (gamet gneiss) is overwhelming. As we shall see, other gamet GEOCHRONOLOGIC INVESTIGATION IN SW NORWAY 123

gneisses from south Rogaland undoubtedly contain old detrital zircon, and this may be the case for Pa66/J too, in principle. There is therefore a strong suspicion that the 467-1,100 m.y. line results from an improper combination of data and has no geological significance.

U-Pb systems which have kept their 'isotopic memory'. - It has been observed that some rock types, typically metasedimentary, contain a detrital zircon keeping an isotopic record of its pre-metamorphic history in granulite facies conditions: quartzites, garnet-sillimanite gneisses, calc-silicate gneisses (Silver 1969). This observation, made in the Adirondack Highlands, is con­ firmed by the geochronological data obtained on Pa69/C and Pa66JF, garnet gneisses, Pa66ji, quartzite, Pa69JH, banded gneiss (Fig. 5). The t207f206 values are of about or higher than 1,200 m.y. The primary ages (if this notion makes sense, in other words: if we are not dealing with mixed populations of different ages) must be still higher and can thus hardly correspond to a metamorphic phase belonging to the Sveco-Norwegian cycle. In the present case it is not possible to extrapolate an upper intercept on the Concordia diagram; the experimental points relative to the different fractions of zircons separated from the same rock remain very dose to each other (this is well illustrated by Pa66fF). The two fractions Pa66JI, Za and Zb illustrate an unsuccessful attempt to split on the base of magnetic susceptibility a bulk zircon which is definitely not magnetic enough for that purpose; as a result, two practically identical fractions were obtained. It thus appears that the criterion 'nature of the rock' is enough to decide whether a given zircon suite is likely to yield a metamorphic age or not in granulite facies conditions. Examination of the zircons under the microscope on the other hand does not provide such a criterion. In practically every case, whatever the nature of the rock and isotopic behaviour of the zircon, ancient cores are observed, corroded and refaceted or surrounded by clear outgrowths or overgrowths. Clear more or less oval-shaped zircon of low radioactivity is observed in augen- or granitic gneisses where there is no evidence of isotopic memory but also in metasedimentary rocks, as for example the quartzite Pa66JI. The uranium-poor, clear, subrounded zircon of quartzite Pa66/I is probably detrital (Poldervaart 1955) though it resembles zircon generated in the granulite facies environment. Only in the banded gneiss Pa69/H does the zircon show little evidence of morphological readjustment, which also seem­ ingly corresponds to its relatively high apparent ages. These observations would correlate with the particular composition of this rock, poor in alkalies, under the likely assumption that permeation with alkaline solutions would favour zircon readjustment (Malcuit & Heimlich 1972).

1,200 m.y. old event. - The three samples, Pa69/E, Pa69/D and Pa66fD, which bear the general features of what is often considered to be 'synkine­ matic anatectic granites', were kept outside the discussion. The zircons from 124 P. PASTEELS & J. MICHOT

Fig. 6. Concordia diagram for zircon, monazite (M), allanite (A) from south Rogaland 'synkinematic granites'. Circles: Lakssvelefjellet birkremite, squares: Søyland (open: Pa69/E, black: Pa69/D). Imprecise data under brackets.

these rocks yield extrapolated 'primary ages' of about 1,200 m.y., a figure which is confirmed to some extent by the Rb-Sr data (Fig. 6). According to the concept of P. Michot, which considers the investigated crustal segment to be monocyclic and monogenic, the 1,200 m.y. event should correspond to metamorphism and deformation in the deep catazone. This would imply a considerable duration for the evolution at a deep crustal level. After a more careful examination of the geochronological data, another interpretation will be proposed. Our feeling, however, is that more detailed investigations (not only geochronological) are necessary in order to solve this controversy. The Lakssevelefjellet birkremite Pa66/D is considered by P. Michot (1960) as the ultimate stage of a progressive anatectic mobilization, practically pure anatectic melt being injected along the fold axes of the second tectonic phase recognized by him. An incipient stage of the same anatexis would be re­ presented by the 'migmatitized norite' Pa66fC. However, such interpretation of the field observations is hardly reconciliable with the geochronological data, the zircon from Pa66/C pointing to a lower age than that of Pa66fD. The anatectic origin of both rocks is currently being reexamined by one of the authors (J. Michot). GEOCHRONOLOGIC INVESTIGATION IN SW NORWAY 125

Since the birkremite is rather poorly exposed, most outcrops being rather severely weathered, another more or less similar rock was selected in the Søyland nappe a little further to the north, for a more detailed investigation. It appears as a lense or pod of rather homogeneous and massive granite, the adjacent rock being a garnetiferous adamellitic gneiss. All zircon data from this second outcrop of 'synkinematic granite', com­ bined (two rock samples, four analysed fractions), yield a 300-1,161 m.y. chord (Fig. 6). A five points Rb-Sr isochron gives 1,286 ± 48 m.y. (2a) with

Ri = 0.7168 (Fig. 7). This isochron age is somewhat uncertain: if sample Pa69 /F2 is discarded, under the assumption that it is not truly representative of the granitic pod (because of its low RbfSr ratio), 1,451 ± 35 m.y. is obtained with Ri = 0.7057 (isochrons computed with the York 1966 pro­ gramme). This higher figure is dose to those obtained by Versteeve (1971). The two relatively large samples from which zircon has been extracted contain monazite yielding 1,000 ± 25 m.y. For the birkremite, considered as the typical 'synkinematic granite' of the region, an extrapolated zircon age is obtained, of 1,222 m.y. One isolated total rock sample yields 1,140 m.y. with Ri = 0.717, 1,260 m.y. with Ri = 0.706. The data from the allanite may be kept outside discussion because the large amount of common lead alters the precision. At face value they would confirm the reality of the 1,000 m.y. old event.

Fig. 7. South Rogaland 'synkinematic granites'. Isochron diagram. TR: total rock, KF: potash feldspar, Q: quartz, Pl: plagioclase, Ap: apatite, Pyr: pyroxene. Stars: SØyland.

Circles with cross: Lakssvelefjellet birkremite. A: 1286 ± 48 m.y. Ri = 0.7168 ± 0.0022

(2a) MSWD = 0.7264. B: 1451 ± 35 m.y. Ri= 0.7057 ± 0.0024 (2a) MSWD = 0.0187. 126 P. PASTEELS & J. MICHOT

Table 2. Rb-Sr isotopic data.

Sample Concentrations (ppm) 87Sr/86Sr 87Rb/86Sr

Rb Sr

Pa69/D, total rock 193 96.3 0.8245 5.84 Pa69/E, total rock 225 136.6 0.8053 4.90 Pa69/F1, total rock 186 135.4 0.7872 4.01 Pa69/F2, total rock 73.5 152.0 0.7423 1.40 Pa69/F3, total rock 171 108.4 0.7996 4.59 Pa66/D, total rock 194 88.4 0.8205 6.43 Pa66/D, microcline 361 142.1 0.8385 7.45 Pa66/D, apatite 1.00 47.3 0.7391 0.062 Pa66/D, quartz + plagioclase 21.2 74.4 0.7575 0.845 Pa66/D, altered pyroxene 97.4 50.1 0.7948 5.67

Thus the general picture is that of the overprinting of a 1,000 m.y. old event on a substrate which has been generated same 1,200 m.y. ago, in not too well known geological conditions. The zircons from all the investigated 'synkinematic granites' are essentially similar to that of other rocks from the gneissic cover. Their rather high uranium content is noticeable. This leaves apen the question of the nature of the 1,200 m.y. event itself (magmatic? metamorphic?). If we refer to the relative chronology established by P. Michot we observe that the anatectic phase which supposedly yielded the birkremite is con­ sidered as contemporaneous with the second phase of falding (the birkremite occurring within the corresponding fold axes). At the same time or earlier (P. Michot 1960), the 'plagioclase magma' which gave rise to the Bjerkrem­ Sogndal lopolith is supposed to have begun its emplacement (this is mainly based on the concept of a horizontally injected magmatic intrusion, which thus should be concomitant with recumbent falding). The Bjerkrem-Sogn­ dal massif has been previously dated at 970 ± 20 m.y. ago (measurements, in progress, on the uranium-poor zircons from the monzonorites not only confirm this figure but would even tend to lower it somewhat). Thus, if the birkremite is 1,200 m.y. old, as seems to be the case, it cannot have been generated by anatexis at the time when the magma at the origin of the Bjer­ krem-Sogndal lopolith started its emplacement. The 1,200 m.y. figure may correspond to a pre-metamorphic event, like the still higher figures obtained by Versteeve (1971) by the Rb--Sr method. The birkremite and similar rocks are, conceivably, orthogneisses instead of synkinematic granites (metamorphosed volcanics or hypabyssal felsites, or even volcano-sedimentary rocks). The reason why these zircons were not reset 1,000 m.y. ago may be that insufficient radiation damage was accumu­ lated in the 1,200-1,000 m.y. time interval (it is well known that zircon with little damaged structure is highly retentive for lead). But this is a tentative explanation only. GEOCHRONOLOGIC INVESTIGATION IN SW NORWAY 127

Let us note, however, that a 1,160-1,200 m.y. age has been obtained by both the U-Pb and Rb-Sr methods, and interpreted as the age of the meta­ morphic climax in the Bamble sector (O'Nions & Baadsgaard 1971) (the metamorphism is there of amphibolite facies). But some of those results concem the hyperites (lens-shaped, concordant, metamorphosed gabbroic bodies) long considered as pre-tectonic. In a previous paper, dealing with K-Ar data, O'Nions, Morton & Baadsgaard (1969) locate the thermal maximum at 1,125 m.y. ago, and thus the possibility that the hyperites may date from the 'geosynclinal stage' apparently cannot be ruled out completely.

The amphibolite facies zone of southern Vest-Agder Only one rock has been investigated from this region of amphibolite facies metamorphism, and this only for comparison with the corresponding gran­ ulite facies rocks of similar composition. Sample Pa66jR, granitic gneiss of possible metaigneous origin, is indeed broadly similar except of course for its mafic constituents, to Pa66jC, Pa66jK, Pa69/J the zircons of which yield extrapolated ages of about 1,000 m.y. We are facing here a very different situation however. The experimental points define a smooth curve concave towards the Pb207radjU235 axis and not, as usual, a straight line. The com­ puted best straight line gives as intercepts, 256 and 1,486 m.y. and is prob­ ably meaningless (Fig. 8).

o,20

0.10

0.05

1,0 1,5 2/J 2,5 3,0 3,5 4,0 2!17Pbrac�/235u

Fig. 8. Concordia diagram for the zircons of the Lyngdal granite gneiss Pa66/R. A: single stage model (chord). B: two-stages model. 128 P. PASTEELS & J. MICHOT

Anyhow, the apparent age so obtained or the crude t207j206 apparent ages are obviously high compared to all other data discussed so far. Since it is well known that zircon does not in general lose all its lead during am­ phibolite facies metamorphism, we feel no necessity to postulate that the rock belongs to a pre-Sveconorwegian basement. Geologically speaking, there are three possibilities to consider (the nation of old basement being thus rejected): the rock is sedimentary and its zircon detrital, it is volcanic and its zircon formed by magmatic crystallization, or it is of mixed origin. Considering the morphology, colour and transparency properties of this zircon population, ane is tempted to favour the third assumption. This zircon has indeed predominantly 'magmatic' features, but same rounded 'detrital' grains are observed in the less magnetic fractions. Their presence would readily explain the distribution along a curved line of the experimental points (the two uppermost points being shifted accordingly to the right). It is quite possible that the few detrital grains present are of Svecofennian age (i.e. about 1,850 m.y. old). The alternate possibility, that this zircon population represents a cogenetic assemblage same 1,600 m.y. old, cannot be ruled out completely, however (Fig. 8). [The curved line B in Fig. 8 corresponds to a very simple theoretical model. Two episodes of lead loss are considered, occurring 1,200 m.y. and O m.y. aga (metamorphism, and 'dilatancy effect' and weathering) (Goldich, Hedge & Stern 1970). The lead retentivity is considered as a constant primary characteristic of each zircon fraction considered, which thus behaves in a similar way during each disturbing event. The latter, however, are more or less 'strong': a stronger event is supposed to have the combined effects of several weaker anes occurring immediately after each other. The curved line B corresponds to a metamorphic event three times 'stronger' (in that sense) than the ultimate loss at zero time. If the age of the disturbing meta­ morphic event is arbitrarily fixed at 1,000 m.y., a poor fit is obtained with the experimental data. This multiple-stage interpretation rests on a 'cogenetic assemblage' assumption, the primary age of the zircon being 1,580 m.y. A poor fit with the data is observed for a higher primary age, even when the other parameters (age and intensity of metamorphism) vary. That the bulk zircon is much older than 1,600 m.y. is thus unlikely.]

Significance of the lower intercepts on the Concordia diagram Same of the zircon suites investigated yield projected lower intercepts on the Concordia diagram which are higher than would be predicted by the continuous diffusion models. This is especially the case for the birkremite zircons (541 m.y.!), but also for those of the augengneisses Pa69/A and Pa69jB considered together (287 m.y.). In the latter case, a combination of continuous diffusive, loss, or loss at zero time, with episodical escape during the Caledonian metamorphism may, GEOCHRONOLOGIC INVESTIGATION IN SW NORWAY 129

theoretically at least, be considered as an explanation of the rather high lower intercept. However, for the birkremite, the indications relating to a possible Cale­ donian influence are not entirely convincing. [In south Rogaland, the highest points in the topography would represent remnants of the sub-Cambrian peneplain (Barth & Dons 1960). If this sur­ face was once covered by Paleozoi:c sediments affected by the same grade of metamorphism as those observed in the adjacent Caledonian subsided block, the depth of burial of the basement undemeath would have been consider­ able. Metamorphism itself is not observed in general. However, same samples (Pa66JC and Pa66/D) from the Lakssevelefjellet exhibit same signs of alteration, perhaps metamorphic. The K-feldspar shows microcline twinn­ ing and the orthopyroxene, barely recognizable as such, is partly replaced by a mineral resembling biotite.] A Rb-Sr mineral investigation of this rock has been undertaken in order to verify whether radiogenic strontium migration had occurred between minerals in Caledonian time (Fig. 7). It is observed that mineral phases did not act as closed systems after the Sveconorwegian metamorphism, but it is impossible to say whether radiogenic strontium or rubidium migration oc­ curred during Caledonian time or more recently.

Comparison with the Adirondacks Comparison of south Norway with the Adirondacks reveals striking simili­ tudes. The U-Pb method, applied to zircon, allows the attribution of a 1,020 ± 10 m.y. age to the Highlands granulite facies metamorphism, which is thus strictly contemporaneous with that recognized in south Rogaland. A 1,070 to 1,100 m.y. old episode, without definite equivalent in Norway, appears to exist also (Silver 1969). In the adjacent Lowlands of lesser metamorphic grade, 1,160 to 1,200 m.y. old metamorphic and igneous events are recorded (Silver 1969). An identical situation is observed in south Norway (Bamble sector: O'Nions & Baadsgaard 1971). Should we consider that granulite-facies metamorphism is superimposed on an earlier, amphibolite facies one? Anyhow, 1,130 m.y. old (measurements on zircon) charnockitic gneisses are considered by Silver as premetamorphic and called by him 'orthogneisses'. This reminds us of the birkremite. Anatectic granites of identical age are known in the adjacent amphibolite facies domain (Bickford & Turner 1971). Finally, the Rb-Sr total-rock isochron method yields apparent ages in the 1,400 m.y. range in the Adirondacks (Spooner & Fairbairn 1970)- and in Rogaland (Versteeve 1971). It has been shown by Gray & Oversby (1972) that, in granulite facies metamorphic conditions, total rock samples are not always rehomogenized on the regional scale for Rb and Sr. The total rock isochron age is not necessarily that of metamorphism. 130 P. PASTEELS & J. MICHOT

Conclusions In south Rogaland, the granulite facies metamorphism, as well as the emplacement of anorthosites and related rocks, took place or ended 1,000 m.y. ago. It was not possible to determine the duration of this metamorphic episode, but there is no geochronologic indication that it has been very long. In that sense, the classical notion of 'very long duration of Precam­ brian orogenies' is not confirmed. The 1,200 m.y. apparent age recorded by some 'anatectic' chamockites remains of uncertain interpretation. It may correspond to a pre-metamorphic event (volcanism?), but this is not the only possibility. The still higher figures obtained, in the range 1,400 to 1,600 m.y. can tentatively be interpreted in terms of premetamorphic ages or else represent geologically meaningless 'mixed ages'. There is no convincing geochronologic indication of the exist­ ence of remnants of an old basement in the area investigated. Under granulite facies conditions of metamorphism, zircon is isotopically 'reset to zero' in granitic gneisses, or augengneisses. It keeps on the contrary an 'isotopic memory' of its previous history in cordierite and gamet bearing gneisses, pure quartzites, and layered quartzo-feldspathic gneisses. The iso­ topic behaviour of the zircon present in metamorphic rocks cannot be de­ duced from a superficial examination of its colour, transparency, and mor­ phologic features.

Acknowledgements. -The present study was carried out as part of the programme of the Belgian Centre for Geochronology. One of us has benefited from a "Charge de Recherches" fellowship from the Belgian National Fund for Scientific Research. The first author is responsible for the geochronological interpretation; the geological frame­ work was provided by the field and petrographic studies of the second author, com­ plementing the impressive earlier work of P. Michot. J. Delhal kindly supervised most of the mineral separation and S. Deutsch took care of the data treatment by computer. The authors are indebted to K. S. Heier for his review of the manuscript, L. Cahen for his criticism and comments, and J. Jedwab for allowing them to publish the stero­ scan pictures presented here. March 1974

REFERENCES

Barth, T. F. W. & Dons, J. A. 1960: Precambrian of Southern Norway. In Holtedahl, O. (ed.), Geology of Norway. Nor. Geol. Unders. 208, 6-67. Bickford, M. E. & Turner B. B. 1971: Age and probable anatectic origin of the Brant Lake gneiss, Southeastern Adirondack Mountains, New York. Geol. Soc. Am. Bull. 82, 2333-2342. Dahlberg, E. H. 1969: Feldspar of Rogaland. Unpublished Ph.D. thesis, University of Utrecht. Deutsch, S., Ledent, D. & Pasteels, P. 1965: Datation par les methodes Sr/Rb et Pb!U au laboratoire de geochronologie du Service de Geologie et Geochimie Nucleaires. Universite libre de Bruxelles. Falkum, T. 1966: Structural and petrological investigations in the Precambrian meta­ morphic and igneous charnockite and migmatite complex in the Flekkefjord area, Southern Norway. Nor. Geol. Unders. 242, 19-25. GEOCHRONOLOGIC INVESTIGATION IN SW NORWAY 131

Goldich, S. S., Hedge, C. E. & Stem, T. W. 1970: Age of the Morton and Montevideo gneisses and related rocks, Southwestem Minnesota. Geo/. Soc. Am. Bull. 81, 3671- 3696. Gray, C. M. & Oversby, V. M. 1972: The behaviour of lead isotopes during granulite facies metamorphism. Geochim. Cosmochim. Acta, 36, 939-952. Malcuit, R. J. & Heimlich, R. A. 1972: Zircons from Precambrian gneiss, Southem Bighom Mountains, Wyoming. Am. Mineral. 57, 1190-1209. Michot, J. & Michot, P. 1969: The problem of anorthosites: the South-Rogaland igneous complex, southwestern Norway. N.Y. State Museum and Science Service, Mem. 18, 399-410. Michot, J. & Pasteels, P. 1968: Etudes geochronologiques du·domaine metamorphique du Sud-ouest de la Norvege (Note preliminaire). Ann. Soc. Geo/. de Belgique, 9111, 93-110. Michot, J. & Pasteels, P. 1969a: The prospects of the Rb-Sr and the U-Pb methods for an advanced geochronological investigation of the Precambrian of southern Norway. Nor. Geo/. Unders. 258, 17-26. Michot, P. 1956a: La geologie des zones profondes de l'ecorce terrestre. Ann. Soc. Geo/. de Belgiqu�, 80, B 19-73. Michot, P. 1956b: Structures tectoniques dans la catazone norvegienne. Acad. Roy. Belgique, Bull. Cl. Se. 5, 209-227. Michot, P. 1957: Phenomenes geologiques dans la catazone profonde. Geo/. Rundschau, 46, 147-173. Michot, P. 1960: La geologie de la catazone: le probleme des anorthosites, la palin­ genese basique et la tectonique catazonale dans le Rogaland meridional, Norvege meridionale. Cong. Int. Geol.-Norden 1960, 21e Sess.-guide A9, 1-54. Michot, P. 1965: Les orogenes fondamentaux. Freiberg Forschungshefte C 190, 49-62. Michot, P. 1969: Geological environments of the anorthosites of South Rogaland, Nor­ way. N.Y. State Museum and Science Service - Mem. 18, 411-423. O'Nions, R. K. & Baadsgaard, H. 1971: A radiometric study of polymetamorphism in the Bamble region, Norway. Contr. Min. Petr. 34, 1-21. O'Nions, R. K., Morton, R. O. & Baadsgaard, H. 1969: Potassium-argon ages from the Bamble sector of the Fennoscandian shield in South Norway. Nor. Geo/. Tidsskr. 49, 171-190. Pasteels, P., Michot, J. & Lavreau, J. 1970: Le complexe eruptif du Rogaland meri­ dional (Norvege). Signification petrogenetique de la farsundite et de la mangerite quartzique des unites orientales; arguments geochronologiques et isotopiques. Ann. Soc. Geo/. de Belgique, 93, 453-476. Pidgeon, R. T. & Aftalion, M. 1972: The geochronological significance of discordant U-Pb ages of oval-shaped zircons from a Lewisian gneiss from Harris, Outer Hebrides. E.P.S.L. 17, 269-274. Pidgeon, R. T. & Bowes, D. R. 1972: Zircon U-Pb ages of granulites from the central region of the Lewisian, northwestern Scotland. Geo/. Mag. 103, 247-258. Poldervaart, A. 1955: Zircons in rocks. I. Sedimentary rocks. Am. l. Sei. 253, 433-461. Silver, L. T. 1969: A geochronologic investigation of the anorthosite complex, Adiron­ dack Mountains, New York. N.Y. State Museum and Science Service, Mem. 18, 399-410. Silver, L. T., McKinney, G. R., Deutsch, S. & Bofinger, J. 1963: Precambrian age determinations in the western San Gabriel Mountains, California. J. Geo/. 71, 196-214. Spooner, C. M. & Fairbairn, H. W. 1970: Strontium 87/Strontium 86 initial ratios in pyroxene granulite terranes. J. Geophys. Res. 75, 6706-6713. Touret, J. 1968: The Precambrian metamorphic rocks around the Lake Vegår (Aust­ Agder, Southern Norway). Nor. Geo/. Unders. 257, 1-44. Touret, J. 1969: Le socle precambrien de la Norvege meridionale (Region de Vegårs­ hei-Gjerstad). Unpublished thesis, Universite de Nancy. Versteeve, A. J. 1971: Progress-report on Rb-Sr investigations in the charnockitic Pre­ cambrian of Rogaland and Vest-Agder, S. W. Norway (abstract). Ann. Soc. Geo/. de Belgique, 94, 134-135. 132 P. PASTEELS & J. MICHOT

Appendix - Localization and brief description of the samples Pa69/A Tveit forest road 6558200-342100 (coordinates of the 1/50,000 scale topographic map of Norway). Massive variety of porphyroblastic gneiss or porphyroblastic gran­ ite. Microcline porphyroblasts of rectangular sections and about 2 cm size are embedded in a matrix composed of heavily sericitized plagioclase, quartz with highly undulose extinction or granulated, slightly perthitip microcline, chloritized biotite with rutile exsolu­ tions, rather abundant allanite, sphene, minor epidote, apatite, opa­ que, zircon. This rock obviously suffered from the Caledonian meta­ morphism (estimated distance under the 'disconformity' = 500 m). Collected in recently blasted road-cut.

Pa69JB Jørpeland, main road to the south 6544850-331150. Granitic augengneiss. This rock, with well-developed foliation, consists of microcline, quartz, oligoclase, biotite and accessorily, apatite, opaque, zircon. The opaque mineral is associated with chlorite, and, as this latter mineral, seems secondary. Plagioclase is sericitized. The 'eyes' are polycrystalline, consisting of microcline with numerous inclusions of quartz and plagioclase, and quartz in elongate denticulate sections. The Caledonian retrograde meta­ morphism is weak though perceptible. Fresh road-cut.

Pa69fJ Leirvik 6461850�375950. Granitic gneiss. Medium-grained rock composed of microcline microperthite and quartz with very minor plagioclase, biotite, opa­ que, apatite, zircon. The rock exhibits a distinct foliation along which elongated lenses of the microcline-quartz (with ribbon struc­ ture) association are developed. Rather fresh road-cut.

Pa66/K Ollestad 6490900-346750. Granitic gneiss. Fine to medium-grained rock composed of equi­ granular perthitic microcline, quartz, oligoclase. Mafic constituents are not very abundant and, biotite excepted, transformed into secon­ dary products. Alteration is perspicuous along fracture zones with the development of chlorite, epidote, etc. . . Accessories are apatite, monazite, zircon and opaque. Slightly weathered sample collected in natura! outcrop.

Pa66jC Lakssevelefjellet southern slope 6501250-824800. Granitic gneiss. Microcline perthite, quartz, oligoclase, altered py­ roxene, apatite, zircon, opaque. The very fine 'hair' perthite ap­ proaches the composition of mesoperthite. Microcline twinning is perspicuous. The original mafic minerals (pyroxene) are nearly GEOCHRONOLOGIC INVESTIGATION IN SW NORWAY 133

entirely replaced by secondary biotite-like or chloritic products. Elongate quartz sections (ribbon structure) or smaller isometric grains often exhibit ondulose extinction. The microcline perthite in­ cludes poecilitically the other minerals. Collected in natural outcrop · but relatively fresh.

Pa66/J Røyiselandsvatn 6505400-322700. Gametiferous, cordierite-bearing gneiss. Perthitic microcline, quartz, plagioclase in very subordinate amount, cordierite, con­ verted into pinite, surrounding an opaque mineral partly converted into chlorite, biotite, altered pyroxene, green spinel, gamet, zircon. Rather fresh road-cut.

Pa69/C Oltedal 6524100/328000. Garnet gneiss. Microperthitic microcline, often poecilitic, quartz with highly ondulose extinction, minor plagioclase, biotite, green spinel, sillimanite, garnet, accessory zircon. The rock exhibits a distinct foliation. Rather fresh road-cut.

Pa66fF Hadland 58°33'55''N 5°44'48"E. Garnet gneiss. Mesoperthite, quartz, minor plagioclase, hyper­ sthene, gamet, opaque, zircon. The foliation is very distinct, grain size varying greatly across structure. Lenses of coarsely crystallized mesoperthite and quartz are observed. Rather fresh road-cut.

Pa66/D Summit of Lakssevelefjellet 6503450-324200. 'Birkremite'. Medium-grained rock consisting of microperthitic microcline, quartz, relatively little oligoclase, altered pyroxene, opaque ( often rimmed with a green amphibole ), allanite, zircon. Biotite, in very small amount, appears to be a primary constituent but is very altered. The alteration product of pyroxene is biotite-like as in Pa66fC. Natura! outcrop, relatively weathered rock.

Pa66/L Liland 6476550-356200. Augengneiss. The K-feldspar porphyroblasts are of moderate size (about 2 cm). The matrix consists of fine-grained oligoclase, cloud­ ed with alteration products, quartz, K-feldspar, brown-greenish amphibole, biotite, opa�ue, apatite, zircon. Quartz and slightly microperthitic orthoclase also occur in coarsely crystallized lenti­ cular aggregates parallel to the rather weak foliation. Natural out­ crop.

Pa66fl Haugeland near Feda 6460500-373600. Augengneiss. The very large porphyroblasts (about 10 cm) of K­ feldspar are embedded in a relatively coarse matrix consisting of 134 P. PASTEELS & J. MICHOT

unzoned andesine, quartz (occasionally with ribbon structure), orthoclase (subordinate), clinopyroxene, biotite, altered orthopyrox­ ene? amphibole (little), opaque, apatite, zircon. Newly open road-cut.

Pa69/H Sandsmork 6484600-344950. Banded noritic gneiss (septum included in the Bj-Sg. lopolith). Fine-grained rock consisting of andesine, quartz, hypersthene, opa­ que, apatite, minor zircon. The light minerals are by far the most abundant, the dark ones being relatively enriched in ribbons of cen­ timetric size which alternate with lighter ones nearly devoid of ma­ fics. Natural outcrop, but fresh.

Pa66fl Moi, Røyiselandsvatn 6505350-322050. Quartzite. Nearly monomineral coarse rock composed of sutured quartz sections. In subordinate amounts, strongly altered plagio­ clase, epidote and chlorite aggregates, etc. . . Road-cut.

Pa69fE Søyiland 6509350-324200. Granite or granitic hornfels. Medium-grained rock of granitic composition, massive. Microperthitic K-feldspar, quartz, unzoned xenomorphic oligoclase, myrmekite. The dark constituents are very minor, consisting of pyroxene(?) altered into chlorite plus carbonate, biotite, garnet, opaque, zircon. Fresh-looking rock, though collected in natural outcrop.

Pa69fD Same location as Pa69fE. Similar to the former, except that the rock is practically holo­ leucocratic and that plagioclase is present in smaller proportion.

Pa66fR Skomvak near Lyngdal 6443700-385950. Granitic gneiss. Microcline, poecilitic quartz with highly ondulose extinction, plagioclase rimmed with albite, clouded with alteration products. The only mafic mineral is biotite, largely replaced by chlorite, sometimes associated with epidote. Opaque, zircon. Arti­ ficial outcrop.