Magma Influx and Mixing in the Bjerkreim-Sokndal Layered Intrusion, South Norway: Evidence from the Boundary Between Two Megacyclic Units at Storeknuten

Lithos, 29 (1993) 311-325 311 Elsevier Science Publishers B.V., Amsterdam

Magma influx and mixing in the Bjerkreim-Sokndal layered intrusion, South Norway: evidence from the boundary between two megacyclic units at Storeknuten

J.C. Jensen a, F.M. Nielsen a, J.C. Duchesne b, D. Demaiffe c and J.R. Wilson a a Geologisk Institut, Aarhus Universitet, 8000Aarhus C, Denmark b Laboratoires associds de GOologie, POtrologie, Gkochimie, Universitk de Likge, B-4000 Sart Tilman, Belgium c Laboratoires associ?s de GOologie-Pktrologie-G~ochronolog ie, UniversitP Libre de Bruxelles, 50,4 venue Ft. Roosevelt, B-1050 Brussels, Belgium (Received March 27, 1992; revised and accepted August 24, 1992 )

LITHOS ABSTRACT Jensen, J.C., Nielsen, F.M., Duchesne, J.C., Demaiffe, D. and Wilson, J.R., 1993. Magma influx and mixing in the Bjerkreim-Sokndal layered intrusion, South Norway: evidence from the boundary between two megacyclic units at Storeknuten. Lithos, 29:311-325.

The Bjerkreim-Sokndal layered intrusion belongs to the Proterozoic anorthositic province in the Ro- galand area of southern Norway. The northwestern part of the intrusion comprises a ca. 6 km-thick Lay- ered Series made up of megacyclic units (MCU) arranged in a syncline; each megacyclic unit reflects the influx of fresh magma into the chamber. The boundary between megacyclic units III and IV has been studied in detail at Storeknuten on the southern flank of the syncline. The megacyclic units can be subdivided into a series of cumulate stratigraphic zones; the interval from the top of zone IIIe to the base of zone IVd is exposed in the Storeknuten area. Modally layered plagioclase-hypersthene-ilmenite-magnetite-augite-apatite cumulates belonging to zone IIIe are overlain by 30 m of massive plagioclase-rich rocks (commonly containing ilmenite and/or hypersthene) constituting zone IVa. The entry of cumulus olivine defines the base of zone IVb (dominantly plagioclase-olivine-ilmenite cumulates) which is about 100 m thick. Many of the olivines are partly or completely replaced by Ca-poor pyroxene/Fe-Ti oxide symplectites. This massive leucotroctolitic zone is overlain by modally layered, laminated plagioclase- hypersthene-ilmenite cumulates of zone IVc. The successive entry of magnetite, apatite (accompanied by Ca-rich pyroxene) and inverted pigeonite defines zones IVd, e and f respectively. The entry of K- feldspar (accompanied by Fe-rich olivine) defines the base of a jotunitic transition zone which passes upwards into mangerites and quartz mangerites. There is a compositional regression through zone IVa. The upper part of zone IIIe has Ca-poor pyroxene with about En68, plagioclase with An44-48 and a Sr-isotope ratio of about 0.7062, while the base of zone IVb has olivine with Fo75 together with En78, An53 and 0.7050 respectively. Similar reversals are shown by the minor element compositions of plagioclase and Fe-Ti oxides. Sr-isotope ratios increase systematically up through zone IVb (reaching 0.7058 in zone IVd) while An% and Sr in plagioclase and Ni and Cr in Fe-Ti oxides decrease. Olivine compositions vary unsystematically and are believed to have changed their Fe:Mg ratios as a result of trapped liquid shift. The magma residing in the chamber when the influx at the base of megacyclic unit IV took place was compositionally zoned, and assimilation of gneissic country rock at the roof had resulted in the St-isotope ratio increasing up through the magma column. The new magma had a Sr-isotope ratio of about 0.7050 while the resident magma had a ratio of 0.7062 at the floor, increasing upwards. The new magma mixed with the basal layer(s) of the compositionally zoned resident magma and crystallization of this hybrid magma during influx and mixing produced the compositional regression in zone IVa. When magma influx

Correspondence to: J.R. Wilson, Geologisk Institut, Aarhus Universitet, 8000 Aarhus C, Denmark.

ceased, olivine-bearing rocks began to crystallize at the base of zone IVb. The leucotroctolites at the base of this zone are the most primitive rocks in the entire intrusion. The systematic increase in Sr-isotope ratios up through zone 1Vb resulted from progressive mixing between new and resident magma. This mixing either took place during magma influx or by the progressive mixing of overlying resident magma layers during crystallization. Calculations based on geochemical modelling, the thickness of cumulate stratigraphy repeated and Sr- isotope ratios indicate that the new magma influx had a thickness of 350-500 m in the Storeknuten section and that the leucotroctolites of zone IVb represent about 20-30% c~stallization of this influx.

Introduction orthosite (Duchesne, 1970) from which it is separated by a thin zone of migmatitic gneisses and older Since the pioneering work by Michot (1960) than anorthosites and leuconorites (Helleren lobe) studies in the Rogaland area of southern Norway of the Haaland-Helleren massif (Michot, 1961 ). have contributed to the debate concerning the ori- The Bjerkreim lobe of the BKSK (Fig. IB) is gin of anorthosites and related rocks (e.g. Michot made up of lithologies ranging from anorthosite to and Michot, 1969; Duchesne and Demaiffe, 1978; quartz-mangerite and dominated by leuconorite. A Demaiffe and Hertogen, 1981; Duchesne et al., series of layered units define a syncline that plunges 1985, 1989). The Bjerkreim-Sokndal layered in- SE at about 35 °. Michot (1960) described the five trusion (BKSK) ( Fig. 1B ) has played an important major rhythmic units of the Bjerkreim lobe as hav- role in this debate, since the entire span of igneous ing an anorthositic or leuconoritic base and becom- rock types normally associated with anorthosites ing more evolved upwards. The uppermost unit occur in the intrusion (Michot, 1965; Wiebe, 1984; grades into a zone of monzonoritic (jotunitic) rocks Duchesne, 1987). BKSK is, however, also well which form a transition to massive mangerites and suited for the study of magma chamber processes. quartz mangerites. Michot recognised several dis- Michot (1960, 1965) divided the northwestern tinctive zones, including an olivine-bearing leuco- part of the BKSK (the Bjerkreim lobe) into five norite (the Svaalestad unit) near the base of major layered units, which he called rhythmic units. rhythmic unit IV. A similar, though less continu- These rhythmic units have been interpreted by Du- ous, olivine-bearing unit occurs at the base of chesne (1972) as reflecting separate influxes of rhythmic unit 11I (Nielsen and Wilson, 1991 ). The magma into the chamber. Lateral variations related sequence from the base of the lowermost anorthos- to the boundary between two of these rhythmic units itic rocks (Unit IA in Fig. 1B) to the top ofthejo- (III and IV) have recently been described by Niel- tunitic transition zone constitutes the Layered Se- sen and Wilson (1991). This article concentrates ries of the Bjerkreim lobe. The Layered Series has a on a detailed profile across this boundary. A com- thickness of about 6 km measured along the axial bination of field and petrographic observations, surface of the syncline, thinning along the limbs microprobe analyses, chemical analyses of mineral partly because the lower units wedge out (Fig. 1B). separates and Sr-isotope analyses allows discussion The BKSK and its envelope are cut by a series of of a wide variety of magma chamber processes, in- broadly monzonoritic dykes (Duchesne et al., 1985 ) cluding the mechanism of magma influx, the role of which are not shown in Fig. 1B. magma mixing and the contribution of assimilation. The geology of the Bjerkreim lobe is currently being investigated by groups from the Universities of Bergen (led by B. Robins), Libge (J.C. Du- The Bjerkreim-Sokndal intrusion chesne) and Aarhus (J.R. Wilson). Work in prog- ress has indicated that Michot's subdivision into five The BKSK massif extends for about 40 km from rhythmic units requires revision, particularly in the Bjerkreim in the northwest to Sokndal in the south lower part of the Layered Series. The boundary be- and was emplaced into granulite facies gneisses tween units III and IV, however, does not require about 955 million years ago (Pasteels et al., 1979). significant modification, and Michot's numbering The massif is younger than the Egersund-Ogna an- system is used in this article. The term megacyclic >. ~r

[-.,"11 x A NORTHOSITE :Z BJERKREIM- LEUCONORITE AND NORITE SOKNDAL iNTRU Si ON MANGERI TE >< • : -.•j ';.'.:: 'X ~ , \ \ \ t~QUARTZ MANGERITE ~] ANORTNOSITE N 1 ~ ?;o ++++++ +++++ '] GNEISS E BJ'ERKREiM"://".,,.'~f+ CF~+ T ÷ : I / EGOG EGERSUND- OGNA MASSIF +++++++++++ HH HAALAND-HELLEREN MASSIF [] ~r [-~ "RHYTHMIC UNITS" ÷+÷÷+.++++ © ~÷+++++++++~++.+ + ~ STRIKE AND DIP OF LAYERING + +÷+÷+++ ÷÷ + +÷+++ ÷ +~ ++. + + ------FAULT • ÷ +

..... ÷.p-~.'p% + + + + . + + + ~ ~ 0 1 2 3 4KM ~ ÷ + + + + ÷ + + + + • J I > .÷ ++ ÷÷+÷÷.÷ ÷ ÷+",--++ .~ .; -< nl rll

o o o ,~+ +.~ o o + ÷' 7z ,TOREKNUTEN ++++++++++ ~ + + + + + .i0101010>1.1..o0 © ):01o1.2°101°1°20 Z

° ° ' -~'qphirn G=GUMULATE p = PLAGEOCLASE +? i ***°***°**°°°°*°°°°: [-~phi C (MCU I'~c) h = Ca-POOR PYROXENE OSLC **°****°°.°°*°°.°°*** poC ± m,I i = ILMENITE °°°°°°°°°°°°°°.°°°°* ~

o* ~ pC/phC -* i m = MAGNETITE o ° °.* ~ _ ~ phimoc C (MCU me) o = OLIVINE °° °' HH °' 4/6 STRIKE AND DIP o = APATITE EGOG 0 OF LAYERING °° °, II1 SAMPLE NUMBERS c = Co-RICH PYROXENE *° o o ° . o

Fig. 1 (A). Locationof Fig. I B in southernNorway. (B). Simplifiedgeological map of the Bjerkreimlobe of the Bjerkreim-Sokndalintrusion, based on fig. 5.8 in Duchesne( 1987). The LayeredSeries consists of five "rhythmicunits", referred to here as megacyclicunits (MCU). The lithologicaland crypticvariation along profile A is shown in Fig. 2. The locationof Fig. I C (Storeknuten,in the centralpart of profileA ) is outlined.(C). Geological map of the Storeknutenarea showing the distributionof major outcrops,lithologies, strike and dip of layering,and samplelocations. The legendexplains the abbreviationsused for the cumulatenomenclature in the text. 314 J.C. JENSEN ET AL. unit (MCU) (Irvine, 1982) is used in this article the basis of their cumulus mineral assemblages. The for the rhythmic units of Michot. thicknesses of the various zones shown in Fig. 2 ap- Most of the Layered Series rocks are character- ply to a profile on the southern limb of the syncline ised by igneous lamination and modal layering (Fig. 1B) from the country rock contact to the en- which are best developed in the upper parts of the try of K-feldspar which defines the base of the jo- MCU's. Cumulate textures can usually be recog- tunitic transition zone. The thicknesses of the zones nised in the field, but in thin section it is apparent up to MCU IVd are based on profile A in Fig. 1B, that the rocks have recrystallised [referred to as but because of poor exposures the thickness data protoclasis by Michot (1965)]. In the limbs of the above this are based on exposures up to about one Bjerkreim lobe there is commomly a steep folia- kilometer to the northwest. The boundary between tion, essentially in the plane of the layering, related MCU's III and IV at Storeknuten is located near the to post-emplacement deformation of the complex. middle of profile A. Possible changes of thickness of megacyclic units during deformation are not believed to have been Profile A important and have been disregarded here. The MCU's can be subdivided into a series of zones on A marginal zone of coarse grained, massive ilmenite-bearing leuconorites (+_ magnetite), in which orthopyroxene is usually poikilitic, occurs in contact with the country rock gneisses at the base of

~x profile A (Fig. 2). These marginal leuconorites are o ~ ~ characterised by the plagioclase being antiperthitic and by the presence of apatite and variable amounts of interstitial quartz. After about 200 m the rocks -IP develop an igneous lamination with prismatic or-

3000- thopyroxene, and modal layering is sporadically present. These phiC (see Fig. I C for explanation) are referred to MCU II which has a thickness of e about 550 m. The laminated leuconorites at the top of MCU II are overlain by piC and pihC belonging 2000- to MCU llIa. MCU Ilia is dominantly massive, but d contains several ilmenite-rich lenses and layers c -- which are usually slumped. MCU Ilia is usually overlain by massive leucotroctolites comprising ..... II MCU IIIb in which Ca-poor pyroxene and/or mag- HI netite are locally additional early phases. However, d ~O00- c in profile A no olivine has been found in this inter- o/b ~, :~:/,5:/; T T val and MCU IIIb is included in MCU lIla in Fig. 2, with a combined thickness of about 90 m. /2" c The entry of cumulus Ca-poor pyroxene, which coincides with the development of an igneous lam-

0 ination and sporadically developed modal layering, l~ ABUNDANTMODAL COARSEGRAINED MASSIVE LAYERING ~ MARGINALZ(~E ROCKS defines the base of MCU IIIc which has a thickness SPORADIC MODAL MZ MARGINALZONE LAYERING ~, :C~ of 110 m in profile A. The arrival of magnetite as a DOMINANTLY MASSIVE JTZ JOI'UNIT~C TRANSITrON ROCK ZONE cumulus phase defines the base of MCU IIId, and SLUM,,to 'LM~N"E- ~ RICH STRUCTURES I P INVERTEDPIOEONITE the entry of apatite defines MCU Ille. MCU IIId and IIIe are 430 m and 110 m thick respectively. The first occurrence of apatite generally coincides Fig. 2. Zonal subdivision and lithology ofmegacyclic units in profile A (Fig. IB) based on cumulus mineral assemblages. with that of Ca-rich pyroxene. MCU IIId and MCU Continuous vertical lines=cumulus status; dashed=cumulus IIIe are laminated and are generally characterised status uncertain. by well developed modal layering. MAGMA INFLUX AND MIXING IN BJERKREIM-SOKNDAL LAYERED INTRUSION 315

The boundary between MCU's III and IV and the and piC, with magnetite and Ca-poor pyroxene also lower part of MCU IV are exposed at Storeknuten occurring as sporadic early phases) which consti- and are described in detail below. The sequence of tute MCU IVa. llmenite-rich lenses and layers, cumulate zones in the lower part of MCU IV is es- which in many cases are slumped, are a character- sentially the same as in MCU III; it is noteworthy istic feature of MCU IVa. Plagioclase compositions that MCU IVa and b are thicker than their counter- are in the range An45-49, and a Ca-poor pyroxene parts in MCU III, while MCU IVc has similar which is interpreted to be a cumulus phase in sam- thicknesses to MCU IIIc in profile A. MCU IV ple 4 has Mg#72. reaches more evolved compositions than MCU lit; The predominantly massive olivine-bearing rocks a thick sequence (ca. 1300 m) of modally layered of MCU IVb [the Svaalestad unit of Michot gabbronorites (phimcaC) belonging to MCU IVe is (1960)] which form the summit of Storeknuten overlain by inverted pigeonite-bearing rocks, defin- contain the most primitive mineral compositions in ing MCU IVf (ca. 210 m). The subsequent pres- the studied sequence. Olivines at the base and top ence of perthitic K-feldspar, which roughly coin- are the most magnesian (Fo75) while there is a cides with the entry of Fe-rich olivine, defines the somewhat erratic variation towards more evolved base of the jotunitic transition zone (Duchesne et compositions (reaching Fo67 ) near the centre. The al., 1987) which grades into massive mangerites and Ca-poor pyroxenes (in which the most primitive quartz mangerites. Michot (1960) recognised a fifth composition reaches Mg~78) are of bronzites in rhythmic unit in the area near the top of profile A, corona structures around olivines; the more or less as shown in Fig. lB. Our field and laboratory stud- parallel variation of Fo in olivines and Mg# in Ca- ies have failed to confirm the presence ofa MCU V poor pyroxenes in Fig. 3E suggests that they are es- in this part of the intrusion and we consider that sentially in equilibrium. The original olivine in MCU IV grades into the overlying mangerites MCU IVb has commonly been partly or completely through the jotunitic transition zone. replaced by a symplectic intergrowth between Ca- poor pyroxene and Fe-Ti oxide, as described by The Storekn uten profile Barton and Gaans (1988). The compositions of plagioclases (An48-53) vary less systematically The section exposed at Storeknuten (Fig. 1C) has than those of the mafic silicates in MCU IVb, but been chosen as the type locality for the boundary the most primitive composition is at the base. The between MCU III and MCU IV because of good ex- mineralogical composition of most of the BKSK posure, representative development and accessibil- Layered Series rocks is notably anhydrous, but MCU ity. Figure 3 shows the lithological variation in a ca. IVb (and MCU IIIb) are characterised by small 300 m thick section, together with compositional amounts of biotite and hornblende. variations of the major silicates. These variations There are two types of magnetite in MCU IVb. are based on 31 samples with an average sample One type occurs as fairly large grains (ca. 1 mm) density of about 1:10 m. From the base of MCU IVa which are about the same size as the ilmenites. This to the top of MCU IVb the sample density is about magnetite, which is considered to be a cumulus l:6m. phase, contains two generations of spinel exsolu- The two cumulus pyroxenes in the modally lay- tions, the first of which occurs as coarse lenses. Thin ered, laminated rocks of MCU IIIe have fairly con- exsolution lamellae of ilmenite can be observed in stant compositions [hypersthene with Mg#opx some grains. A rim of spinelliferous ilmenite is al- ( = 100Mg/(Mg+ Fe) ] values of 68-70 and augite ways developed at the contact between the cumulus with Mg#cpx72-74). In a single sample at the top magnetite and the cumulus ilmenite, indicating of MCU IIIe the pyroxenes show a reversal to con- subsolidus readjustment of chemical composition as siderably more primitive compositions (Mg#opx74; described by Duchesne (1972). The second type of Mg#cpx79 ). Plagioclases show erratic variations at magnetite occurs in the Ca-poor pyroxene-Fe-Ti the top ofMCU IIIe (An43-49) but the most prim- oxide intergrowths and is usually devoid of spinel itive composition is again in the uppermost sample. or ilmenite lamellae. Its amount varies consider- This zone is overlain by about 30 m of plagioclase- ably, depending on the degree of replacement of the rich cumulates (dominant variations between pC olivines. 316 J.C. JENSEN ET AL

A B c D E F G J J i [ , , I i I i i ~ i I [Ed 15! t I I I , i I I I t I ~ I 1

-[- - 14 1200 - Ca:poor px /i y/" ' ~c * Secondary J // o Co-rich px PLAG IO - '/ 13 o OLIVINE CLASE '/

m i;:!i: ,2 100-, F'."::':i 10g mb ~;i:;:."?_ s ? ; "'l:. 5 ..

]~Zo '.:,-,., '.' 4 Lx 0 -+

-- 3

me -- - 2 lsd SEPARATED f -50- y-, PLAGIOCLASE E - --- 1

I i i 80 75 70 65 5Z. 50 45 O~ 7050 0,7054 0,7058 0,7062 100Mg/(Mg* Fe) An */, 87Sr/86Sr

ABUNDANT MODAL LAYERING DOMINANTLY MASSIVE ROCK

-~ SPORADIC MODAL LAYERING MASSIVE ROCKS WITH ILMENITE- RICH LENSES AND LAYERS (OFTEN SLUMPED) Fig. 3. Compositional variations across the boundary between megacyclic units III and IV at Storeknuten. (A). Thickness in meters above and below the MCU I11/IV boundary. ( B ). Cumulate stratigraphic zonal subdivision. (C). Lithology. ( D ). Sample numbers of rocks from which minerals have been separated. Locations are shown on Fig. 1C. (E). Variation of 100Mg/( Mg + Fetot) ratios in Ca-poor pyroxene, Ca-rich pyroxene and olivine. The Ca-poor pyroxenes analysed from MCU IVb are ofbronzites which are a reaction product from olivine. They arc commonly intergrown with Fe-Ti oxides. (F). Plagioclase compositions. (G). 87Sr/ 86Sr ratios in separated plagioclases.

MCU IVb is overlain by laminated phiC that 0.7062, decreasing to 0.7060 in MCU IVa. This is constitutes MCU IVc in which magnetite is absent. followed by a dramatic drop to 0.7050 at the base Magnetite reappears, however, as a cumulus phase of MCU IVb, after which the ratios increase fairly after about 115 m, defining the base of MCU IVd. systematically, reaching 0.7058 in MCU IVd. Modal layering is sporadically present in the lower It appears from Fig. 4, which shows the compo- part of MCU IVc, but becomes well developed in sitional variations in separated minerals, that the the upper part and in MCU IVd. The three samples compatible elements behave in a predictable fash- from these zones show a trend of normal fractiona- ion if MCU III and MCU IV are considered as two tion with Mg#opx varying from 74 to 66 while the separate, closed systems. Minerals in MCU IVa plagioclase composition remains fairly constant at generally have distinctive compositions and, as will An48-49. emerge from the discussion below, it is interpreted 87Sr/86Sr ratios in separated plagioclases (Fig 3G as a hybrid zone. A potential problem with the and Table 1 ) in MCU IIIe are in the range 0.7061- interpretation of analyses of mineral separates is that MAGMA INFLUX AND MIXING IN BJERKREIM-SOKNDAL LAYERED INTRUSION 317

TABLE 1

Chemical analysis of separated plagioclases

Sample Sr CaO KzO Ba 87Sr/S6Sr St. Dev. N (ppm) (ppm) (2a)

JL-I 952 9.36 0.78 261 0.70617 0.00002 8 JL-2 927 8.76 0.87 289 0.70609 0.00002 10 JL-3 941 8.97 0.77 266 0.70621 0.00005 6 JL-4 867 9.30 0.67 274 0.70600 0.00002 18 JL-5 1086 10.02 0.55 266 0.70496 0.00001 15 JL-6 1096 9.84 0.54 301 0.70515 0.00001 14 JL-7 1073 9.71 0.55 280 0.70514 0.00002 8 JL-8 1099 9.49 0.58 280 0.70519 0.00001 9 JL-9 1060 9.49 0.58 302 0.70530 0.00002 9 JL-10 1078 9.64 0.53 255 0.70522 0.00001 8 JL-11 1032 9.77 0.48 248 0.70520 0.00001 6 Jk-12 1036 9.47 0.52 303 0.70527 0.00002 10 JL-13 998 9.60 0.55 288 0.70534 0.00001 9 JL-14 978 9.34 0.598 295 0.70560 0.00003 13 JL-15 976 9.65 0.68 279 0.70582 0.00003 9 they can contain a mixture of cumulus and inter- Both Ni and Cr fall fairly systematically up through cumulus material. However, since most of the BKSK MCU IVb and reach low levels, approaching those Layered Series rocks are extreme adcumulates in MCU IIIe, in MCU IVd. Vanadium in magnetite (Duchesne, 1978; Roelandts and Duchesne, 1979), is highest in MCU life and MCU IVd; the sharp this is not a serious problem here. decrease in MCU IVb may be related to the pres- Strontium in plagioclase is relatively low (927- ence of small amounts of hornblende which has a 952 ppm) in MCU IIIe and relatively high in MCU relatively high partition coefficient for V. IVb (ca. 1100 ppm), decreasing through MCU IVb Compared with Sr, the contents of K20, and par- and c to 976 ppm in MCU IVd. Nickel, Cr and Co ticularly Ba, in plagioclase vary unsystematically. in ilmenite behave in a broadly similar fashion with Potassium roughly follows the An content ( Fig. 3F); Ni showing the most and Co the least systematic it is highest in MCU IIIe (0.78-0.87% K) and de- variation. Nickel increases from < 25 ppm in MCU creases to 0.67% in MCU IVa. In MCU IVb the val- IIIe to ca. 300 ppm at the base ofMCU IVb i.e. an ues vary between 0.48 and 0.58% K20, increasing increase by a factor of > 10. The values then de- to 0.68% K in MCU IVd. Barium in plagioclase var- crease fairly systematically to about 40 ppm in MCU ies unsystematically throughout the entire sequence IVd. Chromium shows a similar pattern of varia- between about 250 and 300 ppm, partly as a result tion, increasing by a factor of about 5 from MCU of the relatively large analytical uncertainty for Ba. IIIe (34-104 ppm Cr) to the base of MCU IV (ca. MCU IVa is characterised by Ni and Cr in ilmen- 500 ppm) and then falling rather irregularly to 111 ire having values intermediate between the under- ppm in MCU IVd. Cobalt likewise increases ab- lying MCU IIIe and the overlying MCU IVb; the ruptly by a factor of about 6 at the base of MCU IV same trend is shown by K in plagioclase. This ten- but shows erratic variation above this. Nickel and dency to have intermediate values was also shown Cr are also compatible in magnetite and show sim- by the 87Sr/86Sr ratio, as outlined above. However, ilar variations (it will be recalled that cumulus the Sr in plagioclase in MCU IVa is the lowest value magnetite is absent in MCU IVa and MCU IVc). in the studied sequence, while the highest level of The Cr evolution in magnetite supports the inter- Co in ilmenite occurs in this zone. pretation of magnetite as a cumulus phase in MCU It was noted earlier that the pyroxenes, in partic- IVb and indicates that the role of secondary mag- ular in the uppermost sample in MCU IIIe, showed netite is minor since it does not affect the cumulus a compositional reversal. This feature is also shown chemical evolution. Nickel increases by a factor of by Cr and Co in ilmenite, Ni in magnetite and about 8 and Cr increases from below detection limit weakly by Sr in plagioclase. This reversal immedi- to ca. 1.5 % from MCU IIIe to the base ofMCU IVb. ately below MCU IV is not a unique feature re- 318 J.C. JENSEN ET AL.

PLAGIOCLASE Sr PLAGIOCLASE K20 PLAGIOCLASE Be MCU Nd

rVc

• •o • • • Nb

• • • ~ e % 900 . 1000 1100 l OiL , 06, , •, , 10, 240 • 280 3?0 320 ILMENITE Ni ILMENITE Cr ILMENITE Co • • • ~d

• • • [SZc

6 • e__ ..... • .~.e ...... °o it" ~a

• • • ]]~.. e

• 100 200 300 • 200 I L00 6010 • 50 100 15~) I I £ I I I MAGNETITE Ni MAGNETITE Cr MAGNETITE V • • • 5Zd

;2c

• • • ~b

~e • • % • % • 200 I 600~ , lO@C,• 05t lOj 15~ 071 08~ 09L ~• 111 12

Fig. 4. Compositional variations of separated minerals; Sr, K2O and Ba in plagioclase; Ni, Cr and Co in ilmenite: Ni, Cr and V in magnetite. Values in parts per million except where indicated. stricted to the Storeknuten profile, but is also pres- resent the first effect of the influx of new magma at ent in other profiles that cover this interval (Nielsen the boundary between MCU III and MCU IV, as and Wilson, 1991 ). This reversal is believed to rep- will be discussed below. MAGMA INFLUX AND MIXING IN BJERKREIM-SOKNDAL LAYERED INTRUSION 319

Discussion Rb. Duchesne (1978) has reported Rb values in plagioclase of < 2 ppm from equivalent levels in There is overwhelming evidence of a composi- the intrusion. This means that the 87Sr/86Sr ratios tional reversal at the base of MCU IV. This is shown in plagioclases with 850-1100 ppm Sr have in- by the occurrence of more primitive mineral assem- creased by 0.00007-0.00009 by the radioactive de- blages, by the Sr-isotope ratios and by the compo- cay of 87Rb since the rocks crystallized about 955 sitions of silicates and oxides. These features are ac- million years ago (Pasteels et al., 1979). The SVSr/ companied by a marked change in rock textures S6Sr values quoted here have therefore not been cor- across the MCU boundary. As has been noted by rected for Rb-decay. previous workers (Duchesne, 1972, 1978; Nielsen Previous workers have reported initial 87Sr/86Sr and Wilson, 1991 ), this reversal reflects the influx ratios of 0.705-0.706 in the BKSK Layered Series of primitive magma into the BKSK chamber at this (Demaiffe et al., 1979) and higher values in the level. This contribution, however, allows consider- range 0.710-0.712 in the mangerites and quartz- ation of processes such as the mechanism of influx, mangerites (Versteeve, 1975; Demaiffe et al., 1979; the degree of contamination of the new and resi- Wielens et al., 1980). In the latter rocks the initial dent magma and the role of magma mixing. ratios are deduced from Rb-Sr whole rock iso- chrons which give significantly younger ages (875 Comparison with previous Sr-isotope results Ma) than the U-Pb zircon age of 930 Ma (Pasteels et al., 1979). Assuming an age of 930 Ma the 878r/ It is assumed here that virtually all the Sr in the 86Sr initial ratio is close to 0.7085 (see discussion rocks studied is present in plagioclase. The rocks are in Demaiffe et al., 1986). Duchesne and Demaiffe notably poor in Rb; our recent analyses show that (1978) interpreted the relatively low 878r/86Sr ra- whole rocks from MCU IVa contain about 1 ppm tios in the Layered Series as reflecting a somewhat

ASSIMILIATION COMPOS]T[ONALLY~/~ ZONED MAGMA ~I e ~x-"':.~-"~-~---J~~ d ~-~S~ ~--/-~IHd A ~.....~-"----~-MODA L~A Y E R~-N G/~_ W y B "/~ll e

Fig. 5. Sketches of the Bjerkreim-Sokndal magma chamber during formation of the MCU III/IV boundary. Modified after fig. 5 in Nielsen and Wilson ( 1991 ). (A). Crystallization of the upper part of MCU Ill. The magma layer parental to MCU IIId is only present in the central, lowest part of the saucer-shaped chamber. MCU IIIe is crystallizing from the overlying magma layers towards the margins. (B). Magma replenishment by fountaining into the lowermost residual magma produces a hybrid magma layer. MCU IVa crystallizes during influx to produce a compositional reversal. The roof is partly pushed up and partly assimilated to accomodate the volume increase. 320 J.C. JENSEN ET AL. contaminated mantle-derived parental magma. The The formation of MCU IVa. nature of the transition from the low values in the Layered Series to the higher values in the overlying Nielsen and Wilson (1991) proposed that the mangerite is being studied by F.M. Nielsen. The high magma which entered the chamber at the base of values in the mangerite have been explained by De- MCU IV was relatively dense and initially mixed maiffe et al. (1979) as reflecting contamination of with the basal magma layers already in the cham- the BKSK magma by 15-30 wt.% gneissic country ber. Mixing was probably achieved by the new rock with an initial 8VSr/86Sr ratio at 950 Ma of magma fountaining into the residual magma, by a 0.7196 (Versteeve, 1975 ). The quartz-mangerites processs suggested by Turner and Campbell ( 1986 ). might also result from the partial melting of the This hybrid magma crystallized during continued country rocks (Wiebe, 1984) or might constitute influx to produce MCU IVa which has composi- one or two separate intrusions on top of the layered tions intermediate between the base of MCU IVb sequence (Wilmart, 1988 ). While the 87Sr/86Sr val- and the underlying unit. The hybrid MCU IVa is ues of 0.705-0.706 reported here are in agreement thickest in the central part of the chamber (Fig. 5 ). with previous results from the Layered Series, the This is a result of the relatively rapid crystallization variations across the MCU III-IV boundary reveal of this hybrid magma during influx, filling out the several interesting features, as will be discussed saucer shaped, central part of the chamber floor. below. The data presented here support the model outlined above for the hybrid nature of MCU IVa. The Evidence for magma strat(~cation. St-isotope ratios and trace-element data from plagioclases and Fe-Ti oxides all show a variation from It has become increasingly apparent in recent evolved compositions in MCU tile to primitive years that magma can become compositionally compositions in MCU IVb. The values in MCU IVa zoned (Turner and Gustafson, 1978; McBirney and are, in general, intermediate between those above Noyes, 1979; Huppert and Sparks, 1984). Nielsen and below. This supports the formation of this zone and Wilson ( 1991 ) consider that the magma in the from a hybrid magma, intermediate in composition Bjerkreim-Sokndal magma chamber was stratified, between those parental to MCU IIIe and the basal at least by the time that MCU III was crystallizing. part of MCU IVb, which became gradually more Since magma stratification is important in the con- primitive upwards as increasing proportions of new text of this paper, their evidence will be briefly sum- magma mixed with the resident magma. marized here. The 87Sr/86Sr ratio in plagioclases in MCU IIIe Nielsen and Wilson ( 1991 ) studied the MCU III/ (0.70616; average value of samples 1-3 in Table 1 ) IV contact in seven profiles (one of which--their is assumed to represent the ratio in the residual profile F--is the Storeknuten section) along its en- magma in the chamber, parental to MCU IIIe, just tire strike length of about 24 kin. They found that before the magma influx responsible for the rever- there is a fairly symmetrical discordance related to sal at the base of MCU lV. The lowest 87Sr/86Sr the boundary between MCU III and MCU IV in that value, at the base of MCU IVb (0.70496; sample MCU IIIe is absent and MCU IVa is thickest in the 5), is assumed to represent the Sr-isotope ratio of central part of the Bjerkreim lobe. The discordance the new magma. Based on the 87Sr/86Sr ratios of the between MCU IIId/e and the base of MCU IV is top sample in MCU IIIe and the bottom sample in explained by the crystallization of compositionally MCU IVb, it is possible to calculate the proportions stratified magma along a saucer-shaped floor, so that of magma with these isotopic compositions which magma parental to MCU IIId was crystallizing in mixed to form sample 4 from MCU IVa. the deepest, central part of the chamber while MCU Using the partition coefficients for the relevant IIIe was crystallizing on the flanks (Fig. 5). It was plagioclase compositions suggested for BKSK by at the stage of evolution illustrated in Fig. 5A that Duchesne (1978) gives values of 410 and 545ppm new magma entered the chamber. This model ex- Sr for the magmas parental to MCU IIIe and the plains the absence of MCU IIIe in the central part basal part of MCU IVb respectively. These values of the chamber and the symmetrically arranged and the determined 878r/86Sr ratios give calculated wedge-shape of MCU IIIe on either flank. mixing proportions of 89% residual magma and 11% MAGMA INFLUX AND MIXING IN BJERKREIM-SOKNDAL LAYERED INTRUSION 3 21 new magma for the analysed sample from MCU IVa. ELEVATED / Sample 4 comes from the central part of MCU IVa RESIDENT / and samples from higher in this unit can be ex- MA y2___/ _ pected to show progressively more influence from the new magma. o NEW An interesting feature is the compositional rever- MAGMA ~ MAGM:6 sal shown by the pyroxenes in the top sample from MCU lIIe which is ca. 2 m below the base of MCU IVa. Nielsen and Wilson ( 1991 ) suggested that this

was formed by reequilibration of the mineral com- ...... ii...... !iii!iiiiii:!iiii ! :: i! positions with primitive magma from the new in- Magma Magma flux which had percolated downwards and ex- ) density )density changed with the more evolved intercumulus melt. An alternative mechanism is related to the eleva- A. PONDING MODEL B. HYBRID MODEL tion of the resident magma column as a result of the Fig. 6. Schematic sections through the magma chamber after influx of dense magma at the floor of the chamber. the formation of the hybrid reversed zone MCU IVa showing Since clinopyroxene remains a cumulus phase after alternative magma density evolutions. (A). Ponding model. the reversal at the top of MCU IIIe, the sloping floor The new magma influx has a thickness of about 500 m (see in the Storeknuten section remained within the text) and did not mix with the resident magma to any significant extent. The base of the elevated resident magma has the magma layer parental to MCU IIIe. While the ele- same density as that parental to the top of MCU IIIe. (B). vation was not sufficient to bring the MCU IIId pa- Hybrid model. Magma mixing during influx resulted in the rental magma from the deeper part of the chamber formation of a hybrid zone several hundreds of meters thick. into contact with this section, it brought the sloping Note that the magma layering of the compositionally zoned floor here into contact with a lower level of the resident magma is not shown. As a first approximation, the Sr-isotope ratio is envisaged as being inversely proportional magma layers parental to MCU IIIe and crystal- to magma density. In (A) the Sr-isotope ratio of the new lized more magnesian pyroxenes. magma is gradually increased by progressive mixing with the overlying resident magma. In ( B ) the Sr-isotope profile is es- The crystallization of MCU IVb sentially established during magma influx and mixing.

Above the base of MCU IVb the 87Sr/86Sr ratios without further mixing. The alternative model in- gradually increase through MCU IVb and MCU IVc volves fairly rapid replenishment with turbulent to MCU IVd. The most obvious source with a rela- mixing throughout. tively high 87Sr/86Sr ratio is the resident magma already in the chamber during the crystallization of A'ponding model for MCU IVb MCU III. Since the 87Sr/86Sr ratio at the top of According to the ponding model, the new, dense MCU lIIe is 0.7062, the resident magma (strictly magma would elevate the resident magma column the basal layer of the compositionally zoned as illustrated in Fig. 6A. This relatively dense, magma) is assumed to have had a similar ratio be- primitive magma had olivine on the liquidus and fore the influx of fresh magma. crystallized fairly rapidly to form the massive leu- There are two possible models for the crystalli- cotroctolites of MCU IVb. Fractional crystalliza- zation of MCU IVb. One involves the dense new tion of this magma would give rise to the mineral magma ponding on the floor after initial mixing to compositional trends illustrated in Figs. 3 and 4. form the hybrid MCU IVa (Fig. 6A). The other as- While the mineral compositional trends can result sumes that the new and resident magma mixed to from fractional crystallization with or without ma- form a hybrid zone which was parental to MCU IVa jor mixing, the Sr-isotope trend can only have de- during influx and to MCU IVb after influx ceased veloped by the resident magma mixing with the new (Fig. 6B). According to Turner and Campbell magma. ( 1986 ), the first would imply that the dense magma Fractionation density considerations involving input from below was initially fairly rapid, forming the removal of olivine, Fe-Ti oxides and plagio- the hybrid IVa, but that it slowed down and ponded clase would decrease the density of the ponded 322 J.C. JENSEN ET AL. magma and when its density reached that of the significantly modified by post-cumulus processes basal overlying layer, the two would mix. Vigorous (Duchesne, 1978; Roelandts and Duchesne, 1979), thermal convection in the rapidly cooling ponded particularly when compatible trace elements are layer would cause effective mixing, so that the com- considered. The smooth and continuous evolution position of the entire ponded layer would become in the Sr content in plagioclase shows a variation slightly more evolved. The ponded layer would ini- from ca. 1090 ppm at the base of MCU IVb down tially have a 87Sr/86Sr ratio close to 0.7050, but the to 1036 ppm at the top. The partition coefficient for successive mixing in of overlying magma layers Sr between plagioclase and liquid can be taken as would gradually increase this value. After about 50 varying from 2.0 at the base to 2.1 at the top, values m of MCU IVb had accumulated, its Sr-isotope ra- well supported by Duchesne's (1978) model de- tio had increased from 0.7050 to 0.7053 (Fig. 3), scribing the evolution in the whole intrusion and in The resident magma (or the magma layer parental agreement with values in the literature (e.g. Hen- to MCU IIIe) is assumed to have had a ratio of derson, 1982). The proportion of plagioclase--the 0.7062. This would require that about 25% resident only St-bearing mineral in the rock--in the cumu- magma was mixed with new magma at this level. In late has been estimated at 75% by point counting an alternative model, the magma mixing took place and norm calculations of whole rock analyses. Solv- during influx and not during crystallization of MCU ing the Rayleigh equation between the base and the IVb. top of MCU IVb [see Duchesne (1978) for details ] gives a fraction of liquid of 0.82 at the top of MCU A hybrid model for MCU IVb IVb, which suggests that ca. 20% of the new influx If magma replenishment was rapid throughout, of magma has crystallized at the base of MCU IVc. turbulent mixing would occur with the resident The evolution of Cr in magnetite also gives inter- magma, resulting in a density gradient in the lower esting information, though rather less constrained. part of the magma chamber. We have argued earlier Point counting on the oxide minerals shows that cu- that the resident magma was compositionally zoned mulus magnetite represents 15 _+ 2% of the oxides. before the formation of MCU IV, and the density The amount of ilmenite in the rock (5%) can be gradient in the lower part of the chamber would be assessed accurately through norm calculations. Par- reinforced by this input (Fig. 6 ), The magma above allel evolution of Cr in magnetite and ilmenite the mixing zone will simply have been elevated. Ac- through MCU IVb allows the ratio of the partition cording to this model the mineral compositional and coefficient (KD) for Cr between magnetite and il- isotope trends resulted from the fractional crystal- menite to be fixed at 22. Introducing these values lization of compositionally zoned hybrid magma. into the Rayleigh equation, together with the fraction of liquid deduced from the Sr evolution in pla- Estimating the volume of new magma emplaced at gioclase, allows calculation of the partition coeffi- the base of MCU IV cient between magnetite and liquid. This value varies between 520 and 710 and is strongly depen- The volume of new magma emplaced at the base dent on the amount of cumulus magnetite. A large of MCU IV can be estimated in a variety of ways. interval of KD values for Cr in magnetite ( 100-620 ) These involve geochemical modelling, the thick- has been reported by Lindstrom ( 1976 ) on the ba- ness of cumulate stratigraphy repeated and Sr-iso- sis of experimental data and a value of 200 has been tope ratios. adopted by McCarthy et al. (1985) in the modelling of magnetite in the Bushveld intrusion. On the Geochemical modelling other hand, Duchesne and Hertogen (1988) used If we assume that MCU IVb crystallized essen- major and trace elements to constrain the compo- tially from ponded magma, the well defined trace- sition of the parental magma to MCU III and IV element patterns in plagioclases and oxides allow its and suggested that a fine grained (chilled) monzo- crystallization to be modelled. As already men- norite found at Tjorn has the appropriate composi- tioned, the cumulus minerals in MCU IVb are pla- tion. The Cr content at Tjorn is 28 ppm which gives gioclase, olivine, ilmenite and magnetite, and there a KD value of 510 for Cr in magnetite, a value close is no indication that their compositions have been to the upper limit found here. MAGMA INFLUX AND MIXING IN BJERKREIM-SOKNDAL LAYERED INTRUSION 323

This internally consistent model thus permits us the base of MCU IVa. If the resident magma was to conclude that the new influx of fresh magma that simply elevated without any mixing, this value gave rise to MCU IVb was approximately five times would represent the thickness of the new magma in- thicker than the MCU IVb rock unit, and therefore flux. Progressive mixing during influx would have was of the order of 500 m thick. However, this mo- two opposing effects: the Sr-isotope ratio in the hy- delling assumes that we are dealing with a closed brid magma would gradually increase; at the same system. As discussed above, the Sr-isotope profile time the resident magma would, in part, be elevated developed as a result of progressive mixing with the and the St-isotope ratio of the lowest magma overlying resident magma, and consequently a layer(s) would decrease by the mixing process. model of assimilation-fractional crystallization Continuous assimilation of gneiss at the roof would (AFC) (DePaolo, 1981 ) would theoretically better gradually increase the ratio in the resident magma. account for the evolution, assuming that the con- The temperature increase associated with magma taminant is the elevated resident magma itself. influx would fairly rapidly propagate up through the In fact, an AFC model brings no further infor- magma column, increasing the rate of roof melting. mation because of the geochemical characteristics However, in view of the relatively rapid process of of the elements considered here. Cr has a high bulk magma influx compared with the assimilation and partition coefficient (ca. 5) and a very low concen- crystallization history of the intrusion, the latter ef- tration in the contaminating magma ( < < 0.2 ppm). fect is thought to be minor compared with the Its behaviour is therefore mainly constrained by former. fractional crystallization and not by assimilation. On Three independent estimates of the thickness of the other hand, Sr has a bulk partition coefficient the new magma influx at the base of MCU IV are close to unity and its concentration in the contami- therefore > 500 m (geochemical modelling), about nant (451 ppm) is similar to that in the fresh magma 500 m (phase layering) and about 350 m (Sr iso- (545 ppm). For high values of the fraction of melt topes). Considering the sources of error in each of (F= 0.7-1.0 ), simple fractional crystallization and these methods of estimation there is reasonably good AFC processes become indistinguishable under agreement; MCU IVb therefore represents 20-30% these conditions. crystallization of the new influx.

Repetition of the cumulate stratigraphy Comparison of the Sr-isotope and mineral cryptic The new influx at the base of MCU IV arrived in variations in MCU IVb. the profile in Fig. 2 after about 100 m ofMCU IIIe had crystallized; the same level of evolution is As discussed above, the gradual increase in Sr- reached in MCU IV after about 500 m. This 500 m- isotope ratios through MCU IVb reflects progres- thick reversal represents the thickness of rocks which sive mixing between the new, primitive magma in- crystallized before the conditions prevailing at the flux and the more evolved resident magma. Trace top of MCU IIIe were reestablished. If there was no elements in Fe-Ti oxides (Fig. 4) indicate progres- mixing during emplacement it would represent the sively more evolved compositions through MCU minimum thickness of the new magma influx since IVb, as do plagioclases (Fig. 3F), although the lat- this would also be parental to more evolved magma ter trend is very erratic. The trend shown by oli- as a result of fractionation. On the other hand, mix- vines (Fig. 3E) is, however, distinct and cannot be ing would produce hybrid magmas which would be explained by progressive mixing and/or fractional parental to MCU IVb, c and d; these are conse- crystallization. The olivines are clearly not in equi- quently thicker than if they crystallized exclusively librium with the coexisting plagioclases. It should from the new influx. be recalled here that the Ca-poor pyroxenes in MCU IVb are secondary reaction products from olivine Sr-isotopes and therefore show the same trend. It therefore Extrapolation at the top of Fig. 3G indicates that seems likely that olivines have not retained their an 87Sr/86Sr ratio of 0.7062, the same as that in the primary compositions. rocks at the top ofMCU llIe before the new influx, A process by which the olivines could have would be present at a height of about 350 m above changed their compositions involves trapped liquid 324 J.C. JENSEN ET AL. shift (Barnes, 1986 ). By this process, cumulus solid Appendix solution minerals reequilibrate towards more evolved compositions by reaction with inte~cumu- Mineral separation. The samples were crushed to lus melt. A trapped liquid shift of about 5% Fo in a grain size of 60-150 mm and split in bromoform. the lower half of MCU IVb, decreasing upwards to The buoyant plagioclase fraction was purified by close to zero at the top, may explain the observed passage through a Frantz isodynamic magnetic sep- trend. Since the rocks contain about 10% cumulus arator. Magnetite was separated by a hand magnet olivine, a shift of 5% Fo would require about 10% and ilmenite by passage through the Frantz separa- trapped liquid (Barnes, 1986). It has been argued tor. Both Fe-Ti oxides were purified using thallium above that MCU IVb crystallized fairly rapidly in formate. Trace element analyses of the separated the initial stages, so that the proportion of trapped minerals were carried out at the Collectif Intersti- liquid would be expected to be high. The amount of tutionnel de G6ochimie Instrumentale, Universitd trapped liquid decreased upwards together with the de Li6ge, by X-ray fluorescence following methods rate of crystallization. While olivines readily re- described in Duchesne ( 1972, 1978). equilibrate, plagioclases do so more reluctantly be- Isotope analyses. Strontium isotope analyses were cause of the more complicated cation exchanges in- performed at the Laboratoires associds de Gdolo- volved; plagioclase zoning is therefore common in gie-Pdtrologie-G6ochronologie, Universit6 Libre de cumulates while olivines are generally homogene- Bruxelles. Plagioclase was dissolved in a 9:1 mix- ous. Plagioclases in MCU IVb are not zoned, but ture of hydrofluoric and perchloric acid and Sr sep- this is believed to be a result of extensive post-cu- arated in an ion exchange column. The Sr isotope mulus recrystallization under granulite facies con- composition was measured using a double Re fila- ditions. It therefore seems likely that the plagio- ment on a Finnigan MAT 260 mass spectrometer. The NBS 987 standard gave a 87Sr/S6Sr ratio of clases originally had slightly Ab-enriched rims 0.710219+_ 0.000024 (2 or). because of trapped liquid shift, although the large modal proportion of plagioclase in these rocks (ca. Microprobe techniques. Microprobe analyses were performed at the Department of Earth Sciences, 80%) means that the effect is less than with olivine. University of Aarhus, using the energy-dispersive The present erratic plagioclase trend in both MCU method. The analyses are usually an average of 4-5 IVa and b is therefore a result of a combination of points from different grains in a polished thin sec- factors. tion. The error bars in Fig. 3 are at the one standard While the olivine trend in MCU IVb is relatively deviation level, based on about ten separate grains systematic in the Storeknuten profile, the pattern is of the mineral in typical thin sections. The values not generally repeated in MCU IVb in other pro- obtained are: olivine 2sd= 1.70% Fo, Ca-poor py- files (fig. 3 in Nielsen and Wilson, 1991 ). The pro- roxene 1.24% Mg:~, plagioclase 2.75% An. Since the cesses responsible for the olivine compositional mineral compositions in Fig. 3 are averages of sev- trends in Fig. 3E were therefore of local character; eral points, the appropriate error bars are consider- this is consistent with varying amounts of trapped ably less than these values. All analyses are with a liquid. focused beam. Pyroxenes usually show exsolution textures and analyses represent subsolidus compositions. Plagioclases are not usually optically zoned, Acknowledgements but grains within a thin section often vary in composition by 2-3 mol% An. Financial support for the Aarhus-based partici- pants to study the Bjerkreim-Sokndal Intrusion was References provided by Danish Natural Science Research Council grants to JRW. The Belgian Fund for Joint Barnes, S.J., 1986. The effect of trapped liquid crystallization Basic Research supported J.C.D. and D.D. We on cumulus mineral composition in layered intrusions. would like to thank Sidsel Grundvig (Aarhus) for Contrib. Mineral. Petrol., 93: 524-531. Barton, M. and Gaans, C.V., 1988. Formation oforthopyrox- help with the microprobe analyses and G. Bologne ene-Fe-Ti oxide symplectites in Precambrian intrusives, and G. Delhaze (Li6ge) for help with the chemical Rogaland, southwestern Norway. Am. Mineral., 73: 1046- analyses and mineral separation. 1059. MAGMA INFLUX AND MIXING IN BJERKREIM-SOKNDALLAYERED INTRUSION 325

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