American Mineralogist, Volume 73, pages 1046-1059, 1988

Formation of orthopyroxenFFe-Ti oxide symplectites in Precambrian intrusives,Rogaland, southwestern Norway

Mrcrr.tnr, Banror.lDepartment of Geology and Mineralogy, The Ohio State University, Columbus, Ohio 43210, U.S.A.

Crrnrs YAN GllNsBilliton Research, B. V., Westervoortsedijk 67d, Postbus 38, 6800 LH Arnhem, The Netherlands

Ansrru.cr

Petrographic and mineral chemical data are presented for orthopyroxene-Fe-Ti oxidesymplectites in three intrusive bodies from southwestern Norway: the Bjerkreim-Sokndallopolith, the Hidra anorthosite, and the Lyngdal hyperite. Detailed studies of the Bjer-kreim-Sokndal lopolith indicate that symplectites are not common, occurring in only sixthin sections out of 1037 studied. Only one of these samples contains olivine, and thesymplectites clearly do not replace or form.pseudomorphs after olivine. The symplectitesin the Hidra anorthosite and Lyngdal hyperite do appear to replace olivine. The propor-tions of orthopyroxene and Fe-Ti oxide in the symplectites are fairly constant and rangefrom 7 3:27 to 66:34 (volo/o). Microprobe data demonstrate that the orthopyroxenes do nothave compositions appropriate for crystallization at or above the solidus. Many of thepyroxenes in these intrusives have re-equilibrated during subsolidus cooling, but theorthopyroxenes in the symplectites show no exsolution and thus must have formed sub-solidus. Pyroxene thermometry indicates temperatures of formation of 700-800 qC atestimated pressures of 5 kbar or less (upper amphibolite4ranulite facies conditions). TheFe-Ti oxide in most of the symplectites is titaniferous magnetite that shows complexexsolution phenomena indicative of extensive re-equilibration during subsolidus cooling.It is virtually impossible to reconstruct the original compositions or to apply oxide ther-mometry.

The symplectites do not result from discontinuous precipitation from a supersaturatedsolid solution, eutectoidal breakdown of a pre-existing phase or isochemical replacementofolivine. It is suggested that the occunence ofsymplectites reflects the overall re-equil-ibration of the original magmatic mineral assemblage during high-grade retrogressivemetamorphism. They form only in the vicinity of orthomagmatic Fe-Ti oxide. The precisemechanism of formation is obscure, but appears to involve co-operative nucleation, pre-dominantly at grain boundaries, and sympathetic growth via a bridging mechanism. Thedevelopment of oriented intergrowths is governed by the requirements of minimal inter-facial and strain energy. Diffusion mainly occurs along grain boundaries or advancinginterfaces and is probably aided by a thin film of intergranular fluid. The supercoolingnecessary for symplectites to form probably reflects the high energy barriers to solid-statenucleation and growth.

INrnorucrtoN

Fine-grained intergrowths (symplectites) are relativelycommon in intrusive igneous rocks, but there is still muchcontroversy about their origin. This is especially true inthe case of orthopyroxene-opaque oxide (magnetite and,/or ilmenite) symplectites that occur in gabbroic-noriticplutonic and hypabyssal rocks. In essence, the controver-sy centers upon the question of whether such symplectitesform by subsolidus processes or whether they form di-rectly from a melt by coprecipitation.

That symplectitic intergrowths can form by subsolidusprocesses is irrefutably demonstrated by their occurrencein metamorphic rocks, and convincing arguments have

been presented that the orthopyroxenmpaque oxidesymplectites in some plutonic rocks formed after solidi-fication was completed (Frodesen, 1968; Starmer, 1969;Esbensen, 1978; Van Lamoen, 1979). These argumentsare largely based upon petrographic or textural evidencesuch as the tendency for the intergrowths to replace oliv-ine in reaction coronas and their occurrence in rocks thatcontain ample other evidence for subsolidus re-equilibra-tion. Attempts to model the formation of the symplectitesquantitatively (see especially Van Lamoen, 1979) indi-cate that this is not a simple isochemical process, but theconsequences of this conclusion have not, to date, beenadequately explored.

It is extremely difficult to prove that orthopyroxenF

0003-004x/88/09 I 0-l 046$02.00 1046

opaque oxide symplectites in gabbroic-noritic intrusionsformed by coprecipitation from a melt. The main reasonfor this is that the long cooling histories ofrelatively largemagma bodies, coupled with the release of volatiles inthe last stages of solidification (see Morse, 1980, p. 328)and the possible access of circulating meteoric water(Taylor and Forester,1979), allow continuous chemicaland textural re-equilibration of the original mineral as-semblage. It may thus be virtually impossible to distin-guish between minerals and textures formed by crystal-lization in residual melt pockets and those formed laterin the presence of deuteric and/or meteoric fluids.

The particular model adopted for the formation ofsymplectites can profoundly influence interpretation ofthe cooling histories ofplutonic rocks and hence influenceinterpretation of the thermal history of the crust. For ex-ample, experience gained by metallurgical engineers sug-gests that symplectites can only grow from melts if thelatter are supercooled so that ifa supersolidus origin forsymplectites is adopted, an explanation of how super-cooling develops in magmas crystallizing at depth withinthe crust is required. Constitutional supercooling (Chal-mers, 1964; Chadwick, 1972) could explain symplectiteformation, but it is necessary to demonstrate that consti-tutional supercooling indeed occurs during crystallizationof magmas in the plutonic environment. If the symplec-tites form by subsolidus reactions, then given diffusiondata for relevant components, together with estimates ofthe conditions of formation, it should be possible to esti-mate the length of time required for formation. Clearly,however, it is first necessary to examine models for theorigin of symplectites, and this is the primary goal of thepresent paper.

In this paper we report the results ofa detailed inves-tigation into the occurrence and mineral chemistry of or-thopyroxene-magnetite-ilmenite symplectites in intru-sive Proterozoic igneous rocks from Rogaland and VestAgder, southwestern Norway. Initially, we concentratedupon the symplectites in the Bjerkreim-Sokndal lopolith,and most of the data presented are for samples from thisintrusion. Subsequently, we have also studied symplec-tites in the Hidra anorthosite massif and in the hyperites(gabbro-norites) from the Lyngdal area, and we includeobservations on samples from these intrusions. All of therocks studied have re-equilibrated to a greater or lesserdegree during prolonged subsolidus cooling.

The Bjerkreim-Sokndal lopolith

The Bjerkreim-Sokndal lopolith is a large layered in-trusion (total volume unknown) associated with anor-thosite massifs (Fig. l). Geochronological studies of thelopolith and the surrounding metamorphic envelope in-dicate intrusion ages between - 1050 m.y. and -950 m.y.(Pasteels et al., 1979;, Priem, 1980; Wielens et a1., l98l).The spread in age is thought to be real and to reflect acomplex intrusive history.

In this paper we give only a brief summary of the ge-ology of the Bjerkreim-Sokndal intrusion. A more de-

lo47

tailed description, including new observations and data,will be presented elsewhere (Voncken and Barton, inprep.). The lower part of the intrusion (hereafter referredto as the anorthositic-leuconoritic phase) consists of atleast five major rhythmic units, each of which has anor-thosite at the base and grades upward via leuconorite tonorite (the latter is not present in the two lowermostrhythmic units). The five rhythmic units are thought toresult from fractional crystallization with superimposedmagma mixing (Duchesne, 1972a;Hermans et al., 1975;Duchesne and Demaife, 1978). The rocks of the lowerrhythmic units (RI, RII, and the lower parts of RIII andRIV) consist essentially of plagioclase, orthopyroxene, andilmenite. Clinopyroxene, titaniferous magnetite, and apa-tite appear as major components in the upper parts ofRIII and RIV, whereas K-feldspar initially appears as acomponent of antiperthite in RV. Olivine occurs onlysporadically near the base of RIV, in what appears torepresent a separate, later intrusion.

The upper part of the intrusion is mangeritic to qllartzmangeritic (P. Michot, 1960, 1968; J. Michot, 1960; J.Michot and P. Michot, 1968) or pyroxene monzonitic tosyenitic (Hermans et al, 1975) in composition. In thepresent paper we refer to the upper part ofthe intrusionas the quartz monzonitic phase (cf. Rietmeijer, 1979).Duchesne (1972a, 1978) and Duchesne and Demaiffe(1978) have proposed that this phase represents the re-sidual liquid, possibly contaminated with crustal material(Pasteels et al., 1979), from crystallization of the lowerlayered series. Rietmeijer (1979), however, has found evi-dence for a structural discontinuity between the upperand lower parts of the lopolith that is reflected in thechemistry of the pyroxenes. This discontinuity suggests atime gap between the intrusion of the anorthositic-leu-conoritic phase and the quartz monozonitic phase that isdifficult, though not impossible, to reconcile with a modelof continuous magmatic differentiation as envisioned byDuchesne (1972a, 1978) and Duchesne and Demaiffe( l e78).

Distribution of orthopyroxene-magnetite-ilrnenitesymplectites in the Bjerkreim-Sokndal lopolith

Representative samples were examined of all of themajor rock types occurring from the base to the top ofthe lopolith. Out of a total of 1037 thin sections studied,symplectites were found in only six. Five of the six sam-ples were from the uppermost part of RV, the sixth wasfrom an olivine-bearing sample near the base of RIV.Thus, with one exception, the symplectites occur at asimilar stratigraphic height and the exception occurs inwhat appears to be a separate, Iater intrusion.

Pnrnocru,pHlc SUMMARY

The rocks in which the symplectites occur are mostlymonzonorites or biotite norites (top of RV). The olivine-bearing sample is a leuconorite. The textures of these rocksare highly variable, even within a single thin section, butare mostly granular or slightly porphyritic or porphyro-

BARTON AND VAN GAANS: ORTHOPYROXENE-Fe-Ti OXIDE SYMPLECTITES

1048 BARTON AND vAN GAANS: ORTHOPYROXENE-Fe-Ti OXIDE SYMPLECTITES

Fig. 1' Geologic map of Rogaland and Vest Agder showing the distribution of the major rock types. The positions of thehypersthene-in and osumilite-in isograds are taken from Hermans et al. (1975). The Hidra anorthosite is the small intrusive SSWof Flekkefiord. Hyperite occurs also to the south of Lyngdal in a small intrusive body that is not shown on the map.

clastic owing to the occurrence of large (up to 4 mm)anhedral feldspar crystals set in a finer-grained matrix.Cumulate textures are absent. In all samples there is evi-dence of deformation, i.e., the minerals show unduloseextinction, strain lamellae, kink bands, and the devel-opment of subgrain boundaries. Some samples showgranulation.

Plagioclase is the dominant mineral (30-70 volo/o) andshows minor zoning (<40lo An). The range in plagioclasecomposition for all samples is An' to Anrr. In two of themonzonorites (R186 and R486), the plagioclase showscomplex exsolution phenomena and intergrowth forms.In addition to antiperthite (not present in all samples),patch-perthite and irregular intergrowths of plagioclaseand alkali feldspar occur. The fledspars have not beenstudied in detail, but all stages of exsolution-from anti-perthite through patch-perthite to complete segregationof two phases-are present.

Orthopyroxene (10-300/o) occurs in various forms: inintergrowths with oxide minerals (described more fullybelow), as relatively large (mostly < I mm, rarely up to5 mm) subhedral crystals that show exsolution and arerimmed by clinopyroxene, as rims around clinopyroxeneand olivine and as small granular crystals associated withnonexsolved equigranular grains of feldspar.

Inverted pigeonite was identified as an accessory phasein only one sample (R486).

All samples contain 5-l5o/o oxides (titaniferous mag-netite and ilmenite) as discrete grains or as inclusions inpyroxenes and as components of symplectites. The oxidesshow complex microstructures that undoubtedly reflectsubsolidus re-equilibration.

Clinopyroxene occurs in five samples; it is absent fromone of the norites (R195). It occurs as subhedral crystals(sl mm) that show exsolution and are rimmed by or-thopyroxene, as rims around orthopyroxene crystals, andas small granular crystals (cf. description of orthopyrox-ene). In general, the exsolution shown by the larger sub-hedral pyroxenes is similar to that described in detail byRietmeijer (1979) and Rietmeijer and Champness (1982)for other samples from the Bjerkreim-Sokndal lopolithand need not be further described here beyond notingthat exsolution is occasionally bleblike and irregular andthat it may coarsen toward and coalesce with an externalrim of exsolved material (cf. Bohlen and Essene, 1978).The pyroxenes also contain platelets of ilmenite orientedalong the (010) and (001) planes.

Other, less abundant, minerals include olivine, apatite,spinel, sulfides, biotite, amphibole, and K-feldspar. In theone sample in which it occurs (GA248), olivine (Fo^) is

LEGEND

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BARTON AND VAN GAANS: ORTHOPYROXENE-Fe-Ti OXIDE SYMPLECTITES t049

Fig.2. (Upper left) Irregular symplectite. The irregular outline suggests that this intergrowth is not a pseudomorph after olivine.(Upper right) A rim of symplectite around clinopyroxene. (Lower left) Symplectite replacing olivine in the Hidra anorthosite. (Lowerright) Association of symplectite and orthomagmatic Fe-Ti oxide. Field of view in all photographs is |.4 x 2.8 mm.

rounded with irregular, resorbed margins and is rimmedby orthopyroxene. Spinel occurs as discrete grains or la-mellae in titaniferous magnetite. Sulfides occur in glob-ules suggestive of formation by liquid immiscibility. Bio-tite and amphibole rim discrete grains of Fe-Ti oxide andpyroxene and also form small, euhedral crystals. K-feld-spar is found as discrete, polygonal crystals and in com-plex intergrowths with plagioclase. Both sanidine and or-thoclase are present.

Description of the symplectites

The symplectites are irregular in shape and in mostcases do not appear to be pseudomorphs (Fig. 2a). Theyform small areas in the core of an orthopyroxene crystalor complete areas l-2 mm in diameter. They sometimesoccur as rims around clinopyroxene (Fig. 2b). Two typesof intergrowth occur: lamellar and vermicular, the lbrmercommonly grading outward into the latter. The width ofthe Fe-Ti oxide component is mostly 5-10 pm but canreach 30 pm. No grain boundary is observed betweendiscrete grains and symplectitic Fe-Ti oxide; optical ori-entation and microstructures are continuous between thetwo. Evidently, the symplectitic and discrete Fe-Ti oxideshave re-equilibrated to a similar degree. The orthopyrox-ene component of the symplectites contains no exsolu-tion lamellae and is identical to the orthopyroxene that

forms rims around olivine and clinoplroxene or occursas discrete granules.

The Fe-Ti oxide component of the lamellar symplec-tites often appears to be oriented such that its (l I l) planeis parallel to the (100) plane of the associated pyroxene.Detailed studies show, however, that these intergrowthsare irregular in three dimensions and that the Fe-Ti oxidelamellae are not continuous along their length. The Fe-Ti oxide in the vermicular symplectites is occasionallyskeletal. In some cases, the Fe-Ti oxides radiate from asmall apatite crystal. Exsolution textures in the Fe-Ti ox-ides are complex. In all but one sample (Rl95) the Fe-Tioxide in the symplectites is predominantly titaniferousmagnetite with thin exsolution lamellae of ilmenite ori-ented parallel to (l I l) and extremely small lenses of spi-nel oriented parallel to (100) and (l I l). Within a singletitaniferous magnetite vermicule, 2-3 generations of il-menite lamellae and l-2 generations of spinel lenses canbe recognized. Microstructures in the FerOr-rich ilmen-ites are more simple. Occasional spinel lenses are ob-served, and in one sample (R195) abundant lamella ofhematite occur.

The proportions oforthopyroxene and Fe-Ti oxide inthe symplectites has been determined by line counting.For each sample, 3-5 areas were measured, and over 4000lines were counted for each area. The precision of the

1050 BARTON AND VAN GAANS: ORTHOPYROXENE-Fe-Ti OXIDE SYMPLECTITES

/R 4 8 6

%d

Fig. 4. Compositions of pyroxenes in the symplectite-bearingsamples plotted in the conventional quadrilateral. Solid symbols: samples from the Bjerkreim-Sokndal lopolith; open circles :sample from the Hidra anorthosite; squares : sample from theLyngdal hyperite.

in rare olivine-bearing leuconorites or in monzonoritesclose to the margins of the intrusion (H. P. Drent, oralcomm., 1982) whereas in the latter they have been foundin rare olivine-bearing biotite norites. Most of the sym-plectites differ from those in the Bjerkreim-Sokndal lopo-lith in that they clearly replace or, in extreme cases, formpseudomorphs after olivine (Fig. 2c). In those cases wherereplacement is incomplete, it is apparent that the devel-opment of symplectites is restricted to the vicinity of or-thomagmatic Fe-Ti oxide (Fig. 2d). Where grains of or-thomagmatic oxide are absent, the olivine is corrodedand is rimmed by orthopyroxene. In other respects, how-ever, the symplectites (and also the discrete pyroxenes,Fe-Ti oxides, etc.) in these intrusions are similar to thosein the Bjerkreim-Sokndal lopolith. In addition to the or-thopyroxene-Fe-Ti oxide symplectites, the hyperites con-tain biotite-orthopyroxene and clinopyroxene-albite in-tergroWhs.

Mineral chemistry

All analyses were obtained using conventional wave-length- and energy-dispersive electron-microprobe tech-

R 2 5

ncf, '7 o +

F "

Fig. 3. Comparison of the compositions of symplectitic (opencircles) and nonsymplectitic (frlled circles) in two samples fromthe Bjerkreim-Sokndal lopolith.

measurements varies from lolo to 5olo (relative). The pro-portions of orthopyroxene and Fe-Ti oxide range from73:27 to 66'.34 (volo/o), i.e., the proportions of the phasesare relatively constant.

In most cases, the symplectites are perfectly fresh, butoccasionally they are replaced by other phases-mainlybiotite and brown pleochroic amphibole. In some cases,the original symplectite is now represented by a relictamphibole-magnetite or biotite-magnetite symplectite.

Symplectites in the Hidra anorthositeand Lyngdal hyperite

Symplectites are not common in the Hidra anorthositeor in the Lyngdal hyperite. In the former, they occur only

TABLE 1. Average compositions of pyroxenes in the symplectite-bearing samples

R1 95 R186

sio,Tio,Al,o3FeO-MgoMnOCaONaro

Total

52.00 .181.40

23.221.50.400.53n.o.

99.2

51.20.201.35

15.910.60.31

20.30.40

100.3

52.8 51.60.15 0.602.55 3.63

16.8 7.0826.2 14.40.28 0.150.65 22.7n.d. 0.68

99.4 100.8

1.927 1 .8990.004 0.0170.1 10 0 .1570.513 0.2181.426 0.7900.009 0.0050.025 0.895n.d. 0.049

4.O't4 4.030

49.7 50.70 .10 0 .100.88 1.88

33.4 15.914.2 10.40.73 0.581.03 20.2n.d. n.d.

100.0 99.8

1.963 1.9530.003 0.0030.041 0.0851 .103 0.5120.836 0.5970.024 0.0190.044 0.833n.d. n.d.4.014 4.002

50.2 51.5n.d. 0.510.43 1.30

36.2 16.812.5 9.30.60 0.370.78 21.1n.d. n.d.

1 00.7 100.9

1.989 1.970n.d. 0.0150.020 0.0s91.200 0.5370.738 0.5300.020 0.0120.033 0.864n.d. n.d.4.001 3.987

5 1 . 0n.o.0.52

33.214.40.640.93n.o.

100.7

Numbsr of cations on the basis of six oxygens

0.0100.8330.0304.017

J I

TiAIFe"MgMnCaNa

Total

1.9610.0050.0620.7321.2090.0130.021n.o.

4.003

1.991n.d.0.o241.0840.8380.0210.039n.d.3.997

1 9620.0060.0610.5100.605

Note: GA249, R195, R53, R186, R486, R2s-Bjerkreim-Sokndal lopolith; H1-Hidra anorthosite; Ll-Lyngdal hyperite; n.d. : not detected.'Total Fe reoorted as FeO or Fe2*.

niques and are fully corrected. Most analyses were per-formed with a focused (0.5 to 2 prm) electron beam, buta broad, defocused spot (>50-pm diameter) was used toobtain bulk analyses of some symplectites and exsolvedpyroxenes.

In each sample, the orthopyroxenes show a restrictedrange of composition, with variations of the order of 3molo/o En and 1.5 mol0/o Wo. There are no systematiccompositional differences between the orthopyroxene inthe symplectites and the host orthopyroxene in largegrains of low-Ca pyroxene, the orthopyroxene that formsrims on clinopyroxene or olivine, and the discrete gran-ular orthopyroxene (Fig. 3). Likewise, most clinopyrox-enes in each sample show a restricted compositional range,corresponding to about 3 molo/o Wo and 2 molo/o En, andfor this reason, average pyroxene compositions are givenin Table 1 and are plotted in the conventional Ca-Mg-Fediagram in Figure 4.

There are, however, significant differences between py-roxenes analyzed with a focused beam and those analyzedwith a broad beam. Reintegrated analyses of pyroxenesfrom the Bjerkreim-Sokndal lopolith will be reportedelsewhere, but from Figure 4 it is obvious that the low-Ca pyroxenes showing exsolution were originally muchricher in Ca than the symplectitic low-Ca pyroxenes andalso that the high-Ca plroxenes that show exsolution wereoriginally much poorer in Ca than the clinopyroxene thatoccurs as granular crystals and rims around the largerorthopyroxene crystals (Fig. a).

Considering all samples, both the orthopyroxenes andthe clinopyroxenes show a wide range in Mg/(Mg +2Fe2*),' from 0.73 to 0.53 and 0.78 to 0.44, respectively.In the case of the Bjerkreim-Sokndal lopolith, this vari-ation can be correlated with location in the intrusion, Mg/

' Total Fe reported as Fe'?t

f eete 1-Continued

opx cpx

1051

En FsFig. 5. Comparison of pyroxene compositions in symplec-

tite-bearing samples with pyroxene crystallization trends in oth-er rocks. (a) Data for other intrusive bodies; Sk: Skaergaard;Bu : Bushveld; BS : Bjerkreim-Sokndal (data from Duchesne,1972b). (b) Generalized solidus (solid lines) and subsolidus(dashed lines) crystallization trends from Huebner (1980). Notethe close correspondence in the compositions of the high-Capyroxenes and Duchesne's crystallization trend and Huebner'ssubsolidus crystallization trend. Symbols as in Fig. 4.

(Mg + 2Fe2*) decreasing with increasing stratigraphicheight. This trend is generally similar to that in basiclayered intrusions such as Skaergaard and to that in theMarcy anorthosite massif (Ashwal, 1982). The orthopy-roxenes are characterized by relatively low AlrO. (<3.0wto/o), TiO, (<0.3 wto/o) and Fe,O, (<2.6 wto/o)' The latterwas calculated by assuming that the sum of the cationsis four on a six oxygen basis. The clinopyroxenes containslightly higher amounts of these oxides: AlrO, up to 3.7wt0/0, TiO, up to 0.8 u4o/o and FerO, up to 3.8 wto/0. Thelow-Ca contents of the orthopyroxenes are remarkable,especially considering the range in Mg/(Mg * )Fe'z*)' Thisis illustrated in Figure 5 where the compositions of thepyroxenes are compared with the solidus trends for theorthopyroxenes in the Skaergaard and Bushveld intru-sions and with the solidus trend established for the Bjer-kreim-Sokndal low-Ca pyroxenes by Duchesne (1972b).The clinopyroxenes are more calcic than the solidus high-Ca pyroxenes in the Skaergaard and Bushveld intrusionsbut are compositionally similar to supposed solidus high-Ca pyroxenes from the Bjerkreim-Sokndal lopolith (Du-

chesne, 1972b). The compositions ofthe orthopyroxenesand clinopyroxenes agree well with Huebner's (1980)

generalized subsolidus trends for quadrilateral pyroxenes

(Fie. 5).In contrast, the reintegrated bulk compositions of

exsolved pyroxenes in the Bjerkreim-Sokndal lopolith,Hidra anorthosite, and Lyngdal hyperite are similar tothose of solidus pyroxenes in other layered intrusions andto Huebner's (1980) solidus trends for quadrilateral py-

BARTON AND VAN GAANS: ORTHOPYROXENE-FE.Ti OXIDE SYMPLECTITES

D i

L1

49.4 50.90 .10 0 .180.65 1 35

37.2 18.211.4 8.980.78 0.380.88 20.2n.d. 0.50

100.4 100.7

1 979 1.9640.003 0.0050.031 0.0611.246 0.5870.681 0.5160.026 0.0120.038 0.835n.d. 0.0374.003 4.019

52.2 50.9n.d. 0.592.93 3.70

19.0 7.7224.9 14.00.39 n.d.0.56 22.1n.d. 0.64

100.0 99.7

1 .915 1 .899n.d. 0.0170.127 0.1630.583 0.2411.362 0.7780.012 n.d.0.022 0.883n.d. 0.0464.021 4.027

53.0 52.40.11 0.292.36 1.96

18.7 7.8025.8 14.3o.24 0.14o.42 21.9n.d. 0.86

100.6 99.7

1.926 1.9530.003 0.0080.101 0.0860.s68 0.2431.398 0.7950.007 0.0040.016 0.784n.d. 0.0624.020 4.026

t052

Tnele 2. Temperature estimates (in €) based upon the com-positions of coexisting pyroxenes

BARTON AND VAN GAANS: ORTHOPYROXENE-Fe.Ti OXIDE SYMPLECTITES

. Temperatures based on s-kbar isothermal section.

roxenes. Nevertheless, from the petrographic descrip-tions, it should be clear that the analyses do not neces-sarily represent the compositions of pyroxenes crystallizedfrom a melt (i.e., some external granule exsolution hasoccurred). It follows that the analyses ofcalcic pyroxenesreported by Duchesne (1972b) cannot represent soliduscompositions.

The temperatures at which the pyroxenes equilibratedcan be calculated from the relationships given by Woodand Banno (1973) and Wells (1977), and estimated bycomparison of the high-Ca pyroxene compositions andthe solvus isotherms presented by Lindsley (1983). Theyare listed in Table 2. Despite the uncertainties involvedin the application of these methods [see, for example,Bohlen and Essene's (1979) discussion of the thermom-eters of Wood and Banno and of Wellsl, it seems safe toconclude that most of the pyroxenes, including those inthe symplectites, equilibrated at temperatures <900 'C,

mostly in the range 700-800'C. It is noteworthy thatthere is no relationship between equilibration tempera-

TreLe 3. Representative analyses of titaniferous magnetites

ture and Mg/(Mg + )Fe'?*) of the pyroxenes, i.e., withstratigraphic height in the intrusion. These relatively lowtemperatures may be compared with those obtained forthe bulk analyses of the exsolved pyroxenes, which are.1000 oC. For those pyroxenes, equilibration tempera-tures correlate with Mg/(Mg * 2Fe'?*).

Most of the samples contain two types of Fe-Ti oxide:a member of the hematite-ilmenite series and a memberof the ulvOspinel-magnetite series. In one sample fromthe lopolith (R53), titaniferous magnetite is the dominantFe-Ti oxide, whereas in another sample (R195) as wellas in the Hidra anorthosite, ilmenite is the dominant Fe-Ti oxide. Both ilmenite and titaniferous magnetite occurin the symplectites in all samples. Representative analy-ses are presented in Tables 3 and 4.

The complex exsolution phenomena described previ-ously make analysis by microprobe techniques exceed-ingly difficult. This is especially true in the case of titanif-erous magnetites, which show considerable compositionalheterogeneity per sample. In R53, for example, TiO, con-tents vary from 0.20 to 7.5 wto/0. This variation undoubt-edly reflects extensive subsolidus re-equilibration, whichalso accounts for the low AlrO, contents of many analy-ses. Rarely, apparently homogeneous, small grains withhigh TiO, (up to 170lo) and AlrO, (up to 290) are found asdiscrete inclusions in subhedral pyroxene crystals (e.g., inR486). Attempts to reintegrate the original bulk compo-sitions of exsolved grains using broad-beam techniqueswere successful only in the case of titaniferous magnetitein a symplectite in sample R25, the composition ofwhichis similar to that of the discrete inclusions in pyroxene inR486. Comparison of the reintegrated titaniferous mag-netite composition and the compositions of magnetite and

Wood and Banno Wells0973) (1e77)

Lindsley'(1983)

GA248R53R186R486R25H1L1

7007208 1 0710700780751

793885914868858842821

8328258438 1 1803855844

GA248 R53 R186

R195 R25 L1H1

sio, 0.50 0.05Tio, 1.30 0.35At,os 0.80 0.65Cr,Os 0.80 0.90Fe,O.' 61.5 65.2FeO" 31.8 31.2MgO 0.40 0.15MnO 0.05 0.05v.o. 0.90 0.95

Total 98.0 99.4

si 0.019 0.002Ti 0.038 0.010Ar 0.037 0.029Cr 0.025 O.O27Feo** 1.799 1.893Fe,*' 1.033 1.002Mg 0.023 0.002Mn 0.002 0.002v 0.025 0.026

Total 3.000 3.000Usp" 0.041 0.011

n.d. 0.35 0.35 n.d. n.d. n.d. 0.390.40 7.40 0.66 3.10 0.25 17.0 0.280.65 3.93 n.d. 0.90 0.10 1.23 0.381.45 n.d. n.d. n.d. n.d. n.d. n.d.

64.4 48.9 67.5 61.6 67.8 33.6 67.331.4 38.9 32.4 33.9 30.9 45.8 31.9

0.10 n.d. n.d. n.d. 0.05 0.26 n.d.n.d. n.d. n.d. 0.10 n.d. 0.30 n.d.

0.70 n.d. n.d. n.d. n.d. n.d. n.d.99.1 99.5 100.9 99.6 99.1 98.12 100.2

Number of cations on the basis of four oxygensn.d. 0.013 0.013 n.d. n.d. n.d.

0.012 0.209 0.019 0.089 0.007 0.4890.030 0.174 n.d. 0.041 0.005 0.0550.044 n.d. n.d. n.d. n.d. n.d.1 .881 1.382 1 .936 1.780 1.981 0.9871 .006 1 .222 1 .032 1 .086 1 .004 1.4640.006 n.d. n.d. n.d. 0.003 0.015n.d. n.d. n.d. 0.003 n.d. 0.010

0.022 n.d. n.d. n.d. n.d. n.d.3.000 3.000 3.000 3.000 3.000 3.000

n.d. 0.39 0.760.95 1.08 0.270.70 0.52 0.26n.d. 1.02 n.d.

66.3 64.5 65.631.9 32.8 31.7

n.d. n.d. 0.160.10 n.d. n.d.n.d. n.d. n.d.

100.0 100.3 98.8

n.d. 0.015 0.0290.027 0.023 0.0080.032 0.031 0.012n.d. 0.031 n.d.1 .914 1 .854 1 .9131.024 1.046 1.028n.d. n.d. 0.0090.003 n.d. n.d.n.d. n.d. n.d.3.000 3.000 3.000

0.028 0.033 0.008

0.0130.0080.015n.d.1.9391.023n.d.n.d.n.d.3.000

0.013 0.254 0.019 0.093 0.007 0.s13 0.008

A/ote.'n.d. : not detected.' FeO and Fe2O3 contents were calculated assuming >Cat : 3.000 on a four-oxygen basis.

'- Ulvdspinel contents were calculated following Stormer (198i1) and are expressed as mole fractions.

BARTON AND VAN GAANS: ORTHOPYROXENE-Fe-Ti OXIDE SYMPLECTITES

TrsLe 4. Representative analyses of ilmenites

1053

GA248 R195 8186 L1

n.d.s3.3

n.d.n.o,0.08

47.3n.o.0.65n.d.

101 .3

sio,Tio,AlrosCrrO"FerO.*FeO'MgoMnOVro.

Total

5 t

TiAICrFe".Fe2+MgMn

Total

RrO".'

n.cl.50.9

n.o.n.o.6.31

39.73.200.3sn.d.

100.5

n.d.0.942n.d.n.d.0.1170.8170.1 170.001n.o.2.000

0.058

n.d.46.10 .100.20

13.040.60 .150.40o.25

100.8

n.o.52.4

n.o.n.d.

0.3246.10.250.65n.d.

99.7

0.2149.7

J.05

n.d.0.24

44.4n.d.0.53n.d.

100.8

0.2251.8

n.d.n.d.

0.5646.3

n.d.0.60n.d.

99.s

0.4649.4

n.o.n.d.

5.9241 .5

1 .620.61n.d.

99.5

o.o120.932n.d.n.d.

0 .1130.8700.0610.013n.d.

2.000

0.056

0.4449.9

n.d.n.d.6 .10

41.02 .140.54n.d.

100.1

0 .0110.932n.d.n.d.0 . 1 1 30.8s30.0790.011n.d.2.000

0.057

Number of cations on the basis of three oxygensn.d. n.d. 0.005 0.0060.873 0.996 0.991 0.9880.003 n.d. 0.162 n.d.0.004 n.d. n.d. n.d.0.243 0.008 0.005 0.0120.859 0.973 0.906 0.9810.006 0.009 n.d. n.d.0.009 0.014 0.01 1 0.0130.005 n.d. n.d. n.d.2.000 2.000 2.000 2.000

0.121 0.004 0.002

n.d.0.999n.d.n.d.0.0020.985n.d.0.014n.o.2.000

0.001

,Vote.'n.d. : not detected.. FeO and Fe2O3 contents were calculated assuming >Cat : 2.000 on a three-oxygen basis.*. R2Oo contents were calculated tollowing Stormer (1983) and are expressed as mole fractions.

ilmenite in the same sample demonstrates that spinel (notanalyzed) is a product ofthe exsolution process.

The ilmenites are much more homogeneous in com-position, and there is little difference between analysesperformed with a broad beam and those performed witha focused beam. The ilmenites in most samples containlow RrO, (0-30/0) and low MgO (<2.00/o), suggesting thatthey too have experienced extensive subsolidus re-equil-ibration, probably to the extent that discrete grains haveformed from an initially homogeneous phase (externalgranule exsolution; Buddington and Lindsley, 1964). Asimilar conclusion was reached by Ashwal (1982) on thebasis of his study of co-existing Fe-Ti oxides in the Marcyanorthosite massif and by Duchesne (1972a), who usedmineral separates, for the oxides in the Bjerkreim-Sokn-dal lopolith. Ilmenites that contain relatively high RrO3(6-13010) occur as inclusions in subhedral pyroxenes intwo samples (GA248 and R486) and as discrete grains inR195. MgO is relatively high in ilmenites in GA248(-3o/o), suggesting that these approach solidus composi-tions, but is low in those in R486 and Rl95 (<20lo), in-dicating some re-equilibration.

Temperatures and oxygen fugacities estimated from theexperimental data ofBuddington and Lindsley (1964) andSpencer and Lindsley ( I 98 I ) are < 630 .C and < I 0-1t barfor most samples, confirming the conclusions reachedabove. The Fe-Ti oxides that occur as discrete inclusionsin subhedral pyroxenes in R486 yield higher tempera-tures (-890 'C) and f", (- 10-'25 bar'), which may becompared with Duchesne's(1972a) estimates of 900-975'C and 10-" atm for samples from a similar stratigraphiclevel. Temperatures estimated from Bishop's (1980) il-menite-orthopyroxene geothermometer are likewise low

for most samples [=680 'C, assuming equilibration at 5

kbar, which is consistent with estimates for the crystal-lization of the quartz monzonitic phase of the lopolith(Rietmeijer, 1979) and with estimates of the conditionsof metamorphism in the aureole surrounding the lopolith(Hermans etal.,1976;Jansen and Maijer, 1980)1. Highertemperatures are calculated for the more magnesian il-menite-pyroxene assemblages in Ga248 (765 'C).

Spinels were analyzed in only two samples, one fromthe Lyngdal hyperite and one from the Hidra anorthosite(Table 5). The spinels are hercynites with negligible Cr'O,(<0.80/o) and low FerO, (<10/0, calculated assuming per-fect analysis and stoichiometry), and there is no differ-ence between the spinel that occurs as lamellae and thespinel that occurs as discrete inclusions in Fe-Ti oxides.Comparison of the co-existing magnetite and spinel com-positions with the solvus data of Turnock and Eugster(1962) suggesis temperatures of <600 "C, providing ad-ditional evidence for re-equilibration ofthe oxide phasesin the intrusive bodies. Kars et al. (1980) have presentedevidence that the spinel-magnetite pairs in metamorphicrocks from the contact aureole ofthe Bjerkreim-Sokndallopolith re-equilibrated to relatively low temperatures(<800 "c).

All of the analyzed olivines (Table 5) are relativelymagnesian (Foro to Fouo) and within any one sample showa restricted range of composition (s2 molo/o Fo). Thedistribution of Mg and Fe between olivine and orthopy-roxene in each sample conforms with the relationshipderived by Medaris (1969) from his experimental study(see also Van Lamoen,1979;Zncket al., 1982).

Finally, the average bulk compositions of symplectitesin one sample from the Bjerkrcim-Sokndal lopolith and

1054 BARTON AND VAN GAANS: ORTHOPYROXENE-FE.Ti OXIDE SYMPLECTITES

TABLE 5. Analyses of spinels and olivines and broad-beam analyses of symplectites

Spinels Symplectites

R25L1L1H1

sio,Tio,Al,03FeO.MgoMnOCaO

Total

osiTiAIFe2*MgMnCa

SumMg/(Mg + >Fe,.)

n.d.n.d.

64.921.513.50.31n.o.

100.2

4n.d.n.d.1.9980.4700.5260.007

3.0010.528

n.o.n.o.

62.725.211 .00.42n.d.

99.3

37.5n.o.n.o.

26.834.80.660.17

99.9

36.0n.d.n.d.

30.932.00.21n.o.

99.1

37.1n,o.n.o.

32.331.80.290.10

101.6

Number of cations on the basis of four or six oxygens

39.31 . 1 22.57

3s.920.90 .180.29

100.3

41.0730.0230.0830.8200.8510.0040.0082.8620.509

34.62 .191.36

52.58.360.540.55

100.0

4 61.046 1.5680.050 0.0750.049 0.073't.328 1.9920.377 0.5660.014 0.0210.018 0.0272.880 4.32',10.221 0.221

4n.d.n.o.

1.9880.5670.44'l0.010n.o.

3.0060.438

41.000n.o.n.o.

0.5981.3830.0150.0053.0000.698

40.988n.d.n.d.

0.7091.3090.005n.d .

3.0120.649

40.996n.o.n.o.0.7251.2730.0070.0033.0040.637

61 .6100.03501241.2301.2760.0060.0134.2940.509

A/otei n.d. : not detected.- Total Fe as FeO or F€F*.

in the Lyngdal hyperite are reported in Table 5. The sym-plectites in these samples are relatively fine grained andare, therefore, readily amenable to analysis using defo-cused-beam techniques. For each sample, at least ninespots were measured per symplectite, and at least threesymplectites were analyzed. The analyses for all samplesshow a relatively small range in composition, and henceaverages are reported in Table 5. Attention is directed tothe low but significant Al2Or, TiOr, and CaO contents ofthe symplectites.

DrscussroN

Origin of the symplectites: Magmatic or subsolidus?

The Fe-Ti oxides in the samples examined in the pres-ent study and also in many other intrusive bodies (e.g.,McSween, 1980; Garrison and Taylor, 1981) have clearlyexperienced complex subsolidus re-equilibration, proba-bly involving oxidation-reduction reactions (cf. Du-chesne, 1972a; Ashwal, 1982) and external granule exso-lution (Buddington and Lindsley, 1964). It is thus notpossible to reintegrate the original (solidus) compositionsof the Fe-Ti oxides with confidence, and measured com-positions are of little help in constraining models for theorigin of symplectites. The data presented in the preced-ing section demonstrate that the oxides in the symplec-tites in the Bjerkreim-Sokndal lopolith, Hidra anortho-site, and Lyngdal hyperite have compositions significantlydifferent than those occurring as inclusions in pyroxenesin the same intrusions. Equilibration temperatures for thelatter are substantially lower than those indicated bybroad-beam analyses of the pyroxenes, suggesting somere-equilibration of even these oxides. Moreover, the rel-atively low Mg contents of the ilmenites that occur asinclusions in pyroxenes in sample R486 confirm the fact

that re-equilibration has occurred. Duchesne (1972a)proposed that re-equilibration of the Fe-Ti oxides wasenhanced by the presence of a volatile-rich fluid phasetrapped after solidification (see also Morse, 1980, p. 328),evidence for which is provided by the occurrence ofau-tometamorphic biotite and amphibole. However, thepossible role of convecting hydrothermal fluids (cf. Tay-lor and Forester, I 979; Norton and Taylor, I 979) cannotbe discounted and must be evaluated through detailedstable-isotope studies. Inclusions of oxides in orthomag-matic pyroxenes were presumably relatively isolated fromthese fluids.

The exsolution textures ofthe feldspars and pyroxenesindicate that the silicates have also re-equilibrated duringsubsolidus cooling. External granule exsolution is in-ferred for most of the pyroxenes (and is also inferred forsome of the plagioclase feldspars). The orthopyroxenes inthe symplectites have low Ca contents and do not showexsolution. There are two possible explanations:

l. The symplectites crystallized from a melt, but atlower temperatures than the subhedral pyroxenes in thesame sample or than pyroxenes in most other layeredintrusions and anorthosites (Bohlen and Essene, 1978;Ashwal, 1982). Comparison of the temperatures estimat-ed for the formation of the symplectitic pyroxenes withthose estimated from bulk analyses of larger, exsolvedpyroxenes leaves no doubt that the latter crystallized atsubstantially higher temperatures than the former (Bar-ton, in prep.). Arculus and Wills (1980) and Wilson et al.(1981) ascribed the presence of magmatic low-Ca pyrox-ene (hypersthene) in plutonic blocks from the Lesser An-tilles and in the Fongen-Hyllingen intrusive complex,Norway, to relatively low-temperature crystallization andsuggested that solidus temperatures and pyroxene crys-tallization temperatures were depressed by the presence

of water. Equilibration temperatures (800-900 "C) esti-mated for the pyroxenes in the symplectite-bearing sam-ples using the thermometers of Wood and Banno (1973)and of Wells (1977) are consistent with crystallization atthe solidus ofhydrous anorthosite at 5 kbar (800'C, Luthand Simmons, 1968). However, the absence of abundantpegmatitic segregations in the Bjerkreim-Sokndal lopo-lith and the absence of abundant orthomagmatic amphi-bole indicates that HrO-saturated residual melts were not,in general, generated during crystallization of the anor-thositic-leuconoritic phase, and we thus consider it ex-tremely unlikely that the symplectites formed by copre-cipitation from a melt.

2. The symplectites formed under subsolidus condi-tions. This explanation is entirely consistent with thecompositional similarity between the symplectitic ortho-pyroxene, that which occurs as rims around olivine andclinopyroxene and that which occurs as discrete granules(which are clearly of subsolidus or metamorphic orign).Furthermore, subsolidus crystallization explains the ab-sence of any relationship between estimated equilibrationtemperatures and Mg/(Mg * )Fe'?*) of the pyroxenes andstratigraphic height in the intrusion. As noted earlier (seeFig. 5), the compositions of the pyroxenes in the sym-plectites conform to established subsolidus trends in theconventional pyroxene quadrilateral, whereas the bulkcompositions ofthe exsolved plroxenes approach solidustrends in other intrusions. Temperatures of formation ofthe symplectites based upon the Lindsley (1983) pyrox-ene thermometer are 700-800 oC and are probably morereliable for granulite-facies conditions than temperaturesobtained using the methods of Wood and Banno (1973)and of Wells (1977) (Stephenson, 1984). These temper-atures are consistent with formation under subsolidusconditions. Finally, it is noteworthy that similar equili-bration temperatures are estimated for co-existing pyrox-enes in the symplectite-bearing samples, in other samplesfrom the anorthositic-leuconoritic phase of the Bjer-kreim-Sokndal lopolith (Barton, in prep.), in samples fromthe Lyngdal hyperite (Knibbeler and Van Zutphen, oralcomm., 1983) and in the noritic Hunnedalen dyke system(Drent, 1982). These temperatures are lower than thoseestimated for the equilibrium mineral assemblages in themetamorphic aureole immediately surrounding the Bjer-kreim-Sokndal lopolith (Jansen and Maijer, 1980), whichindicates regional re-equilibration of pyroxenes in manyof the intrusive bodies in Rogaland-Vest Agder duringpostintrusive cooling (i.e., during the M3 metamorphicphase; Jansen and Maijer, 1980; Kars et al., 1980).

The evidence cited above strongly suggests that the or-thopyroxene-Fe-Ti oxide symplectites described in thisstudy formed during subsolidus cooling. The low Ca con-tents of orthopyroxenes in symplectites in gabbroic intru-sions in other parts of the world suggest that they areprobably also of subsolidus origin, even if formation dur-ing late-magmatic crystallization (Ambler and Ashley,1977; Garrison and Taylor, l98l) has previously beenproposed.

l 055

Formation of the symPlectites

A number of models can be envisaged for the forma-tion of orthopyroxene-Fe-Ti oxide symplectites: exsolu-tion from a pre-existing homogeneous phase, eutectoidalbreakdown of a pre-existing homogeneous phase, reac-tion between primary magmatic minerals during cooling,oxidation or replacement ofolivine, and independent nu-cleation and growth of low-Ca pyroxene and Fe-Ti oxide.

Forrnation by exsolution. The irregularity of the sym-plectites is difficult to explain in terms of an exsolutionmodel, especially when it is recalled that even the appar-ently lamellar symplectites are irregular in three dimen-sions and that the Fe-Ti oxide "lamellae" are not contin-uous along their length. However, exsolution from asupersaturated solid solution by discontinuous or cellularprecipitation (Yund and McCallister, 1970; Chadwick,1972) can produce textures similar to those described here.This process has been invoked to explain the origin ofthe pyroxene-ilmenite symplectites that occur as discretenodules in kimberlites (Dawson and Reid, 1970) as wellas the pyroxene-plagioclase symplectites described by Bo-land and van Roermund (1983) and the symplectitic au-gites described by Boland and Otten (1985). In the caseof the symplectites that are the subject of the presentstudy, there is no logical precursor solid-solution phase.The molar proportions (converted from volume propor-tions) of orthopyroxene:Fe-Ti oxide (avg. 66:34) togetherwith the occurrence of abundant ilmenite in some sym-plectites (e.g., in sample Rl95) rules out formation ofsymplectites from an original, nonstoichiometric pyrox-ene: orthopyroxene crystallized under a range ofP-T con-ditions contains sl.4 wto/o TiO, (Deer et al., I 978), andthis is also true of hypersthene megacrysts in anorthosites(Emslie, 1978; Maquil, 1978). Direct evidence is provid-ed by the bulk analyses of the symplectites in the Lyngdalhyperite and in the Bjerkreim-Sokndal lopolith that de-viate strongly from pyroxene stoichiometry Gable 5). Wethus conclude that the symplectites do not form by exso-lution, although we believe that the fine (-3 pm), ori-ented (parallel to 010 and 00 l) platelets of ilmenite foundin some nonsymplectitic pyroxenes in the samples thatwe have studied formed by exsolution, possibly fromnonstoichiometric hosts (cf. Garrison and Taylor, l98l).As noted by Barink (1982), silicates crystallized at hightemperatures may be nonstoichiometric, and the oxygenreleased upon cooling may combine with Fe2* and Ti toproduce magnetite, hematite, or ilmenite inclusions inthe host phase. This process can only yield relatively mi-nor amounts of Fe-Ti oxide (see Barink, 1982).

Formation by eutectoidal breakdown. Eutectoidal re-actions are known from studies in metallurgical systems(Chadwick, 1972) to produce textures similar to those ofthe symplectites. Ringwood and Lovering (1970) haveproposed that the pyroxene-ilmenite intergrowths thatoccur as discrete nodules in kimberlite diatremes resultfrom eutectoidal breakdown of garnet. This possibilitycan be discounted for the orthopyroxene-Fe-Ti oxide

BARTON AND VAN GAANS: ORTHOPYROXENE-FC-Ti OXIDE SYMPLECTITES

1056

symplectites because there is no evidence that garnet oc-curred as a primary magmatic, xenocrystal, or metamor-phic phase in the rocks under consideration. Additionalevidence against this hypothesis includes the relativelylow Al content of the symplectites and the improbabilitythat garnet transforms to pyroxene and Fe-Ti oxide atpressures that are geologically realistic for layered intru-sions (contrast Ringwood and Lovering,1970). As withthe exsolution hypothesis, there is no other likely precur-sor phase that could form the symplectites by eutectoidaldecomposition.

Reaction between primary magmatic minerals duringcooling. The development of coronas of orthcpyroxeneand amphibole and amphibole-spinel symplectite isthought to result from subsolidus reaction between oliv-ine and plagioclase (Van Lamoen, 1979; Mongkoltip andAshworth, 1983). Such reactions clearly cannot be re-sponsible for the formation of orthopyroxene-Fe-Ti ox-ide symplectites described here. There is absolutely noevidence that these symplectites formed directly by re-action between any pair of orthomagmatic minerals al-though, as shown in the following sections, formation ofthe symplectites must be related to reactions occurringelsewhere in the same rock.

Oxidation or replacement of olivine. Following the sug-gestion of Muir et al. (1957), many workers have pro-posed that orthopyroxene-Fe-Ti oxide symplectites formby the breakdown of olivine according to a reaction ofthe type

3(MgFe)SiO" * t/zO, = 3MgSiO, * FerOo (l)

(see Haggerty and Baker, 1967; Goode, 1974). Petro-graphic data for the Hidra anorthosites and Lyngdal hy-perites can be interpreted in terms of this reaction, whichshould not be confused with the eutectoidal reaction de-scribed above (note that the latter must, by definition, beinvariant). However, the reaction mechanism remainsobscure. Muir et al. (1957) and Yan Lamoen (1979) fa-vored discontinuous replacement reactions whereasJohnston and Stout (1984) proposed a continuous oxi-dation reaction for the formation of orthoplroxene-Fe-Mg ferrite symplectites in lherzolite xenoliths from Kauai,Hawaii. Johnston and Stout (1984) have discussed thedifferent reaction mechanisms in detail, and we note hereonly that major points of difference are (l) which com-ponents are added to or subtracted from the olivine dur-ing replacement and (2) does olivine persist, thoughchanging composition, throughout the reaction? In thefollowing sections we discuss these and other points rel-evant to the formation of the symplectites via oxidationand/or replacement of olivine.

1. Olivine will become unstable during cooling if /o,does not decrease parallel to a T-fo, buffer curve. Thisfollows from the work of Nitsan (1974), which indicatesthat subsolidus decomposition of olivine according toReaction I (or a similar reaction) does not require anabsolute increase in/"r. The results ofthe present study,together with results reported by Duchesne (1972a), in-

BARTON AND VAN GAANS: ORTHOPYROXENE-Fe-Ti OXIDE SYMPLECTITES

dicate that during cooling from 975 to 840 "C,/o, valuesin the Bjerkreim-Sokndal lopolith paralleled those of theQFM buffer. During subsequent cooling, olivine of fixedcomposition will break down to orthopyroxene and Fe-Ti oxide if the rate at which f, decreases with respect totemperature is less than that defined by the QFM buffer.We suggest that influx of water along fractures resultingfrom thermal contraction may alter the 7-/", cooling pathsufrciently to induce oxidation of orthomagmatic olivine.

2. Reaction I is divariant and thus cannot be uniquelybalanced, even if minor components are ignored and theFe-Ti oxide is assumed to be pure magnetite, unless thebulk composition is known (Hensen, 1980). The assump-tion that olivine composition remains constant duringsymplectite formation (Van Lamoen, 1979) is not nec-essarily valid (cf. Grieve and Gittins, 1975), and indeedJohnston and Stout (1984) have demonstrated that con-tinuous oxidation of olivine occurs under at least somecircumstances. However, the symplectites in the Bjer-kreim-Sokndal lopolith, Lyngdal hyperite, and Hidra an-orthosite cannot have formed in this way, since the bulkcompositions of the symplectites differ considerably fromthose of the host olivine or, in the case of the symplectitesin the Bjerkreim-Sokndal lopolith, from olivine stoichi-ometry. Moreover, summing the compositions of the or-thopyroxene and Fe-Ti oxide in symplectites from theBjerkreim-Sokndal lopolith and measured proportions ofthese phases yields unrealistic olivine compositions. ForGa248, the calculated olivine composition is Foo,, where-as the measured olivine composition is Foro, and com-parable compositions for the monzonorites are Fore (cal-culated) and Foro (measured-see Duchesne, 1972b).Theoccunence of spinel and ilmenite in the symplectites to-gether with the measured AlrO, and TiO, contents of thesymplectites demonstrates that the intergrowths cannotform by isochemical replacement of olivine. We concludethat formation of the symplectites in the Bjerkreim-Sokn-dal lopolith, Lyngdal hyperite, and Hidra anorthosite in-volves diffusion of components other than oxygen. More-over, we note that our data indicate that there is no apriori reason to assume that AlrO, or TiOr behave asimmobile components (cf. Mongkoltip and Ashworth,I 983) during symplectite formation.

3. Replacement of olivine involves nucleation andgrowth ofnew phases. The occurrence ofgranular pyrox-enes and low-Ti magnetites in the symplectite-bearingsamples described here is evidence that these phases nu-cleated and grew during retrograde metamorphism. Nu-cleation most probably occurred at grain boundaries orimperfections (e.9., vacancy clusters and dislocations)within orthomagmatic minerals since this is energeticallymost favorable. Evidence from one of the samples fromthe Bjerkreim-Sokndal lopolith suggests that nucleationoccurred on a crystal ofapatite, though this interpretationis debatable since the apatite crystal could have been en-gulfed during erowth (this can be inferred in the case ofalbite-clinopyroxene symplectites in the Lyngdal hyper-ite). Nevertheless, it is thermodynamically improbable

Fig. 6. Illustration of the bridging mechanism described inthe text. (a) Idealized, isolated plate ofa-phase. (b) NucleationofB-phase on the surface ofexisting plate ofa-phase. (c) Growthof a second lamella of a is initiated after bridging at the phaseedge. This mechanism allows the formation of intergrowthswithout requiring continuous nucleation ofeach phase, i.e., theformation of symplectites is a consequence of a specific growthmechanism. Bridging will result in discontinuities in lamellaealong the direction of propagation.

that replacement of olivine is in most cases isochemical.The En content of orthopyroxene, for example, will bedetermined by the Fo content of the olivine according toa relationship such as that determined by Medaris (1969),as shown by the work of Van Lamoen (1979) and, Zecket al. (1982) and the work presented in this paper. Fur-thermore, the orthopyroxene will have Ca and Al con-tents commensurate with equilibrium with phases suchas clinopyroxene and plagioclase occurring in the samerock (or forming at the same time). The same holds truefor high-temperature metamorphic magnetite, the Ti andAl contents of which must be consistent with at leastpartial equilibrium with coexisting spinel and ilmenite.Numerous examples of nonisochemical replacement havebeen described from metamorphic rocks, e.g., sillimaniteby grandidierite (van Bergen, 1980) and osumilite by cor-dierite, alkali feldspar, quartz (Jansen, oral communica-tion, 1983), and it is perhaps surprising that previousworkers have sought to obtain balanced solutions for theformation of symplectites.

4. The formation of the symplectites is critically de-pendent upon the availability of Fe-Ti oxide; where grainsof orthomagmatic Fe-Ti oxide are not available, olivineis partially replaced by orthopyroxene. During symplec-tite formation, nucleation is probably cooperative (Chad-wick, 1972), nucleation of one phase inducing (via su-persaturation) nucleation ofthe second phase. Either phasecould nucleate first, depending upon a number offactorssuch as variations in the local chemical environment andthe degree oflattice coherency between the original andthe new phases.

5. Following nucleation, the two phases propagate viaa sympathetic growth mechanism (Chadwick, 1972) gov-erned by the requirements of minimal interfacial andstrain energy. Chadwick (1972) and Chalmers (1964) havedescribed a bridging mechanism to account for the sym-pathetic nucleation and multiplication of lamellae in aeutectic alloy (Fig. 6); such a mechanism is entirely con-sistent with our observations on orthopyroxene-Fe-Tioxide symplectites. The form of the intergrowths suggests

I 057

+ a , F

l -

Advancing interface ---"'>

Fig. 7. Directions of diffusion required for growth of a sym-plectite. Components must diffuse both parallel and perpendic-ular to the direction ofgrowth (from left to right in the abovediagram). Diffusion probably occurs predominantly along grainboundaries.

nucleation and growth under supercooled conditions,perhaps induced by strong thermal and chemical gradi-ents or, more probably, by the relatively high activation-energy barriers for solid-state nucleation. The precise formof the intergrowth is likely to be governed by a numberoffactors, including the degree oflattice coherency, sur-face- and strain-energy requirements, evolution of latentheat during crystallization, and difusion rates. The lastfactor is important, since the growth of the symplectiterequires diffusion both perpendicular and parallel to theadvancing interface (Fie. 7). Mongkoltip and Ashworth(1983) have illustrated the possible diffusion processesinvolved in symplectite and corona formation. Since vol-ume diffusion is relatively slow for solid-state reactions(Yund and McCallister, 1970; Freer, 1981; Boland, oralcomm., 1983), grain-boundary diffusion is likely to havebeen dominant (note that the reacting interface must havebeen incoherent) and may have been enhanced by thepresence of a thin film of intergranular fluid (note theoccurrence of autometamorphic amphibole and biotite).

6. Our observations strongly suggest that not all sym-plectitic intergrowths of Fe-Ti oxide and orthopyroxeneresult from the replacement of olivine. It thus seems im-possible to avoid the conclusion that such intergrowthscan form in a recrystallizing rock at any point where nu-cleation and growth of Fe-Ti oxide and orthopyroxeneoccurs. This conclusion is consistent with Zeck et al.'s( I 982) description of orthopyroxene-ilmenite symplectitethat apparently replaces olivine and with the occurrencein the Bjerkreim-Sokndal lopolith of rims of symplectitearound orthomagmatic clinopyroxene (from which or-thopyroxene has exsolved) in the vicinity of orthomag-matic Fe-Ti oxide.

In summary, there is no reason to expect that the re-placement of olivine by symplectites is isochemical(though it can be for components other than oxygen);furthermore, not all orthopyroxene-Fe-Ti oxide symplec-tites appear to form by replacement of olivine. We be-lieve that the occurrence ofsymplectites reflects the over-all re-equilibration of a magmatic rock to Ihe P-Tconditions prevailing during retrograde metamorphismand that their rarity results from the complex factors re-

BARTON AND VAN GAANS: ORTHOPYROXENE-Fe-Ti OXIDE SYMPLECTITES

c .b .a .

1058

quired for formation. It is not clear why the proportionsof the phases in the symplectites are relatively constant,though the proportions and the size of the phases may begoverned by diffusion rates and by growth rate. The in-terlamellar spacing of eutectoid alloys cannot be predict-ed simply from theoretical considerations, but a relation-ship of the type trrR ! constant (where X is interlamellarspacing, y is a constant and R is growth rate) is probablyapplicable to metallurgical systems (Chadwick, 1972).However, it is noteworthy that the symplectites in theLyngdal hyperite are much finer grained than those in theBjerkreim-Sokndal lopolith and Hidra anorthosite eventhough all of these intrusives have experienced a similarsubsolidus cooling history. It is thus clear that before theformation of symplectites by solid-state processes can befully understood, detailed studies involving X-ray, reu,and, especially, experimental techniques must be under-taken.

CoNcr,usroNs

The symplectites in intrusive bodies from southwest-ern Norway formed during prolonged subsolidus cooling(equivalent to the M3 phase ofretrograde metamorphismrecognized extensively in the basement of Rogaland andVest Agder). In some samples, the symplectites appear toreplace olivine, but this replacement is not isochemical.The compositions of the component phases of the inter-growths reflect the overall re-equilibration of all phasesin the rocks with falling temperature, and there seems tobe no grounds to believe that any chemical componentsare inert or immobile during symplectite formation. Insome samples, there is no evidence that the symplectitesreplace olivine, and it is concluded that such intergrowthscan form during retrogression where the componentphases are available. Certainly, the presence of discretegrains of orthomagmatic Fe-Ti oxide is critical for theformation of the symplectites. The mechanism of nuclea-tion and growth can be described in a qualitative manneronly. Nucleation is probably cooperative, nucleation ofone phase inducing nucleation of the second phase, andgrowth occurs by a sympathetic mechanism governed bythe requirements of minimal interfacial and strain energy.A bridging mechanism accounts for the multiplication ofthe lamellae once growth is initiated. Integrated mineral,chemical, rer'r, experimental, and theoretical studies arerequired before symplectite formation can be fully under-stood. However, as stressed earlier, an understanding oftheir formation should provide much useful informationabout the cooling histories of plutonic rocks and henceabout the thermal history of the crust.

AcxNowr.nucMENTS

We thank C. Kieft and W. J Lustenhouwer of the Free Universily,Amsterdam, who supplied most of the electron-microprobe data used inthis paper We are extremely grateful to J. Boland and C. Maijer fordiscussions and critical reviews of the manuscript. The electron-micro-probe unit in Amsterdam is supported financially by ZWO (WACOM)Field work in southwestern Norway was financed by the State Universityof Utrecht.

BARTON AND vAN GAANS: ORTHOPYROXENE-Fe-Ti OXIDE SYMPLECTITES

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BARTON AND vAN GAANS: ORTHOPYROXENE-Fe-Ti OXIDE SYMPLECTITES