American Mineralogist, Volume 73, pages 1046-1059, 1988 Formation of orthopyroxenFFe-Ti oxide symplectitesin Precambrianintrusives, Rogaland,southwestern Norway Mrcrr.tnr, Banror.l Department of Geology and Mineralogy, The Ohio State University, Columbus, Ohio 43210, U.S.A. Crrnrs YAN GllNs Billiton Research,B. V., Westervoortsedijk 67d, Postbus 38, 6800 LH Arnhem, The Netherlands Ansrru.cr Petrographic and mineral chemical data are presentedfor orthopyroxene-Fe-Ti oxide symplectitesin three intrusive bodies from southwesternNorway: the Bjerkreim-Sokndal lopolith, the Hidra anorthosite, and the Lyngdal hyperite. Detailed studies of the Bjer- kreim-Sokndal lopolith indicate that symplectitesare not common, occurring in only six thin sections out of 1037 studied. Only one of these samples contains olivine, and the symplectitesclearly do not replace or form.pseudomorphs after olivine. The symplectites in the Hidra anorthosite and Lyngdal hyperite do appear to replace olivine. The propor- tions of orthopyroxene and Fe-Ti oxide in the symplectitesare fairly constant and range from 73:27 to 66:34 (volo/o).Microprobe data demonstratethat the orthopyroxenesdo not have compositions appropriate for crystallization at or above the solidus. Many of the pyroxenes in these intrusives have re-equilibrated during subsolidus cooling, but the orthopyroxenesin the symplectitesshow no exsolution and thus must have formed sub- solidus. Pyroxene thermometry indicates temperatures of formation of 700-800 qC at estimated pressuresof 5 kbar or less(upper amphibolite4ranulite faciesconditions). The Fe-Ti oxide in most of the symplectites is titaniferous magnetite that shows complex exsolution phenomenaindicative of extensivere-equilibration during subsoliduscooling. It is virtually impossible to reconstruct the original compositions or to apply oxide ther- mometry. The symplectitesdo not result from discontinuous precipitation from a supersaturated solid solution, eutectoidal breakdown of a pre-existing phase or isochemical replacement ofolivine. It is suggestedthat the occunence ofsymplectites reflects the overall re-equil- ibration of the original magmatic mineral assemblageduring high-grade retrogressive metamorphism. They form only in the vicinity of orthomagmatic Fe-Ti oxide. The precise mechanism of formation is obscure,but appearsto involve co-operative nucleation, pre- dominantly at grain boundaries, and sympathetic growth via a bridging mechanism. The development of oriented intergrowths is governed by the requirements of minimal inter- facial and strain energy. Diffusion mainly occurs along grain boundaries or advancing interfaces and is probably aided by a thin film of intergranular fluid. The supercooling necessaryfor symplectitesto form probably reflectsthe high energybarriers to solid-state nucleation and growth. INrnorucrtoN been presented that the orthopyroxenmpaque oxide symplectitesin some plutonic rocks formed after solidi- Fine-grained intergrowths (symplectites)are relatively fication was completed(Frodesen, 1968; Starmer, 1969; common in intrusive igneousrocks, but there is still much Esbensen,1978; Van Lamoen, 1979).These arguments controversy about their origin. This is especiallytrue in are largely based upon petrographic or textural evidence the caseof orthopyroxene-opaqueoxide (magnetite and,/ such as the tendencyfor the intergrowths to replaceoliv- or ilmenite) symplectites that occur in gabbroic-noritic ine in reaction coronasand their occurrencein rocks that plutonic and hypabyssalrocks. In essence,the controver- contain ample other evidencefor subsolidusre-equilibra- sy centersupon the question of whether such symplectites tion. Attempts to model the formation of the symplectites form by subsolidus processesor whether they form di- quantitatively (see especially Van Lamoen, 1979) indi- rectly from a melt by coprecipitation. cate that this is not a simple isochemicalprocess, but the That symplectitic intergrowths can form by subsolidus consequencesof this conclusion have not, to date, been processesis irrefutably demonstratedby their occurrence adequatelyexplored. in metamorphic rocks, and convincing arguments have It is extremely difficult to prove that orthopyroxenF 0003-004x/88/09I 0-l 046$02.00 1046 BARTON AND VAN GAANS: ORTHOPYROXENE-Fe-Ti OXIDE SYMPLECTITES lo47 opaque oxide symplectitesin gabbroic-noritic intrusions tailed description, including new observations and data, formed by coprecipitation from a melt. The main reason will be presented elsewhere (Voncken and Barton, in for this is that the long cooling histories ofrelatively large prep.). The lower part of the intrusion (hereafterreferred magma bodies, coupled with the releaseof volatiles in to as the anorthositic-leuconoritic phase) consists of at the last stagesof solidification(see Morse, 1980,p. 328) least five major rhythmic units, each of which has anor- and the possible access of circulating meteoric water thosite at the base and gradesupward via leuconorite to (Taylor and Forester,1979),allow continuouschemical norite (the latter is not present in the two lowermost and textural re-equilibration of the original mineral as- rhythmic units). The five rhythmic units are thought to semblage.It may thus be virtually impossible to distin- result from fractional crystallization with superimposed guish between minerals and textures formed by crystal- magmamixing (Duchesne,1972a;Hermans et al., 1975; lization in residual melt pockets and those formed later Duchesne and Demaife, 1978). The rocks of the lower in the presenceof deuteric and/or meteoric fluids. rhythmic units (RI, RII, and the lower parts of RIII and The particular model adopted for the formation of RIV) consist essentially of plagioclase,orthopyroxene, and symplectites can profoundly influence interpretation of ilmenite. Clinopyroxene,titaniferous magnetite,and apa- the cooling histories ofplutonic rocks and henceinfluence tite appear as major components in the upper parts of interpretation of the thermal history of the crust. For ex- RIII and RIV, whereasK-feldspar initially appearsas a ample, experiencegained by metallurgical engineerssug- component of antiperthite in RV. Olivine occurs only geststhat symplectites can only grow from melts if the sporadically near the base of RIV, in what appears to latter are supercooledso that ifa supersolidusorigin for representa separate,later intrusion. symplectites is adopted, an explanation of how super- The upper part of the intrusion is mangeritic to qllartz cooling developsin magmascrystallizing at depth within mangeritic(P. Michot, 1960, 1968; J. Michot, 1960; J. the crust is required. Constitutional supercooling(Chal- Michot and P. Michot, 1968) or pyroxene monzonitic to mers, 1964; Chadwick, 1972) could explain symplectite syenitic (Hermans et al, 1975) in composition. In the formation, but it is necessaryto demonstratethat consti- presentpaper we refer to the upper part ofthe intrusion tutional supercoolingindeed occursduring crystallization as the quartz monzonitic phase (cf. Rietmeijer, 1979). of magmasin the plutonic environment. If the symplec- Duchesne(1972a, 1978) and Duchesneand Demaiffe tites form by subsolidus reactions, then given diffusion (1978) have proposed that this phase representsthe re- data for relevant components,together with estimatesof sidual liquid, possibly contaminatedwith crustal material the conditions of formation, it should be possible to esti- (Pasteelset al., 1979), from crystallization of the lower mate the length of time required for formation. Clearly, layeredseries. Rietmeijer (1979), however, has found evi- however, it is first necessaryto examine models for the dence for a structural discontinuity between the upper origin of symplectites,and this is the primary goal of the and lower parts of the lopolith that is reflected in the presentpaper. chemistry of the pyroxenes.This discontinuity suggestsa In this paper we report the results ofa detailed inves- time gap between the intrusion of the anorthositic-leu- tigation into the occurrenceand mineral chemistry of or- conoritic phaseand the quartz monozonitic phasethat is thopyroxene-magnetite-ilmenite symplectites in intru- difficult, though not impossible,to reconcilewith a model sive Proterozoic igneous rocks from Rogaland and Vest of continuous magmatic differentiation as envisioned by Agder, southwesternNorway. Initially, we concentrated Duchesne(1972a, 1978) and Duchesneand Demaiffe upon the symplectitesin the Bjerkreim-Sokndal lopolith, (l e78). and most of the data presentedare for samplesfrom this intrusion. Subsequently,we have also studied symplec- Distribution of orthopyroxene-magnetite-ilrnenite lopolith tites in the Hidra anorthosite massif and in the hyperites symplectitesin the Bjerkreim-Sokndal (gabbro-norites)from the Lyngdal area, and we include Representative samples were examined of all of the observationson samplesfrom theseintrusions. All of the major rock types occurring from the base to the top of rocks studied have re-equilibrated to a greater or lesser the lopolith. Out of a total of 1037 thin sectionsstudied, degreeduring prolonged subsoliduscooling. symplectiteswere found in only six. Five of the six sam- ples were from the uppermost part of RV, the sixth was The Bjerkreim-Sokndal lopolith from an olivine-bearing sample near the base of RIV. The Bjerkreim-Sokndal lopolith is a large layered in- Thus, with one exception, the symplectites occur at a trusion (total volume unknown) associatedwith anor- similar stratigraphic height