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Myrmekite Replacing Albite in Prograde Metamorphism

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Myrmekite Replacing Albite in Prograde Metamorphism American Mineralogist, Volume 71, pages 895-899, 1986 Myrmekite replacing albite in progrademetamorphism JorrN R. Asnwonrn Department of GeologicalSciences, Aston University, Aston Triangle, Birmingham B47ET, United Kingdom Ansrnlcr In pelites from the central Menderes massif, Turkey, myrmekite partly or completely replacesalbite in rocks that had developed, at a slightly earlier stageof metamorphism, two plagioclasescoexisting acrossthe peristerite gap. The quartz proportion in the myr- mekite is consistentwith the Al and Si immobility theory of myrmekite origin. This requires exchange of Ca for Na with the rest of the rock (l + x)NaAlSi'Oa I xc-a2+ - Na,-.Ca,Al,*.Si3-,O8 + 4xSiO, + 2xNa*. Myrmekite produced by progradereplacement of albite has the same properties, and develops under the same controls, as myrmekite producedby retrogradereaction from K-feldspar. Poikiloblastic oligoclasecontaining bleb- by quartz may often representcoarsened myrmekite. Myrmekite is important as an indi- cator of Al and Si immobility in a variety of petrological environments. IurnooucrroN over to myrmekite, the proportion of quartz in the inter- Myrmekite is the commonestexample of a symplectite, growth is a function of the plagioclasecomposition: the i.e., an intergrowth of minerals the mutual boundaries of relationship is identical for exsolution (Phillips and Ran- which are approximately perpendicular to a reaction in- som, 1968) and for replacement,as shown by Ashworth terface that moved as the intergrowth formed. The con- (1972). Some of the Si segregatesfrom the Al by counter- stitution of myrmekite is simple and comprises plagio- diffusion within the reaction interface. Carstens (1967) clase intergrown with a smaller amount of quartz, the pointed out that this myrmekite-forming interface is in- latter having a rodlike or wormlike morphology. As a basis coherent,i.e., there is no crystallographiccontinuity across for understandingsymplectites in general,it is important it. The formation of the integrowth is analogousto duplex tounderstandtheoriginofmyrmekiteinitsvariouspetro- cell growth in metals(Yund and McCallister, 1970). logical settings.The study of myrmekite hasa long history, Both of the recognizedmetamorphic myrmekite-form- reviewed by Phillips (1974). Myrmekite resemblessome ing processes(by replacement of and exsolution from textures that arise from crystallization of melts (rod eu- K-feldspar, respectively)are retrograde.The replacement tectics),andingraniticrocksHibbard(1979)hasproposed processis usually part of a hydration reactionthat pro- a melt-grown origin. On the other hand, definitely mag- ducesmica in addition to the plagioclaseand quartz (Phil- matic quartz-feldsparintergrowths (graphic and grano- lips et al., 1972; Ashworth, 1972). K-feldspar may be phyric textures) are usually distinguishable from myr- completelyreplaced(Ashworth,l972,Fig. lb). Ithasbeen mekite (Barker, 1970). It is also found in metamorphic suggestedthat the former presenceof K-feldspar can be rocks (Phillips, 1980a),including metasedimentaryones inferred in polymetamorphic roc(s by the detection of in which a magmatic origin is ruled out (Nold, 1984). coarsenedmyrmekite (Ashworth, 1979). Ashworth (1976) has inferred that replacementof K-feld- Myrmekite is reported by Cooper (1972) in metamor- sparby myrmekite is part of a reaction that also consumes phic rocks without K-feldspar, from relatively low grades residual melt in migmatites. (associatedwith the crest of the peristerite gap); Cooper Myrmekite is usually found adjacent to K-feldspar; (1972) and.Shelley (1973) interpreted these occurrences Phillips (1974) has incorporated this spatial association in terms ofprograde reactions. Cooper (1972) proposed into his definition of mymekite. The intergrowth often a reaction betweenclinozoisite and albite, producing an- has a convex boundary toward the K-feldspar, suggesting orthite component and excesssilica, but this does not that it formed from that mineral. Both exsolution and explain the intimate (symplectic) intergrowth. Shelley replacementwill produce myrmekite from K-feldspar if (1973, p. 336) found a lack of textural indication that Al and Si are immobile relative to other cations (Ash- clinozoisitehad reacted.He also observedgreat variations worth, 1972).Exsolution in a closed system("schwantke in the quartz proportion in the myrmekite, which grades hypothesis") and metasomatic replacement ("Becke hy- into normal poikiloblastic oligoclase.Noting that both pothesis") are thus endmembersof a continuum of mixed Schwantkeand Becke hypothesesin their classicalform reactions in which myrmekite is produced, and in this require K-feldspar as a precursor to myrmekite, Shelley sensemynnekite is polygenetic(Phillips, 1980b).If Al (1973)favoredanalternativeinterpretation,inwhichpre- and Si are quantitatively retained as K-feldspar is made existingquartz was supposedto recrystallizewith an elon- 0003404x/86/0708-0895$02.00 89s 896 ASHWORTH: MYRMEKITE REPLACING ALBITE up to staurolite + kyanite grade (Ashworth and Evirgen, 1985).The reasonfor development of assemblagesof rel- atively high metamorphic grade compared with other areas (in which the peristeritegap closesin the garnet zole; e.9., Cooper, 1972) appearsto be that arro was low, in the presenceof graphite (Ashworth and Evirgen, 1985). The specimensstudied here come from the part of the stau- rolite + kyanite zone mapped by Ashworth and Evirgen (1985, Fig. 1). No granitic intrusions are found nearby. Temperature estimates in the range 520-600"C for this zone (Ashworth and Evirgen, 1985) are obtained from garnet-biotite geothermometry using the calibration of Hodgesand Spear(1982). Pr,acrocr,.qsE TEXTURESAND coMposrrroNs In many pelites from this area,at $ades approximating the crest ofthe peristerite gap, oligoclasecontains ovoid blebs of quartz with dimensions 10-50 pm (Fig. la), dis- tinctly smaller than the quartz grains in the surrounding matrix (which are > 100 pm in size). Other oligoclase Fig. l. Myrmekitesin pelitesfrom the central Menoeres mas- grainscontain groupsof small flakesof mica, mostly mus- sif. (aHc) Transmittedlight, crossedpolars; (d) back-scattered covite. Some of the quartz inclusions are elongate (Fig. electronsErrr image. (a) Plagioclasewith quartzblebs (top left and la); where clearly rodlike, they occur as subparallel swarms top right), showingconvex (center). boundarytoward albite At so that the intergrowh has the internal characteristicsof the top left, the quartz blebs are clearly elongate,so the inter- myrmekite (Fig. lb). To further characteize the plagio- growthis classifiedas myrmekite. (b) Clear example ofmyrmekite (center),in contact with relict albite (in extinction). Nonmyr- clase-quartzintergrowh, three rocks were studied in de- mekitic oligoclaseoccurs at the right, and at the top left where tail. Theseare pelites,with the assemblagequartz-plagio- it is overgrownby the myrmekite.(c) Confusedcontrast due to clase-biotite- muscovite- chlorite- garnet-staurolite - kyanite. overlapofquartz and plagioclasewithin the thin section.Dark They are homogeneousschists with no migmatization or specksare thought to be graphite,on the basisofthe reflected- vermng. light propertiesofthe few that arelarge enough to be identified. Two of these specimenscontain small amounts of al- (d) nsE-modesnv photographof the samearea as in (c),showing bite, in addition to oligoclase,which occursas two textural surfaceoutcrop of quartz (darker gray) and oligoclase(lighter varieties (with and without quartz rods). There are gra- The bright mineralis Black with $ay). biotite. areas bright edges dations, with optical continuity, between the two kinds areholes in the polishedsurface. This is the typeofimage used ofoligoclase, but both have sharp boundaries against al- in the quantitativework to detemine the quartz proportion. bite (Fig. lb). Where there is any definite curvature to the boundary between albite and quartz-bearing oligoclase, gation parallel to the growth direction of the enclosing this is convex toward the albite (Fig. la). The coarseness feldspar. Becauseof the absenceof K-feldspar, Phillips of quartz rods is typified by Figure la, but in a few ex- (1973,1974) did not regardthese examples as being myr- amples,they are thinner in one part ofthe oligoclasegrain, mekite although their internal featuresare indistinguish- always the part adjacent to albite (Fig. 1b). In previous able from classical myrmekite, except perhaps that the studies,this fining ofquartz toward the interfacehas often quartz rods tend to be coarser(Shelley, 1973). beenobserved in myrmekite adjacentto K-feldspar (Phil- In this paper, plagioclase-quartzintergrowths are de- lips, 1974,p. 182).Where relatively coarse (Figs. la, 1c), scribed from another area associatedwith the peristerite the quartz rods tend to be discontinuous, being transi- gap and lacking K-feldspar. It is shown that the inter- tional in shapetoward the quartz blebs in poikiloblastic growths have all the features of true myrmekite and can oligoclase. be attributed to the same fundamental control that pro- Plagioclasecompositions were estimated in mole per- ducesmyrmekite from K-feldspar, the only differencebeing cent from electron-microprobeanalyses for Na, Ca, and that the mineral replacedis albite. K. Albite is approximatelyAno ,. Adjacent quartz-freepla- gioclasesare approximately An,._,u.In these two speci- RncroN,ql, SETTTNG mens, plagioclases with quartz rods are in the range The rocks studied are pelitesfrom the central Menderes
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