Mantled Feldspars and Synneusis

American Mineralogist, Volume64, pages 514-518,1979

Mantledfeldspars and synneusis

RosnnrJ. Srun Departmentof Geology,Califurnia State Uniuersity Los Angeles,Calfornia 90032

Abstract

Synneusis,the drifting togetherand systematicattachment of crystalsin a melt,is involved in threeof the known originsfor mantledfeldspars: (1) overgrowth,(2) exsolution,and (3) filling of skeletalplagioclase. Quartzlatite dikesof the ChocolateMountains, California, contain plagioclase crystals in paralleland twinnedsynneusis. Sanidine mantles consisting of segments,each crystallograph- ically parallelto the adjacentplagioclase crystal, enclose synneusis structures. One-feldspar biotite granite of the Golden Horn batholith, Washington,has mantled feldsparswith complex,multiple albite mantlesformed by synneusisof zonedalkali feldsparsfollowed by subsolidusexsolution. Skeletal plagioclase formed by resorptionis filled with orthoclasein two-feldsparbiotite granite of the Golden Horn batholith. Synneusis,before and after resorption,produced mantled feldspars with systematiccrystal orientations. Wiborgite granite of Finland has anhedralorthoclase enclosed in largenumbers of plagioclasecrystals. An origin by synneusisis suggestedfor this texture.

Introduction (1978)example of peripheralreplacement of sanidine by Mantledfeldspars consist of K-feldsparenclosed in oligoclasewill not be treatedseparately because, with plagioclaseor the reverserelationship. They arecom- regard to synneusis,this processis similar to mon in epizonaland hypabyssalgranites and vol- overgrowth. canicrocks of latite to rhyolitecomposition. Four Analyticaltechniques origins have beenproposed for mantledfeldspars in Feldsparcompositions were determinedwith the igneousrocks: overgrowth (Tuttle and Bowen, 1958), fully-automatedMaterials Analysis Company model exsolution(Gates, 1953),replacement (McDowell, 5-SA3electron microprobe at California Instituteof 1978),and resorprion(Stull, 1978).Each origin im- Technology.Microprobe techniquesand computer plies a different magmatic history. Consequently, programs have been describedby Chodos et al. characterizationand proper interpretation of man- (1e73). tled feldsparSis important to understandingmag- matic processes. Overgrowthand synneusis Interpretationof mantled feldsparsis often com- plicatedby synneusis,the drifting togetherofcrystals Preuiouswork and attachmenton prominentfaces. Vogt (1921)in- Overgrowth of plagioclaseon alkali feldsparhas troduced the term synneusis,and Vance (1969) beenthe most popularinterpretation of mantledfeld- greatlyelaborated on the process.Synneusis may oc- spars (Popoff, 1903;Savolahti, 1956; Tuttle and cur before,during, or after formation of a mantled Bowen,1958; Stewart, 1959). In spiteof the volume feldspar;as a result, the mantled feldsparmay ac- of literature,detailed textural information supporting quirecomplex forms whose origin is not obvious. simpleovergrowth of plagioclaseon K-feldipar hai This paper will illustrate the role of synneusisin not beenpublished. In contrast,reverse mantled feld- formation of mantledfeldspars. A limited numberof spars,with sanidineenclosing plagioclase, are com- exampleswill be presented;these should enablethe mon, providegood evidenceof overgrowth,and illus- petrographerto expand on the possibilitieswhen trate the effectsof synneusis.An exampleof reverse making a petrologic interpretation. McDowell's mantledfeldspars will be described. w03-404x/ 79 /0s05-05 I 4$02.00 STULL: MANTLED FELDSPARS 515

Quartz latite dikes of the ChocolateMountains, Cali- fornia Hypabyssalquartz latite dikes of the Chocolate Mountains, first described by Ehlig and Ehlert (1972),are glomeroporphyriticwith individual feldspar clustersranging up to l5mm. Sanidinemantles enclose single crystals and synneusisgroups of plagioclase.Sanidine is slightlyperthitic adjacentto the sharpfeldspar interface. Plagioclasecrystals within sanidinemantles are commonlysubhedral and havenormal or oscillatory zoning.Each plagioclase within a synneusisstructure hasits individualzoning pattern. Plagioclase crystals Fig. t. Quartz latite dike, ChocolateMountains, California' D are attachedin paralleland joined in parallel Plagioclasecrystals A, B, C, and are most commonly or twinned twinnedsynneusis. Segmented sanidine mantle encloses synneusis synneusis.Plagioclase composition at the interface structure.Section cut parallelto (010)of all grains. with sanidineis Anrr. Minor resorptionat this interface indicatesthat plagioclasecrystallization ceased beforeovergrowth occurred. origin for mantled feldspars.The Waupaca rocks Sanidinemantles (An:Ab:Or : 3:54:43)which en- have clustersand singlecrystals of perthite mantled closeseveral plagioclase crystals are segmented; b and by plagioclasethat is crystallographicallyparallel to c axesof plagioclaseand adjacentsanidine are paral- adjacentalbite exsolutionlamellae. Mantlesrange in thicknessfrom 0.10to 4.lOmm. lel. GoldenHorn batholilh, Washington The outer surfaceof thinner mantlesparallels every irregularityof the plagioclase-sanidineinterface; this One-feldsparbiotite granite of the GoldenHorn parallelismdiminishes with thicker mantles. batholith has mesoperthitemantled by albite which Figure I illustratesan exampleof this texture.Four may be enclosedin an outer mesoperthitezone. The plagioclasecrystals (A, B, C, and D) form a synneusis albitemantle is anhedralor rarelysubhedral and may structureenclosed in a segmentedsanidine mantle. have albite twinning. Albite mantle and adjacent All crystalsare cut approximatelyparallel to (010). stringor patch-typeexsolutions are optically parallel. CrystalsB and C are attachedin Carlsbadtwin rela- Exsolutionlamellae and albite mantle are composi- tionship;(001) cleavages form the exactangle of 53o tionally identicaland fall within the limited rangeof which Turner (1951)found to be characteristicof Anr to Anu. Carlsbadtwins in this orientation.Crystals A and D Mantled feldsparswith two or more partial or are attachedin parallelsynneusis to crystalsB and C completealbite zones are attributedto synneusisand respectively.The entire synneusisstructure is en- exsolution.Figure 2 illustratesa possibleorigin for closedin a sanidinemantle whose individual seg- this texture. Early-formed alkali feldspar crystals ments are crystallographicallyparallel to adjacent floatingin the magmajoin in synneusis.A moresodic plagioclase. zone, destinedto be an albite mantle, enclosesthe Someglomeroporphyritic clusters in dikes of the synneusisstructure. Alkali-feldspar crystallization Chocolate Mountains have only partial mantles. continues,and magma solidificationeventually oc- These result from breaking of mantled synneusis curs.During exsolution,albite is concentratedin the structures.Vance (1969) suggestedthat boundaries more sodic zones of the alkali feldspar.Complex betweencrystals in synneusisare weaker than crystal- albite mantleswithin mesoperthiteresult. lographic planes within single minerals. Dis- Intersectionof open albite mantles (Fig' 2) in- aggregationcould result from mechanicalforces or dicatesthat synneusisoccurred early in magmatic preferentialresorption along synneusisboundaries. history. Synneusisat later magmaticstages resulted in two or more separate,complete albite mantles Exsolutionand synneusis within mesoperthitesynneusis structures. The morphologyof open,intersecting albite man- Preuiouswork tles is identicalto the outer portionsof zonedplagio- Gates(1953), after investigatingthe Waupacain- clasesynneusis structures described by Vance(1969). trusionsof Wisconsin,first proposedexsolution as an This suggeststhat the original alkali feldsparwas 516 STULL: MANTLED FELDSPARS

Plagioclasesynneusis before resorption is common Eorly Synneusis I Growthof in granite. Crystols I Sodic two-feldsparbiotite Figure 3 illustrates t- I LOne interactionof synneusisand developmentof mantled feldspars.Early-formed plagioclase is euhedraland compositionallyzoned. Parallel or twinnedsynneusis occursand growth continues while the magma is deep in the crust. Magmatic ascent of water-under- ffi saturatedgranite melt follows and causespreferential resorptionofcalcic cores;resorption channels and an irregularspace filled with melt appearwithin Contrnued Potch- type skeletal Growt h Albrte Exsolution plagioclase.Plagioclase in granitic magmas commonly ceasescrystallization at shallowcrustal levels (Yoder et al., 1957).Only alkali feldspar,quartz, biotite, and magnetite precipitate from residual magma.These minerals fill the skeletalplagioclase, and a mantled feldsparwith poikilitic inclusionsof magnetite,biotite, and quartzresults. Involvementof synneusiswith mantled feldspars after resorptionis rare in the Golden Horn granite, Fig. 2. Model for the origin of mantled feldsparsin one-feldspar due to rapid crystallizationand progressivesolidi- biotite granite of the Golden Horn batholith, Washington. ficationof magma.Figure 4 showsa brokenmantled Synneusis structure with compositionally-zoned overgrowth experiences subsolidus exsolution and development of albite feldspar structure consisting of a large skeletal mantle. plagioclase(crystal A) partially enclosinganhedral orthoclase;plagioclase and orthoclasehave parallel b zoned and that sodic zones were selectivelyenriched and c axes.Smaller plagioclase crystals with individ- in albite during subsolidusexsolution. ual morphologicalfeatures have joined in parallel and twinned synneusiswith both the plagioclase Resorptionand synneusis mantleand enclosed orthoclase. Crystals A, B, and C are in parallelsynneusis. Crystal D is a Bavenotwin Preuiow work Formation of skeletal plagioclasecrystals by preferential resorption of calcic cores of zoned Zoned I Synneusis Growlh Plog ioc lose plagioclaseduring magmatic ascent of water- under-saturatedmagma is supportedby textural observations(Vance, 1965; Wiebe, 1968) and experi- mentalstudies (Lindsley, 1966). Vance (1965) con- cludedthat patchyzoning in plagioclaseresults from filling of partiallyresorbed, skeletal crystals with a moresodic plagioclase. Stull (1978) foundthat man- H ig h Pressure ------> tled feldsparsin the GoldenHorn batholith,Wash- ington, resultedfrom crystallizationof orthoclase Resorption Infilling plus trace amountsof quartz, biotite, and magnetite from residualmelt within skeletalplagioclase. GoldenHorn batholith, Ihashington Two-feldsparbiotite granite of the Golden Horn batholithhas mantled feldspars, ranging up to lOmm in overall diameter,which formed after partial re- lnlrusion Low Pressure sorptionof plagioclaseand filling of skeletalcrystals with orthoclase(Stull, 1978).Synneusis of plagio- Fig. 3. Model for the originof mantledfeldspars clase,both before and after resorption, in two-feldspar has locally biotitegranite of the GoldenHorn batholith,Washington. See text complicatedthe mantledfeldspar texture. for explanation. STULL: MANTLED FELDSPARS 5t7

of the Banat type with respectto crystalsA and C. This type of Bavenotwin is characterizedby an irregular compositionsurface, prominent reentrant an- gles,(010) on (001),and parallela axes.Crystal E is closeto the Bavenotwin relationship. Mantled feldsparformed by synneusis The foregoingexamples of synneusisall involve attachmentof identicalminerals. If K-feldsparand plagioclasecombine in synneusis,the possibilityex- istsof mantledfeldspars forming by synneusisalone. Vanceand Gilreath's(1967) study of the probability 0 10mm of synneusisbetween unlike mineralsrevealed a ten- dencyof like mineralsto form synneusisand a gen- Fig. 5. Wiborgite granite, Finland. Plagioclase mantle eral antipathy of unlike mineralsto each other. In displaying complex twinned and parallel synneusis encloses contrastto most unlike mineral pairs, Vanceand anhedral orthoclase. Modified from Wahl (1925). Gilreath found that synneusisbetween plagioclase and K-feldsparis common.This suggeststhat man- (1925) report suggeststhat synneusishas played a tled feldsparsmay resultfrom synneusis. majorpart in producingthis texture.Figure 5, modi- Mantles consistingof severalplagioclase crystals fied from Wahl (1925),shows orthoclase with an enclosinga singleorthoclase were first reportedby anhedral,interlocking contact againsta plagioclase Holmquist (1901).Additional exampleshave been mantle. The anhedralform of orthoclasehas been reportedby Wahl (1925)and Terzaghi(1940). Smith- attributedto resorption(Wahl, 1925)and irregular son (1963)described oriented plagioclase inclusions crystalgrowth (Sederholm,1928). The mantlecon- forming partial mantles within a microcline sistsof diversely-orientedcrystals joined in complex megacrystof a porphyriticgranite. twinned and parallel synneusis.Tight nestingof Wahl (1925),in his report on the Precambrian plagioclasecrystals could resultfrom synneusisalone wiborgitegranite of Finland, describedovoid ortho- or additionalcrystal growth. The interlockingcon- clasephenocrysts mantled by up to 60 crystalsof tact betweenorthoclase and mantle suggestscontin- plagioclase.Petrographic observation of examples ued orthoclasegrowth until all availablespace was found in wiborgitegranite and review of Wahl's filled. In wiborgite granite, quartz occurs as large separatecrystals and small anhedral inclusionsin orthoclaseand its mantle. Conclusions ifl,ii"#'iiiiiiilill Problems presentedby mantled feldsparshave been debatedfor nearly a century. Recognitionof ItlMiillilullil synneusisshould facilitate interpretation ofthe origin of mantledfeldspars. Sanidinemantles on plagioclasesynneusis structures in quartz latite dikes of the ChocolateMoun- tainshave features characteristic of overgrowth.The two most distinctivefeatures are: (l) parallelismof the outer surfaceof the mantlewith the plagioclase- sanidineinterface; and (2) presenceof a segmented mantlewith individual sanidinesegments being crystallographicallyparallel to the adjacentplagioclase Fig. 4. Two-feldspar biotite granite, Golden Horn batholith, crystal. Washington. Complex mantled feldspar with all plagioclase Interaction of synneusiswith mantled feldspars Plagioclasecrystals A, crystals cut close to d-normal orientation. formedby exsolutionhas produced open, intersecting parallel synneusis.Crystal D is a Baveno twin of B, and C are in This the Banat type with respect to crystals A and C' Quartz occurs albite mantlesin the Golden Horn batholith. locally at the orthoclase-plagioclase interface. distinctive morphology suggestsearly synneusisof 518 STULL: MANTLED FELDSPARS alkali feldsparcrystals followed by compositionally- Gates,R. M. (1953)Petrogenic significance of perthite.Geol. Soc zoned overgrowth. Albite was selectivelyconcen- Am. Mem.,52,55-69. trated in sodiczones during subsolidusexsolution. Holmquist,P. J. (1901)Rapakivistruktur och granitstruktur. Geol. Fbren. Fdrh., 23, 150-161. Formation of mantledfeldspars by filling of skele- Lindsley,D. H. (1966)Melting relationsof plagioclaseat high tal plagioclasewith orthoclaseplus minor amountsof pressures.Carnegie Inst Wash YearBook,65,204-205. quartz, biotite, and magnetitemay be common in McDowell, S. D. (1978)Little Chief granite porphyry: feldspar epizonalplutons of granitecomposition. Synneusis crystallizationhistory. Bull. Geol.Soc. Am., 89,33-49. may complicatethe textureand renderinterpretation Popoff,B. (1903)Uber Rapakiviaus S[drussland. Trau. Soc. Imp. Natl. St. Petersbourg,3l, 256-260. more difficult. In the Golden Horn batholith,early- Savolahti,A. (1956)The Ahvenistomassif in Finland.Comm. gbol. formed plagioclasecrystals joined in synneusisand FinlandeBull., 174, 1-96. continuedgrowth until magmaticascent caused re- Sederholm,J. J. (1928)On orbiculargranites, spotted and nodular sorptionto produceskeletal crystals. Orthoclase and granites,and on the rapakivitexture. Comm. gbol. Finlande Bull , quartz filled skeletalplagioclase crystals 8.1.l-105. during shal- Smithson, S. B. (1963) Granite studies.IL The PrecambrianFlh low-level crystallization.Synneusis produced rela- Granile, a Ceological and GeophysicalInuestigation Norsk tively larger mantled feldsparswith exterior forms Geol. Undersdk, no.2l9 [not seen;extracted from J. V. Smith typical of plagioclasesynneusis structures. (1974)Feldspar Minerals, Vol. 2. Springer-Verlag,New Yorkl. Synneusisof plagioclaseand K-feldspar is com- Stewart,D. B. (1959)Rapakivi granite from easternPenobscot mon and may be an origin for somemantled feld- Bay, Maine.Rep. Twentieth Int. Geol.Congr., Mexico, 11,293- 320. spars.Although other exampleshave beendescribed Stull, R. J. (1978)Mantled feldspars from the GoldenHorn bath- in the literature,the most notable exampleis in the olith, Washington.Lithos, I I, 243-249. Precambriamwiborgite granite of Finland (Wahl, Terzaghi,R. D. (1940) The rapakivi of Head Harbor Island, 1925).Some orthoclasephenocrysts in wiborgite Maine.Am. Mineral.,25. lll-122. (1951) plagioclase graniteare mantled by up to 60 plagioclasecrystals in Turner, F. J. Observationson twinning of in metamorphicr ocks.A m. M ineral., .t6, 58I -589. complexsynneusis. Tuttle, O. F. and N. L. Bowen(1958) Origin of Granitein theLight of Expertmental Studies in the System NaAlSi"O"-KAlSirO"- Acknowledgments SiO2-HzO.Geol. Soc.Am. Mem. 74. Financialsupport oftwo GeologicalSociety ofAmerica Penrose Vance,J. A. (1965)Zoningin igneousplagioclase: patchy zoning. grants,Sigma Xi, HumbleOil Company,and theNorthwest Scien- J. Geo|..73.636-651. tific Associationmade this and other investigationsof the Golden - (1969)On synneusis.Contrib. Mineral. Petrol.,24,7-29. Horn batholith possible.Perry Ehlig, Peter Misch, and Joseph _ and J. p. Gilreath (1967) The effecr of synneusison Vancehave stimulatedand criticizedmy ideason the origin of phenocrystdistribution patternsin some porphyritic igneous mantledfeldspar textures. Arden Albeeand Art Chodosprovided rocks.Am. Mineral.. 52. 529-536. assistancewith the microprobe.Joseph V. Smith kindly reviewed Vogt, J. H. L. (1921)The physicalchemistry of the crystallization this paper. and magmaticdifferentiation of the igneousrocks. ,/. Geol.,29, 3I 8-350. References Wahl, W. (1925)Die Gesteinedes Wiborger Rapakivigebietes. Fennia,45, l-127. Chodos,A. A., A. L. Albee,A. J. Gancarzand J. Laird (1973) Wiebe,R. A. (1968)Plagioclase stratigraphy: a recordof magmatic Optimization of computer-controlledquantitative analysis of conditionsand eventsin a granitestock. Am. J. Sci.,266,690- minerals. Proc. of 8th Natl. Conf. on Electron Probe Analysis, 703. New Orleans,Louisiana, no. 2334,45A-45C. Yoder,H. S.,Jr., D. B. Stewartand J. R. Smith(1957) Ternary Ehlig, P. L. and K. W. Ehlert (1972)Offset of Miocene Mint fefdspars.Carnegie Inst. Wash.Year Book, 56,206-214. CanyonFormation from volcanicsource along the San Andreas fault, SouthernCalifornia (abstr.). Geol. Soc. Am. Abstractswith Manuscript receiued,June 21, 1978; Programs,4,154. acceptedlor publication,October 26, 1978.