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Mantled Feldspars and Synneusis

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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 gran- ite 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 feld- spar 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 inter- face 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 com- monly 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
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