Mineralogy and Petrology (2007) 89: 1–29DOI 10.1007/s00710-006-0135-7Printed in The Netherlands

Alkali feldspar megacryst growth:Geochemical modelling

E. Slaby1, L. Galbarczyk-Gasiorowska1, R. Seltmann2, and A. Muller2;3

1 Institute of Geochemistry, Mineralogy and Petrology, Warsaw University,Warsaw, Poland2 Natural History Museum, Department of Mineralogy, CERCAMS (Centrefor Russian and Central EurAsian Mineral Studies), London, United Kingdom3 Norges Geologiske Undersøkelse, Trondheim, Norway

Received August 28, 2005; revised version accepted March 27, 2006Published online June 1, 2006; # Springer-Verlag 2006Editorial handling: S. Maaloe

Summary

Alkali feldspar megacrysts from the porphyritic Karkonosze granite (Western Sudetes,Poland) were formed during magma mixing. Barium concentrations in zoned crystals, asensitive indicator of feldspar migration between coeval magmas, serve to reconstructthe crystallization path of the megacrysts. Based on geochemical data, a doublemixing model for the formation of the porphyritic granite and for megacryst growth isconstructed. The feldspar growth model supports megacryst nucleation and earlycrystallization in a hybridized crustal magma of granodioritic composition. The growthmodel gives credibility of the choice of partition coefficients used in the modelling.Insights gained from mixing models based on whole rock composition and mineralzonation allow the recognition of various hybridization events that are reflected in avariety of megacryst crystallization paths within the pluton.

Introduction

The aim of this paper is to construct a feldspar growth model using geochemicaltools. Mantled alkali feldspar megacrysts from the Hercynian Karkonosze porphy-ritic granite on the northern margin of the Bohemian Massif were selected for theinvestigation. The megacrysts show clear evidence of chemical, textural and ther-mal change during their growth in hybridized magmas (Slaby et al., 2002; Slabyand Gotze, 2004). Cathodoluminescence studies on the megacrysts reveal growthmorphologies typical of crystallization in a dynamic, turbulent environment (Slabyand Gotze, 2004). Changes in the conditions of nucleation and crystallization of the

plagioclase inclusions and the K-feldspar hosting them, and their movement withinthe magma chamber, are revealed by luminescence characteristics. These changesallow a precise determination of the genetic affinity of the feldspars to either moremafic or more felsic environments. The formation of megacryst-bearing porphyriticgranite is influenced both by mixing and fractional crystallization, with mixingpredominant (Slaby and Gotze, 2004; Slaby and Martin, 2005). The role of mixingis tested in the present work using whole rock composition and trace elementpatterns (barium concentration) in the megacrysts. Insights derived from modellingare used to reconstruct the crystallization paths of the K-megacrysts and to demon-strate their migration between coeval magmas. The feldspar growth model is alsoused to determine the degree of parental magma contamination at any given timeduring crystal formation and feldspar residence times within the more felsic andthe more mafic magmas.

Mixing of magmas is one of the processes involved in granite formation andevolution. Hybridization is an extended process affecting magma evolution inchambers at depth, both during and after emplacement (Barbarin and Didier,1992; Barbarin, 2005). The process is relatively easy to recognize if the composi-tions of the coeval melts are sufficiently different (Huppert and Sparks, 1988;Barbarin and Didier, 1991) but needs more thorough observation in the case ofmagmas of similar composition (Grogan and Reavy, 2002). Many physical andchemical aspects of mixing related to feldspar formation have been the subjectsof detailed research work. Numerous papers describe feldspar growth morphologyrelated to non-equilibrium or near-equilibrium crystallization conditions (see Smithand Brown, 1987 for a review and, e.g., Grogan and Reavy, 2002). Recently, Baxterand Feely (2002) and Hibbard (1981, 1991, 1995) have recognised that texturesresulting from dissolution and re-growth under conditions of strong undercooling(Loomis, 1982), and mantled textures, are compatible with magma mixing. Dis-solution and re-growth textures specific for dynamic magma systems have alsobeen reproduced experimentally (Ward and Stimac, 1992). Additionally, plagio-clase-mantled K-feldspar crystals can form due to crystallization of granite meltunder conditions involving a marked decrease of pressure, combined with smallchanges in temperature (Nekvasil, 1991; Eklund and Shebanov, 1999).

Changes in feldspar composition during crystallization under magma mixingconditions are a crucial monitor. Dynamic feldspar crystallization has been re-vealed by variance in isotopic composition (e.g. Brown et al., 1999; Davidsonand Tepley, 1997; Gagnevin et al., 2005a, b; Klotzli et al., 2000; Knesel et al.,1999; Tepley et al., 2000; Waight et al., 2000a, b), variance in chemical composi-tion, especially trace element concentrations (e.g. Anderson, 1984; Belkin et al.,1984; Cashman, 1993; Cox et al., 1996; Gagnevin et al., 2005a; Ginibre et al.,2002, 2004; Long and Luth, 1986; Nixon and Pearce, 1987; Singer et al., 1995;Troll and Schmincke, 2002) and by the diffusivity of trace and major elements (e.g.Cherniak and Watson, 1994; Gilletti and Casserly, 1994; Costa et al., 2003).Special attention has been paid to megacrystic feldspars mechanically introducedinto microgranitoid enclaves in granitic bodies (enclave typology after Vernon,1983). Those megacrysts provide evidence of the chemical, mechanical and ther-mal exchanges that occurred during their movement between variably hybridizedbatches (Barbarin, 1990, 2005; Barbarin and Didier, 1991, 1992; Hibbard, 1991;

2 E. Slaby et al.

Vernon, 1983, 1986, 1990, 1991; Waight et al., 2000b). Cathodoluminescencecombined with geochemical modelling has been proposed as a new tool for theelucidation of feldspar crystallization histories in granitic systems of mixed origin(Slaby and Gotze, 2004).

Methods

Chemical point analyses of megacrysts were performed at the Faculty of Geology,Warsaw University (Inter-Institute Analytical Complex for Minerals and SyntheticSubstances) using a Cameca SX-100. Electron microprobe concentrations of Na,Al, Si, P, K, Ca, Ti, Fe, Sr, and Ba in feldspar were determined using an accel-erating voltage of 20 kV, a beam current of 50 nA and a beam diameter of 5 mm.Applying the Monte Carlo simulation, the interaction depth of the electron beamwith the sample was estimated to be less than 5 mm. As alkalis were analysed first,a slight relative enrichment of Si due to the alkali migration occurred duringanalysis. However, any loss of Na and K was not significant over the countingtime of 10 seconds. P, Sr, and Ba were analysed using a counting time on peak of30 seconds, Ti of 20 seconds, and Al, Si, Ca, and Fe of 15 seconds. Representativedetection limits (3�) for minor and trace elements were 50 ppm for P, 61 for Ti, 129for Fe, 176 for Sr, and 130 for Ba.

The whole-rock geochemical data (major elements) were compiled fromBorkowska (1966), Klominsky (1969) and Wilamowski (1998). Trace element con-centrations were estimated using samples kindly provided by Prof. M. Borkowskaand Dr A. Wilamowski. Whole-rock analyses on samples collected by the authorswere done using XRF and ICP-MS. Analyses of major elements (SiO2, TiO2, Al2O3,K2O, Na2O, CaO, MgO, MnO, P2O5, Fe2O3, LOI) and selected trace elements (Ba,Sr, Zr, Cr) were carried out at the Institute of Geological Sciences UAM Pozna�nn(Poland) using an XRF S4 Explorer. Instrument other trace elements (Ni, V, C, Cu,Pb, Zn, Be, Co, Cs, Ga, Hf, Nb, Rb, Ta, Th, U, W, REE) were analysed at the ACMEAnalytical Laboratories, Vancouver, Canada (ICP MS, rare earth and refractoryelements by LiBO2 fusion, base metals by aqua regia digest; standard GSP-2).

Karkonosze pluton – geological setting and general petrology

The Hercynian Karkonosze granite pluton (in Czech, the Krkonose-Jizera pluton)forms the northern margin of the Bohemian Massif known as the Sudetes. Thepluton (Fig. 1) belongs to the western component of the Sudetes that is comprisedof pre-Permian basement fragments involved in the Hercynides. The WesternSudetes and the pluton are considered to belong to the Saxothuringian zone (e.g.Kossmat, 1927; Behr et al., 1984; Matte et al., 1990; Matte, 1991; Franke et al.,1993; Narebski, 1994; Franke and _ZZela�zzniewicz, 2000).

Klominsky (1969) identified many granite and granodiorite varieties within thepluton. The rocks are mostly calc-alkaline (Doma�nnska et al., 2002). The well-known discrimination by Borkowska (1966) of three texturally-different granitefacies – a fine-grained (ridge type) to medium-grained non-porphyritic facies, aporphyritic facies and a granophyric facies – is a good starting point. Slaby andMartin (2005), using geochemical tools to model magma evolution, proposed that

Alkali feldspar megacryst growth 3

Fig. 1. Simplified geological map of the Karkonosze-Izera massif (after Pato�ccka et al.,2000). IG Izera gneisses and granite-gneisses with mica-schist belts, EKC East KarkonoszeComplex, LV Leszczyniec unit, KG Karkonosze Gneisses, SKC South KarkonoszeComplex. SPH Szklarska Poreba Huta, MI Michalowice, K Karpacz, R Rudolfov,F Fojtka quarries

4 E. Slaby et al.

mixing of coeval magmas influenced porphyritic granite formation in the pluton,and fractional crystallization that of the equigranular granite facies. All granodio-rites, displaying many field textures compatible with magma mixing, are viewed bySlaby and Martin (2005) as one hybrid facies. Aplite and lamprophyre veins cutthe pluton everywhere. The aplites are comagmatic with the granite. Although thegenetic relationship of the lamprophyre veins to the granite is not clear, some seemto be composite dikes (Slaby et al., 2005) arguably related to late porphyriticgranite and some to be late mafic dikes younger than the porphyritic granite (Slabyet al., 2004).

The Karkonosze granite is a late- or post-orogenic pluton (Wilamowski, 1998;Diot et al., 1994, 1995; Duthou et al., 1991) of KCG type (Barbarin, 1999) andof mixed origin (Slaby and Martin, 2005). The mode of magma generation andemplacement was first described by Cloos (1925 – see review in Mierzejewski2002). According to Mazur (1995), the magma was emplaced almost immediatelyafter its generation, this being linked to the final deformation event of the extensionalcollapse of the Western Sudetes. The magma pulses have been dated by the Rb–Srwhole rock method (Pin et al., 1987; Duthou et al., 1991) and by the argon–argonmethod on biotite crystals (Marheine et al., 2002). What may be the most reliable datefor the porphyritic granite, 320� 2 Ma, is that obtained by Marheine et al. (2002).The Rb=Sr whole-rock data show scatter which could be a reflection of magmamixing, evidence of which is widespread in the porphyritic-type granite. The youn-gest pulses, the ridge granite, have been dated at 315� 2 Ma (Marheine et al., 2002).

Background to sample selection

The megacryst samples were collected from the porphyritic-granite facies and fromhybrids as recognized by Slaby and Martin (2005). Various hybrid types, resultingfrom the injection of mafic magma into felsic magma, occur within the granite andcan be related to various stages of the pluton crystallization. Textures compatiblewith magma mixing and mingling are easy to recognize over large (<12 km)irregular bodies of granodiorite. Almost homogeneous calc-alkaline granodioritesoccurring near Fojtka in the Czech Republic (Fig. 1) contain ocellar, hornblende-mantled quartz (Fig. 2a). Those show, in addition, narrow zones of mingledmagmas (Fig. 2b). Melanocratic rocks from Rudolfov (Fig. 1) display variousstages of hybridization (mingling) of felsic magma and injected mafic magma(Fig. 2c, d). The compositions of different parts of the hybridization zones rangefrom dioritic (microgranitoid enclaves) to granodioritic. The porphyritic granitecontains numerous microgranitoid enclaves which also show a continuous grada-tion from dioritic to granodioritic compositions (Fig. 2e, f). Early- and late frac-tures in the porphyritic granite are filled by composite dikes and mafic dikes(lamprophyres).

The liquid line of descent as defined by Harker diagrams reveals a co-geneticrelationship between hybrids and the porphyritic granite. The alkali feldsparmegacrysts of the present study are from samples systematically collected soas to be representative of the entire liquid line of descent and, thus, of early,middle and late stages of magma evolution (Fig. 3). The hybridization processis well reflected in micro-textures within the feldspar megacrysts. The megacrysts

Alkali feldspar megacryst growth 5

6 E. Slaby et al.

are zoned with every zone surrounded by trails of mostly plagioclases and biotite(Fig. 2f–h).

Mantled alkali feldspar megacrysts inferred to have grown in hybridized,granodioritic magma were collected in the Fojtka (F) and Rudolfov (R) quarries(Fig. 1). The Fojtka granodiorite is a fine- to medium-grained grey rock with darkgreen spots composed of plagioclase, alkali feldspar, quartz, hornblende, biotite,and accessory minerals (mainly apatite and titanite). Ocellar, hornblende-mantledquartz (Fig. 2a) and mantled K-feldspar perturb the otherwise equigranular texture.Mantled K-feldspar megacrysts are not common and their longest dimensions sel-dom exceed 10 mm. Some of these are glomerocrysts surrounded by a plagioclasemantle comprising a mechanically-juxtaposed plagioclase mosaic. Others showchequered mantles (Fig. 3 – sample F; see Bussy, 1990). Inside the alkali feldspars,there are numerous plagioclase inclusions; many of these have euhedral habits orare checkered. Other plagioclase inclusions form irregular, poikilitic intergrowthswith the K-feldspar. Many of the mantled crystals are ovoid, suggesting resorptionprior to mantling. The plagioclases show complicated growth morphology patternsincluding patchy zoning and cellular texture. Acicular apatite inclusions are com-mon. In numerous samples, the plagioclases show antirapakivi texture. Amphiboleseither mantle nests of felsic minerals, or appear in aggregates (mafic clots) sur-rounded by plagioclase and biotite. Both of the latter textures were deemed toindicate magma mixing by Hibbard (1995).

The Rudolfov granodiorite has a distinct feldspar-porphyritic texture and con-tains numerous dark enclaves. Pink K-feldspar phenocrysts, frequently mantled bydiscrete plagioclase rims, lie in a dark groundmass composed mainly of plagio-clase, alkali feldspar, quartz, biotite and accessory minerals. The concentration ofmegacrysts varies. On outcrop, rock enriched in K-feldspars cross-cuts zones impo-verished in mantled megacrysts (Fig, 2c). Some of the numerous enclaves containforeign introduced feldspars (Fig. 2d). In many places, mingling of felsic and moremafic magma flows is easy to recognise. Locally, diapir-like structures are evident.The mantled Rudolfov megacrysts here are larger (<20 mm) than those in theFojtka outcrops. Like the latter, they contain plagioclase inclusions. Plagioclasemantles are composed of crystals epitaxially grown on the K-feldspar host or ofcrystals mechanically attached to it (Fig. 3).

In the porphyritic granite, both alkali feldspar megacrysts (commonly withrapakivi texture) and plagioclase megacrysts or glomerocrysts lie in an equigranu-lar groundmass composed of plagioclase, alkali feldspar, biotite and quartz.Myrmekite is common in the matrix K-feldspar. Representative samples werecollected from outcrops between Michalowice, Karpacz and Szklarska Poreba

1Fig. 2. Field evidence of magma mixing-mingling. a Mixing in granodiorite from Fojtka(photo by courtesy of B. Barbarin). Arrows show ocellar hornblende-mantled quartz.b Narrow zones of mingled magmas in Fojtka quarry. c, d Mingling – Rudolfovquarry. e Microgranitoid enclave in porphyritic granite – early stage of hybridization(Michalowice quarry). f Microgranitoid enclave in porphyritic granite (Szklarska PorebaHuta quarry). g, h K-feldspar megacrysts in porphyritic granite (Szklarska Poreba Hutaquarry)

Alkali feldspar megacryst growth 7

Huta (Fig. 1). The samples from Michalowice and Karpacz have geochemicalcompositions similar to the average for the porphyritic granite (Fig. 3). The com-position of that from Szklarska Poreba Huta matches the most evolved porphyriticgranite. The alkali feldspar megacrysts (<60 mm) from granites show zonesbounded by plagioclase-biotite lath trails (Fig. 2g and h). Marginal parts of the al-kali feldspar zones close to plagioclase-biotite lath trails, and to rapakivi poikiliticmantles are commonly resorbed (Fig. 3). Plagioclases in the inter-zone screensshow resorbed grain boundaries, truncation of growth zones and rare calcic spikes.Long needles of acicular apatite occur in the plagioclases. Host megacrysts typi-cally contain two or three plagioclase-biotite screens. Mantles always comprisecrystals mechanically attached to the host.

Megacryst geochemistry

All the collected crystals were separated from the rock and cut so that the sectioncrossed the innermost core of each crystal. Feldspar compositions were determinedby electron microprobe in 10 mm steps along selected megacryst profiles. Specialattention was paid to barium concentrations that may be used to reflect degrees ofmelt hybridization (Slaby et al., 2002). Profiles for barium and some major ele-ments are shown in Fig. 3.

Of the samples containing megacrysts, the most silica-poor composition isrepresented by the Fojtka granodiorite, moderate silica enrichment by the Rudolfovgranodiorite followed by the Michalowice–Karpacz granites and the most silica-rich by the Szklarska Poreba Huta granite. The chemical ‘positions’ of the sampleshosting the analysed megacrysts is indicated on the Ba vs SiO2 diagrams (Fig. 3).

Megacrysts from the Fojtka granodiorite display two patterns of Ba distribu-tion. K-feldspar glomerocryst F (Fig. 3; megacryst F) contains many plagioclaseinclusions. Small crystals in the central part of the megacrysts are euhedral. Thelarge plagioclase inclusion in the K-feldspar is irregular and poikilitic. The unevenvariation in barium contents in the central part of the crystal correlates with theposition of plagioclase. Adjacent to inclusions, barium concentrations in the crys-tallizing K-feldspar differ from those of the host formed before inclusion attach-ment. There is no regularity in the spatial distribution of barium; zones of highconcentration are interspersed with zones of lower concentration. Close to therapakivi mantle, K-feldspar barium contents noticeably increase. A second traceelement pattern (not shown on Fig. 3) characterizes smaller crystals. The cores ofthese crystals, which exhibit relatively low barium concentrations, are stronglyresorbed and surrounded by plagioclase inclusions. Further K-feldspar growth ismarked, initially, by a slight increase – followed by a decrease and finally a con-siderable increase in barium content.

1

Fig. 3. Megacryst geochemistry. Simplified Ba vs SiO2 plot, arrow shows position of therock from which the megacrysts were collected. Microphotographs show EMPA traverse(broken lines – sections that are not shown on the EMPA profiles). EMPA (electronmicroprobe analysis) profiles across selected megacrysts: F Fojtka, R Rudolfov, MIMichalowice, SPH Szklarska Poreba Huta

8 E. Slaby et al.: Alkali feldspar megacryst growth

Fig. 3 (continued)

The Rudolfov megacryst profile differs significantly from that of the Fojtkamegacrysts (Fig. 3; megacryst R). The barium profile shows numerous broad zonesof increasing concentration. The zones contain many small plagioclase inclusionsor are separated by lath trails of inclusions. The pattern of barium enrichment ismore or less symmetrical. A decrease in barium content does not invariably accom-pany plagioclase attachment.

The profile of the Michalowice–Karpacz megacryst (Fig. 3; MI) is broadly simi-lar to that of the Rudolfov megacryst. However, there are some important differences.The margins of the megacrysts are strongly impoverished in barium. Within thebarium-enriched zones, additional Ba spikes reflect considerable local increases inbarium content and disrupt the overall simplicity of the barium distribution.

In the megacrysts of the most evolved Szklarska Poreba Huta granite, thebarium shows a more or less clear normal zoning (Fig. 3; megacryst SPH). Thecores show the maximum barium contents. Patterns within the cores are irregularwith many peaks and troughs. Above average contents characterize narrow peaks.In the zones surrounding cores, barium contents change in a stepwise manner,although also with many peaks and troughs. Decreases in barium accompany pla-gioclase lath trails. In general, a single increase in barium content occurs in theinterval between two lath trails. The distribution pattern is not symmetrical. As istypical, the outermost zone abutting the mantle is impoverished in barium.

Geochemical modelling

Whole rock geochemistry and feldspar composition

The compositional relationship between magma evolution and K-feldspar megacrystcomposition is apparent on the liquid line of descent based on Harker diagrams(Fig. 4). The porphyritic granites, with some granodiorites and with some micro-granitoid enclaves, define linear trends for all major and trace elements. The trendextends to involve the lamprophyres (Fig. 4). Such a trend is consistent with mixing(Rollinson, 1993). This trend is termed the ‘main trend’ below. The correlation indexR2 falls in the range 0.9–0.78. The worst correlation (R2¼ 0.4) is obtained for po-tassium. Some hybrid rocks do not fit the main trend. Field evidence indicates thatthese hybrids probably reflect minor mingling events and=or mineral segregation.

The elements involved in feldspar crystallization merit special attention in thiscontext. The major elements define single linear trends throughout the range ofmagma differentiation, although alkalis show some dispersion (Fig. 4a). Bariumand strontium define consistent trends for the porphyritic granite but, for hybrids,the data disperse and split into two roughly orthogonal trends (Fig. 4a). One ofthese is defined by data from the Fojtka granodiorite and from some microgranitoidenclaves. The Rudolfov diorites-granodiorites with distinct mingling texturesdefine a trend which, for barium, is not consistent with the main trend that includesthe porphyritic granite, some Fojtka granodiorites and lamprophyres. It does, how-ever, link into the data for both the porphyritic granite and the lamprophyres.

Sodium, calcium, strontium and barium behaved as compatible elements duringporphyritic granite evolution. Potassium and rubidium concentrated in the meltduring its differentiation. The dispersion in the strontium and barium data seen

E. Slaby et al.: Alkali feldspar megacryst growth 11

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on Fig. 4a could suggest crystal segregation during mixing-mingling. The positionsof alkali K-feldspar and plagioclase as plotted on Fig. 4b do indeed suggest that theperpendicular trends may well reflect plagioclase and alkali feldspar segregationduring hybridization.

Magma mixing models

Persistent linear trends, such as the main trend encompassing porphyritic granite,granodiorite and lamprophyre could suggest mixing as a significant process inthe magma evolution. Enclaves as well as hybrids found in both the less evolvedand highly evolved porphyritic granites are evidence of the continuous supply ofmafic magma to the system during granite and megacryst crystallization. Because

Fig. 5. Mixing models. a Simplified scheme of the possible mixing paths. b Probablescheme of granodiorite formation. Dark arrows mantle-related melt, white arrow crust-related melt, white bars with abbreviations (F, R, MI, K, SPH) – megacryst positions (seetext for explanation). Rock symbols and abbreviations as in Fig. 3

Alkali feldspar megacryst growth 13

of the field evidence for magma mingling-mixing, the mixing model is favored.Preliminary data from mixing models (Slaby and Gotze, 2004) support the hypothe-sis that mixing=mingling between two compositionally different magmas was akey process in the crystallization of the megacryst-bearing granites.

The preliminary models (Slaby and Gotze, 2004) tested two probable mixingpaths (Fig. 5; Table 2, paths 1 and 2). The first path follows the main trend, thesecond the trend defined by the Rudolfov diorite-granodiorite and the porphyriticgranite. Mixing has been evaluated using the Cm ¼ XaCa þ ð1 � XaÞCb massbalance law (where Cm, Ca, Cb represent element concentrations in the mixture,and in both mixed magmas respectively and Xa is the fraction of magma ‘a’ in themixture; Rollinson, 1993). As the least differentiated rocks on the liquid line ofdescent, the Rudolfov diorite and Fojtka granodiorite were selected as the maficend-members. The most siliceous Szklarska Poreba Huta porphyritic granite waschosen as the Si-rich end-member. The hypothesis tested suggests that the averageporphyritic granite was the result of mixing between these end members. Althoughthe result of the calculation was good (Table 2; paths 1 and 2), the selected magmapoles are, of course, uncertain as end-members.

The diorite-granodiorite rocks show signs of hybridization and are themselvesthe products of mixing (Slaby and Gotze, 2004). The evolved granite fromSzklarska Poreba Huta contains numerous microgranitoid enclaves – suggestingthat mixing was involved in magma evolution throughout the entire range of por-phyritic granite compositions. Geochemical modelling shows however, that mixingprevailed during early magma evolution and, together with fractional crystalliza-tion, controlled magma evolution during the final stage of porphyritic granite for-mation (Slaby and Martin, 2005).

Fractional crystallization was also crucially involved in the evolution of theequigranular granite (Slaby and Martin, 2005). Mixing tests based on data from theRudolfov megacrysts-bearing granodiorite are shown as paths 3 and 4 (Table 2).For path 3 (Fig. 5a), the composition of magmas a and b are as for path 1. Theassumed product Cm is silica-rich Rudolfov granodiorite. The derived result doesnot confirm this assumption (Table 2). The Rudolfov granodiorites with rapakivimegacrysts are not a product of any mixture of magmas a and b. The geneticaffinity of the rapakivi megacrysts from the Rudolfov granodiorites with thosefrom porphyritic granite of average composition (Michalowice granite) is, how-ever, positively verified by path 4 (Fig. 5a; Table 2). These results suggest that allof the megacrysts originated within the evolving main trend (Fig. 5a). Any com-plete model for their crystallization requires more precise compositional data onthe coeval magmas.

An additional model (Table 2, paths 5 and 6; Fig. 5a) accounts for mixing alongthe main trend. For this model, the composition of a lamprophyre showing no mine-ral segregation or fractionation was selected as the mafic end-member (Table 1).For the Si-rich end-member, a less-evolved equigranular granite showing no traceof hybridization was selected (Table 1). This composition is probably close to thatof the parental crustal magma. In addition, the cores of some of the smaller andresorbed K-feldspars from Fojtka show trace element patterns consistent with theless-evolved equigranular granite. As the Fojtka granodiorite is an early hybrid, itis likely that some of the feldspars nucleated and grew in pure crustal magma.

14 E. Slaby et al.

Table 1. Chemical analyses of selected granite – granodiorite samples

Lamprophyre Diorite-granodiorite Granite

R-1 R-2 F-1 K MI SPH Eq-gr gr��

SiO2 51.50� 55.77 57.41 71.15 62.18 68.09 70.96 77.07 74.46TiO2 1.30 0.88 1.52 0.26 1.04 0.58 0.49 0.08 0.16Al2O3 16.36 15.21 14.34 15.76 14.77 13.97 12.59 13.17Fe2O3

� 6.40 8.58 1.72 5.71 3.46 3.1 0.74 1.15MnO 0.13 0.16 0.02 0.09 0.06 0.06 0.02 0.03MgO 4.31 3.14 0.40 2.1 1.23 0.96 0.03 0.23CaO 4.38 4.86 0.82 3.86 2.66 2.02 0.73 0.57Na2O 2.0 3.74 3.8 3.13 3.48 3.35 3.46 3.25 3.29K2O 5.6 3.62 3.04 6.33 3.39 3.93 3.63 4.95 5.42P2O5 0.35 0.29 0.08 0.41 0.16 0.15 0.01 0.08

Total 95.94 98.01 99.28 98.02 99.26 98.8 99.47 98.56

V 138 124 16 81 43 30 7 10Co 17.6 22 3 12 7 4 0 5Ni 210 9.4 17 2 35 9 11 10 11Zn 127 111 45 87 60 89 6 19Ga 21.5 23 18 22 21 24 18 24Rb 132.3 142 273 158 167 191 190 297Sr 821.6 253 164 273 221 130 17 46Y 26 56 47 33 43 28 31 26Zr 315 182 199 146 371 204 28 56 85Nb 20 13.9 17 10 14 17 5 15 12Ba 1887 1135 485 856 859 427 522 9 192La 54.5 31 36 32 51 46 10 23.6Ce 98.9 65 67 71 89 89 27 50.2Pr 10.78 7 7 9 10 10 3 5.94Nd 40.7 29 27 35 36 35 12 21.9Sm 6.1 6 4 7 6 7 3 4.7Eu 1.5 1 1 1 1 1 0 0.35Gd 3.77 6 3 6 5 6 3 4.15Tb 0.64 1 1 1 1 1 1 0.76Dy 3.45 5 3 6 5 5 4 3.9Ho 0.61 1 1 1 1 1 1 0.85Er 1.64 3 2 3 3 3 3 2.49Tm 0.25 0 0 0 0 0 1 0.42Yb 1.66 3 2 3 3 3 4 3.1Lu 0.22 0 0 0 0 0 1 0.39Hf 4.8 5 5 8 6 0 3 4.1Ta 0.8 2 1 1 2 nd 4 2.9Pb 4.2 25 12 21 6 34 41 12Th 11.5 12 22 1 24 29 25 28.8U 2.2 7 6 0 6 7 7 9.9

� After Awdankiewicz et al. (2005)�� Eq-gr gr – equigranular granite

Alkali feldspar megacryst growth 15

However, because of the field evidence for continuous mantle activity during por-phyritic granite formation, the possibility that evolved porphyritic granite was aninitial crustal magma cannot be definitely excluded. In tests 5 and 6, the less evolvedporphyritic granite is considered a product of mixing; both options are acceptable(Table 2; paths 5 and 6). Thus, all megacryst-bearing samples could be the productsof mixing of the selected magma end-members. Assuming less evolved equigranu-lar granite as the crustal end-member, the proportions of lamprophyric magma inthe mixtures are X¼ 0.65 for the Fojtka granodiorite, X¼ 0.48 for the Rudolfovgranodiorite and X¼ 0.27 for the Michalowice porphyritic granite respectively.These proportions increase to X¼ 0.72, X¼ 0.55, X¼ 0.35 respectively if silica-rich porphyritic granite is taken to be the crustal end-member. In the former case,R2¼ 0.71–0.95, in the latter R2¼ 0.66–0.94. In both cases, the worst results areobtained for hybrids showing evidence of mineral segregation.

The reconstruction of the feldspar crystallization path

The megacryst- and plagioclase geochemistry points to the involvement of bothmixing and fractional crystallization during magma differentiation. Changes insodium and potassium concentrations during the course of differentiation show thatplagioclase could have been fractionated from the melt, but not alkali feldspar.Megacryst textures and chemical compositions point to the importance of mixingrather than fractional crystallization during their formation. Of the elements ex-tracted from the melt during alkali feldspar crystallization, barium gives the mostsensitive and reliable information about coeval magma compositions. Thereforebarium concentration data is used here to model the feldspar crystallization path.

The same mass balance equation Cm ¼ XaCa þ ð1 � XaÞCb as before enablescalculation of changes in melt barium concentrations during the formation of the

Table 2. Mixing model – calculation results

Mixing path Magma (Cb) Magma (Ca) Mixture (Cm) Derived equation y¼Xax

Previous models (Slaby and Gotze, 2004)1 Silica poor R

granodioriteSilica rich SPHgranite

Average porphyriticgranite

y¼ 0.90xþ 9.72 (R2¼ 0.94)

2 Silica poor F(1)granodiorite

Silica rich SPHgranite

Average porphyriticgranite

y¼ 0.75xþ 2.80 (R2¼ 0.98)

Additional tests3 Silica poor R

granodioriteSilica rich SPHgranite

R granodiorite withrapakivi M-cryst

y¼�0.13xþ 6.47 (R2¼ 0.42)

4 Silica rich Rgranodiorite

Silica rich SPHgranite

Average porphyriticgranite

y¼ 0.5xþ 1.26 (R2¼ 0.84)

New model5 Lamprophyre Less evolved

equigranulargranite

Less evolvedporphyritic granite

y¼ 0.74xþ 1.94 (R2¼ 0.98)

6 Lamprophyre Silica rich SPHgranite

Less evolvedporphyritic granite

y¼ 0.66xþ 5.18 (R2¼ 0.98)

16 E. Slaby et al.

Fig

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18 E. Slaby et al.

megacrysts. As before, Ca and Cb represent element concentrations in the mixedmagmas (Table 2; paths 5 and 6). Cm represents the specific melt composition stimu-lating feldspar crystallization at a particular time. The mantled megacryst compo-sition (barium concentration) estimated along the analysed profiles has been usedto calculate Cm, assuming equilibrium or near-equilibrium partitioning of the ele-ment between melt and crystallizing feldspar.

The choice of partition coefficient Kafs=lBa for the inversion of the feldspar com-

position into melt composition is crucial. The choice was preceded by a roughestimation using Karkonosze bulk geochemical data. That data included the aver-age barium content in matrix alkali feldspar (analyses on feldspar separates byBorkowska, 1966), average barium content in granite megacrysts (analyses onmegacryst separates by Borkowska, 1966) and, from the current work, bariumconcentrations in Ba-rich and Ba-poor megacryst zones and the barium concen-tration in average porphyritic granite. The estimated values mostly range from3.6–11.2. Similar values pertain to granodiorite feldspars.

For the calculation, three values of Kafs=lBa were chosen: Kafs=l¼ 3.7 from Bea

et al. (1994), Kafs=l¼ 6.12 from Pearce and Norry (1979) and Kafs=l¼ 11.4 fromNash and Crecraft (1985). The appropriateness of the chosen partition coefficientwas tested by reference to mixing path 2 (Fig. 6a). The model shows the fraction ofcrust-related magma (Xa) in the hybrid during feldspar crystallization. Only mo-dels using the highest K

afs=lBa value appear to be realistic. A barium distribution

coefficient below 11.4 yielded mainly negative Xa values. The negative Xa valuescharacterize the extremely Ba-rich feldspar zones (e.g., in the Michalowice model

or the Rudolfov megacryst model, Fig. 6a). Thus, Kafs=l¼ 11.4 was chosen for thefinal megacryst crystallization-path calculations.

Two crystallization-path models for all of the investigated megacrysts areshown on Fig. 6b. The first model used end-member magma compositions selectedfor mixing path 5 (less evolved granitic magma and lamprophyric magma). Themodel implies magma evolution involving both mixing=mingling and fractionalcrystallization. All of the domains on Fig. 6b that represent magma that evolved byfractional crystallization have Xa >1. The second model, calculated in the samemanner by reference to mixing path 6, does not involve evolution by fractionalcrystallization. Deep troughs in both models, some of which are shown on theplots, relate to plagioclase inclusions. The discussion in the next section refersto the first, more comprehensive model.

Discussion

Essential questions with granite of mixed origin are the estimation of the composi-tion and mass fraction in a mix of, for example, mantle- and crustal-derivedmagma, and the timing of the mixing process (Huppert and Sparks, 1988; Barbarinand Didier, 1992; Barbarin, 2005). In Karkonosze, megacrysts accompany everyunequivocal hybrid. The megacryst-bearing porphyritic granite facies is itself amore or less well-mixed hybrid. Megacrysts do not appear in Karkonosze granitefacies that evolved by fractional crystallization. The nucleation sites and growthpaths of granitic K-feldspar megacrysts, as products of an interaction of melts, haveseldom previously been the subject of detailed investigation (Gagnevin et al.,

Alkali feldspar megacryst growth 19

2005). Zoning, formed by growth in hybrid liquids, provides information on thecomposition of mixed magmas, and on the geochemical evolution of the mix.

Feldspar crystallization history

The feldspar crystallization path shown (Fig. 6) corroborates Vernon’s (1986,1990), Barbarin’s (1990) and Troll and Schmincke’s (2002) hypotheses for crystaltransfer between co-mingled magma batches of different composition. The modelsuggests that the megacrysts were periodically in contact with coeval magmas. Thehistory of these contacts is different for all of the investigated megacrysts. Thedegree of hybridization of crust-related magma is likely to change not only duringcrystal growth, but also between the environments where the crystals nucleatedand where their growth took place. The rocks from which the megacrysts werecollected defined a sequence of inferred decreasing degrees of crustal magmahybridization from Fojtka granodiorite to Rudolfov granodiorite to Michalowice(Karpacz) granite and, finally, to Szklarska Poreba Huta granite. Taking the modelinto account, the sequence of the megacrysts would be similar. The advantage of themodel is however, its potential for revealing the detailed history of the hybridiza-tion events involved in the feldspar growth – events not commonly or obviouslymirrored by the whole-rock composition.

The model shows changing magma compositions reflected in the compositionsof the megacryst zones. Growth morphologies and zone compositions provide ahistorical record of crystallization events in compositionally different environ-ments. The model shows the process of progressive dilution of the felsic compo-nent by the mafic one. The transfer of feldspars took place between coeval,interacting magmas – variably diluted mafic-magma blobs and variably contami-nated felsic-magma blobs. The model does not reveal the extent of transport. Blobsshowing variable degrees of hybridization may coexist close to each other duringvarious stages of magma evolution and emplacement (Barbarin and Didier, 1992;Barbarin, 2005). Using the model, any particular event can be only approximatelyattributed to processes occurring in the magma chamber at depth or during em-placement. In addition, mixing is followed by differentiation and segregation(Barbarin, 2005).

Even though simplified, the model provides important information. Only theFojtka megacryst seems to have moved between magmas with compositions closeto the assumed end-members. Rudolfov and Michalowice megacrysts grew almostexclusively from hybrid magma and interacted only accidentally with pure crust-related magma. In addition, these megacrysts seems to have nucleated not in a purefelsic melt but in a relatively hybridised magma (Fig. 6b). Some of the zones grewin pure lamprophyric magma. Narrow crystal domains with extremely high bariumcontents (Michalowice megacrysts – see Fig. 7) point to interaction with maficmagma more primitive than assumed for the model mafic end-member. Only thecrystallization of marginal parts close to the rapakivi mantles in the Rudolfov andMichalowice (Karpacz) megacrysts occurred in evolved crustal melt. Similarly,megacrysts from the Szklarska Poreba Huta granite nucleated mainly in hybridmagma. Their cores crystallized in dynamically stirred magmas. The compositionsof zones surrounding the cores point to alternating contact with non-evolved

20 E. Slaby et al.

crustal- and hybrid melts. Outermost zones close to rapakivi mantle grew fromcrustal melt that had evolved by fractional crystallization.

The model shows that crystallization was associated with dynamic changes inmagma composition. However, the crystals did not move rapidly from one magmacomposition to another. The difference in dilution at every step exceeded Xa �0.2on average. To make the picture of the dynamics of mixing more comprehensive,approximate crystal residence times in various compositional environments werecalculated. The real residence time is unknown as is the duration of non-equili-brium events leading to dissolution. The growth rate used in the calculation is fromLong (1978) whose experimental PT conditions, and melt compositions, comparewith those inferred here for the Karkonosze pluton.

For residence-time calculation, the widths of triplets of zones, e.g., a Ba-rich(mafic melt) bordering two Ba-poor (felsic melt), were measured. This parameterdiffers for megacryst cores in granite compared to cores in granodiorite rocks. Themaximum calculated duration of growth for such triplets determined from theRudolfov granodiorite is 1200–1600 years. In the Fojtka, Szklarska Poreba Hutaand Michalowice rocks, the residence time of megacrysts in more hybridisedmagmas seems to have been comparable, or a little shorter than that inferred forthe Rudolfov rock. In these hybridised magmas, growth was influenced by manydissolution events – more so than in the Rudolfov megacrysts. The textural evi-dence revealed by cathodoluminescence study on plagioclase inclusions lath trailsis obvious (Slaby and Gotze, 2004).

The Fojtka megacryst model involves the largest fluctuation in magma compo-sition. The outer zones of the Michalowice (Karpacz) megacrysts grew in a morehybridised magma than their counterparts in the Szklarska Poreba Huta granite.The zones of the Szklarska Poreba Huta megacrysts show clear evidence of migra-tion into a more felsic magma after termination of core growth. After an initial

Fig. 7. Back-scattered electron image of Michalowice megacryst core – dissolution textures

Alkali feldspar megacryst growth 21

residence time about 400 years, a short growth event (40 years) in a more hybri-dised environment preceeded the completion of megacryst formation over the next600 years in a pure melt that had evolved by fractional crystallization. The calcu-lated duration of the formation of crystal zones is fairly accurate, as the appliedmodel includes the main factors. It does not include the duration of dissolution –re-equilibration. However, the calculated durations well illustrate the fluctuationsin migration rates which differ between granodioritic and granitic mediums.

The similarities in the crystallization paths of the cores parts of Rudolfov,Michalowice and Szklarska Poreba Huta megacrysts are corroborated by cathodo-luminescence studies (Slaby and Gotze, 2004). The plagioclase inclusions in alldisplay the same luminescence characteristics. The changing dynamics of K-feld-spar zone growth are reflected in different structure-defect densities (Gotze et al.,2000; Slaby and Gotze, 2004). Plagioclase inclusions in the Fojtka megacrystsshow different cathodoluminescence characteristics. The growth model points toa common heritage for the Rudolfov, Michalowice and Szklarska Poreba Hutamegacrysts, and their growth in the main from variably hybridized crustal melts.The Fojtka megacrysts, in contrast, grew during migration between coeval magmaswith almost pure end-member compositions.

All the megacrysts reflect growth in a hybridised medium resulting from themixing of two melts. The compositions of the hybrids lie on the liquid line ofdescent joining the Fojtka granodiorite and the Szklarska Poreba Huta granite(Table 2; paths 5 and 6). The Harker diagrams show however, that the Rudolfovmegacrysts, though formed from hybrids belonging to the main trend, were sub-sequently introduced as a crystal mush in granitic melt into a mafic melt of similarcomposition to that of the Rudolfov silica-poor granodiorites in what would havebeen three-component mixing (Fig. 5b). The growth model demonstrates that theirgrowth did not continue in this new hybrid; only their plagioclase mantles crys-tallized. Cathodoluminescence study on Rudolfov megacrysts corroborates thisconclusion (Slaby and Gotze, 2004). The luminescence characteristics of the pla-gioclase are similar to those of plagioclases in the Fojtka megacrysts that also grewin almost pure lamprophyric magma.

The model suggests that feldspar compositions preserved information aboutsome hybridization events which are not reflected in the whole-rock compo-sition. This, in turn, suggests the existence of a much more extended range ofcoeval magma compositions than could have been deduced from whole-rockgeochemistry alone. This applys, in particular, to the data bearing on the compo-sition of the mantle-derived melt. As noted above, in some of the Karkonoszemegacrysts (Michalowice), narrow domains extremely enriched in barium do notfit the calculated path even using high Kafs=l values (Fig. 6). These domainsresemble skeletal growths. They could be the result of a kinetically-drivenbarium partitioning between K-feldspar and melt (Lindstrom, 1983; Gagnevinet al., 2005), the product of an anatectic reaction (Morgan and London, 2003)or restite (Klotzli et al., 2002). Examination of many profiles in the megacrystsstudied, and having compared the partitioning of other compatible elementswithin the zones, the first two possibilities seem less likely. Barium incorporationinto the alkali feldspar structure is strongly interrelated to aluminium, the incor-poration of which is the rate-limiting step for alkali feldspar crystallization

22 E. Slaby et al.

(Baker, 1992, 1995). In the light of the highest experimental Kafs=l value ofMorgan and London (2003) and calculated degree of undercooling for Karko-nosze megacrysts (Slaby et al., 2002), it is unlikely that K-feldspar domains richin barium could form from a parental peraluminous melt with a barium concen-tration of 200 ppm.

The hypothesis that the domains might be restite is also not probable althoughin granitic plutons of mixed origin, could preserve some restitic material (Huppertand Sparks, 1988). Exotic, crustal-derived domains within accessory minerals inmafic hybrids are known in Hercynian granitoids (Bea et al., 1999). Barium-enriched domains in megacrysts from granitoid hybrids in the South BohemianPluton are considered by Klotzli et al. (2002) to be restitic on the basis of isotopicdata. The isotopic composition of the Karkonosze megacrysts is unknown but otherdata could support the hypothesis. However, melt inclusions in Karkonosze quartzphenocrysts from hybrids provide some evidence that, in some homogenizedmagmas, the sole restitic phase was K-feldspar (Kozlowski and Slaby, 2004). Thetrace element pattern of those restitic feldspars is unknown. Nevertheless, thedendritic shape of barium-rich domains that perturb regular megacryst zones isdifficult to explain as restite. The domains likely represent patterns of dissolu-tion-replacement due to megacryst reaction with lamprophyric magma. The highbarium content could be related to lamprophyre composition. Lamprophyre dikesin Karkonosze can contain up to 2000 ppm barium (Awdankiewicz et al., 2005).The dissolution-recrystallization hypothesis provided by Bussy (1990) may bepertinent. A feldspathic melt derived from feldspar fusion in mafic magma ishostile to non-feldspar components and can only lead to recurrent feldspar crystal-lization (Bussy, 1990; Watson, 1976, 1982). In Karkonosze, this event is clearlylate; as noted above, the barium-rich zones cut the regular megacryst zones formedearlier.

The model raises many questions not solely concerning the mixing dynamics.Feldspar genesis explained by two- or three-component mixing begs an ultimatequestion regarding the compositions and sources of the parental crustal and mantle-related melts. It also raises a question regarding the genetic relationship betweenthe two latent mafic magmas involved in the formation of the Rudolfov and Fojtkagranodiorites. The liquid line of descent for both, though it shows a similar evolu-tion for the majority of the elements, does not for some, e.g., LILE. The linecontinues toward lamprophyre compositions which are also characterized by var-iable LILE enrichment. This could suggest a common mantle-related magmasource – metasomatised mantle variably enriched in some elements. The enrichmentcould well be modified by, e.g., the partial melting and=or any fluids involved.

Conclusions

The K-feldspar crystallization model provides important insights relating to mega-cryst nucleation and the dynamics of megacrysts movement. In doing so, it pro-vides evidence concerning the source(s) of Hercynian granitoids in the BohemianMassif, many of which are hybrids ranging in composition and usually containingalkali feldspar megacrysts. These megacryst-bearing occurrences vary in volumefrom small plutons down to enclaves (Liew et al., 1989; Vellmer and Wedepohl,

Alkali feldspar megacryst growth 23

1994; Finger et al., 1997; Holub, 1997; Zachovalova et al., 1999; Buda et al., 2000;Gerdes et al., 2000a, b; Janou�ssek et al., 2000; Seltmann et al., 2001; Muller et al.,2002; Muller and Seltmann, 2002). Exact end-member compositions are rarely ifever known. Hybrids enriched in mafic components may give an isotope signaturesimilar to that of crustal melt (see for review Bonin, 2004) or intermediate betweenmantle and crustal melt (Barbarin, 2005). Thus, the role of mantle magmas in thehybridized rocks of the Bohemian Massif plutons may be underestimated(Janou�ssek et al., 1995; Langer et al., 1995; Gerdes et al., 2000). In this contextmegacryst geochemistry could hold valuable information on the evolution historyof Hercynian magmas.

The model shows that megacrysts are not only preserved in mafic magma butthat their crystallization also continued in that magma. This could explain thepresence of megacrysts in both the more mafic hybrids and the more felsic hybridgranitoids in the Bohemian Massif (Zachovalova et al., 1999; Janou�ssek et al., 2000;Buda et al., 2000; Gerdes et al., 2000a; Seltmann et al., 2001; Muller and Seltmann,2002; Muller et al., 2002; Klotzli et al., 2002). The growth model for the Karko-nosze K-megacrysts indicates that those crystals undergo hybridization events notregistered by the whole-rock composition. Thus, the trace element signatures pre-served in feldspars that moved between coeval magmas have the potential to giveunique and precise information about the evolution of their host magmas.

Acknowledgements

We thank H. Martin (Clermont-Ferrand) and G. Worner (Gottingen) for inspiring discussionson geochemical modelling and B. Barbarin (Clermont-Ferrand) for an insightful pre-reviewof this paper. We are grateful to anonymous reviewer, B. Robins and especially, R. Vernon forconstructive and thorough comments, which led to considerable improvements. We appreci-ate the assistance of C. J. Stanley (London) for his time and effort in improving English ofthis manscript, P. Kennan (University College, Dublin) looked closely at English style andgrammar in the final version, correcting both and offering suggestions for further improve-ment. This work was funded by KBN grant 2PO4D00226. LG-G appreciates funding throughthe European Union Sys-Resource scheme that financed her laboratory research visit at NHMLondon.

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Authors’ addresses: E. Slaby (corresponding author; e-mail: E.Slaby@uw.edu.pl),L. Galbarczyk-Gasiorowska (e-mail: legalbar@uw.edu.pl), Institute of Geochemistry,Mineralogy and Petrology, Warsaw University, 02-089 Warsaw, Al. _ZZwirki I Wigury93, Poland; R. Seltmann (e-mail: rs@nhm.ac.uk), Natural History Museum, Departmentof Mineralogy, CERCAMS (Centre for Russian and Central EurAsian Mineral Studies)Cromwell Road, London SW7 5BD, United Kingdom; A. Muller (e-mail: Axel.Muller@ngu.no), Norges Geologiske Undersøkelse, N-7491 Trondheim, Norway

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