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Chemie der Erde 70 (2010) 185–196

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Chemie der Erde

0009-28

doi:10.1

� Tel.

E-m

journal homepage: www.elsevier.de/chemer

Morphology and growth trends of accessory zircons from various granitoidsof the South-western Bohemian Massif (Moldanubicum, Austria)

Robert Sturm �

Brunnleitenweg 41, A-5061 Elsbethen, Austria

a r t i c l e i n f o

Article history:

Received 18 November 2008

Accepted 17 May 2009

Keywords:

Zircon

Morphology

Typology scheme

Growth imaging

Bohemian Massif

Granitoid

Electron microprobe

19/$ - see front matter & 2009 Elsevier Gmb

016/j.chemer.2009.05.001

: +43 662 633321.

ail address: Robert.Sturm@sbg.ac.at

a b s t r a c t

As the main objective of the present study, the morphological development of accessory zircons from

four granitoids (pearl gneiss, fine-grained granite, coarse-grained gneiss, and Weinsberg granite) of the

South-western Bohemian Massif was described in detail. On the one hand, this was realized by the

classical approach, including a statistical evaluation of external zircon morphologies with the typology

scheme. On the other hand, direct insight into the growth of single crystals was established by the

production of crystal sections parallel and perpendicular to the crystallographic c-axis and by their

subsequent imaging with the electron microprobe. Regarding the second method, eventual morpho-

logical trends were represented as a function of the growth rates of single crystal forms. Except for the

coarse-grained gneiss, zircon crystals of the investigated granitoids show similar morphological trends

according to both methods, starting with a dominant {1 0 0} prism and equally sized pyramids. Final

crystal habit, however, is marked by the predominance of {1 1 0} and {2 1 1}. Zircon crystals from

the coarse-grained gneiss run through a completely different development with a change of the prism

habit from {1 1 0} to {1 0 0} and a more or less static growth of {1 0 1} with only slight modifications in

size. Comparison of the results with data from the literature underlines the role of magma chemistry,

magma temperature, and cooling rate as the main factors responsible for growth trends of accessory

zircon.

& 2009 Elsevier GmbH. All rights reserved.

1. Introduction

As demonstrated by numerous studies conducted during thepast decades, the morphological development of accessory zirconmay be evaluated as rather uniform within a given granite type,but, on the other hand, may vary significantly between twodifferent granite types (e.g. Poldervaart, 1956; Larsen andPoldervaart, 1957; Sansoni, 1962; Frasl, 1963; Hoppe, 1962,1963; Pupin and Turco, 1972; Pupin, 1980; Vavra, 1994; Steyrerand Sturm, 2002; Sturm and Steyrer, 2003; Scherer et al., 2007).According to a widely accepted doctrine, formation of a rock-specific crystal habit is chiefly controlled by physical andchemical factors including magma chemistry, content of volatilephases in the magma, and velocity of magma cooling (Pupin,1980, 1985; Sunagawa, 1984; Vavra, 1990, 1994). This circum-stance is impressively underlined by a comparison of, e.g., theI-type granites being characterized by a predominance of zirconcrystals with the flat pyramid {1 0 1} and the petrogeneticallydifferent S-type granites, which mainly include zircon crystalswith the steep pyramid {2 1 1} (e.g. Pupin, 1980; Sturm, 1999).

H. All rights reserved.

Concerning the exact progress of zircon crystal growth duringearly and intermediate stages of magmatic crystallization, variousscientific theories based on external crystal shapes were devel-oped in the past decades (Poldervaart, 1956; Hoppe, 1963; Pupinand Turco, 1972). However, an unequivocal clarification of thiscomplex question was not permitted until the application ofspecific crystal preparation techniques and related microscopicmethods of documentation. Initial theories regarding the mor-phological development of zircon (Poldervaart, 1956; Larsen andPoldervaart, 1957) supposed a somewhat static crystal growthwith constant crystal habit and uniform growth rates throughoutthe whole crystallization process. This rather simple growthmodel was disproved very soon by extensive light-microscopicstudies of granite-specific zircon populations. The investigationsof, e.g., Frasl (1963), Hoppe (1963), Veniale et al. (1968), andKohler (1970) could demonstrate that, contrary to the prevailingdoctrine, the zircon habit may undergo numerous modificationsduring crystal growth. A preliminary statistical evaluation of thewhole number of crystal forms occurring within a granite-specificzircon population was conducted by Pupin and Turco (1972),thereby introducing the so-called typology diagram, where crystalshapes are arranged according to the proportions of the pyramidand prism faces. By extensively applying the diagram to zirconpopulations separated from petrogenetically different granites,

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the authors succeeded in the definition of morphological growthtrends (Typological Evolutionary Trend or T.E.T.) that shouldexpress any changes of the zircon habit occurring during crystal-lization. Pupin (1980) extended this revolutionary model by thehypothesis that, depending on the investigated granites, entiremorphological growth trends indeed may start from differentinitial crystal shapes but ultimately take on similar final shapes(G-type and adjacent subtypes in the typology diagram).

New aspects of zircon crystal growth could be obtained byinnovative preparation methods developed in the 1990s and therecent decade (e.g. Paterson et al., 1989; Vavra, 1990; Benisek andFinger, 1993; Sturm, 1999, 2004). With the help of these partlytime-consuming techniques, single zircon crystals were sectionedparallel or perpendicular to their main crystallographic axes, bymeans of which a direct insight into the growth of pyramids andprisms was realized for the first time. By a carefully directedcathodoluminescence or electron microprobe study of crystalsections produced from representative grains of different zirconpopulations, the development of crystal morphology could bedocumented and quantified on the basis of crystal face-specificgrowth rates (Vavra, 1993; Sturm, 2004). Results of theseinvestigations among others exhibited that the growth trendspropagated by Pupin (1980) have to be extended successively andthat many studies are necessary in future to complete theknowledge in this complex scientific field.

In the work presented here, a detailed insight into themorphological development of zircon crystals from four grani-toids of the South-western Bohemian Massif is provided. Besidesthe ‘classical’ statistical evaluation of external crystal shapesaccording to the scheme of Pupin (1980), zircon growth was alsoquantified exactly by the direct measurement of pyramidal andprism growth bands on respective crystal sections. Possiblediscrepancies between growth trends obtained from the ‘classical’method and those obtained from the modern technique arediscussed.

Fig. 1. Measurement of prismatic and pyramidal growth bands on zircon crystal

sections oriented perpendicular and parallel to the crystallographic main axis.

For an appropriate determination of morphological trends, ri{1 1 0}�ri{1 0 0} and

ri{2 1 1}�ri{1 0 1} are plotted in diagrams against the central distance of a

reference form ({0 1 1}). Through the obtained data points regression lines are

fitted (see Fig. 9).

2. Materials and methods

From the four granitoids considered for the present studysample volumes between 500 and 1000 cm3 were prepared forzircon crystal separation, whereby one or more rock pieces werecollected from a single site. Thereby, separation of zircon crystalswas conducted according to a standard procedure (e.g. Pupin,1980; Sturm, 1995), among others including the crushing, milling,and sieving of the rock material as well as a subsequent mineralseparation in a magnetic field and a high-density liquid (tetra-bromine-ethane, 2.96 g cm�3). For light-microscopic investiga-tions a part of the zircon crystal fraction enriched in this way wasdistributed on glass slides and embedded in a highly refractiveliquid phase (Canada balsam or Cargile Meltmount artificial resin;Frasl, 1963). To study the external crystal morphology more indetail, 20–30 zircon grains of each granitoid were mounted onspecial slides using epoxy resin. After covering the specimenswith carbon, zircon crystals were studied with the electronmicroprobe (type JEOL JXA-8600) at the Institute of Geology andPalaeontology, University of Salzburg, whereby the followingdevice settings were used: secondary electron mode, acceleratingvoltage of 15 kV, and beam current of 3 nA. The statisticalevaluation of external crystal shapes observed by light andelectron microscopies was carried out according to the schemeof Pupin and Turco (1972) as well as Pupin (1980), considering200 grains per sample.

For the investigation of the crystal-specific morphologicaldevelopment, 30 undamaged zircon grains of each sample weresectioned parallel or perpendicular to the crystallographic c-axis

according to previously published preparation procedures(Benisek and Finger, 1993; Vavra, 1994; Sturm, 1995). Photo-graphic documentation of the produced crystal sections was againconducted with the electron microprobe using the following basicdevice settings: backscattered electron mode, accelerating voltageof 15 kV, and beam current of 30–40 nA. Single crystal growthincrements recognizable on the respective sections were com-puted on the basis of the methods introduced by Vavra (1993),thereby plotting differences of the pyramidal and prism growthband widths, ri{2 1 1}�ri{1 0 1} and ri{1 1 0}�ri{1 0 0}, against thecentral distance of a selected reference face. Possible trends ofthe morphological development were determined by calculatingregression lines through the data points. Besides the measure-ment of growth bands according to the scheme of Fig. 1, alsototal growth rates of the main crystal faces were plotted intorespective diagrams, considering 20 zircon grains per sample(see Fig. 9).

3. Petrography and geochemistry of the investigatedgranitoids

The four granitoids of the study presented here are located atthe South-western margin of the Bohemian Massif (Fig. 2) andwere most probably formed under anatectic conditions (Frasl andFinger, 1991). The investigated rocks include, on the one hand,the Weinsberg granite (type II) and the so-called coarse-grainedgneiss (‘Schlierengranit’), both being classified as I-typegranitoids, and, on the other hand, the pearl gneiss and fine-grained granite (‘Zweiglimmergranit’), both of which may beassigned to the S-type granitoids. The petrography andgeochemistry of the rocks coarsely summarized in Fig. 3 aredescribed in detail by Thiele (1962), Fuchs and Thiele (1968), Liewet al. (1989), Finger (1986), Frasl and Finger (1991), and Sturm(1995).

The pearl gneiss is characterized by fine-grained layers ofbiotite and quartz, into which rounded clasts of oligoclase(‘pearls’) with diameters of several millimetres are embedded.The main petrography of the granitoid consists of oligoclase (40–50 vol%), biotite and mica (15–30 vol%), quartz (20–30 vol%) aswell as smaller amounts of K-feldspar and cordierite (both up to10 vol%). In geochemical respects, the rock contains a significantlyenhanced amount of Al, resulting in a higher Al2O3/(CaO+Na2O+K2O) ratio with respect to the other granitoids. The concentrationsof Zr and Nb may be evaluated as moderate, while contents of Y,Sr, and Ba are remarkably increased. The geochemical dataunderline the hypothesis, according to which the pearl gneiss

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Fig. 2. Geological map of the Variscan basement in Central Europe (A) and the Bohemian Massif with its various tectonometamorphic units (B). The detailed map (C)

exhibits the geological units of the South-western margin of the Bohemian Massif as well as the locations of the sample points (modified after Frasl et al., 1965). Samples:

(1) pearl gneiss, (2) coarse-grained gneiss, (3) fine-grained gneiss, and (4) Weinsberg granite.

Fig. 3. Appearance, classification of the magmatic source, petrography, geochemistry (Frasl and Finger, 1991), and geographic details of the investigated granitoids. Sample

volumes used for zircon extraction ranged from 500 to 1000 cm3.

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developed from highly metamorphic gneisses of sedimentaryorigin (Fuchs and Thiele, 1968; Frasl and Finger, 1991).

The calcalkalic coarse-grained gneiss, whose name is derivedfrom the medium- to coarse-grained feldspars (diameter:1–5 cm), also represents a granitoid with anatectic origin. Mainmineral phases of the rock include plagioclase (30–50 vol%),

K-feldspar (20–30 vol%), quartz (20–30 vol%), biotite (10–20 vol%),and in rare cases also amphibole (up to 5 vol%). Regarding itsmajor element geochemistry, the coarse-grained gneiss is con-spicuous due to its enhanced concentrations of alkali elements(mainly K), Ca, and Al, while among the trace elements Rb, Sr, Zr,and Ba exhibit most remarkable concentrations (Fig. 3).

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The fine-grained granite of this study may be assigned to aseries of late- to post-Variscan S-type granitoids, a commoncharacteristic of which is the fine to intermediate granulation.The petrography of the rock consists of K-feldspar (30–35 vol%),plagioclase (25–30 vol%), quartz (25–30 vol%), biotite (ca. 10 vol%),and white mica (up to 5 vol%). In the geochemical respect,the fine-grained granite may be categorized as moderatelyperalumic and shows decreased concentrations of Ba, Zr, andSr (Fig. 3).

The Weinsberg granite, from which two petrogeneticallydifferent types were distinguished in the past, is characterizedby tabular K-feldspars up to 15 cm in size. Besides this with30–40 vol% dominant mineral phase, the Weinsberg granite alsoconsists of plagioclase (20–30 vol%), quartz (ca. 20 vol%), andbiotite (ca. 10 vol%). Accessory minerals include zircon, apatite,titanite, monazite, and xenotime. Concerning its geochemistry,the rock exhibits increased concentrations of Al and K but onlymoderate concentrations of Na and Ca, whereas among the traceelements a significant predominance of Zr and Ba and a sub-ordinate content of Nb and Y is observed. Since K-feldspars of theWeinsberg granite are not marked by any alignment due tomechanical metamorphosis, the granite mainly bears intrusivequalities and is commonly interpreted as a product of continentalcrust melted by anatexis (Frasl and Finger, 1991).

4. Results

4.1. Investigations of the external zircon crystal morphology

According to the results of the light- and electron-microscopicinvestigations summarized in Figs. 4 and 5 most remarkabledifferences of the external zircon crystal morphology arerecognizable as expected between the S-type and the I-typegranitoids. Therefore, zircon crystals separated from the pearlgneiss (Figs. 4A and 5A) are commonly characterized by anunequivocal predominance of the pyramidal form {2 1 1} over thepyramidal form {1 0 1}, whereas the prism form {1 1 0} usuallysurpasses the competing form {1 0 0} in size. Former rockdeformation is indicated by partly extensive rounding of thepyramidal tops and the frequent occurrence of mm-sizedcorrosion pits. The mean length/width ratio of the zircon grainsamounts to 2.35, indicating a somewhat stocky to normal crystalgrowth.

Accessory zircon separated from the coarse-grained gneiss(Figs. 4B and 5B) may be characterized by a habit with a clearpredominance of the forms {1 0 1} and {1 0 0} over {2 1 1} and{1 1 0}. Similar to the pearl gneiss, regional metamorphosis hasalso left its traces on single grains, ranging from weak andmedium superficial damages to grain fractures. The length/widthratio of the zircon crystals is subject to remarkable variations,i.e. besides very stocky grains with respective ratios between 1.0and 1.5 also elongated crystals with ratios greater than 5 can bediscovered in the zircon population.

Regarding their external morphology, zircon crystals occurringin the fine-grained granite exhibit unequivocal similaritiescompared to the crystals from the pearl gneiss (Figs. 4C and 5C).Differences of the zircon habit between both granitoids mainlyconcern the size-specific predominance of the pyramid {2 1 1} andthe prism {1 1 0}, which is even more clearly pronounced in thefine-grained granite. Length/width ratios of the crystals arenormally greater than 3.0.

Zircon crystals separated from the Weinsberg granite arecharacterized by largely balanced size proportions between thepyramidal forms {2 1 1} and {1 0 1} as well as the prism forms{1 1 0} and {1 0 0} (Figs. 4D and 5D). Similar to the other

granitoids, the grains show various damages due to metamorphicprocesses affecting the host rock. As a specific characteristic thefraction of twinned crystals is clearly enhanced compared to thezircon populations of the other rocks (note crystals 3 and 6 inFig. 4D). Length/width ratio of the studied grains varies between2.0 and 8.0, indicating normal to unequivocally elongated crystalgrowth.

Results of the statistical evaluation of the external zirconcrystal shapes according to the typology scheme of Pupin andTurco (1972) are summarized in Fig. 6. In the pearl gneiss,subtypes S1, S6, and S7 occur with relative frequencies of 10–20%,whereas the adjacent subtypes commonly show frequenciesbetween 2% and 10% and therefore are of decreased importance.In the coarse-grained gneiss, a general predominance of thesubtypes S23–S25 and an occurrence of the J-subtypes, indicatingthe complete absence of the prism form {1 1 0}, is noticeable.Similar to the pearl gneiss, also the fine-grained granite is markedby a preponderance of the subtype S1, whereas the Q- andL-subtypes remarkably increase in significance. Regarding theWeinsberg granite, the zircon population is mainly composed ofthe subtypes S7, S12, and S17, whereby a slight tendency towardsthe steep pyramid can be recognized. The typology diagrams ofFig. 6 additionally contain indices of the typological statisticsoutlined by Pupin (1980) and resulting typological trendsrepresented as arrows. Except for the coarse-grained gneiss,where the arrow points from subtype S15 towards subtype S24,computed trend lines run from zircon crystal morphologies withprevailing prism form {1 0 0} and nearly equally sized pyramidalforms towards morphologies with clear predominance of {1 1 0}and {2 1 1}.

4.2. Studies on the internal morphological development

The growth of zircon crystals from the four granitoidspresented here can be approximated using the parallel andperpendicular crystal sections of Figs. 7 and 8. Zircon grains fromthe pearl gneiss commonly dispose of rapidly oscillating growthzones that have developed around a dark or, in rarer cases, a brightinherited core. On the parallel sections, the pyramidal form {2 1 1}generally exhibits a continuous increase in size, resulting in theexternal predominance of the steep pyramid over the flat pyramid(Fig. 7A). Prism growth is frequently characterized by a change ofthe habit, whereby at medium growth stages the form {1 0 0} issuccessively displaced by the form {1 1 0} (Fig. 8A). Zircon crystalsrevealing regular patterns of growth bands are sometimescharacterized by a clearly visible sector zoning (e.g. grain no. 1in Fig. 8A). Phenomena of dissolution and recrystallization causedby high-grade metamorphic processes affect the whole cross-section only in rare cases, but may be observed more frequently atthe marginal areas of the respective sections.

A major characteristic of zircon crystals from the coarse-grainedgneiss is the well-rounded and brightly appearing inherited core,which is enclosed by a rather weakly developed pattern of growthzones. Pyramidal development is often subject to a constantgrowth of the form {1 0 1}, whereas the steep pyramid {2 1 1}shows a decreased importance (Fig. 7B). Concerning prism growth,either a predominance of the form {1 0 0} throughout the entirecrystallization process or a change from {1 1 0} to {1 0 0} duringmedium developmental stages is given (Fig. 8B).

In zircon crystals separated from the fine-grained granite,a clear pattern of dark and bright growth zones enclosing a darkinherited core can be determined. While during more initialgrowth stages the pyramid {1 0 1} may surpass the form {2 1 1} insize, at final growth stages the flat pyramid continuously loses itsinitial significance (Fig. 7C). Longitudinal sections of numerous

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Fig. 4. Light-microscopic images of zircon crystals separated from the studied granitoids: (A) pearl gneiss, (B) coarse-grained gneiss, (C) fine-grained granite, and (D)

Weinsberg granite (bars: 100mm).

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crystals exhibit bright rims in the BSE image, indicating a possibleenrichment of heavy elements (i.e. elements with high Z) inthese areas. Additionally, specific structures caused by dissolutionand recrystallization processes are unequivocally discernibleon several sections. On the cross-sections two differentdevelopmental trends may be distinguished, i.e. the replacementof an initial form {1 0 0} by the form {1 1 0}, and the rather

constant predominance of the form {1 1 0} without any significantchange in proportion during crystal growth (Fig. 8C).

Crystals from the zircon population of the Weinsberg granitefrequently show numerous concentric growth shells representedby dark and bright incremental zones on both the longitudinal andcross-sections. Pyramidal development is commonly character-ized by a continuous increase in size of the form {2 1 1}. In rarer

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Fig. 5. SEM images of selected zircon crystals from the studied granitoids for the detailed determination of the external morphology and the computation of superficial

damages caused by metamorphosis: (A) pearl gneiss, (B) coarse-grained gneiss, (C) fine-grained granite, and (D) Weinsberg granite (bars: 100mm).

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cases pyramidal growth may also take place in favour of the form{1 0 1} (Fig. 7D). The inherited core has a rounded or irregularshape and often shows typical characteristics of metamictization.On the cross-sections different prism growth trends are discern-ible, whereby in most cases a respective form with initialpredominance also determines the external morphology. On somecrystal sections, the conventional growth zoning is added by apartly strongly developed sector zoning (e.g. grain no. 3 in Fig. 8D).

4.3. Quantification of crystal growth

As shown by the results of growth band measurementsummarized in Figs. 9 and 10, for zircon crystals from allinvestigated granitoids significant pyramidal and prismgrowth trends could be computed by linear regression (Fig. 9).As all factors included in the computation may be explainedby a normal distribution, application of the regression

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Fig. 6. Typological statistics of the investigated zircon populations: (A) pearl gneiss, (B) coarse-grained gneiss, (C) fine-grained granite, and (D) Weinsberg granite. The

upper image (Sturm and Steyrer, 2003) represents an overview of the most important crystal types according to Pupin and Turco (1972). The diagrams illustrated below

additionally contain parameters of the typological statistics (Pupin, 1980) as well as respective typological trends (dashed arrows). The parameters I.A. and I.T. describe the

coordinates of the mean point of a given typological trend (white dots), whereby I.A. represents the x-axis of the typology diagram and I.T. the y-axis. Each axis is

subdivided into eight units ranging from 100 to 800 (graph A; Pupin, 1980, Fig. 2). SA and ST denote the respective standard deviations belonging to the mean values

described above, and ST/SA represents the slope of the typological trend. Bold arrows represent those morphological trends derived from the analysis of longitudinal and

transverse crystal sections.

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technique is highly justified. Second, linear trends, althoughnot well explaining the distribution of the data points inall cases, are sufficient for the investigation of growthdevelopments. Concerning the pearl gneiss, the calculated trendlines indicate a change of the prism habit from {1 0 0} to {1 1 0} atmedium growth stages as well as a continuous predominance ofthe steep pyramid {2 1 1}. For the coarse-grained gneiss a prismgrowth trend running contrary to that of the pearl gneiss isobtained, i.e. the average prism habit changes from {1 1 0} to{1 0 0}. Pyramidal development is marked by a nearly horizontaltrend line and thus can be evaluated as static, with {1 0 1}preserving its size over the whole crystallizationprocess. Respective trend lines derived from the growthband analysis of zircon crystals from the fine-grained granite

indicate a significantly increasing predominance of both the prism{1 1 0} and the pyramid {2 1 1}, thereby underlining theobservations described in the preceding section. Regarding theWeinsberg granite, prism growth trend exhibits a morestatic growth with insignificant increase in size of {1 0 0}.Pyramidal growth, on the other side, mainly takes place infavour of {2 1 1}, which is confirmed by the negative slope of therespective trend line. Total growth measurement of pyramidsand prisms, which is simply defined as the sum of growthincrements occurring for each form, represents an alternative wayfor the determination of average external crystal shapes (Fig. 10).The position of the data points essentially confirms theknowledge obtained from microscopic and typologicalinvestigations.

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Fig. 7. BSE images of zircon crystal sections oriented parallel to the main crystallographic axis: (A) pearl gneiss, (B) coarse-grained gneiss, (C) fine-grained granite, and (D)

Weinsberg granite (symbols: closed circles {2 1 1}, open circles {1 0 1}; bars: 100mm).

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5. Discussion

By the use of specific preparation techniques giving an insightinto the growth of single zircon crystals, the scientific hypothesisformulated in the 1960s and studied in detail by Pupin (1980),according to which zircon grains may run through differentmorphological developments during the crystallization process,

can be essentially underlined in the present contribution.Concerning the four granitoids documented here, two differenttrends of the morphological development of the crystal habitcould be determined, using longitudinal and cross-sections ofselected zircon crystals. While accessory zircon separated fromthe S-type granitoids (pearl gneiss, fine-grained granite) and theWeinsberg granite are commonly characterized by an increase in

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Fig. 8. BSE images of zircon crystal sections oriented perpendicular to the main crystallographic axis: (A) pearl gneiss, (B) coarse-grained gneiss, (C) fine-grained granite,

and (D) Weinsberg granite (symbols: closed squares {1 1 0}, open squares {1 0 0}; bars: 30mm).

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size of the steep pyramid {2 1 1} and a frequently occurring changeof the prism habit from early to late growth stages, accessoryzircon separated from the coarse-grained gneiss (I-type granitoid)shows a more static pyramidal and prism growth with constantpredominance of {1 0 1} and {1 0 0}. Only in exceptional cases, asuccessive replacement of the form {1 1 0} by the form {1 0 0} canbe observed on cross-sections of crystals from this granitoid.

Except for the coarse-grained gneiss, nearly identical trends can beobtained by application of the classical typology statistics (Fig. 6;Pupin and Turco, 1972; Pupin, 1980), underlining the significanceof this time-saving technique for the solution of respectivescientific problems. Advantages combined with the productionof zircon crystal sections were discussed in detail by Vavra (1993).According to the author, growth rates of single crystal faces are

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Fig. 9. Quantitative evaluation of prism and pyramidal growth of zircon crystals separated from the investigated granitoids: (A) pearl gneiss, (B) coarse-grained gneiss, (C)

fine-grained granite, and (D) Weinsberg granite. Trend lines obtained from linear regression analysis provide information on the average morphological development of

respective zircon crystal forms (see Fig. 6 and text). x- and y-values of all diagrams have been tested for normal distribution, and respective histograms and parameters (m

mean value, s standard deviation) are added.

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mainly controlled by crystal growth kinetics, thereby mostremarkably reflecting the physical and chemical characteristicsof the crystallization milieu. Furthermore, the morphological

development of a crystal, no matter to which symmetry class itbelongs, may be completely decoded by the quantitative dataobtained from growth band measurement. Vavra (1993) finally

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Fig. 10. Statistical computation of the external crystal morphologies by plotting total prismatic and pyramidal growth. Position of the obtained points chiefly corresponds

very well to the results derived from light and electron microscopy (see Figs. 4 and 5).

Fig. 11. Possible limitations occurring with the use of longitudinal and cross-

sections for the study of the morphological development of a given zircon crystal

population. An appropriate quantification of prism growth (left column) is only

guaranteed by a perfect cut through the prismatic part of the crystal, whereas

respective sections through the pyramidal part are often unusable for growth

diagnoses. A contrary situation is given for the investigation of pyramidal crystal

growth (right column), which may also be carried out on non-median crystal

sections.

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argues that the application of the crystal preparation technique isnot subject to any limitations regarding the number of crystalfaces, so that also minerals with higher crystallographic complex-ity than zircon could be considered for similar questions.

The present contribution seems to be another confirmation ofthe fact that the morphological development of accessory zirconis significantly controlled by magma chemistry. Therefore, inmagmatic milieus with enhanced contents of Al and moderate tohigh concentrations of K, Na, and Ca, preference of the externalcrystal habit {2 1 1}+{1 1 0} is given. At initial stages of the growthprocess, a predominance of the habit {1 0 1}+{1 0 0} is frequentlyobservable, which according to the results presented in theliterature hitherto is continuously replaced by the forms {2 1 1}and {1 1 0} due to the adsorption of growth-blocking elements ormolecules on the respective crystal faces (e.g. Benisek and Finger,1993; Vavra, 1994; Sturm, 2004). Present results obtained for thegrowth of zircon crystals from the pearl gneiss perfectlycorrespond to respective data introduced by Sturm (1999), wheresingle crystals separated from this rock were subject to acombined section parallel and perpendicular to the crystal-lographic main axis. Similar morphological growth trends asthose documented for the S-type granitoids could be alreadydetermined by Pupin (1980) for the high-Al leucogranites of theCentral Massif in France. According to the author, zircon crystalsseparated from these rocks more probably tend to the final habit{1 0 1}7{2 1 1}+{1 1 0} (Fig. 7 in Pupin, 1980), representing amain difference to the present results.

An image being completely contrary to that of the S-typegranitoids is given for the zircon crystal growth in a magmaticenvironment with high content of calcalkalic elements (and K) butdecreased concentrations of Al, where accessory zircon preferen-tially develops the external habit {1 0 1}+{1 0 0}. In many cases,this morphology is already determined at initial stages ofcrystallization, subsequently running through a static growthprocess with nearly constant proportions of the single crystalfaces. Concerning the initial habit {1 0 1}+{1 1 0}, which hasalready been described by Sturm (1999) for zircon crystals ofthe coarse-grained gneiss, a growth-blocking effect contrary tothat described above occurs, i.e. development of {1 0 0} isinhibited (Benisek and Finger, 1993). Vavra (1994) documentssimilar phenomena for zircon crystals from calcalkalic grani-toids of the Budduso pluton on Sardinia, thereby leading backthe significantly decreased growth rates of the prism {1 0 0} to thetemporary supersaturation of Zr in the magma. According to the

author, low supersaturation of this element causes a decreasedgrowth of {1 0 0} and vice versa. Further factors controlling thedevelopment of {1 0 0} could not be decoded hitherto.

The study of morphological trends based on oriented crystalsections has, besides the high sacrifice of time, the essentialdrawback that some crystal sections deviating from the idealmedian position are not appropriate for detailed typologicaldiagnoses. As illustrated in Fig. 11, this restriction is mainly given

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for cross-sections cutting the pyramidal part of the zircon crystal.Longitudinal sections with significant deviation from the medianplane, on the other side, still provide a usable image of the sizeproportions of the pyramidal faces. However, growth ratesmeasured on such sections have to be corrected according toVavra (1993).

From the study presented here it may be concluded that thedetermination of crystal shapes within a zircon population as wellas the decoding of respective morphological trends is madepossible to a certain extent by the application of statisticalevaluations of the external zircon shapes (Pupin, 1980). To obtainhighly reliable predictions for the evolution of zircon morpholo-gies within a given host rock, methods allowing an insight intozircon growth development have to be applied, which are muchmore time-consuming than the light-microscopic techniques.

Acknowledgements

The author is indebted to H.P. Steyrer for his immediatesupport during rock preparation. A. Benisek is thanked for his helpduring crystal preparation and work with the electron microp-robe.

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