GeolCarp_Vol49_No3_151

GEOLOGICA CARPATHICA, 49, 3, BRATISLAVA, JUNE 1998

151160

ZIRCON IN HIGHLY EVOLVED HERCYNIAN HOMOLKA GRANITE,

MOLDANUBIAN ZONE, CZECH REPUBLIC: INDICATOR OF

MAGMA SOURCE AND PETROGENESIS

PAVEL UHER

, KAREL BREITER

, MILAN KLEÈKA

and EDVÍN PIVEC

Geological Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 842 26 Bratislava, Slovak Republic;

geoluher@savba.savba.sk

Czech Geological Survey, Geologická 6, 152 00 Praha 5, Czech Republic

Bohemian Geological Corporation, V Holeovièkách 41, 182 09 Praha 8, Czech Republic

Geological Institute, Czech Academy of Sciences, Rozvojová 135, 165 00 Praha 6, Czech Republic

(Manuscript received November 12, 1997; accepted in revised form March 24, 1998)

Abstract: The Homolka granite stock (southern Bohemia, Czech Republic) is a highly evolved topaz-bearing and phos-

phorus-rich intrusion penetrating the Lásenice and Èímìø S-type granites of the late-Variscan South-Bohemian

(Moldanubian) pluton, subvolcanic dykes of microgranite to granite porphyries are also widespread. Study of accessory

zircon revealed an extensive amount of old inherited zircon in the Homolka granites. On the basis of back-scattered elec-

tron images and microprobe study, the inherited zircon of the Homolka Granite is identical to the zircon from the Lásenice

and Èímìø granites. This older zircon forms transparent crystals with oscillatory zoning and low Hf, Y, U and P contents.

Younger zircon of the highly evolved Homolka Granite exhibits transparent crystals with oscillatory to irregular zon-

ing and metamict grains with high Hf (up to 12.3 wt. % HfO

), often also Y, U and P. If 30 to 50 vol. % of zircon is esti-

mated to be inherited, the calculation of zircon saturation temperature gives T

= 720760

C for the Èímìø and 640

680

C for the Lásenice zircon, and 610650

C for the Homolka zircon. The subvolcanic microgranite-porphyry dyke

gave T

= 690730

C. These zircon data indicate that the Homolka Granite is a product of advanced fractional crystal-

lization from a magmatic source similar to the Èímìø Member of the Eisgarn Suite.

Key words: Czech Republic, South-Bohemian (Moldanubian) pluton, Homolka Granite, zircon saturation temperature,

electron microprobe, BSE, zircon typology, zircon.

Introduction

Accessory zircon of granitic rocks has been used as petroge-

netic indicators for a long time. Its informative value lies

mainly in the relatively high chemical stability of zircon

during subsolidus stages, the dependence of between crystal

morphology and genetic type of parental magma (zircon ty-

pology; Pupin 1980), the variety of internal crystal zoning

as revealed by back-scattered electron images (BSE) and

cathodoluminescence (Vavra 1990), Zr/Hf ratio as a princi-

pal indicator of magmatic fractionation (Fleischer 1955; Ly-

akhovich 1968; Èerný et al. 1985); distribution of other

trace elements, such as Y, REEs, P, U; a possibility to cal-

culate zircon saturation temperature (Watson & Harrison

1983); numerous U-Pb isotope and FT dating, etc.

The aim of this paper is to demonstrate the ability of ac-

cessory zircon to indicate the possible magma source of the

granite using the example of the Homolka highly evolved

rare-element and phosphorus-rich granite.

Geological setting

The studied area is situated in the NW part of the late-

Variscan South-Bohemian or Moldanubian pluton between the

towns of Tábor and Gmünd. The pluton is represented here

mainly by the two-mica Lásenice and Èímìø granites (Fig. 1).

The fine-grained, mostly equigranular two-mica Lásenice

Granite (represented here by sample HO-1) is relatively old-

er and affected by the latest Variscan tectonic movements

(Kleèka & Rajlich 1984). The Lásenice Granite occurs

along the NW contact of the pluton and also forms small

bodies in the Moldanubian paragneisses. The accessory

minerals, represented by andalusite, monazite-(Ce) and il-

menite (Table 1) as well as chemical compositions, very

poor in HFSE and REE (Table 2), both classify the Lásenice

Granite as a typical slightly fractionated metasedimentary

S-type granite.

The younger, post-tectonic Èímìø Granite (HO-3) forms

the major part of the area. The Èímìø Granite is medium-

grained porphyritic, two-mica and relatively older subtype

of the Eisgarn granite group. The ilmenite+monazite-(Ce)

accessory assemblage (Table 1) and chemical composition

(Table 2) indicate their high K-calc-alkaline S-type nature.

Both the Lásenice and Èímìø granites are cut by a ca.

20 km long, N-S trending swarm of more than 30 dykes of

granite porphyries and microgranites (HO-2). The individual

dykes are 220 m thick and up to 1.5 km long. Some thicker

dykes are zoned and exhibit fluidal rhyolitic texture along the

contact, which passes gradually into granite porphyry to-

wards the centre of the dyke.

In the central part of the dyke swarm, close to the Czech-

Austrian border, a stock of leucocratic Homolka Granite

crops out. Its surface of 6 km

is slightly elongated in the N-S

152 UHER, BREITER, KLEÈKA and PIVEC

direction and forms a morphological elevation of the Homol-

ka Hill. Several xenoliths of the Èímìø Granite and granite

porphyry up to a few hundred square metres in area are found

in the central part of the granite body.

The Homolka stock (HO-4 to 8) is composed mainly of

equigranular medium-grained leucocratic alkali feldspar

granite. Only some schlieren of porphyritic variety are devel-

oped in the western part of the body, and they contain up to 1

cm large quartz and K-feldspar phenocrysts in a fine-grained

groundmass. Schlieren and/or dykes of fine-grained aplitic

variety are found in several places. A marginal pegmatite,

stockscheider (HO-7) with large K-feldspars floating in a

Fig. 1. Geological map of the Homolka granite area.

ZIRCON IN HIGHLY EVOLVED HERCYNIAN HOMOLKA GRANITE, MOLDANUBIAN ZONE 153

fine-grained groundmass, develops in some parts of the SE,

W and N contact of the stock. It forms a rim around the ma-

jor xenoliths in the central part of the body.

All textural varietes of the Homolka Granite contain

quartz, albite (An

00-05

), P-rich K-feldspar and Li, Rb, Zn-

and F-rich muscovite. Topaz is frequent, apatite, children-

ite-eosphorite, cassiterite, Nb,Ta-rich rutile, titanian ixiolite

and ferrocolumbite are the main accessory minerals (Breiter

& Scharbert 1995; Frýda & Breiter 1995; Uher et al. in

prep.) Table 1. Post-magmatic processes are restricted to

rare quartz veinlets with greisen rims, patchy chloritization,

veinlets of a barren milky quartz and flakes of U-micas in

fissures (Litochleb et al. 1991; Lochman 1992). The Homol-

ka Granite belongs to the rare type of ore-bearing P-rich per-

aluminous magmatites, highly enriched in Li, Rb, Cs, Sn,

Nb, Ta and F (Table 2) and with a high initial Sr isotopic ra-

tio I

=0.716±0.010 (Breiter & Scharbert 1995). It repre-

sents one of the youngest Variscan magmatic events within

the Moldanubian; the Rb/Sr whole-rock isochrone gave an

age of 319±7 Ma, the muscovite Ar-Ar plateau ages are

317±2 and 315±3 Ma, which indicate a relatively rapid

cooling of the intrusion (Breiter & Scharbert 1995).

Experimental methods

We studied zircon from eight rock samples for their de-

scription see Appendix. Zircon concentrates were obtained

from 510 kg of solid rocks by standard methods: crushing,

sieving, heavy liquid (bromophorm) and electromagnetic

separation.

Back-scattered electron images (BSE) were performed on

the JEOL JSM-840 scanning electron microscope at the

Geological Survey of the Slovak Republic, Bratislava, Slo-

vakia, under an accelerating potential of 25 kV.

Electron microprobe analyses (EMPA) of separated and

polished zircon crystals were carried out on the Cameca

Table 1: Accessory minerals of the Lásenice Granite (HO-1), microgranite-

porphyry dyke (HO-2) Èímìø Granite (HO-3), and Homolka granites (HO-

4 to HO-8). For petrographic types, see Appendix. XX: very abundant, xx:

abundant, x: rare accessory mineral.

HO-1 HO-2 HO-3 HO-4 HO-5 HO-6 HO-7 HO-8

ZIRCON

APATITE

MONAZITE-(Ce)

XENOTIME-(Y)

CHILDRENITE

ANDALUSITE

SCHORL

TOPAZ

CASSITERITE

COLUMBITE

Nb,Ta RUTILE

RUTILE

ANATASE

ILMENITE

GAHNITE

URANINITE

PYRITE

MOLYBDENITE

LÖLLINGITE

Table 2: Chemical analyses of the Lásenice (HO-1), microgranite-por-

phyry dyke (HO-2), Èímìø (HO-3) Granite, and Homolka granites (HO-4

to HO-8). For petrographic types, see Appendix. Main oxides are in weight

%, trace elements in ppm. Determined by XRF (main oxides, Rb, Sr, Y, Zr,

Nb, Ga, Sn and Pb) and INAA (Cs, Ce, Hf, Th, U, Ta and Zn).

HO-1 HO-2 HO-3 HO-4 HO-5 HO-6 HO-7 HO-8

SiO

74.36

73.61

72.46

73.95

73.30

74.67

69.57

73.73

TiO

0.11

0.17

0.24

0.03

0.02

0.06

0.04

0.00

14.43

14.90

15.19

15.18

14.67

17.07

15.16

0.94

1.36

1.61

0.74

0.66

0.88

1.08

0.32

MnO

0.04

0.05

0.04

0.11

0.06

0.07

0.04

0.07

MgO

<0.10

CaO

0.54

0.57

0.14

0.41

0.51

0.36

0.34

0.40

3.36

2.87

2.88

3.59

4.47

3.69

2.71

5.32

4.71

5.12

5.55

4.00

3.50

3.98

7.14

2.92

0.24

0.28

0.25

0.58

0.76

0.47

0.60

0.65

Total

98.73

98.46

98.07

98.60

98.46

98.85

98.59

98.57

249.0

338.0

343.0 1156.0 1312.0

814.0

962.0 1052.0

6.4

14.2

18.6

56.0

69.6

49.4

101.2

53.4

60.0

46.0

60.0

32.0

21.0

13.0

26.0

103.0

23.8

74.3

97.8

5.5

5.3

12.3

6.7

<3.4

13.0

19.0

18.0

27.0

24.0

22.0

19.0

39.0

76.0

105.0

26.0

<5.0

21.0

27.0

<5.0

1.1

2.7

3.6

2.4

1.0

1.2

0.5

<0.4

2.9

11.1

17.6

2.1

0.5

1.9

1.1

0.7

1.5

7.4

4.3

3.4

5.3

4.1

3.4

10.0

14.0

22.0

17.0

45.0

65.0

34.0

48.0

61.0

1.4

2.5

1.6

16.0

24.1

12.2

11.2

41.0

25.0

146.0

108.0

161.0

190.0

144.0

140.0

134.0

14.0

21.0

19.0

29.0

23.0

24.0

30.0

23.0

<10.0

114.0

89.0

58.0

59.0

39.0

36.0

30.0

31.0

16.0

10.0

22.0

16.0

<10.0

SX50 wavelength dispersion instrument at the Department of

Geological Sciences, University of Manitoba, Winnipeg,

Canada. A beam diameter of 12

m was used. An acceler-

ating potential of 15 kV, beam current of 20 nA and count-

ing time of 20 s were applied for P, Si, Zr, Hf, Al, Fe, Sc, Y,

Ca, F and Cl; 20 kV, 30 nA and 40 s, for Th, U, Ce, Sm, Tb,

Dy, Er and Yb. The following standards were used: mona-

zite (for P K

), zircon (Si K

, Zr L

), metallic Hf (Hf M

kyanite (Al K

), almandine (Fe K

), NaScSiO

(Sc K

YAG (Y L

), diopside (Ca K

), fluor-riebeckite (F K

tugtupite (Cl K

), ThO

(Th M

), UO

(U M

), REE3 (Ce

), REE2 (Sm L

, Yb L

), REE1 (Tb L

) and REE4 (Dy

, Er L

). The data were reduced according to the PAP

routine (Pouchou & Pichoir 1985).

Results

Zircon typology

Zircon typology (Pupin 1980) clearly subdivided the stud-

ied rocks into two clusters (Figs. 2, 3):

(1) The Èímìø and Lásenice Granite as well as subvolca-

nic microgranite-porphyry shows a relatively low I.A as

well as I.T values which correspond to the peraluminous

continent-related S-type granites. These values indicate

high Al-activity of the parental magma and a moderate tem-

perature of 650700±50

154 UHER, BREITER, KLEÈKA and PIVEC

(2) The Homolka granite stock reveals similar patterns char-

acterized by medium to high I.A = 520700 and very low

I.T = 190240. All samples lie in the field of Sn-W-F granites

(Pupin 1980) and typology indicates high alkalinity and low

temperature of the parental magma (~600±50

C). However,

some samples show a slighly different pattern with bimodal

subtype distribution (G

vs. L

1-3

, S

1-2

) Fig. 2. This fact can

be explained by contamination of the magma with inherited

older zircon.

Internal texture and zoning

The internal texture of zircon crystals evaluated by BSE

clearly reveal their complex growth history and at least two

different parts in both the older Lásenice and Èímìø, and the

younger Homolka granites (Figs. 4, 5): a core or inherited

zircon and an outer zircon zone.

The Lásenice and Èímìø granites as well as the granite

porphyry dyke have a typical concentric oscillatory zonality

often with a visibly different central part (Fig. 4). These

central parts of oval shape are probably remnants of older

inherited zircon or they represent a partly magmatic corrod-

ed (melted) primary zircon. Locally, inclusions of monazite-

(Ce) appear near the rims of the crystals.

Zircon from the Homolka stock reveals more complicated

textural patterns. All petrographic types in BSE have clear-

Fig. 2. Zircon typological diagrams (Pupin 1980) of the studied granitic rocks, for sample description (bottom left corners) see Appendix.

I.A alkalinity index; I.T temperature index. The two values at the bottom right corners are the coordinates of the mean typological

point, I.A; I.T.

Fig. 3. Zircon typological diagram of the mean typological points

(Pupin 1980) of the studied granitic rocks. I.A, I.T values explana-

tion, see Fig. 2. PAL peraluminous anatectic collision related

granites; CA calc-alkaline mainly subduction related granites;

ALK alkaline post- to anorogenic granites; Sn-W-F field (shad-

ed) granites with Sn, W, F-mineralization; Ab albitized gran-

ites. The numbers correspond to the HO samples, see Appendix.

I.A

I.T

HO-1

300; 365

HO-2

311; 322

HO-3

284; 328

HO-4

678; 200

HO-5

700; 191

HO-6

626; 216

HO-7

523; 213

HO-8

700; 200

2-5

6-10

11-20

>20

Z irco n freq ue ncy (in %):

ZIRCON IN HIGHLY EVOLVED HERCYNIAN HOMOLKA GRANITE, MOLDANUBIAN ZONE 155

ly visible cores of inherited nature. They are always darker

and with lower electron density, mainly due to their higher

Zr/Hf ratio and lower U content (see next paragraph), over-

rimed by lighter rim (Fig. 5AD).

The cores exhibit either homogeneous, diffuse or oscilla-

tory zoning and are often oval in shape (Fig. 5A, B). In oth-

er cases, cores show primary crystal shape but are clearly

incorporated in the outer parts of zircon (Fig. 5C, D). We es-

timate the proportion of the inherited zircon cores to be be-

tween 10 to 80 vol. %, 30 to 50 vol. % on average. Locally,

the inherited zircon cores or whole crystals exhibit oscilla-

tory zoning similar to that of zircons from the Lásenice and

especially Èímìø Granite (cf. Fig. 4A, B and Fig. 5D). Zir-

con of the Homolka medium-grained granite, porphyritic

granite and marginal pegmatite (HO-4, 6 and 7) is transpar-

ent and homogeneous to oscillatory zoned (Fig. 5A, B). On

the contrary, the Homolka aplite and fine-grained granite

zircon (HO-5 and 8) is usually non-transparent and metam-

ict (Fig. 5CF). Small anhedral to euhedral inclusions (up to

m in size) of monazite-(Ce), rarely xenotime-(Y), ferro-

columbite and uraninite occur in both oscillatory and

metamict zircon (Fig. 5). Monazite-(Ce) and xenotime-(Y)

from the Homolka granites occur only as tiny (up to 30

large) inclusions in zircon, often in or near inherited cores (Fig.

5A, C, D), which also indicates their possible partial inherit-

ance into the Homolka rocks. In addition, very similar tiny

monazite-(Ce) inclusions in zircon but also separate monazite-

(Ce) crystals (0.10.3 mm large) occur in the Èímìø Granite.

Chemical composition

Representative microprobe compositions of zircon are

given in Table 3. The Lásenice and Èímìø granite zircon

(HO-1 and 3) both show relatively uniform Hf contents in

their core (1.1 to 1.4 wt. % HfO

). Their P, U and Y contents

are negligible. The outer parts of zircon show generally

higher Hf contents than the cores: 1.4 to 1.6 and 1.9 to 2.6

wt.% HfO

for the Lásenice and Èímìø granites respective-

ly, other trace elements are again low, only uranium of the

Èímìø zircon reaches up to 0.7 wt.% UO

. Zircon from the

subvolcanic microgranite-porphyry dykes has a chemical

composition similar to that found in the host Lásenice and

Èímìø granites in Hf (1.1 to 2.1 wt.% HfO

), but with slight-

ly higher contents of P, U and Y at the margin of crystals.

The two different parts of Homolka zircon which are visi-

ble under BSE, were also well confirmed by EMPA. The

Fig. 4. BSE microphotographs of zircon, locally with monazite-(Ce), rarely xenotime-(Y) inclusions (white): A Lásenice Granite, B

Èímìø Granite, C-D micrograniteporphyry dyke.

156 UHER, BREITER, KLEÈKA and PIVEC

cores show generally low Hf (1.11.5 wt.% HfO

) and other

elements, which are strikingly similar to the Lásenice and

Èímìø zircon. On the other hand, the outer zone could be

characterized by extensive enrichment in Hf, and locally

also P, U and Y. The fine-grained granite (HO-8) and aplite

(HO-5) zircon crystals reach the maximum Hf contents:

4.812.3 and 4.16.4 wt.% HfO

, respectively (Table 3).

The medium-coarse grained granite (HO-4), porphyric gran-

ite (HO-6) and marginal pegmatite (HO-7) display lower

values: up to 3.5 wt.% HfO

. High uranium content is also

characteristic for the outer zircon zone of the Homolka

stock, and reaches 0.3 to 2.8 wt.% UO

The maximal val-

ues are found again within crystals of the fine-grained gran-

ite and aplite.

Fig. 5. BSE microphotographs of zircon from the Homolka stock. ADolder inherited zircon cores overrimed by younger Hf-rich zir-

con with monazite-(Ce), rarely uraninite inclusions (white). Emetamict zircon with ferrocolumbite and uraninite inclusions (white).

Fdetail of E: inclusions of ferrocolumbite (grey) and uraninite (white) in the host zircon (black).

ZIRCON IN HIGHLY EVOLVED HERCYNIAN HOMOLKA GRANITE, MOLDANUBIAN ZONE 157

Zircon saturation temperature

Zircon saturation temperature (T

) was calculated according

to the partly modified formula of Watson & Harrison (1983):

[

C] = {12900/[ln(493000/(Zr

)

ROCK

) + 0.85M +

2.95]} 273,

where M = molar [(2Ca + Na + K)/(Si.Al)]

ROCK

, and Si + Al

+ Fe + Mg + Ca + Na + K + P = 1,

[ppm] total concentration of Zr in the host rock,

[ppm] concentration of Zr in old inherited zircon of

the host rock.

The Zr

value is estimated on the basis of the proportion

of inherited zircon (core zone) on the BSE microphoto-

graphs, assuming that 100 % of the Zr in the host rock is in-

corporated into zircon and Zr concentrations in biotite, mus-

covite and feldspars are negligible. Thus, the (Zr

)

ROCK

value indicates the concentration of Zr in the melt, as re-

quired in the equation of Watson & Harrison (1983). A simi-

lar calculation and suggestion was used by Broska et al.

(1995) for the Southern Bohemian Batholith.

The calculated T

are given in Table 4. On the basis of the

BSE observations, we have estimated the amount of inherit-

ed and/or core zircon to be 30 to 50 % for the Homolka

Granite. The presence of older inherited zircon is less evi-

dent for the Lásenice and Èímìø Granite and microgranite-

porphyry; we evaluate their amount between 20 to 50 % on

average. If we accept these ranges of amounts of inherited

zircon, T

are following: 720 to 760

C for the Èímìø and

640 to 680

C for the Lásenice zircon, the subvolcanic mi-

crogranite-porphyry dykes gave T

= 690 to 730

C and 610

to 650

C for the Homolka stock.

Discussion and conclusions

The zircon typology indicates the crystallization of the

Lásenice and Èímìø granites from an aluminium-rich and

relatively low-temperature magma, which is also confirmed

by their saturation temperature interval (Table 4). Zircon ty-

pology and accessory mineral assemblage with monazite-

(Ce) + ilmenite ± andalusite (Table 1) agree well with their

S-type geochemical trend (Liew et al. 1989; Vellmer &

Wedepohl 1994). Their chemical composition, especially

the weight Zr/Hf ratio near 40 is also characteristic for the

orogenic and calc-alkaline granite suites (Pupin 1992). The

zircon cores (up to 50 vol.%), interpreted as the remnants of

older inherited zircon, are in accordance with crustal pro-

toliths. Broska et al. (1995) estimated the amount of inherit-

ed zircon from the Eisgarn Granite, a continuation of the

Èímìø type in Austria, at 40 to 60 % and T

= 710750

which is in a good agreement with our results.

The zircon typology, internal zonality and chemical

composition as well as the geochemical and mineralogical

characteristics of the microgranite to granite porphyry

dykes are similar to those of the adjacent Èímìø and

Lásenice granites. They do not belong genetically to the

Homolka granites.

The granitic rocks of the Homolka stock intruded older

Lásenice and Èímìø granites. In addition, the latter type also

occurs as xenoliths up to 200 m in size in the Homolka me-

dium to coarse-grained granite (sample HO-3). Zircon ty-

pology, internal texture and chemistry clearly document

some mass contribution of the older adjacent granitic mate-

rial to the younger Homolka magma. The zircon typology of

the Homolka granites also reveals the presence of exotic

zircon types (L

1-2

, S

1-2

), which are practically identical to

the zircon typology of the Lásenice and Èímìø granites (Fig.

2). The Zr/Hf ratio of the inherited zircon in Homolka Gran-

ite is also very similar to the Zr/Hf zircon ratio of the

Lásenice and Èímìø granites (Table 3). The common pres-

ence of older inherited zircons overrimed by younger rims is

a widely accepted idea, documented recently by numerous

BSE, electron and ion microprobes as well as SHRIMP

data, where inherited partly dissolved cores gave apparently

older U-Pb ages than rims (Miller et al. 1992; Paterson et al.

1992). A very similar case of wall-rock derived zircon xe-

nocrysts has been documented in the neighbouring Freistadt

granodiorite in Austria (Finger et al. 1991) and the amount

of inherited zircon from the South-Bohemian pluton varies

between 10 and 70 % (Broska et al. 1995). The BSE study

of the internal texture of the Homolka zircon revealed small

to large inherited cores (10 to 80, 30 to 50 vol.% on aver-

age), often oval in shape, which could indicate their mag-

matic dissolution (Fig. 5), locally also discrete zircon xenoc-

rysts occur (HO-6, 7). The oscillatory zoning, presence of

monazite-(Ce) inclusions and chemical composition of the

cores, especially their low Hf-content, are conspicuously

similar to the zircon of the Lásenice and especially Èímìø

granites (Table 3). Thus, we suggest that zircon cores are

partly dissolved xenocrysts inherited from the mentioned

source rocks. In addition, it is not excluded that at least a

part of the monazite-(Ce) and rarely xenotime-(Y) were also

incorporated into the Homolka magma from the Èímìø

source. A possibility of the inherited nature of monazite-

(Ce) and xenotime-(Y) cores in granites is a less common

phenomenon than in the case of zircon, due to the higher

solubility of these phosphates in granitic melt, however

some BSE and discordant U-Pb data indicate this possibility

(Miller et al. 1992).

Consequently, our zircon study shows that the most prob-

ably source rocks for the Homolka zircon cores were the

Lásenice and/or Èímìø-like granite, however a source from

some older metamorphic rocks of the Moldanubian Unit

(gneisses, granulites) is not excluded. The field and

geochemical evidence reveals spatial and genetic links be-

tween the Èímìø and Homolka intrusions and the inherited

nature of the Homolka zircon cores corresponds to granite

evolution in the South-Bohemian (Moldanubian) pluton.

The oldest Lásenice granite represents the first minimum

melt, very poor in accessory minerals, chemically expressed

by very low contents of HFSE and REE, it did not play a di-

rect genetic role in the origin of the Homolka Granite. The

younger Eisgarn Granite Suite was melted at a higher tem-

perature and fractionated according to the sequence: (1) Eis-

garn Granite s.s. with Èímìø type, (2) moderately fractionat-

ed muscovite granite (Galthof and Lembach types), (3)

158 UHER, BREITER, KLEÈKA and PIVEC

strongly fractionated Homolka Granite (Breiter & Scharbert

1995).

Two basic processes could be suggested to explain the ori-

gin of the Homolka magma: (1) the partial low-degree dise-

quilibrium melting of the adjacent Èímìø Granite with un-

melted portions represented by zircon cores, or (2) advanced

magmatic fractionaction of some Èímìø-like member of the

Eisgarn Granite Suite, eventually with some contamination of

the magma from the already solidified Èímìø s.s. Granite.

Both mechanisms could explain the presence of inherited zir-

con cores in the Homolka Granite.

We prefer the advanced magmatic fractionaction as the

process leading to the origin of the Homolka Granite for the

following reasons:

(1) The origin of REE-depleted S-type leucogranites via

disequilibrium dehydration melting of metapelites with mus-

Table 3: Representative compositions of zircon form the Lásenice Granite (HO-1), microgranite-porphyry dyke (HO-2), Èímìø (HO-3) Granite, and

Homolka granites (HO-4 to HO-8). For petrographic types, see Appendix. Oxides are in weight %, c: cores, r: rims (HO-1 to 3), C: inherited cores in the

Homolka zircon, R: Hf-rich mainly rim parts of the Homolka zircon. Zr/Hf

: weight ratio, Hf #: atomic 100Hf/(Hf+Zr). Note the compositional identity

between c, r and C.

HO-1r

HO-2c

HO-3c

HO-4C

HO-4R

HO-6C

HO-6R

HO-8C

HO-8R

0.11

0.08

0.01

0.00

0.75

0.08

0.71

0.65

0.16

0.75

SiO

31.87

31.83

32.48

32.39

31.42

32.30

31.71

31.32

31.82

30.78

ZrO

65.48

64.95

65.47

66.04

60.55

66.29

61.53

63.37

59.45

52.94

HfO

1.46

1.11

1.43

1.30

5.29

1.37

3.51

1.29

7.60

12.33

ThO

0.02

0.01

0.00

0.04

0.00

0.07

0.00

0.06

0.83

0.00

0.31

0.29

0.43

0.56

0.00

0.01

0.00

0.01

0.00

0.05

0.02

0.00

0.03

0.00

0.02

0.00

0.02

0.00

0.01

0.00

0.03

0.01

0.31

0.17

0.03

0.02

0.04

0.00

0.58

0.00

0.18

0.54

0.00

0.44

0.00

0.03

0.00

0.01

0.00

0.03

0.00

0.01

0.05

0.00

0.03

0.01

0.00

0.02

0.00

0.02

0.05

0.00

0.04

0.06

0.03

0.01

0.00

0.02

0.00

0.05

0.00

0.02

0.05

0.00

0.02

0.05

0.03

0.00

0.03

0.00

0.06

0.01

0.04

0.01

0.02

CaO

0.00

0.02

0.01

0.00

0.01

0.02

0.04

0.10

0.01

0.00

0.06

0.00

0.20

0.01

0.06

0.00

0.01

0.00

Total

99.26

98.11

99.54

99.89

99.68

100.13

98.49

97.81

99.70

97.97

Formulae based on 16 oxygens

0.012

0.008

0.001

0.000

0.080

0.008

0.075

0.069

0.017

0.083

3.953

3.982

4.005

3.984

3.943

3.968

3.964

3.932

4.016

4.017

3.961

3.962

3.937

3.961

3.705

3.971

3.751

3.879

3.658

3.369

0.052

0.040

0.050

0.046

0.189

0.048

0.125

0.046

0.274

0.459

0.001

0.000

0.001

0.000

0.002

0.000

0.002

0.023

0.000

0.009

0.008

0.012

0.016

0.000

0.001

0.000

0.001

0.000

0.007

0.008

0.002

0.000

0.003

0.000

0.002

0.000

0.002

0.000

0.001

0.000

0.003

0.001

0.034

0.019

0.003

0.002

0.003

0.000

0.039

0.000

0.012

0.036

0.000

0.031

0.000

0.001

0.000

0.001

0.000

0.002

0.000

0.001

0.000

0.001

0.000

0.001

0.002

0.000

0.002

0.001

0.000

0.001

0.000

0.002

0.000

0.001

0.002

0.000

0.001

0.002

0.001

0.000

0.001

0.000

0.002

0.000

0.002

0.000

0.001

0.000

0.003

0.001

0.000

0.001

0.003

0.006

0.039

0.004

0.000

0.023

0.000

0.079

0.004

0.024

0.000

0.002

0.000

15.959

15.994

16.000

15.977

16.000

15.921

15.996

15.976

16.000

å cat.

7.989

7.999

8.002

7.995

7.993

7.999

7.975

7.998

7.995

7.993

Zr/Hf

39.1

51.1

40.0

44.3

10.0

42.2

15.3

42.9

6.83

3.75

Hf #

1.30

1.00

1.25

1.15

4.85

1.19

3.22

1.17

6.97

12.0

Table 4: Zircon saturation temperatures of studied granites, cal-

culated for 20 and 50 % (HO-1 to 3) or 30 and 50 % (HO-4 to 7) of

inherited zircon. For details see the text.

Lásenice

HO-1

640-680 °C

felsic dyke

HO-2

690-730 °C

Èímìø

HO-3

720-760 °C

Homolka

HO-4

620-650 °C

HO-6

610-630 °C

HO-7

620-640 °C

covite and/or biotite breakdown is suggested by some authors

(e.g. Nabelek & Glascock 1995) but these granites do not

reach a geochemical specialization (extreme Li, Rb, Cs, Ta

and P contents) similar to that of Homolka. On the contrary,

magmatic fractionaction is suggested as the main process for

ZIRCON IN HIGHLY EVOLVED HERCYNIAN HOMOLKA GRANITE, MOLDANUBIAN ZONE 159

highly evolved P-rich albite-Li-mica-topaz granites (Cuney et

al. 1992; Breiter et al. 1997) with mineralogical and

geochemical analogies to the Homolka Granite (Tables 1, 2).

The Hf-content of zircon from some Homolka members, es-

pecially from the fine-grained granite locally reached very

high values, up to 12.3 wt.% HfO

(Table 3). These unusually

high Hf values and low Zr/Hf ratios reflect an extreme frac-

tionation level of the Homolka granites, similar Hf concentra-

tions are typical only for extremely fractionated rare-element

granitic pegmatites (Èerný et al. 1985) or for highly evolved

rare-element granites. For example, zircon from the Beauvoir

Granite, France, contains up to 19 wt.% HfO

(Wang et al.

1992), zircon from Ta-rich granite related felsic dykes from

the Central Eastern Desert, Egypt, also contains up to 25

wt.% HfO

(Renno 1995) and zircon from the apical part of

the Suzhou granite, China, reaches up to 35 wt.% HfO

(Wang et al. 1996). In contrast, zircon from the marginal peg-

matite (stockscheider) of the Homolka stock is surprisingly

Hf-poor: only 1.72.5 wt.% HfO

in the outer part of the crys-

tals. We explain this fact by the extensive influence of sec-

ondary contamination and the resulting dilution of Hf-richer

pegmatite magma by the adjacent Hf-poor Èímìø Granite.

(2) The zircon typology and saturation temperature (T

) of

the Homolka stock indicate highly alkaline and low tempera-

ture, volatile-rich magma. Calculated T

between ca. 600 and

650

C is interpreted as the temperature of magmatic zircon

saturation from the parental magma. These temperatures are

too low for disequilibrium melting of source rock of

metapelite or similar composition by muscovite or biotite

breakdown, where a temperature around 850875

C is re-

quired for extensive fluid-absent melting and a melt propor-

tion below 850

C is too small to move and form an indepen-

dent pluton (Vielzeuf & Holloway 1988). On the other hand,

low magmatic temperatures are easily compatible with exper-

imental data on the highly evolved P, F, B-rich felsic magmas,

e.g. Macusani glass, Peru, rare-element pegmatites (Picha-

vant et al. 1988; London et al. 1989; London 1992) and high-

ly evolved granites, such as the Beauvoir Granite, France

(Cuney et al. 1992).

Acknowledgements: The autors thank B. Bonin, I. Broska

and an anonymous referee for helpful comments that im-

proved the manuscript. The study was supported by a

NSERC Scientific Grant (Canada) to P. Èerný during PDF

of P.U. and a VEGA Grant No. 4078 (Slovakia) to I. Petrík.

The paper is a contribution to the IGCP Project No. 373.

Appendix: sample description

HO-1: Muscovite-biotite granite, Lásenice type.

HO-2: Felsic granite porphyry dyke, Terezský Dvùr.

HO-3: Muscovite-biotite porphyritic granite, Èímìø type, xenolith in

the Homolka body.

HO-4: Muscovite granite, Homolka type.

HO-5: Aplite dyke, Homolka type.

HO-6: Porphyritic granite, Homolka type.

HO-7: Marginal pegmatite (stockscheider), Homolka type.

HO-8: Fine-grained muscovite granite, Homolka type.

The location of all samples (with the exception of HO-2) is the Homolka
Hill area, 5 km SSE from Lásenice village (Fig. 1).

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