Inferring protoliths of high-grade metamorphic gneisses of the Erzgebirge using zirconology,...

ORIGINAL PAPER

Inferring protoliths of high-grade metamorphic gneissesof the Erzgebirge using zirconology, geochemistry and comparisonwith lower-grade rocks from Lusatia (Saxothuringia, Germany)

Marion Tichomirowa • Sergey Sergeev •

Hans-Jurgen Berger • Dietmar Leonhardt

Received: 11 August 2011 / Accepted: 15 March 2012 / Published online: 3 April 2012

� Springer-Verlag 2012

Abstract Protoliths of highly metamorphosed gneisses

from the Erzgebirge are deduced from the morphology, age

and chemistry of zircons as well as from whole rock geo-

chemistry and are compared with lower-grade rocks of

Lusatia. Gneisses with similar structural appearance and/or

geochemical pattern may have quite different protoliths.

The oldest rocks in the Erzgebirge are paragneisses rep-

resenting meta-greywackes and meta-conglomerates. The

youngest group of zircon of meta-greywackes that did not

undergo Pb loss represents the youngest igneous compo-

nent for source rocks (about 575 Ma). Similar ages and

zircon morphology reflect the subordinate formation of

new zircon grains or only zircon rims in the augengneiss

from Barenstein and Wolkenstein, which probably repre-

sent metamorphic equivalents to Lower Cambrian two-

mica granodiorites from Lusatia. Bulk rock chemistry,

intense fracturing and high U and Th concentrations of

zircons suggest deformation-induced and fluid-enhanced

recrystallisation of zircon grains. Temperatures during

tectonic overprinting—too low to reset zircon ages—indi-

cate mid- or upper crustal levels for shearing recorded in

these augengneisses. Lower Cambrian (*540 Ma) gran-

odiorites are widespread in Lusatia but are exclusively

represented by the Freiberg gneiss dome in the Eastern

Erzgebirge. Ordovician protolith ages were recorded by

zircons from the augengneisses of the Reitzenhain–Cath-

erine dome and the Schwarzenberg dome (Western Er-

zgebirge) documenting significant regional differences

between the eastern and the western Erzgebirge

(*540 vs. *490 Ma). In the Western Erzgebirge, most

meta-volcanic rocks (muscovite gneisses) and meta-gran-

ites (mainly red augengneisses) yield Ordovician zircon

ages, whereas in the Eastern part, similar rocks mainly

recorded Lower Cambrian protolith ages. Zircon over-

printing was highest within discrete tectonic zones where

the combination of fluid infiltration and deformation

induced variable degrees of recrystallisation and formation

of a new augengneiss structure. Variable degrees of Pb loss

caused age shifts that do not correspond to changes in

zircon morphology but may be associated with U and Th

enrichments. Major changes in bulk rock composition

appear to be restricted to discrete zones and to (U)HP

nappes, whereas gneisses with a MP–MT metamorphic

overprint basically show no geochemical modifications.

Keywords Erzgebirge � Zircon ages � Protolith �Gneisses � High pressure metamorphism � Lusatia

Introduction

Rocks that underwent high-grade metamorphism may loose

most of their primary source rock features. In the

Communicated by J. Hoefs.

Electronic supplementary material The online version of thisarticle (doi:10.1007/s00410-012-0742-8) contains supplementarymaterial, which is available to authorized users.

M. Tichomirowa (&)

Institut fur Mineralogie, Technische Universitat Bergakademie

Freiberg, Brennhausgasse 14, 09596 Freiberg, Germany

e-mail: tichomir@mineral.tu-freiberg.de

S. Sergeev

Centre of Isotopic Research at the Russian Geological Research

Institute (VSEGEI), Srednyi Prospect 74, 199106 St. Petersburg,

Russia

H.-J. Berger � D. Leonhardt

Sachsisches Landesamt fur Umwelt, Landwirtschaft und

Geologie, Halsbrucker Str. 31a, 09599 Freiberg, Germany

Contrib Mineral Petrol (2012) 164:375–396

DOI 10.1007/s00410-012-0742-8

Erzgebirge, several metamorphic units with different P-T

histories were identified by petrological studies (e.g.

Schmadicke et al. 1995; Willner et al. 1997; Rotzler et al.

1998). The region represents a tectonic stack of different

crustal segments, recording Variscan subduction and col-

lision of continental crust, followed by exhumation (Kroner

and Gorz 2010). Microdiamonds found in garnets imply a

burial to mantle depth for some of the metamorphic rocks

(e.g. Massonne 1999, 2003). The locations of inferred

tectonic contacts are not recognisable without ambiguity in

the field (e.g. Willner et al. 1997; Rotzler et al. 1998).

Therefore, the assignment of metamorphic rocks to certain

tectono-metamorphic units is done differently by various

research groups (Schmadicke et al. 1995; Willner et al.

1997, 2002; Rotzler et al. 1998; Tichomirowa et al. 2005).

All units contain a variety of zircon-bearing gneisses.

Zircon characteristics (morphology and ages) may help to

unravel the protoliths of these rocks. Previous studies

yielded zircon ages ranging from 3,200 to 340 Ma with

inferred protolith ages between 575 and 480 Ma (Kroner

et al. 1995, 1997; Tichomirowa et al. 2001; Kroner and

Willner 1998; Mingram and Rotzler 1999; Kosler et al.

2004; Mingram et al. 2004). The Variscan overprint caused

a severe disturbance of zircon ages in some ultra-high-

pressure (UHP) gneisses of the Erzgebirge (Tichomirowa

et al. 2005), and thus, there is still an ongoing discussion

which of the gneisses should be assigned to the same

protoliths.

To shed some light on this issue, new zircon ages were

determined for 19 gneiss samples from the Erzgebirge

region. The Pb/Pb evaporation method was combined with

the U/Pb SHRIMP method for several samples. The ages

provide—in combination with CL images, Th/U ratios,

morphological criteria of zircons as well as bulk rock

chemistry—new arguments to identify precursor rocks and

regional correlations with lower-grade rocks from Lusatia.

We will show that the combined use of zirconology and

bulk rock chemistry can be used to identify the pre-meta-

morphic source rocks and that gneisses with similar tex-

tural appearance can have quite different protoliths.

Regional geology and tectonic setting

The Erzgebirge is located at the northern margin of the

Bohemian Massif and belongs to the Saxothuringian

domain of the European Variscan belt. It consists of a high-

grade metamorphic gneiss core surrounded by mica schist

and phyllite units. Recently, the Erzgebirge gneisses

attracted much attention due to findings of microdiamonds

implying burial to mantle depths before exhumation to the

surface (Massonne 1999, 2003). The metamorphic rocks

are divided according to their peak pressure–temperature

(PT) conditions into several tectono-metamorphic units.

High- to ultra-high-pressure (HP, UHP) conditions have

been estimated for microdiamond- and kyanite-bearing

gneisses, garnet peridotites, eclogites and granulites. Three

separate areas of (U)HP units (Fig. 1), differing by their

maximum PT conditions (HP unit 1: 850 �C, [29 kbar, HP

unit 2: 650–750 �C, 24–26 kbar, HP unit 3: 600–650 �C,

20–23 kbar) were defined by Schmadicke et al. (1992,

1995) and Schmadicke (1994). Later, peak metamorphic

conditions were estimated of up to *40–60 kbar and

temperatures of up to *1,000–1,100 �C for these rocks

(Massonne 1999; Hwang et al. 2001). The (U)HP rocks

mostly form small lenses within amphibolite facies gneis-

ses and mica schists. The first two (U)HP nappes are

located within gneisses and contain mainly eclogite,

granulite and amphibolite lenses. Microdiamond- and

kyanite-bearing gneisses and garnet peridotites occur

preferentially in the (U)HP nappe 1 (E1 in Fig. 1). The

third nappe with a lower-peak metamorphic temperature

(600–650 �C) consists of mica schists, gneisses, eclogite

and amphibolite lenses (Willner et al. 1997; Rotzler et al.

1998; E3 in Fig. 1).

No indications for a (U)HP stage have been recognised

in granite gneisses that form distinct domes. All these

gneisses are texturally similar, record considerably lower-

peak pressure values (8–11 kbar, Kroner et al. 1995;

Rotzler 1995) and have been assigned to the medium

pressure—medium temperature unit (MP-MT). This unit

comprises the meta-granodiorites from the Freiberg gneiss

dome in the Eastern Erzgebirge, meta-granodiorites from

the Reitzenhain–Catherine dome, the Schwarzenberg

gneiss dome and a continuous layer called Barenstein-

Wolkenstein augengneiss in the Western Erzgebirge

(Fig. 1).

No relics of a (U)HP stage were reported from fine-

grained gneisses ascribed as meta-greywackes and meta-

conglomerates (Kroner et al. 1995; Willner et al. 1997;

Rotzler et al. 1998). These gneisses enclose the high-

pressure nappes E1 and E2 and contain smaller lenses of

meta-volcanites and meta-granites. The meta-volcanites of

the Griessbach unit form a continuous layer within the

mica schist unit (Fig. 1).

Previous geochronology

The age of the UHP and HP metamorphism is well con-

strained by Sm/Nd isochrons, Ar/Ar phengite ages as well

as U/Pb zircon ages that cluster at about 340 Ma (e.g.

Schmadicke et al. 1995; Kroner and Willner 1998; Werner

and Lippolt 2000; Tichomirowa et al. 2005). Amphibolite

facies rocks yielded Rb/Sr and Ar/Ar metamorphic cooling

ages of about 330 Ma (Tikhomirova et al. 1995; Werner

376 Contrib Mineral Petrol (2012) 164:375–396

and Lippolt 2000) which overlap with the intrusion ages

determined for early post-metamorphic granites and rhyo-

lites of about 330 Ma (Tichomirowa 1997; Kempe et al.

1999, 2004; Kempe 2003; Romer et al. 2007; Forster and

Romer 2010; Tichomirowa and Leonhardt 2010).

Zircon dating during the last two decades provided a

better understanding of the protolith ages (Table 1,

Fig. 1a). A protolith age of 550–540 Ma is accepted for the

Freiberg orthogneiss dome in the Eastern Erzgebirge

(Kroner et al. 1995; Tichomirowa et al. 2001). However,

conflicting age data were obtained for the granite gneiss

from the Reitzenhain-Catherine dome (Table 1). Kroner

et al. (1995) published mean Pb/Pb evaporation ages on

four samples, ranging between 551 and 554 Ma. They

concluded that all augengneisses from the Freiberg and

Reitzenhain-Catherine dome have a single protolith age of

about 550 Ma. In contrast, Kosler et al. (2004) reported

U/Pb zircon ages (LA-ICPMS) of 480 ± 10 Ma, while

Tichomirowa (2003) found a similar age range

(479 ± 15 Ma; Pb/Pb evaporation method) for one sample

from the Reitzenhain-Catherine dome (Table 1).

Other augengneisses in the Western Erzgebirge yielded

quite different zircon ages. An Ordovician age was pub-

lished for the Schwarzenberg augengneiss (Mingram et al.

2004), while the Wolkenstein and Barenstein augengneis-

ses yielded Neoproterozoic ages (Table 1; Kroner et al.

1997; Mingram et al. 2004). Rotzler and Plessen (2010)

interpreted the latter age as dating a metamorphic or

magmatic event at 580–570 Ma. In contrast, Linnemann

et al. (2010) proposed an intrusion age of 536 Ma for the

Fig. 1 Simplified geological map of the Erzgebirge with location of

samples analysed in this study (compare Table 2). Inset shows the

position of the Erzgebirge at the border between Saxony and the

Czech Republic. Zircon ages and references are given in Table 1.

(U)HP units I, II and III are shown according to Schmadicke et al.

(1995) and the occurrence of eclogite lenses (not shown)

Contrib Mineral Petrol (2012) 164:375–396 377

Table 1 Compilation of zircon ages from gneisses of the Erzgebirge

U/Pb Pb/Pb Sample numbers Reference Comments

Large augengneiss bodies

Freiberg dome

550 ± 7 DDR 16 1 Mean of 6 grains; 5 further grains with ages between 1,042 and 2,181 Ma

555 ± 7 DDR 13 1 Mean of 4 grains; 4 further grains with ages between 848 and 1,909 Ma

553 ± 10 CS 18 1 Mean of 3 grains; 5 further grains with ages between 855 and 1,723 Ma

541 ± 4 Ko 92 6 Mean of four almost concordant zircon fractions

541 ± 2 Ko 92, Ko 93, Bo 4, Bo 4a,

Ra 1, Ra 5, DDR 16

6, 7 Combined age: mean of 21 single zircon grains from 6 samples; further ages

up to 2,435 Ma

528 ± 6 Ra 3 7 Mean of 7 SHRIMP analyses from 7 rims of zircon grains; further ages up to

2,740 Ma

Reitzenhain—Catherine dome

551 ± 9 B379/2 1 Mean of 9 small grains

551 ± 6 DDR 14 1 Mean of 2 grains

554 ± 10 B382/1 1 Mean of 10 small grains; one old age: 1,087 Ma

479 ± 15 95-8 7 Mean of 3 grains; 3 further grains with ages between 432 and 540 Ma

480 ± 10 C 9 Laser ICPMS: mean of 8 analyses, 3 older ages around 620 Ma

Schwarzenberg dome

487 ± 7 KE 303 8 Schwarzenberg augengneiss: mean of 4 SHRIMP analyses

Augengneisses from Barenstein and Wolkenstein

575 ± 4 KE 301 3 Barenstein augengneiss: mean of 2 SHRIMP analyses

536 ± 4a KE 488 11 Barenstein augengneiss: mean of 3 youngest grains from (7)

567 ± 7 EZ 21 3, 8 Wolkenstein augengneiss: mean of 2 SHRIMP analyses

Meta-greywackes, meta-conglomerates and similar gneisses

All zircon ages are inherited

Mean of several zircons yielding identical ages, which represent the youngest zircon group

580 ± 20 Ko 277 6 5 nearly concordant youngest ages from the fine-grained ‘‘Outer Gneiss’’

(SHRIMP method), further ages up to 2,800 Ma

576 ± 5 Ra 1a, Ko 195, Ko 277 6, 7 Mean of 10 youngest grains from three samples of the fine-grained ‘‘Outer

Gneiss’’, further ages up to 2,653 Ma

585 ± 5 KE 487 7 Mean of 4 grains; 2 further grains with ages between 2,099 and 2,640 Ma

489 ± 12a EZ 19 2 Only given as mean U/Pb value without presenting any data

The youngest zircon age from scattering age data

[572 Ra 2 7 9 zircon ages from 572 to 1,908 Ma

[552 Ko 282 7 6 zircon ages from 552 to 2,577 Ma

[555 AW 3 7 2 zircon ages from 555 to 1,750 Ma

[509a KE 400 7 6 zircon ages from 509 to 2,476 Ma

Small bodies of meta-volcanites (muscovite gneisses) and meta-granites (red and grey augengneisses)

484 ± 5 Har 1 6 Mean of 7 grains

482 ± 1 DDR 17 4 Mean of 4 grains; 2 further grains with ages between 913 and 1,858 Ma

(Ultra) high pressure nappes

UHP 1: nappe Sayda and Floha zone

489 ± 1 KE 436 8 Mean of 5 grains; muscovite gneiss

479 ± 1 EZ 41 4 Mean of 3 grains; muscovite gneiss

483 ± 6 Mem 1 7 Mean of 7 grains; muscovite gneiss Memmendorf

341 ± 1 EZ 13B 4 Mean of 5 grains; granulite gneiss metamorphic age

Barenstein augengneiss using the youngest Pb/Pb zircon

evaporation ages from Tichomirowa (2003).

The youngest zircon population of fine-grained parag-

neisses yielded a Neoproterozoic age (Tichomirowa et al.

2001: 580–570 Ma; U/Pb SHRIMP and Pb/Pb evaporation

ages). Additional ages of [550 Ma were reported for three

samples of meta-greywackes in the Eastern Erzgebirge and

for one sample in the Western Erzgebirge (Table 1; Pb/Pb

evaporation ages). However, Kroner et al. (1995) reported

a U/Pb age of 489 ± 12 Ma for these gneisses.

Only few Pb/Pb evaporation zircon ages are available

from smaller occurrences of meta-volcanites (muscovite

gneisses) and meta-granites (coarse-grained gneisses)

which are not part of the (U)HP nappes. Within the HP

nappes, most fine-grained muscovite gneisses yielded zir-

con ages between 500 and 470 Ma (Kroner and Willner

1998; Tichomirowa et al. 2001; Tichomirowa 2003;

Mingram et al. 2004, Table 1). The ages of some musco-

vite gneisses from the HP nappe 1 unit were obviously

shifted towards younger ages due to the Variscan meta-

morphic overprint (Table 1).

Materials and methods

Samples

Sample locations are shown in Fig. 1. A short sample

description is given in Table 2. Augengneisses were sam-

pled from the Freiberg dome, the Reitzenhain-Catherine

dome, the Schwarzenberg dome and from Barenstein/

Wolkenstein. One sample (KE 849) was taken from the

augengneisses from the Freiberg dome for comparison with

samples dated earlier (compare Table 1). Sample Non 1

Table 1 continued

U/Pb Pb/Pb Sample numbers Reference Comments

342 ± 1 EZ 14B 4 Mean of 3 grains; granulite gneiss, metamorphic age

340 ± 1 E42x 4 Mean of 4 grains; granulite gneiss metamorphic age; 2 further grains with

ages between 743 and 1,006 Ma

340 ± 1 ggn 1b, 2 10 4 nearly concordant analyses from two samples of granulite gneiss;

metamorphic age; further ages scatter along the Concordia up to about

500 Ma

HP 2: nappe Boden-Hassberg and Medenec

458 ± 1a EZ 23 8 Mean of 3 grains; 2 further grains with ages of 564 Ma; muscovite gneiss

480 ± 8 KE 447 7 Mean of 7 grains; one grain with 340 Ma, medium grained leucocratic red

gneiss

491 ± 10 KE 448 7 Mean of 8 grains; very coarse-grained red gneiss

524 ± 1a CS 16 4 Mean of 4 grains, 5 further grains with ages between 617 and 1,262 Ma;

meta-greywacke

524 ± 10a M 9 Laser ICPMS: mean of 9 analyses; augengneiss Medenec

HP 3: nappe Wiesenthal and Klinovec

484 ± 1 EZ 5 8 Mean of 2 grains; 5 further grains with ages between 587 and 906 Ma;

muscovite gneiss

486 ± 1 EZ 20 8 Mean of 4 grains; 2 further grains with ages between 572 and 726 Ma;

muscovite gneiss

492 ± 4 EG 95-5 5 Mean of 5 grains; muscovite gneiss

476 ± 14 KE 426 7 Mean of 7 grains; one grain with 382 Ma; muscovite gneiss

519 ± 26a K 9 Laser ICPMS: one nearly concordant analysis; 4 further analyses yielded

discordant younger ages; augengneiss Klinovec

Griessbach volcanite complex

485 ± 1 EZ 11 8 Mean of 3 grains; 3 further grains with ages of 1,098 and 1,099 Ma

485 ± 12 EG 95-2 5 Mean of 5 grains

495 ± 5 KE 445 7 Mean of 10 grains; 3 further grains with ages between 520 and 548 Ma

483 ± 12 KE 331 7 Mean of 6 grains; 2 further grains with ages between 564 and 568 Ma

1, Kroner et al. (1995); 2, Kroner et al. (1995); 3, Kroner et al. (1997); 4, Kroner and Willner (1998); 5, Mingram and Rotzler (1999);

6, Tichomirowa et al. (2001); 7, Tichomirowa (2003); 8, Mingram et al. (2004); 9, Kosler et al. (2004); 10, Tichomirowa et al. (2005);

11, Linnemann et al. (2010)a Ages not shown in Fig. 1

from the Reitzenhain-Catherine dome was taken from the

same outcrop as sample DDR 14 (Kroner et al. 1995) to

resolve the age conflict reported in section ‘‘Previous

geochronology’’. Both the Pb/Pb evaporation and the U/Pb

SHRIMP method were applied to zircon grains from this

sample. In order to obtain reliable protolith ages for the

Schwarzenberg dome, two samples (94-3 and KE 548)

were analysed with Pb/Pb and U/Pb methods. For the

augengneisses of Barenstein/Wolkenstein, three samples

were investigated, and the U/Pb SHRIMP method was

applied for two of them (samples KE 488 and KE 549).

The age of meta-greywackes in the Eastern Erzgebirge

is well established by both U/Pb and Pb/Pb zircon ages on

several samples (Table 1). However, only two samples of

these gneisses were dated in the Western Erzgebirge

resulting in conflicting age data (Table 1). Therefore, three

further samples from meta-greywackes and meta-con-

glomerates were analysed for their zircon ages. Seven

additional samples were taken from smaller occurrences of

muscovite and granite gneisses and one sample from the

muscovite gneiss of the Griessbach meta-volcanite unit

(Fig. 1).

Geochemistry

Major and trace element contents for whole rock samples

were analysed at Activation Laboratories (Actlabs Canada;

‘‘4 Litho’’ research analytical protocol) by Fusion-ICP and

Fusion-MS, respectively. Reproducibility is better than

1 % for major elements and better than 5 % for trace ele-

ments based on analyses of certified standards.

Zircon separation and characterisation

Zircon grains were extracted from the whole rock samples

of about 5 kg by standard crushing, heavy liquid and

magnetic separation techniques. More than 100 grains were

obtained from each sample. After optical screening, ca.

30–50 representative grains were selected for a detailed

characterisation by scanning electron microscopy (SEM),

secondary electron (SE) imaging and cathodoluminescence

(CL). The elemental composition of mineral inclusions was

determined by EDX analyses. In addition, mineral inclu-

sions were analysed for the samples OE 7 and OE 9 by

Raman microprobe measurements to determine the

Table 2 Sample description

Sample number Location Location Description

Longitude Latitude

Large augengneiss bodies

KE 849 5648260 5400250 Granite gneiss, Freiberg dome

Non 1 5610681 4587688 Granite gneiss, Reitzenhain-Catherine dome

94-3 5600850 4556300 Granite gneiss, Schwarzenberg dome

KE 548 5698130 4555500 Granite gneiss, Schwarzenberg dome

KE 488 5603220 4564750 Granite gneiss, Barenstein augengneiss

Meta-greywackes and meta-conglomerates

KE 446 5600050 4579900 Fine-grained grey gneiss lense Schmalzgrube

KE 543 5596990 4353620 Meta-greywacke

KE 544 5596990 4553620 Meta-conglomerate within meta-greywacke

Small bodies of meta-volcanites (muscovite gneisses) and meta-granites (red and grey augengneisses)

OE 7 5642720 5413715 Red augengneiss Lockwitzgrund

OE 9 5643100 5414660 Red augengneiss Lockwitzgrund

Ko 345 5648805 4615890 Strongly foliated muscovite gneiss

KE 444 5607770 4569410 Foliated grey augen gneiss Frohnau

KE 484 5608030 4569900 Red augengneiss Annaberg

KE 485 5608140 4569520 Muscovite gneiss Vogelhoehe

KE 518 5599570 4573930 Red augengneiss Kuehberg

KE 519 5601910 4568100 Muscovite gneiss Waltersdorf

KE 520 5605430 4567140 Muscovite gneiss Doerfel

Griessbach meta-volcanite unit

KE 553 5603050 4558260 Muscovite gneiss

composition of feldspars (K-feldspar or plagioclase). These

measurements were performed using a Jobin Yvon (Horiba

Group; formerly Dilor S.A.) LabRAM-HR (focal length

800 mm).

Zircon dating by the evaporation method

The single-zircon evaporation method involves deposition

of Pb and other elements on a second filament and sub-

sequent measurement of Pb isotope ratios in a mass spec-

trometer (Kober 1987). Measurements were carried out at

the Isotope Laboratory in Freiberg on a Finnigan-MAT 262

mass spectrometer.

Evaporation was performed at 1,600 �C in one step to

obtain high signal intensities for measurement. Data

acquisition was carried out by peak switching using a

secondary electron multiplier equipped with an ion counter.

90 207Pb/206Pb and 204Pb/206Pb ratios were obtained per

measurement. Each measured 207Pb/206Pb ratio was cor-

rected with the corresponding 204Pb/206Pb ratio calculated

from the trend line applying the Pb evolution model of

Stacey and Kramers (1975). The obtained 207Pb*/206Pb*

were corrected for mass bias (0.0036 per amu) deduced

from NBS 981 and two zircon standards (zircon 91500

reported in Wiedenbeck et al. 1995 and zircon S-2-87,

Wenham Monzonite, US Geological Survey). The calcu-

lated 207Pb/206Pb age of one zircon measurement and its 2rmean error are based on the mean of all measured scans

(usually 90) of isotope ratios with those omitted lying

outside the 95 % confidence level. A more detailed

description of the method is given in Tichomirowa and

Leonhardt (2010).

In situ zircon dating by the U/Pb SHRIMP method

The in situ U-Th-Pb analyses of zircons were performed

using the SHRIMP II technique (Sensitive High mass

Resolution Ion Microprobe) at the Centre of Isotopic

Research, VSEGEI, St. Petersburg, Russia). Each analysis

consisted of 5 scans through the mass range, spot diameter

was about 18 lm, primary beam intensity was about 4 nA.

The data have been reduced in a manner similar to Wil-

liams (1998, and references therein), using the SQUID

Excel Macro of Ludwig (2000). The standard zircon 91500

was used as the U/Pb ratio and concentration calibration

reference. Corrections for common lead were made using

measured 204Pb and by applying the Pb evolution model of

Stacey and Kramers (1975). Uncertainties given for indi-

vidual analyses (ratios and ages) are at the one r level;

however, the uncertainties in calculated Concordia ages are

reported at the two r level. The inverse Concordia (Tera—

Wasserburg) plot has been prepared using ISOPLOT/EX

(Ludwig 2004).

Results

Bulk rock geochemistry

Results are given in the Supplementary material 1. Fig-

ures 2 and 3 show these data compared with previous data

for known gneiss groups from the Erzgebirge.

Meta-greywackes and meta-conglomerates displayed

the most homogeneous geochemical pattern. These rocks

have almost the same concentrations as estimated by

Rudnick and Gao (2003) for the upper continental crust

(UCC) resulting in flat spidergrams and rare earth ele-

ment (REE) distributions (Figs. 2c, 3c). Calcium and

strontium are only slightly depleted compared with UCC.

Two samples (KE 444, KE 446) well match this geo-

chemical pattern (Figs. 2c, d, 3c, d). The grey augeng-

neisses from the Freiberg and the Schwarzenberg domes

as well as three samples from the Barenstein augeng-

neisses also showed a similar chemical composition with

more pronounced negative anomalies for Ca and Sr

(Fig. 2a, b). Compared with meta-greywackes, all these

samples were slightly enriched in K and Rb, had a small

negative Eu anomaly (Fig. 3a, b) and were depleted to

various degrees in V and Cr (Fig. 2a, b). Therefore, all

these rocks belong to the geochemical group I (Figs. 2h,

3h). In contrast, muscovite gneisses and red augeng-

neisses were obviously different in many element con-

centrations and are assigned to the geochemical group II

(Fig. 2i, 3i). These rocks typically showed a negative Ba

anomaly, lower concentrations of Zr, Hf, Ti, Ca, Sr

(Fig. 2e–g) and of the light rare earth elements (LREE;

Fig. 3e–g). The negative Eu anomaly was clearly more

pronounced (Fig. 3e–g). The augengneiss sample from

the Reitzenhain–Catherine dome (Figs. 2a, 3a) is more

consistent with the element pattern from this second

geochemical group (muscovite gneisses and red aug-

engneisses; Figs. 2i, 3i), while sample Ko 345 (Figs. 2f,

3f) displayed characteristics of both geochemical groups:

high REE concentrations like the geochemical group I

and depletion in several elements similar to rocks from

the geochemical group II.

Zircon morphology

All zircon crystals characterised by scanning electron

microscopy (SEM: 30–50 grains) were used to deter-

mine zircon morphology types as defined by Pupin

(1980) and shown in the Supplementary material 2.

Most samples had a broad variety of zircon morpholo-

gies. Nonetheless, there are distinct differences in zircon

populations.

Meta-greywackes and meta-conglomerates contained

mostly zircons with flat [101] pyramids and a dominant

[100] prism (groups J5, S25 and S20 in Supplementary

material 2) or equally developed prisms [100] and [110]

(group S15 in Supplementary material 2). This group was

also present in the sample of the grey augengneiss (KE

444), in the three samples of the Barenstein augengneiss

and in sample Ko 345 from a muscovite gneiss (Supple-

mentary material 2). Therefore, these samples define the

first group related to zircon morphology (Supplementary

material 2, see j). The augengneiss sample from the Frei-

berg dome also contained this type of zircon yet in a

subordinate amount.

In contrast, the [110] prism was dominant for most

zircon grains in the second morphology group (all zircon

groups \S10 in the upper part of Supplementary mate-

rial 2). The augengneisses from large domes (Freiberg

dome, Reitzenhain–Catherine dome, Schwarzenberg

Fig. 2 Geochemical signatures (spidergrams) of analysed samples

(Supplementary material 1) compared with previous geochemical

data (Tichomirowa 2003) for various gneiss samples from the

Erzgebirge (Table 1) shown as grey bands (n number of samples

contributing to grey bands). a Augengneiss domes: FD Freiberg

dome, RCD Reitzenhain-Catherine Dome, SD Schwarzenberg Dome.

Normalisation to the average composition of the upper continental

crust (UCC) after Rudnick and Gao (2003). KE 446 is the sample

number (compare Table 2)

dome), the red augengneisses, almost all muscovite

gneisses from small occurrences as well as the muscovite

gneiss from the Griessbach meta-volcanite unit belong to

the second group of zircon morphology (Supplementary

material 2, see k). These gneisses also contained zircons

with equally developed [100] and [110] prisms (groups

S11 to S15 in Supplementary material 2—similar to the

first zircon morphology group) but such zircons were

subordinate. The pyramids were differently developed

within this group. Most muscovite gneisses showed a

remarkable homogeneity of zircon crystals with mainly

flat pyramids (groups S5, S10 and to a lower-degree S15

and L5 in Supplementary material 2), while zircons in red

augengneisses and meta-granodiorite gneisses showed a

wider variation of dominant pyramids ranging from flat

(groups L5, S5, S10, S15 in Supplementary material 2) to

steep pyramids (groups S1, S6, S11 in Supplementary

material 2).

Fig. 3 Rare earth element patterns of analysed samples (Supplemen-

tary material 1) compared with previous geochemical data (Ticho-

mirowa 2003) for various gneiss samples from the Erzgebirge

(Table 1) shown as grey bands (n number of samples contributing

to grey bands). a augengneiss domes: FD Freiberg dome, RCD

Reitzenhain-Catherine Dome, SD Schwarzenberg Dome. Normalisa-

tion to the average composition of the upper continental crust (UCC)

after Rudnick and Gao (2003). KE 446 is the sample number

(compare Table 2)

Zircon dating

Pb/Pb evaporation method of single zircon grains

The zircon evaporation method only provides model207Pb/206Pb ages without information about concordance.

Pb loss results in variable 207Pb/206Pb ratios and ages.

Thus, analyses of several grains from one sample are

usually applied to conclude on concordance in case that all

measurements result in the same age within error. Many

studies have shown that the evaporation method can pro-

vide ages that are identical to those obtained by TIMS and

ion microprobe dating methods (e.g. Tichomirowa et al.

2001; Mingram et al. 2004).

147 measurements were obtained on 19 samples. Results

are given in the Supplementary material 3 and shown in

Fig. 4. For each sample, 4–20 grains were dated (except for

sample OE 7, which represents the same rock as sample OE

9). For this method, grains with idiomorphic forms were

chosen to obtain reproducible ages reflecting the igneous

growth stage of zircons. A substantial age range was

obtained for all samples. Both inheritance and Pb loss most

likely contribute to the age scatter. Nonetheless, in many

samples, a weighted mean age could be calculated using a

cluster of nearby ages shown in Fig. 4 and given in the

Supplementary material 3 as bold numbers. Only mean ages

with a MSWD value smaller than 4 were considered for

further interpretation assuming that such apparent ages

closely approximate real geological events. In case of

inheritance, the Pb/Pb evaporation method provides mini-

mum ages.

The apparent age of the sample KE 849 (547 ± 4 Ma)

was within the range of ages formerly obtained for aug-

engneisses from the Freiberg dome (550–540 Ma). Most

zircons from sample Non 1 yielded ages between 470 and

520 Ma. Yet, the age scatter made it impossible to obtain a

precise mean age. Similarly, mainly Ordovician ages were

obtained for most zircons of the two samples from the

Schwarzenberg dome (sample 94-3 and KE 548). In con-

trast, three samples from the Barenstein augengneiss indi-

cate older but scattering ages of about 550–580 Ma. The

three samples of meta-greywackes and meta-conglomerates

gave mean ages between 558 and 584 Ma similar to that of

the grey augengneiss (KE 444: 555 Ma—albeit with a high

MSWD value). All these samples (three samples from the

Barenstein augengneiss, meta-greywackes and meta-con-

glomerates and sample KE 444) had the highest amount of

inherited zircons yielding ages of up to 3.2 Ga. Most red

augengneisses and muscovite gneisses from smaller

occurrences (except for sample Ko 345) yielded Ordovi-

cian ages (from 483 ± 4 to 496 ± 8 Ma) as did the sample

from the Griessbach meta-volcanite unit (KE 553:

498 ± 8 Ma).

In situ U/Pb dating by the SHRIMP method

The results of in situ U/Pb SHRIMP dating are given in

Table 3. Most of these results are presented together with

selected spot locations in Supplementary materials 4–7.

The U/Pb data of most samples yielded scattered ages

along the Concordia as discussed for each sample sepa-

rately below.

The Reitzenhain–Catharine granite gneiss: sample Non

1 The 206Pb/238U ages show the same scatter as the207Pb/206Pb evaporation ages (Supplementary material 4).

Four analyses cluster at about 490 Ma, and these measure-

ments were used to calculate the mean age (Supplementary

material 4: 488.9 ± 4.7 Ma). The obtained Ordovician age

is regarded as most reliable for this sample since at least two

points (15.1, 10.1) reported the highest Th/U ratios (0.24;

0.33) and showed no signs of overprint in CL images. Two

further analyses (spots 5.2, 12.1) yielded higher ages than the

mean 206Pb/238U age but have very low Th/U ratios (0.01)

that are typical for metamorphic zircons with disturbed age

information (Hoskin and Black 2000; Rubatto et al. 2001).

One old core was identified by CL image (Supplementary

material 4: 794 Ma), and another inherited age was found by

the zircon evaporation method recording a minimum age of

about 1,880 Ma.

Schwarzenberg augengneisses: samples 94-3 and KE

548 Both samples contained a subordinate (5–10 %)

fraction of long-prismatic grains although most zircons were

stubby (Supplementary material 5). CL images show that

inherited zircon cores are a common feature of both samples

(Supplementary material 5). Mainly long-prismatic zircons

were dated in sample 94-3 while zircon grains with different

morphology were analysed in sample KE 548.

The age data show considerable scatter along the Con-

cordia. For sample 94-3, the older ages were chosen to

calculate a mean age supposing Pb loss for the younger

ones. In sample KE 548, older ages can be assigned to

inherited cores, detectable in CL images (Supplementary

material 5: 559, 700, 772, 1,845 Ma). Therefore, the

youngest age group was used for calculation. The derived

mean age for sample 94-3 (487.8 ± 4.1 Ma, MSWD =

0.41, n = 6) and that for sample KE 548 (480.6 ± 3.7 Ma,

MSWD = 0.45, n = 7) are identical within errors, and

both samples were pooled. The combined mean SHRIMP

age yielded 487.6 ± 3.0 Ma (MSWD = 0.48, n = 9). This

is in the same range as the calculated mean evaporation

ages (Supplementary material 3: 480 ± 8 Ma, 484 ± 19

Ma) and compares well with the U/Pb SHRIMP age given

by Mingram et al. (2004: 487 ± 7 Ma, n = 4). The Th/U

ratios of these zircons vary from 0.19 to 1.02 that of

Mingram et al. (2004) from 0.10 to 0.51. Both dating

methods recorded the presence of old inherited cores

(533–1,912 Ma).

Barenstein augengneisses: samples KE 488 and KE

549 Long-prismatic zircons are rare in both samples

(\5 %) that yielded a large scatter of their evaporation

and SHRIMP ages (Supplementary material 6; Table 3;

Supplementary material 3). One of the ages at about

620 Ma can be assigned to an inherited core based on the

CL image of that zircon (not shown), pointing to an ori-

ginal age younger than 620 Ma. Further cores yielded ages

of up to 3.16 Ga (Table 3, Supplementary material 3). To

calculate the mean SHRIMP age, we chose four analyses of

sample KE 488 and two additional ones from sample KE

Fig. 4 Probability density plot of zircon evaporation ages for

analysed samples given in Supplementary material 3. a augengneiss

domes: FD Freiberg dome, RCD Reitzenhain-Catherine Dome, SDSchwarzenberg Dome. The calculated mean age for samples given in

Supplementary material 3 is shown in the upper right of each plot and

given in brackets for mean ages with a MSWD value higher than 4.

The age 550 Ma is marked by a broken line for comparison between

diagrams. See text for discussion

Table 3 U-Th-Pb SHRIMP analyses of zircons

Mount-Grain.

206Pb*

232Th/238U 206Pb/204Pb f206a

207Pb*/206Pb*

206Pb*/238U

206Pb/238U age

(±Ma)

Sample Non 1

4-1.1 1,287 75.7 0.06 1,908 0.98 0.0559 (2.6) 0.06781 (0.63) 6 422.9 (2.6)

4-2.1 183 10.7 0.19 1,640 1.14 0.0511 (7.1) 0.06729 (1.4) -41 419.8 (5.6)

4-3.1 3,623 242 0.04 10,389 0.18 0.05708 (0.71) 0.07761 (0.38) 3 481.8 (1.8)

4-5.1 613 43 0.28 4,065 0.46 0.0564 (2.0) 0.08127 (0.94) -7 503.7 (4.5)

4-5.2 404 30.3 0.01 7,791 0.24 0.0579 (1.6) 0.08699 (0.98) -2 537.7 (5)

4-9.1 305 35.9 0.71 445 4.2 0.0662 (8.4) 0.13110 (1.0) 2 794 (7.8)

4-9.2 733 46.4 0.08 3,065 0.61 0.0569 (2.5) 0.07328 (0.93) 7 455.9 (4.1)

4-10.1 168 11.4 0.24 5,844 0.32 0.0551 (3.7) 0.07890 (1.2) -15 489.5 (5.9)

4-10.2 1,974 135 0.05 4,795 0.39 0.05574 (1.3) 0.07936 (0.87) -10 492.3 (4.1)

4-12.1 4,318 319 0.01 93,500 0.02 0.05608 (0.45) 0.08600 (0.57) -14 531.8 (2.9)

4-14.1 1,213 81.9 0.08 18,700 0.1 0.05695 (1.3) 0.07851 (0.89) 0 487.2 (4.2)

4-15.1 164 11.2 0.33 2,968 0.63 0.0528 (2.9) 0.07889 (1.2) -34 489.5 (5.5)

Sample 94-3

4-1.1 249 16.7 0.25 11,687 0.16 0.0573 (2.4) 0.07775 (0.89) 4 482.7 (4.2)

4-1.2 271 18.1 0.31 1,133 1.65 0.052 (7.6) 0.07615 (0.95) -39 473.1 (4.3)

4-2.1 602 40.3 0.45 11,687 0.16 0.0575 (1.8) 0.07779 (0.62) 6 482.9 (2.9)

4-2.2 675 46.4 0.49 12,467 0.15 0.0562 (1.5) 0.07985 (0.59) -7 495.2 (2.8)

4-3.1 211 13.7 0.29 4,921 0.38 0.0553 (3.4) 0.07563 (0.88) -9 470 (4)

4-3.2 387 25.5 0.17 8,500 0.22 0.0559 (2.1) 0.07633 (0.69) -5 474.2 (3.1)

4-6.1 355 23.8 0.13 1,154 1.62 0.0552 (5.5) 0.07666 (0.76) -12 476.2 (3.5)

4-6.2 1,116 73.6 0.29 2,055 0.91 0.0575 (2.5) 0.076 (1.5) 8 472.3 (6.7)

4-7.1 233 15.6 0.53 93,500 0.02 0.058 (2.5) 0.07792 (0.83) 9 483.7 (3.9)

4-7.2 384 25.9 0.13 4,155 0.45 0.0541 (2.6) 0.07823 (0.69) -22 485.6 (3.2)

4-8.1 389 26.6 0.71 8,450 0.22 0.0565 (2.0) 0.07942 (0.68) -4 492.7 (3.2)

4-8.2 502 32.5 0.1 3,979 0.47 0.0564 (2.3) 0.07492 (0.64) 1 465.7 (2.9)

Sample KE 548

4-2.1 582 38.6 0.55 23,375 0.08 0.05803 (1.5) 0.07713 (0.61) 11 479 (2.8)

4-2.2 171 11.6 0.35 2,833 0.66 0.0583 (5.0) 0.07821 (0.99) 11 485.5 (4.6)

3-3.1 723 49.1 1.02 26,714 0.07 0.05666 1.3) 0.07899 (0.53) -2 490.1 (2.5)

4-4.1 148 9.79 0.33 1,406 1.33 0.0499 (8.9) 0.0758 (2.4) -60 471 (11)

4-4.2 800 52 0.08 7,480 0.25 0.0545 (2.3) 0.07548 (0.81) -17 469.1 (3.7)

4-7.1 207 16.3 0.47 1,928 0.97 0.0544 (4.2) 0.09052 (0.85) -31 558.6 (4.5)

4-7.2 502 33.6 0.09 3,896 0.48 0.0536 (2.4) 0.07743 (0.63) -27 480.8 (2.9)

4-9.1 330 23 0.38 7,192 0.26 0.056 (3.2) 0.08095 (0.75) -10 501.8 (3.6)

4-9.2 207 13.4 0.28 14,385 0.13 0.0589 (3.4) 0.07541 (0.99) 20 468.6 (4.5)

4-10.1 546 37.1 0.19 62,333 0.03 0.05747 (1.4) 0.07891 (0.59) 4 489.6 (2.8)

4-13.1 152 11.8 0.27 5,844 0.32 0.0576 (3.6) 0.09028 (0.98) -8 557.2 (5.2)

4-13.2 568 38 0.09 9,842 0.19 0.05592 (1.5) 0.07775 (0.59) -7 482.7 (2.7)

4-16.1 180 19.7 0.37 7,192 0.26 0.0648 (2.1) 0.1272 (0.83) 0 771.8 (6)

4-16.2 297 19.9 0.18 7,792 0.24 0.0557 (2.3) 0.07771 (0.75) -9 482.4 (3.5)

4-17.1 106 10.5 0.9 37,400 0.05 0.0633 (3.4) 0.1147 (1.1) 3 700.1 (7)

4-17.2 397 28.8 0.09 6,926 0.27 0.0555 (2.1) 0.08418 (0.66) -17 521 (3.3)

4-18.1 52 14.9 0.85 5,054 0.37 0.1071 (2.1) 0.3313 (1.2) -5 1,845 (20)

4-19.1 174 11.5 0.39 17,000 0.11 0.0583 (2.7) 0.0765 (0.93) 14 475.2 (4.3)

4-20.1 298 23.4 0.35 3,740 0.5 0.0549 (2.0) 0.09111 (0.79) -27 562.1 (4.2)

Table 3 continued

Mount-Grain.

206Pb*

232Th/238U 206Pb/204Pb f206a

207Pb*/206Pb*

206Pb*/238U

206Pb/238U age

(±Ma)

Sample KE 488

3-1.1 458 37.2 0.18 – – 0.0618 (2.1) 0.0947 (1.2) 14 583.4 (6.5)

3-1.2 440 35.3 0.2 11,687 0.16 0.0563 (1.8) 0.09333 (0.73) -19 575.2 (4.0)

3-2.1 346 26.2 0.42 4,349 0.43 0.0557 (2.6) 0.08786 (0.71) -19 542.9 (3.7)

3-2.2 323 24.5 0.48 8,905 0.21 0.058 (2.1) 0.08793 (0.98) -3 543.3 (5.1)

3-2.3 359 27.6 0.44 46,750 0.04 0.0595 (1.8) 0.08946 (0.73) 6 552.3 (3.9)

3-3.1 513 38.5 0.09 – – 0.05818 (1.3) 0.08737 (0.59) -1 540 (3.0)

3-3.2 445 32.7 0.09 14,385 0.13 0.0584 (2.0) 0.08543 (0.61) 3 528.4 (3.1)

3-4.1 285 24.8 1.01 6,233 0.3 0.0587 (2.1) 0.10097 (0.68) -10 620.1 (4.0)

3-4.2 491 36.5 0.1 37,400 0.05 0.05852 (1.3) 0.08645 (0.58) 3 534.5 (3.0)

3-5.1 643 51.9 0.22 588 3.18 0.551 (9.3) 0.91 (0.72) -26 561.5 (3.8)

4-4.1 237 17.1 0.45 3,339 0.56 0.056 (3.6) 0.08357 (1.1) -12 517.4 (5.6)

4-5.1 258 17.9 0.42 4,921 0.38 0.0568 (3.0) 0.08036 (1.1) -3 498.3 (5.1)

4-6.1 909 70 0.76 15,583 0.12 0.05905 (1.1) 0.08952 (1.0) 3 552.7 (5.5)

4-6.2 251 17.4 0.33 1,545 1.21 0.0558 (4.6) 0.07969 (1.1) -10 494.3 (5.2)

4-7.1 183 14.9 0.31 5,054 0.37 0.0609 (3.7) 0.0946 (1.1) 9 582.4 (6.2)

4-7.2 566 42.4 0.12 10,389 0.18 0.05701 (1.6) 0.08702 (0.94) -9 537.9 (4.8)

4-8.1 220 16.3 0.18 4,675 0.4 0.0573 (2.4) 0.08606 (1.1) -5 532.2 (5.5)

4-8.2 463 35.3 0.11 9,350 0.2 0.05797 (1.7) 0.08859 (0.96) -3 547.2 (5.0)

4-10.1 153 49.6 0.71 46,750 0.04 0.1471 (0.77) 0.378 (1.1) 12 2,067 (19)

4-11.1 620 50.2 0.52 187,000 0.01 0.06004 (1.2) 0.09429 (0.93) 4 580.9 (5.1)

Sample KE 549

3-2.1 672 47.2 0.4 908 2.06 0.06058 (6.9) 0.08011 (0.67) 26 496.8 (3.2)

3-3.1 232 15.7 0.91 2,309 0.81 0.05888 (3.9) 0.07830 (0.82) 16 486.0 (3.9)

3-3.2 789 59.6 0.53 7,792 0.24 0.05985 (1.3) 0.08769 (0.52) 10 541.8 (2.7)

3-4.1 658 38.9 0.8 1,571 1.19 0.05655 (3.6) 0.06794 (0.60) 12 423.8 (2.5)

3-7.1 2,457 98.8 0.96 773 21.42 0.06022 (49.8) 0.03677 (1.64) 16 232.8 (3.7)

3-9.1 221 17.7 0.6 3,169 0.59 0.05501 (3.6) 0.09259 (0.84) -28 570.8 (4.6)

3-10.1 459 39.7 0.39 17,000 0.11 0.05799 (1.5) 0.10074 (0.68) -14 618.8 (4.0)

3-10.2 389 32.2 0.53 1,928 0.97 0.05959 (3.7) 0.09550 (2.02) 0 588.0 (11)

3-11.1 1,288 595.3 0.07 93,500 0.02 0.24723 (0.78) 0.53787 (0.89) 14 2,774 (20)

3-11.2 348 90.4 0.59 4,250 0.44 0.25083 (0.51) 0.30091 (1.62) 88 1,696 (24)

3-12.1 493 34.4 0.35 6,032 0.31 0.5858 (2.2) 0.8093 (0.60) 10 501.7 (2.9)

4-11.1 529 38.8 0.5 3,816 0.49 0.0579 (2.3) 0.08496 (0.61) 0 525.6 (3.1)

Sample OE 9

3-1.1 588 41.2 0.74 181 10.34 0.055 (26) 0.07324 (1.1) -6 455.7 (4.9)

3-1.2 1,754 115 0.38 68 27.55 0.051 (83) 0.05520 (2.2) -34 346.6 (7.4)

2-4.1c 421 33 0.13 3,286 0.57 0.0569 (1.1) 0.07743 (0.81) 0 480.8 (3.8)

2-8.1c 176 15 0.55 1,404 1.33 0.0537 (2.3) 0.07091 (0.83) 0 441.6 (3.5)

2-9.1c 300 22 0.2 251 7.45 0.0526 (4.4) 0.06622 (0.84) 0 413.4 (3.4)

Sample OE 7

4-3.1 246 15.5 0.52 1,255 1.49 0.0526 (5.6) 0.07226 (1.3) -31 449.7 (5.6)

4-3.2 876 61.6 0.2 1,345 1.39 0.0577 (3.6) 0.08071 (0.92) 3 500.3 (4.4)

4-4.1 615 45.3 0.75 9,350 0.2 0.05804 (1.7) 0.08554 (0.94) 0 529.1 (4.8)

4-6.1 408 29.5 0.27 6,926 0.27 0.0589 (2) 0.08393 (1.0) 9 519.5 (5.1)

4-6.2 867 66.3 0.16 2,877 0.65 0.0575 (2.1) 0.08842 (0.91) -6 546.2 (4.8)

4-8.1 193 14.6 0.33 – – 0.0589 (2.2) 0.08774 (1.1) 4 542.1 (5.7)

4-9.1 174 13 0.67 382 4.89 0.0671 (12) 0.08205 (1.3) 64 510.7 (6.5)

549; all from the upper end of the age spread between 580

and 500 Ma. The mean ages of both samples (Supple-

mentary material 6: 579.0 ± 9.2 and 572.6 ± 4.2 Ma) are

identical within error and the combined age yielded

577.6 ± 7.2 Ma. We believe that this value best reflects

the primary age that was not disturbed by Pb loss because

one of the long-prismatic grains showing primary oscilla-

tory zoning record this age (Supplementary material 6:

zircon grain 1 from sample KE 488). Accordingly, we

interpret all ages younger than *580 Ma caused by Pb

loss. The Th/U ratios of zircons used for mean age calcu-

lation vary from 0.18 to 0.60. Mingram et al. (2004)

reported a mean age of 575 ± 4 Ma (n = 2) from a sample

location close to that of KE 549.

The lowest age (233 Ma) was obtained in zircon 7 from

sample KE 549 (Supplementary material 6) that yielded

the highest U content (2,457 ppm), highest Th content

(2,291 ppm) and highest percentage of common 206Pb

(21.4 %, Table 3). The structure of this zircon grain is

porous with a lot of tiny inclusions of quartz, biotite,

K-feldspar and apatite (Supplementary material 6).

Red augengneisses: samples OE 7 and OE 9 While the207Pb/206Pb evaporation ages of sample OE 9 scatter over

an interval of more than 100 Ma, many of the SHRIMP

analyses from sample OE 7 cluster at about 540 Ma

(Supplementary material 7). The mean 206Pb/238U age of

539.6 ± 2.8 Ma was calculated from 13 analyses of sample

OE 7 (Table 3) and is different from the mean evaporation

age of 489 ± 21 Ma based on analyses from sample OE 9

(Fig. 4, Supplementary material 3). The five SHRIMP

analyses from sample OE 9 yielded slightly to considerably

younger U/Pb ages between 347 and 481 Ma (Table 3).

Thus, zircons from the stronger foliated sample OE 9 were

more disturbed in their U-Th-Pb system and were less

suitable for a reliable age reconstruction. High and unstable204Pb/206Pb ratios were observed during all evaporation

measurements (see measurement examples in Tichomirowa

2003). The high 204Pb abundance resulted in an ‘‘overcor-

rection’’ of common Pb and too young evaporation ages. In

contrast, most analyses of the SHRIMP measurements of

both samples did not show elevated common Pb concen-

trations (Table 3). The high amount of common Pb during

zircon evaporation could be caused by mineral inclusions.

EDX analyses were applied to determine the composi-

tion of mineral inclusions. More feldspar inclusions were

found (Supplementary material 7) compared with other

gneiss samples from the Erzgebirge (Tichomirowa et al.

2001; Tichomirowa 2003). The Raman microprobe method

was used to correctly distinguish between K-feldspar and

plagioclase. Supplementary material 7 documents the

abundance of K-feldspar inclusions in zircon crystals that

are minor in other investigated samples. Therefore, it is

proposed that these K-feldspar inclusions contain elevated

common Pb concentrations and led to unreliable zircon

evaporation ages.

Discussion

Gneisses with similar zirconology and geochemistry

Two different groups of zircon morphology and two dif-

ferent geochemical groups can be distinguished in the

Table 3 continued

Mount-Grain.

206Pb*

232Th/238U 206Pb/204Pb f206a

207Pb*/206Pb*

206Pb*/238U

206Pb/238U age

(±Ma)

4-9.2 700 52.4 0.18 1,427 1.31 0.0561 (3.5) 0.08602 (0.92) -14 531.9 (4.7)

4-10.1 285 21.6 0.45 5,500 0.34 0.0557 (3.1) 0.08815 (1.0) -19 544.6 (5.4)

4-11.1 388 29.5 0.36 12,467 0.15 0.0571 (1.9) 0.08849 (0.97) -9 546.6 (5.1)

4-11.2 664 49.9 0.13 6,448 0.29 0.05878 (1.7) 0.08729 (0.93) 4 539.5 (4.8)

4-12.1 360 27.2 0.3 – – 0.05961 (1.5) 0.08792 (0.98) 8 543.3 (5.1)

4-13.1 421 31.8 0.49 15,583 0.12 0.05882 (1.5) 0.08789 (0.97) 3 543.1 (5)

4-14.1 1,030 76.9 0.55 6,679 0.28 0.05829 (1.3) 0.08665 (0.89) 1 535.7 (4.6)

4-14.2 879 61.7 0.15 663 2.82 0.0573 (6.3) 0.07937 (0.94) 2 492.4 (4.4)

4-17.1 236 17.9 0.24 3,896 0.48 0.0543 (3.5) 0.0879 (1.1) -29 543.1 (5.6)

4-17.2 798 59.6 0.26 3,281 0.57 0.0567 (2.1) 0.08645 (0.91) -10 534.5 (4.7)

4-18.1 342 25.8 0.25 17,000 0.11 0.0577 (1.8) 0.08792 (1.0) -5 543.2 (5.2)

2-4ac 421 33 0.13 659 2.84 0.0560 (2.5) 0.07145 (0.81) -2 444.9 (3.5)

2-4bc 151 16 0.44 3,798 0.49 0.0590 (1.6) 0.08712 (0.89) -6 538.5 (4.6)

a Percentage of common Pb, calculated from measured 204Pb and assuming 0 Ma (Stacey and Kramers 1975), average terrestrial Pbb Degree of discordance (%)

studied gneisses (see sections ‘‘Bulk rock geochemistry’’

and ‘‘Zircon morphology’’; Figs. 2, 3 and Supplementary

material 2). Zircon morphology group I dominates in meta-

greywackes and meta-conglomerates (KE 446, KE 543, KE

544), various augengneisses (KE 444, KE 488, KE 516, KE

549) and strongly foliated muscovite gneiss (Ko 345). All

these samples show trace element pattern typical for geo-

chemical group I (except a transitional pattern for Ko 345).

The mean Pb/Pb zircon evaporation ages of 7 samples vary

between 542 ± 6 and 581 ± 20 Ma. SHRIMP spot anal-

ysis yielded an age of 578 ± 7 Ma (2 samples).

All other gneisses belong to the second group of zircon

morphology that is heterogeneous with respect to rock

geochemistry and zircon ages. Calculated mean zircon ages

for gneisses, which fit into the first geochemical group,

yielded a Late Proterozoic age of 547 ± 4 Ma for sample

KE 849 from the Freiberg gneiss dome (Pb/Pb evaporation)

and an Ordovician age of 488 ± 3 Ma for samples KE 548

and 94-3 from the Schwarzenberg dome (U/Pb age and

similar Pb/Pb mean ages). Seven samples from red aug-

engneisses and muscovite gneisses with a typical geo-

chemical pattern of group II yielded Ordovician ages

between 483 ± 4 and 503 ± 10 Ma (Pb/Pb evaporation

ages), while a U/Pb age of 499 ± 5 Ma was recorded for

sample Non 1 from the Reitzenhain-Catherine dome. By

contrast, an Early Cambrian U/Pb age of 540 ± 3 Ma was

determined for sample OE 7 from the small occurrence of

red augengneiss from the Eastern Erzgebirge.

Possible relationship between gneisses and time

equivalent lower metamorphic rocks from Lusatia

Neoproterozoic to Ordovician time equivalent rock suc-

cessions are preserved and exposed in six additional areas of

Saxothuringia: the Schwarzburg Anticline, the Berga

Anticline, the Doberlug Syncline, the North Saxon Anti-

cline, the Lausitz Anticline (further called Lusatia) and the

Elbe Zone (Linnemann and Romer 2002; Fig. 5). The Ca-

domian basement consists of greywackes in all these areas,

formed by erosion of arc-related Neoproterozoic (about

575 Ma) magmatic rocks, intruded by Early Cambrian

granodiorites and Ordovician granites (Linnemann et al.

2000, 2010; Romer et al. 2010). Mingram (1998) has shown

that Early Palaeozoic sediments from the Erzgebirge have

the same geochemical pattern as sediments from other areas

from Saxothuringia. Tichomirowa (2002, 2003) proposed

that the high-grade gneisses from the Erzgebirge represent

time equivalents of Late Neoproterozoic–Ordovician rocks

from Saxothuringia due to similar zircon morphologies,

zircon ages, geochemistry and neodymium isotope values.

Lusatia is the largest well-preserved area with the lowest

Variscan metamorphic overprint within Saxothuringia

(Kroner et al. 2007; Kemnitz 2007; Fig. 5) with an

extensive database (Hammer 1996; Hammer et al. 1999;

Tichomirowa 2002, 2003; Linnemann et al. 2004; Kemnitz

2007). Therefore, geochemistry, zircon morphology and

zircon ages from gneisses were compared with time

equivalent rocks from Lusatia (Fig. 5).

For Lusatia, the geochemical pattern is obviously dif-

ferent for the Ordovician granites, whereas greywackes and

granodiorites share similar geochemical features. The dis-

tinct geochemical pattern of most Ordovician granites and

rhyolites compared with earlier igneous rocks was also

found for rocks of the Schwarzburg anticline (Mingram

et al. 2004) and the Elbe Zone (Gehmlich 2003). Further-

more, greywackes can be distinguished from granodiorites

not only by the ages of their youngest zircon population but

also by their zircon morphology. Zircons from greywackes

show the same abundant morphology group I like meta-

greywackes and meta-conglomerates from the Erzgebirge.

The distinct zircon morphology of the Neoproterozoic

greywackes was also reported for the Schwarzburg anticline

(Tichomirowa 2003) and for the Elbe Zone (Gehmlich

2003) and seems to be a general feature of greywackes from

Saxothuringia. Tichomirowa (2002) has shown that the

change in zircon morphology from greywackes towards

two-mica granodiorites in Lusatia was caused by formation

of new zircon mantles overgrowing pre-existing zircon

crystals. Therefore, the fine-grained two-mica granodiorites

have a large abundance of inherited zircon ages similar to

greywackes. In contrast, there was extensive growth of new

zircon crystals in biotite granodiorites, which yielded ages

of about 540 Ma. These rocks have still inherited zircons or

zircon cores, but their quantity is significantly lower as

compared with the greywackes and two-mica granodiorites.

The Ordovician granites have a well-defined zircon popu-

lation with ages of about 490–480 Ma. We conclude that

these typical features (zircon morphology and rock geo-

chemistry) can be used to infer the protolith nature of highly

metamorphosed gneisses from the Erzgebirge in addition to

zircon ages.

Meta-greywackes, meta-conglomerates and two-mica

granodiorites

Gneisses from the Erzgebirge assigned to these protoliths

should display the following typical features: (1) a large

number of inherited zircon ages (up to Archaean), (2) a high

number of zircons with morphologies of group I or transi-

tional towards group II (like the two-mica granodiorites)

and (3) geochemical pattern most similar to UCC values

with small negative anomalies of calcium and strontium.

Due to Pb loss during the metamorphic overprint, the

age difference of the youngest zircon group between

greywackes (574 ± 5 Ma) and two-mica ganodiorites

(530–580 Ma) may be blurred making a distinction between

them very difficult. However, slight differences occur in

zircon morphology and can be used to infer the pre-meta-

morphic precursor.

Accordingly, the Erzgebirge samples KE 446, KE 543

and KE 544 clearly fit into the group of meta-greywackes

and meta-conglomerates. Their structural appearance is in

line with this conclusion. The zircon ages (Supplementary

material 3) obtained for samples KE 543 (585 ± 6 Ma)

and KE 544 (571 ± 9 Ma) from the Western Erzgebirge

are in good agreement with ages from meta-greywackes

from the Eastern Erzgebirge (Table 1) whereas that of KE

446 (558 ± 6 Ma) is younger. However, all three samples

contain a few zircon crystals with younger ages (Supple-

mentary material 3: from 196 to 558 Ma) pointing to Pb

loss for some of the zircon grains. Therefore, the zircon age

of sample KE 446 should not necessarily be younger than

that of KE 543 and KE 544.

In addition, the ages of the augengneisses from Baren-

stein (KE 488, KE 516, KE 549) and the grey augeng-

neisses from small occurrences (sample KE 444) and two

samples from the Mulda augengneiss best fit to those of the

two-mica granodiorites although their structural appear-

ance more resembles granodioritic rocks. Most zircons

yielded ages between 530 and 580 Ma (both for the Pb/Pb

and U/Pb method), similar to two-mica granodiorites from

Lusatia.

It is difficult to infer the protolith for sample Ko 345.

This leucocratic rock is strongly foliated and was therefore

assigned to the meta-volcanites. However, the zircon

morphology points to greywackes or at least two-mica

granodiorites as protolith rocks. The calculated Pb/Pb mean

age (Supplementary material 3: 542 ± 6 Ma) is not in

conflict with this interpretation. High REE concentrations

corroborate the assignment to such precursor rocks while

most other element concentrations are significantly lower

and similar to the Ordovician igneous rocks.

Cambrian (Cadomian) igneous rocks

Typical features of these rocks are (1) zircon morphology

of group II (clearly different from meta-greywackes) but

with a small portion of group I zircons, (2) geochemical

pattern still similar to UCC with slightly more depleted

concentrations of Ca, Sr, V, Cr and a small Eu anomaly,

(3) a cluster of zircon ages at about 540 Ma and a small

frequency of inherited zircon ages.

The augengneisses from the Freiberg dome conform to

these characteristics and thus possibly represent the meta-

morphic equivalents to Cambrian granodiorites from

Lusatia.

Ordovician magmatites

Typical features of this rock group are (1) predominance of

zircon morphology group II, (2) only minor zircon inheri-

tance, (3) distinct age cluster at about 480–500 Ma and (4)

significantly lower concentrations of several elements

according to geochemical group II (Figs. 2i, 3i).

All of these features are fulfilled by the sample from the

Griessbach meta-volcanite unit (KE 553), all muscovite

gneisses (except sample Ko 345, see discussion above),

two samples of red augengneisses from small occurrences

(KE 518, KE 484) and sample Non 1 (Reitzenhain-Cath-

erine dome), suggesting a genetic relationship to Ordovi-

cian magmatic rocks. Hence, the protolith age of the

Reitzenhain-Catharine dome should be revised as Ordovi-

cian in accordance with data from Tichomirowa (2003) and

Kosler et al. (2004).

The grey augengneisses from the Schwarzenberg dome

display the first three typical features of this group but have a

quite different geochemical pattern that is more similar to

grey augengneisses from the Freiberg dome. Consequently,

the geochemical pattern should not be an exclusive feature

for all Ordovician magmatites, but only for most of them.

Gehmlich (2003) also found a few Ordovician magmatites in

the Wildenfels Zwischengebirge about 40 km north of the

Schwarzenberg dome with a similar geochemical pattern.

Sample OE 7/9 is different in the mean zircon age

(540 Ma) but otherwise shows all features typical of this

Ordovician rock group. It confirms that the geochemical

rock composition alone is not sufficient to assign a rock to

a certain age group (e.g. the igneous rocks of Ordovician

Fig. 5 Typical geochemical pattern (first two columns), zircon

morphology according to Pupin (1980) (third column) and probability

density plots of zircon evaporation ages (fourth column) for rocks

from Lusatia. For greywackes, LA-ICP-MS data from Linnemann

et al. (2008: n = 187) are shown as a grey band. The geochemical

data were normalised to the upper continental crust (UCC) according

to Rudnick and Gao (2003). The black line in all geochemical data

plots represents mean values with the number of analyses given in

parenthesis (e.g. n = 18). These data are from Hammer (1996) and

Hammer et al. (1999). Data for the grey bands in the geochemical

diagrams are from samples analysed by Tichomirowa (2003). Data for

zircon morphology and Pb/Pb zircon evaporation ages are from

Tichomirowa (2002, 2003). The area surrounded by a thick black linein the zircon morphology diagram for greywackes represents the most

abundant zircon morphologies after Hammer (1996). Fields marked

with dark colour in the zircon morphology diagram show the most

abundant morphology types, while lighter colours represent less

abundant zircon crystals. The ages given in the probability density

plot are calculated weighted mean ages for the most abundant zircon

population given together with the number of zircon ages used, and

the calculated mean standard weighted deviation (MSWD). For the

two-mica granodiorites, only an age interval is given (see text for

discussion). The age 550 Ma is marked by a broken line for

comparison between diagrams

Changes caused by metamorphic overprint

Structure and geochemistry

Generally, most of the gneisses still show the typical

structural characteristics of non-metamorphosed rock

equivalents in Lusatia, such as the Lower Cambrian aug-

engneiss from the Freiberg dome or the fine-grained meta-

greywackes. However, locally the metamorphic overprint

led to severe changes. For example, the grey augengneisses

from Barenstein are structurally similar to the grey aug-

engneisses from the Freiberg dome but should be assigned

to Neoproterozoic–Lower Cambrian two-mica granodior-

ites. Wiedemann (1989) already proposed a tectonic origin

of the augengneiss structure of the Barenstein gneiss and

described many small cataclastic shear zones and syn-

deformational recrystallisation of K-feldspar and quartz.

The K-feldspar porphyroblasts in the Barenstein aug-

engneiss were formed due to intense shearing processes

during exhumation of the Erzgebirge gneiss core. Wiede-

mann (1989) suggested that several elements like K, Na

and Ba were mobilised during the tectonic overprint.

Element concentrations of analysed samples and of

additional samples (Tichomirowa 2003; Tichomirowa

et al. 2005) were normalised to mean values of Lusatian

unmetamorphosed rock equivalents in order to estimate

element transport during the metamorphic overprint (not

shown). Generally, the differences were rather small (often

within ±20 %) and rarely exceeded twofold enrichment or

depletion. Similarly, Rotzler and Plessen (2010) docu-

mented very similar geochemical patterns in schists from

HP-HT and HP-LT units of the Erzgebirge and in almost

non-metamorphosed rock equivalents from the Schwarz-

burg Anticline. Therefore, it can be concluded that the

geochemistry of most rocks was not altered during the

(U)HP overprint.

However, several elements show a tendency towards

enrichment or depletion. Some element concentrations (Ba,

Sr, LREE, Ti, V) are obviously lower in fine-grained

varieties of Ordovician meta-igneous rocks located within

the (U)HP nappes, while these elements are significantly

enriched (e.g. Ba by a factor of 2–5) in the coarse-grained

Ordovician varieties. Thus, the formation of K-feldspar

porphyroblasts in coarse-grained gneisses within the (U)HP

nappes was probably related to mobilisation of Ba supplied

from fine-grained rocks. Furthermore, meta-magmatites

located within the (U)HP nappes with a partial or total age

reset (towards the metamorphic age) document significant

mobilisation for Ba, Sr, Ca, LREE, Th, U and some high

field strength elements (e.g. Cr, V, Ti). In contrast, there is

no geochemical difference between coarse-grained and

fine-grained Ordovician meta-magmatites outside (U)HP

nappes. Therefore, mobilisation of elements during

metamorphism is mainly restricted to the (U)HP nappes

and to local zones of intense shearing (e.g. Barenstein

augengneiss and sample Ko 345).

Zircon morphology, ages and chemical composition

The Variscan metamorphism did not cause a change in

zircon morphology. Even in the most severely overprinted

samples from the Barenstein augengneiss, the dominance

of prism [100] was not altered. However, some zircon

grains show clear signs of a tectonic overprint like (1)

intense fracturing (Supplementary material 6: see zircon

grain 34 from sample KE 488) with healed fractures filled

by K-feldspar and quartz (Supplementary material 6: KE

488, zircon grains 6, 7), (2) humpy surfaces and porous

internal structures with a lot of tiny mineral inclusions

(Supplementary material 6: zircon grain 7 from sample KE

488, and zircon 7 from sample KE 549) and (3) a large age

scatter along the Concordia (Supplementary material 6).

Spongy and inclusion-rich zircons like grain 7 from sample

KE 549 (Supplementary material 6) are often assigned to

hydrothermal formation or hydrothermal re-equilibration

where micrometer-sized pores indicate a coupled dissolu-

tion-reprecipitation process of zircons (Tomaschek et al.

2003; Geisler et al. 2007). Thus, an element redistribution,

as described above for the formation of K-feldspar por-

phyroblasts probably occurred also in zircons, because

zircons with the lowest ages (spot 7.1 and 4.1 from sample

KE 549) have the highest concentrations of U, Th and

common Pb, leading to high Th/U ratios (Table 3). The

enrichment of Th and U in zircons is different to the dif-

fusion driven Th loss as usually observed in metamorphic

zircons (Schaltegger et al. 1999; Hoskin and Black 2000;

Tichomirowa et al. 2005). Therefore, under certain condi-

tions a metamorphic overprint accompanied by hydro-

thermal fluids may lead to high Th/U ratios of zircons.

Many gneiss samples in the Erzgebirge yielded scattering

zircon ages (Table 3, Supplementary material 3). The scat-

ter is largest where deformation was accompanied by fluid

movement and element mobilisation, for example, in the

Barenstein augengneiss. In contrast, most gneiss samples

from the HP-LT unit did not show a considerable age shift

(Table 1). Within the (U)HP-HT unit, many orthogneisses

preserved their protolith age (Table 1) or were only partly

reset even when they are located in close vicinity or a few

hundred metre away from granulite lenses (Tichomirowa

et al. 2005). A complete reset to the age of metamorphism

was only documented in granulite gneiss lenses where melt

formation was proposed (Tichomirowa et al. 2005). There-

fore, high temperature with partial melting is a necessary

prerequisite for a complete reset of zircon ages in (U)HP

metamorphic regions as proposed by Schaltegger et al.

(1999), Rubatto et al. (2001) and Tichomirowa et al. (2005).

Geologic units of the gneiss core of the Erzgebirge

The information obtained from the newly investigated

samples was combined with former results to distinguish

geologic units with similar protolith rocks for the gneiss

core of the Erzgebirge (Fig. 6). The oldest protolith rocks,

the Neoproterozoic meta-greywackes, are represented

mainly by fine-grained gneisses both in the Eastern and

Western Erzgebirge. Meta-conglomerates were found in a

few locations within these meta-greywackes. Locally, large

bands and lenses of augengneisses with igneous appearance

occur within meta-greywackes. The formation of K-feld-

spar porphyroblasts and the augengneiss structure in these

gneisses was caused by intense shearing accompanied by

fluid mobilisation. The supposed protolith rocks are Neo-

proterozoic to Early Cambrian two-mica granodiorites that

probably were preferred locations of tectonic shearing and

fluid mobilisation during exhumation.

Large augengneiss domes occur in both eastern and

western parts of the Erzgebirge. The Freiberg gneiss dome

in the Eastern Erzgebirge belongs to Early Cambrian

(Cadomian) meta-granodiorites. In contrast, the gneiss

domes in the Western Erzgebirge were formed by Ordo-

vician granites and granodiorites (Reitzenhain–Catherine

dome and Schwarzenberg dome). In addition, all dated

lenses of meta-granites or meta-volcanites within meta-

greywackes yielded Ordovician ages in the Western

Erzgebirge. So far, no Ordovician rocks were found in the

Eastern Erzgebirge (Figs. 1,6).

The Floha zone, dividing the Eastern Erzgebirge from

the Western Erzgebirge, is regarded as a deep fracture zone

where several (U)HP rocks were found, some of which

containing micro-diamonds. Therefore, this zone is regar-

ded as the high-pressure nappe E1 recording the highest

metamorphic temperatures of up to 1,000–1,100 �C

(Massonne 1999; Hwang et al. 2001). The main rock types

are muscovite gneisses that recorded Ordovician ages,

some of which were partially reset (e.g. Tichomirowa et al.

2005). However, the finding of metamorphic ages of about

340 Ma, assigned to the (U)HP stage of the metamorphism

within this nappe, is restricted to granulite gneisses or

eclogites or the close vicinity to these rocks. No meta-

morphic zircon ages were recorded in the other two HP

nappes. All muscovite gneisses there yielded Ordovician

ages, indicating the importance of these rocks within the

HP nappes. The Griessbach meta-volcanite unit is a band of

muscovite gneisses where all zircons clearly recorded an

Ordovician age (Fig. 1).

Fig. 6 Generalised scheme displaying the most important features typical of gneisses from the Erzgebirge used for derivation of their pre-

metamorphic protoliths

Conclusions

The metamorphic rocks of the Erzgebirge record a complex

tectono-metamorphic evolution. Due to severe overprint-

ing, identification of protoliths is in many cases extremely

difficult. However, a reliable identification of the pre-

metamorphic precursors is possible by a combination of

zircon morphology, zircon ages, zircon chemistry, bulk

rock chemistry and comparison with lower-grade rocks of

Lusatia.

In the Erzgebirge, the oldest rocks are paragneisses

representing meta-greywackes and meta-conglomerates.

These gneisses contain zircon grains with very different

ages recording older (up to Archaean) and younger (up to

Neoproterozoic) sources in their protoliths. The youngest

age component that did not undergo Pb loss (about

575 Ma) represents the youngest igneous component for

source rocks of these paragneisses.

The grey augengneisses occurring as lenses and bands

within meta-greywackes (Barenstein, Wolkenstein, Mulda)

most probably represent metamorphic equivalents to

Lower Cambrian two-mica granodiorites from Lusatia with

only subordinate formation of new zircon rims during melt

formation. Structural (e.g. small cataclastic zones) and

chemical changes, intense fracturing and high U and Th

concentrations of zircons point to tectonic movements

accompanied by fluid mobilisation. The temperature during

shearing was too low to cause a total reset of zircon ages

indicating mid- or upper crustal levels.

Large augengneiss domes have different protolith ages.

Orthogneisses of the Freiberg gneiss dome (Eastern

Erzgebirge) represent metamorphic equivalents of Lower

Cambrian granodiorites from Lusatia. In contrast, zircon

ages of the coarse-grained gneisses of the Reitzenhain–

Catherine dome and the Schwarzenberg dome (Western

Erzgebirge) indicate Ordovician protolith ages, docu-

menting significant regional differences between the east-

ern and western part of the Erzgebirge (*540 vs. *490

Ma). Furthermore, most occurrences of meta-volcanic

rocks (muscovite gneisses) and of meta-granites (mainly

red augengneisses) also yielded Ordovician zircon ages in

the Western Erzgebirge, whereas similar rocks in the

Eastern Erzgebirge mainly recorded Lower Cambrian

protolith ages.

Zircon overprinting was highest within discrete tectonic

zones, where the combination of fluid infiltration and

deformation induced variable degrees of recrystallisation

and the formation of a new rock structure (e.g. the

Barenstein augengneiss). This process often led to U and

Th enrichment in zircons and to variable degrees of Pb loss

that caused age scatter but no changes in zircon morphol-

ogy. Element mobilisation, recorded by bulk rock chem-

istry, was mainly restricted to discrete tectonic zones and

within (U)HP nappes but was mainly absent in lower-grade

metamorphic rocks.

Acknowledgments This research was supported by grants of the

Sachsisches Landesamt fur Umwelt, Landwirtschaft und Geologie.

We thank L. Nasdala for performing the Raman analyses of feldspars.

The authors thank K. Bombach and A. Braun for laboratory assistance

in zircon evaporation analyses and Jorg Matschullat for improving the

English language. Very helpful and constructive comments from two

anonymous reviewers are gratefully acknowledged, as are the Editor0shelpful comments on the manuscript.

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