GeolCarp_Vol60_No3_205

www.geologicacarpathica.sk

GEOLOGICA CARPATHICA, JUNE 2009, 60, 3, 205—212 doi: 10.2478/v10096-009-0014-3

Introduction

Granites  are  important  indicators  of  thermal  and  tectonic
processes occurring in the lower crust and the upper mantle
(Pitcher  1982;  Harris  et  al.  1984;  Pearce  et  al.  1984).  By
their typology they provide some information on the nature
of  the  source  rocks  (Chappell  &  White  1974),  although  we
must be aware of magma modifications that occur, for exam-
ple, through mixing or assimilation. Moreover, granites can
be  very  precisely  dated  by  geochronological  methods.  Gra-
nitic plutons are thus, in a way, windows through which the
deep  infrastructure  of  orogens  and  their  evolution  can  be
viewed.

This short communication covers current research activi-

ties at the University of Salzburg, with the aim of exploring
the geological information potential stored in the Bohemian
Massif granites. As a first step we report here on a newly de-
fined belt of coeval post-collisional granites (Saxo-Danubian
Granite Belt – SDGB), that extends over ca. 400 kilometers
across the south-western Bohemian Massif. Based on the ob-
served plutonic phenomena, and the given geological back-
ground, we argue that the formation of this granite belt was
most likely triggered by a process of delamination of mantle
lithosphere.

Geological background

The Variscan orogen is a collage of microplates (terranes)

that were assembled, between the Devonian and the Carbon-

The Saxo-Danubian Granite Belt: magmatic response to post-

collisional delamination of mantle lithosphere below the south-

western sector of the Bohemian Massif (Variscan orogen)

FRITZ FINGER

, AXEL GERDES

, MILOŠ RENÉ

and GUDRUN RIEGLER

Fachbereich Materialforschung und Physik, Abteilung Mineralogie, Universität Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg,

Austria; Friedrich.Finger@sbg.ac.at

Institut für Mineralogie, Universität Frankfurt, Senckenberganlage 28, D-60054 Frankfurt a. M., Germany; gerdes@em.uni-frankfurt.de

Academy of Sciences, Institute of Rock Structure and Mechanics, V Holešovičkách 41, CZ-18209 Prague 8, Czech Republic;

rene@irsm.cas.cz

(Manuscript received August 11, 2008; accepted in revised form October 23, 2008)

Abstract: On the basis of the synchronicity of geochronological data and the similarity of granite types, it is proposed
that the mid-Carboniferous Fichtelgebirge/Erzgebirge Batholith in the Saxothuringian Zone of the central European
Variscan Fold Belt and the South Bohemian Batholith in the Moldanubian Zone (including the intervening Oberpfalz
and Bavarian Forest granite areas) belong to one coherent and cogenetic, ca. 400 km long plutonic megastructure.
Unlike older (syn-collisional) plutonic structures in the Bohemian Massif, this Saxo-Danubian Granite Belt (nov. nom.)
has developed discordant to the Devonian/Early Carboniferous collision-related tectonic architecture of the Bohemian
Massif. It is argued that the Saxo-Danubian Granite Belt formed in response to a post-collisional detachment of lithos-
pheric mantle below the south-western sector of the Bohemian Massif.

Key words: Variscan orogen, Saxo-Danubian Granite Belt, Bohemian Massif, delamination, granites.

iferous, along the southern margin of the Old Red Continent
(Franke  2000;  Friedl  et  al.  2000;  Winchester  et  al.  2002).
The  central  European  section  of  the  Variscan  orogen,  with
the Bohemian Massif as its main exposure (Fig. 1), includes
several independent collision zones that represent fold belts
active  at  different  times.  Northern  areas  (Rhenohercynian
Zone,  Northern  Phyllite  Zone,  Mid  German  Crystalline
High)  record  the  final  closure  of  the  Rheiic  Ocean  and  the
Carboniferous  collision  of  Variscan  Europe  with  the  Old
Red  Continent.  In  the  central  part  of  the  Bohemian  Massif
(Teplá Barrandian block) a ca. 380 Ma old phase of deforma-
tion  and  regional  metamorphism  is  documented,  and  com-
monly  interpreted  in  terms  of  an  early  Variscan  collision
between  a  Saxothuringian  and  a  Bohemian  terrane  (Zulauf
1997).  In  the  south-eastern  part  of  the  Bohemian  Massif,  a
major phase of collisional crustal thickening and high-grade
regional metamorphism is recorded at ~ 340 Ma, and related
to  the  forceful  docking  of  a  Moldanubian  and  a  Moravian
terrane (Finger & Steyrer 1995; Schulmann et al. 2005). Af-
ter  this  “Moravo-Moldanubian”  orogenic  phase  (i.e.  at  ca.
330 Ma), the Bohemian Massif was more or less established
in its present day tectonic configuration, except for some lat-
eral movements along faults (Edel et al. 2003).

During the Carboniferous, numerous granitic plutons in-

truded all over the Bohemian Massif. These are commonly
treated as a single coherent group of “Variscan granites”
(Franke 2000), although they are of different ages and types.
Elaborating the concept of Finger et al. (1997), it is suggest-
ed that they form at least five independent magmatic systems
with individual tectonothermal backgrounds (Fig. 1):

206

FINGER, GERDES, RENÉ and RIEGLER

Fig. 1.

Sketch map of the Bohemian Massif, mainly after Franke (2000),

showing the distribution of Variscan granites and the attempt

to group these into plutonic belts of different age and ori-

gin.

Triangles

indicate

from

which

positions

the

SDGB

reliab

intrusion

ages

have

been

reported

(only

modern

low-error

zir

con

and

monazite

ages

have

been

considered).

Data

sources

are

given

Table 1.

Arrows

indicate

the

proposed

growth

direction

the

SDGB.

–

Bavarian

Forest,

–

Český

Les,

–

Fichtelgebirge,

–

berpfalz

Forest,

–

Regensburg

For-

est,

SBB

–

South

Bohemian

Batholith,

–

Sudetes,

–

Sauwald.

Inset

illustrates

the

position

the

Bohemian

Massi

within

the

central

European

Variscides.

207

SAXO-DANUBIAN GRANITE BELT: POST-COLLISIONAL DELAMINATION OF MANTLE LITHOSPHERE

1) A “North Variscan Granite Belt” with ca. 330 to

350 Ma I-type granite and granodiorite plutons (e.g. Anthes
& Reischmann 2001; Zeh et al. 2005; Dörr et al. 2006) ex-
tends over almost 500 km along the Mid German Crystalline
High eastward into Poland. This belt is commonly interpret-
ed as a magmatic arc that developed above a southward dip-
ping Rhenohercynian subduction zone (Franke 2000;
Oncken et al. 2000).

2) A similarly dated (360 to 335 Ma) granite belt, dominat-

ed by I-type tonalites and granodiorites, crosses the center of
the  Bohemian  Massif  approximately  in  a  NE-SW  direction
and  is  termed  here  the  “Central  Bohemian  Granite  Belt”.  It
can be followed over ca. 300 km from the Polish Sudetes (Ku-
dowa-Olesnice and Kłodzko-Złoty Stock massifs – Mazur et
al. 2007) to the Bavarian Forest (migmatized I-type granitoids
near  Waldkirchen  –  Propach  2005),  with  the  Central  Bohe-
mian Batholith near Prague (Janoušek et al. 2000) as the major
and  best  studied  exposure.  The  Central  Bohemian  Batholith
includes several generations of granitoids with variable defor-
mation histories (Scheuvens & Zulauf 2000; Bues et al. 2002;
Žák  et  al.  2005).  The  oldest,  Devonian,  I-type  intrusions,  as
well as small bodies of I-type orthogneisses in the metamor-
phic  roof  of  the  batholith,  dated  to  ca.  370 Ma  (Košler  et  al.
1993), have been interpreted by Janoušek & Holub (2007) as
part of a pre-collisional magmatic arc, that formed in connec-
tion  with  the  south-eastward  subduction  of  a  Saxothuringian
ocean. As opposed to this, Zulauf (1997) and Dörr & Zulauf
(2008) argue that all the granitoids (including the mid-Devo-
nian  orthogneisses)  are  post-collisional  with  reference  to  the
Saxothuringian-Bohemian  terrane  collision,  and  intruded  in
several  pulses,  when  the  thickened  crust  collapsed.  Magma
generation  above  a  Moravo-Moldanubian  subduction  system
(eastern  continuation  of  the  Rheiic  suture)  has  been  consid-
ered by Finger et al. (2007).

3) 335 to 340 Ma, high-K to shoshonitic (mela)granites,

granodiorites and syenites/monzonites, commonly grouped as
“durbachite  plutons  (s.l.)”,  are  arranged  along  two  parallel,
NNE-SSW  trending  lines  through  the  Southern  Bohemian
Massif (Klomínský & Dudek 1978). These “durbachite” intru-
sions contain components from an enriched mantle source (Ja-
noušek & Holub 2007), and are typically linked to the steeply
exhumed HP-HT rocks of the Gföhl Unit (Finger et al. 2007).
The magmas obviously used the tectonic uplift channels of the
high-pressure rocks for their own ascent. The Mutěnin syenite
of  the  Český  Les  area  (Dörr  &  Zulauf  2008),  the  Meissen
granitoids near Dresden (Wenzel et al. 1997), and the Niem-
cza granitoids in the Sudetes (Mazur et al. 2007) correlate to
these  Moldanubian  “durbachites”  in  age  and  typology
(Fig. 1). All of them intruded on major faults.

4) The south-western sector of the Bohemian Massif (the

Bavarian  Zone  of  Finger  et  al.  2007; Fig. 1) was invaded by
numerous, mainly crustally derived, granitic magmas between
ca.  330  and  310 Ma,  associated  with  penetrative  LP-HT  re-
gional  metamorphism  and  anatexis.  A  granitic  complex  of
batholithic dimensions developed at the eastern end of the Ba-
varian Zone (South Bohemian Batholith). It will be argued lat-
er  that  these  Moldanubian  granites  form  a  coherent  plutonic
belt with the Saxothuringian granites in the western Erzgebir-
ge and Fichtelgebirge (Saxo-Danubian Granite Belt).

5) Finally, a belt of comparatively younger granitoids

(~ 315 to 300 Ma) including I-type granodiorites, I/S- and
S-type granites (Mazur et al. 2007) occurs in the Sudetic re-
gion (Sudetic Granite Belt – Fig. 1). This belt extends west-
ward into Germany.

The granite belts 1 to 3 and 5 are not further considered in

this paper.

Geochronological data for the Saxo-Danubian

Granite Belt

The state of geochronological research in the Erzgebirge is

summarized in Förster & Romer (2009). These authors state
that  most  of  the  granitic  plutons  of  this  area  were  intruded
between 325 and 318 Ma (see also Table 1). This is corrobo-
rated  by  recent  zircon  dating  work  of  Kovaříková  et  al.
(2007),  which  indicates  an  intrusion  age  of  322  to  323 Ma
for the large Karlovy Vary / Loket pluton in the Czech part of
the  Erzgebirge.  In  the  eastern  part  of  the  Erzgebirge,  some
granite  bodies  show  a  slightly  higher  intrusion  age  of  ca.
327 Ma (Table 1, Fig. 1).

Siebel et al. (2003) published age data from the Fichtelge-

birge and the Oberpfalz Forest (Table 1). They distinguished
between  older  granites  (~ 325 Ma)  and  younger  granites
(~ 310 to 315 Ma). These data imply that the granitic activity
began at the same time as in the Erzgebirge (ca. 325 Ma), but
includes  another  younger  magmatic  pulse  (310  to  315 Ma)
not present in the Erzgebirge. The Bor and Babylon granites,
which intruded at the south-western termination of the Teplá
Barrandian block (Český Les – Fig. 1), are commonly con-
sidered  to  be  equivalents  of  the  older  granites  of  the  Ober-
pfalz Forest (Siebel et al. 1999; Siebel et al. 2003). Dörr &
Zulauf (2008) presented a monazite age of 331± 1 Ma for the
Bor granite, which is slightly higher than the ages of the old-
er granites of the Oberpfalz Forest.

A large number of low-error zircon and monazite ages

have become available for the Bavarian Zone and the South
Bohemian Batholith during recent years (Table 1). The rela-
tively  oldest  granites  (325  to  328 Ma)  are  present  on  the
northern  rim  of  the  Bavarian  Zone,  and  at  the  northern  and
eastern rims of the South Bohemian Batholith (Fig. 1). How-
ever, the main granitic activity took place between 325 and
320 Ma.  Younger  granite  plutons,  dated  to  314  to  317 Ma,
occur near Linz (Mauthausen and Altenberg granites) and in
the  Sauwald  (Schärding  and  Peuerbach  granite).  A  minor
late pulse of magmatism is also recorded in the South Bohe-
mian  Batholith,  ~ 300 Ma  (Freistadt  granodiorite  –  Gerdes
et al. 2003). Some rhyolites in the Erzgebirge have the same
Stephanian age (Förster et al. 2006).

Granite types

Several studies in the Erzgebirge (Förster & Romer 2009,

and references therein), the Fichtelgebirge and Oberpfalz For-
est (Siebel et al. 2003), the Bavarian Forest (Siebel et al. 2008)
and the South Bohemian Batholith (Finger & Clemens 1995;
Gerdes et al. 2000) have pointed out that the majority of the

208

FINGER, GERDES, RENÉ and RIEGLER

granites from these areas are derived through fluid-absent par-
tial melting of lower crustal sources. This mechanism can cre-
ate large volumes of granite melts, if the source region is of
sufficiently high temperature (Clemens & Vielzeuf 1987).

Early, coarse-grained and K-feldspar-phyric granites are

particularly characteristic for the entire SDGB (e.g. Fichtel-
gebirge/Erzgebirge:  G1-granite  Weißenstadt-Marktleuthen,
Karlovy Vary Granite; Oberpfalz Forest and Český Les: Bor,
Babylon and Leuchtenberg Granite; Regensburg Forest: Kri-
stallgranit  I;  South  Bohemian  Batholith:  Schlieren  Granite,
Weinsberg  Granite,  Eisgarn  Granite).  These  include  two-
mica S-type granites as well as a few I-type rocks with horn-
blende,  but  most  are  biotite  granites  with  transitional  I/S
characteristics.  These  early,  coarse-grained  and  K-feldspar-
phyric granites make up more than half of the granite inven-
tory of the SDGB.

In the Erzgebirge, tin-bearing granites play an important

role. Their presence may reflect a local availability of Sn-en-

Table 1: Age determinations referred to in this paper.

riched  crustal  source  rocks  (Förster  &  Romer  2009).  Other
differences  between  the  Erzgebirge  granite  terrain  and  the
South Bohemian Batholith appear to be a matter of a differ-
ent exposure level. Many granitic intrusions in the Erzgebir-
ge  are  felsic  high-level  plutons  with  a  high  degree  of
fractionation, while the exposure level of the South Bohemi-
an  Batholith  is  generally  deeper  and  involves  considerable
amounts of cumulate material (Finger & Clemens 1995).

The younger (310 to 315 Ma) granites of the SDGB are

predominantly, but not exclusively S-type rocks. In the Fich-
telgebirge and the Oberpfalz Forest they are mostly repre-
sented by fine- to medium-grained, muscovite-bearing,
S-type granites (Siebel et al. 2003). The younger granites of
the South Bohemian Batholith (Frasl & Finger 1991) include
both, I-type (Mauthausen granite, Freistadt granodiorite) and
S-type suites (Altenberg, Schärding and Peuerbach granite).

An important observation is that, throughout the SDGB,

small volumes of intermediate to mafic, K

O-rich plutonic

Erzgebirge
Karlovy Vary Pluton

322±2 Ma, Zrn, evap., K07

Karlovy Vary Pluton

323±3 Ma, Zrn, evap., K07

Niederbobritzsch Granite

320±6 Ma, Zrn, evap., T97

Schlemma Granite

322±6 Ma, Mnz, ID-T, F98

Ehrenfriedersdorf Granite

324±4 Ma, Urnt, ID-T, R07

Ehrenfriedersdorf Granite

321±2 Ma, Urnt, ID-T, R07

Eibenstock Granite

321±4 Ma, Mnz, ID-T, F98

Pobershau Granite

321±3 Ma, Mnz, ID-T, F98

Markersbach Granite

327±4 Ma, Zrn, L-ICP, H08

Fichtelgebirge, Oberpfalz Forest, Český Les
Redwitzite Marktredwitz

322±5 Ma, Zrn, evap., S03

Zainhammer Granite

321±1 Ma, Zrn, evap., S03

Marktredwitz G1 Granite

324±4 Ma, Zrn, evap., S03

Leuchtenberg Granite

323±1 Ma, Zrn, evap., S03

Leuchtenberg Granite

324±3 Ma, Zrn, evap., S03

Leuchtenberg Granite

328±1 Ma, Zrn, evap., S03

Liebenstein Granite

315±1 Ma, Zrn, evap., S03

Falkenberg Granite

315±4 Ma, Zrn, evap., S03

Falkenberg Granite

315±2 Ma, Zrn, evap., S03

Flossenbürg Granite

310±3 Ma, Zrn, evap., S03

Steinwald Granite

312±2 Ma, Zrn, evap., S03

Friedenfels Granite

312±4 Ma, Zrn, evap., S03

Mitterteich Granite

310±6 Ma, Zrn, evap., S03

Bor granite

331±1 Ma, Mnz, ID-T, D08

Regensburg Forest
Stallwang Granodiorite

324±2 Ma, Zrn, evap., S06

Kristallgranit I

315±4 Ma, Mnz, ID-T, P00

Trasching Granite

321±3 Ma, Mnz, ID-T, P00

Two-Mica Granite

323±4 Ma, Mnz, ID-T, S06

Granite Porphyry

323±2 Ma, Zrn, evap., S06

Bavarian Forest
Gurlarn Granite

311±2 Ma, Mnz, ID-T, P00

Plöckenstein Granite

325±2 Ma, Zrn, evap., B07

Dreisessel Granite

327±2 Ma, Zrn, evap., B07

Steinberg Ganite

328±2 Ma, Zrn, evap., B07

Lalling Granite

322±6 Ma, Zrn, evap., S08

Metten Granit

321±4 Ma, Zrn, evap., S08

Metten Granite

324±5 Ma, Zrn, evap., S08

Sattelpeilnstein Granite

322±3 Ma, Zrn, evap., S08

Miltach Granite

322±4 Ma, Zrn, evap., S08

Arnbruck Granite

325±2 Ma, Zrn, evap., S08

Lusen Granite

325±4 Ma, Zrn, evap., S08

Finsterau Granite I

324±2 Ma, Zrn, evap., S08

Finsterau Granite II

326±2 Ma, Zrn, evap., S08

Haidmühle Granite

321±2 Ma, Zrn, evap., S08

Haidel Granite

323±3 Ma, Zrn, evap., S08

Saldenburg Granite

315±3 Ma, Zrn, evap., C04

Tittling Granite

323±1 Ma, Zrn, evap., C04

Hautzenberg Granite

320±3 Ma, Zrn, ID-T, K08

Patersdorf Granodiorite

325±3 Ma, Zrn, evap., K08

South Bohemian Batholith
Gebharts Diorite

328±1 Ma, Zrn, ID-T, G03

Eisgarn Granite Aalfang

328±1 Ma, Mnz, ID-T, G03

Sulzberg Granite

326±1 Ma, Mnz, ID-T, G03

Migmagranite Uttendorf

323±1 Ma, Zrn, ID-T, G03

Haibach Granite

316±1 Ma, Zrn, ID-T, G03

Altenberg Granite

315±1 Ma, Mnz, ID-T, G03

Mauthausen Granite

316±1 Ma, Mnz, ID-T, G03

Peuerbach Tonalite

314±4 Ma, Mnz, ID-T, F96

Weinsberg Granite

Kirchberg

328±1 Ma, Mnz, ID-T, G03

Langschlag

323±4 Ma, Mnz, ID-T, F97

Oberneukirchen

323±1 Ma, Mnz, ID-T, G03

Pregarten

323±1 Ma, Zrn, ID-T, G03

Sarleinsbach

322±4 Ma, Zrn, Shrimp, F03

Sarleinsbach (mafic)

323±1 Ma, Zrn, ID-T, F03

Data sources: Breiter et al. 2007 (B07); Chen & Siebel 2004 (C04); Dörr & Zulauf 2008 (D08); Friedl 1997 (F97); Förster 1998 (F98);
Finger et al. 2003 (F03); Gerdes et al. 2003 (G03); Hofmann et al. 2008 (H08); Kovaříková et al. 2007 (K07); Klein et al. 2008 (K08); Pro-
pach et al. 2000 (P00); Romer et al. 2007 (R07); Siebel et al. 2003 (S03); Siebel et al. 2006 (S06); Siebel et al. 2008 (S08); Tichomirova
1997 (T97). Zrn – Zircon; Mnz – Monazite; Urnt – Uraninite; ID-T – U-Pb dating by isotope dilution-thermal ion mass spectrometry;
L-ICP – U-Pb dating by Laser ICP mass spectrometry; evap. – zircon evaporation age.

209

SAXO-DANUBIAN GRANITE BELT: POST-COLLISIONAL DELAMINATION OF MANTLE LITHOSPHERE

rocks  are  associated  with  the  older  coarse-grained  granites.
Such rocks are locally termed “Redwitzite” in the Fichtelge-
birge  and  Erzgebirge  (Siebel  et  al.  2003;  Kovaříková  et  al.
2007) and “Migmagranite” or “group 1 diorites” in the South
Bohemian Batholith (Frasl & Finger 1991). They are gener-
ally  considered  to  contain  melt  components  from  the  en-
riched  mantle  (Krenn  2000;  Siebel  et  al.  2003;  Sapp  2005;
Kovaříková et al. 2007).

Plutonic evolution

The plutonic evolution of the SDGB started by the end of

the Visean, with the formation and ascent of large volumes of
high-T,  lower  crustal  magmas,  that  crystallized  mostly  as
coarse-grained K-feldspar-phyric granites. These coarse “early
granites” intruded more or less simultaneously over the whole
length of the SDGB. Based on the available geochronological
information (Table 1) it can be estimated that two thirds of the
SDGB were created within a fairy short time span of no more
than  8  million  years,  between  328  and  320 Ma.  This  is  a
strong argument for the existance of a powerful and rapidly in-
troduced  heat  anomaly  below  this  area.  A  rapid  temperature
increase in the source region can also be inferred from the ob-
servation that magmas of different melting behaviour (I- and
S-type  granites;  diorites)  intruded  contemporaneously.  Since
other parts of the Bohemian Massif were magmatically quiet
at that time, it would appear that this strong late Visean/early
Namurian heat anomaly had developed only below the south-
western sector of the massif.

The geochronological data give interesting information on

the  growth  history  of  the  SDGB.  It  would  appear  that  the
SDGB came into being with a fairly narrow chain of ca. 325 to
328  Ma  plutons  on  the  north  (see  black  triangles  in  Fig.  1),
and then grew asymmetrically in a roughly south-westerly di-
rection (see arrows in Fig. 1). Granites of the second generation
(310  to  317  Ma)  occur  preferentially  along  the  south-western
periphery of the SDGB.

In search of a tectonic scenario

The extraordinary abundance of late- to post-tectonic gran-

ites  is  a  distinct,  though  still  little  understood  and  much  dis-
cussed  feature  of  the  Variscides  (e.g.  Zwart  &  Dornsiepen
1980; Henk et al. 2000). Various tectonic scenarios have been
invoked to explain it. These include anomalously strong crust-
al  thickening  plus  radiogenic  heating  (Gerdes  et  al.  2000),  a
late Variscan Andean-type subduction setting (Finger & Stey-
rer  1990),  and  post-collisional  delamination  of  mantle  litho-
sphere  (Henk  et  al.  2000).  Worldwide,  delamination  is  now
rated as a newly recognized mechanism capable of explaining
the  formation  of  large  igneous  provinces  (Anderson  2005).
Although discussed largely on a theoretical basis, and without
precise space and time constraints, the idea of delamination is
increasing  popular  for  the  Variscan  fold  belt  (Zulauf  1997;
Schott  &  Schmeling  1998;  Arnold  et  al.  2001;  Massonne
2005;  Medaris  et  al.  2005),  because  it  can  also  explain  the
widely  observed  formation  and  fast  exhumation  of  HP-HT

rocks during the Carboniferous, and their immediate re-sedi-
mentation in foreland basins, as well as the widespread LP-HT
metamorphic overprint of the Variscan basement.

Regarding the Bohemian Massif it was a shortcoming of

many previous studies that the Variscan granites were treat-
ed as a single genetic entity. We suggest here a subdivision
into different (at least five) plutonic belts of slightly but sig-
nificantly different age, each having its individual petrogen-
esis  and  tectonic  environment  (see  section  “geological”
background and Fig. 1). Here we focus on the south-western
sector  of  the  massif  and  the  Saxo-Danubian  Granite  Belt.
The widely synchronous production of high-T lower crustal
melts over a ca. 400 km distance is the more remarkable, as
the western, the central and the eastern sector of the SDGB
are located in different plate tectonic terranes (Saxothuring-
ian,  Bohemian  and  Moldanubian).  From  the  existing  data
(Franke 2000, and references therein) it is clear that the colli-
sional  thickening  histories  of  these  terranes  were  not  uni-
form.  For  example,  crustal  thickening  in  the  south-eastern
Bohemian Massif was effected by the Moravo-Moldanubian
folding  phase  (340 Ma)  whereas  in  the  Bohemian  terrane  it
occurred much earlier (~ 380 Ma). These facts are clearly at
variance with models that designate collisional crustal thick-
ening and radiogenic heat production as the main reasons for
granite  magma  formation  in  the  south-western  Bohemian
Massif.  Since  late  Variscan  subduction  models  have  turned
out  to  be  unlikely  for  the  Bohemian  Massif  (Franke  2000),
we hold the view that a process of post-collisional delamina-
tion  of  lithospheric  mantle  (Bird  1979;  Houseman  et  al.
1981;  Henk  et  al.  2000)  can  probably  best  explain  the  plu-
tonic phenomena observed in the SDGB.

Exploring a delamination model

The basic idea of a delamination model is that the bottom

layer of an orogenically thickened continental lithosphere is
denser  than  the  asthenosphere  and  can  detach  and  sink  into
the  asthenospheric  mantle.  Based  on  numerical  modelling,
Bird (1979) stated that detachment will be facilitated if a tec-
tonic  event  first  creates  an  elongated  conduit  that  connects
the hot asthenosphere with the base of the crust. Such a pro-
cess would create a thermal anomaly with near-linear geom-
etry at the base of the crust, along which voluminous crustal
melting  can  be  expected.  The  observation  that  the  SDGB
was  fairly  narrow  during  its  early  evolution  (see  above),  is
compatible with this scenario. It is feasible that the oldest in-
trusions (black triangles in Fig. 1) trace the “line” where the
lithospheric mantle was initially ruptured.

Once the lithospheric mantle layer is disrupted and penetrated

by a channel of hot asthenosphere, the process of delamination
can proceed further (Bird 1979). In the case of the south-west-
ern Bohemian Massif, one may conclude, from the south-west-
ward migration of the plutonic activity, that mantle lithosphere
has peeled away in this direction (Gerdes et al. 2006).

There are several ways in which instabilities and failures

in the mantle lithosphere below the Bohemian Massif could
have developed. For example, instabilities could occur in re-
sponse to the end of early Variscan subduction (slab break-

210

FINGER, GERDES, RENÉ and RIEGLER

off or slab roll-back), in connection with the gravitational
collapse of the overthickened Teplá-Barrandian block (Dörr
& Zulauf 2008), or due to large-scale plate rotations docu-
mented ca. 330 Ma (Edel et al. 2003; Finger et al. 2007). The
shape of the SDGB in map view, bent around the core of the
Bohemian Massif (Fig. 1), is a puzzling feature in this con-
text, the tectonic significance of which remains unknown.

An interesting but as yet unresolved question is, to what

extent mantle material below the SDGB might have melted.
Theoretically, delamination creates favourable conditions for
the partial melting of both the rising asthenospheric and the
decending lithospheric mantle (Kay & Kay 1993; Anderson
2005). In case that some lithospheric mantle material re-
mained at the crust-mantle boundary (cf. the thermal bound-
ary layer delamination model of Houseman et al. 1981), this
mantle material can also be expected to undergo partial melt-
ing, due to the proximity of the hot asthenosphere.

The scarcity of mafic igneous rocks in the SDGB has often

been taken as an argument against the occurrence of mantle
melting (Gerdes et al. 1996), and also against a delamination
model. However, one should be careful with this conclusion,
as  it  cannot  be  ruled  out  that  large  volumes  of  mantle-de-
rived melts were ponding at the crust/mantle boundary and,
for whatever reason, did not rise into the crust. Vielzeuf et al.
(1990) have argued that lower crustal melting will normally
extract heat from coexisting mantle magmas and cause their
freezing  at  depth.  The  possibility  also  exists  that  consider-
able  amounts  of  mafic  mantle  melts  were  mixed  with,  and
assimilated by, the lower crustal melts. One could speculate
that the abundant I/S transitional granites of the SDGB origi-
nated from such mixing processes. On the basis of the regu-
lar occurrence of small bodies of mafic rocks in all parts of
the SDGB, we suggest that mantle melting was a significant
process  and  not  just  a  local  feature.  The  rise  of  hot  mantle
melts  could  have  considerably  increased  the  rate  of  heat
transport  from  the  mantle  delamination  zone  into  the  crust.
Even  if  the  mantle  lithosphere  were  only  partially  removed
(Houseman et al. 1981; Zulauf 1997; Dörr & Zulauf 2008),
so  that  the  asthenosphere  did  not  make  direct  contact  with
the crust, mafic melts derived from the bottom part of the re-
maining  lithosphere  could  have  effectively  transported  heat
to the base of the crust (magmatic underplating).

Finally, it remains to be mentioned that a delamination

process will normally cause a domal uplift of the overlying
crust  (Kay  &  Kay  1993).  Unfortunately,  the  Variscan  P-T
evolution  of  the  south-western  Bohemian  Massif  is  still
poorly understood. However, at least for parts of the Bavari-
an  Zone,  a  rapid  syn-anatectic  uplift  can  be  indirectly  in-
ferred  from  rapid  cooling  documented  by  monazite,  titanite
and mica geochronometry (Kalt et al. 2000).

Conclusions

There are many reasons to propose that the formation of

the SDGB occurred in connection with a large-scale delami-
nation process of lithospheric mantle beneath the south-
western Bohemian Massif. As shown by numerical models,
this process is capable of quickly (within a few million

years) increasing the temperatures at the Moho to around
1000 °C (Arnold et al. 2001). Voluminous fluid-absent low-
er crustal melting (Clemens & Vielzeuf 1987) would inevita-
bly be the consequence, if the lower crustal rocks were
fertile.

Whether the prominent metamorphic high-T events that

occurred  somewhat  earlier  in  the  Variscan  evolution  of  the
Bohemian  Massif  at  ca.  340  Ma  (HP-HT  metamorphism  in
the Moldanubian Gföhl Unit and in parts of the Saxothuring-
ian) and at ca. 335 Ma (LP-HT metamorphism in the Ostrong
Unit) are also connected with delamination processes (Mas-
sonne  2005;  Medaris  et  al.  2005),  is  open  for  debate.  The
fact  that  plutonism  did  not  reach  batholithic  dimensions  in
the  time  span  between  330  and  340  Ma  (note  that  most
granitoids of the Central Bohemian Batholith formed prior to
340  Ma),  may  argue  against   large  scale  heat  introduction
from the mantle. It needs to be explored if other mechanisms
like  slab  break-off  (Janoušek  &  Holub  2007;  Finger  et  al.
2007), radiogenic heating following crustal stacking (Gerdes
et al. 2000), or heat advection through rising HP-HT granu-
lites (O’Brien 2000), may provide equally good tectonother-
mal solutions in these cases.

Acklowledgments:  Constructive  comments  by  John  Clem-
ens,  Igor  Petrík  and  Gernold  Zulauf  helped  to  improve  this
paper  and  are  gratefully  acknowledged.  An  unpublished
data-CD  for  Bohemian  Massif  granites,  compiled  by  J.
Klomínský (Prague), provided useful information.

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