GEOLOGICA CARPATHICA, JUNE 2009, 60, 3, 205—212 doi: 10.2478/v10096-009-0014-3
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
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
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)
, AXEL GERDES
, MILOŠ RENÉ
and GUDRUN RIEGLER
Fachbereich Materialforschung und Physik, Abteilung Mineralogie, Universität Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg,
Institut für Mineralogie, Universität Frankfurt, Senckenberganlage 28, D-60054 Frankfurt a. M., Germany; email@example.com
Academy of Sciences, Institute of Rock Structure and Mechanics, V Holešovičkách 41, CZ-18209 Prague 8, Czech Republic;
(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):
FINGER, GERDES, RENÉ and RIEGLER
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-
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
Geochronological data for the Saxo-Danubian
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).
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
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
Karlovy Vary Pluton
322±2 Ma, Zrn, evap., K07
Karlovy Vary Pluton
323±3 Ma, Zrn, evap., K07
320±6 Ma, Zrn, evap., T97
322±6 Ma, Mnz, ID-T, F98
324±4 Ma, Urnt, ID-T, R07
321±2 Ma, Urnt, ID-T, R07
321±4 Ma, Mnz, ID-T, F98
321±3 Ma, Mnz, ID-T, F98
327±4 Ma, Zrn, L-ICP, H08
Fichtelgebirge, Oberpfalz Forest, Český Les
322±5 Ma, Zrn, evap., S03
321±1 Ma, Zrn, evap., S03
Marktredwitz G1 Granite
324±4 Ma, Zrn, evap., S03
323±1 Ma, Zrn, evap., S03
324±3 Ma, Zrn, evap., S03
328±1 Ma, Zrn, evap., S03
315±1 Ma, Zrn, evap., S03
315±4 Ma, Zrn, evap., S03
315±2 Ma, Zrn, evap., S03
310±3 Ma, Zrn, evap., S03
312±2 Ma, Zrn, evap., S03
312±4 Ma, Zrn, evap., S03
310±6 Ma, Zrn, evap., S03
331±1 Ma, Mnz, ID-T, D08
324±2 Ma, Zrn, evap., S06
315±4 Ma, Mnz, ID-T, P00
321±3 Ma, Mnz, ID-T, P00
323±4 Ma, Mnz, ID-T, S06
323±2 Ma, Zrn, evap., S06
311±2 Ma, Mnz, ID-T, P00
325±2 Ma, Zrn, evap., B07
327±2 Ma, Zrn, evap., B07
328±2 Ma, Zrn, evap., B07
322±6 Ma, Zrn, evap., S08
321±4 Ma, Zrn, evap., S08
324±5 Ma, Zrn, evap., S08
322±3 Ma, Zrn, evap., S08
322±4 Ma, Zrn, evap., S08
325±2 Ma, Zrn, evap., S08
325±4 Ma, Zrn, evap., S08
Finsterau Granite I
324±2 Ma, Zrn, evap., S08
Finsterau Granite II
326±2 Ma, Zrn, evap., S08
321±2 Ma, Zrn, evap., S08
323±3 Ma, Zrn, evap., S08
315±3 Ma, Zrn, evap., C04
323±1 Ma, Zrn, evap., C04
320±3 Ma, Zrn, ID-T, K08
325±3 Ma, Zrn, evap., K08
South Bohemian Batholith
328±1 Ma, Zrn, ID-T, G03
Eisgarn Granite Aalfang
328±1 Ma, Mnz, ID-T, G03
326±1 Ma, Mnz, ID-T, G03
323±1 Ma, Zrn, ID-T, G03
316±1 Ma, Zrn, ID-T, G03
315±1 Ma, Mnz, ID-T, G03
316±1 Ma, Mnz, ID-T, G03
314±4 Ma, Mnz, ID-T, F96
328±1 Ma, Mnz, ID-T, G03
323±4 Ma, Mnz, ID-T, F97
323±1 Ma, Mnz, ID-T, G03
323±1 Ma, Zrn, ID-T, G03
322±4 Ma, Zrn, Shrimp, F03
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.
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).
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-
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).
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
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
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