GeolCarp_Vol66_No3_197

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, JUNE 2015, 66, 3, 197—216 doi: 10.1515/geoca-2015-0020

Upper Cretaceous to Pleistocene melilitic volcanic rocks of

the Bohemian Massif: petrology and mineral chemistry

ROMAN SKÁLA

1,!

, JAROMÍR ULRYCH

, LUKÁŠ ACKERMAN

, LUKÁŠ KRMÍČEK

1,2

FERRY FEDIUK

, KADOSA BALOGH

and ERNST HEGNER

Institute of Geology of the Czech Academy of Sciences, v.v.i., Rozvojová 269,165 00 Praha 6, Czech Republic;

skala@gli.cas.cz; ulrych@gli.cas.cz; ackerman@gli.cas.cz; krmicek@gli.cas.cz

Brno University of Technology, Faculty of Civil Engineering, Veveří 95, 602 00 Brno, Czech Republic

Geohelp, Na Petřinách 1897, 162 00 Praha 6, Czech Republic

Institute of Nuclear Research, Hungarian Academy of Sciences, Bem tér 18/C, H-4026 Debrecen, Hungary; balogh.kadosa@atomki.mta.hu

Department für Geowissenschaften, Universität München, Theresienstraße 41, D-8033 München, Germany; hegner@lmu.de

(Manuscript received November 27, 2014; accepted in revised form March 12, 2015)

Abstract: Upper Cretaceous to Pleistocene volcanic rocks of the Bohemian Massif represent the easternmost part of the
Central European Volcanic Province. These alkaline volcanic series include rare melilitic rocks occurring as dykes, sills,
scoria cones and flows. They occur in three volcanic periods: (i) the Late Cretaceous to Paleocene period (80—59 Ma)
in northern Bohemia including adjacent territories of Saxony and Lusatia, (ii) the Mid Eocene to Late Miocene
(32.3—5.9 Ma) period disseminated in the Ohře Rift, the Cheb—Domažlice Graben, Vogtland, and Silesia and (iii) the
Early to Late Pleistocene period (1.0—0.26 Ma) in western Bohemia. Melilitic magmas of the Eocene to Miocene and
Pleistocene periods show a primitive mantle source [(

143

Nd/

144

Nd)

= 0.51280—0.51287; (

Sr/

Sr)

= 0.7034—0.7038)]

while those of the Upper Cretaceous to Paleocene period display a broad scatter of Sr—Nd ratios. The (

143

Nd/

144

Nd)

ratios (0.51272—0.51282) of the Upper Cretaceous to Paleocene rocks suggest a

partly heterogeneous mantle source, and

their (

Sr/

Sr)

ratios (0.7033—0.7049) point to an additional late- to post-magmatic hydrothermal contribution. Major

rock-forming minerals include forsterite, diopside, melilite, nepheline, sodalite group minerals, phlogopite, Cr- and
Ti-bearing spinels. Crystallization pressures and temperatures of clinopyroxene vary widely between ~ 1 to 2 GPa and
between 1000 to 1200 °C, respectively. Nepheline crystallized at about 500 to 770 °C. Geochemical and isotopic simi-
larities of these rocks occurring from the Upper Cretaceous to Pleistocene suggest that they had similar mantle sources
and similar processes of magma development by partial melting of a heterogeneous carbonatized mantle source.

Key words: Bohemian Massif, Cenozoic volcanism, melilitic rock, petrology, mineralogy, isotope geochemistry.

Introduction

Melilitic ( > 10 vol. % of modal melilite) and melilite-bearing
(1—10 vol. % of modal melilite – collectively referred to as
melilitic below) olivine rocks generally represent small vol-
ume  volcanic  products  (Dunworth  &  Wilson  1998).  These
rocks are characterized by unusual chemistry and mineralogy
and their origin is subject to debate. According to the model
of Wedepohl (1987), the related olivine nepheline and melilite
magma  in  the  Hessian  Depression  was  formed  at  depths  of
ca.  90 km  in  the  garnet  peridotite  mantle,  at  greater  depths
than  other  basaltic  magmas.  These  data  correspond  to  the
model of generation of the melilitic partial melts from a ther-
mal  boundary  layer  at  the  base  of  the  lithospheric  mantle
(Wilson et al. 1995; Dunworth & Wilson 1998).

Melilitic rocks occur in both oceanic and continental envi-

ronments particularly concentrated in the continental rift set-
ting (e.g. Alibert et al. 1983; Wilson et al., 1995; Keller et al.
2006; Ulrych et al. 2008) and elsewhere. The melilitic rocks
in the continental settings are usually represented by olivine
melilitites, melilite-bearing olivine nephelinites and rare ul-
tramafic melilitic lamprophyres and olivine melilitolites.

The goal of the present paper is to compare the petrology

and mineral chemistry of melilitic rocks of the Bohemian

Massif from Late Cretaceous to Pleistocene periods. Further,
this paper addresses the problem of the melilitic rock tectonic
setting, their magma sources and crystallization history.

Geological setting

Widespread alkaline volcanism in Europe is associated with

the major European Cenozoic Rift System (ECRIS – Prodehl
et al. 1995). It extends for a distance of 1000 km, from Spain to
France, Germany, the Czech Republic and Poland. It is mostly
interpreted as a result of the reaction of the Variscan foreland to
the effects of the Alpine orogeny (e.g. Ziegler 1994; Prodehl
et al. 1995). Rift-related passive asthenospheric upwelling re-
sulted in the generation of large volumes of mantle-derived
magmas (e.g. Wilson & Downes 1991; Lustrino & Wilson
2007; Ulrych et al. 2011). The presence of an active magmatic
source beneath the Bohemian Massif in the western Ohře Rift
area was not confirmed by the seismic studies of Babuška et al.
(2003).The Ohře Rift represents a fundamental Variscan bound-
ary between the Saxothuringian and the Teplá-Barrandian units
in the Bohemian Massif (Ziegler 1994; Babuška & Plomerová
2010). This graben hosts two extensive Tertiary volcanic com-
plexes: the Doupovské hory Mts and the České středohoří Mts.

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Ulrych et al. (2008) interpreted the origin of melilitic rocks

in northern Bohemia from a portion of a depleted mantle
source overprinted by carbonate-rich fluids probably related to
carbonatitic magmatism associated with incipient rifting of the
lithosphere of the Bohemian Massif. Abratis et al. (2009) as-
certained the CO

-dominated type of mantle metasomatism

for melilitic rocks in Vogtland and western Bohemia.

Melilitic volcanic rocks formed in the Bohemian Massif

(Fig. 1) in the Upper Cretaceous and during the whole Cen-
ozoic. Their production culminated in the pre-rift period (Late
Cretaceous  to  Paleocene)  in  the  Ploučnice  River  region  in
northern  Bohemia  (Ulrych  &  Pivec  1997;  Pivec  et  al.  1998;
Ulrych et al. 2008, 2014) and in the late-rift period (Early to
Late  Pleistocene)  in  western  Bohemia  (Ulrych  et  al.  2000a,
2011,  2013).  A  non-melilitic  ultramafic  rock  association  of
Cretaceous age is known from the western part of the Outer
Western  Carpathians  (Szopa  et  al.  2014).  Their  concentra-
tions  are  associated  with  the  junctions  of  grabens  and  fault
zones. In small quantities, melilitic rocks appear in the Ohře
Rift  and  adjacent  areas  of  the  Krušné  hory  Mts/Erzgebirge,
Vogtland (Locality 6 in Fig. 1) and Lusatia, eastern shoulder
of  the  Cheb—Domažlice  Graben  and  the  Labe/Elbe—Odra/
Oder Fault Zone. These rocks are of Eocene to Miocene age

and correspond to the main-rift period of Cenozoic volcanic
activity of the Bohemian Massif (Ulrych et al. 2008).

Metasomatism of the lithospheric mantle beneath the Bo-

hemian Massif was ascertained by several studies suggesting
mantle  enrichment  by  silicate  (e.g.  Ackerman  et  al.  2007,
2014; Puziewicz et al. 2011; Medaris et al. 2014) and/or car-
bonate-rich  melts  (Geissler  et  al.  2007;  Matusiak-Malek  et
al. 2010; Ackerman et al. 2013). The presence of metasomat-
ically  transformed  upper  mantle  is  further  supported  by  the
occurrence  of  phlogopite-bearing  clinopyroxenite  xenoliths
(Ulrych et al. 2008) and those of lherzolite xenoliths with am-
phibole and/or phlogopite (Krammer & Seifert 2000; Geissler
et al. 2008).

Geological characteristics of sampled localities

Pleistocene volcanism associated with the junction of the
Cheb—Domažlice Graben and the Ohře Graben/Rift in the
Cheb Basin area, western Bohemia

Pleistocene volcanic activity in western Bohemia is related

to the Cheb Fault that bounds the Cheb Basin to the west and

Fig. 1. Geological sketch map of the Bohemian Massif showing occurrences of the Upper Cretaceous and Cenozoic melilitic rocks.
BM – Bohemian Massif, OR – Ohře Rift, Ch—DG – Cheb—Domažlice Graben. Sampling sites: 1 – Komorní hůrka Hill at Františkovy
Lázně, 2 – Železná hůrka Hill and Mýtina near Cheb, 3 – Český Chloumek near Karlovy Vary, 4 – Podhorní vrch Hill near Mariánské
Lázně, 5 – Příšovská homolka Hill near Plzeň, 6 – Vogtland, Germany, 7 – Krkavčí skála Hill near Sebuzín, 8 – Pohoř Hill at Odry,
9 – Osečná Complex, near Liberec, 10 – Jiřetín pod Jedlovou and Stožec Hill, 11 – Zeughausgang near Hinterhersmsdorf, Germany,
12 – Pomological Garden in Görlitz, Germany.

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limits the Cheb—Domažlice Graben subparallel to the
Mariánské Lázně Fault Zone in the east.

Pleistocene melilitic rock series

Komorní hůrka (Locality 1 in Fig. 1) at Františkovy

Lázně is a monogenetic Strombolian cinder cone (Hradecký
1994; Gottsmann 1999) with preserved lava-filled conduit
and a single lava flow. Both lapilli and lava are composed of
melilitic rocks (Ulrych et al. 2013). K/Ar dating of the lava
yielded ages ranging from 1.02 Ma (Ulrych et al. 2013) to
0.26 Ma (Šibrava & Havlíček 1980).

Železná hůrka (Locality 2 in Fig. 1) is a cinder cone oc-

curring near Cheb. The Strombolian activity producing strat-
ified  tephra  to  scoria  evolved  into  the  Hawaiian  type  of
eruptions  represented  by  coarse-grained  black  spatter.  The
youngest  sequence  is  formed  by  welded  scoria  (Hradecký
1994;  Schwarzkopf  &  Tobschall  1997).  Melilitic  scoria
yielded a K/Ar age of  ~ 1.0 Ma (Šibrava & Havlíček 1980).

The Mýtina (Locality 2 in Fig. 1) locality of pyroclastic

deposits lies ~ 0.5 km E of the Železná hůrka volcano. Geis-
sler et al. (2008) speculated that the Mýtina “tuff-tephra” de-
posit erupted together with the Železná hůrka scoria cone.
Mrlina et al. (2009) interpreted it as part of an independent
Pleistocene ( ~ 288 ka – Ar/Ar laser dating) maar structure.

Oligocene to Miocene volcanism of the Ohře/Eger Rift, the
Cheb—Domažlice Graben including Vogtland, Saxony and
the Labe—Odra Fault Zone

The Ohře Rift/Graben

The only known locality of melilitic rocks in the Ohře Rift

is Krkavčí skála (Locality 7 in Fig. 1) (28.7 Ma – Lustrino
& Wilson 2007) at Sebuzín in the České středohoří Mts. The
~

100 m long and 6—8 m thick NE—SW-striking dyke(s) of

melilite-bearing olivine nephelinite penetrates the Upper Cre-
taceous sediments in a brecciated zone filled with nepheline
basanite.

The Cheb—Domažlice Graben shoulder, western Bohemia

Relicts of a volcanic edifice of melilite-bearing olivine

nephelinite  to  olivine  nephelinite  composition  near  Český
Chloumek  (Locality 3  in  Fig. 1)  (16.5 Ma  –  Lustrino  &
Wilson  2007)  can  be  associated  with  continuation  of  the
Litoměřice Deep Fault.

Melilite-free olivine nephelinite from the Podhorní vrch

volcano (Locality 4 in Fig. 1) (12.4 Ma – Lustrino & Wilson
2007)  lies  in  the  neighbourhood  of  the  Mariánské  Lázně
Fault.  Here,  the  pegmatoid  segregations  of  ijolite  composi-
tion in the olivine nephelinite parent contain mineral associ-
ation  of  nepheline + diopside + melilite ± olivine,  magnetite,
apatite and sodalite (Ulrych et al. 2000b).

Melilitic volcanism related to the Cheb—Domažlice Gra-

ben continues to Aš in the Cheb Basin area and further to
the South Vogtland Trough in Saxony (Abratis et al. 2009,
2013) where the Lower Miocene (19.5 Ma) melilitic rocks
form dykes, rare plugs and diatremes.

The Příšovská homolka (Locality 5 in Fig. 1) (5.9 to

7.2 Ma – Ulrych et al. 2013) explosive volcano near Plzeň
in the southern part of the Cheb—Domažlice Graben shoulder
produced  two  sequences  of  pyroclastic  products.  Younger
dykes  (0.5  to  1 m  thick)  of  basanite  to  olivine  nephelinite
composition  penetrate  the  tuffites.  The  presence  of  altered
melilite  in  the  rock,  discussed  by  Ulrych  et  al.  (2013),  has
not been confirmed yet.

The Labe—Odra Fault Zone

The occurrences of melilitic rocks in Moravia and Silesia

are very rare. The only currently accessible body is the ba-
saltic dyke of Pohoř Hill at Odry (Locality 8 in Fig. 1)
(32.3 Ma – Ulrych et al. 2013).

Late Cretaceous to Paleocene volcanism of the Ploučnice
River region (the Osečná Complex), northern Bohemia

Dykes of melilitic rocks, including ultramafic melilitic

lamprophyres—polzenites,  occur  in  the  Ploučnice  River
region in northern Bohemia, e.g. Osečná Complex (Locality 9
in  Fig. 1  –  Ulrych  et  al.  2008),  Jiřetín  pod  Jedlovou  and
near Stožec Hill (Locality 10 in Fig. 1) and Zeughausgang
near Hinterhermsdorf (Locality 11 in Fig.1 – Seifert et al.
2008), and Pomological Garden in Görlitz (Locality 12 in
Fig. 1  –  Seifert  et  al.  2008)  mostly  associated  with  the
Lusatian Fault. These rocks are concentrated in the Osečná
Complex  (Locality 9  in  Fig. 1)  situated  at  the  intersection
of  the  Lusatian  Fault  with  the  Ohře  Rift  (Ulrych  &  Pivec
1997;  Ulrych  et  al.  2008,  2014).  The  Osečná  Complex  is
formed by a central lopolith-like intrusion (Ulrych & Pivec
1997).  The  central  part  is  composed  of  medium-,  rarely
coarse-grained  to  porphyritic  olivine  melilitite  with  rare
pods  and  dykes  of  melilitic  pegmatoids,  glimmerites  and
ijolites.  The  dykes  of  the  Devil’s Walls  Dyke  swarm  of
porphyritic melilite-bearing olivine nephelinite to olivine me-
lilitite  composition  are  spatially  associated  with  the  Osečná
Complex.

Analytical procedures

Whole-rock major element concentrations were deter-

mined  using  the  wet  chemical  method.  Analyses  of  the
USGS  international  rock  standard  BCR-2,  and  duplicate
analyses  of  the  samples,  yield  total  procedure  errors  of
± 10 % (2

σ). A quadruple-based ICP-MS (Thermo XSeries)

was used for determination of REE and other trace elements
using  the  methods  outlined  in  Strnad  et  al.  (2005).  The  in-
run  precision  of  the  analysed  elements  was  always  better
than  ± 5 %  (2

σ). The accuracy of the analyses was moni-

tored by replicate analyses of the USGS international refer-
ence material BCR-2 and was better than ± 10 % (2

σ).

Mineral analyses were carried out on a CAMECA SX 100

wavelength-dispersive electron probe microanalyser. Ana-
lytical conditions were as follows: 15 kV accelerating volt-
age, 10 nA beam current and 2 µm beam diameter. Synthetic
phases and natural minerals were used as standards.

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The Sr/Nd isotope compositions were determined according

to the procedures outlined in Hegner et al. (1995). The Sr/Nd
isotopic compositions were determined with a Finnigan MAT
261 using a dynamic triple mass method for

143

Nd/

144

ratios measurements and dynamic double mass method for

Sr/

Sr ratios.

The K/Ar age determinations were carried out using the in-

struments and methods described in Balogh (1985). The accu-
racy  and  reliability  of  the  measurements  were  shown  by  the
results of an interlaboratory calibration project published by
Odin  et  al.  (1982).  The  K/Ar  results  presented  in  this  study
have  been  calibrated  using  standards  LP-6,  HD-B1  and  Asia
1/65, as well as atmospheric argon.

Results

Petrography

Concise petrographic and geological characteristics of the

Upper Cretaceous to Pleistocene melilitic and melilite-bear-
ing  rocks  are  presented  in  Table 1.  The  samples  are  mostly
micro-porphyritic with a fine-grained groundmass and chemi-
cally  homogeneous.  Most  of  them  have  a  common  simple
modal  composition.  A  brief  presentation  of  the  main  rock-
forming minerals in individual rock samples is presented in
Table 1.  The  results  of  the  study  of  the  main  rock-forming
minerals are listed in the following section.

Main rock-forming minerals

Representative analyses of the minerals of the studied

melilitic  rocks  from  the  Bohemian  Massif  are  presented
in  Supplement 1  (Supplements 1—3  available  online  at
www.geologicacarpathica.com).  Analyses  of  rock-forming
minerals  of  melilitic  rocks  from  the  Ploučnice  River  region
published  by  Ulrych  et  al.  (1986,  1988,  1991,  1994)  and
from Vogtland published by Abratis et al. (2009) were used
for comparison. Mutual spatial relationships of selected fun-
damental minerals as shown in optical micrographs and back-
scattered electron images are presented in Supplement 2.

Olivine was observed prevalently in the form of weakly

corroded euhedral to subhedral hopper-like rarely aggregated
phenocrysts  passing  to  rare  grains  of  groundmass  size.  In
scoria and lapilli from the Železná hůrka and Komorní hůrka
cinder cones, olivine occurs as corroded (micro)phenocrysts
with  tiny  glass  inclusions.  Titanian  magnetite  grains  often
concentrate  at  the  contact  between  olivine  phenocrysts  and
groundmass.

Phenocrysts exhibit a normal type of compositional zon-

ing with subhedral Mg-rich cores and Fe-rich rims. Forsterite
(Fo)  contents  usually  vary  between  82  and  89 mol %.  Be-
sides  typical  magmatic  olivine,  variably  corroded  crystals
with high Fo ( ~ 90; up to 92 mol %) were found in cores of
phenocrysts, rarely as independent xenocrystic grains. Com-
positional  trends  of  olivine  phenocrysts  and  xenocrysts  are
visible  in  the  100 Mg/(Mg + Fe)  vs.  NiO  diagram  (Fig. 2).
Olivine  xenocrysts  show  markedly  elevated  NiO  contents
(up  to  0.55 wt. %),  whereas  phenocrysts  follow  a  fraction-

ation trend characterized by decreasing NiO and MgO con-
tents from cores to rims. Low CaO contents (usually below
0.2 wt. %) are characteristic for xenocrysts.

Monticellite occurs as rims of olivine and rarely also oc-

curs as individual grains in ultramafic lamprophyres (polzen-
ites) and olivine melilitolite of the Osečná Complex.
Additionally, this mineral was identified in the altered
polzenite from Jedlová railway station.

Clinopyroxene forms (i) subhedral prismatic (micro)-

phenocrysts or (ii) subhedral to anhedral grains and columns
forming  the  prevailing  part  of  the  groundmass.  In  rare  cases
clinopyroxene  rims  olivine.  Clinopyroxene  analyses  corre-
spond  mostly  to  aluminian,  ferrian,   ± ferroan,  ± titanian,
± chromian, subsilicic diopside following the nomenclature of
Morimoto (1988); see Fig. 3. Phenocrysts show different com-
binations of concentric and rare sector-zoning (e.g. lava from
Komorní hůrka and scoria from Železná hůrka). The majority
of  crystals  display  normal  compositional  zoning:  Ti,  Al,  and
Fe

contents and

[4]

Al /

[6]

Al ratios increase whereas Si, Mg

and Na contents decrease from core to rim. Sodium content in-
creases from centre to rim (Na

O from 0.26 to 0.44 wt. %) in

rare  late  magmatic  clinopyroxenes  in  scoria  from  Železná
hůrka  whereas  those  in  lava  from  the  Komorní  hůrka  show
opposite  trends  (from  0.64  to  0.24 wt. %).  The  analysis  of  a
clinopyroxene  phenocryst  from  Krkavčí  skála  provided  an
exceptionally high K

O content of 0.23 wt. %.

Rhönite was found in homogeneous microphenocrysts

spatially associated with clinopyroxene clusters in the meli-
lite olivine nephelinite of Krkavčí skála and tuffite of Pří-
šovská homolka.

Melilite occurs mostly in the form of lath-shaped 10—400 µm

thick and 0.1—1.3 mm long subhedral to euhedral microphe-
nocrysts in the studied melilitic rocks. Sector zoning is locally
present (e.g. scoria from Železná hůrka). Laths of melilite are
surrounded by nepheline and rimmed by tiny magnetite and

Fig. 2.  Olivine  from  melilitic  rocks  of  the  Bohemian  Massif  in
terms of 100 Mg/(Mg + Fe) vs. NiO (wt. %) compared with the Ha-
waiian high-Ni olivine and MORB olivine, along with olivine frac-
tionation  and  ultradepletion  trend  (compiled  by  Prelević  et  al.
2013).  The  peridotite  melting  box  is  from  Herzberg  (2011),  pyro-
xenite melting box from Straub et al. (2008).

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Fig. 4. Melilite from melilitic rocks of the Bohemian Massif in the
å

kermanite—ferro-åkermanite—soda-melilite (mol %) diagram (a).

Shaded regions illustrate the composition of melilites from common
volcanic rocks as defined by El Goresy & Yoder (1974). Diagram Na
vs. Mg/(Mg + Fe) (b) shows separation of analytical data into several
groups correlated partly with the age. Symbols as in Fig. 2.

rarely perovskite grains. Their bundles in the groundmass fol-
low the fluidal arrangement in some cases. A peg structure of
melilite is emphasized in melilitic rocks from the Osečná
Complex and Pohoř only in association with the late-mag-
matic hydrothermal phase (Ulrych et al. 1991).

The studied melilites show a narrow variation in chemical

composition. Dominant end-members of these melilites are
å

kermanite and soda-melilite accounting for 80 mol % or

more. The contents of the soda-melilite component are higher
in melilites of the youngest rocks and lower in Upper Creta-
ceous to Paleocene rocks. Other components are less impor-
tant,  yet  ferro-åkermanite  displays  the  highest  contents
among  them  in  general  (Fig. 4a).  Aluminium  content  in  T1
site  is  high,  but  does  not  attain  the  level  substantiating  the
presence of alumo-åkermanite (Fig. 4a). The studied melilites
also  commonly  show  a  zoning  pattern  characterized  by  an
increase of the soda-melilite component and Al content and a
decrease  of  Mg  from  core  to  rim.  The  Na  vs.  Mg/(Mg+Fe)
ratio  (Fig. 4b)  shows  a  separation  of  melilites  into  several
groups, partly correlating with age.

Nepheline, haüyne and sodalite are the dominant felds-

pathoids of groundmass, yet they rarely also form microphe-
nocrysts.

Nepheline is a common feldspathoid of the melilitic rocks

studied. It occurs as rare microphenocrysts and/or anhedral
fillings/grains in groundmass rich in Na

O. It locally replaces

melilite. However, it is missing or very rare in scoria from

Fig. 3. Clinopyroxene from melilitic rocks of the Bohemian Massif in the quadrilateral diagram of Morimoto (1988). Symbols as in Fig. 2.

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Fig. 5.  Nepheline  from  melilitic  rocks  of  the  Bohemian  Massif  in
the Ne—Ks—Qz diagram (in mol %). The dashed line identifies “Barth
join” defined by Dollase & Thomas (1978). Tie-lines illustrate lim-
its  of  nepheline  solid  solution  at  the  shown  temperatures  and  ap-
proximate  limit  at  1068 °C  and  10 MPa  (modified  after  Hamilton
1961 and Blancher et al. 2010). Symbols as in Fig. 2.

Fig. 6. Sodalite-group minerals including their alteration products
from melilitic rocks of the Bohemian Massif in K

O—Na

O—CaO

diagram. Field for haüyne (H), sodalite (S), nosean (N), and lazurite
(L) after Lessing & Grout (1971). Symbols as in Fig. 2.

the  Komorní  and  Železná  hůrka.  End-members  in  the  Ne—
Ks—An—Qz  tetrahedron  were  recalculated  adopting  a  proce-
dure  described  in  Blancher  et  al.  (2010).  The  studied
nephelines  are  chemically  quite  variable;  the  content  of
nepheline  end-member  varies  between 65  and  83 mol %,  the
content of kalsilite component between 9 and 29 mol %, silica
end-member between 0.5 and 9 mol %, and anorthite compo-
nent was found to be between 0 and 11.5 mol %. The highest
chemical  variability  was  recorded  for  Upper  Cretaceous—
Paleocene rocks; Pleistocene volcanic rocks display the low-
est variability (Fig. 5). Projection onto the Ne—Ks—Qz plane
shown  in  Fig. 5  demonstrates  that  nephelines  are  mostly
Ne-depleted with a considerable number of data plotting away
from the “Barth join” representing natural nephelines (Dollase
&  Thomas  1978).  The  temperatures  of  nepheline  crystalliza-
tion  concentrated  mostly  between  500  and  700 °C  were  esti-
mated from the isotherms defined in Hamilton (1961).

Analyses of sodalite group minerals are shown in a dia-

gram  after  Lessing  &  Grout  (1971)  in  Fig. 6.  In  microphe-
nocrysts, haüyne forms the centres being rimmed by sodalite
on  margins.  The  intermediate  members  chemically  close  to
noseane  are  present  in  the  olivine  melilitolites  and  micro-
melilitolites of the Osečná Complex (Ulrych et al. 1991) and

melilitic pegmatoids of the Podhorní vrch volcano (Ulrych et
al. 2000b). Corroded and chemically heavily altered haüyne
microphenocrysts are dispersed as a minor phase in ground-
mass of scoria and lapilli from the Železná hůrka and Komorní
hůrka volcanoes; haüyne is missing in the lava from the latter
locality. Haüyne has been also described as microphenocrysts
from melilitic rocks from Vogtland by Abratis et al. (2009).

Following the classification diagram of Tischendorf et al.

(2007),  the  studied  micas  belong  to  the  phlogopite—annite
series  (Fig. 7).  They  occur  rarely  as  irregular  fragments  of
lamellae  in  melilite-bearing  olivine  nephelinites.  Abratis  et
al.  (2009)  reported  subhedral  microphenocrysts  of  phlogo-
pite  from  similar  melilitic  rocks  from  Vogtland.  Phlogopite
was also found as uneven fragments in the porous upper part
of the Komorní hůrka lava flow, concentrated along the rims
of olivine microphenocrysts. These phlogopites are generally
high in Mg (average Mg# = 0.78), Al ( ~ 1.7—3 apfu with av-
erage  ~ 2.6)  and  low  in  Si  ( ~ 4.5—5.8 apfu  with  average
~

5.3; see Fig. 8, Supplement 1).

Phlogopite is present in substantial amounts in the ground-

mass  of  olivine  melilitolites  and  polzenites  of  the  Osečná
Complex. At this locality, phlogopite occurs in at least two
generations, which differ in composition (Pivec et al. 1998).
Early  phlogopite  is  characterized  by  high  Mg#  values  (Mg/
Mg + Fe;  ~ 0.9) and low Ba and Ti contents. It is partly replaced
by  newly  formed  phlogopite,  which  has  a  lower  Mg#  value
(0.81—0.87) and high Ba and Ti contents. High BaO concen-
tration (up to 16 wt. %) consistent with almost 50 mol % con-
tent  of  kinoshitalite  end-member  was  observed  mostly  at  the
margins of phlogopite flakes from the Komorní hůrka lava.

Fig. 7. Classification diagram of Tischendorf et al. (2007) for micas
from melilitic rocks of the Bohemian Massif. Symbols as in Fig. 2.

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Garnet occurs exclusively in olivine melilitolites, their

pegmatoids  and  glimmerites  of  the  Osečná  Complex  as  a
typical late-magmatic phase (Ulrych et al. 1994). Zirconium
rich  melanite  cores  are  rimmed  by  oscillating  zones  of
(F,OH)-bearing  Ti-poor  andradite,  (F,OH)-bearing  titanian
andradite and Ti-rich andradite.

The spinel-group minerals of the melilitic rocks are repre-

sented by the spinel series, consisting of only slightly resorbed
Cr—Al-spinel  cores  overgrown  by  Mg—Al-titanian  magnetite
(cf. Dunworth & Wilson 1998; Abratis et al. 2009), see Fig. 9.
Titanian  magnetite  of  several  generations  occurs  mostly  as
tiny  isometric  euhedral,  subhedral  and  more  rarely  anhedral
grains often concentrated at the contact of olivine phenocrysts
and groundmass. Cr—Al-spinel occurs as euhedral to subhedral
microphenocrysts forming numerous inclusions in the olivine
phenocrysts.  Abratis  et  al.  (2009)  reported  a  similar  spinel-
group  minerals  association  from  melilitic  rocks  from  Vogt-
land, and Seifert et al. (2008) from Görlitz and Zeughausgang.
Spinel grains from olivine melilitolite, polzenite and melilite-
bearing olivine nephelinite of the Osečná Complex show two
distinct  zones;  an  (Mg,Fe)—Al-chromite  core  and  an  Al—Ti-
magnetite  margin.  Spinels  of  the  olivine  micro-melilitolites
display three zones with a transitional “pleonaste” intermedi-
ate zone between core and margin (Ulrych et al. 1986).

Perovskite occurs very rarely (e.g. Pohoř) in the melilite-

bearing olivine nephelinites and olivine melilitites, or is

Fig. 8. Chemical composition of studied micas showing their compositional variability. Symbols as in Fig. 2.

completely missing. Numerous subhedral to euhedral per-
ovskite crystals occur only as microphenocrysts (3—5 vol. %)
in rare olivine melilitite from Vogtland (Abratis et al. 2009).
Abundant perovskite (1—5 vol. %) with normal zoning is
present in all melilitic rocks of the Osečná Complex (Ulrych
et al. 1988). The most abundant late-magmatic perovskite is
characterized by high Nb

(up to 1.2 wt. %) and REE (up

to 1.3 wt. %) contents in olivine melilitolite pegmatoids. The
postmagmatic light-coloured perovskite overgrowths and in-
completely rims titanian magnetites. It is very low in incom-
patible elements, e.g. Nb

content is about 0.05 wt. %.

Geochemical characteristics

Whole-rock geochemistry

Major and trace element analyses of the melilitic rocks

from the Bohemian Massif are given in Supplement 3. The
low SiO

and Al

, medium alkali (Na

O > K

O) and high

CaO and MgO contents correspond to common geochemical
characteristics for melilitic rocks (e.g. Brey 1978; Dawson et
al. 1985; Dunworth & Wilson 1998; Keller et al. 2006; Mel-
luso et al. 2011). The lowest SiO

contents were found in

majority of rocks from the Osečná Complex (down to
29.9 wt. %) whereas the highest SiO

concentrations were

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Primitive mantle-normalized incompati-

ble  trace  element  plots  for  the  melilitic
rocks of the Bohemian Massif are given in
Fig. 12.  The  rare  earth  element  (REE)  pat-
terns  are  similar  for  all  rocks  without  any
significant variations between those of dif-
ferent  age,  but  La

/Yb

ratios are highly

variable between 25 and 71 with the highest
values  found  in  the  ultramafic  xenoliths
near Jiřetín pod Jedlovou. The highest con-
centrations of light REE (LREE) are associ-
ated  with  late-stage  processes  resulting  in
the formation of pegmatoids of the Osečná
Complex, Podhorní vrch Hill and xenoliths
from Stožec Hill near Jiřetín pod Jedlovou.
While all melilitic rocks show distinct neg-
ative  K  anomalies,  extended  trace  element
patterns  reveal  some  important  differences
between rocks of different age. The Eocene
to  Miocene  volcanic  rocks  have  trace  ele-
ment  patterns  similar  to  the  Pleistocene
rocks,  however,  they  display  much  higher
variation  in  concentrations  and  more  pro-
nounced negative Zr anomalies. On the other
hand,  the  Upper  Cretaceous  to  Paleocene
rocks exhibit very high Ba, Nb and Sr con-
tents producing significant positive anoma-
lies in the trace element patterns (Fig. 12).

Sr/Nd isotopic compositions

The Sr/Nd isotopic ratios of the melilitic

rocks from the Bohemian Massif (Table 2)
are similar to those reported for melilitic
rocks throughout the CEVP (e.g. Massif
Central, Vosges, Urach, Hegau – Alibert
et al. 1983; Hegner et al. 1995; Lustrino &
Wilson 2007; see Fig. 13).

The Sr/Nd isotopic ratios of most melil-

itic rocks of the Pleistocene and the
Eocene to Miocene periods show high
(

143

Nd/

144

Nd)

= 0.51280—0.51287 and low

(

Sr/

Sr)

= 0.7034—0.7038 ratios. How-

ever, the volcanic bomb (MR-1) from the
Pleistocene maar locality of Mýtina
(Ulrych et al. 2013) yielded high

Sr/

of  ~ 0.7041. On the contrary, the Late Cre-
taceous  to  Paleocene  melilite-bearing
rocks  (Osečná  Complex)  display  a  broad
scatter  of  Sr/Nd  isotopic  ratios  with
(

143

Nd/

144

Nd)

between 0.51272—0.51282

and (

Sr/

Sr)

of 0.7033—0.7049. The me-

lilite-bearing rocks of the Devil’s Walls
dykes with low

Sr/

Sr (0.7033—0.7034)

and transitional

143

Nd/

144

Nd (0.51283) plot

between the Upper Cretaceous to Paleocene
rocks (Osečná Complex) and younger
Pleistocene and Eocene to Miocene rocks;
see Fig. 13.

Table 2:

Representative Sr/Nd isotopic data for the

melilitic rocks from the Bohemian Massif.

Abbreviations:

HOM

–

haüyne

olivine

melilitite,

NOM

–

nepheline

olivine

melilitite,

MON

–

melilite

olivine

nephelinite,

–

olivine

nephelin

ite,

OME

–

olivine

melilitolite,

UML

–

ultramafic

lamprophyre,

POL

–

clinopyroxene-free

lamprophyre-polzenite,

CPOL

–

clinopyroxene

lamprophyre-alnöite

(“polz

enite”).

Data sources:

–

this

study,

–

Ulrych

al.

(2013),

–

Ulrych

al.

(2008).

Explanations:

Error

(2SE)

refers

the

last

digits

ratio.

εNd

calculated

with

the

parameter

Bouvier

al.

2008.

The

143

Nd/

144

ratios

were

normalized

146

Nd/

144

0.7219

and

147

Sm/

152

0.56081.

The

143

Nd/

144

ratio

the

in-house

Ames

standard

solution

was

0.51214

35),

corresponding

0.511854

the

Jolla

referen

standard

material.

The

εNd(t)

values

were

calculated

with

the

parameters

Jacobsen

Wasserburg

(1980).

Present-day

ratios

for

the

chondrite

uniform

reservoir

(CHUR)

were:

147

Sm/

144

0.1967,

143

Nd/

144

0.512638

(Jacobsen

Wasserburg

1980;

143

Nd/

144

re-normalized

146

Nd/

144

0.7219).

Sr/

ratios

were

determined

with

dynamic

double

mass

method,

monitoring

Rb,

and

normalized

Sr/

0.1194.

The

NIST

987

reference

material

yielded

Sr/

0.710230

22).

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K/Ar geochronology

K/Ar ages, both new and previously presented (Ulrych et

al. 2008, 2014), show that the melilitic rocks of the Bohemian
Massif formed during the broad period from the beginning in
the Late Cretaceous and continuing to the Pleistocene (Ta-

ble 3). The melilite-bearing olivine nephelinite (26.9 ± 1.1 Ma)
from  Krkavčí  skála  at  Sebuzín  in  the  České  středohoří  Mts
and  olivine  nephelinites  from  Podhorní  vrch  at  Mariánské
Lázně (18.3 ± 1.2 Ma – flow and 17.0 ± 0.8 Ma went) are the
only newly dated rock samples of the Eocene to Miocene pe-
riod. The olivine basanite intrusion from the Krkavčí skála is
substantially  younger  (17.1 ± 1.0 Ma).  The  first  K/Ar  data
from ultramafic lamprophyre (polzenite) from a quarry near
the  Jedlová  railway  station  (68.8 ± 3.3 Ma)  and  a  similar
ultramafic  xenolith  in  the  pipe  breccia  from  Stožec  Hill
(60.5 ± 3.3 Ma) in the Lusatian Fault area confirm their affin-
ity to the pre-rift melilitic magmatism.

Discussion

Melilitic rock setting and age considerations

The melilitic volcanic rocks were formed in the Bohemian

Massif over a wide time period of about 80 Ma. The formation
of these rocks culminated during the initial pre-rifting Late
Cretaceous to Paleocene period and in the late-rifting Pleis-
tocene volcanic episode of the Cenozoic volcanism (Ulrych
et al. 2011). The occurrence of melilitic rocks among volca-
nic rocks of the most widespread Eocene to Miocene syn-rift
period is marginal. The volcanism of both periods when me-
lilitic rocks predominantly formed is associated with junc-
tions of the graben structures.

The setting of the Upper Cretaceous—Paleocene melilitic

rocks of the Ploučnice River region represented exclusively
by dykes and solitary lopolith (sill) are concentrated in the
Osečná Complex associated with the intersection of the Ohře
Rift and the regional Lužice Fault. The volcanism of this pe-
riod predates the activation of the Ohře Rift and occurred in
the rift external blocks of the graben, which formed later dur-

Fig. 13. Measured

Sr/

Sr and

143

Nd/

144

Nd isotopic ratios of the

melilitic rocks from the Bohemian Massif compared to data pub-
lished by Lustrino & Wilson (2007), Haase & Renno (2008), Ulrych
et al. (2008, 2013, 2014). Note the very large variation and in some
cases very high

Sr/

Sr indicating melting of enriched mantle sour-

ces. Fields for melilite-bearing rocks from Urach and Hegau (Alibert
et al. 1983; Hegner et al. 1995; Lustrino & Wilson 2007) and Voges
and Massif Central (Alibert et al. 1983) are plotted for comparison.
DMM – Depleted MORB Mantle, EM I – Enriched Mantle
type I, EM II – Enriched Mantle type II (from Lustrino & Wilson
2007). Symbols as in Fig. 2.

Table 3: Representative K/Ar isotopic age data for the melilitic rocks from the Bohemian Massif. Abbreviations: NOM – nepheline olivine
melilitite, ON – olivine nephelinite, OB – olivine basalt, MON – melilite olivine nephelinite, OME – olivine melilitolite, UML – ul-
tramafic lamprophyre, POL – clinopyroxene-free lamprophyre—polzenite, CPOL – clinopyroxene lamprophyre—alnöite (“polzenite”).
Data sources: 1 – this study, 2 – Ulrych et al. (2013), 5 – Ulrych et al. (2008), 6 – Ulrych et al. (2014).

Data

sources

Sample Locality

Rock type

K (wt. %)

Ar (rad)

cc STP/g

Ar (rad)

(%)

Age ± 1σ (Ma)

Early to Late Pleistocene

Ul-Pr-2

Komorní hůrka Hill

NOM

1.854

0.073×10

–6

20.8

1.01 ± 0.1

Mid Eocene to Late Miocene

WB-22a

Podhorní vrch Hill

ON — feeding channel

0.722

4.778×10

–7

45.2

17.0 ± 0.8

WB-22b

Podhorní vrch Hill

ON — flow

1.180

8.405×10

–7

23.7

18.3 ± 1.2

10.1.

Příšovská homolka

MON?–ON — tuff

0.671

0.154×10

–6

33.3

5.89 ± 0.30

10.2.

Příšovská homolka

MON?–ON

0.333

0.094×10

–6

13.2

7.23 ± 0.77

3/13

Krkavčí skála Hill

MON–ON

1.340

1.413×10

–6

56.8

26.9 ± 1.1

4/13

Krkavčí skála Hill

0.896

5.975×10

–7

26.2

17.1 ± 1.0

13.1.

Pohoř Hill at Odry

MON

0.775

0.981×10

–6

51.1

32.3 ± 1.4

Late Cretaceous to Paleocene

POL-119

Osečná, borehole

OME

1.829

4.691×10

–6

56.7

64.8 ± 2.6

POL-57

Děvín Hill at Hamr

UML–POL Vesecite type

0.971

3.067×10

–6

47.7

79.5 ± 3.5

P-2

Modlibohov

UML–POL Modlibovite type

1.318

3.636×10

–6

40.9

69.5 ± 3.0

P-10

Luhov

UML–CPOL Luhite type

1.102

2.672×10

–6

43.9

61.3 ± 2.6

POL-28

Great Devil’s Dyke

MON

1.241

3.053×10

–6

66.9

62.2 ± 2.4

P-4

Mazova horka Hill

MON

1.142

1.142×10

–6

60.7

61.9 ± 2.4

POL-181

Jiřetín pod Jedlovou

UML–POL

1.174

3.200×10

–6

37.1

68.8 ± 3.3

POL-182

Stožec Hill

UML–POL

0.745

1.783×10

–6

30.6

60.5 ± 3.3

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ing the evolution of the rift system. A magmatic event related
to the initial pre-rift period of Cenozoic rifting of the Bohe-
mian  Massif  proceeded  in  the  range  of  80—59 Ma  in  the
Ploučnice River region. On the basis of whole-rock K/Ar de-
terminations,  a  number  of  authors  (e.g.  Lippolt 1983;  Pivec
et  al.  1998;  Ulrych  et  al.  2008,  2011,  2013)  suggested  that
the  intraplate  volcanic  cycle  associated  with  the  Alpine
Orogeny already started in the Late Cretaceous.

The most voluminous syn-rift alkaline volcanism is asso-

ciated with the main stage of the rifting and concentrates in
the  Ohře  Graben.  Individual  melilitic  dykes  are  only  rarely
present  in  the  Ohře  Rift,  within  the  eastern  shoulder  of  the
Cheb—Domažlice  Graben  and  its  continuation  in  Vogtland
and the Labe—Odra Fault Zone.

Cinder cones with melilitic scoria and lava products associ-

ated with the Early to Late Pleistocene final volcanic episode
(1.0—0.26 Ma) occur in the Cheb Basin area where the thick-
ness of the seismic lithosphere is reduced to ca. 80—90 km
(Babuška & Plomerová 2010).

Prominent association of melilitic magmas with regional

faults and block tectonics of the Bohemian Massif suggests
reactivation of deep lithospheric fracture zones. Such old  in-
homogeneities  may  have  facilitate  the  regional  stress  in  in-
traplate  settings  and  thus  contribute  to  generation  of
magmatic  processes  –  asthenospheric  upwelling,  ascent  of
magma  and  migration  of  late-  and  postmagmatic  fluids
(Adamovič & Coubal 1999). The space- and time-dispersed
melilitic  magmas  of  the  Bohemian  Massif  (Table 4)  were
probably  generated  in  specific  conditions  of  adiabatic  de-
compression melting of the mantle associated with astheno-
spheric  upwelling,  which  might  have  been  triggered  by
lithospheric extension (Wilson et al. 1995)

Melilitic magma generation and its sources

Melilitic magma is a typical small volume volcanic prod-

uct characterized by its peculiar chemical composition (Dun-
worth  &  Wilson  1998).  The  geochemical  signatures  of  the
Late  Cretaceous  to  Pleistocene  melilitic  rocks  of  the  Bohe-
mian  Massif  resemble  those  from  continental  intraplate  set-
tings of ECRIS in Western Europe (Alibert et al. 1983; Wilson
et  al.  1995;  Dunworth  &  Wilson  1998;  Lustrino  &  Wilson
2007). These rocks are commonly interpreted as near-primary
melts  originating  by  low  degree  melting  of  heterogeneous
mantle  sources,  including  both  lithospheric  and  astheno-
spheric mantle components (Lustrino & Wilson 2007).

The chemical composition of the melilitic rocks of the Bo-

hemian Massif fully corresponds to common features of me-
lilitic melts characterized by low SiO

, Al

, Na

O > K

contents accompanied by high CaO, MgO, and CO

contents

as well as high (Ca + Na + K)/Al ratio (Wilson et al. 1995; Di
Battistini et al. 2001; Keshav & Gudfinnsson 2004; Ulrych
et al. 2008). High Mg# and broad variations in the contents
of compatible elements were interpreted by Frey et al. (1978)
to reflect primitive, near-primary upper mantle melts which
typically underwent only limited low-pressure fractional
crystallization.

Regardless of their age and place of occurrence in the Bohe-

mian Massif, the melilitic rocks are enriched in both compati-

ble and incompatible elements, which is a characteristic fea-
ture of melilitic rocks worldwide (Dunworth & Wilson 1998).
The highest La

/Yb

ratios ( ~ 70) of xenoliths from Stožec

Hill near Jiřetín pod Jedlovou are comparable with those of
ijolite  xenoliths  ( ~ 55—65)  from  the  Loučná—Oberwiesenthal
Volcanic  Centre  associated  with  the  Ohře  Rift  (Ulrych  et  al.
2005).  The  steep  slope  of  the  REE  patterns  and  the  high
La

/Yb

ratios ( ~ 30—70) of the melilitic rocks, which are

similar to those of OIB, indicate the presence of residual gar-
net  in  the  source  (Wilson  et  al.  1995;  Dunworth  &  Wilson
1998;  Lustrino  &  Wilson  2007).  The  enrichment  in  Nb  rela-
tive to La and Th and an enrichment in La relative to Ce sug-
gests  that  these  rocks  cannot  be  readily  generated  from  a
primitive  mantle  source  but  require  a  metasomatized  source,
enriched  in  strongly  incompatible  trace  elements  (Hofmann
1986).  The  distinct  negative  K,  Rb  and  P  anomalies  on  the
primitive  mantle-normalized  incompatible  element  diagrams
of the melilitic rocks have usually been interpreted as imply-
ing the presence of residual phlogopite and apatite within the
mantle source (Wilson et al. 1995; Dunworth & Wilson 1998).
Nevertheless, the interpretation of Dunworth & Wilson (1998)
suggested that the relative K depletion in these rocks is in part
due to the presence of carbonate in the mantle source, which
enhances the stability of phlogopite (Rogers et al. 1992). This
interpretation of the source of melilitic rocks seems to be real-
istic also in the Bohemian Massif.

Carbonate mantle metasomatism preferentially enriches

LREE relative to Hf (e.g. Yaxley et al. 1991; Rudnick et al.
1993). The lower Hf/Sm ratio in the melilitic rocks ( ~ 0.3—0.6)
compared to the primitive mantle ( ~ 0.70 – McDonough &
Sun 1995) may thus suggest that the source of melilitic volca-
nic rocks can be modified by carbonate-rich melts. The varia-
tions of initial Sr isotopic ratios (

Sr/

Sr = 0.7033—0.7049)

found in the Osečná Complex can also be interpreted as the
result of the late-magmatic to postmagmatic hydrothermal
alteration (Ulrych et al. 2008). The

Sr/

Sr ratio ~ 0.7041

determined for the volcanic bomb from the Pleistocene maar
locality of Mýtina may reflect contamination of the primary
magmas by Variscan phyllites.

The Sr/Nd isotopic ratios of most melilitic rocks of the

Pleistocene and the Eocene to Miocene periods suggest
primitive mantle sources. The high positive initial

values

(3.2—5.1) of the melilitic rocks of the Bohemian Massif are
interpreted as indications of the melting of depleted and
moderately heterogeneous mantle sources precluding signifi-
cant crust contamination.

The primitive nature of the chemical composition of the

melilitite  rock  can  be  used  to  constrain  the  compositional
characteristics  of  the  mantle  sources  (e.g.  Dawson  et  al.
1985; Dunworth & Wilson 1998; Keller et al. 2006; Lustrino
& Wilson 2007; Melluso et al. 2011). The melilitic magma is
generally believed to be formed by partial melting of a car-
bonated mantle peridotite/clinopyroxenite at the base of the
lithosphere  (the  thermal  boundary  layer  of  Wilson  et  al.
1995; Dunworth & Wilson 1998). Similarly Brey (1978) and
Keshav  &  Gudfinnsson  (2004)  concluded  that  melilitites
and  nephelinites  are  partial  melts  of  carbonated  lherzolites
at  3 GPa  (or  higher).  In  the  experiments  of  Gudfinnsson  &
Presnall  (2005),  the  melilitite  melts  resembling  natural  me-

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Abbreviations:

HOM

–

haüyne

olivine

melilitite,

NOM

–

nepheline

olivine

melilitite,

MON

–

melilite

olivine

nephelinite,

–

olivine

nephelin

ite,

OME

–

olivine

melilitolite,

UML

–

ultramafic

lamprophyres,

POL

–

clinopyroxene—free

lamprophyre—polzenite.

Mg#

100

Mg/Mg

for

/Fe

0.15.

Data sources:

1 – this study, 2 – Ulrych

et al. (2013), 3 – Ulrych et

al. (2000b), 4 – Abratis et

al. (2009), 5 – Ulrych

et al. (2008), 6 –

Ulrych et al. (2014), 7

– Seifert et al. (2008).

Notes:

– data for scoria,

– data for lapilli,

– data for olivine nephelinite,

– data for melilite olivine nephelinite.

Table 4:

Comparison

geological,

petrographic

and

geochemical

characteristics

melilitic

rocks

the

Bohemian

Massif.

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lilitite whole-rock compositions were produced only at pres-
sures < 4 GPa; at higher pressures and temperatures they
grade into kimberlitic melts. Modelling of the melting of the
garnet lherzolite phase assemblage containing 0.15 wt. %
CO

indicates that melilitites are produced by < 1 % melting.

In a discrimination diagram of MgO/CaO vs. SiO

/Al

Gudfinnsson & Presnall (2005) our analytical data plot mostly
to or close to a field for melilites near the 3 GPa isobar, yet
some are located outside the field at considerably lower pres-
sures. Falloon & Green (1990) estimated the formation of the
olivine  melilitite  magma  in  equilibrium  with  garnet—phlo-
gopite lherzolite within the dolomite stability field as 1020 °C
at pressures  > 2.5 GPa. The parental magma of the region with
prominent occurrence of melilitic rocks, the Osečná Complex,
was probably derived from a heterogeneous veined(?) meta-
somatically enriched carbonate- and phlogopite-bearing gar-
net  lherzolite  (Ulrych  et  al.  2008).  The  geochemical  and
isotopic similarity of melilitic rocks occurring from the Late
Cretaceous  to  the  Pleistocene  in  the  Bohemian  Massif  sug-
gests that their magma originates from compositionally very
similar mantle sources.

The

143

Nd/

144

Nd and

Sr/

Sr ratios of the melilitic rocks

are similar to common mafic volcanic rocks from the Ohře
Rift. In contrast, the volcanic rocks in the Lusatian Fault area
represented by the Osečná Complex display more radiogenic

Sr/

Sr isotopic composition at given similar

143

Nd/

144

Nd,

which can be explained by mantle sources with decoupled
Sr/Nd isotopic compositions (e.g. due to selective modification
by radiogenic

Sr/

Sr hydrous and/or carbonate-rich fluid).

Crystallization history of melilitic rocks

The ultramafic melilitic volcanic rocks of the Bohemian

Massif are characterized by the early-magmatic mineral asso-
ciation  of  olivine + melilite + Cr—Al-spinels ± clinopyroxene,
which  became  unstable  under  later  hydrothermal  conditions.
Products  of  the  following  main-magmatic  crystallization  are
represented by alkali-rich phases such as nepheline + sodalite—
haüyne and concentrate mostly in the groundmass. The residu-
al fluids of the late-magmatic hydrothermal stage are enriched
in large ion lithophile elements (LILE), high field strength ele-
ments (HFSE) and volatile elements. The number of rare min-
erals such as Ba-rich phlogopite, Zr-bearing (F,OH)-andradite,
perovskite, calzirtite and bartonite crystallized in these stages
(Ulrych et al. 1991).

Olivine with Fo contents of ~ 90 mol %, which we ob-

served as corroded xenocrysts, is typical for mantle xenoliths
from the Bohemian Massif (e.g. Konečný et al. 2006; Acker-
man et al. 2007, 2013, 2014; Špaček et al. 2013). The Mg-rich
olivine may therefore represent xenocrystic cores being
overgrown by Mg-poorer olivine crystallizing from the melt.

The studied olivines exhibit the normal type of composi-

tional zoning, which differs from the predominant reverse
zoning recorded from melilitic rocks of the SW German Ter-
tiary Volcanic Province (Dunworth & Wilson 1998). A char-
acteristic feature observed both in crystallization cores of
olivine phenocrysts and cores corresponding to relicts of xe-
nocrysts is increased NiO concentration positively correlated
with forsterite component content. High Ni contents in oliv-

ine can be explained by partial melting of pyroxenite-rich
mantle domains (Sobolev et al. 2005).

Monticellite

rims around olivine phenocrysts restricted to

rare melilitic rocks from the Osečná Complex and Jiřetín pod
Jedlovou area are related to a late-magmatic stage (cf. exper-
imental data of Yoder 1979). The possible metasomatic ori-
gin is supported by increased LREE, U, Th, Hf contents
(Ulrych et al. 1991).

Clinopyroxene

phenocrysts of the melilitic rocks show

similar  core-to-rim  compositional  trends  illustrated  by  si-
multaneous  increase  in  Al  and  Ti  content.  This  indicates
similar alkalinity of studied melilitic volcanic systems, as the
Al contents of the clinopyroxenes are in general directly pro-
portional to the alkalinity of their parent melts (e.g. Mitchell
&  Bergman  1991).  Typical  “green  cores”  presented  from
Cenozoic basaltic rocks from, for example, Germany (Duda
& Schmincke 1985; Abratis et al. 2009) have not been ascer-
tained.  Occurrence  of  resorbed  cores  of  Cr-rich  diopside
characteristic for melilitic rocks of the SW German Tertiary
Volcanic  Province  (Dunworth  &  Wilson  1998)  implying
their origin from mantle xenoliths was also not found at the
localities  studied.  Dunworth  &  Wilson  (1998)  emphasized
the role of the polybaric crystallization as the low-viscosity,
high-temperature melilitic magmas are likely to have cooled
rapidly  as  they  rose  through  the  relatively  thick  lithosphere
of Central Europe.

Procedure proposed by Putirka (2008) suggests crystalli-

zation pressures for clinopyroxene in a wide range between
2 GPa for crystal cores and 1 GPa for groundmass microphe-
nocrysts. These crystallization pressures are also supported
by clinopyroxene phenocrysts from Krkavčí skála highly en-
riched in K

O (up to 0.23 wt. %) since entry of potassium

into clinopyroxene structure is pressure-dependent. Accord-
ing to the model of Soesoo (1997), most of the clinopyrox-
ene analyses produce crystallization temperatures between
1000 and 1200 °C with majority of them clustering around
the 1150 °C isotherm.

The analysed (Mg,Fe)—Al-chromite cores display high TiO

contents (0.8—2.4 wt. %), which are generally higher than
those reported for primary spinel from peridotite xenoliths in
the Bohemian Massif that usually have only 0.1 to 0.7 wt. %
TiO

(Ackerman et al. 2007, 2014; Medaris et al. 2014).

Titanian magnetites

are characterized by variable contents

of Cr

(0.3—6.6 wt. %), MgO (2.0—9.3 wt. %) and Al

(2.1—9.0 wt. %) (Sebuzín, Český Chloumek, Pohoř) manifest-
ing  most  likely  a  remobilization  from  (Mg,Fe)—Al-chromite
cores.  In  terms  of  the  classification  of  Barnes  &  Roeder
(2001), spinels follow mostly Cr-Al trend modified by incor-
poration  of  a  component  produced  during  fractionation  or
contamination by host magma.

Melilite

belongs to minerals of the early-magmatic phase

of the rock crystallization sequence. Moore & Erlank (1979)
pointed  out  that  melilite  is  unstable  at  the  solidus  tempera-
ture of mafic igneous rocks, although it may be preserved in
volcanic  rocks  if  cooling  is  rapid.  The  composition  of  the
melilite from the melilitic rocks of the Bohemian Massif fits
the trends delineated by El Goresy & Yoder (1974) for vol-
canic rock associations in general and particularly the trends
observed  for  olivine  melilites  from,  for  example,  the

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Rhinegraben—Urach—Hegau volcanic areas presented by
Dunworth & Wilson (1998).

Since the (Ca + Na + K)/Al ratio in the melilitic rocks we stud-

ied is high, melilite formation in them is most possibly associ-
ated with the carbonate—silicate-magma reaction processes as
suggested by Dunworth & Wilson (1998) and Di Battistini et
al. (2001). Mixing of mafic silicate magma and carbonate melt
promoted melilite crystallization. The idea that melilitic rocks
are derived from a Ca-rich melt produced at deep levels of the
upper mantle has been formulated by Rass (2008).

Hamilton (1961; Fig. 5) calibrated the nepheline composi-

tions  so  that  they  can  be  used  as  a  temperature  indicator.
Nepheline  crystallization  temperatures  in  Pleistocene  rocks
cluster around 700 °C, most of Eocene to Miocene nephelines
typically  display  slightly  lower  crystallization  temperatures
(below 700 °C) although those with higher Ne contents show
the  same  temperatures  of  crystallization  as  younger  nephel-
ines. The most scattered values were recorded for nephelines
from  Upper  Cretaceous  to  Paleocene  rocks,  which  display
crystallization  temperatures  in  a  wide  range  from  below
500 °C  up  to  almost  the  limit  of  nepheline  stability  at
1068 °C.  According  to  Abratis  et  al.  (2009),  nephelines  of
melilite-bearing  olivine  nephelinites  from  Vogtland  crystal-
lized at temperatures of about 700 °C.

The studied rocks are characterized by the progressive

manifestation  of  the  low-temperature  hydrothermal  phase
with changing activities of volatile components starting with
high  concentrations  of  chlorine  to  final  phase  with  preva-
lence of sulphur in minerals of the sodalite group. There is a
chemical  zoning  present  in  the  microphenocrysts  following
the  pattern  with  increasing  SO

content compensated by a

decrease in Cl from core to rim.

Early phlogopite crystallized during the late-magmatic pe-

riod.  Intermediate  phlogopite  is  the  reaction  product  of
postmagmatic  fluids  with  olivine,  monticellite  and  early
phlogopite.  The  late-magmatic  processes  (glimmeritization)
of the olivine melilitolite sill from Osečná result in the for-
mation of bimineral rock – garnet glimmerites.

The youngest population of micas in polzenites is represented

by  rims  of  phlogopites  enriched  in  the  tetra-ferriphlogopite
end-member (Pivec et al. 1998). The tetra-ferriphlogopite is
also  present  in  melilitic  dyke  rocks  of  Urach  and  Hegau  in
Germany (Dunworth & Wilson 1998) and the Komorní hůr-
ka lava (Seifert & Kämpf 1994). Edgar (1992), Seifert et al.
(2008) and Abratis et al. (2009) reported the presence of Ba-
rich  phlogopite  from  melilitic  rocks  of  the  West  Eifel,
Komorní hůrka, Görlitz and Vogtland, respectively, suggest-
ing an enrichment of the late-magmatic hydrothermal fluids
in barium.

Low temperature hydrothermal reactions are documented

by the presence of the abundant late-magmatic perovskite
rich in incompatible elements and by postmagmatic light-co-
loured perovskite very low in incompatible elements rim-
ming titanian magnetites (Ulrych et al. 1988).

Dunworth & Wilson (1998) noted that crystallization of

minerals in melilitic magmas is influenced by variable propor-
tions of H

O and CO

and Ca saturation. Late-stage crystalli-

zation of phlogopite and carbonate in melilitic rocks is related
to high contents of H

O and CO

while the crystallization of

melilite is enhanced by low H

O/CO

but high (Ca + Na + K)/Al

in melts (Di Battistini et al. 2001). This interpretation can be
demonstrated in particular in the Osečná Complex.

Conclusions

Melilitic rocks are relatively widespread in the Bohemian

Massif during the Late Cretaceous and Cenozoic. The Osečná
Complex  together  with  the  surrounding  Ploučnice  River  re-
gion  in  northern  Bohemia  located  at  the  intersection  of  the
Ohře  Rift  and  the  Lusatian  Fault,  and  adjacent  territories  of
Saxony and Lusatia host mostly dyke melilitic rocks dated to
the  Late  Cretaceous  to  Late  Paleocene  period  (80—59 Ma).
The  dominant  Mid  Eocene  to  Late  Miocene  (32.3—5.9 Ma)
volcanic period in the Bohemian Massif is very poor in melili-
tic dyke rocks (the Ohře Graben, the Cheb—Domažlice Graben
and  its  continuation  in  Vogtland  and  the  Labe—Odra  Fault
Zone).  Cinder  cones  of  extrusive  melilitic  rocks  (scoria  and
lava)  occur  at  the  junction  of  the  Ohře  Rift  and  the  Cheb—
Domažlice Graben in the Cheb Basin area. They belong to the
Early  to  Late  Pleistocene  volcanic  episode  (1.0—0.26 Ma)  of
the Bohemian Massif.

The mineral, geochemical and Sr/Nd isotopic similarities

of melilitic rocks occurring in the Bohemian Massif from the
Late Cretaceous to the Late Pleistocene suggest that their un-
usual magma evolved from compositionally very similar
mantle sources and those magmas also underwent similar
processes of their formation. Only the melilitic rocks of the
Osečná Complex influenced by late-magmatic and postmag-
matic fluids partly differ in Sr isotopic characteristics, show-
ing more radiogenic

Sr/

Sr values. However, their tectonic

(grabens and fault zones) and geological (dykes, sills, flows,
scoria  cones)  settings  and  petrographic  (melilite  olivine
nephelinite  to  nepheline  olivine  melilitite,  haüyne  olivine
melilitite, ultramafic lamprophyres – polzenite and alnöite,
olivine  melilitolite  and  its  pegmatoid  segregations)  charac-
teristics are partly different.

The ultramafic melilitic rocks are characterized by the pri-

mary olivine + melilite + Cr—Al-spinel/clinopyroxene mineral
association, which became unstable under late-magmatic
conditions. The low Cr

contents in diopside ( < 0.5 wt. %)

and high TiO

contents in Cr—Al-spinels (0.8—2.4 wt. %) do

not  correspond  to  the  primary  composition  of  mantle  xeno-
liths.  The  specific  mineral  association  with  rare  minerals
such  as  Zr-rich  (F,OH)  andradite,  calzirtite  and  bartonite  is
characteristic  only  for  the  Osečná  olivine  melilitolite  intru-
sion strongly influenced by late-magmatic fluids concentrat-
ing LILE, HFSE and volatile elements.

On the basis of major- and trace elements and the Sr/Nd

isotopic characteristics, the melilitic rocks of the Bohemian
Massif  should  be  interpreted  as  melts  originating  by  low
melting  of  heterogeneous  mantle  sources,  including  both
lithospheric  and  asthenospheric  mantle  components.  The
heterogeneous lithospheric source was probably veined car-
bonated mantle peridotite/clinopyroxenite.

Acknowledgments: This research was financially supported
by institutional Project RVO 67985831 of the Institute of

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Geology  of  the  CAS.  We  thank  Věra  Vonásková  and
Ladislav  Strnad  of  Charles  University,  Prague  for  whole-
rock  major-element  analyses  and  ICP-MS  trace-element
analyses,  respectively.  The  K/Ar  dating  was  supported  by
OTKA  Projects  No. T043344  and  M41434  to  Kadosa
Balogh. We are indebted to Vlasta Böhmová for microprobe
analyses  and  Jaroslava  Pavková  and  Jana  Rajlichová  for
technical  assistance.  The  authors  gratefully  acknowledge
critical  and  constructive  comments  by  Jiří  Adamovič  to  the
manuscript  of  this  paper.  The  manuscript  benefits  from  the
constructive reviews of both reviewers.

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