GeolCarp_Vol66_No6_499

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, DECEMBER 2015, 66, 6, 499—514 doi: 10.1515/geoca-2015-0041

Orthopyroxene-enrichment in the lherzolite-websterite

xenolith suite from Paleogene alkali basalts of the Poiana

Ruscă Mountains (Romania)

ZSUZSANNA NÉDLI

, CSABA SZABÓ

and JÚLIA DÉGI

Lithosphere Fluid Research Lab, Department of Petrology and Geochemistry, Eötvös University, Pázmány Péter setány 1/C,

H-1117 Budapest, Hungary; nedlizs@yahoo.com;

cszabo@elte.hu

Geological and Geophysical Institute of Hungary, Stefánia út 14, H-1143 Budapest, Hungary; degi.julia@mfgi.hu

(Manuscript received March 17, 2015; accepted in revised form August 10, 2015)

Abstract: In this paper we present the petrography and geochemistry of a recently collected lherzolite-websterite xeno-
lith series and of clinopyroxene xenocrysts, hosted in Upper Cretaceous—Paleogene basanites of Poiana Ruscă (Romania),
whose xenoliths show notable orthopyroxene-enrichment. In the series a slightly deformed porphyroclastic-equigranular
textured series could represent the early mantle characteristics, and in many cases notable orthopyroxene growth and
poikilitic texture formation was observed. The most abundant mantle lithology, Type A xenoliths have high Al- and
Na-contents but low mg# of the pyroxenes and low cr# of spinel suggesting a low degree ( < 10 %) of mafic melt
removal. They are also generally poor in overall REE-s (rare earth elements) and have flat REY (rare earth elements + Y)
patterns with slight LREE-depletion. The geochemistry of the Type A xenoliths and calculated melt composition in
equilibrium with the xenolith clinopyroxenes suggests that the percolating melt causing the poikilitization can be linked
to a mafic, Al-Na-rich, volatile-poor melt and show similarity with the Late Cretaceous—Paleogene (66—72 Ma) subduc-
tion-related andesitic magmatism of Poiana Ruscă. Type B xenoliths, with their slightly different chemistry, suggest
that, after the ancient depletion, the mantle went through a slight metasomatic event. A subsequent passage of mafic
melts in the mantle, with similar compositions to the older andesitic magmatism of Poiana Ruscă, is recorded in the
pyroxenites (Fe-rich xenoliths), whereas the megacrysts seem to be cogenetic with the host basanite. The Poiana Ruscă
xenoliths differ from the orthopyroxene-enriched mantle xenoliths described previously from the Carpathian-Pannonian
Region and from the Dacia block.

Key words: Paleogene, S Carpathians, mantle xenolith petrology, geochemistry, alkali basalt.

Introduction

Orthopyroxene-rich  xenoliths  represent  only  a  minor  part  of
the lithospheric mantle lithologies (Downes 2001). After ear-
lier interpretation as mantle residues after extensive melt ex-
traction  (Ringwood  1958),  several  different  processes  have
been proposed more recently to explain their formation. These
interpretations suggest that orthopyroxene-rich lithologies can
be  (1)  products  of  interaction  between  mantle  wall  rock  and
aqueous SiO

-rich melts/fluids derived from slab dehydration

or melting (e.g. Kelemen et al. 1992; Arai et al. 2003, 2004),
(2) reaction products of interaction between mantle wall rock
and silica-rich melts of alkali basaltic origin percolating in the
mantle (e.g. Arai et al. 2006; Dantas et al. 2009), (3) mafic-
ultramafic cumulates crystallized at mantle depth (Embey-
Isztin et al. 1989; Santos et al. 2002; Cvetković et al. 2004,
2007; Bali et al. 2007) and (4) products of infiltration-crystal-
lization-reaction processes (Wulff-Pedersen et al. 1996, 1999).

These rocks have been described from several localities of

the Carpathian-Pannonian-Balkan Region (Embey-Isztin et al.
1989,  2001;  Cvetković  et  al.  2004,  2007;  Bali  et  al.  2007,
2008;  Marchev  et  al.  2008;  Embey-Isztin  &  Dobosi  2011;
Berkesi et al. 2012) and show wide variations. Some studies
have  proposed  the  origin  of  these  orthopyroxene-enriched
rocks as mantle-depth crystallization products from boninitic

melts (Embey-Isztin et al. 1989; Cvetković et al. 2007; Bali
et  al.  2007,  2008).  Some  recent  papers  (Cvetković  et  al.
2004,  2010;  Embey-Isztin  &  Dobosi  2011)  suggested  the
possibility  of  orthopyroxene-rich  (poikilitic)  lithologies  as
resulting  from  magmatic  modification  of  the  lithospheric
mantle via infiltrating (alkaline) mafic melt.

In this paper we present the petrography and geochemistry

of a recently collected lherzolite-websterite xenolith series and
of  clinopyroxene  xenocrysts,  hosted  in  Upper  Cretaceous—
Paleogene basanites of Poiana Ruscă (Romania), whose xe-
noliths show notable orthopyroxene-enrichment. We discuss
the  main  characteristics  of  the  lithospheric  mantle  beneath
the  region  and  explain  what  processes  may  be  responsible
for  the  orthopyroxene-enrichment  of  the  studied  mantle,
which  seems  to  be  rather  different  in  geochemical  features
and origin from the orthopyroxene-enriched xenoliths of the
Carpathian-Pannonian-Balkan Region described earlier.

Geological background

The Poiana Ruscă Mountains (Romania) are situated in the

South Carpathians and are largely composed of a low-grade
metamorphic volcano-sedimentary series of Devonian—Lower
Carboniferous age, related to the Rhenohercynian Zone of

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Central-Western Europe (Kräutner 1997). These rocks were
strongly deformed and metamorphosed during the Variscan
orogeny in Carboniferous time and were also involved in
mid-Cretaceous thrusting of the Supragetic unit of the South
Carpathians (e.g. Iancu et al. 2005).

During the Mesozoic collision of the African and Eurasian

plates and a number of smaller continental microplates, the re-
gion  was  dominated  by  the  closure  of  the  western  branch  of
the  Vardar  Zone  and  eastward/north-eastward  subduction  of
the Tethyan oceanic lithosphere beneath stable Europe (e.g.
Ianovici  et  al.  1977;  Karamata  et  al.  1997).  This  tectonic
event  created  a  1000  km  long  belt  of  Late  Cretaceous—Early
Paleogene arc intrusive rocks, regionally referred to as bana-
tites (von Cotta 1864) or as the Banatite-Timok-Srednjegorie
Magmatic  and  Metallogenetic  Belt  (Berza  et  al.  1998),  and
volcano-sedimentary  successions,  appearing  throughout  the
basement  of  the  SW-Carpathians  (Banat,  Poiana  Ruscă,
Fig. 1)  and  Apuseni  Mts  (Ciobanu  et  al.  2002).  The  geo-
chemistry of the banatitic magmatic suites shows an alkaline
to  calc-alkaline  signature  and  suggests  subduction-related
magma  generation  (Rădulescu  &  Săndulescu  1973;  Russo-
Săndulescu  et  al.  1978;  Russo-Săndulescu  &  Berza  1979;
Downes et al. 1995; Berza et al. 1998).

Kräutner (1969) presented the first detailed description of

the  sub-volcanic  alkaline  basaltic  bodies  outcropping  in  the
Poiana Ruscă massif, south of the Apuseni Mts, which post-
dated the banatitic magmatic suites. He divided the magmatic
rocks  geographically  and  geochemically  into  six  groups.
Downes  et  al.  (1995)  reported  K-Ar  data,  petrography  and
geochemistry of the alkaline rocks and the first petrologic and

geochemical  data  on  their  mantle  xenoliths.  They  concluded
that the Late Cretaceous—Paleogene magma emplacement oc-
curred in two phases. The older (66—72 Ma) andesitic magmas
could  have  been  formed  by  subduction-related  volcanism,
from  a  mantle  source  enriched  by  previous  subduction.  The
andesitic  magmatism  was  followed  by  alkaline  basic  vol-
canism  (48—58 Ma),  probably  related  to  deep  lithospheric
fractures  at  the  boundary  between  the  Tisza  and  Dacia  ter-
rains  (Downes  et  al.  1995).  Regarding  the  mantle  xenoliths,
Downes et al. (1995) summarized that they represent a gener-
ally  unmetasomatized,  slightly  deformed  mantle  section  and
the rare occurrence of pyroxenites suggests the passage of ma-
fic magmas through the lithospheric mantle. Recently Tshegg
et al. (2010) studied the younger basanitic activity of Poiana
Ruscă and concluded that its magmatics were formed by a low
fraction  melting  of  a  slightly  CO

influenced garnet-facies

OIB-like asthenospheric source. The magmatic activity can be
related to a post-collision extensional tectonic event, similar to
the Paleocene—Eocene magmatic activity which generated the
xenolith-bearing East Serbian mafic alkaline magmatic rocks
(ES-MAR) (Cvetković et al. 2013).

These Serbian Paleogene mafic alkaline rocks (Fig. 1) form

a north—south line within the Carpatho-Balkan tectonic terrain
as  a  continuation  of  the  volcanic  chain  of  the  Apuseni  Mts
(Cvetković et al. 2004). Jovanović et al. (2001) and Cvetković
et al. (2004, 2007, 2010, 2013) presented detailed descriptions
of  the  magmatism  and  the  mantle  xenoliths.  They  suggested
that the lithospheric mantle underneath East Serbia is more de-
pleted  than  the  normal  European  lithosphere  and  described
two different types of orthopyroxene-rich lithologies. The first

Fig. 1. Sketch map of the Carpathian-Pannonian Region showing lo-
calities of the pre-Neogene xenolith-bearing magmatic rocks (Poiana
Ruscă and East Serbia, Villány Mts and NE Transdanubian region).
The inset shows volcanic fields in Poiana Ruscă after Kräutner
(1969) and locations of analysed samples.

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type of websterites was interpreted by interaction with perco-
lating mafic alkaline melts (Cvetković et al. 2004), the second
type in relation to crystallization of subduction-related pre-
sumably boninitic melts (Cvetković et al. 2007).

Marchev et al. (2006, 2008) also described a pyroxene-

rich  mantle  xenolith  suite  from  the  Oligocene  alkali  basalt-
lamprophyre  series  as  underplated  cumulate  rocks.  A  trace
element study of the most primitive cumulate rocks indicates
that  they  formed  from  a  mantle-derived  parent  melt  similar
to their host alkaline magma. The orthopyroxenite and web-
sterite  xenoliths  are  believed  to  have  been  formed  from  the
same,  but  more  fractionated,  melt  which  may  have  had  a
Mg-Si-rich alkali-basaltic character.

Analytical methods

Xenolith-bearing basanite samples were collected from the

Cerbal and Fintini Mt localities (Fig. 1). From the largest xe-
noliths, 17 double-polished thick sections were prepared for
petrographic  description  and  modal  composition  estima-
tions. The modal composition of the xenoliths was approxi-
mated by digital image analysis of high resolution scanning
of whole thick sections, in the majority of cases including all
the  xenolith,  using  the  image  analysis  software  package  of
Corel X4. The modal proportion of minerals is expressed as
the  percentage  per  total  area  of  rock  surface  corrected  for
grain boundaries. The validity of the calculated modal com-
positions was checked under an optical microscope. The pet-
rographic  study  was  carried  out  at  the  Lithosphere  Fluid
Research Lab, Eötvös University (Budapest, Hungary) using
a Nikon Eclipse E600 POL polarized light microscope.

The microprobe analysis was carried out on representative

thin-sections with a Cameca SX100 Electron Microprobe at
the Natural History Museum, London. Data were collected at

Sample Rock type

Texture Modal composition

Type Equilibrium

T(°C) fO

Ce6-8a

lherzolite

porphy-

ol: 71; opx: 15; cpx: 11; sp: 3

950

–0.55

equiranular

Ce6-10a ol websterite poikilitic ol: 28; opx: 52; cpx: 18; sp: 2

1020 –0.12

Ce6-11a wherlite

porphy-

ol: 87; opx: 1; cpx: 10; sp: 2

1000 –0.48

equiranular

Ce6-16a lherzolite

poikilitic ol: 50; opx: 40; cpx: 7; sp: 3

1000 –1.46

Ce6-16b ol websterite poikilitic ol: 32; opx: 47; cpx: 19; sp: 2

1010 –1,15

Ca6-17a lherzolite

poikilitic ol: 67; opx: 17; cpx: 14; sp: 2

960

–0.93

Ce6-17b lherzolite

poikilitic ol: 51; opx: 27; cpx: 16; sp: 6

970

–0.53

Ce6-18

lherzolite

poikilitic ol: 45; opx: 36; cpx: 18; sp: 1

990

0.17

Ce6-21

lherzolite

poikilitic ol: 63; opx: 14; cpx: 16; sp: 7

990

–0.52

Ce6-12 ol

websterite poikilitic ol: 21; opx: 71; cpx: 7; sp: 1

960

–0.71

Ce6-13

lherzolite

poikilitic ol: 66; opx: 27; cpx: 5; sp: 2

930

–0.72

Ce6-2

lherzolite

poikilitic ol: 78; opx: 14; cpx: 7; sp: 1

1110 2.95

Fi6-3

lherzolite

porphy-

ol: 78; opx: 5; cpx: 15; sp: 2

980

–0.94

equiranular

Ce6-20

lherzolite

poikilitic ol: 73; opx: 13; cpx: 11; sp: 3

Fe-rich 900

0.42

Ce6-15

dunite

poikilitic ol: 84; opx: 6; cpx: 2; sp: 8

Fe-rich 900

0.35

Ce6-7 megacryst

cpx

Ce6-11a megacryst

cpx

Table 1: Modal composition, textures, and equilibrium temperatures of the Poiana Ruscă
xenoliths.

T(°C) – after Brey & Köhler (1990) cpx-opx equilibrium T, fO

– after Ballhaus et

al. (1991).

Fig. 2. Modal composition of xenoliths from Poiana Ruscă.

15 kV,  20 nA  beam  current,  for  100 s  per
analysis,  using  a  Bruker  AXS  4010  XFlash
silicon drift energy dispersive X-ray (EDX)
detector  and  1 µm  spot  diameter.  Natural
and  synthetic  minerals  were  used  as  stan-
dards.  Concentrations  were  averaged  from
2—4 measurements, core to rim homogeneity
was  checked  and  outer  rim  measurement
was avoided.

Laser ICP-MS trace element analysis of sili-

cate  phases  (clinopyroxene,  orthopyroxene
and  olivine)  was  performed  at  the  Natural
History  Museum,  London.  The  LA-ICP-MS
system consists of an Agilent 7500 cs quadru-
pole  ICP-MS,  coupled  to  a  New  Wave  Re-
search UP193FX excimer laser. The synthetic
glass  reference  material  NIST  612  was  used
as the calibration standard, using the average
composition of Pearce et al. (1997). Calcium
(

Ca) was used as the internal standard for

quantification of analysis. The raw data was
processed using the offline Lamtrace soft-
ware. Trace element concentrations were av-
eraged from 2—4 measurements.

Petrography

Xenoliths were collected from two xenolith-bearing basan-

ite  localities  in  Poiana  Ruscă:  most  of  the  samples  are  from
near  Cerbal  and  some  samples  from  Fintini  Mt  (Fig. 1).  The
xenoliths are fresh, their sizes are 1.5—5.0 cm, and they show
variability  in  modal  composition  and  texture.  They  vary
widely between olivine-rich wehrlite, lherzolite and orthopy-
roxene-rich olivine websterite compositions (Table 1, Fig. 2).
One dunite xenolith was also identified. The most distinctive
feature of the modal composition is the high variability in or-
thopyroxene  content  (1—45 vol. %).  No  OH-bearing  minerals
were found in any xenoliths.

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the  presence  of  various  solid  inclusions  in  silicate  phases
(Fig. 3c,e,f).  Orthopyroxene  has  a  typically  larger  grain  size
(up to 3—4 mm) than the other minerals (Fig. 3b,c,d) and often
encloses  olivine  and  rare  clinopyroxene  (Fig. 3e,f).  Two  dif-

Table 2: Olivine major element data of the studied Poiana Ruscă xenoliths.

Sample Ce6-8a Ce6-10a Ce6-11a Ce6-16a Ce6-16b Ce6-17a Ce6-17b Ce6-18 Ce6-21 Ce6-12 Ce6-13 Ce6-2

Fi6-3 Ce6-15 Ce6-20

Type A A A A A A A A A A A B B

Fe-rich Fe-rich

(wt. %)
SiO

40.57 40.36 40.72 40.57 40.52 40.42 40.86 40.33 40.73 40.52 40.62 40.46 40.94 39.42 39.71

TiO

0.00

0.01

0.00

0.01

0.00

0.01

0.00

0.02

0.01 0.01 0.00 0.01 0.01 0.00 0.00 0.02 0.00 0.01 0.00 0.01 0.00 0.01 0.00

0.02 0.01 0.03 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.01 0.03 0.01 0.00 0.01

FeO

9.95

10.45

9.91

10.41

10.29

11.35

8.81

10.91

10.42

9.43

9.69

9.88

9.75

16.24

14.60

MnO 0.15 0.16 0.14 0.12 0.15 0.17 0.13 0.14 0.16 0.13 0.15 0.16 0.16 0.22 0.16
NiO

0.38 0.37 0.35 0.36 0.35 0.34 0.40 0.36 0.37 0.37 0.38 0.35 0.36 0.15 0.23

MgO

48.85

48.63

48.96

48.56

48.64

47.85

49.82

48.12

48.72

49.27

49.65

48.88

49.29

43.78

45.13

CaO

0.06 0.06 0.07 0.06 0.10 0.10 0.05 0.06 0.06 0.05 0.04 0.09 0.05 0.06 0.11

O 0.00 0.01 0.00 0.00 0.01 0.02 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.01

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Total

99.99 100.06 100.18 100.12 100.09 100.27 100.08 99.97 100.48 99.79 100.54 99.88 100.57 99.88 99.98

89.75 89.24 89.80 89.26 89.39 88.26 90.97 88.72 89.29 90.30 90.13 89.82 90.01 82.77 84.64

Table 3: Orthopyroxene major and minor element data of the studied Poiana Ruscă xenoliths. mg# = (Mg/(Mg + Fe)).

Sample Ce6-8a Ce6-10a Ce6-11a Ce6-16a Ce6-16b Ce6-17a Ce6-17b Ce6-18 Ce6-21 Ce6-12 Ce6-13 Ce6-2 Fi6-3 Ce6-15 Ce6-20

Type

A A A A A A A A A A A B

Fe-rich Fe-rich

(wt. %)
SiO

55.04

54.52 55.15

54.60

54.67

54.82

55.64 54.56

54.67

55.32

55.41 54.33 55.93 53.12 54.62

TiO

0.08

0.14 0.09

0.15

0.12

0.12 0.11

0.12

0.05

0.11

0.23 0.15

0.09 0.10

4.43

4.75 3.84

4.72

4.58

3.37 4.63

4.81

3.84

4.02

4.63 3.12

4.90 3.25

0.33

0.31 0.48

0.34

0.33

0.27

0.47 0.38

0.32

0.40

0.37

0.52 0.51

0.07 0.33

FeO 6.34

6.55 6.22

6.43

6.46

7.13

5.56 7.04

6.60

6.01

6.10

6.29 6.16 10.32 9.04

MnO 0.13

0.13 0.13

0.15

0.14

0.17

0.13 0.17

0.15

0.14

0.15 0.15

0.20 0.16

NiO 0.08

0.10 0.08

0.09

0.08

0.11 0.09

0.09

0.08

0.10 0.07

0.03 0.06

MgO 33.21

32.63 33.44

32.68

32.79

32.35

33.75 32.41

32.58

33.71

33.43 32.39 33.80 30.20 31.40

CaO 0.66

0.75 0.75

0.76

0.75

0.69

0.69 0.82

0.74

0.63

0.67

1.12 0.68

0.70 0.71

O 0.09

0.11 0.07

0.12

0.10

0.09

0.09 0.12

0.09

0.06

0.13 0.07

0.02 0.02

O 0.00

0.00 0.01

0.00

0.00 0.00

0.00

0.00 0.00

Total 100.39

99.99 100.26 100.04 100.19 100.30

99.93 100.33 100.17 100.28 100.39 99.89 100.64 99.65 99.69

mg#

0.90

0.91

0.90

0.89

0.92

0.89

0.90

0.91

0.90

0.91

0.84

0.86

(ppm)
Rb n.d.

n.d. 0.01

0.01

0.24 0.04

n.d. n.d. n.d. 0.01

0.02

0.04 0.04

n.d. n.d. 0.07

n.d. n.d. n.d. 0.14

n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.04

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.01

n.d. n.d.

n.d.

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

n.d.

0.01

n.d.

0.02

0.01

n.d.

0.01

0.05

0.01

n.d.

0.01

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.01

n.d.

n.d. n.d. n.d. 0.01

0.01

n.d. n.d. n.d. n.d. 0.01 0.01

0.02

0.01

n.d. n.d.

Ce n.d. 0.01

0.01

0.02

0.03

0.02

0.01 0.01 0.01 0.02 0.04 0.12 0.03

n.d. 0.01

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

n.d.

Sr 0.05

0.12

0.20

0.25

0.35

0.21

0.13 0.09 0.10 0.07 0.17 0.57 0.16

0.08 0.14

Nd n.d.

n.d. 0.04

n.d. n.d. n.d. n.d. n.d. n.d. 0.12

0.06

n.d. n.d.

0.53

1.52

0.96

1.88

1.76

1.45

1.04

0.97

1.48

0.47

1.00

3.04

0.65

Hf 0.02

0.05

0.03

0.07

0.05

0.07

0.05 0.04 0.04 n.d. n.d. 0.09 n.d. 0.03 0.04

Sm n.d. 0.02

n.d.

0.03

0.05

n.d. n.d. n.d. n.d. 0.06

0.02

n.d. n.d.

Eu 0.01

0.01

0.01 0.01 0.01 n.d. 0.01 0.03 0.01

0.01 n.d.

Gd 0.04

0.07

n.d.

0.06

0.05

0.04 0.04 0.04 0.03 0.04 0.08 0.03

0.04 0.04

Tb 0.01

0.01

0.01 0.01 0.01 0.01 0.01 0.02 0.01

0.01 0.00

0.08

0.13

0.09

0.14

0.13

0.08

0.11

0.10

0.07

0.10

0.15

0.08

Y 0.81

1.05

0.78

1.11

0.99

1.08

0.66 0.94 0.92 0.53 0.87 0.90 0.57

0.65 0.48

Ho 0.03

0.04

0.03

0.04

0.03

0.04

0.02 0.03 0.03 0.02 0.03 0.03 0.02

0.02 0.02

Er 0.12

0.14

0.09

0.18

0.13

0.15

0.08 0.13 0.17 0.09 0.12 0.10 0.08

0.10 0.06

Tm 0.02

0.03

0.02

0.03

0.02 0.03 0.02 0.01 0.02 0.02 0.01

0.02 0.01

Yb 0.21

0.27

0.20

0.21

0.26

0.14 0.24 0.24 0.15 0.23 0.15 0.10

0.16 0.12

Lu 0.04

0.05

0.03

0.05

0.04

0.05

0.02 0.04 0.04 0.03 0.04 0.03 0.02

0.03 0.02

ferent grain boundaries are characteristic in the same xenolith:
pyroxenes, particularly orthopyroxenes, have mostly curvilin-
ear boundaries but, close to these grains, straight boundaries
and triple junctions are also observed among smaller minerals

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(mostly olivines) (Fig. 3f). This feature suggests the develop-
ment  of  the  poikilitic  texture  following  the  above  described
porphyroclastic-equigranular texture. Spinel has various grain
sizes and shapes. It occurs either as small, rounded grains in
triple junctions or as inclusion in silicates or as large vermicu-
lar grains at curvilinear connections among silicates (Fig. 3g).
In some cases small spinel grains also show a linear arrange-
ment.  In  a  few  poikilitic  samples  multiphase  sulphide  inclu-
sions  of  approximately  50—300 µm  in  diameter  also  occur
(Fig. 3h).  They  have  spheroidal,  blebby  or  amoeboid  shapes
and are mostly situated in fractures of orthopyroxenes or in the
spaces between spinel and orthopyroxene. The poikilitic sam-
ples  show  no  signs  of  deformation  or  strain.  Petrographic
character  and  modal  composition  are  rather  connected:  por-
phyroclastic-equigranular textured xenoliths have a modal oli-
vine  content  >70 vol. %,  whereas  poikilitic  texture  is
characteristic  of  samples  with  generally  lower  modal  olivine
content (Fig. 2 and Table 1). In correlation with the decreasing
olivine content, the xenoliths become richer in orthopyroxene,
generally  its  modes  are  > 20 vol. %.  In  contrast,  clinopyro-
xene  content,  apart  from  some  xenoliths,  remains  constant
(approximately 15 vol. %) with the increase in orthopyroxene
abundance  in  the  poikilitic  series.  This  feature  indicates  that
the development of the poikilitic texture was accompanied by
orthopyroxene growth at the expense of olivine.

The entire series is generally poor in fluid and/or melt in-

clusions. However, several fluid and/or melt inclusion trails
are  observed,  the  inclusions  are  generally  < 2 µm  and  their
linear  arrangement  close  to  grain  edges  and  in  connection
with  intergranular  spaces  suggests  their  secondary  origin,
therefore we did not take into consideration further studies of
these inclusions.

Mineral chemistry

Mineral chemistry data are presented in Tables 2, 3, 4 and

5, and Figs. 4, 5 and 6. Most of the xenoliths have a very sim-
ilar mineral chemistry and trace element composition with lit-
tle variation. They are interpreted in this study as representing
the common mantle variation beneath the studied area and are
referred to in this paper as Type A xenoliths. A small number
of samples, however, differ from these, especially in trace ele-
ment composition and are referred to as Type B and Fe-rich
xenoliths in this paper. Some clinopyroxene megacrysts were
also found and analysed for comparison.

This grouping was developed especially based on trace el-

ement compositions of pyroxenes. As similar data are miss-
ing in previous studies from the same area direct comparison
with those (Downes et al. 1995; Tschegg et al. 2010) is not
possible. Type A xenoliths are rather homogeneous in their
mineral  compositions.  Olivine  shows  a  restricted  composi-
tion  with  mg#  of  88.3—89.8,  NiO  from  0.34  to  0.39 wt. %
and CaO between 0.04 and 0.10 wt. %. The Mg-value of ortho-
pyroxenes  is  somewhat  higher  than  that  of  coexisting  oliv-
ines with values in the range 89.0—90.3. Their Al

contents

is high, ranging from 3.37 to 4.81 wt. %, whereas CaO is low
(0.66—0.82 wt. %). The most intrinsic feature of clinopyro-
xene compositions is high values of Al

(5.30—7.64 wt. %)

and Na

O (1.49—1.93 wt. %). Spinel is rather homogeneous

within the xenoliths and shows low cr#s of 9—18. For com-
parison, the xenoliths from the same area previously studied
by Downes et al. (1995), are also shown on the figures.
Many of them, mainly clinopyroxene-poor lherzolites, fit the
Type A xenoliths’ trends rather well (Tables 3 and 4, Fig. 4).

Two xenoliths (Ce6-2 and Fi6-3 lherzolites), referred to as

Type B xenoliths, are slightly separate from the main chemical
trends (Table 2, Fig. 4). Orthopyroxene has various Al

contents from 3.12 to 4.63 wt. % and CaO from 0.68 to
1.12 wt. %. Clinopyroxenes have lower Al

(4.71—5.43 wt. %)

and Na

O (1.19—1.36 wt. %) than the Type A xenoliths.

Their spinels are distinct because of the elevated Cr

(25.07—27.01 wt. %) and lower Al

(12.63—40.95 wt. %)

content,  thus  having  the  highest  cr#  (30.67—57.11)  in  the  se-
ries. Ce6-2 lherzolite also shows high FeO (41.10 wt. %) and
low MgO (8.60 wt. %) in spinel (Table 5). About half of the xe-
noliths  described  by  Downes  et  al.  (1995)  are  similar  to  our
Type  B  group  and  differ  in  composition  from  the  Type A
samples, as showed for comparison on Fig. 4a—d.

The compositions of two xenoliths (Ce6-15 and Ce6-20),

referred  to  as  Fe-rich  xenoliths,  differ  significantly  from  the
Type A and B xenoliths. Xenolith Ce6-15 is a dunite, whereas
the  Ce6-20  sample  has  a  lherzolite  modal  composition  and
very similar in mineral composition to the Ce6-15 dunite and
to  the  pyroxenite  sample  analysed  by  Downes  et  al.  (1995).
Similar  Fe-rich  dunites  were  also  reported  from  E-Serbia
(Cvetković et al. 2010). These samples are displaced from the
main residual trend (OSMA) of mantle xenoliths (Fig. 4a) and
show  notably  lower  mg#  in  mantle  silicates  (ol:  82.7—84.6;
cpx: 85.3—87.9; opx: 83.9—86.0, respectively). NiO in olivine
is  low  (0.15—0.23 wt. %),  their  pyroxenes  have  low  Na

(cpx: 0.10—0.52 wt. %; opx: 0.41—0.51 wt. %), but relatively
high Al

(cpx: 3.84—5.60 wt. %; opx: 3.25—4.90 wt. %) and

CaO (cpx ~ 22.2 wt. %; opx: 0.70 wt. %). Their spinel compo-
sition varies over a wide range (cr# = 2—21) (Table 5).

Two clinopyroxene megacrysts are uniform in composition

with low mg# (83.2—83.4) and Cr

content (0.20 wt. %)

and high Al

(8.50 wt. %) and TiO

(1.45 wt. %).

The mineral chemistry of xenolith samples analysed by

Tschegg et al. (2010) is also shown for comparison (Fig. 4a—d).
They are similar in composition to the Type A xenoliths,
showing high Al and Na in pyroxenes and spinel with low cr#.

Trace element chemistry of clinopyroxenes

The trace element composition of clinopyroxenes together

with their mineral chemistry is the basis of the classification
of the xenoliths in this study. A great number of the xeno-
liths (Type A) show similar trace element concentrations
(Table 4, Fig. 5). Clinopyroxenes in Type A xenoliths have a
REY (rare earth elements+Y) content approx. 10

×C1 chon-

drite,  showing  rather  flat  patterns,  apart  from  the  LREEs
(light  rare  earth  elements)  which  are  slightly  depleted  with
low  (La/Lu)

= 0.2—0.5 (Fig. 5a). The REY profiles of

Type B xenoliths are different with steeply downward pat-
terns, with HREE (heavy rare earth elements) concentrations
below 10

×C1 chondrite and with notable LREE enrichment

((La/Lu)

= 2.2—4.3) (Fig. 5c). The Fe-rich xenoliths show a

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convex-upward REY shape with both LREE and HREE-de-
pletion and a maximum at Sm. The Ce6-20 lherzolite has
high (La/Lu)

= 1.2, whereas the Ce6-15 dunite has a ratio

similar to the Type A samples (0.5). Clinopyroxene mega-
crysts have REY patterns roughly similar to Fe-rich xenoliths,
however they are enriched in LREEs and have a high (La/Lu)

ratio (ca. 3.1). The patterns have steeply downward shapes
with maxima at Nd coupled with a slight LREE depletion.

Total REE concentrations in clinopyroxenes are generally

low (Type A: 32—50 ppm; Type B: 37—42 ppm), except for
the Fe-rich xenoliths, which have even lower total REE con-
centrations (22—24 ppm). Megacrysts have slightly higher
REE

tot

concentrations (44—47 ppm).

Extended multi-element patterns of clinopyroxenes in all

types of xenoliths are roughly similar (Fig. 5b) with a nota-
ble  (Ba-)Nb(-Ta)  negative  anomaly  and  sometimes  a  Th-U
trough. Type B and  Fe-rich xenoliths also have pronounced
Zr-Hf negative anomalies. All xenoliths exhibit a Pb negative
anomaly.  Megacrysts  have  a  slightly  different  extended
multi-element pattern, lacking a Nb-Ta anomaly and steeply
downward shape for the HREEs.

Trace element chemistry of orthopyroxenes

Orthopyroxenes from Poiana Ruscă xenoliths have very low

REE contents (0.01—1

×C1 chondrite) (Fig. 6). Patterns of all

types are rather similar to each other and exhibit a continuous
depletion from HREE to LREE ((La/Lu)

= 0.01—0.03).

Discussion

Main physical and chemical characteristics of the litho-
spheric mantle beneath the Poiana Ruscă

Type A xenoliths form the most abundant lithology, hence

the main mantle processes and mineralogy will be discussed
based on them in the following. The Type A xenolith series

shows a significant variation in both modal composition and
texture  (Table 1,  Fig. 2).  However,  the  chemical  composi-
tion  of  these  xenoliths  follows  a  single  chemical  trend.
Therefore,  the  mantle  portion,  represented  by  these  xeno-
liths, regardless of the textural variation, was equilibrated in
a  relatively  narrow  T  range  between  930  and  1020 °C  (Ta-
ble 1). The Poiana Ruscă Type A xenoliths sampled a weakly
depleted mantle segment, which experienced only a low de-
gree ( < 10 %) of basaltic melt removal, according to the low
cr#  (8—23)  of  spinel  and  calculations  (Hellebrand  et  al.
2001), based on the spinel Cr-number (Table 2). The olivine-
spinel  mantle  array  “OSMA”  (Fig. 4a)  and  Y

(N)

—Yb

(N)

dia-

gram (Fig. 7) also show that the Type A xenoliths fall within
the  field  of  common  mantle  (Arai  1994)  and  experienced
only a low degree of melting. Based on the fractional melt-
ing  model  with  Y  and  Yb  contents  in  clinopyroxenes
(Fig. 7),  proposed  by  Norman  (1998),  this  depletion  is

Fig. 6. C1 chondrite normalized (data from McDonough & Sun 1995)
REY patterns of orthopyroxenes.

Fig. 7. Y

vs. Yb

ratios of clinopyroxenes in Type A and Type B

xenoliths with the fractional melting model proposed by Norman
(1998).

< 8 %. High Al- and Na-content and the general low
mg# of the pyroxenes (Fig. 4b,c,d) are also in agree-
ment with slightly different but generally low degrees
of mafic melt extraction.

The Type B xenoliths differ from the Type A ones in

their more depleted character, showing the highest de-
gree  of  partial  melting  in  the  series  (Figs. 4a,  7),  the
highest  olivine  content  (Table 1)  and  also  their  spinel
are  the  richest  in  chromium  (cr# 30—57)  (Table 5).
Nevertheless, the Type B xenoliths have clinopyroxene
with lower Al- and Na-contents and more enriched REY
patterns than the Type A xenoliths (Fig. 5) and show the
highest  equilibrium  temperature  (980—1110 °C).  This
all indicates that the lithospheric mantle represented by
the  Type B  xenoliths  could  have  undergone  a  higher
degree  of  partial  melting,  and  subsequently  experi-
enced small scale metasomatism.

Most Poiana Ruscă xenoliths from the Cerbal and

Panc  localities,  previously  studied  by  Downes  et  al.
(1995),  fit  the  studied  Type A  xenoliths’  trends  well
(Fig. 4a—d).  About  half  of  their  studied  xenoliths,

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however,  are  detached  from  the
principal group, and resemble the
Type B  xenoliths  (Fig. 4a—d).
Therefore, we cannot exclude the
possibility  that  these  more  de-
pleted  and  subsequently  refertil-
ized  xenoliths  are  more  common
in  the  mantle  beneath  the  Poiana
Ruscă Mts, but only poorly sam-
pled  in  our  series.  This  is  also
supported  by  Cvetković  et  al.
(2004)  who  described  an  ex-
tremely  depleted  mantle  in  the
very  vicinity  of  the  studied  area,
underneath  East  Serbia.  One  py-
roxenite, described by Downes et
al.  (1995),  is  similar  to  the  stud-
ied  Fe-rich  xenoliths.  Because
Downes  et  al.  (1995)  did  not
study trace element distributions,
any further comparison with their
samples is rather difficult.

Recently Tshegg et al. (2010)

studied  6  xenoliths  from  Cerbal
and 1 xenolith from Stancesti in
Poiana  Ruscă  (Fig. 1).  They  ob-
served  a  general  occurrence  of

Fig. 8. Partitioning of the rare earth elements between orthopyroxene/clinopyroxene as a function
of cation radius. Equilibrium lines for three different temperatures after (Agranier & Lee 2007).

equilibrium state of the xenoliths, the equilibrium partitioning
of the REE between clinopyroxene and orthopyroxene of each
xenolith  was  calculated.  At  chemical  equilibrium  there  is  a
negative correlation between ratios of REE abundances of or-
thopyroxene/clinopyroxene when plotted as a function of cat-
ion  radii  and  any  deviations  from  this  equilibrium  can  be
considered as a recent metasomatism/contamination (Agranier
& Lee 2007). Orthopyroxene/clinopyroxene REE ratios show
a very monotonous negative correlation of the cation ratio for
each  xenolith  of  the  studied  suite  (Fig. 8),  nearly  parallel  to
the  distribution  trend,  or  more  precisely  to  the  equilibrium
partitioning  model,  developed  for  different  temperatures  by
Agranier & Lee (2007). For our studied xenoliths, the only de-
viation from the equilibrium trends is in La (Fig. 8). This indi-
cates  that  the  xenoliths  could  have  suffered  some  recent
contamination by the host magma, but this enrichment affect-
ed only the most incompatible elements and most of the REE
abundances  reflect  the  primary  characteristics  of  the  mantle
from which the xenoliths were derived.

Only a few xenoliths deviate notably from the equilibrium

distribution  (Fig. 8),  suggesting  different  degrees  of  recent
contamination and therefore their chemistry probably does not
represent primary mantle characteristics. Among these deviat-
ing  xenoliths,  the  Type B  Ce6-2  lherzolite  is  identical  in  its
REY pattern to the host mafic melt (Fig. 9), although it must
have been in equilibrium with the melt calculated. However,
because of its very small dimensions (1—2 cm in diameter) it
was probably significantly contaminated by the host basanite.
Type A Ce6-17b lherzolite also shows some deviation in the
LREE  from  the  equilibrium  lines  (Fig. 8),  which  supports  a
slight  degree  of  recent  contamination  from  the  host  magma.
These samples are excluded from further interpretation.

reaction coronas around ortho- and clinopyroxenes and inter-
preted them as interaction products between mantle minerals
and infiltrated host mafic melt. In fact, they described the xe-
noliths as “thermally affected by the host magmas”. For these
reasons and because we are unable to examine and exclude
the effects of any possible chemical modification of the xe-
noliths by host melt infiltration and/or thermal effects, we
prefer not to use these data systematically for comparison in
our present paper.

The mostly anhydrous character of the mantle can be de-

duced from the absence of OH-bearing minerals in agree-
ment with Downes et al. (1995) who also found no hydrous
minerals in their suite of xenoliths either. The complete ab-
sence of primary fluid and silicate melt inclusions in the
studied xenolith samples also supports this character.

Evaluation of the possible contamination of the xenoliths
by the host melt

Because of the small dimensions of the xenoliths, it is possi-

ble that the chemistry of the smallest xenoliths was modified
by host mafic magma contamination or by recent metasoma-
tism  by  the  pre-eruptive  magmatic  precursor  processes  (e.g.
Stosch  1982;  Zindler  &  Jagoutz  1988;  Witt-Eickschen  &
O’Neill  2005).  To  avoid  utilizing  contaminated  xenoliths  for
the characterization of pervasive geochemical mantle features
beneath Poiana Ruscă, we investigated the possibility of con-
tamination for each xenolith studied. The distribution of REE
between minerals (for example, orthopyroxene—clinopyroxene
or  olivine—orthopyroxene  (Agranier  &  Lee  2007))  clearly
shows  whether  the  xenoliths  escaped  or  went  through  recent
chemical modifications. To test the chemical equilibrium/dis-

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proximately 900 °C), suggesting a different origin. The REY
patterns  of  melts  in  equilibrium  with  xenolith  clinopy-
roxenes were calculated (see details of calculation in the next
section).  The  trace  element  patterns  of  the  calculated  melt
composition  are  different  from  those  of  the  Type A  and  B
xenoliths,  showing  less  LREE-enrichment  and  slightly  de-
creasing REY patterns (Fig. 9a,c). Comparing the REY pat-
terns of melts in equilibrium with clinopyroxenes of Fe-rich
xenoliths  to  the  Poiana  Ruscă  Upper  Cretaceous  basaltic
andesites  (Downes  et  al.  1995)  and  Paleocene—Eocene  host
mafic rocks (Downes et al. 1995) (Fig. 9e), Fe-rich xenoliths
have a REY pattern similar to the basaltic andesites. However
their extended trace element pattern also shows notable dif-
ferences (e.g. significant Th-U positive anomaly and deple-
tions in HFS elements (Nb, Ta, Zr, Hf and Y) (Fig. 5c,d)).

Clinopyroxene megacrysts also show a rather different REY

pattern compared to the Type A xenoliths (Fig. 5c,d, Table 4).
They are more enriched in LREEs and have a steeply down-
ward shape throughout the whole REY pattern. Moreover,
they have a high Th, U and slightly low Nb, Ta concentrations
(Table 4). The composition of the calculated melt in equilib-
rium with the megacrysts resembles the mafic host (Fig. 9),
although there are some differences. For instance, the calcu-
lated melt in equilibrium with megacrysts is very rich in Th-U
and richer in HREE than the host rocks, however they are
very similar in their lack of a Nb negative anomaly.

This suggests that Fe-rich xenoliths and megacrysts are

deep-seated  crystallization  products  of  mafic  magmas,  in
agreement with the widespread interpretations of megacrysts
and  pyroxenites  as  direct  precipitates  from  mafic  alkaline
and the Fe-dunites from high Mg-magmas (e.g. Binns et al.
1970; Wilshire & Shervais 1975; Frey & Prinz 1978; Dobosi
& Jenner 1999; Cvetković et al. 2010). The Fe-rich xenoliths
may have crystallized from the older basaltic andesitic mag-
mas  of  Poiana  Ruscă  or  from  similar  magmas  and  subse-
quently  entrained  by  the  younger  host  basaltic  magmas.  In
contrast, the megacrysts are cogenetic with the host basalts.

The orthopyroxene-enrichment and poikilitic texture for-
mation

Orthopyroxene-rich xenoliths constitute a minor part of the

lithospheric mantle and may originate by distinct process(es).
Such xenoliths have been interpreted as mantle residues after
extensive  melt  extraction  (Ringwood  1958),  however,  this
seems to be unlikely for the studied xenoliths for several rea-
sons. The petrography of the Poiana Ruscă  Type A xenoliths
suggests that a slightly deformed porphyroclastic-equigranular
texture  was  overprinted  by  the  poikilitic  texture  (Fig. 3b,c
and  e),  where  orthopyroxene  was  growing  at  the  expense  of
olivine.  Besides,  > 32 %  modal  orthopyroxene  cannot  be
formed  by  classic  partial  melting  models,  rather  they  can  be
formed by reaction of an olivine-rich peridotite and a siliceous
melt  (Kelemen  et  al.  1998).  Silicate  minerals  in  the  Poiana
Ruscă  xenoliths  have  nearly  constant  mg#  and  olivines  have
constant Ni contents with an increasing orthopyroxene content
(Tables 1 and 2), whereas xenoliths formed by a high degree
of  partial  melting  should  show  a  positive  correlation  of  in-
creasing mg# in silicates and increasing Ni in olivine with the

increasing  modal  composition  of  orthopyroxene,  namely  an
increasing  degree  of  partial  melting.  Moreover,  the  cr#  of
spinels  is  rather  low  (8—23)  (Table 5),  indicating  only  a  low
degree ( < 10 %) of melt extraction as discussed earlier.

Several studies have proposed the origin of orthopyroxene-

enriched rocks via metasomatic reactions between more or
less depleted lithosphere peridotite and percolating melts/flu-
ids for lithologies in a wide range of tectonic settings from all
over the world (e.g. Kelemen et al. 1992; Smith et al. 1999;
McInnes et al. 2001; Santos et al. 2002; Arai et al. 2003, 2004,
2006; Bali et al. 2007, 2008; Rehfeldt et al. 2008; Dantas et al.
2009). Most studies explain orthopyroxene-enrichment of the
mantle rocks either by reaction with aqueous SiO

-rich melts

derived from slab dehydration and melting (e.g. Kelemen et
al. 1992; Arai et al. 2003, 2004) or by evolved, silica-saturated
alkali magmas percolating through the mantle (e.g. Arai et al.
2006; Dantas et al. 2009). Some recent studies from the Car-
pathian-Pannonian-Balkan Region have interpreted the forma-
tion of orthopyroxene-enriched mantle lithologies as resulting
from a percolating melt in the subcontinental lithosphere re-
lated to the melting of subducted slabs (Cvetković et al. 2004,
2007; Marchev et al. 2006; Bali et al. 2007, 2008; Berkesi et
al. 2012) or via magmatic liquids emerging from the astheno-
sphere associated with lithosphere thinning (Embey-Isztin et
al. 1989, 2001; Embey-Isztin & Dobosi 2011).

The petrography and chemistry of the studied poikilitic xe-

noliths  suggest  formation  of  orthopyroxene  by  reaction  of
slightly deformed and depleted peridotitic mantle rocks with a
silicate melt. Their poikilitic texture with large orthopyroxene
crystals, embedding olivines (Fig. 3b,c and e), suggests ortho-
pyroxene growing at the expense of the other silicates, mainly
olivine. The orthopyroxenes and clinopyroxenes in the studied
xenoliths are very rich in Al and Na (Fig. 4, Tables 3 and 4).
However,  Al + Na  cannot  be  derived  from  olivine,  hence  the
percolating  melt  needed  to  be  enriched  in  these  elements.
Therefore,  we  can  suppose  the  modification  of  the  mantle
lithologies  was  by  means  of  an  Al + Na-rich  silicate  melt.
Orthopyroxenes from the Massif Central (Xu et al. 1998), SW
Japan (Arai et al. 2006) and N Patagonia (Dantas et al. 2009),
with similar Al-rich compositions, were also described as re-
sulting  from  a  melt  percolation-reaction-crystallization  pro-
cess in a lherzolite precursor (Xu et al. 1998) and as reaction
products  of  highly  evolved  alkaline  melts  (Wulff-Pedersen
et  al.  1996,  1999;  Arai  et  al.  2006)  or  alkaline-subalkaline
(Dantas et al. 2009) with mantle olivine.

The pyroxenes in the whole series are poor in LREE (Type A

cpx: REE total 30—45 ppm; La/Lu

(N)

= 0.04—0.60) (Table 4),

which  can  be  explained  by  an  incompatible  trace  element
poor  melt  reacting  with  mantle  rock.  Because  volatile-rich
fluids have a great capacity to transport many incompatible
elements  (e.g.  Manning  2004),  such  fluids  were  unlikely  to
have  been  in  contact  with  the  studied  mantle  segment.  The
very  rare  occurrence  of  fluid/melt  inclusions  in  the  studied
xenoliths  also  confirms  the  volatile-poor  feature  of  the  per-
colating and reacting melt.

To test what melt type could have been responsible for the

reaction, we calculated liquid composition in equilibrium with
the clinopyroxenes using crystal/melt partition coefficients af-
ter McKenzie & O’Nions (1991), Hart & Dunn (1993) and

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Fig. 10. Comparative REE patterns of the clinopyroxenes from or-
thopyroxene-enriched xenoliths from the Carpathian-Pannonian-
Balkan Region (Cvetković et al. 2004) and from N-Patagonia
(Dantas et al. 2009).

Green  et  al.  (2000).  The  calculated  melt  compositions  in
equilibrium with the clinopyroxenes of the Type A xenoliths
have  roughly  flat,  slightly  LREE-enriched  and  slightly
HREE-depleted  patterns  (Fig. 9a).  The  calculated  melts  in
the  extended  trace  element  diagrams  show  roughly  similar
patterns  for  all  groups  (Fig. 9b).  All  patterns  are  character-
ized by significant positive Th-U anomaly. Strong Pb nega-
tive  anomaly  (Ce/Pb = 40—170),  with  the  exception  of  the
Ce6-11a  wehrlite,  is  notable  suggesting  that  the  melt  was
rather  depleted  in  Pb.  A  slight  Nb-Ta  negative  anomaly  is
also characteristic. Moreover, Zr shows a slight peak relative
to the Hf. The Type B xenoliths have more LREE-enriched
REY  patterns  than  the  Type  A  ones.  Enrichments  in  LILE
and  depletions  in  high  field  strength  (HFS)  elements  (Nb,
Ta, Zr, Hf, Ti) of calculated melt composition (Fig. 9c and d)
can indicate little similarity to the subduction related fluids/
melts (e.g. Manning 2004).

The geochemistry of the Type A xenoliths and calculated

melt  composition  in  equilibrium  with  the  xenolith  clinopy-
roxenes  suggests  that  the  formation  of  the  poikilitic  texture
and the orthopyroxene-enrichment of the lithospheric mantle
beneath  Poiana  Ruscă  was  linked  to  an  Al-  and  alkali-rich,
volatile-poor mafic melt. Xenolith pyroxenes have an overall
REE-poor and LREE-depleted character. A melt percolating
in the mantle for a long time can react several times with the
peridotitic  wall  rock  and  during  these  metasomatic  events
can lose the majority of its incompatible trace element con-
tent, while retaining its original less incompatible and com-
patible element content. A similar melt to the calculated melt
composition  in  equilibrium  with  the  Poiana  Ruscă  xenolith
clinopyroxenes  was  described  from  N-Patagonia  causing
orthopyroxene-enrichment  in  websterite  xenoliths  (Dantas  et
al. 2009), which are identical in REY pattern to the studied
Type A xenoliths (Fig. 10). Wulff-Pedersen et al. (1999) also
described that the IRC-process (basaltic melt infiltration, reac-
tion  and  crystallization)  can  form  evolved  Si-rich,  REE-poor
melts with low LREE/HREE ratios. Clinopyroxene-rich lher-

zolite (Cpx-L) xenoliths from East Serbia (Cvetković et al.
2004) also show similarities to the Poiana Ruscă xenoliths,
however these xenoliths are more enriched in LREEs (Fig. 10).

Geodynamic remarks

During the Late Mesozoic rifting-spreading-subduction

events (e.g. Schmid et al. 2008), various melts from an origi-
nally  heterogeneous  mantle  could  have  been  formed  within
the  subcontinental  mantle  and  impregnated  it  beneath  the
studied lithospheric mantle section. Some of these melts, re-
lated  to  the  subduction  events  of  the  Severin  and/or  Vardar-
Axios  paleo-oceans  (e.g.  Janković  1997;  Berza  et  al.  1998)
could  have  reached  the  mantle  beneath  Poiana  Ruscă,  after
migrating for a time in the mantle. Cvetković et al. (2013) also
hypothesized  that  the  East  Serbian  magmatic  rocks  of  the
same age (60—70 Ma) could be the results of the magmatic ac-
tivity  immediately  post-dating  the  subduction  and  formation
of Upper Cretaceous magmatic rocks of the Banatite-Timok-
Srednjegorje  Belt  (banatites),  developed  along  the  margin  of
the  European plate. Deep lithospheric fractures at the bound-
ary  between  the  Tisza  and  Dacia  zones  could  permit  these
small  volume  basaltic  melts  to  reach  the  surface  (Downes  et
al.  1995).  This  is  in  agreement  with  the  interpretation  of
Tschegg et al. (2010) of the Poiana  Ruscă magmatic activity
being  asthenospheric  decompression  melting.  Reaction  with
the  subduction-related  early  (66—72 Ma)  andesitic  melts  of
Poiana Ruscă (Herepea, Roscani) can also be a plausible ex-
planation for the formation  of the poikilitization event studied
in  the  mantle  xenoliths,  considering  the  good  agreement  for
LREEs with the calculated liquid composition in equilibrium
with the Poiana  Ruscă Type A xenoliths (Fig. 9a). However
interaction with such a melt hardly justifies the difference of
the calculated liquid in HREEs.

Conclusions

•

Moderate-low mg# of olivines and pyroxenes and low

cr# of spinel suggest that the mantle segment beneath Poiana
Ruscă experienced a low degree ( < 10 %) of mafic melt re-
moval. Furthermore, the Type A xenoliths are generally poor
in overall REEs and have rather flat REY patterns with slight
LREE-depletion;

•

The predominant lithology of the lithospheric mantle be-

neath Poiana Ruscă, represented by the Type A xenoliths,
suggests that a slightly deformed porphyroclastic-equigranu-
lar textured series represents the early mantle characteristics,
which were overprinted by orthopyroxene growth and
poikilitic texture formation at the expense of olivine;

•

Such geochemistry of the Type A xenoliths and calcul-

ated  melt  composition  in  equilibrium  with  the  xenolith  cli-
nopyroxenes  suggest  that  the  percolating  melt,  caused
formation  of  poikilitic  texture  in  the  mantle  portion  repre-
sented by the Type A xenoliths. This melt could have been a
mafic,  Al-Na-rich,  volatile-poor  melt.  Little  similarity  is
seen to the Late Cretaceous—Paleogene (66—72 Ma) subduc-
tion-related andesitic magmatism of Poiana Ruscă;

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•

Subsequent passage of mafic melts in the mantle, with

similar compositions to the older andesitic magmatism of
Poiana Ruscă, is recorded in the pyroxenites (Fe-rich xeno-
liths). These pyroxenites are similar to Cpx-rich lherzolites,
occurring in the E Serbian rocks, whereas the megacrysts
seem to be cogenetic with the host basanite.

Acknowledgments:  The  authors  thank  the  members  of  the
Lithosphere  Fluid  Research  Lab  (Eötvös  University,  Buda-
pest)  for  constructive  discussions  and  indispensable  help  in
sample  collection.  The  authors  would  like  to  thank  Hilary
Downes for her kind advice and useful suggestions. The re-
views by Enikő Bali and Vladica Cvetković were very much
appreciated  for  the  improvement  of  the  manuscript.  Many
thanks  go  to  Géza  Császár  for  making  possible  microprobe
analysis, as well as to Terry Williams, Teresa Jeffries, John
Pratt, Anton Kearsley, Tony Wighton (Natural History Mu-
seum, London) and Nagy Géza (Hungarian Academy of Sci-
ences,  Institute  for  Geological  and  Geochemical  Research,
Budapest) for technical help in microprobe and LA-ICP-MS
analysis.  These  studies  were  supported  by  Grants  from  the
Hungarian  National  Scientific  Research  Foundation  to
Zsuzsanna Nédli and Géza Császár (PF 64020 and K 62468
Projects),  to  Zsuzsanna  Nédli  for  the  Synthesys  Project
(GB-TAF  4146  Project)  and  to  Júlia  Dégi  for  the  TéT
Project. This is the 62

publication of the Lithosphere Fluid

Research Lab, Eötvös University, Budapest.

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