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GEOLOGICA CARPATHICA, JUNE 2007, 58, 3, 211—228


Late Cretaceous basaltic volcanic rocks of the North Mi-
nusinsk Depression Volcanic Field, southern Russia,
(Fig. 1) contain xenoliths both of lower crustal and upper
mantle origin, as well as large (up to several cm) clinopy-
roxene xenocrysts. Earlier studies dealing with the xeno-
liths of this volcanic field (Ashchepkov et al. 1995;
Malkovets et al. 1998, 2000) were focused on a few partic-
ular localities (e.g. Tergesh, Krasnoozerskaya, Bele, Kon-
garovskaya), where garnet-bearing xenoliths (garnet
lherzolite and garnet pyroxenite) occur (Malkovets et al.
1998, 2000). Although these papers give detailed studies
of the investigated localities, a detailed picture of the
lithospheric structure of the North Minusinsk Depression
(NMD) has not been established so far. Detailed petro-
graphic and geochemical work on such xenoliths permits
us to constrain the nature of the subcontinental mantle.

The modal amount of minerals in xenoliths provides

evidence on the degree of depletion and also on enrich-
ment processes acting on a particular mantle region (Frey

Fluid induced melting in mantle xenoliths and some

implications for the continental lithospheric mantle from the

Minusinsk Region (Khakasia, southern Siberia)















Lithosphere Fluid Research Lab, Department of Petrology and Geochemistry, Eötvös University,

Pázmány Péter sétány 1/C, H-1117 Budapest, Hungary;  *


Research School of Earth Sciences, The Australian National University, Building 61, Mills Road, Canberra ACT 0200, Australia;


Institute of Mineralogy and Petrography, United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of Russian

Academy of Sciences, Koptuyga Pr. 3, 630090 Novosibirsk, Russia;


Department of Geological Sciences, Geozentrum, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria;

(Manuscript received September 12, 2005; accepted in revised form December 7, 2006)

Abstract: Eleven representative xenoliths from the Minusinsk Region, southern Russia were studied in order to highlight the
characteristic features of the subcontinental lithospheric mantle beneath the region. Type-I xenoliths show that the lithosphere
underwent various degree of depletion overprinted by enrichment processes leading to LREE-enriched pyroxenes. Estimated
equilibrium temperature for the xenoliths is in the range of 960—1050 

ºC. Type-II xenoliths are the result of crystallization

from a possibly basaltic melt close to the crust-mantle boundary. Three xenoliths in the Type-I series show evidence of incipient
melting such as spongy rims of pyroxenes and interstitial glass. The spongy rims of clinopyroxene consist of clinopyroxene
and glass with modal proportion of approximately 82 and 18 %, respectively. Orthopyroxene rim contains olivine (65 %) and
glass (35 %) with subordinate amounts of clinopyroxene ( < 5 %). Glass within the spongy rims exhibits a clear geochemical
affinity to interstitial glass as both have similarly high Al




, SiO


 and alkali contents. The interstitial glass and the spongy rims

(minerals + glass) display light rare earth (LRE) element and large ion lithophile (LIL) element enriched character. This
indicates that incipient melting of pyroxenes occurred in an open system and was likely triggered by the influx of a Na alkali
silicate melt/fluid. The interstitial glass represents the residual melt after interaction with the pyroxenes. The formation of this
Na-rich silicate melt may represent an earlier stage of the mantle magmatic event that produced the host basalt.

Key words: Minusinsk Region, lithospheric mantle, petrology, major and trace element geochemistry, melting in the
upper mantle.

& Green 1974; Yaxley et al. 1991). Mineral assemblages
of the studied mantle xenoliths will be used for high-
lighting the particular features of the lithosphere beneath
the NMD. The composition of the studied pyroxenes al-
lows determination of the P-T conditions of the xeno-
liths, with implications for the thermal state of the lithos-
phere (Brey & Kohler 1990; Nimis 1995). Furthermore,
pyroxene trace element compositions provide an addi-
tional tool for exploring multiple depletion and enrich-
ment events (i.e. Ionov et al. 2002).

In this study we present modal contents and major ele-

ment composition of minerals in seven Type-I and four
Type-II xenoliths along with P-T estimations. This infor-
mation is used to gain a better insight into the lithospheric
structure of the Minusinsk Region. In the studied series
three Type-I xenoliths show signs of incipient melting and
contain interstitial glass. In order to better understand this
melting event trace and major element analysis of py-
roxenes, their rims, interstitial glass and host basalt are
presented. We discuss the relationships between glass
formed in the rims of pyroxenes with interstitial glass and

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with the host basalt. With this data we are able to show
that incipient melting was triggered by the influx of an al-
kaline metasomatic agent that displays chemical affinities
to the host basalt.

Geological background

The North Minusinsk Region is located on the Salair seg-

ment of the Altay-Sayan fold belt (Fig. 1). The major geo-
logical features are trachybasalt-trachyte-trachydacite and
basanite-phonotephrite volcanic series erupted in Early to
Middle Devonian times (Zubkov 1986; Zonenshain et al.

1990). The genesis of these volcanic series
can be related to the formation of a NW-trend-
ing Devonian rift system. The area is charac-
terized by a thick Paleozoic sedimentary
cover sequence (Fig. 1). Deposition of these
sandstones and limestones are presumably
related to the Caledonian orogeny. Since
then the region has not been affected by
any significant tectonic event until the Late
Cretaceous when considerable thinning of
the lithosphere took place. This thinning is
best documented in alkaline basaltic volca-
nic activity (Zubkov 1986; Zonenshain et
al. 1990).

Formation of these Cretaceous basalt

pipes began with intensive gas releases and
crushing of bedrocks (Kryukov 1964). Then
basanitic melts percolated into the axial
zone of eruptive breccia and fractures (Lu-
chitsky 1960; Kryukov 1964). Subsequent-
ly, these melts reached the surface forming
basalt cones along a NW-SE trending vent
system (Fig. 1). The northwestern segment
of this structure has been moved eastward
along a NE-SW trending fault after the Cre-
taceous (the exact age is unknown). The
northern part of the dyke system includes
five dykes, whereas the southern group con-
sists of 25 dykes (Fig. 1). The dykes are up
to several hundred meters long along strike
with a medium thickness of 3—5 m, which
locally can reach 10—15 meters. The NW-SE
striking dikes are deformed by a number of
NE-SW striking dextral strike-slip faults.
Such patterns of the volcanic dikes are re-
garded as evidence for a tangential NE di-
rected extension and a NW directed
compression that was coincidental with the
penetration of alkali basaltic melt (Ash-
chepkov et al. 1995).

Formation of the NMD basaltic magmas

has been related to decompressional melt-
ing of the upwelling asthenospheric mate-
rial (Litasov et al. 2002). These authors
suggested that the driving force for the de-
compression may have been the geody-

namic rearrangement of the Central Asian lithosphere at
the onset of the India-Eurasia collision. It is known that
the clockwise rotation of the Siberian platform formed
local extensional-compressional environments, which
led to decompressional melting at greater depths (Khra-
mov 1997).

The age of the basaltic rocks from the Bele and

Tergesh (TSH) pipe were determined by the 





dating technique (Malkovets et al. 2003). They found
clearly defined plateau ages of 79 ± 2.0 and 77 ± 1.9 Ma.




Ar ages of 74 ± 5.5 Ma have been found for

the Kongarovskaya (KONG) pipe and of 75 ± 6.2 Ma for
the Sister (SIST) pipe.

Fig. 1. Schematic geological map of the North Minusinsk Depression showing the
most significant basalt pipes (after Malkovets et al. 2003).

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In the summer of 2003, 150 mantle xenoliths were col-

lected from eight distinct volcanic pipes of the NMD:
Tergesh (TSH) 19, Krasnoozerskaya (KZ) 37, Konga-
rovskaya (KONG) 33, Three Brothers (BRO) 9, Sister
(SIST) 10, Dzhirim (DZH) 19, Chebaldak (CH) 14 and
Tochilnaya (TL) 9 (Fig. 1). Eleven samples were selected
for detailed studies.

The modal compositions of xenoliths were determined by

point counting (at least 2000 points) for each sample (Ta-
bles 1 and 2). Two major groups of xenoliths were distin-
guished. One group consists of peridotite xenoliths
containing Cr-diopside (Type-I of Frey & Prinz 1978), in
which modal proportion of olivines always exceeds
40 vol. %. Seven Type-I xenoliths (CHI3, DZH1, KZC1,
KZSI6, SISTI2b, TLC1, TSHII9) have been studied. The other
group includes four xenoliths (BROI1, KONGII1, KZSII4,
SISTII5) and is characterized by a high modal proportion of
pyroxenes (orthopyroxene + clinopyroxene > 80 v/v %) and
the presence of Al-augite (Type-II, of Frey & Prinz 1978).

Type-I xenoliths

The peridotite xenoliths include: three wehrlites

(SISTI2b, CHI3, DZH1), two lherzolites (KZC1, TSHII9),
one harzburgite (TLC1) and one dunite (KZSI6) (see their
modal composition in Tables 1 and 2). The xenoliths usu-
ally exhibit porphyroclastic texture, however, protogranu-
lar texture also occurs subordinately. The grain
boundaries are slightly curved, and internal strain features
are absent, or occur only rarely in the large porphyroclasts
(olivines and mostly orthopyroxenes).

Primary olivine (ol-1) is often found as porphyroclasts (up

to 3.5 mm) or smaller anhedral crystals (0.5—1.0 mm). The
grain boundaries are straight (Fig. 2a and 2b). Newly formed
(secondary) olivine (ol-2), is found only in rare spongy rims
of orthopyroxene (details below, Fig. 2c and 2d). Such newly
formed olivine is elongated and radial to orthopyroxene and
has an average grain size of less than 80  m.

Primary clinopyroxene (cpx-1) is present as fine-grained

crystals with an average grain size of 0.5—1.0 mm. The
grain boundaries are curved and in several peridotites the
clinopyroxene has a spongy rim, which consists of cli-
nopyroxene and glass. The clinopyroxene core is always
clear (Fig. 2a). The proportion of the spongy clinopyrox-
ene and the glass was calculated using backscattered elec-
tron (BSE) images, and it gives consistently 82—88 % area
for the spongy clinopyroxene and 12—18 % area for the
enclosed glass (Fig. 2a and 2b).

Orthopyroxene (opx-1) is frequently observed as por-

phyroclasts (up to 4 mm). Orthopyroxene occasionally has
a spongy rim, which contains small olivine grains and
glass (Fig. 2c and 2d). The proportions of olivine (ol-2)
and glass are 65 % area and 35 % area, respectively. How-
ever, rare newly formed clinopyroxene (cpx-2) was also
recognized in spongy rims (max. 5 % area) (Fig. 2d).
Generally, the spongy rim around orthopyroxene is less
extensive than the spongy rim around clinopyroxene.

Primary spinel (sp-1) occurs as isometric relatively small
(0.5—1.0 mm),  brown coloured grains mostly in interstitial
settings. Occasionally, it can be found as inclusions
within silicate phases. Some spinel rarely contains spongy
rims, as well. Spinel was also identified as dark brown
lamellae in clinopyroxene (sp-1) (DZH1 wehrlite (Fig. 2e)
and KZC1 lherzolite). Secondary spinel (sp-2) was only
found in interstitial glass of SISTI2b wehrlite (details
below), with a grain size of 20—40  m.

Interstitial glass (i.e. not only along grain boundaries)

was found in the wehrlite SISTI2b as amorphous brown
coloured patches ranging in size from 250 to 300  m. The
modal proportion of the glass is considerable (2.4 v/v %).
The interstitial glass is not homogeneous and includes
secondary clinopyroxene ( < 200  m), spinel (20—40  m)
and feldspar ( < 200  m) crystals. Minor interstitial glass
patches were also recognized in KZC1 lherzolite, howev-
er, detailed petrographic observation was not possible due
to the small size of the glass inclusions ( < 100  m). Glass
was also identified along the rims of spongy clinopy-
roxenes and orthopyroxenes (SISTI2b wehrlite, TLC1
harzburgite, KZC1 lherzolite).

Type-II xenoliths

Pyroxenite-rich xenoliths (Type-II) are clinopyroxenites

(KONGII1; SISTII5), plagioclase-bearing ultramafic rock
(KZSII4) and olivine websterite (BROI1) (Table 2). These
rocks display an igneous texture. Tiny elongated spinel
lamellae frequently occur in clinopyroxenes. In addition,
oriented amphibole lamellae can also be observed
(KONGII1 clinopyroxenite), and slightly altered amphibole
patches are also present. Plagioclase was only found in one
xenolith (KZSII4 clinopyroxenite). Vermicular green spinel
inclusions were recognized in this plagioclase (Fig. 2f).


Analytical techniques

Major element compositions of the primary minerals

were determined with a JEOL SUPERPROBE JXA-8600
electron microprobe at the Department of Earth Sciences,
University of Florence, Italy. The operating conditions
were: accelerating voltage of 15 kV, beam current of
10 nA, beam size of 5  m for pyroxene and spinel, 3  m
for amphibole and feldspar, and 40 s of counting time for
all the elements. Standard corrections of Bence & Albee
(1968) were applied.

The compositions of glass and minerals affected by

melting (i.e. spongy clinopyroxene, incongruent orthopy-
roxene and interstitial glass) were obtained by wave-
length-dispersive spectrometry using a CAMECA SX-100
electron probe X-ray microanalyser at the Department of
Geological Sciences, University of Vienna, Austria. Oper-
ating conditions were: accelerating voltage of 20 kV,
beam current 10 nA with a beam size 5  m. Only glassy
patches bigger than 10  m were considered for analysis, in

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Table 1: Major (wt. %), trace element (ppm) and modal (vol. %) compositions of minerals of KZC1 (spinel lherzolite), SISTI2b (wehr-
lite) and TLC1 (harzburgite). Host basalt analysis is also indicated.    Continued on the next page.

Explanations to abbreviations see on the next page.

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Table 1:      Continued.

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Table 1:      Continued from the previous pages.

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Fig. 2. Photomicrographs of fabrics and textural features in the North Minusinsk Depression upper mantle xenoliths. Photos (a), (c), (e) and
(f) were taken in plane polarized light. Photos (b) and (d) are BSE images. cpx-1 – clinopyroxene core; cpx-II – newly formed clinopyrox-
ene relating to the spongy rim of orthopyroxene; fp-1 – feldspar; gl – glass; ol-1 – olivine; ol-II – newly formed olivine relating to the
spongy rims of orthopyroxene; opx-1 – orthopyroxene; sp-1 – spinel. (a) – Melting of clinopyroxene (SISTI2b wehrlite) surrounded by
primary olivine. (b) – Melted (spongy) clinopyroxene on BSE image (KZC1 lherzolite) surrounded by primary olivine. (c) – Incongruent
melting of orthopyroxene (TLC1 harzburgite). Orthopyroxene breaks down into fine-grained assemblages of clinopyroxenes, olivine and
glass. (d) – BSE image of a melted orthopyroxene in KZC1 lherzolite. (e) – Spinel found as dark-brown lamellae in clinopyroxene (DZH1
wehrlite and KZC1 lherzolite). (f) – Vesicular, green spinel inclusions were recognized in plagioclase (KZSII4 clinopyroxenite).

order to avoid significant volatilization. Potential volatil-
ization and loss of Na and K was monitored via plotting
the count rates as a function of counting time. Corrections
have been made where it was necessary. Counting times
for Na and K were 10 seconds and for all other elements 40
seconds with standard ZAF correction procedures applied.

Trace element composition of interstitial glass, host ba-

salt, and minerals were measured by laser ablation, induc-
tively-coupled plasma mass spectrometry (LA ICP-MS) at

the Research School of Earth Sciences, Australian Nation-
al University, Australia. A pulsed 193 nm ArF Excimer la-
ser with 100 mJ energy at a repetition rate of 5 Hz coupled
to an Agilent 7500 quadropole ICP-MS were used for ab-
lation. A spot size of 180  m was needed for silicate phas-
es and host basalt, and a size of 80  m was deployed for
interstitial glass. From the spongy rim of clinopyroxene
and incongruent rim of orthopyroxene only bulk analyses
were available, due to the grain sizes of secondary miner-

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Table 2:

 Major element (wt. %) and modal composition (vol. %) of minerals from additional representative Type-I and Type-II Minusa xeno


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als and glass patches which were much less than the opti-
mal smallest spot size. The counting time was 20—25 s for
the background and 40—45 s for sample analysis. Instru-
ment calibration was against NIST 612 glass, and NIST
610 and BCR glass were used as secondary standards. 



was employed as the internal standard isotope, based on


 concentrations previously measured by electron mi-

croprobe. For the spongy rims of pyroxenes SiO



trations from the calculated mass balance were used as
internal standards (see below) (Table 1).

Major element composition


Regardless of their textural setting (primary ol-1 or newly

formed ol-2 in spongy rim of orthopyroxene) olivines from
the NMD xenoliths have similar major element composi-
tions and display a narrow compositional range.  Mg-val-
ues (mg#) range from 0.87 to 0.91 (Tables 1 and 2).
Olivine in the dunite xenolith (KZSI6) shows the lowest
values, whereas olivine in TSHII9 lherzolite displays the
highest forsterite content. There is no significant variation
between Type-I and Type-II xenoliths in terms of mg#.
The NiO content is in the range of 0.26—0.37 wt. % in the
Type-I series. Only BROI1 (olivine websterite) of the
Type-II group contains olivine, but this has not been anal-
ysed for NiO (Table 2).


Orthopyroxene (opx-1) has a different composition in

the Type-I and olivine websterite (BROI1) xenoliths. Or-
thopyroxene in lherzolites, harzburgites has more Al




(4.4—5.7 wt. %) than in wehrlites and olivine websterite
(2.9—3.8 wt. %) (Tables 1 and 2). Orthopyroxene in Type-I
series and olivine websterite (BROI1) is characterized by
approximately  88—90 En component, 0.30—0.50 wt. % of




 and maximum 0.11 wt. % of TiO

contents. In con-

trast, orthopyroxenes in Type-II xenoliths (KONGII1) of
the NMD have lower Cr




 (0.06 wt. %) and slightly ele-

vated TiO


 content (0.15 wt. %) than the orthopyroxenes

in the Type-I series (Table 2).


Primary clinopyroxenes (cpx-1) in the Type-I xenoliths

has 3.19—6.81 wt. % of Al




 (Tables 1 and 2). Clinopy-

roxene in harzburgite and lherzolites has more TiO







 (0.26—0.41 wt. %, 5.96—6.81 wt. %, respectively)

than in wehrlites and olivine websterite (0.11—0.23 wt. %,
3.19—4.85 wt. %, respectively). The Na


O- and Al





tent of clinopyroxene shows scattered distribution. FeO
and TiO


 display narrower compositional range and also

negative correlation with MgO.

The major element composition of the newly formed cli-

nopyroxene (cpx-2) varies with its textural setting (Ta-
ble 1, Fig. 3). The clinopyroxene incorporated in
interstitial glass is rich in Al




 (5.90—7.67 wt. %), TiO


Fig. 3. Na vs. Al (in a.p.f.u.) diagram for clinopyroxenes from
different textural setting in the North Minusinsk Depression xeno-
liths which exhibit evidence of melting/metasomatism (KZC1
lherzolite, SISTI2b wehrlite, TLC1 harzburgite).

(1.42—2.36 wt. %) and Cr



(0.82—2.98 wt. %) and poor in

MgO (14.6—15.9 wt. %) and SiO


 (47.6—49.1 wt. %). In spongy

rims of orthopyroxene, it is rich in SiO


 (53.7—55.2 wt. %),

MgO (18.7—20.4 wt. %) and Cr




 (0.51—1.66 wt. %), but poor

in Na


O (0.57—0.87 wt. %) and Al




 (0.47—1.28 wt. %). The

spongy rims of primary clinopyroxene (cpx-1) possess low


O (0.36—1.30 wt. %), Al




 (0.58—3.81 wt. %) and high





 (0.46—2.26 wt .%) and CaO (19.9—22.8 wt. %) contents

(Fig. 3).

In the Type-II xenoliths the clinopyroxene has 44.4—54.1 wt. %

of SiO


, 8.91—16.3 wt. % of MgO and 4.20—13.6 wt. % of





. Clinopyroxene, especially in clinopyroxenite- and

plagioclase-bearing ultramafic xenoliths (KZSII1, SISTII5)
show considerably lower amounts of SiO


 and MgO, whereas,

they are enriched in Al




 (Table 2). Clinopyroxene in

KZSII1 clinopyroxenite and SISTII5 plagioclase-bearing ul-
tramafic xenoliths is distinct from that in the BROI1 olivine
websterite and KONGII1 clinopyroxenite (Table 2).


Primary spinel (sp-1) in Type-I xenoliths has 11.0—23.2 wt. %

of Cr




, 42.5—56.0 wt. % of Al




 and mg# of 0.61—0.76

(Tables 1 and 2). Newly formed spinel (sp-II in the intersti-
tial glass of SISTI2b wehrlite) has a 23.3—30.1 wt. % Cr




and 24.1—37.0 wt. % Al




 contents (Table 1). Spinel in

wehrlite and olivine websterite xenoliths has higher cr#
(0.20—0.30) and lower mg# (0.66—0.70) than that in
harzburgite and lherzolite xenoliths (0.12—0.15, 0.75—0.76,
respectively) (Tables 1 and 2). Spinel from Type-II xeno-

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liths shows a wide range in composition, whereas spinel
from olivine websterites (BROI1) overlaps with that of the
Type-I xenoliths (Table 2). In the KONGII1 clinopyroxen-
ite spinel is between Type-I and the other two Type-II rocks,
which show the lowest Cr




 and MgO content (Table 2).

The spinel in olivine websterite (BROI1) is characterized by
the highest Cr-value (cr#) of 0.30.


Feldspar as a primary phase is only found in the plagio-

clase-bearing ultramafic xenolith (KZSII4) of the Type-II
series (Fig. 2f). The feldspar is labradorite and inhomoge-
neous in composition (Table 2). Its anorthite content rang-
es from 61 % to 68 %. However, feldspar as a secondary
phase in interstitial glass of SISTI2b is enriched in Na, and
has an anorthite content of 40—45 % (Table 1).


Glasses in the xenoliths from the NMD are plotted on

the total alkalis vs. silica diagram (Table 1, Fig. 4) (Le Bas
et al. 1986). Interstitial glass in the wehrlite and lherzolite
xenoliths (SISTI2b and KZC1, respectively) display very
high total alkalis and fall in the phonolite-tephriphono-
lite-trachyte field (note that these fields do not mean that
these glasses correspond to any particular volcanic rock,
since in this case it is not used for bulk composition of ig-
neous rocks). However, this glass exhibits very high Al




(23.3 wt. %) and very low CaO (0.77 wt. %) and MgO
(0.82 wt. %) contents (Table 1). Glass in the spongy rim of
orthopyroxene, displays a similarly high alkali content at
an elevated SiO


 content. Glass in the spongy rim of cli-

nopyroxene shows the widest range of composition. The


 content varies between those of interstitial glass and

glass in the spongy rim of orthopyroxene, whereas only
the maximum alkali concentration of glass in the spongy
rims of clinopyroxene is comparable to that of the former
ones (Table 1, Fig. 4).

Trace element geochemistry of glass-bearing Type-I xe-

Representative in situ trace element analyses of clinopy-

roxenes (SISTI2b, KZC1, TLC1), orthopyroxenes (TLC1
and KZC1) and interstitial glasses (SISTI2b) from the melt-
ed Type-I peridotites and those of their host rock are report-
ed in Table 1. Since the grain size of secondary minerals
and glass patches in the rims of clinopyroxene and orthopy-
roxene are much smaller than the optimal smallest spot size
of the LA-ICPMS, only bulk analyses were available from
the spongy rims of these minerals (Fig. 2b,d).


The cores of clinopyroxene (clear clinopyroxene without

any interstitial glass) show different trace element patterns
in the three different rock types (Fig. 5a—b). The overall
REE pattern of clinopyroxene shows an enriched character

(cf. C1 chondrite) with enrichment in LREE and depletion
in HREE (Fig. 5a). Nevertheless, the SISTI2b wehrlite and
TLC1 harzburgite have elevated MREE contents, whereas
the KZC1 lherzolite shows convex upward REE pattern

Fig. 4. Compositions of glass in ultramafic xenoliths from the North
Minusinsk Depression plotted in the TAS diagram (Le Bas et al. 1986).
Glass in lherzolite (KZC1) and harzburgite (TLC1) xenolith displays
trachytic composition and is similar to those glasses related to incon-
gruent melting of orthopyroxene. Glass in wehrlite (SISTI2b) exhibits
lower SiO


 and Na


O + K


O content than those in the harzburgite

(TLC1) xenolith and falls into the phonolite-tephriphonolite field.

Fig. 5. Chondrite (Nakamura 1974) normalized REE and primitive
mantle (McDonough & Sun 1988) normalized trace element distri-
bution of clinopyroxene, orthopyroxene, glass and host basalt of the
three selected North Minusinsk Depression xenoliths showing evi-
dence of melting/metasomatism. Note that for better demonstration
and understanding the log scale is not the same for clinopyroxene
and orthopyroxene (a—d). Also, the shape of the symbol represents
the rock type of the glasses (e—h). (a) – C1 chondrite normalized
REE pattern of clinopyroxenes. (b) – Primitive mantle normalized
trace element pattern of clinopyroxenes. (c) – C1 chondrite normal-
ized REE pattern of orthopyroxenes. (d) – Primitive mantle normal-
ized trace element pattern of orthopyroxenes. (e) – C1 chondrite
normalized REE pattern of interstitial glass from SISTI2b wehrlite.
(f) – Primitive mantle normalized trace element pattern of interstitial
glass from SISTI2b wehrlite. (g) – C1 chondrite normalized REE
pattern of glass from the host basalt. (h) – Primitive mantle normal-
ized trace element pattern of glass from the host basalt.

background image



Fig. 5.

background image



with depletion in MREE. Spongy rims, which consist of
glass and clinopyroxene, show REE patterns similar to the
clean clinopyroxene cores. The rims of SISTI2b wehrlite
display a slight positive Eu-anomaly (Fig. 5a).

The trace element pattern of clinopyroxene shows nega-

tive anomalies (cf. primitive mantle) in HFS- (especially
Ti, Zr an Nb) and fluid mobile elements (especially Pb)
(Fig. 5b). Clinopyroxene exhibits considerable negative
anomaly in LIL elements (especially Cs, Rb and Ba).
Spongy rims display very similar trace element pattern to
cores, apart from being enriched at least 100 times in LIL
elements over the clean clinopyroxene (Fig. 5b).


The REE composition of orthopyroxene shows convex

upward pattern with enrichment in La, Ce, and HREE and
depletion in MREE (Fig. 5c). The rims of orthopyroxenes
(including glass, fine-grained secondary olivine and sec-
ondary clinopyroxene, see Fig. 2d for details) in TLC1
harzburgite are much more enriched in REE (2—12 times
that of the C1 chondrite) than the core (0.01—0.1 times that
of the C1 chondrite) and also displays a slight positive Eu-
anomaly (Fig. 5c).

The orthopyroxene of TLC1 harzburgite shows signifi-

cantly lower trace element concentrations relative to cli-
nopyroxene (0.01—1 times that of the primitive mantle),
with depletion in LIL elements (especially Rb, Ba) and
positive anomalies in P and Ti (Fig. 5d). The orthopyrox-
ene in KZC1 lherzolite shows negative anomalies in Rb,
Ba, Nb, K, La, Pr, Sr and significant positive anomalies in
P, Ti and fluid mobile elements (note that the spongy rim
of orthopyroxene consists also of secondary olivine, glass
and secondary clinopyroxene, Fig. 2d). The orthopyrox-
ene rims in TLC1 harzburgite are enriched in LIL (espe-
cially Cs, Rb) and slightly depleted in HFS (especially Zr,
Ti) elements (cf. primitive mantle), however, the trace ele-
ment concentration is higher than that of the core for each
element (Fig. 5d).

Interstitial glass

Interstitial glass was only analysed for trace elements in

the SISTI2b wehrlite, because other glass patches in KZC1
were too small for appropriate analysis. The interstitial
glass shows a REE pattern (cf. C1 chondrite) enriched in
LREE and relative depletion in HREE (Fig. 5e).

Interstitial glass shows enrichment in LIL (especially Cs,

Rb, Th) and HFS (especially Nb, Zr, Ti) elements. Signifi-
cant positive anomalies were found in Rb, Nb and Zr. Nega-
tive anomalies were observed in Ba, Sr, and Eu (Fig. 5f).

Host basalt

Host rock analyses were carried out using a large spot

size (~180  m) and analysing host basalts of which micro-
phenocrysts were smaller than 50  m. A SiO


 content of

52 wt. % has been used as internal standard. The trace ele-
ment compositions of the host rocks are very similar in all

the three studied xenoliths (Fig. 5g—h, Table 1). The gen-
eral patterns of the REE and trace elements of interstitial
glass look similar to the host basalt, however, the main
differences are the negative Rb anomaly, positive P anom-
aly and the absence of significant positive anomaly in Nb
and Zr in the host rock, in addition a slight positive Eu-
anomaly occurs in the host basalt (Fig. 5g—h).

  P-T estimations

We calculated the equilibrium temperature for the Type-I

xenoliths using the Ca-in-opx geothermometer of Brey &
Kohler (1990). For this calculation cores of primary orthopy-
roxenes were used and we assumed that the equilibrium pres-
sure of the xenoliths was 1.5 GPa. The equilibrium
temperatures of xenoliths from the NMD fall in the range of
960—1050 ºC. The two-pyroxene method of Brey & Kohler
(1990) was also applied and it provided temperatures of
1010—1150 ºC, which is higher than for the Ca-in-opx meth-
od. However, the two-pyroxene method generally overesti-
mates the real temperature. Oxygen fugacity was calculated
using the method of Ballhaus et al. (1990). The oxygen
fugacity varies between FMQ —2 and 0.5  log(fO



Primary clinopyroxenes in Type-II xenoliths fall in the

field of granulite and garnet clinopyroxenite xenoliths on
the Al




 diagram (not shown), which implies pressure

condition corresponding to the lower crust-uppermost man-
tle transition. Approximate pressure values for these xeno-
liths are obtained by using the technique of Nimis (1995).
This method gives 1.2—1.4 GPa (approximately 42—49 km)
for this group of xenoliths.


Some implications for the lithospheric mantle beneath
the NMD

According to the petrographic and geochemical fea-

tures, the xenoliths studied can be divided into two major
groups. Type-I xenoliths and BROI1 clinopyroxene-rich
xenoliths comprise the first group. BROI1 olivine webster-
ite belongs to this group because it displays similar com-
positions to those of Type-I (peridotite) xenoliths. The
relationship between the Fo content in olivines and cr# in
the coexisting spinel of the lherzolite xenoliths from the
NMD is plotted on Fig. 6. Subvertical lines indicate the
degree of partial melting after Jaques & Green (1980).
Note that this method only works for lherzolite xenoliths
with moderate Al




 and CaO content. Type-I xenoliths of

the NMD display a degree of depletion less than 20 %, ap-
plying the diagram of Arai (1994) (Fig. 6).

Type-I xenoliths from the North Minusinsk Depression

show a wide petrological variety from dunite to plagio-
clase-bearing ultramafic rock and also a considerable
geochemical diversity in minerals of different xenoliths. A
depletion event is clearly reflected in the major and trace
element composition of the mineral constituents in some

background image



Fig. 6. Relationship between the Fo content of olivines and
cr# = Cr/(Cr + Al) (in atomic ratio) of coexisting spinel in upper man-
tle xenoliths from the North Minusinsk Depression. OSMA = olivine-
spinel mantle array is shown by dashed lines, after Arai (1994). The
degree of partial melting is shown by subvertical lines (Arai 1994).
Dotted lines indicate the approx. pressure of partial melting (Jaques
& Green 1980).

xenoliths (original depletion in LREE subsequently over-
printed by an enrichment process, high cr# and mg#). This
depleted feature is documented in the composition of por-
phyroclastic pyroxenes and has been labelled “component
A” by Frey & Green (1974). A subsequent enrichment is
represented by a concave upward trace element pattern
predominantly in rim of orthopyroxene and some clinopy-
roxene (i.e. enrichment in LREE (Fig. 5a,c)). This is called
“component B” by Frey & Green (1974). Relatively low
cr# in spinel and the fact that the suspected degree of par-
tial melting is lower than 20 % (Tables 1, 2, Fig. 6) sug-
gest that the continental lithosphere beneath the NMD is
somewhat less depleted than those of areas with thinner
lithosphere (i.e. Szabó et al. 2004). Seven relatively cli-
nopyroxene-rich xenoliths are not enough to claim the
less depleted nature of the lithosphere beneath this area.
However, more than 100 xenoliths, from which these sev-
en were selected for further study, display very similar
modal composition based on macroscopic observations.
The origin of clinopyroxene-rich (i.e. wehrlite such as
SIST12b) xenoliths is commonly thought to be associated
with carbonatite metasomatism (Yaxley et al. 1991). This
is maybe supported by the trace element pattern of py-
roxenes, which also shows enrichment in elements related
to carbonatites (i.e. LREE, U, Th) (Yaxley et al. 1991)
(Fig. 5a—d). Furthermore, Ionov et al. (2005) suggested
that clinopyroxene enrichment in the mantle can also be
achieved by interaction with silica undersaturated melts.
The less depleted character of the lithosphere is in agree-
ment with geological observations that there is no evi-
dence for voluminous volcanic activity before the
Cretaceous basaltic magmatism in the NMD, where only
subordinate Devonian silicic volcanics have been report-
ed. Therefore, the lack of extensive volcanism (Zubkov
1986; Zonenshain et al. 1990) in association with either

the Cretaceous or the Devonian thinning events also pre-
vented the continental lithospheric mantle of the NMD
from a higher degree of depletion.

Clinopyroxenite and plagioclase-bearing ultramafic xe-

noliths (KONGII1, SISTII5, KZSII4) make up the Type-II
group based on their comparable petrographic and
geochemical characters. These rocks of the Minusinsk Re-
gion display high concentrations of basaltic major elements
(i.e. Na, K, Al, Ca, Table 2) in the rock-forming minerals. In
addition, the mineral assemblage in these xenoliths is en-
riched in clinopyroxenes and plagioclase. We suggest that
these rocks crystallized from a basaltic melt close to the
present day mantle/crust boundary beneath the North Mi-
nusinsk Depression (39—42 km) according to our pressure
estimation. This likely indicates that the thickness of the
crust has not changed considerably since the Cretaceous.

Melting and metasomatic phenomena

Textural evidence for melting

Three of the studied Type-I xenoliths display textural evi-

dence of partial melting, which includes spongy rims of cli-
nopyroxene and orthopyroxene, as well as interstitial glass.
Interstitial glass in SISTI2b and KZC1 xenoliths shows no
channels or connections towards the host basalt. Interstitial
glass is in contact with the spongy rim of pyroxenes, which
always contains glass. The extent of melting changes consid-
erably from xenolith to xenolith. KZC1 lherzolite appears to
be the least affected one as the modal proportion of the
spongy rims of pyroxenes is small. The spongy rims are also
too narrow to be analysed for trace elements. Tiny patches of
interstitial glass, however, were observed in KZC1 lherzolite.
TLC1 harzburgite exhibits transitional character with signifi-
cant proportion of spongy rim but without interstitial glass.
The melting process is most profound in the SISTI2b wehr-
lite, in which it is extremely difficult to find unmelted cli-
nopyroxene and the modal proportion of interstitial glass is
significant (2.4 v/v %, Table 1).

Closed or open system melting?

Under upper mantle conditions orthopyroxene melts in-

congruently mostly due to decreasing pressure and/or flu-
id flux. Incongruent melting of orthopyroxene forms
Si-rich melt and olivine at the rims according to the equa-
tion of opx


= olivine+ melt (e.g. Morse 1980; Tracy

1980). However, at the rims of melted orthopyroxene, be-
side the Si-rich glass and the olivine, secondary clinopy-
roxene was also observed (Fig. 2d). The presence of these
newly formed clinopyroxenes cannot be explained solely
by incongruent melting of orthopyroxene. Using mass bal-
ance calculation, the bulk major element composition of
the spongy rim of the orthopyroxene can be estimated
(proportion of olivine consistently 65 % area, glass 35 %
area). This calculation gives excess in Al




, Na


O and



O and deficit in CaO, Cr



(MgO and FeO) for the

spongy rim with respect to porphyroclastic orthopyroxene
(Table 1). This observation suggests that melting is in-

background image



duced by the addition of a metasomatic agent. The pres-
ence of newly formed clinopyroxenes in the incongruent
rim of orthopyroxene is in agreement with metasomatic re-
actions (e.g. Coltorti et al. 2004). This hypothesis is fur-
ther supported by the trace elements and REE’s
(Fig. 5c—d). The trace element and REE patterns show sig-
nificant differences between the orthopyroxene cores and
spongy rims in the TLC1 harzburgite. The cores show de-
pleted character (0.01—1  of the primitive mantle), where-
as the rims are enriched in fluid mobile and LIL elements
and their concentrations are 3—100 fold higher than that of
the primitive mantle. Furthermore, the cores are depleted
in LREE, whereas the rims are enriched in LREE (Fig. 5c).

The melting of the clinopyroxenes under upper mantle

conditions is a solid-solution controlled process (e.g.
Golovin et al. 2000; Carpenter et al. 2002), where the pri-
mary clinopyroxene forms spongy rims, which contain
melt accumulations (Fig. 2b). Similarly to orthopyroxene,
the bulk major element composition of the melted part of
the clinopyroxene can be determined by mass balance cal-
culation. This is possible if the composition of the spongy
rim and the enclosed glass, and their modal proportions
are known. The precise proportions of the spongy rim in-
cluding the glass were calculated using BSE images,
which give consistently 82—88 % area for the spongy rim
and 18—12 % area for the enclosed glass. If the melting
event happened in a closed system then the calculated
bulk composition of the spongy rims should be close to or
the same as that of the core. This is not the case, because
slight depletion in MgO, CaO, Cr




 (and FeO) and signif-

icant enrichment in Na


O, K


O and Al




 were observed

within the spongy rims relative to the unaffected cores in
the three xenoliths (KZC1 lherzolite, SISTI2b wehrlite and
TLC1 harzburgite) (Table 1). The addition of a metaso-
matic agent is also supported by the trace element and
REE pattern of the spongy clinopyroxenes, which show a
slightly enriched character in LREE (Fig. 5a). This enrich-
ment is less profound than in orthopyroxene because cli-
nopyroxene contains a much higher amount of trace
elements. Nevertheless, significantly higher amounts of
fluid mobile elements are observed in the spongy rims of
clinopyroxene than in the cores (Fig. 5b).

In summary, textural evidence, major and trace element

composition all indicate that partial melting of clino- and
orthopyroxene to produce the spongy rims did not occur
in a closed system but was caused by the influx of an ex-
ternal (metasomatic?) agent.

Nature and composition of the metasomatic agent

The spongy rims of different pyroxenes display similar

trace element and REE patterns (Fig. 5a—d), which sug-
gests compositional similarities of the rims. Although the
trace element compositions of the spongy rims of orthopy-
roxene represent a mixture of pyroxene, olivine and melt
composition, the chemical similarity could be the result of
a melting process, which seems to have been triggered by
an agent enriched in LIL, LREE, fluid mobile elements
and alkalis. All these features are also present in the host

basalt and in the interstitial glass (Fig. 5e—h). We, there-
fore, need to evaluate whether either of these two melts
was involved in the partial melting of the pyroxenes. No
fractures or channels were seen where the host magma
could have affected the chemical composition of the
glasses inside the peridotites. The major element composi-
tion of the glass found within the spongy pyroxenes is
characterized by very high Al




 and very low CaO con-

tents and does not resemble the alkaline melt composition
of the host basalt. The LILE pattern of the host basalts
with a negative Rb anomaly is also different from the
LILE patterns found in the spongy rims. As the most in-
compatible elements, such as LILE, are preferentially host-
ed in the melt the bulk analysis of the spongy rims of
orthopyroxene and clinopyroxene provides a good ap-
proximation of the LILE composition of the glass found in
these rims. Although there is a difference in major element
chemical compositions in glass in rims around clinopy-
roxene and orthopyroxene, the general feature with high


 and very high Al




 is common (Table 1, Fig. 4).

The LILE pattern of the interstitial melt also resembles the
LILE pattern in the spongy rims, apart from Ba, which in
some analyses of interstitial glass displays a negative
anomaly. Combining our textural and chemical evidence,
we suggest that the interstitial melt is an appropriate can-
didate for being either the residuum after the addition of
an external metasomatic agent to the melting assemblage
of orthopyroxene±clinopyroxene or the external compo-
nent itself. It is unlikely that there was no interaction be-
tween the metasomatic agent and the melting assemblage,
therefore the former assumption seems to be more feasible.
In the following we will put constraints on the geochemis-
try and origin of the interstitial glass.

In order to characterize the metasomatic agent, we should

also consider whether the quenching process, which led to
the formation of the interstitial glass, had any impact on its
composition as it was pointed out in earlier works (e.g.
Wilkinson 1966; Frey & Green 1974). In our samples small
feldspar and clinopyroxene crystals have been recognized
in interstitial glass which is also in contact with the spongy
rim of both orthopyroxene and clinopyroxene and olivine.
In addition, no rims (i.e. other than spongy rim) or zones
have been observed in minerals adjacent to the interstitial
glass excluding the possibility of any overgrowth related to
the crystallization of this glass. In conclusion, the composi-
tion of interstitial glass has been only modified by the crys-
tallization of minor amount of feldspar and clinopyroxene.
Especially, the crystallization of feldspar might explain the
negative anomaly of Ba, Sr and Eu in some analyses of in-
terstitial melt as feldspar preferentially incorporates these
elements with respect to other trace elements of comparable

At least four possible models have been proposed in the

literature for the origin of interstitial glasses (as either the
residuum after addition of a metasomatic agent to the
melting assemblage of the mantle or the trapped metaso-
matic agent itself) in xenoliths from the upper mantle,
which include: 1 – infiltration and reaction of the xeno-
liths with the host magmas during their transportation to

background image



the surface (e.g. Wulff-Pedersen et al. 1999; Shaw & Klu-
gel 2002), 2 – anhydrous or hydrous partial melting of up-
per mantle rocks during decompression and heating (e.g.
Jaques & Green 1980; Doukhan et al. 1993), 3 – break-
down and melting of pre-existing hydrous phases (amphib-
ole and phlogopite) owing to decompression and heating
(e.g. Frey & Green 1974; Wilson & Downes 1991; Yaxley
et al. 1997, 1999), and 4 – infiltration and reaction of
metasomatic fluid/melts in the mantle prior to entrainment
into the host magma (e.g. Ionov et al. 1994; Szabó et al.
1996; Coltorti et al. 2000; Bali et al. 2002; Demény et al.
2004), where the nature of the agents can be various, and

may include: carbonatites (e.g. Yaxley et al. 1991; Wiechert
et al. 1997; Coltorti et al. 2000), andesites and adakites
(Szabó et al. 1996; Kilian & Stern 2002), Na-rich melt/fluid
(e.g. Beccaluva et al. 2001; Coltorti et al. 2004) and K-rich
melt/fluid (Xu et al. 1996; Coltorti et al. 2000). These latter
two agents (i.e. K and Na alkali silicate melts/fluids) may
be closely related to the host magma, representing either an
earlier incipient stage of melting in the mantle that later
provided the host melt (Schiano et al. 1992; Schiano &
Clocchiatti 1994), or these melts/fluids can represent a late
stage crystallization/differentiation product of an alkaline
basaltic magma (Zajacz et al. 2007).

Fig. 7. Primitive mantle (McDonough & Sun 1988) normalized trace element pattern of mantle glasses with different origins compared
to the studied glass in wehrlite SISTI2b from the North Minusinsk Depression.

background image



Fig. 8. CaO + Na


O vs. TiO


+ K


O composition of glasses in the

Type-I series (KZC1 lherzolite, SISTI2b wehrlite, TLC1 harzburg-
ite) from the North Minusinsk Depression plotted on the discrimi-
nation the diagram of Coltorti et al. (2000), which is completed for
the characteristic fields with data from Beccaluva et al. (2001) and
Coltorti et al. (2004). Fields for adakite, amphibole and host basalt-
related glasses are also indicated.

As shown in the previous section, a simple in situ melting

model cannot explain the excess of Na, Al, K in the rims of
orthopyroxene and clinopyroxene because the rock form-
ing minerals of the peridotites cannot produce such a con-
siderable amount of Na, Al and K even if these major
elements are highly incompatible. Hydrous phases have not
been observed in the Type-I xenolith series of the NMD,
therefore there is no direct evidence that breakdown of pre-
existing hydrous phases during decompression and heating
in a rising magma played a role. This theory is also not sup-
ported by the trace element composition of the interstitial
glass, which is not similar to amphibole-related fluids/melts
(Fig. 7, Demény et al. 2004). Consequently, an external
source is suggested for the interacting agent(s). Textural
and chemical evidence implies that not the host basalt but
another melt was the source of the interstitial glass.

Origin of interstitial glass

The major element composition of interstitial glasses on

the TAS diagram overlaps with the glasses of Wulff-Peder-
sen et al. (1999) and Coltorti et al. (2000), indicating ei-
ther the infiltration of basaltic melt or a Na-K alkali
metasomatism responsible for the formation of interstitial
glass (Figs. 4, 8). Plotting the chemical composition of
these interstitial glasses on a TiO


+ K


O vs. CaO+ N a



discrimination diagram of Coltorti et al. (2000), they fall
in the field depicted for Na-alkali silicate metasomatism,
and only those in the KZC1 lherzolite are in the carbon-
atite field (Fig. 8). Primitive mantle normalized trace ele-
ment patterns of different mantle glasses are compared to
the studied interstitial glass in SISTI2b (Fig. 7). Subducted
slab-related glass exhibits distinctive negative Nb- and
positive Sr-anomaly, whereas amphibole-related glasses
are considerably depleted (100 times) in LIL and LREE
and in U, Th, Nb and K compared to the studied one. Car-
bonatite metasomatism formed glasses show a more or less
similar pattern to the studied one, which is, however, more
enriched in Th and depleted in Pb, Pr and Sr. K- and Na-al-
kali silicate metasomatism- and host basalt infiltration-re-
lated glasses display very similar patterns to the studied
glass, the most similar one of which is the Na-alkali sili-
cate metasomatism-related pattern. The former two cannot
account for the observed positive Nb- and Cs-anomaly,
whereas only the host basalt infiltration-related glasses
can explain the positive Zr-anomaly. The Na-alkali sili-
cate metasomatism-related, nonetheless, is in good agree-
ment with all of these characteristic features of the
interstitial glass in the SISTI2b xenolith. This is also in
good agreement with our previous observations that melt-
ed rims of clinopyroxenes and orthopyroxenes show ex-
cess in Na, Al and K and also in LIL and LREE, which
implies that there may be a link between the melted rims
and the interstitial glasses. In this scenario the interstitial
glass represents the residuum (interaction products) after
melting induced by an external metasomatic agent at the
rims of pyroxenes. We assume that a melt, which was en-
riched in incompatible major (especially Na, K, Al) and
trace (especially fluid mobile and LIL) elements, had

metasomatized the upper mantle prior to the transporta-
tion to the surface. This proposed alkali-rich silicate melt
may be linked somehow to the host basalt, but their exact
relationship is not yet known.

Evidence for earlier metasomatic events

Evidence for metasomatism prior to the studied melting

event has been captured in the trace element composition of
the pyroxenes. Orthopyroxene cores (TLC1, KZC1; Fig. 5c)
display convex upward patterns with enrichment in La, Ce
and Pr. In addition, clinopyroxene cores in KZC1 show a
very similar pattern referring to a similar cryptic metasoma-
tism (Fig. 5a). Enrichment in LREE over MREE and HREE
in clinopyroxene cores is present in each analysed sample.
We assume that the source of the cryptic metasomatism may
have been either the Devonian volcanism or an earlier stage
of the basaltic volcanism that brought up the xenoliths.

Concluding remarks

The studied Type-I xenoliths show variable degrees of

depletion overprinted by an enrichment leading to LREE-
enriched pyroxenes. The lithospheric mantle beneath the
NMD appears to show a lower degree of depletion than
other young areas with thinner lithosphere, which is in
agreement with the lack of extensive volcanism in associ-
ation with either the Cretaceous or Devonian thinning
event, which prevented a higher degree of depletion of the

background image



continental lithospheric mantle. Traces of extensive en-
richment (e.g. presence of hydrous phases as amphibole
and mica) are also absent. The studied Type-I xenoliths
display equilibrium temperatures of 960—1050 ºC. Type-II
xenoliths of the NMD are the product of igneous crystalli-
zation in the subcontinental lithospheric mantle. This is
confirmed both by the high content of basaltic elements in
the constituent minerals and mineral assemblages of the
studied xenoliths. They are probably equilibrated with their
parental magma at 1.2—1.4 GPa (approximately 42—49 km),
which is in good agreement with the present thickness of
the crust beneath the NMD (39—42 km). This suggests that
the thickness of the crust has not changed considerably
since the Cretaceous.

The spongy rims of clinopyroxene containing glass, in-

congruent melting of orthopyroxene and interstitial glass
provide evidence for incipient melting in the Type-I series
of the NMD. We found enrichment in some major ele-
ments (especially Na, K, Al), fluid mobile- and LIL trace
elements in rims of pyroxenes and interstitial glass, which
cannot be explained solely by in situ melting of mantle
silicates. Textural observations, coupled with major and
trace element analysis, suggest that the interstitial glass is
a residuum after a metasomatically-induced partial melt-
ing of pyroxenes. The melting was likely induced by a Na-
alkali silicate melt/fluid. This melt/fluid is not the direct
result of the host basalt infiltration, but it rather represents
an earlier event, which could be in association with the
basaltic volcanism that generated the host basalt.

Acknowledgments:  The authors express their honor to Or-
lando Vaselli and †Filippo Olmi (University of Florence)
for major element work. We are also grateful to György
Falus (Lithosphere Fluid Research Lab, Eötvös Universi-
ty) for his helpful discussions. Members of the field team
(J. Dégi, K. Kóthay, M. Pető, Z. Siklósy, Z. Zajacz) are ac-
knowledged for their help in sample collection. The au-
thors are grateful to Dmitri Ionov, Andy Beard, Marian
Janák, an anonymous reviewer and Heather Sparks for
their constructive criticism which substantially improved
an earlier version of the manuscript. This work greatly
benefited from the fruitful discussions with David H.
Green. The Russian Academy of Sciences and Hungarian
Academy of Sciences are thanked for financial support
based on the Project #52. This work is partially supported
by an A. E. Ringwood Memorial Scholarship and an Aus-
tralian International Postgraduate Research Scholarship to
I. Kovács. This is the 22


 publication of the Lithosphere

Fluid Research Lab of the Department of Petrology and
Geochemistry at Eötvös University, Budapest, Hungary.


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