GEOLOGICA CARPATHICA, DECEMBER 2006, 57, 6, 447—460
www.geologicacarpathica.sk
Chemical composition of spinels from Mesozoic alkali basalts
of the Western Carpathians: implications for sources of
detrital spinels in flysch sediments
TOMÁŠ MIKUŠ
1
, JÁN SPIŠIAK
1
, MILAN SÝKORA
2
and RASTISLAV DEMKO
3
1
Geological Institute Slovak Academy of Sciences, Severná 5, 974 01 Banská Bystrica, Slovak Republic;
mikus@savbb.sk; spisiak@savbb.sk
2
Faculty of Natural Sciences, Comenius University, Mlynská dolina, 842 15 Bratislava, Slovak Republic; sykora@fns.uniba.sk
3
Geological Survey of Slovak Republic, Kynce ovská 10, 974 01 Banská Bystrica, Slovak Republic; demko@gssrbb.sk
(Manuscript received October 16, 2004; accepted in revised form October 5, 2006)
Abstract: Cr-spinel is a relatively widespread accessory heavy mineral in Mesozoic alkali volcanic rocks in the Central
Western Carpathians. According to their chemical composition, spinels from these rocks (basalts, submarine
hyaloclastites) can be divided into several groups (volcanic – Cr-spinel and Fe-Ti spinel, and peridotitic spinel,
altered). Volcanic spinels crystallized in the plumbing system, and peridotitic spinels ultimately originated from
peridotitic xenoliths, entrapped and brought to the surface by magmas. Melt/silicate inclusions (clinopyroxenes,
plagioclases, melt) were found only in Cr-spinels of volcanic origin. Alteration processes are similar in all studied
samples (enrichment in Ti
4+
, Fe
2+
, Fe
3+
and depletion in Mg
2+
and Al
3+
) . The chemical composition of Cr-spinels from
Cretaceous alkali basalts from the Western Carpathians is different to those of detrital Cr-spinels of volcanic origin
from the Albian Poruba Flysch Formation. Mesozoic alkali volcanites were not important source for widespread
detrital spinels of volcanic origin in Cretaceous sediments (mainly in the Tatric and Fatric Tectonic Units).
Key words: Cretaceous volcanism, Cr-spinel, silicate melt inclusions, clinopyroxene, alteration.
Introduction
Spinel is an important accessory mineral of Mesozoic alkali
volcanic rocks because it is resistant to mechanical break-
down and low-grade alteration. Mantle peridotite- and vol-
canic rock-derived spinels are indicative of magmatic and
tectonic evolution and spinel chemistry is diagnostic of
melt composition, crystallization conditions and tectonic
setting. Although Cretaceous basaltic volcanites were
studied mainly from the geochemical point of view in
many previous works (e.g. Hovorka & Spišiak 1988; Spi-
šiak & Hovorka 1997; Hovorka et al. 1999), no attention
was paid to the chemical composition of spinels. There-
fore, we analysed spinels from these rocks in order to ob-
tain information about their petrogenetic nature. With the
use of discrimination criteria for spinel compositional
variations (Lenaz et al. 2000; Kamenetsky et al. 2001) ge-
netically different spinel types were distinguished. Melt
inclusions entrapped within Cr-spinels and clinopyroxene
phenocrysts were used to decipher the geochemical char-
acter of Cretaceous basaltic rocks of the Central Western
Carpathians (CWC). Clinopyroxene inclusions in Cr-spinel
were used for thermobarometric calculations. Parental melt
composition was recalculated from the association of
daughter phases and residual silicic glass composition of
melt inclusions entrapped in Cr-spinel.
Many authors have been looking for the source area of a
rather widespread detrital Cr-spinel occurring in Creta-
ceous flysch sediments (e.g. Mišík et al. 1980; Jablonský
et al. 2001). For this reason, we considered Cretaceous ba-
salts as a potential important source of flysch-hosted detri-
tal spinel. Thus we compared the composition of spinels
from Cretaceous basaltic volcanites with that of detrital
spinels from siliciclastic flysch sediments of the Tatric and
Fatric Tectonic Units.
Geology and tectonic settings
Mesozoic (Lower Cretaceous) alkali rocks of various
types occur in the External (EWC) and Central Western
Carpathians (CWC). In the CWC, Mesozoic alkaline vol-
canism is known from the Tatric (envelope) and Fatric Su-
perunits (Krížna, Manín and Klape Units, Plašienka 1999).
On the basis of stratigraphic data, the age of the volcanics
under consideration is Barremian—Aptian, which is proved
also by the K/Ar method applied to hornblende and cli-
nopyroxene concentrate, and the whole rock (102 Ma,
Spišiak & Balogh 2002). The K/Ar method applied to
hornblende and clinopyroxene concentrate and the whole
rock proved the Aptian—Albian age (116 ± 6.5 and
106.2 ± 1.7 Ma) of volcanic activity in the Krížna Nappe
(Bujnovský et al. 1981). The products of the volcanic ac-
tivity are low differentiated basalts/basanites, or very rare-
ly picrites. Volcaniclastic rocks are present in substantial
amounts (hyaloclastites, etc.). The majority of Mesozoic
alkaline rocks (except for picrites) are characterized by the
presence of a fine-grained devitrified matrix (up to
40 vol. %). Olivines are completely replaced by chlorite-
serpentine-carbonate aggregates. Clinopyroxenes are dom-
448
MIKUŠ, SPIŠIAK, SÝKORA and DEMKO
inating minerals in all rock types. Besides phenocrysts of
various shape and size, they also form microlites in the de-
vitrified matrix. Kearsutite, albite, apatite, analcime,
pseudoleucite, spinel, ore minerals, and Ti-biotite are rarely
present (Spišiak & Hovorka 1997). Based on alkali charac-
ter of volcanics, Cretaceous volcanism of CWC as well as
EWC is associated with short-living rifting (Hovorka & Spi-
šiak 1988). The studied rock types are represented mostly
by hyaloclastites and autometamorphosed basalts.
In this contribution we studied Cr-spinels and clinopy-
roxene phenocrysts from the following localities in the
CWC: Podmanín, Košeca, Dobrá, Slopná, Štepnica, Oso-
bitá and Čebra (Fig. 1). A majority of the studied locali-
ties are situated in the Strážovské vrchy Mts (Podmanín,
Slopná, Dobrá and Košeca). The Štepnica locality is situ-
ated in the Javorníky Mts, Čebra belongs to the Chočské
vrchy Mts, and Osobitá is located in the Tatry Mts. Hyalo-
clastite bodies in Podmanín and Slopná belong to the
Manín Tectonic Unit. The surrounding rocks are Lower
Albian cherty limestones of the Jelenia skala Formation
and thin-rhythmic flysch sediments of the Praznov Forma-
tion (Cenomanian—Middle Turonian). The stratigraphic
age of the hyaloclastite body is Early Albian (Mello et al.
2005). The basalt body near Dobrá is situated between or-
ganodetrital limestones with interbeds of cherts (Baremi-
an—Lower Aptian) and patchy marls with foraminifers of
Late Albian—Early Cenomanian age. This sedimentary se-
quence with the volcanic body belongs to the Fatric Supe-
runit (Krížna Nappe). The stratigraphic age of the volcanic
rock is probably Early Albian. Another Fatric basalt hy-
aloclastite body is situated near Košeca. The surrounding
rocks are marly limestone of Late Jurrasic—Early Creta-
ceous age. The hyaloclastite contains clasts of Hauterivian
and Barremian limestones. The assumed age of the vol-
canism is probably Albian. Volcanics in Čebra (Krížna
Nappe) are represented by basaltic breccias and massive
autometamorphosed basalts. The sedimentary sequence
and the age of the volcanic rock are similar to those of
the Košeca locality. The hyaloclastite body in Štepnica
is embedded in marls of the Púchov Formation (Lower
Albian—Lower Campanian). This formation is part of
the Pieniny Klippen Belt and belongs to the Czorsztyn
Tectonic Unit, Púchov – Jarmuta Group (Mello et al.
2005). Several hyaloclastite bodies in Osobitá are situat-
ed in Tithonian Sobótka Limestones (Lefeld 1985). Super-
incumbent beds of volcanites are found in the Osobitá
chert limestones (Valanginian—Aptian). This rock se-
quence belongs to the Tatric Tectonic Unit.
Petrographic description
The size of spinel grains (octahedral crystals and frag-
ments) is in average about 0.3—0.5 mm. Spinel phenoc-
rysts are of black colour. They are clearly homogeneous
in the reflected light. Cr-spinels of volcanic origin (see
Fig. 1. Geological position of the studied localities in the simplified geological map of the Western Carpathians. Localities studied: 1 – Dobrá,
2 – Košeca, 3 – Štepnica, 4 – Slopná, 5 – Podmanín, 6 – Čebra , 7 – Osobitá.
449
CHEMICAL COMPOSITION OF SPINELS FROM BASALTS OF THE WESTERN CARPATHIANS
spinel composition) from Podmanín and Čebra have
very similar characteristic features: a – oscillatory zoning
(Fig. 2a); b – silicate/melt inclusions (Fig. 2b,c).
Silicate and melt inclusions trapped in spinel were
found only in volcanic Cr-spinel from Podmanín and Če-
bra . The shape of the inclusions is dominantly convex
oval and their size is up to 40 m. The inclusions with
cracks are altered and filled with secondary chlorite and
calcite (Fig. 2b). Most of the inclusions contain two phas-
es: daughter clinopyroxene (cpx) + plagioclase (Fig. 2b)
or daughter cpx crystal + glass + shrinkage vapour bubble
(Fig. 2c). Single-phase inclusions contain cpx or glass.
Clinopyroxene inclusions in Cr-spinel host were ob-
served as single individual euhedral crystals, usually oc-
curring along the inclusions contact with the host spinel.
Generally, daughter cpx inclusions are surrounded by re-
sidual glass containing circular cells which are most
probably a result of vapour exsolution from the en-
trapped melt (Fig. 2c).
Fe-Ti spinels are associated with clinopyroxene (cpx)
phenocrysts and apatite in the Dobrá and Štepnica locali-
ties (Fig. 2d,e,f).
Fig. 2. a – Primary magmatic zoning of Cr-spinel of volcanic origin with melt inclusions from Podmanín; b – Inclusion of plagioclase
(pl) with clinopyroxene (cpx) in Cr-spinel (sp) of volcanic origin and inclusion filled by secondary minerals (Čebra locality); c – Inclu-
sions of basaltic melt with clinopyroxene (cpx) and shrinkage bubbles in volcanic spinel (Podmanín); d – Euhedral clinopyroxene phe-
nocryst with inclusion of apatite (ap) and Fe-Ti spinel (ulvöspinel-usp) from Dobrá locality; e – Impregnations of Fe-Ti spinel (usp) in
clinopyroxene (cpx) grain from Štepnica; f – Inclusions of apatite (ap) in Fe-Ti spinel (usp) from Štepnica. BSE images.
450
MIKUŠ, SPIŠIAK, SÝKORA and DEMKO
Altered spinels were recognized among spinels of vol-
canic origin. The textures of altered spinels are heteroge-
neous (Fig. 3b,c,d) and porous (Fig. 3b,d). Enriched rims
were observed very often (enrichment in TiO
2
, Fe
2
O
3
or
Cr
2
O
3
, Fig. 3a). The strongest changes in spinel chemis-
try due to the alteration process were observed in Košeca,
where almost all the spinel grains are altered. Alteration
affects spinel grains from their margins and alteration
progresses faster in spinel grains containing microfrac-
tures.
Clinopyroxene phenocrysts were found in heavy miner-
al fractions from Štepnica, Dobrá and Podmanín. The eu-
hedral phenocrysts are up to 1.5 mm in size and are brown
(Štepnica, Dobrá) or green (Podmanín) in colour. Oscilla-
tory zonation is quite usual in cpx phenocrysts from Štep-
nica and Dobrá (Fig. 4). Several oscillatory zoned cpx
grains consist of an anhedral homogeneous core with dif-
ferent chemical composition (Fe-rich cores). The boundary
between core and zoned mantle is sharp (Fig. 4a,b) and the
cores are magmatically corroded.
Fig. 3. Textures of altered spinels from Podmanín (a), Košeca (b,e), Slopná (c), Dobrá (d), Štepnica (f). BSE images. a – Euhedral
Cr-spinel grain with altered rim. b – Heterogeneous Cr-spinel with porous structure. Pores are filled with secondary minerals. Altered
phase is located around the pores. c and d – Decomposition of heterogeneous Cr-spinel structure. e – Almost completely altered
Cr-spinel grain. White phase corresponds to Cr-magnetite. f – Heterogeneous Fe-Ti spinel. Lighter phase is enriched in TiO
2
.
451
CHEMICAL COMPOSITION OF SPINELS FROM BASALTS OF THE WESTERN CARPATHIANS
Analytical methods
Spinel and clinopyroxene were hand-picked from 10—15 kg
of crushed volcanic rock (fraction < 2 mm). Individual crystals
were mounted in epoxy resin, polished and coated with carbon.
Spinel, silicate melt inclusions and clinopyroxene were
analysed with a wave-dispersion (WDS) electron micro-
probe and photographed in back-scattered electrons (BSE)
at the Department of Mineralogy in the Natural History Mu-
seum, London (UK). The microprobe used was a Cameca
SX50 probe. The operating conditions were as follows:
20 kV accelerating voltage, 20 nA beam current, beam di-
ameter 2—5 m, ZAF corrections, standards (n – natural,
sy – synthetic) – TiO
2
(sy) and CaTiO
3
(sy) for Ti, V (sy)
for V, wollastonite (n) for Ca, Cr
2
O
3
(sy) for Cr, Mn (sy) for
Mn, hematite (sy) for Fe, Co (sy) for Co, Ni (sy) for Ni,
sphalerite (sy) for Zn, corundum (sy) for Al, diopside (n) for
Si, MgO
2
(sy) for Mg, jadeite (n) for Na, KBr (sy) for K, ha-
lite (sy) for Cl. A defocused beam could not be used for the
melt inclusions analyses due to their small size. The Cpx
phenocrysts were analysed with a Cameca SX100 probe at
the Geological Survey of the Slovak Republic (Bratislava)
under the following conditions: 15 kV accelerating voltage,
20 nA beam current, beam diameter 2—5 m, ZAF correc-
tions, standards (n – natural, sy – synthetic) – albite (n)
for Na, wollastonite (n) for Si and Ca, Al
2
O
3
(sy) for Al, or-
thoclase (n) for K, rodonite (n) for Mn, hematite (n) for Fe,
MgO (sy) for Mg, TiO
2
(sy) for Ti, nickel (sy) for Ni,
chromite (n) for Cr.
Fe
2+
and Fe
3+
in spinels were calculated assuming an ide-
al stoichiometry. Fe
3+
in clinopyroxenes was calculated ac-
cording to charge balance proposed by Ryburn et al. (1976)
and Papike (1974).
Thermometric calculation with a combination of two
thermometers was used for clinopyroxene inclusion in
Cr-spinel. A QUILF software written by Andersen et al.
(1995) was used for cpx—sp pairs. The results from QUILF
equilibrium are compared with a single clinopyroxene ther-
mometer proposed by Mercier (1976). For pyroxene barom-
etry a barometer proposed by Nimis (1995) was used.
Microprobe analyses of residual glass were normalized
to 100 %. Modal proportions of cpx and residual glass
were calculated in the first step using BSE images. It is
not possible to estimate the volume of spinel crystallized
from the entrapped melt. Therefore 1, 3, 6 and 9 % of
spinel component was added to homogenized compo-
nents (glass+cpx). Final melt composition was plotted on
a model curve in the TAS (total alkali vs. silica) diagram.
The estimated primary melt composition is influenced by
the following factors:
Volumetric phase ratios do not respect the third di-
mension.
Fig. 4. Oscillatory zonation of cpx phenocrysts. Several cpx grains contain Fe-rich core with chemical composition different from that
of younger zoned cpx (a,b). Numbers correspond to analyses in Table 4.
452
MIKUŠ, SPIŠIAK, SÝKORA and DEMKO
The contents of alkalis are underestimated due to a
focused electron beam used in the WDS analyses.
Results
Spinel composition
The spinels from Mesozoic alkali volcanics show a sig-
nificant variation in chemistry, mainly in terms of the
most important parameters such as Mg# (Mg/Mg + Fe
2+
),
Cr# (Cr/Cr + Al), TiO
2
and Fe
2+
/Fe
3+
(Table 1). These vari-
ations suggest multiple sources of the spinels. To dis-
tinguish between spinels from these sources (hereafter,
peridotitic and volcanic), the most useful variables are
TiO
2
content with a combination of the Fe
2+
/Fe
3+
ratio
in spinels (Lenaz et al. 2000; Kamenetsky et al. 2001).
“Mantle” spinels (from ophiolitic peridotites and mantle
xenoliths) have statistically ( > 95 %) lower TiO
2
( < 0.2 wt. %) and higher Fe
2+
/Fe
3+
( > 3) over the whole
interval in Al
2
O
3
(6—56 wt. %) than volcanic spinels
(Kamenetsky et al. 2001). Volcanic spinels with
TiO
2
< 0.2 wt. % are uncommon (some suites of low-Ti
MORB, arc tholeiites and boninites) and those with
TiO
2
< 0.1 are exceptionally rare (some low-Ca boninites).
Lenaz et al. (2000) have set a compositional boundary
between the peridotitic and volcanic spinels at
TiO
2
= 0.2 wt. %. Volcanic spinels tend to have the Fe
2+
/Fe
3+
ratio usually up to four (Kamenetsky et al. 2001).
Several principal compositional groups have been dis-
tinguished according to chemical composition (Fig. 5):
1.
mantle peridotitic spinel (Podmanín, Dobrá and Slopná
localities); 2. volcanic spinel: a – Cr-spinel (Podmanín,
Dobrá, Slopná, Košeca and Čebra localities); b – Fe-Ti
spinel (Podmanín, Dobrá, Štepnica and Osobitá localities);
3.
altered spinel (all localities studied).
Peridotitic spinel is characteristic of the highest Al
2
O
3
content (up to 60 wt. %, e.g. Podmanín). TiO
2
content is
usually very low ( < 0.10 wt. %). Mg# (Mg/Mg + Fe
2+
) val-
ues are 79—81 mol % and Cr# (Cr/Cr + Al) ranges between
7 and 12 mol %. Peridotitic spinels from Dobrá show the
lowest content of Al
2
O
3
(36—48 wt. %) in this composi-
tional group (Table 1, Fig. 6). An interesting spinel phase,
with up to 50 wt. % Cr
2
O
3,
was recognized as tiny inclu-
sions (up to 10 µm) within Cr-diopside phenocrysts from
Podmanín.
Among volcanic spinels, those from Podmanín and Če-
bra show very similar chemical compositions. Al
2
O
3
con-
tent ranges from 24 to 39 wt. % and TiO
2
content is
between 0.9 and 1.1 wt. %. Mg# values range between 71
and 74 mol % and Cr# values correspond to 28—35 mol %
(Table 1). Oscillatory zoning of volcanic Cr-spinels from
Podmanín and Čebra is caused by different content of
trivalent cations (Al
3+
, Cr
3+
) in individual zones. Au-
tometamorphosed basalt from Čebra contains only volca-
nic Cr-spinel, which contains the highest TiO
2
content (up
to 1.84 wt. %).
Fe-Ti spinels (titanomagnetites) have been found in
Podmanín, Dobra, Štepnica and Osobitá (Table 2). The
major constituent is magnetite. The amount of the ul-
vöspinel (usp) component is between 8 and 38 mol %. Al-
most all studied samples of Fe-Ti spinels show Al
2
O
3
content about 3 wt. % in average. A complete chromian
spinel—titanomagnetite (Fe-Ti spinel) solid solution has
not been observed.
Detrital Cr-spinels of volcanic origin from the Poruba fly-
sch Fm in general are different in terms of Al
2
O
3
content,
Mg# and Cr# to those of alkali basalts. Detrital Cr-spinels
have lower Al
2
O
3
content (6.11—30.58 wt. %) and most
Fig. 5. Nomenclature and composition of spinels from Mesozoic al-
kali volcanic rocks based on the classification of Deer et al. (1992).
Fig. 6. Al
2
O
3
vs. TiO
2
compositional relationships in spinel from
the Mesozoic alkali volcanic rocks. Compositional fields of
spinels from volcanic environments (MORB – mid-ocean ridge
basalt, OIB – ocean-island basalt, LIP – large igneous provinc-
es, ARC – island arc magmas, BABB – back-arc basin basalt)
are compared with spinels from mantle (SSZ and MORB – dot
line) peridotites (according to Kamenetsky et al. 2001). BABB
field is restricted by a dashed line from Lenaz et al. (2000). Anal-
yses of detrital spinels of volcanic origin (black circles) from mid-
Upper Cretaceous flysch sedimets in the Western Carpathians are
plotted for comparison.
453
CHEMICAL COMPOSITION OF SPINELS FROM BASALTS OF THE WESTERN CARPATHIANS
Table 1: Representative microprobe analyses of Cr-spinels from Mesozoic alkali volcanics.
454
MIKUŠ, SPIŠIAK, SÝKORA and DEMKO
Table 2:
Representative microprobe analyses of Fe-Ti spinels from Mesozoic alkali volcanics.
of them have higher Cr# (60—87 mol %) and
lower Mg# (34—63 mol %) (Table 3).
Inclusions within Cr-spinel
Clinopyroxene inclusions
Cpx inclusions within Cr-spinel from
Podmanín and Čebra correspond to aug-
ite and diopside according to the classifi-
cation by Morimoto (1989). In comparison
to cpx phenocryst from Štepnica, Dobrá
and Podmanín, the cpx inclusions have
higher TiO
2
(3.08—4.26 wt. %) and Al
2
O
3
(7.50—11.61 wt. %) contents. The Na
2
O con-
tent is of about 0.70 wt. % in average (Ta-
ble 4). A higher content of Cr
2
O
3
in some
cpx inclusion analyses is caused by host-
spinel contamination.
The QUILF program (Andersen et al. 1995)
and a single cpx thermometer proposed by
Mercier (1976) which refer to hypothetical
spinel (as an associated crystallized phase)
were used for thermometric calculations
based on Mg-Fe partitioning between host
spinels and cpx inclusions in Podmanín.
The QUILF data show the temperature of cpx
crystallization between 1143—1183 ºC corre-
sponding to a pressure of 0.50—0.68 GPa
(Nimis 1995). The minimum temperature
ac-cording to Mercier’s thermometer corre-
sponds to 1023—1175 ºC.
Plagioclase and melt inclusions
Plagioclase was found only as inclusions
in Cr-spinel. Albite end-member is domi-
nant (81.40 mol %) in the plagioclase inclu-
sion from Čebra . This albite is probably
not primary magmatic material, but it could
be a product of secondary hydrothermal al-
teration of the basalt body.
Most of the residual glass analyses project
on the border of trachybasalt—basalt field in
the TAS diagram (Fig. 7). One analysis corre-
sponds to basanite—tephrite. All the residual
glass analyses are situated above the bound-
ary defining alkali basalt (Irvine & Baragar
1971). The bulk melt compositions show a
drift to lower Na
2
O + K
2
O values into the ba-
salt field. Addition of x % (x = 1—9%) spinel
into the phase assemblage of the silicate melt
and cpx inclusions will cause a decrease in
SiO
2
content and drift to picrobasalt or
basanite fields. The composition of parental
magma could be derived from bulk melt
composition. The residual glass and bulk melt
compositions suggest alkali character of the
magma. The bulk melt composition shows the
455
CHEMICAL COMPOSITION OF SPINELS FROM BASALTS OF THE WESTERN CARPATHIANS
Table 3:
Representative microprobe analyses of Cr-spinels of volcanic origin from the Poruba Flysch Formation.
Al
2
O
3
/(CaO+ Na
2
O+ K
2
O) ratio in the range
of 0.70—0.97 and the Al
2
O
3
/(Na
2
O + K
2
O)
ratio in the range of 3.45—4.75, thus corre-
sponding to metaluminous melt. Selected
electron-microprobe analyses of the residu-
al melt are listed in Table 5.
Clinopyroxene phenocrysts
Cpx phenocrysts from Štepnica and Do-
brá correspond to diopside according to
the classification by Morimoto (1989).
Their composition is slightly different
from that of cpx inclusions in Cr-spinel.
Cpx phenocrysts have a lower content of
TiO
2
(up to 2.16 wt. %) and a lower Al
2
O
3
content (2.2—5.94 wt. %). Their zonation
is a consequence of small differences in
Al
2
O
3
(e.g. 2.31—3.65 wt. %), TiO
2
(e.g.
1.16—1.73 wt. %), FeO and MgO contents
in individual zones. The Al
VI
/Al
IV
ratio is
always less than 1. Calculated pressures
according the pyroxene barometer by Ni-
mis (1995) correspond to 0.41—0.69 GPa.
The distinct compositional differenc-
es in terms of TiO
2
, FeO, MnO, MgO
and Na
2
O contents have been observed
between homogeneous cores and zoned
rims. Cores are enriched in FeO
(10.44 wt. %), MnO (0.54 wt. %) and
Na
2
O (1.94 wt. %) in comparison with
rims. The Al
VI
/Al
IV
ratio is always
above 1. On the other hand, zoned cpx
rims are enriched in TiO
2
(2.16 wt. %),
and MgO compared to cores (Fig. 4, Ta-
ble 4). Barometric calculation for one of
these cores yielded the value of 1.15 GPa.
Cpx phenocrysts with exsolved lenticu-
lar cpx lamellas occur rarely in Dobrá. This
cpx has the highest Al
2
O
3
content
(6.70 wt. %) and the lowest FeO content
(2.56 wt. %). The exsolved cpx contains
up to 4.88 wt. % Al
2
O
3
and 1.99 wt. % of
CaO. These cpx correspond to enstatite
(En
85—88
mol %, Fs
11
mol %, Wo
1—4
mol %).
The cpx phenocrysts from Podmanín
have the lowest TiO
2
(up to 0.81 wt. %)
and Al
2
O
3
(0.62—1.44 wt. %) contents. On
the other hand, they have remarkable con-
tents of Cr
2
O
3
(0.66—2.02 wt. %) and
could be classified as Cr-diopside. Na
2
O
content is usually the highest (up to
2 wt. %). Cr-diopsides have the highest
Al
VI
/Al
IV
ratio (1.56—3.30). Cr-diopside
phenocrysts are rarely heterogeneous
(small differences in SiO
2
, Al
2
O
3
, TiO
2
,
FeO contents). The calculated pressures us-
ing the barometer of Nimis (1995) corre-
spond to 1.34—1.48 GPa.
4
5
6
MIKUŠ, SPIŠIAK, SÝKORA and
DEMKO
Table 4: Representative microprobe analyses of clinopyroxenes from Mesozoic alkali volcanics.
457
CHEMICAL COMPOSITION OF SPINELS FROM BASALTS OF THE WESTERN CARPATHIANS
Table 5: Representative microprobe analyses of glass inclusions
and albite in volcanic spinels.
Discussion
Spinel substitutions
Although there is nearly complete solid solution between
the magnetite-ulvöspinel spinels and the chromate and alu-
minate spinels at magmatic temperatures (Sack & Ghiorsio
Fig. 7. Total alkali silica (TAS) diagram (Le Maitre et al. 1989) with
projection points of melt inclusions in Cr-spinels from Podmanín.
Dashed arrows indicate bulk melt composition with added different
spinel content (1—9 %). Fields of Cretaceous basalts from the EWC
and CWC are after Hovorka & Spišiak (1988), Hovorka et al.
(1999), solid line—boundary between alkaline rocks (ALK) and
tholeiites (TH) after Irvine & Baragar (1971), dashed line – after
MacDonald & Katsura (1964). Other abbreviations: q – normative
quartz; ol – normative olivine.
1991), no continuous solid solution has been observed in
studied samples. In Mesozoic alkali basalts both discrete
phases (Cr-spinels and Fe-Ti spinels) are present. Cr-spinel
(peridotitic and volcanic) compositional variations follow
the effects of tetrahedral Mg
Fe
2+
substitution (Fig. 8a)
and octahedral Cr
Al substitution (Fig. 8b), whereas
2Fe
3+
Fe
2+
+Ti
4+
exchange takes place in Fe-Ti spinels
(magnetite—ulvöspinel series) (Fig. 8c). Cr
Al substitu-
tion is a dominant mechanism for Cr-spinel chemical vari-
ability. The described substitution trends are identical with
those reported for Madeira Island alkaline lava (Mata &
Munhá 2004).
Fe-Ti spinels from CWC alkalic rocks are somewhat
poorer in Ti (Usp
8—38
) than those from other alkali basalts
occurrences (e.g. Usp
52—71
, Corner & Maury 1980; Price &
Taylor 1980, etc.). Ti-depletion of the studied Fe-Ti
spinels could be explained in terms of alteration processes
(Fig. 3f). Fe-Ti spinels in alkali basalts are richer in minor
components (Al and Mg) than those from tholeiitic rocks
and andesites. The distinctive feature of Fe-Ti spinels from
alkalic rocks is their tendency toward high Al
2
O
3
content
(Frost & Lindsley 1991).
Cr-spinel alteration
Alteration processes were recognized on the basis of
chemistry and textural changes (Fig. 3). Alteration pro-
cesses mainly affected volcanic chromium spinels. In
comparison to unaltered volcanic spinels, altered Cr-
spinels typically have a higher TiO
2
content (up to
8.4 wt. %). Al
2
O
3
contents are lower (17—24 wt. %), and so
are Cr
2
O
3
contents (12—20 wt. %) and MgO (usually be-
tween 8—12 wt. %, rarely ~ 3 wt. %). Altered spinels are en-
riched in FeO (15—26 wt. %) and Fe
2
O
3
(15—35 wt. %).
Two distinct trends of Al
2
O
3
and Cr
2
O
3
loss are displayed
in Fig. 9. The Al/Cr loss ratios are rather similar (1.56—1.77
respectively, which means a Al/Cr loss rate of 3 : 2) and
depend on the initial composition of unaltered Cr-spinels.
The average Al
2
O
3
and Cr
2
O
3
loss in the first trend is
34.5 % and 22.2 % respectively. The second trend shows
Al
2
O
3
loss of 30.7% and Cr
2
O
3
loss of 26.2 %. Generally,
dissolution of MgO, Al
2
O
3
and enrichment in FeO, Fe
2
O
3
and TiO
2
were recognized. Changes (usually enrichment)
in MnO, ZnO and V
2
O
5
content are not so evident. Altered
spinels have only a slightly higher content of these oxides.
Implication for the source of detrital spinel
We compared the composition of spinels from Creta-
ceous alkali volcanics with detrital spinels of volcanic
origin from Cretaceous sedimentary rocks (Fig. 6, Ta-
ble 3, Fig. 10). The most detrital volcanic spinels (fol-
lowing the discrimination criteria proposed by Lenaz et
al. 2000; Kamenetsky et al. 2001) in CWC Cretaceous
sediments were found in the Poruba flysch Formation
(Albian) in the Tatric and Fatric (Krížna Nappe) Units.
Some localities in the Poruba Flysch Formation show
37 % of volcanic spinels in the whole spinel population.
Detrital volcanic spinels have a different composition
458
MIKUŠ, SPIŠIAK, SÝKORA and DEMKO
(usually are richer in Cr
2
O
3
and TiO
2
) from those of alka-
li volcanic rocks and most of them correspond to the
spinels from back-arc basin basalts (BABB) or mid-ocean
ridge basalts (MORB) according to the classification by
Kamenetsky et al. (2001). Volcanic spinels from Creta-
ceous alkali volcanics correspond to intra-plate alkali
basalts (Fig. 10). Therefore, Mesozoic alkali rocks most
probably could not be an important source of detrital
spinel in Cretaceous sediments of the Tatric and Fatric
Tectonic Units. Another argument for this suggestion
could be knowledge that Fe-Ti spinels (titanomagne-
tites), which are quite usual on some alkali volcanic out-
crops, were not found among detrital spinels. The source
of detrital spinel in the Poruba Formation remains un-
known.
One of the solutions for the source rock of detrital vol-
canic Cr-spinel from the Poruba Flysch Formation could
be back-arc basin basalts (BABB) of the Meliata Ocean.
The opening of the Meliata Ocean is in general interpreted
as a result of back-arc rifting due to Paleotethys subduc-
tion under the Euroasian plate (e.g. Stampfli et al. 1998).
The initial stages of the Meliata Ocean opening as a back-
arc basin (with BABB) with later conversion to typical
MORB is suggested by Ivan (2002).
The composition of peridotitic spinels from Mesozoic
alkali volcanic rocks is similar to those of spinels from
type I peridotites, according to Dick & Bullen (1984) and
they show lherzolite character. In some cases (Podmanín)
this type of spinel could be a part of disintegrated upper-
mantle xenoliths, entrapped in magma of low viscosity.
Fig. 8. Correlation of major elements in spinels from Mesozoic al-
kali basalts of Slovakia.
Fig. 9. Relationship between Al and Cr during alteration of
spinels from Mesozoic alkali basalts of the Western Carpathians.
Fig. 10. Cr# vs. TiO
2
compositional relationship in volcanic Cr-spin-
els from the Mesozoic alkali volcanic rocks in comparison with
detrital spinels from Cretaceous flysch sediments of the Western
Carpathians (black circles). Compositional fields are after Arai
(1992). Symbols as in Fig. 3.
459
CHEMICAL COMPOSITION OF SPINELS FROM BASALTS OF THE WESTERN CARPATHIANS
Cr-diopside phenocrysts could be part of these upper-man-
tle xenoliths as well. However, there is one contradiction.
The Cr-rich spinel inclusions from Cr-diopside correspond
to the spinels from harzburgite. In this case, mantle rocks
should represent both rock types.
Melt composition
Cpx inclusions associated with residual glass crystal-
lized (1143—1183 ºC and 0.50—0.68 GPa) within the host
spinel from the entrapped melt as a phase in equilibrium
with spinel (cpx + sp). The composition of the residual
glass is slightly different from that of whole rock analyses
reported from CWC by Hovorka & Spišiak (1988) and Ho-
vorka et al. (1999) (Fig. 7). However, the composition of
bulk melt (residual glass + daughter cpx + X % spinel) part-
ly overlaps the field of Cretaceous basalts from the CWC.
The alkali character of the melt is apparent from the TAS
diagram. Tectonic affinity of the basaltic volcanites is
shown in a discrimination diagram after Mullen (1983).
Residual glass and bulk melt compositions are situated in
the ocean island alkaline basalt field (Fig. 11). The bulk
melt composition is the same as reported by Hovorka &
Spišiak (1988) and Hovorka et al. (1999) from the CWC.
Cpx phenocryst
Three essential types of cpx phenocrysts were observed
in alkali basalts from the CWC: (1) oscillatory zoned (Ti-
rich) diopside phenocrysts, (2) Fe-rich diopside cores with
oscillatory zoned Ti-rich rims and (3) Cr-diopsides. Ac-
cording to the Al
VI
/Al
IV
ratio, Na
2
O, TiO
2
content and py-
roxene barometry (Nimis 1995), high- and low-pressure
cpx phenocrysts can be distinguished. In general, high-
pressure cpx has a higher Al
VI
/Al
IV
ratio ( > 1) along with a
higher Fe/Fe + Mg ratio, higher Na
2
O content and lower
TiO
2
content (Dobosi & Horváth 1988). The Cr-diopsides
and Fe-rich cores could be high-pressure phases (high
Al
VI
/Al
IV
ratio, higher Na
2
O content) originated in the up-
per mantle. Oscillatory zoned diopsides with higher TiO
2
content, low Na
2
O content and low Al
VI
/Al
IV
ratio are sup-
posed to be low-pressure cpx, which could crystallize in a
crustal magmatic reservoir or during magma ascent.
Conclusions
Volcanic spinels, peridotitic spinels and altered
spinels were distinguished according to spinel chemical
composition.
Peridotitic spinels are supposed to originate in mantle
peridotites and most likely represent relics of upper-man-
tle xenoliths, which had been entrained during the erup-
tion of alkali volcanites.
Mesozoic alkali volcanites were not a principal source
for detrital Cr-spinels in Cretaceous flysch sediments.
Cpx phenocryts reflect at least two stages of magma
evolution (upper-mantle cpx
1.34—1.48 GPa and Ti-rich
zoned crustal cpx
0.41—0.69 GPa).
Acknowledgments:
We thank Dr. V. Hurai, Dr. D. Lenaz
and an anonymous reviewer for critical reviews and con-
structive comments on a preliminary draft of this paper
which greatly improved the final version. We are thankful
to J. Spratt, A. Kearsley and Dr. T. Williams from the Natural
History Museum in London for help with works on elec-
tron microprobe. This study also represents a partial out-
put from the Grants APVT-51-012504, APVV-51-046105,
VEGA 1/2031/05 and VEGA 2/6092/26.
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