GEOLOGICA CARPATHICA, AUGUST 2005, 56, 4, 369378
www.geologicacarpathica.sk
Reaction coronas around quartz xenocrysts in the basaltic
andesite from Detunata (Apuseni Mountains, Romania)
NICOLAE HAR
Department of Mineralogy, Babeº-Bolyai University, Kogãlniceanu Str. 1, 3400 Cluj Napoca, Romania; har@bioge.ubbcluj.ro
(Manuscript received May 20, 2004; accepted in revised form March 17, 2005)
Abstract: Olivine-bearing basaltic andesite from Detunata (Apuseni Mts, Romania) contains xenolithic material of
sedimentary origin. Fragments of sandstone as well as isolated xenocrysts of quartz originated from sandstone, were
identified. Reaction coronas consisting of pyroxene, tridymite, quartz fragments and glass are developed around some of
the sandstone fragments and quartz xenocrysts, as a result of interaction between the xenolithic material and the host
basaltic magma. Coronas can be divided into two distinctive parts. The outer one consists of a glassy matrix containing
prismatic crystals of augite. The inner part of the coronas consists of isolated crystals of augite, fragments of quartz and
tridymite in a glassy groundmass. Tridymite is also present along fractures in the quartz xenocrysts. Electron probe
microanalysis and Raman spectroscopy were used to investigate the corona pyroxenes, glass, and silica polymorphs. The
corona pyroxenes have chemical compositions typical for augite. The glass is highly siliceous (SiO
2
= 72.076.8 wt. %)
as a result of quartz dissolution, high in alkalis (Na
2
O = 1.383.22 wt. %; K
2
O = 4.726.23 wt. %) and aluminum (Al
2
O
3
=
9.3112.18 wt. %). Tabular and twinned crystals of tridymite have a high content of alkalis (Na
2
O+K
2
O = 0.300.39 wt. %)
and alumina (Al
2
O
3
= 0.540.86 wt. %) as compared to quartz xenocrysts. Raman spectra of tridymite show the most
representative peaks at 403 and 422 cm
1
. The geochemistry of reaction corona, diffusion processes, and temperature
were the controlling factors in the genesis of the newly-formed minerals in the reaction zones. The xenolithic material
was partly assimilated, and represented an important source of Si
4+
, Al
3+
, Ca
2+
, Mg
2+
, Fe
2+
and K
+
. The spatial relation-
ship between reaction pyroxenes and host basaltic andesite suggests that magma could also be the source for cations such
as Ca
2+
, Mg
2+
and Fe
2+
. The relatively higher content in alkalis of the volcanic glass from the basaltic andesite ground-
mass as compared to the glass of the corona suggests an enrichment in alkalis of the late differentiated magmatic melt
which could represent the main source of alkaline cations (Na
+
and K
+
). The genesis of the corona took place at low
pressure, during the eruption of the basaltic magma. The presence of tridymite in the corona indicates temperatures
higher than 870 °C while the presence of calcite in some xenoliths points to temperatures below 920 °C.
Key words: basaltic andesite, quartz xenocrysts, coronas of reaction, pyroxene, tridymite.
Introduction
The youngest NeogeneQuaternary calc-alkaline volcanic
rocks cropping out in the Apuseni Mountains (Romania)
(Fig. 1) are represented by the olivine-bearing basaltic andes-
ites from Detunata (7.6±0.4 Ma; Pécskay et al. 1995). They
occur as two distinctive bodies: Detunata Goalã (developed
as polygonal columns) and Detunata Flocoasã. The bodies of
olivine-bearing basaltic andesites were intruded into the fly-
sch-type sedimentary deposits (SantonianMaastrichtian) of
the Bucium Unit. The magmatic rock contains xenoliths de-
rived from the flysch. Different degrees of xenolith assimila-
tion can be noticed. Some of them still preserve the textural
and compositional features of the initial sedimentary rocks.
Most of the xenoliths were fractured and partly assimilated
by the basaltic magma. Thus, isolated xenocrysts of quartz
occur in the resulted basaltic andesite rock. Millimeter-sized
quartz xenocrysts (sometimes up to 10 mm) are randomly
distributed in the rock. At the contact between quartz xenoc-
rysts and basaltic magma, reaction coronas have developed.
The coronas consist of pyroxene, glass, and SiO
2
polymor-
phs. Sato (1975), Grove et al. (1988), Backer et al. (1991),
Luhr et al. (1995) and Har & Rusu (2000) described similar
reaction products developed at the contact between quartz
xenocrysts and basaltic melt.
Mãldãrescu (1978), based on microscopic data, reported the
presence of pyroxene, glass and silica polymorphs (tridymite,
metacristobalite and cristobalite) developed in connection
with the assimilation of the xenoliths of quartz, in basaltic
andesite from Detunata.
The aim of the present study is to investigate, using modern
techniques, the morphology of reaction coronas developed
around quartz xenocrysts, as well as their chemistry and min-
eralogy, in order to assess the genetic conditions of the corona
phases (minerals and glass) related to the reaction processes
between basaltic magma and quartz xenocrysts.
Analytical techniques
Thin sections were investigated under a polarized micro-
scope in order to study the reaction coronas developed around
the quartz xenocrysts. Scanning electron microscope was used
for imaging the corona pyroxenes and silica polymorphs, their
mutual spatial relationships, and those with quartz xenocrysts
and volcanic glass.
370 HAR
Pyroxenes, silica polymorphs, and the glass from the
groundmass were analysed using a Cameca Camebax electron
microprobe, with accelerating voltage of 15 kV, in the Depart-
ment of Earth Sciences at Bristol University, UK. The take-off
angle was 40° and the beam current was 15 nA for pyroxene,
25 nA for silica, and 10 nA for glass. The silica polymorphs
were investigated also using a Raman spectrometer (HeNe
laser 633 nm, laser spot <2 µm) in the Physics Department
of Bristol University.
Petrography and geochemistry of olivine-bearing
basaltic andesite
The olivine-bearing basaltic andesites have porphyritic tex-
tures with intergranularintersertal groundmasses. Due to the
Table 1: Major element geochemistry of the basaltic andesite from
Detunata (Har 2001) and its recalculated composition without xeno-
lithic material.
52
54
298
38
277 278
Average
Average
recalcu-
lated
SiO
2
55.29 55.52 53.51 54.67 54.45 54.85 54.72
51.99
TiO
2
0.84 0.85 0.93 0.89 0.90 0.93 0.89
1.09
Al
2
O
3
15.75 16.55 15.56 15.05 15.18 15.52 15.60
15.29
Fe
2
O
3
6.80 7.05 7.31 6.90 6.79 6.90 6.96
8.56
MnO 0.13 0.10 0.13 0.14 0.11 0.14 0.13
0.16
MgO 6.09 5.96 6.33 6.81 6.46 6.45 6.35
7.81
CaO 9.49 9.58 9.52 9.84 9.38 9.74 9.59
8.54
Na
2
O 2.71 1.96 2.53 2.77 2.70 3.01 2.61
3.21
K
2
O 1.39 0.98 1.32 1.32 1.54 1.30 1.31
1.05
P
2
O
5
0.21
0.22 0.24 0.24 0.25 0.23
0.28
H
2
O 0.69
1.57 1.02 1.25 0.47 1.00
1.23
LOI
0.85
0.12
0.14
Total 99.39 99.40 98.93 99.65 99.00 99.56 99.51
99.35
Table 2: Chemical composition of the volcanic glass from the
groundmass of the basaltic andesite (wt. %).
Fig. 1. Location of the Apuseni Mountains on the map of Romania
(above) and position of the Neogene volcanism (+) and Detunata
Massif in the Apuseni Mountains (below).
partial assimilation of the sedimentary material, specific com-
positional (hybrid) features of the volcanic rocks from Detuna-
ta can be observed. The following are present in the rock:
l
minerals of igneous origin: plagioclase feldspars 41 %
(phenocrysts 83.6 % An51.9 % An, microlites 70.2 % An
58.6%An), augite 31 %, olivine (75 % Fo) 3 %, oxidized
amphibole 6 %, titanomagnetite 5 %, secondary miner-
als (chlorite, calcite) 2 %, and volcanic glass 7 % etc.
l
phases related to the assimilation process of the xenoliths
5 %: quartz xenocrysts (corroded by magma), minerals of
reaction (pyroxene, silica polymorphs) and glass developed at
the contact between basaltic rock and quartz xenocrysts.
The chemical composition of the volcanic rock from Detu-
nata is typical for basaltic andesite (Table 1). However, its
present composition, especially the SiO
2
, Al
2
O
3
, K
2
O and
CaO contents, has been modified due to the assimilation of xe-
nolithic material. Mineralogical features of the basaltic andes-
ite corroborated with the observations on the xenoliths indi-
cate that about 10 % of the present composition of the basaltic
andesite is the result of the incorporation of xenolithic materi-
al into the magma; half of it was assimilated and the other half
is still preserved as fragments of sandstone, quartz xenocrysts
and new phases of the reaction corona. The total enrichment
of the basaltic andesite in the main oxides, originated from the
xenolithic material, is estimated as follows: SiO
2
5 %,
1
2
3
4
5
6
7
8
9
10
SiO
2
67.86 66.24 63.64 67.95 67.55 67.00 65.62 67.78 67.97 67.68
TiO
2
1.73 1.38 1.19 1.79 1.72 1.56 1.51 1.81 1.69 1.36
Al
2
O
3
13.03 13.89 16.47 11.43 12.52 12.77 13.66 11.23 11.85 13.27
Cr
2
O
3
0.04 0.01 0.00 0.00 0.04 0.00 0.04 0.05 0.00 0.00
FeO 3.08 2.66 2.41 3.17 2.93 3.00 2.83 3.35 3.12 2.52
MnO 0.04 0.04 0.06 0.13 0.12 0.11 0.09 0.07 0.10 0.02
MgO 0.34 0.29 0.29 0.37 0.28 0.36 0.33 0.50 0.39 0.26
CaO 1.89 2.20 4.09 0.90 2.01 1.37 2.07 0.95 1.08 1.89
Na
2
O 3.77 3.49 3.84 3.25 3.51 3.61 3.79 3.07 3.20 3.43
K
2
O 4.80 5.83 4.32 6.19 4.85 6.21 5.39 6.34 6.04 5.72
Total 96.58 96.01 96.32 95.18 95.53 95.98 95.33 95.14 95.43 96.14
REACTION CORONAS IN THE BASALTIC ANDESITE FROM DETUNATA (ROMANIA) 371
Al
2
O
3
2 %, K
2
O 2 %, and CaO 1 %. Thus, on the ba-
sis of these data, the chemical composition of the volcanic
rock was recalculated without the participation of the xeno-
lithic material and the result is presented in Table 1.
The volcanic glass (aprox. 7 wt. %) is present in the ground-
mass of the basaltic andesite. The chemical composition of the
volcanic glass is presented in Table 2. It is acidic in composi-
tion (SiO
2
= 63.6367.95 wt. %) with total alkalis (Na
2
O +
K
2
O) between 8.179.81 wt. %.
Petrography of the sedimentary xenoliths
The basaltic andesite from Detunata pierced the flysch-type
deposits of the Bucium Unit, which consists of conglomerates,
polygeneous breccias, sandstone and marls. During its ascen-
sion, the basaltic magma incorporated fragments of sedimen-
tary rocks as xenoliths. Depending on the depth of incorpora-
tion and the temperature of the magma, the xenoliths
underwent different degrees of transformation. Minor transfor-
Fig. 2. Xenolithic material incorporated by the basaltic andesite from Detunata. a Fragments of sandstone in volcanic material. b Mi-
crophotograph showing a sandstone xenolith at the contact with the host basaltic andesite (B). The sandstone consists of quartz, micas (M),
and calcite (Cc). The isotropic groundmass (G) is the result of partial melting of the xenolith. c Fragment of quartzite originated from
sandstone, into the isotropic groundmass (G). d Isolated quartz (Q) into isotropic groundmass (G), as the result of partial melting of the
sandstone.
mations are typical for the xenoliths from the margins of the
magmatic body. They still preserved some of the textural (e.g.
stratification) and mineralogical features of the sedimentary
rocks. The south eastern part of the Detunata Goalã contains
the only outcrop where fragments up to 5 cm length of sedi-
mentary xenoliths can be found (Fig. 2a). Macroscopically
they are light grey and yellowish in colour. Under the micro-
scope they show a porphyritic texture. Fragments of sand-
stone, isolated angular quartz and quartzite originating from
sandstone are surrounded by an isotropic groundmass, which
sometimes represents up to 50 % of the rock (Fig. 2b,c and d).
Fragments of sandstone are well preserved and consist of
quartz, lithic fragments of quartzite, and micas (muscovite, bi-
otite sometime opacitized or transformed into chlorite) as ter-
rigenous components, having quartziferous and calcitic ce-
ment. The isotropic groundmass consists of glass and opaque
material. The presence of glass in the groundmass is the result
of partial melting of the xenoliths after their incorporation by
the basaltic magma as well as of the rapid quench of the mag-
ma at the contact with the xenolithic material.
372 HAR
The xenoliths were partly assimilated and thus isolated frag-
ments of sandstone and quartz are present in the host magmat-
ic rock. Both fragments of sandstone (consisting of quartz, mi-
cas and calcite) and individual crystals of quartz, which appear
as xenocrysts in the magmatic rock, reacted with the basaltic
magma generating coronas of reaction (Fig. 3).
Description of the coronas
The best-developed are the coronas which surround the
quartz xenocrysts. The anhedral form of quartz xenocryst (ir-
regular in shape and corroded by the host magma) is the result
of the dissolution processes, which took place at the contact
with the basaltic melt. The coronas consist of different phases,
most of them being the result of quartz dissolution and reac-
tion with the basaltic magma (Fig. 4a,b). The coronas are vari-
able in width. In some cases they are absent, but sometimes
the width is up to 1 cm.
Coronas developed between quartz xenocrysts and the ba-
saltic rock can be divided into two distinctive zones (Fig. 4b):
Fig. 3. a Reaction corona consisting of pyroxene (Px) and glass
(Gl) at the contact between basaltic andesite (B) and xenolithic
sandstone (S). The sandstone consists of quartz (Q) and calcite
(Cc). b Isolated quartz xenocryst (Q) with reaction corona con-
sisting of radial crystals of pyroxene (Px) and glass (Gl).
l
Zone I (inner) consists mainly of glass associated with
crystals of silica polymorphs, isolated pyroxenes, and fragments
of quartz. The aggregates of tabular crystals of silica polymor-
phs are concentrated at the edge of the quartz xenocrysts, in a
glassy groundmass. Locally, at the edge of quartz some isotro-
pic and colourless silica is also present. Some grains of pyrox-
ene are developed as individual crystals in the glassy ground-
mass, but some of them grew up on the quartz xenocrysts.
l
Zone II (outer) is composed of well-developed py-
roxenes in a glassy groundmass. The prismatic crystals of py-
roxene are developed mainly perpendicular to the interface be-
tween the corona and the basaltic host-rock. In some cases
there is only glass present in the outer part of the corona.
The quartz xenocrysts were partly broken and assimilated
by the basaltic melt. This is evident from the presence of small
fragments of quartz in the coronas (Fig. 5) as well as from the
higher content of SiO
2
of glass of the corona as compared to
the volcanic glass of the basaltic andesite. Due to the high
Fig. 4. a Detailed microphotograph (ordinary light) of the reaction
corona (C) with pyroxene (Px) and glass (Gl) developed between ba-
saltic andesite (B) and quartz xenocrysts (Q). b The zonality of the
coronas: zone I consisting of tabular crystals of silica polymorph into
a glassy groundmass (Gl) and zone II where pyroxenes are developed
at the contact with the basaltic andesite (B), in the glass (Gl) of the co-
rona. Due to the partial dissolution of the quartz xenocrysts (Q), iso-
lated fragments of quartz are also present in the corona.
REACTION CORONAS IN THE BASALTIC ANDESITE FROM DETUNATA (ROMANIA) 373
Fig. 5. SEM images of the inner part of the corona, close to the anhedral fragments of quartz xenocrysts (Q). a Small fragments of
quartz coated by the newly-formed skeletal pyroxene into the glassy groundmass (Gl) of the corona. b Tabular and twinned crystals of
tridymite (Tr) at the margin of the quartz grain (Q). The marked points represent the spots where the analyses were performed.
temperature of the basaltic magma, the dissolution processes
took place along the quartz margins. Within the resulted sili-
ca-saturated melt, crystals of silica polymorphs were formed.
Along internal fractures, where clusters of silica polymorphs
crystals were also identified, quartz probably recrystallized.
Pyroxenes are developed mainly in the outer part of the co-
ronas, at the contact with the basaltic rock (zone II), but they
are present as isolated crystals in the inner part as well (zone
I). The pyroxenes of zone II are prismatic in shape and form
radial aggregates (Fig. 3b), usually perpendicular to the con-
tact between the basaltic rock and the reaction rim.
In the inner part of the coronas the pyroxenes are prismatic
or isometric in shape, isolated and unoriented, randomly dis-
tributed in the glassy groundmass. Nucleation of the pyroxene
started homogeneously as individual crystals or on small pre-
existing fragments of quartz (Fig. 5a).
Composition of the coronas
Quartz xenocrysts and tabular crystals of silica polymorphs,
glass, and pyroxenes from coronas were analysed by electron
microprobe. The points selected for analyses are shown in
Figs. 5 and 6.
Table 3: Chemical composition of the quartz xenocrysts (wt. %).
Quartz and tabular silica polymorphs
The analyses of quartz were performed on the primary
grains of the xenocrysts as well as on the small fragments from
the inner parts of the coronas (zone I), especially on the isolat-
ed grains representing the substrate of the newly-formed py-
roxene (e.g. Fig. 5a: 37-1sil 6, 37-1sil 7, 37-1sil 8 etc.). The
results are reported in Table 3.
Significant differences in alkalis (Na
2
O+K
2
O) and Al
2
O
3
contents can be noticed when comparing the analyses per-
formed within the quartz grains (477-8, 37-1sil 2 and 37-
1sil 5) with those from the margins, as well as from the isolat-
ed grains of the inner zone (I) of the coronas. Low alkali
contents (Na
2
O+K
2
O = 00.03 wt. %) and Al
2
O
3
(0.01
0.08 wt. %) are typical for the measured points inside the
quartz grains. The external rims of the quartz xenocrysts, as
well as the small quartz grains present in the inner area of the
coronas are characterized by important increases of alkalis and
Al
2
O
3
contents (Fig. 7).
The analysed crystals of silica polymorphs are tabular in
shape or twinned (triangular shape in section) and developed
at the edge, or within fractures in quartz grains. Five tabular or
twinned crystals of silica polymorphs were analysed: 477-9,
477-10, 477-11, 477-17 and 477-18 (Figs. 5b and 6a). The re-
477-8
477-12
477-13
37-1sil1
37-1sil2
37-1sil3
37-1sil4
37-1sil5
37-1sil6
37-1sil7
37-1sil8
SiO
2
97.70
97.28
98.15
97.99
97.99
96.67
98.55
99.23
94.31
98.97
98.34
TiO
2
0.03
0.11
0.15
0.16
0.00
0.11
0.21
0.02
0.37
0.02
0.00
Al
2
O
3
0.08
0.56
0.47
0.50
0.01
0.72
0.47
0.02
0.81
0.06
0.03
Cr
2
O
3
0.13
0.09
0.00
0.00
0.00
0.00
0.00
0.01
0.00
0.00
0.00
FeO
0.16
0.17
0.15
0.10
0.00
0.12
0.02
0.00
0.95
0.09
0.17
MnO
0.00
0.07
0.02
0.00
0.00
0.03
0.00
0.07
0.06
0.00
0.01
MgO
0.15
0.01
0.00
0.04
0.00
0.00
0.01
0.00
0.05
0.03
0.00
CaO
0.04
0.01
0.03
0.01
0.01
0.01
0.00
0.00
0.13
0.01
0.08
Na
2
O
0.01
0.19
0.22
0.22
0.00
0.13
0.19
0.00
0.26
0.15
0.10
K
2
O
0.02
0.15
0.11
0.12
0.00
0.17
0.10
0.00
0.32
0.02
0.00
Total
98.32
98.64
99.30
99.14
98.01
97.96
99.55
99.35
97.26
99.35
98.73
374 HAR
sults are typical for tridymite (Table 4). The analysed crystals
of tridymite show high contents of Na
2
O, K
2
O, and Al
2
O
3
as
compared to the quartz xenocrysts (Fig. 7). Moreover, the al-
kalis are more concentrated in the isolated quartz grains of
zone I, in the outer rims of the quartz xenocrysts, as well as in
the glass of the coronas.
Raman spectra of the silica polymorphs
In order to identify the silica polymorphs, Raman investiga-
tions in the range 01000 cm
1
were performed, although the
range between 380 and 480 cm
1
is the most representative for
the structures with six-membered rings, such as quartz and
tridymite (Matson et al. 1986). Raman spectroscopy was per-
formed on quartz xenocrysts as well as on tabular crystals of
tridymite from the inner parts of the coronas. The Raman
spectra of quartz show the most intense peaks at 128,
206, 265, 355, 464, 511, 696, 796, and 808 cm
1
(Fig. 8). The
Raman patterns of tridymite contain peaks at 115, 403, 422,
537 and 593 cm
1
(Fig. 8), but all of them show smaller rela-
tive intensities compared with α-quartz. Raman measurements
performed on silica polymorphs fit well with the data of King-
ma & Hemley (1994) for α-quartz and only partly for tridym-
Fig. 6. Quartz xenocrysts (Q) and their reaction corona consisting of glass (Gl), silica polymorph (tridymite) and pyroxenes of reaction
(Px), with the location of the investigated spots in the main phases of the corona.
ite (403 and 422 cm
1
peaks, which seem to be the most repre-
sentative).
Glass
Glass is one of the main components and represents the
groundmass of the coronas. Fifteen electron microprobe anal-
yses of the glass were performed in different parts of the diffu-
477-9
477-10
477-11
477-17
477-18
SiO
2
97.50
97.26
96.93
97.36
97.49
TiO
2
0.07
0.11
0.12
0.03
0.07
Al
2
O
3
0.55
0.77
0.86
0.54
0.68
Cr
2
O
3
0.00
0.00
0.05
0.08
0.00
FeO
0.15
0.06
0.30
0.04
0.12
MnO
0.00
0.01
0.00
0.00
0.00
MgO
0.00
0.03
0.06
0.01
0.03
CaO
0.02
0.00
0.08
0.01
0.01
Na
2
O
0.23
0.25
0.28
0.26
0.16
K
2
O
0.14
0.19
0.19
0.13
0.14
Total
98.66
98.68
98.87
98.46
98.70
Table 4: Chemical composition of the tridymite (wt. %).
REACTION CORONAS IN THE BASALTIC ANDESITE FROM DETUNATA (ROMANIA) 375
Fig. 7. Variation in alkalis and Al
2
O
3
contents in the quartz xenoc-
rysts, rim of the quartz xenocryst, the isolated quartz grains of the
corona and tridymite.
distributed crystals as well as in the outer zones at the contact
with the basaltic host, where the crystals are usually prismatic
in shape and form radial aggregates.
Electron microprobe analyses on the corona pyroxenes were
performed in two different ways (Figs. 5a and 6a,b,c, and d):
l
on profiles from the core to the rim of the crystal (in two
differently oriented profiles on two different crystals). One
profile was choosen longitudinally (along the c axis) of the
crystal (Fig. 6a) and the second one transversally (from core to
rim in a thin section perpendicular to the c axis; Fig. 6d) in or-
der to illustrate the chemical changes during crystal growth
(Table 6).
l
as isolated measurements on different crystals of the coro-
nas (Table 7).
The profiles for both crystals revealed very homogeneous
compositions with no significant variations. Gentle variations
in some elements (e.g. for SiO
2
) could be the result of the local
geochemistry and diffusion rates within the reaction zone,
even if the increase in SiO
2
fits very well with the normal evo-
lution of the crystallization process, as well as the dissolution
processes of the quartz xenocrysts.
All the chemical analyses presented in Table 7 were per-
formed in the cores of isolated crystals. The results show a
very high homogeneity of the analysed crystals. Small differ-
ences in chemical composition may be the result of different
rates of cation exchange in different parts of the diffusion co-
ronas.
In order to identify the corona pyroxene species, their com-
position were recalculated in terms of Wo, En, and Fs contents
(Table 8). According to the diagram of Morimoto et al.
(1988), all the analysed crystals correspond to augite.
Discussion
Incorporation of the sedimentary xenoliths by the basaltic
magma led to local geochemical changes. According to the
depth of incorporation and the magma temperature, parts of
the xenoliths were assimilated. The resulting quartz xenoc-
rysts were also partly dissolved by the host magma and thus,
the enrichment in SiO
2
of the surrounding melt took place. As
compared to the volcanic glass of the basaltic andesite, the
glass of the corona is more acidic and with smaller amounts of
alkalis (Fig. 9).
The chemical composition of the glass from the coronas
also shows an enrichment in alkalis and Al
2
O
3
as compared to
Fig. 8. Raman spectra of quartz xenocryst and tridymite from De-
tunata.
sion coronas (Figs. 5a and 6a,b,c and d). The results are pre-
sented in Table 5.
The glass of the coronas is highly siliceous in composition
(SiO
2
= 70.3576.87 wt. %) and also contains high levels of
alkalis (Na
2
O = 1.393.22 wt. %, K
2
O = 4.736.23 wt. %)
and Al
2
O
3
(9.3212.18 wt. %). A high content of alkalis is
typical for Si-rich glass from such coronas (Sato 1975).
Pyroxenes
Clinopyroxene is also an abundant phase of the coronas. It
is present in the inner reaction zones as isolated and randomly
477gl1 477gl2 477gl3 477gl4 477gl5 477gl6 477gl7 477gl8 477-gl21 477-gl22 371gl1 371gl2 37-1gl3 37-1gl4 37-1gl5
SiO
2
73.89
74.90
73.29
72.28
75.31
74.29
72.03
72.99 72.37
76.87
70.35
72.99
72.00
73.59
74.62
TiO
2
0.36
0.45
0.51
0.82
0.64
0.76
0.78
0.33 0.70
0.52
0.89
0.81
0.81
0.83
0.61
Al
2
O
3
10.09
9.32
9.68
11.68
9.73
10.60
12.18
10.61 12.00
9.83
12.17
10.13
11.85
10.52
9.57
Cr
2
O
3
0.46
0.09
0.41
0.01
0.04
0.17
0.13
0.00 0.05
0.02
0.04
0.14
0.00
0.38
0.04
FeO
2.12
2.52
3.27
2.61
1.91
2.37
2.63
3.64 1.94
1.88
3.12
2.67
1.47
2.63
2.26
MnO 0.58
0.00
0.00
0.00
0.00
0.00
0.00
0.00 0.00
0.06
0.10
0.13
0.17
0.00
0.00
MgO 0.17
0.00
0.27
0.13
0.12
0.08
0.09
0.46 0.04
0.03
0.20
0.10
0.03
0.10
0.11
CaO
0.43
0.53
0.68
0.71
0.46
0.58
0.74
1.03 0.42
0.26
0.81
0.40
0.45
0.44
0.47
Na
2
O 2.85
2.64
2.50
3.22
2.15
2.48
2.96
2.71 2.14
1.39
3.21
2.66
2.31
2.44
2.04
K
2
O
4.73
4.83
4.93
5.67
5.15
4.91
5.35
4.64 5.30
4.96
5.80
5.11
6.23
5.37
5.33
Total 95.68
95.28
95.54
97.13
95.51
96.24
96.89
96.41 94.96
95.82
96.69
95.14
95.32
96.30
95.05
Table 5: Chemical composition of the glass in the corona (wt. %).
376 HAR
the silica polymorphs of the corona. Analyses of the glass re-
veal average concentrations of alkalis (7.73 wt. %) and Al
2
O
3
(10.66 wt. %) about 30 times the average level observed in
quartz and tridymite (0.23 wt. % and 0.34 wt. % respectively)
of the coronas. Both, the basaltic magma and the xenoliths, es-
pecially the phyllosilicate phases (muscovite, biotite), repre-
sent the source of alkalis and Al
2
O
3
. Alkali enrichment of
tridymite is mainly dictated by the high partition coefficient
for Na and K in silica-rich melt (derived from quartz dissolu-
tion) relative to the dominant basaltic melts of the host lava
groundmass. Sato (1975) reported similar enrichments of alka-
li elements in the diffusion coronas developed around quartz
Table 6: Chemical composition of the coronas pyroxenes on lon-
gitudinal and transverse profile (wt. %).
Table 7: Chemical composition of isolated crystals of pyroxenes in corona (wt. %).
Sample Wo En
Fs Sample Wo En
Fs
37-1pr1
42.85 41.61 16.52 477-1
42.81 41.85 15.33
37-1pr2
41.14 43.17 15.67 477-2
43.76 41.33 14.89
37-1pr3
40.73 43.64 15.62 477-3
42.93 41.22 15.84
37-1pr4
41.43 43.36 15.20 477-4
42.46 41.15 16.37
37-1px5
41.87 42.64 15.48 477-5
41.91 41.11 16.97
37-1px6
39.39 44.49 16.10 477-6
42.11 39.89 17.99
37-1px7
40.76 44.27 14.96 477-7
43.02 38.40 18.56
37-1px8
41.46 42.88 15.65 477-14 43.88 39.95 16.15
37-1px9
40.71 43.45 15.82 477-15 41.40 41.77 16.82
37-1px10 40.12 44.11 15.68 477-16 42.50 41.09 16.40
37-1px11 41.14 43.176 15.67
Table 8: The recalculated Wo, En and Fs end members of coro-
na pyroxenes (%).
xenocrysts from basaltic rocks of the Tertiary volcanic region
in northeastern Shikoku in Japan.
Tridymite crystallized into the silica-saturated melt of the
corona. The presence of Na and K in the corona melt favoured
the crystallization of tridymite, which takes place more rapidly
in the presence of such mineralizing agents (Heaney 1994).
A positive correlation between Al
2
O
3
and Na
2
O+K
2
O in the
tridymite from coronas can be noticed. Such a correlation is
typical for tridymite and it is likely to be due to the substitu-
tion of atomic Al to charge balance the substitution of Na, K,
and Li in the silica polymorphs (Smith & Steele 1986). The
relatively rapid quenching of the melt around the cool
quartz xenocrysts, proved also by the presence of glass, inhib-
ited the inversion of the tridymite to quartz.
Nucleation of the pyroxenes took place either homoge-
neously to form individual crystals, or on quartz grains, espe-
cially in the inner zone of the coronas. The crystals are invari-
ably in contact with the glassy groundmass. The spatial
association with the glass and the euhedral crystals of the co-
rona pyroxenes, as well as their high compositional homoge-
neity suggest that the newly-formed crystals grew from melts
that later quenched rapidly to glass. All clinopyroxenes from
the corona are low in Al (Al
2
O
3
= 0.210.85 wt. %) as com-
pared with the pyroxene phenocrysts of the olivine-bearing
basaltic andesite (Al
2
O
3
= 2.392.54 wt. %; Har 2001). The
low Al content reflects the high silica content of the local envi-
ronment of the corona. The spatial position of the pyroxenes in
the reaction zone developed between the host basaltic rock and
the quartz xenocrysts suggests that both the basaltic melt and
the glass of the corona represents the source of Ca, Mg, Al, and
Fe, whereas the quartz xenocrysts represent the source of Si.
Fig. 9. SiO
2
vs. K
2
O+Na
2
O in the volcanic glass of the host basal-
tic andesite as compared to the glass of the corona.
477-1 477-2 477-3 37-1px1 37-1px2 37-1px3 37-1px4
SiO
2
52.75 52.22 51.74 50.77
52.88
52.84
52.74
TiO
2
0.11 0.16 0.21 0.33
0.42
0.35
0.42
Al
2
O
3
0.52 0.46 0.53 0.47
0.77
0.85
0.74
Cr
2
O
3
0.00 0.00 0.00 0.06
0.00
0.04
0.00
FeO 9.24 9.06 9.50 9.70
9.57
9.72
9.44
MnO 0.43 0.25 0.36 0.45
0.38
0.31
0.28
MgO 14.83 14.52 14.40 15.01
15.39
15.72
15.58
CaO 21.12 21.39 20.87 20.56
20.41
20.41
20.71
Na
2
O 0.34 0.34 0.30 0.30
0.27
0.30
0.30
K
2
O 0.00 0.00 0.02 0.01
0.01
0.01
0.05
Total 99.34 98.40 97.93 97.66
100.10
100.55
100.26
477-4 477-5 477-6 477-7 477-14 477-15 477-16 37-1px5 37-1px6 37-1px7 37-1px8 37-1px9 37-1px10 37-1px11
SiO
2
51.51 52.40 52.06 51.65 51.96 52.09 52.60 52.74
52.61
52.38
52.48
52.58
52.47
52.44
TiO
2
0.13 0.17 0.11 0.06 0.23 0.39 0.37 0.34
0.34
0.27
0.35
0.37
0.36
0.45
Al
2
O
3
0.21 0.38 0.23 0.41 0.35 0.58 0.41 0.44
0.46
0.52
0.54
0.49
0.47
0.62
Cr
2
O
3
0.00 0.15 0.03 0.00 0.05 0.00 0.03 0.00
0.12
0.01
0.00
0.04
0.03
0.00
FeO
9.90 11.18 11.57 12.75 9.75 10.27 10.11 9.55
9.71
9.05
9.71
9.76
9.72
9.27
MnO
0.32 0.45 0.61 0.63 0.41 0.34 0.33 0.36
0.25
0.38
0.33
0.28
0.33
0.40
MgO
14.41 14.47 13.91 13.22 15.12 15.80 15.69 15.32
15.46
15.67
15.44
15.49
15.90
15.64
CaO
20.70 20.52 21.44 20.62 21.28 20.41 21.14 20.94
21.05
21.08
20.77
21.19
21.08
21.02
Na
2
O 0.32 0.33 0.30 0.33 0.32 0.34 0.31 0.31
0.24
0.30
0.29
0.30
0.30
0.29
K
2
O
0.00 0.06 0.03 0.02 0.04 0.01 0.02 0.00
0.01
0.02
0.00
0.00
0.01
0.03
Total
97.50 100.11 100.29 99.69 99.51 100.23 101.01 100.00
100.25
99.68
99.91
100.50
100.67
100.16
REACTION CORONAS IN THE BASALTIC ANDESITE FROM DETUNATA (ROMANIA) 377
It is very difficult to appreciate the intensity of diffusion of
some cations from the magma as long as it must have been
partly crystallized when the xenoliths were incorporated. Sev-
eral evaluations regarding the participations of phases in reac-
tion corona indicate the following aproximative amounts:
glass 55 %, pyroxene 40 % and tridymite 5 %. Tak-
ing their participation into account the weighted average
chemical composition of the corona was calculated: SiO
2
=
66.17 wt. %, TiO
2
= 0.47 wt. %, Al
2
O
3
= 6.09 wt. %, Cr
2
O
3
=
0.08 wt. %, FeO = 5.33 wt. %, MnO = 0.18 wt. %, MgO =
6.10 wt. %, CaO = 8.66 wt. %, Na
2
O = 3.91 wt. %, and K
2
O =
2.79 wt. %. The results point out an enrichment of the corona
in SiO
2
, K
2
O and Na
2
O as compared with the host basaltic
andesite, similar values of MgO, and depletion in Al
2
O
3
.
Conclusions
Olivine-bearing basaltic andesites from Detunata (Apuseni
Mountains, Romania) contain xenoliths of sedimentary rocks
(sandstone) and xenocrysts of quartz as the result of partial as-
similation of the xenoliths. Both xenoliths and xenocrysts of
quartz, randomly distributed in the basaltic andesite rock, un-
derwent various processes:
l
partial melting of the xenoliths and dissolution of quartz
led to the enrichment in SiO
2
and alkalis of corona melt;
l
tridymite crystallized into high silica environment of the
corona;
l
reactions with the basaltic melt led to the formation of co-
rona pyroxenes (augite);
l
assimilation, resulting in compositional changes of the
magma.
The glass, which represents the groundmass of the reaction
zone, was formed at the contact between the basaltic melt and
cold quartz xenocrysts. Thus, surrounding the quartz xenoc-
rysts, coronas consisting of glass, fragments of quartz, tridym-
ite, and augite are present.
During the formation of the coronas, a cations exchange be-
tween the basaltic magma and the melt of the corona took
place. The cation exchange was controlled by the specific
physical (especially temperature, pressure, time from incorpo-
ration of xenoliths and the growth of the corona pyroxenes)
and geochemical conditions existing in the reaction zone.
Luhr et al. (1995) concluded that for a longer reaction time,
Al-poor pyroxene crystallizes from centres on the outer coro-
na margins. Such spatial distribution of corona pyroxenes of
reaction is very common at Detunata (Fig. 3b). The dissolu-
tion rate of quartz in a basaltic melt is strongly controlled by
the crystallization state of the melt (Donaldson 1985). A high-
er dissolution rate is present if the basaltic melt is superheated
(above the liquidus temperature) than in the case of a partly
crystallized melt. The presence of olivine and pyroxene as
phenocrysts in the basaltic andesite from Detunata suggests
that the melt was partly crystallized when quartz was incorpo-
rated. On the other hand, the presence of glass in the ground-
mass of the host-rock indicates a high rate of post-eruptive
cooling. Thus, the reaction process seems to be mainly con-
trolled, beside geochemical conditions, by the time interval
between the incorporation of the quartz xenocrysts and the
eruption of the magma. The presence of xenolithic sand-
stone, quartz xenocrysts, with and without reaction coronas
at Detunata indicates variable depth of quartz incorporation
by the melt.
The diffusion of Si
4+
from the lattice of quartz into the coro-
na generates the acidic composition of the melt in the reaction
zone. During dissolution, Si
4+
diffused from the quartz xenoc-
rysts towards the neighbouring melt. K
+
and Al
3+
diffused in
the opposite direction (from the melt towards the quartz xe-
nocrysts) as shown by the high amounts of K
2
O and Al
2
O
3
at
the external zone of quartz, as compared to those in the centre
(see Fig. 7).
Different processes have controlled the geochemistry of the
reaction corona:
1. Partial melting of the sandstone xenoliths followed by the
quartz dissolution. The fusion of the hydrous phyllosilicate
phase (muscovite, biotite) could be an important source of K
+
.
2. Metasomatic processes which took place at the contact
between the quartz xenocrysts and the basaltic melt:
Si
4+
was released from the quartz into the reaction zone;
The cation exchanges between the magma and the melt
of the corona. The diffusion between the two liquids (the ba-
saltic vs. the corona melt) induced a permanent change in the
chemical composition of the melt in the neighbourhood of the
quartz xenocrysts. The process decreased in intensity progres-
sively as the pyroxene crystals, constituting a diffusion barri-
er, started to form; in parallel, the chemical heterogeneity of
the two liquids decreased too.
The highest values of K
2
O and Na
2
O in the corona also
suggest the existence of an alkaline front diffused from the ba-
saltic melt into the corona zone.
3. The crystallization of tridymite and pyroxenes of reaction
from the melt. The presence of alkalis in the corona favoured
the crystallization of tridymite from the silica-saturated melt
next to the quartz xenocrysts and thus, a high content of alka-
lis (Na
2
O+K
2
O = 0.300.47 wt. %) and aluminum oxide
(Al
2
O
3
= 0.540.86 wt. %) in the tridymite can be noticed.
Schneider (1986) explained the relationship of the alkalis and
aluminum oxide in the tridymite as the result of the entry of
sodium and potassium into the structural channels and voids
of the tridymite. Aluminum substitution for silicon in the oxy-
gen tetrahedra, in the neighbourhood of alkali ions centers was
required to maintain charge balance due to alkali ion entry into
the structure of the tridymite. The glassy groundmass of the
corona is also enriched in alkalis (Na
2
O = 1.393.21 wt. %;
K
2
O = 4.646.23 wt. %), aluminum oxide (Al
2
O
3
= 9.32
12.18 wt. %), and SiO
2
(70.3576.87 wt. %). Dissolution of
quartz is responsible for the acidic nature of the glass, while
the fusion of hydrous phyllosilicate phases (e.g. muscovite, bi-
otite) as well as the diffusion of K
2
O, and Na
2
O from the ba-
saltic melt led to the enrichment of glass in these components.
The calcite from xenoliths represents the source of Ca
2+
. The
diffusion of Si
4+
, Al
3+
, Ca
2+
and Mg
2+
towards the pyroxene
nucleation centres leads to the growth of pyroxene crystals
(augite) in the acidic melt. The pyroxenes were formed mainly
at the contact with the basaltic magma. Isolated crystals of
augite were also formed inside the melt of the diffusion coro-
na, via nucleation as individual pyroxene crystals, or as over-
growth on quartz fragments (Figs. 5a and 6b).
378 HAR
The genesis of the reaction corona took place at low pres-
sures and at temperatures around 900 °C. The reaction pro-
cesses took place while the basaltic magma erupted. Such con-
ditions indicate a low pressure of the magma xenolithic
material system. The presence of tridymite in the reaction co-
rona indicates temperatures higher than 870
°C, while the
presence of calcite in some xenoliths with typical reaction co-
ronas point out temperatures below 920940
°C. Higher tem-
peratures could be reached by the earlier incorporated xeno-
liths which underwent partial melting, the quartz xenocrysts
representing practically the unmelted part of the xenolith (e.g.
restite).
Acknowledgments: I acknowledge the support of the Europe-
an Community Access to Research Infrastructure action of the
Improving Human Potential Program, contract HPRI-CT-
1999-00008 awarded to Prof. B.J. Wood (EU Geochemical
Facility, University of Bristol). All my thanks are due to Dr.
Stuart Kearns and Dr. John Dalton for their assistance during
electron microprobe analyses and Raman spectroscopy at the
University of Bristol (UK).
I gratefully acknowledge Dr. Patrik Koneèný and Dr. Peter
Luffi for the careful review of the manuscript and their perti-
nent observations. Special thanks to Dr. Ben Williamson
(NHM London UK, Department of Mineralogy) and Dr.
Dana Pop (Babeº-Bolyai University, Department of Miner-
alogy) for their careful review of the English version of the
manuscript.
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