-
ORIGINAL PAPER
Globule-rich lavas in the Razjerd district, Qazvin, Iran:a
unique volcanic fabric
A. Asiabanha & J. M. Bardintzeff
Received: 10 November 2012 /Accepted: 11 January 2013# Saudi
Society for Geosciences 2013
Abstract A hypocrystalline silica-rich (63–67 wt.% SiO2,dacitic
composition) lava flow (called G-lava) in the subaerialeruptive
sequence of the Alborz Mountains (Razjerd district,Qazvin Province)
of northern Iran, contains abundant (40–50 vol.%) 0.1- to 5.0-cm
globules set in a matrix of rathersimilar composition and
microtexture. Numerous globuleshave coalesced, showing triple-point
junctions with 120°angles. Both phases in the G-lava (globules and
matrix) con-tain similar microphenocrysts (plagioclase, ortho- and
clino-pyroxene and magnetite) in a trachytic groundmass.
However,their mesostasis differ in colour, in composition, in the
amountof glass and their amount of volatiles and silica: in the
globulesthe mesostasis is darker and richer in SiO2 but is volatile
poor.Other volcanic materials in the same unit are very similar
incomposition to the G-lava. The globular fabric was formedwith two
phases: one poor in volatiles (the globules), the otherrich in
volatiles (the matrix). The globules are slightly moresilicic (66.9
against 64.6 wt.% SiO2), more potassic (3.7againt 2.8 wt.% K2O) and
more viscous (of the order of 10
3
to 104) than the matrix outside the globules. It seems that
thetwo phases (globules and matrix) with different silica
andvolatiles contents and thus different vesicularities,
viscositiesand densities, were produced in the dacitic melt due
to
temperature and pressure drop and magmatic degassing inthe
volcanic conduit involved fluid-melt exsolution processes.Some of
the volatile-rich melt was probably frothy duringeruption,
producing volcanic bombs and scoria.
Keywords Globular texture . Fluid-meltexsolution .Dacite .
Qazvin . Iran
Introduction
In the northern heights of Qazvin Province, situated on
thewestern Alborz zone of North Iran, there is a narrow strip(about
5×15 km in area) of post-Eocene volcanic rocks inthrust-fault
contact with Eocene volcanic rocks (Fig. 1). TheEocene volcanic
succession in the area initiates with sub-aqueous
volcano-sedimentary deposits and then subaerialmafic-felsic lava
flows were produced with potassic calc-alkaline to shoshonitic
affinities related to a continentalcollision regime (Asiabanha et
al. 2009). The post-Eocenesubaerial volcanic succession (∼200 m in
thickness) showsthree fresh and unaltered volcanic facies (Fig. 1)
that arerhyolitic ignimbrite sheet, shoshonitic basaltic and
trachy-basaltic lava flow, latitic and andesitic lava flow
(Asiabanhaet al. 2012). In the northwestern part of this
post-Eocenestrip (near Razjerd), the latitic-andesitic facies
changes to acurious dacite, the topic of this paper, that shows a
hetero-geneous fabric. Here, it exhibits two distinctive and
easilyseparable phases: brown spherical masses
(hereafter“globules”), 0.1 to 5 cm in size, in a pink–grey matrix
of arather similar composition (Fig. 2a, b). This dacitic lava
isherein referred to as a “G-lava” (G standing for globules).
We observe great similarities (textural, mineralogical andeven
chemical) between the globules and the host. Becauseof these
similarities, previous workers (e.g. Hosseini 1988)were not able to
provide a satisfactory explanation for thisunusual fabric.
A. Asiabanha (*)Department of Geology, Faculty of Science,Imam
Khomeini International University, Qazvin, Irane-mail:
[email protected]
J. M. BardintzeffIUFM, Université de Cergy-Pontoise, 95000
Cergy-Pontoise,Francee-mail:
[email protected]
J. M. BardintzeffLaboratoire de Pétrographie-Volcanologie and
équipePlanétologie, UMR CNRS IDES 8148, Bât. 504,Université
Paris-Sud,91405 Orsay Cédex, France
Arab J GeosciDOI 10.1007/s12517-013-0842-4
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An apparent lack of similar cases in other parts of theworld is
just one reason for uncertainties in explaining thephenomenon. In
this paper, we describe an example of thisglobular texture from
Iran, discuss different hypotheses forgenesis and propose the
tentative explanations of its originbased on field, petrographic
and chemical data.
Geological setting
Geologically, the study area is part of the post-Eocenevolcanic
succession in the Alborz magmatic assemblage ofnorthern Iran.
According to Asiabanha et al. (2012), thesuccession was produced by
subaerial explosive eruptionsfollowed by effusive eruptions. (1)
The rhyolitic ignimbriticsheet underlain by a thicker lithic
breccia is the product ofthe gas-rich explosive eruptions. (2) Lava
flows, including
shoshonitic basalt and trachybasalt, latite and andesite,
over-lie the first event products across a reddish soil
horizon.
Asiabanha et al. (2012) concluded using mineral chemistrydata
and petrographic evidence that the magma chamber hadbeen evolved by
differentiation, magma mixing and vesicula-tion. Andesites of this
succession are the main products ofsuch chemical
disequilibrium.
The exceptional globule-bearing outcrop (G-lava), themain
subject of this paper, of dacitic composition, is seenin the same
level as those containing the andesitic lava flowsin the
neighbouring district (Abazar).
Field relations and volcanic fabrics
Two dominant volcanic units are seen in the Razjerd area:(1) A
brown pyroxene-phyric shoshonitic basalt composed
Fig. 1 Geological map of theRazjerd district, Iran
Fig. 2 Field photographs ofglobule-rich lava in the
Razjerddistrict. a A hand specimen of aglobule-rich lava; b outcrop
ofthe same lava; c coalescence ofglobules in the direction offlow;
and d two ellipsoidal-shaped volcanic bombs in theG-lava unit
Arab J Geosci
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of phenocrysts of zoned plagioclase (An33–74), zoned ensta-tite,
augite, olivine (partially weathered into iddingsite) andFe–Ti
oxides (total phenocrysts, >50 vol.%) in a trachytic
ormicrolitic matrix containing the same minerals together
withscarce microlites of sanidine and a hyaline mesostasis.
(2)Shoshonitic basalt is overlain by a thick (about 100
m)grey–black unit of dacite lava that contains lenses of
scori-aceous material (Fig. 3). Although the same unit,
mainlyandesitic, is found throughout the area, it has a unique
fabricand chemical composition (dacitic) only in the
Razjerddistrict. In Razjerd, the lava contains 0.1–5.0 cm
sphericalglobules (40–50 vol.%) embedded in a matrix with nearlythe
same composition and texture, and with little apparentdifference
between globules and matrix (Fig. 2a, b). Most ofthe globules have
coalesced together in the direction of flow(Fig. 2c). In the same
area, abundant ellipsoidal-shapedvolcanic bombs (up to 50 cm long)
are also found(Fig. 2d) that are chemically more similar to the
matrix ofthe G-lava rather than globular masses (see below).
Analytical methods
Many samples were prepared for microprobe analyses, espe-cially
from the G-lava, because an initial analysis of handspecimens
suggested that the two distinctive phases (globulesand matrix) may
contain different minerals. The minerals and
mesostases in the volcanic rocks were assessed and analysedusing
a CAMECA SX 100 (15 kV, 10 nA) electron microprobeat the Université
Pierre et Marie Curie, Paris VI, France(Tables 1, 2, and 3). Kα
lines were used. The analysed stan-dards were diopside for Si, Ca
and Mg; Fe2O3 for Fe; MnTiO3for Ti andMn; Cr2O3 for Cr; albite for
Na; and orthoclase for Kand Al. Counting times were 10 s for both
peaks and back-ground, with a 5-μm defocused beam.
SEM (Philips XL 30) analyses (60 s counting time, 1
nA,water-free analyses, recalculated to 100) and back
scatteredelectron microphotographs were made at the
UniversitéParis-Sud Orsay, CNRS-IDES, France (Table 4).
Reflection infrared (IR) spectrometry analyses were per-formed
with a Bruker Vector 22 FTIR spectrophotometerattached to a Bruker
Hyperion 2000 IR microscope with a×15 Cassegrain objective,
numerical aperture of 32, at theCentre de Spectroscopie Infrarouge
of the Muséum Nationald’Histoire Naturelle, Paris, France. The IR
spectra wereobtained in reflection mode on polished thin
sections.
For whole-rock geochemical analysis, samples were se-lected from
G-lava, scoria and volcanic bombs and wereanalysed using
inductively coupled plasma (ICP)–massspectrometry at the Actlab
Laboratory, Canada (Table 5).Samples were crushed and pulverised in
an agate mill, andanalysed using the lithium metaborate/tetraborate
fusionICP Whole-Rock Package. A portion of sample pulp wasmixed
with flux (lithium metaborate, LiBO2) to lower themelting point.
The mixture was then heated in a mufflefurnace until molten. After
cooling, the fused mass wasdigested in 5 % HNO3, and the resulting
clear solutionwas analysed.
Petrography and mineral chemistry
The main lava flows of the study area are described
heretexturally and mineralogically.
Globule-bearing hypocrystalline lava
As noted earlier, a level of andesitic composition is
foundthroughout the wider region, but it is only in the study
areaaround Razjerd that the lava has a dacitic composition
andcontains abundant (40–50 vol.% and locally up to 60
vol.%)spherical globules. Despite the apparent distinctive
differ-ences between globules and matrix in outcrops and
handspecimens, their petrographic characteristics, including
tex-ture and mineral assemblage, crystal size and crystal amountare
very similar. The only difference is in the colour of theglass
(Fig. 4). Indeed, we observed that the micropheno-crysts of
plagioclase (An25–67), enstatite, diopsidic augite(Tables 1 and 2;
Fig. 5) and magnetite, set in a trachytichypocrystalline matrix,
are seen in both phases (globules
Fig. 3 Scoriaceous inliers (about 30 cm thick) in the
globule-rich lavathat is bordered by white lines
Arab J Geosci
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Tab
le1
Chemical
compo
sitio
nsof
selected
mineralsfrom
glob
ules
ofG-lava
Label
8687
9091
103
104
106
109
158
137a
102
108
7071
7274
9596
97Cpx
-cCpx
-mCpx
-cCpx
-rOpx
-cOpx
-rOpx
Opx
Opx
Plag
Plag
Plag
Plag-c
Plag-m
Plag-r
Plag
Plag-c
Plag-r
Plag
Mineral
SiO
251
.09
52.38
51.79
52.44
53.95
53.61
54.37
52.33
54.40
68.21
54.49
53.20
55.23
53.06
55.37
57.24
53.60
65.46
61.20
TiO
20.52
0.45
0.33
0.38
0.25
0.24
0.18
0.30
0.23
0.16
0.00
0.01
0.00
0.02
0.00
0.00
0.02
0.15
0.00
Al 2O3
2.36
2.45
1.53
1.92
1.19
1.43
1.28
1.66
0.90
18.83
27.50
29.43
27.99
29.42
27.84
26.24
28.60
19.78
22.78
Cr 2O3
0.08
0.05
0.02
0.00
0.06
0.02
0.00
0.06
0.03
0.01
0.04
0.05
0.03
0.01
0.00
0.05
0.00
0.02
0.00
Fe 2O3
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.61
1.04
0.70
0.97
0.73
0.73
0.90
0.85
0.74
1.00
FeO
8.51
8.84
8.23
8.30
16.40
16.21
16.95
18.61
16.53
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
MnO
0.41
0.44
0.45
0.44
0.71
0.63
0.75
0.68
0.63
0.00
0.00
0.01
0.05
0.01
0.00
0.04
0.03
0.01
0.00
MgO
14.80
14.06
15.58
15.66
24.70
24.97
24.56
23.63
24.81
0.02
0.07
0.07
0.10
0.05
0.05
0.05
0.06
0.03
0.05
CaO
20.74
20.69
20.52
20.16
1.43
1.47
1.38
1.70
1.41
3.14
10.40
12.38
10.59
12.00
10.78
9.08
11.38
4.40
6.48
Na 2O
0.26
0.36
0.28
0.31
0.00
0.00
0.02
0.04
0.01
6.04
5.29
4.47
5.63
4.64
5.42
6.29
4.98
5.96
5.94
K2O
0.02
0.05
0.01
0.00
0.02
0.01
0.00
0.02
0.01
2.95
0.48
0.28
0.38
0.37
0.46
0.66
0.39
2.07
1.89
P2O5
0.02
0.01
0.02
0.04
0.01
0.04
0.03
0.01
0.01
0.05
0.03
0.04
0.03
0.03
0.01
0.04
0.04
0.00
0.06
NiO
0.04
0.01
0.00
0.00
0.00
0.01
0.00
0.04
0.03
0.00
0.00
0.04
0.06
0.04
0.02
0.00
0.02
0.05
0.02
SrO
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.08
0.00
0.00
0.07
0.00
0.10
0.00
0.00
BaO
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.06
0.39
0.00
0.07
0.04
0.00
0.00
0.09
0.00
0.00
0.11
Cl
0.00
0.02
0.00
0.01
0.01
0.00
0.00
0.02
0.00
0.01
0.01
0.02
0.00
0.00
0.00
0.00
0.00
0.01
0.01
F0.10
0.04
0.21
0.00
0.20
0.06
0.02
0.00
0.23
0.00
0.07
0.00
0.13
0.11
0.00
0.03
0.00
0.04
0.13
Total
98.89
99.83
98.87
99.66
98.84
98.67
99.53
99.10
99.19
100.42
99.37
100.84
101.16
100.44
100.76
100.70
100.07
98.70
99.60
Si
1.91
1.95
1.93
1.94
1.98
1.97
1.99
1.94
1.99
3.00
2.48
2.40
2.48
2.40
2.49
2.57
2.43
2.93
2.75
Ti
0.02
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.01
0.00
Al
0.10
0.11
0.07
0.08
0.05
0.06
0.06
0.07
0.04
0.98
1.48
1.56
1.48
1.57
1.47
1.39
1.53
1.04
1.21
Cr
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Fe3
+0.08
0.01
0.13
0.03
0.02
0.00
0.00
0.04
0.02
0.02
0.04
0.02
0.03
0.03
0.03
0.03
0.03
0.03
0.03
Fe2
+0.18
0.27
0.13
0.23
0.48
0.50
0.52
0.54
0.49
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Mn
0.01
0.01
0.01
0.01
0.02
0.02
0.02
0.02
0.02
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Mg
0.83
0.78
0.86
0.87
1.35
1.37
1.34
1.31
1.35
0.00
0.01
0.01
0.01
0.00
0.00
0.00
0.00
0.00
0.00
Ca
0.83
0.83
0.82
0.80
0.06
0.06
0.05
0.07
0.06
0.15
0.51
0.60
0.51
0.58
0.52
0.44
0.55
0.21
0.31
Na
0.02
0.03
0.02
0.02
0.00
0.00
0.00
0.00
0.00
0.51
0.47
0.39
0.49
0.41
0.47
0.55
0.44
0.52
0.52
K0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.17
0.03
0.02
0.02
0.02
0.03
0.04
0.02
0.12
0.11
P0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Ni
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Sr
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Ba
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Cl
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Arab J Geosci
-
and matrix) (Fig. 4a, b). In the matrix, the finer microlites
ofplagioclase and thin rims around the larger micropheno-crysts are
richer in potassium (up to Or20). Interestingly,across the borders
between globules and matrix, the flowdirection indicated by the
microlites continues unchanged(Fig. 4a, b). In some cases, the same
crystal is situatedexactly between globule and matrix that is to
say with onepart in a globule and the other part in the matrix.
Moreover,numerous globules have coalesced, and are even
embeddedtogether (Fig. 2c). Sometimes, triple point junctions
withangles of 120° are displayed (Fig. 4c). Of note, all
theminerals and even the mesostasis are fresh and
unaltered.Furthermore, vesicles and cavities are not occupied by
sec-ondary materials. A few inliers of scoriaceous material, as
acontinuous unit, with similar mineralogical and
chemicalcompositions to the host, are seen in the G-lava (Fig.
3).
Scoria
The occurrence of scoria lenses in the G-lava unit (Fig. 3)
isinterpreted to be related to an increase in volatile contents
inthe chamber at a near surface level, and the consequentformation
of foam-rich horizons in the melt. The petro-graphic considerations
show that the vesicles in the scoriahave not been occupied by
secondary minerals (Fig. 4d).
Groundmass in G-lava
The less content of glass in the globule with respect to
thematrix in G-lava is worth noting. The colour of the meso-stasis
(as reflected in the refractive indices) is darker in theglobule
(Fig. 4a, b) than in the matrix. To elucidate thesituation in the
G-lava, the mesostasis of both the globulesand matrix were analysed
using a microprobe (Table 3) anda SEM (Table 4, Fig. 6).
Glass of the matrix, analysed with a microprobe (Table
3),contain 73–74 wt.% SiO2, that correspond to 37–38 wt.%
ofnormative quartz and 11–12 wt.% Al2O3. Totals are about94–95 wt.%
that witness of 5–6 wt.% of fluids, as thedifference between the
analytical total and 100 is consideredto represent volatiles,
especially water (Anderson 1979).Glasses analysed in bombs are
nearly the same. It wasimpossible to analyse globule groundmass
with the micro-probe as the size of the phases are too small (less
than5 μm).
SEM observations have been made (Fig. 6; Table 4).Globules are
Si and K richer than the matrix.
The matrix contains glass phases 10–20-μm wide (77–79 wt.% SiO2
and 14–15 wt.% Al2O3 when recalculated to100). These analyses are
consistent with those obtained withmicroprobe presented in Table
3.
The groundmass phases in the globules have small sizes(less than
5-μm wide) and are in small amount. They areTa
ble
1(con
tinued)
Label
8687
9091
103
104
106
109
158
137a
102
108
7071
7274
9596
97Cpx
-cCpx
-mCpx
-cCpx
-rOpx
-cOpx
-rOpx
Opx
Opx
Plag
Plag
Plag
Plag-c
Plag-m
Plag-r
Plag
Plag-c
Plag-r
Plag
F0.01
0.01
0.03
0.00
0.02
0.01
0.00
0.00
0.03
0.00
0.01
0.00
0.02
0.02
0.00
0.00
0.00
0.01
0.02
Sum
3.99
4.01
4.01
3.99
3.99
4.00
3.99
4.00
4.01
4.85
5.03
5.00
5.04
5.03
5.01
5.02
5.00
4.87
4.95
AlIV
0.09
0.05
0.07
0.06
0.02
0.03
0.01
0.06
0.01
AlV
I0.01
0.06
0.00
0.03
0.03
0.03
0.04
0.01
0.03
Wollaston
ite45
.19
44.05
45.24
42.20
2.96
3.01
2.83
3.50
2.90
Enstatite
44.86
41.64
47.79
45.63
71.50
71.08
70.05
68.20
71.44
Ferrosilite
9.95
14.31
6.97
12.17
25.54
25.91
27.12
28.30
25.66
Ortho
clase
19.95
2.79
1.59
2.15
2.18
2.56
3.72
2.27
13.96
11.51
Albite
62.15
46.56
38.91
47.99
40.26
46.41
53.57
43.25
61.07
55.22
Ano
rthite
17.90
50.65
59.50
49.85
57.57
51.03
42.70
54.48
24.97
33.26
Calculatio
nsarebasedon
four
catio
nsforpy
roxenesandeigh
tox
ygensforplagioclases
ccore,m
middle,rrim,Plagplagioclase,Cpx
clinop
yrox
ene,Opx
orthop
yrox
ene
aSam
ple13
7=microliteless
than
15μm
long
Arab J Geosci
-
Tab
le2
Chemical
compo
sitio
nsof
selected
mineralsfrom
thematrixof
G-lava
Label
114
117
118
121
160
162
151
124
125
126
128
140a
143a
144a
145a
148
150a
155a
Opx
Opx
-cOpx
-rOpx
Opx
-cOpx
-rCpx
Plag-c
Plag-r
Plag
Plag
Plag
Plag
Plag
Plag
Plag
Plag
Plag
Mineral
SiO
253
.83
54.37
54.41
54.42
54.45
53.87
50.74
53.56
53.96
64.31
53.47
54.63
64.09
57.60
68.97
51.81
56.71
59.03
TiO
20.08
0.20
0.12
0.18
0.19
0.12
0.37
0.00
0.04
0.20
0.04
0.00
0.09
0.10
0.27
0.00
0.03
0.00
Al 2O3
0.72
0.71
0.70
1.54
0.62
1.13
1.99
28.71
28.79
21.03
28.80
28.71
20.44
24.72
16.78
29.51
26.20
24.30
Cr 2O3
0.01
0.00
0.04
0.04
0.06
0.05
0.06
0.00
0.00
0.03
0.05
0.02
0.02
0.05
0.03
0.00
0.00
0.03
Fe 2O3
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.82
0.70
0.87
0.84
0.90
0.82
0.89
1.15
0.86
0.69
0.77
FeO
16.49
16.27
15.98
16.20
16.46
16.06
8.43
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
MnO
0.62
0.62
0.62
0.61
0.67
0.77
0.29
0.02
0.02
0.00
0.00
0.02
0.05
0.00
0.00
0.00
0.01
0.00
MgO
25.56
25.54
25.46
24.23
25.55
25.05
15.08
0.07
0.06
0.03
0.09
0.05
0.01
0.06
0.07
0.06
0.02
0.04
CaO
1.40
1.47
1.53
1.73
1.50
1.46
20.02
11.51
11.65
6.79
11.62
10.85
4.26
7.19
2.36
12.41
8.61
7.62
Na 2O
0.00
0.06
0.03
0.02
0.02
0.03
0.31
4.81
4.81
4.38
4.69
4.73
6.43
6.84
5.81
4.01
6.38
6.28
K2O
0.01
0.00
0.00
0.00
0.01
0.00
0.01
0.33
0.33
1.94
0.27
0.44
1.94
0.73
2.61
0.32
0.57
1.15
P2O5
0.01
0.02
0.00
0.00
0.00
0.00
0.02
0.02
0.00
0.08
0.03
0.02
0.07
0.02
0.07
0.04
0.07
0.06
NiO
0.02
0.05
0.00
0.01
0.00
0.07
0.00
0.02
0.02
0.00
0.05
0.02
0.00
0.01
0.00
0.01
0.05
0.00
SrO
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.07
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
BaO
0.00
0.18
0.01
0.00
0.00
0.00
0.00
0.03
0.08
0.23
0.10
0.05
0.30
0.04
0.10
0.00
0.14
0.24
Cl
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.02
0.00
0.03
0.00
0.01
0.00
F0.11
0.00
0.00
0.00
0.00
0.11
0.00
0.27
0.00
0.00
0.00
0.03
0.00
0.00
0.00
0.01
0.15
0.11
Total
98.81
99.47
98.89
98.98
99.52
98.67
97.31
100.12
100.45
99.87
100.06
100.46
98.54
98.24
98.25
99.02
99.58
99.60
Si
1.98
1.99
1.99
1.99
1.99
1.98
1.93
2.43
2.44
2.86
2.43
2.46
2.88
2.63
3.07
2.38
2.57
2.67
Ti
0.00
0.01
0.00
0.01
0.01
0.00
0.01
0.00
0.00
0.01
0.00
0.00
0.00
0.00
0.01
0.00
0.00
0.00
Al
0.03
0.03
0.03
0.07
0.03
0.05
0.09
1.54
1.53
1.10
1.54
1.52
1.08
1.33
0.88
1.60
1.40
1.29
Cr
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Fe3
+0.04
0.00
0.00
0.00
0.00
0.01
0.05
0.03
0.02
0.03
0.03
0.03
0.03
0.03
0.04
0.03
0.02
0.03
Fe2
+0.47
0.50
0.49
0.50
0.50
0.49
0.21
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Mn
0.02
0.02
0.02
0.02
0.02
0.02
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Mg
1.40
1.39
1.39
1.32
1.39
1.37
0.85
0.01
0.00
0.00
0.01
0.00
0.00
0.00
0.01
0.00
0.00
0.00
Ca
0.06
0.06
0.06
0.07
0.06
0.06
0.82
0.56
0.56
0.32
0.57
0.52
0.21
0.35
0.11
0.61
0.42
0.37
Na
0.00
0.00
0.00
0.00
0.00
0.00
0.02
0.42
0.42
0.38
0.41
0.41
0.56
0.61
0.50
0.36
0.56
0.55
K0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.02
0.02
0.11
0.02
0.03
0.11
0.04
0.15
0.02
0.03
0.07
P0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Ni
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Sr
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Ba
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.01
0.00
0.00
0.00
0.00
0.00
Cl
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Arab J Geosci
-
silica rich (93.5–97 wt.% SiO2 and 2–4 wt.% Al2O3 insample R1b;
88–91 wt.% SiO2 and 6–7 wt.% Al2O3 insample R2b).
A border of quenched glass (5–15 μm wide) separatesglobules from
their matrix. In detail, it presents a heteroge-neous chemical
composition from core to rim: 69 to 59 wt.%SiO2, 19 to 7 wt.% Al2O3
and 7 to 26 wt.% FeO (Table 4).These values are in the range of
those of the whole rock G-lava analyses, except for iron. Note that
the border ofquenched glass is wider (15 μm) in sample R1b that
containlarger (3–5 mm wide) globules, than in sample R2b (4–10 μm)
that contain smaller globules (1 mm wide). Notethat some crystals
are situated in part in globule and in partin matrix, cross the
border.
No vesicles are observed inside globules and matrix butvesicles
(10–100 μm) appeared in the matrix close to theglobules and all
around them (Fig. 6). Note that thesevesicles are not spherical,
that could be explained by thedeformation of the whole molten rock
by fluidality, asevidenced by microliths orientation.
Samples were analysed by reflection IR spectrometry(Fig. 7).
Glass of matrix spectra presents two bands at1,092 and 785 cm−1.
This is typical of volcanic glass andlooks like obsidian (Lipari,
Italy) spectrum with a band at1,085 cm−1.
Quenched border spectrum seems typical of a glass dueto two
reflection bands at 1,046 and 793 cm−1 and moreovera phonon mode at
1,110 cm−1. This asymetric band shape isknown from amorphous and
vitreous siliceous phases.
For globule, analyses are more difficult because of thevery
small size of the groundmass phases (less than 5 μm).So, obtained
analyses (spot size=40×40 μm) mainlycorrespond to mixing between
several phases. We observetwo large bands: one around 1,000 cm−1
that could correspondto microliths, the other around 1,120 cm−1
that couldcorrespond to the silica phase analysed with SEM.
Thissecond band is concordant with silica phase as
crystoballite,tridymite and lechatelierite.
Whole-rock chemistry
Major element chemistry
Figure 8 shows the chemical compositions of the samples onthe
TAS diagram of Le Maitre et al. (2002). Some importantobservations
are as follows:
1. Although the chemical composition of the andesitic unitin the
adjacent area (i.e. Abazar district) falls in theandesitic field
(Fig. 8), the G-lava from the study area(Razjerd district) occupies
the dacite field. Note, how-ever, that the modal compositions of
the samplesTa
ble
2(con
tinued)
Label
114
117
118
121
160
162
151
124
125
126
128
140a
143a
144a
145a
148
150a
155a
Opx
Opx
-cOpx
-rOpx
Opx
-cOpx
-rCpx
Plag-c
Plag-r
Plag
Plag
Plag
Plag
Plag
Plag
Plag
Plag
Plag
F0.01
0.00
0.00
0.00
0.00
0.01
0.00
0.04
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.02
0.02
Sum
4.01
4.00
3.98
3.98
4.00
3.99
3.99
5.05
4.99
4.81
5.01
4.97
4.88
4.99
4.77
5.00
5.02
5.00
AlIV
0.03
0.01
0.01
0.01
0.01
0.02
0.07
AlV
I0.01
0.02
0.02
0.06
0.01
0.03
0.02
Wollaston
ite2.87
2.93
3.09
3.61
3.02
2.97
43.28
Enstatite
72.87
71.52
71.65
70.10
71.25
71.66
45.35
Ferrosilite
24.26
25.55
25.26
26.30
25.73
25.37
11.36
Ortho
clase
1.89
1.89
13.58
1.61
2.60
12.66
4.30
19.50
1.83
3.26
6.70
Albite
42.27
41.93
46.54
41.49
42.98
63.97
60.54
65.71
36.18
55.39
55.84
Ano
rthite
55.83
56.18
39.88
56.90
54.42
23.38
35.16
14.79
61.99
41.35
37.46
Calculatio
nsarebasedon
four
catio
nsforpy
roxenesandeigh
tox
ygen
forplagioclases
ccore,rrim,Plagplagioclase,Opx
orthop
yrox
ene
aSam
ples
140,
143,
144,
145,
150and15
5=microlites
less
than
30μm
long
Arab J Geosci
-
(plag≈25 vol.%, px≈3 vol.% and opaque of ≤2 vol.%)are not so
different than those of an andesite. It seemsthat such a
compositional shift is caused by magmadifferentiation near the
surface.
2. In the G-lava, globules contain more SiO2 and K2O(that
confirm SEM analyses), and a lower LOI thanthe matrix (Table 5,
samples G1 and GF1). This resultis confirmed by the occurrence of
silica phase inglobules observed with SEM.
3. The chemical compositions of other volcanic productsfrom the
study area (including volcanic bombs and
scoria) do not differ from the G-lavas (Table 5;Fig. 8). The
high values for LOI in the scoria(4.30 wt.% in sample R8; Table 5)
confirm the highvolatile contents of the scoria, that witness of
foam-richhorizons.
Trace element chemistry
Table 5 shows the abundance of trace elements in
differentvolcanic products from the study area. These data are
plot-ted as spider diagrams (Fig. 9) that enable comparisons of
Table 3 Chemical microprobe analyses and CIPW normative
compositions of mesostasis in matrix of G-lavas and in bombs
Label 123 141 142 147 204 206Matrix Bombs
SiO2 74.18 73.60 73.26 72.97 71.10 66.39
TiO2 0.50 0.47 0.52 0.35 0.33 0.19
Al2O3 12.05 11.23 11.94 11.32 14.29 16.81
Cr2O3 0.02 0.01 0.06 0.03 0.00 0.01
FeO 0.63 0.81 0.62 0.93 0.94 1.17
MnO 0.00 0.00 0.00 0.00 0.00 0.01
MgO 0.03 0.01 0.01 0.16 0.07 0.11
CaO 0.30 0.17 0.71 0.56 1.72 3.15
Na2O 2.96 2.91 3.33 2.47 3.97 4.98
K2O 4.75 5.09 4.07 5.00 3.74 2.81
P2O5 0.09 0.07 0.03 0.04 0.08 0.12
NiO 0.02 0.01 0.01 0.00 0.04 0.06
SrO 0.00 0.00 0.00 0.00 0.00 0.00
BaO 0.01 0.00 0.10 0.00 0.06 0.15
Cl 0.08 0.07 0.05 0.08 0.05 0.04
F 0.04 0.11 0.11 0.00 0.27 0.00
Total 95.66 94.56 94.82 93.91 96.66 95.88
CIPW norm
Quartz 38.40 37.06 36.92 38.19 30.18 20.07
Corundum 1.71 0.79 0.84 0.92 0.78 0.14
Orthoclase 28.04 30.05 24.03 29.51 22.08 16.59
Albite 25.02 24.60 28.15 20.88 33.56 42.10
Anorthite 0.91 0.39 3.33 2.52 8.01 14.84
Diopside
Hypersthene 0.07 0.02 0.02 0.40 0.17 0.41
Magnetite 0.19 0.32 0.93
Ilmenite 0.52 0.69 0.52 0.67 0.63 0.36
Hematite 0.38 0.49 0.37 0.43 0.33
Apatite 0.20 0.15 0.07 0.09 0.17 0.26
Densitya (kg/m3) 2,092 2,043 2,057 2,032 2,157 2,166
Viscositya (Pa.s) 1.77E+04 1.15E+04 1.22E+04 9.32E+03 1.85E+04
1.02E+04
Viscosityb (Pa.s) 4.78E+03 9.69E+02 1.32E+03 5.52E+02 9.89E+03
2.73E+03
Density calculated according to Bottinga and Weil (1972); and
viscosity calculated according to Bottinga and Weil (1972) and Shaw
(1972)a Bottinga and Weil (1972)b Shaw (1972)
Arab J Geosci
-
Tab
le4
SEM
glassanalyses
(recalculatedto
100)
Sam
ple
R1b
R1b
R1b
R2b
R2b
R1b
R1b
R2b
R1b
R1b
R1b
R1b
Globu
leMatrix
Border
Point
No.
A6
A7
A11
B40
B41
A13
A16
B39
A18
A32
A33
A34
Precise
Scan3×3μm
Scan5×5μm
Core
Rim
SiO
293
.59
95.19
97.01
88.46
90.72
78.12
77.35
78.95
67.27
59.09
68.52
61.35
TiO
20.27
0.22
0.11
0.12
0.00
0.63
0.30
0.53
0.75
0.18
0.81
0.20
Al 2O3
4.25
3.30
2.00
7.40
6.21
14.85
14.07
14.01
18.72
7.55
14.67
9.02
FeO
0.27
0.18
0.33
0.36
0.33
0.79
0.82
0.58
7.08
26.12
9.65
22.96
MnO
0.00
0.18
0.00
0.00
0.00
0.11
0.00
0.00
0.21
0.43
0.23
0.00
MgO
0.07
0.11
0.15
0.05
0.05
0.07
0.11
0.05
2.77
2.24
2.93
2.55
CaO
0.13
0.13
0.02
0.31
0.33
0.22
0.25
0.17
0.75
0.86
0.90
0.96
Na 2O
0.33
0.40
0.20
0.62
0.64
1.48
2.14
1.30
0.24
0.33
0.20
0.48
K2O
1.09
0.29
0.18
2.68
1.72
3.73
4.96
4.41
2.21
3.20
2.09
2.48
Total
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
CIPW
norm
Quartz
87.19
91.25
94.97
73.93
79.71
54.72
45.28
54.13
50.91
33.20
52.00
38.37
Corun
dum
2.29
2.09
1.44
2.92
2.70
7.98
4.73
6.79
14.58
1.98
10.45
3.80
Ortho
clase
6.43
1.71
1.06
15.82
10.15
22.02
29.28
26.03
13.05
18.89
12.34
14.64
Albite
2.79
3.38
1.69
5.24
5.41
12.51
18.09
10.99
2.03
2.79
1.69
4.06
Ano
rthite
0.64
0.64
0.10
1.54
1.64
1.09
1.24
0.84
3.72
4.26
4.46
4.76
Hyp
ersthene
0.17
0.58
0.37
0.12
0.12
0.17
0.27
0.12
8.67
20.02
9.72
17.05
Magnetite
0.28
0.11
5.65
18.56
7.84
16.98
Hem
atite
0.21
0.26
0.26
0.05
0.51
0.44
0.37
Ilmenite
0.14
0.42
0.15
0.21
0.84
0.57
0.44
1.43
0.34
1.54
0.38
Density
a(K
g/m
3)
2,27
12,27
62,26
32,28
42,27
82,32
52,31
92,31
32,53
42,89
32,56
928
00
Viscosity
a(Pa.s)
1.84
E+11
3.36
E+11
4.35
E+11
1.75
E+10
5.32
E+10
5.51
E+08
1.72
E+08
6.92
E+08
2.04
E+06
2.24
E+03
8.47
E+05
7.05
E+03
Viscosity
b(Pa.s)
1.29
E+06
1.55
E+06
1.75
E+06
8.35
E+05
1.08
E+06
3.73
E+05
4.56
E+05
3.77
E+05
3.72
E+04
5.01
E+03
7.11E+04
8.99
E+03
aBottin
gaandWeil(197
2)bShaw
(197
2)
Arab J Geosci
-
Table 5 Chemical whole-rock analyses and CIPW normative
compositions of volcanic rocks from the Razjerd area, Qazvin,
Iran
Sample No. G1 GF1 R1 R2 R5 R8 R13Globule Matrix (outside the
globule) G-Lava Bomb Scoria Shoshonitic Basalt
SiO2 66.90 64.60 64.80 65.40 65.40 63.00 45.20
TiO2 0.39 0.40 0.38 0.39 0.38 0.39 1.66
Al2O3 15.80 15.45 15.55 15.80 15.50 15.85 17.15
Cr2O3
-
the trace element patterns for globules and matrix from the
G-lava (Fig. 9c, f), G-lava and shoshonitic basalt (Fig. 9a,
d),scoria, volcanic bomb and G-lava (Fig. 9b, e).
As shown in these plots, there are similarities in the
trace-element patterns for materials from the G-lavas
(globules,matrix, bombs, scoria and globule-free lava), including
en-richment in LREEs compared with HREEs (by a factor ofabout 10),
negative anomalies of Eu, Ba, Ta and Ti andpositive anomalies of Th
and Ce.
The spider diagram pattern for shoshonitic basalt differsfrom
the patterns for the G-lava and related rocks (globules,scoria and
bombs). As shown in Fig. 9d, the G-lava showsmore depletion in
MREEs than the shoshonitic basalt.
Physical parameters
Different physical parameters (temperature, pressure andvolatile
content) could significantly modify density andviscosity of a melt
(e.g. Bardintzeff 1992; Wohletz 1999).In that way, Kushiro et al.
(1976) have shown that an
addition of 4 wt.% of fluids in an andesitic melt wouldreduced
its viscosity by a factor of 20.
The tentative calculated results of density and viscosity inthe
volcanic materials of the Razjerd district by the 2010improved
MAGMA program (Wohletz 1999) are presentedin Table 6. As shown, the
globules are dramatically moreviscous (about 103–104 orders in
magnitude) and denser(about 100–200 kg/m3 more) than the matrix
outside theglobules. Also, other ejecta, such as bombs and scoria
havemore or less viscosities similar than those of the matrix
ofG-lavas (outside the globules). As a comparison, theshoshonitic
basaltic neighbouring lava has a higher densityfor a strong lower
viscosity (Table 6; Fig. 10).
Discussion
Spherical figures, at different scales, are exhibited in
thevolcanic rocks of different magmatic provinces around theworld
in very various conditions including: weathering,
Table 5 (continued)
Sample No. G1 GF1 R1 R2 R5 R8 R13Globule Matrix (outside the
globule) G-Lava Bomb Scoria Shoshonitic Basalt
Ta 0.8 0.8 0.9 0.8 0.8 0.9 1.2
W 2 2 2 2 2 3 1
Tl
-
hydrovolcanism, nucleation, magma mixing/mingling, mag-matic
immiscibility and fluid-melt exsolution.
Weathering
At first glance, it might have been thought that the
easilyseparated globules in the Iran samples are secondary inorigin
(possibly the result of weathering or devitrification,e.g. Lasaga
and Kirkpatrick 1981) or that they differ incomposition from the
matrix. But the detailed petrographi-cal observations reported here
show otherwise. It means thatthe absence of devitrified products,
such as spherulites andor axiolites and also unfilled vesicles
(Fig. 4d) even in thescoria horizons show that these samples are
completely
fresh and unaltered without any post-solidification
effects.Also, we did not observe the secondary minerals, such
aschlorite and secondary silica after hyaline mesostasis and
orsericite after feldspar. Still, the colour contrast in the
glassymesostasis of the globules and the matrix (Fig. 4a, b)
havebeen well preserved. Because of this, it can be concludedthat
the volcanic rocks in the succession were notundergone the deuteric
and or low-temperature alterations(e.g. weathering, diagenesis and
so on).
Hydrovolcanism
Spherical features are produced by hydrovolcanic process-es at
different scale, as pillow-lavas (Wohletz and
Fig. 4 Microphotographs ofvolcanic rocks from the
Razjerddistrict (PPL). a Two globulesembedded in the matrix withthe
same texture andmineralogy (note thedifferences in colour
betweenthe mesostasis in the globulesand matrix); b trachytic
texturethat continues across theboundary between matrix andan
embryonic globule; c threecoalesced globules with triplepoint
junctions meeting atangles of 120°; and d vesiculartexture in the
scoriaceoushorizon with its unfilledvesicles and cavities
Fig. 5 Representative chemicalcompositions of feldspars (a)and
pyroxenes (b) in G-lavas.The compositions of mineralsin both phases
(globules andmatrix) are rather similar (seetext)
Arab J Geosci
-
Fig. 6 Backscattered electronSEM images (scale isindicated). a
Globules insidematrix. Note the occurrence ofvesicles all around
the globules;b detail of the matrixgroundmass (glass is dark);
cdetail of the globulegroundmass (silica phase isdark); d detail of
a globule(lower left) and matrix (upperright) and quenched
glassbetween; e K elementcartography of (d) (1.25 hcounting time).
Light areasindicate greater amount of K; fSi element cartography of
(d)(1.25 h counting time). Lightareas indicate greater amountof Si;
g vesicles in the matrixbetween two globules. Note thesame
orientation of microliths(trachytic fluidal texture) inglobule and
matrix. h detail ofthe quenched glass and vesiclesbetween a globule
(lower left)and matrix (upper right)
Fig. 7 Reflection infrared spectrometry of groundmass of matrix
andglobule. Normalized reflectance scale in arbitrary unit vs. wave
num-ber; 666 cm−1 is atmospheric CO2 band. a Spectra of glass of
matrixand of glass of the border between globule and matrix (spot
size=100×25 μm; spectra were acquired between 1,300 and 650 cm−1;
and
spectral repetition, 120 times) and obsidian of Lipari (Italy,
sample F.Fröhlich) for comparison. b Spectra of groundmass of
globule (spotsize=40×40 μm; spectra were acquired between 1,350 and
650 cm−1;and spectral repetition, 500 times) and of lechatelierite,
Libyan DesertGlass (Fröhlich 1989) for comparison
Arab J Geosci
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McQueen 1984), accretionary lapillis (Schumacher andSchmincke
1995) and peperites (Corsaro and Mazzoleni2002; Donaire et al.
2002; Jerram and Stollhofen 2002).
But it is clear that our case study is very different, as
Iranianglobules are not vitreous as pillows and are one magnitude
ofsize less; they do not present concentric structures as
accre-tionary lapillis and have the same chemical composition
astheir matrix that differ from the peperites.
Nucleation
Spherules, also named varioles, mm to cm in size, sometimesmore,
are globular structures widespread in komatiites and insome
basalts, especially in Archean volcanic sequences (Gélinaset al.
1977; Arndt and Nisbet 1982; Fowler et al. 1986, 2002;Arndt and
Fowler 2004). Different types of varioles have beendescribed,
linked to different origins (alteration, magma min-gling and
immiscibility?).
Moreover, formation of varioles could be explained bynucleation
in magma with high volatile contents (Arndt andFowler 2004). This
could involve crystals growth accordingto cooling of superheated
liquids and/or loss of volatiles,and then form spherical
structures.
It is possible that variations of volatile contents in
liquid,involving nucleation and crystal growing have played a
rolein the formation of Iranian G-lavas.
Fig. 8 Chemical compositions of rock samples from the
Razjerddistrict, as shown in the TAS diagram of Le Maitre et al.
(2002). Rocks(shoshonitic basalt, shoshonitic trachybasalt and
andesite) from neigh-bouring Abazar district (Asiabanha et al.
2012) are plotted for com-parison. Abbreviations: A andesite, B
basalt, BA basaltic andesite, BTAbasaltic trachyandesite, D dacite,
T trachyte, TA trachyandesite, TBtrachybasalt, TD trachydacite, R
rhyolite. Analyses were recalculatedto 100 % on a LOI-free
basis
a
b
c
d
e
f
Fig. 9 Spider diagrams forvolcanic products from theRazjerd
district in MORB-(Pearce 1983) and chondrite-normalised (Boynton
1984)plots
Arab J Geosci
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Magma mixing/mingling
Some globule-bearing rocks have been interpreted in termsof
magma mixing and mingling (e.g. Clark et al. 1987;Eichelberger
1980; Freundt and Schmincke 1992). Freundtand Schmincke (1992)
defined a general type of texture asan “emulsion texture” in which
sharply defined roundishbodies of one component are suspended in
the other coherentcomponent.
Schreiber et al. (1999) described one example fromWesterwald in
Germany where a globule-rich horizon isfound at the contact between
a latitic dyke and a trachytichost. On its border with the
trachyte, the latite forms severalfinger-like smaller dykes. With
increasing distance from themain body of latite, the small fingers
turn into schlieren thatare finally dispersed as small spherical
inclusions or glob-ules. The prominent mineralogical and chemical
contrastsbetween globules and their matrix are worth noting
(forexample, enrichment of Nb, Rb, Th and Zr in the globulesand
depletion in Ca, Mg, Ti, Fe, Sr and Ba, globules show adepletion in
MREEs). Moreover, Schreiber et al. (1999)stated that the globules
were developed in the magma cham-ber before eruption. Because of
the apparent chemical con-trasts between the globules and the host
rock, Schreiber et al.(1999) concluded that they formed as a result
of magmamingling. On contrary, Iranian globules studied in
thispaper have the same chemical composition as the matrix
allaround.
Magmatic immiscibility
Immiscibility is the contrary of the mixing: a
single-phaseliquid is becoming unstable and separated spontaneously
intwo immiscible phases. Such a phenomenon has been ob-served by
Creig since 1927. It is well known in glassinclusions, especially
in an andesitic context (West Indies,Kamtchatka). In the groundmass
of andesitic lavas of MonteArci, Sardinia, red brown vitreous
spheres, less than 3 μm indiameter, TiO2 (17 wt.%) and P2O5 (7
wt.%) rich but silica(30 wt.%) poor are scattered inside a clear
glass of daciticcomposition (Clocchiatti 1979). By experimentation,
an im-miscibility field has been defined (Philpotts 1979;
Roedder1951). At a larger scale, such phenomenon is invoked
toexplain the genesis of peculiar magmas as carbonatites(Hamilton
et al. 1979; Peterson 1990).
According to Anderko and Pitzer (1993) and Botcharnikovet al.
(2004), increasing activity of H2O in the shallow-depthcrustal
magmatic settings resulted into increasing immiscibilitytend in the
fluids. Visser and Koster van Gross (1979) notedthat minor
additions of nonsilicate anions have significanteffects on liquid
immiscibility.
Roedder and Stalder (1988) stated that the pressure dropwill
certainly affect the onset of immiscibility in the ascendingmagma
through the surface. Moreover, Giggenbach (1987)and Shmulovich and
Churakov (1998) observed that subsur-face magmatic degassing can
occur at low, near-surface pres-sures if the magma chambers are
connected to the surface.
Table 6 Computational water content, viscosity and density
values of volcanic samples of the Razjerd area with MAGMA program
(Wohletz1999)
Volcanic materials SiO2 (wt.%) H2O (wt.%) Viscositya (Pa.s)
Viscosityb (Pa.s) Densityb (kg/m3)
Globule 66.90 0.85 1.70E+06 5.01E+04 2,495
Matrix 64.60 3.84 9.36E+03 7.99E+03 2,386
Bomb 65.40 3.34 1.98E+04 1.82E+04 2,399
Scoria 63.00 5.34 1.88E+03 4.22E+03 2,341
Shoshonitic basaltic lava 45.20 2.79 1.06E+02 2.13E+02 2,806
a Shaw (1972)b Bottinga and Weil (1972)
Fig. 10 Variation diagrams ofviscosity (in Pascal
seconds)against silica (a) and H2O (inweight per cent) (b) based
onthe results of Table 6. Asshown, the H2O content andviscosity of
globules (filledcircle) in G-lava are less andmore than matrix
(open circle),respectively. Other symbols:cross, bomb; star,
scoria; andrectangle, shoshonitic basalt
Arab J Geosci
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The forming of two immiscible melts by magmatic degass-ing in
the Campanian Ignimbrite of central Italy was reported bySignorelli
et al. (2001). Elsewhere, Kratzmann et al. (2009)attributed colour
differences in glassy bands of pumice lapilliin the Hudson volcano
of Chile to variations in vesiculation. Asimilar interpretation was
made by Davidson et al. (2004) forbanded pumice clasts from the
Damavand volcano in Iran.
Although chemical compositions of the samples studiedin this
paper could show some evidence of magmatic im-miscibility, the
chemical differences between the globulesand matrix of the G-lava
are not considerable. Globule andmatrix contain respectively 67.6
and 67.4 wt.% of SiO2, 8.9and 9.5 wt.% of
(FeOt+MgO+MnO+TiO2+CaO+P2O5)and 23.5 and 23.1 wt.% of
(Al2O3+K2O+Na2O). Note thatthese compositions are very close to the
immiscibility fielddescribed by Roedder (1951). But G-lava differs
from mostcases study of magma immiscibility, which result in
twoclearly different in composition melts (Roedder 1978,
1979;Charlier and Grove 2012). Thus, it seems that it is
notprobable that the G-lavas could result from a
magmaticimmiscibility process.
Fluid-melt exsolution
Although G-lavas occur in the same stratigraphic level than
theandesitic lavas located in the neighbouring area (Asiabanha
etal. 2012), the whole rock chemical composition of G-lava in
theRazjerd district correspond to a dacite. The two lavas
(andesiteand dacite) exhibit rather similar petrographic
characteristics.We note an increase of SiO2, K2O and H2O and a
decrease ofFe2O3, MgO and CaO from andesite to dacite. According
tothese three arguments (field, petrography and geochemistry),
itcan be envisaged that the andesitic melt was locally
differen-tiated into the dacitic liquid. During such a process,
theevolving melt might be oversaturated in the volatile
contents.Thus, the occurrence of scoriaceous horizons and also
theabundant volcanic bombs can be explained by the vesiculationof
magma followed by an increase in volatile contents inthe chamber at
the near surface levels, and the consequentformation of foam-rich
horizons in the melt.
We note that there is not any H2O-bearing mineral phasethat is
likely due to volatile vesiculation in the shallow levels.
For explaining the distinctive differences in H2O contentsof
globules and matrix, it could be envisaged that the fluid-melt
exsolution occurred in the volcanic conduit. Veksler(2004)
suggested that the aluminosilicate melts at pressuresbelow 1 GPa
and temperatures less than 900 °C may haveexsolved two aqueous
fluids of contrasting density andsalinity. It seems that the dacite
magma in the Razjerddistrict was experienced a magmatic degassing
due to sig-nificant pressure drop. The silica-richer liquid
separatedfrom the matrix as the spherical masses. Moreover,
theconsequential contrast in viscosities might have contributed
to the formation of two distinct phases: the globules andtheir
matrix.
Aqueous fluid exsolution from silicate melts in the late-stageof
crystallisation process was proposed firstly by Brögger(1890) and
later by Jahns and Burnham (1969) for generationof granitic
pegmatites. Also, the role of fluxing components(including H2O, B,
F and P) in reduction of melting/crystallisa-tion temperatures and
also the immiscibility fields was stated byLondon (2005).
On the other hand, Merritt (1924a, b) advocated viscoussilicate
gels as the pegmatite-forming medium, and gel-based models for
pegmatites have been proposed againmore recently (e.g. Merino 1999;
Taylor et al. 2002).According to London (2005), the concentration
of H2O orother fluxing components needed to form gels.
Moreover, Halter and Webster (2004) stated that phaseseparation
occurs when the solubility of the volatile phasesin the silicate
melt is exceeded. So, the bulk system enters animmiscibility domain
where two or more phases (typically asilicate melt and an aqueous
fluid) is favoured over a singlemixed liquid.
Different evidences in the Razjerd district confirm thatthe
G-lava might be formed by exsolution of a silicatemelt/gel
(globules) from the aqueous fluid (matrix): enrich-ment of globules
in SiO2 relative to matrix; the highercrystallinity of the globules
than the matrix; the molten stateof globules during eruption; the
chemical distinctivechanges across the quenched border of
globules.
Concluding remarks
Razjerd district lavas situated on western Alborz
Ranges,Northern Iran, present an unusual and peculiar case study
inwhere a hypocrystalline dacitic lava (G-lava) contains abun-dant
0.1- to 5.0-cm spherical masses (or globules) set in amatrix of
nearly similar composition and microtexture. TheG-lava level
contains scoriaceous lenses, ellipsoidal-shapedejecta or volcanic
bombs. Stratigraphically, this level isunderlain by shoshonitic
basaltic lava.
Spherical globules of the G-lava are arranged in thedirection of
lava flow, and many have coalesced. The lineararrangement of
microlites of plagioclase across the contactsof globules and matrix
and also triple point junction withangles of 120° confirms that the
globules were in a liquidstate during eruption and during flowing
of lava. Theirabundance (up to 60 vol.%) in particular horizons
presentsa stratified appearance.
The globules and their matrix are very similar in mineral-ogy
and texture. In both phases, the microphenocrysts ofplagioclase
(An25–67), augite, enstatite and magnetite are em-bedded in a
trachytic groundmass. However, because of dif-ferences in colour
between the mesostasis of the globules
Arab J Geosci
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(darker) and matrix (clearer), it is evident that they are
notentirely of the same composition.
The field, petrographic and chemical considerations us-ing
whole-rock chemistry, microprobe analysis, SEM anal-ysis and
reflection IR spectrometry show that:
1. Despite of apparent similarities, the globules are a
fewricher in Si and K and poorer in LOI components.
2. The globules were covered by a quenched border (5–15 μm in
thickness) that is heterogenous in composi-tion. It means that the
Si and Al proportions are de-creased and Fe content is increased
from core to rim(Fig. 11).
3. The size of glassy mesostasis in the globules is smaller(
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Arab J Geosci
Globule-rich lavas in the Razjerd district, Qazvin, Iran: a
unique volcanic fabricAbstractIntroductionGeological settingField
relations and volcanic fabricsAnalytical methodsPetrography and
mineral chemistryGlobule-bearing hypocrystalline
lavaScoriaGroundmass in G-lava
Whole-rock chemistryMajor element chemistryTrace element
chemistryPhysical parameters
DiscussionWeatheringHydrovolcanismNucleationMagma
mixing/minglingMagmatic immiscibilityFluid-melt exsolution
Concluding remarksReferences
![Without boundary : [brochure] seventeen ways of looking ... · Born in Qazvin, Iran. Lives and works in New York. Speechless. 1996. RC print and ink, 66 x 52 Vi" (168 x 133 cm).](https://img.dokumen.tips/doc/110x75/60b551ea72df3b6bb153e612/without-boundary-brochure-seventeen-ways-of-looking-born-in-qazvin-iran.jpg)
