Flowage differentiation in an andesitic dyke of the
Motru Dyke Swarm (Southern Carpathians,
Romania): Evidence from AMS, CSD and
geochemical investigations
COLLIN NKONO1, OLIVIER FÉMÉNIAS1*, HERVÉ DIOT2, TUDOR BERZA3, DANIEL DEMAIFFE1
1
Laboratoire de Géochimie Isotopique et Géodynamique Chimique, DSTE, Université Libre
de Bruxelles (CP 160/02) 50, av. Roosevelt 1050 Bruxelles, Belgium.
2
Université de La Rochelle, av. Crépeau, 17402 La Rochelle cedex 1, France. UMR CNRS
6112, UFR de Sciences et Techniques, BP 92208, 44322 Nantes Cedex 3 France.
3
Institutul Geologic al Romaniei, Bucarest 78344, Romania.
*
CORRESPONDING AUTHOR OLIVIER FÉMÉNIAS
[email protected]
Tel : 00 32 2 650 22 54
Fax : 00 32 2 650 22 26
This paper is composed of 9 figures and 2 tables.
Abstract
Two dykes of different thickness (5.5m for TJ31 and 23m for TJ34) from the late Pan-African
calc-alkaline Motru Dyke Swarm (S. Carpathian, Romania) have been studied by electron
microprobe (mineral chemistry), Crystal Size Distribution (CSD), Anisotropy of Magnetic
Susceptibility (AMS) and whole rock geochemistry. All the physical and chemical variations
observed along dyke’s with point to concordant result and show that the variations of both
modal and size of the amphibole and biotite micro-phenocrysts inside the dykes deduced from
the classical CSD measurements are the result of a mechanical segregation of suspended
crystals during magmatic transport. These variations can not be linked to different pulses of
flow during ascent of the magma. Despite a pene-contemporaneous regional tectonic, the flow
induced differentiation in the large dyke, characterised by the concentration of pre-existing
crystals Ti-rich pargasite-tschermakite, clinopyroxene and plagioclase in the core and an
extracted differentiated liquid near the walls, contributed to a chemical differentiation. The
core is of andesitic basalt composition whereas the margins are andesitic. Thus the chilled
margins appear as slight more evolved liquid with a Newtonian behaviour when compared to
the average composition of the dyke. The localisation of the liquid on both sides of the dyke
has certainly facilitate the ascent of the central part of the dyke that behaved as a Binghamian
mush.
Key-words: Dyke, CSD, ASM, Flowage differentiation, Romania
1
1. Introduction
Understanding of the dyke emplacement mechanism is of major importance to study the
feeder processes of volcanic eruptions and the magmatic evolution of lavas during their
transport through the crust. Numerous observations have shown the complex and various
petrographic textures inside a dyke (i.e. Helmut & Winkler, 1948). More over several
mechanical and/or thermal processes have been proposed to explain the position of mineral
phases, and their size variations, within a dyke. In many cases, the crystals are coarser grained
in the core of the dyke than near its walls. Both asymmetrical and symmetrical fabrics have
been observed and related to contemporaneous regional tectonic activity (Correa-Gomes et
al., 2001; Féménias et al., 2004).
Nevertheless, the interpretation of these fabrics is based on two mean approaches that are
rarely discussed together:
i-) The modal and size mineral variations inside a dyke are the result of a mechanical
segregation (Bagnold effect) during the magmatic transport. The magmatic flow during
the dyke emplacement may indeed generate interaction between solid particles (the carried
crystal) and between these particles and the wall for a Newtonian mush that is a magmatic
medium containing from 8 to 30% of crystals. (Bagnold, 1954; Bhattacharji and Smith,
1964; Bhattacharji, 1967; Bhattacharji and Hehru, 1972; Komar, 1972a; 1972b; 1976;
Barrière, 1976)
ii-) The modal and size mineral variations inside the dyke result from the timing of
crystallization during the cooling of the magma, which is it self function of the thickness
of the dyke. The thermal window favourable for the crystallization of a given phase stays
open much longer in the core zone of a dyke than near the rims dyke (Winkler, 1919;
Helmut and Winkler., 1948; Cashman and Marsh, 1988; Cashman, 1993; Marsh, 1988;
1998; Higgins, 2002; Zieg and Marsh, 2002). In all these Crystal Size Distribution (CSD)
interpretations it is supposed that the magma does not contain on suspended crystals
before cooling.
This study is focused on two s (TJ 31 and TJ 34) of the Romanian Motru Dyke Swarm
(MDS). Each dyke has been the subject of mineralogical, petrological, chemical as well as
structural (by CSD and ASM= Anisotropy of magnetic susceptibility) investigations moreover
in view of our results, we propose to reconsider the geological interpretation of the CSD in a
dynamic point of view by linking the mineral segregation induced by the flowing to the
chemical signature of the rocks which can significantly vary along the width of the dyke.
2. Geological setting
The two dykes studied in this work belong to a large Late Pan-African calc-alkaline dyke
swarm, the Motru Dyke Swarm (MDS) that occurs in the Danubian window of the South
Carpathian mountains in Romania. The south Carpathian crystalline units were thrust upon
the Moesian cratonic plat-form during the Upper Cretaceous to Tertiary. After erosion, they
have been partly covered by Cainozoic sediments. Various Danubian nappes have been
recognized and subdivided in Upper and Lower Danubian (Berza et al., 1983; 1994) on the
basis of their Mesozoic cover (Stanoiu, 1973; Kräutner et al., 1981). The Motru Dyke Swarm
represents the last (pre-Silurian) magmatic event, in the area defined by Berza and Seghedi
(1975) and recently revisited by Féménias (2003).
The swarm, contains numerous dykes. On the basis of their size, mean orientation, and
magnetic fabric, two populations of dykes have been identified (Féménias, 2003):1) small
width (<1m) they have a N80° mean wall direction; 2) thicker dyke (>1 m to several meters)
2
they have a N-S mean wall direction. The two dykes studied in this paper (TJ31 and TJ34)
belong to the second population.
3. Sampling and analytical procedures
For the two dykes, the contacts with the wall rocks are well exposed. Sampling intervals
across dyke’s width have been adapted according to their thickness. The 23 m thick TJ34
dyke has been sampled at 24 stations and the 5.5 m thick TJ31 dyke at 22 stations. Sampling
has been performed with a portable drilling machine and, at each station, an oriented sample
(25mm-diameter) has been cored.
The AMS measurements were performed on the upper part of each drill core, cut into two or
three 22mm-high cylinders, using a Kappabridge KLY-3S susceptometer working in a weak
alternative field whose resolution is better than 10–8 SI. For each cylinder, the measurement
provides the magnitudes of the three principal, mutually orthogonal, axes of the AMS
ellipsoid (K1 ≥ K2 ≥ K3), as well as their azimuth and inclination with respect to the specimen
frame. For each AMS station, the calculated averages of the magnitudes and orientations of
the principal axes in the geographical frame have been reported in Table 1.
The modal proportions (in vol. %) and Crystal Size Distribution (CSD) of amphibole and
biotite were determined by image analysis on thin polished sections (30 µm). The principal
CSD parameters are reported in Table 1. Major element compositions of amphibole have been
determined by electron microprobe (Cameca SX50 instrument) at the Centre d’Analyses par
Microsonde en Sciences de la Terre (CAMST, University of Louvain) using a combination of
natural and synthetic mineral standards. The operating conditions involve an accelerating
voltage of 15 kV, a beam current of 20 nA and a count time of 10 s for Fe, Mn, Ti and Cr, 16
s for Si, Al, K and Mg, 24 s for Na.
For the TJ 34 dyke, samples selected (n=12) for whole-rock major and trace elements have
been crushed and ground in a stainless steel mortar. Whole-rock X-ray fluorescence (XRF)
spectrometric analyses of major elements were made on fused glass discs at the Liege
University (Collectif de Géochimie instrumentale). Samples were pre-ignited at 1000°C
during 3 hours, so major elements are reported on a volatile-free basis and total iron is
reported as Fe2O3 (tot). The XRF measurements were calibrated using 22 international
standards, the typical precision and accuracy (illustrated by the Relative Standard Deviation,
RSD) for the major elements XRF analyses range from 0.1 to 1%. Some trace elements (Rb,
Sr, Zr, Y, Nb, Th, Pb, Cu, Ni, Co, Zn and Ga) were measured by X-ray fluorescence
spectrometry on pressed powder pellets. Data are reported in Table 2.
4. Petrology of the dykes
The MDS is essentially composed of andesitic to dacitic dykes, andesitic basalt and rhyolite
are less common. The textures are aphyric to micro-porphyritic. Porphyritic andesites contain
euhedral tchermakite and/or plagioclase micro-phenocryst, while porphyritic dacites have
euhedral plagioclase, magnesiohornblende and/or partly resorbed quartz. Plagioclase is often
deeply retrogressed to epidote +/- calcite +/- white mica and appears commonly as ghost. The
groundmass consists of tiny quartz, plagioclase, ferromagnesian phases (amphibole and rarely
clinopyroxene), oxides and devitrified glass. Mn-rich ilmenite is the main oxide; it occurs
either as euhedral to subhedral crystals forming glomeromorphic assemblages (mean grain
size: 100-400 µm) or as single tiny (few tens to hundred µm) anhedral to skeletal grains.
Deuteritisation and propylitisation occur during and/or just after the dyke emplacement and
induce a low temperature-low pressure secondary paragenesis. Locally, Ti-bearing silicate
phases have been pseudomorphosed to a leucoxene assemblage; chlorite, calcite, talc, epidote
3
and pyrrhotite-pyrite (altered to goethite and hematite) developed locally as millimetre-size
euhedral grains.
TJ31 is a micro-porphyritic andesitic basalt with amphibole micro-phenocrysts
(<1mm) and rare small clinopyroxene micro-phenocryst. Modal proportion of amphibole
varies from ~5% vol. near the wall to ~30% vol. in the core. The core of the body ,defined by
the highest modal proportion of amphibole shows that the dyke is not symmetrical, it has a
short eastern half-dyke and a long western half-dyke (Fig. 1). Clinopyroxene is present all
along the dyke as a fresh phase or as chlorite pseudomorphs; plagioclase (<2mm) is always
deeply retrogressed. They have not been analysed nor counted for CSD.
TJ34 is a fine-grained, biotite- and amphibole-bearing, andesitic basalt to andesite
with scarce sulphides and oxides in the core of the body.
TJ31 is mineralogically homogeneous along its width whereas TJ34 shows
heterogeneous distribution of oxide and ferromagnesian phases between core and rim, and
inside the body itself. TJ34 is biotite-bearing along its margin, with a modal proportion
varying from 0% (near the wall) to ~5% its central micro-porphyritic zone is composed of
~12% of amphibole, ~5% plagioclase and ~13% of strongly altered clinopyroxene in a finegrained groundmass. Biotite is absent of the core zone of the dyke, but a dm-wide transitional
zone with coexisting biotite and amphibole have been observe on each side of the amphibolebearing central zone.
The two dykes have chilled margins implying a high level (subvolcanic) emplacement.
These margins consist of few micro-phenocrysts (amphibole and clinopyroxene in TJ31
plagioclase and biotite in TJ34) in a secondary (=deuteritic) groundmass.
4.1. Mineral associations
The TJ31 and TJ34 dykes are characterised by a primary (magmatic) association composed of
brown amphibole, clinopyroxene, plagioclase, biotite (in TJ34 only) and Mn-rich ilmenite and
by a secondary association composed of white mica, epidote, chlorite, oxide etc that result
from the deuteritic and/or propylitic syn-emplacement alteration. Only the primary mineral
association will be studied with some details to constrain the acquisition of the magmatic
texture recorded during emplacement and the cooling of the dykes.
The principal ferromagnesian phase is euhedral brown amphibole: it is a Ti-rich pargasitetschermakite according to the IMA classification (Leake et al., 1997;) that is retrogressed to
magnesio-hornblende and actinolite toward its rim (Fig. 2). P-T conditions of amphibole
crystallisation can be estimated on the base of the Al-content that depends of the pressure
(Hammarstron and Zen, 1986; Hollister et al., 1987; Johnson and Rutherford, 1988; Schmidt,
1992; Blundy and Holland, 1990) and of the Ti-content that is temperature related (Heltz,
1973; 1976; 1979; Otten, 1984). Using the empirical model calibrations of Schmidt (1992) for
the pressure and of Otten (1984) for the temperature (both applicable to the Motru
amphiboles, regarding the mineral association), it is shown that the Ti-rich pargasitetschermakite crystallized at 900°C<T<1100°C and 0.45<P<0.7 GPa these conditions (Fig 2)
obviously to those prevailing in a deep magma chamber, before the emplacement and cooling
of the dyke in subvolcanic environment. Clinopyroxene micro-phenocryst is present in TJ31
and in the amphibole-bearing central zone of TJ34 it is strongly chloritised but scarce nonaltered crystals have a diopside-augite composition (Wo42-47En47-49Fs6-11), which is classical of
the MDS series (Féménias, 2003). Euhedral plagioclase micro-phenocryst are strongly
retrogressed. Using backscattered image analyses, it can be shown that the scarce non-altered
area in plagioclase ghost have an intermediate composition (An~50) (see discussion in the
geochemical section). The biotite observed in the andesitic margin of TJ34 has the
4
composition (Phl51-65Ann35-40Sid0-12). Experimental data on micas (Osborn, 1962; Eugster and
Wones, 1962; Wones and Eugster, 1965; Wones, 1972) show that the Fe content of mica is
strongly dependent of the temperature. According to the calibration of Wones and Eugster
(1965) the biotites from TJ34 have crystallized at 600°C<T<680°C which is significantly
lower than the temperature recorded by the amphibole and could possibly be related to inside
the dyke.
4.2. Crystal Size Distribution (CSD)
The thin sections of the dykes TJ31 and TJ34 have been studied using numerical images. The
size distribution of amphibole micro-phenocryst, considered as a typically magmatic phase
because of its low degree of weathering has been determined in the two dykes. The CSD of
the biotite in TJ34 has also been measured for the same reason. The number of crystals of
each phase has been measured in each thin section, by size category. The crystal length (L)
was determined as the greatest axis. Data have been statistically compiled after the method
proposed by Zieg and Marsh (2002), except for the mean length (Lm) which is calculated
after the method proposed by Higgins (2000). The total number of grains Nt has been deduced
from the modal composition.
The complete statistical data set for amphibole and biotite crystals from the two dykes are
presented in Figure 3 and Table 1. In these diagrams each sample is represented by a line
defined by its slope (S) and its intercept (n°) for a total number of crystals per volume unit.
This total number comprised between 224 and 8019 for the amphibole in TJ 31. The slope is
proportional to the deviation of the population around the mean crystal size (Lm). The
intercept is the extrapolated number of grains of a population when the crystal size (L) tends
to zero. Each line correspond to a given sample distribution of a phase in a size, dyke is thus
represented by a fan of lines.
In TJ31, the slope (S) and the intercepts (n°) of a CSD line for amphibole both
increase (Table 1 and Fig. 3) with decreasing crystal size that is for samples situated near the
margin of the dyke (this effect could be partially enhanced by the limitations of the
measurement procedure for very small size or/and by the presence of quench crystals as
suggested by Cashman and Marsh, 1988). On the contrary, slope and intercept both decrease
when the crystal size increase in samples from the core of the dyke. This fan line distribution
is classically observe for igneous bodies such as laccoliths, lavas flows, sills and dykes (i.e.
Cashman and Marsh, 1988; Marsh, 1998; Mangan, 1990; Cashman, 1992; Mangan and
Marsh, 1992; Resmini and Marsh, 1995; Higgins, 1996; Zieg and Marsh, 2002; Mock et al.,
2003). The good correlations between the line parameters (S and n°) fro a sample and the
position of this sample inside the dyke are well illustrated in Figure 4. These diagrams show
the strong decrease of the slope from the samples close to the margins (S=161 near the
western wall and S=72 near the eastern wall) to the sample of the core of the dyke (S~20).
The intercept (n°) decreases respectively from ~14 to ~7. Nevertheless, the lowest values of S
and n° correspond to the petrographic core deduced from the modal abundance of amphibole
(see Fig 1) and not to the geometric centre of the dyke at a width of ~275cm. The variations
of those parameters are thus not a simple function of the distance to the wall of the dyke but
are linked to other processes related to the magmatic emplacement of the dyke. So the modal
proportion of amphibole, and the parameters S and n° define an asymmetrical magmatic
fabric. The mean crystal length (Lm) and the total number of crystals per volume unit (NT)
present opposite evolutions along the profile of the dyke. The total number of amphibole
grains (NT) displays the same evolution trend as the line parameters S and n° (Fig.4). The
highest values of Nt characterize the two margins (8019 on the western rim and 5435 on the
5
eastern rim); the lowest value Nt= 224 is recorded in the "petrographic core". Correlatively,
the mean length (Lm) of the amphibole micro-phenocrysts increases from the rim (<0,017
cm) to the core of the dyke (0,09 cm); this variation trend is similar to the variation of the
modal proportion of amphibole (Figs.1 and 4).
All the CSD parameters show extreme values, near the western margin of the dyke
probably linked to a quenching effect on the wall. The parameters are not extern on the
western wall: to on the eastern this is possibly due to the too large sampling interval and to the
lower quality of the contact with the country rocks that did not allow to image cm-scale
variations. Nevertheless, the available data set clearly show that the eastern half-dyke did not
extend considerably over the last sample.
In the TJ34 dyke, we measured the CSD of amphibole in the core zone and of biotite
in the two margins. The two phases present different fan line distributions (Fig. 3). The
amphibole population is roughly homogeneous: each line= each sample indeed shows similar
slope and intercept: all the lines are sub-parallel, with S~40 and n°~11. The biotite population
by contract is characterized by a more classical fan line distribution with significative
variations in the slope and intercept values from sample to sample. The amphibole population
only displays slight variation in the total number NT: (from 1877 to 4292) and mean length
Lm: (from 0.048 to 0.030) from the core zone to the transition zone with biotite (Table 1 and
Fig. 4). The amphibole-bearing zone also displays a significative asymmetry between its
western and eastern part. By contrast, the biotite population is characterised by higher S and
n° values near the margins of the dyke and lower S and n° values near the central amphibolebearing zone. These CSD variations of the biotite population are correlated with the variations
of biotite content. The smallest grains (Lm=0.022cm) have been observed symmetrically on
both margins of the dyke (Fig. 4), the highest value (Lm=0.036) is observed close to the
transition zone with the amphibole-bearing core. The total number of grains, Nt decreases in a
roughly symmetric way, naturally from the margin (NT~4000) to the transition with the
amphibole-bearing zone (NT~900).
5. AMS investigation
5.1. Short presentation of the method
The kinematics of lava emplacement recorded in the solidified rock as foliation and lineation,
gives information on the late stage of fabric history (Fernandez and Laporte, 1991) and on
flow direction in dykes (Archanjo et al., 2000). Dyke emplacement in a given tectonic setting
has recorded differential stress involving the combined effects of both external and internal
stress fields as observed by Correa-Gomes et al., (2001) for sheet like bodies and by Féménias
et al. (2003) for dykes in the Motru Dyke Swarm. Deuteritic fine-grained assemblages and/or
micro-porphyritic sub-volcanic textures do not easily permit to estimate fabrics or subfabrics
from image analysis. Moreover, microscopic and macroscopic flow indicators are scarce (i.e.
ordered orientation and/or gathering of crystals). Fortunately field or petrographic studies of
shape preferred orientations (SPO) of crystals can be completed by magnetic subfabric survey
deduced from measurements of the Anisotropy of Magnetic Susceptibility (AMS). This
method is indeed sensitive enough to determine the subtle fabrics in lava flows even when
they are poorly (only a few percent) anisotropic (Knight and Walker, 1988; Cañón-Tapia et
al., 1996; Archanjo et al., 2000; Callot et al., 2001). Magnetic fabric has also been
investigated in numerous cases and appears to be a good subfabric of the real magmatic fabric
(bulk shape-preferred orientation of crystals in a magmatic medium).
5.2. AMS results
6
Magnetic susceptibility and Anisotropy
The bulk magnetic susceptibility magnitude Km is given by the arithmetic mean of the K1, K2
and K3 lengths of the AMS ellipsoid (Km = (K1 + K2 + K3)/3). In the Motru dyke, Km values
range from 317 to 1192 µSI (Table 1) with a mean value of 465 µSI, there is a bimodal
frequency distribution (Féménias et al., 2003). This range of magnitudes is representative of
paramagnetic phases (amphibole and biotite) as the principal contributors to susceptibility,
Km variations from sample to sample are related to variation of the amphibole and biotite
modal proportions. Nevertheless, the Km high values (> 500 µSI) of some samples could
presumably be due to the contribution of secondary (propylitic) sulphide and associated
hematite. So, the variations of Km from rim to core in the two dykes can be attributed to the
variations of the modal proportions of amphibole and/or biotite microphenocrysts and to the
late-emplacement fluid percolation along channelled conduits that induced the crystallization
of propylitic iron sulphide and associated hematite. The pyrrhotite-pyrite development and its
localization in the amphibole-bearing central zone of TJ34 is presumably related to the iron
remobilisation during retrogression of the primary ferromagnesian silicate phases; this process
could have enhanced the primary magnetic signal due to the amphibole. According to this
phenomenon and the lack of single domain magnetite of inverse fabric, this secondary
mineralogy mimics the first magnetic fabric acquired during the dyke emplacement.
Intensity and shape of the magnetic fabric
The parameters P’ and T, defined by Jelinek (1981) were calculated for the different AMS
stations. These parameters are defined by the following expressions:
P ' = exp
2
2
2



K  
K 
K 

2   ln 1  +  ln 2  +  ln 3  
 K 
 K 
 K 

m 
m 
m  




and
T =
2 (ln K 2 − ln K 3
ln K − ln K
1
3
)−
1
where P’ describes the strength, or anisotropy degree, of the magnetic fabric; it varies from
P’ = 1 for an isotropic, spherical AMS ellipsoid to infinity. The shape parameter T varies from
a prolate shape for –1 ≤ T < 0 (cigar-shaped for T = -1, i.e. K2 = K3) to an oblate shape for 0 <
T ≤ 1 (pancake-shaped for T = 1, i.e. K1 = K2) (Hrouda, 1982).
These parameters were calculated for the different ASM stations of the two Motru dykes. The
range of P’ values is quite large (1.01 to 1.36; Table 1 and Fig. 5), with an average of 1.1 for
TJ34 and 1.04 for TJ31. Most P’ values are in the narrow range 1.01 to 1.1, which is similar
to the values reported for basaltic dykes (Raposo and D’Agrella-Filho, 2000). For TJ34,
which displays the largest range of variations, there is a good correlation between the position
of the samples in the dyke and the P’ value (Fig. 5), whereas for TJ31 the P’ values are nearly
constant. In TJ34, the P’ value regularly increases from the western rim to the core (from 1.03
to 1.36), then it suddenly decrease to 1.03 and start again to increase very slowly toward the
eastern rim. In fact there seems to be a sharp boundary, (amphibole bearing) inside the central
zone suggesting a kinematics step between the eastern and the western parts of the dyke.
The T values range from –0.40 to 0.63 (Fig. 5 and Table 1) with an average of 0.27-0.28 for
the two dykes, indicating a planolinear to oblate AMS ellipsoid for the whole set of samples.
P’ and T parameters vary together through the dykes from high T low P’ (large part of TJ31
and rim of TJ34) to low T high P’ values (mainly the centre of TJ34) which correspond to the
zones with high modal proportions of crystals. In TJ34, the difference between the biotitebearing external zones and the amphibole-bearing central zone is enhanced. Moreover, the
7
structural central limit defined by P’ variations correspond also to a step regarding T values
(Fig. 5).
Magnetic foliation and lineation
The axes of the AMS ellipsoids have very regular orientations in the two dykes. The
stereographic projections of the foliation and lineation have been plotted on (Fig 5) after
tilting the wall to a vertical position. The patterns of magnetic foliations K1K2 (perpendicular
to K3 magnetic direction) and lineations K1 are thus roughly homogeneous. The two dykes are
however characterized by oblique sigmoid foliation trajectories (Fig. 5). These foliations and
lineations are unusual for dykes, that generally show imbricated foliation near the walls and
inverse fabric in the core (Callot et al., 2001; Geoffroy et al;, 2002). The trajectory observed
for the Motru dykes is indicative of a sinistral transcurrent movement according to CorreaGomez et al,(2001). Local concave or convex morphologies near the dykes boundaries have
been observe in the field (penetrative and curved cleavage along the walls in the chilled
margin). Moreover, the lineations do not correspond to the deep line in the magnetic plane
everywhere (the plunge varies from 52° to 88° for TJ34 and from 59° to 83° for TJ31) which
implies an horizontal component in the direction of the magmatic flow. Some of the low
plunge values (< 60°) are closed to dyke boundaries of TJ31 and, to a lesser extent, of the
eastern rim of TJ34 also. In the central part of the dykes, the values of plunge are less
homogeneous and probably correspond to variations of flowing regime and of fabric intensity
between core and rim (Féménias et al., 2003).
6. Geochemistry of the TJ 34 dyke
Bulk-rock chemical analyses (major and some trace elements) have been obtained for
representative samples of the amphibole- and/or biotite bearing andesitic basalt dyke TJ34.
(Table 2). Twelve samples have been chosen covering both the petrography and magnetic
heterogeneities observed along the profile.
6.1. Lateral chemical variations along the profile
Major element variations
Plot of the major element variations against distance in the dyke (Fig. 6) illustrate the
significant differences between the amphibole-bearing central zone and the biotite-bearing
external margins. All the major oxides display significant variations: the core has lower SiO2,
Al2O3, Na2O and K2O contents than the rims, but it has higher Fe2O3t, MgO and CaO,
contents. The mean TiO2 contents are roughly comparable in the two zones. These chemical
variations are well-correlated with the petrographic observations: the high Fe2O3t, MgO, and
CaO contents in the core directly reflect the presence of primary amphiboles, plagioclases and
clinopyroxenes, whereas K2O enrichment on the margins is due to the presence of biotite. We
can interpret these variations as a slightly lesss differentiated comprosition of the core zone
(andesitic basalt) than the two margins (andesitic compositions). The few samples from the
transition zone with an intermediate composition between the amphibole and biotite-bearing
zones content both amphibole and biotite. In the central amphibole-bearing core, the
distribution of most of the oxides is asymmetrical: less or more well marked step occurs at the
with of 1480 cm, and is correlated with the structural limit deduced from the AMS survey.
Trace element variations
8
Some trace elements concentrations have been measured by X-ray fluorescence spectrometry.
Two groups can be distinguished: 1) The transition elements (Co, Ni, Cu, Zn) that are
compatible in amphibole and clinopyroxene; and 2) the other element (Zr, Rb, Ga, Sr, Y, Nb,
Pb and Th) that are incompatible and thus theoretically enriched in the crystal poor
(quenched) margin of the dyke.
To illustrate the contrasting behaviour of these two groups of elements, we have chosen to
represent only two elements per group: the transition elements Ni and Co and the
incompatible elements Rb and Zr (Fig 7). The others, elements reported in Table 2 will be
discussed by comparison with these four elements.
The incompatible elements are significantly enriched in the margins of the dykes (i.e Rb:
65ppm; Zr: 155ppm) when compared to the core (Rb: 35ppm; Zr: 120ppm). These elements
are thus positively correlated with the SiO2 content that range from 56% in the core to 59% in
the margin. This is in favour of a magmatic differentiation by fractional crystallisation.
The transition elements (Co, Cu, Ni and Zn) all show, excepted Zn, a significant enrichment
in the central amphibole-bearing zone (i.e. 5ppm Ni and 20-25ppm Co in the margins to 1520ppm Ni and 30-35ppm Co in the core). These elements also positively correlated to the
MgO, Fe2O3t and CaO contents and, by extension to the amphibole (pargasite-tschermakite)
and clinopyroxene (diopside-augite) modal proportions. These minerals probably
concentrated in the core zone of the dyke by a mechanical process
In the central amphibole-bearing core, most trace elements (incompatible or compatible)
display an asymmetrical distribution with a step at a width of 1480 cm, that is, nevertheless,
less well-marked than for the major elements.
6.2. Magmatic differentiation
Chemical variations of major and trace element contents along the dyke width suggest a small
but significant differentiation that occurred inside the dyke body itself. The margins indeed
appear as slightly more differentiated liquid, whereas the core is enriched in amphibole and
clinopyroxene micro-phenocryst. These data are in favour of a fractional crystallisation
process and of the migration of the crystals inside the dyke by extraction of the liquid from
the central mush and/or concentration of pre-existing phases in the core zone. This process
can be tested quantitatively by a simple mass balance calculation. Our objective is to check if
the average chemical composition of the margin of the dyke can be obtained by mechanical
segregation of amphibole, plagioclase and clinopyroxene in the core of the dyke. It is also
possible with this modelling to estimate the global composition of the fractionated solid
phases and the respective modal proportions of solid and liquid in the central mush.
The modal composition of the theoretical cumulate represented by Star in Fig. 8 is given by
a×Cpx + b×Am + c×Pl
with a+b+c=1
The composition of the real mush, illustrated by the core of the dykes, could be written for
each element by
a×Cpx + b×Am + c×Pl + d×Li
with a+b+c+d=1
where d×Li is the proportion of interstitial material
In the various oxides vs SiO2 diagrams (Fig 8), the range of whole-rock composition of the
main micro-phenocrysts (amphibole, clinopyroxene, and plagioclase microprobe data from
Féménias 2003) have been reported. The coefficients a, b, c and d can be easily obtained by a
classical least square inversion method. The calculation is mathematically feasible and the
result is statistically usable because Σr²~0.55 r corresponds to the residue that is the
difference: %oxidemeasured - %oxidecalculated. The model ends up with a composition of ~12%
amphibole, +~5% of plagioclase, +13% of clinopyroxene (that is 30% of solids= crystal)
+70% of interstitial liquid represented by the mean rim composition of the margins
interpreted as a quenched liquid. This composition is in good agreement with the petrographic
9
observation and with the core composition. The composition of the margin of the dyke is thus
compared to the composition of the interstitial material of the central zone (calculated from
the whole rock core composition by subtracting the solid phases). So this margin composition
can reasonably be considered as an extracted liquid. The high proportion of liquid near the
margin of the dyke contributed to a steady state flow domain characterised by a Newtonian
behaviour of the fluid near the wall as long as the liquid / glass transition is not reached. This
Newtonian behaviour is no more possible in the zone where the percentage of suspended
particles (= crystals) reach the threshold of 30-40%. The margin domain is thus a lubricated
channel where the flow is localised (Fig. 9).
7. Discussion and conclusion
The 5.5m thick TJ31 dyke is an asymmetrical body: the crystal size distribution (CSD) of
amphibole (Ti-rich pargasite-tschermakite) micro-phenocrysts shows that the modal
proportion varies along the dykes width and that this distribution is asymmetrical describing a
long western half-dyke and a short eastern half-dyke. The AMS results are also asymmetrical
and show an oblique sigmoid magnetic foliation by the participation of an external dyke-wall
kinematics contemporaneous with the dyke emplacement. The asymmetrical disposition of the
amphiboles concerning as well their proportion (Fig.1) as their size (Fig.4) and the HT-HP
conditions of crystallisation (900°-1100°C; 0.45-0.7Gpa) show that a Bagnold effect (flowage
differentiation) is responsible of the crystal distribution inside the dyke. The higher
concentration of crystals near the eastern boundary (Fig.1) could be explained by an
acceleration of the flowing velocity (and induced lateral shearing and deformation "epsilon")
near the western margin (Fig. 9). The lack of a clear inverse fabric argues for a fabric that is
representative of the flow (Féménias et al; 2003) the South to North horizontal component of
the magma flow can be inferred from direction and plunge of lineations during an horizontal
sinistral component of the tectonic movement of the wall deduced from the asymmetrical
foliations. As a consequence, the concentration of crystals during pre to syn-emplacement
processes occurs in or near the lower shearing zone during the flowing process (Bagnold,
1954 ; Komar, 1972a ; 1972b ; Barrière, 1976). In this zone, the arrangement of the platy
crystals in the flow plane contributes to increase the concentration of particles. The CSD of
amphibole (Ti-rich pargasite-tschermakite) displays classical patterns and fan line distribution
that oblige to reconsider the classical interpretation of the CSD parameters. In our opinion, the
intercept number n° could not be only an approximation of the nucleation rate during static
cooling. As the strain intensity and importance of the Bagnold effect (Bagnold, 1954) is a
function of the distance to the wall dyke, the mean crystal length (Lm), the total number of
grains (NT) and the line parameters (S and n°) change naturally along the profile.
To confirm this hypothesis of a possible migration of suspended crystals during the dyke
emplacement, we have studied a second much thicker (23m) dyke (TJ34). This dyke, is
petrographically heterogeneous; the two margins have very fine-grained textures and contain
biotite whereas the core zone shows a higher cristallinity and correspond to a mush composed
to micro-phenocrysts of brown amphibole, strongly altered clinopyroxene and plagioclase in a
secondary fine-grained matrix. The transition between the two petrographic domains is
continuous biotite has locally been observed with amphibole. The brown amphibole of the
core is a HT-HP Ti-rich pargasite-tschermakite as in the TJ31 dyke, but its CSD does not
show the classical fan line distribution (Fig.3). The populations of crystals are homogeneous
from sample to sample with roughly constant S and n° whatever the position of the sample
inside the dyke. Nevertheless, a slight asymmetrical distribution of NT and Lm parameters is
observed (Fig 4), and tentatively corroborated by the AMS asymmetrical characteristics of the
dyke. The petrographic heterogeneity is clearly correlated to a chemical heterogeneity. The
10
chemical trends observed along the dyke width (Figs 6 and 7) can be linked reasonably to a
differentiation process by fractional crystallisation involving the migration of the suspended
crystals (amphibole, clinopyroxene and plagioclase) from the rim to the core and/or the
concomitant extraction of the interstitial liquid from the central mush to the margin (Fig 9).
These mechanisms imply a migration of material inside the dyke that is in opposition with the
static development of the textures. The biotite grains of the marginal zones of the dyke have
crystallised at significantly lower T° (600-680°C) than the amphibole of the central zone
suggesting that they formed during the cooling of the dyke in subvolcanic environment. So
the CSD of the biotite can reasonably be linked to the static crystallisation occurring late in
the cooling history of a more evolved Newtonian liquid. The localisation of this Newtonian
liquid on both margins of the dyke has certainly facilitate the ascent of the central Binghamian
mush during the dyke emplacement. The structural step observed in the central part of the
body would correspond to the last kinematics effects of lava emplacement recorded in an
almost completely solidified dyke.
Acknowledgments
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13
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FIGURE CAPTIONS
Fig.1. Variation of the modal proportion (vol %) of amphibole micro-phenocryst along the
TJ31 (andesitic basalt) profile.
Fig.2. P-T crystallisation conditions of amphiboles from the TJ31 and TJ34 dykes. T°
calculated with the thermometer of Otten (1984); P estimated from Schmidt (1992).
Fig. 3. The Crystal Size Distribution of amphibole and biotite (CSD) in the TJ31 and TJ34
dykes show typical fans: a) CSD of amphibole in TJ31, b) CSD of amphibole in TJ34, c) CSD
of biotite in TJ34. For a and c, the position of a line in the fan is strongly correlated with the
position of the sample in the dyke.
Fig.4.Variation of some CSD parameters (S, n°, NT and Lm) within the two dyke bodies. The
grey curves underline the variation of a given parameter for a given phase.
Fig.5.Anisotropy of Magnetic Susceptibility (AMS) results. a) Stereographic representation of
the foliation and lineation in each dyke. Note the obliquity between the wall direction and the
magnetic foliation.
b) Variations of the magnetic parameters T and P’ versus position of the sample, in the dyke.
Fig.6.Major element oxides (in wt.%) vs. width (in cm) for whole rock samples of the TJ34
andesitic basalt dyke.
Fig.7. Trace elements (in ppm) vs. width (in cm) for whole rock samples of the andesitic
basalt dyke TJ34.
Fig.8. Major element oxides versus SiO2 (in wt.%) for the main mineral phases
(clinopyroxene, amphibole and plagioclase; data from Féménias, 2003) and whole rock
samples for the dyke TJ34. The dashed triangle limits the composition domain of the total
solid phases involved in the fractional crystallisation process. The dashed grey line illustrates
the mass balance calculation allowing the evolution of the dyke from the central zone (core
sample) to the margin liquid by subtraction of theoretical cumulate or solid phases (white star
illustrating the composition of the assemblage clinopyroxene+amphibole+plagioclase).
Fig.9. Evolution of crystals distribution during the emplacement of the dikes. below each
sketch, diagrams give a representation of the amount of crystals (dashed line) and the
evolution of the deformation (solid line) through the dike; the asymmetry of the flow into
TJ31 is responsible of the asymmetry of the distribution in accordance with the structural data
(AMS); For both the "maturity" of the dike (right part of the figure) and the Bagnold effect is
responsible of the concentration of the crystals in the core and crystal-free margins (in grey).
TABLE CAPTIONS
Table 1: Measured Crystal Size Distribution (CSD) and some Anisotropy of Magnetic
Susceptibility (AMS) parameters for the two dykes TJ31 and TJ34. X (in cm) represents the
distance to the western wall-dyke; Bt: biotite-bearing; Am: amphibole-bearing; S: slope; n°:
intercept; Lm: mean length; NT: total number of crystals; Km: mean magnetic susceptibility;
P’: anisotropy of magnetic ellipse; T: the shape parameter; K1az: azimuth of the maximum
14
susceptibility axis; K1inc: inclination of the maximum susceptibility axis; K3az: azimuth of the
minimum susceptibility axis; K3inc: inclination of the minimum susceptibility axe.
Table 2: Bulk rock analyses of representative samples from the TJ34 andesitic dyke (Bt:
biotite-bearing; Am: amphibole-bearing).
15
Nkono et al. Fig. 1
35
Modal proportion
of amphibole en TJ 31
[vol.%]
30
25
20
15
10
5
Western Eastern
half-dyke half-dyke
0
0
100
200
300
Width [cm]
400
500
Nkono et al. Fig. 2
T (°C)
700
800
900
1000
1100
0.0
TJ 34
TJ 31
0.1
late-emplacement
alteration of
Ti-pargasite
P (GPa)
0.2
0.3
0.4
0.5
0.6
0.7
Magmatic phases
0.8
Nkono et al. Fig. 3
16
-4
Ln(n) [cm ]
Amphibole TJ 31
Amphibole TJ 34
Biotite TJ 34
dyke margins
12
core of the dyke
core of the dyke
8
core of the dyke
dyke margins
4
0
a
0
b
c
L (Crystal size) [cm] 0.25 / 0 L (Crystal size) [cm]0.25 / 0 L (Crystal size) [cm] 0.25
Nkono et al. Fig. 4
Amphibole TJ 31
Amphibole and Biotite TJ 34
13
140
13
120
12
100
11
80
10
60
9
12
100
11
80
10
60
9
40
8
20
7
0
0.1
0.08
0.07
0.06
0.05
1000
0.04
0.03
0.02
NT
L mean
0.01
0
0
100
200
100
300
Width [cm]
400
500
8
Bt Intercept n°
Bt Slope
Amp Intercept n°
Amp Slope
20
6
10000
0.09
40
0
0.1
14
Intercept n° [cm-4]
120
Intercept n°
Slope
Biotite zone
(east)
Amphibole zone
160
Total number NT
Mean crystal size Lm [cm]
-1
Slope S [cm ]
140
15
Biotite zone
(west)
14
-1
Slope S [cm ]
Western Eastern
half-dyke half-dyke
Intercept n° [cm-4]
160
Mean crystal size Lm [cm]
180
15
7
6
10000
0.09
0.08
0.07
0.06
0.05
1000
0.04
0.03
Bt NT
Bt L mean
Amp NT
Amp L mean
0.02
0.01
100
0
0
500
1000
Width [cm]
1500
2000
Total number NT
180
Nkono et al. Fig. 5
aTJ 31
Wall great circle
Wall great circle
TJ 34
AMS
Lineations
AMS
Lineations
AMS
Foliations
Wall
Wall
AMS
Foliations
b1
0.6
1
1.4
P’
T
0.8
Biotite zone
(west)
0.8
1.35
Biotite zone
(east)
Amphibole zone
0.6
1.3
1.3
0.4
1.2
1.15
-0.4
1.25
0.2
0
1.2
-0.2
P’
0
-0.2
P’
1.25
0.2
T
0.4
T
1.4
1.35
1.15
-0.4
Western Eastern
half-dyke half-dyke
-0.6
-0.8
-1
1.1
1.05
1
0
100
200
300
Width [cm]
400
500
1.1
-0.6
P’
T
-0.8
-1
0
500
1000
Width [cm]
1.05
1
1500
2000
Nkono et al. Fig. 6
60.0
9.00
Biotite zone
(west)
59.5
Biotite zone
(east)
Amphibole zone
Biotite zone
(west)
Biotite zone
(east)
Amphibole zone
59.0
58.5
8.00
7.00
%SiO2
58.0
57.5
6.00
57.0
5.00
%CaO
56.5
56.0
4.00
55.5
55.0
19.0
3.00
8.00
%Al2O3
18.5
7.00
18.0
6.00
17.5
5.00
17.0
4.00
16.5
3.00
%MgO
16.0
2.00
15.5
1.00
15.0
7.2
0.00
4.50
%Na2O
7.0
4.00
6.8
3.50
6.6
%Fe2O3
6.4
3.00
2.50
6.2
6.0
0.95
2.00
%TiO2
0.90
%K2O
0.85
1.75
1.50
0.80
0.75
1.25
0.70
1.00
0.65
0.60
0.75
0.55
0.50
0.50
0
500
1000
Width [cm]
1500
2000
0
500
1000
Width [cm]
1500
2000
Nkono et al. Fig. 7
25.0
70.0
Rb [ppm]
65.0
Ni [ppm]
20.0
60.0
55.0
15.0
50.0
10.0
45.0
40.0
5.0
35.0
170.0
30.0
0.0
40.0
Zr [ppm]
160.0
Co [ppm]
35.0
150.0
30.0
140.0
130.0
25.0
120.0
20.0
110.0
100.0
15.0
0
500
1000
Width [cm]
1500
2000
0
500
1000
Width [cm]
1500
2000
Nkono et al. Fig. 8
Theoretical cumulate Core composition
Am
TJ 34 (margin)
Pl
TJ 34 (core)
Cpx
Amphibole
Liquide
Plagioclase
=margin
Clinopyroxene
4.0
3.5
3.0
25
%MgO
20
2.5
15
%TiO2
2.0
10
1.5
1.0
5
0.5
0
25
0.0
30
%CaO
%Al2O3
25
20
20
15
15
10
10
5
5
0
18
0
6
%Na2O
%Fe2O3
16
5
14
12
4
10
3
8
6
2
4
1
2
0
2
1.8
0
0.35
%K2O
%MnO
0.30
1.6
0.25
1.4
1.2
0.20
1.0
0.15
0.8
0.6
0.10
0.4
0.05
0.2
0.00
0.0
40
42
44
46
48
50
52
%SiO2
54
56
58
60
42
44
46
48
50
52
%SiO2
54
56
58
60
Nkono et al. Fig. 9
TJ31
5%
11%
27%
e
11%
TJ34
e
11%
11%
20%
11%
2%
27%
2%
2%
27%
2%