magma chambers in the oman ophiolite: fed from the top and the … · magma chambers in the oman...
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EPSL ELSEVIER Earth and Planetary Science Letters 144 (19961239-250
Magma chambers in the Oman ophiolite: fed from the top and the bottom
Frarqoise Boudier * , Adolphe Nicolas, Beno”it Ildefonse
Laboratoire de Tectonophysique, CNRS UMR 5568, ISTEEM, Uniuersite Montpellier It, 34095 Montpellier cede.x 05, France
Received 8 January 1996; revised 22 July 1996; accepted 30 July 1996
Abstract
Recent models of magma chambers at fast-spreading ridges are based on the idea that the entire gabbro section of the
oceanic crust crystallizes from a thin melt lens located just below the sheeted dike complex. The shape of the lens has been deduced from seismic reflection data at fast-spreading ridges. On the basis of structural studies in the Oman ophiolite, we suggest that the accretion of the lower crust may not proceed entirely in this way. We emphasize the contrast between: (1)
upper level gabbros characterized by a magmatic foliation which, from a flat attitude at depth, rapidly steepen upward and tend to become oriented parallel to the sheeted dikes; and (2) lower gabbros, flat-lying, magmatically deformed, and more or less strongly layered. Wehrlite layers and lenses which contribute to the layering of these gabbros have previously been interpreted as sills. We suggest here that the modally graded bedding, which is an important feature of the lower layered gabbros, may have similarly originated as sills. This is deduced from the fact that, above mantle diapirs, the several hundred metre thickness of the transition zone contains sills of layered gabbros, commonly organized in modally graded sequences.
These sills, which are interlayered with dunite or harzburgite, contain gabbros which are shown here to be structurally similar to those in the layered gabbro unit at all scales. If this interpretation is correct, the gabbro section of the oceanic crust
in Oman is built up by crystallization, both along the walls and the floor of the perched magma lens, followed by subsidence, and also in sills intruded either in the subsiding foliated gabbros or in the mantle dunites of the Moho transition zone. Supply from the perched melt lens generates the upper foliated gabbros, and supply by sills emplaced near Moho level gives rise to the basal layered gabbros and the gabbro-troctolite lenses of the transition zone. Feeding of the perched melt lens by vertical dikes and feeding of the Moho horizon by sills may correspond to successive stages of a basaltic melt injection episode.
Kewords: Oman; Semail ophiolite; magma chambers; spreading centers; gabbros
1. Introduction has significantly improved, thanks to new data ob-
During the past ten years, our understanding of the geometry and dynamics of fast-spreading centres
* Corresponding author. Fax: +33 67 14 36 03. E-mail:
tained on the East Pacific Rise. Multichannel seismic data on the EPR have allowed the identification of a perched magma lens (I-4 km wide, 50-200 m thick), overlying a low velocity zone (6-8 km wide, 2-4 km thick), with a poorly constrained shape, usually interpreted by geophysicists as a hot zone
0012-821X/96/$12.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PII SOOl2-821X(96)00167-7
240 F. Boudier et al. /Earth and Planetary Science Letters 144 (1996) 239-250
with a few percent of melt [1,2]. The thin perched magma lens is continuous along-strike, except for
gaps occurring at major discontinuities (overlapping
spreading centres or transforms) and is probably permanent on a scale of a few million years.
The Oman ophiolite is interpreted as an ancient
fast-spreading ridge which appears to have largely escaped emplacement-related deformation [3,4], and
which displays structures related to the dynamics at a fast-spreading ridge. Extensive structural analysis and
mapping along this 500 km long ophiolite shows that the spreading centre geometry and the melt delivery
to the crust are controlled by mantle diapirism [3,4] Recent models of ridge magma chambers aim to
reconcile constraints provided by seismic experi- ments and those resulting from structural and petro-
logical studies in the gabbro section of the Oman
ophiolite. The gabbros are interpreted either as the result of successive sill intrusions from the perched
magma lens [5], or from crystallization along the floor of the perched magma lens and subsequent subsidence. During subsidence they are stretched, as a result of ridge spreading [6,7].
In a related model, the crystal mush subsides from the bottom of the perched magma lens and crystal-
lizes along the dipping walls of the low-velocity
zone beneath, in conjunction with large-scale motion related to underlying mantle flow [8,9]. In these
models it is generally assumed that the plutonic
section is built from the perched magma lens, except for wehrlitic intrusions, and sills incorporated into the gabbro layering, which indicate that lower parts of the section may not be derived entirely from the
perched magma lens [6]. The role of gabbro sills in the construction of the layered gabbro pile was pro- posed for the Oman [lo], and Bay of Islands ophio- lites [ 111.
We wish to favour here the sill injection concept,
and we relate it to a common component of the
layering in the basal gabbros: the modally graded layers. Our analysis is based on a structural and textural study of the gabbros in the Oman ophiolite
and their relationship with the Moho transition zone. We address the question of feeding of the crustal gabbro section via dikes and sills emplaced at differ- ent levels, possibly in connection with the dynamics of melt extraction from the mantle.
2. Gabbro sills in the Moho transition zone
The Moho transition zone in Oman has a variable
thickness [9]. Above mantle diapirs, the Moho transi-
tion zone is several hundred metres thick [12]; this zone is characterized by gabbro layers alternating
with residual peridotites, depleted harzburgites, and dunites. Identification of this unit as a mantle com-
ponent is based on recognition of the residual charac- ter of the ultramafic layers (mantle fabrics and oc-
currence of residual orthopyroxene). The transition from diffuse plagioclase-clino-
pyroxene impregnations in the normally sterile peri- dotites (Fig. 1 a> to gabbro layers is described in [ 121
from microstructural scale to outcrop scale. Lens- shaped impregnations display a foliation and a lin-
eation, resulting in a strong fabric anisotropy. Lo-
cally, gabbro dikes branch into gabbro sills in a metric-scale interconnected network (Fig. lb),
demonstrating that both contributed to magma trans- port at this structural level. Sills thinner than 50 cm are generally massive or only poorly layered and foliated (Fig. la). Sills display a modal layering
which commonly appears above a lower horizon of massive gabbro. In particular, the modally graded
sequences (Fig. lc,d) are typically observed in sills several metres thick. Some sills reach 100 m in
thickness. Laterally, they pinch out over short dis-
tances, generally by interfingering with the surround- ing dunites (Fig. 1~). The gabbro sills display a strong magmatic flow structure, with foliation and
lineation parallel to those in the surrounding peri- dotites where solid-state conditions prevailed. Thus, melt was injected in these sills during the horizontal flow carrying these upper mantle formations away
from the ridge axis. Away from mantle diapirs, the thickness of the
Moho transition zone is reduced to tens of metres or less. We note an inverse correlation between the
thickness of the Moho transition zone and that of the layered gabbro section [9].
3. The gabbro section
The gabbro section in the Oman ophiolite has been largely described from a petrological point of
F. Boudier ef al. / Earth and Planetary Science Letters 144 (1996) 239-250 241
Fig. 1. Layered gabbros from sills in Moho transition zone (a-d), and from the gabbro section (e and f). (a) Gabbro lens, 10 cm thick,
showing diffuse contacts with the enclosing impregnated peridotite, wadi Shafad. (b) Branching gabbro sills (concordant with a steep
foliation, N-S on the photograph) and dikes (E-W on the photograph), wadi Fayd. (c) Layered gabbro sills in the Moho transition zone,
pinching in the enclosing dunite, Maqsad area. (d) Close-up of graded bedding in (cl. (e) Composite layering in the lowermost iayered
gabbros, wadi Halhal. (f) Close-up of (e), concordant wehrlite bands showing sharp contacts with enclosing gradually layered sequence,
wadi Halhal.
242 F. Boudier et aI./Earth and Planetary Science Letters I44 (19961239-250
view [ 13,141. Particular attention has been paid to the modal, occasionally graded, layering which is domi-
nant in the lower part of the gabbro section. Recur-
rent variations in mineral compositions, and the lack
of simple geochemical correlations between individ-
ual phase variations led to the conclusion that the
gabbros crystallized in a continuously replenished open system [ 13-151, or in successive small magma
chambers hundreds of metres thick [161.
3.1. Typical facies and associated structures
Structural analysis and mapping have shown that
the gabbros structure results from large-scale mag-
matic flow [ 17,181 and that the gabbro section may be divided into two levels.
The lower level consists of layered gabbros. The lowermost gabbros, overlying the Moho transition
zone, usually display a very contrasted and spectacu- lar layering, with alternating layers of gabbro,
anorthosite, melagabbro and wehrlite (Fig. le>, often including modally graded units (Fig. If). Ultramafic
bands, ranging in thickness from millimetres to tens of metres, are commonly grouped in levels a few
tens of metres thick, and of limited lateral extent, in the range of 50- 100 m [ 131. These ultramafic bands
have been interpreted as intrusive wehrlite sills
[6,10,193. Up-section, the frequency of ultramafic
layers rapidly decreases, and the graded layering tends to disappear (Fig. 2). The upper part of this
layered gabbro unit usually has a more gneiss-like appearance, with a less contrasted and more lenticu-
lar layering mainly marked by the presence of thin
anorthosite lenses. The layered gabbros have a foliation defined by
the fabric of plagioclase laths and elongated olivine
aggregates. This foliation has been shown to be of
magmatic origin, and lies close to the flow plane [18,20]. In the foliation plane, a generally weak mineral lineation is marked by elongation of olivine aggregates and clinopyroxene. Foliations and lin- eations in the lower gabbros and the underlying mantle peridotites have been mapped throughout the entire ophiolite. They are systematically parallel above and below the Moho [3,4,17]. Dynamic struc- tures disturbing the gabbro layering are commonly observed at various scales: boudinage, mullions, iso- clinal folds and normal and occasionally conjugate
In ” Upper G I
G 1 G
0
Jebel Oih; sections
Fig. 2. Histogram showing the respective thickness of graded
bedding (black) within the layered gabbro unit (dark grey) as a
function of gabbro thickness (distance to the Moho) in cross-sec-
tions from Jebel Dimh. aq = wadi Aqsaybah: ka = wadi Kadir;
fa = wadi Fara kh = wadi Khafifah, gi = wadi Gideah; sa = wadi Saq; zi = wadi Zib; na = wadi Nassif.
shear bands [ 181. Wispy terminations and lens-shaped layers become more common up-section. overturned
folds and normal faults on a scale of around 100 m are interpreted to be the result of large-scale slumps.
All these structures were developed within the magma
chamber, during viscous flow, as shown by related microstructures which hardly present any sign of
intracrystalline deformation of minerals. Finally, at the edge of the magma chamber, intruded wehrlite
bodies locally truncate and fold the gabbro layering. The upper level is represented by foliated gab-
bros. Olivine-rich layers, which are responsible for the contrasted layering in the lower unit, tend to
disappear. In fact, even in the gabbro groundmass, the olivine content decreases progressively. The
magmatic structures, foliation, and especially lin- eation, become difficultto observe in the uppermost gabbros. The difficulty in identifying these structures explains why these gabbros have also been called ‘isotropic’. Isotropy becomes real at the top of the sequence where the gabbros were affected by hy- drous anatexis and hydrothermal recrystallization un- der static conditions. The transition from layered to foliated gabbros is commonly located in the horizon where the dip of the foliation steepens, becoming,
F. Boudier et al. /Earth and Planetary Science Letters 144 (1996) 239-250 243
upwards, more or less parallel to the overlying
sheeted dikes [17,21]. The tent-shape model of the magma chamber is based on the systematic mapping
4. Textures and fabrics
4.1. Textural evolution in a gabbro section
of this type of magmatic foliation in the upper A constant feature of the Oman gabbros is the
gabbros, in a paleo-ridge frame [4]. nearly complete absence of intracrystalline plastic
Fig. 3. R’ epresent ative tex tures in gabbro section (all at the same scale and the same orientation perpendicular to th
lineatic In, E-W c KI the pl ates). (a) Mosaic texture in layered gahbro, 2.5 km above Moho, wadi Farah, # 89 0
superir np Nosed on mosaic texture in lowermost layered gabbro, 100 m above Moho, west fork of wadi Abda, #
fluidal te ‘xture in foliated I gabbros, 500 m below sheeted dikes, wadi Samad, # 94 OB 28. (d) Subdoleritic te2 croppi] % out as s creen in sheeted dikes, wadi Andam, # 95 OA 160. Plane polarized light; scale bar: 1 mm.
e fol iati on a md p
A 1; ‘A. (b) Plas
95 ( IA 165 ‘. Cc) :ture in foli ated
arallel to
tic strain
Tabular
244 F. Boudier et al./Earth and Planetary Science Letters 144 (1996) 239-250
deformation, except in olivine grains. The field struc- tures and microstructures were acquired during the magmatic and submagmatic stages by suspension flow in the presence of a limited melt fraction [ 18,221.
The lower layered gabbros have a homogeneous grain size (l-3 mm), with a weak to moderate fabric marked by the orientation of elongated plagioclase crystals. In the major part of the layered gabbro
Fig. 4. Textural evolution of olivine as a function of olivine content, in modally graded layers from gabbro lens in the Moho transition zone
(a-c) and from the layered gabbro section (d-f). Septate texture: concave grains of olivine with black serpentine network, clinopyroxene as
lensoid grey grain, and plagioclase as white grains. (a) # 90 OA 65A (see Fig. le), Maqsad area, and (d) # 90 OA 1 lOC, wadi Falaj, lower
graded layer. (b) #95 OF 88B, wadi Mazari, and (e) # 88 OA 52A, wadi Halhal (see Fig. If), central graded layer. (c) # 94 MA2, Maqsad area (see Fig. le), and (f) # 88 OA 53A, wadi Halhal (see Fig. If), upper graded layer. Sections cut in plane perpendicular to foliation and
parallel to lineation, E-W on the plates. Plane light, scale bar: 2 mm.
F. Boudier et al. /Earth and Planetar?, Science Letters 144 (1996) 239-250 245
section, the textures are mosaic, comparatively
coarse, with planar boundaries, meeting at triple junctions; this is especially conspicuous in anorthosite aggregates (Fig. 3a). Pyroxene grains are globular;
olivine is interstitial (see next section). The textures reflect active grain-boundary migration, indicating
that solidus to hypersolidus conditions prevailed for a comparatively long time in the lower magma
chamber. The lowermost levels of the layered gabbros may
exhibit traces of plastic strain, marked by serrated
plagioclase-plagioclase grain boundaries, undulose extinction, and development of tapered twins (Fig.
3b), a textural type also seen in the layered gabbros
from sills in the Moho transition zone. This over- printing of plastic strain at Moho level records the
mechanical coupling between the plastic mantle and
the magmatic gabbros operating on a limited thick-
ness outside the magma chamber in the solidifying lower gabbros.
The foliated gabbros and norites from above the
a 90oA65A [1oo]
HMax.Densltv = 0.70
b 90OA110C 0001
. Max.Dens~ty = 5.11
layered gabbros exhibit tabular fluidal textures (Fig.
3c) grading upwards to subdoleritic (Fig. 3d). Plagio- clase crystals are tabular (1 .O X 0.2 mm on average) and exhibit a strong shape fabric with development
of (010) rational face. The clinopyroxene is intersti- tial and poikilitic. A subdoleritic texture appears very
close to the root of the sheeted dikes in amphibole- bearing gabbros; plagioclase occurs as hypidiomor- phic crystals, presenting tight magmatic twins and a
compositional zoning of normal concordant type (see
also [13]); it is embayed in a matrix of interstitial
clinopyroxene and/or amphibole.
4.2. Textural evolution of ohine in modally graded layers
The same textural types are observed in the lay- ered gabbro from the basal gabbro section and in
sills from the Moho transition zone. A common origin of modally graded layers in both Moho transi-
tion zone and the gabbro section is reinforced by the
HMax.Denatv = 10.71
n MaxDensity = 9.72
w n Max.Density = 7.56
Fig. 5. (a) and (b) Olivine lattice preferred orientations in textural types of Fig. 4a and d. respectively. Compiled from U-stage
measurements of 100 grains, in lower hemisphere of projection, structural reference system with foliation vertical E-W, lineation horizontal
E-W. Maximum density compiled from [23].
246 F. Boudier et al. /Earth and Planetary Science Letters 144 (1996) 239-250
similar textural features of olivine in the two occur- rences (Fig. 4). In the basal ultramafic layers (Fig. 4a,d), where the olivine fraction is 60-80%, this mineral forms a continuous network enclosing monocrystalline lenses of clinopyroxene or plagio- clase; this network is interconnected in three dimen- sions forming a septate texture. Sharp (100) sub- boundaries and strong crystallographic fabrics in olivine are indicative of high plastic strain accompa- nied by high-temperature annealing. Olivine lattice fabrics (Fig. 5) exhibit slight obliquity relative to the foliation plane and lineation, typical of intracrys- talline slip; they result from activation of the high- temperature [ 1001 (010) slip system. The fabric in- tensity [23] is stronger in the specimens from gabbro lenses in the Moho transition zone (Fig. 5a) than in specimens from the gabbro section (Fig. 5b).
There is a continuous evolution between this basal zone texture and that observed in the central part of the graded layer (Fig. 4b,e), which has about 30% olivine fraction. Here, olivine forms contorted strings of elongated (3-5 X 1 mm) interstitial skeletal ag- gregates with concave boundaries. Examination of these aggregates shows that they are composed of elongated olivine grains with a strong lattice pre- ferred orientation similar to the one measured down-section. Olivine grains are also cut by trans- verse, sharp (100) sub-boundaries, and polygonal grains having common rational (100) grain bound- aries (Fig. 6). In plastically deformed rocks, this typical association is derived through a process of
strain-induced recrystallization, by progressive mis- orientation of subgrain boundaries [24] in highly strained olivine porphyroclasts. In the upper part of the graded layer where olivine fraction is about 15% (Fig. 4c,f), elongated aggregates of olivine are sim- ply more dispersed in the foliation plane, with a slight grain size reduction. Otherwise, in the central and upper part of graded layers, plagioclase and clinopyroxene aggregates are of the mosaic type.
5. Discussion
5.1. Modally graded layers as sill components in the lower gabbros
The contrasted layering of the basal gabbros is made of wehrlite bands and layered gabbro se- quences, often modally graded in the layered gabbro unit. Emplacement as sills has been invoked for gabbro and wehrlite layers [lO,l 11. For the latter, this origin was demonstrated from the field observation of branching relationships between wehrlite dikes and layers.
The above description has emphasized a complete structural and textural similarity between modally graded sequences in the lower crustal gabbros and in the Moho transition zone, where modally graded gabbros are interlayered with residual mantle [12], and are regarded as sills. We suggest that the modally graded gabbro sequences of the lower gabbro unit in
Fig. 6. Enlargement of Fig. 4b, showing strained olivine aggregates. (a) Strained olivine crystal with high-T annealed sub-boundaries
(indicated by white arrows). (b) Polygonal grains derived by subgrain rotation (120’ triple junction indicated by white arrow). Polarized plane light, scale bar: 1 mm.
F. Boudier et al. /Earth and Planetary Science Letters 144 Cl 996) 239-250 247
Oman were also emplaced as sills. A number of
mechanisms have been proposed for the origin of
modally graded layering in gabbros [25-281 but, whatever the mechanism, the absence of graded bed- ding in the upper levels of the Oman section could
be symptomatic of distinct accretion processes for the lower and the upper igneous crust.
5.2. Rapid crystallization of the upper level gabbros
The upper foliated gabbros have structural and
textural features different from those of the lower
layered gabbros; their magmatic foliation systemati-
cally steepens upwards; their layering is poor, and they display fluidal tabular to subdoleritic magmatic
textures. Other striking differences are the occur-
rence of tight magmatic twins, and of zoning in the plagioclase laths. This plagioclase zoning, also en-
countered in gabbros drilled at Hess Deep [29], is
indicative of crystallization in a chemically evolving
closed system, followed by rapid cooling. The dis- tinct characters of the foliated gabbros and their
upper structural position are consistent with the hy-
pothesis that they crystallized at the floor and along
the walls of the perched magma lens. A rapid and regular drop of crystals onto the floor of the thin
melt lens may hinder crystal segregation, resulting in
these poorly layered and relatively homogeneous gabbros.
5KM
5.3. Magma chamber model
From the above observations and discussion, we
suggest that the magma chamber is fed by a dual melt delivery. The foliated gabbro partly crystallizes
at the floor of the perched magma lens, subsides as a magmatic mush from that elevated position and ac- quires its magmatic foliation during this subsidence
and subsequent motion. This mechanism has also
been discussed by Quick and Denlinger [6] as poten-
tially significant in the formation of the gabbro
section. The lower layered gabbro is a composite in which the mass of foliated gabbros is intruded by a
variable number of sills issued from melt ponding at
the Moho level (Fig. 7). These sills could be the wehrlite sills described in previous papers [ 10,191,
but also gabbro sills within which modally graded
layers similar to those developed in the Moho transi-
tion zone would be segregated. The contrasted layer-
ing is accentuated by the tectonic transposition re-
lated to the drag caused by the fast asthenospheric flow beneath [ 181. Through this process, large defor-
mation in the magmatic and submagmatic stage
would transpose features such as graded bedding
units, mechanical segregations of crystals, wehrlite
sills and dikes, into parallel layers and lenses.
This model is an alternative to the recent models which imply that all structures observed in the lay- ered gabbros are generated during the subsidence of
0 SKM ,
Fig. 7. Dual feeding model for magma chamber model (see text for further discussion).
248 F. Boudier et al. / Earth and Planetary Science Letters 144 (1996) 239-250
the mush crystallized at the floor of the perched
magma chamber [30], or result from the combination of such subsidence and wehrlitic intrusions [6]. The contribution of gabbro sills to the build-up of the
gabbro-troctolite sequence has also been inferred
from field studies in the Bay of Island ophiolite [ 1 I].
5.4. Feeding by dikes and/or sills
It has recently been proposed [3 1,321 that the
accreting crust in the Oman paleoridge was fed by both dikes and sills. Dikes in the upper mantle
parallel to the sheeted dikes were ascribed to hydro-
fracturing controlled by the lithospheric tensional stress prevailing at the ridge (minimum principal
stress normal to the ridge trend), and the sills to the
deeper seated stress field related to mantle diapirism. After a melt burst opening and filling lithosphere
fractures, the tensional ridge stress relaxes and the subsequent melt injection preferentially fills sills. A
similar interpretation has been proposed for Iceland
rifting [33]. In this model, the first melt injected rises
to the perched melt lens and replenishes it, also disrupting the overlying lid, creating a new metre- sized diabase dike in the sheeted dike unit, and
feeding basaltic flow at the sea bottom. In contrast, the subsequent melt injected ponds within the plu-
tonic section of the crust and at the Moho level as
sills. The level of sill injection deepens with time, in response to: (1) lithospheric stress relaxation; and (2)
a drop in melt pressure as the hydrofracture rooting
into the melting asthenosphere closes at depth and
drains progressively shallower levels. We assume that the melt trapped in the sheeted
dike complex, and replenishing the underlying melt lens, is the pristine melt, only modified at depth by possible crystal segregation and chemical reactions
along hydrofracture conduits. This model appears not
to be consistent with geochemical data, which imply that the melt generating the upper level gabbros and overlying lid is highly evolved and has segregated a large volume of crystals at shallow depth. This argu- ment is developed by Kelemen and co-workers [34],
within an analysis complementary to ours, and lead- ing to a similar conclusion regarding the occurrence of gabbro sills in the Oman ophiolite. These authors propose that the primary melt ponds into the sills,
where it crystallizes massively. The fractionated liq- uid issued from this crystallization is then injected
into the perched melt lens and the overlying lid. An alternative interpretation which takes into ac-
count the geochemical constraint would be to re- verse, in the crust, the dike/sill chronological se-
quence proposed above. The melt issued from the
rising asthenosphere would reach, via ridge-parallel dikes, a level of the layered gabbros and pond there
as sills, possibly in response to the stress field devel-
oped at the top of a mantle diapir [32]. After partial crystallization within these sills, the fractionated melt
is reinjected toward the perched melt lens and the overlying lid in response to a tensional failure of the
lid when the ridge-spreading stress exceeds the yield
strength of the lid. In this model, the gabbro sill
injection acts as a relay between two hydrofracturing processes, which might be connected: hydrofractur-
ing caused by melt pressure, which extracts melt
from its mantle source [35], and tensional fracturing caused by the spreading of lithosphere, which would
be dominant in the upper crust.
6. Conclusion
Gabbros in Oman display several striking struc-
tural and petrological differences between the lower
layered unit and the upper foliated one. However,
very contrasted layering, often defined by graded
bedding, is observed both in the lower layered gab- bros and in the gabbro sills found below in the Moho transition zone. This layering is macroscopically and
texturally similar in both occurrences. Thus, the layering may originate in the same way and at least part of the layered gabbros may be derived from sills
which intrude the crystallizing mush in the magma chamber and which are progressively intermingled with gabbros subsiding from above as deformation progresses. This mechanism has been proposed for wehrlite sills emplacement [6,10,19] for which field evidence of feeding by dikes is available. It has been invoked for gabbros [ 10,111, although such rooting evidence has not been reported; we specifically ap- ply this process to the modally layered gabbros of the Oman ophiolite.
F. Boudier et al. /Earth and Planetary Science Letters 144 (1996) 239-250 249
This conclusion suggests a dual feeding of the
magma chamber, first via sills emplaced at the Moho level, contributing to the well defined layering of the lower gabbros (Fig. lc,e), and via dikes observed
both at these deeper levels, and up-section, where they fill the perched magma lens and the sheeted
dike complex. This model is consistent with earlier results [32], showing that the feeding system at the
Oman paleo-ridge was composed of sills and ridge-
parallel dikes, active during the same episode of melt injection.
Finally, our model helps to reconcile the existence of a large magma chamber, inferred from the obser-
vations in Oman and the geophysical evidence for the concurrent occurrence of melt localized at the top
of the igneous crust (perched magma lens) and at the Moho level, the two locations being separated by a
large volume of seismically attenuated rocks. The
occurrence of sills at, and immediately above, the Moho level in the Oman ophiolite is indeed consis-
tent with PmS reflection data at the EPR [36-381,
ascribed by the authors to thin melt lenses, which extend up to 25 km off the ridge axis.
Acknowledgements
Stimulating discussions with P.B. Kelemen are at the origin of this paper, which also owes much to N.
Shimizu. Based on our Oman data, the results pre- sented here belong to all members of our field
expeditions. We also acknowledge the support of the
Ministry of Petroleum and Minerals of the Sultanate
of Oman, and the financial help from the Centre
National de la Recherche Scientifique (CNRS-INSU).
[FAI
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