Journal of Volcanology and Geothermal Research 184 (2009) 351–366
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Journal of Volcanology and Geothermal Research
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s
A late-Ordovician phreatomagmatic complex in marine soft-substrate environment:
The Crozon volcanic system, Armorican Massif (France)
Martial Caroff ⁎, Muriel Vidal, Antoine Bénard, Jean-René Darboux
Université Européenne de Bretagne, Université de Brest, CNRS IUEM, UMR no. 6538 « Domaines Océaniques » et Département des Sciences de la Terre, 6 avenue Victor Le Gorgeu, CS 93837,
F-29238 Brest cedex 3, France
a r t i c l e
i n f o
Article history:
Received 9 October 2008
Accepted 4 May 2009
Available online 7 May 2009
Keywords:
volcanic breccias
phreatomagmatism
peperite
Ordovician
Armorican Massif
a b s t r a c t
The mafic lavas and the diabases of Crozon (Armorican Massif, France), belong to an anorogenic Ordovician
volcanic complex, emplaced on a rifted passive margin in North Gondwana. Magma passed through syn-volcanic
soft sedimentary substrate, which is today mostly composed of alternating sandstones and mudstones, from
Llanvirn to Ashgill in age. Field observations together with microscopic studies and geochemical analyses of
magmatic rocks lead us to propose a model of volcano formation which combines hydromagmatic processes,
peperitic intrusions, a shallow submarine tephra settling, eruption-fed turbidity currents, and a pillow lava
effusion. The Crozon outcrops can be used to reconstruct a complete cross-section from the root of the volcanic
complex to the lavas and breccias emplaced on the sea floor. The sites expose: (i) a hypabyssal breccia containing
mud chunks and coarse-grained diabase clasts with amoeboidal fine-grained magmatic material; (ii) bulbous
peperitic sills and pillow-like lobes bearing a great quantity of sediment-derived enclaves of fluidal morphology;
(iii) volcaniclastic breccias containing near-spherical magmatic clasts that resulted from the complete
fragmentation of sills in the ductile regime; (iv) a rhythmic peperitic breccia interpreted as the product of
mingling between thin lava flows and soft calcareous sediment. The Crozon volcanic form, resulting from
explosive interaction with subsurface/surface water, was probably a subaqueous collapsed tuff cone. This upper
part of the system is synchronous with an Ashgill carbonate sedimentation, which overlies an Ordovician
siliciclastic succession deposited in shelf environments.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Processes and products of magma–sediment mingling (peperite)
and maar–diatreme volcanism have been the subjects of two Special
Volumes of Journal of Volcanology and Geothermal Research, respectively Skilling et al. (2002a) and Martin et al. (2007). This illustrates
the recent renewal of most of the concepts associated with both
themes. The application of fuel–coolant interaction (FCI) theory is
appropriate for interpretation of both peperite and maar–diatreme
structures (Wohletz, 2002). Magma/wet sediment interactions
involve heat transfer over a wide range of rates from mingling to
explosive fragmentation. Fluidal peperite results from the development of a vapor-film layer at the magma–sediment interface, acting as
an insulating barrier. Explosive fragmentation occurs when the vapor
film becomes unstable. The behavior of the vapor film is mainly
controlled by the sediment characteristics, the hydrology, and the
mass interaction ratio of wet sediment and magma. Wohletz (2002)
suggested that the higher the ratio then the more unstable will be the
⁎ Corresponding author.
E-mail address: caroff@univ-brest.fr (M. Caroff).
0377-0273/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jvolgeores.2009.05.002
vapor films around the hot magmatic clasts. According to Skilling et al.
(2002b) and Templeton and Hanson (2003), fluidization is more
efficient and vapor films are more stable when magma interacts with
fine-grained, well-sorted, and loosely packed sediment. Some peperites
are inferred to record frozen FCI coarse mixing-stage and thus, the
mixing geometry recorded might be related to the processes governing
the pre-explosive coarse mingling phase of FCI explosions (Hooten and
Ort, 2002).
Inverted-cone-shaped diatremes are substructures typical of
subaerial phreatomagmatic centers. They are filled with clastic debris
derived from the surrounding rocks, juvenile clasts and subsided
blocks, and they are typically cut by intrusive magmatic bodies (Martin
et al., 2007). The volcanic forms classically associated with diatremes
in subaerial environments are maars (tuff rings). Following White
(1996), below sea level eruptions mainly result in either tuff cones
(vigorous magmatic injection, persistent interaction of magma with
sediment-laden coolant and explosivity) or pillow lavas (weak
magmatic effusion, persistent interaction of magma with pure-water
coolant and no explosivity). Although not excluded, diatreme formation in subaqueous environment has never been clearly established.
Within the early Palaeozoic sedimentary succession of the North
Gondwana margin, the Upper Ordovician of the Medio-North-Armorican
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M. Caroff et al. / Journal of Volcanology and Geothermal Research 184 (2009) 351–366
Fig. 1. Geological maps and location of the studied outcrops. (a) Simplified structural sketch of the Crozon Peninsula (Armorican Massif, France). (b) Geological map of the southern
part of the Crozon Peninsula (modified after Chauris and Plusquellec, 1975, 1979), with location of the volcanic outcrops.
Domain (Western France) has been affected by an anorogenic volcanism,
which is particularly well exposed in the Crozon Peninsula (Fig. 1). The
four main interesting aspects of the Ordovician hypabyssal/volcanic
exposures of Crozon consist in (i) the existence of several outcrops
displaying features characteristic of the ancient volcanic substructure,
such as normal faults, hydrothermalism, hypabyssal breccias and
intrusions, (ii) the occurrence of a very well-developed set of fluidal
peperites, (iii) the presence of unusually abundant sediment-derived
enclaves and clasts within sills and breccias, respectively, and (iv) the
existence of calcareous levels, located at key positions in the volcanic
succession, which offer valuable pieces of information on the deposit
environment and depth.
2. Geological context and lithostratigraphy
The Crozon Peninsula displays a remarkable Palaeozoic sedimentary
succession, from Brioverian (Upper Precambrian–Lower Palaeozoic) to
upper Devonian. It is made up of two structural domains, the North
Crozon domain locally overthrusting the South Crozon one (Darboux
and Rolet, 1979; Rolet et al., 1984; Fig. 1a). Both domains were clearly
part of the same palaeogeographic area and recorded the same
palaeontological and sedimentary events (Robardet et al., 1994; Paris
et al., 1999), except at the base and the top of the sedimentary piles
where differences exist. The northern zone was probably located less
than 30 km from the southern one during Ordovician, as shown by a
palaeotectonic reconstruction (unpublished). In the South Crozon area,
the unconformable Palaeozoic succession starts with conglomerates
and red siltstones, underlying the Armorican Sandstones (Fig. 2),
whereas in the North, this latter formation rests directly on Brioverian.
In both domains, Armorican Sandstones are overlain successively by
Postolonnec (mudstones) and Kermeur (mainly sandstones) Formations (Llanvirn to Caradoc ages, sensu Fortey et al., 1995). Volcanics
and limestones of the Rosan Formation (Ashgill) outcrop in the
South domain only, overlain by Hirnantian sandstones (Plusquellec
et al., 1999; Fig. 2). In the North area, sandstones of the Kermeur
Formation are directly overlain by Hirnantian glacio-marine deposits
(Bourahrouh, 2002). From Middle (mainly Llanvirn) to Upper (Ashgill)
Ordovician, the South Crozon sedimentary succession contains many
hypabyssal and volcanic rocks (Fig. 2): breccias and sills in the
Postolonnec Formation (mainly Llanvirn), sills and dykes in the
Kermeur one (Caradoc), and breccias, sills, and pillow lavas in the
Rosan Formation (Ashgill).
The present study focuses mainly on four sites located on Fig. 1b:
i) Kerdreux, where sills and a hypabyssal breccia crop out within the
M. Caroff et al. / Journal of Volcanology and Geothermal Research 184 (2009) 351–366
353
Fig. 2. Lithostratigraphic column of the southern part of the Crozon Peninsula, up to the end of the Ordovician, with global and British stratigraphic stages (assigned from Paris, 1990;
Bourahrouh, 2002; Webby et al., 2004). The (non-graduated) x-axis correlates schematically to grain size.
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M. Caroff et al. / Journal of Volcanology and Geothermal Research 184 (2009) 351–366
mudstones of the Postolonnec Formation; ii) Postolonnec, where sills
and normal fault-related hydrothermal breccias can be observed in
mudstones; iii) Aber (Rosan Formation), which is a locality displaying
the greatest range of volcanic breccias; iv) Lostmarc'h Headland
(Rosan Formation), for pillow lavas and a rhythmic peperitic breccia.
Additional dykes and sills from Kerdra (Kermeur Formation), Morgat
and Trégarvan (Rosan Formation) have also been analyzed for major
and trace elements.
The studied area was deformed during the Variscan orogeny, leading
to folds and cleavage development under anchizonal metamorphic
conditions (Paradis et al., 1983). Structural analysis of coastal regions as
well as regional mapping also gives evidence for some major strike–slip
and reverse faulting (Rolet et al., 1986; Fig. 1b). The magmatic sills
and the volcanic successions are either steeply dipping (Kerdreux,
Postolonnec, and Aber sites) or vertical (Lostmarc'h Headland).
The sedimentary succession enclosing the hypabyssal and volcanic
products represents unquestionably true marine accumulated deposits,
as shown by the presence of fossils of stenohaline marine fauna as
chitinozoans, trilobites, and echinoderms (Paris et al., 1999). Sedimentary structures such as shell beds, hummocky cross stratifications, and
ripples attest that the entire Ordovician succession was deposited in
shelf environments dominated by storm wave action (Paris et al., 1999;
Dabard et al., 2007). Within or in the vicinity of the volcanic succession
of Rosan, there are three calcareous bioclastic levels that have been
carefully examined. At the top of the Kermeur succession of the Aber site,
there is a condensed level formed by an oolithic iron bed overlain by a
calcareous shell bed, with mostly brachiopods and bryozoa (L1 in Fig. 3).
A few limestone beds containing a great amount of large disarticulated
shells of brachiopods lie between the volcaniclastic deposits and the
pillow lavas of the Aber site in the Rosan Formation (L2 in Fig. 3).
Hummocky cross stratifications have been identified in this part of the
section (A. Loi, pers. comm., 2009). Finally, volcanics are sealed at the
Table 1
Chemical data (wt.% oxide, ppm element) on whole rock dyke samples.
Fig. 3. Lithostratigraphic column showing the Ashgillian volcanic succession from
Crozon (Rosan Formation) at the Aber and Lostmarc'h sites. Letters along the left side
denote the facies as defined in Fig. 4. L1, L2, and L3 correspond to three calcareous
bioclastic levels, located at key positions in the volcanic succession of Rosan, which offer
pieces of information on the deposit environment and water depth (see text). Grain size
(x-axis) is schematic. Photographs illustrate some details of the succession.
Location
Morgat
Kerdreux
Kerdreux
Kerdra
Trégarvan
Sample
CNM1
CNX1
CNX3
Ka2
Ga1
Affinity
Tholeiitic
Tholeiitic
Alkalic
Tholeiitic
Transitional
SiO2
TiO2
Al2O3
Fe2Ot3
MnO
MgO
CaO
Na2O
K2O
P2O5
L.O.I.
Total
Sr
Sc
V
Cr
Co
Ni
Y
Nb
La
Ce
Nd
Sm
Eu
Gd
Dy
Er
Yb
Th
46.80
1.13
14.85
12.55
0.17
7.85
9.70
1.88
0.62
0.14
3.53
99.22
247
25
192
328
57
157
20
7.6
7.0
15.0
9.0
2.75
0.99
2.9
3.4
1.9
1.71
0.95
50.90
2.20
13.02
15.60
0.18
3.58
7.92
2.82
0.04
0.29
2.85
99.40
215
29
258
20
41
16
30
13.7
12.0
27.5
18.0
5.35
1.90
6.0
5.7
2.6
2.34
1.35
48.00
2.81
14.05
13.30
0.25
5.60
6.65
3.88
0.13
0.60
4.03
99.30
495
22
210
182
45
124
28
29.5
23.0
51.5
34.0
8.3
2.46
7.7
5.7
2.1
1.58
1.95
50.25
2.07
13.95
14.80
0.18
4.13
6.25
4.75
0.68
0.21
2.37
99.64
242
34
302
60
44
37
31
14.0
12.0
25.5
15.8
4.5
1.54
5.4
5.6
2.7
2.60
1.85
49.25
1.78
14.95
12.75
0.18
5.25
7.80
4.48
0.23
0.23
2.90
99.80
265
28
240
224
36
77
24
15.6
12.5
26.5
16.0
4.1
1.49
5.1
4.4
2.0
1.91
2.00
ICP-AES analyses (analyst: J. Cotten, Brest). Data for Kerdra and Trégarvan from Juteau
et al. (2007).
M. Caroff et al. / Journal of Volcanology and Geothermal Research 184 (2009) 351–366
Table 2
Description and interpretation of the breccias.
Table 2 (continued)
Lithotype
F
355
Lithotype
A
Hypabyssal breccia
– Structureless.
– Size of clasts: mm to dm.
– Components: angular coarse-grained magmatic clasts; sedimentary quartz-rich
argillaceous clasts; fluidal juvenile magmatic clasts; composite fluidal clasts with a
core formed by a coarse-grained magmatic/sedimentary argillaceous fragment,
surrounded by a fine-grained juvenile magmatic rim.
Interpretation: coarse mixing of magma with sediment coolant associated with local
phreatomagmatic disruption or fragmentation induced by phreatomagmatic explosions
in the root zone of the volcanic system.
Locality: Kerdreux.
Figures: 2, 4a, 5a b, 6a, 12.
B Crudely stratified volcaniclastic deposits (calcareous matrix)
– Base of the BCD succession.
– Weakly normally graded, poorly sorted 1–2 m-thick beds.
– Size of the clasts: b5 cm.
– Clasts: abundant fluidal and vesicular sediment-derived chloritic clasts; sparse
angular, plagioclase-phyric, slightly vesicular volcanic fragments.
– Matrix: calcareous bioclasts with volcanic shards and chloritic sediment-derived
micro-fragments.
Interpretation: deposits of water-supported gravity mass flows and/or result of high
concentration suspension sedimentation derived from subaqueous concentrated tephra
jets. Probable incorporation of unconsolidated fossil-rich lime mud into the erupting
vent.
Locality: Aber.
Figures: 3, 4b.
C Crudely stratified volcaniclastic deposits (silicic matrix)
– Interbedded with facies D.
– Poorly sorted 1–2 m-thick.
– Size of the clasts: b5 cm.
– Clasts: abundant fluidal and vesicular sediment-derived chloritic clasts; sparse
angular, plagioclase-phyric and slightly vesicular volcanic fragments; sparse
armored lapilli.
– Silicic bioclastic matrix with volcanic shards and chloritic sediment-derived
micro-fragments.
Interpretation: deposits of water-supported gravity mass flows and/or result of high
concentration suspension sedimentation derived from subaqueous concentrated tephra
jets. Probable incorporation of unconsolidated fossil-rich (secondary silicified) lime
mud into the erupting vent. The armored lapilli are inferred to have been formed in
subaqueous conditions from a steam envelope that developed above the vent due to
magma–water interaction.
Locality: Aber.
Figures: 3, 4c, 8a, 10, 12.
D Thin-bedded volcaniclastic deposits
– Interbedded with facies C.
– Sometimes lenticular, well-sorted thin beds (from a few millimeters to a few
centimeters in thickness).
– Abundant fluidal sediment-derived chloritic clasts and sparse angular volcanic
clasts in a mixed matrix, composed of ash, chlorite and bioclast-bearing siliceous
patches.
– Sub-facies D1: lithic fragments b 1 cm in length; planar matrix-poor beds.
– Sub-facies D2: lithic fragments b 1 mm in length; planar, wavy or convolute
matrix-rich bedding; elongated volcanic microclasts in a parallel direction with the
laminae; mm-sized load structures.
Interpretation: submarine product of dilute, lithic-poor turbulent flows or eruption-fed
turbidity currents around the periods of eruptive rest.
Locality: Aber.
Figures: 3, 4d, 6b, 8b, 10, 12.
E Non-stratified peperitic breccias
– Coarse breccias enclosing fluidal magmatic clasts.
– Matrix: Highly fluidal (sediment-derived) chloritic and magmatic fragments
surrounded by siliceous microbioclastic cement. Sparse small angular magmatic
fragments.
– Sub-facies E1: weakly graded breccia bearing fluidal magmatic and sedimentderived chloritic clasts, less than 2 cm in length.
– Sub facies E2: near-spherical magmatic clasts up to 30 cm in diameter.
Interpretation: ductile disintegration of sills during their intrusion into soft BCD-type
volcaniclastic breccias.
Locality: Aber.
Figures: 3, 4e f, 6c.
F Calcareous conglomerate
– Upon bioclastic limestone beds, laterally equivalent to pillow lavas.
– Components: plagioclase-phyric volcanic and sediment-derived chloritic rounded
pebbles, from millimeter to centimeter in diameter, associated with biogenic debris
(crinoids and brachiopods).
(continued on next page)
Calcareous conglomerate
– Matrix: exclusively formed of biogenic microdebris and recrystallized calcite.
Interpretation: post-eruption remobilization of volcaniclastic debris and sedimentation
with bioclasts.
Locality: Aber/Rosan.
Figures: 3, 4g.
G Chaotic monomict basaltic breccias
– Associated with pillow lavas.
– Structureless.
– Jigsaw-fit angular fragments or lobate clasts.
– The matrix resulted from the crushing of similar material (in local association with
bioclasts and/or recrystallized calcite).
Interpretation: fragmentation of the Lostmarc'h pillow lavas, either during their
emplacement (autobrecciation) or after flowing (talus accumulation).
Locality: Lostmarc'h.
Figures: 3, 4h, 9c.
H Rhythmic fining-up graded peperitic breccia
– Upon the Lostmarc'h pillow lavas and beneath a thick bioclastic limestone bed (L3
in Fig. 3).
– Well-developed fining-up graded sequences, from a few centimeters to a few
decimeters in thickness.
– Clasts: highly fluidal plagioclase-phyric altered volcanic clasts, from 1 mm to 2 cm
in length, with rounded, curvilinear or finger-like margins.
– Matrix: calcareous biogenic fragments (crinoids, brachiopods, and bryozoans)
cemented with recrystallized calcite.
Interpretation: ductile disintegration of basalt by mingling during flowing within wet,
unconsolidated sediment, followed by settling of the largest clasts toward the base of
each sequence.
Locality: Lostmarc'h.
Figures: 3, 4i, 6d, 9a, 11, 12.
Lostmarc'h site by a thick bioclastic limestone bed (L3 in Fig. 3), named
the Porzhig limestone by Paris et al. (1999), which has yielded
conodonts of Ashgill age (Paris et al., 1981), bryozoa and crinoids.
Taking account of the stratigraphic correlations proposed by these
authors, the succession comprised between the layers L1 and L3, which
brackets all the Crozon subaqueous volcanic activity, corresponds to less
than four millions years in duration (from the chronostratigraphic chart
proposed by Webby et al., 2004).
The majority of the magmatic rocks from Crozon have been slightly
hydrothermalized or weathered. We present in Table 1 five geochemical
analyses (major and selected trace elements) of moderately/weakly
altered sills, those with L.O.I.b 4 wt%. Most of the intrusions, lavas and
volcanic clasts from Crozon are mafic. The only intermediate compositions that have been analyzed are the Kerdra sill, which is trachybasaltic
in composition. A large amount of the clasts occurring in the
volcaniclastic deposits from Aber is vesicular, colourless, and has high
silica contents (SiO2 N 60 wt.%), suggesting that they might be rhyolitic
pumices. However, their high Ni and Cr values (75 b Ni b 121 ppm;
182 b Crb 275 ppm) are not consistent with such a hypothesis (Juteau
et al., 2007). These latter authors have interpreted the corresponding
clasts as silicified basaltic fragments. All the magmatic products from
Crozon have anorogenic affinities. They plot either in the within-plate
field or in the ocean-floor one in the Pearce and Cann (1973) diagram
(not shown). Trace element data can be used to identify alkalic and
tholeiitic rocks, the first group being enriched in Light REE with respect
to tholeiites (Table 1). The Trégarvan sample Ga1 has REE contents
characteristic of a transitional magmatism. The geographic repartition of
the alkalic versus tholeiitic rocks seems to be random.
3. Hypabyssal facies
The hypabyssal facies from Crozon include a breccia and intrusions.
The hypabyssal breccia is only observable in a narrow Kerdreux
outcrop while the intrusions, commonly sills, occur in every site.
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M. Caroff et al. / Journal of Volcanology and Geothermal Research 184 (2009) 351–366
Fig. 4. Photographs and sketches illustrating the volcaniclastic facies of Crozon. (a) Hypabyssal polymict breccia from Kerdreux (facies A). Pen for scale. (b) Drawing (from a thinplate microphotograph) of details of the crudely stratified volcaniclastic deposits from Aber with fine calcareous bioclastic matrix (facies B). Black: lava clasts; grey: sediment-derived
chloritic clasts; white: calcareous matrix. (c) Crudely stratified volcaniclastic deposits from Aber with silicic matrix (facies C). 20 cm-long rule for scale. (d) Thin-bedded
volcaniclastic deposits from Aber (facies D). One graduation for 1 mm. Inset: drawing (from a thin-plate microphotograph) of the contact between sub-facies D1 (lithic fragments
b 1 cm in length; planar matrix-poor beds) and D2 (lithic fragments b 1 mm in length; planar, wavy or convolute matrix-rich beds; elongated volcanic microclasts in a parallel
direction with the laminae; mm-sized load structures). Black: lava clasts; dark grey: sediment-derived chloritic clasts; pale grey: fine composite matrix; white: bioclastic matrix.
(e) Weakly graded peperitic breccia from Aber bearing fluidal (sediment-derived) chloritic and volcanic clasts less than 2 cm in length within a bioclastic matrix (sub-facies E1).
20 cm-long rule for scale. (f) Non-stratified peperitic breccias from Aber with near-spherical magmatic clasts up to 30 cm in diameter (sub-facies E2). Matrix corresponds to the subfacies E1. Pen for scale. Inset: drawing (from a thin-plate microphotograph) of the contact between a near-spherical magmatic clast and the composite E1-type matrix. Black:
magmatic clasts; grey: chloritoid-rich sediment-derived chloritic clasts; white: bioclastic matrix. (g) Drawing (from a thin-plate microphotograph) of details of the Aber/Rosan
calcareous conglomerate, bearing centimeter-sized volcanic and vesicular sediment-derived chloritic rounded pebbles, associated with crinoid and brachiopod debris (facies F).
Black: lava pebbles; grey: sediment-derived chloritic pebbles; white: calcareous bioclasts. Inset: microphotograph of an area rich in bioclasts. The long chips are brachiopod
fragments. (h) Pillow-lava-derived chaotic monomict breccia from Lostmarc'h (facies G). 20 cm-long rule for scale. (i) Peperitic breccia from Lostmarc'h exhibiting repeated fining-up
graded sequences, and comprising highly fluidal plagioclase-phyric volcanic clasts within a bioclastic calcareous matrix (facies H). Scale is graduated in millimeters.
M. Caroff et al. / Journal of Volcanology and Geothermal Research 184 (2009) 351–366
357
Fig. 5. Examples of sedimentary incorporation within magmatic breccias or bodies. (a) Ancient mud chunks within the hypabyssal breccia from Kerdreux (Facies A of Fig. 4a). Pen for
scale. (b) Fine-grained magmatic fluidal clasts having a core formed by a coarse-grained magmatic fragment or a sedimentary argillaceous one. (c) Sediment invading cooling cracks
of a sill from the Postolonnec site. Pen for scale. (d) Isolated amoeboid pillow-like lobe from the Postolonnec site, including fluidal sedimentary enclaves. Pen for scale. (e) Pipe
vesicles and vesicle veins at the base of a sill in the Kermeur Formation, Aber site. Hammer for scale. (f) Fluidal sediment-derived chloritic enclaves within a sill from Aber. Pen for
scale.
3.1. Hypabyssal breccia
At the Kerdreux site, the lower part of the Postolonnec Formation
(Fig. 2) is cut by several basaltic intrusions (Fig. 1b). Between a large
coarse-grained sill and the Postolonnec mudstones, there is a complex
brecciated zone, ca. 10 m in extent, including a range of angular or
rounded polylithologic clasts of various sizes (from millimeter to
decametre). The present dipping of the sharp breccia/diabase contact
(ca. 70° SE) suggests it was gently sloping when the volcanic system was
active. The breccia/mudstone contact is not visible. Recognized as Facies
A in Table 2 and Fig. 4a, this polymict breccia is chaotic (Fig. 2). It includes
angular coarse-grained magmatic fragments, mm-to-dm in size, having
a texture close to that of the adjacent sill. These components probably
result from the fragmentation of the diabase. The second group of
components is constituted by quartz-rich argillaceous clasts. They are
present in great quantity everywhere in the breccia, sometimes
prevailing. The largest ones are angular, but most of them, less than
5 cm long, are typically rounded and elongated. The smallest clasts
display a fluidal morphology, with lobate margins (Fig. 5a), and can be
interpreted as ancient mud chunks (in the sense of White,1991; Németh
and Martin, 2007). A very few argillaceous clasts contain andalusite (Fig.
6a). The third component of the breccia consists of crystallite-bearing
cryptocrystalline/fine-grained magmatic fluidal clasts, basaltic in
composition. Their microtexture is similar to that of adjacent small
lobate igneous bodies, a few meters in extent. These fluidal magmatic
components are contemporaneous with the breccia formation. A few
fine-grained magmatic fluidal clasts have a core formed by a coarsegrained magmatic fragment or a sedimentary argillaceous one (Fig. 5b).
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M. Caroff et al. / Journal of Volcanology and Geothermal Research 184 (2009) 351–366
Fig. 6. Photomicrographs of Crozon breccia lithofacies. (a) Andalusite within an argillaceous clast in the hypabyssal breccia from Kerdreux (facies A of Fig. 4a). (b) Detail of a sample of
the thin-bedded volcaniclastic breccia from Aber (sub-facies D1 of Fig. 4d). (c) Detail of a sample of the weakly graded peperitic breccia from Aber (sub-facies E1 of Fig. 4e). Note the
presence of chloritoid within the chloritic clasts. (d) Plagioclase-bearing fluidal volcanic clast within the peperitic breccia from Lostmarc'h (Facies H of Fig. 4i).
Matrix of the Kerdreux breccia is an intimated mix of the three
macroscopic components, i.e. it is formed by crushed coarse-grained
diabase mixed with fluidal argillaceous bits and amoeboidal finegrained magmatic material.
3.2. Diabases
Dykes and especially sills can be observed in many places (Fig. 1b)
intruding through various lithologic formations (Fig. 2). They intruded
both the basement of the ancient volcano(es) (Postolonnec and
Kermeur Formations) and the volcaniclastic deposits themselves
(Rosan Formation). The largest intrusions form the Kerdreux and
Kerdra headlands, in the Postolonnec and Kermeur Formations,
respectively (Fig. 1b). The first one is bordered with the A-type
breccia (see earlier in the text). Both are coarse-grained sills, from
30 m to 100 m in thickness, intruded by dykes and cut by hercynian
E–W-trending strike–slip faults. Fracturing is especially severe near
the contact with the A-type breccia.
Fig. 7. Photomontage showing a lobate peperitic sill intruded into the C/D volcaniclastic lithofacies (Rosan Formation, Aber). Length of the rule: 80 cm. Inset: pillow-like lobes at a sill
tip.
M. Caroff et al. / Journal of Volcanology and Geothermal Research 184 (2009) 351–366
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Fig. 8. Details of the volcaniclastic breccias from Aber. (a) Armored lapilli in the C-type breccia, formed by lapilli-sized volcanic rock fragments concentrically coated by ancient
bioclastic mud mixed with ash-sized glass shards and sediment-derived chloritic clasts. (b) Thin plane-parallel beds of facies D. Sparse scoriaceous blocks form overlying wave
structures in the stratification (sub-facies D1 of Fig. 4d). The beds are planar, wavy laminated or slightly convolute in sub-facies D2 (Fig. 4d).
In the Postolonnec cliffs, there are a number of sills of smaller
thickness, intruded into the Postolonnec Formation. Their margins
tend to have a fluidal geometry on any scale (Fig. 2). Cracks
perpendicular to their margins are sometimes invaded by sediment
(Fig. 5c). Some isolated amoeboid basaltic bodies, including fluidal
sedimentary enclaves, can also be observed (Fig. 5d). Some fractures,
cutting the cliffs, are filled-up either with crushed diabase or with
quartz-bearing hydrothermal breccia, enclosing sedimentary and
igneous pieces. The fractures are clearly pre-folding normal faults,
which predate the Hercynian tectonic events. They were used as
conduits by magmatic intrusions.
At the Aber site, we observe the passage from the Kermeur to the
Rosan Formations. Both are intruded by fluidal sills (Fig. 2). An
example of fluidal sill intruded into the Rosan volcaniclastic succession is shown as a photomontage in Fig. 7. Some of the sills change into
pillow-like lobes at their tips (Fig. 7 inset). Pipe vesicles are
sometimes visible close to both margins of the sills in the Kermeur
Formation (Fig. 5e). Vesicle veins have been observed above the pipe
vesicles at the basal margin of one sill (Fig. 5e). All the sills contain lots
of fluidal sediment-derived chloritic enclaves, generally less than 2 cm
in length (Fig. 5f).
4. Volcanic breccias and pillow lavas
The volcanic breccias from Crozon are diversified. They can be
divided into two groups according to the presence or not of peperitic
features. The non-peperitic group can be observed only at the Aber
site whereas the peperitic one occurs both at the Aber area, above the
non-peperitic breccia succession, and at the Lostmarc'h site, just over
pillow lavas (Fig. 3). All the breccia lithofacies are presented in Table 2,
where listed are their codes, descriptions, interpretations, and
reference figures or photos.
4.1. Non-peperitic volcanic breccias
4.1.1. Stratified polymict volcaniclastic deposits (BCD lithofacies association)
This lithofacies association is exposed at the Aber site and
comprises Facies B, C, and D (Fig. 3). Facies B, occurring just above
the limestone bed L1, forms the base of the succession and facies C is
interbedded with facies D (Fig. 3). Facies B consists of block-rich
massive units, less than 2 m thick. In thin section, the rocks of facies B
contain abundant fluidal and vesicular sediment-derived chloritic
clasts together with a few angular, plagioclase-phyric and slightly
vesicular volcanic fragments in a fine calcareous bioclastic matrix
(Fig. 4b). Facies C breccia corresponds to thick (1–2 m), internally
massive or weakly normally graded, poorly sorted beds (Fig. 4c). It
contains predominantly fluidal sediment-derived chloritic clasts
together with minor angular, vesicular to scoriaceous volcanic clasts
in a siliceous matrix. We note also the occurrence of sparse armored
lapilli, formed by lapilli-sized (more rarely block-sized) volcanic rock
fragments concentrically coated by ancient bioclastic mud mixed with
ash-sized glass shards and sediment-derived chloritic clasts (Fig. 8a).
The matrix of the C-type breccia might correspond to an ancient
limestone, secondarily silicified, as suggested by some textural
similarities between facies B and C (comparable clasts/matrix
proportions; similar bioclastic fragments of identical size in both
matrix). Facies C resembles (1) the lapilli breccia described by McPhie
(1995) in a Pliocene shoaling basaltic seamount from Fiji (stratified;
thick; internally massive; moderately sorted; but matrix very minor
with respect to the Crozon facies C); (2) the facies A of Rinaldi and
Venuti (2003), defined for a 1.7 Ma eruption of the Bombarda volcano,
Cyclades, Greece (massive to normally graded pumice and lithic
breccia, even though pumices are very unusual in Crozon); and (3) the
facies LT 2 of Martin et al. (2004), from the Cretaceous MIT Guyot,
West Pacific (poorly sorted, structureless matrix-rich layers with
volcanic and limestone clasts). The three facies were interpreted as
proximal/medial (re)deposition facies of eruptive debris in a shallow
submarine setting. Facies D consists of thin (from a few millimeters to
a few centimeters in thickness) plane-parallel beds, with rare local
cross-beddings. Sparse scoriaceous small blocks form overlying wave
structures in the stratification (Fig. 8b). Facies D can be divided into
two sub-facies. Sub-facies D1 (resembling facies B of Rinaldi and
Venuti, 2003, but without erosive scours, and facies LT1 of Martin et
al., 2004) consists of millimeter-sized abundant fluidal chloritic and
sparse angular volcanic clasts in a mixed matrix, composed of ash,
chlorite and bioclast-bearing siliceous patches (Figs. 4d, 5b). Subfacies D2, resembling facies C of Rinaldi and Venuti (2003), is
characterized by similar components in same proportions, but the
lithic fragments are less than 1 mm in length. The beds are planar,
wavy laminated or slightly convolute (Fig. 8b). Beds with internally
discontinuous lenses are common, but cross bedding is an unusual
feature. The elongated chloritic microclasts are in a parallel direction
with the laminae (Fig. 4d). In the sample shown in Fig. 4d, there is a
one-millimeter-wide load structure between each D2-type bed and its
overlying D1-type bed.
4.1.2. Calcareous conglomerate
Above the BCD lithofacies association, the Aber seashore succession is continued with coarse peperitic breccias (facies E of Fig. 4e, f;
see Section 4.2.1.), then with conglomerate (facies F, Fig. 4g), which is
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exposed at the top of the Aber breccia pile. In a neighboring quarry,
the conglomerate rests upon the bioclastic limestone beds L2 and it is
laterally equivalent to a small number of pillow lavas (Fig. 3). This
lithofacies F consists of volcanic and chloritic rounded pebbles, from
millimeter to centimeter in length or diameter, associated with
biogenic debris. Bioclasts are fragments of crinoids and brachiopods.
The volcanic clasts are plagioclase-phyric and the chloritic ones,
probably derived from semi-consolidated mudstones, are generally
vesicular. A few of these latter clasts also contain chloritoids (see later
in the text) that attests to a greenschist metamorphism of the
sedimentary rocks at their origin. The matrix is exclusively formed of
biogenic microdebris and recrystallized calcite. From brachiopods,
Mélou (1990) assigned this part of the Rosan Formation, i.e. his level
16 above pillows in the same quarry, to Ashgill age, i.e. Cautleyan–
Rawtheyan or upper Katian in the new international stratigraphic
scale (Webby et al., 2004).
4.2. Peperitic volcanic breccias
Peperitic breccias are generated by mingling and fragmentation of
magma in contact with typically wet sediment (White et al., 2000;
Corsaro and Mazzoleni, 2002; Skilling et al., 2002b and references
herein; Galerne et al., 2006). Fluidal peperite is characterized by the
presence of lobate surfaces in the juvenile clasts, at any scale. Such
lobate or fluidal clasts form through coarse mixing during fuel–
coolant-interactions (FCIs), involving contact between a sedimentladen vaporizable liquid (impure coolant) and magma (fuel) (White,
1996; Hooten and Ort, 2002). Insulation, due to the persistence of a
stable vapor film between melt and water-rich sediment, allows the
magma to develop complex globular forms without, at least initially,
undergoing brittle fragmentation (White, 1996; Martin and Németh,
2007). Thus, the lobate sills described in Section 3.2. (as, to a lesser
extent, the hypabyssal breccia of Kerdreux) can also be termed fluidal
peperites. Peperitic breccias can contain blocky and/or fluidal juvenile
clasts, the proportion of which is a function of several factors, such as
the viscosity conditions during the cooling, the extent of wet
sediment–magma mingling (Galerne et al., 2006), or the strain rate
during explosions (higher rates will favor brittle clasts: Skilling, pers.
comm., 2009).
In Crozon, the peperitic volcanic breccias divide into two main
groups: chaotic peperitic breccias, containing near-spherical magmatic clasts up to 30 cm in diameter (Aber), and a sequenced and
fining-up graded peperitic breccia, with volcanic clasts less than 2 cm
in length (Lostmarc'h).
4.2.1. Non-stratified peperitic breccias
Above the BCD lithofacies association and below the F-type
conglomerate, the Aber seashore displays a succession of coarse
breccias enclosing fluidal magmatic clasts, termed facies E (Fig. 3).
Sub-facies E1 consists of locally weakly graded breccia bearing fluidal
magmatic and sediment-derived chloritic clasts, less than 2 cm in
length (Fig. 4e). Only a few small magmatic clasts are angular. Subfacies E2 is characterized by the presence of chloritic enclave-bearing
near-spherical magmatic clasts up to 30 cm in diameter (Fig. 4f). The
E2-type breccia is generally chaotic at the scale of the outcrops. One
cross-section displays the passage from the E1- to the E2-type breccia.
The matrix of both sub-facies is comparable. Highly fluidal chloritic
fragments and fluidal/angular magmatic ones are surrounded by
Fig. 9. Photographs of volcanic and peperitic features from the Lostmarc'h Headland (Rosan Formation). (a) Contact between the peperitic graded breccia (facies H of Fig. 4i) and
the overlying L3 limestone (Fig. 3). Limestone is found as enclaves within the breccia. (b) Basaltic pillow lavas with interstitial bioclastic limestone (Fig. 3). (c) Sill intruded into
the G-type monomict breccia (Figs. 3 and 4h). (d) Calcitic plume in a lava block, issued from a thin carbonated level (see text for explanation).
M. Caroff et al. / Journal of Volcanology and Geothermal Research 184 (2009) 351–366
siliceous microbioclastic cement (Fig. 6c). Fragments of crinoids and
bryozoa have also been recognized. Both fluidal chloritic and
magmatic clasts enclose bioclastic fragments within their amoeboidal
convolutions (Figs. 4f, 6c). Magmatic fragments are plagioclasephyric. Most of the chloritic clasts are chloritoid rich (Figs. 4f, 6c), a
feature characteristic of facies E and F (Fig. 3). The borders of the
chloritic clasts are generally diffuse, as the chloritic material tends to
mix with the bioclastic cement to produce a composite matrix.
4.2.2. Rhythmic fining-up graded peperitic breccia
The Lostmarc'h Headland is mainly formed by pillow lavas and
derived monomict breccia (facies G of Fig. 4h) within a calcareous
matrix (see the next section). At the top of the Lostmarc'h volcanic
succession, we observe an 8 m-thick rhythmic succession of peperitic
breccia sequences exhibiting well-developed, mainly closed-packed,
fining-up graded bedding (facies H of Fig. 4i). Each graded sequence is
made up of highly fluidal plagioclase-phyric magmatic clasts, from
1 mm to 2 cm in length, within a bioclastic calcareous matrix. The
clasts are systematically altered to chlorite. Most of them display
rounded, curvilinear or finger-like margins (Fig. 6d). Sparse fragments
also have angular morphologies. Each graded sequence has a thickness varying from a few centimeters to a few decimeters. The calcareous matrix contains biogenic fragments (crinoids, brachiopods,
and bryozoans) cemented with recrystallized calcite. The graded
breccia sequences are topped by the L3 bioclastic limestone bed
(Fig. 3), the apparent thickness of which (ca. 5 m) is a minimum value
because the Ordovician succession is here interrupted by a thrust-fault
(Fig. 1b). The contact between the breccia and the overlying limestone
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is irregular, contorted. Limestone can be found as enclaves within the
breccia (Fig. 9a), and vice-versa.
4.3. Pillow lavas and associated chaotic monomict breccia
Basaltic pillow lavas occur in the Lostmarc'h Headland beneath the
rhythmic peperitic breccia (facies H), with a bioclastic calcareous
matrix displaying petrographical and palaeontogical features close to
those of the L3 limestone bed that overlies the H-type breccia (Figs. 3
and 9b). The pillow lavas are locally entirely fragmented to form a
chaotic monomict breccia (facies G: Fig. 4h; Fig. 9c). Some lobate or
pillowed sills have intruded the G-type breccia (Fig. 9c). The
monomict breccia shows no structure and is composed of both
jigsaw-fit angular and arched fragments (Fig. 4h). Fig. 9d displays a
lava block enclosing a calcitic plume issued from a thin carbonated
level. Such a feature might denote locally intense carbonate
vaporization as a result of the lava emplacement onto wet unconsolidated subaqueous carbonated sediment bed.
5. Magma–sediment mingling processes
To discuss the formation of the peperites described in Sections 3
and 4.2, we distribute them within two groups: (1) intrusion-related
peperites, i.e. peperites formed either at the contact between sills/
dykes and sediment or by ductile disintegration of sills/dykes;
(2) effusion-related peperites, i.e. peperites formed by disintegration
of hot pillow lavas/lava flows. This subdivision is transversal with
respect to the previous one used to describe the facies (hypabyssal
versus volcanic products).
5.1. Formation of the intrusion-related peperites
Fig. 10. Model of sill intrusion into unconsolidated wet volcaniclastic deposits of
repeated C/D facies type, followed by its progressive disintegration into pillow-like
clasts. The near-spherical magmatic clasts of the E2-type non-stratified peperitic
breccia from Aber could have been formed through such a mechanism. The smaller
peperitic clasts observed in the E1-type breccia might also result from a comparable
mechanism, but with a higher degree of disintegration, perhaps associated with local
phreatomagmatic disruption along the intrusive bodies (not shown). Modified after
Martin and Németh (2007).
5.1.1. Hypabyssal breccia
The Postolonnec cliffs expose normal faults which were used by
magma to upraise before setting as sills. The Kerdreux breccia, typically
structureless, lies at the contact between diabase and Postolonnec
mudstones (see Section 3.1). The breccia comprises angular coarsegrained diabase fragments and blocky/fluidal sediment-derived clasts,
both locally enclosed by coherent fine-grained juvenile magma,
sometimes fragmented into spherical clasts (Fig. 5b). The presence of
siliceous nodules and quartz grains, together with distinctive fossils
and typical sedimentary features, makes evident that the greatest part
of the mud clasts came from the immediately surrounding Postolonnec
Formation. The fluidal-shaped morphology of a number of them attests
that the mud was semi- or unconsolidated. However, some sedimentary fragments have obviously another origin. The occurrence of
andalusite within a few argillaceous clasts (Fig. 6a), unknown in the
regional Lower Palaezoic succession, strongly suggests the incorporation into the breccia of components derived from deeper levels, and
probably of Brioverian age.
The narrowness of the outcrop area, the fact that the breccia/
mudstone contact is masked, which make impossible any characterization of the possible steepness of the contact — a key to infer or not
the existence of a diatreme breccia (Ross et al., 2005; Auer et al., 2007;
Lorenz and Kurszlaukis, 2007) — and the lack of other occurrences of
such a facies in Crozon make unclear the significance of the breccia.
We interpret the lack of organization of the breccia and the presence
of both blocky and fluidal country-rock components together with
juvenile clasts as a consequence of either (i) a pre-detonation coarse
mixing of magma with sediment coolant (“frozen” FCI) associated
with local phreatomagmatic disruption or (ii) a fragmentation
induced by more general phreatomagmatic explosions in the root
zone of the volcanic system (White, 1996).
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5.1.2. Sills and detached pillow-like lobes
Sills intruding the Postolonnec and Kermeur sedimentary Formations or the overlying Rosan volcaniclastic succession display
undulating lobate contacts (Fig. 7). They are generally associated
with pillow-like lobes, still attached or not to their parent sill,
exhibiting intimate sediment–magma mingling features (Figs. 5d, 7
inset). Such peperitic morphologies indicate an emplacement into a
soft substrate (Skilling et al., 2002b). This implies that the oldest
observed formation intruded by peperitic sills (i.e. the Postolonnec
one dated as Llanvirn age) was incompletely lithified during the
Ashgillian volcanism activity, about 15 Ma later (using the Ordovician
timescale calibration proposed by Sadler and Cooper, 2004).
The pods or the balls characteristic of the E2-type breccia have
near-spherical forms similar to the pillow-like lobes, and both contain
numerous highly fluidal chloritic enclaves. The formation of both
structures is thus likely comparable. They probably result from ductile
disintegration of the coherent magma during its intrusion either into
fine sediment (Martin and Németh, 2007) or into unconsolidated
volcaniclastic breccia, rich in fine matrix. Such intrusions of peperitic
pods into submarine polymict volcaniclastic breccia have also been
proposed by Templeton and Hanson (2003) for Jurassic arc-apron
metavolcanic deposits from Sierra Nevada in northern California.
A model of sill intrusion into soft BCD-type volcaniclastic breccias,
followed by its progressive disintegration into pillow-like clasts, is
depicted in Fig. 10. The host volcaniclastic material is remobilized and
a fluidized halo forms around the sills. Fluidal magmatic fragments
detach from the main body and mingle with the BCD-type breccias.
The E2-type near-spherical clasts would correspond to a complete
disintegration of the initial sills/dykes whereas the pillow-likes lobes
observed at the tips of lobate sills (Fig. 7 inset) would reflect the
initiation of the process, as shown in Fig. 10. The smaller peperitic
clasts observed in the E1-type breccia would form similarly, but with a
higher degree of disintegration, perhaps associated with local
phreatomagmatic disruption (Martin and Németh, 2007).
Almost all the sills observed in Crozon and all the E2-type nearspherical clasts contain chloritic enclaves (Fig. 5f). These enclaves are
fluidal and exhibit morphologies very similar to those of the fluidal
mud clasts identified within the Kerdreux hypabyssal breccia. Some of
them also resemble the inclusions of fluidal sandy matrix in igneous
clasts described by Petry et al. (2007) in the Lower Cretaceous Paraná
Basin peperites, southern Brazil (their Fig. 7). The presence of such
fluidal enclaves cannot be easily explained. Their great amount (they
locally represent more than 30 vol.% in some diabases) and their
occurrence in the central part of the largest intrusive bodies (N15 m
thick) exclude incorporation of surrounding sediment, except near the
contact. The possibility of introducing a great quantity of soft
surrounding sediment into a large intrusion by passive incorporation
during its progression should be rapidly limited by thermal and
mechanical constraints (e.g., Bohrson and Spera, 2001; Costa et al.,
2007; Petry et al., 2007). Similar fluidal sediment-derived fragments
also occur within the matrix of BCDE-type breccias, interpreted as
tephra (re)deposited into shallow submarine sediment. Subsurface
thermohydraulic explosions, which fragment the semi- or unconsolidated country sediment, then incorporation of the resulting clasts
first into explosive breccias, then into subsequent intrusive magmas
passing through the brecciated zone might satisfactorily account for
all the observed features (Auer et al., 2007). Most of the chloritic
enclaves/clasts correspond probably to ancient argillites. This strongly
suggests that the greatest part of them originates from the Kermeur
and Postolonnec Formations, which are intruded by dykes and sills
and directly underlie the submarine volcaniclastic deposits (Rosan
Formation, Fig. 2). The chloritoid-bearing sediment-derived clasts
typical of the E-type breccias might be interpreted as fragments from
deeper Brioverian levels.
Fig. 11. (a) Model of lava flow disintegration during emplacement into soft sediments. Modified after Brown and Bell (2007). The Lostmarc'h H-type graded peperitic breccia
sequences are thought to have resulted from such a process. (b) Lava block isolated within limestone, with a train of small clasts behind. This feature is interpreted as the consequence
of the sinking of a large volcanic fluidal fragment into the lime mud after in situ brecciation of the lava flow.
M. Caroff et al. / Journal of Volcanology and Geothermal Research 184 (2009) 351–366
5.2. Formation of the effusion-related peperites
The pillow lavas from Lostmarc'h, together with the associated Gtype monomict breccia, are overlain by the H-type graded peperitic
breccia sequences (Fig. 3). The presence in this latter facies of
numerous fluidal-shaped basaltic clasts, showing delicate finger-like
structures (Fig. 6d), and the lack of cross-stratification definitely
excludes any epiclastic remobilisation or pyroclastic phenomenon.
Streamflow mechanisms, such as high bulk density mass flows, which
may not have evidence of traction structures, could transport delicate
clasts without damage (e.g. Allen and McPhie, 2001). However, such
mechanisms would be unable to produce neither thin ordered
sequences nor sinking structures as shown in Fig. 11 and discussed
below in this section.
Waichel et al. (2007) have proposed a model in which the
interaction between lavas flows and wet fine-grained sediments
generates grossly graded fluidal peperite breccias. Below a massive
lava flow, the mingling process forms non graded, closed-packed
peperite close to the contact, then dispersed peperite toward
the interior of the sediment layer. Such a model is nevertheless
unable to explain the formation of the Lostmarc'h peperitic breccia,
because 1) dispersed peperites are rare and 2) the structure of the
Lostmarc'h breccia sequences, graded and rhythmic, is much betterordered than that of the central Paraná Flood Basalt peperites
(Waichel et al., 2007).
The model proposed by Brown and Bell (2007) for the graded
peperitic breccia from the Palaeogene Carraig Mhór Bed (CMB),
Scotland, better fits with our own observations. These authors
explain the formation of the ~ 10 m thick stratiform, graded CMB
through disintegration of basalt by mingling during flowing within
wet, unconsolidated sediment, followed by settling of the largest
clasts toward the base of each unit. Following Brown and Bell
(2007), the vertical variation in clast morphology and size within
the CMB developed as a consequence of magma being emplaced
onto wet sediment, producing a sub-horizontal alignment of clasts.
The magma behaved firstly in a ductile fashion, fragmented as large,
fluidal-shaped clasts that sank through sediment. As heat began
to dissipate from the system, magma–sediment interaction
decreased and local collapses of the vapor film at the melt–sediment
interfaces caused small explosions, quench-fracturing the lava into
sub-angular clasts (Fig. 11a). In the Brown and Bell (2007) model,
contrary to the Waichel et al. (2007) one, the causative flow is
essentially consumed during the interaction. As shown in Fig. 11,
such a model can be used to explain the formation of the Lostmarc'h
peperitic breccia, with one notable difference: the small thickness of
the graded sequences (a few centimeters/decimeters) is not
consistent with disruption of thick lava flows. Entire brecciation of
thin sheets of fluid lava, less than one meter thick, alike — but more
pillowed — than the continental pahoehoe sheet flows described by
Hon et al. (1994), could more satisfactorily account for the
observations. The presence of a lava block, 30 cm in diameter,
isolated within limestone, with a train of small clasts dragged
behind (Fig. 11b), is consistent with the hypothesis of sinking of the
largest volcanic fragments into the lime mud after in situ brecciation. The contact between the pillow lavas (plus G-type monomict
breccias) and the overlying rhythmic peperites is undulating and
irregular. The passage from the first facies to the second one might
correspond to a decline in the magmatic effusion rate. When the
magma budget was high with respect to the sediment production,
pillow lavas formed. When the magma budget decreased, thin
pillowed lava sheets flowed onto the sediment surface separately
from each other. They invaded the sediment, mingled with it, up to
be entirely disintegrated. At the top of the Lostmarc'h section,
the contact between the peperitic breccia and the Porzhig limestone
(L3) is lobate and contorted too. This feature can be explained
through peperitic contacts in a rich water environment setting. It
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does not imply any erosion period, with induration followed by
karstification, as previously suggested by Paris et al. (1981). The
Ashgill Porzhig limestone, quasi devoid of volcanic fragments, dates
the end of the volcanic activity.
6. Volcano structure and palaeoenvironment
The Crozon outcrops are interpreted as a cross-section from the
root to the sea floor through a fairly unusual soft-substrate volcanic
system. The root zone displays polymict breccias in close proximity to
intrusive rocks, normal faults, hydrothermal breccias, feeder dykes,
and sills. The prevalence of fluidal sedimentary clasts within the
hypabyssal breccia, in association with a few angular diabase
fragments and juvenile magmatic clasts, is consistent with a
phreatomagmatic process, involving water-rich unconsolidated sediment (White, 1996; Martin et al., 2007). The fluidal chloritic fragments
occurring in the matrix of the volcaniclastic deposits, within the
greatest part of the shallow sills and also within the sill-derived
pillow-like lobes can be interpreted as mud chunks split up by
thermohydraulic explosions within the wet unconsolidated sediment.
The occurrence of andalusite- and chloritoid-bearing argillaceous
clasts in the A-, E-, and F-type breccias highly suggests that the
corresponding fragments are issued from the deep pre-hercynian
metamorphic substratum, probably Brioverian in age. Relative to the
syn-eruptive surface, these clasts came from a depth greater than
1500 m (Fig. 2).
The structural characteristics of the Crozon volcanic deposits on
the palaeo-surface are not typical of those encountered in subaerial
tuff rings (surge-dominated eruptions) or subaerial tuff cones (fallout-dominated eruptions). The volcaniclastics do not exhibit features
such as impact structures, U-shaped channels or dune/antidune
beddings (Auer et al., 2007). Lack of impact sags is a good evidence
of subaqueous emplacement, though there is some depth of very
shallow water in which sags could still form (e.g. Sohn and Park,
2005). Likewise, no indication of emergent surtseyan eruption has
been found in Crozon: alternation of cross-stratified beds with
discontinuous laminae, indicative of pyroclastic surges, and/or regular
beds of even thickness with continuous internal stratification,
suggestive of subaerial tephra fallout (White and Houghton, 2000;
Cole et al., 2001).
The presence of fossiliferous marine beds throughout the stratigraphic succession of Crozon is indicative of submarine environment.
Although water depth of the depositional setting is not easy to
constrain, the three calcareous levels L1, L2, and L3 (Fig. 3), located at
key positions in the volcanic succession of Rosan, offer valuable pieces
of information on the deposit environment and water depth. At the
base of the Aber volcanic succession, the oolithic (non reworked) iron
bed (base of the L1 level) is a condensed layer, probably quite deep at
the end of the condensation. The overlying calcareous shell bed
(summit of the L1 level) gives evidence of the development of a shelf
benthic fauna, with the first significative occurrence of bryozoa. The
L2 limestone beds are located between the E-type peperitic breccias
and the F-type conglomerate at the Aber site. They contain a great
amount of non-eroded, non-fragmented but disarticulated large
brachiopod shells. This taphonomic grade is usually related to the
upper offshore, between the fair weather wave base and the storm
wave base (Botquelen et al., 2006). The hummocky cross stratifications identified a few meters below in the Aber section are
characteristic of the proximal upper offshore (30–60 m). Finally, in
the thick bioclastic L3 limestone bed which seals the effusive activity
at Lostmarc'h, the association of bryozoa and crinoids suggests a
deepening of the environment with respect to that characterized by
brachiopod assemblages, as the L2 level or comparable Moroccan
occurrences (Alvaro et al., 2007).
The main non-peperitic volcanic breccias from Crozon (the BCD
lithofacies association and the material surrounding the peperitic
364
M. Caroff et al. / Journal of Volcanology and Geothermal Research 184 (2009) 351–366
Fig. 12. Schematic representation of the evolution of the volcanic system of Crozon.
(a) Eruption, transport and depositional mechanisms thought to have formed the A-type
hypabyssal breccia (Kerdreux), the groundlevel volcaniclastic succession (BCD lithofacies
association of Aber) and the intrusive peperites (E-type breccias and intrusions from
Postolonnec/Aber). Dykes and sills intruded the surrounding soft substrate and the
volcaniclastic deposits after they had incorporated sedimentary bits in the course of their
progression. The morphology of the peperitic intrusions at different palaeo-depths
suggests that the substrate was less hydrated and more cemented deep down, but not
completely lithified. The volcano(es) had probably the structure of one (or several)
subaqueous tuff cone(s). If subaqueous eruption within a soft-sediment environment is
unlikely to produce a highly positive constructive volcanic form (Belousov and Belousova,
2001; Martin et al., 2004), the lack of curved contacts and truncated beds and the absence
of abrupt changes in dip inclination within the Crozon volcaniclastic succession do not
allow proposing a negative form either. (b) Volcano section just after the end of the activity.
Pillow lavas, associated F- (not shown) and G-type breccias and H-type effusive peperitic
breccia postdate the phreatomagmatic events. Depths have been estimated from
sedimentological and palaeontological/taphonomic observations in the L1, L2, and L3
limestone beds (Fig. 3).
clasts in the lithofacies E) display features consistent with submarine
deposition following phreatomagmatic eruptions, as the examples of
Greek rhyolitic volcanoes described by Allen and McPhie (2000) and
Rinaldi and Venuti (2003) or the example of the shallow-marine MIT
Guyot (Martin et al., 2004): interbedding of weakly normally graded
lapilli beds (facies B/C) with sequences of plane-parallel ash-rich
laminae (facies D) and presence of fossiliferous sediment within the
breccias. The abundance of highly vesicular clasts in the BCD
succession is consistent with the hypothesis of emplacement at a
relatively low depth. We interpret the facies B/C either as the deposits
of water-supported gravity mass flows derived from the collapse of
subaqueous primary volcanic piles (Rinaldi and Venuti, 2003) and/or
as the result of high concentration suspension sedimentation derived
from subaqueous concentrated tephra jets (Chough and Sohn, 1990;
Martin et al., 2004). The C-type breccias containing armored lapilli are
inferred to have been formed in subaqueous conditions from a steam
envelope that developed above the vent due to magma–water
interaction (White and Houghton, 2000; Martin et al., 2004). Facies
D could be the product of dilute flows or turbidity currents around the
periods of eruptive rest (Martin et al., 2004). Occasionally, bloc
settlings perturbed the beddings.
The Crozon volcano(es) resulting from explosive interaction with
subsurface and surface water had probably the structure of one (or
several) subaqueous tuff cone(s) (vigorous magmatic injection,
persistent interaction of magma with sediment-laden coolant,
explosivity, shallow surface water, infrequent surges: White, 1996;
Sohn and Park, 2005). If subaqueous eruption within a soft-sediment
environment is unlikely to produce a highly positive constructive
volcanic form, as suggested by Belousov and Belousova (2001), the
lack of curved contacts and truncated beds and the absence of abrupt
changes in dip inclination within the Crozon volcaniclastic succession
do not allow proposing a negative form either (crater), as Martin et al.
(2004) do for the MIT Guyot. Seamounts, which are isolated steep
cone-shaped volcano, usually found rising from a seafloor of 1000–
4000 m depth, and guyots, flat-topped seamounts, are deep-sea
volcano structures and thus cannot be invoked here. Tuff rings
(maars) correspond exclusively to subaerial phreatomagmatic landforms (White, 1996).
A schematic illustration of the Crozon volcanic system during the
first stages of its activity is represented in Fig. 12a. The depicted
processes are thought to have formed the A-type hypabyssal breccia
(Kerdreux), the groundlevel volcaniclastic succession (BCD lithofacies
association of Aber) and the intrusive peperites (E-type breccias and
intrusions from Postolonnec/Aber). Dykes and sills intruded the
surrounding soft substrate and the volcaniclastic deposits after they
had incorporated sedimentary bits in the course of their progression.
The morphology of the peperitic intrusions at different palaeo-depths
suggests that the substrate was less hydrated and more cemented
deep down, but not completely lithified. Pillow lavas, associated
monomict G-type breccia and H-type effusive peperitic breccia
postdate these events (Fig. 12b).
Occurrence of both tholeiitic and alkalic volcanism in restricted
continental areas, sometimes in a single edifice, is typical of
magmatism either in active continental rift zones (e.g., Bailey, 1983;
Kabeto et al., 2001; Lustrino et al., 2002) or in already rifted passive
margins (Maury et al., 2003). This latter palaeogeographic context is
in good agreement with the tectonic and sedimentary scenario
proposed by Hammann (1992, p. 45), where rifting takes place during
the Ordovician in front of the North Gondwanan shelf, including the
Armorican Massif. This event initiates the opening of the Rheic Ocean,
then marked by Ashgillian volcanic episodes along the Gondwanan
passive margin.
In the Crozon Peninsula, the development of carbonate sedimentation appears to be synchronous with the volcanic episode. The
situation is comparable in the Central Iberian Domain (Buçaco
Syncline, Portugal), where some sections expose tuffs, breccias, and
sills in similar lithostratigraphic positions (Paris, 1981; Young, 1988).
The resemblances between the Crozon and Buçaco Ordovician
successions have already been shown by several authors (compiled
in Robardet et al., 1990), based on sedimentary events and
conspecific benthic faunas. The study of the Upper Ordovician in
the Buçaco Syncline is in progress to compare precisely both volcanic systems and provide new arguments for strict continuity or
not.
M. Caroff et al. / Journal of Volcanology and Geothermal Research 184 (2009) 351–366
Acknowledgements
Detailed and constructive comments by Drs Ian Skilling and Karoly
Németh helped us a lot to improve the manuscript. We thank Dr Lionel
Wilson for his editorial assistance. We are also grateful to Mélanie
Dain for her field assistance.
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