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Journal of Structural Geology xx (2006) 1e16
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Intrusion mechanisms in a turbidite sequence: The Voetspoor
and Doros plutons in NW Namibia
Cees W. Passchier a,*, Rudolph A.J. Trouw b, Ben Goscombe c,
David Gray d, Alfred Kröner a
a
Institut für Geowissenschaften, Johannes Gutenberg University, 55099 Mainz, Germany
Instituto de Geociencias, Universidade Federal do Rio de Janeiro, 21949-900 Rio de Janeiro, Brazil
c
Continental Evolution Research Group, Department of Geology and Geophysics, Adelaide University, Adelaide, S.A. 5005, Australia
d
School of Earth Sciences, The University of Melbourne, Vic. 3010, Australia
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Received 14 February 2006; received in revised form 20 September 2006; accepted 22 September 2006
Abstract
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Two syntectonic plutons of Cambrian age intruded Neoproterozoic metaturbidites in Namibia at the junction of the NS trending Kaoko and
EW trending Damara belts. Sinistral transpression in the Kaoko Belt produced km-scale upright D1 folds overprinted by minor D2 folds. D3 is
associated with NeS shortening in the Damara Belt. The plutons show two main pulses of intrusion: hornblende syenite intruded late during D1
or during D2 and biotite granite during D3. Each tectonic event produced a strain shadow defined by the shape of folds and the foliation trend
around the plutons. The internal igneous fabric and the arrangement of wall rock xenoliths that locally make up 50% of the intrusion mass suggest that the plutons have a disk or wedge shape. A marginal shear zone indicates that one of the syenite intrusions descended during emplacement with respect to the wall rock. Emplacement is therefore inferred to be at least partly accommodated by descent of the intrusion floor. The
biotite granite intruded into mega-strain shadows and tension gashes alongside and in the syenite during D3. The plutons show evidence of sinistral solid state rotation with respect to the wall rocks in response to D1eD2 transpression.
Ó 2006 Published by Elsevier Ltd.
Keywords: Granite emplacement; Namibia; Lower Ugab domain; Neoproterozoic
1. Introduction
Emplacement mechanisms of igneous bodies and especially
the related space problem are of considerable interest in crustal genesis and evolution. Plutons emplaced during regional
deformation are important because they provide a means of
dating deformation phases and to establish a link with magma
generation elsewhere in the crust or mantle (Brun and Pons,
1981; Paterson and Vernon, 1995; Vigneresse, 1995; Cruden
and McCaffrey, 2001). Models of magma ascent consider diapirism and related mechanisms, where a pluton rises through
the crust, pushing aside the wall rock (Marsh, 1982; Bateman,
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* Corresponding author. Tel.: þ49 6131 3923217; fax: þ49 6131 3923863.
E-mail address: [email protected] (C.W. Passchier).
1984; Mahon et al., 1988; Paterson and Vernon, 1995) or ascent through dykes (Clemens et al., 1997; Petford et al.,
2000), commonly associated with faults or shear zones in
the host rock (D’Lemos et al., 1993; Petford et al., 1993;
Clemens et al., 1997). Emplacement of large plutons is problematic since space has to be created in the crust for the intrusion (Petraske et al., 1978; Paterson and Fowler, 1993).
Material transfer mechanisms such as stoping (Marsh, 1982;
Paterson and Fowler, 1993) do not solve the large-scale space
problem. Presently, most generally accepted and well-documented emplacement models are (1) ballooning or forceful
emplacement through fluid pressure in the intrusion, displacing the wall rocks sideways (Bateman, 1985; Marre, 1986;
Castro, 1987; Courrioux, 1987; Ramsay, 1989), (2) nested diapirism without lateral expansion (Paterson and Vernon,
1995), (3) uplift of the overlying sequence as in sills and
0191-8141/$ - see front matter Ó 2006 Published by Elsevier Ltd.
doi:10.1016/j.jsg.2006.09.007
Please cite this article in press as: Cees W. Passchier et al., Intrusion mechanisms in a turbidite sequence: The Voetspoor and Doros plutons in NW Namibia,
Journal of Structural Geology (2006), doi:10.1016/j.jsg.2006.09.007
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with a nearly identical sequence of events. They are also similar in composition and tectonic age to other plutons in the
area, which have not yet been studied in detail, including
the large Omangambo intrusion (Fig. 1). The geological setting and excellent exposure in the Namibian desert constitute
a favourable reference frame to study the structures associated
with pluton emplacement. The area around the plutons was
mapped on scale 1:10.000 and over 4000 structural measurements stored in a MapInfo Database.
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2. Geological setting
The plutons and the surrounding metasedimentary successions of the Zerrissene Turbidite System (Swart, 1992) are localised in the southern Kaoko Zone (Miller, 1983; Miller et al.,
1983; Hoffman et al., 1994; also called Ugab Zone, Goscombe
et al., 2003a,b), a tectonic unit localised where the NeS trending Kaoko Belt merges into the NEeSW trending Damara
Belt (Fig. 1). The limits of the southern Kaoko Zone are
poorly defined because of its cover by late Palaeozoic and
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laccoliths (Jackson and Pollard, 1988; Roman-Berdiel et al.,
1995; Morgan et al., 1998) possibly including fracture systems
that isolate part of the roof (Corry, 1988; Benn et al., 2000)
and (4) footwall collapse, where space is created by sinking
of the footwall along monoclines or a fault system (Cruden,
1998; Petford et al., 2000; Cruden and McCaffrey, 2001).
Although it may seem easy to reconstruct emplacement
mechanisms in well-exposed areas, the problem has been
a topic in the geological literature for over a hundred years,
and is still only partly resolved. Reasons are that three dimensional exposure is usually limited, and that the geometry of an
intrusion only represents the last stage of development (Paterson and Vernon, 1995; Grocott et al., 1999). Also, wall rocks
are commonly hornfelsed and may have lost the delicate small
scale structures that are the main tools of structural geology.
We analysed two plutons that invaded a belt of Neoproterozoic turbidites in Namibia, the Zerrissene Turbidite System,
deformed during the Panafrican Orogeny of late Neoproterozoic and Cambrian age (Swart, 1992; Fig. 1). Both have a comparable composition and intruded the metaturbidite sequence
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Fig. 1. Map of the Neoproterozoic rocks in the Lower Ugab and Goantagab Domains. Position of the Doros and Voetspoor plutons is indicated with squares.
Please cite this article in press as: Cees W. Passchier et al., Intrusion mechanisms in a turbidite sequence: The Voetspoor and Doros plutons in NW Namibia,
Journal of Structural Geology (2006), doi:10.1016/j.jsg.2006.09.007
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et al., 2002; Goscombe et al., 2003a). The main folds, labelled
D1, have a wavelength of 50e500 m depending on their tightness. The axes of D1 folds are subhorizontal and trend predominantly NeS (Fig. 1). D1 folds show a large-scale
gradient in asymmetry associated with a cleavage fan; in the
west of the area, axial planes dip gently east and folds verge
westwards; close to the two plutons described in this paper,
axial planes are subvertical and folds are upright and symmetric; east of the plutons, and especially in the Goantagab Domain, axial planes are west-dipping to subhorizontal and D1
folds become isoclinal and recumbent (Passchier et al.,
2002; Fig. 2). The variable dip of the axial planes and the
asymmetry of the folds is associated with a second phase of
deformation D2, which also produced minor (D2) folds and
S2 crenulation foliations throughout the area; in the west
and centre, these D2-structures are weak and only locally developed, but east of the investigated plutons in the Goantagab
Domain, upright D2 folds refolding recumbent isoclinal D1
folds become a dominant deformation feature (Fig. 2-profile;
Passchier et al., 2002). The Goantagab Domain is an anticlinorium that structurally underlies the Lower Ugab Domain
(Passchier et al., 2002).
Fold axes of D1 and D2 structures are normally parallel,
and metamorphic conditions of formation similar, indicating
that D1 and D2 are related phases of one continuous tectonic
event. The two phases may even be diachronous (Passchier
et al., 2002).
D1 and D2 folds show evidence for stretching parallel to
the fold axes, mostly through fibrous fringes around pyrite
and boudinage of pelitic beds with EeW trending quartz veins
filling the necks. Asymmetry of the boudin necks indicates
a component of sinistral strike slip flow late during D1 and
during D2. In fact, D1eD2 seems to have been a phase of
transpressional deformation throughout the Lower Ugab Domain, which may have initiated, rotated and stretched the
D1 folds (Passchier et al., 2002). In the Goantagab Domain,
the stretching component of D1eD2 is stronger, and as a result
NeS to NWeSE stretching lineations of D1eD2 age developed parallel to the fold axes (Fig. 3). Local peak metamorphic conditions prevailed during D1eD2 since biotite and
rare garnet grew during these phases: included oblique foliation traces in garnet porphyroblasts and a slight outbowing
of the external foliation S1 in the adjacent matrix show that
garnet grew during D1.
D3 is a locally important refolding phase with upright folds
and steep foliations that overprints D1eD2 structures at a large
angle throughout the area (Fig. 2). The tightness and orientation of D3 folds and presence of an S3 foliation is variable
over the area. Where D3 folds and foliations are ENEe
WSW trending, local NNWeSSE shortening seems to be coaxial. NEeSW to NNEeSSW trending folds and foliations
also occur, and these structures indicate a component of sinistral shear. This implies that D3 has been a phase of NeS bulk
shortening. Metamorphic conditions during D3 were probably
lower than during D1 and D2 since cordierite and andalusite
porphyroblasts had broken down to biotite and chlorite, and
the resulting aggregates were deformed. However, locally
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Mesozoic sedimentary rocks and volcanics, both to the north
and to the south (Fig. 1). This part of the Kaoko Belt was subdivided into three tectonic domains (Hoffman et al., 1994); the
poorly exposed mylonitic Ogden Rocks Domain in the west,
the Lower Ugab Domain occupying the central part of the
belt and Goantagab Domain in the northeast (Fig. 1; Hoffman
et al., 1994; Passchier et al., 2002). The plutons are localised
in the Lower Ugab Domain, close to the contact with the
Goantagab Domain (Fig. 1). The Zerrissene Turbidite System
in the Lower Ugab Domain is composed of a succession of
metasedimentary rocks of about 1600 m minimum thickness.
The basement and the top are not exposed. The succession
consists of two turbiditic marble deposits separating three formations of pelitic and psammitic metaturbidite (Fig. 1). The
tubiditic marble units consist of a lower, 15e20 m thick uniformly banded formation (Brandberg West Formation) and
an upper w200 m thick alternation of thinly banded marble
and metapelite, including an upper massive marble unit
(Gemsbok River Formation). To the east of the studied plutons, the stratigraphy changes; diamictite, coarse sandstone
and quartzite lenses appear, and the marble units are more
massive, indicating a sedimentary environment more proximal
to the craton. The age of the metaturbidites has not been
clearly established. Regional scale mapping, however, suggests that the Brandberg West and Gemsbok River marbles
correspond to cap carbonates, which represent the end of the
Sturtian and Marinoan glaciations, now dated at ca. 710 and
635 Ma (Kendall et al., 2004).
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Metamorphism in the Ugab Domain is of middle to upper
greenschist facies, as indicated by the presence of abundant biotite in almost all rock types, besides albite/oligoclase, chlorite, carbonate and white mica. Garnet is rare, probably due
to relatively low pressure and rock composition. Amphiboles,
mainly actinolite and possibly actinolitic hornblende, form
poikiloblastic porphyroblasts in calcsilicates and impure marble. Superposed contact metamorphism around the granitic
bodies produced cordierite and andalusite porphyroblasts, generally replaced by biotite muscovite chlorite aggregates. In
calcsilicate layers hornblende, diopside and garnet grew, and
wollastonite occurs locally along contacts between marble
and the intrusions. Goscombe et al. (2004) give estimates of
540e570 C and 2.5e3.2 kbar for the Ugab domain and this
corresponds well with our estimates from the mineral parageneses. Goscombe et al. (2004) claim to recognise pervasive contact metamorphism in the Ugab Domain, attributed to
extensive partly hidden granite intrusions similar to the plutons investigated here.
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4. Deformation
The principal deformation phase (D1eD2) that affected the
Turbidite System produced a sequence of upright to inclined
tight to open large-scale folds, accompanied by the development of a penetrative slaty or spaced cleavage (Passchier
Please cite this article in press as: Cees W. Passchier et al., Intrusion mechanisms in a turbidite sequence: The Voetspoor and Doros plutons in NW Namibia,
Journal of Structural Geology (2006), doi:10.1016/j.jsg.2006.09.007
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Fig. 2. Schematic presentation of the orientation of foliations in and surrounding the Voetspoor and Doros plutons. Dip values of S1 and S2 measurements are
indicated with different font and size on lines presenting their strike trends. Where metasedimentary rock panels are present in the plutons, the orientation of
S1-foliation within them is indicated.
recrystallisation of biotite has been observed. The intensity
and orientation of D3 structures is mostly associated with
the presence of granite plutons; the S3 foliations tend to concentrate between plutons, and to wrap around them. However,
there are local high strain zones that are not associated with
outcropping plutons, and these may be associated with buried
or eroded plutons or to high-strain zones in the basement.
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5. The Voetspoor and Doros plutons
The Lower Ugab Domain is intruded by a number of similar plutons, two of which, the Voetspoor and Doros plutons,
are the subject of this study (Figs. 2e4). The Doros pluton
is a roughly circular intrusion that is well exposed in the south,
but less so in the NW where it is partly covered by drift, while
in the SE it is partly covered by volcanic rocks of the Mesozoic Doros Mafic complex (Fig. 4a). The Voetspoor pluton
is an elongate intrusive body, so named because of its resemblance to a giant footprint (‘‘voetspoor‘‘ in the Afrikaans language). It is well exposed except in its central southern part
(Fig. 4b). The Voetspoor and Doros plutons are nested plutons
both composed of two main components, hornblende syenite
and biotite granite (Fig. 4). It was found that 60e70% of
each pluton is composed of hornblende syenite with a range
of compositions from quartz-syenite to gabbroic. The biotite
granite occurs in the SW part of both plutons, along the western and southern rims, and in dykes in the centre. A minimum
intrusive age of the hornblende syenite in the Voetspoor pluton
Please cite this article in press as: Cees W. Passchier et al., Intrusion mechanisms in a turbidite sequence: The Voetspoor and Doros plutons in NW Namibia,
Journal of Structural Geology (2006), doi:10.1016/j.jsg.2006.09.007
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Fig. 3. Orientation of fold axes of D1 folds, and D1eD2 aggregate lineations around the Voetspoor and Doros plutons. Lithological contacts are traced for reference. A representative selection of measurements of S1 in metasedimentary rock panels and of igneous foliation in the syenite is shown.
was obtained at 530 2 Ma by Pb evaporation on single zircons (Seth et al., 2000) and by conventional U-Pb dating of
sphene at 541 6 Ma (Jung et al., 2005). For the biotite granite, four idiomorphic zircon grains were evaporated individually and yielded comparable Pb isotopic ratios that provide
a mean 207Pb/206Pb age of 513 1 (Table 1, Fig. 5) and that
we interpret to reflect the time of granite emplacement. For
the Doros pluton there is a whole rock Rb-Sr age of
573 33 Ma (Kröner, 1982).
5.1. Hornblende syenite
Hornblende syenite with a matrix grain size of 1e3 mm
and a strong composition gradient is the main component of
both plutons. In the NE of the Voetspoor pluton and the centre
of the Doros pluton, dioritic or composition dominates with
30e40% hornblende and up to 5% clinopyroxene. Towards
the SW in the Voetspoor pluton and external parts of the Doros
pluton, pyroxene disappears, the percentage of hornblende
Please cite this article in press as: Cees W. Passchier et al., Intrusion mechanisms in a turbidite sequence: The Voetspoor and Doros plutons in NW Namibia,
Journal of Structural Geology (2006), doi:10.1016/j.jsg.2006.09.007
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Fig. 4. Detailed maps of the plutons. (a) Geological map of the Doros pluton and surrounding units. (b) Geological map of the Voetspoor pluton and surrounding
units. The maps are partly overlapping. Metasediment xenoliths in the intrusions cannot be attributed to individual formations. Location of both maps is indicated
in Fig. 1. Mesozoic dolerite dykes have been omitted from the maps.
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gradually decreases to about 20% and idiomorphic K-feldspar
phenocrysts gradually increase in size up to 3 cm in length.
Composition changes gradually to a quartz-syenite or quartzmonzonite for the most felsic lithotype. All lithotypes plot
as peraluminous, alkalic to subalkalic magmas with normative
olivine and nepheline in the darkest phases and normative
quartz in the lighter rocks (Jung et al., 2005). Enclaves of
more hornblende and pyroxene-rich material, from cm to
100 m diameter are common, especially towards the northern
part of the intrusions. These enclaves usually have sharp contacts with the more light-coloured syenite. The distribution of
K-feldspar phenocrysts is variable; in darker parts of the
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intrusion they are fully absent, whereas in the lighter rocks
they constitute up to 70 vol%. Locally, a Rapakivi structure
with plagioclase rims around microcline was recognised.
The range in composition observed for the hornblende syenites
can be explained by simple crystal-liquid fractionation of Kfeldspar, plagioclase, clinopyroxene and amphibole from
a mantle derived alkaline magma (Jung et al., 2005). The contacts between the hornblende syenite and the wall rock are
sharp and usually parallel to bedding or a foliation in the
wall rock. Jogs, sharp corners and thin apophyses invading
the wall rock parallel to the fabric are common, suggesting
that the viscosity of the magma was low during intrusion
Table 1
Isotopic data from single grain zircon evaporationa
Sample number
Zircon colour
and morphology
Grain #
Mass scansb
Evaporation
temp. in C
Mean 207Pb/206Pb ratioc
and 2-s error
Pb/206Pb age
and 2-s error
NA 20/9
Stubby to long prismatic,
light brown, idiomorphic
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2
3
4
1e4
84
127
149
108
468
1589
1587
1598
1588
0.057559 45
0.057562 29
0.057558 25
0.057580 38
0.057564 16
513.0 1.7
513.1 1.1
513.0 1.0
513.8 1.4
d
513.2 ± 1.0
Mean of four grains
207
a
Single zircons were analyzed on a Finnigan-MAT261 mass spectrometer at the Max-Planck-Institut für Chemie in Mainz, Germany, using the evaporation
technique developed by Kober (1987). Analytical procedures are detailed in Kröner and Hegner (1998). Data acquisition was by magnetic peak switching using
the electron multiplier. The calculated ages and uncertainties are based on the means of all ratios evaluated and their 2-sigma (mean) errors.
b
Number of 207Pb/206Pb ratios evaluated for age assessment.
c
Observed mean ratio corrected for non-radiogenic lead. Errors based on uncertainties in counting statistics.
d
Error of combined mean age (in bold) is based on reproducibility of internal standard with error in 207Pb/206Pb ratio of 0.000026 (2s).
Please cite this article in press as: Cees W. Passchier et al., Intrusion mechanisms in a turbidite sequence: The Voetspoor and Doros plutons in NW Namibia,
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the Doros pluton, the main foliation in the panels is locally
overprinted by a steep second foliation, which could be an
in-pluton foliation or S2. Because of these observations, and
the different orientation of S2 and S3 around the plutons,
the foliation in the panels is interpreted to be regional S1.
In the Voetspoor syenite the total volume of metasediment
xenoliths is negligible, but in the central southern part of the
Doros syenite up to 50% of the outcrops consist of metasediment panels (Fig. 4a). The syenite sheets in between the panels
usually have a strong igneous fabric parallel to the contacts.
Cathodoluminescence images of zircons from the hornblende
syenite show that all contain inherited cores with small overgrowths (R. Schmitt, pers. comm.). This could imply that contamination with wall rock material has taken place.
At least three sets of dykes are associated with the hornblende syenite, invading both the main body of the pluton
and the wall rock. In the pluton, their orientation is variable
but in the wall rock the veins are normally parallel to bedding
or S1, and at a small angle to the contacts of the pluton. The
principal type of dykes are phenocryst-bearing light-coloured
granite, pegmatite and bimodal dykes, consisting of a mafic
core and a light rim.
5.2. Biotite granite
The south-western part of both plutons is made up of medium grained red light-coloured biotite granite that is intrusive
in the hornblende syenite (Fig. 4). In the Voetspoor pluton, the
granite is clearly distinct from the hornblende syenite by its
higher quartz content (71e75 wt% against 47e60 wt% in
the Voetspoor pluton) and lack of hornblende, although the
difference in the Doros pluton is less pronounced. The biotite
granite is more homogeneous and has fewer enclaves and xenoliths than the hornblende syenite, but xenoliths of both the
metaturbidite wall rock and the hornblende syenite occur.
The contact with the hornblende syenite and the metasediment
enclaves is sharp. Angular contacts and thin dykes invading
the wall rock indicate that the magma had a low viscosity (Petford et al., 1993, 2000). Chilled margins are lacking. According to Jung et al. (2005), the biotite granite may be
fractionated from syenite, but is unlikely to have formed
from the syenites in a closed system crystal fractionation scenario because of its distinct isotope composition.
The northern part of the Voetspoor hornblende syenite is
cut by pink granite dykes of up to 40 m thick, mainly oriented
NWeSE to EeW, approximately orthogonal to the longest dimension of the granite body on the map. Some of the veins are
clearly arch-shaped (Fig. 4b). Intrusive contacts are sharp and
vertical or dipping steeply inward, but since the vertical outcrop relief is less than 60 m, it is uncertain what the larger
scale geometry of the contact is. These major veins are inferred to be of the same age as the biotite granite in the southwest because of their composition and relation with
deformation in the wall rocks. Minor veins of biotite granite
and associated pegmatite and aplite occur throughout the pluton and in the wall rocks, where they are mostly parallel to the
foliation.
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Fig. 5. Histogram showing distribution of radiogenic lead isotope ratios derived from evaporation of four zircon grains from biotite granite sample NA
20/9, Voetspoor Intrusion. The spectrum plotted has been integrated from
468 ratios. Mean age is given with 2-sigma (mean) error.
(Figs. 3, 4; Petford et al., 1993, 2000). There are no chilled
margins. The contacts are locally deformed, as discussed
below.
K-feldspar phenocrysts have locally a strong preferred orientation interpreted to result from magmatic flow, because of
the idiomorphic shape of the phenocrysts and lack of evidence
for solid state deformation. The fabric is too weak to recognise
a linear component although this may be present. The igneous
fabric forms a more or less concentric pattern inside both syenite plutons and dips inwards (Fig. 2). In the more mafic part
of the intrusion, hornblende can show a similar igneous fabric.
The hornblende syenite contains xenoliths and panels of
metasediment up to 500 m in length and 100 m thick, mainly
hornfelsed micaschist with some layers of marble (Fig. 4).
These panels are oriented parallel to the igneous fabric, and
have internal bedding and a foliation parallel to their elongate
shape on the map. In most panels, the foliation is parallel or at
a small angle to bedding. In the Doros pluton, tight to isoclinal
folds were observed in bedding. Since there is no indication of
strong ductile deformation in the panels after intrusion of the
syenite, these folds are interpreted as predating intrusion. In
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Please cite this article in press as: Cees W. Passchier et al., Intrusion mechanisms in a turbidite sequence: The Voetspoor and Doros plutons in NW Namibia,
Journal of Structural Geology (2006), doi:10.1016/j.jsg.2006.09.007
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Field observations and thin section studies indicate that neither the hornblende syenite nor the biotite granite in both plutons underwent significant ductile deformation over most of
the pluton volume. In the centre of both plutons, ductile strain
does not exceed 10% shortening, manifested as minor undulose extinction in quartz (fabric Type 1; Hibbart, 1986; Hutton,
1988; Paterson et al., 1989; Cruden et al., 1999a). In the Doros
pluton, refolding of some sediment panels occurs by open
folds, some with a foliation, and this is attributed to D3. The
wall rocks, however, were relatively strongly deformed during
and after intrusion, as seen from the deformation of veins associated with the main intrusions. This contrast between a relatively rigid intrusion and ductile wall rocks may be due to the
high percentage of feldspar and hornblende in the plutons,
which would be relatively rigid at the deformation temperatures (<500 C) manifest during D3 in the area.
The regional tectonic pattern changes considerably when
approaching the two investigated plutons (Figs. 2, 3). S3 and
vertical axial planes of D3 folds are deflected around the plutons, creating a D3-strain shadow at the SW corner of both bodies (Fig. 2). D3 deformation is strong between the plutons, as
inferred by the tightness of D3 folds and the development level
of S3. D3 folds between the two plutons are the result of coaxial NWeSE shortening, since the asymmetry of D1 quartzfilled boudin necks is symmetrically disposed around local
D3 folds. However, D3 was a phase of non-coaxial flow on
the west side of the Doros pluton; shear sense indicators of
D3 age are all sinistral here. Moreover, SW of the Doros pluton
S3 wraps around the pluton into a NWeSE trending orientation
that is unique in the Lower Ugab Domain and is transected by
a NS trending vertical foliation of the same style and metamorphic grade, which we labelled S3b (Fig. 2). This foliation is
only present here and can be explained by rotation of S3 into
the shortening field of bulk non-coaxial flow (Fig. 2). The
strong D3 fabric on this side of the pluton grades to the SW
into the D3-Bushman fold, also with evidence for sinistral
non-coaxial flow (Passchier et al., 2002).
The effect of the plutons on D1eD2 structures is less clear
because of the D3 overprint. D1 folds become E-vergent or
vertical close to the plutons (Fig. 2). Along the south side of
the Voetspoor pluton, a D1 synclineeanticline pair outlined
in the rocks of the Gemsbok River Formation is cut by the pluton and folds have an unusual open geometry (Figs. 1, 4b).
The interlimb angle of this structure decreases away from
the pluton to the west and south, while S1 and the trend of minor folds are axial planar to the large fold, and run up to the
contact without deflection (Figs. 2, 4b). On the NW side of
the Voetspoor pluton, folds in the Gemsbok River Formation
form a refolded pattern (referred to as the ‘‘heron-structure’’:
Figs. 4b, 6) where isoclinal D1 structures with NE fold vergence, are refolded by upright NEeSW trending D3 folds
(Figs. 2, 3, 6). The three boomerang-shaped structures in the
position of the ‘‘wings’’ of the heron (Figs. 4b, 6) are D3-refolded tight NE-vergent D1 anticlines. The D1-fold that forms
the neck of the heron structure spreads out towards the
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6. Deformation adjacent to the plutons
Fig. 6. Detailed map of the ‘‘heron-structure’’ NW of the Voetspoor pluton.
The block diagrams show the structure of folded bedding in three dimensions.
S e D1 syncline. A e D1 anticline. Numbers refer to parts of the fold structure
discussed in the text.
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Voetspoor pluton into a star-shaped group of isoclinal anticlines and synclines (2 in Fig. 6) increasing the wavelength
of the major fold structure from 1 km to more than 2 km.
The plunge of D1 fold axes steepens from subhorizontal in
the neck of the heron structure to vertical in this star-shaped
cluster over a distance of a few hundred metres (Figs. 2, 3,
6). From the star-shaped cluster of isoclinal folds, bedding
trends parallel to the pluton contact in a vertical sheet with isolated asymmetric isoclinal folds with vertical axes. In the
north, one of these connects to a single isoclinal anticline
with NE plunging axis (1 in Fig. 6), from which the bedding
trends NW with shallowing dip. To the south (3 in Fig. 6), bedding runs parallel to the contact until it is transected by the
pluton. This creates a fan-shape of D1 folds and S1 towards
the pluton (Figs. 2, 3). If D3 shortening is removed from the
‘‘heron structure’’, its geometry is similar to that of the open
D1-folds in the SW of the Voetspoor pluton.
All around the Voetspoor pluton and on the south side of
the Doros pluton D1 folds, which on a regional scale have subhorizontal fold axes, show steepening of the fold axes close to
the plutons in a narrow rim monocline (Fig. 3); over a distance
of a few hundred metres, fold axes change from subhorizontal
to subvertical. The sense of deflection indicates a relative
downward motion of the pluton with respect to the wall
rock. This deflection causes a map pattern with D1 fold closures close to the contact of the pluton (Fig. 4). Unfolding
the D1 fold structures would place the surface of the pluton
at least several hundred metres above its present level.
A subvertical mylonitic ductile shear zone with a width of
100e400 m lies along the entire NE rim of the hornblende
Please cite this article in press as: Cees W. Passchier et al., Intrusion mechanisms in a turbidite sequence: The Voetspoor and Doros plutons in NW Namibia,
Journal of Structural Geology (2006), doi:10.1016/j.jsg.2006.09.007
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structures close to the pluton grade into closed folds further away.
6. Along the highly indented NW contact of the syenite
a steep EeW trending S1 in the country rocks is cut approximately orthogonally by the equally steep NeS syenite contact, showing that D1 deformation largely
precedes the syenite intrusion.
7. Along the NE rim of the Voetspoor pluton, many irregular
D2 folds become tighter and more regular towards the pluton, with axial planes dipping at a shallow angle.
8. In metasedimentary rock panels in the Voetspoor and
Doros plutons S1 is usually parallel to bedding that may
show isoclinal D1 folds.
9. Minor folds with axial plane foliation deform the main foliation interpreted as S1 in metasedimentary rock panels
of the Doros pluton.
10. Porphyroblasts associated with contact metamorphism of
the intrusions are common in metapelitic hornfelses.
These are mostly retrogressed to micaequartzefeldspar
aggregates, but from their shape and some remnants can
be deduced to represent cordierite and andalusite. In
a few cases, andalusite preserved deviation patterns of
S1 that indicate porphyroblast growth late syn-D1 (Fig. 7).
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syenite of the Voetspoor pluton (Fig. 3). This mylonite zone
mostly affects the hornfelsed wall rocks and granite veins in
it, but also a strip of hornblende syenite up to 100 m from
the contact. Deformation is clearly solid-state, with strong deformation and recrystallisation of quartz, and ductile deformation of feldspar (Type 4 fabric; Hibbart, 1986; Hutton, 1988;
Paterson et al., 1989; Cruden et al., 1999a). This mylonite
zone may have even extended further south, since the younger
biotite granite intruded along the contact, and may have erased
traces of the mylonite zone. The foliation in the mylonite zone
is parallel to the contact, while a strong stretching lineation is
steeply SW plunging all around the pluton, both in the wall
rock and in the syenite (Fig. 3). Boudinaged dykes form shear
band boudins which, together with local shear band cleavage
and sigma-type mantled porphyroclasts of feldspar, indicate
relative downward motion of the intrusion with respect to
the wall rock. The shear zone contains deformed dykes of phenocryst-bearing hornblende syenite, but is cut by bimodal
dykes and by red granite, aplite and pegmatite dykes, which
seem to be related to the biotite granite.
7. Relative age of intrusion and deformation
7.1. Hornblende syenite
From these observations we conclude that the hornblende
syenite intruded late during D1 to early D2 in both plutons.
7.2. Biotite granite
The biotite granite in the Voetspoor pluton is isotropic with
few xenoliths. There is no shear zone or other deformation in
the steep contact zone with the host rock. The biotite granite
cuts through bedding and D1 structures and postdates the
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It is difficult to date the intrusion of syenite and granite in
the plutons with respect to deformation in the wall rock. The
main reason is that contact metamorphism destroyed delicate
foliation overprint structures in the wall rock, while a syn-intrusive shear zone affected the Voetspoor pluton along most
of the contact with the wall rock. Intrusive relations with veins
can be established further away from the main body of the intrusions, but such veins are usually fine grained and of slightly
different composition as the main lithotypes. It is therefore
usually not possible to attribute a vein unambiguously to one
of the intrusions. In all, the most reliable means of establishing
relative age of intrusion and deformation in the wall rock is the
large-scale geometry of deformation structures further away
from the intrusions. The following relations have been observed in the field mainly around the well-exposed Voetspoor
pluton:
1. A major D1 fold is cut at almost right angles by the main
hornblende syenite in the SE.
2. Dykes of phenocryst-bearing hornblende syenite cut D1
folds.
3. A marginal shear zone affects the north side of the hornblende syenite in the Voetspoor pluton, with steep lineations and relative downward motion of the syenite
against the wall rocks.
4. Undeformed bimodal dykes cut the contact shear zone
along the edge of the syenite in the Voetspoor pluton.
The bimodal dykes probably belong to the hornblende syenite; similar dark phases were never found in dykes associated with the biotite granite.
5. D1 structures form large strain shadow geometries on the
W and SE side of the Voetspoor pluton; relatively open
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Fig. 7. Photomicrograph of porphyroblasts of andalusite in the Amis River
Formation. Note andalusite crystal with chiastolitic inclusions in the upper
left quarter; other porphyroblasts are substituted by quartz mica aggregates.
Horizontal foliation S1 is included in some of the porphyroblasts and bends
around them, indicating that the porphyroblasts grew late during D1. Hence,
the Voetspoor Pluton must have intruded during D1 at this site. Width of
view 5 mm. Plane polarised light. Location 100 m W of the central western
contact of the Voetspoor pluton.
Please cite this article in press as: Cees W. Passchier et al., Intrusion mechanisms in a turbidite sequence: The Voetspoor and Doros plutons in NW Namibia,
Journal of Structural Geology (2006), doi:10.1016/j.jsg.2006.09.007
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8. Granite intrusion, general considerations
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Commonly, elliptical to circular granite intrusions have
been attributed to a diapiric origin and subsequent ballooning
(Marsh, 1982; Bateman, 1984; Cruden, 1990; Paterson et al.,
1996). This is mostly due to the fact that intrusion contacts
are steep to vertical in many plutons (Cruden, 1998; Cruden
et al., 1999a), with concentric patterns of magmatic foliation
(Brown and McClelland, 2000) and the fact that the floor of
plutons is rarely exposed, suggesting a dome shape. However,
strain in the wall rocks of elliptical plutons is usually insufficient to allow an interpretation of ductile diapirism and ballooning, especially in high-level intrusions (Cruden, 1998;
Benn et al., 1999; Brown and McClelland, 2000). Also, there
is increasing evidence that the 3D shape of most plutons with
circular and elliptic map-pattern is not spherical or balloonshaped, but rather tabular or wedge-shaped. Some intrusions
are exposed as tabular, subhorizontal sheets (Coleman et al.,
1995; Scaillet et al., 1995; Sisson et al., 1996; Grocott et al.,
1999) or such a shape can be inferred from magnetic susceptibility fabrics (Cruden and Launeau, 1994; Aranguren et al.,
1997; Benn et al., 1999). Other plutons with a circular or elliptical shape on the map may have a bath-tub shape (Hamilton,
1988; Brown and McClelland, 2000) or more commonly
a wedge-shape in 3D with floors dipping shallowly towards
one or more conduits (McCaffrey and Petford, 1997; Cruden,
1998; Dehls et al., 1998; Benn et al., 1999; Grocott et al.,
1999; Petford et al., 2000; Cruden and McCaffrey, 2001).
Characteristic for many plutons with inferred tabular or
wedge-shape is down-folding of older wall-rock structures towards the contacts of the pluton in a rim monocline (Hamilton
and Myers, 1967; Bridgewater et al., 1974; Brun et al., 1990;
Grocott et al., 1999). Shear zones with down dip stretching lineations in the margin of these plutons, which have been active
during emplacement, commonly indicate displacement of the
granite downward rather than upward with respect to the
wall rock (Tobisch and Cruden, 1995; Glazner and Miller,
1997; Dehls et al., 1998; Sylvester, 1998; Cruden et al.,
1999b).
Based on a large number of described examples, the development of tabular and wedge-shaped granite bodies is presently imagined as follows (e.g. Grocott et al., 1999; Petford
et al., 2000; Cruden and McCaffrey, 2001). Granitic magma
may have a viscosity low enough to be transported to higher
crustal levels along dykes (Dingwell et al., 1993; Pitcher,
1993; Clemens and Petford, 1999) and this is probably how
tabular and wedge-shaped plutons are filled from below
(Clemens and Mawer, 1992; Clemens et al., 1997; Petford
et al., 2000). At a certain level, either depth related or due
to some suitable lithology, intrusion of a sill-like body starts,
which is then increasing in thickness and may grow laterally
(Corry, 1988; Tobisch and Cruden, 1995; Cruden, 1998).
This growth may occur by addition of km-scale tabular
sheet-like intrusions by several magma pulses (Benn et al.,
1999; Brown and McClelland, 2000; Petford et al., 2000).
Commonly, the oldest sheets are at the bottom and the youngest at the top in such sequences (Cruden et al., 1999b; Brown
and McClelland, 2000). Vertical inflation of individual intrusion sheets is common (Brown and McClelland, 2000; Petford
et al., 2000). This leads to uplift of the wall rock to form a laccolith, depression of the floor to form a lopolith, or both. Laccolith formation is due to bending of the roof rocks (Dixon and
Simpson, 1987; Jackson and Pollard, 1988; Roman-Berdiel
et al., 1995; Morgan et al., 1998) or uplift along faults (Corry,
1988). Laccoliths dominantly form at shallow crustal conditions where surface uplift and erosion is easy, but can also
form at greater depth if magma pressure is high (Petraske
et al., 1978; Corry, 1988; Roman-Berdiel et al., 1995; Benn
et al., 1999, 2000).
Most granites intruded at depth greater than 5 km have a lopolith rather than a laccolith form, testified by gravimetric data
(Vigneresse, 1995), and in-bending of wall rocks (Myers,
1975; Paterson et al., 1996; Cruden, 1998). This may have
happened by depression of the floor during intrusion: a magma
reservoir lower in the crust may collapse when granite is transported upwards and induce sagging of the intermediate crust,
creating space for the intruding granite (Brown and Walker,
1993; Cruden, 1998). Cruden (1998) showed that if sagging
of the floor is by radial simple shear, larger plutons require
slower ductile accommodating strain rates than smaller ones.
Strain rates needed to accommodate granite intrusion at
a speed sufficient to avoid solidification in the channel, are
not exceeding those proposed for ductile crustal deformation
(Pfiffner and Ramsay, 1982). Accumulated crystalline material
at the bottom of the magma chamber may have sunk together
with the wall rock to form a layered basal part of the intrusion
(Wiebe and Collins, 1998).
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hornblende syenite and its marginal shear zone as indicated by
crosscutting relations. The relationship with D3 structures is
more complex. The biotite granite cuts through a major EeW
trending D3 synform and apophyses of biotite granite and late
light-coloured granite and pegmatite veins cut small D3 folds
in the wall rock without being deformed. On the other hand,
some biotite granite veins are folded by D3, and S3 can be
seen to bend sharply around the pluton and the southern biotite
granite of the Voetspoor pluton (Figs. 2, 4). This can be due to
forceful intrusion of the biotite granite, or by ductile deformation of the wall rocks around the solidified pluton during D3.
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9. Intrusion mechanisms in the Voetspoor
and Doros plutons
9.1. Ballooning as a possible intrusion mechanism
At first sight, diapiric intrusion and ballooning seems an attractive mechanism for intrusion of the hornblende syenite in
the Voetspoor and Doros plutons. Metasedimentary rock
panels and igneous foliation are arranged in circular patterns
parallel to the edges of the hornblende syenite. At the southern
rim of the Voetspoor pluton, D1 folds open towards the pluton
and the shape of the ‘‘heron-structure’’ along the west side of
the Voetspoor pluton is also suggestive of ballooning. One
could consider the option that originally tight D1 folds further
Please cite this article in press as: Cees W. Passchier et al., Intrusion mechanisms in a turbidite sequence: The Voetspoor and Doros plutons in NW Namibia,
Journal of Structural Geology (2006), doi:10.1016/j.jsg.2006.09.007
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indications that the darker parts of the intrusion locally crosscut the more leucocratic parts, although the opposite was also
observed. In the Voetspoor pluton sudden changes in dip of the
igneous fabric (indicated as a dotted line in Fig. 4b), coincide
with panels of metasedimentary rocks oriented parallel to the
igneous fabric, and with asperities in the contact with the wall
rock (Fig. 4b). Although no intrusive relationship has been observed on a small scale, these sudden breaks in the igneous
fabric are interpreted as intrusive contacts between separate intrusive sheets of the hornblende syenite in the pluton. It is uncertain if the older sheets were completely solidified when the
younger ones intruded. In the Voetspoor pluton, at least three
such sheets can be recognised. Apparently, hornblende syenite
in the Voetspoor pluton did not intrude in a single event or as
a simple diapir-like mass of magma, but rather as a series of
repeatedly intruding planar bodies. In the Doros pluton, the
outcrop quality is insufficient to distinguish sudden changes
in igneous fabric orientation, but the presence of numerous
parallel concentric panels of metasediment suggests a similar
situation (Fig. 4a).
Presently, most of the metasediment panels and included S1
and bedding in both the Voetspoor and Doros plutons are dipping towards the centre of the intrusion, parallel to the igneous
fabric. In the Voetspoor pluton, most dips are over 60 , but in
the Doros pluton, dips of the igneous fabric and metasedimentary rock panels are relatively gentle on the outside (20e30 ),
and increase in steepness inwards (Fig. 3). This orientation of
the panels must have formed during intrusion, since the syenite itself is hardly deformed in the solid state. The orientation
of S1 and bedding away from the intrusions is mostly steeply
dipping in tight folds. This would suggest that the sediment
panels were rotated from steep to gentle dips after they were
incorporated in the intrusion.
Relative downward movement of an intruded granite with
respect to its wall rock as observed for the Voetspoor pluton,
with a rim monocline or shear zone has been observed adjacent to other plutons as well (Glazner and Miller, 1997; Dehls
et al., 1998; Sylvester, 1998; Grocott et al., 1999; Cruden
et al., 1999b). This phenomenon was explained by Glazner
and Miller (1997) as an effect of sinking of a pluton after solidification because of a higher density than the wall rocks, and
by other authors to sinking of the floor of the intrusion (Cruden et al., 1999b; Grocott et al., 1999; Petford et al., 2000).
Sinking of the intrusions due to density contrast is unlikely,
since the density contrast with the wall rock is small and metamorphic grade is low. Other possible mechanisms not mentioned by these authors are the effects of ballooning, or later
deformation in the wall rock, which is folded around and
over the existing rigid pluton. An often neglected problem
of intrusion by floor-descent is that this can only be relevant
if a magma chamber is present at limited depth below the pluton. Partial melting in the lower crust or upper mantle directly
below the pluton may not be able to create enough volume for
the floor of an intrusion to descend significantly. The strong
composition gradient of the syenite intrusion, probably due
to differentiation, favours the presence of such a magma
chamber below the plutons. With the presently available
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away from the pluton were opened by horizontal stretching of
the wall rock around an inflating pluton to form the present geometry. Judging from the geometry of the folds around the
Voetspoor pluton, the wall rock would have to be stretched
by 200e300% parallel to the contact to create the present arrangement. However, intrusive relations indicate that the syenite intruded after S1 had developed. Ballooning of the pluton
would have led to folding of this foliation and development of
a foliation parallel to the contact, in addition to considerable
vertical extension of the wall rock in the contact zone. It is difficult to conceive how this could have led to the gradually tapering geometry of the open folds in the SW of the pluton, to
relative descent of the granite with respect to the wall rocks,
and development of a narrow shear zone rather than a broad
zone of intense deformation. The only foliation that runs parallel to the plutons and wraps around them to some extent is
S3. This foliation postdates the intrusive phase of the hornblende syenite. We therefore favour a model whereby the pluton intruded into a relatively open fold, which then closed
away from the pluton, while parts of this fold south of the pluton were protected in a large scale strain shadow. In the case of
the Voetspoor pluton, D1 folds were still relatively open during intrusion, but the Doros pluton may have intruded later, after folds had tightened further; this is attributed to the lack of
open D1 folds close to the Doros pluton and the parallel or
subparallel orientation of S1 and bedding in metasedimentary
rock panels in that pluton (Fig. 4a). D2 then further tightened
and rotated D1 folds, especially around the Voetspoor pluton,
and locally formed a new foliation and D2 folds where earlier
structures had rotated into the shortening field. Therefore,
some inflation of the pluton may have taken place during intrusion of the syenite, but this must have been of minor
importance.
In the case of the biotite granite on the south side of the plutons, one could argue that the deviation of S3 around the pluton is the result of forceful intrusion postdating D3, but this
would have led to associated stretching in the wall rocks.
However, this does not explain the presence of similar S3 further away from the pluton. More likely, regional scale D3 deformation postdating intrusion created the D3 folds and their
deflection around the biotite granite-syenite plutons.
The biotite granite occurs in an unusual arrangement, in
a crescent-moon shaped main body along the south side, and
as EeW to NWeSE trending dykes in the central and northern
part of both plutons. This arrangement can be explained by
syn-D3 intrusion. If D3 is indeed due to local NWeSE compression, as indicated by the geometry of S3, the arrangement
of the biotite granite is that of a void-filling alongside a rigid
object, in this case the older hornblende syenite. The curved
dykes can be interpreted as tension gashes that fit the inferred
orientation of the D3 stress field.
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9.2. Intrusion mechanism of the hornblende syenite
In both the Voetspoor and Doros plutons, the composition
of the hornblende syenite shows a gradient, which can be attributed to crystal-liquid fractionation. Also, there are
Please cite this article in press as: Cees W. Passchier et al., Intrusion mechanisms in a turbidite sequence: The Voetspoor and Doros plutons in NW Namibia,
Journal of Structural Geology (2006), doi:10.1016/j.jsg.2006.09.007
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9.3. Development of the hornblende syenite intrusions
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Based on the geometrical data presented above, the following scenario can be envisaged for intrusion of the hornblende
syenite in the Doros pluton (Fig. 8).
A differentiated magma-chamber existed below the future
site of the plutons, where pyroxene, hornblende and K-feldspar phenocrysts had segregated to the bottom and top of the
chamber, respectively. First intrusion was sill-like into a gently
folded sequence of metaturbidites transecting the level of the
Gemsbok River formation (Fig. 8a). Unlike other models
where a single planar sill is envisaged (Brown and McClelland, 2000; Cruden et al., 1999a,b; Grocott et al., 1999), the
steep S1 foliation and bedding in D1 fold limbs in the metaturbidites may have caused sill-stepping and isolation of elongate
wall rock panels close to where the feeder dyke joins the sill
feeder dyke (Fig. 8a,b). Subsequent pulses of intrusion may
have penetrated the initial sill and may have intruded along
the roof, while the bottom of the pluton sagged downwards
into the emptying magma chamber (Fig. 8a,b; cf. Cruden
et al., 1999a,b; Grocott et al., 1999; Brown and McClelland,
2000). The isolated metasedimentary rock panels may have
been pushed into the outer parts of the intrusion suffering boudinage, bending and rotation in the process to reach their present position and orientation. Subsequent intrusion pulses were
intruding into the upper reaches of the pluton, while the lower
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data, it is difficult to decide if floor-descent is the only or dominant mechanism responsible for emplacement of the investigated plutons, or if other mechanisms have played
a significant role as well. As discussed in Section 9.1, ballooning does not seem to be the main mechanism of emplacement.
The rim monoclines of the plutons are very narrow, and dykes
cutting the rim mylonite zone seem to indicate that intrusion
was still in progress during the downward motion. Late ductile
deformation in the wall rock is insufficient to explain the
structures; a wider deformation aureole would be expected,
and no dominant displacement direction, since the pluton contacts are vertical. Most likely, therefore, is that both the rim
monoclines and the marginal shear zone are mainly due to
sinking of the floor of the pluton during intrusion.
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Fig. 8. Schematic presentation of the intrusion of hornblende syenite in the Doros (a, b) and Voetspoor (c, d) plutons. (a, c) SW-NE cross-sections at several stages
during development of the plutons; (b, d) planar cross-sections. In the Doros pluton (a) intrusion detaches many panels of metasedimentary rock parallel to foliation, which are subsequently transported to the outer parts of the developing pluton. Space for the intruding magma is made mainly by descent of the floor into
a composition-layered magma chamber at depth. In the Voetspoor pluton (c, d) development is similar but descent and intrusion are asymmetric. The rotation of the
plutons in D1 is omitted for simplicity.
Please cite this article in press as: Cees W. Passchier et al., Intrusion mechanisms in a turbidite sequence: The Voetspoor and Doros plutons in NW Namibia,
Journal of Structural Geology (2006), doi:10.1016/j.jsg.2006.09.007
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structures indicative of forced intrusion was encountered.
Stoping is unlikely as a mechanism since xenoliths are rare
in the biotite granite. The syn-D3 age of the intrusion makes
it likely that space for the half-moon shaped biotite granite
bodies along the SW side of both plutons was made by detachment of the wall rock from the hornblende syenite in the direction of D3 extension. This is remarkably similar to formation
of small strain shadows around rigid objects that are common
in many deformed rocks (Passchier and Trouw, 2005). The intrusion of dykes also fits a setting with NWeSE compression
and NEeSW extension in the hornblende granite. Because of
the subdued topography, we presently do not know the vertical
arrangement of the syenite and the biotite granite. If the crescent-shaped dykes are feeder dykes, the biotite granite may
have occupied the roof of the pluton. This would fit the observation that the Goantagab domain underlies the Ugab domain
structurally, so that the exclusive presence of the biotite granite to the south side of both plutons can be attributed to tilting,
enhanced uplift and erosion in the NE.
The similar distribution of biotite granite in the Doros and
Voetspoor plutons could imply that they are somehow connected at depth; both plutons are separated by a major D3
syncline.
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parts continued to sag and rotate downwards. It is uncertain,
however, to what extent the roof of the intrusion was raised.
From theoretical and experimental considerations, both elevation and sagging are possible at the depth at which the pluton
intruded, depending mainly on magma pressure (Petraske
et al., 1978; Benn et al., 2000). The final geometry is
a wedge-shaped structure with increasing steepness of igneous
fabric and metasedimentary rock panels inwards, and also increasing hornblende content and decreasing K-feldspar phenocryst content inwards (Fig. 8a,b). The floor of the pluton may
have descended along faults oblique to bedding. The feeder
dyke or vent may be located below the central part of the
pluton.
For the Voetspoor pluton, the situation may have been
slightly different. The intrusion may be slightly older than
the Doros pluton, cutting more open D1 folds. The youngest
and most mafic rocks are in the NE corner of the elliptical
Voetspoor pluton, and not in the centre as in the Doros pluton.
The marginal shear zone lies in the NE of the Voetspoor pluton, and this suggests the floor of the intrusion may have detached from the wall rocks along the marginal shear zone
and sunk deepest below the NE part of the intrusion, where apparently the feeder vent was located (Fig. 8c,d).
9.4. Location of the hornblende syenite
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10. Rotation of the plutons
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Syenites with an inferred mantle source, like the Voetspoor
and Doros plutons are commonly associated with steep shear
zones that act as channels for the rising magma (Seth et al.,
2000; Jung et al., 2005). However, such shear zones have
not been recognised in the Lower Ugab and Goantagab domains despite detailed mapping (Figs. 1e4). More likely is
that the intrusion is associated with D1 transpression, while
the dominance of steep foliations may also have played
a role. Tension faults could have opened oblique to D1 fold axial planes parallel to the instantaneous shortening direction
and feeder dykes may have formed either along S1 in between
such faults, or at the junction of S1 with such faults (Fig. 8a,b).
Plutons similar to the Voetspoor and Doros pluton are located
along the southern and eastern margins of the Ugab Domain.
A brief reconnaissance has shown that these have a similar bimodal nature and relation with regional deformation as the
two plutons described in this paper. Domains with porphyroblasts similar to those in the contact aureoles of the hornblende
syenite are found throughout the Lower Ugab Domain, and the
local metamorphism is clearly at least in part a low pressure
contact metamorphism with narrow anticlockwise PeT path
(Goscombe et al., 2004). It is therefore likely that plutons similar to the Voetspoor and Doros plutons intruded throughout
the area, but are now buried or eroded away in the central
Lower Ugab Domain.
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9.5. Intrusion of the biotite granite
The biotite granite is interpreted to intrude late syn-D3 on
the SW side of the Doros and Voetspoor plutons, and in dykes
inside the hornblende syenite. No evidence for ballooning or
It is not unusual that granite plutons are affected by horizontal ductile shearing (McCaffrey, 1992; Vigneresse, 1995;
Aranguren et al., 1997). Many plutons have an asymmetric
shape that can be attributed to deformation of the entire pluton
by intrusion into a major shear zone, deformation related to
a later shear zone, or rotation of the entire pluton (Djouadi
et al., 1997; Gleizes et al., 1997). The Voetspoor and Doros
plutons show evidence for rotation.
Bedding and S1 in included pelitic metasedimentary rock
fragments in the south of the hornblende-syenite of the Voetspoor pluton trend EeW (Figs. 2, 3, 4b), while the regional
trend is NNEeSSW (Fig. 1). There are also hook-shaped folds
of bedding on the south and SE side of the pluton, and a NWe
SE trending major D1 syncline on the SW side (Figs. 1, 3, 4b).
Since the aberrant D1 orientation is clearly associated with the
presence of the pluton, we think that it may be due to anticlockwise rotation of the entire Voetspoor pluton with respect
to the bulk S1 orientation over 45e60 (Fig. 9). The orientation of bedding and S1 in the ‘‘heron structure’’ in the NW,
the open syncline in the SW, and the very tight anti- and synclines of the Gemsbok River Formation in the east can all be
satisfactorily explained by such rotation (Figs. 4b, 9).
The situation in the Doros pluton is less clear because it is
partly covered by younger strata. However, the pattern of inclusions in the pluton and deflection of bedding along the
southern rim (Figs. 1, 4a) also support some anticlockwise rotation for this pluton. The tight D3 folds between the Voetspoor and the Doros plutons are interpreted as an effect of
compression of material between the two plutons as they
were brought closer together during D3 (Fig. 2).
Please cite this article in press as: Cees W. Passchier et al., Intrusion mechanisms in a turbidite sequence: The Voetspoor and Doros plutons in NW Namibia,
Journal of Structural Geology (2006), doi:10.1016/j.jsg.2006.09.007
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Fig. 9. Schematic presentation of the development of the Voetspoor and Doros plutons in map view. This series of images shows how after initial intrusion of the
Voetspoor pluton in open D1 anticlines (A) and synclines (S), further D1 and D2 shortening associated with a sinistral shear component caused tightening of D1
folds and rotation of the pluton. The Doros pluton may have intruded at this stage. The shear zone (SZ) along the northern side of the Voetspoor pluton is due to
intrusion and may be aided by EeW shortening. D1 folds to the west of the Voetspoor pluton are tightened into an asymmetric strain shadow due to ongoing D1
shortening and rotation of the plutons. During D3, the biotite granite intruded (dark grey) and D1 folds were slightly refolded.
Acknowledgements
The area in and around the plutons was mapped with the
help of Janneke Salemans, Martine Vernooi, Fábio Paciullo,
André Ribeiro and Jirka Konopásek. Their help is gratefully
acknowledged. Jean-Louis Vigneresse is thanked for enlightening discussions on intrusion mechanisms, and Renata
Schmitt for discussion on chemical data. We are grateful to
Stefan Jung for providing us with a preprint of an unpublished
paper on the Voetspoor Pluton. Excellent reviews by JeanPierre Burg, Jean-Louis Vigneresse and Keith Benn are gratefully acknowledged. The Geological Survey of Namibia is
gratefully acknowledged for logistic support and help during
the fieldwork. We thank the Schürmann Foundation and
DFG for financial support for this project. DRG acknowledges
travel costs to Namibia on Australian Research Council (ARC)
Large grant A00103456.
11. Conclusions
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The arrangement of S3 around the plutons is clearly asymmetric, and this could be used as an argument to attribute rotation to D3. However, on a regional scale D3 is a phase of
coaxial NeS shortening. It is more likely that the rotation is
of D1eD2 age. The arrangement of structures around the plutons mostly predates D3, notably the steepening of fold axes,
and the marginal shear zone of the hornblende syenite. Moreover, N of the Voetspoor pluton D2 structures are arranged in
W-vergent folds that correspond well with the rotational motion (Figs. 2, 9). Kinematically, rotation of the plutons is possible if they are relatively rigid bodies with a wedge or disk
shape as indicated by our field data.
The Doros and Voetspoor plutons are composed of two
compositional phases of intrusion, hornblende syenite and biotite granite, each of which intruded during a period of tectonic activity. The syenite apparently intruded in sill-like
bodies, mainly by displacing the footwall downwards, disrupting existing D1 folds and foliation in the metaturbidites and
including large panels of the metasedimentary wall rock, probably by several pulses of intrusion. Since the intrusion occurred during ductile transpression in the metaturbidites,
strain shadows and deflection of structures developed similar
to those around porphyroblasts on a microscopic scale. Another interesting effect is that the older syenite intrusion seems
to have rotated considerably after consolidation in response to
ductile transpression. A late phase of biotite granite intrusion
probably used the same channels and intruded along tensional
fractures in the syenite, and along the southern contacts in
strain shadows related to D3. The geometry of intrusion of
the biotite granite strongly resembles veins that open alongside
rigid objects in microscopic fringe structures.
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Journal of Structural Geology (2006), doi:10.1016/j.jsg.2006.09.007
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