CONSTRAINTS TO INTRUSION MECHANISMS OF GRANITE IN A TURBIDITE
SEQUENCE: THE VOETSPOOR AND DOROS GRANITE PLUTONS IN NW
NAMIBIA
PASSCHIER, C.W. TROUW, R., KRÖNER, A., GOSCOMBE, B., GRAY, D.
INTRODUCTION
Intrusion mechanisms of igneous bodies and especially the related space problem
are of considerable interest in crustal genesis and evolution. Of special interest are granite
plutons that are emplaced during large-scale crustal deformation, but these also pose
considerable dificulties in their interpretation (references, JSG, 1998). There are two main
problems; the plutons must create space in the wall rock into which they intrude; and, once
present, may influence deformation in the same wall rock. Most complex is the situation
where plutons are emplaced during active deformation, and this case is considered here. A
large number of intrusion mechanisms for granite plutons has been proposed, most of them
documented by well studied examples. This includes diapirism and related mechanisms,
where a pluton rises as a rounded diapir through the crust, pushing aside the wall rock;
mechanisms that uplift the overlying sequence as in sills and laccoliths, possibly including
fracture systems that isolate part of the roof; footwall collapse, where space is created by
sinking of the footwall along monoclines or a fault system; and forcefull emplacement trough
fluid pressure in the intrusion, forcing away the wallrocks sideways. All possible gradations
and intermediate cases of such mechanisms have been proposed in the literature (refs...).
Although it may seem easy to reconstruct intrusion 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 vertical exposure is usually limited, and that the present
geometry of an intrusion only represents the last stage of development, after solidification of
the magma; the intrusion may have had quite a different shape during earlier stages of its
development, wthout leaving much traces in the rocks. Also, wall rocks are commonly
hornfelsed and have lost the delicate small scale structures that are the main tool of structural
geology to reconstruct deformation sequences and kinematics.. Therfore, reconstruction of
granite intrusion mechanisms is some of the most difficult subjects in structural geology.
We analysed a granite plutons that invaded planar, continuous stratification of a belt of
Neoproterozoic turbidites in Namibia (Swart, 1992), deformed during the Panafrican Orogeny.
Thegeneral setting constitutes an ideal reference frame to study the structures produced by
syntectonic granite intrusions. The turbidite sequence was folded during a first deformation
phase (D1-D2) and refolded by a regional D3 deformation to produce map scale interference
patterns (Fig. 1, 2; Miller et al., 1983). A small number of granite plutons intrude the
Zerrissene Turbidite sequence before deposition of the late Palaeozoic cover sediments,
apparently partly during ductile deformation of the host rock (Fig. 2). Of these, we studies two
small intrusive bodies, known as the Voetspoor and Doros plutons, located about 50 km
southwest of Khorixas (Fig. 2-4). The Voetpoor pluton is an elongate intrusive body, so
named because of its resemblance to a giant footprint, is well exposed except in its central
southern part. The Doros pluton is a circular intrusion that is well exposed in the south, but
less so in the NE. Both plutons intruded a metaturbidite sequence with a nearly identical
sequence of events, apparently during deformation of the wall rocks.
This paper describes the structure around and in the plutons, and discusses the
probable evolution of these bodies and their interference with tectonic events in the area.
GEOLOGICAL SETTING
The plutons and the surrounding metasedimentary successions of the Zerrissene Turbidite
System (Swart, 1992) are localised in the Lower Ugab Domain (Hoffman et al., 1994), also
called southern Kaoko Zone (Miller, 1983), a tectonic unit defined in the area where the N-S
trending Kaoko Belt merges into the NE-SW trending Damara Belt (Fig. 2). The limits of the
Lower Ugab Domain are poorly defined because of its cover by late Proterozoic and
Mesozoic sediments and volcanics both to the north and to the south. The belt was
subdivided into three tectonic domains (Hoffman et al., 1994) the Ogden Rocks Domain in the
west, the Lower Ugab Domain occupying the central part of the belt and the Goantagab
Domain in the northeast (Fig. 2). The granite plutons are localised in the Lower Ugab
Domain, close to the contact with the Goantagab Domain (Fig. 2).
The Zerrissene Turbidite System in the Lower Ugab Domain is composed of a
succession of metasediments of about 1600 m minimum thickness. The basement and the top
are not exposed. The succession was subdivided into five formations (Swart, 1992), from
bottom to top (Fig. 2,3):
Zebrapüts Formation (>350 m), metasandstone and metapelite
Brandberg West Formation (15-20 m), turbiditic marble, calcsilicate and metapelite
Brak River Formation (~500 m), metasandstone and metapelite with dropstones
Gemsbok River Formation (~200 m), turbiditic marble, calcsilicate and metapelite
Amis River Formation (>550 m), metasandstone and metapelite with rare marble.
In spite of the regional metamorphism and the deformation, bedding can usually be recognised
and primary sedimentary structures, such as cross lamination and flute casts are locally well
preserved.
The Goantagab domain, to the east of teh lower Ugab doman contains different
lithologies, including sandstones, quartzite and diamictite, while carbonate and dolomite
breccais arecommon inteh limestone sequnece. Detailed mapping in the Goantagab domain
has reveiled that the same formationsare present there as in the lower Ugab Domain, but with
different, more proximal facies.
METAMORPHISM
The regional metamorphism is of middle to upper greenschist facies, as indicated by the
presence of abundant biotite in almost all rock types. Garnet related to the regional
metamorphism was only found at two locations, one in the SW of the area and the other one
west of the Voetspoor Granite. This scarcity may be due to insufficient temperature or to
relatively low pressure. Microstructures of included oblique Si in garnet porphyroblasts, with
a slight outbowing of Se (= S1) in the matrix show that garnet grew during D1. Other
metamorphic minerals in the metasandstones and metapelites are albite/oligoclase, chlorite,
carbonate and white mica. Amphiboles, mainly actinolite and possibly actinolitic hornblende,
form often conspicuous poikiloblastic porphyroblasts of more then one centimetre length in
calcsilicates and impure marble. Superposed contact metamorphism around the intrusive
granitic bodies produced dark spots that may contain biotite porphyroblasts or biotite
muscovite chlorite aggregates, interpreted, because of their shape, as pseudomorphs of both
cordierite and andalusite. In calcsilicate layers hornblende, diopside and garnet grew in
consequence of the contact metamorphism.
DEFORMATION
The principal deformation phase (D1) that affected the Zerrissene Turbidite System produced
a sequence of upright to inclined tight to open megascopic folds, accompanied by the
development of a penetrative slaty or spaced cleavage (Passchier et al 2003). A curious fact
about this deformation phase is that the relatively thin veneer of about 1.600 meters of known
stratigraphy are repeated by the folding along an E-W section of more than a hundred
kilometres without exposing their basement. The axes of D1-folds are subhorizontal and trend
predominantly N-S (Fig. 2,3). D1 folds tend to be asymmetric and form 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 granite plutons, axial planes are subvertical and
folds are upright and symmetric; while east of the plutons, and especailly in the Goantagab
domain, axail planes are west-dipping to subhorizontal (Fig.5). The change in attitude of the
axial planes and the asymmetry of the folds is associated with the development of second
phase (D2) folds and foliations troughout the area; in the west and centre, these D2-structures
are of minor importance, but east of the granite plutons D2 folds are upright structures
refolding D1 folds, and become the dominant deformation features (Fig.5).
The shape of D1 folds permits an estimation of about 40-70% E-W shortening during
that phase, and even higher perentages can be reconstructed for D2 folding east of the granite
plutons. Metamorphic circumstances during D1 and D2 can be estimated as middle to upper
greenschist facies, as indicated by contemporaneous growth of biotite and garnet. Fold axes of
D1 and D2 structures are normally parallel, and metamorphic conditions of formation similar,
indicating that D1 and D2 are related events grading into each other and apparently not much
separated in time. The two phases may even be diachronous and temporary on a large scale.
D1 and D2 folds show evidence of stretching parallel to the fold axes. West of the
granite plutons, this is indicated by local presence of fibrous fringes around pyrite, and by
boudinage of pelitic beds with EW-trending quartz veins filling the necks. Asymmetry of the
boudin necks indicates that there may be a component of sinistral strike slip flow late during
D1 or D2. East of the Voetspoor and Doros granite plutons, in the Goantagab domain, the
stretching component is more pronounced, and stretching lineations of D1-D2 age developed
parallel to the fold axes (Fig. 6). Here, shear sense markers indicated dominant thrusting to
the north during D1, while D2 seems to have been a phase of EW shortening and folding in
the west of the Goantagab domain, with increasing componnt of sinstral shear towards teh
east side of that domain. Aroundthe granite plutons, there may have been a component of
sinistral shear accompanying dominant EW shortening during D2.
D3 is a locally important refolding phase with upright folds and steep foliations that
overprints D1-D2 structures thorughout the area (Fig. 5). The intensity and orientation of D3
structures is very patchy over the area; where D3 folds and foliations are EW trending, local
shortening seems to becoaxial and NS, but NE-SW trending folds and folations also occur,
and there structures seem to develop in sinstral shear. This implies that D3 must be a phase of
NS large scale shortening. D3 folds are generally upright and open to tight depending on
intensity of D3 deformation. Metamorphic circumstances during D3 were probably somewhat
lower than during D1 and D2 since generally no mineral growth along S3 is present. However,
in various places recrystallisation of biotite took place. The intensity and orientation of D3
deformation is strongly variable over teh lower Ugab domain, and is clearly associatedwith the
presence of granite intrusions; the folaitons tends to concetrate between intrusion, and to wrap
around them. However, there are local high strain zones that are not associated with
outcropping intrusions, and these may be assocaied with burried intrusions or to high-strain
zones in the basement.
THE PLUTONS
Hornblende granite
The Voetspoor and Doros plutons are both composed of two main components (Fig. 3, 4),
hornblende granite, and biotite granite. 60-70% of each pluton is composed of hornblende
granite with a composition around the intersection point between the fields of quartz-syenite,
quartz-monzonite and granite (table with modal composition). The hornblende granite has a
strongly variable composition with a clear gradient from NE to SW. In the NE ofthe
Voetspoor pluton and the centre of the Doros pluton, a dark variety dominates with 30 to 40
% hornblende and up to 5% pyroxene. Towards the SW and external parts of the pluton,
pyroxene disappears, the percentage of hornblende gradually decreases to about 20 % and
idiomorphic K-feldpar fenocrysts increase in size up to 3cm in length. Most samples in the
centre of the pluton have a matrix grainsize of 1-3 mm and contain between 25 and 30 %
hornblende. In some places a Rapakivi structure, with plagioclase rims around microcline,
was recognised, supporting the idea that these are essencially A-type granites.
K-feldspar fenocrysts have a strong preferred orientation which represents a flow
fabric since ductile deformation is minor. This flow fabric forms a concentric pattern inside
both granite plutons and dips inwards (Fig. 5).
The hornblende granite contains enclaves and panels of metasediment up to 500m in
length, mainly hornfelsed micaschist with some layers of marble (Fig. 3,4). These panels are
oriented parallel to the flow fabric, and have internal bedding and S1 parallel to the panels
long axis. In most panels, bedding is parall to S1. In the Doros pluton, isoclinal D1 folds have
been observed and in some panels, and S1 is rarely overprinted by a steep second folation,
interpreted as S3. In the voetspoor granite, the total volume of these sediments is negligable,
but in the Doros granite up to 50% of the pluton surface consists locally of metasediment
panels (Fig. 4). The granite sheets in between sediment panels usually have a strong flow
fabric parallel to the screens.
Analysis of zircons from the hornblende granite has shown, that all contain
inherited cores with small overgrowths. This could imply that considerable contamination
with wall rock materail has taken place (check this with chemical analyses).....
At least three sets of dykes are associated with the honblende granite, both in the main
body of the pluton and in the wall rock. These include fenocryst bearing leuogranite;
pegmatite; and bimodal dykes, consisting of a core of dark magma and a light rim. The
contact between the light magma and the wall rock is a sharp intrusive contact, but between
the ligh and dark phases in the dykes, the contact is sharp but loboid , with numerous cuspate
structures. This suggest that the dykes were intruded with a bimodal magma.
Biotite granite
The southwestern part of both granite plutons is made up of medium grained red biotite
granite that is intrusive in the hornblende granite (Fig.3, 4). This biotite granite is more
homogeneous and has fewer enclaves than the hornblende granite. The contact with the
hornblende granite and the sediment enclaves in it is sharp. The northern part of the
hornblende-granite is cut by pink aplitic dykes of up to 40m thick, mainly oriented NWSE(215/90) to E-W, approximately orthogonal to the longest dimension of the granite body on
the map. Some of the veins are clearly arch-shaped (Fig.4). Intrusive contacts are vertical or
dipping steeply inward, but since the vertical outcrop relief is less than 50 metres, is is
uncertain what the larger scale geometry of the contact is. Minor veins of biotite granite and
associated pegmatite and aplite occur through the pluton and in the wall rocks.
Deformation close to the granite plutons
Thin section studies have shown that neither the hornblende granite nor the biotite granite in
both plutons underwent significant ductile deformation, except mylonitisation of the
hornblende pluton in a narrow rim alongh the contact with the wall rock. In the centre of both
plutons, ductile strain is minor, with a maximum of 10% ductile shortening, manifested as
minor undulous extinction in quartz. 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 strongly deformed during and after intrusion, as seen from the deformation of
veins associatedwith the main intrusions. This contrast between relatively rigid granite 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 low deformation temparatures manifest in the
area. Below, we describe deformation in the wall rocks of the plutons, starting with the
younger phases where the pattern is undesturbed by overprint.
The regional tectonic pattern of D1-D2 and D3 as descibed changes considerably when
approaching the two granite plutons (Fig.5,6), as follows. D3 structures are clearly deflected
by both plutons. S3 and vertical axial planes of D3 folds deflect around the pluton, creating a
D3 -strain shadow at the SW corner of both plutons (Fig. 5). Despite the presence of the
Mesozoic Doros crater, which covers part of the turbidites betwene both plutons, it is clear
that D3 is relatively strong between the two plutons. Both plutons have relatively weakly
developed D3 structures on the NE side. D3 folds seem to form by coaxial NW-SW
shortening between the two plutons, since the asymmetry of D1 quartz-filled boudin necks is
symmetrically disposed around the D3 folds. On the other hand, D3 deformation seems to be
strongly non-coaxial on the west side of the Doros pluton; shear sense indicators that are of
D3 age as all sinistral here. Moreover, SW of th Doros pluton S3 wraps aroundthe nearly
circular pluton into a NW-SE trending orientation that is unique in the lower Ugab domain.
Specifically here, S3 is transected by a NS trending vertical foliation of the same style and
metamrophic grade, which we labelled S3b (Fig. 5). This folaition has not been fond
anywhere else in the fieldwork area. This local S3b foliation can be explained by rotation of
S3 into the shortening field of bulk non-coaxial flow , forming new folds and a cleavage
(Fig.5).
D1-D2 structures are also influenced by the presence of the plutons, but their geometry
is les easily interpreted because of the D3 overprint. Nevertheless, the following statements
can be made. D1 axial planes tend to become E-vergent or vertical close to the plutons (Fig.
5). Also, several tight D1 folds seem change to more open folds close to the west- and southside of the Voetspoor pluton (Fig.3, 4). Because of poor outcrop conditions, the same cannot
be confirmed for the Doros pluton. Along the south side of the Voetspoor pluton, an D1
syncline-anticline pair occurs with very open geometry, unique in the lower Ugab turbidite
belt (Fig.3,4). The interlimb angle of this structure, however, decreases away from the pluton
to the west, suggesting that the pluton intruded into a relatively open structure, which then
closed further away from the pluton, while parts of the fold close to the pluton were protected
by the relatively rigid granite. Another intreaging structure occurs on the NW side of the
Voetspoor pluton; here, folds in the Gemsbok River Formation form a strange, refolded
pattern close to the granite which for reference purposes and a certain likeness we will refer
to as the "Heron-structure" (Fig. 3, 7). Inspection of foliations associated with folds in the
"Heron-structure" shows that they are isolclinal D1 structures with NE fold vergence, refolded
by upright NE-SW trending D3 folds (Fig. 5-7). Despite the refolding by D3, an isoclinal D1anticline in the Gemsbok River Formation (1 in Fig.7) is connected to other isoclinal folds in
the centre of the star-shaped structure (2 in Fig.7), and then to a long vertical band parallel to
the edge of the pluton (3 in Fig.7). Detailed mapping allows a reconstruction of the 3D shape
of this structure. Fig. 7 Shows that a set of tight to isoclinal E-vergent D1 synclines and
anticlines decreases in amplitude and increases in tighness to the NE, away from the granite,
with gently plunging foldaxes, not much different from that in the reast of the area (Fig. 6;
upper block in Fig. 7). These folds are refolded around NE-SW axes (refolding axis in Fig. 6)
and show marked steepining and opening of the fold structures towards the granite, untill
alongside the granite foldaxes plunge steeply and parallel to the granite contact (Fig. 6,7). This
creates an extreme fan shape of D1 folds and S1 towards the granite.
The steepening of D1 foldaxes in the structure NW of the voetspoor granite is not
unique to the Heron-structure. All around the Voetspoor pluton, and on the south side of the
Doros pluton can be observed that D1 folds, which on a regional scale have subhorizontal fold
axes, show abrupt steepening of the foldaxes close to the plutons (Fig. 6); over a distance of a
few hundred metres, foldaxes change from subhorizontal to subvertical. The sense of
deflection indicateds a relative downward motion of the pluton with respect to the wallrock.
This phenomenon explains a curious feature on the map pattern where many D1 fold closures
can be observed very close to the contact of the granites and the wall rock (Fig. 3,4).
Unfolding the D1 fold structures would place the surface of the granite at least several
hundred metres above its present level .
Apparently associated with this phenomenon is a unique structural feature in the area.
Along the entire NE rim of the hornblende granite of the Voetsoor pluton lies a mylonite zone
with a width of 100-400m, which mostly affects the hornfelsed wall rocks and granite veins in
it, but also a narrow strip of hornblende granite (Fig. 6). The foliaton in this zone is parallel to
the contact, while a strong stretching lineation is steeply SW plunging all around the pluton
(Fig.6). Boudinaged dykes form shear band boudins which, together with local shear band
cleavage indicate relative downward motion of the granite with respect to the wall rock. The
shear zone contains deformed dykes of phenocryst bearing hornblende granite, but is cut by
bimodal dykes, and by red granite, aplite and pegmatite dykes which seem to belong to the
biotite granite. If the bimodal dykes belong to the hornblende granite, this implies that the
shear zone is of the same age as the intrusion of that earlier granite.
Absolute age of the intrusions
A preliminary age of the hornblende granite was estimated by B. Seth (personal
communication) as 530 +/- 2 Ma, by Pb-Pb evaporation in single zircon grains. ....
On of us (AK) dated zircons from the biotite granite by the evaporation method. Resultsare
shown in Table 1. This gave date of 513 ±1 Ma, significantly younger that the hornblende
granite.
Table 1. Isotopic data from single grain zircon evaporation.
__________________________________________________________________________________________
Sample
Zircon colour
Number
and morphology
Grain
Mass
Evaporation
Mean 207/Pb/206Pb
#
scans1
temp. in °C
ratio2 and 2-m error
207Pb/206Pb age
and 2-m error
__________________________________________________________________________________________
NA 20/9
stubby to long-
1
84
1589
0.057559±45
513.0±1.7
prismatic, light
2
127
1587
0.057562±29
513.1±1.1
brown,
3
149
1598
0.057558±25
513..0±1.0
idiomorphic
4
108
1588
0.057580±38
513.8±1.4
1-4
468
0.057564±16
*513.2±1.0
mean of 4 grains
__________________________________________________________________________________________
1Number of 207Pb/206Pb ratios evaluated for age assessment. 2Observed mean ratio corrected for non-radiogenic Pb
where necessary. Errors based on uncertainties in counting statistics. *Error of combined mean age (bold print) is based
on reproducibility of internal standard with error in 207Pb/206Pb ratio of 0.000026 (2).
Discussion
The deformation patters around the Voetspoor and Doros plutons can be used to put contrains
on possible mechanisms of intrusion and interaction with deformation in the wall rock.
Relative age of intrusion and deformation
It is difficult to date the intrusion of both granites in the plutons with respect to deformation in
the wall rock. The reasons are, that contact metamorphism destroys delicate foliation overprint
structures in the wall rock, while a syn-intrusive shear zone affects the granite directly along
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 finegrained and of slightly
different composition as the main granites, and it is therefore usually impossible to attribute a
vein definitively to one of the intrusions. In all, the most reliable means of establishing
relative age of granite 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 - major open D1 folds are cut by the hornblende granite
2 - dykes of phenocryst-bearing hornblende granite cut D1 folds
3 - undeformed bimodal dykes cut the contact shearzone along the edge of the syenite granite
in the Voetspoor pluton
4 - D1 structures seem to form pressure shadow geometries on the west side of the Voetspoor
pluton; relatively open structures close to the pluton grade into closed folds further away.
However, along the highly indented NW contact a steep E-W trending S1 in the country rocks
is cut approximately orthogonally by the equally steep N-S granite contact, showing that most
of the D1 deformation precedes the intrusion of the granite (Fig. 3). Along the NE rim many
irregular D2 folds become tighter and more regular, with axial planes dipping at a low angle
outside the granite, suggesting that the intrusion of the hornblende granite must here have
been syn-D2.
5 - pophyroblasts associated with contact metamorphism of the intrusions are common in
metapelites. Presently, these are mostly retrogressed to biotite aggregates, but from their shape
they can eb inferred to have been cordierite and andalusite. In a few cases, cordierite is
preserved and inclusion paterns indicate porphyroblast growth during late D1 or D2.
The biotite granite appears to be roughly contemporaneous with D3, since some veins
derived from the granite cut through minor D3 folds, while others are folded by D3.
Moreover, there is a clear deviation of S3 axial surfaces around both granite plutons, as
discussed above (Fig. x).
Possible Ballooning
In many plutons, ballooning of the intrusion and forceful expulsion of wall rock outwards has
been suggested as an important mechanism to create space for the intrusion. At fird sight, this
could apply for the intrusion of the hornblende granite. The shape of the "heron-structure" in
the NW, and the open natrue of folds in the south.close to the granite could be a resut of such
ballooning. In order to explain these structures in this way, the wall rock would have been
stretched parallel to the contact by 100-500%. Such strong deformation in the dominantly
pelitic metasediments surroundingthe pluton would have given rise to strond folations and
subhorizontal stretching lineations parallel to the contact. There is indeed a shearzone along
the contact of the granite, but as we have seen above, this is relatively thin and contains steep,
not horizontal stretching lineations. The geometry of this shear zone indicates relative motion
of blocks in a vertical sense, and does not agree with a ballooning-type geometry. The only
foliation that runs parallel to the plutons and wraps around them to some extend is S3. This
foliation has been shown to be relatively late, of the age of the biotite granite, and not
assocaited with the intrusive phase of the hornblende granite. Moreover, the foliation is not
everywhere parallel to the contact. In conclusion, some ballooning may have take place during
intrusion of the hornblende granite, but it must have been of minor importance.
Intrusion mechanism of the hornblende granite
In both plutons, the composition of hte hornblende granite is strongly variable, but shows a
clear gradient from relativel quartz-feldspar rich on the outside, to more hornblende rich in the
core. Also, there are indications that the darker parts of the intrusion crosscut the more
leucocratic parts.
In the Voetspoor pluton, there are sudden changes in dip of the flow fabric, and these
coincide with panels of sediment oriented parallel to the flow fabric, and with asperities in the
contact with the wall rock (Fig.3,4). Although no intrusive relationship has been observed on
a small scale, these sudden breaks in the flow fabric are interpreted as intrusive contacts
between separate intrusions of the hornblende granite in the pluton. In the Voetspoor pluton,
at least three such intrusions can be recognised. In the Doros granite, the outcrop quality is
insufficent to distinguish sudden changes in flow fabric orientation, but the intrusion consists
of numerous sheets of hornblende granite, separted by screens of sediment. Both in the
Voetspoor and in the Doros pluton, these sediment screens and strictly parallel to each other,
and some are extremely long and thin strips. This type of geometry cannot form if panels of
sediment float freely in a liquid magma, but ar similar to structures observed in veins, where
fracturing along the contact between vein and wall rock separates narrow strips of wall rock
from to be included in vein material. The strips of sediment in both plutons can be explained
ina similar way: if the pluton formed by repeated intrusion alongthe contact of wall rock and
older intrusive material, thin strips of wall rock could have become isolated and included into
the growing pluton (Fig.8). If each pulse of intrusion had similar composition as its
predecessor, contacts within the granite would not be obvious.
Concluding, it seems likely that at least the hornblende granite in each pluton did not
intrude in a single event or as a simple diaper-like mass of magma, but rather as a series of
repeatedly intruding planar bodies along the contacts of the magma chamber with the wall
rock.
Presently, most of the sediment panels in both the Voetspoor and Doros plutons are
dipping towards the centre of the intrusion, parall to the flow fabric. In the Voetspoor granite,
most dips are over 60°, but in the Doros granite, dips of the flow fabric and sediment panels
are relatively gentle on the outside (20-30°), and increase in steepness inwards.This
orientation of the panels must have formed during intrusion, since the granite itself is hardly
deformed in the solid state. The enveloping surface of bedding away from the intrusions is
mostly gently dipping. This would suggest that the sediment panels were rotated to a steeper
orientation after they were incorporated in the intrusion. The only way in which this could
have taken place is by rotation of the intrusive sheets, including the panels from a gentle dip to
steeper dips during the intrusion process. Interesting in this respect is also that apparently the
darker, central part of the hornblende granite is younger than than the more leucocratic outer
part. This would imply that the outer sediment panels were emplaced earlier than the internal
ones. If this is the case, the panels can ony have split of the roof of the intrusion, and not from
the bottom or sides.
Detailed study of the sediment panels in the Doros pluton have shown that in
them, a foliation interpreted as S1 is commonly parall to bedding. In some cases, thigh to
isoclinal D1 folds were observed in bedding. Since there is little indication of strong
ductiledeforamtion in the panels after intrusion of the granite, this must be an original feature
of the panels. This, in turn, implies that they were included after the asymmetry of D1 folds,
with subparallel orientation of bedding and S1 in the long limbs of the folds, was established.
Keeping in mind that we can only see a two-dimensional section through both plutons,
the following scenario for their formation seems most likely during intrusion of the
hornblende granite (Fig.9).
A differentiated magma-chamber existed not far below the future site of the intrusive
granite, where hornblende and K-feldspar fenocrysts had seggragated to the bottom and top of
the chamber respectively. First intrusion was sill-like into a gently folded sequence of
metaturbidites at the level of the Brak-, Gemsbok- and Amis River formations (Fig. 9a).
Subsequent pulses of intrusion may have penetrated the formed sill and have mainly intruded
alongthe roof, while the bottom of the formed pluton, including earleir intruded material
sagged downwards into the emptying magma chamber (Fig. 9b,c). Subsequent intrusions
phases were of more and more dark material, intruding into the upper reaches of the pluton,
while the lower parts continue to sag and rotate downwards. The final geometry is a funnel-
shaped structure with increasing steepnesss of flow fabric and sediemnt panels inwards, and
increasing hornblende content and decreasing K-feldspar fenocryst content inwards (Fig.9d,e)
Contact shearzone of the voetspoor granite
An important aspect in determining the intrusion machanism is the contact of the granite with
the wall rock. Contacts of the voetspoor pluton with the wall rocks are steep to subvertical and
clearly intrusive in both the Voetspoor and Doros plutons. In the Voetspoor pluton, these
contacts are everywhere affected by a thin (up to 300 m wide) ductile shear zone with steep Splunging lineation and a foliation parallel to the contact. Shear sense indicates relative uplift
of the wall rock with respect to the granite. Most dykes pertaining to the hornblende granite,
and the granite itself in the contact zone are deformed by this shear zone, but some
undeformed bimodal dykes are seen to cut the shear zone. The biotite granite transects the
shearzone without deformation. This would mean that the shear zone was active late during
intrusion of the hornblende granite, unless the bimodal dykes belong to the biotite granite; in
that case, the shearzone could postdate hornblende granite intrusion, and predate the biotite
granite.
Relative downward movement of a granite with respect to its wall rock has been
observed in many plutons (refs..). It has been attributed to regional shortening around a stiff
intrusion, pushing wall rocks up with respect to the granite, or to cooling of granite in the
pluton and volume loss; or to absolute sinking of an entire pluton trough wall rocks due to a
density contrast. The last two mechanisms seem unlikely for the plutons discussed here. If
volume change were involved, the Doros and Voetspoor plutons would be expected to give
similar structures, but no shear zone is developed around the Doros pluton. Sinking of the
intrusion due to density contrast is unlikely, since the density contrast with the wall rock is
small and metamorphic grade is low. Two possible explanations remain; the shear zone could
be associated with collapse of the magma chamber below the intrusion, if it is
contemporaneous with intrusion; or it could be due to deformation in the wall rock.
Relative downward motion of the intrusion has been established all around the
Voetspoor pluton, and on the south-side of the Doros pluton. D1-D2 deformation could
explain such relative motion on two opposite sides of an intrusion, but not on all sides. It is
therefore liley that the downward motion is associated with the intrusion mechanism, probably
with collapse of the magma chamber below the granite at a later stage of intrusion (Fig. 9).
The contact shear zone may be associated with this motion, but could also be a later effect,
associated with long-range deformation in the wall rocks (Fig. 10). In fact, its orientation and
kinmatics fit well with the development of the heron.structure and of the distribution of D1
structures around the Voetspoor pluton (Fig. 10).
Intrusion of the Biotite granite
The biotite granite is isotropic with few xenoliths, and has steep contacts with the country
rock. There is no shear zone or other deformation in the contact zone. The biotite granite cuts
through bedding and D1 structures and postdates the hornblende granite, as indicated by the
main contact between the granites, and by crosscutting dykes. The contact shearzone of the
hornblende granite is cut by the biotite granite. The relationship with D3 structures is more
complex. The biotite granite cuts through a major E-W trending D3 synform. At the outcrop
scale, various apophyses of the biotite granite can be seen to cut through minor D3 folds. On
the other hand, minor biotite granite veins, and late leucogranite and pegmatite veins cut D3
structures without being deformed, and S3 can be seen to bend sharply around the pluton and
the southern biotite granite (Fig. 5). One could argue that the deviation of S3 around the
pluton might be the result of forcefull intrusion postdating D3, but the orientation of the
deviation, coherent with D3 strain, favours the interpretation of a late syn-D3 intrusion.
The biotite granite occurs in an unual arrangement, in a crescent-moonshaped main
body along the south side, and as EW to NW-SE trending diykes in the central and norhern
part of in both plutons. This arrangement can be explained by syn-D3 intrusion; if D3 is
indeed due to local NW-SE compression, as seems to be indicated by geometry of S3, the
arrangement of the biotite granite is in the southern D3 strain shadow of the older hornblende
granite, and in tension gash arrangements in the older intrusion (Fig. 10).
The crosscutting relation of the biotite granite with its country rocks demonstrates that
stoping was apparently the main intrusion mechanism. No evidence for ballooning or
structures indicative of forced intrusion were encountered.
Rotation of the plutons
The Voetspoor Pluton has a curious relation with the surrounding deformation fabric (Fig.35). Bedding and S1 in included pelite fragments in the south of the hornblende-granite trends
EW (Fig. 3), while the regional trend of S1 is NNE-SSW (Fig. 2). There are also curious
hook-shaped folds of bedding on the south and SE side of the pluton, and a NW-SE trend of a
major D1 syncline to the SW of the pluton (Fig. 2,3). In fact, the major D1-syncine in the
south of the Voetspoor Pluton is bent in an unusual way. Since the abberant D1 orientation is
clearly associated with the presence of the pluton, we think that it may be due to sinistral
ROTATION of the entire Voetspoor Pluton with respect to the bulk S1 orientation over 4560° (Fig. 10). The orientation of bedding and S1 in the "herons structure" in the NW, the
open syncline in the SW, and the very tight anti- and synclines of Gemsbok River Formation
in the east can be explaned by rotation of the entire pluton (Fig. 4, 10).
The situation in the Doros Pluton is less clear, since it has been covered in the east by
the Doros mafic complex and exposure is poor in the north where it is partly covered by
Permian and Mesozoic sediments; however, the pattern of inclusions in the pluton and
deflection of bedding along the southern rim (Fig. 2) also support some sinistral 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. 5). A major D3 fold also occurs on the west side of the Doros pluton,
grading into the Bushman fold to the south (Fig. 3).
A difficult problem is the age of the rotational motion. The arrangement of S3 around
the pluton is clearly asymmetric, and sinistral shear sense indicators have been found
associated with D3. On the other hand, there are some indications that the rotation may be of
D2 age. The arrangement of structures around the pluron seems to predate D3 in many cases,
notably the steepening of fold axes, and the contact shear zone of the hornblende grantie.
Moreover, NE ofthe pluron D2 structures are arranged in shallow dipping folds that would
correpond well with the rotational motion (Fig. 10)
CONCLUSIONS
ACKNOWLEDGEMENTS
REFERENCES