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THE VOETSPOOR GRANITE IN NW NAMIBIA - staff.uni-mainz.de

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THE VOETSPOOR GRANITE IN NW NAMIBIA - staff.uni-mainz.de

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 NW-SE(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 south-side 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 D1-anticline 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.

    __________________________________________________________________________________________

    207206207206 Sample Zircon colour Grain Mass Evaporation Mean /Pb/PbPb/Pb age

    12 Number and morphology # scans temp. in ?C ratio and 2-;m error 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

    mean of 4 grains 1-4 468 0.057564?16 *513.2?1.0

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    1207206 2Number of Pb/Pb ratios evaluated for age assessment.Observed 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

    207206on reproducibility of internal standard with error in Pb/Pb 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

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