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Characterisation and tectonic implications of the Early Cretaceous, Skeleton Coast Dyke Swarm, NW Namibia
Accepted Manuscript Characterisation and tectonic implications of the Early Cretaceous, Skeleton Coast Dyke Swarm, NW Namibia M. McMaster, J. Almeida, M. Heilbron, E. Guedes, M.A. Mane, J.H. Linus PII:
S1464-343X(18)30352-2
DOI:
https://doi.org/10.1016/j.jafrearsci.2018.11.010
Reference:
AES 3365
To appear in:
Journal of African Earth Sciences
Received Date: 20 May 2018 Revised Date:
8 November 2018
Accepted Date: 12 November 2018
Please cite this article as: McMaster, M., Almeida, J., Heilbron, M., Guedes, E., Mane, M.A., Linus, J.H., Characterisation and tectonic implications of the Early Cretaceous, Skeleton Coast Dyke Swarm, NW Namibia, Journal of African Earth Sciences (2018), doi: https://doi.org/10.1016/j.jafrearsci.2018.11.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Characterisation and Tectonic Implications of the Early Cretaceous, Skeleton Coast Dyke Swarm, NW Namibia McMaster, M.a,*, Almeida, J.a, Heilbron, M. a, Guedes, E. b, Mane, M.A. a, Linus, J.H.
c
a
TEKTOS - Geotectonics Research Group, Faculdade de Geologia, Universidade do Estado do Rio de Janeiro, Rua São Francisco Xavier 524/4006-A,Maracanã, 20559-900 Rio de Janeiro, RJ, Brazil b Museu Nacional/UFRJ, Quinta da boa Vista S/N, São Cristovão, 20940-040 Rio de Janeiro, RJ, Brazil c Regional Geoscience Division, Geological Survey of Namibia, Windhoek, Namibia
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The Early Cretaceous, Skeleton Coast Dyke Swarm (SCDS) was intruded into Permo-Carboniferous
11
sediments of the Karoo Supergroup, Pan-African granites, Neoproterozoic metasediments, and
12
Archean to Mesoproterozoic gneisses of the Kaoko Belt, NW Namibia. Aeromagnetic data and
13
satellite imagery has been used to map the distribution of the generally coast parallel mafic dykes
14
along with a significant number of dykes that cut across Pan-African structures of the Kaoko Belt.
15
Dykes of the SCDS were mapped for some 500km along the Skeleton Coast of NW Namibia and up to
16
200km inland, and with a likely continuation into Angola. The geochemistry and published
17
geochronological data suggest that the majority of the dykes are related to the Etendeka volcanics.
18
These volcanics, together with the basalt and rhyolite lava flows and associated mafic dyke swarms
19
of S-SE Brazil, form the Paraná-Etendeka magmatic province of Early Cretaceous age. Geochemical
20
analysis of 26 dykes show that the majority of the dykes are Low-TiO2 quartz tholeiites of two types:
21
1) qtz-tholeiites enriched in LREE [(La/Yb)N - 3.43 to 6.46]; & 2) qtz-tholeiites with REE patterns
22
similar to E-MORB [(La/Yb)N < 3.4]. The former typically strike N15W whilst the latter strike N70E /
23
N85E. By applying the principles of kinematic analysis and through the identification of asymmetrical
24
features such as zigzagging, branching and en échelon dykes, and bridges between dyke segments,
25
we were able to identify that in addition to extension perpendicular to dyke walls a significant
26
number of dykes display lateral displacements of their walls consistent with oblique extension. This
27
information was used to determine the shear sense and estimate the direction of maximum and
28
minimum stress of the regional stress field active during dyke intrusion. Three dyke generations have
29
been identified within the SCDS: 1) NNW-SSE dykes associated with normal ENE-WSW extension; 2)
30
WNW-ESE to NW-SE trending dykes with sinistral or dextral components related to NNE-SSW
31
extension; & 3) ENE-WSW dykes often with a sinistral component indicating NW-SE extension. When
32
South America is restored to its pre-break-up position, the principal direction of the Skeleton Coast
33
and the Florianopolis dyke swarms are equivalent. Multiple generations of dykes are also recognised
34
in the Florianopolis dyke swarm, but we interpret that the majority of the dykes of the SCDS & FDS
35
were intruded during initial rifting that was characterised by ENE-WSW extension.
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Keywords: Kinematic Analysis. Paraná-Etendeka. West Gondwana. Conjugate margins
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1. Introduction
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the early Cretaceous (119-139Ma - Almeida et al., 2013; Renne et al., 1992; Turner et al., 1994 ) as a
43
precursor to the break-up of Gondwana and subsequent opening of the South Atlantic Ocean
44
(Heilbron et al., 2000; Marsh et al., 2001; Will & Frimmel, 2013). This magmatism is represented by
45
both mafic and acid volcanics and intrusives, including a number of predominantly mafic dyke
46
swarms in Brazil, South Africa, Namibia and Angola. Dyke swarms have important tectonic
47
implications, and using the principles of kinematic analysis it is possible to evaluate the conditions of
48
stress that were active during their emplacement (Ernst and Buchan, 2003; Correa-Gomes et al.,
49
2001; Martinez-Poza & Druguet, 2016; Will & Frimmel, 2013). A dyke swarm can be defined as a
50
concentration of dykes emplaced in the same igneous episode (e.g. Speight et al., 1982). This
51
definition implies that all of the dykes within a specific dyke swarm are of a similar age and
52
consequently were emplaced under the same stress regime. However, more recent publications
53
(e.g. Gaggero et al., 2007; Guedes et al., 2005 & 2016, Reid et al., 1991; Trumbull et al., 2004) have
54
characterised dyke swarms that include dykes of varying composition and orientation, emplaced
55
across longer intervals of up to 20 Ma. We have adopted this latter usage of the term and define a
56
dyke swarm as a concentration of dykes emplaced during the same tectono-magmatic event which
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consequently could include several generations of dykes.
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The voluminous tholeiitic magmatism of the Paraná–Etendeka Magmatic Province developed during
Previous studies have identified Early Cretaceous mafic dykes along the south-western
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margin of Africa (Reid, 1990; Reid et al., 1991; Salomon et al., 2017; Trumbull et al., 2004; Will &
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Frimmel, 2013), however their distribution, orientation, age and composition has not been as well
61
documented as Cretaceous age dyke swarm in Brazil. In Namibia, mafic dykes have been mapped
62
parallel to the coast, within the NNW-trending Kaoko and Gariep belts, as well as oblique to the
63
coast sub-parallel to the NE-trending Damara belt (Salomon et al., 2017; Will & Frimmel, 2013;
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Trumbull et al., 2004). These latter dykes extend up to 500km inland and were mapped in detail
65
using high-resolution aeromagnetics by Trumbull et al. (2004) in defining the Henties Bay-Outjo dike
66
swarm (HOD). Similar high-resolution aeromagnetics, and satellite imagery, was used in this study to
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map in detail for the first time the distribution of the generally coast parallel mafic dykes along the
68
NW margin of Namibia, here named the Skeleton Coast Dyke Swarm (SCDS- Fig.1). The
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interpretation of magnetic data and satellite imagery was supplemented by comparison with
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published geological maps and through ground-truthing including structural analysis and sampling
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during field work in September 2015.
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Figure 1 - Map of the Skeleton Coast Dyke Swarm (SCDS) and part of the Henties Bay Outjo Dyke Swarm (HOD) in the Kaoko and Damara Belts of NW Namibia. The dykes of the SCDS are shown in red and were mapped from aeromagnetic data and satellite imagery and verified during field work. The green rectangles show the coverage of the high-resolution aeromagnetic data provided by the Geological Survey of Namibia (GSN). The black lines are the major faults and shear zones which divide the Kaoko Belt into a number of tectonic zones (WKZ - Western Kaoko Zone; CKZ - Central Kaoko Zone; EKZ - Eastern Kaoko Zone) from Miller (1983) and Goscombe & Gray (2008). The inset map shows the simplified geology and tectonic setting of NW Namibia.
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The main objectives of this paper are to characterise the distribution and geometry of the SCDS and
82
to determine the orientation of the regional stress field active during the intrusion of the dykes.
83
Preliminary geochemical results are also presented, which allows the comparison of the dyke
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geochemistry with known magma compositions of the (Paraná)-Etendeka province.
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ACCEPTED MANUSCRIPT Tectonic reconstructions of Western Gondwana suggest that Namibia and Southern Brazil
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were adjacent prior to break-up (Almeida et al., 2013 & 2014; Hein et al., 2013; Moulin et al., 2010;
87
Salomon et al., 2017; Torsvik et al., 2009; De Wit et al., 2008), placing northwest Namibia next to
88
Santa Catarina, Brazil. Five dyke swarms have been described along the south-eastern margin of
89
Brazil (Almeida et al., 2013; Coutinho, 2008; Guedes et al., 2005; Piccirillo et al., 1990; Raposo et al.,
90
1998; Renne et al., 1996). Three of these dykes swarms, the NNW-trending Vitória–Colatina (VCDS)
91
and Resende-Ilha Grande (RIGDS) dyke swarms as well as the NW-trending Ponta Grossa–Guapiara
92
(PGDS) dyke swarm are orientated obliquely to both the coast and in relation to Precambrian
93
structures. Two of these dykes swarms, the NE-trending Serra do Mar dyke swarm (SMDS) and the
94
NNE-trending Florianopolis Dyke Swarm (FDS), are orientated parallel to the coast. The dykes of the
95
SMDS strike parallel to the regional trend of the Ribeira Belt, however being subvertical they cut
96
across the variably dipping Brasiliano foliations. The results of this study are compared with the
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geochemistry (Florisbal et al., 2014) and paleostress indicators (Almeida et al., 2013) of the Early
98
Cretaceous, Florianopolis Dyke Swarm (FDS). The same techniques of kinematic analysis have been
99
applied to both dyke swarms, allowing a direct comparison of the regional stress fields active during
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dyke intrusion. Some insight is also gained into the geodynamic evolution of this portion of Western
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Gondwana during continental break-up and the subsequent opening of the (proto) South Atlantic
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Ocean.
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2. Geological Setting
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Kaoko Belt of NW Namibia were deformed and metamorphosed during the Neoproterozic to
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Cambrian amalgamation of Western Gondwana (Miller, 1983; Dürr & Dingeldey, 1996; Goscombe et
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al., 2003a, 2003b; Goscombe & Gray, 2008; Konopásek et al., 2005). The Kaoko Belt is the northern
110
branch of the Damara orogeny resulting from the collision of the Kalahari, Angola and Rio de la Plata
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cratons during the Pan-African orogenic cycle. The Kaoko belt has been correlated with the Dom
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Feliciano and Ribeira Fold Belts of South America (Chemale et al., 1994; Trompette & Carozzi, 1994).
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The Neoproterozoic metasediments and Mesoproterozoic to Archean basement of the
Miller (1983) subdivided the transpressive Kaoko Belt into five tectonic zones of contrasting
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metamorphic and deformational styles. The three central zones; the Western Kaoko Zone (WKZ), the
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Central Kaoko Zone (CKZ) and the Eastern Kaoko Zone (EKZ) are separated by crustal-scale, NNW
116
orientated shear zones. The Ugab and Kunene zones occur in the extreme north and south of the
117
Kaoko Belt. Later work by Goscombe et al. (2003a, 2003b) resulted in the identification of new shear
118
zones and to a better understanding of the tectonic and thermal evolution of the Kaoko belt, leading 4
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these authors to propose a further subdivision of the WKZ into the Coastal Terrane and Orogen Core
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(Fig. 1). The Coastal Terrane is considered to be an exotic terrane that was accreted onto the margin
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of the Angolan Craton at around 580 Ma (Goscombe & Gray, 2007). This terrane is characterised by
123
gneisses and metagranites of the upper amphibolite facies. The metagranites display primitive arc-
124
like geochemical signatures with ages between 656-630 Ma (Goscombe & Gray, 2007; Goscombe et
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al., 2005a, 2005b; Masberg et al., 2005). Whilst, S-type granites related to the collision of the Coastal
126
Terrane are found throughout the WKZ and range in age from 576-549 Ma (Seth et al., 1998; Franz
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et al., 1999; Kröner et al., 2004; Goscombe et al., 2005b; Goscombe & Gray, 2007).
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The geometry of the Kaoko Belt was produced by progressive sinistral shearing during the
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transpressional deformation and shear zone development between 570 and 550 Ma (Goscombe et
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al., 2005b). An early, sub-horizontal, bedding-parallel schistosity was folded and locally transposed
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during the development of the shear zones and associated steeper, mylonitic foliation within the
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Western and Central Kaoko Zones (Goscombe et al., 2003a; Goscombe & Gray, 2007). These shear
133
zones and associated mylonitic foliation represent inherent crustal weakness within the Kaoko Belt
134
that may have been reactivated by extension during the break-up of Gondwana, particularly where
135
subvertical as within the orogen core (Salomon et al. 2015a & 2017).
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The voluminous tholeiitic magmatism of the Paraná–Etendeka Magmatic Province
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developed during the early Cretaceous (119-139 Ma / 127-137 Ma: Almeida et al., 2013 & Turner et
141
al., 1994) and is thought to be related to the impact of the head of the Tristan da Cunha plume on
142
the base of the lithosphere of Northwest Namibia at ca. 136 Ma (Connor & Duncan, 1990; Peate et
143
al., 1990; Ryberg et al., 2015). The Walvis Ridge has been interpreted to represent the offshore
144
continuation of this plume related magmatism albeit at a lesser scale (O’Connor & Duncan, 1990;
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Ewart et al., 1998, Renne et al., 1996b).
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The distribution of igneous rocks of the Paraná-Etendeka Magmatic Province displays a
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marked asymmetry relative to the Atlantic Rift (Peate et al., 1990; Turner et al., 1994 - Fig. 2). The
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Paraná volcanics presently cover some 1.2 x 106 km2 of the South American continent, with a
149
preserved thickness of up to 1,700m (Melfi et al., 1988; Peate et al., 1990; Peate, 1997). In contrast,
150
the Etendeka volcanics of NW Namibia display a maximum thickness of 900m and an aerial extent of
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approximately 78,000 km2 (Ewart et al., 2004). This difference in preserved volume and areal extent
152
has been attributed to differences in paleotopography (Peate et al., 1990) and to variations in post5
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154
et al., 2002; Trumbull et al., 2004). However a lack of preservation alone does not explain the
155
significant volume of quartz-latites (rhyolites) in NW Namibia, where they represent nearly 50% of
156
the preserved volume of the Etendeka Province (Ewart et. al., 2004). In contrast, the rhyolite lava
157
flows of the Paraná Province make up only 3% of the total eruptive volume (Bellieni et al., 1984,
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1986). The quartz-latites and basalt lava flows of the Etendeka volcanic sequence have been dated
159
to between 136-130Ma (Renne et al., 1996; Dodd et al., 2015) and are associated with the ca. 132-
160
123 Ma old Damaraland Intrusive Complexes (Allsopp et al., 1984; Milner et al., 1995; Renne et al.,
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1996; Wigand et al., 2004). Within the study area, this magmatism was also accompanied by the
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intrusion of predominantly NNE trending tholeiitic dykes that parallel the trend of the Kaoko Belt,
163
and together form the SCDS, characterised in this study (Fig. 1 & 2). Trumbull et al. (2005) used high-
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resolution aeromagnetics to define the NE-trending Henties Bay-Outjo dyke swarm (HOD) within the
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Damara Belt to the south of the study area (Fig. 2).
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SMDS
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RIGDS
PGDS
SCDS
FDS HOD
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Figure 2 - Tectonic reconstruction of part of Western Gondwana at ca. 139Ma prior to the opening of the South Atlantic Ocean and showing the Jurassic-Cretaceous tholeiitic dyke swarms: Vitória-Colatina (VCDS), Resende-
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Ilha Grande (RIGDS), Serra do Mar (SMDS), Ponta Grossa-Guapiara (PGDS) Florianópolis (FDS), Skeleton Coast (SCDS), Henties Bay-Outjo (HOD). Modified from Almeida et al. (2014). Note: The South American continent has been rotated 40°W to its pre-break up position whilst the African continent remains fixed.
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According to published K-Ar ages the tholeiitic dykes of NW Namibia range were emplaced between 116-144 Ma (Siedner & Mitchell, 1976; Hunter & Reid,
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1987). More precise Ar-Ar dating also support an Early Cretaceous age for the tholeiitic dykes with Ewart et al. (1984) reporting
174
plagioclase of 125–130Ma from the Horingbaai dolerite dikes and Renne et al. (1996) obtaining an 40Ar–39Ar plagioclase age of 132 ± 0.7 Ma from a gabbro
175
sill in the Huab region within the southern portion of the Etendeka Province (Table 1).
40
Ar–39Ar ages for
Table 1- Summary of published radiometric ages for the tholeiitic dykes and associated intrusions of northwest Namibia and southern Brazil. Dyke Swarm
Lithologies
Orient.
Age (Ma)
Ponta Grossa, Brazil
PGDS
basalt, basaltic andesites, andesite, trachyandesite, rhyolite
N04E-N84E & N71W-N20W
120.7 ±1.3- 131.4
40
Santa Catarina Island, Brazil
FDS
Dolerites
116.4 ±3.9 - 129.4 ±0.3
Santa Catarina Island, Brazil
FDS
Basalt
119.0 ±0.9 - 128.3 ±0.5
FDS
Diabase, Trachyandesite
FDS
Basalt, Basaltic trachyandesite
Damara Belt, Namibia
HOD
Dolerites
Horingbaai, Namibia
HOD
Dolerites
Damaraland Intr. Compl.
-
Ne-syenites, Comendite
No. of samples
Reference
39
Plagioclase
17
Renne et al., 1996a
40
39
plagioclase
2
Deckhart et al., 1998
40
Ar / Ar
39
plagioclase
5
Raposo et al., 1998
ID-TIMS U–Pb
baddeleyite / zircon
3
Florisbal et al., 2014
Ar / Ar
39
plagioclase
2
K - Ar
whole Rock
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Ar / Ar Ar / Ar
N30E & N50E
119 - 139
N.D
116 - 144
N.D
125 - 130
40
39
plagioclase
2
Veronez & Tomazzoli, 2016 Siedner & Mitchell, 1976 Ewart et al., 1984
126.9 ±0.6 - 137.0 ±0.7
40
39
whole Rock
3
Milner et al., 1995
-
132.0 ± 0.7
40
39
plagioclase phlogopite, kaersutile plagioclase
1
Renne et al., 1996b
2
Wigand et al., 2004
1
Kirstein et al., 2005
plagioclase hornblende / biotite
1
Kirstein et al., 2005
1
Stewart et al., 1996
-
40
Ar / Ar Ar / Ar
Huab Sill, Namibia
-
Erongo Complex, Namibia
-
Basanite-tephrite
-
130.8 ±1.0 -132 ± 1.0
40
39
Huab Sill, Namibia
-
Gabbro
-
130.5 ± 0.8
40
39 39 39
Ugab River, Namibia
SCDS?
Messum complex, Namibia
-
Gabbro
134.7 ±0.3 - 133.9 ±0.7
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Silveira & Pinheira Beaches, Santa Catarina, Brazil Ponta do Pasto, Santa Catarina Island, Brazil
N15E & N30W N10W & N15E-N55E
Analysed material
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Location
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Ar / Ar Ar / Ar Ar / Ar
Dolerite
N.D
129.5 ± 1.5
40
Syenite
-
127.2 ±1.2 -129.1 ±1.4
40
Ar / Ar Ar / Ar
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Florisbal et al. (2014) by ID-TIMS U-Pb dating of baddeleyite and zircon extracted from three dykes
179
of the Florianopolis Dyke Swarm (FDS). However, it should be noted that all three of the dated
180
samples were taken from dykes of the dominant NNE-trending family. Previous work by Raposo et
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al. (1998) and Deckart et al. (1998) identified at least two dyke generations of differing orientations
182
within the FDS. 40Ar-39Ar dating by the same authors suggest a longer interval of between ca.129 to
183
116 Ma for the intrusion of the FDS and recent 40Ar-39Ar dating by Veronez & Tomazzoli (2016) also
184
obtained similar results with age ranging between 139 to 119 Ma and identified at least three
185
generations of dykes. A summary of published radiometric ages for the tholeiitic dykes and
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associated intrusions of northern Namibia and southern Brazil is presented in Table 1.
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The geochemical classification of magma types and their distribution within a Large Igneous
188
Province (LIP) can provide insights into the eruptive history of the igneous province. Marsh et al
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(2001) consolidated the previous work by Erlank et al. (1984) and defined 8 types of basic magmas
190
and 16 types of acid magmas for the Etendeka province, based on the geochemical and petrographic
191
characteristics of over 1000 samples of the Etendeka volcanics and associated intrusives. The basic
192
magmas include the High-TiO2 (TiO2 > 2.2% & Sr > 450ppm) Khumib basalts which are prevalent in
193
the north of the Etendeka province and are considered to be equivalent to the Urubici-type basalts
194
of the Paraná province (Marsh et al., 2001; Peate, 1997). However, majority of the mafic magmas of
195
Paraná-Etendeka magmatic province are Low TiO2 and in Namibia 7 types of Low-TiO2 magma have
196
been identified: Tafelberg, Kuidas, Horingbaai, Huab, Tafelkop, Albin and Esmeralda. The
197
characteristics of the five principle magmas: Khumib, Tafelberg, Esmeralda, Horingbaai are
198
summarized in Table 2 and discussed in more detail below. However it should be noted that,
199
Trumbull et al. (2004) highlighted the importance of the Tafelkop magma type, as despite their small
200
areal extent at the base of the Etendeka volcanic sequence, the distinctive geochemistry of these Fe
201
& Mg-rich basalts indicate a strong influence of the Tristan plume (Ewart et al., 1998; Gibson et al.,
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2000; Thompson et al., 2001).
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Table 2- Geochemical characteristics of the principal, basic magmas of the (Paraná-) Etendeka Province 1,2
Province High-TiO2
Etendeka Khumib
Parána Urubici
Mg# 29 - 63
Low-TiO2
Tafelberg
Gramado
26 - 65
TiO2% > 2.2
Sr -ppm > 450
< 2.2
< 450
2
2
Zr/Y 4.9 - 9.8
Ti/Zr 47 - 82
2.6 - 7.1
33 - 91
Esmeralda Esmeralda 26 - 46 2.8 - 4.4 57 - 162 Horingbaai 37 - 77 2.8 - 5.4 57 - 128 1) Magnesium number: Mg# = 100x[MgO mol% /(FeO mol% + MgO)]; 2) Values for the Etendeka Province are based on geochemical data published by Ewart et al. (1998, 2004), Jerram et al. (1999), Marsh et al. (2001), Marsh & Milner (2007), Thompson et al. (2001), Trumbull et al. (2007)
9
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The Tafelberg type is the dominant magma of the Etendeka Province and can be correlated with the Gramado basalts (Marsh et al., 2001). Peat and Hawkesworth, (1996) suggested that this
209
magma composition resulted from the crustal contamination of a lithospheric mantle melt. The
210
Esmerald type also occurs on both sides of the Atlantic and is considered to be derived from the
211
mixing of a Tafelberg/Gramado type magma with a depleted asthenospheric melt (Peate and
212
Hawkesworth, 1996). An asthenospheric source has also been proposed for the Horingbaai magma
213
which displays MORB-like geochemical and isotopic characteristics (Erlank et al., 1984; Ewart et al.,
214
2004; Marsh et al., 2001). Igneous rocks of the Horingbaai magma type are olivine normative and
215
occur as dykes and sills intrusive within Neoproterozoic metasediments, Karoo sediments. In some
216
places have been observed to cut lava flows of the Etendeka Group (Marsh et al., 2001). This
217
indicates that they post-date the main eruptive phase of the Paraná-Etendeka Magmatic Province.
218
Horingbaai type magmas have not been described within the Paraná Province (Marsh et al., 2001;
219
Thompson et al., 2001). Comparisons between the geochemical characteristics of the dykes of the
220
SCDS sampled during this study and the aforementioned magma types are made in sections 6.1 &
221
6.2 of this paper.
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4. Methodology
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4.1 Aeromagnetic data processing and interpretation
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High resolution aeromagnetic data is an important tool for mapping mafic dykes as they create
227
distinctive, linear magnetic anomalies due to the generally high contrast in magnetic susceptibility
228
between the dyke and their country rocks. For the purpose of this study, four areas of high
229
resolution magnetic data was acquired from the Geological Survey of Namibia (GSN),covering a total
230
of 19,665 km of NW Namibia, - Fig. 1.
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The aeromagnetic data was collected during aerial surveys undertaken by GPX Surveys and
232
Geoterrex between 1995 and 2010 as part of a regional acquisition program under the supervision
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of the GSN. These surveys were flown with a nominal terrain clearance of 80-100m with the
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exception of the survey over the Brandberg massif which was flown at 2895 metres above sea level,
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which corresponds to an altitude some 300m higher than the highest elevation within the survey
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area. The surveys were flown along N-S or E-W traverse lines generally spaced at 200m with the
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exception of one of the earlier surveys which was flown with 400m spacing. Tie lines were flown at
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2500m intervals. The data was corrected during acquisition to remove regional variation by
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subtracting the 2005 model of the International Geomagnetic Reference Field (IGRF) to obtain the
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residual total magnetic intensity (RTMI). Some post-acquisition interpolation was undertaken by the
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GSN to produce integrated grids for each of the areas with spatial resolutions of 50m. RTMI maps
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were produced for the four areas of high-resolution data and integrated with lower resolution RTMI
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maps available through GSN's Earth Data Namibia website, to produce a composite RTMI map of NW
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Namibia. A synthesis of steps taken during the processing of the magnetic data during this study is
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presented in Fig. 3. The high-resolution data was initially reduced to pole to centre the anomalies
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over their sources; however this procedure created false E-W anomalies likely to be the result of the
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amplification of noise in the data that was not removed by levelling between survey traverse lines.
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The unreduced magnetic data was compared with satellite imagery and it was noted that there was
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little difference in location between outcropping dykes and their corresponding magnetic anomaly.
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Since the bipolarity of the magnetic anomalies in the data is also not pronounced, it was assumed,
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that the local effect of magnetic declination and inclination of the magnetic field wouldn't greatly
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impact on the mapping of magnetic anomalies.
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Figure 3 - Synthesis of the processing of the high-resolution aeromagnetic data as exemplified by the Area 4 (Torra Bay) data, modified from Ruy et al., 2006. RTMI - Residual Total Magnetic Intensity; RTP - RTMI reduced to the magnetic pole; dZ - first vertical derivative; dX - first horizontal derivative in X (E-W of the RTMI; dY - first horizontal derivative in Y (N-S) of the RTMI; AS - Analytical Signal; THG- Total Horizontal Gradient of the RTMI; EULER DECONV. - Solutions to the Euler Deconvolution Process.
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and the two horizontal derivatives (dX & dY) for each area. Further processing resulted in maps of
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the Analytic Signal (AS) and Total Horizontal Gradient (THG). The Euler Deconvolution process was
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applied to the data to estimate the depths of the magnetic anomalies' sources. The maps of AS, THG
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and dZ were compared with Google Earth and Landsat 8 imagery, and with digital elevation models
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(SRTM and AsterGDEM) to manually map the magnetic anomalies/dykes within the areas with the
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high-resolution aeromagnetic data. In the rest of the study area dykes were mapped through the
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comparison of the lower resolution map of RTMI with satellite imagery and digital elevation models.
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4.2 Geochemistry
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Samples were taken of relatively fresh, unweathered material from 26 dykes and 3 lava flows of the
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Etendeka group. In some localities dykes outcrop as a line of cobbles, in which case care was taken
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to remove weathered material using a diamond saw prior to geochemical analysis. This was
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undertaken at Rio de Janeiro State University by the Geology Faculty's Sample Preparation
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Laboratory (LGPA-FGEL/UERJ). Selected fragments of the samples were sent to Activation
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Laboratories Ltd. (ACT Labs -http://www.actlabs.com) in Ontario, Canada, for further sample
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preparation and major and trace element geochemistry. The samples were crushed to a nominal
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minus 10 mesh (1.7 mm), mechanically split to obtain a representative sample (250g) and then
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pulverized to at least 95% minus 150 mesh (105 microns) in a mild steel mill to avoid Cr and Ni
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contamination. Smaller volumes (3g & 5g) of each sample were separated for lithium metaborate /
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tetraborate fusion. The resulting molten bead is rapidly digested in a weak nitric acid solution. This
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technique ensures that the entire sample is dissolved and guarantees that the major oxides including
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SiO2, refractory minerals, Rare Earth Elements (REE) and other high field strength elements are put
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into solution. Major element geochemistry was then determined by inductively coupled plasma
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optical emission spectrometry (ICP-OES) and the concentrations of 45 trace elements, including REE,
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were determined by inductively coupled plasma mass spectrometry (ICP-MS). The results of this
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analysis are presented in section 5.2 of this paper.
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4.3 Kinematic analysis of dykes
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Linear dyke swarms possess a special tectonic significance as their geometry varies according to the
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orientation of the regional stress field active during their emplacement (Fialka & Rubin, 1999; Jolly &
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Sanderson, 1995; Pollard, 1987). Such dyke swarms are considered to be the physical manifestation
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of crustal extension and thus tend to be orientated perpendicular to the direction of minimum stress
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(σ3) (Anderson, 1951; Hoek, 1991). According to Pollard (1987), dykes propagate as extensional
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these structures present a favourable orientation. Asymmetrical features such as zigzagging,
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branching and en echelon dykes and bridges between dyke segments indicate an oblique
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component to extension during dyke emplacement. In this case additional criteria, such as variations
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in dyke width or offset of extensional markers, must be used to determine the direction of
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extension. The above criteria and the kinematic analysis of asymmetrical features allow the
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estimation of the orientation of the regional stress field active during dyke intrusion (Rickwood,
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1990; Corrêa Gomes, 2001) - Figure 4.
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Figure 4 - Asymmetrical features that indicate oblique extension and shear movement during dyke emplacement: a) Zigzag b)En echelon dykes; c) Ramifications; d) steps with variation in dyke thickness; e) rotated enclave; f)other features which indicate the shear sense 1 .& 2. steps with variation in dyke thickness, 3. bridge between dyke segments, 4. branches. Modified from Corrêa Gomes et al. (1996).
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The orientation of internal fractures can also be used to determine the orientation of the
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local stress field during emplacement of the dyke, as immediately following crystallization recently
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nucleated fractures tend to orientate themselves parallel to the three stress axes (σ1>σ2>σ3).
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Corrêa Gomes et al. (1996) identified three families of internal fractures within a hypothetical
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vertical dyke emplaced under conditions of normal extension: 1) longitudinal fractures (fcl) -
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subvertical & parallel to the dyke walls; 2) transversal fractures (fct) - subvertical & perpendicular to
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the dyke walls; 3) basal fractures (fcb) - subhorizontal - Fig. 5a. In this case, the orientation of the
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longitudinal and transversal fractures represent the direction of maximum and minimum stress (σ1
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& σ3) respectively. Variation from this fracture pattern may indicate an oblique component during
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dyke emplacement (Corrêa Gomes et al., 1996) - Fig 5b.
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5. Results and Interpretations
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Figure 5 a) Internal fractures of a hypothetical N-S dyke with normal extension: longitudinal fractures (fcl - grey lines) are subvertical and parallel the dyke walls, transversal fractures (fct - black lines) are also subvertical, but orientated perpendicular to the dyke walls, and the basal fractures (fcb - white lines) are subhorizontal. Note in the stereogram that the poles to the longitudinal indicate the direction of extension (σ3). b) Zigzagging dyke with internal fractures oblique to the walls of the central segment, indicating the influence of the regional stress field during dyke emplacement. Modified from Corrêa Gomes et al., 1996
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5.1 Dyke distribution and orientation Aeromagnetic data and satellite imagery were used to map some 4579 dykes of the SCDS across north-west Namibia - Fig 1. The orientation of these dykes was extracted in ArcGIS using the COGO tool resulting in some 27,283 linear dyke segments. The majority of the dykes are coast parallel; NNW-SSE (N25W) orientated, and occur up to 200km from the coast. Some of these dykes are greater than 50m wide and extend for kilometres along strike. Many authors (e.g. Jourdan et al., 2006; Salomon et al. 2017; Trumbull et al. 2004 & 2007; Will & Frimmel, 2013) stress the influence of pre-existing structures on dyke orientation. However, whilst the majority dykes of the SCDS trend sub-parallel to the principal structures of the Kaoko Fold Belt (Fig. 6), they are subvertical and are often observed cutting across Pan-African shear zones and associated mylonitic foliations. The fact that the dykes of SCDS are almost exclusively subvertical suggests that the direction of minimum stress (σ3) was subhorizontal during their emplacement. The Pan-African structures are of a similar orientation (N15W) to the dominant dyke direction (N25W). However, the dip of these structures varies from subhorizontal in the coastal terrain and foreland of the Kaoko Belt, to subvertical in the orogen core. Field observations indicate that extensional reactivation of these structures was largely restricted to the orogen core. ENE-WSW dykes were also mapped, these dykes are thinner (0.5-5m wide), occur with greater frequency close to the coast (Fig. 7a), and cut across the older N25W orientated dykes. Less common N80W to N45W orientated dykes were also mapped, and whilst cross-cutting relationships were not clear these dykes may represent a transitional phase (T2) between the dominant NNW-trending dykes (T1) and the younger ENE-trending dykes (T3). The geometry and kinematics of the dykes of the SCDS are discussed in detail in section 5.3 of this paper. 14
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Figure 6 - Rose diagrams comparing the directions of Eo-cretaceous dykes of the SCDS and the principal Pan-African structures of the Kaoko Belt. a) Dykes (n=27283, intervals de 10°, max. freq. 11.5% - 3126 dyke segments); b) Pan-African structures (n=2279, 10° intervals, max. freq. 23.7% - 541 segments).
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Figure 7 - Rose diagrams comparing the directions of dykes of the SCDS: a) <50 km from the coast (n=20892, 10° interval, max. freq. 11.2% - 2340 dyke segments); and b) >50 km from the coast (n=1790, 10° intervals, max freq. 25.1% - 449 dyke segments). Note: The rose diagrams do not include the dykes from the Ugab zone of the Kaoko Belt where dyke display greater variation in orientations possibly related to local changes in the regional stress field associated with the Damaraland intrusive complexes.
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5.2 Geochemistry and Petrography Previous geochemical and petrographic studies of the Cretaceous dykes of north-west Namibia have generally focussed on dykes outcropping along the coast (e.g. Thompson et. al., 2001) or on the dykes of the Henties Bay-Outjo dyke swarm (Trumbull et. al., 2004 & 2007). Some comparisons are made between the compositions of the dykes of the SCDS, the Henties Bay-Outjo Dyke Swarm (HOD) and with principal magmas of the Paraná-Etendeka province. A more detailed discussion, including a comparison with the compositions of the dykes of the FDS, is presented in section 6.1 of this paper. 29 samples were taken of relatively fresh, unweathered material from 26 dykes and 3 lava flows of the Etendeka group during field work undertaken in September 2015.. Major and trace element geochemistry of 26 samples of dykes and 3 samples of lava flows of the Etendeka group, was determined by ICP-OES and ICP-MS respectively by Activation Laboratories Ltd. (ACT Labs) in 15
ACCEPTED MANUSCRIPT Ontario, Canada. Sample preparation procedures and analytical techniques are described in section 4.2 of this paper. Sample coordinates lithologies and concentrations of major elements, selected trace elements and Rare Earth Elements (REEs) are listed in Table 3. The majority of the dykes sampled were classified as subalkaline basalts and basaltic andesites (Figure 8a) and they form part of the tholeiitic series (Figure 8b). Two dacite dykes were also encountered; these dykes are porphyritic with altered, fine to medium grained phenocrystals of K-Feldspar within a fine grained, intergranular matrix composed of K-Feldspar and quartz. Two dykes can be classified as alkaline rocks (Fig. 8), the first being a N30W-trending nephelinite dyke (NA024A) outcropping to the north-west of the Brandberg igneous complex, and the other is an olivine bearing, alkaline basalt dyke (NA026) from the same region and with a similar orientation (N50W) as the nephelinite dyke.
395 396 397 398 399 400 401 402 403 404 405 406 407
Figure 8 a) Total alkali-silica diagram (Cox et al 1979) shows that the dykes sampled in this study range in composition from basaltic to dacitic, but the majority of the dykes are subalkaline basalts and basaltic andesites. b) The sampled dykes and lava flows are part of the tholeiitic series according to the AFM diagram of Irvine & Baraga (1971). The dykes are represented by blue circles, whilst the orange squares represent the three lava flows of the Etendeka Group that were also sampled during this study.
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Table 3- Whole rock geochemical analysis of the Skeleton Coast Dyke Swarm and Etendeka Group lavas (1).
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SiO2 40.12 45.82 64.32 63.03 46.31 45.71 51.31 47.81 46.44 47.13 49.75 55.74 54.54 54.28 53.17 52.44 52.67 53.01 52.34 52.12 54.23 48.29 52.72 52.00 50.00 51.78 51.43 52.53 51.31
Al2O3 10.40 14.86 12.85 12.19 12.31 12.26 14.20 13.55 13.95 13.96 14.00 13.20 13.97 13.12 12.15 12.71 13.26 13.33 13.17 12.32 13.78 14.33 12.97 13.38 14.22 12.80 15.05 13.63 13.48
Fe2O3 (T) 17.92 15.29 8.61 10.02 12.03 12.41 12.51 12.33 12.11 12.96 11.16 11.94 12.65 11.97 16.05 15.62 14.19 14.45 14.41 15.93 12.94 11.85 13.22 13.53 11.90 14.11 13.52 13.80 13.14
MnO 0.21 0.19 0.10 0.13 0.18 0.18 0.14 0.16 0.17 0.18 0.17 0.16 0.19 0.16 0.20 0.19 0.22 0.20 0.20 0.22 0.18 0.18 0.20 0.19 0.18 0.21 0.21 0.18 0.20
MgO 9.65 8.56 0.67 2.04 14.64 14.42 4.82 5.68 10.97 9.99 7.10 4.29 4.51 4.35 3.58 3.97 4.40 4.36 4.17 4.05 4.96 8.60 5.62 5.78 7.69 5.93 4.75 5.54 5.97
CaO 9.11 8.95 3.10 3.87 10.21 10.44 8.19 8.78 10.28 11.71 11.91 7.73 8.49 7.92 6.97 7.64 8.44 8.06 8.12 8.47 8.34 11.99 9.59 8.35 12.64 10.41 9.79 9.46 10.68
Na2O 4.39 3.18 2.30 3.46 1.87 1.61 2.67 2.33 1.81 2.12 2.53 2.53 2.95 2.39 3.15 2.70 2.81 2.75 2.83 2.70 2.72 2.08 2.26 2.92 2.09 2.45 2.71 2.44 2.22
K2O 1.02 0.61 5.11 3.70 0.25 0.09 1.78 1.24 0.52 0.28 0.34 1.83 1.03 1.95 1.54 1.34 1.39 1.63 1.45 0.87 1.56 0.22 1.10 1.54 0.28 0.47 0.94 0.95 0.51
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Geochem. Alkaline Alkaline Acid dykes Acid dykes Olivine Thol. Olivine Thol. Kh- High TiO2 Kh- High TiO2 QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (E-MORB) QT (E-MORB) QT (E-MORB) QT (E-MORB) QT (E-MORB) QT (E-MORB) QT (E-MORB) QT (E-MORB)
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Lithology Nephen. Alk. Basalt Dacite Dacite Basalt Basalt 1 Basalt 1 Basalt Basalt Basalt Basalt 1 Andesite Bas. and. Bas. and. Bas. and. Bas. and. Bas. and. Bas. and. Bas. and. Bas. and. Bas. and. Basalt Bas. and. Bas. and. Diabase Basalt Basalt Bas. and. Basalt
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Long. (°E) 14°05'17'' 14°04'31'' 14°41'16'' 12°43'31'' 14°04'56'' 14°05'00'' 12°45'32'' 12°45'35'' 14°05'42'' 14°03'53'' 14°04'58'' 13°00'03'' 12°55'56'' 12°55'57'' 12°43'49'' 12°55'34'' 12°50'45'' 12°50'45'' 12°52'42'' 12°53'28'' 13°09'06'' 14°05'00'' 13°17'16'' 13°17'18'' 12°43'49'' 12°42'19'' 12°43'25'' 13°25'59'' 13°14'47''
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Lat (°N) -20°48'22'' -20°46'54'' -21°05'22'' -19°20'43'' -20°52'31'' -20°48'57'' -18°08'34'' -18°08'33'' -21°02'31'' -20°54'47'' -20°52'17'' -19°55'14'' -19°46'31'' -19°46'35'' -19°23'42'' -18°57'23'' -18°52'10'' -18°52'09'' -18°51'28'' -18°49'50'' -18°18'03'' -20°48'57'' -20°20'38'' -20°20'35'' -19°23'42'' -19°22'13'' -19°20'44'' -19°11'14'' -19°15'53''
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Sample NA024A NA026 NA011B NA044 NA022 NA023B NA093A NA095 NA016 NA020 NA021 NA032 NA034 NA035 NA038C NA069A NA078A NA079 NA081C NA082 NA099 NA023A NA029D NA030 NA038D NA040 NA045D NA053 NA066
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TiO2 3.67 2.15 0.91 1.21 1.11 0.93 3.29 3.53 0.95 1.29 1.41 1.34 1.45 1.36 1.95 1.83 1.75 1.81 1.75 1.77 1.42 1.13 1.12 1.19 0.97 1.22 1.45 1.40 1.83
P2O5 1.03 0.28 0.32 0.17 0.12 0.09 0.44 0.45 0.13 0.13 0.13 0.20 0.19 0.19 0.29 0.22 0.26 0.27 0.27 0.21 0.17 0.11 0.15 0.15 0.12 0.14 0.15 0.16 0.19
LOI 1.94 -0.05 1.89 0.92 0.73 1.34 1.22 4.00 1.51 0.83 0.90 0.82 0.56 1.21 1.29 1.08 1.18 0.48 0.71 0.90 0.39 0.78 0.74 0.81 -0.14 0.17 0.73 0.54 0.46
Total 99.46 99.84 100.2 100.7 99.75 99.49 100.6 99.86 98.84 100.6 99.42 99.79 100.5 98.9 100.3 99.75 100.6 100.3 99.41 99.58 100.7 99.55 99.7 99.85 99.95 99.7 100.7 100.6 99.99
Major elements express in wt. %, trace elements and REE elements in ppm. LOI - Loss of ignition. Nephen. - nephelinite, Alk. basalt - alkaline basalt, Bas. and. - basaltic andesite, Ol. Thol. Tholeiite, Kh - High Ti, High Ti02 Khumib basalts, QT (Enrich) - enriched quartz-tholeiites, QT (E-MORB) - 'primitive' quartz-tholeiites similar to E-MORB. The detection limits for each element are listed on the Activation Laboratory's webpage. http://www.actlabs.com)
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Table 3- cont. Lithology Geochem. Sr Y Zr La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Nephen. Alkaline 1553 26 386 85.7 180 19.9 76 14.4 4.46 11.6 1.5 7.3 1.1 2.6 0.32 1.8 0.23 Alk. Basalt Alkaline 490 20 124 14.2 34.4 4.42 20.5 5.2 1.83 5.3 0.8 4.8 0.9 2.3 0.31 1.9 0.29 Dacite Acid dykes 155 72 668 90.1 184 21 82.6 16.7 3.34 15.3 2.3 13.9 2.7 7.7 1.13 7.4 1.07 Dacite Acid dykes 148 42 311 65.6 139 15.4 59.8 11.7 1.64 10.1 1.6 9.1 1.7 4.9 0.72 4.6 0.66 Basalt Olivine Thol. 192 15 65 6.6 15.8 2.14 10.1 3 1.05 3.4 0.5 3.3 0.6 1.8 0.26 1.6 0.23 Basalt Olivine Thol. 180 13 50 4.3 10.8 1.51 7.8 2.2 0.85 2.8 0.4 2.7 0.5 1.4 0.21 1.3 0.2 1 Basalt 1 Kh- High TiO2 698 27 283 44.1 93.2 11.4 47.2 10.4 3.26 9.1 1.3 7.1 1.2 3.3 0.43 2.7 0.35 1 Basalt Kh- High TiO2 688 28 292 44.2 95.4 11.7 48.5 10.8 3.43 9.6 1.4 7.5 1.3 3.3 0.45 2.7 0.37 NA095 Basalt QT (Enrich.) 169 19 81 9.5 21 2.61 11.8 3.1 1.03 3.7 0.6 4 0.8 2.3 0.36 2.3 0.35 NA016 Basalt QT (Enrich.) 179 18 68 4.2 11.4 1.71 8.7 2.8 1.11 3.6 0.6 3.5 0.7 1.9 0.27 1.7 0.25 NA020 Basalt QT (Enrich.) 211 19 91 6.7 17 2.47 12.1 3.7 1.3 4.5 0.8 4.7 0.9 2.5 0.35 2.2 0.32 NA021 1 Andesite QT (Enrich.) 199 28 168 26.7 56.3 6.55 26.8 6 1.58 6 1 5.9 1.2 3.2 0.47 3.1 0.45 NA032 Bas. and. QT (Enrich.) 216 27 147 22.2 47 5.44 22.3 5.4 1.53 5.7 0.9 5.6 1.1 3.1 0.45 2.9 0.44 NA034 Bas. and. QT (Enrich.) 208 28 140 21.3 45.7 5.29 21.6 5.4 1.57 5.8 0.9 5.5 1.1 3.1 0.45 2.9 0.44 NA035 QT (Enrich.) 193 34 176 25.2 54.4 6.63 27.7 6.8 1.98 7.3 1.2 7.1 1.4 4 0.58 3.7 0.53 NA038C Bas. and. QT (Enrich.) 199 32 151 19.7 43 5.39 23.2 6 1.83 6.8 1.2 7.2 1.5 4.1 0.6 3.9 0.59 NA069A Bas. and. QT (Enrich.) 282 28 191 29.5 61.2 7.31 29 6.6 1.9 6.7 1.1 6.4 1.2 3.6 0.5 3.1 0.49 NA078A Bas. and. Bas. and. QT (Enrich.) 281 29 195 31.6 64.2 7.49 30.2 6.9 1.96 6.9 1.1 6.8 1.3 3.7 0.54 3.5 0.51 NA079 QT (Enrich.) 288 31 178 30.6 62.3 7.49 30.3 6.7 1.97 7 1.1 6.6 1.3 3.6 0.53 3.3 0.51 NA081C Bas. and. Bas. and. QT (Enrich.) 203 31 151 20.1 43.2 5.42 22.9 6 1.81 6.7 1.1 7.1 1.4 4.1 0.59 3.8 0.57 NA082 Bas. and. QT (Enrich.) 233 23 133 21.5 45.8 5.5 22.5 5.2 1.58 5.5 0.9 5.6 1.1 3.1 0.44 2.9 0.42 NA099 QT (E-MORB) 203 16 70 5.8 14.1 1.95 9.8 3 1.06 3.5 0.6 3.5 0.7 1.9 0.28 1.7 0.25 NA023A Basalt QT (E-MORB) 146 25 106 12.9 28.5 3.48 15 4.1 1.25 4.9 0.8 4.9 1 2.8 0.43 2.8 0.4 NA029D Bas. and. Bas. and. QT (E-MORB) 173 26 104 13.1 28.4 3.43 15.1 4 1.24 4.7 0.8 5.1 1 2.9 0.44 2.7 0.42 NA030 QT (E-MORB) 182 16 60 5.6 13.2 1.78 8.9 2.7 0.97 3.3 0.5 3.3 0.7 1.8 0.26 1.7 0.24 NA038D Diabase Basalt QT (E-MORB) 135 25 85 7.4 17.4 2.36 11.1 3.4 1.28 4.5 0.8 5 1 2.9 0.42 2.7 0.42 NA040 QT (E-MORB) 169 27 98 9.8 21.9 2.87 13.4 4 1.34 5.2 0.8 5.5 1.1 3.1 0.45 2.9 0.43 NA045D Basalt Bas. and. QT (E-MORB) 213 26 121 15 33.5 4.08 17.8 4.5 1.39 5.1 0.9 5.5 1.1 3 0.45 2.9 0.42 NA053 Basalt QT (E-MORB) 282 23 132 14.8 33 4.24 18.9 4.9 1.72 5.7 0.9 5.7 1.1 3.1 0.45 2.9 0.45 NA066 Major elements express in wt. %, trace elements and REE elements in ppm. LOI Loss of ignition. Nephen. - nephelinite, Alk. basalt - alkaline basalt, Bas. and. - basaltic andesite, Ol. Thol. Tholeiite, Kh - High Ti02, High Ti02 Khumib basalts, QT (Enrich) - enriched quartz-tholeiites, QT (E-MORB) - 'primitive' quartz-tholeiites similar to E-MORB. The detection limits for each element are listed on the Activation Laboratory's webpage. http://www.actlabs.com)
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The alkaline basalt dyke (NA026) contains fine grained olivine phenocrystals within a very fine grained matrix composed of pyroxene, plagioclase and abundant opaque minerals. Petrographic analysis and the CIPW normative mineralogy of Hutchison (1974, 1975), indicated that two dykes (NA022 & NA023B), can be classified as olivine-tholeiites and are considered to form a distinctive geochemical group. These olivine basalts are fine grained, holocrystalline with subhedral phenocrystals of plagioclase, pyroxene and olivine and display subophitic and poikilitic textures. It is worth noting that these dykes were encountered within the Ugab zone in the extreme south of the Kaoko Belt. The rest of the dykes can be classified as quartz-tholeiites (basalts and basaltic andesites) with normative quartz and hypersthene. These dykes are generally fine grained and contain plagioclase, augite and in some samples, pigeonite. The majority of the basaltic andesites are porphyritic, with plagioclase and pyroxene phenocrystals which often form glomerocrysts. Some plagioclase phenocrystals are zoned indicating fractional crystallisation. All of the quartz-tholeiite dykes can be classified as Low-TiO2 according to the criteria defined by Marsh et al. (2001), as they contain <2.2% TiO2 and < 450ppm Sr. The variation in the concentration of chondrite-normalised REEs was used to subdivide the Low-TiO2 quartz-tholeiites into two geochemical groups: enriched quartz-tholeiites and 'primitive' quartz-tholeiites (Fig. 9). The former display relative enrichment in LREEs (Fig. 9a) as exemplified by La/Yb ratios which vary from 3.4-6.5 (Fig. 10). These dykes also display large negative Eu anomalies indicating significant fractional crystallisation of plagioclase. The latter group of quartz-tholeiites display less enrichment with La/Yb ratios less than 3.4, and smaller negative Eu anomalies (Fig. 9b & 10). The enriched quartz-tholeiitic dykes are more widespread and generally strike NNW, whilst the quartz-tholeiitic dykes with more primitive compositions are more common near the coast and display orientations that vary between 250-315°N (ESE-NW). b)
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Figure 9- Chondrite normalized REE (McDonough and Sun, 1995) for basalts of the Skeleton Coast dyke swarm. a) Enriched quartz tholeiites in orange; b) 'Primitive" Quartz tholeiites in dark blue. The light grey area represents the Tafelberg magma composition from Ewart et. al. (1998 & 2004), Marsh et al. (2001), Marsh & Milner (2007) Thompson et. al. (2001) and Trumbull et. al. (2007). The dark grey area represents the compositional range of the Horingbaai dykes from Marsh et. al. (2001), Thompson et al. (2001) and Trumbull et al. (2007). Compositions of ca. 74 Ma basalts from DSDP Leg 74 525A (29°04'14"S 2°59'7"E) drilled within the southern extension of the Walvis Ridge (Hoernel et al., 2015), as well as N-MORB & E-MORB (Sun & McDonough, 1989) are also plotted for comparison.
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Figure 10- (La/Yb)N vs. MgO (wt%) of the quartz-tholeiite dykes(circles) of the SCDS are compared with the two High-TiO2 basalt lava flows of the Khumib formation (Etendeka Group). References: 1. Walvis Ridge (DSDP Leg 74 525A -29°04'14"S 2°59'7"E) - Hoernel et al. (2015); 2. N-MORB & E-MORB - Sun & McDonough (1989); 3. MgO% N-MORB - Hart et al. (1999); 4. MgO% E-MORB Klein et al. (2004).
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455 5.3 Kinematics of the SCDS
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As outlined in section 4.3 of this paper, the application of the principles of kinematic analysis to the
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geometry of individual dykes and in particular to asymmetrical features such as branches, en
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echelon dykes and bridges between dykes segments, allows the estimation of the orientation of the
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regional stress field active during dyke intrusion (Rickwood, 1990; Corrêa Gomes, 2001). The
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dominant NNW-trending dykes of the SCDS (T1) are generally rectilinear, with only occasional
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branching segments. The relative absence of asymmetrical features and the angular relationship
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between the dyke walls and internal fractures suggests that these dykes were intruded under
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conditions of normal extension with the direction of minimum stress (σ3 - ENE-WSW - N65E)
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perpendicular to the strike of the dykes. In contrast, ENE-WSE trending dykes (T3) often present
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asymmetrical features such as steps, and bridges between dyke segments that indicate a sinistral
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strike-slip component related to NW-SE (315-135°) extension during their intrusion. These dykes
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occur with greater frequency closer to the coast and were observed to cut across the NNW-striking
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dykes (Fig. 11).
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Figure 11 - a) ENE-trending dyke (green) cutting across an earlier NNW-trending diabase dyke intruded under conditions of normal extension (T1 - σ3 - 65-245°). b) Asymmetrical features along the later basalt dyke such as bridges between dyke segments indicate a sinistral component to extension with the direction of minimum stress (σ3) perpendicular to the walls of the bridge (T3 - σ3 - 315-135°). Local: NA038 - 19°23'42.00"S, 12°43'49.84” E (Mowe Bay)
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The directions of (horizontal) minimum and maximum stress were calculated for 29 different
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dykes of the SCDS (Fig. 12). The interpretation of these results, together with cross-cutting
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relationships observed in the field and in satellite imagery, indicates the presence of three dyke
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generations (T1, T2 & T3) related to different extensional events that affected north-west Namibia
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during the late Cretaceous as summarized in Table 4.
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Table 4- Extensional events related to the intrusion of the dykes of the SCDS, NW Namibia
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The first event (T1) is characterised by ENE-WSW (65-245°) extension, which resulted in the intrusion of the dominant NNW-trending dykes. These dykes display dacitic to basaltic compositions and are cut by ENE-WSW orientated dykes. These younger ENE-trending dykes display a sinistral component related to NW-SE (315-135°) extension and generally display more 'primitive' compositions with less enrichment in LREEs similar to E-MORB. Dykes which vary in orientation from N45W to N80W were also observed in the field, but were not mapped in significant numbers on the regional aeromagnetic data. These dykes display a sinistral or dextral component depending on their orientation and are associated with NNE-SSW (25-205°) directed extension. The cross-cutting relationships between these NW to WNW-trending dykes and the ENE trending dykes were not clearly observed in the field. Nonetheless, we suggest that these NW to WNW-trending dykes represent a transitional phase (T2) between the dominant NNW-trending dykes (T1) and the later 'primitive' ENE-WSW trending dykes (T3). The tectonic implications of these results are discussed in section 6.2 of this paper.
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6. Discussion
6.1 Relationship between the SCDS and the Paraná-Etendeka Volcanics Some comparisons can be made between the composition of the dykes of the SCDS sampled in this study and the principal magma types of the Paraná- Etendeka province, which were described in section 3 of this paper. Trumbull et al., (2004, 2007) showed that the majority of the dykes of the Henties Bay-Outjo dyke swarm (HOD) are Low-TiO2 tholeiites and show similarities with the Tafelberg, Esmeralda and Horingbaai magma types (Fig. 13). These authors considered that the concentrations and ratios of High Field strength, incompatible elements (e.g. Zr, Ti, Zr) are the most 22
ACCEPTED MANUSCRIPT useful to discriminate between magma types as the concentration of these elements is not strongly affected by the crystallization of olivine nor by minor rock alteration. The small variation in composition between the dykes of the HOD and the Etendeka basalts led the Trumbull et al (2004,) to suggest these dykes represent the feeder dykes to now eroded southern extension of the Etendeka basalts. However, these same authors and others (e.g. Ewart et al., 1998; Thompson et al., 2001) also observed that mafic dykes have been observed to cut across the Etendeka basalts suggesting a younger age for at least some of these dykes. For example at their type locality along the Skeleton Coast to the south-east of the Messum and Brandberg Intrusive complexes, the more primitive Horingbaai dykes and sills are intrusive in the 125-132 Ma Tafelberg basalts (Ewart et al., 1984). As outlined in section 5.3 of this paper, three dyke generations have been recognised within the SCDS, and based on cross-cutting relationships, the dominant NNW-striking dykes are the oldest. The majority of these dykes are quartz-tholeiitic basalts and andesitic basalts with relative enrichment in LREEs and (La/Yb)n ratios between 3.43 and6.56. The dykes show geochemical similarities with Tafelberg and Esmeralda basalts (Fig. 13 & 14). These dykes often occur sub-parallel to the NNW-trending, closely spaced, listric faults that cut across the Etendeka volcanics within the coastal structural domain of the Etendeka province defined by Marsh et al. (2001). ENE-WSW dykes are clearly observed cutting across these earlier NNW-SSE orientated dykes. These younger quartz-tholeiitic dykes are characterised by more 'primitive' geochemistry with less enrichment in LREEs and are similar in composition to Horingbaai and Esmeralda-type basalts (Fig. 13 & 14). These ENE-striking dykes are assumed to register a third extensional event (T3) that affected NW Namibia during the Early Cretaceous, whilst NW to WNW-trending dykes represent a transitional phase (T2) between the dominant NNW-trending dykes (T1) and these later 'primitive' ENE-WSW trending dykes (T3). The NW to WNW-trending dykes display both 'enriched; and 'primitive' geochemical compositions.
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Figure 13- Zr/Y & Ti/Zr discrimination diagram of the 'basaltic' dykes of the SCDS (Si02 < 60 wt%) compared with the composition of the basaltic dykes of the Henties Bay-Outjo Dyke Swarm (HOD -1) and the principal Etendeka flood basalt types (Tb - Talfberg, Kh - Khumib, Tk - Tafelkop, E- Esmeralda, Horingbaai). Modified
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ACCEPTED MANUSCRIPT from Trumbull et al. (2004). The composition of two basalt lavas flows of the High Ti, Khumib Formation sampled during this study are also shown. Note: 1. HOD - Trumbull et al., (2004 & 2007); 2. Walvis Ridge Hoernel et al. (2015); 3. N-MORB e E-MORB - Sun & McDonough (1989).
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Figure 14- Zr/Nb & La/Nb discrimination diagram of the 'basaltic' dykes of the SCDS (Si02 < 60 wt%) compared with the composition of the dykes of the HOD and the principal Etendeka flood basalt types (Tb - Talfberg, Kh Khumib, Tk - Tafelkop, E- Esmeralda, Horingbaai). The composition of OIB of the South Atlantic, the Kudu basalts of offshore Orange Basin and basalt clasts of the syn-volcanic Albin conglomerate are also shown. Modified from Jerram et al. (2015). Note: 1. HOD - Trumbull et al., (2004 & 2007); 2. Walvis Ridge - Hoernel et al. (2015); 3. N-MORB e E-MORB - Sun & McDonough (1989); 4. Chondrite - McDonough & Sun (1995).
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6.2 Tectonic Implications of the Skeleton Coast Dyke Swarm A tectonic reconstruction of part of Western Gondwana demonstrates a spatial correlation between the SCDS of North-west Namibia and the Florianopolis Dyke Swarm (FDS - Fig. 2). It can be observed that the continuation of the Ponta Grossa-Guapiara Dyke Swarm (PGDS) points toward the start of the Walvis Ridge at approximately 18°S along the Skeleton Coast close to the mouth of the Cunene River which marks the border between Namibia and Angola. Several authors (e.g. O'Connor & Duncan, 1990; Peate et al., 1990; Ryberg, et al., 2015; Thompson and Gibson, 1991) have suggested that the Tristan plume head impacted the base of the lithosphere in this region during the Early Cretaceous. This hypothesis is supported by the recent magnetotelluric imaging of the NW Namibian margin (Jegen et al., 2016) and the results of a seismic refraction survey which identified a Highvelocity Lower Crustal (HVLC) Body beneath the Kunene region of NW Namibia. This HVLC body was interpreted to represent mafic-ultramafic intrusives or underplating of the lower crust at the site of plume impact (Ryberg et al., 2015; Heit et al., 2015). The plume impact is likely to have influenced magma supply and the conditions of stress during initial rifting, and if we compare the width of the two dyke swarms it can be observed that the dykes of the FDS are restricted to a 50km wide coastal strip of Santa Catarina state, whilst the dykes of the SCDS are found up to 250km from the Skeleton coast of NW Namibia. Coast-parallel block faulting of the Etendeka basalts along the NW Namibia margin is common (Milner and Duncan, 1987; and Stollhofen, 1999). In contrast, in their recent study which 24
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compared the paleostress fields of the NW Namibia -S Brazil conjugate margins, Salomon et al. (2015b) failed to identify major extensional faults associated with Atlantic rifting along the studied section of the southern Brazilian coastline. However, it should be noted that their study area did not encompass the outcrop area of the FDS along the Santa Catarina coast. Nonetheless based on their observations, Salomon et al. (2015b) concluded that during the opening of the South Atlantic, extension was focused between the current Brazilian coastline and the Continental-Ocean Boundary. This hypothesis is supported by offshore seismic data which indicates extensive listric normal faulting along both margins (Gladczenko et al., 1997; Blaich et al., 2011). It is also worth noting that Early Cretaceous volcanic rocks have been identified offshore of the NW Namibian margin as seaward dipping reflectors (SDRs) underlying post-rift sediments in the Walvis Basin. These SDRs were faulted during Aptian-Albian rifting leading to the creation of structural highs and local depocentres within the offshore Walvis Basin (Holtar & Forsber, 2000). The majority of the dykes of the FDS strike to the NNE (N25E), but dykes of other directions also occur (Fig. 15a & b). Field observations allowed the calculation of the directions of minimum and maximum stress active during the intrusion of 36 different dykes of the FDS (Almeida et al. 2013). The results confirmed that the dominant direction of the FDS is associated with WNW-ESE (285-105°) extension (Almeida et al., 2013). Nonetheless, NE-SW dykes can also be observed cutting across these earlier NNE-trending dykes and are associated with NW-SE extension (Fig. 15c).
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Figure 15 - Rose diagrams of the FDS from: a) field observations (n=70, 12° interv., max. freq. 20% - 14 dykes); and b) directions of rectilinear dyke segments extracted in ArcGIS (n=609, 10° interv., max. freq. 32.5% - 198 segments); c) directions of maximum (σHmax) and minimum (σ3) stress for 36 dykes of the FDS (n=36, 10° intervals, principal σ3 - 285-105°, principal σHmax - 15-195°). Modified from Almeida et al. (2013).
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ACCEPTED MANUSCRIPT A recent study by Veronez & Tomazzoli (2016) identified at least three dyke generations. A NW-trending dyke presented an Ar-Ar age of 119 Ma and an older N30E trending dyke was dated at 139 Ma (Veronez & Tomazzoli, 2016)). These ages vary from the results of ID-TIMS U–Pb baddeleyite/zircon dating of dykes from the FDS published by Florisbal et al. (2014). These authors suggested a restricted interval of between 134.7±0.3 and 133.9±0.7Ma for the intrusion of the dykes of the FDS. However, this interpretation is based on the dating of only three dykes; two NNEtrending, High-TiO2 basalt dykes and a trachyandesite dyke of the same direction. Florisbal et al. (2014) observed that the majority (90%) of the basalt dykes of the FDS are High Ti-Sr and suggested that the FDS represents feeder dykes for the High-TiO2 Urubici and Khumib basalt lava flows of S Brazil and NW Namibia respectively. However, dykes other directions within the FDS are generally Low-TiO2, cut across the dominant NNE-trending dykes and have not been dated by ID-TIMS U–Pb baddeleyite/zircon dating. It is worth noting that in the present study; only two samples were taken of High-TiO2 basalts, both being of Khumib lava flows. All of the dykes sampled were Low-TiO2 and some of them display compositions very similar to the Low-TiO2 dykes of the FDS (Fig 16). We assume that the majority of the dykes of the SCDS and the FDS were intruded during an early stage of rifting between ~133-139 Ma (Veronez & Tomazzoli, 2016; Florisbal et al., 2014).
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Figure 16 - Comparison of the Ti/Y ratios and Sr content of the dykes of the FDS and the 'basaltic' dykes of the SCDS (Si02 < 60 wt%) with the principal mafic magmas of the Paraná-Etendeka igneous province (after Peate, 1997). The composition of the two samples of High-TiO2 Khumib basalt lava flows are also shown. Modified from Florisbal et al., 2014. Note. 1. Composition of the three dykes dated by Florisbal et. al., 2014; 2. Walvis Ridge - Hoernel et al., 2015; 3. N-MORB e E-MORB - Sun & McDonough, 1989.
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If we restore South America to its position at ~139Ma, prior to the break-up of Gondwana, the principal directions of the SCDS and Florianopolis dyke swarm (FDS) are almost equivalent (Figure 17a). By applying a rotation of 40°W we restore the majority of the dykes of the FDS to their prebreak-up orientation. After restoration the principal direction extension of both the FDS and the
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SCDS is essentially the same (Fig. 17b & c). This supports the hypothesis that the majority of the dykes of both the FDS and SCDS were intruded under the same stress regime with ENE-WSE (65245°) directed, subhorizontal extension, during the initial rifting that led to the break-up of Gondwana.
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b) FDS*
c) SCDS
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Figure 17 - a) Pre-break-up tectonic reconstruction of the conjugate margins of S Brazil and NW Namibia. Note the position of landward portion of the Walvis Ridge and the correlation between the SCDS and FDS. Other tholeiitic dykes swarms: Ponta Grossa-Guapiara (PGDS), Henties Bay-Outjo (HOD). Rose diagrams of the directions of minimum (σ3) and maximum (σHmax) stress active during the intrusion of the dykes of b) FDS rotated 40W to their pre-break orientation; and c) SCDS; indicating the principal directions of the extension related to multiple dyke generations. For key to the geological units refer to the legend in Figure 14.
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ACCEPTED MANUSCRIPT This extensional event was also identified by Salomon et al. (2015b) through the kinematic analysis of faults that cut the Etendeka basalts of NW Namibia. A younger event with subhorizontal NNE-SSW (25-205°) directed extension was also identified by the same authors. This corresponds to the direction of minimum stress (σ3) of the second generation of dykes (T2) of the SCDS (Figure 17c). This event has not been clearly defined within the FDS, nonetheless the younger, ~119 Ma, NWtrending dykes (present day orientation), of the FDS may be equivalent to the third generation (T3) of dykes within the SCDS. It should be observed that the South American continent had probably already undergone some rotation between the intrusion of the first generation of dykes at ~133-139 Ma and the intrusion of these later dykes at ~119 Ma. Therefore, the directions of minimum stress for this event do not exactly correspond on either side of the South Atlantic (Figure 17 b & c).
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7. Conclusions Satellite imagery and aeromagnetic data have allowed the mapping of over 4000 mafic dykes intrusive in the Neoproterozoic Kaoko Belt and adjacent Angolan craton. These dykes define an important NNW-SSE trending, linear dyke swarm, the Skeleton Coast Dyke Swarm. The SCDS is some 150-200km wide and extends for more than 400km parallel to the NW Namibian coast with likely extension into SW Angola. Three dyke generations were identified within the SCDS: 1) dominant NNE-trending dykes related to ENE-WSW (65-245°) extension; 2) NW to WNW-trending dykes related to NNE-SSW (25-205°) extension; 3) ENE-WSW orientated dykes related to NW-SE (315-205°) extension. The majority of the dykes sampled in this study are Low-TiO2, basalts and andesitic basalts, with the exception of a nephelinite dyke and two dacitic dykes which also trend to NNE-SSW. The dominant NNE-SSW orientated dykes display relative enrichment in LREEs and display geochemical similarities with Tafelberg and Esmeralda basalts. This suggests that this earlier generation of dykes may represent feeder dykes for now eroded Etendeka basalt lava flows. Greater lithospheric thickness during initial rifting may explain the apparently greater crustal contamination of this first generation of dykes.. In contrast, the third generation of dykes tend to display more 'primitive' compositions similar to the Horingbaai dykes and sills which are interpreted to have been derived from a depleted asthenosphere source during the final break-up of Gondwana when the lithosphere had already undergone significant extension (Ewart et al, 1984; Duncan et al., 1990; Marsh et al., 2001). The NW to WNW-trending dykes display both 'enriched' and 'primitive' compositions and are assumed to represent a transitional phase characterised by NNE-SSW directed extension. This extensional event was also identified by Salomon et al. (2015b) through the kinematic analysis of faults that cut the Etendeka basalts of NW Namibia. When South America is restored to its position at ~139 Ma, prior to the break-up of Gondwana, the principal directions of the Skeleton Coast and Florianopolis dyke swarms are equivalent. The majority of the dykes of the FDS are High-TiO2 basalts similar in composition to the Urubici/ Khumib basalts, however younger dykes of varying orientations and Low-TiO2 geochemistry have also been recognised within the FDS. 40Ar-39Ar dating suggests a prolonged period of dyke intrusion within the FDS between 139-119 Ma (Deckhart et al., 1998; Raposo et al., 1998; Veronez & Tomazzoli, 2016). However, it is assumed that the majority of the dykes of the SCDS and FDS were intruded during an early stage of rifting occurring between ~133-139 Ma. This rifting was characterised by ENE-WSW extension that led to coast-parallel block faulting of the Etendeka basalts in Namibia (Salomon et al., 2015b). The second generation of dykes identified in the SCDS has been not recognised within the FDS. However, the younger (~119 Ma) NW-trending dykes (present day orientation) of the FDS may be equivalent to the third generation (T3) of dykes within the SCDS. This later extensional event may be related to offshore Aptian-Albian rifting within the Walvis and Pelotas Basins of NW Namibia and S Brazil, (Holtar & Forsberg, 2000; Dias et al., 1994; Bueno et al., 2007). Precise 40Ar-39Ar dating of
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Acknowledgements The authors would like to acknowledge the Geological Survey of Namibia (GSN) for providing the high-resolution aeromagnetic data that was used in this study and in particular Deputy Directors Anna Nguno and Kombada Mhopjeni of the Regional Geosciences and Geo-Information Divisions respectively, for providing logistical support for our fieldwork. Special thanks also to our colleagues, Henrique Bruno and Aimee Guida from the Rio de Janeiro State University; and Marcelo Motta and Felipe Frai from the Pontifical Catholic University of Rio de Janeiro who participated in the fieldwork. Financial support for field expenses and geochemical analyses was provided by the Fundação de Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) through grant number 26/010.002995/2014 and CNE_05/2015 - Cientista do Nosso Estado - 2015. Finally we thank Prof Fred Kamona and Dr. Antony Mamuse for their reviews of this manuscript.
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Table 4- Extensional events related to the intrusion of the dykes of the SCDS, NW Namibia Extension (σ3)
Orient.
Cinem.
T1
N15W
Normal
T2
N80W-N45W
Dextral to Sinistral
Lithology
ENE-WSW (75Dacites, Basalts - Basaltic Andesites 255)
NNE-SSW (25205)
Geochem.
No.
Enriched
15
"Primitive" & Enriched
8
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Generation / Event
Basalts - Basaltic Andesites
Sinistral
NW-SE (315315)
"Primitive"
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N70E - N85E
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T3
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ACCEPTED MANUSCRIPT Characterisation and Tectonic Implications of the Early Cretaceous, Skeleton Coast Dyke Swarm, NW Namibia McMaster, M.a,*, Almeida, J.a, Heilbron, M. a, Guedes, E. b, Mane, M.A. a, Linus, J.H. c
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Highlights -
1. Geophysical, structural and geochemical characterisation of the Skeleton Coast Dyke Swarm. 2. Cinematic evolution of dyke emplacement related to the opening of the South Atlantic Ocean.
3. Skeleton Coast Dyke Swarm correlated with the Florianopolis Dyke Swarm across the South
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S1464-343X(18)30352-2
DOI:
https://doi.org/10.1016/j.jafrearsci.2018.11.010
Reference:
AES 3365
To appear in:
Journal of African Earth Sciences
Received Date: 20 May 2018 Revised Date:
8 November 2018
Accepted Date: 12 November 2018
Please cite this article as: McMaster, M., Almeida, J., Heilbron, M., Guedes, E., Mane, M.A., Linus, J.H., Characterisation and tectonic implications of the Early Cretaceous, Skeleton Coast Dyke Swarm, NW Namibia, Journal of African Earth Sciences (2018), doi: https://doi.org/10.1016/j.jafrearsci.2018.11.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Characterisation and Tectonic Implications of the Early Cretaceous, Skeleton Coast Dyke Swarm, NW Namibia McMaster, M.a,*, Almeida, J.a, Heilbron, M. a, Guedes, E. b, Mane, M.A. a, Linus, J.H.
c
a
TEKTOS - Geotectonics Research Group, Faculdade de Geologia, Universidade do Estado do Rio de Janeiro, Rua São Francisco Xavier 524/4006-A,Maracanã, 20559-900 Rio de Janeiro, RJ, Brazil b Museu Nacional/UFRJ, Quinta da boa Vista S/N, São Cristovão, 20940-040 Rio de Janeiro, RJ, Brazil c Regional Geoscience Division, Geological Survey of Namibia, Windhoek, Namibia
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The Early Cretaceous, Skeleton Coast Dyke Swarm (SCDS) was intruded into Permo-Carboniferous
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sediments of the Karoo Supergroup, Pan-African granites, Neoproterozoic metasediments, and
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Archean to Mesoproterozoic gneisses of the Kaoko Belt, NW Namibia. Aeromagnetic data and
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satellite imagery has been used to map the distribution of the generally coast parallel mafic dykes
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along with a significant number of dykes that cut across Pan-African structures of the Kaoko Belt.
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Dykes of the SCDS were mapped for some 500km along the Skeleton Coast of NW Namibia and up to
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200km inland, and with a likely continuation into Angola. The geochemistry and published
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geochronological data suggest that the majority of the dykes are related to the Etendeka volcanics.
18
These volcanics, together with the basalt and rhyolite lava flows and associated mafic dyke swarms
19
of S-SE Brazil, form the Paraná-Etendeka magmatic province of Early Cretaceous age. Geochemical
20
analysis of 26 dykes show that the majority of the dykes are Low-TiO2 quartz tholeiites of two types:
21
1) qtz-tholeiites enriched in LREE [(La/Yb)N - 3.43 to 6.46]; & 2) qtz-tholeiites with REE patterns
22
similar to E-MORB [(La/Yb)N < 3.4]. The former typically strike N15W whilst the latter strike N70E /
23
N85E. By applying the principles of kinematic analysis and through the identification of asymmetrical
24
features such as zigzagging, branching and en échelon dykes, and bridges between dyke segments,
25
we were able to identify that in addition to extension perpendicular to dyke walls a significant
26
number of dykes display lateral displacements of their walls consistent with oblique extension. This
27
information was used to determine the shear sense and estimate the direction of maximum and
28
minimum stress of the regional stress field active during dyke intrusion. Three dyke generations have
29
been identified within the SCDS: 1) NNW-SSE dykes associated with normal ENE-WSW extension; 2)
30
WNW-ESE to NW-SE trending dykes with sinistral or dextral components related to NNE-SSW
31
extension; & 3) ENE-WSW dykes often with a sinistral component indicating NW-SE extension. When
32
South America is restored to its pre-break-up position, the principal direction of the Skeleton Coast
33
and the Florianopolis dyke swarms are equivalent. Multiple generations of dykes are also recognised
34
in the Florianopolis dyke swarm, but we interpret that the majority of the dykes of the SCDS & FDS
35
were intruded during initial rifting that was characterised by ENE-WSW extension.
36 37 38
Keywords: Kinematic Analysis. Paraná-Etendeka. West Gondwana. Conjugate margins
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1. Introduction
42
the early Cretaceous (119-139Ma - Almeida et al., 2013; Renne et al., 1992; Turner et al., 1994 ) as a
43
precursor to the break-up of Gondwana and subsequent opening of the South Atlantic Ocean
44
(Heilbron et al., 2000; Marsh et al., 2001; Will & Frimmel, 2013). This magmatism is represented by
45
both mafic and acid volcanics and intrusives, including a number of predominantly mafic dyke
46
swarms in Brazil, South Africa, Namibia and Angola. Dyke swarms have important tectonic
47
implications, and using the principles of kinematic analysis it is possible to evaluate the conditions of
48
stress that were active during their emplacement (Ernst and Buchan, 2003; Correa-Gomes et al.,
49
2001; Martinez-Poza & Druguet, 2016; Will & Frimmel, 2013). A dyke swarm can be defined as a
50
concentration of dykes emplaced in the same igneous episode (e.g. Speight et al., 1982). This
51
definition implies that all of the dykes within a specific dyke swarm are of a similar age and
52
consequently were emplaced under the same stress regime. However, more recent publications
53
(e.g. Gaggero et al., 2007; Guedes et al., 2005 & 2016, Reid et al., 1991; Trumbull et al., 2004) have
54
characterised dyke swarms that include dykes of varying composition and orientation, emplaced
55
across longer intervals of up to 20 Ma. We have adopted this latter usage of the term and define a
56
dyke swarm as a concentration of dykes emplaced during the same tectono-magmatic event which
57
consequently could include several generations of dykes.
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The voluminous tholeiitic magmatism of the Paraná–Etendeka Magmatic Province developed during
Previous studies have identified Early Cretaceous mafic dykes along the south-western
59
margin of Africa (Reid, 1990; Reid et al., 1991; Salomon et al., 2017; Trumbull et al., 2004; Will &
60
Frimmel, 2013), however their distribution, orientation, age and composition has not been as well
61
documented as Cretaceous age dyke swarm in Brazil. In Namibia, mafic dykes have been mapped
62
parallel to the coast, within the NNW-trending Kaoko and Gariep belts, as well as oblique to the
63
coast sub-parallel to the NE-trending Damara belt (Salomon et al., 2017; Will & Frimmel, 2013;
64
Trumbull et al., 2004). These latter dykes extend up to 500km inland and were mapped in detail
65
using high-resolution aeromagnetics by Trumbull et al. (2004) in defining the Henties Bay-Outjo dike
66
swarm (HOD). Similar high-resolution aeromagnetics, and satellite imagery, was used in this study to
67
map in detail for the first time the distribution of the generally coast parallel mafic dykes along the
68
NW margin of Namibia, here named the Skeleton Coast Dyke Swarm (SCDS- Fig.1). The
69
interpretation of magnetic data and satellite imagery was supplemented by comparison with
70
published geological maps and through ground-truthing including structural analysis and sampling
71
during field work in September 2015.
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Figure 1 - Map of the Skeleton Coast Dyke Swarm (SCDS) and part of the Henties Bay Outjo Dyke Swarm (HOD) in the Kaoko and Damara Belts of NW Namibia. The dykes of the SCDS are shown in red and were mapped from aeromagnetic data and satellite imagery and verified during field work. The green rectangles show the coverage of the high-resolution aeromagnetic data provided by the Geological Survey of Namibia (GSN). The black lines are the major faults and shear zones which divide the Kaoko Belt into a number of tectonic zones (WKZ - Western Kaoko Zone; CKZ - Central Kaoko Zone; EKZ - Eastern Kaoko Zone) from Miller (1983) and Goscombe & Gray (2008). The inset map shows the simplified geology and tectonic setting of NW Namibia.
81
The main objectives of this paper are to characterise the distribution and geometry of the SCDS and
82
to determine the orientation of the regional stress field active during the intrusion of the dykes.
83
Preliminary geochemical results are also presented, which allows the comparison of the dyke
84
geochemistry with known magma compositions of the (Paraná)-Etendeka province.
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86
were adjacent prior to break-up (Almeida et al., 2013 & 2014; Hein et al., 2013; Moulin et al., 2010;
87
Salomon et al., 2017; Torsvik et al., 2009; De Wit et al., 2008), placing northwest Namibia next to
88
Santa Catarina, Brazil. Five dyke swarms have been described along the south-eastern margin of
89
Brazil (Almeida et al., 2013; Coutinho, 2008; Guedes et al., 2005; Piccirillo et al., 1990; Raposo et al.,
90
1998; Renne et al., 1996). Three of these dykes swarms, the NNW-trending Vitória–Colatina (VCDS)
91
and Resende-Ilha Grande (RIGDS) dyke swarms as well as the NW-trending Ponta Grossa–Guapiara
92
(PGDS) dyke swarm are orientated obliquely to both the coast and in relation to Precambrian
93
structures. Two of these dykes swarms, the NE-trending Serra do Mar dyke swarm (SMDS) and the
94
NNE-trending Florianopolis Dyke Swarm (FDS), are orientated parallel to the coast. The dykes of the
95
SMDS strike parallel to the regional trend of the Ribeira Belt, however being subvertical they cut
96
across the variably dipping Brasiliano foliations. The results of this study are compared with the
97
geochemistry (Florisbal et al., 2014) and paleostress indicators (Almeida et al., 2013) of the Early
98
Cretaceous, Florianopolis Dyke Swarm (FDS). The same techniques of kinematic analysis have been
99
applied to both dyke swarms, allowing a direct comparison of the regional stress fields active during
100
dyke intrusion. Some insight is also gained into the geodynamic evolution of this portion of Western
101
Gondwana during continental break-up and the subsequent opening of the (proto) South Atlantic
102
Ocean.
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2. Geological Setting
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Kaoko Belt of NW Namibia were deformed and metamorphosed during the Neoproterozic to
108
Cambrian amalgamation of Western Gondwana (Miller, 1983; Dürr & Dingeldey, 1996; Goscombe et
109
al., 2003a, 2003b; Goscombe & Gray, 2008; Konopásek et al., 2005). The Kaoko Belt is the northern
110
branch of the Damara orogeny resulting from the collision of the Kalahari, Angola and Rio de la Plata
111
cratons during the Pan-African orogenic cycle. The Kaoko belt has been correlated with the Dom
112
Feliciano and Ribeira Fold Belts of South America (Chemale et al., 1994; Trompette & Carozzi, 1994).
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The Neoproterozoic metasediments and Mesoproterozoic to Archean basement of the
Miller (1983) subdivided the transpressive Kaoko Belt into five tectonic zones of contrasting
114
metamorphic and deformational styles. The three central zones; the Western Kaoko Zone (WKZ), the
115
Central Kaoko Zone (CKZ) and the Eastern Kaoko Zone (EKZ) are separated by crustal-scale, NNW
116
orientated shear zones. The Ugab and Kunene zones occur in the extreme north and south of the
117
Kaoko Belt. Later work by Goscombe et al. (2003a, 2003b) resulted in the identification of new shear
118
zones and to a better understanding of the tectonic and thermal evolution of the Kaoko belt, leading 4
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these authors to propose a further subdivision of the WKZ into the Coastal Terrane and Orogen Core
120
(Fig. 1). The Coastal Terrane is considered to be an exotic terrane that was accreted onto the margin
122
of the Angolan Craton at around 580 Ma (Goscombe & Gray, 2007). This terrane is characterised by
123
gneisses and metagranites of the upper amphibolite facies. The metagranites display primitive arc-
124
like geochemical signatures with ages between 656-630 Ma (Goscombe & Gray, 2007; Goscombe et
125
al., 2005a, 2005b; Masberg et al., 2005). Whilst, S-type granites related to the collision of the Coastal
126
Terrane are found throughout the WKZ and range in age from 576-549 Ma (Seth et al., 1998; Franz
127
et al., 1999; Kröner et al., 2004; Goscombe et al., 2005b; Goscombe & Gray, 2007).
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The geometry of the Kaoko Belt was produced by progressive sinistral shearing during the
129
transpressional deformation and shear zone development between 570 and 550 Ma (Goscombe et
130
al., 2005b). An early, sub-horizontal, bedding-parallel schistosity was folded and locally transposed
131
during the development of the shear zones and associated steeper, mylonitic foliation within the
132
Western and Central Kaoko Zones (Goscombe et al., 2003a; Goscombe & Gray, 2007). These shear
133
zones and associated mylonitic foliation represent inherent crustal weakness within the Kaoko Belt
134
that may have been reactivated by extension during the break-up of Gondwana, particularly where
135
subvertical as within the orogen core (Salomon et al. 2015a & 2017).
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The voluminous tholeiitic magmatism of the Paraná–Etendeka Magmatic Province
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developed during the early Cretaceous (119-139 Ma / 127-137 Ma: Almeida et al., 2013 & Turner et
141
al., 1994) and is thought to be related to the impact of the head of the Tristan da Cunha plume on
142
the base of the lithosphere of Northwest Namibia at ca. 136 Ma (Connor & Duncan, 1990; Peate et
143
al., 1990; Ryberg et al., 2015). The Walvis Ridge has been interpreted to represent the offshore
144
continuation of this plume related magmatism albeit at a lesser scale (O’Connor & Duncan, 1990;
145
Ewart et al., 1998, Renne et al., 1996b).
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The distribution of igneous rocks of the Paraná-Etendeka Magmatic Province displays a
147
marked asymmetry relative to the Atlantic Rift (Peate et al., 1990; Turner et al., 1994 - Fig. 2). The
148
Paraná volcanics presently cover some 1.2 x 106 km2 of the South American continent, with a
149
preserved thickness of up to 1,700m (Melfi et al., 1988; Peate et al., 1990; Peate, 1997). In contrast,
150
the Etendeka volcanics of NW Namibia display a maximum thickness of 900m and an aerial extent of
151
approximately 78,000 km2 (Ewart et al., 2004). This difference in preserved volume and areal extent
152
has been attributed to differences in paleotopography (Peate et al., 1990) and to variations in post5
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154
et al., 2002; Trumbull et al., 2004). However a lack of preservation alone does not explain the
155
significant volume of quartz-latites (rhyolites) in NW Namibia, where they represent nearly 50% of
156
the preserved volume of the Etendeka Province (Ewart et. al., 2004). In contrast, the rhyolite lava
157
flows of the Paraná Province make up only 3% of the total eruptive volume (Bellieni et al., 1984,
158
1986). The quartz-latites and basalt lava flows of the Etendeka volcanic sequence have been dated
159
to between 136-130Ma (Renne et al., 1996; Dodd et al., 2015) and are associated with the ca. 132-
160
123 Ma old Damaraland Intrusive Complexes (Allsopp et al., 1984; Milner et al., 1995; Renne et al.,
161
1996; Wigand et al., 2004). Within the study area, this magmatism was also accompanied by the
162
intrusion of predominantly NNE trending tholeiitic dykes that parallel the trend of the Kaoko Belt,
163
and together form the SCDS, characterised in this study (Fig. 1 & 2). Trumbull et al. (2005) used high-
164
resolution aeromagnetics to define the NE-trending Henties Bay-Outjo dyke swarm (HOD) within the
165
Damara Belt to the south of the study area (Fig. 2).
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SMDS
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RIGDS
PGDS
SCDS
FDS HOD
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Figure 2 - Tectonic reconstruction of part of Western Gondwana at ca. 139Ma prior to the opening of the South Atlantic Ocean and showing the Jurassic-Cretaceous tholeiitic dyke swarms: Vitória-Colatina (VCDS), Resende-
6
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Ilha Grande (RIGDS), Serra do Mar (SMDS), Ponta Grossa-Guapiara (PGDS) Florianópolis (FDS), Skeleton Coast (SCDS), Henties Bay-Outjo (HOD). Modified from Almeida et al. (2014). Note: The South American continent has been rotated 40°W to its pre-break up position whilst the African continent remains fixed.
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According to published K-Ar ages the tholeiitic dykes of NW Namibia range were emplaced between 116-144 Ma (Siedner & Mitchell, 1976; Hunter & Reid,
173
1987). More precise Ar-Ar dating also support an Early Cretaceous age for the tholeiitic dykes with Ewart et al. (1984) reporting
174
plagioclase of 125–130Ma from the Horingbaai dolerite dikes and Renne et al. (1996) obtaining an 40Ar–39Ar plagioclase age of 132 ± 0.7 Ma from a gabbro
175
sill in the Huab region within the southern portion of the Etendeka Province (Table 1).
40
Ar–39Ar ages for
Table 1- Summary of published radiometric ages for the tholeiitic dykes and associated intrusions of northwest Namibia and southern Brazil. Dyke Swarm
Lithologies
Orient.
Age (Ma)
Ponta Grossa, Brazil
PGDS
basalt, basaltic andesites, andesite, trachyandesite, rhyolite
N04E-N84E & N71W-N20W
120.7 ±1.3- 131.4
40
Santa Catarina Island, Brazil
FDS
Dolerites
116.4 ±3.9 - 129.4 ±0.3
Santa Catarina Island, Brazil
FDS
Basalt
119.0 ±0.9 - 128.3 ±0.5
FDS
Diabase, Trachyandesite
FDS
Basalt, Basaltic trachyandesite
Damara Belt, Namibia
HOD
Dolerites
Horingbaai, Namibia
HOD
Dolerites
Damaraland Intr. Compl.
-
Ne-syenites, Comendite
No. of samples
Reference
39
Plagioclase
17
Renne et al., 1996a
40
39
plagioclase
2
Deckhart et al., 1998
40
Ar / Ar
39
plagioclase
5
Raposo et al., 1998
ID-TIMS U–Pb
baddeleyite / zircon
3
Florisbal et al., 2014
Ar / Ar
39
plagioclase
2
K - Ar
whole Rock
10
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Ar / Ar Ar / Ar
N30E & N50E
119 - 139
N.D
116 - 144
N.D
125 - 130
40
39
plagioclase
2
Veronez & Tomazzoli, 2016 Siedner & Mitchell, 1976 Ewart et al., 1984
126.9 ±0.6 - 137.0 ±0.7
40
39
whole Rock
3
Milner et al., 1995
-
132.0 ± 0.7
40
39
plagioclase phlogopite, kaersutile plagioclase
1
Renne et al., 1996b
2
Wigand et al., 2004
1
Kirstein et al., 2005
plagioclase hornblende / biotite
1
Kirstein et al., 2005
1
Stewart et al., 1996
-
40
Ar / Ar Ar / Ar
Huab Sill, Namibia
-
Erongo Complex, Namibia
-
Basanite-tephrite
-
130.8 ±1.0 -132 ± 1.0
40
39
Huab Sill, Namibia
-
Gabbro
-
130.5 ± 0.8
40
39 39 39
Ugab River, Namibia
SCDS?
Messum complex, Namibia
-
Gabbro
134.7 ±0.3 - 133.9 ±0.7
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Silveira & Pinheira Beaches, Santa Catarina, Brazil Ponta do Pasto, Santa Catarina Island, Brazil
N15E & N30W N10W & N15E-N55E
Analysed material
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Location
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Ar / Ar Ar / Ar Ar / Ar
Dolerite
N.D
129.5 ± 1.5
40
Syenite
-
127.2 ±1.2 -129.1 ±1.4
40
Ar / Ar Ar / Ar
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178
Florisbal et al. (2014) by ID-TIMS U-Pb dating of baddeleyite and zircon extracted from three dykes
179
of the Florianopolis Dyke Swarm (FDS). However, it should be noted that all three of the dated
180
samples were taken from dykes of the dominant NNE-trending family. Previous work by Raposo et
181
al. (1998) and Deckart et al. (1998) identified at least two dyke generations of differing orientations
182
within the FDS. 40Ar-39Ar dating by the same authors suggest a longer interval of between ca.129 to
183
116 Ma for the intrusion of the FDS and recent 40Ar-39Ar dating by Veronez & Tomazzoli (2016) also
184
obtained similar results with age ranging between 139 to 119 Ma and identified at least three
185
generations of dykes. A summary of published radiometric ages for the tholeiitic dykes and
186
associated intrusions of northern Namibia and southern Brazil is presented in Table 1.
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The geochemical classification of magma types and their distribution within a Large Igneous
188
Province (LIP) can provide insights into the eruptive history of the igneous province. Marsh et al
189
(2001) consolidated the previous work by Erlank et al. (1984) and defined 8 types of basic magmas
190
and 16 types of acid magmas for the Etendeka province, based on the geochemical and petrographic
191
characteristics of over 1000 samples of the Etendeka volcanics and associated intrusives. The basic
192
magmas include the High-TiO2 (TiO2 > 2.2% & Sr > 450ppm) Khumib basalts which are prevalent in
193
the north of the Etendeka province and are considered to be equivalent to the Urubici-type basalts
194
of the Paraná province (Marsh et al., 2001; Peate, 1997). However, majority of the mafic magmas of
195
Paraná-Etendeka magmatic province are Low TiO2 and in Namibia 7 types of Low-TiO2 magma have
196
been identified: Tafelberg, Kuidas, Horingbaai, Huab, Tafelkop, Albin and Esmeralda. The
197
characteristics of the five principle magmas: Khumib, Tafelberg, Esmeralda, Horingbaai are
198
summarized in Table 2 and discussed in more detail below. However it should be noted that,
199
Trumbull et al. (2004) highlighted the importance of the Tafelkop magma type, as despite their small
200
areal extent at the base of the Etendeka volcanic sequence, the distinctive geochemistry of these Fe
201
& Mg-rich basalts indicate a strong influence of the Tristan plume (Ewart et al., 1998; Gibson et al.,
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2000; Thompson et al., 2001).
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Table 2- Geochemical characteristics of the principal, basic magmas of the (Paraná-) Etendeka Province 1,2
Province High-TiO2
Etendeka Khumib
Parána Urubici
Mg# 29 - 63
Low-TiO2
Tafelberg
Gramado
26 - 65
TiO2% > 2.2
Sr -ppm > 450
< 2.2
< 450
2
2
Zr/Y 4.9 - 9.8
Ti/Zr 47 - 82
2.6 - 7.1
33 - 91
Esmeralda Esmeralda 26 - 46 2.8 - 4.4 57 - 162 Horingbaai 37 - 77 2.8 - 5.4 57 - 128 1) Magnesium number: Mg# = 100x[MgO mol% /(FeO mol% + MgO)]; 2) Values for the Etendeka Province are based on geochemical data published by Ewart et al. (1998, 2004), Jerram et al. (1999), Marsh et al. (2001), Marsh & Milner (2007), Thompson et al. (2001), Trumbull et al. (2007)
9
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The Tafelberg type is the dominant magma of the Etendeka Province and can be correlated with the Gramado basalts (Marsh et al., 2001). Peat and Hawkesworth, (1996) suggested that this
209
magma composition resulted from the crustal contamination of a lithospheric mantle melt. The
210
Esmerald type also occurs on both sides of the Atlantic and is considered to be derived from the
211
mixing of a Tafelberg/Gramado type magma with a depleted asthenospheric melt (Peate and
212
Hawkesworth, 1996). An asthenospheric source has also been proposed for the Horingbaai magma
213
which displays MORB-like geochemical and isotopic characteristics (Erlank et al., 1984; Ewart et al.,
214
2004; Marsh et al., 2001). Igneous rocks of the Horingbaai magma type are olivine normative and
215
occur as dykes and sills intrusive within Neoproterozoic metasediments, Karoo sediments. In some
216
places have been observed to cut lava flows of the Etendeka Group (Marsh et al., 2001). This
217
indicates that they post-date the main eruptive phase of the Paraná-Etendeka Magmatic Province.
218
Horingbaai type magmas have not been described within the Paraná Province (Marsh et al., 2001;
219
Thompson et al., 2001). Comparisons between the geochemical characteristics of the dykes of the
220
SCDS sampled during this study and the aforementioned magma types are made in sections 6.1 &
221
6.2 of this paper.
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4. Methodology
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4.1 Aeromagnetic data processing and interpretation
226
High resolution aeromagnetic data is an important tool for mapping mafic dykes as they create
227
distinctive, linear magnetic anomalies due to the generally high contrast in magnetic susceptibility
228
between the dyke and their country rocks. For the purpose of this study, four areas of high
229
resolution magnetic data was acquired from the Geological Survey of Namibia (GSN),covering a total
230
of 19,665 km of NW Namibia, - Fig. 1.
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The aeromagnetic data was collected during aerial surveys undertaken by GPX Surveys and
232
Geoterrex between 1995 and 2010 as part of a regional acquisition program under the supervision
233
of the GSN. These surveys were flown with a nominal terrain clearance of 80-100m with the
234
exception of the survey over the Brandberg massif which was flown at 2895 metres above sea level,
235
which corresponds to an altitude some 300m higher than the highest elevation within the survey
236
area. The surveys were flown along N-S or E-W traverse lines generally spaced at 200m with the
237
exception of one of the earlier surveys which was flown with 400m spacing. Tie lines were flown at
238
2500m intervals. The data was corrected during acquisition to remove regional variation by
239
subtracting the 2005 model of the International Geomagnetic Reference Field (IGRF) to obtain the
240
residual total magnetic intensity (RTMI). Some post-acquisition interpolation was undertaken by the
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GSN to produce integrated grids for each of the areas with spatial resolutions of 50m. RTMI maps
242
were produced for the four areas of high-resolution data and integrated with lower resolution RTMI
243
maps available through GSN's Earth Data Namibia website, to produce a composite RTMI map of NW
244
Namibia. A synthesis of steps taken during the processing of the magnetic data during this study is
246
presented in Fig. 3. The high-resolution data was initially reduced to pole to centre the anomalies
247
over their sources; however this procedure created false E-W anomalies likely to be the result of the
248
amplification of noise in the data that was not removed by levelling between survey traverse lines.
249
The unreduced magnetic data was compared with satellite imagery and it was noted that there was
250
little difference in location between outcropping dykes and their corresponding magnetic anomaly.
251
Since the bipolarity of the magnetic anomalies in the data is also not pronounced, it was assumed,
252
that the local effect of magnetic declination and inclination of the magnetic field wouldn't greatly
253
impact on the mapping of magnetic anomalies.
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Figure 3 - Synthesis of the processing of the high-resolution aeromagnetic data as exemplified by the Area 4 (Torra Bay) data, modified from Ruy et al., 2006. RTMI - Residual Total Magnetic Intensity; RTP - RTMI reduced to the magnetic pole; dZ - first vertical derivative; dX - first horizontal derivative in X (E-W of the RTMI; dY - first horizontal derivative in Y (N-S) of the RTMI; AS - Analytical Signal; THG- Total Horizontal Gradient of the RTMI; EULER DECONV. - Solutions to the Euler Deconvolution Process.
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262
and the two horizontal derivatives (dX & dY) for each area. Further processing resulted in maps of
263
the Analytic Signal (AS) and Total Horizontal Gradient (THG). The Euler Deconvolution process was
264
applied to the data to estimate the depths of the magnetic anomalies' sources. The maps of AS, THG
265
and dZ were compared with Google Earth and Landsat 8 imagery, and with digital elevation models
266
(SRTM and AsterGDEM) to manually map the magnetic anomalies/dykes within the areas with the
267
high-resolution aeromagnetic data. In the rest of the study area dykes were mapped through the
268
comparison of the lower resolution map of RTMI with satellite imagery and digital elevation models.
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4.2 Geochemistry
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Samples were taken of relatively fresh, unweathered material from 26 dykes and 3 lava flows of the
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Etendeka group. In some localities dykes outcrop as a line of cobbles, in which case care was taken
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to remove weathered material using a diamond saw prior to geochemical analysis. This was
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undertaken at Rio de Janeiro State University by the Geology Faculty's Sample Preparation
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Laboratory (LGPA-FGEL/UERJ). Selected fragments of the samples were sent to Activation
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Laboratories Ltd. (ACT Labs -http://www.actlabs.com) in Ontario, Canada, for further sample
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preparation and major and trace element geochemistry. The samples were crushed to a nominal
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minus 10 mesh (1.7 mm), mechanically split to obtain a representative sample (250g) and then
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pulverized to at least 95% minus 150 mesh (105 microns) in a mild steel mill to avoid Cr and Ni
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contamination. Smaller volumes (3g & 5g) of each sample were separated for lithium metaborate /
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tetraborate fusion. The resulting molten bead is rapidly digested in a weak nitric acid solution. This
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technique ensures that the entire sample is dissolved and guarantees that the major oxides including
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SiO2, refractory minerals, Rare Earth Elements (REE) and other high field strength elements are put
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into solution. Major element geochemistry was then determined by inductively coupled plasma
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optical emission spectrometry (ICP-OES) and the concentrations of 45 trace elements, including REE,
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were determined by inductively coupled plasma mass spectrometry (ICP-MS). The results of this
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analysis are presented in section 5.2 of this paper.
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4.3 Kinematic analysis of dykes
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Linear dyke swarms possess a special tectonic significance as their geometry varies according to the
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orientation of the regional stress field active during their emplacement (Fialka & Rubin, 1999; Jolly &
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Sanderson, 1995; Pollard, 1987). Such dyke swarms are considered to be the physical manifestation
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of crustal extension and thus tend to be orientated perpendicular to the direction of minimum stress
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(σ3) (Anderson, 1951; Hoek, 1991). According to Pollard (1987), dykes propagate as extensional
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these structures present a favourable orientation. Asymmetrical features such as zigzagging,
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branching and en echelon dykes and bridges between dyke segments indicate an oblique
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component to extension during dyke emplacement. In this case additional criteria, such as variations
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in dyke width or offset of extensional markers, must be used to determine the direction of
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extension. The above criteria and the kinematic analysis of asymmetrical features allow the
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estimation of the orientation of the regional stress field active during dyke intrusion (Rickwood,
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1990; Corrêa Gomes, 2001) - Figure 4.
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Figure 4 - Asymmetrical features that indicate oblique extension and shear movement during dyke emplacement: a) Zigzag b)En echelon dykes; c) Ramifications; d) steps with variation in dyke thickness; e) rotated enclave; f)other features which indicate the shear sense 1 .& 2. steps with variation in dyke thickness, 3. bridge between dyke segments, 4. branches. Modified from Corrêa Gomes et al. (1996).
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The orientation of internal fractures can also be used to determine the orientation of the
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local stress field during emplacement of the dyke, as immediately following crystallization recently
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nucleated fractures tend to orientate themselves parallel to the three stress axes (σ1>σ2>σ3).
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Corrêa Gomes et al. (1996) identified three families of internal fractures within a hypothetical
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vertical dyke emplaced under conditions of normal extension: 1) longitudinal fractures (fcl) -
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subvertical & parallel to the dyke walls; 2) transversal fractures (fct) - subvertical & perpendicular to
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the dyke walls; 3) basal fractures (fcb) - subhorizontal - Fig. 5a. In this case, the orientation of the
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longitudinal and transversal fractures represent the direction of maximum and minimum stress (σ1
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& σ3) respectively. Variation from this fracture pattern may indicate an oblique component during
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dyke emplacement (Corrêa Gomes et al., 1996) - Fig 5b.
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5. Results and Interpretations
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Figure 5 a) Internal fractures of a hypothetical N-S dyke with normal extension: longitudinal fractures (fcl - grey lines) are subvertical and parallel the dyke walls, transversal fractures (fct - black lines) are also subvertical, but orientated perpendicular to the dyke walls, and the basal fractures (fcb - white lines) are subhorizontal. Note in the stereogram that the poles to the longitudinal indicate the direction of extension (σ3). b) Zigzagging dyke with internal fractures oblique to the walls of the central segment, indicating the influence of the regional stress field during dyke emplacement. Modified from Corrêa Gomes et al., 1996
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5.1 Dyke distribution and orientation Aeromagnetic data and satellite imagery were used to map some 4579 dykes of the SCDS across north-west Namibia - Fig 1. The orientation of these dykes was extracted in ArcGIS using the COGO tool resulting in some 27,283 linear dyke segments. The majority of the dykes are coast parallel; NNW-SSE (N25W) orientated, and occur up to 200km from the coast. Some of these dykes are greater than 50m wide and extend for kilometres along strike. Many authors (e.g. Jourdan et al., 2006; Salomon et al. 2017; Trumbull et al. 2004 & 2007; Will & Frimmel, 2013) stress the influence of pre-existing structures on dyke orientation. However, whilst the majority dykes of the SCDS trend sub-parallel to the principal structures of the Kaoko Fold Belt (Fig. 6), they are subvertical and are often observed cutting across Pan-African shear zones and associated mylonitic foliations. The fact that the dykes of SCDS are almost exclusively subvertical suggests that the direction of minimum stress (σ3) was subhorizontal during their emplacement. The Pan-African structures are of a similar orientation (N15W) to the dominant dyke direction (N25W). However, the dip of these structures varies from subhorizontal in the coastal terrain and foreland of the Kaoko Belt, to subvertical in the orogen core. Field observations indicate that extensional reactivation of these structures was largely restricted to the orogen core. ENE-WSW dykes were also mapped, these dykes are thinner (0.5-5m wide), occur with greater frequency close to the coast (Fig. 7a), and cut across the older N25W orientated dykes. Less common N80W to N45W orientated dykes were also mapped, and whilst cross-cutting relationships were not clear these dykes may represent a transitional phase (T2) between the dominant NNW-trending dykes (T1) and the younger ENE-trending dykes (T3). The geometry and kinematics of the dykes of the SCDS are discussed in detail in section 5.3 of this paper. 14
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Figure 6 - Rose diagrams comparing the directions of Eo-cretaceous dykes of the SCDS and the principal Pan-African structures of the Kaoko Belt. a) Dykes (n=27283, intervals de 10°, max. freq. 11.5% - 3126 dyke segments); b) Pan-African structures (n=2279, 10° intervals, max. freq. 23.7% - 541 segments).
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Figure 7 - Rose diagrams comparing the directions of dykes of the SCDS: a) <50 km from the coast (n=20892, 10° interval, max. freq. 11.2% - 2340 dyke segments); and b) >50 km from the coast (n=1790, 10° intervals, max freq. 25.1% - 449 dyke segments). Note: The rose diagrams do not include the dykes from the Ugab zone of the Kaoko Belt where dyke display greater variation in orientations possibly related to local changes in the regional stress field associated with the Damaraland intrusive complexes.
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5.2 Geochemistry and Petrography Previous geochemical and petrographic studies of the Cretaceous dykes of north-west Namibia have generally focussed on dykes outcropping along the coast (e.g. Thompson et. al., 2001) or on the dykes of the Henties Bay-Outjo dyke swarm (Trumbull et. al., 2004 & 2007). Some comparisons are made between the compositions of the dykes of the SCDS, the Henties Bay-Outjo Dyke Swarm (HOD) and with principal magmas of the Paraná-Etendeka province. A more detailed discussion, including a comparison with the compositions of the dykes of the FDS, is presented in section 6.1 of this paper. 29 samples were taken of relatively fresh, unweathered material from 26 dykes and 3 lava flows of the Etendeka group during field work undertaken in September 2015.. Major and trace element geochemistry of 26 samples of dykes and 3 samples of lava flows of the Etendeka group, was determined by ICP-OES and ICP-MS respectively by Activation Laboratories Ltd. (ACT Labs) in 15
ACCEPTED MANUSCRIPT Ontario, Canada. Sample preparation procedures and analytical techniques are described in section 4.2 of this paper. Sample coordinates lithologies and concentrations of major elements, selected trace elements and Rare Earth Elements (REEs) are listed in Table 3. The majority of the dykes sampled were classified as subalkaline basalts and basaltic andesites (Figure 8a) and they form part of the tholeiitic series (Figure 8b). Two dacite dykes were also encountered; these dykes are porphyritic with altered, fine to medium grained phenocrystals of K-Feldspar within a fine grained, intergranular matrix composed of K-Feldspar and quartz. Two dykes can be classified as alkaline rocks (Fig. 8), the first being a N30W-trending nephelinite dyke (NA024A) outcropping to the north-west of the Brandberg igneous complex, and the other is an olivine bearing, alkaline basalt dyke (NA026) from the same region and with a similar orientation (N50W) as the nephelinite dyke.
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Figure 8 a) Total alkali-silica diagram (Cox et al 1979) shows that the dykes sampled in this study range in composition from basaltic to dacitic, but the majority of the dykes are subalkaline basalts and basaltic andesites. b) The sampled dykes and lava flows are part of the tholeiitic series according to the AFM diagram of Irvine & Baraga (1971). The dykes are represented by blue circles, whilst the orange squares represent the three lava flows of the Etendeka Group that were also sampled during this study.
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Table 3- Whole rock geochemical analysis of the Skeleton Coast Dyke Swarm and Etendeka Group lavas (1).
409 410 411 412
SiO2 40.12 45.82 64.32 63.03 46.31 45.71 51.31 47.81 46.44 47.13 49.75 55.74 54.54 54.28 53.17 52.44 52.67 53.01 52.34 52.12 54.23 48.29 52.72 52.00 50.00 51.78 51.43 52.53 51.31
Al2O3 10.40 14.86 12.85 12.19 12.31 12.26 14.20 13.55 13.95 13.96 14.00 13.20 13.97 13.12 12.15 12.71 13.26 13.33 13.17 12.32 13.78 14.33 12.97 13.38 14.22 12.80 15.05 13.63 13.48
Fe2O3 (T) 17.92 15.29 8.61 10.02 12.03 12.41 12.51 12.33 12.11 12.96 11.16 11.94 12.65 11.97 16.05 15.62 14.19 14.45 14.41 15.93 12.94 11.85 13.22 13.53 11.90 14.11 13.52 13.80 13.14
MnO 0.21 0.19 0.10 0.13 0.18 0.18 0.14 0.16 0.17 0.18 0.17 0.16 0.19 0.16 0.20 0.19 0.22 0.20 0.20 0.22 0.18 0.18 0.20 0.19 0.18 0.21 0.21 0.18 0.20
MgO 9.65 8.56 0.67 2.04 14.64 14.42 4.82 5.68 10.97 9.99 7.10 4.29 4.51 4.35 3.58 3.97 4.40 4.36 4.17 4.05 4.96 8.60 5.62 5.78 7.69 5.93 4.75 5.54 5.97
CaO 9.11 8.95 3.10 3.87 10.21 10.44 8.19 8.78 10.28 11.71 11.91 7.73 8.49 7.92 6.97 7.64 8.44 8.06 8.12 8.47 8.34 11.99 9.59 8.35 12.64 10.41 9.79 9.46 10.68
Na2O 4.39 3.18 2.30 3.46 1.87 1.61 2.67 2.33 1.81 2.12 2.53 2.53 2.95 2.39 3.15 2.70 2.81 2.75 2.83 2.70 2.72 2.08 2.26 2.92 2.09 2.45 2.71 2.44 2.22
K2O 1.02 0.61 5.11 3.70 0.25 0.09 1.78 1.24 0.52 0.28 0.34 1.83 1.03 1.95 1.54 1.34 1.39 1.63 1.45 0.87 1.56 0.22 1.10 1.54 0.28 0.47 0.94 0.95 0.51
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Geochem. Alkaline Alkaline Acid dykes Acid dykes Olivine Thol. Olivine Thol. Kh- High TiO2 Kh- High TiO2 QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (Enrich.) QT (E-MORB) QT (E-MORB) QT (E-MORB) QT (E-MORB) QT (E-MORB) QT (E-MORB) QT (E-MORB) QT (E-MORB)
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Lithology Nephen. Alk. Basalt Dacite Dacite Basalt Basalt 1 Basalt 1 Basalt Basalt Basalt Basalt 1 Andesite Bas. and. Bas. and. Bas. and. Bas. and. Bas. and. Bas. and. Bas. and. Bas. and. Bas. and. Basalt Bas. and. Bas. and. Diabase Basalt Basalt Bas. and. Basalt
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Long. (°E) 14°05'17'' 14°04'31'' 14°41'16'' 12°43'31'' 14°04'56'' 14°05'00'' 12°45'32'' 12°45'35'' 14°05'42'' 14°03'53'' 14°04'58'' 13°00'03'' 12°55'56'' 12°55'57'' 12°43'49'' 12°55'34'' 12°50'45'' 12°50'45'' 12°52'42'' 12°53'28'' 13°09'06'' 14°05'00'' 13°17'16'' 13°17'18'' 12°43'49'' 12°42'19'' 12°43'25'' 13°25'59'' 13°14'47''
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Lat (°N) -20°48'22'' -20°46'54'' -21°05'22'' -19°20'43'' -20°52'31'' -20°48'57'' -18°08'34'' -18°08'33'' -21°02'31'' -20°54'47'' -20°52'17'' -19°55'14'' -19°46'31'' -19°46'35'' -19°23'42'' -18°57'23'' -18°52'10'' -18°52'09'' -18°51'28'' -18°49'50'' -18°18'03'' -20°48'57'' -20°20'38'' -20°20'35'' -19°23'42'' -19°22'13'' -19°20'44'' -19°11'14'' -19°15'53''
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Sample NA024A NA026 NA011B NA044 NA022 NA023B NA093A NA095 NA016 NA020 NA021 NA032 NA034 NA035 NA038C NA069A NA078A NA079 NA081C NA082 NA099 NA023A NA029D NA030 NA038D NA040 NA045D NA053 NA066
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TiO2 3.67 2.15 0.91 1.21 1.11 0.93 3.29 3.53 0.95 1.29 1.41 1.34 1.45 1.36 1.95 1.83 1.75 1.81 1.75 1.77 1.42 1.13 1.12 1.19 0.97 1.22 1.45 1.40 1.83
P2O5 1.03 0.28 0.32 0.17 0.12 0.09 0.44 0.45 0.13 0.13 0.13 0.20 0.19 0.19 0.29 0.22 0.26 0.27 0.27 0.21 0.17 0.11 0.15 0.15 0.12 0.14 0.15 0.16 0.19
LOI 1.94 -0.05 1.89 0.92 0.73 1.34 1.22 4.00 1.51 0.83 0.90 0.82 0.56 1.21 1.29 1.08 1.18 0.48 0.71 0.90 0.39 0.78 0.74 0.81 -0.14 0.17 0.73 0.54 0.46
Total 99.46 99.84 100.2 100.7 99.75 99.49 100.6 99.86 98.84 100.6 99.42 99.79 100.5 98.9 100.3 99.75 100.6 100.3 99.41 99.58 100.7 99.55 99.7 99.85 99.95 99.7 100.7 100.6 99.99
Major elements express in wt. %, trace elements and REE elements in ppm. LOI - Loss of ignition. Nephen. - nephelinite, Alk. basalt - alkaline basalt, Bas. and. - basaltic andesite, Ol. Thol. Tholeiite, Kh - High Ti, High Ti02 Khumib basalts, QT (Enrich) - enriched quartz-tholeiites, QT (E-MORB) - 'primitive' quartz-tholeiites similar to E-MORB. The detection limits for each element are listed on the Activation Laboratory's webpage. http://www.actlabs.com)
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Table 3- cont. Lithology Geochem. Sr Y Zr La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Nephen. Alkaline 1553 26 386 85.7 180 19.9 76 14.4 4.46 11.6 1.5 7.3 1.1 2.6 0.32 1.8 0.23 Alk. Basalt Alkaline 490 20 124 14.2 34.4 4.42 20.5 5.2 1.83 5.3 0.8 4.8 0.9 2.3 0.31 1.9 0.29 Dacite Acid dykes 155 72 668 90.1 184 21 82.6 16.7 3.34 15.3 2.3 13.9 2.7 7.7 1.13 7.4 1.07 Dacite Acid dykes 148 42 311 65.6 139 15.4 59.8 11.7 1.64 10.1 1.6 9.1 1.7 4.9 0.72 4.6 0.66 Basalt Olivine Thol. 192 15 65 6.6 15.8 2.14 10.1 3 1.05 3.4 0.5 3.3 0.6 1.8 0.26 1.6 0.23 Basalt Olivine Thol. 180 13 50 4.3 10.8 1.51 7.8 2.2 0.85 2.8 0.4 2.7 0.5 1.4 0.21 1.3 0.2 1 Basalt 1 Kh- High TiO2 698 27 283 44.1 93.2 11.4 47.2 10.4 3.26 9.1 1.3 7.1 1.2 3.3 0.43 2.7 0.35 1 Basalt Kh- High TiO2 688 28 292 44.2 95.4 11.7 48.5 10.8 3.43 9.6 1.4 7.5 1.3 3.3 0.45 2.7 0.37 NA095 Basalt QT (Enrich.) 169 19 81 9.5 21 2.61 11.8 3.1 1.03 3.7 0.6 4 0.8 2.3 0.36 2.3 0.35 NA016 Basalt QT (Enrich.) 179 18 68 4.2 11.4 1.71 8.7 2.8 1.11 3.6 0.6 3.5 0.7 1.9 0.27 1.7 0.25 NA020 Basalt QT (Enrich.) 211 19 91 6.7 17 2.47 12.1 3.7 1.3 4.5 0.8 4.7 0.9 2.5 0.35 2.2 0.32 NA021 1 Andesite QT (Enrich.) 199 28 168 26.7 56.3 6.55 26.8 6 1.58 6 1 5.9 1.2 3.2 0.47 3.1 0.45 NA032 Bas. and. QT (Enrich.) 216 27 147 22.2 47 5.44 22.3 5.4 1.53 5.7 0.9 5.6 1.1 3.1 0.45 2.9 0.44 NA034 Bas. and. QT (Enrich.) 208 28 140 21.3 45.7 5.29 21.6 5.4 1.57 5.8 0.9 5.5 1.1 3.1 0.45 2.9 0.44 NA035 QT (Enrich.) 193 34 176 25.2 54.4 6.63 27.7 6.8 1.98 7.3 1.2 7.1 1.4 4 0.58 3.7 0.53 NA038C Bas. and. QT (Enrich.) 199 32 151 19.7 43 5.39 23.2 6 1.83 6.8 1.2 7.2 1.5 4.1 0.6 3.9 0.59 NA069A Bas. and. QT (Enrich.) 282 28 191 29.5 61.2 7.31 29 6.6 1.9 6.7 1.1 6.4 1.2 3.6 0.5 3.1 0.49 NA078A Bas. and. Bas. and. QT (Enrich.) 281 29 195 31.6 64.2 7.49 30.2 6.9 1.96 6.9 1.1 6.8 1.3 3.7 0.54 3.5 0.51 NA079 QT (Enrich.) 288 31 178 30.6 62.3 7.49 30.3 6.7 1.97 7 1.1 6.6 1.3 3.6 0.53 3.3 0.51 NA081C Bas. and. Bas. and. QT (Enrich.) 203 31 151 20.1 43.2 5.42 22.9 6 1.81 6.7 1.1 7.1 1.4 4.1 0.59 3.8 0.57 NA082 Bas. and. QT (Enrich.) 233 23 133 21.5 45.8 5.5 22.5 5.2 1.58 5.5 0.9 5.6 1.1 3.1 0.44 2.9 0.42 NA099 QT (E-MORB) 203 16 70 5.8 14.1 1.95 9.8 3 1.06 3.5 0.6 3.5 0.7 1.9 0.28 1.7 0.25 NA023A Basalt QT (E-MORB) 146 25 106 12.9 28.5 3.48 15 4.1 1.25 4.9 0.8 4.9 1 2.8 0.43 2.8 0.4 NA029D Bas. and. Bas. and. QT (E-MORB) 173 26 104 13.1 28.4 3.43 15.1 4 1.24 4.7 0.8 5.1 1 2.9 0.44 2.7 0.42 NA030 QT (E-MORB) 182 16 60 5.6 13.2 1.78 8.9 2.7 0.97 3.3 0.5 3.3 0.7 1.8 0.26 1.7 0.24 NA038D Diabase Basalt QT (E-MORB) 135 25 85 7.4 17.4 2.36 11.1 3.4 1.28 4.5 0.8 5 1 2.9 0.42 2.7 0.42 NA040 QT (E-MORB) 169 27 98 9.8 21.9 2.87 13.4 4 1.34 5.2 0.8 5.5 1.1 3.1 0.45 2.9 0.43 NA045D Basalt Bas. and. QT (E-MORB) 213 26 121 15 33.5 4.08 17.8 4.5 1.39 5.1 0.9 5.5 1.1 3 0.45 2.9 0.42 NA053 Basalt QT (E-MORB) 282 23 132 14.8 33 4.24 18.9 4.9 1.72 5.7 0.9 5.7 1.1 3.1 0.45 2.9 0.45 NA066 Major elements express in wt. %, trace elements and REE elements in ppm. LOI Loss of ignition. Nephen. - nephelinite, Alk. basalt - alkaline basalt, Bas. and. - basaltic andesite, Ol. Thol. Tholeiite, Kh - High Ti02, High Ti02 Khumib basalts, QT (Enrich) - enriched quartz-tholeiites, QT (E-MORB) - 'primitive' quartz-tholeiites similar to E-MORB. The detection limits for each element are listed on the Activation Laboratory's webpage. http://www.actlabs.com)
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The alkaline basalt dyke (NA026) contains fine grained olivine phenocrystals within a very fine grained matrix composed of pyroxene, plagioclase and abundant opaque minerals. Petrographic analysis and the CIPW normative mineralogy of Hutchison (1974, 1975), indicated that two dykes (NA022 & NA023B), can be classified as olivine-tholeiites and are considered to form a distinctive geochemical group. These olivine basalts are fine grained, holocrystalline with subhedral phenocrystals of plagioclase, pyroxene and olivine and display subophitic and poikilitic textures. It is worth noting that these dykes were encountered within the Ugab zone in the extreme south of the Kaoko Belt. The rest of the dykes can be classified as quartz-tholeiites (basalts and basaltic andesites) with normative quartz and hypersthene. These dykes are generally fine grained and contain plagioclase, augite and in some samples, pigeonite. The majority of the basaltic andesites are porphyritic, with plagioclase and pyroxene phenocrystals which often form glomerocrysts. Some plagioclase phenocrystals are zoned indicating fractional crystallisation. All of the quartz-tholeiite dykes can be classified as Low-TiO2 according to the criteria defined by Marsh et al. (2001), as they contain <2.2% TiO2 and < 450ppm Sr. The variation in the concentration of chondrite-normalised REEs was used to subdivide the Low-TiO2 quartz-tholeiites into two geochemical groups: enriched quartz-tholeiites and 'primitive' quartz-tholeiites (Fig. 9). The former display relative enrichment in LREEs (Fig. 9a) as exemplified by La/Yb ratios which vary from 3.4-6.5 (Fig. 10). These dykes also display large negative Eu anomalies indicating significant fractional crystallisation of plagioclase. The latter group of quartz-tholeiites display less enrichment with La/Yb ratios less than 3.4, and smaller negative Eu anomalies (Fig. 9b & 10). The enriched quartz-tholeiitic dykes are more widespread and generally strike NNW, whilst the quartz-tholeiitic dykes with more primitive compositions are more common near the coast and display orientations that vary between 250-315°N (ESE-NW). b)
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Figure 9- Chondrite normalized REE (McDonough and Sun, 1995) for basalts of the Skeleton Coast dyke swarm. a) Enriched quartz tholeiites in orange; b) 'Primitive" Quartz tholeiites in dark blue. The light grey area represents the Tafelberg magma composition from Ewart et. al. (1998 & 2004), Marsh et al. (2001), Marsh & Milner (2007) Thompson et. al. (2001) and Trumbull et. al. (2007). The dark grey area represents the compositional range of the Horingbaai dykes from Marsh et. al. (2001), Thompson et al. (2001) and Trumbull et al. (2007). Compositions of ca. 74 Ma basalts from DSDP Leg 74 525A (29°04'14"S 2°59'7"E) drilled within the southern extension of the Walvis Ridge (Hoernel et al., 2015), as well as N-MORB & E-MORB (Sun & McDonough, 1989) are also plotted for comparison.
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Figure 10- (La/Yb)N vs. MgO (wt%) of the quartz-tholeiite dykes(circles) of the SCDS are compared with the two High-TiO2 basalt lava flows of the Khumib formation (Etendeka Group). References: 1. Walvis Ridge (DSDP Leg 74 525A -29°04'14"S 2°59'7"E) - Hoernel et al. (2015); 2. N-MORB & E-MORB - Sun & McDonough (1989); 3. MgO% N-MORB - Hart et al. (1999); 4. MgO% E-MORB Klein et al. (2004).
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455 5.3 Kinematics of the SCDS
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As outlined in section 4.3 of this paper, the application of the principles of kinematic analysis to the
458
geometry of individual dykes and in particular to asymmetrical features such as branches, en
459
echelon dykes and bridges between dykes segments, allows the estimation of the orientation of the
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regional stress field active during dyke intrusion (Rickwood, 1990; Corrêa Gomes, 2001). The
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dominant NNW-trending dykes of the SCDS (T1) are generally rectilinear, with only occasional
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branching segments. The relative absence of asymmetrical features and the angular relationship
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between the dyke walls and internal fractures suggests that these dykes were intruded under
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conditions of normal extension with the direction of minimum stress (σ3 - ENE-WSW - N65E)
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perpendicular to the strike of the dykes. In contrast, ENE-WSE trending dykes (T3) often present
466
asymmetrical features such as steps, and bridges between dyke segments that indicate a sinistral
467
strike-slip component related to NW-SE (315-135°) extension during their intrusion. These dykes
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occur with greater frequency closer to the coast and were observed to cut across the NNW-striking
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dykes (Fig. 11).
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Figure 11 - a) ENE-trending dyke (green) cutting across an earlier NNW-trending diabase dyke intruded under conditions of normal extension (T1 - σ3 - 65-245°). b) Asymmetrical features along the later basalt dyke such as bridges between dyke segments indicate a sinistral component to extension with the direction of minimum stress (σ3) perpendicular to the walls of the bridge (T3 - σ3 - 315-135°). Local: NA038 - 19°23'42.00"S, 12°43'49.84” E (Mowe Bay)
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The directions of (horizontal) minimum and maximum stress were calculated for 29 different
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dykes of the SCDS (Fig. 12). The interpretation of these results, together with cross-cutting
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relationships observed in the field and in satellite imagery, indicates the presence of three dyke
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generations (T1, T2 & T3) related to different extensional events that affected north-west Namibia
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during the late Cretaceous as summarized in Table 4.
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ACCEPTED MANUSCRIPT Figure 12- Rose diagram showing the directions of minimum (green) and maximum (red) stress during the intrusions of 29 dykes of the SCDS, indicating three different extensional events (T1, T2, T3).
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Table 4- Extensional events related to the intrusion of the dykes of the SCDS, NW Namibia
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The first event (T1) is characterised by ENE-WSW (65-245°) extension, which resulted in the intrusion of the dominant NNW-trending dykes. These dykes display dacitic to basaltic compositions and are cut by ENE-WSW orientated dykes. These younger ENE-trending dykes display a sinistral component related to NW-SE (315-135°) extension and generally display more 'primitive' compositions with less enrichment in LREEs similar to E-MORB. Dykes which vary in orientation from N45W to N80W were also observed in the field, but were not mapped in significant numbers on the regional aeromagnetic data. These dykes display a sinistral or dextral component depending on their orientation and are associated with NNE-SSW (25-205°) directed extension. The cross-cutting relationships between these NW to WNW-trending dykes and the ENE trending dykes were not clearly observed in the field. Nonetheless, we suggest that these NW to WNW-trending dykes represent a transitional phase (T2) between the dominant NNW-trending dykes (T1) and the later 'primitive' ENE-WSW trending dykes (T3). The tectonic implications of these results are discussed in section 6.2 of this paper.
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6. Discussion
6.1 Relationship between the SCDS and the Paraná-Etendeka Volcanics Some comparisons can be made between the composition of the dykes of the SCDS sampled in this study and the principal magma types of the Paraná- Etendeka province, which were described in section 3 of this paper. Trumbull et al., (2004, 2007) showed that the majority of the dykes of the Henties Bay-Outjo dyke swarm (HOD) are Low-TiO2 tholeiites and show similarities with the Tafelberg, Esmeralda and Horingbaai magma types (Fig. 13). These authors considered that the concentrations and ratios of High Field strength, incompatible elements (e.g. Zr, Ti, Zr) are the most 22
ACCEPTED MANUSCRIPT useful to discriminate between magma types as the concentration of these elements is not strongly affected by the crystallization of olivine nor by minor rock alteration. The small variation in composition between the dykes of the HOD and the Etendeka basalts led the Trumbull et al (2004,) to suggest these dykes represent the feeder dykes to now eroded southern extension of the Etendeka basalts. However, these same authors and others (e.g. Ewart et al., 1998; Thompson et al., 2001) also observed that mafic dykes have been observed to cut across the Etendeka basalts suggesting a younger age for at least some of these dykes. For example at their type locality along the Skeleton Coast to the south-east of the Messum and Brandberg Intrusive complexes, the more primitive Horingbaai dykes and sills are intrusive in the 125-132 Ma Tafelberg basalts (Ewart et al., 1984). As outlined in section 5.3 of this paper, three dyke generations have been recognised within the SCDS, and based on cross-cutting relationships, the dominant NNW-striking dykes are the oldest. The majority of these dykes are quartz-tholeiitic basalts and andesitic basalts with relative enrichment in LREEs and (La/Yb)n ratios between 3.43 and6.56. The dykes show geochemical similarities with Tafelberg and Esmeralda basalts (Fig. 13 & 14). These dykes often occur sub-parallel to the NNW-trending, closely spaced, listric faults that cut across the Etendeka volcanics within the coastal structural domain of the Etendeka province defined by Marsh et al. (2001). ENE-WSW dykes are clearly observed cutting across these earlier NNW-SSE orientated dykes. These younger quartz-tholeiitic dykes are characterised by more 'primitive' geochemistry with less enrichment in LREEs and are similar in composition to Horingbaai and Esmeralda-type basalts (Fig. 13 & 14). These ENE-striking dykes are assumed to register a third extensional event (T3) that affected NW Namibia during the Early Cretaceous, whilst NW to WNW-trending dykes represent a transitional phase (T2) between the dominant NNW-trending dykes (T1) and these later 'primitive' ENE-WSW trending dykes (T3). The NW to WNW-trending dykes display both 'enriched; and 'primitive' geochemical compositions.
538 539 540
Figure 13- Zr/Y & Ti/Zr discrimination diagram of the 'basaltic' dykes of the SCDS (Si02 < 60 wt%) compared with the composition of the basaltic dykes of the Henties Bay-Outjo Dyke Swarm (HOD -1) and the principal Etendeka flood basalt types (Tb - Talfberg, Kh - Khumib, Tk - Tafelkop, E- Esmeralda, Horingbaai). Modified
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ACCEPTED MANUSCRIPT from Trumbull et al. (2004). The composition of two basalt lavas flows of the High Ti, Khumib Formation sampled during this study are also shown. Note: 1. HOD - Trumbull et al., (2004 & 2007); 2. Walvis Ridge Hoernel et al. (2015); 3. N-MORB e E-MORB - Sun & McDonough (1989).
544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570
Figure 14- Zr/Nb & La/Nb discrimination diagram of the 'basaltic' dykes of the SCDS (Si02 < 60 wt%) compared with the composition of the dykes of the HOD and the principal Etendeka flood basalt types (Tb - Talfberg, Kh Khumib, Tk - Tafelkop, E- Esmeralda, Horingbaai). The composition of OIB of the South Atlantic, the Kudu basalts of offshore Orange Basin and basalt clasts of the syn-volcanic Albin conglomerate are also shown. Modified from Jerram et al. (2015). Note: 1. HOD - Trumbull et al., (2004 & 2007); 2. Walvis Ridge - Hoernel et al. (2015); 3. N-MORB e E-MORB - Sun & McDonough (1989); 4. Chondrite - McDonough & Sun (1995).
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6.2 Tectonic Implications of the Skeleton Coast Dyke Swarm A tectonic reconstruction of part of Western Gondwana demonstrates a spatial correlation between the SCDS of North-west Namibia and the Florianopolis Dyke Swarm (FDS - Fig. 2). It can be observed that the continuation of the Ponta Grossa-Guapiara Dyke Swarm (PGDS) points toward the start of the Walvis Ridge at approximately 18°S along the Skeleton Coast close to the mouth of the Cunene River which marks the border between Namibia and Angola. Several authors (e.g. O'Connor & Duncan, 1990; Peate et al., 1990; Ryberg, et al., 2015; Thompson and Gibson, 1991) have suggested that the Tristan plume head impacted the base of the lithosphere in this region during the Early Cretaceous. This hypothesis is supported by the recent magnetotelluric imaging of the NW Namibian margin (Jegen et al., 2016) and the results of a seismic refraction survey which identified a Highvelocity Lower Crustal (HVLC) Body beneath the Kunene region of NW Namibia. This HVLC body was interpreted to represent mafic-ultramafic intrusives or underplating of the lower crust at the site of plume impact (Ryberg et al., 2015; Heit et al., 2015). The plume impact is likely to have influenced magma supply and the conditions of stress during initial rifting, and if we compare the width of the two dyke swarms it can be observed that the dykes of the FDS are restricted to a 50km wide coastal strip of Santa Catarina state, whilst the dykes of the SCDS are found up to 250km from the Skeleton coast of NW Namibia. Coast-parallel block faulting of the Etendeka basalts along the NW Namibia margin is common (Milner and Duncan, 1987; and Stollhofen, 1999). In contrast, in their recent study which 24
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compared the paleostress fields of the NW Namibia -S Brazil conjugate margins, Salomon et al. (2015b) failed to identify major extensional faults associated with Atlantic rifting along the studied section of the southern Brazilian coastline. However, it should be noted that their study area did not encompass the outcrop area of the FDS along the Santa Catarina coast. Nonetheless based on their observations, Salomon et al. (2015b) concluded that during the opening of the South Atlantic, extension was focused between the current Brazilian coastline and the Continental-Ocean Boundary. This hypothesis is supported by offshore seismic data which indicates extensive listric normal faulting along both margins (Gladczenko et al., 1997; Blaich et al., 2011). It is also worth noting that Early Cretaceous volcanic rocks have been identified offshore of the NW Namibian margin as seaward dipping reflectors (SDRs) underlying post-rift sediments in the Walvis Basin. These SDRs were faulted during Aptian-Albian rifting leading to the creation of structural highs and local depocentres within the offshore Walvis Basin (Holtar & Forsber, 2000). The majority of the dykes of the FDS strike to the NNE (N25E), but dykes of other directions also occur (Fig. 15a & b). Field observations allowed the calculation of the directions of minimum and maximum stress active during the intrusion of 36 different dykes of the FDS (Almeida et al. 2013). The results confirmed that the dominant direction of the FDS is associated with WNW-ESE (285-105°) extension (Almeida et al., 2013). Nonetheless, NE-SW dykes can also be observed cutting across these earlier NNE-trending dykes and are associated with NW-SE extension (Fig. 15c).
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Figure 15 - Rose diagrams of the FDS from: a) field observations (n=70, 12° interv., max. freq. 20% - 14 dykes); and b) directions of rectilinear dyke segments extracted in ArcGIS (n=609, 10° interv., max. freq. 32.5% - 198 segments); c) directions of maximum (σHmax) and minimum (σ3) stress for 36 dykes of the FDS (n=36, 10° intervals, principal σ3 - 285-105°, principal σHmax - 15-195°). Modified from Almeida et al. (2013).
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ACCEPTED MANUSCRIPT A recent study by Veronez & Tomazzoli (2016) identified at least three dyke generations. A NW-trending dyke presented an Ar-Ar age of 119 Ma and an older N30E trending dyke was dated at 139 Ma (Veronez & Tomazzoli, 2016)). These ages vary from the results of ID-TIMS U–Pb baddeleyite/zircon dating of dykes from the FDS published by Florisbal et al. (2014). These authors suggested a restricted interval of between 134.7±0.3 and 133.9±0.7Ma for the intrusion of the dykes of the FDS. However, this interpretation is based on the dating of only three dykes; two NNEtrending, High-TiO2 basalt dykes and a trachyandesite dyke of the same direction. Florisbal et al. (2014) observed that the majority (90%) of the basalt dykes of the FDS are High Ti-Sr and suggested that the FDS represents feeder dykes for the High-TiO2 Urubici and Khumib basalt lava flows of S Brazil and NW Namibia respectively. However, dykes other directions within the FDS are generally Low-TiO2, cut across the dominant NNE-trending dykes and have not been dated by ID-TIMS U–Pb baddeleyite/zircon dating. It is worth noting that in the present study; only two samples were taken of High-TiO2 basalts, both being of Khumib lava flows. All of the dykes sampled were Low-TiO2 and some of them display compositions very similar to the Low-TiO2 dykes of the FDS (Fig 16). We assume that the majority of the dykes of the SCDS and the FDS were intruded during an early stage of rifting between ~133-139 Ma (Veronez & Tomazzoli, 2016; Florisbal et al., 2014).
611 612 613 614 615
Figure 16 - Comparison of the Ti/Y ratios and Sr content of the dykes of the FDS and the 'basaltic' dykes of the SCDS (Si02 < 60 wt%) with the principal mafic magmas of the Paraná-Etendeka igneous province (after Peate, 1997). The composition of the two samples of High-TiO2 Khumib basalt lava flows are also shown. Modified from Florisbal et al., 2014. Note. 1. Composition of the three dykes dated by Florisbal et. al., 2014; 2. Walvis Ridge - Hoernel et al., 2015; 3. N-MORB e E-MORB - Sun & McDonough, 1989.
616 617 618 619
If we restore South America to its position at ~139Ma, prior to the break-up of Gondwana, the principal directions of the SCDS and Florianopolis dyke swarm (FDS) are almost equivalent (Figure 17a). By applying a rotation of 40°W we restore the majority of the dykes of the FDS to their prebreak-up orientation. After restoration the principal direction extension of both the FDS and the
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SCDS is essentially the same (Fig. 17b & c). This supports the hypothesis that the majority of the dykes of both the FDS and SCDS were intruded under the same stress regime with ENE-WSE (65245°) directed, subhorizontal extension, during the initial rifting that led to the break-up of Gondwana.
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b) FDS*
c) SCDS
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Figure 17 - a) Pre-break-up tectonic reconstruction of the conjugate margins of S Brazil and NW Namibia. Note the position of landward portion of the Walvis Ridge and the correlation between the SCDS and FDS. Other tholeiitic dykes swarms: Ponta Grossa-Guapiara (PGDS), Henties Bay-Outjo (HOD). Rose diagrams of the directions of minimum (σ3) and maximum (σHmax) stress active during the intrusion of the dykes of b) FDS rotated 40W to their pre-break orientation; and c) SCDS; indicating the principal directions of the extension related to multiple dyke generations. For key to the geological units refer to the legend in Figure 14.
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ACCEPTED MANUSCRIPT This extensional event was also identified by Salomon et al. (2015b) through the kinematic analysis of faults that cut the Etendeka basalts of NW Namibia. A younger event with subhorizontal NNE-SSW (25-205°) directed extension was also identified by the same authors. This corresponds to the direction of minimum stress (σ3) of the second generation of dykes (T2) of the SCDS (Figure 17c). This event has not been clearly defined within the FDS, nonetheless the younger, ~119 Ma, NWtrending dykes (present day orientation), of the FDS may be equivalent to the third generation (T3) of dykes within the SCDS. It should be observed that the South American continent had probably already undergone some rotation between the intrusion of the first generation of dykes at ~133-139 Ma and the intrusion of these later dykes at ~119 Ma. Therefore, the directions of minimum stress for this event do not exactly correspond on either side of the South Atlantic (Figure 17 b & c).
639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676
7. Conclusions Satellite imagery and aeromagnetic data have allowed the mapping of over 4000 mafic dykes intrusive in the Neoproterozoic Kaoko Belt and adjacent Angolan craton. These dykes define an important NNW-SSE trending, linear dyke swarm, the Skeleton Coast Dyke Swarm. The SCDS is some 150-200km wide and extends for more than 400km parallel to the NW Namibian coast with likely extension into SW Angola. Three dyke generations were identified within the SCDS: 1) dominant NNE-trending dykes related to ENE-WSW (65-245°) extension; 2) NW to WNW-trending dykes related to NNE-SSW (25-205°) extension; 3) ENE-WSW orientated dykes related to NW-SE (315-205°) extension. The majority of the dykes sampled in this study are Low-TiO2, basalts and andesitic basalts, with the exception of a nephelinite dyke and two dacitic dykes which also trend to NNE-SSW. The dominant NNE-SSW orientated dykes display relative enrichment in LREEs and display geochemical similarities with Tafelberg and Esmeralda basalts. This suggests that this earlier generation of dykes may represent feeder dykes for now eroded Etendeka basalt lava flows. Greater lithospheric thickness during initial rifting may explain the apparently greater crustal contamination of this first generation of dykes.. In contrast, the third generation of dykes tend to display more 'primitive' compositions similar to the Horingbaai dykes and sills which are interpreted to have been derived from a depleted asthenosphere source during the final break-up of Gondwana when the lithosphere had already undergone significant extension (Ewart et al, 1984; Duncan et al., 1990; Marsh et al., 2001). The NW to WNW-trending dykes display both 'enriched' and 'primitive' compositions and are assumed to represent a transitional phase characterised by NNE-SSW directed extension. This extensional event was also identified by Salomon et al. (2015b) through the kinematic analysis of faults that cut the Etendeka basalts of NW Namibia. When South America is restored to its position at ~139 Ma, prior to the break-up of Gondwana, the principal directions of the Skeleton Coast and Florianopolis dyke swarms are equivalent. The majority of the dykes of the FDS are High-TiO2 basalts similar in composition to the Urubici/ Khumib basalts, however younger dykes of varying orientations and Low-TiO2 geochemistry have also been recognised within the FDS. 40Ar-39Ar dating suggests a prolonged period of dyke intrusion within the FDS between 139-119 Ma (Deckhart et al., 1998; Raposo et al., 1998; Veronez & Tomazzoli, 2016). However, it is assumed that the majority of the dykes of the SCDS and FDS were intruded during an early stage of rifting occurring between ~133-139 Ma. This rifting was characterised by ENE-WSW extension that led to coast-parallel block faulting of the Etendeka basalts in Namibia (Salomon et al., 2015b). The second generation of dykes identified in the SCDS has been not recognised within the FDS. However, the younger (~119 Ma) NW-trending dykes (present day orientation) of the FDS may be equivalent to the third generation (T3) of dykes within the SCDS. This later extensional event may be related to offshore Aptian-Albian rifting within the Walvis and Pelotas Basins of NW Namibia and S Brazil, (Holtar & Forsberg, 2000; Dias et al., 1994; Bueno et al., 2007). Precise 40Ar-39Ar dating of
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ACCEPTED MANUSCRIPT the multiple dyke generations identified in this study should be undertaken to better constrain the kinematic history of NW Namibia during the break-up of Gondwana and subsequent formation of the proto-South Atlantic.
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Acknowledgements The authors would like to acknowledge the Geological Survey of Namibia (GSN) for providing the high-resolution aeromagnetic data that was used in this study and in particular Deputy Directors Anna Nguno and Kombada Mhopjeni of the Regional Geosciences and Geo-Information Divisions respectively, for providing logistical support for our fieldwork. Special thanks also to our colleagues, Henrique Bruno and Aimee Guida from the Rio de Janeiro State University; and Marcelo Motta and Felipe Frai from the Pontifical Catholic University of Rio de Janeiro who participated in the fieldwork. Financial support for field expenses and geochemical analyses was provided by the Fundação de Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) through grant number 26/010.002995/2014 and CNE_05/2015 - Cientista do Nosso Estado - 2015. Finally we thank Prof Fred Kamona and Dr. Antony Mamuse for their reviews of this manuscript.
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Table 4- Extensional events related to the intrusion of the dykes of the SCDS, NW Namibia Extension (σ3)
Orient.
Cinem.
T1
N15W
Normal
T2
N80W-N45W
Dextral to Sinistral
Lithology
ENE-WSW (75Dacites, Basalts - Basaltic Andesites 255)
NNE-SSW (25205)
Geochem.
No.
Enriched
15
"Primitive" & Enriched
8
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Generation / Event
Basalts - Basaltic Andesites
Sinistral
NW-SE (315315)
"Primitive"
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N70E - N85E
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T3
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ACCEPTED MANUSCRIPT Characterisation and Tectonic Implications of the Early Cretaceous, Skeleton Coast Dyke Swarm, NW Namibia McMaster, M.a,*, Almeida, J.a, Heilbron, M. a, Guedes, E. b, Mane, M.A. a, Linus, J.H. c
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Highlights -
1. Geophysical, structural and geochemical characterisation of the Skeleton Coast Dyke Swarm. 2. Cinematic evolution of dyke emplacement related to the opening of the South Atlantic Ocean.
3. Skeleton Coast Dyke Swarm correlated with the Florianopolis Dyke Swarm across the South
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Atlantic.