[Tectonics]
Supporting Information for
[Structural and geochronological constraints on the Pan-African tectonic
evolution of the northern Damara Belt, Namibia]
[Jérémie Lehmann1,2, Kerstin Saalmann1,3, Kalin V. Naydenov1,4, Lorenzo Milani1,
George A. Belyanin2, Horst Zwingmann5,6, Guy Charlesworth1, and Judith A. Kinnaird1]
[1 EGRI, School of Geosciences, University of the Witwatersrand, PVT Bag 3, Wits, 2050, South Africa
present address: Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa
3 Geological Survey of Norway (NGU), Postboks 6315 Sluppen, 7491 Trondheim, Norway
4 Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl. 24, 1113 Sofia, Bulgaria
5
CSIRO, Earth Science and Resource Engineering, Bentley, WA 6102, Australia
6 present address: Division of Earth and Planetary Sciences, Kyoto University, Kyoto, 606-8502, Japan]
2
Contents of this file
Captions for Tables S1, S2 and S3
Text S4
Figure S4 and caption
Table S4 and caption
Additional Supporting Information (Files uploaded separately)
Tables S1, S2 and S3
Introduction
This supporting information file contains four different items. Table S1 to S3 are
uploaded separately and in excel format. Table S1 is a database of published and
unpublished radiometric ages used for the computations of the age histograms in Figure
2 of our manuscript. Table S2 contains the U-Pb age dating results for reference
materials Plešovice obtained during this study by LA-SF-ICP-MS method. Table S3
presents the EMP analyses performed on three of the dated samples by 40 Ar/39Ar
(N187B and N226B) and K-Ar methods (N226A and N226B). The microstructural
characterization of chemically analysed individual phengites can be found in this table.
Auxiliary material S4 deals with the numerical modelling of radiogenic argon diffusion
through time. Text S4 presents the possible constraints for the T-t evolution of our dated
material, explains the modelling method and describe the results of the modelling that
are presented in Figure S4 and Table S4.
1
Table S1. Uploaded separately.
Radiometric ages used for histograms presented in Figure 2.
Table S2. Uploaded separately.
U–Pb age dating results for reference materials Plešovice obtained during this study by
LA-SF-ICP-MS.
Table S3. Uploaded separately.
55 chemical analyses from white micas of samples N187B, N226A and N226B.
Microstructural observations for individually analysed grains and interpretations are
reported.
Text S4.
Our data show that different mica fractions retained different 40Ar/39Ar and K-Ar ages,
even in the case of the same sample being analysed by both methods (i.e. N226B). A
first age group, given by the 40Ar/39Ar data on 100-300 µm fraction, clusters in between
600 and 580 Ma, whereas a second one, given by the K-Ar data on < 2 µm fraction,
clusters at 480–460 Ma. In both groups, the Ar reservoir is interpreted as metamorphic
phengites on the basis of microstructural and petrological investigations. Few older steps
in the 40Ar/39Ar spectra cluster at 630–625 Ma. Based on microstructural and chemical
data, as well as on the correlation with detrital U-Pb ages from the same geological units
of Foster et al. [2015], they are interpreted as the age of detrital phengites.
The tentative controls on the thermal evolution of the dated rocks are as follow.
(i) The post Pan-African cooling path is only weakly constrained by the Cretaceous
apatite fission track ages from Raab et al. [2005], although deep parts of the Damara
Belt had already reached the surface by Cambrian times [Grotzinger and Miller, 2008],
since the Cambrian molasse basin (Nama Group) on the Kalahari Craton contains
detrital garnets supplied from the north [Blanco et al., 2011]. (ii) The pelitic rocks of the
northern Damara Belt remain below the garnet isograd during the 140–120 Ma-long
thermal history and were heterogeneously deformed during D1, D2 and D3 in the biotite
stability field of the greenschist facies. (iii) The samples were gathered in the Kuiseb
Formation which has a maximum depositional age of 622 ± 27 Ma (207Pb/206Pb age on
detrital zircon) [Foster et al., 2015]. Hence, the 40Ar/39Ar ages of 600–580 Ma indicate
that these rocks were buried and heated within a maximum of ~ 40 Ma. (4) Another
control comes from applying the concept of closure temperature for muscovite (i.e.
temperatures below which the mineral grain is effectively closed to argon diffusion
[McDougall and Harrison, 1999]). The maximum pressure recorded in the Ugab Zone
and western Northern Zone is 4–5 kbar [Goscombe et al., 2004]. The closure
temperature at 5 kbar pressure varies from 440 °C to 370 °C for a cooling rate and grain
radius ranging from 1 to 10 °C/Ma and 100 to 500 µm, respectively [Harrison et al.,
2009].
Based on this complex thermal history, a potential thermal overprint on the ages
of the dated phengite fractions was evaluated by basic Ar diffusion calculations. The
general theory of Ar diffusion is summarized by Crank [1975] and Watson and Baxter
[2007], and its geological application to the K-Ar and 40Ar/39Ar methods can be found in
McDougall and Harrison [1999]. Only limited Ar diffusion parameters for white mica have
been published [Robbins, 1972; Wijbrans and McDougall, 1986; Harrison et al., 2009].
We evaluated the influence of potential Ar diffusion on the phengite fractions by a
thermal overprint via a basic Ar diffusion model described by Huon et al. [1993] using
2
200 m and 2 m grain fractions, temperature range from 200 to 600 °C and a
timeframe of 1 to 125 Ma. Because the shape of fine grained clay minerals (fibres vs.
plates for example) will strongly influence diffusion, cylindrical, plane and sphere
geometric shapes were applied. As diffusion calculations of fine-grained clay minerals
are not common in the literature; we used the original parameters listed in Huon et al.
[1993, p. 174] in this study with Do and Ea values 6.03 x 10-7 cm2/s and 40 x 103 cal/mol,
respectively [Wijbrans and McDougall, 1986]. We assume that the thermal anomaly
generating the 600–580 Ma ages lasted for 10 Ma  any longer heating would narrow
the possible T–t range recorded for the dated samples.
The results of our calculations are summarized in auxiliary material Table S4 and
auxiliary material Figure S4 where four different temperature paths were tested on 200
m (Figure S4b) and 2 m grain size fractions (Figure S4c). For the 200 m fraction,
path (a) corresponds to heating to 500 °C in 10 Ma and will lead to 100 % loss of
radiogenic 40Ar content. Path (b), that simulates heating during the same time frame (10
Ma) but only reaching 400 °C, would reduce the radiogenic 40Ar content by
approximately 40 %. Path (c) models a thermal overprint of 300 °C over 125 Ma, and
would decrease the radiogenic 40Ar content by 10 %. Finally, path (d) simulates 125 Ma
of constant heating at 200 °C. This would not affect the radiogenic 40Ar retained in a 200
m grain size muscovite. We subsequently tested the same T–t paths on the 2 m grain
fraction (Figure S4c). Paths (a), (b) and (c) led to complete loss of 40Ar in less than 5 Ma
while path (d) would reduce it by only 25 % after 125 Ma of heating at 200 °C.
Our numerical simulation of Ar diffusion implies that the system never reached a
temperature higher than ~ 350 °C for a period of ≥ 10 Ma after that the 600–580 Ma
plateau ages were obtained (Figure 13). This is consistent with the above-mentioned
closure temperatures of Harrison et al. [2009]. These plateau ages, however, constrain
the initial temperature perturbation at 600–580 Ma to be no less than 400 °C, and
probably at least of 450 °C in order to produce our Ar plateaux. After 580 Ma, the rocks
must have remained above the 200 °C isograd for a time frame of 125 Ma. This is
compatible with D2 and D3 being active in the biotite stability field of the greenschist
facies. If they were below 200 °C, the K-Ar ages on 2 m grain fractions would be
systematically older than 480–460 Ma.
In summary, information from the geology and Ar diffusion modelling is
consistent with an initial thermal anomaly prior to 580 Ma bracketed between 400 and
500 °C (Figure S4a). This was followed by limited cooling in between 350 and 200 °C
until 480–460 Ma with final cooling below 200 °C after 460 Ma.
3
Figure S4. Argon diffusion modelling in 200 and 2 µm white mica fractions. (a) Possible
T–t range for the northern Damara Belt with T–t paths (a) to (d) used for argon diffusion
modelling in white micas. Results of Ar modelling in (b) 200 µm and (c) 2 µm white
micas fraction, with percentage of radiogenic Ar loss plotted against the time-scale of
heating event. The diagrams show the impact of different heating temperatures from 200
to 600 °C.
grain
size
[µm]
Time
[Ma]
Temperature [°C]
200
240
280
2
200
10
10
5.8
0.0
320
360
400
460
500
540
580
Average rad 40Ar loss [%]
39.5 98.8 100.0 100.0 100.0 100.0 100.0 100.0 100.0
0.0 0.1
0.3
1.8
7.9
32.0 61.9 93.0 100.0
2
200
125
125
26.6
0.0
31.4 94.5 100.0 100.0 100.0 100.0 100.0 100.0 100.0
0.1 0.3
2.5
11.8 33.3 89.8 100.0 100.0 100.0
Table S4. Argon diffusion results for different geometrical shapes of 2 and 200 μm white
micas particles at temperatures between 200 and 600 °C for maximum heating time
scale of 125 Ma.
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