Publisher: GSA
Journal: GEOL: Geology
DOI:10.1130/G39344.1

Southwestern Africa on the burner: Pleistocene carbonatite
volcanism linked to deep mantle upwelling in Angola
Andrea Giuliani 1,2,3,*, Marc Campeny 4,5, Vadim S. Kamenetsky 6, Juan Carlos Afonso 2,7,
Roland Maas 1, Joan Carles Melgarejo 5, Barry P. Kohn 1, Erin L., Matchan 1, José Mangas 8,
Antonio O. Gonçalves 9, José Manuel 9
1 School of Earth Sciences, The University of Melbourne, Parkville, VIC 3010, Australia
2 ARC Centre of Excellence for Core to Crust Fluid Systems and GEMOC, Department of
Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia
3 Department of Earth Sciences, Vrije Universiteit Amsterdam, 1081 HV Amsterdam, The
Netherlands
4 Natural History Museum of Barcelona, 08003 Barcelona, Catalonia, Spain
5 Department Mineralogia, Petrologia i Geologia Aplicada, Universitat de Barcelona, 08028
Barcelona, Catalonia, Spain
6 School of Physical Sciences, University of Tasmania, Hobart, Tasmania 7001, Australia
7 Centre for Earth Evolution and Dynamics, Department of Geosciences, University of Oslo
8 Department Física, Instituto de Oceanografía y Cambio Global, Universidad de Las Palmas
de Gran Canaria, Campus universitario de Tafira, 35017 Las Palmas de Gran Canaria, Spain
9 Departament Geologia, Universidade Agostinho Neto, 815 Luanda, Angola
*E-mail addresses: andrea.giuliani@unimelb.edu.au; andrea.giuliani@mq.edu.au;
telephone: +61-3-83449094

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ABSTRACT
The origin of intraplate carbonatitic to alkaline volcanism in Africa is controversial. A
tectonic control, i.e., decompression melting associated far-field stress, is suggested by
correlation with lithospheric sutures, repeated magmatic cycles in the same areas over
several Myr, synchronicity across the plate, and lack of clear age progression patterns.
Conversely, a dominant role for mantle convection is supported by the coincidence of
Cenozoic volcanism with regions of lithospheric uplift, positive free-air gravity anomalies
and slow seismic velocities. To improve constraints on the genesis of African volcanism,
here we report the first radiometric and isotopic results for the Catanda complex, which
hosts the only extrusive carbonatites in Angola. Apatite (U-Th-Sm)/He and phlogopite
40Ar/39Ar ages of Catanda aillikite lavas indicate eruption at ~500–800 ka, >100 Ma after
emplacement of abundant kimberlites and carbonatites in this region. The lavas share
similar HIMU-like Sr-Nd-Pb-Hf isotope compositions with other young mantle-derived
volcanics from Africa (e.g., Northern Kenya Rift; Cameroon Line). The position of the
Catanda complex in the Lucapa corridor, a long-lived extensional structure, suggests a
possible tectonic control for the volcanism. The complex is also located on the Bié Dome, a
broad region of fast Pleistocene uplift attributed to mantle upwelling. Seismic tomography
models indicate convection of deep hot material beneath regions of active volcanism in
Africa including a large area encompassing Angola and northern Namibia. This is strong
evidence that intraplate late-Cenozoic volcanism, including the Catanda complex, resulted
from the interplay between mantle convection and pre-existing lithospheric
heterogeneities.

INTRODUCTION
Cenozoic volcanism is widespread in Africa, with prominent examples such as the Eastern
African Rift System (EARS), the Cameroon Line, Darfur, Hoggar, Tibesti and the Moroccan
Atlas (Fig. 1) (e.g., Wilson and Guiraud, 1992; Burke, 1996; Ebinger and Sleep, 1998; Njome
and de Wit, 2014). The magmatic products commonly derive from the mantle and range
from carbonatitic and mafic alkaline to differentiated silica-rich magmas (e.g., Wilson and
Guiraud, 1992; Bailey and Woolley, 2005). Most magmatic centers are located along suture
zones between tectonic terranes (e.g., Thorpe and Smith, 1974). A profound lithospheric
control on intraplate volcanism, whereby melts are generated by decompression-induced
melting in (or below) the lithosphere due to extensional stress, is indicated by 1)
association of volcanism with terrane, boundaries and/or thin lithosphere; 2) cyclicity of
magmatic activity in the same regions over >100 Myr; 3) synchronicity across the African
plate; and 4) absence of clear age progression patterns (Bailey and Woolley, 2005). The

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only notable exceptions are Late Cretaceous and Paleogene alkaline volcanic centers in
southern Namibia, possibly related to the passage of hot spots currently in the South
Atlantic (Reid et al., 1990). Regions of active and recent volcanism coincide with some of
the several swells (i.e., topographic highs) that characterize the surface morphology of the
African continent (e.g., Burke, 1996). These zones are characterized by positive free-air
gravity anomalies (e.g., Burke, 1996; Al-Hajri et al., 2009) and slow seismic velocities in the
mantle (e.g., Forte et al., 2010; French and Romanowicz, 2015), suggesting that
sublithospheric convection plays a significant role in sustaining the high topography of the
swells and triggering intraplate volcanism (Fishwick and Bastow, 2011). With evidence for
tectonic, lithospheric and convective controls, the ultimate trigger(s) of mantle-derived
Cenozoic intraplate volcanism in Africa remains unresolved.
To contribute to this discussion, we report the first geochronological and radiogenic
isotope results for aillikite (i.e., carbonate-rich ultramafic lamprophyre) lavas from the
Catanda volcanic complex in central Angola (Fig. 1 and Fig. DR1 inSupplementary
Material). This complex also includes the only extrusive carbonatites documented in
Angola to date (Campeny et al., 2014, 2015). The well-preserved volcanic morphology and
ongoing hydrothermal activity in the Catanda area (i.e., small mud volcanoes and abundant
travertine occurrences; Fig. DR2) suggest that volcanism is considerably younger than a
nearby nephelinitic dyke which was dated at 92 ± 7 Ma by whole rock K-Ar dating
(Torquato and Amaral, 1973).
Catanda is located on the Bié Dome, along the NW margin of the Lucapa corridor(Fig. 1), a
>1000 km-long NE-SW oriented graben structure. The corridor records magmatic
(including kimberlites and intrusive carbonatites) and tectonic activity in the
Neoproterozoic, Permo-Triassic and in the Cretaceous between 145 and 100 Ma (e.g.,
Allsopp and Hargraves, 1985; Jelsma et al., 2009, 2013).
Earthquakes of magnitude up to 5.1 M (including three in the Catanda area between 1989
and 2014; https://earthquake.usgs.gov/) suggest ongoing tectonism in the corridor (Fig.
DR3). The Bié Dome is a large plateau (~1000 km in diameter; ~2500 m of elevation)
formed during major phases of uplift in the Oligocene and Pleistocene (including unusually
fast (~2 mm/yr) uplift in the last 1 Myr), probably triggered and supported by mantle
upwelling (Al-Hajri et al., 2009; Walker et al., 2016). Mantle convection models indeed
show active upwelling beneath an area straddling Angola and northern Namibia (Forte et
al., 2010). Several lines of geophysical evidence indicate that the Catanda area is underlain
by thinner lithosphere (140 km) than inland Angola (>160 km; Fig. 1) (Globig et al., 2016).
The Catanda complex therefore provides an excellent opportunity to test the possible

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interplay between mantle convection and pre-existing lithospheric structures in the
formation of intraplate magmas.

GEOLOGICAL SETTING
In the Lucapa corridor carbonatitic volcanism is restricted near the village of Catanda (Fig.
1). The Catanda complex consists of a cluster of small volcanic edifices with maar and tuff
ring morphologies covering an area of 50 km2. The complex comprises pyroclastic rocks
with subordinate calciocarbonatite, altered natrocarbonatite and aillikite lavas (Fig. DR1),
which together form volcanic successions up to 100 m thick (Campeny et al., 2014, 2015).
Fresh surface samples from three lava flows (AC-21, -24 and -25) in the Ungongué volcanic
succession (Fig. DR1), whose petrography, mineral and bulk-rock compositions were
reported by Campeny et al. (2015), were selected for this study. The samples exhibit a
porphyritic texture and contain variable proportions of 0.5–2.0 mm-sized phenocrysts of
fluorapatite, titanomagnetite, Ti-rich clinopyroxene, Ti-rich phlogopite partially altered to
vermiculite, and serpentinised olivine in a calcite-rich groundmass (Fig. DR4).
Based on their compositional and mineralogical features, these lavas are classified as
aillikites. The lavas contain occasional glimmerite (i.e., phlogopite-dominated) xenoliths
probably sourced from the lithospheric mantle.

RESULTS
Age Constraints
Initial attempts to constrain the age of Catanda lavas by apatite U-Th-Pb and phlogopite RbSr dating did not provide conclusive results because of limited radiogenic ingrowth (see
Supplementary Material). Robust age constraints are provided by (U-Th-Sm)/He (AHe)
dating of nine apatite aliquots from lavas AC-21 and 25. Given the expected young age (and
low 4He content) of the apatite grains, most of the AHe analyses were carried out on multigrain samples (see Methods in Supplementary Material), which yield weighted (U-ThSm)/He ages of 640 ± 130 ka and 650 ± 90 ka (uncertainties at 95% confidence level) for
samples AC-21 and -25, respectively (Table 1).
40Ar/39Ar dating of phlogopite grains (see Tables DR4-DR6 and Methods) from samples
AC-24 and -25 yield similarly young results, which were pooled to produce an inverse
isochron age of 558 ± 22 ka (2σ; n = 6; Fig. 2). By contrast, phlogopite grains from sample
AC-21 yield generally older apparent ages across all heating steps, and produce a weighted

4

mean age of 741 ± 44 ka calculated by pooling concordant 40Ar*/39Ar results from the five
analyzed grains (Fig. DR5). The corresponding inverse isochron for these data (Fig. 2)
yields an age of 776 ± 81 ka. Although the age difference between samples AC-21 and 24/25 is statistically significant, partial alteration of phlogopite to vermiculite, coupled
with the possible occurrence of mica xenocrysts (as suggested by glimmerite xenoliths) in
Catanda lavas warrant cautious treatment of these results.
Nevertheless, the phlogopite 40Ar/39Ar ages overlap with the apatite AHe ages, and
support a Middle Pleistocene age for the volcanism.
Radiogenic Isotopes
Clinopyroxene, apatite and groundmass fractions of the Catanda lavas have high
206Pb/204Pb (>20), low 87Sr/86Sr (0.7035), and moderately high 143Nd/144Nd (0.51278–
0.51286), reminiscent of HIMU-type ocean island basalts (OIB) such as St. Helena (Fig. 3,
Figure DR6 and Table DR1). Isotopic compositions transitional between the HIMU and
‘enriched’ mantle (EM) components, are also found in other Cenozoic mantle-derived
volcanic suites across the African continent (e.g., EARS carbonatites from the Northern
Kenya Rift, Cameroon Line volcanics, Western Cape olivine melilitites; Fig. 3). We therefore
suggest that aillikite (and carbonatite) magmas at Catanda represent a further example of
mantle-derived, HIMU-like magmatism in Africa. A mantle origin is also supported by C
isotope data for Catanda carbonates (Campeny et al., 2015).

DISCUSSION AND CONCLUSIONS
The young (~500–800 ka) radiometric age of the Catanda lavas is consistent with other
evidence for recent volcanic and geothermal activity in the Catanda area, including wellpreserved volcanic landforms and ongoing hydrothermal activity (Fig. DR2). These
observations imply a recent resumption of volcanic activity in western Angola, some 90 Ma
after emplacement of mafic alkaline mafic dykes in the area (Torquato and Amaral, 1973;
Marzoli et al., 1999). The position of the Catanda complex in the Lucapa corridor suggests
that the local lithospheric architecture facilitated melt ascent to the surface.
Recent seismic activity (Fig. DR3) might support a tectonic trigger (i.e., far-field stress) for
volcanism. However, the rapid Pleistocene uplift (Walker et al., 2016), the occurrence of
positive free-air gravity anomalies (Fig. 1), seismic tomography and mantle convection
models (Forte et al., 2010; French and Romanowicz, 2015), all support active mantle
upwelling beneath the area and make unlikely a purely tectonic trigger for mantle-derived

5

magmatism. Seismic tomography models (e.g., Ritsema et al., 1999; Forte et al., 2010;
French and Romanowicz, 2015) suggest that a large dome of mantle material with slow
seismic velocities, sourced from the core-mantle boundary beneath central-southern Africa
(termed the “South African Superplume”) ascends beneath the Kalahari and Congo cratons
and splits into multiple branches once it reaches the upper mantle. The larger eastern
branches of this deep structure feed the EARS, whereas the smaller western branches
upwell beneath the Cameroon Line and a large area beneath northern Namibia and centralsouthern Angola, where Catanda is located. We speculate that rapid uplift of the Bié Dome
and attendant Catanda carbonatitic-aillikitic volcanism in the Pleistocene might be related
to a finger-like structure stemming from a branch of the South African Superplume that is
upwelling beneath southwestern Africa. The recent global seismic tomography model of
French and Romanowicz (2015, their Extended Fig. 3a) indeed shows a narrow structure
with low surface-wave velocity beneath central Angola, which is connected to the top of a
larger upwelling. A similar model was proposed to explain volcanism in the northern part
of the EARS (Civiero et al., 2015) and Cenozoic intraplate magmatism in 500 km-large
zones of localized uplift across Europe (e.g., Wilson and Patterson, 2001). Thinned
lithosphere beneath the Lucapa corridor and especially its western sector (Fig. 1) would
provide a preferential pathway for focusing mantle upwelling as previously proposed by
Fishwick and Bastow (2011) for other intraplate volcanic regions in northern Africa (e.g.,
Hoggar, Tibesti; Fig. 1).
Late Cretaceous mantle upwelling activity is suggested by mafic alkaline rocks with trace
element and Sr isotope affinity to the St Helena OIBs (and similar 87Sr/86Sr values to the
Catanda lavas), which emplaced at 90–95 Ma along the central Angolan margin (Fig. 1)
(Marzoli et al., 1999). Similar Sr-Nd isotope compositions for Catanda lavas, the 116–133
Ma Lunda Norte kimberlites (Castillo-Oliver et al., 2016) and Cretaceous carbonatites
(Alberti et al., 1999) in the Lucapa corridor (Fig. 1) may indicate that Cretaceous
kimberlites and carbonatites, and Pleistocene Catanda carbonatite-aillikite magmas have
tapped similar (convective) mantle sources.
While the Catanda complex and other expressions of late Cenozoic intraplate volcanism in
Africa (e.g., EARS, Cameroon Line, Darfur, Hoggar, Tibesti, Moroccan Atlas) correlate with
pre-existing lithospheric discontinuities, they also show evidence of being affected by
active mantle upwelling. We therefore argue that intraplate carbonatitic and alkaline
magmatism result from the complex interplay between multi-scale mantle convection and
pre-existing lithospheric structures, as previously proposed for Cenozoic volcanism in the
North China Craton (Guo et al., 2016), rather than being controlled by tectonic processes
alone (e.g., continental break-up, changes in direction and speed of plate motion). We
recognize that current models of mantle convection do not readily explain the recurrence

6

of volcanism in specific areas of Africa over >100 Myr, for example in the Rungwe zone of
the EARS (Bailey and Woolley, 2005), in the continental sector of the Cameroon Line
(Njome and de Wit, 2014) and in the Lucapa corridor (this study). Future models of mantle
dynamics will need to incorporate these observations to address the question of whether
upwelling of asthenospheric material can be the main driver of intraplate carbonatitic and
alkaline magmatism in Africa and elsewhere.
ACKNOWLEDGMENTS
This work was supported by the consolidated research group SGR-444 and -1661 of the
Catalonia Government, the Spanish Government (research project n. CGL2009–13758), the
Society of Economic Geologists (Hugh E. McKinstry fund), and the Australian Research
Council (DE150100009 to AG,DP130100257 to VSK and DP160103502 to JCA). We thank
José Fortuna and Felipe Correia from Catanda village for their assistance during field-work;
Mark Kendrick, David Phillips and Gareth Davies for informal reviews of an earlier version
of this manuscript; and Louis Moresi, Ting Yang and Mike Sandiford for insightful
discussions. This manuscript was significantly improved by constructive reviews from
Cynthia Ebinger, Phil Janney and an anonymous reviewer, and the encouraging and helpful
editorial handling of Dennis Brown.

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FIGURE CAPTIONS
Figure 1. Elevation map of the African plate from the ETOPO1 global model (Amante and
Eakins, 2009) showing major tectonic and volcanic features. Major structural features of
the East Africa Rift System (EARS) are from Chorowicz (2005). Inset shows location of the
Catanda Complex, Lucapa Corridor, Kwanza seamounts, and major Triassic and Cretaceous
kimberlite, carbonatite and alkali basalt occurrences (Marzoli et al., 1999; Jelsma et al.,
2013; Castillo-Oliver et al., 2016). Grey shaded area indicates free-air, crust-corrected
positive gravity anomaly above 15 mGal (after Forte et al., 2010). Lithospheric thickness
estimates (dashed red lines) are from Globig et al. (2016).
Figure 2. 40Ar/39Ar phlogopite inverse isochrons for Catanda lavas. The chart includes the
inverse isochrons for data combined from samples AC-24 and AC-25 (blue
symbols),assuming a J-value of 0.00104689 ± 0.037% (AC-25-PHL value); and for AC-21
results with concordant 40Ar*/39Ar ratios (red symbols; J-value of 0.000105030 ±
0.036%). Rejected steps (i.e., low-T heating steps from grain AC-21PHL-1) are shown in
gray in the inset.
Figure 3. (a) 87Sr/86Sr vs 143Nd/144Nd, and (b) 206Pb/204Pb vs 87Sr/86Sr
compositions of Catanda lavas compared to East Africa Rift System (EARS) carbonatites
from the Northern and Southern Kenya Rifts (NKR, SKR) (Bell and Tilton, 2001), lavas from
the Cameroon Line (Lee et al., 1994), Western Cape melilitites (Janney et al., 2002), HIMU
OIB lavas from St Helena and enriched-mantle (EM) OIB end-members (Stracke, 2012). The
following solution-mode multi-collector ICP-MS analyses are reported in each panel: (a)
clinopyroxene (3 analyses), apatite (3); and (b) apatite (1), combined groundmass (Pb
isotopes) and clinopyroxene (Sr isotopes) from the same sample (3).

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1GSA Data Repository item 2017xxx, xxxxxxxx, is available online at
http://www.geosociety.org/datarepository/2017/ or on request from
editing@geosociety.org

TABLE 1. (U-TH-SM)/HE AGES OF APATITE
GRAINS IN SAMPLES AC-21 AND -25 FROM THE
CATANDA COMPLEX
Sample 4He (ncc) Th/U
AC-21_1
0.047 11.86 690 ± 40
AC-21_2
0.078 12.88 890 ± 50
AC-21_3
0.166 12.26 580 ± 30
AC-21_4
0.208 13.29 570 ± 30
AC-21_5
0.035 15.75 650 ± 40
*Mean 64 ± 130
AC-25_1
0.017
AC-25_2
0.044
AC-25_3
0.075
AC-25_4
0.029
AC-25_5
0.068
*Mean 650 ± 90

13.34
8.82
11.60
11.39
11.35

640 ± 40
†1330 ± 80
630 ± 40
760 ± 50
620 ± 40

Notes: *Weighted mean age calculated at 95% c.i.
†Analysis excluded from calculation of weighted mean age. Between 1 and 7 grains were
analyzed for each sample. See Supplementary Material for further information

12

Figure 1

13

Figure 2

14

Figure 3

15

Supplementary Material for GSA Data Repository

SUPPLEMENTARY FIGURES
Figure DR1

Supplementary figure DR1. Schematised geological map of Angola and the Catanda volcanic
area and detailed volcano-stratigraphic succession of the Ungongué succession (modified
from Campeny et al., 2014 Lithos). The location of the volcanic flows examined in this study is also
included.

16

Figure DR2

Supplementary figure DR2. Surface features of the Catanda volcanic complex. (a), (b) Wellpreserved morphology of the eruptive centres (volcanic cones, maars – see dotted lines). (c)
Travertine deposits up to 30 m thick, associated with present-day hydrothermal systems in the
Catanda area. (d) Products of recent, small-scale carbonate mud (mini-) eruption.

17

Figure DR3

Supplementary figure DR3. Elevation map of Angola from the ETOPO1 global model (Amante and
Eakins, 2009) showing the location of historical eartquakes (i.e after 1900) of magnitude above
2.5M (source https://earthquake.usgs.gov/). The approximate boundaries of the Lucapa corridor are
also shown. Grey shaded area indicates free-air, crust-corrected positive gravity anomaly above 15
mGal (after Forte et al., 2010). Lithospheric thickness estimates (dashed red lines) are from Globig et
al. (2016).

18

Figure DR4

Supplementary figure DR4. Outcrop, hand specimen and petrographic images of Catanda
carbonatitic-aillikitic lavas. (a) Photograph of lava flow in the upper part of the Ungongué
succession; (b) porphyritic sample of lava from the Ungongué volcanic sequence; (c), (d) transmittedlight photomicrographs of porphyritic lavas with apatite (ap) and spinel (sp) micro-phenocrysts
hosted in a groundmass dominated by calcite (cal); (e), (f) scanning electron microscope (SEM), backscattered electron (BSE) images of lava groundmass, which contains apatite (ap), perovskite (prv),
calcite (cal), brucite (bru) and accessory euhedral crystals of pyrochlore (pcl).
19

Figure DR5

Supplementary figure DR5. 40Ar/39Ar phlogopite age spectrum for grain 21PHL-1.

20

Figure DR6
0.5131
0.5130

0.2831

a

143Nd/144Nd

St Helena

0.5129

0.2830

Cameroon Line

0.5128

b

176Hf/177Hf

EARS carb - SKR

0.2829

EARS carb - NKR

0.5127

Western Cape

0.5126

0.2828

Catanda
EM1

0.5125

EM2

0.2827

EM2

EM1

0.5124
206Pb/204Pb

0.5123
17.0

18.0

19.0

20.0

0.2826
0.5123

21.0

0.2831

0.5127

0.2831

c

176Hf/177Hf

0.5125

143Nd/144Nd

0.5131

d

St Helena

176Hf/177Hf

0.2830

0.5129

Cameroon Line

0.2830

Western Cape
0.2829

Catanda

0.2829

EM2

0.2828

EM2

0.2828

0.2827

0.2827

EM1

EM1

206Pb/204Pb

87Sr/86Sr

0.2826
17.0

18.0

19.0

20.0

0.2826
0.7025 0.7030 0.7035 0.7040 0.7045 0.7050 0.7055 0.7060

21.0

15.9

41.0

e

207Pb/204Pb

15.8

15.7

St Helena
Cameroon Line
EARS carb - SKR
EARS carb - NKR
Western Cape
Catanda

40.5

40.0

EM2

15.6

15.5

f

208Pb/204Pb

39.5

EM2

39.0

EM1

EM1

206Pb/204Pb

15.4

206Pb/204Pb

38.5
17.0

18.0

19.0

20.0

21.0

17.0

18.0

19.0

20.0

21.0

Supplementary figure DR6. Radiogenic isotope compositions of Catanda lavas (additional charts).
The Catanda values are compared to the isotopic compositions of Eastern Africa Rift System (EARS)
carbonatites from the Northern and Southern Kenya Rifts (NKR, SKR) (Bell and Tilton, 2001 J Petrol),
lavas from the Cameroon Line (Lee et al., 1994 EPSL; Ballentine et al., 1997 Chem Geol), Western
Cape melilitites (Janney et al., 2002 J Petrol), HIMU OIB lavas from St Helena (Stracke, 2012 Chem
Geol), and enriched-mantle (EM) OIB end-members represented by the most extreme compositions
of lavas from Pitcairn (EM-1) and Samoa (EM-2) (Stracke, 2012). The following solution-mode multicollector ICP-MS analyses are reported in each panel: (a, e, f) bulk groundmass (3 analyses), apatite
(1); (b) groundmass (3) and clinopyroxene (1); (c) groundmass (3); (d) clinopyroxene (2). In (c), the
terrestrial array is from Vervoort et al. (2011 GCA).
21

SUPPLEMENTARY MATERIAL

Methods
Apatite AHe dating
(U-Th-Sm)/He (AHe) dating was carried out at the University of Melbourne. Apatite was
handpicked under a binocular microscope, immersed in ethanol and examined under
polarised light to select the most suitable grains (with the least number of mineral impurities).
AHe dating is usually performed on clear, euhedral, unfractured grains, but the Catanda
apatites are markedly anhedral and fractured. This precludes accurate estimates of grain
geometry required for α-ejection corrections (Farley et al., 1996). The selected grains were
thus mechanically abraded using silicon carbide grit (300-425 µm) in a Krogh (1982)-style
abrasion cell to remove the outer ~25-30 µm (i.e. greater than the typical α-ejection distance)
of the grains (e.g., Spiegel et al., 2009). The process was halted periodically to monitor
abrasion progress. Abrasion of the grain periphery eliminates consideration of issues related
to change of radiogenic 4He concentrations by α-ejection or α-implantation. Therefore,
following removal of at least ~20 m there is no the need to apply ejection corrections, which
can be considered close to unity in rapidly cooled samples (Min et al., 2006).
Abraded apatite grains were transferred to small Pt-capsules and outgassed under
vacuum at ~900˚C for 5 minutes, using a semiconductor diode Coherent Quattro FAP laser
(820 nm) with fibre-optic coupling to the sample chamber to provide optimal coupling with
samples and heating without melting, ablation or fusion. 4He contents were determined by
isotope dilution with a pure 3He spike, using a Balzers QMS 200–Prisma quadrupole mass
spectrometer. A hot blank was run after each gas extraction to verify complete outgassing of
the apatite grains. Most samples yielded negligible amounts of gas after the first reextraction, and for all samples a second re-extraction invariably contributed <0.5% of the
total measured 4He.
Following the He outgassing step, the apatite grains were dissolved (within their Pt
capsules) in HNO3 for determination of U-Th-Sm concentrations using an Agilent 7700x
ICPMS. USGS basalt BCR-2 was used as primary standard. Total uncertainty in measured
4He/(U+Th+Sm)

for abraded apatite is 3% (±1). USGS basalt BHVO-2 and Durango apatite

were analysed to monitor data quality for U-Th-Sm concentrations and AHe ages,
respectively. The results obtained for BHVO-2 are consistent with the 5-year averages (Sm
6075 ppb [± 0.5%; ± 1sd], U 424 ppb [± 0.36%], Th 1209 ppb [± 0.35%]; n=124), while four

22

AHe ages of Durango apatite acquired with the Catanda samples average 31.2 ± 1.0 Ma (±
2Table A2), consistent with an AHe age of 31.02 ± 1.01 Ma for a set of 24 Durango
apatite determined at the Caltech He laboratory (McDowell et al., 2005).
Phlogopite 40Ar/39Ar dating
The Catanda carbonatites are not ideal candidates for 40Ar/39Ar geochronology given the
common alteration of phlogopite to vermiculite (Tauler et al., 2014) and the presence of
glimmerite xenoliths and xenocrystic micas in the studied samples. As an exploratory
exercise to determine whether any meaningful 40Ar/39Ar age information could be extracted
from the samples, a small number of apparently microphenocrystic phlogopite grains from
lava samples AC-21, -24 and -25 were selected under binocular microscope for 40Ar/39Ar
geochronology. The AC-21 grains appeared to be pervasively altered to vermiculite, but the
least altered grains were selected for this study. Following soaking in 5% HNO3, rinsing in
water and then acetone, the samples were wrapped in aluminium foil packets, stacked in a
quartz-glass vial along with fluence monitor Alder Creek Rhyolite sanidine (1.18144 ±
0.00068 Ma, Phillips et al., 2017) and irradiated in the CLICIT facility at the Oregon State
University TRIGA reactor for 0.4 MWhr (UM#61).
The 40Ar/39Ar analyses were undertaken in the Noble Gas laboratory at the University
of Melbourne, using a multi-collector Thermo Fisher Scientific ARGUSVI mass
spectrometer linked to a stainless steel gas extraction/purification line and a Photon Machines
Fusions 10.6 CO2 laser system (e.g., Phillips and Matchan, 2013). Single-grain step heating
analyses were performed on the AC-21 phlogopite grains (n = 5). In the case of samples AC24 and -25, grains were combined for analysis due to small sample size (two aliquots for AC24, one for AC-25). Grains were heated using a 6 mm homogenized laser beam over a range
of 3–30% laser power, following initial outgassing at 0.5% laser power to remove adsorbed
gas.
Due to the short irradiation time, it was not feasible to include Ca/K/Cl salts/glasses in
the same package as the samples. Therefore, correction factors determined for K-glass and
Ca-salts contained in package irradiated close in time in the CLICIT facility were used. These
are the employed interference correction values: (36Ar/37Ar)Ca

= 2.570 (± 0.002) x 10-4;

(39Ar/37Ar)Ca = 6.62 (± 0.08) x 10-4; (40Ar/39Ar)K = 0.001210 (± 0.000016). Age spectra and
inverse isochron diagrams were generated using ISOPLOT/Ex.3.75 (Ludwig, 2012). Plateau
ages are defined as including at least 60% of the 39Ar, distributed over a minimum of three

23

contiguous steps and with

40Ar/39Ar

ratios within agreement of the mean at the 95%

confidence level (e.g., Lanphere and Dalrymple, 1978). An atmospheric (40Ar/36Ar)i value of
298.56 ± 0.62 (Lee et al., 2006) is assumed.

Phlogopite Rb-Sr dating and Sr-Nd-Hf-Pb isotope analyses
Rb-Sr isotope dilution data for phlogopite, as well as Sr-Nd-Hf-Pb isotope data for apatite,
clinopyroxene and groundmass samples were acquired using multi-collector ICP-MS at the
University of Melbourne (e.g. Maas et al., 2015; Giuliani et al., 2015). Phlogopite grains
were leached with 0.5M HNO3 (40oC, 2 min) or 2M HNO3 (40oC, 2 mins) to gently remove
easily soluble Sr-rich impurities and generate higher Rb/Sr ratios in the residual phlogopite
(Brown et al., 1989; Maas, 2003). The residues were dissolved on a hot plate (3:1 HF-HNO3,
6M HCl). The resulting solutions, and the 2M HNO3 leachates for each of the three
phlogopite samples, were equilibrated with a 85Rb-84Sr tracer, followed by extraction of Rb
and Sr using cation exchange and Eichrom Sr resin.
Apatite was cleaned with hot water and dissolved with 5M HNO3, while
clinopyroxene (cleaned with hot 2M HNO3) and chips of groundmass material were
dissolved with 3:1 HF-HNO3 and 6M HCl. Small splits of each solution were reserved for
trace element analyses on an Agilent 7700x quadrupole ICPMS; the remaining solution was
used to extract Pb, Sr, Nd and Hf as required.
Isotopic analyses were carried out on a Nu Plasma multi-collector ICP-MS. Mass bias
in unspiked Sr, Nd and Hf isotope analyses was corrected by internal normalization to
86Sr/88Sr

= 0.1194, 146Nd/144Nd = 0.7219 and 179Hf/177Hf = 0.7325 using the exponential law;

final data are reported relative to SRM987 = 0.71023, La Jolla Nd = 0.511860 and JMC475 =
0.282160 and have internal precisions (2se) of ±0.000020, ±0.000010 and ±0.000008,
respectively. External precision (2sd) is ±0.00004 (Sr), ±0.000020 (Nd), ±0.000015 (Hf).
Mass bias in Pb isotope analyses was corrected by thallium-doping (Woodhead, 2002;
SRM981 is assumed to have 206Pb/204Pb = 16.935, 207Pb/204Pb = 15.489 and 208Pb/204Pb =
36.701) and corrected ratios have external precisions of ±0.04-0.09% (2sd). Results for
USGS basalt BCR-2 acquired at the time the Catanda samples were analysed average
87Sr/88Sr

= 0.705004 ± 39 (n=10),

143Nd/144Nd

= 0.512644 ± 24 (n=11),

176Hf/177Hf

=

0.282878 ± 13 (n=9), 206Pb/204Pb = 18.759 ± 0.039%, 207Pb/204Pb = 15.621 ± 0.064% and
208Pb/204Pb

= 38.730 ± 0.087% (2sd, n = 22; BCR-2 not acid-leached); the JNd-1 Nd standard

averaged 0.512115 ± 11 (n = 12). These results are consistent with TIMS and MC-ICPMS
reference values.

24

87Rb/86Sr

obtained by isotope dilution (for phlogopite residues and leachates) have an

external precision of ±0.5% (2sd). Rb-Sr isotope data for several standards (e.g., SRM607
feldspar, GLO-1 glauconite) are consistent with reference values. Likewise, biotite and
hornblende from Mt Dromedary monzonite (the source of the GA-1550 and MD-2 reference
biotites) produce a model isochron age of 98.63 ± 0.58 Ma (using the 87Rb decay constant of
Villa et al., 2015), within error of biotite 40Ar-39Ar age constraints (Renne et al., 1998; Spell
and McDougall, 2003; Phillips et al., 2017).
Parent-daughter ratios for apatite, clinopyroxene and groundmass were calculated
from trace element concentrations measured for splits of the sample solutions (see above) and
are assigned uncertainties of ±2% (2sd), based on the long-term reproducibility of standard
rocks analysed on the same instrument. Modern CHUR is assumed to have 147Sm/144Nd =
0.196, 143Nd/144Nd = 0.512632 176Lu/177Hf = 0.0336 and 176Hf/177Hf = 0.282785 (Bouvier et
al., 2008).

Additional results
Rb/Sr dating of Catanda phlogopite
Preliminary trace element data for phlogopite phenocrysts in samples AC-21, -24 and -25
indicate high Sr contents (2000-4000 ppm) and Rb/Sr<1, presumably related to Sr-rich
impurities (apatite, carbonate). Mild acid leaching, employed to remove easily soluble
impurities, reduced Sr contents to 115-227 ppm and raised Rb/Sr to 1.8-4.5 (87Rb/86Sr =
5.16– 12.95; Table A2). The analysed leachates (2M HNO3) have lower Rb/Sr ratios (0.050.28, 87Rb/86Sr = 0.13–0.81). Despite the dispersion in Rb/Sr, measured 87Sr/86Sr in the
phlogopite residues is low and relatively uniform (0.70331-0.70397; Table A2). The
leachates (0.70346-0.70348; Table A2) and apatite and clinopyroxene phenocrysts from the
same samples (0.70309-0.70354; Table A1) have similar present-day 87Sr/86Sr. In the Rb-Sr
isochron diagram (not shown), the data do not define linear arrays. This suggests that the RbSr systems are very young.
40Ar/39Ar

dating of Catanda phlogopite

Results are summarised below with the full dataset located in Tables A4-A6 A summary of
age results is provided in Table A5. Uncertainties are stated at the 2σ level unless otherwise
specified. Although samples were too small to measure 37Ar precisely, this has a negligible
impact on the results given the large error on apparent age calculations resulting from
extremely low

40Ar*

yields. For this reason, we prefer not to correct for Ca-induced

25

interferences (see Table A4 for both versions of the dataset). The large uncertainties in
40Ar*/39Ar

ratios (Table A4) are due to extremely small 39Ar contents (small sample size),

and high levels of atmospheric argon in the samples.
Phlogopite grains from samples AC-24 and AC-25 yield similar argon isotopic data,
with atmospheric argon dominating the 40Ar signal (40Ar* ≤ 17% of total 40Ar; Table A4).
Mid- to high-temperature heating steps with meaningful 39Ar measurements (>1 fA 39Ar; n =
6) yield apparent ages ranging from 582 ± 26 ka (24PHL-1, step 2) to 1.35 ± 0.23 Ma
(24PHL-2, step 2). These data (n = 6) are combined for inverse isochron analysis (J-values
overlap at the 1σ-level), revealing the presence of a small amount of excess argon
((40Ar/36Ar)i = 302.01±0.88; MSWD = 0.5, p = 0.73) and an inverse isochron age of 558 ± 22
ka (Fig. A4a). In comparison, the AC-21 grains yield older apparent ages across all heating
steps and exhibit higher levels of atmospheric argon contamination (40Ar* ≤ 3% of total
40Ar).

A weighted mean age of 741 ± 44 ka (MSWD = 1.4, p = 0.22) is calculated by pooling

the concordant 40Ar*/39Ar values (n = 7) from all five AC-21 grains. The corresponding
inverse isochron (Fig. A4b) yields an age of 776 ± 81 ka, with an atmospheric (40Ar/36Ar)i
value of 298.18 ± 0.75. The age spectrum for 21PHL-1 subtly increases with increasing
temperature, ranging from 520 ± 210 to 780 ± 160 ka, suggesting that this sample may have
experienced 40Ar* loss but the large analytical uncertainties (13–40%; 2σ) preclude any
further comment. A plateau age of 690 ± 63 ka (MSWD=1.5 p=0.19, 84% of 39Ar; Fig. A4c).

References

Bouvier, A., Vervoort, J., Patchett, J.P., 2008. The Lu-Hf and Sm-Nd isotopic composition of
CHUR: Constraints from unequilibrated chondrites and implications for the bulk
composition of terrestrial planets. Earth and Planetary Science Letters 272, 48-57.
Brown, R.W., Allsopp, H.I., Bristow, J.W., Smith, C.B., 1989. Improved precision of Rb-Sr
dating of kimberlitic micas: An assessment of a leaching technique. Chemical Geology 79,
125-136.
Farley, K.A., Wolf, R.A., Silver, L.T., 1996. The effects of long alpha-stopping distances on
(U-Th)/He ages. Geochimica et Cosmochimica Acta 60, 4223-4229.
Giuliani, A., Phillips, D., Woodhead, J.D., Kamenetsky, V.S., Fiorentini, M.L., Maas, R.,
Soltys, A., Armstrong, R.S., 2015. Did diamond-bearing orangeites originate from
MARID-veined peridotites in the lithospheric mantle? Nature Communications 6:6837
doi: 10.1038/ncomms7837.

26

Krogh, T.E., 1982. Improved accuracy of U-Pb zircon ages by the creation of more
concordant systems using an air abrasion technique. Geochimica et Cosmochimica Acta
46, 637-649.
Lanphere, M.A., Dalrymple, G.B., 1978. The use of 40Ar/39Ar data in evaluation of disturbed
K-Ar systems, Open-File Report VL. U.S. Geological Survey Open File Report.
Lee, J.-Y., Marti, K., Severinghaus, J., Kawamura, K., Yoo, S.-S., Lee, J., Kim, J., 2006. A
redetermination of the isotopic abundances of atmospheric Ar. Geochimica et
Cosmochimica Acta 70, 4507–4512.
Ludwig, K.R., 2012. User’s Manual for Isoplot 3.75: A Geochronological Toolkit for
Microsoft Excel. Berkeley Geochronology Center Special Publication No. 6, 75 p.
Maas, R., 2003. Acid leaching of micas: improved Rb-Sr geochronology of disequilibrated
rocks from zones of alteration and deformation. Journal of the Virtual Explorer 13,
doi: 10.3809/jvirtex.2003.00085.
Maas, R., Grew, E.S., Carson, C.J., 2015. Isotopic constraints (Pb, Rb-Sr, Sm-Nd) on the
sources of Early Cambrian pegmatites with boron and beryllium minerals in the Larseman
Hills, Prydz Bay, Antarctica. The Canadian Mineralogist 53, 249-272.
McDowell, F.W., McIntosh, W.C., Farley, K.A., 2005. A precise 40Ar-39Ar reference age for
the Durango apatite (U-Th)/He and fission-track dating standard. Chemical Geology 214,
249-263.
Min, K., Reiners, P. W., Wolff, J. A., Mundil, R., Winters, R. L., 2006. (U-Th)/He dating of
volcanic phenocrysts with high-U-Th inclusions, Jemez Volcanic Field, New Mexico.
Chemical Geology 227, 223–235.
Phillips, D., Matchan, E.L., 2013. Ultra-high precision 40Ar/39Ar ages for Fish Canyon Tuff
and Alder Creek Rhyolite sanidine: New dating standards required? Geochimica et
Cosmochimica Acta 121, 229–239.
Phillips, D., Matchan, E.L., Honda, M., Kuiper, K.F., 2017. Astronomical calibration of
40Ar/39Ar

reference minerals using high-precision, multi-collector (ARGUSVI) mass

spectrometry. Geochimica et Cosmochimica Acta 196 IS -, 351–369.
Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., DePaolo, D.J., 1998.
Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chemical
Geology 145, 117-152.
Spell, T.L. and McDougall, I., 2003. Characterization and calibration of 40Ar/39Ar dating
standards. Chemical Geology 198, 189-211.

27

Spiegel, C., Kohn, B., Belton, D., Berner, Z., Gleadow, A., 2009. Apatite (U-Th-Sm)/He
thermochronology of rapidly cooled samples: The effect of He implantation. Earth and
Planetary Science Letters 285, 105-114.
Tauler. E., Campeny, M., Melgarejo, J.C., Bambi., A., Mangas, J., Manuel., J., 2014.
Patrones de formación y de alteración de la vermiculita en las carbonatitas volcánicas de
Catanda (Angola). Macla 19, Proceedings of the XXXIV Conference of the Spanish
Society of Mineralogy.
Villa, I.M., De Bièvre, P., Holden, N.E., Renne, P.R., 2015. IUPAC-IUGS recommendation
on the halflife of 87Rb. Geochimica et Cosmochimica Acta 164, 382-385.
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MC-ICP-MS. Journal of Analytical Atomic Spectrometry 17, 1-6.

28

Supplementary Tables
Table A1. Sr-Nd-Hf-Pb isotope compositions in minerals and groundmass of Catanda lavas
Sample
Fraction

AC-21
apatite

AC-21
cpx

Rb ppm
Sr ppm
87Rb/86Sr

0.04
3539
0.00003

6.20
601
0.0299

87Sr/86Sr

AC-21
groundmass

AC-24
apatite

AC-24
cpx

0.18
4274
0.00012

AC-24
groundmass

0.30
147
0.0059

AC-25
apatite

AC-25
cpx

0.62
3996
0.00045

AC-25
groundmass

95.20
2861
0.0962

0.703363

0.703525

0.703460

0.703101

0.703537

0.703086

Sm ppm
Nd ppm
147Sm/144Nd

79.5
536
0.0897

6.80
40.60
0.101

28.01
220
0.0771

169
1155
0.0885

3.86
21.40
0.109

27.08
211
0.0775

123
839
0.0887

3.30
16.27
0.1230

22.35
183
0.0739

143Nd/144Nd

0.512782

0.512803

0.512790

0.512784

0.512855

0.51

0.512783

0.512810

0.512796

eNd
Lu ppm
Hf ppm
176Lu/177Hf

2.9

3.3
0.08
12.10
0.00094

3.1
0.32
2.94
0.0155

3.0

4.4

3.6
0.32
3.10
0.0147

2.9

3.5
0.04
6.67
0.00081

3.2
0.24
2.95
0.0115

0.282752

0.282806

0.282807

0.282827

0.282802

1.5

176Hf/177Hf

eHf

0.7

0.8

U ppm

2.07

6.77

6.43

4.97

Th ppm

27.51

18.53

18.43

14.14

0.53
261

2.34
193

1.37
314

2.74
20

Pb ppm
238U/204Pb

-1.2

0.6

232Th/204Pb

3585

547

931

354

206Pb/204Pb

20.576

20.504

20.502

20.093

207Pb/204Pb

15.788

15.808

15.809

15.784

208Pb/204Pb

39.994

40.176

40.167

39.866

206Pb/204Pbi

20.550

20.485

20.470

20.091

207Pb/204Pbi

15.627

15.689

15.615

15.772

208Pb/204Pbi

39.879

40.158

40.137

39.855

trace element abundances and parent/daughter ratios based on trace element analysis of splits taken from the sample solutions
age corrections (for 0.65 Ma) are trivial and not listed, except for Pb isotope results
cpx: clinopyroxene

29

Table A2. Rb-Sr isotope dilution results for Catanda phlogopites (plus apatite and clinopyroxene results)
Rb ppm

Sr ppm

87Rb/86Sr

87Sr/86Sr

AC-21
AC-21
AC-21

phl res 1
phl res 2
phl L2

513
210
431*

115
118
1672*

12.95
5.164
0.746

0.703969
0.703458
0.703471

AC-21
AC-21

apatite
cpx

0.04
6.20

3539
601

0.00003
0.0299

0.703363
0.703525

AC-24
AC-24
AC-24
AC-24
AC-24

phl res 1
phl res 2
phl L2
apatite
cpx

475
378
169*
0.18
0.30

227
174
3748*
4274
147.0

6.047
6.292
0.1301
0.00012
0.0059

0.703380
0.703451
0.703463
0.703460
0.703101

AC-25
AC-25
AC-25
AC-25
AC-25

phl res 1
phl res 2
phl L2
apatite
cpx

409
459
66*
0.62
95.2

196
169
236*
3996
2861

6.026
7.859
0.814
0.00045
0.0962

0.703425
0.703307
0.703475
0.703537
0.703086

Abbreviations: phl=phlogopite, phl res1 and phl res2 refer to sets of phlogopite grains picked from same separate,
phl res 1 was leached with 0.5M, phl res 2 was leached with 2M HNO3; phl L2 refers to 2M HNO3 leachates; cpx is clinopyroxene
all phlogopite residue and leachate analyses by isotope dilution; Rb/Sr for apatite and clinopyroxene from trace element analysis, see Table A1
*denotes nanograms of Rb and Sr in phl L2 leachate fractions: ppm could not be calculated because leachate equivalent masses are unknown

30

Table A3. (U-Th-Sm)/He ages of Catanda apatite grains
Sample no.

Lab. no.

No. of
crystals
analysed

He no.

AC-21
AC-21
AC-21
AC-21
AC-21

8971
9009
9069
9070
11669

3
4
7
6
4

27059
27145
27193
27195
36352

0.047
0.078
0.166
0.208
0.035

11.86
12.88
12.26
13.29
15.75

AC-25
AC-25
AC-25
AC-25
AC-25

8989
9012
11673
11674
9072

1
6
4
6
5

27083
27156
36364
36367
27199

0.017
0.044
0.075
0.029
0.068

13.34
8.82
11.60
11.39
11.35

1
1
1
1

27204
36180
36289
36400

1.308
5.357
3.867
21.672

17.32
15.27
15.18
17.25

4He

(ncc)

Th/U
ratio

aAge

(Ma) Error (± 2)
(Ma)

0.69
0.89
0.58
0.57
0.65
b 0.64

0.04
0.05
0.03
0.03
0.04
0.13

0.64
0.63
0.76
0.62
b 0.65

0.04
0.08
0.04
0.05
0.04
0.09

32.4
31.6
31.7
29.7
b 31.3

1.9
1.9
1.9
1.8
1.0

c1.33

Durango apatite - standard
Durango
Durango
Durango
Durango

aNote

9074
11658
11675
11684

grains were abraded and more than 20 m of the outer surface removed, so no -ejection correction was applied.
mean age (uncertainties at 95% confidence level) calculated using Isoplot v. 4.15 (Ludwig, 2012).

bWeighted
cAnalysis

excluded from calculation of weighted mean age.

31

Table A4. ARGUSVI 40Ar/39Ar Analytical Results for Catanda phlogopite samples 21PHL, 22PHL, 24PHLa,b,c,d
Data are corrected for mass spectrometer backgrounds, discrimination, radioactive decay and interferencese
Sample
ID

Step
No

1
2
3
4
5
6
7

Laser
Power
(%)
J-Value =
(one grain)
3%
5%
7%
9%
12%
16%
30%

Sample 21PHL-2
21PHL-2a
1
21PHL-2b
2

Sample 21
Sample 21PHL-1
21PHL-1a
21PHL-1b
21PHL-1c
21PHL-1d
21PHL-1e
21PHL-1f
21PHL-1g

40Ar

±1σ

(fA)

39Ar

±1σ

(fA)

38Ar

±1σ

(fA)

37Ar

±1σ

(fA)

36Ar

±1σ

39Ar
(x10-14
mol)

Ca/K

±1σ

(fA)

%40Ar*

40Ar*/39Ar

±1σ

Cum.%
39Ar

Age
(Ma)

±1σ

0.000105030 ±0.000000038
796.33
860.80
324.39
116.56
218.19
152.01
12.30

0.28
0.23
0.08
0.05
0.04
0.03
0.03

1.607
1.825
1.621
0.904
2.076
2.100
0.116

0.022
0.017
0.032
0.023
0.023
0.023
0.019

0.5000
0.5400
0.2016
0.0714
0.1324
0.0906
0.0076

0.0005
0.0009
0.0003
0.0003
0.0006
0.0004
0.0001

1.52
0.07
0.07
0.07
1.14
2.47
0.07

1.83
0.15
0.15
0.15
1.61
1.45
0.15

2.6524
2.8649
1.0698
0.3790
0.7023
0.4808
0.0405

0.0029
0.0050
0.0017
0.0014
0.0030
0.0019
0.0007

0.0057
0.0065
0.0058
0.0032
0.0074
0.0075
0.0004

1.66
0.07
0.08
0.14
0.96
2.06
1.11

2.00
0.14
0.16
0.28
1.36
1.21
2.22

0.56
0.63
1.54
2.91
3.91
5.58
1.67

2.76
3.00
3.09
3.75
4.11
4.04
1.77

0.56
0.83
0.32
0.49
0.44
0.27
1.73

15.68
33.48
49.30
58.12
78.38
98.87
100.00

0.52
0.57
0.58
0.71
0.78
0.76
0.34

0.11
0.16
0.06
0.09
0.08
0.05
0.33

(one grain)
3%
30%

601.21
711.12

0.09
0.19

1.182
5.505

0.021
0.030

0.3764
0.4357

0.0004
0.0006

0.21
0.53

1.58
2.16

1.9970
2.3112

0.0021
0.0032

0.0042
0.0195

0.31
0.17

2.34
0.69

0.83
2.97

4.22
3.83

0.55
0.18

16.70
94.49

0.80
0.73

0.10
0.03

Sample 21PHL-3
21PHL-3a
1
21PHL-3b
2

(one grain)
3%
30%

813.86
398.07

0.11
0.11

1.378
2.073

0.015
0.012

0.5100
0.2453

0.0011
0.0004

2.73
0.07

2.00
0.15

2.7055
1.3013

0.0060
0.0021

0.0049
0.0074

3.47
0.06

2.54
0.13

0.75
2.40

4.44
4.61

1.30
0.31

39.93
100.00

0.84
0.87

0.25
0.06

Sample 21PHL-4
21PHL-4a
1
21PHL-4b
2

(one grain)
3%
30%

523.53
1,364.20

0.14
0.31

0.387
6.026

0.009
0.022

0.3285
0.8480

0.0007
0.0013

2.10
1.52

1.52
1.42

1.7428
4.4985

0.0035
0.0068

0.0014
0.0214

9.49
0.44

6.85
0.41

0.61
1.55

8.31
3.51

2.75
0.34

6.04
100.00

1.57
0.66

0.52
0.06

Sample 21PHL-5
21PHL-5a
1
21PHL-5b
2
21PHL-5c
3

(one grain)
3%
10%
30%

256.29
1,843.37
32.23

0.07
0.63
0.02

0.068
2.341
0.102

0.025
0.026
0.028

0.1607
1.1596
0.0197

0.0002
0.0014
0.0001

1.60
3.77
2.34

0.87
1.32
1.55

0.8526
6.1517
0.1046

0.0011
0.0074
0.0007

0.0002
0.0083
0.0004

41.49
2.82
40.25

27.38
0.99
28.89

0.68
0.36
3.06

25.84
2.87
9.69

10.76
0.98
3.34

2.69
95.94
100.00

4.89
0.54
1.83

2.03
0.19
0.63

Sample 24
J-Value = 0.000104859 ±0.000000038
Sample 24PHL-1 (one grain, thin flake)
24PHL-1a
1
3%
26.88
0.02
24PHL-1b
2
30%
83.17
0.03

0.556
4.603

0.026
0.020

0.0158
0.0436

0.0001
0.0002

0.20
0.01

1.26
0.01

0.0837
0.2311

0.0008
0.0010

0.0020
0.0163

0.64
0.00

3.97
0.01

7.05
17.04

3.41
3.08

0.45
0.07

10.78
100.00

0.64
0.58

0.09
0.01

Sample 24PHL-2 (two grains)
24PHL-2a
1
3%
24PHL-2b
2
7%
24PHL-2c
3
30%

0.603
2.635
7.646

0.019
0.012
0.020

0.6503
0.5847
0.1092

0.0009
0.0010
0.0004

0.07
0.97
2.75

0.15
1.97
1.44

3.4501
3.1020
0.5793

0.0050
0.0052
0.0021

0.0021
0.0094
0.0271

0.22
0.64
0.63

0.43
1.31
0.33

1.39
1.99
12.55

24.09
7.14
3.25

2.67
0.61
0.08

5.54
29.75
100.00

4.55
1.35
0.61

0.50
0.12
0.02

1.575
4.118
1.729

0.039
0.036
0.039

0.1048
0.1469
0.0187

0.0003
0.0004
0.0001

0.74
0.07
1.74

2.80
0.15
1.24

0.5560
0.7796
0.0992

0.0015
0.0020
0.0007

0.0056
0.0146
0.0061

0.82
0.03
1.77

3.11
0.06
1.26

3.63
6.13
16.19

3.97
3.69
3.31

0.30
0.15
0.14

21.22
76.71
100.00

0.75
0.70
0.62

0.06
0.03
0.03

1,044.58
944.95
197.78

0.36
0.43
0.09

Sample 25
J-Value = 0.000104689 ±0.000000039
Sample 25PHL-1 (three grains, thin flakes)
25PHL-1a
1
3%
172.26
0.05
25PHL-1b
2
7%
247.95
0.07
25PHL-1c
3
30%
35.33
0.03

a Atmospheric argon composition of Lee et al. (2006) is assumed for mass-discrimation corrections and for trapped argon composition.
a Red text indicates meaningless 39Ar measurement (<1 fA, i.e. <10x background), step excluded from total gas age calculation.
c Age errors are one sigma uncertainties and exclude uncertainties in the J-value.
d J-values are calculated assuming an age of 1.18144 ±0.00068 Ma (1s; Phillips et al., 2017.) for Alder Creek sanidine.
e Interference corrections values are: (36Ar/37Ar)Ca = 2.570 (± 0.002) x 10-4; (39Ar/37Ar)Ca = 6.62 (± 0.08) x 10-4; (40Ar/39Ar)K = 0.001210 (± 0.000016).
f Data are corrected for mass spectrometer backgrounds, discrimination and radioactive decay.
g Data are not corrected for Ca-interference due to small sample size and negligible quantities of 37Ar.

32

Table A4. cont.
37Ar set to zero, no Ca-interferece correction appliedf,g

±1σ

±1σ

39Ar
(x10-14
mol)

Ca/K

±1σ

2.6528
2.8649
1.0698
0.3791
0.7026
0.4814
0.0405

0.0028
0.0050
0.0017
0.0014
0.0030
0.0018
0.0007

0.0057
0.0065
0.0058
0.0032
0.0074
0.0075
0.0004

N/A
N/A
N/A
N/A
N/A
N/A
N/A

N/A
N/A
N/A
N/A
N/A
N/A
N/A

0.54
0.63
1.54
2.90
3.87
5.45
1.62

2.69
2.99
3.08
3.75
4.06
3.94
1.72

0.56
0.83
0.32
0.49
0.43
0.27
1.73

15.68
0.51
33.48
0.57
49.30
0.58
58.11
0.71
78.37
0.77
98.87
0.75
100.00
0.33
Total gas age: 652 ± 18 ka (2σ)

0.0
0.0

1.9970
2.3113

0.0021
0.0031

0.0042
0.0195

N/A
N/A

N/A
N/A

0.83
2.96

4.21
3.82

0.54
0.17

16.70
0.80
0.10
94.47
0.72
0.03
Total gas age: 740 ± 90 ka (2σ)

0.0
0.0

0.0
0.0

2.7062
1.3013

0.0060
0.0021

0.0049
0.0074

N/A
N/A

N/A
N/A

0.73
2.40

4.29
4.60

1.30
0.31

39.96
0.81
0.24
100.00
0.87
0.06
Total gas age: 850 ± 270 ka (2σ)

0.0007
0.0013

0.0
0.0

0.0
0.0

1.7433
4.4989

0.0035
0.0068

0.0014
0.0214

N/A
N/A

N/A
N/A

0.58
1.54

7.86
3.49

2.72
0.34

6.06
100.00

1.49
0.66

0.49
0.06

0.1608
1.1598
0.0198

0.0002
0.0014
0.0001

0.0
0.0
0.0

0.0
0.0
0.0

0.8530
6.1527
0.1052

0.0011
0.0074
0.0005

0.0002
0.0083
0.0004

N/A
N/A
N/A

N/A
N/A
N/A

0.63
0.35
2.51

23.64
2.74
7.80

9.84
0.98
2.64

2.73
95.89
100.00

4.47
0.52
1.48

1.70
0.18
0.40

0.03
0.02

0.0158
0.0436

0.0001
0.0002

0.0
0.0

0.0
0.0

0.0837
0.2311

0.0007
0.0010

0.0020
0.0163

N/A
N/A

N/A
N/A

7.00
17.04

3.38
3.08

0.42
0.07

10.79
100.00

0.64
0.58

0.08
0.01

0.60
2.64
7.65

0.02
0.01
0.02

0.6503
0.5848
0.1093

0.0009
0.0010
0.0004

0.0
0.0
0.0

0.0
0.0
0.0

3.4501
3.1022
0.5800

0.0050
0.0052
0.0020

0.0021
0.0094
0.0271

N/A
N/A
N/A

N/A
N/A
N/A

1.39
1.98
12.44

24.07
7.11
3.22

2.67
0.61
0.08

1.58
4.12
1.73

0.04
0.04
0.04

0.1048
0.1470
0.0188

0.0002
0.0004
0.0001

0.0
0.0
0.0

0.0
0.0
0.0

0.5562
0.7796
0.0996

0.0013
0.0020
0.0006

0.0056
0.0146
0.0061

N/A
N/A
N/A

N/A
N/A
N/A

3.60
6.13
15.81

3.93
3.69
3.23

0.27
0.15
0.12

40Ar

21PHL-1a
21PHL-1b
21PHL-1c
21PHL-1d
21PHL-1e
21PHL-1f
21PHL-1g

1
2
3
4
5
6
7

3.00%
5.00%
7.00%
9.00%
12.00%
16.00%
30.00%

796.33
860.80
324.39
116.56
218.19
152.01
12.30

0.28
0.23
0.08
0.05
0.04
0.03
0.03

1.61
1.82
1.62
0.90
2.08
2.10
0.12

0.02
0.02
0.03
0.02
0.02
0.02
0.02

0.5000
0.5400
0.2017
0.0715
0.1324
0.0907
0.0076

0.0005
0.0009
0.0003
0.0003
0.0006
0.0003
0.0001

0.0
0.0
0.0
0.0
0.0
0.0
0.0

0.0
0.0
0.0
0.0
0.0
0.0
0.0

21PHL-2a
21PHL-2b

1
2

3.00%
30.00%

601.21
711.12

0.09
0.19

1.18
5.50
6.69

0.02
0.03

0.3764
0.4357

0.0004
0.0006

0.0
0.0

21PHL-3a
21PHL-3b

1
2

3.00%
30.00%

813.86
398.07

0.11
0.11

1.38
2.07

0.02
0.01

0.5101
0.2453

0.0011
0.0004

21PHL-4a
21PHL-4b

1
2

3.00%
30.00%

523.53
1,364.20

0.14
0.31

0.39
6.03

0.01
0.02

0.3286
0.8480

21PHL-5a
21PHL-5b
21PHL-5c

1
2
3

3.00%
10.00%
30.00%

256.29
1,843.37
32.23

0.07
0.63
0.02

0.07
2.34
0.10

0.03
0.03
0.03

24PHL-1a
24PHL-1b

1
2

3.00%
30.00%

26.88
83.17

0.02
0.03

0.56
4.60

24PHL-2a
24PHL-2b
24PHL-2c

1
2
3

3.00%
7.00%
30.00%

1,044.58
944.95
197.78

0.36
0.43
0.09

25PHL-1a
25PHL-1b
25PHL-1c

1
2
3

3.00%
7.00%
30.00%

172.26
247.95
35.33

0.05
0.07
0.03

(fA)

37Ar

±1σ

Laser
Power
(%)

(fA)

38Ar

±1σ

Step
No

(fA)

39Ar

±1σ

Sample
ID

(fA)

36Ar

(fA)

%40Ar*

40Ar*/39Ar

±1σ

Cum.%
39Ar

Age
(Ma)

0.10
0.16
0.06
0.09
0.08
0.05
0.32

5.54
4.55
0.50
29.75
1.34
0.11
100.00
0.61
0.02
Total gas age: 800 ± 80 ka (2σ)

21.22
76.70
100.00

0.74
0.70
0.61

Total gas age: 690 ± 60 ka (2σ)

33

±1σ

0.05
0.03
0.02

Table A4. cont.
Background Correction
Blank no.

40Ar

ARGUSVI Detector Sensitivity and Discrimination Corrections
±1σ

(fA)

39Ar

±1σ

(fA)

38Ar

±1σ

(fA)

37Ar

±1σ

(fA)

36Ar

±1σ

(fA)

H1/Ax
[40]

H1/L1
[40]

H1/L2
[40]

AX
(1amu)

L1
(1amu)

L2
(1amu)

H1/CDD

±1σ
(%)

1.00121689
1.00121689
1.00121689
1.00121689
1.00121689
1.00121689
1.00121689

0.99890499
0.99890499
0.99890499
0.99890499
0.99890499
0.99890499
0.99890499

0.99378531
0.99378531
0.99378531
0.99378531
0.99378531
0.99378531
0.99378531

0.99279355
0.99279355
0.99279355
0.99279355
0.99279355
0.99279355
0.99279355

0.99079307
0.99079307
0.99079307
0.99079307
0.99079307
0.99079307
0.99079307

0.99352078
0.99352078
0.99352078
0.99352078
0.99352078
0.99352078
0.99352078

320.402846
320.402846
320.402846
320.402846
320.402846
320.402846
320.402846

0.09594157
0.09594157
0.09594157
0.09594157
0.09594157
0.09594157
0.09594157

EXB#57
EXB#57
EXB#58
EXB#58
EXB#59
EXB#59
EXB#60

2.481
2.481
2.547
2.547
2.185
2.185
2.575

0.028
0.028
0.024
0.024
0.016
0.016
0.026

0.077
0.077
0.086
0.086
0.056
0.056
0.068

0.016
0.016
0.020
0.020
0.018
0.018
0.016

-0.026
-0.026
-0.065
-0.065
-0.021
-0.021
-0.069

0.040
0.040
0.026
0.026
0.026
0.026
0.013

0.002
0.002
0.021
0.021
-0.011
-0.011
0.019

0.013
0.013
0.007
0.007
0.012
0.012
0.023

0.01591
0.01591
0.01670
0.01670
0.01573
0.01573
0.01591

0.00028
0.00028
0.00026
0.00026
0.00009
0.00009
0.00035

EXB#62
EXB#62

1.934
1.934

0.013
0.013

0.035
0.035

0.019
0.019

-0.068
-0.068

0.024
0.024

0.016
0.016

0.019
0.019

0.01324
0.01324

0.00031 1.00121689 0.99890499 0.99378531 0.99279355 0.99079307 0.99352078 320.402846 0.09594157
0.00031 1.00121689 0.99890499 0.99378531 0.99279355 0.99079307 0.99352078 320.402846 0.09594157

EXB#63
EXB#63

1.849
1.849

0.030
0.030

0.077
0.077

0.008
0.008

-0.035
-0.035

0.010
0.010

-0.005
-0.005

0.012
0.012

0.01362
0.01362

0.00026 1.00121689 0.99890499 0.99378531 0.99279355 0.99079307 0.99352078 320.402846 0.09594157
0.00026 1.00121689 0.99890499 0.99378531 0.99279355 0.99079307 0.99352078 320.402846 0.09594157

EXB#63
EXB#64

1.849
2.632

0.030
0.014

0.077
0.027

0.008
0.016

-0.035
-0.034

0.010
0.017

-0.005
0.025

0.012
0.013

0.01362
0.01619

0.00026 1.00121689 0.99890499 0.99378531 0.99279355 0.99079307 0.99352078 320.402846 0.09594157
0.00029 1.00121689 0.99890499 0.99378531 0.99279355 0.99079307 0.99352078 320.402846 0.09594157

EXB#65
EXB#65
EXB#65

1.975
1.975
1.975

0.014
0.014
0.014

0.033
0.033
0.033

0.021
0.021
0.021

-0.093
-0.093
-0.093

0.005
0.005
0.005

0.008
0.008
0.008

0.008
0.008
0.008

0.01356
0.01356
0.01356

0.00028 1.00121689 0.99890499 0.99378531 0.99279355 0.99079307 0.99352078 320.402846 0.09594157
0.00028 1.00121689 0.99890499 0.99378531 0.99279355 0.99079307 0.99352078 320.402846 0.09594157
0.00028 1.00121689 0.99890499 0.99378531 0.99279355 0.99079307 0.99352078 320.402846 0.09594157

EXB#65
EXB#66

1.975
1.813

0.014
0.025

0.033
0.048

0.021
0.007

-0.093
-0.015

0.005
0.012

0.008
0.029

0.008
0.018

0.01356
0.01340

0.00028 1.00121689 0.99890499 0.99378531 0.99279355 0.99079307 0.99352078 320.402846 0.09594157
0.00021 1.00121689 0.99890499 0.99378531 0.99279355 0.99079307 0.99352078 320.402846 0.09594157

EXB#66
EXB#66
EXB#66

1.813
1.813
1.813

0.025
0.025
0.025

0.048
0.048
0.048

0.007
0.007
0.007

-0.015
-0.015
-0.015

0.012
0.012
0.012

0.029
0.029
0.029

0.018
0.018
0.018

0.01340
0.01340
0.01340

0.00021 1.00121689 0.99890499 0.99378531 0.99279355 0.99079307 0.99352078 320.402846 0.09594157
0.00021 1.00121689 0.99890499 0.99378531 0.99279355 0.99079307 0.99352078 320.402846 0.09594157
0.00021 1.00121689 0.99890499 0.99378531 0.99279355 0.99079307 0.99352078 320.402846 0.09594157

EXB#67
EXB#67
EXB#67

2.091
2.091
2.091

0.029
0.029
0.029

0.061
0.061
0.061

0.032
0.032
0.032

0.009
0.009
0.009

0.016
0.016
0.016

0.006
0.006
0.006

0.011
0.011
0.011

0.01542
0.01542
0.01542

0.00042 1.00121689 0.99890499 0.99378531 0.99279355 0.99079307 0.99352078 320.402846 0.09594157
0.00042 1.00121689 0.99890499 0.99378531 0.99279355 0.99079307 0.99352078 320.402846 0.09594157
0.00042 1.00121689 0.99890499 0.99378531 0.99279355 0.99079307 0.99352078 320.402846 0.09594157

34

Table A5. Summary of Ar-Ar age results calculated for Catanda phlogopite (samples 21PHL, 24PHL and 25PHL)
Inverse isochron results

Age spectrum/weighted mean age results
Plateau age
E (ka, 2σ)
Steps
39Ar (%)
(ka)
690
±63
2–7
84.3

Total gas ages
MSWD

p

Age (ka)

E (ka, 2σ)
±180

(40Ar/36Ar)i

E (2σ)

Age (ka)

E (ka, 2σ)

MSWD

p

21PHL-1

1–6

297.62

±0.72

777

±98

0.56

0.69

1.5

0.19

650

-

-

-

-

-

-

-

-

-

-

-

-

-

740

±90

21PHL-3

-

-

-

-

-

-

-

-

-

-

-

-

-

850

±270

21PHL-4

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

21PHL-5

21PHL

Steps included

21PHL-2

Sample no.

-

-

-

-

-

-

-

-

-

-

-

-

-

-

298.18

±0.46

762

±66

1.70

0.07

Weighted
mean age
(ka)

E (ka, 2σ)

Steps

MSWD

p

-

-

ratios

298.18

±0.76

776

±81

1.50

0.19

741

±44

n=7

-

1.4

0.22

-

-

Aliquant no.

All data where
21PHL-1, -2, -3, -4, -5

39Ar>1

fA (n=12)

Data with concordant
21PHL-1, -2, -3, -4, -5

40Ar*/39Ar

(n=7)
24PHL

24PHL-1,-2

n=3 (errorchron)a

302.2

±1.1

553

±24

0.20

0.65

-

-

-

-

-

-

800

±80

25PHL

25PHL-1

n=3 (errorchron)a

301.2

±1.8

588

±58

0.53

0.47

-

-

-

-

-

-

690

±60

0.50

0.73

-

-

-

-

-

-

-

-

24PHL +
24PHL-1,2 and 25PHL-1b
n=6
302.01
±0.87
558
±22
25PHL
aClassified as errorchron, rather than isochron, due to use of 3 data points only, uncertainty is 95% CI.
bJ-value of 0.00104689 ± 0.037% (25PHL value) was used to calculate age for 24PHL-25PHL composite inverse isochron.

35

Table A6. ARGUSVI

Sample
ID

Step
No

40Ar/39Ar

Analytical Results for Alder Creek Rhyolite sanidine fusionsa,b

Laser
Power

40Ar

±1σ

(fA)

39Ar

±1σ

(fA)

38Ar

±1σ

(fA)

±1σ

37Ar

(fA)

36Ar

±1σ

39Ar
(x10-14
mol)

Ca/K

(fA)

±1σ

%40Ar*

40Ar*/39Ar

±1σ

Sample AC8 (three-grain aliquots)
AC8-1
1
35%
AC8-2
1
35%
AC8-3
1
35%
AC8-4
1
35%
AC8-5
1
35%

489.30
473.33
606.62
576.50
527.63

0.04
0.13
0.12
0.18
0.11

72.03
69.62
89.83
89.09
76.95

0.05
0.04
0.05
0.03
0.02

0.0257
0.0251
0.0300
0.0132
0.0309

0.0003
0.0002
0.0002
0.0002
0.0001

2.6374
0.0729
3.6744
0.7241
0.0730

1.8899
0.1458
2.7247
2.5232
0.1459

0.1363
0.1333
0.1591
0.0698
0.1638

0.0014
0.0010
0.0012
0.0009
0.0008

0.2557
0.2471
0.3189
0.3163
0.2732

0.064
0.002
0.072
0.014
0.002

0.046
91.68
6.2283
0.0073
0.004
91.59
6.2274
0.0058
0.053
92.17
6.2240
0.0053
0.050
96.38
6.2371
0.0044
0.003
90.73
6.2214
0.0037
Weighted mean: 6.2272±0.0044 (2σ); MSWD=2.0, p=0.1

Sample AC9 (three-grain aliquots)
AC9-1
1
35%
AC9-2
1
35%
AC9-3
1
35%
AC9-4
1
35%
AC9-5
1
35%

384.71
520.09
600.46
351.94
512.39

0.08
0.10
0.11
0.10
0.28

59.09
79.17
90.01
52.67
71.62

0.02
0.03
0.05
0.03
0.05

0.0090
0.0150
0.0229
0.0141
0.0414

0.0001
0.0002
0.0001
0.0002
0.0002

4.4979
2.3338
1.0941
2.1657
4.4124

2.0392
1.6513
1.4692
2.2360
1.5657

0.0478
0.0795
0.1213
0.0750
0.2197

0.0006
0.0010
0.0007
0.0011
0.0009

0.2098
0.2811
0.3195
0.1870
0.2542

0.133
0.052
0.021
0.072
0.108

0.060
96.29
6.2696
0.0042
0.036
95.44
6.2690
0.0045
0.029
93.97
6.2689
0.0044
0.074
93.64
6.2576
0.0072
0.038
87.20
6.2387
0.0070
Weighted mean: 6.2679±0.0047 (2σ); MSWD=0.8, p=0.5

a Data

are corrected for mass spectrometer backgrounds, discrimination and radioactive decay. Atmospheric argon composition of Lee et al. (2006) is assumed.
corrections values are: (36Ar/37Ar)Ca = 2.570 (± 0.002) x 10-4; (39Ar/37Ar)Ca = 6.62 (± 0.08) x 10-4; (40Ar/39Ar)K = 0.001210 (± 0.000016).

b Interference

36