Abstract
Using a recent compilation of African alkaline igneous rocks and
carbonatites, we show that nearly 90% (28 of 32 occurrences) of
nepheline syenite gneisses and deformed carbonatites are concentrated
within known or inferred Proterozoic suture zones. Given the
well-established intracontinental rift setting for these rocks and the
likely continental collisional setting for their subsequent deformation,
we suggest that deformed alkaline rocks and carbonatites (DARCs)
represent the products of two well-defined parts of the Wilson Cycle.
DARCs mark the places where vanished oceans have opened and then closed.
We further postulate that DARCs taken into the mantle lithosphere to ca.
100 km depths at collision could provide source material for later
alkaline magmatic activity. This could account for the observation of
recurrent alkaline magmatic activity over hundreds of m.y. in provinces
such as that of southern Malawi.
INTRODUCTION: ELUSIVE ANCIENT SUTURE ZONES
The record of the dissipation of Earth's internally generated heat jy
plate tectonic processes over tne past 4 txy. is mainly to be found in
the evidence, preserved within continental crust, of the operation of
the Wilson cycle (Wilson, 1968; Burke and Dewey, 1974). It is critical
for recognizing the past operation of the Wilson cycle to identify
ancient suture zones marking the places where now-vanished oceans have
opened and closed. Intense collision and erosion have led to uneven
preservation of dismembered ophiolites, ultrahigh-pressure metamorphic
rocks, rocks with reset isotopic systems, and postcollisional granites,
which are the commonly accepted indicators of continental collision.
In places in which sutures are cryptic (Brown and Coleman, 1972),
evidence of collision may consist only of the juxtaposition of
reactivated and unreactivated continental rocks. Mapping ancient sutures
has proven understandably difficult, and in some areas consensus about
suture locations has not yet been reached.
We report here on a newly recognized indicator of continental collision.
We have found that occurrences of deformed alkaline rocks and
carbonatites (DARCs) – in Africa, mainly nepheline syenite gneiss and
deformed carbonatite – are concentrated in suture zones of various
ages (Fig. 1). We have used the newly identified association between
DARCs and suture zones in Africa to suggest locations for poorly mapped
examples, and to confirm the locations of some that are well known.
PROCEDURE
The best-defined of all associations of igneous rocks with a specific
tectonic environment is the association of nepheline syenites, their
volcanic equivalents, and carbonatites with rifts (Bailey, 1974, 1977,
1992). Linking more abundant rock types such as basalts with particular
environments is much more difficult, calling for subtle geochemical
and isotopic interpretation (e.g., Pearce and Cann, 1973; Zindler and
Hart, 1986). Woolley (2001) published a comprehensive catalogue of
African alkaline rocks and carbonatites that confirmed the association
of nepheline syenites, carbonatites, and rifts. Acquaintance with the
Wilson cycle led us to conjecture that DARCs could be associated with
the ancient collisional plate boundaries whose former existence
can be discerned in suture zones. We therefore tabulated (Table 1) and
plotted on a map of Africa (Fig. 1, inset) the 32 DARCs of Woolley's
catalogue. We also plotted (Fig. 1) the distribution of known suture
zones that represent continental and arc-continental collisions. Our
analysis was restricted to suture zones that record collisions involving
at least one continent, because nepheiine syenites ana caroonar;;
have, with very few exceptions, been emplaced into continental crue*
RESULTS
Of the 32 DARCs (Fig. 1), 10 are within mapped suture zones, and 18 are
close to the western margin of the Mozambique belt in a pattern that has
led us to suggest locations for hitherto elusive suture
zones separating the Congo and Kalahari cratons from the
Pan-African-aged Mozambique belt. The four remaining DARCs (4, 7, 2'1,
and 28 in Fig. 1 and Table 1) are not clearly linked to suture zones.
Suture Zones at the Western Margin of the Mozambique Belt
Kennedy (1965, Fig. 1) placed the western boundary of the Mozambique
belt of Holmes (1951) at the margins of the Congo and Kalahari cratons
(Fig. i). Although it was soon recognized thai Kennedy's craton
boundaries were close to places where oceans had closed during
Pan-African time (Burke and Dewey, 1972), locating southern Mozambique
belt suture zones with any exactitude has remained a challenge.
In a new attempt at locating those elusive structures, we have plotted
(Fig. 1) the distribution of DARCs in an area between 20°S and the
equator that straddles the western border of the Mozambique
belt. We have used the DARC distribution within that area together with
regional geologic and geophysical data to tentatively suggest possible
locations for suture zones. Figure 1 shows the suture-zone pattern that
we have discerned.
In Figure 1 the suture zone within the Damara-Lufilian-Zambezi fold
belt, which separates the Kalahari and Congo cratons, carries two DARCs
(31 and 32, both deformed carbonatites). The Kalahari craton was almost
completely surrounded by collision zones during the interval
1.3-1.0 Ga, and until better ages are determined it remains uncertain
whether the two DARCs shown in Figure 1 were thrust onto the Kalahari
craton ca. 1 Ga or ca. 0.5 Ga. The Pan-African suture extends eastward
toward Tete (Fig. 1), where the regional strike has long been recognized
to change from due east to due north (e.g., Evans et al., 1999; Fig. 1).
We interpret a 300-km-wide cluster of 18 DARCs that crop out between
13°S and 17°S as recording the suturing of Mozambique belt
rocks against the Congo and Kalahari cratons.
Southward from Tete we used a line of six DARCs (13, 22, 23, 14, 15, and
25) to map the location of a Pan-African suture that links with two
other sutures at Nsanje (Fig. 1). From Nsanje, one suture
continues southward and in a few kilometers becomes completely buried
under Phanerozoic cover. The other suture, which is marked by six DARCs
(24, 20, 19, 18, 17, and 16), extends northward in a curve
Figure 1. Alkaline rocks, carbonatites, cratons, Pan-African fold belts,
and suture zones in southeast Africa. Circles – nepheline syenites;
squares – carbonatites; open symbols – Mesozoic occurrences; filled
symbols – Pan-African occurrences. Only deformed alkaiine
rocks and carbonatites examples (DARCs) are numbered.
Dashed lines indicate regional strike of Pan-African rocks in
Damara-Lufilian-Zambezi (DLZB) and Mozambique (MB) fold belts.
Phanerozoic sedimentary rocks south of Nsanje are shown with dotted
pattern. Sutures (lines with bars) indicate locations of ocean closure
during Pan-African time. Suture pattern in area in vicinity of Lake
Malawi is based on distribution of DARCs, regional geology,
and geophysics. Line passing through DARCs 28 and 29, and along
eastern shore of Lake Tanganyika, is Ubendian suture (question mark
indicates uncertainty about its continuation). Pan-African nepheline
syenites and DARC 4 in northwest corner are possibly on
Kibaran (ca. 1.25 Ga) suture. Inset map of Africa shows DARC locations
with respect to cratons (Kennedy, 1965) bounded by sutures that mark
sites of ocean closure during Pan-African time (Burke and
Dewey, 1972). Line carrying two DARCs inside Congo craton shows where
Tanzanian craton was incorporated into Congo craton in Ubendian time
(ca. 1.85 Ga). Dashed line within Kalahari craton
marks 1.2 Ga Natal-Namaqualand suture zone and its postulated
continuation.
A. 1000? Ma
First alkaline rocks formed,
continental rupture
B. 700? Ma
Ocean open, alkaline rocks not active
C. 550 Ma
Pan-African collision, deformtion of
alkaline rocks
D. 450-140 Ma
Lithospheric stability, DARCs in crust
and mantle lithosphere
E. 140 Ma
Cretaceous rifting, renewed alkaline
igneous activity
Figure 2. Repeated episodes of alkaline intracontinental magmatism,
example from Malawi. A: Nepheline syenites and carbonatites (black
filled circles) were emplaced into intracontinental rift ca. 1 Ga. B:
Those rocks were later preserved at rifted continental margin. C: During
Pan-African collision, alkaline rocks from rifted margin developed
gneissic fabrics, becoming nepheline syenite gneisses and
deformed carbonatites (DARCs). D: During long period of lithospheric
stability, DARCs remained preserved both in crust and in mantle
lithosphere at depths of -100 km. E: At beginning of Cretaceous time,
renewed episode of rifting led to adiabatic-decompression
melting of DARCs in mantle lithosphere, producing new generation of
nepheline syenites and carbonatites. ~500 km. We have projected that
suture to the north-northwest under Lake Malawi to pass through a group
of DARCs (10, 8, 9, 30, and 29) and to reach the Congo craton border at
9°S, 33°E. From that point to
as far as the equator, the margin of the Congo craton is a cryptic
suture.
To complete our picture of Pan-African suturing in the region of Lake
Malawi, we have drawn a suture northward from Tete through DARCs
11 and 12 to close a loop of sutures at ~11°S (Fig. 1). We used
regional
geologic and geophysical maps in addition to DARCs in plotting these
sutures, but make no claims for great accuracy. Incorporating additional
multidisciplinary data sets could lead to an improved suture pattern. We
claim that the anastomosing pattern of the sutures that we have drawn is
stylistically of the kind to be expected in a continental
collision zone.
The Congo craton, ca. 1.85 Ga, incorporated what had been a distinct
Tanzanian craton (represented by rocks now cropping out east of Lake
Tanganyika) along a Ubendian suture that Daly (1988) interpreted to have
been an ~100-km-wide strike-slip fault zone. We recognize two DARCs (29
and 28) on that boundary, which we show as
a curved line in Figure 1 that continues northward along the shore of
Lake Tanganyika. Where the suture goes to the north of 4°S is
unknown.
Nepheline syenites and carbonatites of Pan-African age, including one
DARC (4), crop out between 1°S and 2°S in the Congo. It seems
possible that these bodies may be on a Kibaran (ca. 1.25 Ga)
suture.
TABLE 1. OCCURRENCES OF DEFORMED ALKALINE ROCKS AND CARBONATITES IN
APR!"
No. Locality
2 Algeria
3 "">'Ouzzal
4 Cameroon
5 Nkonglong
~> Lolodorf
7 Conao (DRCi
8 Numbi
tf Ghana
10 Somanya, Kpong & Pore
11 Dufo to Jirawde
12 Madagascar
13 Makaraingobe
14 Malawi
15 llomba
16 uiindi
r7 Chikangawa
18
Ifl
20
21
22
23
24
25
26
27
28
29
JU
31
32
Chipaia and Chipala East
Kasungu
Ncheu
Tambani
Nsanje Area
Mozambique
Unnamed
Meponda
Unnamed
Unnamed
Unnamed
Unnamed
Unnamed
Unnamed
Chiperone and Derre
Lulwe
South Africa
Bull's Run
"i anzani.
Lungolo
Sangu-lkola
Mbozi
Nachendezwava
Zimbabwe
Dande-Doma
Kapfrugwa
Lat
23°37'N
02°46'N
03°23'N
01°46'S
06WN
06°07'N
17°52'S
09°31'S
09°31'S
11°52'S
12"59'S
13°03' S
14°43'S
15°43'S
17°01'S
12°26'S
13°27'S
14°14'S
14°14'S
14°29'S
14°45'S
15°07'S
15°43'S
16°32'S
17°02'S
28°45'S
05°27'S
06°48'S
09°08'S
09°30'S
16°20'S
16°28'S
Long
03°13'E
10°01'E
10°58'E
28°54'E
00°08'E
00°11'E
45°40'E
33°11'E
33°14'E
33°48'E
33°28'E
33°27'E
34°37'E
34°27'E
35WE
35°07'E
34°54'E
35°3VE
36°1 1 'E
36°25'E
40WE
34"33'E
34°21'E
35°45'E
35°05'E
31°26'E
38°02'E
30°31 :E
32°46'E
33°12'E
30°21'E
32°09'E
Rock
t
Age*
brimarv
(Mai
2090 [31
1994 ± 20 [31
2890 ± 45 [2]
N.D.5
830 ± 51 [2]
N.D.I
N.D.I
N.D.s
N.D.5
N.D.s
N.D.s
N.D.s
N.D.s
N.D.S
N.D.s
N.D.S
N.D.S
755 ± 115(2]
N.D.s
N.D.S
N.D.s
N.D.5
N.D.S
N.D.S
N.D.S
N.D.S
1140 ± 35 [3]
1100 ± 40 [3]
N.D.5
N.D.5
N.D.s
655 [3]
N.D.5
N.D.5
Aae* Suture Suture
reactivation name a--
(Mal if...
255 ± 8 [4] W. African 550
564 ± 64 I4] craton
529 ± 15 [1] Congo 550
N.D.s craton
648 ± 17 [2] Kibaran? 1100
N.D.s w. African 550
N.D.5 craton
N.D.5 Unknown ?
508 ± 12 [1]
490 r i2 jij
685 ± 62 [2]
686 ~ 62 [2]
410 ± 16(1]
650 ± 40 12}
N.D.s
N.D.S
N.D.S
587 ± 72 [3]
542 [3]
N.D.S
N.D.5
538 [3]
N.D.5
N.D.S
N.D.5
N.D.S
N.D.5
N.D.S
N.D.5
N.D.S
Mozambique 550
belt
900 [1] Namaqua- 1100
Natal oe..
N.D.5 Usagaran? ?
N.D.S Ubendian 1800
743 ± 30 [1] Ubendian 1800
•45 ± 45 M]
N.D.s Mozambique 550
belt
N.D.s Kalahari 1000?
N.D.5 craton 550?
t\ev reTererKOuzegane
et al. (1988)
Kornprobst et al. (1976)
Kornprobstet al. (1976)
Kampunzu et al. (1988)
Holm (1974)
Allen and Charslev (1968)
Welter (1964,
Wooiley et ai. (199o
Ebyetal. (1998)
Gaskell (1973)
Ebyetal. (1998)
Ebyetal. (1998)
Walshaw(1965)
Bloomfield (1968)
Alien and Charsley (1968)
Institute Nacional de Geologia (1987)
Lulin et al. (1985)
Institute Nacional de Geologia (1987)
Institute Nacional de Geologia (1987)
Institute Nacional de Geologia (1987)
Institute Nacional de Geologia (1987)
Institute Nacional de Geologia (1987)
Institute Nacional de Geologia (1987)
Cilek (1989)
Instituto Nacional de Geologia (1987)
Scogings and Forster (1989)
Kempe (1968,
van Straaten (1989)
Basu and Ikingura (1984)
Nelson et al. (1988)
Barber (199-1;
Barton et al. (199')
Note: Data taken from Wooiley (2001 :
*c = carbonatite, n = nepheline syenite, g
;i]K-Ar, [2] Rb-Sr, [3] U-Pb,
â€" peralkaline aranne
4] apatite fission track.
^N.D. = not determined.
Tectonic Explanation of Bailey's Observation
Bailey (1974, 1977, 1992) drew attention to the repeated emplacement
over intervals of as long as hundreds of millions of years of nepheline
syenites and carbonatites within some relatively small regions of
Africa. This relationship is best seen in Malawi (Fig. 1), where
Cretaceous nepheline syenites and carbonatites were emplaced among
numerous DARCs of Pan-African age (ca. 550 Ma). Our recognition of the
association of DARCs with suture zones leads us to a tectonic
explanation of Bailey's observation of repeated eruptions in the same
area over eons intervals (Fig. 2).
The findings of ultrahigh-pressure minerals in rocks that have been
involved in continental collisions shows that material from Earth's
surface
is commonly carried to depths of 90-120 km in collision zones (e.g.,
Smith and Lappin, 1989). We suggest that nepheline syenites and
carbonatites have been carried to depths of â€"100 km in collisions'
zones such as that that developed in Malawi ca. 550 Ma. The outcrops of
DARCs among the Malawi suture zones indicate that suitable material for
emplacement within the mantle lithosphere was being subducted
at the time of the Fan-African collision. If subducted nepheline syenite
was incorporated into the mantle lithosphere at â€"100 km beneath
Malawi ca. 550 Ma, then that rock would have been in a position to
respond to pressure-relief melting when the overlying Shire rift
formed in Malawi 400 m.y. later, at the end of Jurassic time (ca. 140
Ma) Woollev. 1991).
Isotopic compositions of nepheline syenites and carbonatites show
that they represent products of mantle-derived magmatism, although
the relative roles of primary mantle melting, fractional
crystallization,
.no liquid immiscibility are currently debated (e.g., Hall, 1996). There
is abundant evidence from experimental petrology that carbonatitic melts
would form from mantle peridotite in the presence of carbonate minerals,
at solidus temperatures considerably below those that would yield basalt
(e.g., Lee and Wyllie, 2000). Similarly, experiments to
35 kbar in the quartz-albite-nepheline system (Boettcher and Wyllie,
1969) indicated that nepheline-normative melts would remain
silicaundersaturated
throughout the melting interval, unless substantial aqueous fluid was
present, which seems unlikely. Available data, therefore, allow the
possibility that the parental magmas of DARCs were derived
from previously formed alkaline igneous rocks that were subducted to
â€" 100 km depths. For carbonatites, Sr, Pb, and to a lesser degree
Nd isotope signatures show temporal evolution compatible with the idea
of DARCs as sources. Such DARCs could have been incorporated into
the mantle lithosphere as long ago as 3.0 Ga (Bell and Tilton, 2002).
CONCLUSIONS
We have found a high concentration of DARCs along African suture zones
and few DARCS in other tectonic environments. We attribute this to the
emplacement in intracontinental rifts of alkaline rocks and
carbonatites, and their later deformation during continent-continent and
arc-continent collisions. Concentration of subducted nepheline syenite
and carbonatite in mantle lithosphere as a result of collision generates
a reservoir of source rocks for alkaline magmas and carbonatites
and a plausible explanation for Bailey's observation that magmatic
activity in alkaline rock provinces within continents has recurred
episodically over periods of hundreds of millions of years.
ACKNOWLEDGMENTS
We thank Judith Kinnaird and Paul Nex. who kindly loaned us their copy
of the
Woolley catalogue. Peter Wyllie and Sharad Master shared their expertise
on petrogenesis and African geology, respectively. John Dewey and Tim
Kusky provided helpful reviews of the manuscript. The South African
National Research Foundation generously provided
funds for a visit by Burke to Wits University. This work was carried out
during that visit.
"I just wanted you to know"
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