 |
|
|
Magmatism
Within Africa: Lithophere Control and Global
Tectonics
|
|
|
| |
|
Since the original concept of plumes as an explanation for
intraoceanic volcanism, they have become a popular choice for the cause
of intracontinental igneous activity, where the most numerous eruptions,
in time and space, are small volume melts in the alkaline igneous-carbonatite-kimberlite
range. Such activity is most intense in interior Africa, which has been
anorogenic for 550 Ma, with a lithosphere record > 3 Ga. Because continental
alkaline magmatism is characteristically repetitive through the same segment
of the lithosphere over geologic time, this long record is ideal for defining
constraints on processes of magma generation.
Plume models for African magmatism are numerous and highly
divergent, but in the absence of hot spot tracks none can claim the primary
attribute of plumes, namely, lithosphere independence. On the contrary,
the typical pattern in Africa is that the magmatism is sited on pre-existing
structures in the lithosphere, notably rifts and rift intersections. Some
have a record of repeated activity over periods of 2 Ga, during which
time the African plate has moved long distances over the deep mantle.
|
|
When the canvas is broadened to other parts of the plate
the constraints are multiplied, because with increasing age data it is
becoming apparent that in many cases igneous episodes in different provinces
are synchronous (Figure 1). Repeated, synchronous magmatism at the same
sites across Africa is incompatible with any plume model. As many of the
eruptives carry ocean island basalt (OIB) type chemical signatures, it
follows that these are not exclusive to plume melting. Petrogenetic options
are still further constrained when the African igneous episodes are found
to coincide with external, global events.
|
|
Figure 1: Synchronous repetition of CALK
(Carbonatite-Alkaline igneous-Kimberlite activity) in widely scattered
provinces across the African plate. Since Gondwana break-up the
activity peaks E-K, L-K, E-T, T-R in Figure 2, are registered in
29 provinces, most of which have already yielded records of two
or three of the four episodes. This space-time distribution requires
that triggering of CALK eruptions must be a plate-wide episodic
phenomenon, exploiting pre-existing anisotropies in the lithosphere.
(For localities and references see Bailey, 1992). T is the Tanzania
craton. The stippled area (Z) south of L. Tanganyika shows the approximate
position of the Precambrian upland bounded by the rift zones of
Luangwa-Zambesi-Malawi. |
Since the late Precambrian, the carbonatite/alkaline
igneous age histogram for Africa reveals spikes of activity, which until
140 Ma are of similar amplitude, but between 130-80 Ma and 40-0 Ma, there
were unprecedented levels of activity (Figure 2). The Cretaceous “storm”
is also signaled in kimberlite activity peaks, especially in southern
Africa. The clearly-defined surge in igneous intensity during the Cretaceous
correlates with the Magnetic Quiet Zone (CN superchron), and is matched
by major magmatic bursts in other plate interiors, and by changes in sea
floor spreading rates, sea levels, sedimentation and chemical stratigraphy
(Figure 3).
Figure 2. Frequency vs. age histogram of
CALK activity across Africa (A&L combined). Episodes: P-A, Pan-African;
E-C, Early Caledonian; L-C, Late Caledonian; A, Armorican; G, Gondwanaland
starts to break up. Africa/Europe start to collide: O, according
to Olivet et al. (1987) D, according to Dewey et al. (1989). C-N
shows span of CN superchron. Cretaceous-Recent peaks: E-K; L-K;
E-T; T-R. From Bailey & Woolley, 1995. Details on igneous ages,
data, methods, sources, in Woolley (2001). |
|
Figure 3. Age correlations of magmatic events
with the CN Superchron (time span shown at top). Two main age
peaks of the large igneous provinces of Kerguelen and Ontong-Java
plateaux (Pringle et al., 1995; M. Coffin, pers. comm. 1995; A.D.
Saunders, pers. comm. 1995), and key dates in Emperor-Hawaiian
line. Igneous ages histogram for CALK across Africa (sources as
for Figure 2). Closure rates for Africa/Europe collision (calculated
from Dewey et al, 1989: Figure 1, east end of Mediterranean).
D and O as in Figure 2.
|
Globally, the magmatic outbursts
marking Early and Late Cretaceous are neither exclusively alkaline,
nor continental. Both of the biggest oceanic Large Igneous Provinces,
the Ontong-Java and Kerguelen plateaux, have their main eruption
dates around 120 Ma and 85 Ma. Such correlations are consistent
with the geophysical inference that the CN superchron marks a critical
perturbation in mantle dynamics over this period. Further details
may be found in Bailey & Woolley (1995,1999).
Petrogenetic hypotheses currently in vogue are
clearly inapt for the new scenario emerging from the improving chronology
of magmatism, tectonics and other worldwide phenomena. African magmatic
episodes, therefore, must be manifestations of larger, global processes,
capable of re-activating old zones of weakness in plate interiors.
Each re-opening of channels allows a new flux of volatile and incompatible
elements to move into the lithosphere, with all the potential for
metasomatic enrichment, and ultimately for near-solidus melting
and typical alkaline magmatism. No matter what melt mechanism is
preferred, magmatism within Africa declares that the final control
is in the plate structure, and the process cannot be lithosphere
independent.
|
|
-
Bailey,
D.K., Episodic alkaline igneous activity across Africa: implications
for the causes of continental break-up. In: Storey et al., 1992, “Magmatism
and the Causes of Continental Break-up”. Geol. Soc. Spec. Pub., 68, 91-98, 1992.
-
Bailey. D.K. and Woolley, A.R., Magnetic quiet
periods and stable continental magmatism: can there be a plume dimension?
in Anderson, D.L., Hart, S.R., and Hofmann, A.W., Convenors, Plume
2, Terra Nostra, 3/1995, 15-19, Alfred-Wegener-Stiftung,
Bonn, 1995.
-
Bailey, D.K., and Woolley, A.R., Episodic rift
magmatism: the need for a new paradigm in global dynamics, Geolines,
9, 15-20, Czech. Acad. Sci., 1999.
-
Dewey, J.F., Helman, M.L., Turco, E., Hutton, D.H.W.
and Knott, S.D., Kinematics of the western Mediterranean, in, Coward,
M.P., Dietrich, D. & Park, R.G. (eds.) “Alpine Tectonics”.
Geol. Soc. Spec. Pub., 45, 265-283, 1989.
-
Olivet, J.-L., Goslin, J., Beuzart,
P., Unternehr, P., Bonnin, J. & Carre, D., The break-up and dispersion
of Pangea, Coedition Elf Aquitaine (Pau) and IFREMER (Brest) (Wall
map, with text on reverse), 1987.
-
Piper, J.D.A., Palaeomagnetism and the continental
crust. Open University Press, Milton Keynes, UK, 434 pp, 1987.
-
Pringle, M.S., Mitchell, C., Fitton, J.G., and
Storey, M., Geochronological constraints on the origin of Large Igneous
Provinces: examples from the Siberian and Kerguelen flood basalts.
in Anderson, D.L., Hart, S.R., and Hofmann,A.W., Convenors, Plume
2, Terra Nostra, 3/1995, 120-121, Alfred-Wegener-Stiftung,
Bonn, 1995.
-
Woolley, A.R., Alkaline rocks
and carbonatites of the world. Part 3: Africa. Geological
Society/Natural History Museum, London, 2001.
|
|
|
MECHANISMS 
