GEMOC ARC National Key CentreKIMBERLITES, ACCELERATED EROSION AND EVOLUTION OF THE LITHOSPHERIC MANTLE BENEATH THE KAAPVAAL CRATON DURING THE MID-CRETACEOUS
R.W. Brown1, K. Gallagher2, W.L. Griffin3, C.G. Ryan4, M.C.J. de Wit4,
D.X. Belton1, R. Harmon2
1. Vict. Institute Earth & Planetary Sci., La Trobe,
2. Dept of Geology, Imperial College, London, 3. GEMOC, Macquarie and
CSIRO EM, 4. De Beers Consolidated Mines, South Africa
Background
Global surface heat flow data (Pollack et al. 1993) indicate that cratons
are presently characterised by some of the lowest geothermal gradients
on Earth (generally ~10 °C.km-1). Thermobarometry data from mantle
xenoliths hosted by on-craton dimondiferous kimberlites, mostly of Paleozoic
to Mesozoic age, generally document geotherms consistent with the low present
day surface heat flow for these terrains. Furthermore, Archaean and Proterozoic
ages inferred for diamonds from various localities and Re-Os isotopic data
from sub-cratonic mantle xenoliths make a strong case for the extreme longevity
of these low thermal gradients. Overall the available data indicate that
low thermal gradients have persisted within many cratonic regions, in some
places since the Archaean, and at least the Neoproterozoic more generally.
In addition to the abundant evidence for the extreme longevity of cold
and thick cratonic lithosphere there is also mounting evidence which indicates
that the these ancient cratonic roots can be destroyed by later tectonothermal
events. For example, geochemical data for mantle xenoliths and alkaline
magmas from the Sino-Korean craton indicate a dramatic thinning of the
cratonic lithosphere from 180-220 km during the Palaeozoic to <120 km
by beginning of the Tertiary (Griffin et al., 1998a). A similar history
of thinning is suggested for the Wyoming craton in North America (Eggler
et al., 1988). Geochemical and fission track data from South Africa point
to an analogous history of thinning for the lithospheric mantle beneath
the Kaapvaal craton during the mid-Cretaceous.
Kimberlites & lithospheric mantle evolution
Proton microprobe trace element analyses of >700 garnet concentrate
garnets have been obtained from a wide range of kimberlites (n=18),
ranging in age from 140Ma to 80Ma, from the Kaapvaal craton in South Africa.
These data suggest that a major change in the composition and thermal structure
of the lithosphere occurred beneath the craton over a short period at about
90 Ma ago (Figure 1).
Figure 1. Lithospheric sections beneath
the Kaapvaal craton for two time slices (before 90Ma (left and after 90Ma
(right)), constructed from data on concentrate garnets and xenoliths.
The lithosphere base corresponds to the 1250 °C isotherm; the
increase in geotherm associated with lithosphere thinning at ca 90 Ma also
has driven the graphite-diamond transition to greater depth.
Figure 1. Lithospheric sections beneath the Kaapvaal craton for two time slices (before 90Ma (left and after 90Ma (right)), constructed from data on concentrate garnets and xenoliths. The lithosphere base corresponds to the 1250 °C isotherm; the increase in geotherm associated with lithosphere thinning at ca 90 Ma also has driven the graphite-diamond transition to greater depth.
Kimberlites erupted prior to 90 Ma sampled a harzburgite- rich (especially between 140-180km) lithosphere ca 210-220 km thick, which had a geotherm near the 34 mW.m-2 conductive model and was only mildly affected by melt-related metasomatism near the base. Kimberlites erupted after 90 Ma sampled a strongly modified lithosphere: about 80% of the volume at depths >170 km was affected by melt-related metasomatism, the proportion of harzburgite was reduced significantly by metasomatic processes (Griffin et al., 1998b), the geotherm had risen to near a 40mW.m-2 conductive model (as seen in many xenolith suites) and the lithosphere thickness had been reduced by approximately 40 km.
Fission track results & erosion history
Apatite fission track (FT) data from the Kaapvaal craton region indicate that over extensive areas the present land surface exposes rocks which resided at significantly elevated palaeotemperatures (in some places > ~110°C) as recently as 100-80 Ma ago. However, the palaeotemperatures recorded by the FT data are not uniformly distributed across the craton. The current data indicate a general trend of increasing palaeotemperature towards the eastern and northern margins, and there are some regions that have not cooled significantly since the early Palaeozoic. For reasonable estimates of the palaeothermal gradient the distribution of palaeotemperature estimates imply substantial amounts of mid-Cretaceous denudation. In particular, FT results from the deep BK-1 bore hole (1.5 km) within the interior of the Kaapvaal craton document a mean cooling of 44±5°C and a mid-Cretaceous palaeogeothermal gradient of 13±5°C, and the best estimate of the time of cooling is 90±10 Ma. If the eroded material had similar thermal properties to the underlying basement then 3.4±1.4 km of mid-Cretaceous denudation is inferred for the BK-1 site. The data also confirm that over the last ~500 Ma the maximum near surface (? ~10km) thermal gradient has never exceeded the present day value of ca 15 °C.km-1.
The high rates of mid-Cretaceous denudation implied for the Kaapvaal
craton (100-300 m.Ma-1) are corroborated by a dramatic peak in the rate
of clastic sediment accumulation within the adjacent Mozambique basin (eastern
margin) as well as increased rates within the Orange basin (western margin)
(Figure 2). Onshore, this period of enhanced erosion
is recorded by the wide occurrence of the mid-late Cretaceous Malvernia
Formation along the northwestern margins of the craton: an accumulation
of poorly sorted sandstones and pebble and boulder conglomerates comprising
coalesced alluvial fans and major fluvial channel/floodplain systems (Botha
and de Wit, 1996). Mid-Cretaceous denudation of the craton interior is
also supported by the abundance of eroded on-craton kimberlite intrusions
(and related alkaline rocks) with intrusive ages of 95-85 Ma. However,
the preservation of terrestrial gravels at Mahura Muthla (near Lichtenburg),
dated as Late Cretaceous (T.C. Partridge pers. comm., 1997), indicates
that major erosion had essentially ceased in the northwestern interior
region of the craton by the end of the Cretaceous.
Figure 2. Comparison of offshore sedimentation
rates for three boreholes (Macia-1, Sunray-1, Palmeira-1) within the proximal
Mozambique basin and the average onshore erosion rate estimated from FT
data for the Kaapvaal craton.
Figure 2. Comparison of offshore sedimentation rates for three boreholes (Macia-1, Sunray-1, Palmeira-1) within the proximal Mozambique basin and the average onshore erosion rate estimated from FT data for the Kaapvaal craton.
Conclusions
We suggest that the enhanced rates of denudation were caused, in part
at least, by regional uplift of the Kaapvaal craton at approximately 90
Ma. The uplift was probably driven by bouyancy forces arising from a decrease
in the mean density of the underlying lithosphere. The erosion of the buoyant
Archean lithosphere and its replacement by denser asthenospheric
material would effectively increase the density of the column, if the process
were isothermal (Griffin et al., 1998c). However, the concentrate
data indicate that the lithosphere thinning was accompanied by an overall
(transient?) rise in the geotherm, accompanying the thinning of the mechanical
boundary layer by 40 km. This heating could provide the density decrease
required for uplift, and it may have been enhanced by compositional changes
related to metasomatic processes. We therefore believe that the mid-Cretaceous
geomorphological history of the Kaapvaal craton and the eruption
of the main phase of Group I kimberlites are both genetically linked to
the thermo-chemical changes that took place within the underlying lithospheric
mantle approximately 90 Ma ago.
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