The following main lithostratigraphic units have been distinguished in
the Domes Area. The Kibaran basement complex composed of gneisses,
migmatites with amphibolite bands and metagranites is exposed in dome
structures; metamorphic features of Kibaran age have been almost
completely obliterated by extensive Lufilian reactivation. The
post-Kibaran cover sequence is subdivided into the Lower Roan Group
consisting of well-preserved quartzites with high Mg content,
talc-bearing, extremely foliated schists intercalated with
pseudo-conglomerates of tectonic origin and the Upper Roan Group
including dolomitic marbles with rare stromatolites, metapelites and a
sequence of detrital metasediments, with local volcano-sedimentary
components and interlayered banded ironstones. The sediments of the
Lower Roan Group are interpreted as continental to lagoonal-evaporitic
deposits partly converted into the talc-kyanite + garnet assemblage
characteristic of ``white schists''. The dolomites and metapelites of
the Upper Roan Group are attributed to a carbonate platform sequence
progressively subsiding under terrigenous deposits, whilst the detrital
metasediments and BIF may be interpreted as a basinal sequence, probably
deposited on oceanic crust grading laterally into marbles. Metagabbros
and metabasalts are considered as remnants of an ocean-floor-type
crustal unit probably related to small basins. Alkaline stocks of
Silurian age intruded the post-Kibaran cover. Significant ancestral
tectonic discontinuities promoted the development of a nappe pile that
underwent high-pressure metamorphism during the Lufilian orogeny and all
lithostratigraphic units.
Rb-Sr and K-Ar and U-Pb data indicate an age of 700 Ma for the highest
grade metamorphism and 500 Ma for blocking of the K-Ar and Rb-Sr system
in micas, corresponding to the time when the temperature dropped below
350-degrees-400-degrees-C and to an age of about 400 Ma for the
emplacement of hypabyssal syenitic bodies.
A first phase of crustal shortening by decoupling of basement and cover
slices along shallow shear zones has been recognized. Fluid-rich
tectonic slabs of cover sediments were thus able to transport fluids
into the anhydrous metamorphic basement or mafic units. During the
subsequent metamorphic re-equilibration stage of high pressure,
pre-existing thrusts horizons were converted into recrystallized
mylonites.
Due to uplift, rocks were re-equilibrated into assemblages compatible
with lower pressures and slightly lower temperatures. This stage occurs
under a decompressional (nearly adiabatic) regime, with P(fluid)
almost-equal-to P(lithostatic). It is accompanied by metasomatic
development of minerals, activated by injection of hot fluids. New or
reactivated shear zones and mylonitic belts were the preferred conduits
of fluids. The most evident regional-scale effect of these processes is
the intense metasomatic scapolitization of formerly plagioclase-rich
lithologies. Uraninite mineralization can probably be assigned to the
beginning of the decompressional stage.
A third regional deformation phase characterized by open folds and local
foliation is not accompanied by significant growth of new minerals.
However, pitchblende mineralization can be ascribed to this phase as
late-stage, short-range remobilization of previously existing deposits.
Finally, shallow alkaline massifs were emplaced when the level of the
Domes Area now exposed was already subjected to exchange with meteoric
circuits, activated by residual geothermal gradients generally related
to intrusions or rifting. Most of the superficial U-showings with
U-oxidation products were probably generated during this relatively
recent phase.