Mesoproterozoic rocks of Namibia and their plate tectonic setting
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Date
2006Author
Becker, T.
Schreiber, U.
Kampunzu, A.B.
Armstrong, R.
Publisher
Elsevier www.elsevier.com/locate/jafrearsciType
Published ArticleMetadata
Show full item recordAbstract
Two main Mesoproterozoic provinces occur in southern Namibia: (1) The high-grade Namaqua Metamorphic Complex (NMC) composed
of a supracrustal sedimentary succession and interpreted as a passive margin sequence in the west of the Kalahari craton; (2) The
Sinclair Group and its northeastern correlatives, composed of two main magmatic and metamorphic units, reflecting northeast-directed
subduction, which started before 1.37 Ga and lasted until about 1.1 Ga. These two units were tectonically juxtaposed during the 1.1–
1.03 Ga Namaqua orogenic event.
The Kairab–Kumbis Metamorphic Complex comprises metasedimentary and metavolcanic rocks intruded by the 1.37 Ga arc-related
Aunis tonalite. The mafic volcanic rocks from this complex have geochemical features of island arc calcalkaline basalts; they were
emplaced and metamorphosed along an active margin before 1.37 Ga. The 1.2–1.1 Ga low-grade unmetamorphosed volcanic and immature
sedimentary rocks of the Sinclair Group and its northwestern equivalents rest disconformably on the Kairab–Kumbis Complex.
They occur in fault-bounded depocenters defining a regional arc-shape structure up to 100 km-wide and with a minimum length of
2000 km. The plate tectonic setting of this arc is best constrained by the composition of volcanic rocks from the 1.2 Ga Barby Formation
and coeval granitoids; they comprise high-K calcalkaline rocks suggesting emplacement in an active continental margin setting. The
final stage of this continental arc evolution is recorded in the <1.1 Ga tholeiites of the Opdam Formation. High Ti-content and flat REEpatterns
in the tholeiites suggests an extensional event, whereas high Th/Ta and La/Nb ratios, low Ce/Pb values and negative anomalies
for Nb–Ta suggest a subduction-related setting for the mantle source from which the mafic magmas were derived. Docking of continents
led to the slab detachment, allowing interaction between the asthenospheric mantle and the mantle wedge enriched during the subduction
process. The magmatic underplating related to this event induced the genesis of large-scale batholitic granitoid bodies in the NMC and a
1.1–1.0 Ga high-grade LP/HT metamorphism, with mineral assemblages indicating an anti-clockwise P–T–t path.