A continental back-arc setting for the Namaqua belt: Evidence from the Kakamas Domain

Geoscience Frontiers

A.F.M. Kisters¹  C. Spencer²  C. Tinguely²  H.F.G. Moen³  A. Nguno⁴  E. Muvangua⁴  C.W. Lambert⁴  C.A. Groenewald⁴  H. Smith⁴  J.A. Miller⁴  P.H. Macey⁴  R.J. Thomas⁴  M. Angombe⁵  R.A. Armstrong⁶  J.F.A. Diener⁶  H. Minnaar⁶  P. le Roux⁷  D. Frei⁷  G. Shifotoka⁷  J. Indongo⁷  S. Doggart⁷ 

[1] Corresponding author.;Department of Earth Sciences, University of the Western Cape, South Africa;Department of Geology, University of Free State, South Africa;Council for Geoscience, P.O. Box 572, Bellville, South Africa;Department of Geological Sciences, University of Cape Town, South Africa;Department of Geology, University of Stellenbosch, South Africa;Geological Survey of Namibia, Windhoek, Namibia;

关键词: Kakamas;    Namaqua;    Geodynamics;    Geochronology;    Hot orogeny;    Mesoproterozoic;   

DOI  :  

来源: DOAJ

【 摘 要 】

A study of the NW Kakamas Domain in South Africa/Namibia provides a new, unified lithostratigraphy and evolutionary history applicable to the whole Namaqua Sector. The Mesoproterozoic history ranges from ∼1350 Ma to 960 Ma, but isotopic evidence suggests it was built upon pre-existing Paleoproterozoic continental crust that extended west from the Archaean Craton. In eastern Namaqualand, early rift-related magmatism and sedimentation at ∼1350 Ma occurred in a confined ocean basin. Subsequent tectonic reversal and subduction at ∼1290–1240 Ma led to establishment of the Areachap, Konkiep and Kaaien Domains. In the Kakamas Domain, widespread deposition of pelitic sediments occurred at ~1220 Ma (Narries Group). These contain detrital zircons derived from proximal crust with ages between ∼2020 Ma and 1800 Ma (western Palaeoproterozoic domains) and 1350–1240 Ma (eastern early Namaqua domains), suggesting pre-sedimentation juxtaposition. The pelites underwent granulite grade metamorphism at ∼1210 Ma (peak conditions: 4.5–6 kbar and 770–850 °C), associated with voluminous, predominantly S-type granitoid orthogneisses between ∼1210 Ma and 1190 Ma (Eendoorn and Ham River Suites) and low-angle ductile (D2) deformation which continued until ∼1110 Ma, interspersed with periods of sedimentation. This enduring P-T regime is inconsistent with the expected crustal over-thickening associated with the generally-accepted collision-accretion Namaqualand model. Rather, we propose the Namaqua Sector is a ‘hot orogen’ developed in a wide continental back-arc with subduction west of the present-day outcrop. The observed high geotherm resulted from thinned back-arc lithosphere accompanied by an influx of mantle-derived melts. Ductile D2 deformation resulted from “bottom-driven” tectonics and viscous drag within the crust by convective flow in the underlying asthenospheric mantle. This extended tectonothermal regime ceased at ∼1110 Ma when SW-directed thrusting stacked the Namaqua Domains into their current positions, constrained in the Kakamas Domain by late- to post-tectonic I-type granitoids intruded between ∼1125 Ma and 1100 Ma (Komsberg Suite). The thermal peak then shifted west to the Bushmanland and Aus Domains, where voluminous granites (1080–1025 Ma) were associated with high-T/low-P granulite facies thermal metamorphism and mega-scale open folding (D3). Unroofing of the Namaqua Sector is marked by large-scale, NW-trending, sub-vertical transcurrent dextral shear zones and associated pegmatites and leucogranites at ∼990 Ma.

【 授权许可】

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