Abstrakt
Granitoid intrusions of the Boundary Igneous Complex separate segments with different ages of high-grade metamorphism in the Kaoko Belt, NW Namibia. Two granitoids of this complex were dated at 575 +- 10 Ma (secondary ionization mass spectrometry; SIMS) or 571 +- 9 Ma (laser ablation inductively coupled plasma mass spectrometry; LA-ICP-MS) and 562 +- 11 Ma (SIMS) or 572 +- 4 Ma (LA-ICP-MS), respectively.
The age of granulite-facies metamorphism in the eastern part of the Western Kaoko Zone was established at 549 + 5 Ma (SIMS) by analysing metamorphic overgrowths of older (c. 1850-1000 Ma) zircons from melt segregations in amphibolites. The coastal part of the Western Kaoko Zone consists of horizons of migmatitic metasedimentary rocks that are intercalated with fine-grained orthogneisses and amphibolites resembling metamorphosed sequences of bimodal volcanic rocks.
Zircons from fielsic members of two bimodal suites have SIMS ages of 805 +- 4 Ma and 810-840 Ma, respectively, that are interpreted as dating their respective igneous protoliths. Melt segregations in the mafic member of the lower bimodal suite contain two populations of zircon dated at 650 +- 5 Ma (SIMS) or 645 +- 5 Ma (LA-ICP-MS) and 629 +- 6 Ma (SIMS) or 630 +- 5 Ma (LA-ICP-MS), respectively.
The later age is indistinguishable from the age of 630 +- 4 Ma (SIMS) or 625 +- 10 Ma (LA-ICP-MS) obtained from melt patches present in overlying metagreywackes. The available age data suggest that the Boundary Igneous Complex masks the suture between the Coastal Terrane and the rest of the Kaoko Belt.
Ages of granitoid intrusions in this igneous complex are indicative of magmatic activity between 580 and 550 Ma.