Neoproterozoic igneous complex emplaced along major tectonic boundary in the Kaoko Belt (NW Namibia): ion probe and LA-ICP-MS dating of magmatic and metamorphic zircons

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 felsic 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.

纳米比亚西北部卡科克造山带（Kaoko Belt）内，边界火成杂岩（Boundary Igneous Complex）的花岗岩质侵入体，分隔了不同形成时代的高级变质岩段。该杂岩中的两处花岗岩体分别被定年为：575 ± 10 Ma（二次离子质谱法；SIMS）或571 ± 9 Ma（激光剥蚀电感耦合等离子体质谱法；LA-ICP-MS），以及562 ± 11 Ma（SIMS）或572 ± 4 Ma（LA-ICP-MS）。通过分析角闪岩中熔体分凝体里的老锆石（约1850~1000 Ma）的变质增生边，确定了西卡科克带东部的麻粒岩相变质时代为549 ± 5 Ma（SIMS）。西卡科克带的沿海区域由混合岩状沉积变质岩地层构成，这些地层与细粒正片麻岩和角闪岩互层，类似双峰式火山岩的变质序列。两套双峰式岩系的长英质组分中的锆石，其SIMS定年结果分别为805 ± 4 Ma和810~840 Ma，该年龄被解释为对应其火成原岩的形成时代。下伏双峰式岩系的镁铁质组分中的熔体分凝体，包含两批锆石群体，定年结果分别为650 ± 5 Ma（SIMS）或645 ± 5 Ma（LA-ICP-MS），以及629 ± 6 Ma（SIMS）或630 ± 5 Ma（LA-ICP-MS）。较晚的年龄与上覆变质杂砂岩中熔体斑块测得的630 ± 4 Ma（SIMS）或625 ± 10 Ma（LA-ICP-MS）年龄无法区分。现有定年数据表明，边界火成杂岩掩盖了海岸地体（Coastal Terrane）与卡科克造山带其余部分之间的缝合带。该火成杂岩内的花岗岩质侵入体时代指示了580~550 Ma期间的岩浆活动。