Magmatic indicators of subduction initiation: The bimodal Goas intrusive Suite in the Pan-African Damara Belt of Namibia

Abstract

Many orogens contain granitic intrusions with minor, contemporaneous, mafic bodies (gabbros, diorites and ultramafic rocks). Such bodies are typically small, may form ring complexes and may accompany or precede more voluminous crust-derived magmatism. The Goas Intrusive Suite, in the Damara Belt of central Namibia, forms a cluster of tabular plutons and ring complexes with gabbros, diorites and associated granitic rocks. The plutons form the oldest suite of intrusive rocks (580–555 Ma) in the belt, and their emplacement coincided with the onset of subduction of the oceanic crust of the Khomas Sea, which separated the Kalahari and Congo Cratons in the late Neoproterozoic. None of the silicic magmas formed through fractional crystallization or AFC of the mafic to intermediate magmas. Like their mafic counterparts, these silicic magmas display chemical and isotopic diversity inherited through partial melting of different crustal components, probably at different depths. Nevertheless, if silicic and mafic magmas coexisted, as in the present case, high-level magma mingling (mesoscopic mechanical dispersion) and limited mixing (chemical reaction and diffusion) can occur. The bimodal composition, early timing and intrusion along the leading edge of the Congo Craton are consistent with emplacement of the Goas Intrusive Suite during subduction initiation at ca 580–575 Ma. Passive-margin collapse and detachment of the gravitationally unstable Khomas oceanic plate was probably facilitated by the Okahandja Lineament Zone, which marks the southern edge (former passive margin) of the Congo Craton. Sinking of the oceanic plate and replacement by asthenospheric mantle is proposed to have caused decompression melting that resulted in underplating of the lithospheric mantle and under- and intra-plating of the overlying and extending fore-arc crust. Globally, the commonly more voluminous silicic members of such complexes appear to represent groups of crustally derived magmas that were formed in episodes of crustal heating brought about through the intrusion of the mafic magmas, at depth. These dioritic complexes therefore represent high-level expressions of mantle melting events that also triggered crustal melting, with production of highly diverse suites of magmas with origins at various levels in both the crust and mantle, complicated by multi-stage magma-crust interactions.