Mingling with minimal mixing: Mafic-silicic magma interactions in the Oamikaub ring complex, Namibia

Abstract

The ca. 560 Ma Oamikaub ring complex (ORC) is a member of the Goas Intrusive Suite in the Damara Belt of Namibia. The volumetrically dominant granitic rocks contain a variety of microgranular magmatic enclaves (MEs), commonly with lobate and cuspate borders against their enclosing host rocks. Some also have mafic selvages and evidence for internal melt segregation. These features indicate magma mingling and deformation during host-magma flow and emplacement, and a small degree of chemical mixing with the host magmas. The spatial diversity and mineralogical characteristics of the MEs show that mechanical hybridism occurred prior to mingling at emplacement level. Despite one group of MEs forming near-linear arrays in chemical plots, the mineralogical and isotopic data show that these do not form a mixing series. A second group represents samples of chilled mafic magmas that chemically resemble the mafic rocks that form the earliest-emplaced components of the ORC. However, their fine-grained textures and isotopic characteristics show that these were a separate, later pulse of mafic magmas. There were multiple episodes of mingling and the younger pulses of mafic ORC magmas were generally more hybridised with evolved crust. Magma mingling and limited marginal hybridisation of MEs produced impressive outcrop features. However, apart from initial, source-level hybrid formation, magma mixing had little or no role in producing compositional variations in either the MEs or their host rocks. Globally, such relationships are common in enclave suites from granitic rocks, and their study can provide insights into processes and materials in magma source regions. In the present case, enriched mantle melting, crustal melting (of mafic to silicic rocks) and fractionation of tholeiitic parental magmas may all have played their parts in forming the ME suite of the ORC.