Petrogenesis of the synorogenic alkaline Doros intrusion: Insights into geodynamic processes and lithospheric mantle heterogeneity beneath the Damara belt, Namibia

The ∼530 Ma synorogenic alkaline Doros intrusion is characterized by a wide range of SiO₂ contents (48.5–75.3 wt%) and includes nepheline-normative monzodiorites to quartz-normative syenites, monzonites and granites. The K₂O + Na₂O contents are high and the MgO contents are low to moderate. Incompatible elements of the alkaline rocks are enriched (LILE: Sr and Ba up to 3460 and 4290 ppm, respectively) and Rare Earth Element patterns are strongly fractionated ([La/Yb]_N: 22–64) with no or minor Eu anomalies. Strontium and Nd isotopic compositions are moderately evolved (initial ⁸⁷Sr/⁸⁶Sr: 0.7049–0.7068, initial εNd: −2.7 to −4.9) and the Pb isotope ratios are moderately radiogenic (²⁰⁶Pb/²⁰⁴Pb: 17.48–18.62, ²⁰⁷Pb/²⁰⁴Pb: 15.58–15.67, ²⁰⁸Pb/²⁰⁴Pb: 37.41–37.93). The magma of the Doros intrusion evolved by combined AFC processes. Relative depletions of Ba and Sr with increasing differentiation imply fractionation of K-feldspar and plagioclase. Zr/Hf and Nb/Ta ratios are superchondritic in the primitive samples and decrease with increasing SiO₂ mainly as a result of amphibole and biotite fractionation coupled to some crustal assimilation. Systematic isotopic variations indicate 10–20 % assimilation of highly evolved basement material. The Doros intrusion likely resulted from partial melting of metasomatized lithospheric mantle material that had been enriched in a subduction zone environment, as is evident from relative depletions of Rb, NbTa, K, Pb, P, ZrHf and Ti (primitive mantle-normalized multi-element diagram) in less evolved rocks. Metasomatic enrichment could have occurred in the Palaeoproterozoic (∼1.7–1.9 Ga) during subduction of juvenile crustal material or during subduction of evolved crustal material shortly before alkaline melt generation. Distinct differences in the Sr, Nd and Pb isotope signatures of the synorogenic alkaline intrusions of the Damara orogen (Doros, Voetspoer, Otjimbingwe alkaline complex) reflect large-scale heterogeneity of the lithospheric mantle beneath the Damara belt during Pan-African times as a result of spatial variations in the nature of the subducted material. Transtensional reactivation of shear zones in the Ugab Zone during the Damara orogeny facilitated melting of the lithospheric mantle and magma ascent for the Doros and Voetspoer intrusions.