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Free keywords:
Synorogenic alkaline magmatism
Metasomatized mantle
Mantle heterogeneity
Damara orogen
Sr-Nd-Pb isotopes
Abstract:
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, Nb–Ta, K, Pb, P, Zr–Hf 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 syn-
orogenic 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.
