Neo-Proterozoic rift-related syenites (Northern Damara Belt, Namibia): Geochemical and Nd-Sr-Pb-O isotope constraints for mantle sources and petrogenesis

Major element, trace element and Nd-Sr-Pb-O isotope data for a suite of Neo-Proterozic, pre-orogenic, rift-related syenites from the Northern Damara orogen (Namibia) constrain their sources and petrogenesis. New U-Pb ages obtained on euhdreal titanite of inferred magmatic origin constrain the age of intrusion of the Lofdal and Oas syenites to ca. 750 Ma compatible with previous high-precision zircon analyses from the Oas complex. Major rock types from Lofdal and Oas are mildly sodic nepheline-normative and quartz-normative syenites and were primarily generated by fractional crystallization from a mantle-derived alkaline magma. Primitive samples from Lofdal and Oas show depletion of Rb, K and Th relative to Ba and Nb together with variable negative anomalies of P and Ti on a primitive mantle-normalized diagram. Evolved samples from Oas develop significant negative Ba, Sr, P and Ti anomalies and positive U and Th anomalies mainly as a function of crystal fractionation processes. The lack of a pronounced negative Nb anomaly in samples from Lofdal suggests that involvement of a crustal component is negligible. For the nepheline-non-native samples from Lofdal, the unradiogenic Sr and radiogenic Nd isotope composition and low delta O-18 values suggest derivation of these samples from a moderately depleted lithospheric upper mantle with crustal-like U/Pb ratios (Sr-87/Sr-86: 0.7031-0.7035, epsilon Nd: ca. +1, delta O-18: 7%., Pb-206/(204) Ph: ca. 18,00, Pb-207/Ph-204: 15.58-15.60). Primitive samples of the Oas quartz-normative syenites have identical isotope characteristics (Sr-87/(86) Sr: 0.7034, epsilon Nd: ca. +1, delta O-18: 6.5%, Pb-206/Ph-204: ca. 18.00, Pb-207/(204) Ph: 15.59) whereas more differentiated samples have higher Sr-87/Sr-86 ratios (0.709-0.714), slightly higher delta O-18 values (7.0-7.1%), less radiogenic epsilon Nd values (-1.1 to -1.4) and more radiogenic Pb-206/Ph-204 ratios up to 18.27. These features together with model calculations using Sr-Nd-Pb isotopes suggest modification of a primary syenite magma by combined AFC processes involving ancient continental crust. In this case, high Nb abundances of the parental syenite liquid prevent the development of significant negative Nb anomalies that may be expected due to interaction with continental crust. (C) 2006 Elsevier B.V. All rights reserved.