Synorogenic melting of mafic lower crust: constraints from geochronology, petrology and Sr, Nd, Pb and O isotope geochemistry of quartz diorites (Damara orogen, Namibia)

Abstract

Quartz diorites represent the earliest (ca. 540 Ma) and most primitive plutonic rocks in the Pan African Damara belt and they pre-date the main phase of high-T regional metamorphism. Two suites of synorogenic quartz diorites are unusual among Damaran intrusive rocks in their elemental and isotopic features. Comparison of the diorite compositions with melts from amphibolite-dehydration melting experiments points to a garnet-bearing meta-tholeiite, probably enriched in K2O, as a likely source rock. Partial melting processes generated mafic (ca. 50 wt% SiO2) quartz diorites in the deep crust at temperatures of between 1,000 and 1,100 °C, based on comparison with experimental results and similar temperature estimates based on P2O5 solubility in mafic rocks. Subsequently, the quartz diorites evolved by multistage, polybaric differentiation processes including fractional crystallization of mainly hornblende and plagioclase and assimilation of felsic basement gneisses. Although their chemical characteristics (high LILE, low HFSE) resemble those of other quartz diorites with calc-alkaline affinities, they differ in their enriched Sr (initial 87Sr/86Sr: 0.70943–0.71285), Nd (initial e Nd: –9.1 to –15.2 ) and O (δ18O: 6.8–8.1‰) isotope compositions. Neodymium model ages (TDM) that range from 1.7 to 2.2 Ga and large variation in 207Pb/204Pb relative to 206Pb/204Pb indicates involvement of ancient crustal material. Lead (206Pb/204Pb: 17.08–17.23, 207Pb/204Pb: 15.53–15.62, 208Pb/204Pb: 37.71–38.16) isotope compositions are strongly retarded, indicating that the source underwent a pre-Pan-African U/Pb fractionation and U depletion. It is proposed that the quartz diorites originated by synorogenic high temperature melting of mafic lower crust. This contrasts with previous suggestions favouring an origin of these rocks by melting of an enriched mantle during Pan-African times with characteristics modified by subduction of oceanic crust and sedimentary rocks.