The role of crustal contamination in the petrogenesis of nepheline syenite to granite magmas in the Ditrău Complex, Romania: evidence from O-, Nd-, Sr- and Pb-isotopes

Abstract

The Ditrău Complex of the Carpathian Mountains in Romania is a Mesozoic igneous complex (~ 200–230 Ma) generated in a continental rift environment. Felsic rocks of the Ditrău Complex consist of nepheline syenite, syenite, quartz syenite, quartz monzonite, monzonite and granite. The Ditrău rocks have mantle-like ɛNd values that range from + 0.8 to + 5.5 ‰. High-temperature equilibrium O-isotope fractionations between minerals are generally preserved, although some subsolidus O-isotope re-equilibration occurred. Magma δ18O values estimated from quartz, feldspar and amphibole (5.7–11.7‰) are higher than those estimated from zircon. We suggest that this difference results from continuous crustal contamination, with zircon recording the early, high-temperature δ18O values, and quartz and the other silicate δ18O values, reflecting a combination of subsequent crustal contamination and deuteric alteration. Negative correlations between calculated magma δ18O values and Na2O and Al2O3 content and eNd are consistent with the spectrum of felsic rocks from nepheline syenite to granite resulting from an increase crustal input. Both O- and Nd-isotope compositions are consistent with a dominantly mantle origin of hornblendites, diorites and nepheline syenites. The Nd- and O-isotope composition of the silica-oversaturated rocks can be explained by the assimilation of 20–60% upper crustal melts into the re-injected mafic alkaline parent magma to generate the Ditrău syenites, quartz syenites, quartz monzonites and granites.