Petrochronological Evidence for a Three-Stage Magmatic Evolution of the Youngest Nepheline Syenites from the Ditrau Alkaline Massif, Romania

Abstract

The Ditr«u Alkaline Massif (DAM) is an igneous massif in the Eastern Carpathian Moun-tains of Romania. Numerous geochronological and geochemical studies have proposed a long formation history (ca. 70 m.y.) of the DAM from Middle Triassic to Cretaceous times, which is hardly reconcilable with geochemical evolutionary models and the geotectonic environment during the Mesozoic in this part of the Eastern Carpathian Mountains. In order to put tighter age constraints on the igneous processes forming the DAM, two nepheline syenites from the so-called Ghidut¸ and L«zarea suites were investigated. Based on field and geochemical evidence, the two rock suites represent the younger part of the DAM intrusives. Detailed zircon characterization, in situ zircon SIMS U-Pb dating, and geochemical modelling were used to establish the timing of zircon crystallization and thus to set time constraints on the igneous formation of these parts of the DAM. The intrusion of the dated Ghidut¸ suite sample took place at 232 ± 1 Ma in the Karnium, whereas the L«zarea suite nepheline syenite sample was intruded at 225 ± 1 Ma in the Norium. Together with published geochemical and geochronological data, three different magmatic events can thus be identified: Ghidut¸ suite at 231.1 ± 0.8 Ma, Ditr«u suite at 230.7 ± 0.2 Ma, and L«zarea suite at 224.9 ± 1.1 Ma. Although the ages of the events 1 and 2 are statistically indistinguishable, the combination of geochemical and petrochronological data certainly favor independent intrusion events. Thus, the igneous events forming the younger parts of the DAM encompassed a time span of ca. 13 m.y. Additionally, each igneous event can tentatively be divided in an older syenitic stage and a younger nepheline syenitic one, each with an age difference of some 100,000 years. No indication of any post 215 Ma igneous or hydrothermal activity was found. The new data and interpretation significantly improve our understanding of the temporal and geochemical evolution of the DAM and of alkaline complexes as such, demonstrating that the underlying igneous processes (melt generation, assimilation, fractionation, and the duration of plumbing systems) work on the same time scale for both sorts of magmatic rock suites.