The short duration and anorogenic character of anorthosite magmatism: U [sbnd]Pb dating of the Rogaland complex, Norway

Abstract

U Pb dating of zircon and baddeleyite extracted from orthopyroxene megacrysts yield emplacement ages of 929 ± 2, 932 ± 3, and 932 ± 3 Ma for the Egersund-Ogna, Helleren, andÅna-Sira massif-type anorthosites, respectively. This surprisingly short period of magmatic activity also includes the injection of 931 ± 5 Ma old jotunite dikes. Intrusion of the ilmenite norite, which constitutes the Tellnes Fe Ti deposit took place about 10 Myr later at 920 ± 3 Ma. In the margin of massif anorthosites secondary baddeleyite was formed at 915 ± 4 Ma, in association with later deformation produced by the diapiric mode of emplacement. Zircon cannot have formed in the primary magmas and more differentiated, quartz-mangeritic liquid within mega-orthopyroxene aggregates seems to be the most plausible liquid to precipitate zircon. The magmatic event at 931 ± 2 Ma postdates latest regional deformation by about 60 Myr and, therefore, anorthosite genesis is clearly anorogenic in nature, representing a thermal pulse that followed the Sveconorwegian-Grenville orogeny. Inherited zircon components and initial Pb isotopic compositions substantiate that the anorthosite parent magmas were generated to a large degree by melting of old continental material having ages of about 1690, 1450, and 1240 Ma. The most likely source lithology for these crustal melts are gabbroic rocks that were successively emplaced into the lower crust during the Meso-Proterozoic, prior to anorthosite formation.