Episodic anorthosite-mangerite-charnockite -granite (AMCG) plutonism in Rogaland, South Norway: the 1.24 Ga Gloppurdi and Botnavatn fayalite-bearing charnockitic intrusions

This study addresses the episodic nature of Proterozoic anorthosite-mangerite-charnockite-granite (AMCG) plutonism in the Sveconorwegian orogen in South Norway. Field, petrographic, major and trace element data are reported on the Gloppurdi and Botnavatn intrusions, dated together at 1236 ± 8 Ma (U–Pb zircon, 3 samples). These two small, foliated plutons are dominated by fayalite-bearing charnockitoid (mangerite, quartz mangerite and charnockite), and emplaced some 300 Myr before the large and nearby 932–915 Ma AMCG complex, known as Rogaland Anorthosite Province (RAP). The characteristic fayalite-bearing charnockitoid is ferroan (Mg# < 0.1), alkalic and metaluminous. The large variation in composition (60 < SiO2 < 75 wt.%) is explained by variable clustering of mafic minerals. The rocks display major element compositions similar to the fayalite-bearing quartz mangerite of the Bjerkreim–Sokndal (BKSK) layered intrusion in the RAP. Zr, Th, U and REE compositions link the group to Zr-rich jotunite, a typical kindred rock type of AMCG plutonism. This suggests the occurrence of concealed magma chambers at 1.24 Ga in which anorthosite and related rocks possibly developed. Episodic AMCG plutonism thus possibly punctuated the regional evolution between 1.24 and 0.85 Ga.