Petrology and geochemistry of the Lyngdal granodiorite (Southern Norway) and the role of fractional crystallisation in the genesis of Proterozoic ferro-potassic A-type granites

Abstract

Abstract In south-western Norway, the Sveconorwegian orogenic thickening (1024–970 Ma) is followed by an important post-collisional magmatism (950–930 Ma), divided in two suites (Vander Auwera et al., 2003): the Anorthosite–Mangerite–Charnockite suite (AMC suite) and the Hornblende–Biotite Granitoids suite (HBG suite). The HBG suite displays a continuous trend from gabbronorites to granites. This paper presents the petrography and geochemistry (major and trace elements, Sr–Nd isotopes) of the Lyngdal granodiorite and associated massifs (Tranevag and Red Granite massifs) which belong to the HBG suite, although being very close to the anorthosite massifs. Mafic microgranular enclaves (MME), resulting from magma mingling, can be abundant and probably correspond to the parent magma of the studied plutons. The Lyngdal granodiorite and associated massifs are subalkaline, metaluminous A-type granitoids with high FeO t /(FeO t +MgO) ratio and K 2 O content, forming a ferro-potassic A-type continuous trend from quartz monzodiorite to granite (∼56–72 wt.% SiO 2 ). In Harker diagrams, the Lyngdal–Tranevag plutons share the Proterozoic rapakivi granites trend that they extend to lower SiO 2 content. Major and trace element modelling, as well as Sr–Nd isotopes, show that fractional crystallisation—without assimilation—is the differentiation process for Lyngdal–Tranevag. Fractionating minerals are clinopyroxene, hornblende, plagioclase, oxides, biotite, apatite, zircon and allanite. The Red Granite (71–75 wt.% SiO 2 ) does not belong to this trend probably due to a different initial magma composition. This study shows that ferro-potassic A-type granitoids can be derived by fractional crystallisation from mafic magmas.