Spürgin, and 2 co-authors, reported in 2009 that four stages of mineral growth are distinguished in the Berdalsbandet pegmatites: 1) an initial pegmatitic stage with mineral growth from the melt and formation of a metasomatic biotite rim by interaction of the pegmatitic melt with the wallrock; 2) a hydrothermal stage, leading to the formation of massive quartz cores during the final dyke solidification; 3) a stage of allanite-(Ce) recrystallisation, and 4) a subsolidus, late- or post- Caledonian stage, where local zeolitisation of the dykes took place. The age of a secondary copper mineralisation remains uncertain.
The 950 ± 1 Ma Berdalsbandet Pegmatite Swarm (BPS) in Hurrungane, in the Jotun Nappe Complex, SW Norway, consists of ≤2 m wide subparallel granitic dykes emplaced in a sinistral non-coaxial shear zone, constraining the age of local Sveconorwegian deformation. The abyssal class dyke swarm is interpreted to be related to local Sveconorwegian anatexis; mineralogy and REE characteristics indicate low degree partial melting.
The pegmatites are symmetrically zoned and typically consist of an outer zone of feldspar + allanite-(Ce) + biotite, a transitional zone of feldspar + quartz + schorl + Fe-Ti-oxides, an inner zone of quartz + schorl + garnet + beryl and a core of hydrothermal quartz. A metasomatic rim of biotite is locally developed along contacts to the amphibolite wallrock. Secondary zeolites are suggested to relate to late- to post-Caledonian alteration.
The dykes contain various rare-earth minerals, predominantly allanite-(Ce). Its composition, determined by electron microprobe (EMP) and expressed by representative analyses from three samples The BPS belongs to the abyssal class of pegmatites (AB-LREE), supporting a genetic link to coeval, granitic anatexite present in the field area. A strong enrichment in LREE content of the BPS, inferred from allanite- (Ce) compositions and relative abundance in the BPS, indicates a low degree of partial melting consistent with field observations. The geometric relationship of the BPS to its amphibolite host rock and to the surrounding shear zones, along with detailed observations of minerals in the BPS, lead us to conclude that the BPS was emplaced in an active sinistral, non-coaxial shear zone, and that the pegmatites date Sveconorwegian shearing to 950 ± 1 Ma.
The authors proposed that a strong Caledonian overprint in the area surrounding the amphibolite lens hosting the BPS constitute reactivation of Sveconorwegian shear zones, though field observations point to a limited amount of displacement despite the locally pervasive metamorphic overprint.
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