NJG89-4-06
2009
Mineralogy and geological setting of allanite-(Ce)-pegmatites in western Hurrungane, Jotun Nappe Complex, Norway: an EMP and ID-TIMS study
89
4
pp 341-356
978-82-92394-57-1

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, is: (Ca0.86REE0.80Mn0.09Th0.01Na0.03)?1.79(Al1.79Fe2+ 0.99Mg0.16Ti0.10)?3.04(Si3.17P0.01)O12(OH), (Ca1 .12REE0.72Mn0.03Th0.01Na0.02)?1.90(Al1.73Fe2+ 0.66Fe3+ 0.33Mg0.26Ti0.03)?3.01Si3.07O12(OH) and (Ca1.10REE0.74Mn0.02Th0.01Na0.01)?1.88(Al1.75Fe2+ 0.69Fe3+ 0.26Mg0.30Ti0.04) ?3.04Si3.08O12(OH). The ratio of Fe3+/(Fe3++Fe2+) is variable. Dominant substitution mechanisms are Ca2+ + M3+ = REE3+ + M2+ and 3Ca2+ = 2REE3+. Isotope dilution thermal ionisation mass spectrometry (ID-TIMS) documents excess 206Pb reflecting high Th/U ratios, showing that the allanite largely retains its magmatic Pb isotopic signature despite the regional Caledonian upper greenschist- to lower amphibolite facies metamorphism. Local compositional heterogeneities in the allanite crystals are suggested to reflect Caledonian annealing of metamict domains accompanied by minor Pb-loss.

029-196X
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