Bulletin of the Geological Society of Finland (Dec 2007)
Intracratonic Palaeoproterozoic granitoids in northern Finland: Prolonged and episodic crustal melting events revealed by Nd isotopes and U-Pb ages on zircon
Abstract
Zircons from twelve Palaeoproterozoic granitoids from the Central Lapland Granitoid Complex (CLGC) and from the Hetta Complex (HC) in northern Finland were dated using the NORDSIM ion microprobe. In addition, U-Pb age determinations on zircons from two samples were made using TIMS. Results reveal a wide range of U-Pb ages from 2.13 Ga to 1.76 Ga. The oldest samples include the porphyritic, weakly deformed Nilipää granite, which has an ion probe concordia age of 2126 ± 5 Ma, compatible with the previously published TIMS age of 2136 ± 5 Ma. The Tohmo granodiorite provided a slightly younger ion microprobe age of 2105 ± 4 Ma. The age of these two granitoids overlaps the depositional age of Karelian cratonic metasediments, suggesting that they represent an unusual tectonic setting for granitoid magmatism in the Fennoscandian Shield. The Ruoppapalo granodiorite, which is also porphyritic and weakly deformed, yielded an ion probe zircon concordia age of 1905 ± 5 Ma, in accord with the conventional TIMS result. The strongly deformed Molkoköngäs granite has an ion probe concordia age of 1855 ±13 Ma, which is within error limits the same as the TIMS age, 1843 ± 23 Ma. The Pernu monzogranite has a concordia age of 1813 ± 6 Ma. All other granitoids in the CLGC, including those of appinitic affinity, as well as a leucosome from the Kappera migmatite in the Hetta Complex, are dated at 1.79 – 1.76 Ga, although many granitoids have older inherited zircons. The abundance of deformed granitoids of this age show that intensive ductile deformation, metamorphism and melting occurred at around 1.79 – 1.76 Ga in the northern Fennoscandian Shield, coeval with post-collisional magmatism in the southern part of the Shield. Most granitoids in the Central Lapland Granitoid Complex have strongly negative εNd values (1.8 Ga), ranging from -8 to -5. It is evident that they have a major Archaean component in their source, indicating derivation from Archaean crust during crustal reworking in an intracratonic tectonic setting.
Keywords
