The 3.5 Ga Siurua trondhjemite gneiss in the Archaean Pudas-

• Main Authors: Järvi Granulite Belt, Northern Finl, Tapani Mutanen, Hannu Huhma
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Corresponding author; Rua; Fennoscandian
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.597.9918; http://www.geologinenseura.fi/bulletin/Volume75/MutanenHuhma.pdf

In the Archaean Pudasjärvi Complex the pyroxene-bearing rocks are considered to form a belt, the Pudasjärvi Granulite Belt (PGB). The major rock types of the PGB are meta-igneous mafi c and felsic granulites, and trondhjemite gneisses. Red alaskites, white leu-cogranites and trondhjemitic pegmatoids are locally abundant. Ion microprobe U-Pb anal-yses on zircons suggest a magmatic age of ca. 3.5 Ga for the trondhjemite gneiss in Siu-rua, considered the oldest rock so far identifi ed in the Fennoscandian Shield. The old age is supported by the Sm-Nd depleted mantle model age of 3.5 Ga, and by convention-al U-Pb zircon data, which have provided a minimum age of 3.32 Ga. The U-Pb sims-da-ta on the Siurua gneiss are, however, heterogeneous and suggest several stages of zircon growth, mostly at 3.5–3.4 Ga. An inherited core in one crystal provided an age of 3.73 Ga, whereas the youngest two analyses yield ages of 3.1 and 3.3 Ga. Metamorphic mon-azite formed in the Siurua gneiss ca. 2.66 Ga ago, roughly contemporaneously with the high-grade metamorphism recorded by zircon in a mafi c granulite. Magmatic zircons from a felsic high-grade rock provide ages of ca. 2.96 Ga, but no zircons coeval with the 2.65 Ga metamorphism were detected by ion-microprobe. As a whole the PGB seems to be a tectonic block-mosaic containing rocks with Sm-Nd crustal formation ages ranging from 3.5 to 2.8 Ga.