Bulletin of the Geological Society of Finland (Dec 2003)
The 3.5 Ga Siurua trondhjemite gneiss in the Archaean Pudasjärvi Granulite Belt, northern Finland
Abstract
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 mafic and felsic granulites, and trondhjemite gneisses Red alaskites, white leucogranites and trondhjemitic pegmatoids are locally abundant. Ion microprobe U-Pb analyses on zircons suggest a magmatic age of ca 3.5 Ga for the trondhjemite gneiss in Siurua, considered the oldest rock so far identified in the Fennoscandian Shield. The old age is supported by the Sm-Nd depleted mantle model age of 3.5 Ga, and by conventional U-Pb zircon data, which have provided a minimum age of 3.32 Ga. The U-Pb sims-data 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 monazite formed in the Siurua gneiss ca. 2.66 Ga ago, roughly contemporaneously with the high-grade metamorphism recorded by zircon in a mafic 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.
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