The Neoarchaean
Nurmes paragnei
ss belt,
situated between the Ilomant
si and Kuhmo granite–green
stone terrain
s in ea
stern Finland compri
se
s mainly turbidite wacke-derived, migmati
sed paragnei
sse
s with minor amphibolite intercalation
s and younger granitoid intru
sion
s. The average chemical compo
sition of typical biotite–plagiocla
se gnei
ss me
so
some
s of the paragnei
sse
s i
s nearly identical with the global average for Neoarchaean greywacke
s. The paragnei
ss me
so
some
s typically contain graphite, which exhibit
s δ<
sup>13
sup>C value
s from −36.0‰ to −14.2‰, around a mean of –22.6 ± 5.6‰. The
se low
δ<
sup>13
sup>C value
s and the
stratiform occurrence of graphite within di
screte layer
s containing iron
sulphide
s, are indicative of that it had a biogenic origin. The intercalated amphibolite
s corre
spond to oceanic tholeiite
s with flat to mildly LREE enriched chondrite-normali
sed di
stribution
s, and have
<
sub>Nd
sub> (2.7 Ga) value
s around +1.6 pointing to derivation from depleted mantle.
After allowing for analytical uncertainties (2σ), U–Pb age data (SHRIMP, TIMS) on zircon grains from the paragneiss mesosomes and crosscutting granitoid plutons constrain deposition of the protolith wackes to between 2.71 Ga and 2.69 Ga. This estimate for the timing of sedimentation is also supported by a six-sample Sm–Nd isochron of 2756 ± 89 Ma obtained for the associated amphibolites. A relatively short average crustal prehistory for the sediment source terrains is implied by the clustering of <sub>Ndsub> (2.7 Ga) values of the mesosomes around zero. Trace element and U–Pb data suggest that the source terrains comprised mainly 2.75–2.70 Ga TTG and/or sanukitoid-type plutonic and mafic volcanic rocks, for which the adjacent Ilomantsi terrain is the most likely source candidate.
The data presented here indicate that the Nurmes paragneisses closely resemble wackes in the linear metasedimentary belts of the Superior Province of Canada, in terms of depositional age and detrital zircon provenance ages, as well as chemical composition.The postdepositional metamorphic–plutonic histories of the Nurmes and Canadian belts also seem to have been strikingly similar. These similarities lend further support to proposals advocating that the Karelia and Superior provinces were juxtaposed during the Neoarchaean. However, in contrast to the wackes in the Canadian belts, which are usually interpreted as deposits in accretionary prisms, the presence of MORB-type volcanic intercalations in Nurmes wackes suggests they were deposited in a back arc or intra-arc setting.