Baltic shield

The Baltic (Fennoscandian) shield is an
Archean craton that traditionally is divided into three distinct
parts. The northern, Lapland-Kola province, mainly consists of
several previously dispersed Archean crustal terranes that
together with the different Paleoproterozoic belts have been
involved in a collisional-type orogeny at 2.0 billion to 1.9 billion
years ago. A central, northwest-trending segment known
as the Belomorian mobile belt is occupied by assemblages of
gneisses and amphibolites. This part of the Baltic shield has
experienced two major orogenic periods, in the Neoarchean
and Paleoproterozoic. The Neoarchean period included several
crust-forming events between 2.9 billion and 2.7 billion years
ago that can be interpreted in terms of first subduction-related
and later collisional orogeny. In the end of the Paleoproterozoic
at 1.9–1.8 billion years ago, strong structural and thermal
reworking occurred during an event of crustal stacking and
thrusting referred to as the Svecofennian orogeny caused by
overthrusting of the Lapland granulite belt onto the Belomorian
belt. Although the Svecofennian high-grade metamorphism and
folding affected all of the belt, its major Neoarchean crustal
structure reveals that early thrust and fold nappes developed
by 2.74–2.70 billion years ago. In contrast, the Karelian
province displays no isotopic evidence for strong Paleoproterozoic
reworking. The Karelian craton forms the core of the
shield and largely consists of volcanic and sedimentary rocks
(greenstones) and granites/gneisses that formed between 3.2
billion and 2.6 billion years ago and were metamorphosed at
low-grade. Local synformal patches of Paleoproterozoic 2.45
billion to 1.9 billion year old volcano-sedimentary rocks
unconformably overlie the Karelian basement. To the southwest
of the Archean Karelian craton, the Svecofennian domain
represents a large portion of Paleoproterozoic crust developed
between 2.0 billion and 1.75 billion years ago.
Although tectonic settings of the Karelian Archean
greenstone belts are still a matter of debate, there are some
indications that subduction-accretion processes similar to
modern-day convergent margins operated, at least, since 2.9
billion years ago. However, a large involvement of deep mantle-
plume derived oceanic plateaus in Archean crustal growth
processes remains questionable in respect to subduction style.
Further Reading
Shchipansky, Andrey, Andrei V. Samsonov, E. V. Bibikova, Irena I.
Babarina, Alexander N. Konilov, K. A. Krylov, Aleksandr I.
Slabunov, and M. M. Bogina. “2.8 Ga Boninite-Hosting Partial
Suprasubduction Zone Ophiolite Sequences from the North Karelian
Greenstone Belt, NE Baltic Shield, Russia.” In Precambrian
Ophiolites and Related Rocks, Developments in Precambrian
Geology, edited by Tim Kusky. Amsterdam: Elsevier, 2004.
Post a Comment

Post a Comment