DEFINITION OF 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.

See also KOLA PENINSULA

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