Early Proterozoic Metavolcanic Suites of the Northernmost Part of the Baltic Shield

Summary Metavolcanic rocks of Proterozoic age occur in three major supracrustal belts in the northernmost part of the Baltic Shield, in Norway and the USSR. In the west, the Kautokeino Belt , and its continuation into the tectonic windows of Finnmark, contains thick sequences of tholeiitic metabasalt the age of which is poorly constrained at present. In the Repparfjord Window, these basalts are strongly enriched in Fe, Ti, LIL and LREE and are chemically similar to modern continental flood basalts. Stratigraphic equivalents in the Alta area and Kautokeino Belt are less enriched in incompatible elements, and more like MORB in composition, although a within-plate setting is indicated by the field evidence. These differences are believed to reflect heterogeneity of the mantle source region, with a locally ‘enriched’ source in the Finnmark windows. The Karasjok Belt of central Finnmark forms the northward continuation of the Kittilä Belt of northern Finland. Amphibolitic rocks derived by metamorphism of tholeiitic basalts dominate the tectonostratigraphy of the greenstone belt. These are associated with metamorphosed komatiitic rocks which have recently yielded a Sm-Nd age of 2.1 Ga. The Karasjok Belt is interpreted as an early Proterozoic supracrustal sequence thrust westwards as an allochthonous sheet during uplift of the Lapland Granulite Complex in the Svecokarelian Orogeny at about 1.9 Ga. The Pechenga Complex of the USSR continues westward into Finnmark, and overlies the Archaean gneisses of the Kola Complex. The Kola Superdeep Borehole penetrated a metamorphosed supracrustal sequence 7 km thick, dominated by basalts and andesites of calcalkaline composition. The MORB-normalized geochemical patterns of these lavas indicate that they were erupted in a mature continental volcanic arc. The geochemical data support geotectonic models invoking rifting of pre-existing continental crust in the early Proterozoic. Komatiitic liquids were locally able to reach the earth’s surface, but probably only in the rift zones suffering greatest extension. Calcalkaline arc magmatic suites along the western edge of the Kola Craton reflect subduction of‘oceanic’ lithosphere, although no trace of this is preserved along the suture now marked by the Karasjok Belt.