PROVENANCE ANALYSIS FROM DETRITAL ZIRCONS IN LOWER PALEOPROTEROZOIC SILICICLASTIC SEDIMENTS - INDICATIONS FOR CRUSTAL EVOLUTION OF THE BALTIC SHIELD

We present U-Pb age and Hf isotope data on detrital zircons in combination with geochemical whole rock data from Paleoproterozoic siliciclastic sedimentary rocks. The samples were drilled in the Pechenga and Imandra-Varzuga Greenstone belts of the Baltic Shield in NW Russia and comprise five formations, from the Seidorechka (2.43 Ga) to the Kolasjoki Formation (1.9 Ga). U-Pb age results range from 3.5 to 1.9 Ga and show coinciding age groups, which overlap with global phases of crustal growth at c. 3.2, 2.7 and 1.9 Ga. Hf isotope analyses on dated zircon grains are in progress and will further constrain the crustal evolution recorded by the sampled formations.

The Baltic Shield underwent a complex growth history starting at c. 3.5 Ga with the formation of new continental crust. The following Saamian (3.1-2.9 Ga) and Lopian orogenies (2.9-2.6 Ga) mark stages of crustal growth, and after accretion of Neoarchean cratons at c. 2.7 Ga the Baltic Shield became part of the supercontinent Kenorland. This period (3.5-2.6 Ga) is reflected by 73 % of all dated zircons. In contrast, ages of c. 2.5 Ga, deriving from the Archean Baltica basement, and of 2.4-2.1 Ga are rare (5 and <2 %), the latter group marks a period of orogenic quiescence. Younger zircons with ages of 2.1-2.0 Ga (9 %) and 2.0-1.9 Ga (10 %) mirror the break-up of Kenorland after 2.05 Ga, initiated by rift processes, and the following Svecofennian orogeny (<2.0 Ga), marked by microcontinent and juvenile arc accretion.

The geochemical results of the Paleoproterozoic whole rock samples are comparable to those of late Archean sedimentary rocks (see Taylor & McLennan, 1985). Normalized to chondrite, rare earth element patterns are similar for almost all samples, showing enriched LREE, lower HREE abundances and a minor negative Eu anomaly. These Archean distribution patterns in Paleoproterozoic sedimentary rocks indicate derivation from Archean crust. Only samples from the upper Kolasjoki Sedimentary Formation (1.9 Ga) show a strongly negative Eu anomaly compared to the lower Kolasjoki Sedimentary Formation (c. 2.05 Ga), indicating different provenances for the respective parts of this formation. Combined geochronological and geochemical results also point to multiple source areas for all sampled formations. Their nature will be further constrained by Hf isotope data.