Detrital Zircon Geochronology: New Evidence for an Old Model for Accretion of the Southwest Baltic Shield.

Abstract

Disruption of the original crustal architecture as a result of tectonic events often complicates studies of the crustal evolution of Precambrian cratons. Ion-probe U-Pb dating of detrital zircons is one way to circumvent this problem. It yields a "fingerprint" of the source of the metasedimentary host that can be used to tie the host to a counterpart elsewhere. In this study, ion-probe U-Pb dating of detrital zircons from segments of the late Mesoproterozoic Southwest Scandinavian Domain (SSD) of the Baltic Shield provides new evidence for an old model of crustal evolution and tectonic development. Zircon ages for both the pebble and matrix fraction of the Seljord conglomerate, Telemark sector, Norway, show a 1.9-1.7-Ga frequency maximum, as was previously recognized for metasediments from the adjacent Bamble sector. However, the Telemark metasediments lack evidence for significant input of 1.4-1.6-Ga-old crustal material, characteristic of some of the Bamble metasediments. A maximum depositional age of ca. 1450 Ma is inferred for the Seljord conglomerate and a quartzite from the Bamble sector. Definite evidence for significant amounts of pre-1.6-Ga crust in the SSD is absent. Instead, provenance from the early Mesoproterozoic Svecofennian Domain for the 1.9-1.7-Ga crust is proposed, supported by the omnipresence of small amounts of Archean zircons and the distribution of Nd model ages of these and other SSD metasediments. It is proposed that the SSD originally formed a single, continuous belt, fringing the Baltic Shield, with the northernmost segments transported to their current position west of the Oslo rift along north-south-trending shear zones, i.e., at odds with the commonly held view of progressive westward growth of the Baltic Shield. The detrital zircon ages from the Faurefjell quartzite, Rogaland-Vest Agder sector, cluster around 1.65 Ga and reveal a maximum depositional age of 1244+/-40 Ma. Its deviating depositional setting indicates that the Rogaland sector evolved in a different setting, shielded from the crust of Svecofennian age. Instead, its ages allow for a source terrain within the Mesoproterozoic crust of South Norway.