Provenance of zircon of the lowermost sedimentary cover, Estonia, East-European Craton

Bulk and accessory mineral composition of fresh and weathered crystalline rocks, and sedimentary deposits overlying the crystalline-sedimentary unconformity have been examined in core samples from 28 drill holes in Estonia. Before the Late Vendian to Early Cambrian regional subsidence and sedimentation, the region represented a flat plateau within the Svecofennian Domain. Palaeo- and Mesoproterozoic crystalline rocks, regardless their different initial mineral composition, subcrop under the Upper Vendian/Lower Cambrian sedimentary cover as usually intensely weathered rocks (saprolites) composed of residual quartz, altered micas and prevailing clay minerals mainly of the kaolinite group. Thus, the bulk mineral composition of any basement crystalline rocks imparts no specific inherited rock-forming minerals into the covering sedimentary rocks. From the variety of accessory and opaque minerals of crystalline rocks, only zircon populations survived in saprolites. Crystalline rocks of different origin yield different zircons. Relationships between the zircon typology of the basement rocks having specific areas of distribution and the sedimentary rocks immediately overlying those crystalline rocks were the main subject of this study. The result is that siliciclastic sedimentary rocks covering weathered crystalline rocks only in places inherited zircons with typological features characteristic of specific basement areas. In northeastern Estonia, local lenses of the Oru Member (the earliest Upper Vendian sedimentary rocks in Estonia resembling the debris of weathered crystalline rocks) yield accessory zircon which in a 1-2 m thick layer above the basement surface is similar to the zircons of the underlying weathering mantle of certain crystalline rocks. In the next unit, the Moldova Member, up to 43 m above the basement surface, a mixture of zircons resembling those of various local basement rocks has been found. Further upwards, in the Vendian and Lower Cambrian sequence, zircons resembling those of local basement sources are very rare or absent. Obviously, basal Vendian/Cambrian sedimentary rocks sealed off the basement as a source of zircon. Therefore a distant source, probably outside the Svecofennian Domain, could be supposed for the bulk clastic minerals and zircons of the upper part of the Vendian and the lower part of the Cambrian. Probably, studies of isotopic ages of different typological varieties of zircons, both of obviously local and distant origin, could provide new information on respective source rock ages and areas, and on the general palaeogeographic pattern of the Vendian and Cambrian epicratonic sedimentary basins of the East-European Craton.