Long-lived LREE mobility in the cratonic, rift and foredeep to foreland sedimentary cover at the western margin of the Karelia Province

Abstract

The western margin of the Karelia Province of the Fennoscandian Shield consists of Archean crust covered by remnants of Paleoproterozoic (c. 2.5–1.9Ga) metasedimentary rocks. The study area in the eastern part of the margin shows 2.3–2.1Ga cratonic sedimentation followed by 2.1–2.05Ga rift sedimentation and c. 1.91–1.92Ga foredeep to foreland sedimentation related to continent–arc/continent collision. Extreme LREE mobility (loss up >95% of La) is recorded by many samples from these units. Monazite, bastnäsite and, in some cases, allanite have been the prealteration LREE host phases in the LREE depleted samples. Monazite was altered to Th-silicates with variable loss of the LREE, the remaining part of which was incorporated in Th-poor monazite.The depositional age of the sedimentary units was studied by dating (LA-MC-ICPMS) detrital zircon grains, and the age of LREE mobility by dating monazite and xenotime and by whole-rock Sm–Nd modeling. The maximum deposition age of the foredeep to foreland basin is 1.91–1.92Ga based on detrital zircon data and the minimum age of the main foliation in the study area is 1.87–1.86Ga based on the younger age limit of late-to post-tectonic xenotime and the ages of few probable metamorphic zircons. Coeval bastnäsite, with xenotime in a LREE enriched sample, suggests that this was also one important stage of LREE mobility. Monazite crystallizations have occurred 1.76–1.78Ga ago while the Rb–Sr whole rock data suggest that the closure of this isotope system took place 1732±21Ma ago. The Sm–Nd modeling gives ages at 0.4–1.0Ga for LREE depletion. The Sm–Nd age of 412±27Ma can be considered as the age of the latest major LREE depletion event.We propose that alkali-bearing oxidizing fluids started to form in the cratonic sequences due to diagenetic reactions maybe as early as 2.3–2.1Ga. Alkalinity of fluids was later increased by interaction with alkaline 2.05Ga source materials before and during the c. 1.91–1.92Ga foredeep to foreland basin stage. Basin inversion and metamorphic crystallization occurred at 1.91–1.87Ga. Episodes of fluid migration at 1.78–1.73Ga and afterwards were focused along fracture zones and in microfracture networks. The 0.4Ga stage is the most prominent of the <1.7Ga events and is best explained by assuming shield-scale foreland basin related to the Caledonian orogenic front, causing heat redistribution and also fluid circulation in the underlying crystalline basement. The 2.1Ga formed marginal fault has been a very important shield-scale tectonic element and a pathway for fluid circulation for almost 2billion years.