Tracking coupling and decoupling during lithosphere evolution with geochemistry and geochronology: a case history from Arctic Norway

Recent studies integrating mantle and lower crustal geochronology on deep xenoliths, isotopic data from crustal zircons worldwide, and seismic tomographic imaging of deep lithosphere domains, suggest that >70% of the deep lithosphere formed by 3 Ga. Subsequent tectonism modified the lithospheric mantle causing crustal reworking. The Bockfjord area (NW Spitsbergen, Arctic Norway) provides an ideal natural laboratory to track crust/mantle evolution and tectonism over >3.2Ga. Quaternary alkali-basalt volcanism yields abundant mantle and crustal xenoliths across a majortranslithospheric N-S fault. Zircons from lower-crustal xenoliths (both sides) show mainly Neoarch./Paleoprot. or Paleozoic U-Pb ages and some Hf model ages >3.2 Ga. Metasomatic minerals are common in mantle xenoliths from east of the fault but are rare to the west. Re-Os analysis of sulfides in xenoliths west of the fault show TRD model ages to 3.3 Ga; major populations are 2.4-2.6 Ga, 1.6-1.8 Ga and 1.2-1.3 Ga. However, sulfides in xenoliths east of the fault show maximum TRD of 2.3 Ga with major peaks at 900-1100 and 400-500 Ma, identical to the spectrum of zircon ages of protoliths for exposed gneisses and schists east of the fault. The striking differences in the SCLM on either side of the fault suggest majortranscurrent movement, juxtaposing lithospheric sections that evolved discretely. Archean lower crust overlying Archean SCLM west of the fault suggests coupling of the crust and mantle for 3 Ga.