Coupling, decoupling and metasomatism: a saga of crust-mantle relationships beneath NW Spitsbergen (Arctic Norway)

Recent studies integrating mantle and lower crustal geochronology on xenolith samples, isotopic information from crustal zircons worldwide, and seismic tomographic imaging of deep lithosphere domains, suggest that over 70% of the present deep lithosphere formed by about 3 Ga. Subsequent tectonism has modified the lithospheric mantle and caused crustal reworking. The Bockfjord area of NW Spitsbergen (Norwegian Arctic) provides an ideal natural laboratory to track crust/mantle evolution and tectonism over >3.2Ga. Quaternary alkali-basalt volcanism provides abundant xenoliths of mantle and crustal rocks from both sides of a major translithospheric N-S fault. Zircons from lower-crustal xenoliths (from both sides of the fault) have mainly Neoarchean/ Paleoproterozoic or Paleozoic U-Pb ages; several show ages and/or Hf model ages >3.2 Ga. Mantle-derived peridotite xenoliths east of the fault contain common metasomatic minerals rare in those west of the fault. Re-Os analysis of sulfides in xenoliths west of the fault show T (super RD) 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, with rare Caledonian ages. However, sulfides in xenoliths east of the fault show maximum T (super RD) 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 These data demonstrate a major disjunct, on both sides of the B-B fault, between the Archean lower crust and a Proterozoic-Paleozoic upper crust; this suggests that the original Archean upper (and middle) crust was detached from the lower crust and replaced by thrust sheets of younger material, probably during the major overthrusting of the Caledonian orogeny. The striking differences in the SCLM on either side of the B-B fault suggest major transcurrent movement, juxtaposing lithospheric sections that evolved discretely at some distance from one another. West of the B-B fault, the presence of Archean lower crust overlying Archean SCLM suggests coupling of the crust and mantle for > or =3 Ga