Petrogenesis of the mantle sequence of the Jormua Ophiolite (Finland): Melt migration in the upper mantle during Palaeoproterozoic continental break-up

Upper-mantle rocks are exposed as several fault-bonded blocks within the Palaeoproterozoic Jormua Ophiolite, northeastern Finland. These blocks differ from other on the basis of degree of partial melting of the peridotites, type of intrusive mantle dykes (dyke cumulates), and spatially associated ophiolite units. Detailed field mapping, and geochemical and Sm-Nd isotopic studies, together with U-Pb zircon dating, indicate that these blocks represent an almost contiguous sequence across an ancient ocean-continent transition, recording the accretion of a mantle diapir to the sub-continental, most probably Archaean, lithospheric mantle during the formation of an incipient oceanic basin similar to 1950 Ma ago. Rupture of the subcontinental lithosphere was preceded by OIB-like magmatism and coeval emplacement of clinopyroxene + amphibole +/- garnet mantle dykes as well as pervasive metasomatism of subcontinental peridotites (western block peridotites) at depths of 30-50 km, and emplacement of peralkaline granites in the crust. Soon after, parts of the recently accreted suboceanic mantle diapir (eastern mantle blocks) were uplifted and intruded by gabbroic mantle dykes, 'upper-level' ferrogabbros and plagiogranites, and the sheeted dyke complex--all being related and progressively shallower expressions of oceanic crust-forming magmatism. After a relatively quiescent period of similar to 50 Ma within the newly formed passive continental margin, fragments of oceanic crust and mantle and continental mantle were obducted and transported over the Archaean craton to the present location of the Jormua Ophiolite.