PORTRAIT OF A GIANT MAGMATIC CONDUIT SYSTEM AT THE ROOTS OF THE CRUST: THE SEILAND IGNEOUS PROVINCE, NORTHERN NORWAY

The Seiland Igneous Province (SIP) consists of >5,000 km²of mafic, ultramafic and alkaline melts that were emplaced into the lower continental crust (25-30 km depth) in <10 Ma (570-560 Ma) during mantle plume upwelling. We argue that the SIP was the deep plumbing system of a large igneous province, making the region a key location in which to study the ascent, emplacement and modification of dense mantle melts enroute to more shallow igneous systems.

Loci of the main conduit system features ultramafic complexes dominated by peridotitic cumulates that occupy 1/3 of the area of which, the Reinfjord Complex is an excellent example.

The picritic to komatiitic melts in the Reinfjord Complex were emplaced into gabbros in three major pulses punctuated by several smaller replenishment events. The first two pulses, the lower and upper layered series (LLS + ULS) comprises modally layered ol – cpx ±opx cumulates. The final phase, the central series (CS), comprises dunitic cumulates in the centre of the intrusion. The CS intruded into a crystal-melt mush of the ULS. The CS-forming melt was saturated with ol and assimilated ULS cpx to form discordant replacive dunites. Super-imposed upon these major events, cryptical zonation of ol and cpx reveal several replenishment episodes. We have identified 15 events over only 700 metres of cumulates. Field observations document smaller replenishment events of pyroxenitic melts. They occur as irregular dykes that intersect semisolid cumulates before dissipating in mushy melts higher up in the stratigraphy. One such event is associated with the formation of a 5 metres thick PGE-Ni reef with 0.8 ppm Pt, Pd and Os. We also observe that the melt-mushes of CS were intruded by several events of alkaline CO₂-H₂O rich melts forming veinlets of feldspatoids, amphibole, carbonates, opx and cpx.

Together, this rich diversity of igneous rocks documents the complexity of melts that are produced during plume assisted emplacements of very large volumes of mafic-ultramafic melts. Not least, in Reinfjord we learn to appreciate the importance of melt modification (mixing-assimilation-fractionation etc.) of mantle derived melts in the deep crust before the “homogenized” products are emplaced in shallow magma chambers or flows.