Portrait of a giant deep-seated magmatic conduit system: The Seiland Igneous Province

Abstract

The Seiland Igneous Province (SIP), Northern Norway, contains >5000km2 of mafic and ultramafic intrusions with minor alkaline, carbonatite and felsic rocks that were intruded into the lower continental crust at a depth of 25 to as much as 35km.The SIP can be geochemically and temporally correlated to numerous dyke swarms throughout Scandinavia at 560–610Ma, and is linked to magmatic provinces in W-Greenland and NE-America that are collectively known as the Central Iapetus Magmatic Province (CIMP).Revised mapping show that the SIP exposes 85–90% layered tholeiitic- alkaline- and syeno-gabbros, 8–10% peridotitic complexes, 2-5% carbonatite, syenite and diorite that formed within a narrow (<10Ma) time frame in the Ediacaran (560–570Ma). Large peridotite complexes were emplaced into the still hot and unconsolidated gabbro (no dating available) and are regarded as the main-conduit systems.Gravimetric data implies an average thickness of igneous rocks of 4–5km and also features six deep lithospheric roots of ultramafic rocks extending min 9km into the crust. Together, the root structures represent the main volcanic conduits conveying thousands of km3 of mafic-ultramafic melts from the asthenosphere to the lithosphere.The ultramafic complexes were predominantly emplaced into the layered gabbros at four major igneous centres, respectively, Nordre Brumandsfjord, Melkvann, Kvalfjord and Reinfjord. All complexes are situated in a right-way-up position and are steep sided forming large plugs. A marginal hybrid zone forms at the contact with country-rock and transitions gradually from olivine-mela-gabbro over pyroxenites that grades in to an olivine-clinopyroxenite zone, which is followed by a wehrlite zone and, finally, the centre of the complexes comprises pure dunite. From pyroxenite to dunite, olivine changes from Fo72 to Fo85 and clinopyroxene from Di80 to Di92 i.e. the complexes observe a reverse fractional crystallisation sequence with time.Parental melt compositions modelled from early dykes indicate komatiitic to picritic melts with 16–22wt% MgO, Cr of 1594ppm and Ni of 611ppm, which were emplaced at 1450–1500°C. Melt compositions calculated from clinopyroxene compositions from Reinfjord are OIB-like with LREE enriched over HREE.The high abundance of carbonatites and lamproites demonstrates the volatile-rich nature of the mantle source region and is further corroborated by the unusually high abundance of magmatic sulphides (0.5–1%) and carbonated and hydrous assemblages (c. 1%) throughout the region. In Reinfjord, they are also closely associated with PGE-Cu-Ni reef deposits.Essentially, the ultramafic complexes in the SIP comprises deep-seated transient magma chambers that facilitated mixing and homogenisation of a rich diversity of fertile asthenospheric melts en route to the upper parts of the continental crust.