Origin of the Magmas of the Vestfold Lava Plateau

Abstract

The Vestfold lava plateau is the largest (about 600 km²) area of volcanic rocks preserved in the Oslo Paleorift. It also contains the longest, stratigraphically-continuous record of eruptions. The upper half is unique because of its diverse lithologies and petrochemistries: Between rhomb porphyry flows RP11 to RP26 occur four basaltic flows (B3-B6) and four trachytic flows, ranging from latitic to rhyolitic in composition. A genetic model to explain this compositional diversity must be constrained by the Ramberg [2] gravity anomaly analysis, which pictures a lens-shaped gabbro body, ten km thick, under the major part of the region and some 22 km below the surface. A corresponding basaltic magma chamber must have had a number of differentiation trends, to produce the complex Vestfold lava plateau and later plutonics. More than 100 complete wet silicate analyses and XRF trace element analyses (Rb, Sr, Y, Zr, and Nb) illuminate the complicated magmatic relations. The excluded elements Zr, Nb, and Y indicate:

Rhomb porphyries make up a rather concetrated cluster and are clearly different from the basalt group lineage, so that the surface basalts do not seem to represent the mother magmas for the rhomb porphyries. Trachytes make up four groups. T3 and T4, rich ni Zr and Nb, may be derived from the RP magamas by fractionation. A second group with very low contenets, can not have developed by fractionation from basaltic magmas of compositions similar to the surface basalts, but need a more primitive mother magma, perhaps like the Kolsaas Bl tholeiite of Weignad [1]. A third group with aberrant values comparable to those of rhomb porphyry flows is of uncertain parentage.

These conclusions are supported by computer mixing models of possible crystal fractionation paths.