Geochemical stratigraphy of the lavas of the Solund-Stavfjord Ophiolite Complex, W. Norway, and magma-chamber dynamics

Abstract

The stratigraphy and geochemistry of the uppermost 200–300 m of the metabasalt sequence of the Solund-Stavfjord Ophiolite Complex of western Norway has been investigated over a lateral distance of ~6 km. Volcanologically, this sequence was constructed in a cyclic manner. The lowest part of a volcanic cycle is characterised by sheet flows and/or large pillows followed by lavas in which pillows become progressively smaller upwards. In several of the cycles, pillow lavas are differentiated Fe–Ti basalts at the base, and successive flows become gradually more primitive stratigraphically upwards. The compositional differences between the bottom and top of a cycle can be substantial (e.g. TiO2 is 2.98–1.00 wt%, Zr 227–76 ppm, and Cr 125–520 ppm respectively). Nd isotopic and other data suggest that all the basaltic magmas were generated from a uniform source. Concomitant with the upward decrease in the Ti and Fe contents of the metabasalts through a volcanic cycle, estimated magma densities also decrease. We attribute the geochemical stratigraphy of the metabasalts to mainly reflect magma mixing in a frequently replenished magma chamber. Hybrids in the chamber repeatedly mixed with inflowing magmas which were more primitive and less dense. During the time intervals between cycles, the magma chamber was essentially closed and extensive fractional crystallisation took place. In some sequences, however, volcanic cycles and stratigraphical geochemical trends are poorly defined. We tentatively propose that in such cases the volcanics were erupted from more than one magma chamber.