Evolution of lavas in the Late Ordovician/Early Silurian Solund‐Stavfjord Ophiolite Complex, West Norway

Abstract

The geochemistry of the volcanic sequence of the Late Ordovician/Early Silurian Solund‐Stavfjord Ophiolite Complex (SSOC) of the Western Norwegian Caledonides has been investigated along 12 stratigraphic sections spaced over a lateral distance of ∼60 km. The metabasalts commonly show a cyclic organization, comprising sheet flows followed by pillow lavas and with volcanic breccias at the top of some volcanic units. The proportions of the volcanic rock types vary considerably along‐axis, even within short distances. The robust nature of the sheeted dike complex, the proportion and regional distribution of the various volcanic rocks, the aphyric nature of the metabasalts, as well as the predominance of Fe‐Ti basalts suggest that the SSOC formed at an intermediate to fast spreading ridge, probably within a back‐arc basin. The variations in the concentrations of the incompatible (e.g., Zr) and compatible (e.g., Cr) elements in the lavas are substantial, i.e., from 49–384 ppm and 66–443 ppm, respectively. Nd isotopic ratios show only minor variations, suggesting that the lavas were generated from an isotopically uniform source. The variations in the incompatible elements (represented by Zr) define stratigraphic units ∼25 to 150 m thick, with either gradual decreases or increases in Zr concentrations, while Cr may show different trends. The different Zr‐Cr covariations are interpreted in the light of a numerical model in which fractional crystallization and mixing of various magmas are the principal processes. Through upwardly Zr‐increasing units, Cr generally decreases and estimated magma densities increase, compatible with progressive fractional crystallization. However, through upwardly Zr‐decreasing units, Cr and estimated magma densities either increase or decrease, trends that are attributed to hybridization in frequently replenished magma chambers. Within short lateral distances (<1 km), the geochemical stratigraphy changes dramatically, excluding eruption from a homogeneous, axially continuous magma chamber. Instead, we propose that the volcanic sequence was fed from small, separate magma lenses that evolved independently of each other.