Magmatic evolution of the Løkken SSZ Ophiolite, Norwegian Caledonides: Relationships between anomalous lavas and high‐level intrusions

Abstract

AbstractThe Lower Ordovician Løkken ophiolite fragment shows a tripartite subdivision of its thick volcanic sequence into: (1) an Upper Volcanic Member (UVM) of transitional MORB/IAT character, comprising basalts or, in a related, thrust‐bounded sequence, a bimodal basalt‐rhyolite assemblage; (2) a Middle Volcanic Member (MVM) derived from widely variable, generally MORB‐related magmas; and (3) a Lower Volcanic Member (LVM) of N‐type MORB basalts which locally passes into an underlying sheeted dyke complex. The UVM and LVM were fed from relatively deep‐seated magma chambers, and lavas were erupted at moderate flow rates to form predominantly pillow lavas. The whole sequence is interpreted as having formed in a marginal basin setting.The MVM is characterized by abundant voluminous sheet basalts thought to have been erupted at high flow rates. The volcanites had their source, through extensive dyke swarms, in high‐level magmas which intruded the sheeted dyke complex and parts of the LVM, possibly in a near‐axial seamount setting. The magmas differentiated to form plagiogranitic melts, with a wide range of conjunctive, mafic to intermediate cumulates. This plutonic assemblage, possibly in crystal mush form, was subsequently intruded by new, mafic magma, leading partly to large‐scale remelting or assimilation of cumulus phases. These processes are reflected in the MVM by common hybrid flows, as well as local intermediate lavas having the geochemical characteristics of cumulates, erupted during a period of intense faulting. Major tapping of the primary, mafic magma occurred toward the end of the period of MVM volcanism. A predominance of voluminous, apparently fluid, massive flows in the MVM is mainly ascribed to the shallow depth of magma reservoirs.