The Leka Opholite Complex, central Norwegian Caledonides: field characteristics and geotectonic significance

Abstract

The Leka Ophiolite Complex (LOC) of Lower Ordovician age (U-Pb zircon age of 497 ± 2 Ma) contains the following components: (1) A lower, strongly deformed harzburgite unit, representing the depleted upper mantle. This unit becomes progressively richer in dunite towards (2) a sequence of ultramafic cumulates, ‘unconformably’ overlying the harzburgite tectonite. The crystallization sequence in the ultramafic cumulates is: olivine→chromite→clinopyroxene→orthopyroxene. In the south-western part of the ultramafic cumulate sequence, layers of metagabbro appear interlayered with banded dunite/wehrlite. The above-mentioned rock sequences may be in tectonic contact with (3) layered metagabbros which give way upwards to laminated and vari-textured metagabbro. Within the laminated/vari-textured metagabbro is the first appearance of (4) metabasalt dykes, which increase upwards in abundance from scattered single dykes to a 100% sheeted dyke swarm. Associated with the vari-textured metagabbro and metabasalt dykes are minor acid intrusions. Within the upper part of the dyke swarm is the first appearance of (5) pillow lavas, which are associated with thick volcaniclastic metasediments at apparently higher stratigraphic levels. Geochemically the metabasalt dykes and associated pillow lavas range from predominant island arc tholeiite (IAT) to MORB character, and typical metaboninites are also present. This range of geochemical composition, as well as the crystallization sequence of the cumulates, strongly suggests that the first stages in the formation of the LOC was by spreading above a subduction zone. The pillow lavas associated with volcaniclastic rocks, are of MORB or WPB type, and these probably represent the later spreading stages of the LOC in a back-arc basin, but at sufficient distance to be unaffected by the geochemical ‘fingerprints’ of the subduction process.