An early proterozoic ophiolite — the jormua mafic-ultramafic complex, Northeastern Finland

Abstract

The 1.96 Ga old Jormua complex comprises a fault-bounded unit (2–5 × 20 km) in the centre of the Kainuu Schist Belt at the eastern margin of the early Proterozoic Svecokarelides. The complex was dismembered and metamorphosed to lower amphibolite facies during the Svecokarelian orogeny. In the intact complex the basal unit was formed by serpentinites, the middle unit of mafic dykes, and the uppermost extrusive unit of basaltic pillow lavas. Gabbroic rocks are present as intrusive bodies in the serpentinites and narrow interdyke screens within the dyke unit. Trondhjemitic rocks occur as minor segregations in the gabbroic rocks or as cross-cutting narrow dykes in the mafic dyke unit. The dyke unit is locally composed exclusively of subparallel metabasalt and metadolerite dykes; in places interdyke metagabbro and serpentinite screens are abundant. Well-preserved chilled margins are common and many dykes are split and intruded by younger dykes. The chemical composition of the serpentinites indicates derivation from harzburgitic-dunitic mantle peridotites. The gabbroic rocks, which represent obvious tholeiitic cumulates, show extreme differentiation similar to gabbros of the high-Ti ophiolites and gabbros dredged from present midocean ridges. In chemistry, the trondhjemitic rocks are close to granitic rocks from mid-ocean ridges and high-Ti ophiolites. The similar chemical composition of the dykes and lavas of the complex indicates that they are cogenetic, both being tholeiitic metabasalts with chondrite-normalized REE patterns ranging from slightly LREE depleted to slightly LREE enriched ones. Compared with modern rocks, the Jormua metabasalts most closely resemble (transitional) midocean ridge basalts. It is suggested that the Jormua complex and related mafic-ultramafic bodies in eastern Finland originated as divergent-margin ophiolites concomitantly with the break-up of the Archaean craton in the extensional stage of the Svecokarelian orogeny, which culminated in the opening of a major ocean after 1.97 Ga. The tectonic emplacement of these ophiolites into their present locations occurred in connection with thrusting during the compressional stage of the orogeny about 1.9 Ga ago.