The Resfjell ophiolite fragment, southwest Trondheim region, central Norwegian Caledonides

Abstract

The Resfjell Complex of Meldal, southwestern Trondheim region, provides an almost ideal crosssection through the uppermost part of an ophiolite. The stratigraphically lowest rocks exposed are metagabbro wich are followed by a sheeted dyke complex, a transition zone composed of hydroclastites, pillow lavas and 10-30% dykes. These are succeeded by two lava units, lower and upper, which are separated by a sequence of volcaniclastic sediments in the eastern part of the area. The assemblage og ophiolitic rocks is exposed over a.c. 5 km N-S section. The entire complex is inverted with moderate to step dips, and has tectonic contacts towards both the south and the north. Subordinate, non-economic sulphide mineralizations occur in zones parallel to dykes in all units below the upper pillow lavas. Geochemical data show that the dykes in all units below the upper pillow lavas. Geochemical data show that the dykes and lavas are tholeiitic and of typical ocean floor or extensional basin type basalt. The upper pillow lavas, however, are somewhat more evolved. This is interpreted as due to extensive fractional crystalli- zation. Based on information from neighbouring ophiolite fragments, where subduction-related volcanic rocks are interbedded with tholeiitic metabasalts correlated with the Løkken ophiolite fragment, it is considered that the ocean floor type of metabasalts of Resfjell and nearby Grefstadfjell represent magma- tism of a very early extensional stage above newly subducted oceanic crust, or alternatively in a very wide marginal basin. It is tentatively suggested that these basalt were formed before the underlying mantle wedge became enriched in mobile LIL elements from the subducted oceanic lithosphere.