Geochemistry of Precambrian basic igneous rocks between St. Jonsfjorden and Isfjorden, central western Spitsbergen, Svalbard

Abstract

Two types of basic igneous rocks have been mapped in the middle Hecla Hoek succession in the area between St. Jonsfjorden and Isfjorden, central western Spitsbergen: the metadiabase-gabbros from the calc-argillo-volcanic formation and the Trollheimen volcanics from the quartzite-shale formation; both formations are older than the Eocambrian tilloid formation. The former has diabasic and gabbroic textures and occurs as thin discontinuous lensoid masses, while the latter exhibits definite extrusive structures with large amounts of pyroclastics and tuffs and relatively small amounts of solid lavas with abundant amygdules. Both basic rocks have been metamorphosed under the greenschist facies conditions, characterized by actinolite-epidote-biotite-albite assemblage. The metadiabase-gabbros are moderate-high Fe tholeiites and the Trollheimen volcanics are mainly Na-alkaline, accompanied by a small amount of calk alkaline rocks. The immobile minor and trace element contents indicate that the metadiabase-gabbros are oceanic, similar to the MORB, while those of the Trollheimen volcanics show non-oceanic chemical characteristics and occur in shallow marine, shelfshelfedge sediments. The calc-argillo-volcanic formation reveals a shallow marine sedimentary environment and this does not conform with the oceanic nature of the metadiabase-gabbros, occurring in the formation. To overcome this disagreement, an idea of changing tectonic position is postulated to bring a shallow marine sedimentary regime onto a mid-oceanic tectonic zone. This hypothesis supports the concept of active consuming margin tectonics of the Svalbard Caledonides and explains, for example, the occurrence of the high-P metamorphic rocks in the western part of the present area.