The role of crustal contamination in magma evolution of Neoproterozoic metaigneous rocks from Southwest Svalbard

Abstract

Late Neoproterozoic metavolcanic rocks occur along the Southwest coast of Svalbard. The protoliths of the metavolcanic rocks from Wedel Jarlsberg Land and Nordenskiöld Land are mainly diabase, basalt and felsic tuff of tholeiitic affinity associated with continental magmatism. We investigate the magma evolution of the metavolcanic rocks paying particular attention to processes of magma-crust interaction and assess potential sources of crustal contamination. These goals are achieved by employing trace element geochemistry, as well as Sr and Nd isotope geochemistry. Metavolcanic rocks from the South (Orvindalen and Werenskiöldbreen) have higher LREE, LILE and Th compared to rocks from the North (Nordenskiöld Land), which are relatively enriched in Sr. Incompatible element ratios like Th/La, Th/Nb, La/Nb, Th/Yb and Nb/Yb also decrease from South to North. The 143Nd/144Nd(635 Ma) ranges from 0.511396 to 0.512356 and increases from South to North. For Sr isotopes, the metavolcanic rocks show a wide range, however in the South we observe 87Sr/86Sr(635 Ma) of 0.70407–0.73043 and in the North 87Sr/86Sr(635 Ma) of 0.70410–0.71028. Energy Constrained – Assimilation and Fractional Crystallization (EC-AFC) modelling indicates that the extent of magma contamination is highest in the South. Additionally the modelling suggests fractional crystallization and assimilation of granulite or amphibolite followed by shale for the metavolcanic rocks in the South and for the North mixtures of carbonate and shale contributed. This geographical pattern of assimilation reflects the upper crustal metasedimentary sequences, where phyllites are common in the South (Orvindalen and Werenskiöldbreen) and carbonates are more common in the North (Nordenskiöld Land). Density contrasts and impermeable layers within the continental crust would likely have acted as barriers to ascending magma, forcing it to stall and providing opportunities for magma-crust interaction.