The composite xenoliths from Spitzbergen : Evidence of the circulation of MORB-related melts within the upper mantle

Category Petrology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Gregoire, Michel۱; Chevet, June۲; Maaloe, Svan۳
Holding Date 29 September 2008

The Sverrefjell quaternary volcano in Spitsbergen (Svalbard archipelago, Norway) contains numerous mantle peridotite xenoliths. Some of them are composite xenoliths showing spinel lherzolite wall rocks cross-cut by websterite veins. These two rock-types are characterised by similar major and trace element compositions of clinopyroxenes but also of major element compositions of olivine, orthopyroxene and spinel. The clinopyroxenes of both rock types mostly display globally upward convex or spoon shaped REE patterns with a systematic enrichment in La over Ce (CeN/YbN:0.72-1.32; SmN/YbN: 0.86-1.93 and LaN/CeN: 1.27-1.93, excepted for one sample (SV-69), in which clinopyroxene show a LREE pattern (CeN/YbN: 0.33-0.35). These characteristics point out that metasomatic processes appears to be the most reasonable origin to form the lherzolites/websterites association. In such a model websterites probably represents channels of focused melt percolation, and spinel lherzolites the host rock metasomatized by reactive porous flow. Based on the petrological characteristics of the studied xenoliths and on comparison with non composite mantle xenoliths from Spitzbergen and Patagonia and abyssal mantle websterites we could propose that the Spitsbergen mantle has underwent at least two metsomatic events:
(1) a sub-alkaline (tholeiitic) metasomatism followed by (2) an alkaline metasomatic event.