Petrology of upper mantle and lower crustal xenoliths from the northwestern Spitsbergen

Upper mantle and lower crustal xenoliths in Quaternary basanites from the continental rift of northwestern Spitsbergen are classified, on the basis of rock chemistry and mineralogy, into peridotites and pyroxenites, which comprise a number of groups. It was demonstrated that nodules from the stratovolcano Sverre and the adjacent Halvdan and Sigurd diatremes are different. The rocks are thoroughly described petrographically, and their P-T conditions are determined. The pyroxenite rock groups crystallized at T = 965-1033 degrees C, and are higher temperature rocks than the peridotites (T = 877-939 degrees C). The revealed genetic conditions of the Spitsbegen nodules indicate that the peridotites are restites, and the pyroxenites show genetic links with picritoid melts separated from the mantle. The relationships between the peridotites and pyroxenites examined in composite xenoliths from Spitsbergen, compositional trends of the spinel, and geothermometry of the partial melting zones provide an insight into the evolution of the thermal regime beneath Spitsbergen. The mantle beneath the Spitsbergen was discovered to have an elevated geothermal gradient, which was caused by the opening of a modern continental rift and basalt formation during the closing stages of this process.