Alain Demant2


During Ocean Drilling Program Leg 152, a 779-m-thick volcanic sequence was drilled at Hole 917A, on the feather edge of the seaward-dipping reflector sequence. The volcanic succession, which erupted under subaerial conditions, was subdivided into three series (Upper, Middle, and Lower) and 92 flow units. The Upper Series comprises picrites and olivine-phyric basalts. The Middle Series basalts are more evolved, and dacitic lavas and tuffs were erupted at the base of this series. Lavas from the Lower Series show a wider compositional range. A detailed mineralogical study of the different petrographical types shows that olivine is the principal phenocryst phase in the basalts, whereas clinopyroxene and plagioclase are the main components of the groundmass. High-Mg olivines (up to Fo92.4) with chrome-spinel inclusions are abundant in the picrites and olivine-phyric basalts. Fo-content of the olivine decreases as a function of the composition of the whole rock except in some aphyric basalts with ophitic textures where the olivine underwent further re-equilibration during cooling. Plagioclase exhibits a large compositional range from bytownite (An91) in the more primitive basalts to oligoclase (An21) for the late crystallizing feldspar in the ophitic textures. Plagioclases from the Upper Series lavas are very poor in K2O. The presence of bytownite phenocrysts (An86–66) associated with sodic sieve-textured plagioclase (An36–26) in the dacite (Unit 54) clearly demonstrates that this lava is a mixed lava. Pyroxenes are mainly clinopyroxene, but pigeonite and orthopyroxene were found in the matrix of some differentiated basalts with ophitic textures. The low Ti contents of the pyroxenes and the evolution trends are typical of tholeiitic lavas. The development of sector zoning in pyroxene microphenocrysts of the more differentiated basalts has a scattering effect on the distribution of the points on the pyroxene quadrilateral. Opaque oxides crystallize late and are mainly titanomagnetite. Ilmenite is also present in some lavas.

The volcanic succession drilled at Hole 917A presents many petrographical and mineralogical similarities to the flood basalts of the Scoresby Sund region in East Greenland. These thick basaltic sequences, which characterize the North Atlantic volcanic rifted margins, were emplaced during a short time interval during the phase of continental breakup. The Lower and Middle Series have evolved in crustal magma chambers. The frequency of picrites in the Upper Series emphasizes an easier access of more primitive magmas to the surface during the final phase of rifting.

1Saunders, A.D., Larsen, H.C., and Wise, S.W., Jr. (Eds.), 1998. Proc. ODP, Sci. Results,152: College Station, TX (Ocean Drilling Program).
2Laboratoire de Pétrologie Magmatique, URA CNRS 1277 and FU 17, CEREGE BP 80, 13545 Aix-en-Provence, Cedex 04, France. ADEMANT@ARBOIS.CEREGE.FR