Mineral chemistry of volcanic sequences from Hole 917A, southeast Greenland Margin

Abstract

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 Fo 92.4 ) 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 (An 91 ) in the more primitive basalts to oligoclase (An 21 ) for the late crystallizing feldspar in the ophitic textures. Plagioclases from the Upper Series lavas are very poor in K 2 O. The presence of bytownite phenocrysts (An 86− 66 ) associated with sodic sieve-textured plagioclase (An 36−26 ) 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.

Low-pressure melting studies of basalt and basaltic andesite from the Southeast Greenland continental margin and the origin of dacites at Site 917

“…As was observed by Thy et al (1998), the primitive flows from Hole 917A in the Lower and Upper Series plot away from the experimentally determined multiply-saturated cotectic. This is consistent with the phenocryst assemblages found in these lavas (Larsen, Saunders, Clift, et al, 1994;Demant, 1998). Thy et al (1998) used inferred shifts in the locations of the olivine-plagioclase cotectic to suggest A B Figure 6.…”

“…The selected sample from the Middle Series (Section 152-917A-27R-2) is compositionally a basaltic andesite with 5.6 wt% MgO, while the two other samples (Sections 152-918D-108R-1 and 163-989B-10R-7) are basalts with MgO contents of 8-8.2 wt% ( Table 1). The basaltic andesite sample (Section 152-917A-27R-2) contains small amounts (~2%) of plagioclase (An 53-65 ) and trace amounts of augite phenocrysts in a groundmass of fine-grained pyroxene, plagioclase, and mesostasis (Larsen, Saunders, Clift, et al, 1994;Demant, 1998). The sample from the oceanic succession (Section 152-918D-108R-1) is aphyric with <1% plagioclase phenocrysts (An 72-86 ) and a fine-grained groundmass of plagioclase, augite, Fe-Ti oxide minerals, and mesostasis (Larsen, Saunders, Clift, et al, 1994).…”

Major element heterogeneity in the mantle source of the North Atlantic igneous province

“…Regarding continental £ood basalts, there is an additional caveat. Southeast Greenland samples, for example, include large, euhedral olivine phenocrysts with Fo 90X2À92X4 [54]. Thy et al [30] inferred that the high-MgO lavas (V18 wt% MgO) with such high-Fo olivines are a possible primary melt.…”

Geodynamic nature of magmatic sources of North-West Pacific: an interpretation data on isotope composition of Sr and Nd in rocks dredged at Stalemate ridge, Ingenstrem depression, and Shirshov Rise

“…2). Этот спектр составов отвечает вариациям состава оливина в пикритовых базальтах, образованных в различных геодинамических обстановках (Dietrich et al, 1981;Eggins, 1993;Demant, 1998;Tree, 2016). Пироксен в пикритовых базальтах ст.…”