6:15 PM - 7:30 PM
[SVC47-P18] Emplacement and solidification process of off-axis large submarine lava field from the Oman Ophiolite
Keywords:Oman Ophiolite, V3, Large Lava Flow, emplacement process, chemical variation, geochemistry
Low-T hydrothermal alteration and weathering slightly modified the bulk compositions as indicated by moderately albitized plagioclase, completely replaced olivine by clay minerals and partially replaced titanomagnetite and augite by titanite and actinolite, respectively. However, HFSEs and REEs show mutual positive correlations and relatively good correlations with some major elements besides LILEs and Pb, indicating that these elements were less mobile and preserve primary characteristics. V3 flow is hawaiitic-mugearitic dolerite and has intermediate characteristics between OIB and E-type MORB. TiO2 shows a moderate increase with decreasing MgO from 8 to 5 wt%, and then decreases with the decrease in MgO down to 4 wt%, whereas Yb ranges from 2.12 to 4.56 ppm.
Whole-rock major and trace element variations through a stratigraphical transect at a distance of 8.7 km from the feeder dike indicate fractionation of augite, plagioclase and magnetite. By contrast, other V3 samples show highly scattered whole-rock compositions, suggesting internal mixing of variably differentiated magmas. Yb concentrations of the basal crust increase downflow to a distance of 4.5 km from the feeder dike, and then decrease further downflow with a spike at 7 km. Because the basal crust is the first lava that came to rest at that place, samples farther away from the feeder were extruded and emplaced later in the eruptive event. The downflow variations show extrusion of differentiated lava in the middle stage of the eruption and less differentiated lava in early and late stages.
Width/length ratio of groundmass plagioclase at 6 km from the feeder, where V3 flow is thickest, is higher in the Upper flow than in the Lower flow. Stratigraphic variations of Yb shows a decrease from the basal crust to a height of 26 m in the core, and then increase to a height of 83 m in the upper crust and decrease to the top of the Lower flow. The minimum Yb in the core is close to that of the latest lava shown by the basal crust. This can be reconciled with the model that the core is formed by the last intruded lava. On the contrary, the variation in Yb from the height of 83 m to the top of the Lower flow is correlatable to that of the basal crust at distances from 6 km to 8 km, suggesting that the upper crust consists of piled-up and welded lava lobes.