2:15 PM - 2:30 PM
[SCG54-08] Fine-grained gabbroic layers in the lower and middle crustal sections of the Oman Ophiolite (Holes GT1A and GT2A), ICDP Oman Drilling Project
Keywords:Layered gabbro, Mid-Ocean Ridge, Sheeted sill model, Gabbro glacier model
Both Holes GT1A and GT2A are mainly composed of olivine gabbro and olivine-bearing gabbro, with minor amount of other lithologies. Igneous layering caused by modal and grain-size variations was observed in the both drilled cores.
Fine-grained (about 0.5 mm in size) gabbroic layers are interbedded into medium-grained gabbros in some sections of the GT1A (e.g., GT1A_45Z-1, 47Z-1, 47Z-2 and 48Z-3). The contact between them is microscopically sharp. Both the modal composition and grain size change discontinuously at the contact. In the contact boundary, an interfingering texture between clinopyroxene of the fine-grained olivine gabbro and olivine of the medium-grained olivine gabbro is observed. The original modal olivine contents of the fine-grained gabbroic layers are 4 to 5 vol%, generally much lower than those of the surrounding medium-grained olivine gabbros.
We also found similar fine-grained gabbroic layers in Hole GT2A. Some of the fine-grained gabbros contain much larger plagioclase crystals than the matrix crystals. The large plagioclase “phenocryst” commonly displays a distinct zoned structure. The fine-grained gabbroic layers may suggest repeated injections of basaltic liquids into the oceanic crust.