Hydrothermal metamorphism of metabasalt xenoliths of the inner layered Gabbro series, Cuillins complex, Isle of Skye, Scotland

This thesis concerns metabasalt xenoliths in the Inner Layered Gabbro Series of the Cuillin Complex, Isle of Skye, Scotland. These rocks have been hydrothermally metamorphosed by meteoric waters leading to elemental, mineralogical and isotopic shifts in the xenoliths and host gabbros. In the first part of this thesis, the elemental and mineralogical shifts are documented and the metamorphic reactions that occurred are determined. This is accomplished by sampling across hydrothermal veins in the metabasalt xenoliths and comparing unaltered rock to altered rock. The three reactions are as follows 1 labradorite+0.237 olivine+.258c px+0.04ilmeni te+.474H 2O ⇒0.81bytownite +.164hornblende +.14H4SiO4 +0.015magneti te 2.25 olivine+1.75 H4SiO4aq ⇒1.00tal c+0.52magneti te+1.92H 2O+0.58 H2 2.82 plagioclase+7.37olivine+4.32 cpx+.04ilmenite+0.64Fe⁺⁺ +1.01H⁺+9.83 H2O ⇒ 2.06chlorite+ 2.10actinolite+ .13magnetite+ .94Na⁺+ .68Ca⁺⁺ ; The second part documents the shifts in δ18O of the xenoliths and host gabbros. It is shown that the magma intruded as a low δ 18O magma. The magma exchanged oxygen locally with the xenoliths that had previously interacted with meteoric water. The xenoliths were recrystallized by the hot magma. After the magma crystallized, the ILGS and xenoliths were again subjected to hydrothermal metamorphism by low δ18O meteoric water, especially along veins.; The final section of this thesis examines the mechanisms of oxygen isotope exchange. At high temperatures, exchange is modeled as diffusion from an intergranular fluid into spherical grains. At lower temperatures, growth of hydrous minerals caused the fluid to be excluded from the rock except along veins. Exchange along the veins is modeled in several different ways and the relative merits of each mechanism are discussed. The most favorable mechanism is diffusion from a thin layer of low δ18O material into the surrounding rock.