Origin and metamorphism of xenoliths in the Cuillin Center, Isle of Skye, Scotland

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Honors Thesis




Igneous rocks, Igneous rocks-Inclusions-Scotland-Skye (Isle of), Earth sciences, Geology-Scotland-Skye (Isle of), Igneous petrology, Xenoliths, Geosciences, Layered mafic intrusion, Isle of Skye Scotland


The Cuillin Center, a large mafic and ultramafic intrusion on the Isle of Skye, contains abundant fine-grained xenoliths within the Outer Bytownite Gabbros and Inner Gabbros. Xenoliths collected in several areas have been classified into six different types based on their locations, major and trace element geochemistry, textures, mineral proportions and compositions: East, South Ridge, Summit, Druim Hain, dikes within xenoliths, and Coruisk. These xenoliths are roughly basaltic in composition (45-49% SiO2) and have the same mineral assemblages in varying proportions of plagioclase, clinopyroxene, orthopyroxene, olivine, magnetite, and ilmenite. Magnetite is abundant in many xenoliths, and commonly forms rims along contacts between the xenoliths and the gabbro. This abundant magnetite suggests that the xenoliths were derived from heavily oxidized protoliths that differ from unaltered basaltic lavas, presumably because the lavas were hydrothermally altered to mineral assemblages containing chlorite and epidote. Most xenoliths show evidence of complete textural and chemical equilibration, with the exception of the Druim Hain xenoliths. The Druim Hain xenoliths were probably near roof of the intrusion and were metamorphosed for a shorter length of time, leading to incomplete equilibration. With the exception of the Coruisk xenoliths, most of the xenoliths contain abundant amygdules. These amygdules suggest protoliths of country rock lavas. These basaltic country rocks at the contact of the intrusion, and all of the xenoliths, are depleted in incompatible elements (Ba, Rb, K, P, Zr, Ti, and Y ) compared to the country rock dikes, cone sheets, and lavas further from the contact zone. Every xenolith was metamorphosed at temperatures well above the wet basalt solidus. The high temperatures and the depletion of incompatible elements are strong evidence for partial melting of the xenoliths and extraction of this melt into the magma body. Whole-rock strontium concentrations differentiate the mafic lavas and dikes of Skye into two types, which can be recognized among the xenoliths; at strontium concentrations >200 ppm are alkaline lavas and dikes, while at strontium concentrationslavas, dikes, and cone sheets. East and South Ridge xenoliths are alkaline, contain abundant amygdules, and have layering structures interpreted as flow boundaries, suggesting origins from the Skye Main Lava Series (SMLS). The Summit xenoliths are alkaline, contain abundant amygdules, and have exceedingly high proportions of olivine (>32%), suggesting origins from a picritic lava sequence within the SMLS. Druim Hain xenoliths are tholeiitic, and contain abundant amygdules, suggesting an origin from lavas, similar to, but more evolved than, the Preshal More Basalts; these more evolved lavas have since been eroded away from the country rocks. The tholeiitc and alkaline dikes within xenoliths have tholeiitc and alkaline dike protolithss, respectively. Coruisk xenoliths do not share the same traits as xenoliths with lava protoliths; they lack amygdules, and have Ca-rich plagioclase with composition (An83) similar to those of the layered troctolites of the Cuillin, suggesting a fine-grained troctolite protolith. Xenoliths with lava protoliths follow the same stratigraphy seen in the country rocks, suggesting a lack of movement within the magma body. Coruisk xenoliths are distributed through a layer that runs for several kilometers across the Outer Bytownite Gabbros, implying an anomalous event within the history of the Cuillin Center’s formation. These observations support the model of formation of the Cuillin Center by repeated emplacement of small-volume magma sheets or sills into the intrusion, with the injection of magma generating little displacement of the country rocks, except in the case of a major intrusive episode that spread the Coruisk xenoliths over distances of kilometers within the intrusion.




viii, 91 pages : color illustrations, maps. Honors project, Smith College, 2016. Includes bibliographical references (pages 73-77)

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